Post on 19-Mar-2018
ÅBO AKADEMI
KEMISK-TEKNISKA FAKULTETEN Processkemiska forskargruppen
FACULTY OFCHEMICAL ENGINEERING
Process ChemistryGroup
REPORT 01-03
Ash Forming Matter in Biomass Fuels
Maria F.J. Zevenhoven-Onderwater
Academic Dissertation
Combustion and Materials Chemistry Lemminkäinengatan 14-18 B
FIN-20520 Åbo, Finland http://www.abo.fi/instut/pcg/
Åbo Akademi
Ash Forming Matter in Biomass Fuels
by
Maria Zevenhoven-Onderwater
Akademisk avhandling som för avläggande av teknisk doktorsexamen vid Åbo Akademiförsvaras vid offentlig disputation .......
Fakultetsopponent:
Avhandlingen försvaras på engelska
Department of Chemical EngineeringProcess Chemistry Group
Biskopsgatan 820500 Åbo, Finland
Åbo Akademi
Ash Forming Matter in Biomass Fuels
by
Maria Zevenhoven-Onderwater
Department of Chemical EngineeringProcess Chemistry Group
Biskopsgatan 820500 Åbo, Finland
Preface
I
PREFACE
The work as described in this thesis was done within the Process Chemistry Group at Åbo
Akademi University.
5
I especially want to thank my supervisor, Prof. Mikko Hupa, for the “challenge and
opportunity” to be a member of his group. Bengt-Johan Skrifvars and Rainer Backman are
thanked for their patience and enthusiasm and encouragement to continue and finish this
work. Dr. Flemming Frandsen is gratefully acknowledged for never-ending encouragement.
10
Prof. Brian Scarlett from the University of Technology in Delft is gratefully acknowledged for
teaching me “thinking”. Prof. Kaj Karlsson is warmly thanked for introducing me in the
world of inorganic chemistry. He taught me “to walk on the ice of research” without hurting
myself badly.
15
I gratefully want to thank all my coauthors for their contribution to this work.
Prof. Mikko Hupa is thanked for carefully reading and discussing the manuscripts; Dr. Rainer
Backman is thanked for providing a solid basis for the thermodynamic equilibrium
calculations as used in this work; Dr. Bengt-Johan Skrifvars is thanked for reading and20
discussing manuscripts as well as developing the “star” system for ranking fuels (See paper
II); Dr. Patrik Yrjas is thanked for discussing and commenting paper I; J-P Blomqvist is
thanked to carry out part of the experiments as described in paper II.
My coauthors Prof. Risto Laitinen, from the University of Oulu, is acknowledged for carefully25
reading and commenting paper I; Laura Nuutinen is thanked for the lion part of the analysis
of the chemical fractionation and SEM analysis as described in paper I
My coauthors Dr. Marcus Öhman and Dr. Anders Nordin from the Energy Technology
Center, Piteå, Sweden are thanked for taking care of the lab- scale experiments and SEM30
Preface
II
analysis as described in paper VI;
Truls Liliendahl, Christer Rosén, Dr. Krister Sjöström from The Royal Institute of Technology
(Sweden) and Klas Engvall and Dr. Anders Hallgren from TPS Termiska Prosesser AB
(Sweden)are thanked for taking care of the gasification experiments as described in paper5
V.
Fortum Power and Heat oy is acknowledged for kindly sharing fractionation results.
My special thanks go to Linda Fröberg, Johan Werkelin and Clifford Ekholm who carried10
out almost all of the experimental work considering fractionation and SEM analysis as
described in this thesis.
Dr. Don Hardesty, Prof. Larry Baxter ,currently at Brigham Young University in Utah, and
Gian Sclippa from Sandia National Laboratories, Livermore, California are gratefully15
acknowledged for their hospitality during the winter 1999-2000.
Sonja Enestam and Kristoffer Sandelin are acknowledged for sharing their thoughts about
“ash chemistry at equilibrium”. All remaining group members have contributed in several
ways to the completion of this thesis. Especially my colleagues in “Axelia” have been20
supporting me during these long years.
