Understanding slagging and fouling during pf combustion
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lEA COAL RESEARCH Understanding slagging and fouling during pf combustion
Transcript of Understanding slagging and fouling during pf combustion
lEA COAL RESEARCH
Understanding slagging and fouling during pf combustion
Understanding slagging and fouling in pf combustion
Gordon Couch
IEACRn2 August 1994 lEA Coal Research, London
Copyright © IEA Coal Research 1994
ISBN 92-9029-240-7
This report, produced by IEA Coal Research, has been reviewed in draft form by nominated experts in member countries and their comments have been taken into consideration. It has been approved for distribution by the Executive Committee of IEA Coal Research.
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Abstract
This report covers the basics of boiler design, the factors which affect it, the nature of the ash-forming materials present and their transformations in the boiler. The mechanisms of deposition and the properties of the deposits formed are discussed. A number of empirical indices which attempt to predict the slagging or fouling propensity of a coal are described. Work is reported on advanced analytical techniques for both raw coal and for the intermediates and deposits formed. These results can be applied both to improving the capabilities of existing indices, and to computer models of the complex situation inside the boiler. Other methods of studying and predicting the behaviour of coal in a boiler include pilot-scale tests, and drop tube furnace work. The various methods of reducing boiler deposition are discussed, including optimising the sootblowing cycle and more extensive monitoring of the boiler operating conditions to give early warning of problems. It is concluded that general predictions of the behaviour of coals in a boiler are not yet secure, although considerable progress has been made. Both designers and operators can be more confident when trouble-shooting, or making technical assessments or decisions about coal-fired boilers, as a result of the increased understanding of the behaviour of the inorganics during and after combustion.
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Contents
List of figures 7
List of tables 9
Acronyms and abbreviations 11
1 Introduction 13
2 Coal-fired boilers 15 2.1 Boiler design 16
2.1.1 A comparative study 20 2.2 Boiler configurations 23 2.3 What happens inside a boiler'? 23
2.3.1 The effects of changes in operating conditions 24 2.4 Combustion 25
2.4.1 Combustion equipment 26 2.5 Ash deposition 26
2.5.1 The effects of deposition 27 2.5.2 Corrosion and erosion 28
2.6 Investigative methods 29 2.6.1 Laboratory techniques 29 2.6.2 Pilot-plant rigs 30 2.6.3 Full-scale boiler trials 31
3 Ash formation 34 3.1 Ash-forming materials in coal 34
3.1.1 Organically-associated material 36 3.1.2 Minerals 36
3.2 Distribution of minerals in pf 37 3.3 Characterisation of the inorganics in coal and ash 38
3.3.1 New and developing techniques 39 3.3.2 Computer controlled scanning electron microscopy (CCSEM) 40 3.3.3 III situ ash characterisation 41
3.4 Inorganic transformations 41 3.5 Ash formation mechanisms 46
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4 Deposition processes and deposit properties 50 4.1 Deposition mechanisms 50
4.1.1 Inertial deposition 50 4.1.2 Condensation 52 4.1.3 Thermophoresis 52 4.1.4 Chemical reaction 53 4.1.5 Particle 'stickiness' 53
4.2 The requirements for deposit formation 54 4.3 Slagging 55 4.4 Fouling 57 4.5 Deposit properties 59
5 Predictive indices, modelling and scale-up from tests 62 5.1 Predictive indices 62 5.2 Boiler modelling 63
5.2.1 International Flame Foundation work 68 5.2.2 The Busbar model 69 5.2.3 CQE/CQIM 70
5.2.4 Advanced indices 72 5.3 Scale-up from tests 76
6 Reducing boiler deposition 78 6.1 On-line cleaning techniques 78
6.1.1 Optimising sootblower use 80 6.2 Coal supply and coal cleaning 81
6.2.1 The effects of coal cleaning 8] 6.2.2 The use of models and of advanced indices 83
6.3 Boiler monitoring and changing operating conditions 83 6.4 Use of additives 84 6.5 Boiler design 85 6.6 Dealing with low NOx combustion conditions 86
7 Case studies 87 7.1 Experience in the Netherlands 87 7.2 Utility use of advanced techniques to assess alternative coal supplies 87 7.3 Use of pilot-scale tests to predict ash deposition impacts on commercial boiler performance 89 7.4 The use of low rank coals in the USA 90 7.5 Neyveli lignite use in India 9] 7.6 Monitoring fouling and optimising sootblowing at Teruel, Spain 92 7.7 South African experience at Kriel 93 7.8 Brown coal use in Australia and the fouling problems at Loy Yang 95 7.9 Clinker formation at the Bayswater plant, NSW, Australia 100
7.9.1 Burner modelling 100 7.9.2 Copper oxychloride dosing 101 7.9.3 Clinker observation 101 7.9.4 Ash adhesion tests 101
7.10 Experience at the Comanche power station, Pueblo, CO, USA 101
8 Discussion 104 8.1 Empirical approach 104 8.2 Mechanistic approach 105 8.3 Computer models and advanced indices 105
9 Conclusions 107
10 References 110
Figures
The age profile of boilers in selected countries/areas 15
12 Relative emissivity of an ash deposit from
13 Regression analysis of the ash chemistry
2 Arrangements of heat transfer surfaces and regions of slagging and fouling in typical boilers 17
3 Comparative boiler sizes for burning different coals 18
4 Effect of flue gas recirculation, through the convection section of a boiler 19
5 Circulation system and typical heat flux profile in a boiler 19
6 Typical boiler configurations 22
7 Generalised time temperature cycle 23
8 Typical cyclic changes in heat flux between sootblowing operations 25
9 Organic coal with its associated inorganic constituents 34
10 Coal breakage and mineral matter distribution 37
II Critical temperature points of the ash fusion test 38
a western US coal 10 and 45 minutes after deposition 42
of size and gravity fractions of eight eastern US coals 43
14 Thermal decomposition of major coal rillnerals 44
15 Equilibrium phase diagram FeO-Ah03-Si02 44
16 Phase diagram of changes in US eastern coal under reducing conditions 45
17 Mechanisms involved in fly ash formation 47
18 Ternary diagrams showing the redistribution of inorganics during lignite combustion 48
19 Potential fouling and slagging mechanisms 51
20 Inertial impaction mechanism on a tube 52
21 Condensation on a tube 52
22 Thermophoretic deposition on a tube 52
23 Comparison of coal ash particle size distributions 53
24 Processes contributing to ash deposit growth and removal 54
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25 Schematic of slag build-up 56
34 Comparison of the predicted and measured deposit
36 CQE/CQIM validation tests at Watson 4 unit showing the variation
39 PSI PowerServe slagging model. Schematic
44 Distribution of iron in the non-magnetic phases of raw and washed
48 Typical results using an optical temperature monitor,
49 Typical output from infrared imaging cameras
51 Heat flux through waterwall panel for two coals
56 Ash deposition chemistry in Loy Yang unit 1, as at November 1986 98
57 The influence of ash fouling on boiler operation,
58 Fold-out diagram of Loy Yang furnace showing
59 VDU display at Comanche power plant showing
26 Structure of a fouling deposit 58
27 Tube spacing and arrangement 58
28 Deposit build-up on tubes 59
29 Variation in the thermal shock parameter at different porosity values from 0 to 50 vol% 60
30 Various mechanisms of heat transfer 60
31 Expected trends in deposit properties during growth 61
32 Modelling of the relative fouling tendency of a coal at MIT 66
33 Schematic of the Sandia National Laboratory (ADLVIC) model 67
compositions near the furnace exit of a 600 MWe unit 67
35 The role of the Busbar model in coal purchase evaluation 70
in FEGT at different excess air levels, and the differences between the coals used 71
37 C-QUEL information flow 72
38 Eutectic temperature distribution for two different coal ashes 73
showing the relationship between the sub-models 74
40 Comparison of furnace wall heat absorption decay under different operating conditions 77
41 Antelope Valley No 1 station, USA, showing position of sootblowers and tube layouts 79
42 Details of heat flux sensor in a locally thickened tube wall section 80
43 Graph showing trial results, comparing optimised sootblowing with standard cycle 80
Kentucky No 9 coal and Texas lignite, from various locations in the pilot plants 82
45 Fold-out screen showing deposits on the furnace walls 83
46 Changes in heat transfer in the radiant superheater 84
47 The build-up of fouling deposits in the economiser 84
showing the effect of sootblowing on the FEGT 88
monitoring wall emissivity showing the effect of sootblowing 88
50 Deposit collection efficiency for various bituminous coals from drop tube furnace work 89
in the combustion engineering pilot unit 90
52 Furnace heat flux measurement principles used at Teruel, 92
53 Temperature distribution in Kriel boilers before and after burner modifications 94
54 Ash deposit chemistry for Loy Yang coal, fired on the pilot combustor 96
55 Observed fouling rate on test bank 1 in the pilot combustor 97
Loy Yang unit 1 leading to November 1986 outage 99
impaction points of solid inert particles coming from the main burners 100
changes in surface cleanliness in various parts of the boiler 102
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Tables
Basic boiler design parameters used by German manufacturers in the early 1980s 19
2 Main specifications of the coals used in the boiler design study 20
3 Basic boiler dimensions for firing the six coals 21
4 Design furnace exit gas temperature for the six coals 21
5 Pf sizes for coals of different rank 23
6 The effect of varying coal rank 24
7 Mineral matter in coals 35
8 Coal ash analyses 37
9 Thermal expansion coefficients for selected materials 55
10 Summary of coal ash indices 64
11 A comparison of slag and coal ash chemistry 64
12 Coal composition and advanced fouling indices developed by EERC 75
13 Cape Breton coal analysis 89
14 Top ten US steam electric plants in 1990, ranked by unit operating cost 91
15 Effectiveness of remedial measures for controlling ash deposition 91
16 Composition of Neyveli lignite 'laboratory' ash 92
17 Coal specification for boiler design at Kriel 93
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Acronyms and abbreviations
ASTM ATRAN I BHEL BSE CCSEM CQE CQIM EERC EPRI FEGT FTIR IR LEADER MIT pf SEM SNL
American Society for Testing and Materials Ash Transformations Version I Bharat Heavy Electricals Ltd back-scattered electron/s computer controlled scanning electron microscopy Coal Quality Expert Coal Quality Impact Model Energy and Environmental Research Centre Electric Power Research Institute furnace exit gas temperature Fourier transform infrared emission spectroscopy infrared low temperature engineering algorithm of deposition risk Massachussetts Institute of Technology pulverised fuel scanning electron microscopy Sandia National Laboratory
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1 Introduction
All coals have a significant content of ash-forming inorganic material which cannot be economically removed before combustion. This amount can range from below 3% in a clean low ash feedstock, to over 40% in some low grade coals. Most commonly, the ash-forming material represents between 10 and 25% of the feed coal.
Uncontrolled or unexpected deposits on the heat transfer surfaces in and around the boiler can interfere with operation, and cause unplanned shut-downs or reduced output and efficiency. The deposits are derived from the mineral matter in coal and its other inorganic components. They can be difficult to remove, and major incidents of internal damage due to fused ash material falling to the bottom of the boiler have occurred. The deposits interfere with heat transfer. With partial blockage between tube banks, increased gas velocities elsewhere are often associated with erosion. Corrosion may also occur underneath a developing deposit. All these factors affect the efficiency and availability of a plant for electricity generation and hence the power cost.
While boiler manufacturers design their equipment to minimise the effect of deposit build-up, problems can arise when the behaviour of the inorganic matter in the boiler environment is insufficiently understood, when the coal feed is changed, or when emissions standards and regulations require changes in operating conditions. Relatively few plants operate for long periods with a coal feedstock from the same source, which is of consistent quality and gives trouble-free operation.
The main deposits are described as being either slagging or fouling, and various definitions are used. For the purpose of this report, slagging refers to deposits within the furnace, in areas which are directly exposed to flame radiation such as the furnace walls and some widely spaced pendant superheaters. Fouling refers to deposits in those areas not directly exposed to flame radiation, such as the more closely spaced tubes in the convection sections of the boiler.
Slagging takes place in the hottest parts of the boiler, while fouling takes place as the flue gases and suspended fly ash cool down.
The behaviour of mineral matter is difficult to predict under the complex conditions which arise in a coal-fired boiler and its associated heat transfer equipment. The analysis of ash-forming components and tests to establish its behaviour, have in the past been carried out under conditions that are very different from those which occur in various parts of the boiler. Numerous empirical indices have been developed for assessing the slagging or fouling potential of a coal, and they have been widely used. While the indices covering slagging and fouling factors have been widely quoted, they frequently give misleading results, and are, at best, somewhat umeliable. Hence there has been a substantial effort to establish more reliable methods of assessing slagging and fouling tendencies; to understand what causes ash deposition, and to develop test methods that more closely parallel the conditions in the boiler.
Recent growth in the international trade in thermal coals has meant that power stations in Europe have used feedstocks from Australia, Canada, China, Colombia, Indonesia, Poland, South Africa, the USA and countries of the fonner USSR, among others. Since they had previously burned exclusively local European coals, adjustments have been needed to boiler design or to operating conditions to cater for the different inorganics present in the traded coals. The range of internationally traded coals is available to any coal importer, and there is forecast to be substantial growth in demand around the Pacific Rim, among other places (Daniel, 1991). In addition, in the USA, there is pressure to meet environmental standards by using western (low sulphur, lower rank) coals to replace some high sulphur bituminous coals from the eastern mining areas.
There has been considerable effort in research and development into the reasons for ash deposition and ways of
13
Introduction
minimising the effects. Overviews have been presented by Reid (1983); Raask (1983); Bryers (1992); Jones and others (1994).
This report considers the basics of boiler design and operation; the nature of ash-forming materials in coal and the transformations that take place in the boiler. The processes of deposition are discussed, and investigative methods are assessed. Laboratory- and pilot-scale testing can provide useful information, but it is not possible to simulate all the conditions of a full-size boiler on a small scale.
New work is attempting to model the behaviour of ash-forming materials in the boiler in an attempt to improve the predictive capability of traditional indices. Various routes are available for reducing slagging and fouling in boilers, either by avoidance, or by improved control of operating conditions.
The difficulties involved in predicting ash behaviour and the changes in patterns of coal use, highlight the need to understand the mechanisms involved in ash deposition, so that ash-related problems can be assessed accurately. The object of this report is to outline progress.
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2 Coal-fired boi lers
Worldwide, there is approximately 900 GWe of installed capacity for coal-fired generation. Most of the plant is listed and detailed in World coal-fired power stations (Maude and others, 1994a, b, c). There are about 4000 pulverised coal-fired boilers with over 50 MWe capacity designed at different times. During the past 20-30 years, boiler size has increased, to a maximum of over 1000 MWe. Many large units built from the mid-1970s onwards have been around the 500-600 MWe size.
Many boilers were designed and built during 1960-1980, although some older plant is still in operation. This means that different design criteria have been used. With the application of rigorous emission controls on new plant, there
250 -.
200
III... ~ 150 • Japan.8
Australia'0 ... D EuropeQ) ..c
-E 100 USA :::l Z
50
o •
is an incentive in many countries, to extend the life of operating units. Some have been modified since construction, for example to reduce NOx emissions. The age profile of boilers in selected countries is shown in Figure 1.
There is considerable growth in coal use for power generation in developing countries, such as China, India, Indonesia and elsewhere around the Pacific rim. In addition, new plant is needed to replace older and smaller units in most major coal-using countries. In the USA, for example, there are over 300 boilers which were built during the 1950s.
New capacity was being built around the world, at the rate of a little over 10 GWe/y, in the early 1990s. This consists of
.J0 J, .. ..I ...JB .l ~] .~ 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80-84 85-89 90+
Commissioning year
Figure 1 The age profile of boilers in selected countries/areas (Maude and others, 1994a,b,c)
15
Coal-fired boilers
about 30 boilers/year, which is less than I% of the total number in operation.
A wide variety of coals are fired, from high grade bituminous coals with low ash and sulphur contents, to lignites with as much as 60% moisture, or sometimes 40% ash, and possibly with a high sulphur content. Most coals are used within the country where they are mined, and only about 10% of coal is traded internationally. Of the 10%, nearly half is coking or metallurgical coal, although the amount and proportion of steam coal being traded is rising quite sharply (Daniel, 1991).
2.1 Boiler design Boilers are designed and tested to meet performance guarantees based on a 'specification' coal; to operate with maximum efficiency and availability, and with minimum maintenance.
Boiler design has several objectives, and allows a number of methods for dealing with the necessary adjustments to the relative heat transfer rates in the radiant and convective sections of the boiler to meet the steam specification required for optimum turbine operation. There are limits imposed by the mechanical design and materials of construction used, by the capacity of pulverisers and of other equipment.
The objectives include:
safe and reliable operation; to produce the specified amount of steam to run the turbines; to operate economically, generally over a period in excess of 25 years; to have a good turn-down ratio, which facilitates load following when needed; to operate predictably, with a system for planned maintenance and shut-downs; to use different coals without difficulty.
Of increasing importance for new plant are the additional requirements that boilers should produce the least amount of pollutants possible, and be able to burn a range of coals. With the incorporation of flue gas desulphurisation and NOx
reduction equipment, boiler plant has to operate with more restraints. Operational flexibility is considerably reduced, as it is necessary to control operating conditions within tighter limits. There is a detailed discussion of the many factors involved in Stultz and Kitto (1992); Singer (1991).
Ultimately the heat released from the fuel in the boiler is transferred in the steam to the turbine. It is then converted into electrical energy. The gas side consists of the combustion chamber where radiant heat transfer dominates, followed by tube bundles where the flue gases cool down and convective heat transfer is more important. On the steam side, the feed water at high pressure passes first through the economiser, where it is heated to close to boiling point. Most of it is converted to steam in the tube walls of the furnace. The saturated steam is then superheated in tube banks (normally two). Water may be injected to control the final steam temperature to the design value, and this adjustment is
called attemperation. The superheated steam is expanded in a high pressure turbine.
Exhaust steam from the high pressure turbine is taken back to the boiler where it is reheated and returned to pass through a separate turbine section. Modern systems often include more than one reheat stage. Tube banks are referred to in terms of their function, ie superheating or reheating.
There are various stages in the development of a functional design for a large power station boiler:
the required electrical output, and flexibility of operation are agreed; steam turbine conditions and boiler feed water temperature are specified, and on this basis the heat input required from the coal is calculated; the designer then estimates boiler efficiency, based on the flue gas temperature leaving the air preheater; using purchasing specifications, or the results from coal deposit exploration, a specification will be agreed. This should include acceptable ranges, and the typical analysis will normally become the 'performance coal' providing the basis for the boiler performance guarantee by the contractor; data on grindability and moisture content will determine the type, size and number of pulverisers required, and the design of any flue gas recirculation circuit. With low rank coals the standard grindability tests are less reliable; burner design depends on a number of factors including, coal reactivity, ash characteristics, moisture content and the need to reduce NOx formation. It will depend on the basic boiler configuration being used, on the air volume needed for complete combustion, together with the proportion of excess air; the combustion system will require tight control of the introduction of fuel and air to achieve high burnout; minimal deposition; controlled gas temperatures; low NOx formation and minimal furnace tube damage. boiler furnace dimensions are based on the heat transfer area needed, taking the various slagging and fouling indices into account. These also affect the burner locations and design heat inputs in various parts of the boiler. The key figures are maximum heat release rates per unit area of the furnace wall and per unit volume of the burner zone, both based on past experience with similar coals.
Furnace height depends on minimum clearance and on the maximum allowable furnace exit gas temperature (FEGT) consistent with achieving adequate burnout. The tube arrangement and materials of construction depend on the FEGT, the mass flow and amount and type of ash. There must be adequate tube spacing and sootblowing capability to prevent the excessive accumulation of ash anywhere in the boiler, while maintaining high rates of heat transfer and limiting both erosion and corrosion.
During the 1970s, boiler manufacturers recognised the trend towards the wider use of internationally traded coals with their variable characteristics. They recognised that even with 'conservative' designs (including relatively low heat release
16
Coal-fired boilers
(a) Tower boiler configuration-------"""T'"----__ convective surface for water heating
9
to air preheater convective heat ESP and stack exchange surface for superheating and reheating steam
combustion H-__ chamber lined
with tubes for raising steam
burners
, ash
Main locations of ash deposition: 1 Ash hopper (bridging) 2 Ash slope (mechanical damage) 3 Burner (eyebrows) 4 Wall slag 5 Division wall slag - where approriate 6 Platen (birdnesting) 7 Convection bank (bonded deposits) 8 Economiser (bonded deposits) 9 Air heater (gas inlet fouling)
(b) Conventional pf-fired boiler configuration
convective heat exchange surface for superheating and reheating steam
combustion chamber lined with tubes for raising steam
\~_9=--_~
to air preheater ESP and stack
burners
fouling, slagging
ash
Figure 2 Arrangement of heat transfer surfaces, and regions of slagging and fouling in typical boilers
rates in the furnace, low flue gas velocities and low FEGTs), not all coals could be fired equally successfully. This was due to the variable mineral matter, volatiles and macerals in the different coals. The development of a boiler design in which all risks of uncontrollable deposition were eliminated would be prohibitively expensive, even if theoretically possible.
Key data for boiler design are the specific coal properties, in terms of the range of heating values, combustion characteristics and ash characteristics. Sulphur content affects the need for downstream removal equipment.
Furnace size, fuel preparation, firing systems and the amount of heat transfer surfacl? required in the boiler are all fuel-dependent. In particular, furnace size depends mainly on the heating value of the coal, its grinding characteristics, and an assessment of ash behaviour in terms of its slagging and fouling potential. Newer boiler designs also take into account the formation of NOx and of unburned volatiles. Staged firing systems, to achieve lower NOx levels, are associated with lower flame temperatures and reduced burning rates. Hence even larger combustion chambers are needed and/or a consistently finer pulverised feed.
In order to understand the effects of ash deposition in boilers in the form of slagging and fouling deposits, it is necessary to consider basic factors affecting their design and operation. The regions where slagging and fouling occur in boilers are shown in Figure 2.
The effects of coal characteristics on boiler heat transfer and hence on steam conditions are complex. When the coal composition changes, the radiative characteristics of the reacting gases and of the suspended particles change, along with the characteristics of wall and tube deposits. The properties of the deposited ash which have the greatest effect on heat transfer in the combustion zone are its emissivity and thermal resistance.
A large boiler of 500 MWe capacity requires, typically, some 7000 m2 of heat transfer surface in the boiler wall tubes and a further 30,000 m2 in the superheater and reheater tubes (derived from Reid, 1983). The figures depend on the heating value of the coal, the deposition characteristics of its ash, and on the steam conditions. Coals with difficult slagging or fouling characteristics need to be fired in larger boilers where more of the heat transfer takes place to the boiler walls and the FEGT can be controlled to a figure at the low end of the range.
A well established view of the effect of coal type on boiler size is shown in Figure 3. It illustrates the increased volume required when firing a high fouling lignite with a high moisture content. Where the feed moisture content is over 45% it is commonly pre-dried using recirculated flue gases. This increases the mass flow in the combustion area so that a larger volume is needed (see Couch, 1989). In other situations, flue gas is recirculated through the convection section of the boiler to increase gas velocity and hence increase heat transfer (see Figure 4). With staged firing systems, an additional 5 to 10% of height is needed in the combustion chamber.
17
• •
Coal-fired boilers
(a) The influence of sla99in9 potential
+ Intial sootblowers 1 • future sootblower
• • • + • + • +
66.4m
• • • • • • • + • + • +
+ • + • + • • + • + • +
+ • + • + • • + • + • +
Lignite severe slagging
46.9m
I
Bituminous low/medium slagging
Subbituminous high slagging
(b) The effect of coal rank and sodium-in-ash
Plan area W
= 1.00
0
r HL
h
-t__,,-,, Medium volatile
bituminous
PlanPlanPlan area
Plan area 1.29 Warea 1.26 Warea 1.08 W 1.16 W = = 115
1.060
r 115 h
L 1.05 H
1 \ High volatile
bituminous or subbituminous
1.63= 156= 1.25
1080 1.240 1.260
r 1.17 h
l 1.07 H
U Low sodium
lignite
r 1.52 h
l 1.30 H
Medium sodium lignite
r 2.1 h
l 1.45 H
1 \ High sodium
lignite
Figure 3 Comparative boiler sizes for burning different coals (Singer, 1991; Stultz and Kitto, 1992)
Deposits are removed on a regular basis by sootblowers. These are mechanical devices used periodically. They involve the insertion of a tube into the boiler. On the end of the tube (or lance), there are nozzles to direct the cleaning medium, and the tube rotates. In principle, knowledge of the amount and type of ash allows the number and location of sootblowers to be decided. Ash behaviour, and the tendency to form slagging and fouling deposits, and whether they can
be easily removed are key factors in determining a design. As discussed in this report, it is not currently possible to anticipate these aspects precisely, and there are many uncertainties associated with boiler design, and particularly with ash behaviour.
The deposition of ash on heat transfer surfaces affects gas side conditions and therefore those on the water-steam side.
18
Coal-fired boilers
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reheater
pr'lfnary superheater
o 10 20 30 40 50 60
Gas recycle, %
Figure 4 Effect of flue gas recirculation, through the convection section of a boiler (Stultz and Kitto, 1992)
Thus the design of water-steam tubes is affected by the slagging and fouling properties of a coal, and particular configurations may restrict the range of coals which can be used. Two circulation systems are commonly used. These involve natural circulation, and forced or pumped circulation. In some cases a combination of these approaches is used.
The most effective way to produce high pressure steam is to heat water or steam in relatively small diameter tubes. As heat is added, steam forms, and the water-steam mix is of lower density than that of water alone, so it tends to rise in the tubes. At the top of the unit there is a large steam drum for separating the water and steam. In forced circulation circuits, a pump assists the flow through the system. At pressures below the critical point of 21.8 MPa, a drum is necessary, but above that pressure, there is no phase distinction between steam and water.
Natural circulation is most effective where there are large density differences between the vapour and liquid. Forced circulation designs may be used over the whole range of
steam drum and separators
feedwater
furnace tubes
steam-water mixture
downcomer L
steam-free subcooled
water
supplies
Figure 5 Circulation system and typical heat flux profile in a boiler (Stultz and Kitto, 1992)
pressures. Natural circulation can be used at pressures up to about 19.7 MPa (design), equivalent to 17.7 MPa (operating), as above this level, the system becomes too large to be cost effective (French, 1993). The various designs are discussed by Stultz and Kitto (1992); Singer (1991).
A typical heat flux profile in a boiler is shown in Figure 5, showing the intense level of heat transfer in the combustion zone. A natural circulation system is also shown.
The advantages of a once-through boiler design incorporating forced circulation were highlighted by Kautz (1983). Basic design parameters for the once-through system are set out in Table 1. These were further developed for future designs to limit the burner heat release to 0.7-0.8 MW/m2. In addition, a minimum distance of over 15 m is needed between the upper burners and the first pendant heat exchangers to ensure adequate bum-out of the particles. It is important to avoid contact between burning coal particles and heat exchange surfaces, and to avoid reducing conditions near the boiler walls. This can usually be achieved with a sufficiently large plan area where the heat release is below 5.4 MW/m2.
Table 1 Basic boiler design parameters used by German manufacturers in the early 1980s (Kautz, 1983)
Furnace Burner region Furnace volumetric FEGT, DC Range of volatiles heat release, MW/m2 heat release, MW/m3 in the coal, %
Corner-fired 1.10 0.12 1230 25-38 Opposed-fired 1.25 0.084 1250 19-33 Opposed-fired with dividing wall 0.837 0.089 1150 28-38
19
Coal-fired boilers
Steam temperatures were limited to just below 550°C to reduce fireside corrosion. The final superheaters and reheaters were arranged in an area where flue gas temperatures are about 1000°C, protected (or preceded) by tube bundles containing cooler steam and hence with lower surface metal temperatures.
Even with these design figures, experience showed that in a large comer-fired boiler the coal range was limited to 6-10% ash and 25-38% volatiles. After 5000 h operation, there was what was described as slight slagging on the furnace walls, but the FEGT had risen from 1230 to 1400°C. Additional wallblowers were reported to have solved this problem.
2.1.1' A comparative study
In a study supported by The Electric Power Research
Institute (EPRI) in the USA, three boiler manufacturers were asked to prepare outline designs for a series of hypothetical 500 MWe base-load units firing each of six different coals. Detailed coal analyses were provided (see Table 2). The comparative results are shown in Table 3. The base case was taken as coal A, which was a non-slagging eastern US bituminous coal. The relative FEGT figures, shown in Table 4, are expressed in relation to the design figures for each manufacturer for the base case conditions.
The results illustrated the fact that different manufacturers used markedly different criteria for designing important boiler characteristics. The approach used was essentially simplistic, in that it was assumed that the units would fire only the design fuel. At the manufacturers suggestion the results were expressed in relative terms, but the baseline fuel chosen was a relatively trouble-free coal for which it would
Table 2 Main specifications of the coals used in the boiler design study (Barrett, 1990)
Type of coal
Source A B C D E F
Proximate analysis, %
Moisture Volatile matter Fixed carbon Ash
7.00 28.16 50.65 14.19
7.60 38.02 41.36 13.02
7.00 36.21 42.87 13.91
25.60 30.50 40.00
3.90
31.80 32.50 30.80
5.00
37.60 27.80 27.60
7.00
Ultimate analysis, %
Carbon Hydrogen Nitrogen Chlorine Sulphur Ash Oxygen
71.97 4.75 1.33
1.24 15.26 5.44
66.80 4.83 1.11 0.14 4.43
14.09 8.60
67.46 5.07 1.33
4.23 14.96 6.23
72.00 5.10 1.00
0.50 5.20
16.20
69.30 5.20 0.90
0.50 7.30
16.80
64.10 4.30 0.90
0.80 11.20 18.70
Heating value, as-received, MJ/kg 27.7 26.0 26.0 21.5 19.0 15.5
Ash analysis, %
Si02 Ah03 Ti02 Fe203 CaO MgO K20 Na20 P20S S03
55.93 24.74
1.10 10.24 0.83 1.47 3.43 0.39 0.18 0.92
43.06 19.96 0.89
28.90 1.43 1.05 2.25 0.35 0.60 1.89
44.08 18.18 0.91
23.91 4.54 0.87 1.99 0.39 0.26 5.92
29.40 20.00
0.60 8.00
13.00 3.50 0.80 6.10
16.80
28.80 13.00 0.70 9.00
25.00 6.50 0.40 1.20
18.00
28.40 11.00 0.40
14.00 18.00 5.00 0.70 3.60
19.80
Sulphur forms, %
Pyritic Sulphate Organic
0.33 0.25 0.66
2.43 0.36 1.64
2.15 0.73 1.35
Hardgrove grindability index 58.0 52.5 56.1 48.0 55.0 55.0
Nature of ash, based on Fe203 compared to CaO +MgO Bituminous Bituminous Bituminous Lignitic Lignitic Lignitic
20
Coal-fired boilers
Table 3 Basic boiler dimensions for firing the six coals' (Barrett, 1990)
Type of coal
Boiler dimensions A B C D E F
Furnace waterwall area Manufacturer J 1.00 1.07 1.07 1.09 1.01 2.12 Manufacturer K 1.00 1.16 1.13 1.75 1.29 1.46 Manufacturer L 1.00 1.44 1.34 1.20 1.03 0.98
Plan area Manufacturer J 1.00 1.12 1.12 1.17 1.06 1.63 Manufacturer K 1.00 1.16 1.15 1.50 1.25 1.32 Manufacturer L 1.00 1.00 1.00 1.00 1.00 1.00
Furnace height Manufacturer J 1.00 1.01 1.01 1.01 0.99 1.60 Manufacturer K 1.00 1.06 1.05 1.35 1.09 1.17 Manufacturer L 1.00 1.22 1.17 1.10 1.01 0.99
Burner-zone volume Manufacturer J 1.00 1.01 1.01 0.83 0.87 1.22 Manufacturer K 1.00 1.17 1.14 1.43 1.20 1.25 Manufacturer L 1.00 1.00 1.00 1.00 1.00 1.00
Above-burner volume Manufacturer J 1.00 1.20 1.20 1.44 1.06 4.71 Manufacturer K 1.00 1.43 1.32 3.25 1.64 2.04 Manufacturer L 1.00 1.44 1.34 1.20 1.03 0.98
Number of wallblowers Manufacturer J 1.00 1.31 1.31 1.31 1.00 1.64 Manufacturer K 1.00 1.13 1.06 1.88 1.11 1.72 Manufacturer L 1.00 3.00 3.00 6.82 6.82 6.82
* all figures are expressed as ratios, related to the base case, coal A
Table 4 Design furnace exit gas temperatures for the six coals (Barrett, 1990)
Type of coal
Source A B C D E F
Manufacturer I 1183 1160 1160 1160 1183 1066 Manufacturer 2 1210 1155 1170 1038 1127 1082 Manufacturer 3 1149 1027 1054 1093 1143 1160
Nature of ash, based on: Fez03 compared to CaO + MgO Bituminous Bituminous Bituminous Lignitic Lignitic Lignitic
be expected that the vendors would produce similar designs. Even if this assumption is not strictly accurate, the variations between designs illustrate the empirical nature of much of the available information.
FEGT is a major parameter affecting boiler design since it strongly influences the condition of fly ash entering the convective sections. If the design figure is too high, then molten, plastic and sticky particles will be more likely to deposit on the tubes, possibly attracting other fly ash particles to stick as well. If the design FEGT is reduced, additional wall surface must be provided since otherwise the unit may be unable to heat the steam to the required superheat temperature.
In Table 4 it can be seen how the different manufacturers used the available data on ash fusion temperatures and coal characteristics. Manufacturer I used almost the same FEGT for all the coals except the North Dakota lignite. Manufacturer 2 made significant adjustments to the FEGT, and used higher figures for all the bituminous coals. Manufacturer 3 chose a lower FEGT for the slagging bituminous coal than for the low rank coals.
In relation to furnace dimensions, the first step is to determine the waterwall surface area required in order to achieve the design FEGT. Once this is established there are various ways of arranging the furnace dimensions. In the
21
Coal-fired boilers
exercise, Manufacturer 3 used the same plan area for all the coals and adjusted the boiler height to achieve design conditions. Manufacturer 1 used virtually the same boiler height for all the coals except the lignite, while Manufacturer 2 varied both plan area and boiler height when designing for the different coals.
There was probably more agreement between the designs on the question of the volume above the burners than on any other furnace parameter. On the question of the provision of wallblowers, Manufacturers 1 and 2 varied the number by a ratio of less than 2: 1. Manufacturer 3 varied the provision by as much as 3: 1 for the slagging bituminous coals and had nearly seven times as many for the low rank coals.
(a) (b)
(a) front wall-fired (b) opposed wall-fired
t t
(e)
The manufacturers clearly recognised the differences in coal properties when preparing the designs. They made adjustments to increase boiler volume and added wallblowers to reduce or eliminate the possible effects of slagging or fouling. However, they seemed to have quite different assessments of the degree of possible problems with the specified coals. It is clear that they were relying more on their own experience than on any fundamental understanding of ash deposition processes (Barrett, 1990). They may also have different approaches to the economic trade-offs between increased capital cost and increased operating costs. These latter can be affected by using more wallblowers, by higher maintenance and by accepting more outages. Costs are also affected by the tube material used, especially in the slagging (radiant) region.
(
(c) (d)
(c) corner-fired/tangential (d) downshot burners (e) downshot with staged air
addition for anthracite fuel
(a)
(b)
staged air
(c)
---[> burners
Figure 6 Typical boiler configurations (modified from Hjalmarsson, 1990; Radulovic and Smoot, 1993; Bryers, 1994)
22
Coal-fired boilers
2.2 Boiler configurations Typical dry bottom boiler configurations for pulverised coal with different burner positions, are shown in Figure 6. The basics of the process are the same. Pulverised coal, commonly 70% below 75 f!m size, is blown into the combustion chamber with primary preheated air. The variation in pf (pulverised fuel) size for coals of different rank is shown in Table 5. The variations arise from the different coal properties, the greater reactivity and sometimes fibrous nature of lower rank coals, and the different mills used. The objective is to achieve maximum carbon burnout.
Table 5 Pf sizes for coals of different rank (Singer, 1991)
Coal rank* -74 !-Lm, +297 !-Lm, wt% wt%
Lignite and subbituminous C coal 60-70 2.0
Subbituminous A and B high volatile bituminous C 65-72 2.0
Bituminous high volatile bituminous A and B low and medium volatile 70-75 2.0
* ASTM classification
Secondary and tertiary air may be added to control the combustion rate and to reduce NOx formation. In wall-fired boilers, the burners are fixed, with limited possibilities of adjustment. In tangentially-fired units (the design preferred by ABB Combustion Engineering), the angle of flow from the burners can be adjusted according to pre-set patterns, so that the combustion zone and its swirl can be adjusted both upwards and downwards.
Although not discussed in this report, one of the approaches used to minimise the effects of deposition, and in particular of fouling by low rank coals has been the use of cyclone-fired boilers. These operate with an intense combustion regime at high temperature just outside the main boiler chamber. Much of the ash-forming material forms a liquid slag which collects on the chamber walls. It flows down from the burner, and out through the bottom hopper. Cyclone furnaces are not commonly specified now, because the combustion temperature is higher. This is associated with the formation of larger amounts of NOx which have to be removed from the t1ue gas before discharge. In addition, more of the ash-forming material will vaporise, and this sometimes results in fouling problems. Cyclone boilers have limited design t1exibility, are associated with high maintenance costs, and are applicable only to particular coals. They are discussed by Couch (1989); and Barsin (1990).
2.3 What happens inside a boiler? In a 500 MWe unit, the coal feed will be about 5000 tid (using a typical UK bituminous coal). Just under 50,000 tid of air will be supplied to the boiler (Hart and Lawn, 1977). If the ash content is 15%, then some 750 tid of ash passes
through the boiler, and it only needs a tiny proportion of this to be deposited to cause serious problems.
The coal feed consists of a range of particle sizes, including liberated mineral matter as a result of the milling process. The amount of 'free' mineral matter will depend on its distribution in the coal matrix. Some particles will consist solely of organic material, others will be associations (in various proportions) of organic and inorganic components.
The particles enter a high turbulence, high temperature combustion zone. They receive shock heating by both radiative and convective transfer, and are heated by the exothermic reactions taking place on and inside the particles themselves. The environment is an extraordinary 'witches brew'. The gas consists of nitrogen, water vapour, oxygen, carbon oxides (CO and C02), together with various ions, radicals and volatilised species. These include hydrogen chloride, sulphur, sulphur oxides, sodium and potassium compounds, trace element radicals, sulphides, chlorides and oxides. The particles are a mixture of partly decomposed coal and of minerals and of agglomerations of coal and minerals (Jackson, 1987).
The conditions experienced by the inorganic components are closely related to those which arise both during and as a result of the exothermic oxidation (combustion) of the carbon. The heating rates of the coal particles are known to be of the order of 104 to 106 °C/s.
A generalised time temperature cycle seen by individual particles is shown in Figure 7. The time spent in the boiler depends on particle size, which burner the particle passes
The top curve represents finer particles of 0.1 11m and the bottom curve larger particles of
1500 100 J.!m size.
!;> ~ ;1000
~ E ~
500
10 20
Time,s
in the combustion and heat exchange sections for suspended particles
particles deposited on a surface
Figure 7 Generalised time-temperature cycle (based on Wibberley and Wall, 1983; Raask, 1986)
23
Coal-fired boilers
through (whether it is high up or low down in the boiler), and on the effects of turbulence. Residence times are typically between 2 and 5 seconds (Wibberley and Wall, 1983). Cooling rates through the convective section can be of the order of 500°C/s.
The time temperature cycle shown is an average. Individual particles may see something very different. One which heats very rapidly, becomes sticky, impacts on the waterwall and sticks, will cool suddenly, and then may reheat gradually and melt, and run down the boiler wall. Others will experience completely different conditions.
The amount of ash handled in a notional 500 MWe unit can vary from as little as 200 tid using some Indonesian coals, to well over 2000 tid with some Indian or Bulgarian coals. Most of the ash leaves in the flue gases and is taken out in the electrostatic precipitators or in a baghouse. Some ash leaves the system from the boiler bottom or from collection hoppers under various tube banks.
While ash content is commonly expressed on a weight per cent basis, account must also be taken of the heat input associated with the coal. The contrast between three coals of different rank, having the same ash content but different moisture contents can be seen in Table 6. The amount of ash passing through the boiler would be about three times as much for the lignite as for the bituminous coal. It is simply because of its lower effective heating value, due to a high moisture content, a higher coal throughput is needed to achieve a given rating.
Table 6 The effect of varying coal rank (Stultz and Kitto, 1992)
Rank High volatile Subbituminous Lignite bituminous
Moisture, % 3.1 23.8 45.9 Volatile matter, % 42.2 36.9 22.7 Fixed carbon. % 45.4 29.5 21.8 Ash, % 9.4 9.8 9.6 Heating value, MJ/kg 29.6 20.2 10.4 kg ash/GJ 3.5 5 10
Ash deposition affects heat transfer in the boiler, and can reduce it to a half (compared to a clean tube) in as little as an hour, and to a quarter, in 24 hours (Reid, 1983). These figures are indicative simply of the scale of the effect of slagging. Deposit coverage can vary widely at different points on the furnace walls and on tube banks.
Adjustments to compensate for variations in heat transfer can be made by:
attemperation; flue gas recirculation, commonly used with high moisture content coals to dry the incoming feed before combustion. It can also be used to increase the mass flow rate, and hence heat transfer, through the convective section of the boiler firing system adjustments which alter the proportion of heat taken up in the furnace. In all units the coal firing
rate may be biased vertically, while in tangentially-fired units there is also the possibility of varying the burner tilt; sootblowing; changes in the amount of fuel supplied to the boiler.