Further I want to acknowledge my financial supporters, without whom this thesis never could
be finalised. I gratefully thank the Finnish National Combustion and Gasification Research
Programme LIEKKI 2; The CODE programme; The Academy of Finland; the Finnish25
Technology Agency (TEKES); the Värmeforsk Project No B8-803; EU/Joule III “Improved
technologies for the gasification of energy crops” JOR3-CT97-0125 (DG12-WSMN) project;
Foster Wheeler Energia OY, Karhula, Finland; Brista Kraft Sigtuna Energi, Rävsta, Märsta,
Sweden; Skellefteå Kraft AB, Skellefteå, Sweden; Falun Energi, Falun, Sweden; Söderenergi,
Södertälje, Sweden, C-4 Energi, Kristianstad, Sweden; Fortum Power and Heat oy, Vantaa,30
Preface
III
Finland and Kvaerner Pulping oy, Power division, Tampere, Finland.
Finally, I want to thank Pia and Liza, my daughters, for welcome distraction and love and
my parents, although living a long way south, always interested and supporting my life and
work here in the north.5
And last but not least there is Ron......
”ilman sinua elämäni olisi hävinnyt kuin tuhka tuuleen...”
10
Turku, ........2001
15
Maria Zevenhoven
Abstract
IVIV
ABSTRACT
Ash-forming matter in biomass fuels can be present in different ways, i.e. as soluble ions,
organically associated, as included minerals or as excluded minerals. The way ash-forming
matter is present can have consequences for the behaviour of a fuel in a fluidised bed boiler.5
Deposit formation and possible bed agglomeration are dependent on the release of ash-
forming matter from the fuel.
In this thesis the ash-forming matter in biomass fuels has been studied by means of
traditional fuel analysis and an extended fuel characterisation consisting of chemical
fractionation analysis and in some cases SEM/EDX analysis.
The extended fuel characterisation was carried out for 16 fuels. Clear differences have been
shown in the distribution of ash-forming elements in the different fuels. In the older fuels
more ash-forming elements were present as included or excluded minerals. In relatively15
young fuels up to half of the ash-forming elements was organically associated or present as
easily soluble salts or as included minerals.
The results of the extended fuel analyses were used to predict fuel behaviour in fluidised bed
combustion and gasification. Deposit formation tendencies were predicted for 23 fuels using
the combination of chemical fractionation with Thermodynamic multi-Phase multi-
Component Equilibrium(TPCE) calculations. This work shows that easily leached elements
form the main constituents in the fine fly ash, and are consequently reasonable
approximations of the fly ash compounds. The ranking of the fuels as studied in this work
was presented as less-problematic < problematic: coal< peat < wood derived fuels <25
annual crops < agricultural waste. This corresponds well to the general practical experiences
with these fuels.
TPCE calculations as presented in this work were used as guidelines for predicting bed
agglomeration. Modelling showed that the presence of an excess of dolomite/calcite
Abstract
VV
decreases the amount of alkali components in the bed due to an increase in the amount
volatilised. An excess amount of silicates increased the amount of alkali retained in the bed,
forming low melting alkali silicates, leading to bed agglomeration. At atmospheric pressure
the amount of melt formed could be smaller, when compared to high pressures, indicating
a lower risk for bed agglomeration.5
Chemical fractionation results revealed that when firing woody biomass fuels potassium and
calcium present in a bed coating are originating from the reactive fraction in the fuel, i.e.
leachable with water or ammonium acetate and/ or present as included small minerals as
pointed out by SEM/EDX analysis.