2.3.1 The effects of changes in operating conditions
In addition to the various physical design parameters, boiler operating conditions can have a considerable effect on the amount of ash deposition. Operating conditions affect temperatures, gas velocities and particle movement within the boiler. Whether or not there is flue gas recirculation, will have a significant effect. Recirculation may be used either to dry the incoming coal feed, or to increase the mass flow through the convective section.
Key factors include:
the amount of excess air, and hence the extent of oxidising conditions; variability in the coal feed; deposition on heat transfer surfaces. This in tum affects the amount of further deposition; the distribution of both fuel and air through the various burners; the boiler load (hence the coal and air feedrates), which will affect the mass flow through the boiler and local turbulence patterns.
In many utility boilers, comparatively little is known about the conditions inside. With boilers using lower rank coals it may be possible to see inside from various inspection ports. This becomes increasingly difficult with higher rank coals because, among other things, higher temperatures are involved resulting from greater combustion intensity. Problems have been reported in supercritical, pressurised furnaces, where visibility is very poor and large clinkers can bridge over the hopper outlet without being noticed (Johnson and others, 1991).
The level of instrumentation is limited. The environment inside the boiler is harsh, involving high temperature and turbulent flow patterns carrying both solids and liquids which may be both hot and sticky or possibly corrosive. This affects both routine monitoring and investigational work. Both sampling and measurements are difficult.
The coal feed may have varying properties. These are rarely monitored on-line, although new possibilities for doing this are emerging (Kirchner, 1991). Mill performance and the distribution of the coaUair mixture to the various burners is not monitored. although occasional surveys may be carried out. Both ultrasonic and microwave techniques are being developed for the on-line measurement of pulverised coal distribution and mass flow in burner feed pipes. Work has been carried out on a rig, and field trials at a power station are planned for 1994 (Cutmore and others, 1993). Temperature measurement in the combustion zone is difficult and most figures are based on derived information based on
24
Coal-fired boilers
100
"-
....... -'\.
90 '\. \ \
~ 0 .... \::J
80 '. ":;::><
".'. "-III Gl .....:\ .r:.
70 III '\:-e I:
><::J
:;:: 60-III Gl :I:
50 slag panel 1
--- slag panel 2
40 ------_. slag panel 3
30
0 4 8
~" /: \\ . \ '\
\ "",.........
~r ~r I
12 16 20 24
Time, h
Figure 8 Typical cyclic changes in heat flux between sootblowing operations (Phol, 1993)
measurements made of the flue gases in the economiser together with information about the heat transferred to the steam. There is careful monitoring of the flue gases to ensure that emission limits are met, and measurement of CO and C02 levels gives a measure of combustion efficiency and of 'excess air' levels.
Given the lack of instrumentation and information, it is perhaps remarkable that boilers generally operate so well. As the requirements tighten in terms of emission standards, demands for increased efficiency together with general economic pressures, there is scope for the appropriate application of more measurement and control techniques to improve boiler operation. New techniques are being investigated and developed, and there has been some work on comprehensive boiler monitoring which is discussed in Section 6.3.
Boiler operation is controlled by an estimate of the FEGT normally based on the flue gas temperature in the economiser. The FEGT would commonly be held at a figure between 1100 and 1400°C, depending on the ash properties.
Boilers are increasingly being asked to handle cycling duties where there are patterns of load changes. One positive outcome of this pattern of operation is that sometimes the deposit breaks loose, and either falls off, or is more easily removed by sootblowing. This is due to the differential movement resulting from thermal contraction or expansion of the metal tubes and the deposits they are can)'ing. Other major variations in local conditions in the boiler are caused
by the operation of sootblowers. These blast the local deposits off by a stream of water, steam or air. Sootblowing regularly and deliberately distorts the flow patterns on a local basis. The build-up of deposits on heat transfer surfaces is accompanied by changes in the flow pattern. Sometimes deposits build up sufficiently to plug sections of the boiler, reducing the area available for handling the gas flow, and increasing the gas velocity elsewhere sufficiently to cause local erosion by fly ash particles. Typical variations in heat flux meter readings are shown in Figure 8 for a series of cycles with periodic sootblowing.
There will be changes in boiler performance and combustion behaviour due to slow changes that take place in mill performance, principally due to gradual wear or to feed variations. The tendency is for an individual mill to wear gradually, and therefore feed an increasingly coarse material into the classifier. Eventually this may affect both throughput and size distribution. The products from the mills are classified aerodynamically. As the density of most of the inorganics is greater than that of the combustible coal, the particle size of the denser (mineral-rich) particles is generally lower than that of the relatively cleaner coal particles (Jackson, 1987). This will have an effect on particle behaviour, and hence on deposition tendency in the boiler.
2.4 Combustion Pulverised coal combustion has been discussed by Morrison (1986). Some processes take place in milliseconds. Some depend on whether the environment is oxidising or reducing.
25
Coal-fired boilers
Some depend on the nature of the coal. The thermal decomposition processes in coal are important, because they relate to:
the rate at which combustion proceeds; the rate at which oxygen is consumed; the rate and form of evolution of nitrogen, sulphur and other species; the state of mineral inclusions, and their subsequent reactions; the release of chemically-combined inorganics.
The local temperature in the combustion zone is normally between 1300 and 1700°C.
During combustion the inorganics in coal are transformed into a number of intermediates which include gases and liquids as well as residual solids which may have a sticky surface. The nature of the intermediates depends on the original ash-forming material present; on the reaction temperatures and other conditions it has seen. The intermediates see peak temperatures in the combustion zone, and are then moved into cooler parts of the boiler where they condense and solidify. They may impinge on relatively cold surfaces, even in the combustion zone.
The mechanisms involved are extremely complex. Organically-associated elements interact with the grains of mineral matter. The properties of deposited material and of the fly ash that passes through to the particulates removal unit depend on:
the nature of the original coal; the boiler conditions; all the processes taking place, such as char fragmentation, mineral fragmentation, phase changes, partial reactions, coalescence and partial coalescence (Benson and others, 1993).
The materials are in an oxidising environment only where there is excess air. Inside or immediately around individual particles, the conditions will be reducing, because of the predominant effect of the carbon which reacts with the locally available oxygen. High moisture content coals will tend to have lower combustion temperatures because of the quenching effect of the water-steam.
The first stage of heating is devolatilisation. Most coals soften and become plastic in form as the gases produced from within the particle erupt from the surface as bubbles. Some mineral particle separation may take place at this stage.
The volatile species involved include tars, oils and gaseous hydrocarbons, CO, C02, and H20. The tars and higher hydrocarbons may be broken down to smaller structures and are subsequently oxidised in gas phase reactions. These are not well understood even though they are important to combustion zone stability (hence flame stability) and to the behaviour of mineral matter components which form both fly ash and surface deposits.
Oxidation of the residual char which is left after the loss of
the volatiles can take anything from a few milliseconds to 1 second, and itself involves complex reactions. Much depends on coal type, particle size and shape, and its local environment in terms of both temperature and the availability of oxygen.
2.4.1 Combustion equipment
Practically all types of coal from anthracite to lignite, can be burned by pulverised firing. This includes low grade coals with an as-received heating value as low as 6 MJ/kg. A primary objective during combustion is to minimise the loss of unburned carbon.
In a conventional system firing bituminous coal, primary air at about 60-90°C takes the pf to the burners, at a rate determined by the heat requirement. Secondary air at around 300°C is fed to the windboxes surrounding the burners. Modern systems control the addition of secondary air, and may introduce some of it higher up the furnace. This is to minimise NOx formation.
Coals with a low volatile matter content, such as anthracite, which is difficult to ignite, require different designs, and one such was shown in Figure 6. A long flame is required to ensure carbon burnout while minimising the risk of impingement. For coals with a moisture content of over 45%, hot flue gas from the upper part of the boiler is recirculated and mixed with combustion air, to pre-dry the incoming coal.
Various factors affect burner performance, and these in turn, affect ash-forming behaviour. These are mainly:
flow control; air-coal mixing; flame stability; air-coal mixture temperature.
2.5 Ash deposition Ash deposition has been extensively studied, but in spite of this, the chemical and physical transformations of the inorganic components are not fully understood (Benson and others, 1993). Ash formation is discussed in Chapter 3.
The extent of ash-related problems in coal-fired boilers depends on:
the quantity, association and nature of the inorganic constituents present in the coal; combustion and plant operating conditions; the system geometry.
Not all boiler problems are properly documented, and part of the knowledge, including some solutions developed by various operators are regarded as proprietary information. There have, however, been a number of studies which have attempted to assess the extent of the losses due to ash-related problems.
In a recent paper the possible savings that could be made by better control of ash deposition in coal-fired boilers in the
26
Coal-fired boilers
USA were quoted as being some US$400 million/year. This was based on achieving a 1% decrease in heat rate and a 1% improvement in availability (Helble and others, 1990). Earlier estimates of the ash-related losses on a single 500 MWe unit using a high fouling lignite could be as high as US$8 million/year (Honea and others, 1981). In another estimate. Pennsylvania Power and Light Co, with 12 coal-fired plants of 8000 MWe capacity, assessed their losses due to coal quality at approximately US$5 million/year (Johnson and others, 1991). While difficult to assess accurately, it is clear that the costs due to poor coal quality and to slagging and fouling in particular, are considerable. US coal-fired capacity represents about one third of the world total, so if the possible savings estimated there are typical, then worldwide the savings from reductions in costs due to slagging and fouling would be in the region of US$1000-1500 million/year. They are likely to be at or above the upper end of this range, because of the proportion of low grade (high ash content, low heating value) coals used in parts of Eastern Europe and in China and India.
In an extensive study of large boilers in the UK and USA, out of a total of 58 units of over 300 MWe, 15 were reported with frequent slagging problems and 9 with frequent fouling. 22 boilers had occasional slagging, and 11 occasional fouling problems (Barrett and others, 1989).
A well documented example of deposition problems is that at Kreil in South Africa where severe slagging developed on the walls of a newly commissioned 500 MWe horizontally opposed-fired boiler. To overcome the difficulties, a major redesign of mill operation, burner location, flame recirculation pattern and economiser tube area was required. The effect was to reduce flame temperature which meant that the ash deposit was less strongly attached to the walls. (Unsworth and others, 1991). The experience at Kreil is discussed as a case study in Section 7.7.
A survey in Australia at J2 power plants burning 15 different coals showed that Australian operators using bituminous coals had fewer problems with ash deposition than those elsewhere. This was thought to be due to the use of conservative boiler designs and operating procedures (Phong-anant and others, 1991). A further factor was that the boilers were designed for indigenous coals on the basis of considerable previous experience. Until recently, many operated on a coal feed from a single nearby mine. Supply is now determined more by price, and supplies may come from elsewhere. There are still some slagging problems at Australian utilities, such as those at Bayswater, NSW, and others in Queensland, and these have been extensively investigated. The Latrobe Valley brown coals have had a long history of fouling problems.
These studies and examples illustrate the widespread and variable boiler problems associated with ash deposition. The problems are sufficiently serious around the world, in terms of their cost implications, to justify a considerable research and development effort to improve the understanding of the behaviour of inorganics in the boiler. This should improve the possibilities of predicting and quantifying the effects of feedstock changes.
In recent assessments from both boiler manufacturers and utility operators viewpoints, there was complete agreement that the reliable prediction of ash deposition and its effects continues to be one of the most difficult tasks. Current methods for predicting slagging and fouling behaviour are inadequate (Borio and others, 1992; Bull, 1992). Most of the work described in this report is intended to help overcome or mitigate these undesirable effects.
2.5.1 The effects of deposition
There are a number of undesirable effects associated with the deposition of ash on heat transfer and other surfaces. These are principally:
reduction of heat transfer due to solid or liquid deposits. This leads to a reduction in the amount of heat exchanged between the combustion gases and the circulating water-steam. It therefore leads to an increase in gas temperature (as it is not being cooled so effectively) which readily leads to an increase in the rate or amount of deposition. This also results in continually changing conditions in the boiler; the formation of sticky surfaces which then collect other particles; the fouling of surfaces in the convective section of the boiler by the condensation of volatile species; the formation of large clinkers on heat transfer tubes. These can weigh several tonnes and can physically distort the tubes (possibly leading to premature failure) and be a substantial hazard when attempts are made to remove these deposits during a shut-down; increased rates of corrosion and erosion which can either be direct effects of the deposition or due to the sootblowing which is necessary; large slag falls during operation.
Deposition can and does take place in various parts of the boiler. The effects are different, depending on where deposition occurs, and in many situations it may be possible to control deposition by sootblowing. It is where there is a long-term accumulation which is not easily removed, or where there is a very rapid short-term accumulation, that there are potential problems. The main locations are shown in Figure 2, and the numbers refer to those in the diagram (Jackson and Jones, 1983).
1. Ash hopper bridging is a major cause of unplanned outage. It is usually caused either by slag running down the boiler walls and solidifying or by large sintered deposits falling off the tube platens high up in the boiler and falling into the hopper. The incidence of bridging is largely unpredictable. but coals with high iron content and low ash fusion temperatures are particularly susceptible. High heat content resulting in high flame temperature can also have an effect. Slag bridges may be removed by thermal shock (from a load reduction or water lancing), by mechanical prodding or, ultimately during a shut-down. In severe cases the bridge may have to be removed with the help of explosives.
2. Accumulations on the hopper slope may be due to other
27
Coal-fired boilers
accumulations higher up in the boiler coming loose and dropping down. This may damage the tubes. If an ash slab on the slope breaks loose, it may slide down and bridge over the hopper exit, as mentioned above.
3. Burner 'eyebrows' can form above or below the burner mouth. These can distort the flow pattern from the burner, and in severe cases cause quarl damage and flow blockage. They can develop into large lumps of slag hanging onto the burner tip. The problem is difficult to diagnose and correct, and extensive experience in tackling the problem has been reported from the Bayswater plant in NSW, Australia. This is discussed in Section 7.9.
4. Wall slag can occur where coals with a low ash fusion temperature and/or high heating value are burned. The interaction between burner type and boiler dimensions can be critical, and the degree of swirl on short flame turbulent burners is critical. A further factor is minimising local reducing conditions by ensuring a sufficient air supply. Particle size is important, and a coarse grind may result in local slagging.
5. In certain boiler designs, slag can form on internal division walls within the furnace. This has caused considerable problems in certain UK stations, and in some cases the walls had to be removed.
6. 'Birdsnesting' in tube platens is due to deposits of sintered/fused ash. These build-up first on the bottom of the platens, and may be removed fairly easily. Larger accumulations resist on-load cleaning and can become considerably harder with age. They can eventually bridge across the tube bank and cause a major distortion of flow patterns. This results in erosion and increased pressure drop, and may cause tube distortion. Larger accumulations can eventually fall off, possibly damaging burners and the main boiler hopper.
7. Convection bank bonded deposits are often the result of condensing alkali metal sulphates. Hard thick deposits can form as other particles stick to the surface. The initial deposits may be difficult to remove, and can be the cause of tube corrosion. Some deposits may be removed by differential expansion during load variations, especially when the tubes are made of austenitic steels.
8. Some boilers have finned tube economisers, and these are particularly vulnerable to the build-up of bonded dust. A high calcium content in the ash can exacerbate this problem, although this may not be true for coals with lignitic rather than bituminous type ash (Hein, 1991). A knock-on effect may result from sootblowing in the higher temperature sections of the boiler because most of the resulting debris is carried forward with the flue gas. Much of this is collected in the economiser hopper located at the bottom of the back pass of the boiler in a conventional arrangement. Difficulties in clearing this material are sometimes encountered.
9. There are sometimes difficulties with ash deposition at the air heater gas inlet. This may be due to large particles
being dislodged elsewhere by sootblowing bypassing the earlier collection hopper. A material known as 'popcorn ash' sometimes accumulates in air preheaters. It has a low density, and may have been deposited temporarily, and then removed by sootblowing or by load changes. It then moves further along with the flue gases (Benson, 1994).
The reduction in heat transfer that results from deposition has a number of significant effects on the overall performance of a power station. In particular, for a given output and level of availability, these effects include:
increased maintenance cost; a reduction in boiler efficiency, and hence in the amount of fuel needed, and in the amount of C02 formed; increased likelihood of unplanned shut-downs; increased capital cost (for new plant).
For existing plant, there is a trade-off between the cost of a particular coal supply, and its effects on the operating costs of the plant. This is discussed by Skorupska (1993).
2.5.2 Corrosion and erosion
Both corrosion and erosion in boilers cause costly damage, and are associated with ash behaviour and deposition. They are not, however, the central focus of this report, which is looking primarily at the mechanisms of slagging and fouling, and their effects on heat transfer.
Corrosion can be aggravated or accelerated by the chemical make-up of the initial deposits on a tube, particularly if the first layer is not removed by sootblowing. Additionally, various species can migrate through the various layers as softening, melting or chemical reaction takes place.
Corrosion in heat transfer tubes can cause rupture, steam leaks and boiler shut-down. It is another major cause of unplanned outages. It takes place when metal from the tube wall reacts with a component from an ash deposit to form an iron compound. Reactive components are Cl, S, Na, K and to a limited extent, AI.
Provided that there is regular maintenance, together with the correct application of control strategies, corrosion is not currently a prime cause of unplanned shut-downs. However, the remedial measures are quite costly, and current efforts seek to reduce these by substantially extending maintenance intervals. The mechanisms involved are not well understood. Strategies to limit NOx formation can increase the likelihood of corrosion due to the enlargement of the flame zone, and extended reducing environment.
Chlorine attack has always been regarded as potentially serious, and is at its worst under reducing and alternately reducing/oxidising conditions. The precise concentration at which it becomes troublesome is uncertain. While the corrosion is believed to be due to CO, chlorine is believed to accelerate the attack by creating a permeable scale.
During the 1950s, it became accepted that in the USA, coals with more than 0.3% of chlorine on a dry coal basis were
28
Coal-fired boilers
potentially troublesome (Reid, 1971). A similar situation arose in the UK. As there are substantial reserves of coal with chlorine contents over 0.3%, pressure may well grow to develop the technology to facilitate their use. The trend towards staged combustion for NOx control will make this more difficult (Bryers, 1994).
Erosion is due to the impact of hard particles on tube surfaces. It is mainly due to quartz, hematite or a combination of magnetite and corundum. It tends to occur in the high velocity sections of the convective part of the boiler, and is exacerbated when there are partial blockages due to fouling deposits. When these occur, flow patterns are distorted and flow velocities increased. The erosion caused by high ash content (30 to 50%) Indian coals in utility boilers is discussed by Krishnamoorthy and others (1993). Erosion is recognised as a major cause of boiler tube failures. Fly ash is more erosive that the coal from which it has been formed, because of the absence of the soft organic fraction.
Erosion often occurs where the flue gas changes direction, as at the top of the furnace, when it passes through the pendant tube banks, and then into the economiser section. Quartz particles above a certain size are influential in the erosion process, and the time temperature cycle seen by particles also plays an important role in determining the erosive characteristics of the particles. If their edges are rounded off. by whatever mechanism, they will tend to have less erosive effect.
2.6 Investigative methods The principal indication that there is a deposit build-up is that the total amount of heat transferred declines or alternatively, in order to maintain the steam output from the boiler, more coal is required. The increase in the amount of attemperation is a strong indication of a reduction in the amount of heat transfer through the furnace walls. The steam flow, or the inlet temperature to the superheater, drops. Because of the reduced amount of heat removed from the gases through the furnace walls there is a reduction in output, and the boiler control increases the fuel supply. Increasing the amount of fuel in order to maintain output because higher driving forces are needed to maintain the heat flux drives up the FEGT. The temperature of the gases in the superheater region rises (if other adjustments are not made). Unfortunately this has the effect of increasing the tendency of the inorganics to be deposited.
If the build-up of a deposit is localised, there might be little effect on overall heat transfer, except, possibly, across a particular set of tubes. Large hanging deposits and burner eyebrows will probably not be detected by changes in attemperation or in the firing rate, but can cause considerable damage lower in the boiler if they fall off. Heavy deposits may cause tube distortion and eventually result in leakage.
In an operating unit, ash deposits can sometimes be seen through the various ports provided. This is easier in boilers operating with a high moisture content feed where the flame temperature may be only l300°e. It is considerably more difficult to 'see' much in bituminous coal-fired units where
the fireball temperature will be some 1600-1700°e. The various access/viewing ports in boilers are small, and give only a restricted view.
The main methods available for studying ash deposition are:
testing in the laboratory to assess coal properties and ash composition under standardised conditions. This data may be used to compare different coals, or to relate a coal to a standard specification. Synthetic mixtures of various kinds, for example of ash components, are used for some studies. Coals or mixtures of minerals can be heated under controlled and reproducible conditions. The effects can be both monitored and observed; dynamic coal testing can be carried out in laboratory equipment that simulates combustion conditions such as the drop tube furnace; pilot-scale work using combustion test rigs to compare the behaviour of different coals. These rigs can simulate some of the conditions inside a boiler, and commonly use a representative burner. They also use panels inserted into the gas stream to assess slagging and fouling deposits. Again, coal feed characteristics are related to deposit characteristics. Careful attention to the coal used is necessary to ensure that its properties are representative; sampling and analysing the ash and deposits formed in an operational boiler, and relating the information gained to operating conditions and coal feed characteristics. This can include sampling from the combustion zone and flue gases using probes and extensive monitoring of heat fluxes, temperatures and other boiler conditions.
Pilot testing requires around 5-10 t for a unit with a feed rate of, say 20--40 kg/h (equivalent to 150 to 300 kW heat input). Full-scale testing on a large utility boiler requires anything up to about 20,000 t of coal.
2.6.1 Laboratory techniques
Laboratory techniques for studying the behaviour of coals have been discussed by Carpenter and Skorupska (1993). The use of thermal gravimetric analysis and of heated wire grids were assessed, along with the use of drop tube furnaces and other methods. Some methods were principally directed towards studying the behaviour of the organic material, and of the char which forms as an internlediate.
For studying the behaviour of the inorganic materials in coal, and the mineral matter in particular, laboratory studies have provided a range of infornlation about the behaviour and properties of various mixtures. Phase diagrams for mixtures such FeO-Si02-Ah03 and CaO-Si02-Ah03 have been developed. These, and more complex diagrams, contribute to the interpretation of the mechanisms involved. Samples of the original coal, and of the ash formed are taken from the boiler (and from pilot and drop tube furnace work) and the mechanisms clarified by laboratory work on model mineral mixtures under carefully controlled conditions. Examples of such studies are those of Falcone and Schobert (1986); Biggs and Lindsay (1986); and Vorres and others (1986). In the first study, 10 model mixtures which simulated the mineral
29
Coal-fired boilers
matter characteristically found in low rank coals in the USA were heated to 125°C, 750°C and 1000°C. The transformations taking place were studied. In the second paper, the minerals known to be present in low temperature ash were heated from 25°C to l400°C and observed under a microscope. Changes were observed, and characterisation of the mixtures was carried out by x-ray diffraction. Heating was carried out under an inert atmosphere, so that reactions sensitive to the presence of oxygen would be inhibited. In the third study the viscosities of molten slags with various compositions were measured at temperatures between 1300 and l550°C. The materials tested included Si02, Ah03, CaO, MgO and FeO.
In other laboratory studies, the behaviour of coal ash samples taken from boilers or pilot plant has been studied. In a study by Kalmanovitch and Williamson (1986), the crystallisation behaviour of samples of ash melts taken from a power plant in South Africa (at Matla), and from a plant in the UK (Drax) were examined. The equilibrium system CaO-FeO-Ah03-Si02 was shown to be able to model the initial crystallisation behaviour of their samples.
Studies on a laboratory-scale have also looked at the thermal properties and heat transfer characteristics of various slags and deposits, based on model or synthetic mixtures of minerals.
Drop tube furnaces On a laboratory-scale, drop tube furnaces provide the opportunity to perform closely controlled, repeatable tests to determine the effects of oxygen levels, coal feed rates, temperatures and residence times on inorganic transformations and the tendency of materials to deposit. The advantages are that only small amounts of coal are needed, and a large number of controlled tests can be performed relatively quickly and inexpensively. A drawback is that test procedures for this apparatus have not yet been standardised.
In a drop tube furnace size graded coal particles are fed into the axis of a heated vertical tube with a strictly controlled preheated gas stream. It is possible to simulate combustion conditions. including, high heating rates of 104105°C/s. The pyrolysis and combustion products are collected, and ash formation and deposition can be investigated by sampling.
Drop tube furnaces have been used to produce 'intermediate' species under controlled conditions. A cyclone collecting device is used at selected positions down the reactor. Hot particles are generally quenched with liquid nitrogen as they are collected.
The work that can be carried out in a drop tube furnace or laboratory ashing furnace is limited, but nonetheless can be extremely useful. It provides the possibility of providing closely controlled and reproducible conditions, similar to those which the coal and char particles will encounter inside the boiler. In conjunction with advanced analytical techniques, and in conjunction with a database of experience, results can help in both explaining and therefore solving ash deposition problems, and predicting the behaviour of 'new' coals or blends (Carpenter and Skorupska, 1993).
2.6.2 Pilot plant rigs
Pilot-scale testing has considerable advantages. In comparison with full-scale boiler trials, it is relatively inexpensive, and assessments can be carried out under closely controlled conditions. The value of testing on this smaller scale depends on the validity of comparisons between the sample coal and known reference coals. The reference coals can be tested in the pilot plant, and ideally the results can be compared with full-scale plant experience.
Pilot-scale testing in a controlled setting is designed to assess and compare the fireside behaviour of different coals. It is possible to control the temperature profiles and heat fluxes in a pilot unit, to provide the basis for comparison. Nevertheless, pilot combustors involve a substantial investment both in terms of capital and operating costs, and the skilled staff needed to run them. However. such investment can often be justified by boiler manufacturers, large research groups and by the larger utilities.
The difficulties of extrapolating from pilot-scale results to full-scale behaviour are well recognised (Juniper and Pohl, 1990; Thompson and Giovanni, 1993). This is because it is difficult to apply appropriate and rigorous scale-up procedures to the complexities of pilot- and full-scale boiler conditions. Additionally the performance of a coal at full-scale may be atlected by inappropriate boiler design and inadequately controlled or measured operating conditions, and these could obscure the results.
Two possible procedures have been recommended:
multiple-point correlations from small-scale results, where performance parameters are measured for a wide range of coals. The relative performance of test and reference coals can be compared and assessed. Estimates of the extent of various effects is possible. The approach allows for the evaluation of the relative performance criteria for a test coal compared with a number of reference coals, under reproducible conditions, without the need for a rigorous and consistent scale-up method; single-point correlations, where the pilot-scale results for the test coal and reference coals are translated to full-scale conditions by mathematical simulations. This relies on the ability of the mathematical models to describe correctly the most sensitive performance parameters.
There are quite a number of pilot-scale facilities around the world. They are operated in much the same way, and with broadly the same objectives. They use either a vertical or horizontal combustion chamber, simulating flame conditions in the boiler. followed immediately by a section which has similar conditions to the radiant section of the boiler, and in which cooled panels are introduced to assess deposition under slagging conditions. This is followed by a further section where flow conditions and velocities simulate the convective boiler section and cooled panels can collect and assess fouling deposits. Such facilities are operated to satisfy a number of objectives, not all related to deposition. These
30
Coal-fired boilers
can include assessing and comparing flame stability, thermal characteristics, erosion, and flue gas emissions.
One of the difficulties in using the results is that every facility is unique. There is no commonly accepted standard for the design or operation of pilot plants. They may have a combustion chamber fired vertically upwards, or vertically downwards, while some are horizontally-fired. The choice depends more on the space available, on the range of fuels to be tested, and on the views of the particular operator. Ducting and deposition probes downstream of the combustion chamber will be differently arranged, even though they are intended to get much the same kind of data. Each needs correlating and validating with the coals used and boilers of the particular utilities for whom tests are being run.
Tests at pilot-scale would normally be designed to evaluate all the significant impacts of a given coal on power plant performance. Some might be carried out with narrower objectives. It is important that any work is capable of extrapolation to the conditions in full-scale boilers.
In some of the work in the early 1980s, it was reported that correlations had been established between pilot and full-scale results for the effect of coal properties on flame stability, NOx formation and ESP performance. Correlating the results for combustion efficiency, slagging and fouling was considerably more difficult (Payne and others, 1989).
In a recent paper, two high volatile bituminous coals were compared by tests using all three methods - in a 250 MWe wall-fired Riley boiler, in a vertical pilot combustor, and in a drop tube furnace (Allan and others, 1992). The full-scale testing was carried out in Sep-Nov 1990 as part of the EPRI Coal Quality Expert (CQE) programme. Pilot-scale tests were conducted on the ABB Combustion Engineering upward-fired test facility which has a complete fuel handling system, coal pulveriser and air preheater. The drop tube furnace work was carried out at the Energy and Environmental Research Centre (EERC) in North Dakota, USA.
The two coals tested had quite similar characteristics, and while this probably represents a common situation, it meant that the differences being investigated were relatively marginal. In the discussion, the results were presented starting with those from the boiler. These were broadly that the alternative coal being tested had slightly worse slagging characteristics but caused similar convective pass fouling. Increased slag coverage and fluid slag were observed, along with higher FEGTs, increased sootblowing and lower slag viscosities for the alternative coal.
It was said that the results from the drop tube furnace work supported the conclusion about the lower slag viscosity by producing stronger and more sintered deposits with the alternative coal. Pilot-scale testing showed the alternative coal to have lower heat flux values, confirming its increased slagging tendency, but it showed no appreciable difference in fouling behaviour.
The results from the three different scales of test were shown
to be reasonably consistent. However, this was with quite similar coals. The next stage of development in using these techniques is to use the smaller-scale tests in a genuinely predictive manner and then check the predictions against experience. This is being done in a number of situations, but results are not always publicly available.
2.6.3 Full-scale boiler trials
Carrying out large-scale tests on a boiler involves installing a considerable amount of instrumentation not normally used. It means an extended sampling programme, probably including the insertion of probes into the boiler. It involves skilled people and additional laboratory work. Some effects, such as the build-up of deposits may take several days to show up. Much of the boiler is inaccessible, and in many parts it is difficult to see what is going on. The use of special optical probes can help in certain instances.
In an operating plant it is not usually possible to maintain the same firing conditions, heat absorption and temperature profiles over any long period. While in one sense, the only absolute test of the performance of a fuel is one in the unit in which it is going to be burned, it is preferable to make preliminary assessments of its slagging and fouling potential on a smaller scale. Large-scale tests mean ordering thousands of tonnes of the test coal, and handling it separately, together with the risk of a major outage if the test should result in serious deposition or other damage. As deposition can cause tube damage or bottom hopper blockage and cause a forced outage, a boiler operator would always want to minimise such a risk.
One of the more difficult aspects of coal quality impact testing is separating the influence of boiler design and of operating practices, from those of the coal itself.
A number of operational problems can bias the results of a plant-scale test burn of a coal (Thompson and Giovanni, 1993). These include:
operation at lower excess air levels than normal; lack of combustion uniformity; factors connected with control equipment and the location of sensors; unit geometry and burner design; equipment malfunction.
In addition, there can be problems associated with the use of sampling probes for collecting both solids (char/ash) and gas. Neither procedure is without its problems, and it is not always easy to interpret what happens to the rapidly cooled material in the probe, and relate it back to what was happening in the area from which the sample was taken.
A number of steps can be taken to minimise the effects of operational variations. A proper sampling and analytical programme of the coal being fed to the boiler is essential, to assess variability. This is even more important when blends are being used. The equipment being used to check the gases for 02 content needs to be recalibrated before each test with the filters and sample lines checked for leaks or blockage.
31
Coal-fired boilers
The need for more 02 analysers to obtain representative readings from more places under different operating conditions needs to be considered. The amount of 02 and its change through the boiler should be compared for consistency during the test runs.
If either fuel or air distribution between the burners is uneven, then local conditions may enhance ash deposition in particular locations. Consequently, measurements of air flow, coal flow and coal size distribution in individual burner feed pipes are recommended. An air heater leakage test is essential, to find out if leaky seals are allowing combustion air to bypass the boiler.
The stability and operation of all control systems should be checked. Diagnostic testing is dependent on boiler type, its specific design and other site-specific issues. One of the most common problems with equipment malfunction is ineffective sootblower operation. Most problems here are maintenance-related, and caused by damaged tips, low pressure or incorrect positioning.
Other areas of monitoring which can provide valuable data about the comparability of test runs are superheat and reheat attemperator spray quantities, boiler heat absorption characteristics and boiler efficiency figures.
EPRI have developed a series of Fireside Test Guidelines as a basis for a detailed coal quality impact test, to assess a broad category of issues commonly termed 'fireside impacts'. These include slagging, fouling, corrosion and erosion, and the short-term effects on heat transfer, together with longer term effects on boiler life and availability.
The methods include:
FEGT measurements; the use of slagging and fouling probes; heat flux measurements; direct observations and the use of video cameras; the use of chordal thermocouples; furnace wall atmosphere measurements; ash deposit sampling.
It is estimated that a comprehensive field test programme to evaluate two test coals on a full-size boiler using most of the techniques in the guidelines, is around US$0.5 million per test unit. This illustrates the amount of effort involved.
In an investigation in Australia to determine the effects of mineral matter on heat transfer, a 500 MWe unit was instrumented to provide steam, gas and tube metal temperatures throughout the boiler (Lowe, 1989a). Extensive use was made of thermocouples. The prime intention was to gather data to help with the design of new plant.
The calculation of thermal performance for design purposes was considered in three stages:
the prediction of heat absorbed by the waterwalls. This governs the rate of steam production, the FEGT and ultimately the final steam temperature;
with the FEGT determined, the heat pick-up over each tube bank can be predicted; on the basis of local steam and gas conditions, local heat flux and metal temperatures must be predicted to allow suitable metal selections to be made.
The test was designed to take measurements in each of these areas, and mathematical models were developed to allow extrapolation to different boiler designs and conditions.
Data were collected on the temperature and gas composition at specific locations throughout the combustion chamber, and on the local heat flux at various points on the chamber wall. The effect of ash or slag layers on heat absorption efficiency is assessed. For consistency, measurements of the bulk furnace efficiency were made requiring data on heat input, gas flow rates and the FEGT.
Where radiation does not predominate, the heat transfer to the tube banks was more easily obtained, based on steam conditions at the inlet and outlet, and on gas temperature, composition and flow rate through the bank.
In the high temperature sections of the boiler thermocouples can give erroneous readings due to radiation loss. The analysis is therefore based on a gas temperature measurement at the economiser inlet. At any point upstream, gas temperature is calculated by heat balance. Where radiation is involved, the assessment must include information on the species in the gases and their radiating properties.
Each test lasted some 10-14 days, and at some stages a crew of 25 people was required, even though much of the data collection was automated. Special arrangements had to be made to take meaningful and representative coal samples, and there was no provision at the station for sampling the as-fired coal. Samplers were therefore fitted to all eight feeders to the boiler. Considerable commitments of both manpower and money were required to undertake this work. Its justification was in the potential increase in availability of both new and existing plant as a result of the knowledge gained. The account illustrates the amount of work involved.
Some of the results are reported by Lowe (l989b). The boiler was fired with a number of high ash bituminous coals with generally high ash fusion temperatures. Heat absorption was shown to be sensitive to the coal characteristics. A relatively small increase in ash fusion temperature (based on 'laboratory' ash) correlated with a significant increase in absorbed heat. As tube metal temperature increases with increased heat flux, it was concluded that coals with high ash fusion temperatures would require higher grade materials for the tubing.
The work has been continued, and similar approaches used for tackling operational problems. An example of this is the investigation of the formation of clinker in the burner region of the Bayswater plant in NSW, Australia (Boyd and Foreman, 1992) which is discussed as a case study in Section 7.9.
Temperature measurement in boilers presents many
32
Coal-fired boilers
problems, and quoted figures of temperatures in the combustion zone and of the FEGT are always a 'best estimate'. A recent development is that of the acoustic pyrometer. This offers the possibility of a direct measurement of the FEGT, compared with the derived/calculated figure normally used. An average temperature measurement can be made across the line-of-sight path between an acoustic transmitter/receiver mounted on the outside walls of a boiler. By using several transceivers in a horizontal plane on three or four of the walls it is possible to produce a spatial temperature map.
The principle is that the speed of sound in a gas changes with temperature, and with gas composition. The practical range for the technique is from 500 to 2000 Hz. However, at furnace temperatures, acoustic velocities can be up to 1000 m/s with wavelengths of about 1 m. Consequently the signal must be resolved to a fraction of a wavelength to obtain practical resolution and accuracy. This requires sophisticated digital signal processing techniques. Where fuel composition varies, there can be errors due to uncertainties of the molecular weight of the gas.
During boiler start-up a direct FEGT measurement could be of particular value. It would replace conventional retractable probes. Superheater tube failures caused by overheating during start-up could be reduced. Malfunctioning pulverisers could be detected at an early stage, and it would be possible to optimise burner operation with a great deal more confidence (Yori and others, 1991). The device has only been used on an experimental basis so far.
Another method which has been tried for measuring temperatures inside the boiler is infrared (IR) sensing. One device which is claimed to work in the range from 370-1200°C was first tested in 1988 at the George Neal station in Iowa, USA. It was mounted outside the boiler in an
observation door. It had a depth of focus fixed at about 10 m, and it was claimed that the ash in the flue gas had little effect on the readings. The transmitter was air-cooled. An accuracy of about lOoC was claimed (Bretz, 1989).
New techniques are being developed to facilitate studies of the basic mechanisms of combustion and inorganics transformations in boilers. One such is a pulverised coal sampler which can collect a representative sample from 64 points across a pipe cross section. Supplemented by separate air flow measurements it can be used in conjunction with test work to identify the effects of pulveriser changes and changes in coal quality. It has been used at the Watson plant in the USA in conjunction with the CQE programme. This is discussed later, under modelling. A simple conventional temperature measuring device has been used in conjunction with the sampler to produce a temperature profile along the flame axis to define the point of coal ignition for different size distributions (Frompovicz and others, 1993).
Modern burner management and control systems are designed to maintain safe burner operation. Flame scanners are tuned to specific emission wavelengths and are mounted on each burner to check the presence of a stable flame. In addition, the control system is designed to maintain the desired air/fuel ratio. During start-ups and load swings, this ratio can be as much as 20% different from the average at individual burners. Advanced systems combining chemically specific imaging of selected flame radicals in the flame zone with neural network image processing may provide much closer control in the future. While the main purpose of this control is in relation to emissions and carbon burnout, it may provide useful information which can be used to minimise ash deposition as well (Allen and others, 1993). Video cameras are also being increasingly used to monitor deposits where access is possible.
33
3 Ash formation
The key things in relation to the problems which are associated with ash deposition are to 'know your coal', and to be able to monitor and control the conditions in the boiler. In particular, it is essential to know about the inorganics present in the coal, either from experience, comparing it with other similar coals, or from detailed analysis and characterisation. Other factors, such as the amount of moisture present and the volatiles content will affect combustion characteristics and flame temperature. They will thus affect the behaviour of the inorganics.
Ash formation is affected by boiler design, and by operating conditions such as the amount of excess air, and the distribution of coal through different burners. These were discussed in Chapter 2.
It is not yet possible to predict exactly what happens to the inorganics during combustion and subsequent cooling, because of limitations in analytical methods, and lack of detailed knowledge of the exact conditions prevailing in the boiler.
3.1 Ash-forming materials in coal Strictly, coal does not contain ash. 'Ash content' is, however, a convenient and widely used term which quantifies the solid residue that remains after the coal is burned. Coal contains both organically-bound inorganic elements, and mineral matter which result in ash formation during combustion. The chemical analysis and characteristics of the ash residue depend both on the raw material and the conditions under which it is formed.
Unfortunately, in the literature, this is not always made clear. There is additional confusion, because the ash which forms during the standard method of testing does not necessarily represent the materials and intermediates which form in a boiler, and which are involved in deposition mechanisms. The ash obtained by laboratory test is the material formed
under closely prescribed conditions, but which are not in any way representative of the time temperature cycle experienced by particles in the boiler. This material is described in this report as laboratory ash.