Table of contents
VI
TABLE OF CONTENTS
1 Introduction 1-11.1 The use of biomass in heat and power generation 1-11.2 Producing heat and power from biomass fuels 1-21.3 Ash-related operational problems 1-31.4 The objective of this work 1-3
2. Literature review 2-12.1 Fuel characterisation 2-12.1.1 Differences in solid fuels 2-12.1.2 Traditional fuel analysis 2-32.1.3 Chemical fractionation 2-52.2 Fluidised bed reactors 2-82.2.1 Combustion in atmospheric systems 2-92.2.2 Combustion in pressurised systems 2-112.2.3 Gasification 2-122.3 Ash behaviour in fluidised beds 2-132.3.1 Conversion of fuel particles 2-132.3.2 Deposit formation 2-172.3.3 Deposit formation: Field and laboratory experiences 2-242.3.4 Bed agglomeration: The mechanism 2-292.3.5 Bed agglomeration: Field and laboratory experiences 2-31
3 Results 3-13.1 Biomass fuel characterisation 3-13.1.1 Traditional fuel analysis 3-13.1.2 Chemical fractionation 3-23.1.3 SEM/EDX analysis of biomass fuels 3-63.2 Ash behaviour in (P)FBG and FBC 3-83.2.1 Prediction of ash deposit formation 3-83.2.2 Prediction of bed agglomeration 3-16
4 Conclusions 4-14.1 Fuel characterisation 4-14.2 Deposit formation 4-14.3 Agglomeration 4-3
5 References 5-1
Table of contents
VII
PUBLICATIONS
I. “Searching for improved characterization of ash forming matter in biomass”, MariaZevenhoven, Bengt-Johan Skrifvars, Patrik Yrjas , Mikko Hupa,Laura Nuutinen,RistoLaitinen, (Paper 73) accepted for presentation at the 16th International Conferenceon Fluidised Bed Combustion, May 2001, Reno, Nevada, USA
II. “The prediction of behaviour of ashes from five different solid fuels in fluidised bedcombustion, Maria Zevenhoven, Jan-Peter Blomqvist, Bengt-Johan Skrifvars, RainerBackman, Mikko Hupa, Fuel, 79(9), (2000), 1353-1361
III. “Predicting the ash behaviour of different fuels in fluidised bed combustion”, Bengt-Johan Skrifvars, Maria Zevenhoven, Rainer Backman, Mikko Hupa,(Paper 113)accepted for presentation at the 16th International Conference on Fluidised BedCombustion, May 2001, Reno, Nevada, USA
IV. “The ash Chemistry in Fluidised bed gasification of biomass fuels: Part I- Predictingthe chemistry of melting ashes and ash-bed material interaction”, Maria Zevenhoven,Rainer Backman, Bengt-Johan Skrifvars, Mikko Hupa, Fuel in press
V. “The ash Chemistry in Fluidised bed gasification of biomass fuels: Part III-Ashbehaviour prediction versus bench scale agglomeration tests”, Maria Zevenhoven,Rainer Backman, Bengt-Johan Skrifvars, Mikko Hupa, Truls Liliendahl, ChristerRosén, Krister Sjöström, Klas Engvall and Anders Hallgren, Fuel in press
VI. “Effect of fuel quality on the bed agglomeration tendency in a biomass fired fluidisedbed boiler”, Maria Zevenhoven, Marcus Öhman, Bengt-Johan Skrifvars, RainerBackman, Anders Nordin, Mikko Hupa, to be submitted
VII. “The prediction of deposit formation in combustion and gasification of biomassfuels”, Maria Zevenhoven, report 01-02, Åbo Akademi University, in press
Chapter 1: Introduction
1-1
1 INTRODUCTION
1.1 The use of biomass in heat and power generation
Biomass is the ecological term for organic material, both above and below ground and both5
living and dead, such as trees, crops, grasses, tree litter and roots. The types of biomass used
in power generation production include energy crops, agricultural and agro-industrial wastes,
sewage sludges, municipal solid wastes, black liquor and peat. This diversity and ready
availability make biomass a strong alternative to fossil fuels for future energy requirements
around the world.10
Efforts to develop ways of producing and using biomass resources for heat and power
generation are currently supported by various national and international stimulation
programmes. Interests in bioenergy vary from country to country, but can be generalised for
developed and developing countries. Governments of developed countries are searching for15
ways to reduce both the emissions (especially CO2) produced by combustion of traditional
fuels and the amount of municipal solid waste, sewage sludge, etc. requiring disposal.