The solid inorganics are commonly described in the literature as mineral matter or simply as minerals. While most are present in crystalline form (either as a single compound, or as interwoven mixtures), some may exist as amorphous phases, of various sizes and combinations. Organically-bound elements such as Na, K and Ca, are commonly found in lower rank coals. There may also be small quantities of dissolved inorganic salts in the water both in the pore
coal particle organicallyassociated
included (locked) C elements
minerals
~ • _ excluded
.:. (liberated) minerals
/ ~ / ~ o O-Ca++-O 0 I
"iiii'8 'mineralS with some <.. attached organic ij~j coal
Figure 9 Organic coal with its associated inorganic constituents (modified from Benson and others, 1993)
34
Ash formation
Table 7 Mineral matter in coals (modified from Renton, 1982; Bryers, 1992)
Major Silicates Clay minerals
Carbonates
Disulphides
Minor Sulphates
Feldspars
Sulphides
Oxides
Other mineral matter that may be found
Witherite Sylvite Halite Garnet Hornblende Apatite Zircon Epidote Biotite Augite Prochlorite Diaspore Lepidocrocite Magnetite Kyanite Staurolite Topaz Tourmaline Hxmatite Penninite
Kaolinite Illite Chlorite Mixed layer*
Quartz
Calcite Dolomite Ankerite Siderite Pyrite Marcasite
Coquimbite Szmolnokite Gypsum Bassanite Anhydrite Jarosite
Plagioclase Orthoclase
Sphalerite Galena PYlThotite
Rutile
BaC03 KCI NaCI 3CaO Ah03Si02 CaO 3FeO 4Si02 9CaO 3P20S CaF2 ZrSi04 4CaO 3Ah03 6Si02 H20 K20 MgO Ah03 3Si02 H20 CaO MgO 2Si02 2FeO 2MgO Ah03 2Si02 2H20 Ah03H20 Fe203 H20 Fe304 Ah03 Si02 2FeO SAh03 4Si02 H20 (AIFhSi04 H9Ab(BOHhSi40 19 Fe203 SMgO Ah03 3Si02 2H20
AhSi20s(OH)4 KAh(ShAI)OlO(OHh (MgFeAI)s(SiAI)401O(OH)g
Si02
CaC03 Ca, Mg(C03)2 Ca(FeMg)C03 FeC03 FeS2 (cubic) FeS2 (orthorhombic)
Fe2(S04)3 9H2O FeS04 H2O CaS042H20 CaS04 1/zH20 CaS04 KFe3(S04h(OH)6
(NaCa)AI(AISi)ShOg KAIShOg
ZnS PbS FeS
Ti02
* mixed layered clays are usually randomly interstratified mixtures of illitic lattices with montmorillonitic and/or chloritic lattices
structure and on the particle surface. In this report, mineral are shown in Table 7. The data in the table are based on the matter is taken as referring to everything present, except for minerals commonly found by x-ray diffraction analysis of the the water, and the elements directly bound to the organic low temperature 'ashes' of subbituminous, bituminous and matter. anthracite coals in the USA (Renton, 1982). It relates to
indigenous coals which originated from plant debris in the The association of the inorganic constituents with the organic place where the plants grew. The data in the table are coal is shown in a simplified form in Figure 9, and some of supplemented by a list from Bryers (1992) which illustrates the most common forms of mineral matter in various coals the variety of species that may be present.
35
Ash formation
Investigational work in relation to slagging and fouling is directed towards a detailed assessment of the inorganics in the raw coal together with studies of the deposits formed in the boiler, and of the chemical intermediates that are likely to be encountered during the complex transformations. These are related to the operating conditions. Extensive experimental work has been carried out to clarify the mechanisms and interactions between the various components present, and to assess their properties (and those of intermediates) under the conditions arising inside the boiler. This includes their physical state at different temperatures under both oxidising and reducing conditions, and measurements of properties such as viscosity, stickiness, and conductivity. Experimental work is carried out both at benchand pilot-scale.
3.1.1 Organically-associated material
The proportion of the inorganic components that are organically-associated varies with coal rank. The lower rank coals have high levels of oxygen (commonly some 20-25%), and part of this is in the form of carboxylic acid groups. These act as bonding sites for cations such as sodium, magnesium, calcium, and potassium. Some trace elements such as strontium and barium may be associated with the coal in this way. Other oxygen-containing groups include methoxyl, hydroxyl, ethers and ketones (Couch, 1990).
Some of the elements may be in the form of chelate coordination complexes with pairs of adjacent oxygen functional groups. The cations involved originate from the plant material from which the coal was formed, and from the groundwater filtering through the seam.
In the lowest rank coals the organically-associated inorganic elements can comprise up to 60% of the total inorganic content, while in higher ranked coals the inorganics consist mainly of minerals. This results in substantial differences in the behaviour of the ash-forming components.
Sulphur, which is involved in some of the inorganic reactions affecting slagging and fouling characteristics, is present both as iron sulphide and as organically combined sulphur. The iron associated with much of the inorganic sulphur can itself have a major effect on ash behaviour. Organic sulphur in coal is the subject of a recent report by Davidson (1993).
3.1.2 Minerals
The major groupings of mineral matter in coals include silicates, oxides, carbonates, sulphides, sulphates and phosphates. Some of the most common minerals present in various coals are shown in Table 7.
The inorganics which are found as minerals were introduced as:
constituents of the vegetation from which the fuel is derived. Woody tissue contains 1-2% of inorganic material, while leaves and bark contain 10-20%; detrital matter which was blown on to or washed into the deposits;
36
crystalline deposits from groundwater (mainly iron, calcium, magnesium and chloride minerals).
Detrital minerals that were transported by wind and water during the first stages of coalification are mainly silicates, including clay minerals and quartz. They are often the most abundant minerals present. Minerals which accumulate during the next stage of the formation of the coal bed, are called syngenetic and include carbonates, sulphides, oxides and phosphates. Those forming during the later stages of coalification (called epigenetic), consist mainly of the carbonates, sulphides and oxides that form in the fissures, cleats and cavities within the coal matrix. The epigenetic minerals form in a way which makes them more likely to be intimately mixed and finely dispersed within the organic coal material (Benson and others, 1993).
Coals were formed under widely differing conditions, and there are huge differences in their properties. The youngest coals are the lignites/brown coals. The various bituminous coals are older, and are widely distributed around the world. The major coalfields are described in detail by Walker (1993). Some, such as those in India and South Africa, have finely disseminated mineral matter throughout. Others have larger mineral particles which may be removed more easily by conventional coal cleaning processes based on density separations. The broad differences between the Carboniferous coals largely from the northern hemisphere and the Permian coals from the southern hemisphere are important. This is because, historically, most users in Europe and the USA have used local Carboniferous coals, but now commercially competitive coals from the southern hemisphere (Gondwana region) are taking an increasing share of the market in internationally traded coals.
Carboniferous coals generally contain less mineral matter. In Permian coal the mineral matter present is the result of the more varied sedimentary conditions during formation. There have sometimes been igneous intrusions into some Permian coals such as those found in South Africa, and these have introduced large amounts of localised mineral together with a heat effect on the surrounding coal. The rank of Permian coals (from southern Africa, Australia, India and Brazil) lies broadly in the subbituminous to mid-bituminous range. The older and more mature Carboniferous coals (from the eastern US, Poland, parts of Germany, and the UK) generally range from mid/low volatile bituminous to anthracitic (Ham and van der Riet, 1992).
In addition, there are very significant quantities of low rank coals, subbituminous and lignites. These are generally used near where they occur. These have been formed during the past 200 million years, and some deposits are less than 100 million years old. There are major deposits in Australia (Latrobe Valley), Germany, eastern Europe, Russia (Kansk-Achinsk), Thailand, Turkey and the USA (Fort Union and Powder River basins). They are characterised by having a high moisture content, ranging from 35 to 65%; a highly variable mineral matter content, and significant quantities of chemically-bound inorganics such as Ca and Na. The nature of the deposits is discussed by Couch (1988). The variation in the inorganics in different coals using standard laboratory
Ash formation
Table 8 Coal ash analyses (Pacer and Duzy, 1982; Anderson and others, 1988; Singer, 1991)
Colombia Poland Australia United States
Tarong Curragh Morwell Loy Yang North Dakota Texas Illinois West Virginia Oliver Milam Randolph Marion
Hvbb Subb Hvbb Subb Bituminous Brown coals Lignite High rank
Ash (as-received), % 8.8 7.4 11.5 24.4 28.0 16.0 0.9 0.4 % Si02 61.8 37.2 46.8 55.0 72.3 51.1 5.3 15.6 24.5 45.3 47.4 42.0
% Ah03 21.1 31.5 21.8 24.1 23.3 22.8 3.9 13.5 14.5 18.2 19.8 25.5
% Fe203 6.6 7.4 9.6 9.3 0.9 14.2 12.3 5.9 11.0 10.3 24.0 19.0
% CaO 2.2 7.8 5.8 3.4 0.1 3.9 27.7 6.0 32.1 21.5 4.5 8.0 %MgO 2.1 3.8 3.5 1.5 0.2 1.4 19.8 12.1 11.1 2.2 0.9 1.5 % Na20 1.1 1.6 0.8 1.1 0.1 0.2 4.6 16.5 6.0 0.1 0.8 1.3 %K20 2.4 1.2 3.1 1.7 0.3 0.9 0.2 0.2 0.3 0.9 1.8 1.7 % Ti02 0.9 0.7 1.1 1.4 1.0 0.1 0.2 0.5 1.5 1.0 1.0
% P20S 0.2 0.3 - 0.1 1.5 % S0 3 1.6 9.5 6.6 2.8 0.1 1.2 25.5 29.7
Base/acid ratio 0.17 0.32 0.33 0.21
FusibilityOC Initial 1248 1231 1290 1248 Softening 1303 1305 1206 1345 Fluid 1413 1371 1344 1470 Fusion 1325 1150 1110 1180 Deformation 1485 1175 1285 1336
ash analyses which show all the constituents as oxides, is illustrated in Table 8. The lignites and most subbituminous coals are not internationally traded.
The differences between northern and southern hemisphere coals are substantial. Various studies have shown that the minerals in North American bituminous coals are largely similar to those in European bituminous coals, although there are differences in their distribution. However, virtually all Australian coals contain less than 1.3% of sulphur, and have very little chlorine. Pyrite and marcasite are the primary sources of iron in northern hemisphere coals. These are present only in small amounts in Australian coals where iron also appears as siderite and ankerite. In the Australian brown coals there is some organically-bound iron, which frequently occurs in concentrations exceeding that of aluminium. This is not a situation encountered in northern coals (Bryers, 1992).
3.2 Distribution of minerals in pf The distribution of minerals in pf, after milling, depends largely on how much of the inorganics are organically-associated with the coal, and on the size and distribution of the mineral matter. If the minerals are large enough, they will be liberated during the milling process. Some will still be attached to combustible material. If the minerals are finely dispersed through the coal, as in some Gondwana coals, breakage will not affect their distribution (see Figure 10).
In the older coals, some of the organically-bound inorganics become oxides, and precipitate out in cracks and fissures. When the coal is pulverised, this mineral matter can be
(a) Minerals weakly bonded to the coal are liberated
handling~
(b) Particle densities are redistributed with liberated particle little complete liberation
liberated particles
mineral matter
(c) No liberation
Figure 10 Coal breakage and mineral matter distribution (Torak and others, 1988; Lockhart and others, 1988)
37
Ash formation
liberated. This can mean that the bulk of the inorganics are in the form of discrete, extraneous particles whose behaviour will be somewhat different from that of the same material if it remains attached to a particle of organic coal.
Another factor which will affect the transformations taking place during combustion is the association of minerals in the coal matrix. If minerals are interwoven or situated very close to each other, then there is a greater chance of their interacting during shock heating and the burning of the char from around them.
As a general trend, the effects of mineral liberation, and of the different behaviour of extraneous mineral matter particles will be greatest in the higher rank coals (Bryers, 1994). Because of their higher density compared with organic material, both liberated minerals and coal particles with a high mineral matter content will tend to be ground to a finer size than other particles. This is because of the effect of the mill classifier which recycles heavier particles (Unsworth and others, 1991). Particle size is also affected by the hardness of the mineral matter, and so the effect will depend on the particular coal being used.
Coal cleaning can reduce the proportion of heavier components, mainly minerals. and wet washing may remove some soluble material such as the salts of Ca, Mg and Na. If dense-medium separation techniques are used then there will be small quantities of magnetite present.
3.3 Characterisation of the inorganics in coal and ash
Particular care must be taken because samples used for analysis are small. The difficulties of obtaining truly representative or typical samples of a coal are well known, but not often discussed when quoting analytical results. Sampling problems are less significant when investigational or development work is being undertaken. They can be of great importance when considering the use of large quantities of coal as a utility feed since if the samples are not representative, wrong and possibly costly decisions may be made. Relevant information on the sample should always be given, such as whether it is taken from the mine, during transport or after storage. Also, whether it is a fresh or weathered sample, and whether it is obtained from a moving source such as a conveyor belt or a stationary stockpile or coal seam. This essential information is not commonly quoted.
A number of parameters are used to evaluate the behaviour of the inorganics, and their possible effects on deposition both on the furnace walls, and on tube surfaces. These include:
laboratory ash analysis; standardised ash fusibility temperatures; the base/acid ratio, iron/calcium ratio and/or silica/alumina ratio in laboratory ash.
In spite of shortcomings in these measurements in relating to deposition potential, they are still widely used.
The standard method of ash analysis is to use ASTM D 3174 or an equivalent, where a pulverised sample is completely oxidised under carefully controlled conditions in a ceramic crucible at 750°C, over a period of some 2 h. A residue is left, and its analysis is expressed as the mass per cent of each equivalent oxide, where its composition is represented by:
Si02 + Ah03 + Fe203 + CaO + MgO + Na20 + K20 + Ti02 +P20S + (S03) (Singer, 1991).
Most ash analyses are quoted in this form, although in some coals it is necessary to include additional components such as BaO and MnO. It should be noted that there are a number of different national and international standards for making ash fusion temperature measurements. Standards have been set by the International Standards Organisation (ISO). There are also American (ASTM), Australian, British, Chinese, German (DIN), and South African standards. These have been reviewed by Hough (1990). There are substantial differences between the methods, both in terms of standard practice, and the chosen characteristic temperature. This means that the tests are not strictly comparable.
For many years this has been the principal basis of assessing the probability of ash deposition in a boiler. However, the laboratory ash forms much more slowly than in a boiler and absorbs both S03 from sulphate present and oxygen from ferric oxide, which does not, generally, happen in the boiler.
The ash fusion test measures the softening and melting behaviour of the laboratory ash which is formed in the standard method described above. The temperature range involved is from 1050-1650°C. It involves observing the profiles of carefully shaped cones of material held together by a binder. The cones are gradually heated in a furnace under either oxidising or reducing conditions, until the ash softens and melts, as shown in Figure II.
2 3 4 5 IT 8T HT FT
1 cone before heating 2 IT (or 10) initial deformation temperature 3 8T softening temperature (H=W) 4 HT hemispherical temperature (H= 1/2W) 5FT fluid temperature
Figure 11 Critical temperature points of the ash fusion test (Singer, 1991; ASTM 01857)
The four cone shapes are defined as follows:
initial deformation, with the rounding of the cone tip; softening temperature, where the height equals the width of the cone; hemispherical temperature, where the height equals one half of the width; fluid temperature where the height equals one sixteenth of the width.
38
Ash formation
Under reducing conditions the fusion temperatures are lower, since the iron is present as ferrous ions which have a greater t1uxing action than when the iron is in the ferric form under oxidising conditions. These are not precise measurements, and the reproducibility of ash fusion temperatures is quoted as being about 55°C in ASTM Dl957 (Barrett and others, 1989). Across the range of national and international standards, repeatability and reproducibility tolerances are large. Tolerances on the measurement for the initial deformation temperature range from 30-60°C for repeatability and 50-80°C for reproducibility. This kind of difference could result in a large difference in the provision of heat transfer surface in a boiler, affecting capital cost and/or affecting boiler operating conditions (Hough, 1990).
In an effort to improve the repeatability and reproducibility of the standard ash fusion test, Australian Coal Industry Research Laboratories (ACIRL) have developed a modified procedure. The new method uses most of the equipment from the old test. The test measures the separation of two standard square tiles which are kept apart by a pillar of ash at each comer. Four ash pellets are used, and the response of the ash to being heated is determined by photographing the tiles at 20°C intervals from 1000 to 1600°C. The separation of the tiles is measured with a microscope. Ash fusion properties are derived from the rate of change of separation for a carefully controlled and steady rate of temperature increase. The test is claimed to give considerably more information about the behaviour of the laboratory ash than the standard method (Combustion News, 1994).
A method involving electrical resistance measurements on the ash has been used to reduce the subjectivity involved in the standard test. When the ash is still at temperatures low enough to ensure that it is all in the solid state, its resistivity is high. As it begins to melt, its resisitivity drops sharply. Early work on this method was reported by Cumming and Sanyal (1983). There has been development of the technique since then, combining measurements resistivity with linear shrinkage of the sample as it is heated under controlled conditions (Gibson and Livingston, 1992).
3.3.1 New and developing techniques
There are a number of new analytical techniques which allow mineral matter to be identified in much greater detail than is possible from traditional measurements both in its raw state, and as deposited material. Together with equipment such as the drop tube furnace, these open the possibility of investigating the detailed mechanisms of mineral matter transformations under the conditions which occur inside the boiler. The drop tube furnace is a dynamic testing device which comes close to reproducing part of the time temperature cycle seen by particles in the boiler. It is described and discussed by Carpenter and Skorupska (1993).
The analytical techniques include mineralogical methods which have been well developed such as:
optical microscopy, which is the traditional tool of both geologists and petrographers for mineral identification. It can provide some positive identification on the nature
and distribution of minerals, although much of the finely divided material cannot be identified. It is time consuming, and not amenable to automation. Quantifying the minerals present is difficult; differential thermal analysis has been used to identify minerals. This is done using reference materials, but the results are not quantitative, and there may be problems due to overlapping curves for different minerals; electron microscopy, where prepared samples are scanned to identify the mineral species present and to measure the particle size and distribution. The technique has been developed in conjunction with the measurement of the x-rays formed during electron bombardment, and a computer controlled scanning system. The measurements and their assessment can be automated. The method is discussed in more detail in the next section, and has also been described by Skorupska and Carpenter (1993); x-ray diffraction can be used to identify individual minerals, and is capable of giving quantitative results relating to the main crystalline minerals found in coal with an accuracy of about ±6%. It is less accurate for the clays and other inorganics which have a less organised structure.
Analytical methods for assessing the mineral matter in coals have been reviewed (Harvey and Ruch, 1986; Robbins, 1991).
Newer methods include:
extended x-ray absorption fine structure spectroscopy; electron energy loss spectroscopy; Fourier transform infrared (FTIR) spectroscopy; Mossbauer spectroscopy; nuclear magnetic resonance spectroscopy; chemical fractionation, for quantifying the organically-associated inorganics in low rank coals.
Both the x-ray and infrared methods listed here involve the need to separate the mineral matter from the coal before analysis. Usually this is done by radio-frequency-stimulated oxidation, or low temperature ashing, of the coal. This may take up to 100 h to complete and can promote minor mineralogical transformations, although these are not thought to be important. Any separation procedure such as this does, however, introduce a degree of uncertainty.
Some of the techniques can be used to characterise the nature not only of the mineral matter in the coal, but also the nature of both deposits and t1y ash. Using them, the picture of the complex transformations taking place can be clarified. Minerals can be well characterised by infrared or ret1ectance methods, dependent on their molecular or crystal lattice stmcture. As the temperature of the sample in the detector is raised above that of the IR source (typically around 1000°C) the same frequencies may be detected as radiation emitted from the sample. IR emission may be developed to offer the first practical method of determining boiler deposit composition in situ (Finnie and others, 1992). The use of Fourier transform techniques has revolutionised the application of IR spectroscopy. In particular the large increase in sensitivity and speed has allowed the development of microscopes which can examine an area as small as 10 11m
39
Ash formation
square. The technique is of particular use in the analysis of glassy matrices in slags or of the bonded interi'aces between adjoining ash particles (Phong-anant and others, 1991).
X-ray diffraction techniques depend on the effect on the x-rays of different spacings in the crystalline structure. This produces characteristic diffraction patterns at various sanlple orientations. It gives semi-quantitative information about the crystalline parts of substances, and gives a positive identification of the individual minerals. Amorphous materials do not yield x-ray diffraction data since there are no discrete planes of atoms to provide resolution, but diffractograms can give an indication of the presence of amorphous phases.
Mossbauer spectroscopy is based on the resonant recoilless emission of low energy gamma rays, of the order of 10 to 100 keV. 57Fe Mossbauer spectroscopy is one of the best methods available for the quantitative analysis of iron-bearing phases in complex multi-phase samples (Huffman and Huggins, 1983).
Nuclear magnetic resonance spectroscopy is a powerful tool to measure the active nuclei which have a characteristic and identifiable nuclear spin such as 27Al and 29Si. The signal is proportional to the amount of nuclei present. Both Al and Si are common ash components, hence the technique can be used to follow various thermal transformations (Phong-anant and others, 1991).
Chemical fractionation is used to extract organically-associated elements from the coal, based on solubility. The technique involves extracting the coal with water to remove water-soluble elements such as the sodium in sodium sulphate, and the elements associated with the groundwater in the coal. The next stage is an extraction with ammonium acetate, to remove elements that may be bound as the salts of organic acids, such as Na, Ca and Mg. The residue from that procedure is extracted with HCl to remove acid-soluble species such as Fe and Ca which may be in the form of hydroxides, oxides, carbonates and other organically coordinated species. Components remaining in the residue from this process are assumed to be associated with insoluble mineral species (Benson and others, 1992).
3.3.2 Computer controlled scanning electron microscopy (CCSEM)
The method into which much of the current development work and effort is being directed, is scanning electron microscopy. While its use in relation to ash deposition is in the development stage, most investigations into ash deposition behaviour now use a CCSEM analysis of the minerals present in the raw coal, together with a similar study of some of the ash formed. Attempts are also being made to derive engineering design parameters for boilers, based on CCSEM analysis of the potential coal feed rather than laboratory ash analyses (Kalmanovitch, 1992, 1994).
The technique is discussed by Skorupska and Carpenter (1993), and is particularly relevant to the
developing understanding of slagging and fouling phenomena. CCSEM quantifies particle size, shape and composition of mineral matter on a two dimensional basis (it is looking at a polished section). As a result, it is possible to quantify both the mineralogy and size distribution of the mineral matter in coal, and that in fly ashes. It also shows the amount and nature of the liberated mineral matter in the coal feed to the boiler, which it is important to take account of when predicting the transformations which may take place.
The scanning electron microscope (SEM) uses a beam of electrons a few hundred nm (or Angstroms) in diameter. The beam is commonly generated from a fine tungsten point and systematically sweeps over a prepared specimen. In the case of coal this is normally a pulverised sample which has been set in an epoxy block, carefully polished and covered with a thin protective coating to eliminate electrostatic effects. Various detectors measure the number of electrons that are scattered back or emitted from the surface. Others measure the spectrum of the dispersed x-rays which are fornled as electrons in the target atoms are raised to higher energy states. This is a development of the basic procedures for CCSEM and facilitates the identification of many of the mineral species in coal. It is referred to by some workers as electron probe microanalysis.
The back-scattered electron (BSE) signal is entirely dependent on the average effective atomic number of the material under the beam. The higher this number, the brighter is its appearance. The sensitivity of the BSE image to variations in the mean atomic number greatly facilitates the search for mineral grains containing heavy elements. Minerals with mean atomic number differing only slightly can sometimes be identified by adjusting the image contrast, but positive identification is sometimes difficult.
During analysis, each particle is individually located and distinguished from the mounting medium by its BSE brightness threshold. Material that is brighter than the threshold is classified as mineral matter. It is scanned by a grid of points, and the BSEs detected. The particle image is built up point-by-point. The image analyser is a software-based system with associated electronics which controls the SEM beam, image amplification and thresholding. It allows the particle boundaries to be established, after which the electron beam can either be directed towards the particle centre, or towards a pre-determined pattern of points within the particle boundaries.
Digital image analysis allows the information obtained from the electron detector unitls to be transferred to numeric form for handling in the computer which controls the operation of the electron beam and the selective collection of x-ray spectra.
Those electrons already forming part of an inner shell in the atomic structure can be elevated to a higher energy level. As these electrons revert to their stable condition they produce x-radiation of specific wavelengths.
The process has been successfully automated. A series of
40
samples can by examined automatically, and this is described as QEM*SEM (quantitative electron microscopy*scanning electron microscopy) in Australia and SEM-AIA (scanning electron microscopy, automated image analysis) or SEMIEPMA electron probe microanalysis in the USA (Gottlieb, 1992; Steadman and others, 1992; Straszheim and Markuszewski, 1992). There is an automatic method of recalibrating the BSE brightness and measuring the electron beam current on a regular basis.
In a recent paper from Ames Laboratory, Iowa, USA, it was stated that the technique involving the analysis of thousands of coal and mineral particles in a particular sample can currently be completed using some 15 hours of instrument time (Straszheim and Markuszewski, 1992).
The method of analysis is statistically based in that thousands of individual particles are examined, and their elemental composition determined. The data are examined to infer the mineralogical characteristics of the sample. Various classifications are used, and the mineral names given to chemical groups cannot be ascribed too precisely. Neither oxygen nor carbon are measured. Hydrated crystal forms are not distinguished. Some CCSEM systems cannot detect certain light elements, and some chemical isomers cannot be identified precisely. The technique is chemical/elemental specific, and not mineral specific. Results from CCSEM give the ratio of the main elemental components but there is some ambiguity where elemental ratios are difficult to distinguish. For true mineral identifications, crystallographic or petrographic confirmation is needed, and x-ray diffraction can be used to confirm CCSEM identifications.
The technique does, however, provide an enormous amount of information not otherwise available. It provides sufficient information to make a major contribution to the understanding and prediction of ash formation and deposition tendencies in boilers. For most practical purposes it may not be necessary to know the precise mineral identification of every particle present. Aluminosilicate species, such as the clay minerals are particularly difficult to categorise.
A number of different analysers are being used, and techniques are developing, based on experience and on increased computer power and more sophisticated software for controlling the particle interrogations. People use different magnifications and different pre-set patterns for gaining an overall view of the mineral matter present. They also collect different amounts of x-ray data on individual particles.
Much work remains to be done in developing the technique, and in particular in standardising procedures for sample preparation, sample examination and data collection and interpretation. Sample preparation is described as being an extremely difficult part of all SEM analyses (Steadman and others, 1992). Yang and Baxter (1992) have looked at the detailed factors affecting the results from CCSEM analysis. They highlight the need to use standard reference samples which are chemically and morphologically stable. Comparative tests to detect differences in sample preparation techniques and instrument variations are currently being conducted by groups around the world (Zygarlicke and
Ash formation
others, 1993). Until these have been carried out, and a regular procedure put in place for maintaining comparability, CCSEM results should be treated with some care.
3.3.3 In situ ash characterisation
A recent development of Fourier transform infrared (FTIR) emission spectroscopy allows the deposits to be characterised as they form. Work has been carried out on a pilot-scale combustor under conditions which are broadly representative of those in a boiler. It has been possible to identify the presence of silica, sulphates and of silicates.
The advantage is that analysis can be carried out without disturbing the deposit. When deposits are removed for analysis, they may change, and time is needed for analysis by traditional methods. FTIR emission spectroscopy provides information on the species present on the surface of deposits as they form.
The technique has been used on a Powder River Basin subbituminous coal in the USA. A cooled rotating probe in the pilot combustor collected deposits under conditions similar to those in the convective pass of a boiler. The emission spectra are collected by a series of off-axis parabolic mirrors, and analysed using an FTIR interferometer. The probe surface temperature was maintained at 450°C during the tests. Obtaining meaningful emission spectra from the relatively cool surface immersed in hot combustion gases with entrained particles was described as a 'challenge' (Baxter and others, 1993). Sixty-four individual scans were averaged to obtain a single spectrum. Spectrum collection time under these conditions is approximately 3 minutes. However, the results obtained were in reasonable agreement with those from samples extracted during the test and analysed by traditional methods.
Use of FTIR was also reported by Vassallo and others (1992), who used the technique to look at kaolinite and gypsum as well as at a sample of ash deposit.
The technique has potential for direct application and for use as a boiler diagnostic to distinguish chemical transformations on deposit surfaces as they form and change. Progress in making quantitative analyses and identifying specific chemical species will depend on developing methods of distinguishing and discriminating between chemically and morphologically complex materials. Work is at an early stage. An example of the results is shown in Figure 12 where the relative emissivity of an ash deposit after 10 and 45 minutes during a test is plotted. The data are compared with the known spectra of silica, silicates and sulphates, however, the interpretation of the results is not easy.
3.4 Inorganic transformations The transformations taking place in the boiler depend on the inorganics present, and on the conditions. While some variations in the inorganics tend to be rank-related, there is no uniform trend. The chemistry of deposit formation is difficult to unravel, as a significant proportion of the deposits are amorphous and hence less susceptible to some analytical
41
Ash formation
0.84
.....
slica, slicates, sulphates
0.83
~ .:;; .~ 0.82
'E .\............ /-SilicatesQl
, ..... """~ iiii 0.81 a::
0.80
western coal deposit
10min •••••••••• 45 min
......... '.. """",,,.
............. '"
............
900 800
1.04
1.02
1.00
0.98
0.96
0.94
0.92
0.79 ---L------,----------,----------,-------------,-----------1
1200 1100 1000
Wavenumbers,"iT [cm·1]
Figure 12 Relative emissivity of an ash deposit from a western US coal 10 and 45 minutes after deposition (Baxter and others, 1993)
techniques. Perhaps more important is that with the rapid changes taking place amongst so many inorganic components, it is difficult to follow and assess all the intermediate stages involved.
Coals are classified as having a bituminous type ash if the sum of [CaO + MgO] is less than Fe203 in the laboratory ash. For lignitic type ash, the reverse applies (Unsworth and others, 1991). This classification arose from the tendency of ashes from lower rank coals to have higher calcium contents than those from bituminous ones. It is, however, possible for a bituminous coal to have a 'lignitic type ash' and vice versa. Using this empirical distinction means that a number of bituminous coals with a high calcium content are misleadingly classified as having a lignitic type ash.
It should be noted that calcium and magnesium are usually present in lignites as organically-bound carboxylates rather than in the minerals which is their normal form in a bituminous coal. Because ash behaviour depends on the form and distribution of the calcium and magnesium rather than on the laboratory ash equilibrium melt composition, it cannot be assumed that ashes from bituminous coals high in calcium and magnesium will behave in the same way as those from lignites.
Most of the inorganics in coals with a bituminous type ash, are present as discrete mineral particles, some of which are mixtures of minerals. The amount of organically-associated material is small, although there will be some organically-bound sulphur. This results in high proportions of Si02 and Ah03 in the laboratory ash. There is likely to be a significant amount of Fe203, but only small amounts of CaO, MgO, K20 and Na20, and a small amount of S03. Minerals
with associated iron include pyrite, some clays such as illite and chlorite, and carbonates such as siderite and ankerite. Some sodium may be present as halite (NaCl).
Coals with lignitic type ash commonly have significant amounts of chemically-combined salts of humic or carboxylic acids. The alkali metals present (Ca, Na and K) are commonly in this form. Sodium may also be present as montmorillonite or halite (NaCl). In lignites, iron is normally present mainly as pyrite or its weathered products such as iron sulphates and oxyhydroxides. The proportions of Si02 and Ah03 as determined by laboratory ash analysis tend to be considerably lower. The difference is mainly made up with CaO, MgO, NaO and K20, together with a higher proportion of S03.
The effects of pulverisation are important, in that the particles have widely differing compositions and densities. In some extensive work on eastern US coals a correlation between the ash softening temperature and base/acid ratio was identified within a pf feed. The work was carried out at pilot-scale by firing various size fractions and gravity fractionated cuts from the selected coals. The results are shown in Figure 13. The pf was separated into four sizes, and each size of coal was further separated into four gravity fractions. The data points represent the fractionated species. The spread in the results and the scatter in figures for different size fractions from the same coal shows the distribution of softening temperature amongst the particles. It illustrates the inadequacy of a single ash softening temperature figure for a bulk sample. A visual qualitative assessment of the deposits on the slagging probes indicated that slagging was most severe where there was the greatest degree of liberation of the pyrite from the coal (Bryers, 1986).
42
---
-------------1.....--.----11 - -- -- --- -- -- -- --- -- --t--9-----..
The reactions and changes which take place start during the earliest stages of heating. Some are irreversible. The first reaction is dehydration, followed and accompanied by structural rearrangements and the generation of porosity as swelling takes place and gases are released. The final thermal transformation will either be vaporisation or the melting and homogenisation of materials. On cooling, the rate of cooling can have a considerable effect on the structure of the products formed.
The complexity of mineral transformations, and the different temperatures at which reactions take place is discussed by Unsworth and others (1991); Kuhnel and Eylands (1992). The details of the chemistry of mineral decomposition is complicated, and in spite of intensive study, much remains to be understood. The interactions between the various intermediates are also poorly understood.
During heating, pressure conditions can change drastically in pores due to the release and removal of gases. Gaseous constituents are the most mobile and can therefore affect transformations in a heterogeneous system. There is effectively a 'microclimate' in and surrounding a reacting particle or gas bubble - and the external influences on it are simultaneously changing rapidly as it moves around. Some components in the system will have catalytic effects in
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• Lower Kittaning
x
.x~ • ~ 1 It: tl!l --.l"
curve is generated from least squares fit: '-----y...- __....I
y = 2877 - 36.0x + 0.353x 2 (fJ (J) OJ c:
Ash formation
certain circumstances. These can cause significant deviations in the changes taking place compared with what would happen in their absence.
While the mineral matter will vary from coal to coal, clay minerals (such as kaolinite and illite) are often dominant, followed by quartz and possibly by pyrite.
The alkali elements are relatively easily volatilised, and at typical flame temperatures around 1300-1500°C organically-bound components would certainly have vaporised. These volatile components may then condense on the surface of fly ash particles as soon as they enter a cooler area. Alternatively they may condense on cooler heat transfer surfaces, either in the combustion zone or in the convective transfer area.
During combustion, the environment in the flame area can be considered as reducing, in the sense that the stable ionic form of the iron will be ferrous. As soon as particles leave the flame region then the excess oxygen present makes the environment an oxidising one where the iron will be ferric in form.
The temperatures at which changes and reactions take place among the minerals commonly present in coals are
-------------1 , - - - - - - - - - - - - - - - - --I
0 20 40 60 80 100 ~
Per cent basic
Figure 13 Regression analysis of the ash chemistry of size and gravity fractions of eight eastern US coals (Bryers, 1986)
43
• •
Ash formation
metakaolinite mullite + amorphous quartz (A12 Si2 07) (A1 6 Si 20 13 ) + (Si02 ) ... ...
... illite
... calcite
(CaC0 3 )
... pyrite
(Fe S2)
gypsum
amorphous aluminosilicates • ...
lime (CaO)
• ... haematite!magnetite
(Fe2 0 3 )!(Fe 3 0 4 ) ...• anhydrite
(CaS04 2H 2 O) (CaS04 ) ... ..I I I I I I I I I I I I I I I I 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Temperature, °C
Figure 14 Thermal decomposition of major coal minerals (Unsworth and others, 1991)
summarised in Figure 14. The heating rate is so high that even in small particles there will be temperature gradients and time lapses, such that changes do not always proceed in a straightforward sequence. For example, the aluminosilicate (clay) minerals undergo dehydration, dehydroxylation, lattice degradation and phase transformation as their temperature rises in a fraction of a second. Vitrification and softening may occur before all the other changes are complete.
Quartz (Si02) may dissolve into an aluminosilicate melt at temperatures over lOOO°C or if it is not associated with clays, will begin to volatilise at about 1650°e.
Kaolinite loses water at 400°C to form metakaolinite which in turn changes to form mullite and amorphous quartz above 900°e. Illite (which is typical of most iron, magnesium, potassium or sodium containing clays), loses water and decomposes to form aluminosilicates above 400°e.
Calcite (CaC03) decomposes, losing C02 at around 800°C to form lime (CaO). Other carbonates decompose to form the relevant oxide and C02 at different temperatures, for example, siderite (FeC03) at about 500°C, and dolomite (MgC03 CaC03) at 750°C.
Iron has been found to be a key element affecting the formation of deposits, and much effort has been directed to observing the behaviour of iron compounds under the conditions that might apply in boilers. Under drop tube conditions, pyrite (FeS2) decomposes at 300°C, losing sulphur, to form pyrrhotite (Fel·xS) and then oxidises at 500°C to form haematite (Fe203) and magnetite (Fe304). Liberated sulphur oxidises to S02. Much of the iron in coals is present as pyrite, particularly in bituminous coals. Because the oxidation rate is low, the pyrite may only be partially oxidised in the flame, forming a dense low melting point eutectic FeSlFeO. Some pyrrhotite may still be present even at temperatures over l100°C.
Any finely dispersed iron that is in contact with the coal matrix during combustion may be reduced to iron carbide which subsequently reacts with silica. Both these reactions
evolve carbon monoxide, and the release of this can lead to the formation of cenospheres and plerospheres.
In discussion of the various transformations that take place, phase diagrams are commonly used (Huffman and others, 1981). A number of ternary diagrams may be used, including FeO-Si02·Ah03, CaO-Si02-Ah03 and K20-Si02-Ah03.
In a study of the behaviour of 18 US coals, the high temperature behaviour of the ash-forming materials was studied. This made early use of advanced techniques such as CCSEM and Mossbauer spectroscopy. It concentrated on reducing conditions, where the melting of most of the ashes investigated can be qualitatively understood by reference to the phase diagram in Figure 15. Most of the bulk ash compositions fall in the mullite region, but the reactions leading to melting occur in the iron-rich corner. The first liquid phase-forming will probably be of low melting
Si02
1723
-1590
corundum
Figure 15 Equilibrium phase diagram FeO-AI203-Si02 (Huffman and Huggins, 1983)
AI2 0 3
-2020
44
Ash formation
100 glass
90
80
Ul Ql 70Ul ell
..c: Q.
u 60 kaolinite, illite '2
ell chlorite Cl
-...
0
0 50 \' , fayalite.!: \ ,
hercynite\ ''"' aluminosilicates40 \c: -
Ql \U \...
Ql 30 \c. quartz \ quartz1-':"""".,....-.,....-...,..... - - - ~ ....:..---------_-"
pyrite, siderite, \ wustite20 -t-'-iro"""n-su'-:l-p;""'ha"'7 - - ~ ...;.;.,:;.:.;;..:.:...._---tes
~ glass
10 calite CaS, CaO
0
700 900 Temperature, DC
Figure 16 Phase diagram of changes in US eastern coal under reducing conditions (Huffman and Huggins, 1986)
1100 1300
eutectics in the iron-rich corner. Subsequent particle capture moves the composition towards the shaded area.
Among other things, Figure 16 shows the approximate dependence on temperature of the percentages of iron contained in the various iron carrying phases, as observed by Mossbauer spectroscopy. The diagram was based on equilibrium conditions at relatively low heating rates, and care must be taken in its application to the shock heating situation in a boiler. The study was on the original minerals present in the coal under reducing conditions and the investigation of shock-cooled samples. The relationships applied broadly to the 15 eastern US bituminous coals in the study. Below about 900°C, the phases observed in the ash samples have a direct correspondence to the minerals in the coal. The wustite and iron-rich ferrite phases (principally Fe304 with substitution of calcium and magnesium) are derived from the iron-rich minerals (pyrite, siderite and iron sulphates). The amount of iron in glass observed below 900°C is approximately proportional to the iron in the potassium-containing clay mineral, illite in the coal. This is thought to be due to the numerous low eutectic points in the K20-Si02-Ah03 phase diagram. Between 900 and 1000°C wustite and other iron-rich oxides react strongly with quartz and kaolinite to form melts. Between 1000 and 1200°C the fluxing action of iron is retarded by the formation of hercynite and other iron aluminates. Over 1200°C all the iron has been incorporated into a viscous silicate liquid.
The amount of glass observed at a given temperature in an oxidising atmosphere is significantly less. In both reducing and oxidising atmospheres, partial melting takes place at temperatures well below the temperature identified by standard testing as the initial deformation temperature in ash fusion temperature measurements. It is not uncommon to
observe up to 50% of the ash in the form of a glass at up to 400°C below the initial deformation temperature. Such partial melting is important in deposit formation (Huffman and Huggins, 1986).
It was recognised that even with these insights into the mechanisms it was premature to attempt any coal comparisons at that time. The work needed to be quantitative, in terms of measuring the percentage of melted ash, and the ashes of the high Ca content coals in the study exhibited distinctly different melting behaviour (Huffman and others, 1981). Nonetheless the basis was established for much of the subsequent work reported.
It is clear that iron has a dominating influence on the slagging characteristics of coals with bituminous type ash (Stultz and Kitto, 1992). The reduced forms of iron have significantly lower melting temperatures than the oxidised forms. Consequently particles containing iron in its reduced form are much more likely to adhere to surfaces and promote deposition.
The sodium present will either react with minerals, or vaporise at the flame temperature, and the vapours will later condense as they move to cooler parts of the boiler. Most potassium becomes bound into aluminosilicate glasses. As the quantities are relatively small, they are not associated with deposition problems in bituminous coals as much as they are in coals with lignitic type ash.
Organic sulphur will be released into the gases as S02 during the burnout of the char. Under rapid heating and reducing conditions, pyrite will fuse and then undergo partial decomposition to FeS. When it then encounters oxidising conditions the FeS tends to form iron oxides and releases sulphur into the gases as S02. Most of the coal sulphur will
45
Ash formation
be present in the gas phase and is therefore available for subsequent reactions. In the char. iron compounds may interact with silicate ash to form low melting point iron silicate fly ash particles.
Alkali silicate combinations would be molten at the temperatures of interest, and hence where alkali metals condense on the surface, they may well make the particle sticky. Below 1100°C, alkali oxides and chlorides react rapidly with SOz and Oz, or with S03 to form condensed sulphates. This takes place on both fly ash particles and heat transfer surfaces. Because of their low melting points, these sulphates form strongly bonded deposits and are corrosive. The melting points of the two most easily formed sulphates NazS04 and KZS04 are 820°C and 1075°C respectively. The minimum melting point of sulphate mixtures is 830°C. If the local S03 content is sufficiently high then pyrosulphates KZSZ07 and NazSz07 may be formed, and these melt at 400°C and 300°C respectively. Sulphur reactions, and the effects of sulphate formation and deposition were reviewed by Reid (1971).
Calcium-rich aluminosilicates can also form a high proportion of fouling deposits in a boiler using a coal feed with a lignite type ash (Huffman and Huggins, 1986).
3.5 Ash formation mechanisms
The early mechanisms involve decomposition, followed by the softening and fusion of most of the mineral components together with the partial volatilisation of species such as Na, Ca, Mg, Fe and AI. Organically- associated elements such as Sand Cl are vaporised during the early stages of burnout (Hurley and Schobert, 1993).
Inherent mineral matter can reach peak temperatures above those of the surrounding gas, because of the exothermic reactions in the char. The inherent minerals are also physically close to each other (and to the organically-bound inorganics in low rank coals), and they can therefore react easily together. Extraneous mineral matter will reach lower temperatures, and will not be affected by a locally reducing environment. Thus the transformations in liberated material may be significantly different, and it may behave differently with regard to deposition.
During char burnout, inherent minerals often appear as molten particles on a receding char surface. They may also appear as a lattice network inside the particle. As burnout proceeds, the minerals tend to coalesce within a single particle. Some char particles will fragment in the early stages, and the precise mechanisms will be coal-specific, as with most coal behaviour. Agglomeration can take place when particles collide, or when they come into contact on the surface of a deposit on a boiler wall or tube.
During combustion, the inorganics can undergo fusion, agglomeration, separation, fragmentation, vaporisation and condensation. These processes may be sequential or simultaneous, and are determined by spatial relationships, the time temperature cycle seen by particles together with their 'chemical' environment.
46
The process of disintegration and subsequent coalescence of ash from the burning char greatly influences the particle size distribution of the fly ash formed. This directly affects the surface area available for alkali-ash reactions and the condensation of vapours on the surface and thus affects the deposition of fly ash on the boiler tubes. If the char retains its integrity, and bums as a shrinking sphere, then the ash will tend to emerge as a single coalesced particle. Conversely at high heating rates, the char may disintegrate. and a large number of small mineral particles may be released into the furnace gases. There may also be a very large number of submicron particles arising from the vaporisation and subsequent condensation of volatile components (Wibberley and Wall, 1983). At various stages, this condensation mechanism involves the formation of aerosols, submicron size droplets which may stick to other particles, or to tube surfaces, before solidifying or reacting.
Some of the major mechanisms affecting ash formation are that (Miller and Schobert, 1993):
once the coal volatiles have reacted, and oxygen reaches the char surface, pyrrhotite is oxidised to iron oxide. Subsequent disintegration releases many small particles; organically-bound cations (mainly in the low rank coals) react with the minerals present to form new species, or are volatilised; clays and quartz are the most abundant minerals in most coals. Illite coalesces rapidly with other minerals while kaolinite does so rather more slowly; the low melting aluminosilicates which form, are derived from the clays; extraneous quartz can be highly reactive in the presence of alkali and alkaline earth vapours (for example in the flame zone); there are differences of view about whether large extraneous quartz particles tend to fragment; silica vaporisation is only possible at temperatures over 1300°C; extraneous carbonates tend to fragment due to gas evolution on rapid heating; siderite and ankerite particles tend to fragment into FeO
particles below 1 11m size; pyrite oxidises to produce SOz, and an intermediate molten Fe-S-O phase, and also becomes iron oxide in the deposited ashes. Under reducing conditions it is said that the pyritic iron can vaporise, and form submicron particles on condensation. The behaviour of pyritic iron under reducing conditions is subject to debate, and it is difficult to identify this particular mechanism (Benson, 1994). If the pyrite is associated with clay materials it may form iron aluminosilicate glass; submicron aerosols or particles are fonned when the volatilised/vaporised species are condensed.
As the coal char particles fragment, the fused mineral inclusions may melt and coalesce. Coupled with the formation of gas bubbles, this effect results in the formation of hollow spherical shapes up to 250 11m in diameter, called cenospheres and plerospheres. Cenospheres are almost complete while plerospheres consist of a sector or side of a sphere. They are quite important forn1ations, as they are
• •••
• •
•••••••
relatively large, and may capture smaller particles on their surface as they move through the system. Fly ash can contain up to 2% of these lightweight particles.
Although quartz has a high fusion temperature of over 1500°C, the formation of SiO vapour from Si02 is enhanced by the presence of carbon and of other constituents. Fuel-borne sulphur can react with Si02 to form SiS vapour.
Mineral matter may present a physical barrier through which oxygen must pass in order to react with part of the carbon. This effect will be the greatest towards the end of the combustion process, and for coals which contain large quantities of mineral matter.
Most of the inorganics in the coal will end up in the residual fly ash formed during combustion or during subsequent cooling. A few volatile species may leave in the flue gases, notably As, Hg and Se. If the mixed vapours condense on
Ash formation
heat transfer surfaces or if particles are sticky and adhere to these surfaces or impinge on surfaces which are themselves sticky, then deposits build-up and inhibit heat transfer.
The mechanisms of ash formation and the variety of intermediates and products is illustrated in Figure 17. In the USA a comprehensive programme involving a multidisciplinary study (Helble and others, 1992b, c) has addressed the questions:
what determines the size and composition of individual ash particles? what determines whether or not they deposit? how do combustion conditions, including reactor size affect the processes?
Seven US coals were included in the programme, four bituminous, one subbituminous and two lignites. In addition two Australian low rank coals were studied. In the second
HEATING COOLING
~ 1O-90"m
some vaporisation condensation
'4mceo", mloeml,. /
•• . {pyrite 1100 0 e solidification.<70/-lm -- fUSion and clays 13000 e -----I... •+--='='''==''------I~
• • fragmentation quartz 15500 e •
expansion. "" ~@ ceoo,pl"'" ---C--------.~
- - - - - - - reducing environment
Figure 17 Mechanisms involved in fly ash formation (modified from Skorupska, 1993)
47
Ash formation
Ca AI Ca AI
phase of the assessment both coal blends and highly cleaned coals were used. Eight more coals were tested. These included chemically cleaned coals with <1 % ash content and a coal cleaned by oil agglomeration to 3.5% ash.
A number of conclusions were reached, including:
under all the conditions tested, a broad distribution of ash particle compositions was obtained, suggesting that 'mean' ash composition is a poor indicator of the behaviour of individual particles; the most important factors determining the size and chemical composition of the ash which forms are mineral size, mineral type, the coalescence of included minerals and the fragmentation of excluded minerals; char fragmentation has little effect on the ash particle size distribution for all but coals with less than 5% ash; calcium-rich aluminosilicate particles form where there is a significant amount of organically-associated calcium. Iron aluminosilicate formation occurs to a very limited extent; the amount of sodium condensing on the smallest ash particles is affected by the quantity of aluminosilicates present. Hence fouling is dependent on the sodium to aluminosilicate ratio, and not the total amount of sodium present; pyrite reacts to form several molten intermediates before finally oxidising to magnetite and haematite. It is the intermediates which contribute to iron-based slagging; calcium aluminosilicate particles have the stickiest surface and cause problems due to inertial impaction.
The redistribution (and change) in the inorganics during combustion of a US Beulah lignite is shown in Figure IS. The tests were carried out by the State Electricity Commission of Victoria in Australia and PSI PowerServe in the USA, at a flame temperature of l300°C and with 7% oxygen at the combustor exit. The ternary diagrams show (a)
(a) Minerals in the coal
Si
the original mineral matter and (b) the fly ash formed. Each point represents at least one mineral or ash particle with its identified (normalised) composition. The height of the peaks represents the relative number of particles of any particular composition. All particles where Ca+Al+Si form >SO mole% of the total are shown. It shows that the process of ash coalescence results in the formation of mixtures not existing in the parent coal. The involvement of organically-bound Ca is highlighted with the formation of calcium-alumino-silicate phases (Helble and others, 1992a).
Work at the EERC, has investigated the behaviour of the volatile organic alkali carboxylates, primarily sodium. The organically-bound alkali constituents readily volatilise under flame conditions, and then condense downstream after the furnace exit. The volatilised alkalis can condense on the surface of fly ash particles, forming a low melting point. sticky, surface. Alternatively, the volatiles can condense to form extremely fine particles (of less than 0.2 flm) which are difficult to capture. Laser techniques have been used to study the details of the changes taking place (Zygarlicke and Katrinak, 1992).
A laboratory-scale study has been carried out on the influence of both coal rank and of pretreatment (cleaning) on ash particle size (Kramlich and Newton, 1994a, b). It is suggested that pf combustion usually produces a bi-modal ash size distribution. Most coal ash forms particles of over 0.5 flm size.
The finest particles of about 0.1-0.2 flm diameter are thought to arise from the vaporisation of inorganics from within the char particles. In the locally reducing conditions, non-volatile metal oxides (Ah03, Si02, MgO, CaO and Fe203) may react to form volatile suboxides or elements. These vaporise, and then reoxidise to the non-volatile foml after leaving the char particle, to form aerosol particles of 0.1-0.2 flm size. The mechanism may be augmented by accelerated iron release
(b) Ash formed from the combustion
Si
- - - - - ,-,--- ~
'-,- ------,-'~~.-.~--
Each point represents at least one mineral or ash particle with its identification (normalised) composition. The height of the peak represents the relative number of particles of any given composition.
All particles such that Ca + AI + Si is >80 mole% of the total are shown.
Figure 18 Ternary diagrams showing the redistribution of inorganics during lignite combustion (Helble and others, 1992)
48
which can occur at the surface of inclusions. Coals which have alkali metal-rich mineral matter or organically-bound alkali metals are likely to generate larger amounts of aerosol.
Later in the furnace, these fine particles become a preferred site for the condensation of other volatile metals. These include some of the toxic elements. Although the aerosol may account for only about 1% of the coal ash, its enrichment in these toxic components and the difficulty of removing these particles from the flue gases, means that it is an issue of some concern.
Earlier work indicated that for bituminous coals, the removal of excludedlliberated minerals by froth flotation markedly increased the yield of fine aerosol particles. Cleaning of a North Dakota lignite did not change the emission of aerosol.
The effect in the bituminous coals would indicate that the separation of the minerals in some way suppresses the vaporisation, although the mechanism by which this happens is not understood. Some of the included minerals may act as a sink for mineral vapours which would otherwise escape and form an aerosol as they cool or oxidise. When they are liberated they can no longer do this. Alternatively some minerals may moderate the melting temperature of the
Ash formation
intermediates which are formed. If the fusion temperature is increased by the removal of some mineral, and the remaining minerals only act as a sink when they have melted, then it is easier for the vapours to escape. Yet another possible mechanism is that the presence of mineral in a char particle tends to reduce its temperature. As aerosol formation is dependent on temperature, this suppression may reduce the formation of vapours. These mechanisms, and the presence or otherwise of condensible vapours in various parts of the boiler are likely to affect deposition behaviour.
The influence of coal cleaning and coal grinding on ash yields in the 0.5-8 /lm size range was also studied. These particles are also difficult to remove from flue gas streams, and represent a much higher proportion of the ash by weight due to classification at the mill. They will also have an effect on deposition behaviour. Fourteen coals from Australia, Canada, Indonesia and the USA were studied, ranging in rank from anthracite to lignite. Two of the US coals were subject to physical cleaning by froth flotation.
The number of ash particles at around 2 /lm size seemed to be unaffected by coal cleaning, which suggests that they arose from inherent inorganics in the coal feed, probably small mineral particles.
49
4 Deposition processes and deposit properties
The processes which result in deposition depend directly on the nature of the inorganics in the coal, as discussed in Section 3.1. They also depend on the conditions inside the boiler of temperature. residence time, fluid dynamics, and of the locally oxidising or reducing environment. The transformations in the inorganics are strongly influenced by the effects of both heating and cooling. Deposition is largely the result of movement to a heat transfer surface with either sudden or gradual cooling of the inorganic intermediates. It happens as particles pass out of the flame zone, and lose heat by radiation and by convection to the flame gases. It occurs as the inorganics cool down. Particle stickiness is a major factor, as is particle size and momentum (Bryers, 1994). If the tube or wall surface is itself sticky, as it commonly is in the slagging region of the boiler, then virtually all incident particles will stick.
In most locations there is a regular pattern of deposition, and of the partial or complete removal of the deposit by sootblowing. The material which is removed may fall into the bottom ash hopper, or it may be transferred downstream with the combustion gases. It can then redeposit elsewhere, for example in a cooler section of the boiler where the heat transfer tubes are closely spaced. The smaller particles which arise from the cleaning operation may pass through the boiler and leave with the flue gases.
4.1 Deposition mechanisms For ash or volatiles to cause problems, they have to be transported to and held on the heat transfer surfaces. Some of the specific mechanisms related to ash deposition are represented in Figure 19. The mechanisms depend on the physical state of the materials (solid, liquid or vapour), and their chemistry, together with local temperature and flow patterns. Solid or liquid particles with sufficient inertia may break through the boundary layer adjacent to solid surfaces, and impact on cool solid surfaces. Gravitation may be important in some boiler locations. Electrophoresis can affect
the deposition behaviour of particles with high conductivities. Small particle diffusion mechanisms also playa part. The mechanisms also depend on the physical state of the surface with which the particles are interacting.
In a review by Samms and Watt (1966), the factors affecting deposition were described in terms of the transport processes within the gas of particles. droplets and gas molecules. In addition, there were the mechanisms of deposition on to a surface, and the factors affecting the retention of the particles deposited.
Transport mechanisms within the gas include:
molecular and Brownian diffusion: thermal diffusion; eddy diffusion; gravity effects; electrostatic effects.
In a more recent analysis (Baxter, 1993) the four principal mechanisms involved in deposition have been described as:
inertial deposition; condensation; thermophoresis; chemical reaction.
4.1.1 Inertial deposition
Inertial deposition is more common for the larger particles (over 10 11m size). It tends to result in a coarse-grained deposit. In relation to deposits on tubes, impaction rates are highest on the leading edge of the tube where the air flow is at its slowest. As the angle of impact decreases, particles will tend to bounce off the surface unless extremely sticky. This is illustrated in Figure 20. For smaller particles, the drag forces acting on them are sufficient to keep them in the gas
50
Deposition processes and deposit properties
Mechanism Schematic Description with example
·······
Adherence to a sticky Particle impacts with a sticky or fluid surface and is held there. It surface tends to coalesce with the material on the surface. o -----Condensation on ash Vapour condenses on non-reactive ash particle (P) on cooling. particles Liquid-coated particle sticks to cool target. Example: Particle = quartz;o -------() H!./
~ vapour = Na2S04.
Surface reaction on ash Condensate on surface reacts with reactive ash particle (R) to form particles plastic product. Example: Particle = AI203.6Si02; condensate = NaC!.
Plastic flow Plastic particle (P) deforms to conform with surface and becomes mechanically attached. It may react later. The mechanism is well established for glass-like ash with viscosities between 104 and 250 poise. Alternatively, there can be a temperature rise on deformation at high velocity and high impact angle eg CaS04 at 540-815°C. The deposition boundary moves to lower temperatures at high velocities, and higher temperatures at low angles of impact.
Cementation on surface Non-sticking particles (I) impact the surface followed quickly by plastic (P) or liquid (L) particles to cement them together. Cementation also occurs by the condensation of vapour species, especially by alkali sulphates.
(li)~)O{0 ---
Surface condensation Vapour species condense on surface to provide a sticky surface for 810 entrapment and cementing of non-sticking particles (I). Observed for 0'""0)---80 tUbe) boiler tubes where the vapour is Na2S04. Build-up occurs only for
8~) / high impact angles.
Sintering on the surface Sintering between similar or dissimilar particles (R1 and R2) to "" . increase deposit strength. Increases adherence by increasing R.;J resistance to thermal shock and erosion. Sintering may include the
tube. Very sensitive to alkali content. Independent of the ash fusion and softening temperatures measured on gross sample.
Reaction with target tube Target reacts with impacted particle (R) to increase bond strength. Examples: metal tube where particle = Si02: Fe203 + Si02 Fe203.Si02; ceramic tube where tube = Si02 on surface and particle =CaS04: CaS04 + Si02 CaO.Si02 + S03.
Incomplete reaction due Inadequate residence time in the combustor for complete reaction to short residence time leaves sticky surface. Example: incomplete homogenisation of silicate
glasses leaving low viscosity, sticky regions; also incompletely burned coal particles with surface tars.
o --·et Reducing reactions Reduction by carbon particles (C) will lower the viscosity if iron (or ©--'g other reducible oxide) is present. Typical reduction of fouling
temperature is 90°C.
Deposition/erosion Mildly fouled surface by glassy siliceous particles (P) of high viscosity (eg 104 poise) may be eroded by semi-inert particles (I) of high abrasiveness such as mullite or quartz. Temperature at surface of deposit increases to near gas temperature as build-up occurs.
slagglngTemperature gradients in For constant tube temperature, surface sintering increases with gas the deposit temperature.
fouling---Figure 19 Potential fOUling and slagging mechanisms (modified from Hsu and others, 1984)
51
--------
Deposition processes and deposit properties
flow stream as it changes direction. In typical utility boilers, gas velocities are in the range 10-25 mls.
Another effect which results in impaction is due to the eddies formed behind a tube situated in the gas flow. Deposits can build-up on the back of a tube, particularly where gas velocities are increased (by decreasing the cross-section of
Flow pattern
-
captured
particle ~
Impacting e------particles
Figure 20 Inertial impaction mechanism on a tube (modified from Baxter and DeSollar, 1991)
tacky
depOSit ~
----Figure 21 Condensation on a tube (Baxter and DeSollar,
1991)
the ducting) in the lower temperature regions of the boiler, such as the economiser.
4.1.2 Condensation
Condensation takes place when vapours pass over coal heat transfer surfaces, and various components condense from the boundary layer. They tend to form a thin uniform layer. This may well be sticky, as the surface sees radiative heat from the furnace, and a sticky surface will have a marked effect on the pick-up of other solids. This is illustrated in Figure 21 (Baxter and DeSollar, 1991).
Vapours may condense either on the cool heat transfer surfaces, or on entrained or deposited particles. Condensation also takes place as the gases cool, and the aerosols or submicron particles formed may attach to (or form on) other particles before they impact. Condensed materials may make the surface sticky.
4.1.3 Thermophoresis
Steep temperature gradients in the gases close to the walls or tube surfaces, may encourage migration to the surface. This effect is called thermophoresis. This is particularly significant for the smallest particles. In the combustion zone, the temperature driving force for heat transfer can be of the order of IOOO°C. This means sharp changes in the physical properties of the gas through the boundary layer. If the particles are sticky when they impact, or if there is a rough surface or chemical reaction, they may well adhere. The formation of a rough deposit tends to break up the boundary layer and accelerate deposition.
In general the forces act in the direction opposite to that of the temperature gradient, although they can result both from the differences in gas temperature or from the temperature gradient in the particle itself. The deposits are more finely grained and evenly distributed than those from impact. The process is illustrated in Figure 22. Electrophoresis may also contribute to deposit forn1ation. Particles may be held on the surface by a combination of Van der Waals and electrostatic forces (Raask, 1986b).
---~-
Figure 22 Thermophoretic deposition on a tube (Baxter and DeSollar,1991)
52
Deposition processes and deposit properties
4.1.4 Chemical reaction
Once a particle reaches the tube or deposit surface, some bonding will be chemically dependent. Chemical reaction is the mechanism which can determine whether particles stick and whether the deposits grow.
Among the most important chemical reactions with respect to ash deposition are:
the formation of low temperature eutectics from the interaction of iron, sodium or calcium, and the alumina and silica; sulphation; alkali absorption; oxidation.
The principal sulphating species are compounds containing sodium or potassium in the fOIl11 of condensed hydroxides or possibly as chlorides.
Silica absorbs alkali materials to form silicates, which melt at lower temperatures than silica. These transformations can induce sintering and result in significant changes in deposit properties.
Residual char often deposits with inorganic materials, but it tends to oxidise away leaving, typically, less than 2% of carbon in a fouling deposit, and virtually none in a slagging deposit.
Chemical reactions are strongly temperature dependent. Thus there are considerable variations in deposits depending on where they are in the boiler and the local temperature. Reactions also proceed at different speeds, and the sulphation of sodium and potassium is relatively quick compared with the absorption of alkali materials by silica.
4.1.5 Particle 'stickiness'
One of the important mechanisms concerns the 'stickiness' of a particle, and/or the 'stickiness' of the surface which a particle may touch. Many particles will have a glassy, viscous, surface. The tendency of a particle to deposit is affected by its size and the velocity and angle of impact. A great deal of the fly ash formed during combustion is amorphous, and consists of solidified silicate glass. This is an inorganic fusion product which has cooled to a rigid condition without crystallising. It behaves like a liquid with an extremely high viscosity. The behaviour of glass/liquid intermediates is important because it influences the way in which the deposit becomes stronger through sintering. Silicate glass formation is affected by the presence of iron. It is encouraged by the presence of ferric iron (Fe+++), as Fe203 which can take part in building the necessary three-dimensional random networks. The variations in the silicate structures in the glasses fOIl11ed are discussed by Benson and others (1993).
A laboratory study was carried out using pure materials, and coal (Srinivasachar and others, 1990). This was a systematic attempt to assess the effect of ash particle surface composition and surface state in order to understand
deposition and to lead towards the possibility of prediction. It was intended to test the following hypothesis:
a) if a liquid film is present on a particle, some of its kinetic energy is lost on impact, deforming that film;
b) if the particle viscosity is low, rebound is minimised, and surface tension forces will cause the particle to stick;
c) if the viscosity is too low, the particle will completely deform on impact, and flow over the surface; but if
d) the viscosity is too high, the particle will rebound on impact, without sticking.
The hypothesis suggests the existence of a range of particle surface viscosities over which sticking will occur. The experiments on pure glass spheres of known viscosity (as a function of temperature) showed a critical figure for particle stickiness and adhesion in the range 105-107 poise.
As fly ash particles cool, their viscosity rises sharply. The surface viscosity may also be affected by condensation. Experiments conducted with non-sticky magnesium oxide in the presence of sodium vapour showed that deposition could occur at temperatures of up to 300°C below the bulk melting temperature of the solid oxide. This is due to the sticky surface layer of condensed material.
Coal-based experiments demonstrated the adhesion of ash particles under conditions similar to those in the pure glass experiments. Loose friable deposits were formed at ash particle viscosities of 106 to 107 poise, within the critical range defined above. During the experiments, the different ash size distributions for a bituminous coal and a lignite were determined. These are shown in Figure 23, where the larger proportion of particles below 0.1 /lm size can be seen. This means a huge increase in the number of particles. With the bituminous coal there are many more large particles over 10 /lm size.
Erickson and others (1991), investigated the interactions of
1.0
- NO Beulah lignite 08 - - - Kentucky No 11 bituminous ,-----~
Temperature = 1250°Cr::: 0
:;::: u 0.6III...-l/Il/IIII E 0.4 .r:. l/I<l:
0.2 ~---
0
0 0.01 0.1 1.0 10.1 1000
Particle diameter, ~m
Figure 23 Comparison of coal ash particle size distributions (Srinivasachar and others, 1990)
53
Deposition processes and deposit properties
sodium, sulphur and silica between 900 and 1500°C and from 0.1 to 2.4 s residence time. The deposition of synthetic mixtures illld coal was studied. The experiments focused on sodium, since sodium-containing materials are thought to provide a matrix which binds particles together, particularly in fouling deposits. They showed that fly ash formation was characterised by fragmentation followed by coalescence. with more fragmentation taking place at higher temperatures. The formation of sodium silicates was favoured by higher temperatures and longer residence times. Similar studies have been reported by Baxter (1991) and by Hurley and Schobert (1992) from pilot-scale work. While they draw conclusions which would be in line with general expectations, they also quantify the amount of change (fragmentation and sodium silicate formation, for example) under different conditions.
Drop tube furnace systems have also been used to study the formation of deposits. A deposition probe is placed within the ash trajectory. Abbott and others (1989) assessed the sticking potential of minerals. Ash deposits were collected from an accelerated gas stream (around 2 m/s) on a water-cooled probe at gas temperatures from 1000 to 1200°C. The apparent contact angle and adhesion forces to the probe were measured using microscopic techniques to evaluate sticking potential. Iron-rich drops provided the best adhesion to oxidised carbon steel, and marcasite and pyrite began sticking at temperatures as low as 200°C. Adhesion was enhanced if the drop contained silicon, alumina and possibly a fluxing agent such as potassium or calcium. The influence of pyrite particle size, gas temperature, oxygen concentration and the extent of fragmentation of the coal particle was investigated by Srinivasachar and others (1990). The deposition tendency of ash particles on a cooled probe varied significantly with gas temperature. Coals could be categorised as good to bad with respect to slagging, but the severity of slagging was not quantifiable. One of the main problems with this type of study is that deposits tend to fall off the probe before significant amounts have formed.
A great deal of work took place during the 1980s to investigate the factors which control ash deposition. The reactions and transformations taking place in coal mineral matter at elevated temperatures were discussed by Huffman and Huggins (1986). They highlight the utilisation of phase diagrams in following the mechanisms involved, and the importance of the reactions of the volatile alkalis to fouling phenomena. Falcone and Schobert (1986) discuss the transformations in US low rank coals from both the Fort Union and Gulf Coast regions. It was shown that organically-bound cations were more reactive in producing new mineral species with existing minerals than the cations present in the minerals themselves.
Biggs and Lindsay (1986) carried out work on minerals known to be present in the low temperature ash extracted from coals and heated them under a microscope. Various combinations of minerals were observed in detail and it was shown that the reaction temperature between calcite and pyrite was lowered by the presence of clays. The migration of iron was also studied. Raask (l986a) looked at the
vitrification and sintering and vitrification of silicate-based ash since silicate species are the largest mineral component in many coals. Kalmanovitch and Williamson (1986) reported work on the crystallisation of coal ash melts.
These papers looked in depth at the mechanisms involved, and the effects of the presence of different inorganic species in low and high rank coals. The importance of the reactions of silicate compounds illld derivatives was highlighted, and several investigations covered these in some depth. Much of the work was laboratory-based.
There has been ongoing work in Australia to study the mechanisms of deposition, including stickiness. Early work centred on studying the alkali/ash reactions (Wibberly and Wall, 1982). The experiments investigated the formation of sodium silicates in reactions between silica particles and a synthetic furnace gas containing Na, Cl, S and water vapour. The presence of Cl tends to suppress the reaction at temperatures below 100011 100°C, while S suppresses it at higher temperatures. The layer of low viscosity silicate has a thickness ranging from 0.02 flm to 0.31 flm, but it allows particles to stick to a cool metal surface.
4.2 The requirements for deposit formation
A typical deposit build-up is shown in Figure 24. A clean surface collects only sticky particles, and as long as the deposit is thin, these solidify. As the deposit thickens, its surface temperature increases, and some particles may remain sticky, thus holding on to dry incident particles. Some of the deposited particles will be removed by erosion, and in many situations, the build-up is controlled by regular sootblowing.
o0 0 sticky slag o droplets0
sticking tube
shedding
~
solidified droplets
dry ash still sticky
Figure 24 Processes contributing to ash deposit growth and removal (Benson and ohters, 1993)
54
Deposition processes and deposit properties
The four requirements for deposit formation are that (Wibberley, 1985a):
the vapours and fly ash penetrate the boundary layer of the tube or wall surface, and touch the cold metal; the material adheres to the surface: there is sufficient cohesion to prevent the material from detaching as a result of local turbulence, vibration, sootblowing or temperature cycling; the thermal and chemical compatibility of the steel surface and depositing material.
The first deposit layer may result from different mechanisms, including:
surface attraction between the fine ash and the tube, which may result from the ash particles and tube walls carrying opposite electrostatic charges; inherent tube roughness due to oxide growth; liquid phases on the tube surface; themlOphoresis; condensation.
Given that deposits will form in most parts of the boiler at some stage, the mechanisms of growth are of even greater importance. Deposits wiJI grow if the surface is sticky, as it is likely to be when slag has formed.
As the deposit thickens. the temperature at its outer surface increases by some 30-100°Clmrn, depending on its thermal conductivity and the local heat flux (Wibberley, 1985b). The temperature gradient can exceed 200°C/min (Jackson, 1987). With increasing temperature, the viscosity of any liquid phase present decreases, which increases the stickiness so that more
Table 9 Thermal expansion coefficients for selected materials (Benson and others, 1993)
Linear expansion coefficient, 0-1000°C, (cm/cm K x 106)
Deposits Ah03 8.8 MgO 13.5 Si02 Muilite 5.3 Spinel 7.6 Soda-lime-glass 9.0 Sodium aluminosilicate glass* 6.4-7.0 K20-Fe203-Si02t 10.3 CaO-Ah03-Si02+ 4.5-8.3 Na20-Fe203-Si021l 11.6-14.6 Fe-Oxides 8-10
Boiler construction materials Stainless steel 16-18 Mild steel 11-12 Fireclay refractory 5.5
* at temperature 2573 K t K20 = 16.6%, Fe203 = 16.67%, Si02 =66.66%.
at room temperature to 700 K + at 300-573 K, expansion coefficient depends upon composition 'I! at 323-623 K, expansion coefficient depends upon composition
fly ash particles are retained when they impinge. The deposit tends to consolidate, by sintering and sulphation.
The deposit surface often becomes increasingly irregular particularly in a fouling deposit, as the size of the fly ash which is retained increases. Continued growth depends on simultaneous capture and consolidation, and the surface temperature gradually increases towards that of the local gases (Skorupska. 1993). Equally with a fluid slag, the surface may become smooth due to the glassy components on the surface. Fluid deposits will flow, and are affected by both gravitational forces and by local boundary layer effects.
The thermal characteristics of deposits are important. If the thermal expansion coefficients of the surface and the depositing material are similar, then the deposit will not break loose easily when the unit is cycled. On the other hand, if there is a significant difference between thermal expansion coefficients, then the deposits will shed more easily when the load is changed.
Thermal expansion coefficients for some of the materials involved are shown in Table 9. Chemical bonding is also important if it takes place. A high level of iron in the glass is a good indicator that the deposit will form a strong bond with a steel heat transfer surface.
4.3 Siagging Slagging refers to the deposition taking place in the section of the boiler where radiant heat transfer is dominant. These are the parts which can 'see' the main flame zone and include the burners, the main boiler waterwalls, the bottom hopper and, usually, the bottom of the first bankls of heat exchange tubes.
The deposition of ash particles to form a slagging deposit depends on a series of mechanisms. The waterwall surfaces are typically at a temperature of 200 to 400°C which is too low for mineral matter to melt. The deposits often appear to start with fine dry powdery material which sticks, and then more particles deposit so that it grows 'fingers' in a downwards direction. This is illustrated in Figure 25, for a situation where the tubes are horizontal which is more common in once-through type boilers. It may also apply in areas around a bumer port or to superheater platens in natural circulation. The fingers continue to grow, and the deposit insulates the walls so that the surface temperature increases. The thicker the deposit the greater the temperature gradient across it. Eventually the surface is at a high enough temperature for the new material to remain soft until finally it forms a running layer on the surface. The running fluid may consist of dissolved or molten fly ash, or it may be a glassy phase loaded with small solid particles. It may partially vaporise near the centre of the combustion zone. In some places the slag may freeze and build up a hard glassy deposit. For satisfactory boiler operation, there should be no long term build-up of slag deposits.
Many slagging deposits form from coals that have pyrite as a major mineral component. Another type of slagging deposit has sodium and/or calcium as the primary fluxing agent
55
Deposition processes and deposit properties
(a) (b) (e)
lii;'~1 porous, dry molten
Figure 25 Schematic of slag build-up (Heap and others, 1986)
rather than iron. Where there is iron, it tends to react with the alumino-silicates (clays) to form low melting point or low viscosity particles which stick to the walls or tubes. These deposits tend to be amorphous (dark, solid and glassy), or vesicular (glassy, but with trapped air bubbles and hence a sponge-like appearance). If the pyrite is not oxidised, then the presence of iron can result in deposits with a metallic lustre (Hart, 1990).
There is evidence that, in some situations, the initial layer deposited is enriched in sulphate materials and depleted in iron, while the outer layer at the higher temperature is iron-rich (Heap and others, 1986). The initial layers are often of very fine particles which can be highly reflective. They can also consist of simple oxides such as CaO, MgO, FeO, Fe203 and Fe304 as well as alumina and silica. The primary bonding mechanisms are dominated by silicate liquid phases. The fine reflective layer can considerably reduce heat transfer. Slagging is often caused by low melting FeS-FeO intermediates from pyrite oxidation.
Once an initial deposit has formed, its surface tends to become sticky, and most incident particles will be held on the surface, and cause a build-up. Because of this dominant mechanism, slagging deposits often have an overall composition close to that of the bulk fly ash in the region, particularly near the surface. As with other deposition phemonena, the composition depends on the coal being burned.
Boiler design normally allows for the presence of flowing slag on the walls. On the heat-exchange tubes, the liquid (or sticky-solid) deposits tend to build-up and with the cooling from the tube surface, tend to solidify. If the deposit remains as a liquid, it will build up into larger liquid drops. As these get bigger, they tend to fall off. If it is solid, then its surface temperature rises. It may become sticky, and then catch other solid particles, thus tending to grow and grow as conditions in the boiler cycle on a short-term basis.
In the Electric Power Research Institute (EPRI) Coal Quality Impact Model (CQIM), slag deposits are classified as follows:
running slag; a light deposit of <2.5 em; a medium deposit of between 2.5 and 7.5 em thick; a heavy deposit of over 7.5 em.
The physical nature of deposits is described as (Afonso and Molino, 1991):
A. very t1uid and runny;
B. molten and slowly creeping downward;
C. fused with rounded surface or molten with no apparent downward movement;
D. dry deposits with sharp edges;
E. other.
In order to maintain the steam production when deposition has taken place, the boiler flame temperature needs to be increased slightly. This can result in deposition elsewhere, and the slagging deposit spreads.
In a recent investigation in Australia into the slagging problems at two plants, a wide range of the techniques discussed here were used (Pohl and others, 1993). The investigation at plant AI, later identified as Callide, included trials with four coals from various opencast mines. These coals had widely differing iron contents, measured as Fez03 and ranging from 7 to 17%.
The investigation included:
coal, fly ash and deposit analyses; use of normal and low NOx burners; variations in the distribution of pulverised coal and air, and the degree of swirl: use of the normal boiler logs and records; particulate, temperature and gas concentration profiles (02, CO, C02, and NO); wall temperature measurements, and assessing wallblowing effectiveness; boiler modelling.
Sample analyses included:
both optical and scanning electron microscopy; also hot stage microscopy; standard ash fusion tests and analyses; the use of x-ray diffraction; energy dispersive x-rays; and Mossbauer spectroscopy; thermal/mechanical studies of deposit strength, and float/sink separations of the feed.
The tests showed that only one of the four coals produced any significant slagging. This was the one with the highest Fe203 content of 16.5 to 17%, although one of the other coals had 15.8 to 17.3% and did not cause a problem.
56
Deposition processes and deposit properties
The study concluded that slagging is sensitive to the heterogeneous deposit chemistry and the prevailing temperature. In this instance it was caused by the deposition of low viscosity materials on the wall. These had a high iron content and were molten at the prevailing surface temperature. One of the materials was iron corderite, which melts at about 1200°C. Initially, the low viscosity materials on the surface ingest other impacting particles until the whole deposit becomes partly fluid. These deposits were uniform throughout, and not layered in structure. They are difficult to remove with either sootblowing or wallblowing.
The iron may be responsible for deposition by either of two mechanisms, depending on its juxtaposition with other mineral species, or with carbon. Coarse, liberated, pyrite free of other mineral species can be responsible for slag which is virtually 85% Fe203, as a result of the deposition of partially combusted FeS2. It may also deposit as a silicate, as a result of the interaction of FeS2 with clays. The resulting slag would be a mixture of Fe203 or FeO and silicates, or of intermediate compounds (Bryers, 1994).
The outcome of this investigation was to pinpoint the fact that of four possible feedstocks for the plant, one was likely to cause slagging. The mechanism of the deposit formation and the reason for the difficulties in removing it have been much more clearly explained than was possible before the study.
The view of the investigators was that slagging was so sensitive to the individual chemistry of particles, and boiler temperature, that the use of coal analyses and norn1al power plant data only would be insufficient to predict deposition behaviour with any confidence. Specific and detailed information on the coal and on the conditions inside the boiler at the time of slag formation is needed to determine its cause and enable operators to take preventative action.
A major collaborative research programme is under way in the UK to address all aspects of slagging in pf furnaces. It includes full-scale plant trials, pilot plant work and laboratory-scale testing. Its intent is to enhance the existing methods for the prediction of slagging, accounting for the interaction of boiler design, operation and of coal composition. Four full-scale trials on 500 MWe boilers are included, three with low NOx burners. Preliminary results were presented by Gibb and Jones (1993).
These trials have used the resources of PowerGen, Diamond Power, Babcock Energy. British Coal, Bristol University and Imperial College, London as well as National Power. Three British coals from Bentinck (low slagging), Daw Mill (medium-slagging) and Silverdale (high slagging) were used. The number of organisations involved and the range of the work shows the extent of the testing needed to help understand and explain slagging behaviour. The main project was started in 1991, and is due to be completed in 1994.
In earlier test work, the same coal was used in two 500 MWe boilers, one of which was front wall-fired, and the other tangentially-fired (Jackson and Jones, 1989). Operational parameters such as excess oxygen, coal fineness, burner tilt
and supplementary oil firing were varied. Ash behaviour was assessed by using air-cooled deposition probes.
It appeared that decreasing the coal fineness by adjusting the classifier speed at the mill, had a small effect on deposition rate, with the deposition rate increasing as the coal feed became coarser. Both deposition and the degree of fusion increased as the excess oxygen was reduced from the normal operating value of 3.5%. It was accompanied by an increase in the proportion of ferrous iron in the deposits collected.
Changes in the burner tilt from +5° to 5° in the tangentially-fired unit produced unexpected results, in that the tilt upwards should have produced higher temperatures at certain probe positions, whereas lower temperatures were observed. The reasons for this were unclear, but it may have been due to changes in the lateral gas temperature distribution as the tilt was varied. Using oil burners immediately adjacent to operating pf burners exacerbates any slag formation tendency, due to high local CO concentrations and possibly the presence of vanadium. The observations illustrate the complexities of boiler operating conditions.
Later trials in the UK programme have looked at the effects of retrofitting low NOx burners. Four trials, each of five days duration are being undertaken, firing coals of varying slagging propensity. Some of the results are reported by Gibb and Jones (1993). The programme includes not only the trials on 500 MWe units, but also sootblower studies, use of a single burner test rig, ash deposition studies on a pilot rig, and drop tube furnace work. In addition, the coals used are being thoroughly characterised, and the various elements of the programme will be incorporated into a model in three stages:
to assess and predict the formation of fly ash particles, depending on the minerals in the feed coal, and operating conditions; to predict particle trajectories based on computational fluid dynamics; to develop an ash deposition model based on the temperature, heat flux, trajectory. and characteristics of the particulate matter arriving at the deposition sites.
Significant differences were observed in the nature of the deposits formed from each of the coals, but none caused an uncontrollable increase in slagging (Gibb and others, 1994). Comparing the results with previous experience, it did not seem that low NOx firing had any adverse effects. For all three coals, the initial deposit layer was markedly enriched in iron and calcium, relative to the bulk ash. This was due to the preferential deposition of pyrite-derived particles, and differs from the results of Heap and others (1986) discussed earlier.
Less pronounced chemical partitioning was found between the bulk deposit samples and the fly ash. The deposits from Bentinck and Silverdale coals were enriched in iron and depleted in silica and alumina. The Bentinck deposits alone were enriched in calcium. By contrast, the Daw Mill deposits were enriched in silica, depleted in alumina and with no change in the iron or calcium (compared with the fly ash).
57
Deposition processes and deposit properties
4.4 Fouling
Fouling takes place in the cooler convective heat transfer sections of the boiler, and results from the behaviour of various components as the gases cool down. In high temperature fouling, which takes place in the superheater and reheater regions, deposit bonding is commonly dependent on the presence of silicate liquids. In low temperature fouling in the economiser region, deposits occur when sulphur oxides from the flue gas combine with alkali components to form suplhates. These sulphate salts act as a glue, bonding ash particles together when they impact.
Fouling is commonly initiated by the deposition of a thin layer of material consisting of condensed vapours. The composition of this is typically high in alkali metals (Ca, Na or K) and may consist principally of their sulphates. The alkali metals form oxides or hydroxides which react with the S03 in the gas phase to form sulphates. They can also form low melting point eutectics leading to sticky deposits on the surface. Generally, sodium and calcium compounds dominate the initial deposit layers. As the deposits build up, they can sinter into a strong fused mass. Because of the insulating effect, the deposit surface temperature increases, and this can result in the formation of a liquid (or sticky) surface which will capture almost any impacting particle. This results in rapid build-up in the direction of the gas flow. This may completely encapsulate various deposited ash particles. The deposits are not uniform, and are built of successive layers differing in particle size and chemical composition. They are thus quite unlike slagging deposits.
Further deposition is of typical fly ash particles, many of which have a layer of condensed vapours on the surface and hence tend to link together after deposition. A typical build-up of a fouling deposit is shown in Figure 26. Deposits also form on the downstream side of a tube, due to eddy effects. It is the sodium chloride in coals with a bituminous type ash and the organically-bound sodium in coals with
direction of gas flow
deposit development t t t
tube
agglomerates of glass and melt phase with a few unreacted
inner sinter layer, discrete particles with
very little bonding of
outer sinter layer,
flow----.. initial fine particle layer
minerals
----. initial deposit
~ ----. ~ final wedge~ shaped deposit particles
inner white layer, rich in Na 2 S04
lignitic type ash, that are most commonly the cause of fouling (Osborn, 1992). Calcium is another major contributor.
Boiler manufacturers design the units so that as the flue gases cool down, the fly ash can either bypass the tubes or the deposits formed can be removed by sootblowing. At higher temperatures where there may be more sticky particles, the tubes are widely spaced, but as the flue gas temperature drops. it is necessary to increase the velocity and turbulence in order to maintain adequate heat transfer as the temperature driving force is reduced. Where the tubes are closely spaced it is easier for deposits to bridge across, and in extreme cases to fonn a major blockage. At lower temperatures most of the particles should be 'non-adhesive'. Progressive tightening of the tube spacing as the gases cool is illustrated in Figure 27, and an example of the build-up of deposits in Figure 28.