Developing countries face pressures to build energy systems that supply heat and power to
rural areas. There is no shortage of biomass on a global scale. The total energy content of
biomass reserves equals the proven oil, coal and gas reserves combined; However, most20
of this biomass energy is held in trees and the biomass source is regenerated slower than as
it is harvested, thus depleting the resource. (Kendall et al.,1997)
The European Union aims at increasing the use of bioenergy to 90 million toe (tons of oil
equivalent) by the year 2010 (EU15).15 million toe should come from the exploitation of25
biogas which is emitted from landfills to the atmosphere today. 30 million toe should be
retrieved from agricultural and forest residues. The last 45 million toe should come from
cultivated energy crops such as Salix (willow), Eucalyptus and Miscanthus. (Communication
from the Commission, 1997)
30
The use of bioenergy in Finland has increased by more than 60% since 1980 and was
Chapter 1: Introduction
1-2
almost 7 million toe in 1996 . The use of peat is 2,1 million toe and the rest consists mainly
of wood and wood-based fuels. The share of bioenergy in the primary energy consumption
is more than 20%. The target of Finnish energy policy is to increase the bioenergy use by
25% by the year 2005.This increases to 1.5 million toe. The target for the year 2010 is to
increase with 3.5 million toe from the 1995 level. The interest in growing energy crops such5
as Reed Canary Grass, Turnip rape and Willow, or methanol and ethanol production from
barley and wheat in Finland is very low due to the abundant resource of woody biomass
and peat for producing heat and power. The potential of those two is far much higher than
that of energy crops (Communication from the Commission, 1997).
10
1.2 Producing heat and power from biomass fuels
The energy conversion technologies that are interesting from the energy crop’s point of view
are production of pyrolysis oils and combustion or gasification. Combustion means direct
burning in presence of oxygen to produce heat. Gasification means conversion under sub-15
stoichiometric conditions producing combustible gases. These can, for example, be burned
in a gas turbine producing heat and electricity. In power generation, the heat produced is
generally applied in boilers which produce steam to drive turbines. For coal and biomass,
combustion technology is well understood, fully commercial and widely used for district
heating and power production. Gasification technology for biomass fuels is in a more20
experimental stage.
The performance of bioenergy technologies depends on local circumstances such as power
production requirements, availability of fuel and delivery costs, as well as on the chemical
and physical characteristics of the fuel (Faaij, 1997). Research on the reduction of25
atmospheric emissions has come a long way in recent years. Although emissions are already
substantially lower than with fossil fuels, efforts to reduce them further have been successful.
Assuming complete regeneration, biomass is a more CO2-neutral fuel when compared to
coal, only releasing what it has taken up during growth when fired.
30
Chapter 1: Introduction
1-3
1.3 Ash-related operational problems
Conversion of biomass fuels in fluidised bed reactors (FB) seems a most promising way to
produce electricity due to their high fuel flexibility. The relatively long residence times and
good mixing can ensure high conversion efficiencies (Clean Coal Technologies, 1993).5
Technical inefficiencies, which are often ash-related and pollution-related still exist today.
Ash-related problems in fluidised bed combustion (FBC) and fluidised bed gasification
(FBG) could lead to deposit formation and defluidisation. Both types of problems depend
on geometry, process conditions and ash chemistry. These are often reasons for
unscheduled shut-down. The ash chemistry is fuel-related, whereas the other factors will10
depend strongly on boiler design and operation. Fluidised beds are useful due to the good
mixing and relative low process temperatures preventing many of the ash-related problems
that might occur in other furnace types. However, ash-related problems during combustion
or gasification of biomass fuels can still lead to operational problems.
15
Much research has been carried out on FBC and FBG failure due to ash-related problems.
The ASTM /DIN standard ash fusion test is often employed for predicting bed agglomeration
(DIN 51730, 1984). It has, however, been reported to be a poor indicator of ash related
operational problems with biomasses and energy crops. Another predictive method is the
ash pellet compression strength measurement as used, among others by Skrifvars (1994).20
Many FB test rig and pilot plant tests are carried out both under reducing and oxidising
conditions. Test rig experiments are time-consuming.
1.4 The objectives of this work
25
Ash-forming matter in biomass fuels can be present in different ways, i.e. easily soluble salts,
organically associated compounds, as included minerals or as excluded minerals. The way
ash-forming matter is present can have consequences for the behaviour of a fuel in a
fluidised bed boiler. Deposit formation and possible bed agglomeration are dependent on
the release of ash-forming matter from the fuel.30
Chapter 1: Introduction
1-4
In this thesis the ash-forming matter in biomass fuels has been studied by means of
traditional fuel analysis and an extended fuel characterisation consisting of chemical
fractionation analysis and in some cases SEM/EDX analysis.
The extended fuel characterisation of 16 fuels is described. The results of the extended fuel5
analyses were used to predict the behaviour of 23 fuels in fluidised bed combustion and
gasification. Deposit formation tendencies and agglomeration are predicted for 30 fuels
using the combination of chemical fractionation with Thermodynamic multi-Phase multi-
Component Equilibrium(TPCE) calculations.