/, r' close spaced non-adhesion convection
/) 0 r ,,, /10 /10/10rOl °°°°°°°° zone heatingr01l01 rOl rOHOl °°°° surface
( ( ( intermediate101 101 10 spaced
r"1 convection
0 l heating r"1 r" ° ° \01 10i 10 0 surface
SLrOJ rOJ ~ ') ~ ') CO 0 0T
--1 ST r-
radiant panels high 'platinised' deposition rate heating
surface
SL = longitudinal spacing gas flow ST = transverse spacing
Surface spacing perpendicular to gas flow (plan view)
1372mm 610mm 305mm 229mm 114mm 114mm
gas flow ---{>
pendantplaten pendant secondary horizontal banks primary superheater reheater (reheater and primary radiant superheater)
superheater
furnace ~ convection pass
IE~ 1 ~,"ge Q" tempe,":"
~ outlet ~ ------..........:::..:::.:.:.:
Surface location
Figure 26 Structure of a fouling deposit (Sondreal and Figure 27 Tube spacing and arrangement (Richards, 1978; others, 1977; Wall and Lindner, 1988) Stultz and Kitto, 1992)
58
Deposition processes and deposit properties
Detailed work at the EERC looked at the fouling occurring in boilers burning high calcium low sulphur coals from the western US (Hurley and others, 1994). Sampling was carried out from five utility boilers, and supporting laboratory-scale studies were carried out. Advanced methods of analysis were used. The reported work concentrated on the mechanisms responsible for deposit fonnation.
The main sampling programme looked at five plants taking coals from four different mines. The plants included both cyclone (wet bottom) and dry bottom pulverised coal-fired units, and investigated the effect of load changes. A new probe which could be left in the boiler for long periods to acquire deposit samples was designed with a sample collector that retracted into an electrically heated tube furnace. The developing sample could thus be removed from the furnace during sootblowing cycles whilst minimising thennal shock. The outer surface of the probe was air-cooled to simulate a local steam tube surface temperature. It was planned to use the probe in connection with remotely controlled cameras to monitor deposit build-up.
It has been found that the low temperature deposits generally contain large amounts of sulphur. The sulphation occurs after deposition at temperatures below 1050°C where calcium sulphate is preferred over the silicate or oxide. The sulphate deposits fonn on the leading edge of tubes and can fonn a massive fin over 0.3 m long. They harden rapidly, and are difficult to remove by sootblowing. Such deposits may promote corrosion as well as interfering with heat transfer.
Deposits on the trailing edge of tubes tend to be of calcium-rich silicon-depleted material and result from particles of less than 3 ~m size, forming a thin enamel. Alternatively they may result from somewhat larger particles up to 10 ~m size deposited from the eddy currents. They are
well spread over most of the tubes, they are thicker, and fonn a significant barrier to heat transfer. On vertical tubes they tend to come off under their own weight.
Laboratory work on development of deposits showed that SO:? in the flue gases increases their compressive strength.
The EERC work illustrates the increasing knowledge of the mechanisms of deposition. Many phenomena specific to a given coal or power plant were observed. This comment is significant in relation to this report. It illustrates the diversity of the effects within a tightly controlled experimental study on a particular group of coals with broadly similar characteristics. Also methods of prediction and prevention arising from this work have yet to be fully assessed and reported.
4.5 Deposit properties The chemical and physical characteristics of the deposits found in utility boilers vary widely. They are of critical importance. In particular the key aspect is whether or not they can be easily removed. In addition, their thermal properties are significant, and also whether or not they are likely to promote corrosion in the areas where they are present.
The development of deposit strength is largely due to sintering of the deposit. This involves liquid phase (or viscous phase) flow over deposited particles such that some of the mobile material solidifies. It then acts as a 'glue' holding the deposit together. The resultant formation is denser and more difficult to remove. In fouling deposits, sintering can take place as particles coated with a liquid phase are in contact, and tend to fuse together. A number of mechanisms contribute to the phenomenon which is discussed by Benson and others (1993).
top view
There is no agreed method of testing deposit strength. Some investigators simulate sootblower action in their pilot plant in order to assess whether a deposit will be readily removable. ABB Combustion Engineering also use a mechanical tester to establish the physical force needed to detach a deposit. This is another area where the ultimate test has to be under boiler conditions, because any simulation is imperfect. Testing needs to be as far as possible at the temperature of the deposit in the boiler.
side view
superheater and reheater tubes
furnace
Figure 28 Deposit build-up on tubes (Hatt and Rimer, 1989)
A characteristic of some of the ash formed in the combustion zone is that it is composed of spherical particles, whereas the original mineral matter in the coal is of angular rough surfaced particles. This is because either the whole particle or its surface have become molten at some stage, or components may have vaporised and recondensed. The smaller alumino-silicate particles vitrify in the flame, and are partially recrystallised on cooling. Large quartz particles will only be vitrified on the surface. Typically in pf combustion, up to 25% of the coal quartz may remain in its original fonn in the fly ash. Quartz needs to reach a temperature from 1700 to 1900°C to vitrify while kaolinite and the potassium alumino-silicates vitrify in the 1400 to 1700°C range. Kaolinite contains some sodium, calcium and iron in the
59
Deposition processes and deposit properties
crystalline structure, and vitrifies at the upper end of the range indicated.
Attempts have been made to study the crystallisation behaviour of ash melts. Acidic components such as silica and ferric oxide tend to increase the viscosity. Basic components such as the alkali and alkali earth oxides decrease the viscosity. There are indications that the more viscous phases produce a weaker deposit, and as a result these are more easily removed.
Important changes in materials may take place after deposition. This is because a droplet or particle may stick on a relatively cold surface. Other particles may stick around it. Its surface is subject to radiative heat transfer on one side which may cause it to soften or even melt, forming a running slag. Thus the material may be shock heated, suddenly cooled on contact, and then reheated by radiant transfer and t10w to another part of the boiler.
In an extensive investigation of the properties of slags, samples were taken from eight UK boilers burning a variety of UK coals. In addition, one came from a pilot rig using a UK coal and one from a Dutch utility burning an eastern US coal (Wain and others, 1992). There are little fundamental data on the nature of slags, the rate at which deposition and growth occurs. or their properties relevant to removal by sootblowing. It was thought that an understanding of the relevant thermal and mechanical properties of the deposits could improve the current subjective approach to the selection and location of sootblowers in a boiler. This should lead to increased unit availability. Various mechanical properties were measured, such as thermal conductivity, the coefficient of thermal expansion, compressive strength and elastic modulus, porosity and crystallinity. All these measurements were carried out at ambient temperatures. In addition, thermal shock parameters were calculated.
Crack propagation in both glassy and crystalline materials occurs on rapid change in the surface temperature. Thermal stress leads to crack initiation and propagation. Below the glass transition temperature, boiler slags exhibit all the
5000
~ 4500
ci 4000
~ 3500
~ 3000 CIl c. 2500
..II::
g 2000 .c til
Ci'i E Qj ~
15001000 _:c:::;~"",,==.::r-~::::::::~--_
500
--o o 5 10 15 20 25 30 35 40 45 50
Porosity, vol%
Figure 29 Variation in the thermal shock parameter at different porosity values from 0 to 50 vol% (Wain and others, 1992)
characteristics of brittle materials. A method of ranking various materials against each other was developed, in terms of the temperature change (or energy) required to generate sufficient stress to initiate crack propagation. The indices depend on the elastic modulus, the temperature change and on the Poissons ratio, which relates transverse to longitudinal strain for either compression or tensile loading. This parameter was not measured, but a value of 0.25 was taken, as being typical of most glasses and ceramic materials.
It was shown that the thermal shock resistance was strongly affected by the degree of porosity (see Figure 29). Crack propagation occurs more readily in highly porous, glassy slags. Resistance to fracture increases with decreasing porosity and increasing crystallinity.
Ash deposits in boilers are significant because of their effects on heat transfer. The properties which influence these effects are dependent on the chemical and physical nature of the deposits. These in turn affect the absorptivity of the surface for radiative transfer, its roughness for convective transfer and its ability to conduct heat to the tube walls for water-steam heating.
The various mechanisms of heat transfer are shown in Figure 30. In the slagging region, radiative transfer predominates, but in the fouling region convective transfer is the larger component. The cycle of changes is commonly interrupted on a regular basis by sootblowing. Since the processes of heat transfer take place both in parallel and in series, the concept of an 'effective' conductivity of a deposit under particular conditions is commonly used.
Heat transfer depends on:
emittance from the flame; absorption/reflection at the surface; convective transfer (which is a small contributor in the slagging region).
The fly ash formed in the combustion zone is itself an important emitter of radiation. Together with the triatomic gases and unburned char, the fly ash affects the magnitude and spectral distribution of the thermal radiation incident on the boiler walls and tubes. The deposits on the walls reduce the absorption of this incident radiation and form a barrier
steam/
convective
incident radiation
tube deposit wall
flame water
transfer
Figure 30 Various mechanisms of heat transfer (modified from Wall and others, 1994)
60
Deposition processes and deposit properties
-----------------. Time I Growth I Consolidation I Maturity
I ), depos~ tube
10 final
0.2
(a) (b) (c) (d) (e) (1) Clean Fine Coarse Thickening, Rough Siagging tube ash ash consolidation surface
layer
Figure 31 Expected trends in deposit properties during growth (Wall and others, 1993)
steady state
intermediate stead~ state
06
08
0.4
0.2
0.6
~k/x (kW/mOK) 0.4 conduction coefficient
a absorptivity
across which the absorbed heat has to be transferred. Total absorptivity is a function of both surface temperature and the source radiation temperature.
There have been many studies on heat transfer effects, and, for example, measurements of the spectral emittance for particulate ash were made by Wall and Becker (1984). They showed that ash deposits have a low emittance and therefore high reflectance, at wavelengths below 6 !-Lm.
The changes in deposit properties during formation are illustrated in Figure 31. The data are illustrative of the kind of changes taking place. The deposit is notionally in the slagging region, and the two main parameters are:
the total absorbence, which depends on the spectral distribution of the incident flame radiation and also on the physical state of the surface; the conduction coetlicient.
For the clean wall (or tube), the absorbence is high at around 0.8. The initial deposit layer is likely to be either condensible salts or fine ash transported by thermophoresis. For fine particulates of (say) 2 !-Lm, the absorbence may drop to as low as 0.3. As the layer thickens, larger particles start to deposit and the absorbence increases. As the deposit thickens and consolidates, the temperature gradient through the deposit continually increases, so that the surface temperature rises. Eventually this will result in sintering and fusion. As
the layer builds and the surface temperature increases further a liquid slag will develop with increased absorptivity.
The conduction coefficient through the deposit drops rapidly as the deposits form. The surface temperature will continuously increase. Sintering and fusion will result. When consolidation starts the conduction coefficient and absorptivity may increase associated with the rough surface formed. When liquid slag develops, the conduction coefficient may increase or decrease compared with its value for a sintered deposit.
Pilot-scale work at the EERC has measured the temperature gradients in ash deposits (Benson and others, 1990). The work was carried out on two lignites which had just over 30% moisture and about 10% ash content, by proximate analysis. Externally manipulated thermocouples were placed in deposits while they were forming, taking precautions to avoid disturbing natural deposit growth. The measurements quantify gradual decrease in temperature of the lower layers as they become more insulated. Differences of as much as 170°C were measured in various parts of the deposit. The different layers were characterised using CCSEM techniques. The inner layers were found to be rich in calcium sulphate. In some cases the outer layers were rich in calcium as well, in others, as a result of the higher temperature, more complex alkali alumino-silicate phases formed (such as mullite and plagioclase).
61
5 Predictive indices, modelling and scale-up from tests
Boiler design and the choice of a coal for an existing unit has always been, and still is, largely based on empirical indices derived from past experience. Design depends principally on experience with other coals which are thought to behave in a similar fashion, or on the same coal which it is assumed, will continue to have the same characteristics. Boiler designers would like to have more secure and accurate indices for the prediction of ash deposition.
However, the bigger problem to be tackled is the provision of guidelines and information to the boiler operator, to assess the effect of burning a coal feed with different ash-forming components from that with which he is familiar. He needs to know what overall effect on the boiler operation the new feedstock will have.
During the past 10 years, a number of techniques have been tested and developed which can help with both diagnosis, in a boiler where there are problems, and with the prediction of unwanted ash deposition.
These techniques are principally:
measurements and samples taken from inside a boiler; test work using drop tube furnaces and pilot combustors; the addition of CCSEM and other advanced analytical data to the coal and laboratory ash analyses on which existing indices are based; the direct measurement of the result of deposition by heat flux metering; the modelling of the flow pattems and combustion behaviour of the various components.
The main purposes of predictive indices, modelling and of test work, are:
the assessment of alternative coal supplies for a boiler; to provide data so that engineering and/or operational
solutions can be found to particular slagging and fouling problems in an operating unit; to provide the basis for the design of new boilers.
5.1 Predictive indices A number of indices have been used to predict ash behaviour and deposition tendencies. They are based on experience, with different coals in a number of different boilers. These indices have provided the basis of virtually all boiler design, and coal feedstock assessment for existing boilers. They are still widely used, in spite of their shortcomings, and probably remain the most secure basis for decision making, if used in conjunction with pilot plant testing.
In an extensive EPRI study during the early 1980s to evaluate the predictive indices being used, it was estimated that about 80% of deposition problems could be predicted with reasonable confidence using a knowledge of the laboratory ash chemistry, and of combustion intensity. Many of the indices apply to particular coal types. It should be noted that the 80% confidence level was achieved largely in US utilities burning indigenous coals, backed by a great deal of experience. If the study had been able to include the low grade coals used in China, eastern Europe, and India, the predictive indices might have been even less successful.
In a report prepared for the working group of the International Union of Producers and Distributors of Electricity, on the chemical aspects of slagging and fouling in pf boilers, it was concluded that the predictive capability of current methods achieved only about 60% success rate at best. The methods are relative. While methods based on chemical composition are the easiest to apply, physical methods can give a closer simulation of the combustion process. However, they require expert interpretation, and are unlikely to become standard for some time (Snel, 1988).
In a study on 60 boilers in China it was reported that the
62
prediction of fireside behaviour based on laboratory ash analyses and the use of conventional slagging indices was unreliable. When the indices were combined in a correlation with boiler heat release rate, the possibility of accurate prediction increased to about 80% (Unsworth and others, 1991).
A widely used descriptor for the prediction of deposition behaviour, based on laboratory ash analysis is the base-to-acid ratio, where 'base' and 'acid' are simply the sums of the weight percentages of the basic and acidic oxides:
Acid = Si02 + Ah03 + Ti02
In order to explain and predict behaviour more completely. it has been necessary to develop more detailed data.
Each boiler manufacturer, and many major operators, have developed their own indices to provide criteria for various aspects of design and operation. Some of the indices have been published, but much of the detail is regarded as proprietary information. All are based on the laboratory ash analysis or fusion temperatures. The principal indices in use are summarised in Table 10. These are based on a number of indicators, which include:
the viscosity of laboratory ash; various ratios, such as iron-calcium, and silica-alumina in laboratory ash; sodium-in-ash content; total alkali metal content in laboratory ash.
The fluxing effect of certain oxides is taken into account in some of the equations. Increasing concentrations of the basic oxides of iron, calcium and magnesium tend to lower the ash viscosity and increase slagging tendency.
For coals with a bituminous type ash, the base/acid ratio is often used as an indication of the tendency of the ash to form a low melting point eutectic. It is then liable to be more sticky at a lower temperature. The low melting point eutectic is formed at a base/acid ratio of 0.75, and since the base/acid ratio for most bituminous type ashes is below this, a high base/acid ratio is an indication of increased deposition potential.
A standard slagging index for a coal with a bituminous type ash is the product of the base/acid ratio and the sulphur content. The sulphur content is an indication of the quantity of pyritic iron in the mineral matter and this influences the degree of oxidation of iron in the slag, affecting its slagging behaviour.
The slagging index for a coal with a lignitic type ash is commonly based on ash fusion temperatures.
Attempts have been made to correlate slagging behaviour with the composition of the higher density fractions in pulverised coal. These fractions (for example ov"j3 2.9) are often enriched in iron, associated with the presence of a
Predictive indices, modelling and scale-up from tests
high proportion of pyrite. The data in Table II show a strong similarity between the 2.9 gravity fraction and the slag formed in the case of a lignite. The relationship for a bituminous coal is not particularly striking. This illustrates the coal-specific nature of most work in this field, and the need to develop a more realistic base than that of the laboratory ash composition of a bulk coal sample.
A standard fouling index for a coal with a bituminous type ash is the product of the base/acid ratio and the sodium oxide content of laboratory ash. This attempts to estimate the tendency for particles to adhere either because of their inherent stickiness, or the effect of the condensation of volatile sodium compounds on surfaces. An alternative approach is to use total alkali content.
For a coal with a lignitic type ash, fouling tendency is judged mainly on sodium content. The use of all these indices for bituminous and lignitic type ashes is based on empirical practice (Unsworth and others, 1991).
The indices have been used to good purpose over many decades, but were initially established under different conditions from those now applying. Boilers were smaller, and the steam temperature and pressure conditions were less arduous. Coal sources were more limited. In particular, many boilers were probably designed more conservatively, as capital cost was not scrutinised as carefully as it is today.
The various empirical methods were developed from limited data sets, in response to specific problems, and it is not surprising that they have limitations when the framework on which they were based is changed. Until many of the newer methods under development move boiler design and operation beyond empiricism and into the realm of science and engineering, it would seem to be sensible to develop and refine the existing indices. This can be done against new data sets and a wide range of design and operating parameters (Smouse, 1994). The assessment of new techniques and indices should be made, taking this background into account.
5.2 Boiler modelling Models are being developed to help in the diagnosis and correction of operational problems due to deposition and also in the prediction of the fouling or slagging propensity of a particular coal.
Because of the substantially increased power of computers and the availability of advanced analytical data, it is possible to contemplate the development of models to describe extremely complex situations. Potentially, these computer simulations can give insights into the phenomena occurring inside the combustion fireball area and subsequently during flow through the banks of heat transfer tubes. There are, however, formidable difficulties, because of the complexity of the mechanisms and lack of precise data on conditions. Consequently there are widespread doubts about the applicability of such models to the area of ash deposition. This is based on the belief that the process cannot be accurately modelled while the basic mechanisms are not clearly understood. It is recognised, however, that modelling
63
Predictive indices, modelling and scale-up from tests
Table 10 Summary of coal ash indices (Skorupska, 1993)
Index Factors
Ash descriptor Base-acid ratio (BfA)
Ash viscosity T250 of ash, °C (OF) Silica ratio
Slagging propensity Base-acid ratio (BfA) (for lignitic ash: %CaO + %MgO >%Fe203) Slagging factor (for bituminous ash: %CaO + %MgO <%Fe203) Iron-calcium ratio Silica-alumina ratio
Slagging factor, °C
Viscosity slagging factor
Fouling propensity Sodium content
Fouling factor
Total alkaline metal content in ash (expressed in equivalent %Na203)
% chlorine in dry coal
Strength of sintered fly ash, Psi
Temperature when ash viscosity = 250 poise %Si02f(Si02 + Fe203 + CaO + MgO)
(BfA)(S dry)
%Fe203f%CaO %Si02f%Ah03
Maximum hemispherical temperature + 4(minimum initial deformation temperature) 5
T250 (oxid) - T lllOOO (red) 975 Fs
(Fs ranges from 1.0-11.0 for temperature range 1037-1593°C)
%Na203 (for lignitic ash: %CaO + %MgO >%Fe203) (for bituminous ash: %CaO + %MgO <%Fe203)
BfA(%Na20 in ash) (for bituminous ash: %CaO + %MgO <%Fe203) BfA(%Na20 water soluble/low temperature ash) %Na20 + %K20 (for bituminous ash: %CaO + %MgO <%Fe203)
oxid red
oxidising conditions reducing conditions
Table 11 A comparison of slag and coal ash chemistry (Bryers, 1988)
Lignite
Slag, Slag, Bulk -2.9 gravity inner layer outer layer ash fraction
Bituminous coal
Slag Bulk ash
-2.9 gravity fraction
Si02 6.9 12.5 47.9 4.9 17.7 47.5 30.3 Ah03 8.9 8.5 30.6 2.8 10.9 23.6 3.8 Ti02 0.2 0.3 1.0 0.1 0.5 2.0 0.1 Fe203 79.2 73.7 8.3 86.3 55.7 23.6 29.9 CaO 1.3 2.5 3.4 0.1 4.5 9.8 23.2 MgO 1.0 1.3 1.5 0.8 0.8 1.6 2.4 Na20 1.8 2.6 0.2 1.4 2.7 2.3 0.7 K20 0.2 0.4 1.4 0.2 0.8 1.2 0.5 S03 3.5 5.0 2.9 5.4
Ash fusion temperature 1540 1285 1540 1315 1340 1160 (reducing) °C
can be helpful in the diagnosis and correction of particular providing an 'overall' model of the whole process with deposition problems. predictive capabilities. It is probably best to regard the
development and validation of particular models in the Many utilities remain sceptical about the possibilities of context of the particular units, operating conditions and coals
64
Predictive indices, modelling and scale-up from tests
Table 10 - continued
Tendencies/values
Low
>1302
Medium
1399-1149 Viscosity proportional to silica ratio
High
1246-1121
Severe
<1204
<0.5 0.5-1.0 1.0-1.75
<0.6 0.6-2.0 2.0-2.6 >2.6
<0.31 or >3.00 Low
>1343 1232-1343
0.31-3.0
1149-1232 <1149
High
0.5-0.99 1.0-1.99 >2.00
<2.0 <0.5 <0.2 <0.1 <0.3
2.0-6.0 0.5-1.0 0.2-0.5 0.1-0.24 0.3-0.4
6.0-8.0 1.0-2.5 0.5-1.0 0.25-0.7 0.4-0.5
>8.0 >2.5 >1.0 >0.7 >0.5
<0.3 0.3-0.5 >0.5
<1000 1000-5000 5000-16,000 >16,000
with which they have been tested. Given the shortcomings of existing predictive indices relating to ash deposition, it would seem likely that using better analytical data about the mineral matter in coal, and with a better understanding of transformation mechanisms, the prospects for prediction will improve.
There have been attempts to validate some of the models against the results from small-scale combustion test rigs, and a few modellers have attempted validation against full-scale boiler operating conditions.
The development of models to evaluate all or part of the coal-to-electricity chain have been discussed by Skorupska (1993). Models are of varying complexity, ranging from very large models which emphasise the basic science of combustion, and may neglect the realities of the boiler, to simply a collection of 'rules-of-thumb' which are often too simplistic to be useful (Rees and Symonds, 1993). Some models look particularly at traded coals, and attempt to address the issues involved in choosing the 'least cost' coal/coal blend. Examples of this type include the International Coal Value Model, and the Least Cost Fuel
System both used in Australia. In addition there is Perfectblend and the Steam coal blending pial! used in the USA. They do not, generally, look at the cost of slagging or fouling, at variations in plant availability nor on the impact of coal quality on gross power station heat rate and efficiency. One such model which does include the effects of ash deposition, the Busbar model, is described below.
In addition, there are both component evaluation models, and whole plant models. These attempt the evaluation of coal quality impact on subsystems (such as the boiler chamber) or on a whole operating unit (including coal handling, milling, the boiler and flue gas clean-up). Most of these address the likelihood and impact of ash deposition on boiler operation, and examples are discussed below. The amount of computer power and the time taken to run any test condition on a model (particularly an integrated plant model) is enormous.
An example of a comprehensive model is that of the Massachusetts Institute of Technology (MIT), USA, where there has been a long-standing programme to look at fly ash deposition tendencies (Beer and others, 1992a, b; Barta and others, 1993, 1994). This has increasingly used the advanced
65
Predictive indices, modelling and scale-up from tests
CCSEM of size classified coal
stereological correction chemical composition correction
mineral size distribution mineral chemical composition distribution
viscosity distribution (estimate coalesceable mineral matter fraction)
urn model (particle to particle variation of coalesceable mineral matter)
combustion a) shrinking core/percolative fragmentation b) cenospheric (swelling) particle burning
random coalescence of mineral matter
fly ash size and chemical composition distribution
calculate density and viscosity distribution
impaction coefficient sticking coefficient
ash mass/heating value of coal
Figure 32 Modelling of the relative fouling tendency of a coal at MIT (Beer and others, 1992a)
techniques available. Currently the work is attempting to integrate a number of computational codes. These cover the characterisation of coal minerals by CCSEM, predictions of fly ash size and composition, and the probability of particle impaction and retention. The interrelation between the various parts of the model is shown in Figure 32. Several submodels are involved, and these have been tested, submodel by submodel. MIT have also compared the combined models with some experimental data. When it has been thoroughly validated, it is a possible tool for determining the relative fouling tendencies of different coals, or of blends, comparing them with a coal of known deposit-forming characteristics.
The only experimental-analytical data required are the CCSEM figures for the raw coal. A stereological correction programme is applied, based on a statistical method to assess three-dimensional size distributions from two-dimensional data. The 'coalesceable fraction' is the proportion of mineral
matter of sufficiently low viscosity to coalesce, calculated from mineral size and temperature distribution.
Calculations of particle to particle variation of the coalesceable fraction provides mean values and variances of individual chemical species concentrations. The framework is set by the application of a combustion model and the implications of the char combustion mechanism for the movement and relative positions of mineral particles.
The random coalescence model provides information on fly ash size and composition. This gives density distributions which are fed into impaction model calculations. The composition figures can be used to estimate the distribution of slag viscosity for a given temperature and hence a 'sticking coefficient'.
A relationship between ash weight per cent, coal heating value, the impaction efficiency and the sticking coefficient
66
40
30
20
10
o
Predictive indices, modelling and scale-up from tests
gives a figure for the relative fouling tendency for the coal in question. Some experimental work to validate the model has been carried out on industrial-size flames (2 MW) in the MIT combustion research facility. Five coals or coal blends have been tested and the results classified to produce a ranked order with respect to the deposition behaviour of their fly ash in a given boiler.
A factor not considered is that some deposited fly ash may subsequently be eroded; it may detach when a certain thickness of deposit is reached, and there may be further transformations in the deposit while it is attached. These effects are considered to be of second order and while they should not be ignored, MIT suggest that they are unlikely to change the relative fouling tendency of fly ash as determined by the model.
The outline above illustrates the large number of interlinking assumptions that are necessary in order to develop a predictive, largely theoretical, model. One of the more difficult requirements is to assess the time temperature history of groups of particles, and hence the length of time during which they may have the necessary viscosity characteristics affecting their tendency to stick.
The MIT work has been integrated with that of a number of other investigators in a range of programmes in the USA, notably in the US DOE study on Tran~formations of inorganic coal constituents in combustion systems (Helble and others, 1992c). Work on advanced predictive indices at the EERC and by PSI PowerServe is discussed in Section 5.2.4.
Work on developing an understanding of the mechanisms of ash deposition has been going on for several years at the Sandia National Laboratories, Livermore, USA. While no generally applicable or validated model of ash deposition in a boiler has been developed, a recent paper discussed progress towards that goal (Baxter and others, 1992). Some quantitative data on ash deposit properties as a function of coal type and operating conditions have been published for several pilot-scale tests (Chow and Lexa, 1987; Griffith and others, 1988; Loehden and others, 1989; Helble and others, 1989). A few papers have attempted to relate pilot-scale tests to behaviour in full-size boilers (Baxter, 1991).
Sandia has developed a reasonably simple model that aims to predict the elemental composition of ash deposits. It is based on both first-principle derivations, and a series of experimental results. The model is compared with combustion systems of several different sizes and coals of different rank. Comparisons of the predictions with experimental data are discussed (Baxter and others, 1992). A schematic of the model is shown in Figure 33. Primary inputs are shown on the left. The model is designed to predict the path of particle clouds through the boiler and the response of the particles to the changing environments encountered. The solutions to the numerous differential equations involved are used to calculate deposit properties such as elemental composition, viscosity and emissivity.
To compile a database for testing the model, combustors of
six different sizes were used. The largest was a 600 MWe unit in Illinois, USA under test with a western coal. Figure 34 illustrates the results of predictions and measurements at the furnace exit in the 600 MWe unit. Predictions are claimed to be in reasonable agreement, although calcium is significantly over-predicted, and silicon and aluminium are somewhat under-predicted. The predictions represent an improvement over ASTM coal ash approximations to deposit composition. The model predictions were typically within ten relative per cent of measured values, but it was also recognised that there is ample opportunity for improvement of the model. Rates of condensation and heterogeneous chemical reaction need to be finalised and validated within the model codes. Also, particle capture efficiency needs to be established in more fundamental terms. Finally, deposit properties such as emissivity, viscosity, removability and thermal conductivity need to be related to elemental composition.
------..,
coal specifications
iteration on boiler size and type
specifications coupled differential
equation solver for particles operating conditions
initial conditions
Figure 33 Schematic of the Sandia National Laboratory (ADLVIC) model (Baxter and others, 1992)
'i" "C ')( 0 til t'Cl "C Gl
~ til t'Cl Gl .s 'E Gl U
iii a. til til t'Cl
::i:
50 • predicted
[3illI observed
Figure 34 Comparison of the predicted and measured deposit compositions near the furnace exit of a 600 MWe unit (Baxter and others, 1992)
67
Predictive indices, modelling and scale-up from tests
The results suggest that ways of determining the modes of occurrence of calcium need to be developed and refined. In addition, more data need to be gathered on ash deposit properties from full-scale boilers under carefully controlled and well characterised operating conditions - with a range of coal feeds. The discussion and results illustrate the complexities involved in using models to predict the behaviour of ash-forming materials in boilers.
Modelling work at the Brigham Young University, Provo, USA, is focused on the development and integration of submodels to describe ash formation and deposition into comprehensive models/codes which simulate turbulent combustion. Two such codes are being developed, one two-dimensional (pulverised gasification/combustion - two dimensions PCGC-2), and the other three-dimensional (pulverised gasification/combustion - three dimensions PCGC-3).
The combustion code is used to predict the flow field, temperature field, radiation field and particle transport, in an axi-symmetric combustor for different coals at different firing rates. The deposition model is then used to examine the effect of different coals and operating conditions on deposit growth, thermal properties, porosity and composition. The calculations are applicable to the slagging deposits formed in the radiant section of a boiler.
In these models the gas-phase flow field is calculated with the use of steady-state finite difference techniques, together with a turbulence model. Gas properties are calculated by integrating the instantaneous properties over a probability density function based on turbulence statistics. The code also includes a description of energy transport (by convection and radiation) for both particle and gas phases, as well as transfer between phases. The models can be used to calculate local compositions, temperatures, velocities and heat fluxes, all of which are important in understanding ash formation and deposition.
Early work on modelling deposition in conjunction with the two-dimensional code was done by Jamaluddin and Smith (1990) who utilised an eddy impaction model to approximate impaction rates. Recently a more sophisticated deposition model has been added to the code (Richards and others, 1993). This uses a stochastic description of the particles, to approximate impaction rates at the wall.
The model then attempts to determine:
which of the particles transported to the surface will stick; the local value of both the porosity and thermal conductivity of the deposit; the changing emissivity of the surface during deposit growth; the fraction of liquid and crystalline phases in the deposit.
Particle sticking was determined on a particle-by-particle basis for a representative number of particles, based on the physical characteristics (viscosity/stickiness) both of the impacting particle, and of the surface. No attempt was made to simulate the detailed morphology of the deposit surface.
The particles which formed the deposit were assumed to approach their equilibrium composition if their viscosity at the local deposit temperature was below a critical value. The local phase composition was determined, and used to estimate porosity.
A feature of this model is that it includes the coupling between furnace conditions and deposit properties. The model has been applied to an experimental system studied at ABB Combustion Engineering under the CQE programme. The work involved a blend of Wyoming and Oklahoma coals, and a blend of Wyoming and a cleaned Oklahoma coal. Two firing rates were used. The results were discussed by Richards and others (1993). Qualitative agreement was observed between predicted and observed deposition behaviour. The local thermal conductivity of the deposit was predicted to increase sharply with the formation of a liquid phase. The effective conductivity in fact remained relatively constant, indicating that the inner deposit layer dominated the thermal resistance.
The three-dimensional code (PCGC-3) was recently evaluated by comparing predictions with experimental data from an 85 MWe boiler (Hill and Smoot, 1993). Measured and predicted temperatures and species concentrations were in good agreement in most regions of the furnace. Predicted particle trajectories provided insight into areas prone to deposition. The model did not, however, correctly indicate the exact location of ignition, leading to some discrepancies in the near-burner region. Discrepancies between measured and predicted values occurring near the furnace walls may have been due to difficulties in obtaining accurate measurements. Efforts are continuing to develop the three-dimensional code. and when it has been established, the deposition submodels will be integrated in a similar way to their use with PCGC-2.
In Australia, a comprehensive three-dimensional furnace model capable of predicting gas flow, coal combustion and radiative heat transfer has been used. The model was developed by Pacific Power and the University of Sydney. It has been validated using operational data from 500 MWe tangentially-fired units at Liddell, NSW. Problems cUlTently under investigation using the model include the reduction of ash deposition in the burner quarls of a 660 MWe unit (Boyd and Kent, 1994).
5.2.1 International Flame Foundation work
In recent years, detailed experimental measurements of temperatures, velocities, gas compositions and heat fluxes have been calTied out in coal flames at a number of sites. These have included the International Flame Research Foundation in the Netherlands, the Ris0 National Laboratory in Denmark and Imperial College in the UK. Single pulverised coal burners up to about 1 MWt output have been used, in cylindrical test furnaces. Several computational tluid dynamics (CFD) codes have been used to model these flames. These involve the numerical solution of the mass and momentum balance equations for the flow, based on a computational grid based on the geometry of the system. Various submodels represent processes such as radiation,
68
Predictive indices, modelling and scale-up from tests
particle transport, coal combustion and the production of potential pollutants such as sax and NOx.
In the course of computing these flame characteristics, the accuracy of the various submodels has steadily improved, and it is thought that predictions are now reasonably accurate for these relatively simple geometries and at this small scale (Fuell and others, 1993).
In full-size furnaces. burner thermal input can be of the order of 40 MWt. Virtually no validation data are available at this scale. In addition, boilers are fitted with multiple burners, and their interaction is not well understood. Finally, the computation of three-dimensional multi-burner geometries is extremely expensive if each of the burner near fields is to be resolved with accuracy. As a result, successful predictions of conditions in full-scale furnaces using these codes have not yet been demonstrated.
Work is continuing at the International Flame Research Foundation as part of the lEA Coal Combustion Sciences Annex 2 programme with detailed measurements on a 12 MWt coal burner. Over the past two years, PowerGen in the UK has carried our extensive measurements using a 5 MWt multi-burner Furnace Modelling Facility. As a result, valuable information is being obtained for code validation at larger scales and with more complex geometries.
There are two main obstacles to the successful computation of detailed flow fields in a multi-burner furnace:
the difficulties introduced by the complex geometry and the degree of resolution required where burner overlap occurs; the limited accuracy of the various empirical submodels being used.
To use a fine mesh pattern of computation points for resolution and extend it throughout the area to take account of all the burners would involve computer capacity far beyond that currently available. Some workers have developed robust algorithms for transitions from a region of one mesh density to a coarser or finer one. However, these are not available for the codes commonly used in the UK.
To make the computation practicable, two different simplifications have been used. These are the 'effective burner' strategy and the 'solution matching' strategy.
In the first of these, which is the simpler of the approaches, the accurate resolution of the near-burner zones is replaced with a coarse effective burner representation. As a result, only the far field prediction, away from the burner zone, yields any useable information. The processes of devolatilisation, gaseous combustion and the formation of NOxand sax are neglected as they take place in the near-burner zone. Char-oxidation, radiation and wall heat removal can all be assessed with a reasonable expectation of accuracy.
The availability of some corroborative validation data is crucial to the use of the 'effective burner' approach. Adjustments can then be made and properly tested.
In the second simplification, the near-burner zone is computed on a fine point mesh for each buruer separately. Data from appropriate planes in the resulting converged solutions are taken and interpolated on to a coarser mesh for the whole furnace. The disadvantage of this is that the near-burner zones and the furnace far field are not coupled, and therefore boundary conditions must be specified at each of the interfaces. The magnitude of the error in assessing these conditions is critical to whether the 'solution matching' strategy can produce useful engineering predictions.
While the study is incomplete, it illustrates the difficulties involved in the modelling approach to assessing and predicting the behaviour of materials in a boiler furnace. Using some simplifications, it is possible to predict aspects of the furnace flow field away from the burner area. Velocity measurements made using a laser doppler velocimeter showed that significant secondary flows penetrate the burner <UTay close to the front wall to satisfy the entrainment requirements of the inner burners. The 'solution matching' approach has shown that these flows can be resolved using a combination of symmetry and pressure boundary conditions (Fuell and others, 1993).
5.2.2 The Busbar model
In connection with the development of a large new power plant at Paiton, Java. Indonesia, a model has been developed to help negotiate the coal supply contracts for the first two 400 MWe units. It was anticipated that the subbituminous and low volatile bituminous coals from southern Sumatra and eastern Kalimantan would be used.
Northwest Mine Services Inc from the USA developed a simple computer model to provide an estimate of the impact of coal quality on the power plant (Rees and Symonds, 1993). It was called the Busbar Model, and was designed to run on readily available spreadsheet software. Its application is illustrated in Figure 35. It was based on production costs for US power plants using subbituminous coals.
It was recognised that the operating and maintenance component of the production cost was difficult to estimate, in terms of making realistic comparisons between different coals. The assumption was made that these costs would be proportional to some measure of 'coal quality'. To validate this, Northwest looked at the reported O&M costs at eight power plants in the USA, and established that about 30-40% of total production costs could be described as fuel sensitive. The data showed only a slight trend towards increased costs for poorer fuel quality, and that year-on-year variability in O&M costs can be substantial. In the absence of other data, a correlation of production costs against ash loading in the feed coal was used, from over 30 plants. The data are for US plants and US subbituminous coals. They show that at a level below 3.6 kg ash/GJ, there is a 17-22 chance that production costs will be below US$20/kW. Above that value there is only a 1-9 chance.
Ash deposition in the boiler and its effect on heat transfer and hence the plant heat rate was taken into account by estimating an average tendency to slag or foul. Five methods
69
__
GEOLOGY
TECHNICAL EVALUATION
OF MINE
RESERVES
MINING CAPABILITY
Predictive indices, modelling and scale-up from tests
COAL DATA
B_O_IL_E_R_D_A_JA_---'I--~,
CONTRACT TRANSPORTATIONCOAL SELECTIONNEGOTIATIONS
COAL PURCHASE
Figure 35 The role of the Busbar model in coal purchase evaluation (Rees and Symonds, 1993)
(or indices) were taken into account to estimate slagging potential. A similar approach was taken for fouling, and the common indices based on alkali-metal content in laboratory ash were used. This approach indicated a relatively low tendency to slag, and a moderate potential to foul. An operating penalty is assessed, according to the amount of steam used for sootblowing.
It was recognised that the model will need upgrading, based on operating experience in the new boilers. As yet it is untried. The first units are expected to be on-line in 1994. The model provides a useful insight into the realities of forecasting and predicting the behaviour of inorganics in boilers in situations where there is little or no experience with particular coals. It is necessary to extrapolate from experience elsewhere with coals that are regarded as broadly similar. In this particular example, it would appear that extensive characterisation of the mineral matter in the coals was not considered to be worthwhile, beyond standard laboratory ash analysis. It will be interesting to see how close the Busbar Model comes to predicting costs. It is typical of many models in that it is empirically-based, it deals with probabilities and uses laboratory ash analysis. It reflects the practicalities of using current knowledge to make decisions.
5.2.3 CQE/CQIM
The most comprehensive model is that developed by EPRI together with Black and Veach in the USA where the Coal Quality Impact Model (CQIM) is a submodel of the Coal Quality Expert (CQE). CQE is being developed by CQ Inc which is a subsidiary of EPRI. These models aim to use information about the coal being used (or proposed to be used), to make predictions about the resultant performance of the boiler, and in particular to assess the cost implications.
Part of the driving force involved is the need to assess the effect of changing the coal feed to existing boilers. Given the increasingly stringent regulations governing the emission of particulates, SOx and of nitrogen oxides, the need for a predictive tool has increased. Utilities generally have the option of adding-on emissions controls or of coal switching, which can be done more quickly with minimum capital cost. A CQIM gives utilities the opportunity of comparing the overall economic effects over a period of years.
In particular, a CQIM may be used to pre-screen coals for test burns, and to predict changes in equipment performance. It can allow for the adjustment of operating conditions so as to maximise the positive impacts and minimise the negative ones for a particular coal (Stallard and others, 1993). This may be its most important current use, since a detailed programme of test burns is still an essential stage before committing a long-term contract for coal supplies (Palvish and others, 1992).
CQIM was introduced in 1989, and has been used by about 100 utilities, engineering firms and coal companies. Users have input their own data and adapted the model to their own conditions and experience. This has been mainly in the USA, but it has also been applied internationally. Models have been built for tangentially-fired, wall-fired, opposed-fired and cyclone-fired units, of capacities ranging from 100 to 1000 MWe. It is the most widely used systems-engineering model.
The CQIM provides a method of estimating the overall costs of changes in coal quality. The most recent version CQIM386 allows the user to establish 'break even' costs for a number of possible coals.
CQIM uses detailed equipment models, based on general
70
Predictive indices, modelling and scale-up from tests
engineering principles for all key systems. Thus most kinds of plant can be accommodated. A CQIM applies to a particular unit and needs to be carefully calibrated and validated against measured and recorded operating characteristics for a baseline coal. Once the validation is complete, the engineering models can be used to predict the impacts of using a variety of different coals. It is also designed to consider the range of acceptable values for key variables.
The prediction of slagging and fouling in a boiler is not the central aim of either CQIM or CQE, although these are important factors which affect boiler performance and economics. Two submodels which address slagging and fouling are being developed jointly by EERC, PSI PowerServe and ABB - Combustion Engineering. These interact with each other, and also with the boiler performance model to account for changes in coal feed properties and operating conditions (Erickson and others, 1993, 1994). The Stagging AdvisOI·™ is discussed in Section 5.2.4.
It must be emphasised that at this stage. the information fed to the CQIM is that about laboratory ash. As a result, and in relation to predictions about ash deposition, its operation is in a sense compromised. The submodels discussed above are attempting to integrate and use the more advanced analytical data now potentially available.
The part of CQIM which relates to the slagging and fouling impacts of coals is discussed in the EPRI Fireside Test Guidelines. These describe the tests and instrumentation needed. The main options and requirements are:
FEGT measurement, and other temperatures through the system; slagging and fouling probes; radiant heat flux measurements; direct observations, possibly by camera or video; furnace wall atmosphere measurements; coal feed and ash deposit sampling.
The CQIM can be implemented at a number of different levels, depending on available information, and the operating/economic problems perceived at different plants.
Six coal-burning utility sites in the USA are being used for full-scale combustion testing to assess and validate the CQIM/CQE. One set of tests was carried out at the Mississippi Power Companies Watson unit 4 of 250 MWe capacity (Lowe and others, 1993; Vitta and others, 1993). Two coals were used, with a 4 week test burn each. The study did not address maintenance and availability effects, as these were thought to be more long-term. Although the test plan called for a detailed evaluation of combustion uniformity, waterwall and reheat tube leaks interfered with the combustion pattern.
Design information and results from prior tests were fed into the model. The special runs involved firstly several levels of excess air. This was followed by three days of steady-state trials to establish the performance of the boiler, pulverisers and the precipitator. There were special slagging tests
~ or
.~:;1300 10 ~ .........a. E , ralternative coal Gl
;1250 CIl ~ . ')( ~ T"',~'A~ •til
GI Gl
:61200 l baseline coal E :::l U.
I I I I I 2.5 3 3.5 4 4.5 5
Economiser exit 02' % dry
Figure 36 CQE/CQIM validation tests at Watson 4 unit showing the variation in FEGT at different excess air levels, and the differences between the coals used (Vitta and others, 1993)
designed to measure ash deposition rate at various locations in the upper furnace regions at different load conditions of 2551260 and 265 MWe. The FEGT was measured with a water-cooled high velocity thennocouple along a traverse line in front of the boiler nose. Measurements were taken every 61 cm (2 ft) along the probe insertion path, When the excess air was low, the gas temperatures were high, due to the delayed mixing and longer burning times for coal particles. Higher excess air levels were associated with lower FEGTs due to rapid mixing and burnout lower down in the furnace. This is illustrated in Figure 36 which also shows a marked difference between the two coals being compared.
Following the test burn, the new data were used to calibrate the CQIM under real rather than design conditions. Once the model was calibrated for the baseline coal, tests were carried out to compare predictions with the results from firing the alternative coal. In general, predictions of heat rate, boiler efficiency, furnace temperature profiles and NOx levels at full load, were fairly accurate. The principal differences between the baseline and alternative coal were - moisture 6.5/11.5%; ash content 8.3/6.9% and HHV 29.2127.5 MJ/kg. Ash fusion temperatures under oxidising conditions were about 15-20°C lower for the alternative coal. The coals were thus not particularly different. Because of the relatively short test burn time, assessing economics was not the main thrust of this programme, although it forms an important part of CQIM.
The costs of establishing a CQIM A comprehensive test programme to evaluate two test coals using most of the techniques mentioned can cost over US$0.5 million, including pulveriser and precipitator testing. This represents a substantial investment (Thompson and Giovanni, 1993). As a result of the costs involved, many utilities will select particular test methods and boiler conditions so as to maximise the information gained at minimum cost. If existing instrumentation can be used in conjunction with a test programme, the costs can be substantially reduced. There is also a strong incentive to develop the use of pilot-scale test work, provided this can be related to what happens in a full-scale unit. This is discussed in the next section.
71
Predictive indices, modelling and scale-up from tests
access 'value' of action, efficiency, cost, and
risk impact
outstanding 'actions'
prioritise and
display actions
verify consistency with other outstanding
actions previously defined within C-OLIEL
information flow within C-OUEL
:-------;~ I !+f--------.t
I identify situations I
where non-optimum I matches between
coal quality, I operations, and I
design exist I I I I
coal quality data
plant status, process information,
key operating parameters
operator
on-line coal analyser
monitoring systems
objectives constraints
economic factors load requirements
operation strategies action(s) taken
perform analysis in context of specific problems
or opportunities
COIM performance and economic models
viable actions
consider potential 'action'
Figure 37 C-QUEL information flow (Stallard and others, 1993)
Coal quality evaluator C-QUEL A further development of CQIM is C-QUEL, the EPRI Coal Quality Evaluator, where an on-line analyser of coal quality is used to relate this information to boiler operating conditions, and identify any adjustments needed to maximise performance while meeting emissions standards. C-QUEL is a suite of computer programmes which can be used as a basis for controlling various parts of the power station operation. The information flow in C-QUEL is shown in Figure 37, but the system is in the early stages of development. On-line analysis of coal is discussed in detail by Kirchner (1991), but is not yet applied to many utility boiler plants.
5.2.4 Advanced indices
Much of the current work is to develop indices based on CCSEM data on the minerals present. together with a knowledge of the transformations. As has been described, the mechanisms are extremely complex, and the conditions are difficult to measure. It is likely to be a considerable time before such indices are widely used, partly because of difficulties in validation on enough different boilers.
A recent proposal for such advanced information to play its part in boiler design is that by Kalmanovitch (1992, 1994). It is based on on-site evaluations of full-scale plant. These include a review of operating parameters, the extent of deposition and an assessment of the impact on operations, maintenance and availability. Samples of feed coal, deposits and fly ashes are collected and analysed. The data are combined to develop a unit-specific model of the impact of ash behaviour on the boiler. The model can then be used to
establish and determine the effects of various changes, including fuel switching, improved mill performance, changing burner operation, sootblower coverage, and the use of additives. These will impact both operation and the design of future units.
The approach involves a multi-disciplinary team including field service and performance design engineers. A novel aspect of the approach is in the interpretation of the analysis of the ashes. A method has been developed to determine the eutectic temperature of the ash samples on a particle by particle basis. The eutectic is the melting point of the particular mixture. Individual particles can melt at a temperature well below that of the bulk mixture.
A distribution curve is developed which shows the proportion of the ash which will have molten phases present at a given temperature. This is a measure of the likelihood of adhesion on impact. The information is combined with the unit-specific data, and the model is used to predict conditions for minimum impact of ash on the system.
Differences between the eutectic temperature distribution for two ashes are shown in Figure 38. Ash B has a greater assemblage of lower temperature eutectics than ash A. At 1200°C, 80% of the particles of B would have a melting point below that figure. For ash A, 60% of the particles would have a melting point below its bulk eutectic temperature of 1350°C. It is claimed that the differences in properties could be applied to develop relationships which would assess deposition potential. This in tum could be used to determine tube spacing to maximise heat transfer while
72
Predictive indices, modelling and scale-up from tests
minimising fouling, although no account is taken of the strength of a deposit.
An interesting approach is being pursued in Australia. This involves developing some deposition indices based on the mineral matter size distribution in the coal and the operating conditions. It is proposed that the net iron content present (as vapour and aerosol) in the gas phase be used as the index for slagging. Similarly, the net sodium and potassium content present (as vapour and aerosol) in the gas phase should be used as the index for fouling. The quantities are expressed per unit quantity of coal consumed. The indices take into account the coal chemistry, the mineral matter content and operating conditions, and can be estimated from thermal equilibrium calculations. The effect of mineral matter size distribution can be taken into account by means of an 'availability coefficient' for each species (Gupta and others, 1994). Three coals have been studied, with widely differing characteristics. These were Newlands (Australian), Daido (Chinese) and Miike (Japanese).
Initially, thermodynamic studies were carried out. The availability coefficient was defined as the ratio of the mass of a reactive layer that participates in establishing the equilibrium to the total mass of the particle and is assigned to different species, depending on their presence in fractions of differing sizes. The thickness of the reactive layer increases with temperature, and in the case of silica particles is only 0.1 to 0.2 ~m at temperatures of around 1300°C. The basic data come from CCSEM analysis.
The particle temperature was varied from L325 to 1925°C and at stoichiometric ratios (ie excess air) from 0.8, reducing, to 1.2, oxidising. It should be noted that the particle temperature can be anything from 50 to 250°C higher in temperature than the surrounding flame.
The thermodynamic studies showed that a comparatively large amount of fume/vapours is generated by the Daido and Miike coals. In order to operate at similar 'fume' levels, they would have to be combusted at lower furnace temperatures and higher excess air levels than the Newlands coal.
':~ --()- Ash A
-AshB
Q; 'tl c: ::l 60 ~ ~ 0
Ql
.::: § ::l 40 E ::l 0
20
o o 500 600 700 800 900 1000
The basic oxides are retained in the molten phase as silicates and aluminosilicates. All the MgO and CaO is converted, irrespective of coal type, excess air or temperature. The retention of sodium and potassium in the molten phase decreases from 95% at l325°C to about 20% at 1925°C. As ash particles are depleted of the alkalis, they are expected to become less sticky.
Some preliminary experimental work was carried out to check the various forecasts. The fouling index was found to have some validity. The slagging index was not as useful, because the fumes/vapours are responsible only for the initiation of slagging, and the build-up is strongly affected by the presence of sticky particles. It would need to be combined with other factors to make it useful.
PSI Slagging Ad~'isorTM
PSI PowerServe have developed a model called the SlaKfjing Advisor™, to compare slag fonnation in coals and coal blends. It uses CCSEM measurements of the coal mineralogy combined with traditional characterisation data. An ash formation model predicts the size and composition of ash particles allowing for char fragmentation and mineral coalescence. From these predictions. a viscosity model for the range 105 to 107 Pa (critical for deposition) is used to calculate particle viscosities. Simplified boiler flow modelling predicts particle transport to the walls, where deposition may occur if the critical viscosity criterion is met. Deposit amounts and location can be predicted and comparisons made between different coals.
Laboratory work has confirmed that with three bituminous and one low rank coal, the prediction of the particle size and composition of ash particles was reasonably good (Helble and others, 1992d). Work is under way to validate the model under boiler operating conditions, but it is recognised that this will require extensive work because of the complexities involved.
As currently formulated, the Advisor compares the slagging tendencies of different coals. It does not claim to be able to predict the impact of a coal on boiler operation. PSI are
I I I I I II I I
1100 1200 1300 1400 1500 1600 1700
Temperature, DC
Figure 38 Eutectic temperature distribution for two different coal ashes (Kalmanovitch, 1993)
73
Predictive indices, modelling and scale-up from tests
planning an extension to the Advisor by incorporating it into a fundamentally-based model for describing the effects of deposits on boiler performance. Such a model would have to combine a deposit growth model and a deposit removal model with a complete boiler heat balance. It would have to include a particle by particle simulation of the deposit dynamics, and incorporate the mechanisms responsible for deposit growth, strength and removal. The reduction in heat transfer due to the deposit would also have to be taken into account.
Such a model could be used by boiler operators to determine the effect of changes in load, excess air levels and sootblowing frequency. It could also be used to quantify the cost of deposition. However, a great deal of work would need to be done before such a model is developed and validated. The relationship between the various submodels is shown in Figure 39.
gas temperature profile calculated
mineral matter transformation
pyrite model model
compilation of particles in gas ~ ---.J
by location (particles
reaching the wall)
arrival models
,------I~ deposit temperature calculated
deposit reflectivity viscosity calculated calculated
'----- --1 stickiness ~__---.J
calculated
sintering
Figure 39 PSI PowerServe slagging model. Schematic showing the relationship between the sub-models (Helble and others 1992d)
EERC work The EERC have been working on a model to predict fly ash particle size and composition. Two complementary approaches have been used. The first is stochastic, based on random variables. It combines the initial coal inorganics and
their distribution in the coal, and predicts the size and composition of the fly ash formed. The second is a so-called 'expert' system, derived from a knowledge base together with some inferential rules related to ash formation. This approach has the advantage that as the knowledge base increases, and the inferential rules are clarified, the same basic programme can be used. The more results that are fed in, the more refined will be the model.
The approach uses advanced analytical data, and in particular CCSEM results, and chemical fractionation analyses. Given the vast amount of data involved, its organisation is central to its practical use within the modelling processes. The extent of the data is illustrated by the fact that the database is expected to hold some 20-30 coal and ash analyses, with approximately 2000 analysed particles per coal and ash sample. Data compression is discussed by Zygarlicke and others (1992).
This work has been developed in order to derive some advanced indices for Powder River basin coals in the USA. These indices can be used in conjunction with the computer model known as ATRAN 1 (Ash Transformations Version 1) which predicts the ash particle size and composition. This is a stochastic model based on combining the coal inorganics in a random manner. A second model, ASHPERT (Expert system), gives a first order estimate of fly ash size and composition, based on a large empirical database. These data are combined and used in LEADER (low temperature engineering algorithm of deposition risk) which is designed to predict low temperature fouling potential. It requires specific inputs about the amount and associations of the inorganic components in the coal derived from advanced analysis.
The two indices are:
a low temperature index, relating to fouling in the lower-temperature regions of the boiler; a high temperature index related to deposition in areas such as the secondary superheater or reheater.
The indices are such that a higher value is indicative of greater deposition propensity in the boiler. The criteria used in developing these indices are:
sodium content, as organically-bound sodium quickly enters the vapour phase as a hydroxide or sulphate, and interacts with silica or aluminosilicates; calcium content, as organically-bound calcium reacts with aluminosilicates and quartz to form sticky materials within the char particles; mineral content, which refers to discrete minerals which may have a diluting content on deposit strength if the ratio of mineral content to organically-bound content is greater than 1: 1; organically-bound content, which refers to inorganics which can react inside char particles and may form fine-size fly ash when it condenses in cooler parts of the boiler; the quartz content below 4.6 /lm size. This is more likely to be carried to the back end of the boiler and then react with Na or Ca to form sticky material;
74
Predictive indices, modelling and scale-up from tests
excluded quartz content, which is less likely to react with sticky) liquid phases if combined with the
organically-bound components; organically-bound cations such as K, Na and Ca.
calcite content, including discrete mineral grains which
may act as a diluting agent for deposit strength; Results from various coals are shown in Table 12. From
clay content, which includes all the aluminosilicate these data, the coals with the poorest boiler performance in
minerals which may form lower melting point (and the low temperature region would be D and F. In the higher
Table 12 Coal composition and advanced fouling indices developed by EERC (Benson and others, 1993)
Minerals, % A B C D E F G
Quartz 33.5 21.8 25.1 18.2 19.9 34.8 32.1 Iron oxide 1.4 0.1 1.0 0.1 0.7 2.1 1.3 Rutile 1.0 0.1 0.4 1.3 0.2 2.2 1.3 Alumina 0.0 0.0 0.0 0.1 0.0 0.6 0.2 Calcite 6.8 24.6 4.4 23.7 6.6 1.1 4.0 Dolomite 0.7 0.8 0.4 2.6 0.1 0.4 0.5 Ankerite 0.0 0.0 0.0 0.0 0.0 0.3 0.0 Kaolinite (Kaol) 26.0 18.8 ll.8 16.4 27.3 7.6 25.5 Montmorillonite 2.5 0.7 5.4 1.4 3.8 1.5 1.2 K AI-Silicate 0.6 9.4 1.0 0.2 12.6 1.5 0.6 Fe AI-Silicate 1.2 0.2 0.5 1.6 0.3 0.8 4.9 Ca AI-Silicate 0.7 1.5 0.2 0.4 0.5 1.4 0.2 Na AI-Silicate 0.0 0.0 0.0 0.0 0.0 0.1 0.0 Aluminosilicate 2.4 1.0 3.1 0.2 0.5 2.2 0.9 Mixed Silicates 0.2 0.2 0.2 0.9 0.2 0.3 0.5 Fe Silicates 0.0 0.0 0.0 0.1 0.1 0.5 0.1 Ca Silicate 0.4 0.2 0.0 0.2 0.1 1.9 0.0 Pyrite 0.7 9.6 24.0 1.5 16.9 6.0 2.2 Pyrrhotite 0.0 0.0 0.0 0.0 0.1 0.0 0.1 Gypsum 0.7 0.4 1.5 0.6 0.7 3.1 5.1 Barite 0.4 0.9 1.3 0.0 1.3 1.2 0.0 Apatite 0.1 0.0 0.0 0.0 1.4 0.0 0.0 Ca AI-P 7.8 0.5 4.3 3.5 0.5 6.0 4.3 GypsumJBarite 0.0 0.0 0.0 0.0 0.0 0.0 0.1 Gypsum/AI-Silicate 0.1 0.1 0.4 0.2 0.1 0.3 0.6 Si-Rich 1.2 2.7 5.0 1.2 1.9 5.1 2.5 Ca-Rich 0.7 0.5 0.8 2.0 0.5 0.6 1.3 Unknown 10.9 5.8 9.2 23.8 3.7 18.6 10.6
Organically-bound Al 5.6 0.8 6.4 4.6 1.6 1.4 8.1 Fe 0.0 1.0 1.5 4.6 4.8 1.2 0.2 Ti 0.1 0.1 0.5 0.6 0.4 0.9 0.5 P 0.1 0.4 0.5 1.0 0.1 0.4 0.0 Ca 21.0 22.0 24.0 31.5 15.9 28.4 27.2 Mg 5.7 2.2 5.7 5.7 3.9 7.1 5.0 Na 3.0 2.3 1.3 1.0 1.2 3.3 0.0 K 0.0 0.1 0.2 0.0 0.0 0.0 0.0
Total Ash, wt% 5.1 12.3 5.6 6.7 9.0 4.6 6.1
% Minerals 1.8 7.2 3.4 3.2 6.9 1.3 2.4 % Organically-bound 1.8 3.6 2.3 3.3 2.5 2.0 2.5 % Mineral & Organically-bound 3.7 10.7 5.7 6.5 9.4 3.2 4.9 % Quartz & Kaol :0;4.6 ~m 27.2 18.9 11.3 17.0 18.4 23.6 19.5 % Quartz :0;4.6 flm 15.9 10.8 8.3 10.4 9.2 21.7 14.2 % Quartz :0;10 ~m 9.3 3.7 5.9 3.1 3.2 4.7 6.4 %NazO 1.9 1.5 0.8 0.6 0.8 2.1 0.0 Ratio Mineral/Organically-bound 1.0 2.0 1.5 1.0 2.7 0.6 1.0 Qtz :O;4.6/0rg (Ca+Na) 0.7 0.9 0.5 0.3 1.5 0.4 0.5 Excluded Quartz % 25.4 9.6 9.8 0.9 13.2 3.4 4.1 Excluded Clays % 10.6 3.7 11.9 1.4 8.5 5.5 8.5
Low Temperature Fouling Index 8 5 13 122 270 4 High Temperature Fouling Index 9 18 12 5 6 7 2
75
Predictive indices, modelling and scale-up from tests
temperature region, coals Band C show the greatest tendency to deposit.
The indices are in the development stage, and it is recognised that other factors need to be considered which may outweigh the influences of inorganic composition. Nonetheless, it provides an example of the way in which predictive indices may be improved, and in which advanced analytical data can be used (Zygarlicke and Katrinak, 1992).
The results from ATRAN I have been verified in full-scale testing in pf boilers, and LEADER has been tested on five utility boilers firing subbituminous coals (Benson and others, 1993).
5.3 Scale-up from tests A survey was reported by EPRI of utilities around the world who had used pilot-scale testing in an effort to understand and predict the impact of coal characteristics on boiler operations (Johnson and Sotter, 1988). While many aspects of boiler operation were included in the questions, it was concluded that provided the combustion conditions during ash formation were representative of those in the full-scale boiler, pilot testing produces a good indication of deposition tendency. Superheater deposits were well simulated in tenus of their physical and chemical characteristics, but it was not possible to predict deposit growth rates. In connection with fouling, deposit strength measures which indicate their ability to resist on-line cleaning are often a more appropriate indicator than deposit growth rates.
The assessment of slagging tendency was less reliable than that of fouling tendency, probably because the complex ash particle dynamics are not easy to simulate in the combustion zone in a pilot-scale test.
Work in Australia on improving predictive capabilities from small-scale work was reported by Phong-anant and others (1991). Although Australian bituminous coals have fewer slagging and fouling problems than many others, some difficulties remains with particular coals. There is a need to provide better understanding of ash deposition mechanisms both in terms of power station operation in Australia and in the various export markets.
The work involved both laboratory analysis using advanced techniques for mineral characterisation and tests to simulate ash deposition using a drop tube furnace. It involved pilot-scale work, and the collection of coal feed and fly ash samples from a number of operating boilers.
It was demonstrated that both mineral composition and boiler operating conditions have a major influence over ash formation, deposition processes and the characteristics of deposits. For these particular bituminous coals the key minerals responsible for initial slagging appeared to be the finely dispersed silica, clays, alkalis (mostly calcium) and pyrite. At lower temperatures, the fluxing action of both iron and alkalis on the quartz and aluminosilicates, together with molten iron mineral particles appeared to be one of the main mechanisms for the formation of soft and sticky particles.
Large particles did not readily fuse. As surface temperatures increased, most iron-based droplets and larger free quartz particles slowly dissolved into the glass matrix. If the slag ran into a cooler region before complete diffusion and mixing of the iron-rich and silica-rich materials, they would recrystallise into iron- and silica-based crystals respectively. In addition, a calcium-rich glass formed on the surface of some unfused particles, and was responsible for their sticking to the walls.
In terms of predicting behaviour from information obtained by small-scale testing, the best parameters for these Australian coals were still based on laboratory or low temperature ash figures.
These were:
total Fe203 + CaO, to be <10% for low slagging; total iron or calcium containing nnnerals to be <16% for low slagging.
Earlier work has shown that ash samples having identical laboratory ash fusion properties may have different melting and crystallisation characteristics. They could then behave in quite different ways when slagging. The approach of predicting slagging propensity from high temperature phase equilibria data was shown to be promising. It was found that simple ternary phase equilibrium diagrams such as CaO-FeO-Ab03-Si02 can be used to predict ash-melting characteristics such as liquidus temperatures, eutectic temperature and the composition of the major crystalline phase in the ash deposit. The fouling potential for these coals was minimal, so that comparisons were not of value in this study.
The study concluded that a great deal more work was needed before it would be possible to predict ash deposition behaviour in full-scale plant from smaller-scale test work. Among other things detailed comparisons between pilot-scale and full-scale operating conditions would be needed, as would the systematic collection of coal feed samples, and deposits from different parts of the boiler.
Consol Coal in the USA has used data from a pilot-scale combustor to assess slagging and fouling factors to be incorporated into its Coal Quality/Power Cost model. Work has been reported on the use of the combustor to measure heat fluxes and assess sootblower effectiveness (Abbott and Bilonick, 1992). Recently eight US coals were tested, ranging from a high sulphur eastern bituminous to a western subbituminous coal. Coal blends were also used, and part of the objective of the particular study was to assess the effects of coal switching at US plants to meet emissions limits, particularly of S02.
The pilot combustor was designed to simulate the time/temperature cycles experienced in most boiler types commonly used. There are slag deposit collection panels, and fouling probes, with load cells to monitor the rate of build-up. Apart from using simulated sootblowing with controlled peak pressures, the mechanical strength of a deposit is not measured. Two correlations were developed
76
Predictive indices, modelling and scale-up from tests
from the pilot-scale combustor slagging and fouling data. These were incorporated into the Consol model, and generally good agreement with commercially available data on full-size plant was claimed. The model demonstrates the significant impact of coal selection on power plant costs (Abbott and others, 1993, 1994).
One test procedure has been evolved to relate pilot-scale slagging results to full-scale field tests. Pilot-scale testing in the slagging region frequently involves monitoring the heat absorption on a test panel, firstly as the deposits build-up, and then after simulated sootblowing. It is recognised that the pilot testing is under 'ideal' and relatively uniform conditions which are often quite different from those inside the boiler.
In order to relate results to plant-scale behaviour, the approach has been to run some short-term 'accelerated' slagging tests on the plant under both normal and adverse conditions. Changes in heat absorption with time are measured and compared under different operating conditions.
The heat absorption of the furnace wall is proportional to the increase in water temperature from inlet to outlet d-dT (provided the How is maintained). As ash deposits build-up on the walls, the d-dT decays at a measurable rate (in °C/h). Operation at lower 02 levels tends to accelerate ash deposition, and the rate of decay of water temperature rise through the walls is more rapid (see Figure 40).
~ nominal O2
h
°Closs slopem =-h
low O2 or max load
Time, h
Figure 40 Comparison of furnace wall heat absorption decay under different operating conditions (Thompson and Giovanni, 1993)
This kind of assessment has only been carried out at two sites in the USA so far (Thompson and Giovanni, 1993). It does, however, show considerable promise. It provides a relatively low cost method of comparing ash deposition behaviour in the slagging region of a boiler under different operating conditions. It can be used to compare different coals. It can be correlated with pilot-scale results, but has the advantage that it is carried out in a full-size unit. The approach could have other applications in boiler operation, and might be used as an indicator of the adequacy of sootblowing operations.
One engineering analysis procedure recently referred to has been developed and used in the USA by the Energy and Environmental Research Corporation (Maly and others, 1993). This involved a pilot-scale combustor to simulate the radiant and convective regions of a utility boiler. The results were used in conjunction with a computer model based on boiler geometry and heat release patterns in the boiler. The model calculates burnout and heat transfer so that temperature profiles and FEGT value can be derived. The engineering analysis model can identify regions of the boiler that are above the initial ash deformation temperature. It is not clear how much success the engineering analysis has had, although some in the area of estimating burnout, and heat transfer rates is claimed.
The use of the Combustion Engineering Fireside Test Facility in connection with boiler design is discussed as a case study in Section 7.3.
Over the years it will be necessary to build up a body of experience with the small-scale predictive techniques to establish for what coals, and for which boilers they can effectively be used. With the advanced analytical techniques now available, this goal may be achievable, but it will take time, particularly in view of the variety of coals being fired. It should be recognised that it is extremely difficult to extrapolate these small-scale results because of the number of variables involved. The results from pilot plant require validation with those from full-scale boilers. Essentially the results are only true for the particular pilot plant and full-scale plant involved. As time goes on, it should be possible to extrapolate results to other similar plant and other similar coals with greater confidence, although it may never be possible to forecast behaviour with absolute certainty. The increased level of confidence achievable should enable operators to optimise their operations and reduce costs, even though absolute confidence in extrapolation is never achieved.
77
6 Reducing boiler deposition
The objective of most of the work reported in this review is to reduce the adverse effects of ash deposition. This can be done by changing the operating conditions, and/or changing the coal feed. Engineering changes such as the redesign of tube banks and the installation of additional sootblowers may be required to change the operating conditions. A better understanding of the mechanisms involved, and of the factors which affect deposition will help both operators and designers to avoid or reduce uncontrolled ash deposition. In this chapter a number of methods of reducing deposition are discussed, along with some factors associated with low NOx combustion conditions which are being increasingly used.
6.1 On-line cleaning techniques Most boilers have sootblowers to remove deposits. The fluid used may be superheated steam, saturated steam, water or compressed air (Stultz and Kitto, 1992).
The provision of sootblowers involves both capital and operating costs. The mountings for long tubes take up a considerable amount of space outside the boiler. The use of steam can be up to 1% of the total generated (Hurley and Schobert, 1993).
Fixed position blowers can be used to remove light dusty deposits in the cooler parts of the boiler, and have the advantage of lower cost. Retractable wallblowers have short tubes, and simply blow the wall area around their location. Most blowers are mechanically complex, and are retractable, with a rotating tube of varying length, to direct the jet of cleaning medium.
In many situations, superheated steam is used in preference to saturated steam (where the water droplets present may cause erosion). Compressed air is also widely used in large boilers. Air and steam have generally similar effects when used as a cleaning medium. The tubes need to be warmed-up before
use, and are slightly less flexible than water-based systems. The use of steam affects the operation in other ways, as it diverts energy from the basic combustion - steamturbogenerator route on an intermittent basis.
Where a deposit is plastic or strongly sintered, water may be the preferred medium, providing greater thermal shock. Water lances tend to be physically larger than steam or air blowers, and do not need to be warmed up before operation. They can therefore be operated one at a time which facilitates 'blowing on demand' when there is a perceived problem. Pressurisation of the system is almost instantaneous, and requires only a small pump. Particular examples where water lancing has been found to be more effective are in lignite-fired units and in those where the ash is more reflective (Lucas, 1994).
A typical pattern is to clean an oval area with an axis of around 3 x 3.5 m. Air and steam blowers have a smaller effective area of operation than water lances. Long retractable blowers are used to clean the boiler tube bundles, and because of their size, these are considerably more expensive.
An important aspect of boiler design is to decide where to install sootblowers, and where to make provision for the installation of blowers which can be provided if they prove necessary. An example of a boiler firing a North Dakota, USA, lignite which had caused serious fouling problems in earlier plant, is the Antelope Valley No 1 unit. The provision of over 300 wallblowers, and of 138 retractable sootblowers in the superheater, reheater and economiser regions is shown in Figure 41. The addition of 10 more blowers was allowed for.
As with all mechanical equipment, regular maintenance is necessary to ensure satisfactory performance. Intractable deposit build-up can occur when sootblowers are out of action for one reason or another.
78
• • • • • • • • • •
-- -
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
• • • • • • • • • • • • •
Reducing boiler deposition
tube lining around the main boiler shell
burners --------1-+'-1
I ~ I-I ~ ~
1-'I-I
~ 1-[ ~ I-I ~ I-I ~ I-I I ~ I-I I
L.~ -~ .- I
--0o. • • • • •
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----,
-- --.J
----, 0
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1------, ---..---~1-----, ~ ___ ----J
12
138
10
311
2
retractable sootblower
retractable sootblower (juture)
wallblower
temperature probe
Figure 41 Antelope Valley No 1 station, USA, showing position of sootblowers and tube layouts (Burbach and Janssen, 1984)
79
Reducing boiler deposition
6.1.1 Optimising sootblower use It is difficult to determine the optimum pattern of use on a conventional plant. A sequence is normally developed on an
Sootblowers tend to be used at fixed time intervals, based on empirical basis and modified with experience, to keep the operating experience with the boiler firing a known coal. build-up of deposits under long-tenn control. The They may also be used on spurious criteria. The shift optimisation of sootblowing sequences at the plant at Teruel operator may want to hand over a 'clean' boiler to the next in Spain is described as a case study in Section 7.6. shift. and blowers may be operated simply on that basis, possibly unnecessarily. One approach to optimising sootblower operation is
discussed by Davidson (1994). It is based on the use of heat flux sensors, and extensive trials have been carried out on four 500 MWe front wall-fired boilers in the UK. The heat thermocouples
gas seal flux sensors involved use the thermally guarded cylinder
\ thermal guard technique for measurement. This makes use of the temperature gradient along a cylinder of known thermal conductivity when heat is applied at one end, while the other is connected to a heat sink (see Figure 42). Careful calibration is needed for each sensor, however, preliminary indications from the utility involved are that the sensors have an adequate life-expectancy.
The results from a trial where the sensor was placed near a water lance sootblower are shown in Figure 43. The basic, unoptimised, setting of the blower was cyclic operation according to a time schedule. The operation was optimised so that the sootblower was activated when the heat flux fell to 120 kw/m2. It was claimed that compared with the previous hll~:e!llil standard pattern of use there was found to be a 33% increase
thermally guarded YV '\j v ) in the heat flux, with a 66% reduction in sootblower requirement. sensor body radiant heat flux
/ It is thus possible to identify an 'optimum sootblowing time' by use of these techniques. The trials showed that using such local heat flux data would increase boiler efficiency, even where there are no apparent slagging problems. This is because existing units tend to be operated inefficiently, with the overuse of sootblowers. There could be considerable financial savings from improving furnace heat transfer,
Figure 42 Details of heat flux sensor in a locally thickened reducing attemperator spray requirements and maintaining a tube wall section (Davidson, 1994) lower FEGT. It also introduces the possibility of identifying
500
total heat flux for total heat flux for the period~2800 kW/m 2 the period=4200 kW/m2
400 /
/
/ /
//
~ /\-----,-/ " / \
/ \.-, / \
----.....~-/- -,~
/ /
~ 300
x --~------f-F------::::I
;;:: /, / I \.......10 200 Ql
----' ---:.'-/ .............. ,. /J:
- - -\'-,-/- - - -
100 t t t 'of
t t t t t 0
I 1I I I I I I 0 3 6 9 12 15 18 21 24
Time, h
--- unoptimised sootblowing t sootblow optimised sootblowing
Figure 43 Graph showing trial results, comparing optimised sootblowing with standard cycle (DaVidson, 1994)
80
, radiai heat transfer
and tackling localised problems in particular parts of the boiler.
Further work to develop the use of this instrumentation, and the techniques for its use should be encouraged. It is, however, important to minimise the total costs to the operator which will include the fuel cost, cost of sootblower provision and operation, and the cost of boiler maintenance, resulting from either blowing or of failure to blow.
6.2 Coal supply and coal cleaning Coal users in many parts of the world have become accustomed to receiving a feedstock which is a blend from different seams in a pit, or which is a blend of coals, designed to meet the specification for ash content, sulphur content or moisture. Coal washeries may take their supplies from several pits. Generally the coals blended will have been of similar rank. Many coals which are internationally traded are blends, to meet specifications for sulphur content, ash, moisture and heating value. Even in countries with a large indigenous mining industry, like the UK, the coal supply to a particular power plant over several years may come from several different pits.
With the recent tightening of emission regulations, power utilities in many paI1S of the world are having to consider the alternatives of adding flue gas cleaning equipment or of changing their feedstock. Consequently both switching and/or blending the feed to meet the regulations is increasingly being considered. This is particularly true in the USA, where there are large reserves of low sulphur, low rank coals available from the Powder River Basin. It also applies, however, to most importing countries, and the increasing supplies of low sulphur, low ash coals from Indonesia introduces new possibilities for coal blending to achieve particular specifications.
Some of the factors to be taken into account when considering coal switching are discussed by Rupinskas and Hiller (1993). These include many other characteristics in addition to ash deposition behaviour. The first is the handleability and friability of the coal. Lower rank coals will have a tendency toward spontaneous combustion. If the heating value is lower, then the mass flow rate needed to maintain output is increased. Its pulverising characteristics may be different. Moisture content, volatiles content, and ash content and characteristics, all affect boiler behaviour and performance, and have operational implications. Factors affecting coal behaviour in power stations were also discussed by Skorupska (1993).
Coal switching and blending introduce new variables to boiler operation, and hence possible problems. This means that a better understanding of slagging and fouling is needed. Prediction of the physiochemical behaviour of the ash in a blend, cannot be handled by averaging current indices. In terms of deposition behaviour, the fly ash has a composition consisting of lower concentrations of both the best and worst components of the separate ashes. The condensation of volatile alkali components will occur on some fly ash not normally subjected to this, and its behaviour is then difficult ---------------.---------
Reducing boiler deposition
to predict. The emissivities and thermal conductivity of intermediates and of fly ash may well be substantially altered (Bryers, 1994). Predictions are particularly difficult when coals with dissimilar ashes are blended, including both bituminous type and lignitic type ashes, but blends do not necessarily bring unacceptable deposition. Pilot-scale testing is necessary.
Utility experience with blends of western and eastern USA coals was reported by Gunderson and others, (1993). A survey covered 12 units, and the results from one particular boiler were presented. These were thought to be representative of the experience of others. Furnace wall slagging was controllable with slightly more wall sootblowing. Increased slagging was tackled successfully in the 12 plants by:
improved maintenance. This improved sootblower availability; adding blowers where needed; increased frequency of blowing.
Fouling in the convective pass was tackled in the same way, again, generally successfully.
Work at pilot-scale in the UK on indigenous coals showed that when a non-slagging coal was blended with a highly slagging one, the deposition behaviour and slagging performance was dominated by the slagging coal, even when only 10% is present. This demonstrated that the various indices used, which are subject to reservations that have been discussed earlier, are not additive in nature. There should be considerable caution about applying the indices to blends, even of bituminous coals from within the same country. The form and association of the iron present may be the controlling factor (Barnes and others, 1994).
6.2.1 The effects of coal cleaning
One of the ways of reducing the amount of inorganics passing through the boiler is to clean the coal more thoroughly before combustion. This can have a substantial positive effect on boiler performance. It reduces the total solids load in the boiler, and some of the components particularly associated with deposition or erosion may be partly removed. It is, however, possible to alter the chemistry of the deposit-forming components so that slagging or fouling becomes a greater problem in the cleaned coal. This deterioration in performance is a danger particularly when coals are deep cleaned in such a way as to substantially alter the chemical composition of the inorganics. If dense medium separations are used during cleaning, there is an increase in magnetite levels in the coal, as not all of it is removed. The presence of iron compounds tends to increase slagging tendency.
With moderate amounts of cleaning, savings in operating costs would be expected, but it is quite difficult to quantify the overall effects. Cleaning involves additional cost, and some loss of carbon. This has to be balanced against reductions in the operating cost of the boiler. The evaluation is currently being attempted using the EPRI Coal Quality
81
i l: C) III E t: 0 l:
.!: Cll u..
fC,
0 lI
outer WW (clean)
L0
+
o +
+ V' WW (Texas lignite)
outer WW (rom)
I initial WW
initial SH
o o
I 0.2
I 0.4
I 0.8
I 0.0
Fe++ IFe+++ ratio for non-magnetic phases
WW = waterwall SH = superheater
I 1.2
I 1.4
I 1.6
Reducing boiler deposition
Impact Model for particular units on which the model has been validated.
In a study on British coals, it was thought that coal cleaning would significantly reduce slagging difficulties as well as reducing erosion (Raask, 1983). It was estimated that for both UK and USA coals, the optimum ash content in bituminous coals for pf firing was between 12 and 15%. It was not thought that further reduction in mineral content would bring any additional reduction in slagging. The exception to this would be high chlorine content coals, commonly found in the UK. Where there is a high ratio of NaCl/ash in the feed coal there is an increased risk of boiler fouling. Reduction in sulphur and other mineral matter should marginally reduce fouling by British coals.
In a detailed study of the effects of cleaning on the behaviour of the inorganic constituents in a Kentucky No 9 (bituminous) coal and a Texas lignite, the observed changes in the deposits and their formation were minor for the bituminous coal, but more significant for the lignite. This was because in the lignite the compositions and therefore the properties of the intermediate phase mixtures were altered (Huggins and others, 1990).
The inorganic constituents in both raw and washed samples of the coals were studied, together with samples taken from within the flame, and deposit samples from inside the Fireside Performance Test Facility at Combustion Engineering, at Windsor, USA. The tests were at pilot-scale,
w lignite deposits
~ \ x
V'~ x x
8
)( x
x x x
~ fC,
+ 0 0 ~ 0 70 fC, +4en Cll Ul III
..c: Co 60
0 fC, +
in-flame solids
()
and a variety of advanced analytical techniques were used to characterise the samples, including CCSEM, and Mossbauer and x-ray absorption fine structure spectroscopy. In particular the behaviour of the iron present in the different feeds was studied.
Samples of the in-flame solids from the Kentucky coal consisted mainly of loose aggregates of spherical fly ash particles. Much of the aggregation probably occurred as a result of the collection procedure. Compositions of the particles varied greatly, and appeared to correspond mostly to the mixtures of minerals in the coal, rather than to individual mineral particles. There were occasional particles of calcium sulphates. The structure of the highly agglomerated material found at the centre of some of the aggregates resembled the structure of some of the waterwall deposits. This suggested that it might be material that was temporarily deposited and then recycled as it became detached from the walls or tubes. The nuclei of unburned char tended to be enriched in S, and sometimes with both Fe and S.
The in-flame solids from the lignite tests were examined by x-ray absorption fine structure spectroscopy, and showed that much of the Ca was present in the form of a silicate glass, while there was little evidence for the presence of calcium sulphate.
Figure 44 shows the distribution of iron in different forms at various locations, in the non-magnetic phases, plotted against the ferrous/ferric ratio.
+ Kentucky No 9 medium cleaned
o Kentucky No 9 deep cleaned
fC, Kentucky No 9 run-of-mine
x Texas lignite run-of-mine
V' Texas lignite cleaned
Figure 44 Distribution of iron in the non-magnetic phases of raw and washed Kentucky No 9 coal and Texas lignite, from various locations in the pilot plants (Huggins and others, 1990)
R?
Reducing boiler deposition
Cleaning of both coals altered the mineralogy and chemistry of the ash-forming components by:
removing much of the mineral matter 10 11m size; preferentially removing the minerals with the coarser size (pyrite for the Kentucky coal and quartz/silicates for the lignite); pick-up of magnetite which reduced the beneficial effect of cleaning in the Kentucky coal.
The changes reduced the base/acid ratio for the Kentucky coal, but increased it for the lignite. Apart from the volumetric reduction, the effect of cleaning on the bituminous Kentucky coal was not thought to have any great effect on deposit formation. In the case of the lignite, because the base/acid and Ca/Si ratios were changed, different melts fornled having different properties. Iron-rich species appeared to playa major role in the formation of initial superheater deposits with the Kentucky coal while the lignite deposits were enriched in CaS04 which appeared to be playing a similar role.