10
This thesis consists of a short literature review on biomass fuel characterisation, ash deposit
formation and bed agglomeration in fluidised beds (Chapter 2); a summary of the results
obtained from studying 30 biomass fuels (Chapter 3); conclusions which could be drawn
from the work described in chapter 3 (Chapter 4); references (Chapter 5); six papers and
one report (referred to as I-VII)15
Paper I describes the use of the chemical fractionation procedure in the characterisation of
biomass fuels. SEM/EDX analyses of the untreated biomass fuels, leached fuels and
laboratory ashed fuels complete the fuel characterisation.
20
In paper II and III the combination of chemical fraction and TPCE calculations for the
prediction of deposit formation is described and demonstrated for several biomass fuels.
Predictions are compared to bench-scale and full-scale measurements.
Papers IV and V describe the use of TPCE calculations for prediction of bed agglomeration25
in biomass fired fluidised bed gasification; a comparison between 1) the thermodynamically
modelled chemical behaviour of four biomass fuels in pressurized and non-pressurised
gasification experiments as carried out in two test rigs and 2) the thermodynamic modelled
chemical behaviour and SEM/EDX analysis from bed material retrieved from these test rigs.
The small-scale experiments were carried out in the pressurized FBG at the Royal Institute30
of Technology and in an atmospheric FBG of Termiska Processer AB (TPS) both in Sweden.
Chapter 1: Introduction
1-5
In Paper VI chemical fractionation of biomass fuels is used to explain bed agglomeration in
fluidised bed combustion. SEM/EDX analyses of bed samples retrieved from bench scale
experiments are used to determine which kind of ash forming elements are responsible for
the formation of agglomerates. The bench scale experiments were carried out at ETC in
Umeå, Sweden.5
In Report VII the approach as described in Papers II through VI is used to predict ash
deposition and agglomeration of biomass fuels studied at Sandia National Laboratories,
Livermore, California. Chemical fractionation results for these fuels were described by Miles
et al. (1995a through c). This report summarises the work as carried out during winter10
1999-2000 at Sandia National Laboratories, Livermore, California.
Chapter 2: Literature review
2-1
2 LITERATURE REVIEW
2.1 Fuel characterisation
2.1.1 Differences in solid fuels5
Naturally occurring solid fuels include fuels such as biomass, peat, lignite, bituminous coal
and anthracite coal. In addition to carbon and hydrogen constituents solid fuels contain
significant amounts of oxygen, water, ash-forming elements, nitrogen and sulphur. The
oxygen is chemically bound in the fuel and varies from 45 wt% for wood to 2 wt% for10
anthracite coal on a dry ash-free basis. Moisture can exist in two forms in the fuel- as free
water between cell walls in wood or in the larger pores of low grade coal or as bound water
held by physical adsorption. Green wood typically consists of 50 wt% water. Lignite contains
between 20-40 wt% moisture most of which as free water. Bituminous coals contain about
5 wt% moisture as bound water (Borman and Garland 1998).15
Ash is the inorganic residue remaining after the fuel is completely burned. Wood usually has
only a few tenths percentage of ash, while coal typically contains 10 wt% or more ash. Ash
characteristics play an important role in boiler design in order to minimise deposit formation,
erosion and corrosion and defluidisation.20
Coal has a structure and composition which differ widely from that from biomass fuels.
However in both coal and biomass ash-forming matter can be present in four general forms
(see Figure 2.1):
1) As easily leachable salts25
2) As inorganic elements associated with the organic matter of the biomass. This is
defined as organic associated matter in this thesis.
3) As minerals, included in the fuel structure, so-called included minerals.
4) As inorganic material, typically sand or salt or clay from harvesting the biomass fuel
or deposited in plant debris as discrete particles of foreign material. According to30
Chapter 2: Literature review
2-2
Figure 2.1: Schematic of the different forms of ash-forming matter
in coal and biomass
Reid (1984) inorganic material
in coal may also have been
deposited later by mineral-
laden water percolating through
the coal seam. This ash-5
forming matter is called
excluded minerals.