The whole test, and the detailed assessment of the results in the paper cited, illustrates the complexities involved in trying to identify all the mechanisms involved
6.2.2 The use of models and of advanced indices
The fouling and slagging tendency is currently assessed using various indices based on laboratory ash analysis. As discussed in the previous chapter, there have been extensive efforts to establish models of the combustion/deposition process, and to establish advanced indices, based on more secure data.
This work has made considerable progress, considering the complexity of the conditions being studied. The results as yet can only be applied to a narrow range of coals, comparing a new coal with one whose behaviour is well known. Validation of the various submodels and models involves substantial effort.
The work is continuing, based on better knowledge of the minerals and other inorganics in the coal, and of operating conditions in the boiler. It should be possible to extend the application of advanced indices, but this will only happen gradually, and will involve a lot of work.
6.3 Boiler monitoring and changing operating conditions
The most common methods of mitigating and controlling ash deposition are by reducing the boiler/power output, to limit the FEGT, or to increase the excess air in order to extend the oxidising environment. Derating involves obvious penalties, although the alternative is often a premature forced outage. The use of more air carries the penalty of lower thermal efficiency.
Partly due to the hostile environment inside the furnace, to
design restraints and to a lack of appreciation of the potential value of the information, relatively few boiler parameters are routinely monitored. During the current programme to validate the CQE, a number of utility boilers are the subject of extensive monitoring. The CQE model was discussed in Section 5.2.3.
With the increased use of computers linked to plant instrumentation, it has become possible to provide the boiler operator with more comprehensive diagnostic information. A system is available which can provide a quantitative indication of furnace wall and convective tube cleanliness. This is based on a heat transfer model of the boiler, and an assessment of gas temperatures and flows throughout, together with an assessment of the heat input to various sections of the water-steam tubing. The flue gas composition leaving the economiser is also analysed. The system only works under reasonably steady-state conditions, and is not well suited to a coal feed of varying properties. An example of the use of such a system is outlined in one of the case studies in Section 7.10.
During the 1980s, a method for integrated monitoring in boilers was developed, together with appropriate instrumentation (Wynnyckyj and others, 1990). The system incorporated the normal plant instrumentation together with heat flux meters, pyrometers and thermocouples. These were used in conjunction with a microcomputer, data acquisition unit and operator displays. The intention was to monitor
Boundary Dam Unit 3
E s w N
c=Jclean moderate load 150
light tilt -31 0 _ heavy
Figure 45 Fold-out screen showing deposits on the furnace walls (Wynnyckyk and others, 1990)
83
0.5
0
I .. ..... I ~ .. ,...., '_I
Reducing boiler deposition
50
40
"C
~ 30 ,g "if..:' Ql III'E o 5 20 u w
10
o o 3 6 9 12
September 12
I
,~
• • "WI ~,;'r'
I • ~ I '(1"'"pl'
15 18 21 24 27 30
Time, h
boiler performance, and the onset and growth of ash deposition on the furnace walls and boiler tube banks so as to develop strategies for sootblowing and possibly for feed blending.
In an installation at the Boundary Dam No 3 unit in Saskatchewan, Canada, the furnace wall monitoring system involved 82 dirty heat flux meters directly installed on the waterwall tubes and 13 clean heat flux meters which are protected from slag deposition by being installed in observation door openings. The 'dirty' meters were welded to the wall surface and became fouled just like the wall
2
0
1:.... "") 1.5 clean..Il:
~
-,~_('.. ~ ,...... ~ ........."'-..' .... Jrll
..:' ..,""<\..,..'..,... 1 ..._r ~,,"'" "~ .....' : rf'-"~'" Ql
III c: co.::: 1ii Ql J:
sootblows
4 8 12 16 20 24
Time, h
Figure 46 Changes in heat transfer in the radiant superheater (Wynnyckyk and others, 1990)
surface. The 'clean' meters were kept clean by purging air and provided a reference base to measure the heat flux available.
Convection section instrumentation included infrared pyrometers, tube metal thermocouples and various existing instruments. Tube metal thermocouples were installed on 18 of the 66 pendant tubes to obtain internlediate steam temperatures between the two stages of the secondary superheater. Existing instrumentation was used to provide a complete steam side heat balance for each tube bank.
Typical results showing different degrees of fouling on the furnace walls are shown in Figure 45. The changes in heat transfer in the radiant superheater are shown in Figure 46 where the effects of sootblowing can be followed, and the progressive build-up of a fouling deposit in the economiser is shown in Figure 47.
A more recent example of the use of heat flux meters was outlined by Davidson (1994). They were being used as part of a programme to optimise the sootblowing cycle on four 500 MWe units, as discussed in Section 4.8.
6.4 Use of additives Various additives have been proposed and used to try to mitigate ash deposition problems in boilers. This is based on the idea of modifying the ash chemistry in some beneficial way. Additives are principally applied to tackle particular problems where other solutions have not succeeded.
Figure 47 The build-Up of fouling deposits in the economiser (Wynnyckyk and others, 1990)
84
Reducing boiler deposition
Additives may be used in the system prior to coal pulverisation, to improve coal handling. For example to prevent freezing, sodium, calcium or magnesium chlorides may be added. or oil-surfactant blends, or glycol-based chemicals. All of these may impact on ash deposition in the boiler, particularly the alkali metal chlorides.
Various additives are proposed to minimise the blockage of both ash hoppers and slag taps. Ash hopper blockage may be reduced by neutralising additives, such as calcium or magnesium carbonates, or the injection of some NH3 into the economiser. Slag tap blockage may be reduced by limestone, sodium, iron or boron compounds. The additives may also have negative effects.
The cost of additives and their accurate metering must always be considered. Widely differing views are held on their effectiveness. As with most parameters relating to boiler operation, the issues are complex. The only way of proving something is to carry out well monitored and long term comparative tests. The net economic benefit is difficult to quantify, and additives are not widely used for controlling slagging or fouling.
Early work looked at the possibility of using copper oxychloride to alleviate slagging in a number of boilers in the UK (Kiss and others, 1972). Some stations reported a marked improvement in combating slagging. It was considered that it should be used on a regular basis only when other methods of dealing with slagging problems had failed.
During the 1980s, there was an extensive EPRI-sponsored investigation, reported by Radway (1990). This considered additives such as:
Mg(OH)2 in a water-based dispersion; MgO in an oil-based dispersion: limestone; manganese organometallic compounds.
The list was presented in recommended order for consideration, based on depth of experience, costs and secondary advantages and disadvantages. The magnesium additives were added in the upper furnace. Limestone and the manganese organometallics were added via the burners. The report commented that there was a lack of information relating to coal-fired units, and much of this was related to flue gas conditioning and not to fireside problems.
The addition of calcium-based sorbents in the upper part of coal-fired boilers to absorb S02 is practised in a number of places. The sorbent can alternatively be added to the coal feed and injected through the burners. The presence of these sorbents will affect the behaviour of various ash-forming components both in the combustion zone and through the various heat exchangers. It will consequently affect ash deposition.
It has been shown that both CaO and partly sulphated CaO tend to form sintered deposits on superheater surfaces. The sintering effect is strongly dependent on flue gas composition and is greatest with the presence of higher proportions of
C02. The results of laboratory work and of field tests on a 500 MWe unit with injection into the upper furnace area are described by Skrifvars and others (l991). Short-term deposit samples from the boiler showed that the strongest deposits were obtained at flue gas temperatures of around 800°e. Deposit formation is enhanced if there are local reducing conditions.
One of the remedial measures adopted to reduce fouling by Saskatchewan lignites was the intermittant dosing with limestone, and this was subsequently adopted by other plants both in Canada and the USA (Selle and others, 1986).
It may be that with the more advanced techniques now available for studying the chemistry of ash and deposit formation that the effect of additives on slagging and fouling behaviour can be more thoroughly assessed.
In a recent study (Bryant and others, 1992; Vuthalum, 1992), a drop tube furnace was used to assess the effects of additives on the ash chemistry, and its deposition behaviour. Three pre-treatment processes were used on a Loy Yang coal which was high in both Na and Cl:
using soluble aluminium compounds to substitute both Na and Cl; washing, to reduce both Na and Cl; treatment with aluminium lactate.
The experiments simulated combustion conditions. Samples of 45-70 ~m size were combusted with 20% excess air at temperatures of 1000, 1200 and 1400°e. Product gases and ash particles were collected by means of a water-cooled quench probe.
Coal treatment with Al showed a sharp reduction of typically 60-80% in the amount of submicron ash formed, indicating a substantial decrease in fouling tendency. Treatment with soluble Al compounds appeared to be more effective in reducing fouling tendency. A reduction in slagging tendency would also be expected. The assessment showed that the addition of some 2-3% of kaolin or alumina with a particle size of J0-20 ~m would be required to control deposition problems. The use of additives at the Bayswater plant in Australia is discussed as a case study in Section 7.9.2.
6.5 Boiler design Various methods may be adopted at the design stage to minimise and prevent ash-related problems. These include:
increasing the furnace size, when the ash composition or relevant indices indicate increased slagging or fouling risk; increasing the heat transfer surface to compensate for low rates of heat transfer associated with deposition, and with the use of wider spacings between tubes ancl/or lower gas temperatures; the use of more costly steels in the construction of tube walls and banks; the installation of (and provision for) more sootblowing equipment.
85
Reducing boiler deposition
Economics dictate that steam generators must be designed for minimum capital, operating and maintenance costs, together with maximum efficiency and fuel flexibility. In this context, the characterisation of coals which may be fed to the boiler during its lifetime, and the possibilities of developing more advanced indices than those based on laboratory ash analysis assumes considerable importance (Jones and others, 1994). These criteria present both designer and operator with some interesting challenges.
6.6 Dealing with low NOx combustion conditions
To reduce the NOx content of the flue gases, the combustion pattern in a boiler is altered. There are various approaches, including:
operational measures, such as decreasing the excess air; staged air input using low NOx burners and/or over-fire air; staged coal input injecting some fuel into a secondary combustion zone to consume some of the NOx already formed; flue gas recirculation.
When the combustion air is introduced in stages, the coal particles do not experience such an oxygen-rich environment, although the same total amount of air is used to ensure good carbon bum-out. The reduction in NOx content is achieved, generally, by using burners with a careful aerodynamic design resulting in a controlled distribution of the oxygen relative to the flow of coal particles. If deposits form and build-up near to the burners, or around the burner tips then this pattern can be disrupted (ten Brink and others, 1992a, b).
With the changed conditions, the time temperature cycle of the particles travelling from the combustion zone to other parts of the boiler will be modified. This will modify the behaviour of the ash, and affect its slagging and fouling tendencies. In particular the proportion of boiler volume that is affected by reducing conditions tends to increase, and this is likely to worsen slagging behaviour and result in the need for more frequent sootblowing (Makansi, 1993). Alternatively it will result in a decrease in heat transfer in the boiler.
Some coals which are high in sulphur and iron contents (such as those from the eastern USA) are reported to have caused major problems when staged combustion was introduced. In particular, the sulphur contributes to the formation of corrosive compounds, especially under the reducing conditions which appear in various parts of the combustion zone. Iron can contribute to a significant reduction in the ash fusion temperature, resulting in uncontrolled slagging (LaRue and Cioffi, 1987).
A result of the need to meet low NOx emission standards is that the range of acceptable operating conditions in the boiler is restricted. For example, slagging behaviour can be
alleviated by increasing excess air, to provide more oxidising conditions, but this also tends to increase emissions of NOx.
Detailed work has been undertaken in the Netherlands, where there is particular interest in the effects of low NOx burning conditions on the nature of the fly ash formed. This is because most fly ash is used in applications where spherical particles are preferred. Part of the investigation looked at the build-up of slag deposits on the burners themselves, thought to be due to reduced iron from the pyrite present. The role of calcite in burner slagging was also investigated, together with the role of quartz as a 'sand-blasting' agent in removing deposits from tubes.
The tests were on bituminous coals, one silica-rich, one calcite-rich and one pyrite-rich. The test facility was designed to simulate the time temperature history of particles in low NOx combustion, although it was recognised that this was not known for a large furnace. It was therefore decided to simulate conditions in a semi industrial-scale unit where the time temperature history was known. To compare behaviour of the minerals with low NOx conditions, the burner was also fired conventionally, with excess oxygen. Coal in the 38-45 f.l.m and 53-63 f.l.m size ranges was used for the tests. CCSEM was used for mineral characterisation. This investigation showed that in the silica-rich coal the silica was present only as micron-sized inclusions. In the pyrite-rich coal, about half the pyrite was present as separate (liberated) particles, while the other half was in clusters of inclusions in the coal. Virtually all the calcite was present as large separated mineral particles in the calcite-rich coal. The slag deposit probe was situated at distances from the burner corresponding, typically, to 30 to 60 ms residence time, but up to 125 ms. Reacting particles were extracted from the hot zone using a helium-fed quench probe which provides instantaneous freezing of the reactions in the particles.
It was found that for the included minerals, their form depended on the structure of the intermediate char. This had its structure completed within 10 ms. During this time, pyrolysis and devolatilisation were proceeding, with gases leaving the particle at high speed. These drive the surrounding gases away, so that oxygen only reaches the particle surface after combustion of the volatiles, which takes about 25 ms. Thus the form of the char, and hence the ash form, is much the same in both staged (low NOx), and unstaged combustion. It was also found that the reducing conditions do not, of themselves, create tenacious deposits from pyrite (ten Brink and others, 1993). The calcite breaks up into micron-sized fragments, after about 40 ms at flame temperature.
It was recognised that these results from small-scale work are in contrast to the differences observed in the ash-form from staged compared with unstaged firing in large furnaces. This may be due to lower flame temperatures. More work is clearly needed to clarify the mechanisms involved, and move towards a testing procedure that can become predictive of what will happen in a utility-scale boiler with a different coal feed under staged combustion conditions.
Rn
7 Case studies
Although operational problems due to ash deposition have been widespread, and occur unpredictably in some situations, they often go unreported in the literature. Published papers and information cover the work carried out to clarify the mechanisms of deposition, but often omit discussion of parallel results from boiler operation. Some operational data are regarded as proprietary by boiler designers and by operators, and they will only divulge limited amounts of information. In addition, boiler operation is subject to a large number of factors, not just ash deposition. Poor performance and outages may be due to a whole range of factors, whereas the analytical and test work reported here is focused specifically on the effects of deposition.
A number of different aspects are discussed in this chapter from reported experience. They vary from long-term attempts to cope with heavily slagging or fouling coals, to the utilisation of pilot plants and the use of advanced techniques, for predicting the behaviour of different coals in a boiler. They are intended to illustrate and amplify some of the subjects discussed earlier, and provide the opportunity to relate some of the theory to practical experience.
7.1 Experience in the Netherlands There are some 4000 MWe of installed coal-fired capacity in the Netherlands. Most of the coal used comes from Australia and the USA, and is generally fired in blends with little tendency to slag. A smaller quantity comes from Poland, and this is rated as being moderately slagging.
In general, the introduction of low NOx burners has altered the morphology of the fly ash formed, but does not appear to have had much effect on deposition.
With the coal blends used in 600 MWe units, there have been no slagging or fouling problems. The chlorine content of the blends has a specified maximum of 0.2%, while the S content is about 0.7%.
With two smaller tangentially-fired units, taken out of operation in 1991, there was a persistent deposition problem, which was kept in check by the use of a commercial additive fed to selected burners. The additive consisted of sodium, copper, and zinc salts. In two other smaller wall-fired units, taken out of operation in 1992, the flame temperature was over l600°C, and the problem was kept in check by adding copper oxychloride (Bolt, 1990). Even though deposit samples were taken, both from the walls, and by using probes, no firm conclusions were drawn about the action and mechanisms of the copper-oxychloride additive.
Part of the reason for the relative success in the Netherlands in terms of boiler operation with minimal ash deposition problems is their use of supplies from consistent sources with known low slagging and fouling tendencies.
7.2 Utility use of advanced techniques to assess alternative coal supplies
New England Power in the USA is evaluating the use of low sulphur coals as part of its strategy to meet Acid Rain Legislation in 1995. In addition, they wish to explore the economic opportunities offered by the availability of other cheaper coals.
Candidate coals which deviate from the purchasing specifications are being systematically evaluated using both laboratory analysis, and field testing techniques. The methods include chemical fractionation, CCSEM, laboratory-scale combustion with ash deposition testing and field test burns using optical temperature monitors and deposit imaging cameras.
The assessment of two candidate coals is described by Afonso and others (1993). Pocahontas coal from Virginia was tested at Brayton Point and Cape Breton coal from
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• •
Case studies
1450 -
1400 P eli'.. .a ~ 1350 Q) Q.
E ~
1300 • sootblower operation
17.5 18.5 19.5 20.5 21.5 22.5
Time, h
Figure 48 Typical results using an optical temperature monitor, showing the effect of sootblowing on the FEGT (Afonso and others, 1993)
Canada was tested at Salem Harbour. Each test bum included testing a baseline coal and the candidate coal during a two to four week period. It began by fully characterising the full load conditions on the plant and optimising operating conditions. During the initial week, steady-state tests were conducted, while varying the excess air, and/or the load, within their normal operating ranges.
To evaluate the effects of coal quality on ash deposition, furnace deposit diagrams are used, based on frequent furnace observations. The deposit diagrams are then related to boiler performance data, flue gas composition and sootblowing records. As the effect of slag deposits on furnace heat absorption can be quantified by monitoring the FEGT, an optical temperature monitor, was used during test bums. A typical plot of results is shown in Figure 48 where the immediate effect of sootblowing can be seen, and also the signs of gradual build-up of deposits. As the FEGT rises, attemporation spray consumption also rises to control the boiler steam temperature. FEGT measured in this way for the two coals provided a good indicator of the effects of coal quality on slag deposition.
The test procedure called for establishing a 'clean furnace' FEGT by blowing all the wallblowers in rapid succession. The clean furnace value will be different for different coals, because it depends on the deposit coverage on waterwalls in regions where there are no sootblowers. Next, the furnace was allowed to slag, until the FEGT became stable. The dirty furnace FEGT value is critical, because it must not exceed the value at which the unit runs out of attemperation spray capacity. The FEGT data, together with observations of the walls can be used to optimise the sootblowing schedule. If the FEGT does not change after sootblowing, the slag deposits are not being removed. A small drop in FEGT indicates that only partial removal has been achieved.
Infrared imaging cameras were mounted at available viewports to provide a quantitative measure of wall emissivity. Figure 49 shows the results from the imaging camera taken during a typical test. By combining the results from the temperature monitor with those from the cameras, a more complete measure of the impacts of coal quality on
100
90
50
40
sootblower operation
I I 07:12 09:36 12:00 14:24 16:48 19:12
Time of day
Figure 49 Typical output from infrared imaging cameras monitoring wall emissivity showing the effect of sootblowing (Afonso and others, 1993)
boiler operations could be obtained. It was reported that the data were reasonably consistent.
One of the coals tested was from Cape Breton in Nova Scotia, Canada. On the grounds of price and the close proximity of supply, it had potential advantages, although conventional analysis showed that it had an unacceptably low ash fusion temperature of 1060D C under reducing conditions. Its ash content, however, was low, at about 5%. CCSEM analysis showed that the minerals were predominantly clays and pyrite, with pyrite making up 35% of the mineral volume and 50% of its weight. Its size ranged evenly from 1 to 46 /-lm size, and most of the pyrite over 10 /-lm size was liberated during pulverisation.
Detailed evaluation and test work involved drop tube furnace comparisons, use of the PSI Slagging Advisor™ model, and field tests on a wall-fired 155 MWe unit. The Cape Breton coal was found to deposit more slowly than the other coals tested, but tended to sinter and melt more readily. Hence the deposits could be more tenacious and require more frequent
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Case studies
045 ] + .......
.......0.40 Coal: rIl-.. " Cape Breton -"'b,.T
~. --+._- A
0.35
D
-1\ "-. \ "§"
\ . \ "\ .
'\ ". '\ "".'+" . ....... '-.
+ ....... '-. .......
D ........ +........
--0-- Pocahontas
'C .S! '(jj 0 0.30 a. Cll 'C c: 0
:;:::; 0.25u ..ell
u..
0.20 -"-' D'-.-.- .. ........ --0.15
0.10 I
1350 1250 1150 1050
Temperature, °C
Figure 50 Deposit collection efficiency for various bituminous coals from drop tube furnace work (Afonso and others, 1993)
sootblowing. The results of deposition tests in the drop tube Table 13 Cape Breton coal analysis (Afonso and others, furnace are shown in Figure 50. In the field tests the Cape 1993)Breton coal had a higher rate of slagging on the burner wall and on the leading edge of the secondary superheaters, Cape Breton New England
coal Power coal compared with the baseline coal. The conclusion from the specificationtest work was that the coal could be an acceptable alternative
feedstock. Potential 'problem' areas were identified, and it Proximate analysis, %was established that these could be catered for by adjusting Total moisture 0.93 :<;8.0
operating conditions with an optimised sootblowing schedule. Ash 4.75 :<;10.0 Volatile 36.31 26-36
The specification of the Cape Breton coal lay outside the Fixed carbon (by diff) 57.99 standard parameters of the purchasing specification in that its ash fusion temperature under reducing conditions was nearly Ultimate analysis, %
300°C below the 'set' figure (see Table 13). However, its Moisture 0.93
detailed assessment using the advanced techniques described Carbon 81.94
in this report, including CCSEM analysis, drop tube furnace Hydrogen 5.12 Nitrogen 1.44testing and field trials with additional instrumentation and Sulphur 1.64diagnostics, showed that it was a potentially acceptable fuel. Ash 4.75It will be interesting to see whether these findings are Oxygen (by diff) 5.10
confirmed by long-term operational experience.
As-received HHV, MJ/kg 34.0 :<;30.2 While the cost of such testing is relatively high, over a Sulphur, kg/GJ 0.5 :<;0.5 period, there could be substantial savings in operating costs Hardgrove Grindability Index 60 :<;60 associated with the use of a less expensive fuel.
Ash softening temperature 1057 :<;1343 (reducing) DC7.3 Use of pilot-scale tests to predict
ash deposition impacts on commercial boiler performance and operating conditions. Pilot-scale testing was carried out
at various increasing firing rates to determine if conditions ABB Combustion Engineering has used the results from their were generated which would inhibit the removal of deposits pilot Fireside Performance Test Facility to predict the impact by conventional sootblowing. of deposition in two different situations (Borio and others, 1992). The pilot test results were fed into a boiler Operational parameters generated include furnace performance assessment programme, which was configured temperatures, gas velocities in the convective pass and for each particular boiler design to calculate unit performance attemperator spray requirements. If any of the calculated
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10
Case studies
parameters exceed limits that are set by pilot-scale tests or equipment design considerations, then the boiler operating conditions are changed, for example by decreasing the boiler load, to ensure that critical conditions are not exceeded. Since boiler thermal conditions and ash deposit amounts are interdependent, it is essential that information on ash deposit effects is correctly fed into the iterative calculation process.
Using this approach for a 500 MWe unit firing an eastern USA bituminous coal, it was possible to compare the use of two alternative feedstocks. In the unit, the superheater outlet temperature is controlled by firing rate and attemperator water. The reheat outlet temperature is controlled by burner tilt and by a further attemperator spray.
The alternative coals were tested and assessed in the pilot combustor. Coal Al showed greater slagging tendencies than coal A2, while neither had significant fouling tendency. The thermal impact of slagging as determined in the pilot plant by plots of the waterwall heat absorption rates is shown in Figure 51. Both the physical and thermal properties of the deposits were strongly affected by the firing intensity as measured by local gas temperature and furnace heat flux. Resistance to sootblowing increased with increased furnace temperature. Maximum allowable furnace temperatures were approximately 55°C higher for coal A2 compared with coal AI.
Predictions using the pilot results were that the unit would require a capacity reduction to 60% of the design figure using coal Al to avoid experiencing slagging limitations. As a result of operating at this reduced level, upper furnace and convection path temperatures would be reduced significantly. This would create a problem in reaching the required reheat temperature. To compensate, the burner nozzles would be tilted upwards to raise furnace outlet temperatures. Additional attemperator spray would be required to control superheater temperature. The predictions with coal A2 indicate the capability to operate up to full design capacity without reaching ash deposition design limitations. However, a burner tilt of 30° is also needed to ensure adequate superheat and reheat temperatures.
Coal A2 Firing rate: 3.8 GJ/h Furnace temperature: 1595°C
o 3 6 9 12 15
Cumulative time, h
Figure 51 Heat flux through waterwall panel for two coals in the combustion engineering pilot unit (Borio and others, 1992)
In a second example, an alternative feedstock for a 350 MWe unit designed to fire subbituminous coal was assessed. Tests in the pilot unit indicated that firing coal B I reduced the convection tube deposit build-up by approximately 25% compared with the baseline coal at similar gas temperatures. However, the deposits were more tenaciously bonded, and because of this the potential advantage of coal B I was negated.
It is recognised that some boilers are able to operate regularly at conditions in excess of the critical conditions for a given coal because they drop load at regular intervals, thereby shedding deposits which might otherwise have been a problem. It is also recognised that this method of assessment requires more time, more coal and costs more than bench-scale derived indices. However it is probably more reliable, and provides a quantitative assessment of necessary levels of derating.
7.4 The use of low rank coals in the USA
The use of low rank coals in the USA provides an example of the progress made with understanding the mechanisms of deposition, and avoiding the economic penalties involved with the use of coals that have a high slagging or fouling potential.
Coals from the Dakotas, Wyoming, Montana and Utah in the Fort Union and Powder River basins have been associated with serious ash deposition problems. These were major barriers to sustaining high loads on boilers burning the high sodium, low rank coals. The problems have been substantially reduced by improvements in coal selection, boiler design, on-line cleaning of heat transfer surfaces, the control of operating conditions and the use of additives.
While problems still exist, they now occur mainly where boiler designs and operating practices have not been sufficiently matched to the variable properties of the low rank coals. There has been an enormous amount of effort made to study and understand the mechanisms of slagging and fouling in these coals. Recent advances are reported by Benson and others (1992). More advanced and, it is hoped, more reliable indices for predicting both high and low temperature fouling have been developed and are being validated.
Even before the latest work using CCSEM and chemical fractionation for characterising the ash-forming species, the top ten steam/electric plants in the USA based on unit operating costs used these troublesome low rank coals (see Table 14). A basic reason for this was the low cost fuel supply coming from smface mines with thick seams. However, the operational disadvantages of high moisture content, lower heating value and the tendency to produce difficult ash deposits in the boiler clearly did not make the use of these coals uneconomic. The ash deposition problems were successfully contained, although there is scope for further substantial improvement.
The state of the art in boiler design for highly fouling lignites
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Case studies
Table 14 Top ten US steam electric plants in 1990, ranked by unit operating cost (Sondreal, 1993)
Plant Capacity, Operator Fuel source Fuel cost, Production cost, MWe cents/GJ mills/kWh
Operating Total
Milton R Young 673 Minnesota Power ND lignite 46.1 9.38 16.1 Laramie River 1710 Basin Electric Power WY subbituminous 57.6 9.70 21.1 Nebraska City 616 Omaha Public Power WY subbituminous 63.2 11.06 Dave Johnston 750 Pacific Power and Light WY subbituminous 58.4 11.11 Coal Creek 1100 Cooperative Power Assn ND lignite 66.8 11.13 Hunter 1339 Pacific Power and Light UT subbituminous 82.7 11.28 Huntingdon 893 Pacific Power and Light UT subbituminous 82.8 11.85 Iatan 726 Kansas City Power & Light WY subbituminous 77.2 11.92 Antelope Valley 870 Basin Electric Power ND lignite 61.4 11.95 37.4 George Neal 1686 Iowa Public Service WY subbituminous 99.6 11.97
Operating cost includes fuel, on-plant manpower, maintenance and any other on-going costs of operations. Total production cost includes operating cost plus fixed costs for capital depreciation, taxes, interest and insurance.
has been important locally for nearly 20 years. The boilers Table 15 Effectiveness of remedial measures for
there, currently of 210 MWe size have been adversely controlling ash deposition (Sondreal, 1993) affected by serious ash-related problems, and in particular by
Reported effectiveness heavy slagging. As a result, Bharat Heavy Electricals Ltd (BHEL) initiated a major research programme to develop an
Remedial measure Fouling, % Slagging, % understanding of the various impacts of fuel quality on plant performance.
Altered tube area or spacing 100 Load reduction and cycling 86 100
Although many of the ash-related problems at Neyveli have Coal change 91 79
been attributed to specific design features of the units, the Reduced furnace exit temperature 80 80 general fuel characteristics of the lignite are at the root of theAdded steam or waterwall blowers 55 95
Added retractable tube blowers 91 problems. In particular, the lignite supplied from Mine 2 is Increased excess air 60 80 often heavily contaminated with both pyrite and marcasite. Boiler additives 67 63 The resultant slagging leads to high FEGT values; Burner changes 64 53 temperatures up to 1190°C have been reached compared with Increase coal fineness 50 64 the design value of 1043°c. The problems in the earlier units Hand lancing 66 47 were so severe that 36% of the heating surface area in the
reheaters was removed soon after commissioning in order to reduce the attemperation water injection rates. Even after the
is represented by large boilers designed to very conservative modifications the capacity of the sprays was inadequate to standards. The most important design parameters are the maintain full load. FEGT, the height from the burner to the arch, and furnace heat release rates. Wide spacing of the tubes in the The heavy accumulations of ash also resulted in occasional convective transfer sections and extensive use of both wall slag falls which resulted in violent explosions in the chamber and retractable sootblowers are characteristic of such plants. when lumps of hot slag fell into the water seal at the base of
the boiler ash hopper. The most effective remedial measures for reducing uncontrolled deposition in an existing boiler are listed in The slagging was further aggravated in 1989 by: Table 15. They include physical modifications to the tube layout, the provision of more sootblowers, modifications to co-firing oil with the lignite when some mills were the burners and a variety of operational changes (Sondreal, unavailable; 1993). the sustained use of the boiler at full load in order to
meet electricity demand when it should probably have It is hoped that the increasing understanding of the been derated; mechanisms of slagging and fouling, and knowledge of the high lignite feed rates because of poor quality fuel; effects of various mineral components, will enable plant choking of the recycle flue gas duct which caused burner operators to pinpoint the cause of problems more easily, and slagging due to the reduced velocity of the fuel/air to correct them appropriately. stream;
poor maintenance resulting in the unavailability of some wallblowers.7.5 Neyveli lignite use in India
The use of the large reserves of lignite at Neyveli in India A number of modifications were implemented to the mills
91
Case studies
and to operating procedures to alleviate the problems. These included:
making the feed a little coarser, thereby retarding combustion and moving the flame front away from the furnace walls. The fuel/gas velocity was also increased; maintaining symmetric firing and a uniform heating load in the furnace at all loads; correcting air imbalances in the burners; installing additional wallblowers and increasing the frequency of operation; avoiding mixed firing of oil and lignite; using some cold flue gas to reduce the temperature of the recirculated flue gas for lignite drying.
Even after these modifications, slagging is still a significant operational problem, and full load operation cannot be sustained in these boilers. Following shut-down and extensive cleaning, the FEGT rose from 850°C to 950°C over a period of 45 days, and after 60 days it rose to 1000°C, necessitating a reduction in load to about 80% of its maximum rating because of insufficient attemperation spray capacity.
Based on this experience, BHEL modified the design of the boilers for new units installed near Mine 2. Various design changes were incorporated, including:
the surface area of the radiant section was increased by 6% by adding 1.2 m of height, and thereby reducing the FEGT. The design figure was reduced from 1143°C to 1109°C; the superheater surface area was decreased by 13% to reduce attemperator spray requirements; the economiser surface area was increased by about 30%.
The Neyveli lignite has widely varying properties. From 500 samples collected, the range of ash content was from 2 to 13%, and of volatile matter from 22 to 33%. Its heating value varied from 10.7 to 14.7 MJ/kg. The broad range of chemical compositions of the laboratory ash can be seen in Table 16.
With assistance from the USAID programme, a fuels evaluation test facility was built at the BHEL site, Trichy, Tamil Nadu. In addition, large samples of Neyveli lignite were shipped to the Pittsburgh Energy Technology Centre for testing in the fuels evaluation facility there. However, in the test facility, the slagging deposits formed were powdery, and easily removed with a brush. No fouling was observed. These results were not in line with the observations on the plants.
Table 16 Composition of Neyveli lignite 'laboratory' ash
Analysis, wt% Minimum Maximum Average
Si02 24.0 72.9 42.7 Fe203 3.2 19.2 6.9 Ah03 3.8 23.5 13.2 CaO 7.3 27.8 13.6 MgO 1.2 9.6 4.3 Na20 0.2 1.7 0.6 K20 0.02 0.5 0.2 Ti02 0.1 0.9 0.6 S03 1.4 26.6 14.6
However, as the samples tested were low in pyrite and marcasite, the results supported the view that slagging on the plant is strongly associated with the presence of these particular minerals. The results also support the view that the empirical approaches did not reflect the slagging tendencies of Neyveli lignite. Although a high propensity to slag was predicted by the empirical indices, there was little evidence of this in the tests (Smouse and others, 1992).
Even in the new units built more recently, output is restricted to some 160 MWe (compared with the 210 MWe designed).
These events illustrate the difficulties in developing both designs and operating procedures to cope with difficult and varying fuels, even when there is considerable experience. It would appear that the difficulties and uncertainties associated with providing a 'representative' coal sample for test work contributed to the problems.
7.6 Monitoring fouling and optimising sootblowing at Teruel, Spain
The Teruel power plant consists of three identical 350 MWe units. The boilers are front wall-fired, with natural circulation. Each has 56 steam wallblowers and 54 long, retractable sootblowers in the later tube banks and panels. The plant is normally operated with a cyclic load.
The coal fuel is a mix of two high sulphur black lignites, a washed lignite and a low sulphur bituminous coal. Spanish black lignites are of similar rank to subbituminous coals. The black lignites are always blended with the bituminous coal.
Boiler furnace deposits are monitored by the use of heat flux meters. These are claimed to have a useful life of more than five years. The instrumentation is illustrated in Figure 52. A network of 52 sensors was used in unit 1, monitoring the whole network of sootblowers in the burner region. To confirm the validity of the measuring grid, the sum of the individual measurements was compared with the heat balance over the walls, and a good correlation obtained.
thermocoupleradiative transfer • measuring heat flux in tube crown
FLAME SIDE
COLD SIDE
I
t
tube
thermocouple wires on protected side
Figure 52 Furnace heat flux measurement principles used at Teruel (Cortes and others, 1993)
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Case studies
To obtain a measure of 'dirtiness', the absorbed heat flux is compared with the maximum attainable value at that location under clean conditions. One approach to this is to have a number of meters which are in fact kept 'clean', to provide the baseline figures. However, if this is not possible, then they are of little use, and they are certainly not applicable in cases of severe slagging.
At Teruel, the maximum heat flux available was calculated from a number of standard plant data. Observed maximum values under a number of operating conditions were correlated. This produced a model of furnace heat transfer patterns. Only drastic changes in the coal blend were found to affect the correlation. The influence of burner combination and excess air was not significant, although these will not vary much under full load. Monitoring was necessary over an adequate period of time, to include both 'dirty' and 'cleaned' conditions in order to provide sufficient data to establish the correlations.
The heat flux device used is only applicable to tubes in the furnace wall. Consequently, monitoring of the deposits elsewhere can only be carried out on an indirect basis through a heat balance on tube banks. Again, on-line heat transfer is compared with a calculated or observed baseline. As in other plant, FEGT and other temperatures are estimated from temperatures measured at the air heater inlet, and an upstream, back calculated, heat balance is applied. Other plant instruments only provide accurate information on the steam-side absorption of heat.
The use of the fouling monitoring system helped the operators to optimise the sootblower use. In particular regular sootblowing in areas with no deposits was avoided. Firstly a pattern of readings was established. Two sootblowing ratings are ascribed to every surface block. The percentage of necessity to sootblow is the opposite of the percentage of cleanliness at the time of a scheduled sootblow. Percentage of effectiveness is calculated be assessing the short-ternl change in heat absorbed after blowing.
By seeking the twin objectives of achieving necessity and effectiveness in all areas, the operator can improve sootblower use. The first objective is achievable, and should ensure that clean areas are not blown. The second objective raises the question of whether repeated wallblowing should be tried. Often it is better to wait for a thicker deposit to build-up before attempting removal. Also, repeated blowing on clean surfaces can increase tube failures. As a result of the information available, sootblowing sequences have improved, and a programme developed to evaluate further the necessity and effectiveness criteria.
In order to optimise sootblowing, a cost-benefit analysis is needed. However, there are a substantial number of variables. Deposition is affected by both coal quality and operational variables. In some situations there may be anomalous deposition behaviour. The fall in heat transfer, and the response to sootblowing need to be modelled as a function of measured plant parameters. At Teruel this is being attempted with the use of neural network techniques. It is hoped that this will lead to the on-line estimation of boiler efficiency,
with improvements resulting from the optimisation of the use of sootblowers.
In the Teruel work it is recognised that the cost of the instrumentation itself needs optimisation. It was estimated that in this particular case, only 30 heat flux instruments were needed to get a good correlation with the boiler heat balance, compared with the 52 installed. Adequate spacing was estimated as about 25 m2/sensor. However, the degree of coverage needed is difficult to define, and there is an unmeasurable link between the cost of instrumentation, and its use, and the potential reductions in operating costs (Cortes and others, 1993).
7.7 South African experience at Kriel The well documented story of the design, operation and modification of the 500 MWe boilers at KrieL makes it a useful case study. Although the plant was designed in the early 1970s, and much more is now known about boiler design, what happened illustrates many problems that can still arise.
When the contract was let, the only information on the coal was that shown in Table 17. No ash analysis was included. The early information on Kriel coal was based on borehole samples, and it was compared to what was thought to be a similar South African coal.
Table 17 Coal specification for boiler design at Kriel (Energy Developments, 1979)
Coal Typical Basis for boiler low volatile perfonnance coal guarantees
Values as-received Heating value (gross), MJ/kg 23.3 22.2 Heating value (net), MJ/kg 2\.5
Proximate analysis Surface moisture, % 5.0 5.0 Inherent moisture, % 3.6 4.5 Ash. % 19.6 19.7 Volatiles, % 18.0 2\.9 Fixed carbon, % 53.8 48.9
Ultimate analysis Total moisture, % 9.5 Ash, % 17.9 Total carbon, % 56.70 Sulphur - organic, % 0.38 Sulphur - inorganic, % 0.73 Hydrogen, % 3.01 Nitrogen, % 1.38 Oxygen, % 8.60 Chlorine, % <0.02
Ash fusion Basis for boiler temperature performance
guarantees
Initial deformation, DC 1225 Hemispherical, DC 1265 Fluid, DC 1305
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Case studies
a) Before b) After 2,5 h after sootblowing 4 h after sootblowing
,-- ...."" ..."" ..."" ...
~1300 ~1250
"'" ' / , \
I \ \ I \ \
I \ I I
/
/-t+tI
II
I /
/
II
I
II
/
\
/ I
I , I
III
\ \\ ,
\\
\\
\ , ' ....
~1330 : I ~1430 I I
I ' I\
I\ , , ""
/
'-' - .,,"
~1200 ~1180
Output 500 MWe Excess air ratio 1:35
Figure 53 Temperature distribution (Oe) in Kriel boilers before and after burner modifications (Energy Developments, 1980)
The design was based on experience with 400 MWe units built at Scholven for a German coal. Certain differences were identified, in particular:
the ash content at Kriel was 20% compared with 12% for the German coal; the ash 'hemisphere' temperature was lower at l265°C compared with l350°C; the differential between the ash initial deformation temperature and the fluid temperature was only 80°C for the Kriel coal, which is small.
In view of these factors, the boiler designer decided to make a drastic reduction in the heat release in the burner zone, and hence reduce the heat flux and the maximum flame zone temperature.
The initial coal sample on which the comparison was based came from one drill hole. Subsequently, samples came from as many as 200 drill holes, and on the basis of analyses, a new assessment was made of the expected coal properties. When the mine was opened, it was found that 95% of the
coal had properties (and generally unwanted properties) outside those which had been predicted. In addition, the mineral matter in the South African coal was much more finely disseminated through the coal than that in the German coal with which it had been originally compared. There was, however, no realistic alternative but to using the local coal for the power plant.
The boiler design was for a horizontally opposed firing system, with six pulverisers and 36 burners in three horizontal rows, and a central dividing wall of water tubes which was not cleaned by sootblowers. The burners were a well-established design with movable vanes to adjust coal-air mixing, and secondary air velocity.
During initial operations, the boiler was shown to be operating as designed. However, output was quickly limited to 400 MWe due to the formation of layers of slag up to above the last row of burners. This was in a doughy, molten state. Large lumps of slag dropped off, and overloaded the water seal and hopper underneath the boiler. With sootblowing cycles every four hours and excess air increased
94
Case studies
from 1.25 to 1.35, output could be increased to 425 MWe. It was possible to observe the build-up of deposits. Above 400 MWe, equivalent to a furnace temperature of l400°C the whole process from initial sticking to a substantial build-up took less than an hour. There were, however, no signs of fouling, even when the FEGT went as high as 1350°C.
Various optimisations were attempted, but with little success, and it was decided that in spite of conservative design parameters, the heat release in the burner zone was still too high. There was an extensive investigation, and a great deal of effort was expended in trying to understand what was causing the slagging. These are described in a series of articles in Energy Developments (1979,1980,1981).
Eight burners out of the 36 were moved to a higher level in the boiler, reducing the heat release from 0.9 to 0.7 MW/m2. The angle of the burner quarls was changed from 50° to 40° because of wall contact, and as a precautionary measure 26 additional wall-mounted sootblowers were added. The change in temperature distribution before and after the burner modifications is shown in Figure 53. After the modifications it was possible to run the plant at a continuous load equivalent to 500 MWe output, but the air ratio still had to be kept at 1.35. Consequently further tests and modifications were made. Important changes were made to the flame pattern to reduce impaction by soft or mblten particles. The air inlet temperature was reduced from 310 to 260°C thus reducing the secondary air velocity at the burner throat by 9%. This reduced the recirculation effect. A reduction of the excess air from 1.35 to 1.24 reduced the air velocity by 11 %, again reducing recirculation.
These changes resulted in such a pronounced reduction in slagging that even at the excess air ratio of 1.24, the sootblowers only needed to be operated twice a day.
7.8 Brown coal use in Australia and the fouling problems at Loy Yang
There has been an enormous amount of experience in Australia in burning the Latrobe Valley brown coals. Usage on a substantial scale started during the 1920s. After World War II, when the largest individual boilers were of 20 MWe capacity, larger plant was built with Yallourn E at 129 MWe, Hazlewood at 200 MWe, Yallourn W at 350 MWe (in the 1970s), and most recently Loy Yang with 500 MWe units.
The Hazelwood plant, commissioned during the 1960s, ran into serious problems due to fouling, and a considerable amount of work was undertaken to minimise the effects on operation. In particular, water sootblowers were introduced, and boiler operation was improved so that the operating period between off-load cleaning was between 3000 and 4000 h (Clark, 1984). At Yallourn W tower type boilers were used. There were serious operating problems associated with the milling of the woody and fibrous coal feed. There were also problems with the maintenance and reliability of the precipitators. Development work concentrated on these areas, and by comparison, fouling was a less important issue at
Yallourn. Long-term fouling tests, and the effects of soluble aluminium were reported by Anderson (1989). This paper includes a description of the pilot plant which can bum about 12 kg/h of coal and be used for 1000 h tests.
During the 1980s, plans were finalised to develop a new open cut at Loy Yang, to supply up to four 500 MWe generating units. The coal deposit was extensively explored, and detailed maps were made of the distribution of the main ash-forming materials. In view of the extensive programme of research and development work undertaken by the State Electricity Commission of Victoria, and in particular by the Herman Research Laboratories, it might have been expected that these units would operate with minimal difficulty. The fact that there have been problems, including excessive fouling which has recently restricted operating periods between off-load cleaning to less than 1000 h, illustrates the complexities involved, and the difficulties of forecasting ash behaviour.
The resource characterisation was carried out on the basis of a 400 m grid of boreholes, equivalent to the testing of 1 Mt of coal. The minerals and inorganics were analysed for every 6 m of depth of each borehole. This provided a huge database of information about th~deposit, on the basis of which the distribution of coal quality could be mapped. Extensive combustion tests were carried out in both short-term pilot testing covering the ranges of Na20/ash ratio and long-term (1000 h) pilot testing. These tests meant that the fouling propensity of the coal could be both modelled and forecast
.(Anderson and others, 1990; Woskoboenko and others, 1992).
It was recognised in 1984, when the first Loy Yang unit came into use, that the inorganic make-up of the coal was not fully understood, and that there was much to be learned about its behaviour during combustion. The minerals and other inorganics at Loy Yang were different from those at both Morwell and Yallourn, and different from any other coal in use in the world. The ash consists of mainly sodium sulphate and chloride, silica and alumina, and compounds containing those constituents. The more usual clay particle remnants, iron oxides and calcium and magnesium are only present in very small amounts. The presence of acid soluble aluminium in the upper layer of the coal tended to result in a highly refractory, low fouling fly ash. This gave rise to the possibility of blending this top level with high sodium, high fouling coal lower down, to produce a low fouling mixture. It was not easy to produce a mining plan to achieve this, and the feed rate to each boiler of 600 t/h would make blending an expensive operation (Anderson, 1989). The brown coal is not stocked. Only enough for about 12 h of operation is held between the open cut and the boiler, because of the risk of spontaneous combustion.
An example of the ash deposit chemistry derived from the long-term test work is illustrated in Figure 54. These are expressed in terms of Niggli values which are based on a recalculation of the elemental composition from percentage weight to a relative molar basis. The composition of each component is expressed as a molar percentage of the sum of the basic oxides (CaO and MgO in this case). It provides a basis for a direct assessment of the enrichment and depletion of the various components.
95
Case studies
200
150
100
50
o
100 ______ MgO 0'
-------~-----_._~-------~-------....o
II o 50 CaOCl
----e-~
+ o 0 III
~ Ul Q) :::l
co > g 250l
z 200
150
100
50
o
Coal Slag panel Test bank 1 Test bank 2 Precipitator ash
~ '-
Furnace Test bank 1 Test bank 2 Air heater 1000DC 800DC
Figure 54 Ash deposit chemistry for Loy Yang coal, fired on the pilot combustor (Anderson and others, 1990)
The Niggli values illustrate the following: The Na is due to condensation, and S to enhanced sulphation of Na, Ca and Mg.
the uniform ratio of Ca, Fe and Mg in the raw coal, deposits and the final fly ash; Ash deposits were analysed using advanced methods such as the enrichment of Si and Al on the slag panel, and the CCSEM and x-ray diffraction. In addition ash samples were depletion of Sand Na; subject to thermo-mechanical analysis. This is a sensitive and enrichment of Na and S in the convection pass deposits. objective method for measuring the detailed sintering and
96
• •
Case studies
6
5
:E Cl
4 Cl .5 "'5 o-'0 Q) 3 1ii... "C
~ Q) I/) 2 ..c o
o
/
• / / .
/ / ./
/ /
/ /
/ /
/ /
/ /
/ /'
• /
/' /'
./ /'./
/'./ /'
./ /'./ /'
./ ./ ./~..- ./
./ ./
./ ./ Regression equations
Rate 1 (g/h) = -0424 + 0.172 Na2 0/ash r = 0.85 (n = 24) Rate 1 (g/h) = -0849 + 0 126 Na2 0/ash + 0263 Na/AI sol r = 0.96 (n = 17)
•
/'
• ./
/ /. /.
/ • A
./
o 10 20 30
Percentage sodium oxide in ash
Figure 55 Observed fouling rate on test bank 1 in the pilot combustor (Anderson and others, 1990)
fusion properties of ash samples. Ash pellets are heated at a constant rate whilst subject to a small constant load via an alumina probe. Movement of the probe is monitored by a linear voltage displacement transducer, and changes can be detected at different temperatures.
The ash fouling rate could be correlated with the Na20/ash ratio in the coal, based on data from the first test bank in the pilot combustor. The correlation is shown in Figure 55.
Following the start-up of the first Loy Yang unit in 1984, there was an extensive study to compare the behaviour of ash in the boiler with that measured in the pilot combustor. During a shut-down in 1986, samples were taken from various parts of the boiler (see Figure 56) and these broadly confirmed the validity of the results from the pilot-scale work. The influence of ash fouling on boiler operation, and the kind of monitoring that is needed is illustrated in Figure 57. This shows high sodium contents after 480 days and 550 days operation leading to changes in the FEGT, sufficient to force a boiler outage.
Although a great deal had been learned about the effects of changes in the coal feedstock on boiler operation, and in particular on fouling, the Loy Yang units encountered serious problems during 1992-93 when the mining operation ran into some high sodium areas of coal without having low sodium coal to blend it with. The loss in availability was as high as 12%, compared with a target loss of below 2%. Units needed to be shut-down for cleaning after as little as 800 h of operation.
During an intensive investigation, it was shown that the ash and silica contents were significantly higher than had been
inferred from the borehole samples. Operational problems were acute particularly with hopper bridging at the bottom of the boiler. As much as 100 t of deposit solidified, on occasion, and had to be blasted to remove it. The bridging was associated with the higher then expected silica levels, together with the sodium. There were explosions in the bottom hopper water seal, which needed strengthening. A whole series of measures were taken, such as combustion tuning to reduce the flame temperature, and in particular the temperature in the hopper. More waterblowers were installed, and the maintenance programme strengthened to ensure that blowers were operational. Blower effectiveness was optimised. Innovative on-line cleaning and diagnostic methods were tried, including hopper slope rappers, high temperature thermal imaging of deposits and the use of additives such as alumina to weaken deposits. Efforts to improve coal quality by intensive pre-sampling of blocks to be mined, control of the mining operation, and cutting the time taken to clean the boiler when it is shut down, have all brought about a significant improvement in availability. By mid-1993 the interval between shut-downs was normally over 1600 h. The sodium content of the feed coal is forecast to fall from a weighted average of 0.22% in 1992-93 to 0.17% during 1994-95. Together with the various engineering and operational steps taken, it is hoped to reduce the availability loss to below 2% once more.
Support work to investigate the causes of the deposition at Loy Yang has been carried out by the Herman Research Laboratory, near Melbourne. Among other things, they developed a computer model to predict particle trajectories within the furnace. These were used to track the fuel/ash particles from the burner to their point of impact on the furnace wall. Chemical additives that might be injected to
40
97
Case studies
minerals 500 ~ 400
300
200
100
~'-----------' :----~~--- ;....------'~---~------;~----------o
s o... II o Cl
:2E + o III
~ 1/1G> ::J iii > 'El Cl Z
inorganicsy--- ¥. -----y~- - - -Y- - - -Y- - - -y. - - -Y- - - -y. - - -y- - - - MgO
50
o
150
100
50
o
i~ Ic= =:::::J II I I I
o 0
... o S 'is.. '0 ~ c..
o 0 o 0
'\ / '\ /
e--:J Figure 56 Ash deposition chemistry in Loy Yang unit 1, as at November 1986 (Anderson and others, 1990)
98
Case studies
mitigate deposition (such as bauxite) could also be tracked. It entering through the main burners is shown in Figure 58. The was thus possible to determine the optimum injection points, particles came mainly from the opposite side of the furnace. injection velocities and particle size for maximum effectiveness. This has provided a useful starting point for The example illustrates that even with a great deal of full-scale test work to confirm the predictions where the background experience and knowledge, practical problems additives are likely to have a beneficial effect. A diagram still require a combination of investigational work and showing the predicted impaction points of solid inert particles modifications to operating conditions, as well as monitoring
1250 boiler performance
regression equation
1200 p ~ ::::J
'la ~ 1150 Q.
E .2! f/) tll Cl ~ 1100 >< Ql Ql o tll l:
~ 1050
1000
25
~ 200
£ 15f/)
!:! 0 10 '" tll
Z 5
0
8
6 g
~ 4
z 2
0
> 9 tll
:E! 6..r:::s: Cl 3
0
340 370 400 430 460 490 520 550 580 610 640
Day
Figure 57 The influence of ash fouling on boiler operation, Loy Yang unit 1, leading to November 1986 outage (Anderson and others, 1990)
99
Case studies
gas split 42/58 (main/vapour) primary jet level 3
particles exiting from burners 1, 2, 3, 4, 5, 6, 8
2 8 6 5 4
particle diameter 1000 11m particle density 100 kg/m 3
Figure 58 Fold-out diagram of Loy Yang furnace, showing impaction points of solid inert particles coming from the main
6
5
4
3
2
o o o =. D
. ~ / ~. D . ' ~.. cf= ..
." .......;.",.:'" .~"''''.. .(:/f7:~::~~,l~:T"~ .I---_-.--,-;.-----,-~-.-:'.~?':....-• •":',r')-----\.
~ .. i:.
:
o D' o o a D',: ,
,r4.... ....... .... ....."[r ~.". = .\= = =
n ....;;.c.~~~..,...~.f .= . D· .
6
5
4
3
2
1
burners (SEC, 1993)
their effectiveness. It is not yet possible to anticipate and prevent operational losses.
7.9 Clinker formation at the Bayswater plant, NSW, Australia
The Bayswater power plant consists of four 660 MWe boilers. They are opposed wall-fired units, and each has 28 dual register swirl burners, allowing tuning for low NOx
operation. The boilers were built to a conservative specification, incorporating the best practice and experience of the 1980s. The coal used during the early years of operation had high ash fusion temperatures, and was also used at the nearby Liddell plant. Liddell has four 500 MWe tangentially-fired boilers, and had experienced no clinker or slagging problems.
At Bayswater. there has been a persistent history of operational problems related to the formation of ash deposits in and around the burner throats. The formation of clinkers in this region has caused (from time to time), ash hopper explosions, bottom hopper blockages and flame scanner interference. This is due to the large size of the clinkers, which grow, hanging downwards until they drop off due to their weight. Clinkers have grown to such a size as to cause flame distortion and subsequent burner tripping out, because the flame scanners are unable to detect the flame. On occasion, the clinker formation has been such that when oil guns are used to re-ignite the burner. interference with the
spray has resulted in fuel oil entering the windbox and igniting there.
Although the units were of identical design, it appeared that units 3 and 4 had more problems than units 1 and 2.
The problems have been thoroughly monitored and investigated, and are currently being kept in check (Boyd and Lowe, 1990; Boyd and Foreman, 1992). The efforts required provide an insight into what may be needed, even on a well-designed modem plant, in order to cope with unexpected interactions between boiler design and ash behaviour.
The first stage of the investigation programme involved aerodynamic modelling of the burner, the development of a clinker observation technique, and assessment of the effect of copper oxychloride dosing of the coal. The second stage involved additional aerodynamic modelling, burner re-tuning (on-site), both laboratory and in situ fly ash adhesion tests, the monitoring of quarl refractory temperatures, and of clinker growth rates and patterns.
7.9.1 Burner modelling
A computational fluid dynamics model was used to find out if clinker formation was related to burner aerodynamics. It was based on a two-dimensional axisymmetric finite difference flow prediction programme. The study investigated the effects of parameters such as the degree of inner, outer and primary swirl, quarl angle and entrained gas temperature.
:' :.. '.: :....~?:~~~< .' .
o o o
--<01~ ....:--,:... ~ .;. ~•.. , .. , Dt
. ~..... ~
., D~,
~,!
"
. ... :..., ."
100
It found that burner flow patterns and coal ignition points (flame distance from the burner) were extremely sensitive to burner swirl. Operation with low swirl values was more likely to incur particle deposition around the burners.
Theoretical predictions were verified by a series of gas temperature and oxygen concentration measurements along the burner centrelines. Over 20 burners were checked, with a large range of swirl values. There was a strong correlation between flame light off distance and swirl. Subsequent work investigated the effect of changing the quarl profiles at a range of burner swirl settings. Two new profiles were modelled, and compared with the existing profile. In addition, an attempt was made to measure the sensitivity of quarl temperature to the degree of burner swirl. Although the results were not entirely repeatable, in general higher swirl produced lower quarl temperatures.
A more comprehensive three-dimensional furnace model has been used recently (Boyd and Kent, 1994). It was able to elaborate on the aerodynamic mechanisms which might lead to ash deposition around the burners. Particle trajectories were found to be sensitive to both the burner levels being used, and the degree of swirl imposed. The results tended to confirm that increased swirl would reduce ash particle deposition. The model was then used to predict the overall impact of the increased burner swirl levels.
7.9.2 Copper oxychloride dosing
Based on earlier work in the UK, dosing with copper oxychloride at 2 ppm of the coal feed was carried out for a four week period during 1989. The test failed to produce any quantitative or qualitative evidence of reduction in the formation of clinker around the burners. This may have been due to the ash chemistry of the particular coal feed, the dosage may have been insufficient (in the UK, up to 14 ppm was required to have an effect in certain coaliboiler combinations), or clinker formation may be primarily due to aerodynamic effects, rather than ash chemistry.
7.9.3 Clinker observation
One of the problems in studying clinker formation is that because of the large boiler size (associated with conservative design, and the high coal ash content) it is not possible to see the burners from the side wall during firing. As a result, a commercially available prismatic borescope with an air and water-cooled probe has been modified for the work at Bayswater. The viewing head is located in the secondary air stream, via a flame scanner access port, and permits a view of a burner during firing. While the angle-of-view takes a little while to get used to, the probe has been successfully used to undertake systematic surveys of clinker build-up, and to investigate the effects of changes in burner tuning.
7.9.4 Ash adhesion tests
Attempts have been made to investigate the relationship between refractory surface temperature and the propensity for ash/refractory bonding. Laboratory tests were carried out on a number of refractories.
Case studies
Tests showed that fly ash adhesion to all the tiles started at 900°C. From 900-1300°C adhesion was described as light, while at 1400°C there was firm adhesion on all the tiles.
Thermocouple measurements on burners with different refractories showed quarl temperatures of approximately 700°C for an in-service burner, and 950°C for an out of service burner. There was no discernable difference between the different materials. It appears that the light adhesion observed in the laboratory may be sufficient to initiate clinker formation when the burner is out of use. Attempts are now being made to find the temperature below which clinker will not form on the various refractories under conditions found in the furnace.
It is interesting to note that in the discussion on laboratory work the reference is to 'testing the adhesion of fly ash'. It is not quite clear whether the material used was what has been called in this report laboratory ash, fly ash collected from the ESPs, or fly ash samples removed from the burner region. The point is that these materials are chemically different, and these differences might affect the results of tests.
This is all part of a thorough and competent investigation into clinker formation which has resulted in a considerable reduction in operating problems. It therefore illustrates the difficulties of undertaking rigorous work under controlled and well-defined conditions. In practice it may not matter much which ash was used, and in a great deal of reported work similar doubts would apply. The most important work repOlted is that carried out inside the boilers during operation.
Longer-term tests are being undertaken to monitor a boiler fitted with modified-swirl burners, and re-tuning of the burners will be carried out on all the boilers, and monitored. This will check not only clinker formation but also the effects on furnace heat transfer, combustion efficiency and on NOx formation. The situation is fmther complicated as it is now expected that the coal feed will be purchased at minimum cost from a wider possible range of suppliers. This may introduce feed coals with different ash chemistry.
7.10 Experience at the Comanche power station, Pueblo, CO, USA
The Comanche station is about 20 years old and has two 360 MWe units. One is a Combustion Engineering tangential-fired boiler and the other a Babcock and Wilcox opposed wall-fired boiler. The Combustion Engineering boiler has had the staggered tubes in the economiser replaced by in-line tubes and this had reduced the effect of fouling.
They have used the same coal source throughout, with a long-term supply contract. It is a western subbituminous coal with about 30% moisture and 5-6% ash. Coal quality does not vary much. It is thoroughly sampled and analysed only once a month, although sulphur, ash and heating value measurements are made on a sample from each train-load.
Major boiler outages for maintenance and cleaning are every two years, but other outages are necessary after 9-12 months
101
Case studies
to remove deposits. Both boilers suffer from reduced output because of ash deposition and when the plant was visited, output on the Babcock unit was about 300 MWe instead of 356.
One thing that has been tried is to spray the walls and tubes with MgO after cleaning, but there was no conclusive advantage. Additives to alter the ash chemistry have not been tried.
The coal flowing through each mill (4 or 5, depending on the boiler) is measured, but not that to individual burners. Combustion air flow and its temperature are measured. The boiler is run at 20% excess air (or around 2.5-3% excess 02).
The key temperature measurement is at the economiser exit. This varies, depending on the load. It is 455°C maximum, (limited by the duct construction) and is normally around 440°C. It can drop to 280°C at 125 MWe. When the plant was visited, the economiser exit temperature was 380°C at 309 MWe output, and the calculated FEGT was reported to be 1290°C.
The loading on the fan is monitored by measuring the amps used. In addition, draught readings are taken by probes around the boiler, every day once deposits have started to build-up, in order to check their effects.
For the past two years, all these readings have been fed into a computer programme which also uses the log mean temperature difference across the main steam tube banks. These give a measure of the heat transferred to the steam and can be used to back-calculate or infer the gas temperature at the feed points to each bank, and the state of cleanliness of the tubes. The cleanliness figures (and the heat balance) are only meaningful under steady running conditions ±1.5% for over 30 minutes. The figures are displayed on a VDU for the different tube banks, and an example of this is shown in Figure 59. This shows surface cleanliness in each part of the boiler.
Surface cleanliness is a measure of how well heat is being transferred in the different parts of the boiler. As deposits form, the tube is insulated, reducing heat transfer and boiler efficiency. Continued deposition can inhibit gas tlow through
Comanche Generating Station OTIS On-line Thermal Information System
OPERATOR AWARENESS INDICATOR
SURFACE CLEANLINESS
1.5
0.5 W WALL SEC-SH RE-HT PRI-SH ECON
BEST RESULTS WILL BE OBTAINED DURING STEADY LOAD CONDITIONS
UNIT 2
308.913 Gross MW 10148 h Btu/nkWh 0.33 h deY % 99.18 $/shift
Boiler Eff 85.5 %
ECON In 477 deg F ECON Out 567 deg F ECON Exit Gas 716 deg F SEC-SH Out 986 deg F PRI-SH Out 769 deg F Hot Reheat 999 deg F Cold Reheat 550 deg F FEGT 2361 deg F Main Steam 2405 psig Hot Reheat 449 psig Cold Reheat 499 psig Drum 2541 psig
SH Spray 0 kpph RH Spray 62 kpph Main Steam 2009 kpph Feed Water 2040 kpph Total Coal 339 kpph Total Oil 0 kpph Total Gas 0 ksc/h Excess O 2.7 %
2
Control Summary What - if Unit Trend Input Report Shift
26 May 93 14 37 5 C2-0AI-GRAF
Figure 59 VDU display at Comanche power plant, showing changes in surface cleanliness in various parts of the boiler
102
Case studies
parts of the boiler, diverting it to other parts which may cause hot spots to develop where the tubes are clean. The deposition can also increase pressure drop through the boiler so that the induced draft fan has to work harder to maintain the gas flow.
In the diagram, each bar for each section represents a reading for a 10 minute period. The most recent measurement is that on the left hand end. For example, for the waterwalls, the bar on the left represents the latest position. There was little change during the previous two 10 minute periods but about half an hour ago (the fourth bar from the left) there was a measurable decline in cleanliness. By contrast, in the reheat section, cleanliness improved at the same time, probably due to sootblowing a little earlier. It takes about 15 minutes to see a change after a section has been blown.
A key operating parameter is the FEGT. The fusion temperature of laboratory ash from the coal is about 13 15°C. As the accuracy of the FEGT measurement is thought to be ±55°C, any displayed temperature between 1260 and 1370°C could represent a temperature above the measured fusion temperature of the laboratory ash. This illustrates the difficulties involved in boiler control, in view of the sh0l1comings of fusion temperature measurements discussed elsewhere in the report, together with the margin of error in assessing the FEGT.
Deposition is controlled by sootblowing, but there are limitations as excessive blowing can cause tube erosion or
might generate hot spots in parts of the boiler. Sootblowing can be virtua]]y continuous on the waterwalls and may be 2-3 times per shift using lances on the tubes banks. When the boiler is getting plugged-up it is desirable to reduce output to 120 MWe for a time to try to displace deposits by thermal shock and differential contraction/expansion. The waterwalls are then even more thoroughly 'blown', mainly on the night shift.
The surface cleanliness indicators are only valid under reasonably steady operating conditions. After an outage for cleaning, it is necessary to recalibrate the bar graphs. It should be noted that the use of such programmes requires a good basic understanding of the heat transfer taking place in various parts of the boiler and what affects it.
Among the things that are needed for better operational monitoring are:
reliable and accurate temperature measurements at higher temperatures (up to the FEGT); on-line draught probes for continuous measurement.
One interesting observation was that during the first 5-6 years of operation there had been little or no deposition problems. Coal quality has ostensibly remained the same. Apparently there has been a similar experience at another nearby station, Rawhide. The plant personnel spoke highly of the monitoring programme which assesses surface cleanliness, and regard it as an effective aid to boiler control.
103
8 Discussion
Ever since the introduction of coals as a means of generating electricity at the turn of the century, the impurities in the fuel have had a major impact on the design of plant and on its availability and capacity. A huge amount of extraneous material passes through the boiler and although only a tiny proportion forms deposits, these can cause significant longer-term operating problems.
From a boiler operators point of view, ash-related problems can be thought of as impacting either unit capacity (an immediate effect), or maintenance costs, which is a longer term effect. Unit capacity is affected when a deposit cannot be removed by sootblowing, and thermal resistance builds up. This causes FEGT values to increase beyond a level at which the superheat and reheat temperatures can be controlled with acceptable rates of attemperation (Borio and others, 1992). For the designer, ash-related behaviour can be thought of as primarily affecting capital cost in order to accommodate the anticipated behaviour of ash-forming materials.
Predicting the behaviour of a coal under combustion conditions is complex, and involves several stages and approaches. These include detailed analytical work on representative coal samples, dynamic testing under near combustion conditions in a drop tube furnace, studies in a pilot-scale combustor and full-scale field trials. Because of the relative complexity of these studies, and the time and cost involved, relatively few such studies have so far been attempted.
Two basic approaches have been applied to the investigation of ash deposition phenomena. The first is empirical, and the second is theoretical or mechanistic.
Much of the research and development over the years has been in response to major practical needs. Most commonly it has been because boilers have not operated as expected, due to ash deposition and its effects. This has affected the economics of operation, such that both operators and
designers have wanted to minimise current and future problems. It should be noted that the driving force, and hence the main area of technical interest, has been the reduction of slagging in countries such as Australia and the UK as well as in coal importing countries. In the USA, the principal focus has been on fouling, because of the many operating problems encountered with indigenous coals. Australia also has a particular interest in fouling, associated with the Victorian brown coals. Both slagging and fouling are encountered in the use of German coals. The ditferent emphasis in different countries is reflected in the way that work is written up, although the focus is not always clearly identified.
8.1 Empirical approach Empirical approaches have been driven by immediate commercial need, and are still the ones mainly used in practice. They have been based on the somewhat superficial laboratory characterisation of coal ash, and measurements of its ash fusion temperatures. Even if the methods are successful there is virtually no possibility of extrapolating the results beyond the particular coals tested, and there are dangers in transferring the findings from one boiler to the next if there are significant design differences.
The empirical approach to studying slagging and fouling has been pursued both by assessing the overall performance of large-scale plants, and by undertaking pilot plant work. The main development in the empirical approach is the possibility of correlating behaviour with more meaningful analysis of the inorganics present than has been possible with the standard laboratory ash figures. The new analytical methods include CCSEM, IR spectroscopy, x-ray diffraction and chemical fractionation. The use of this information has meant that the correlations can be based on a more realistic foundation.
Pilot-plant work offers the possibility of comparing the behaviour of different coals at comparatively low cost. The
104
Discussion
results are now used in conjunction with the latest analytical techniques for characterising both coal and deposits. However, different designs are used. Some units are fired vertically, and others horizontally. Many have a refractory lined combustion chamber. Residence times vary. Thermal and aerodynamic simulations are often compromised by either economic or technical considerations. It is not possible to simulate all the conditions in a full-size boiler. The relative aerodynamics and distribution of time temperature cycles in the combustion gases and flue gases are very different from those in a full-size furnace. It is also difficult to generalise from deposition results on a single probe or panel.
The pilot plant is essentially an empirically calibrated furnace. Its use is limited to the range of coals over which it has been tested. Data interpretation is not easy. In some cases the results have been successfully extrapolated to full-scale facilities. In others, they have produced results that appeared quite contrary to what happened in the full-scale boiler. It is essential to understand the differences in conditions that produce such results. In many cases, pilot plants are dedicated to the use of a relatively narrow range of coals (Bryers, 1992).
The empirical approach has been used to look at the results from operating units. In one of the largest surveys, 131 boilers around 300 MWe were studied. A statistical analysis of the empirical slagging and fouling indices commonly used produced an 80% correct prediction of behaviour. This survey was carried out in the early 1980s, and confirmed the need for additional information and a more rigorous approach. In particular it pointed to the need to take into account the behaviour of the inorganics, boiler design and the operating conditions.
8.2 Mechanistic approach The mechanistic approach attempts to both explain and predict the chemical and physical changes that take place in the boiler. This aims to cover the whole range of inorganics present, through the high temperature transf011llations followed by rapid quenching. Through an understanding of the entire transformation/deposition process it is hoped to explain all the various differences in behaviour. A central weakness in this is that both the transformations and the exact boiler conditions are insufficiently understood and are difficult to measure.
There have been extensive efforts to undertake detailed studies of the mineral matter present in coals, and to a lesser extent the chemically-combined inorganics. Significant advances have been made. In addition there have been detailed studies which have explored the mechanisms of the various transformations that take place.
Some of the basic studies were reported in the early 1980s; in particular, the transformations in the mineral matter in bituminous coals by Huffman and Huggins (1986); interactions between the minerals by Biggs and Lindsay (1986) and the vitrification and sintering characteristics of silicate by Raask (1986a).
More recently, studies have related coal characteristics as established using advanced techniques, with the behaviour of various coals in full-scale units. Five subbituminous coals were tested, and an index developed which attempts to rank the coals according to their performance. It was acknowledged that the index which takes account of the inorganics which might be expected to int1uence combustion and ash deposition was speculative (at this stage). Although there was some success in relating to plant results, it was recognised that controlled bench-scale combustion testing of these coals was needed. This could develop ash stickability and deposit strength figures which are currently lacking. These data might be generated with the use of a drop tube furnace (Weisbecker and others, 1992). It was intended to extend these tests to include a wider range of coals.
Perhaps the interesting aspect of these studies is that while they demonstrate how much more is known about transformations in the inorganics in coal, they also illustrate how difficult it is to relate this knowledge to the complex situation in a boiler. The knowledge can help explain what has happened both in pilot combustors and in full-scale plant. However, it still does not enable operators or designers to make accurate predictions about slagging and fouling, although it will have improved the reliability of predictions in certain cases.
8.3 Computer models and advanced indices
Attempts are being made to do this in the form of comprehensive models, but there is a long way to go, before such models can be used with confidence, except where used with a narrow range of coal types and boiler designs. They use huge amounts of computer capacity. In addition, models generally address a 'steady-state' condition, and a principal problem with boiler operation is that ash deposition and build-up change the conditions. The change in t10ws and temperatures is sometimes significant and occasionally disastrous, in terms of maintaining output and availability. Modelling cannot yet address these mechanisms.
A range of approaches to modelling have been described. Fly ash formation models are in various stages of development by several workers. The intended output of such models is a description of the size and elemental composition distributions of the entrained particles, together with the formation and subsequent condensation of vapours.
It must be recognised that the process of combustion and mass and heat transport throughout the system is already extremely complex. When the interactions of a matrix of both inorganic and organically-bound mineral species interacting with each other and with the carbon and sulphur present are all taken into account, the complexity tends to get out of hand. At the moment, this approach cannot be extrapolated far beyond the particular coals and conditions under which it is validated. However, mainly during the past five years or so, there have been attempts to adopt a methodical approach on a large enough scale to permit the possibility of extrapolating the results beyond the scope of the fuels tested
105
Discussion
(Bryers, 1992). This includes some of the complex modelling work which has been discussed in the report. The results of this approach are beginning to appear, but their practical implications have yet to be evaluated.
In terms of deposition, the complexity can be reduced by recognising perhaps a dozen main mineral species, and studying their interactions. In addition. the process has been broken down into small fragments to permit manageable. controlled laboratory studies. Unfortunately these end up as fragments which apply only to the limited range of coals and conditions tested. There is a clear need to integrate the information with an interpretation in terms of engineering principles. In some way the gap between research and applied engineering design needs to be bridged (Bryers, 1992). This has not yet happened, and it may be a considerable time before it is.
There are differing views about the value of models. Some people take the view that the situation being analysed is so complex that the only real source of data is results from full-scale testing. Others say that models can help clarify the mechanisms which can lead to cost-saving decisions over operating conditions or coal feed choice. The difficulty will always be that of validation and optimisation of the models.
Much depends on:
how the model is constructed; how various submodels interact; what data the models consider; whether the model can cope with the changes caused by deposition.
The development and validation of a model involves substantial effort, and one of the dangers, as with small-scale combustion testing, is that the use of models might even lead to spurious conclusions. On the other hand, model work and the resultant clarification of probable mechanisms and effects has shown itself to be potentially useful in a number of situations.
The CQIM is probably the best tested model available but is applicable to particular units where there has been thorough calibration and validation against recorded operating conditions for well characterised coals. It is cost orientated, and most applications will use the standard (inadequate) indices when assessing the cost effects of deposition. It is expensive to validate, and a test on an individual boiler to compare two coals can cost around US$O.5 million. It does, however, address the overall issues and their interaction.
In an extensive review of the current status of the
possibilities of predicting ash behaviour in coal-fired boilers, Benson and others (1993), say that current techniques are limited to certain types of coal and to certain parts of the boiler. This is based on extensive work on the mechanisms of ash deposition over many years. Most advanced techniques have been applied to particular types of coal in which there is considerable current interest. These include Australian coals, with export potential in mind, as well as domestic, US low sulphur coals, for compliance with emission regulations and UK coals to minimise operating costs.
As an example, the EERC in the USA, report an advanced index to predict the fouling propensity of Powder River Basin subbituminous coals. Work is continuing on the verification of the index with a wider range of coals, in a wider range of combustion systems.
The models being developed, and most of the laboratory- and pilot-scale work result in comparisons between the behaviour of different coals. For example, the PSI Slagging Advisor™
does not claim to be able to predict the impact of a coal on boiler operation. What it does is to compare or 'rank' different coals. While an extension to the Advisor is being planned, this would have to combine a deposit growth model and a deposit removal model with a complete boiler heat balance. It would have to include a particle by particle simulation of the deposit dynamics, and incorporate the mechanisms responsible for deposit growth, strength and removal. The reduction in heat transfer due to the deposit would also have to be taken into account (Helble and others, 1992d). A great deal of work would need to be done before such a model is developed and validated.
Particular areas where relatively little work seems to have been done, and where information has not been integrated with ash deposition studies include:
mineral variability in a given coal, and assessing how representative of the inorganics present is a properly taken coal sample. It should be noted that this will vary from coal to coal; the effects of beneficiation on mineral matter composition: the effects of coal blending or switching on a given boiler (with the possible exception of work on blending eastern and westem coals in the USA); predicting deposit strength, and whether or not it will be readily removable; developing comprehensive boiler monitoring systems and better instrumentation to improve the understanding of operating conditions and to provide information to help the operator understand and minimise ash deposition problems.
106
9 Conclusions
Ash formation from the inorganic impurities in coals, and its subsequent deposition, causes problems in utility boilers all over the world. Much of it is unreported. The ash arises from the presence of mineral matter, and, particularly in the lower rank coals, from organically-bound elements. Although only a tiny proportion of the ash formed actually deposits, the effects on operation can be serious, and are usually costly to rectify.
There are problems in many of the existing 4000 pulverised coal-fired boilers around the world. Many of these were built using outdated design criteria. Large numbers of boilers are required to operate on a coal feed that has different characteristics from the 'specification' coal for which the boiler was originally designed. This may be either because the coal comes from a different source, or because of quality changes within a deposit or because mining methods have changed during the life of the plant.
Many operators are facing economic pressure to change their coal supply. This may be because cheaper internationally traded coals are available, or because of emission limits on sulphur. To meet sax emission limits, the choice may be between coal switching to use a compliance fuel with low sulphur content, and installing flue gas desulphurisation plant. To meet NOx emission limits, the necessary operational changes using staged combustion and low excess air tend to aggravate slagging and fouling with some coals.
Most boilers have only a limited amount of instrumentation and control equipment. In view of this it is perhaps remarkable that they work so well. However, when it comes to reducing the operational problems associated with ash deposition, this lack of instrumentation and hence of precise information about conditions in the boiler is a major problem.
Fouling and slagging are very complex phenomena, and most predictions of ash behaviour have been based on empirical data which are not entirely satisfactory.
There are a large number of variables, including:
the different mineral species involved and their juxtaposition and particle size; the inorganic components which are organically-bound; the effects of milling, prior to combustion; the shock heating. and the variable time temperature cycle seen by different particles; the multiple phase changes and chemical reactions taking place, with rapid transformations, most of which take place under non-equilibrium conditions; the effects of the aerodynamics of the bulk gas flow, and the boundary layer around the boiler walls and the heat transfer tubes; the effects on radiant heat transfer of the ash material which is formed, particularly in the combustion zone; operating conditions, including boiler cycling and load following.
In order to minimise the deposition, and associated corrosion and erosion, engineers have tried to characterise the inorganics present in the different coals, and also to understand the mechanisms that are involved. This is with a view to changing the operating conditions where possible, or to choosing a coal feedstock which will minimise the difficulties.
Standard laboratory analyses of coal ash involve procedures which are very different from the time temperature cycles seen by the ash-forming components in a boiler. The indices of slagging and fouling tendency based on laboratory ash analysis have been shown to be about 80% accurate for US coals, according to figures from over 100 plants. Such indices are likely to be even less accurate or secure when applied to the range of coals traded and used internationally. Other assessments, based on a wider range of coals, suggest a figure of only around 60% success in the predictive capabilities of the standard indices in relation to the inherent slagging and fouling propensities of coals.
107
Conclusions
In spite of the complexity, development work has established:
new analytical techniques to look at ash-forming materials in raw coal, and the composition of inorganic intermediates and of the deposits and t1y ash formed; empirically-based correlations from drop tube furnace and pilot work to plant experience; new modelling possibilities, associated with increased computer power.
There have been significant advances in recent years in characterising the inorganics in a coal. The techniques, however, tend to use very small samples. While the detailed knowledge is good for development work, questions about how representative the sample is have been largely ignored. The use of unrepresentative samples for all kinds of test work causes problems, since the findings can be misleading or even invalid, when related back to the operating conditions experienced in practice.
Possibly the most important development has been that of computer controlled scanning electron microscopy (CCSEM), although this remains a high technology laboratory procedure, and there are not many such instruments. Standard sample preparation and test procedures have not yet been adopted with CCSEM, although efforts are being made to do this.
Drop tube furnaces are being increasingly used to compare the properties of different coals, and in particular their tendency to form sticky deposits. These are dynamic laboratory devices which come close to replicating some of the more important conditions seen by coal particles in a boiler.
Many large research institutes, boiler manufacturers, and a few utilities and coal companies, have pilot combustors to test the behaviour of coal ash under combustion and post combustion conditions. Again the results enable operators to produce a ranking of coals in terms of their fouling or slagging tendencies. However, none of the results is absolutely secure, in that a given coal might behave differently in a real boiler, and one of the difficulties in using the results is that every pilot facility is unique. Many results are regarded as proprietary knowledge.
Over the years it will be necessary to build up a body of experience with the small-scale predictive techniques to establish for what coals, and for which boilers they can be used effectively. The results from drop tube furnaces and pilot plants require validation with those from full-scale boilers. With current work, the number of results is steadily growing, even though not all are published. Essentially the results are only true for the particular pilot plant and full-scale plant involved. As time goes on, it should be possible to extrapolate results to other similar plant and other similar coals with greater confidence, although it may never be possible to forecast behaviour with absolute certainty. The increased level of confidence achievable should enable operators to optimise their operations and reduce costs.
Progress has:
greatly improved the possibilities for 'trouble-shooting' and investigational work; improved the possibilities of identifying a 'rogue' coal which would cause trouble; made some steps towards more secure predictions of deposition behaviour on units where small-scale tests and/or large models have been thoroughly validated.
Some full-scale work has been carried out, but it is both expensive and time-consuming. Some models have been validated on particular plant, but it is hazardous to extend their application beyond the narrow range of coals tested in particular boilers.
A number of groups are working on developing improved indices, or of comprehensive models from which ash behaviour can be predicted. Some are beginning to use data from advanced analytical techniques. Considerable progress has been made, but much of the work has been based on small-scale tests. As these models are more widely validated and more full-scale tests carefully monitored, they will become more useful. They are looking at a huge number of variables in addition to those associated with ash deposition.
It should be noted that:
many of the new techniques are focused on limited aspects of the overall deposition process, to permit manageable and controlled studies; most models are still dependent on indices based on laboratory ash; current design methods depend on traditional indices, experience, and small-scale test work; the greatest benefit in the short-term is probably the refinement of the well-established indices; the models being developed, and most of the laboratoryand pilot-scale work result in comparisons between the behaviour of different coals. They do not claim to quantify the effects of ash deposition on boiler operation, other than on a comparative basis; there is a long way to go before general predictions will be secure.
Currently, slagging and fouling are tackled by a number of preventive and remedial measures which include:
a substantial increase in boiler size (and hence cost) for coals containing particular extraneous components, or combinations of components; an increase in the heat transfer surface to compensate for lower rates of heat transfer caused by deposition or by lower gas temperatures; increased levels of excess air to maintain oxidising conditions. This, however, results in loss of efficiency, and may increase NOx levels; the installation of increasing numbers of sootblowers; the closer monitoring and testing of conditions in a boiler to check factors such as the even distribution of fuel to different burners;
108
Conclusions
accepting the need for frequent scheduled outages. Under certain circumstances this can be the optimum course of action; thorough cleaning during an outage. This may mean the use of high pressure water jets, or even the use of dynamite, to remove deposits; derating the generator, deliberate load cycling to remove deposits as there is differential expansion and contraction of the heat transfer tubes and the deposits; the selection of alternative coal feedstocks with tight monitoring and control of their properties and characteristics, together with the use of coal blending where appropriate.
Because of the complexities involved, it is doubtful whether any predictive technique will ever be able to anticipate and explain all deposition behaviour. With the intensive studies being carried out, of which modelling forms a part, people's understanding of many of the factors affecting deposition has improved. Predictions of behaviour should steadily improve for types (or groups) of coals with similar characteristics.
The work outlined in this report has been responsible for considerable advances in the understanding of the mechanisms of ash deposition. The result is that the effects can be more readily understood, and the possibilities of both prediction and reduction have been improved.
109
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