In biomass the included and
organically associated ash-10
forming matter makes up for
the major part of the total ash-
forming matter
Bryers (1996) summarised the15
occurrence of ash-forming
elements in biomass in a review
article. The two principal forms
of sulphur in plants are
sulphates or organic sulphur.20
The former increases with
increasing sulphate in the nutrient supply. Chlorine in biomass appears as a chloride ion. Its
concentration is closely related to the nutrient composition of the soil. Phosphorous exists
in its most oxidised form in biomass fuels and is not reduced during plant’s metabolism. It
is primarily introduced as H3PO4 and either remains in the inorganic form or is incorporated25
in organic structures by forming esters or pyrophosphates. Silicon is introduced in the plant
by absorption of silicic acid from the soil. Silicon is deposited as a hydrated oxide usually
in an amorphous form, but occasionally in crystalline forms. Potassium is the next most
important element in the plants. Potassium occurs as ion that is highly mobile with little
structural function. Potassium uptake is highly selective and correlates with the plants30
Chapter 2: Literature review
2-3
Figure 2.2: Schematic of traditional fuel analysis after Hannes,
(1996)
metabolic activity. Thus, potassium is often found in regions where plant growth takes place.
2.1.2 Traditional fuel analysis
Since solid fuels are not homogeneous, an elementary description is not possible. Due to the5
different ages and origins of many fuels their composition will vary widely. Standard testing
and analysis of coal are prescribed by ASTM standards or DIN standards. The proximate
analysis, ultimate analysis and heating value will be described below shortly. (See Figure
2.2)
1) With the proximate analysis, the amount of moisture, mineral residues (ash), volatile10
matter and fixed carbon are determined (ASTM D3172-89, DIN 51718, DIN
51719). Although the determination is done quite accurately, the name “proximate”
indicates the empirical nature of the method; a change in procedure can change the
results. A sample of coal (or biomass fuel) is crushed and dried in an oven at 105 to
110/C to constant weight to determine residual moisture. The sample is then heated15
in a covered crucible (to prevent oxidation) at 900/C to constant weight. The weight
loss is referred to as volatile matter. The remaining sample is then placed in an oven
at 750/C with the cover off so that the sample is combusted. The weight loss upon
combustion is termed fixed carbon or char. The remaining residue is ash. The
components of a proximate20
analysis are rather arbitrary.
There is no sharp distinction
between free water and water
chemically bond to the fuel. The
split between volatile matter and25
fixed carbon depends on the rate
of heating as well as the final
temperature. Some of the ash
can be volatilised during the char
determination. Nevertheless, the30
Chapter 2: Literature review
2-4
Figure 2.3: Typical volatile matter, fixed carbon and higher heating
value for wood, peat and coal on a moisture and ash free basis (after
Borman and Garland, 1998)
proximate analysis provides a useful comparison between fuels. The proximate
analysis for biomass is limited to 600/C (Borman and Garland, 1998).
2) The ultimate analysis (ASTM D 3176) does not distinguish between the origin of an
element coming from fixed carbon or volatile matter. The analysis determines the
content of the elements, carbon, hydrogen, oxygen nitrogen and sulphur. The5
analysis provides the major elemental composition of the fuel, usually reported on
a dry ash-free basis. Carbon and hydrogen are determined by burning the sample
in oxygen in a closed system and quantitatively analysing the combustion products.
The carbon includes organic carbon as well as carbon from the mineral carbonates.
The hydrogen includes the organic hydrogen as well as any hydrogen from the10
moisture of the dried sample and mineral hydrates. The extraneous carbon and
hydrogen are usually negligible. Nitrogen and sulphur are determined chemically.
Oxygen is usually determined as the difference between 100 and the sum of the
percentages C, H, N and S. Sometimes chlorine is included in the ultimate analysis.
3) The ash analysis is mainly used to get a better inside in ash related problems15
Analysis takes place after ashing of the fuel sample. Today a lower ashing
temperature is used when analysing biomass when compared to coal in an attempt
t o a v o i d u n w a n t e d
volat i l i sat ion of alkali
components and subsequently20
an underestimation of the
alkali amount in the fuel
(ASTM E1755-95). Elemental
analysis of the ash-forming
matter often is expressed as25
oxides (DIN 51719-A, ASTM
D 271-68, Borman and
Garland, 1998)).
30
Chapter 2: Literature review
2-5
Table 2.1: Average fuel composition of wood, husk/shells, grass/plant, coal and Finnish peat
(after Phyllis, 2000) (daf = dry, ash free)
Figure 2.3 and Table 2.1 clearly show differences between the composition of fuels. Figure
2.3 shows a typical volatile matter, fixed carbon, and heating value for wood, peat and coal
on a moisture and ash free basis. Table 2.1 shows the composition of different groups of
biomass fuels, peat and coal as taken from ECN’s database Phyllis (2000)
5
2.1.3 Chemical fractionation
The chemical fractionation is a method based on selective leaching by water, ammonium
acetate and hydrochloric acid. The method was originally developed by Benson et al. (1985)
for the characterisation of coal. Baxter (1994) modified the method for the characterisation
of biomass fuels. The method can be used to distinguish how ash-forming elements are10
bound in the fuel. A simplified scheme is given in Figure 2.4, which shows that the chemical
Chapter 2: Literature review
2-6
Figure 2.4: A schematic of the fractionation procedure as
developed by Benson et al., (1985) and Baxter (1994)
f r a c t i ona t i o n t e c h n i q ue
distinguishes different types of
ash-forming matter in the fuel
according to their solubility in
different solvents. Increasingly5
aggressive solvents, i.e. water
(H2O), 1M ammonium acetate
(NH4Ac) and 1M hydrochloric
acid (HCl) leach samples into a
series of four fractions (including10
the unleached rest) for analysis.
T y p i c a l a s h - f o r m i n g
components, which are leached out by water are alkali sulfates, carbonates and chlorides.
Elements leached out by NH4Ac are believed to be organically associated, such as Mg, Ca15
as well as K and Na. HCl leaches the carbonates and -sulfates of alkaline-earth and other
metals. Silicates and other minerals remain in the insoluble rest.
The amounts of leaching agents as suggested by Baxter (1994) and Benson et al. (1985)
cannot be used in case of leaching dry biomass fuels. It is difficult to achieve proper mixing20
for most fuels. Instead, an excess of water should be used in the first step, firstly to achieve
proper wetting of the samples, secondly to achieve proper leaching. These problems are not
encountered in the other leaching steps, since the solid samples here are already thoroughly
wet from previous steps and extensive washing (See I). Miles et al. (1995a through c)studied
the chemical fractionation technique to characterise seven fuels and their inorganic25
composition. These fuels were almond hulls, almond shells, olive pits, paper, rice straw,
switch grass and wheat straw (See also 3.1 and VII).
Chapter 2: Literature review
2-7
c)
g)
Figure 2.5: Fractionation results as retrieved by
Miles et al. (1995 a through c)
a) almo nd hulls white = water leached
b) almo nd shells striped = acetate leached
c) olive pits grey = hy droch loric
d) paper acid leached
e) rice straw black = residue
f) switch grass
g) wheat straw
e)
a) b)
f)
d)
Chapter 2: Literature review
2-8
Figure 2.6: Schematic of different stages of fluidisation
Figure 2.5 a through g show the distribution of ash-forming elements over the different
leaching agents. In all fuels the major part of the refractory materials, Si, Al, Fe, Ti was found
in the residual fraction only a minor part was leached by the other leaching agents. The
silica content of the grasses was high. It plays an important role in the sturdiness of the
plants. These elements are believed to play only a minor role in photosynthesis and are5
supposedly present as inorganic granules. According to Miles et al (1995a through c) the
alkali and alkaline earth materials (K, Na, Ca and Mg) occur in organic structures or very
mobile inorganic components. The major part of the alkali materials was found in the water
soluble fraction. Miles states calcium is present in the cell wall and will be organically
bonded as easily ion exchangeable material, thus found in the acetate fraction. Potassium10
and sodium were also found in the residual fraction as components of mineral soil
contamination of the fuels. Non-metallic materials (S, P, Cl) occur as plant nutrients. The
fractionation results showed that all chlorine could be present in an easily soluble form.
2.2 Fluidised bed reactors (FB)15
When a gas is passed upwards through a bed of particles, the degree of disturbance is
determined by the velocity of the gas. At low velocities there is only little particle movement.
As the velocity increases, individual particles begin to be forced upwards until they reach the
point at which they remain suspended in the gas stream (the minimum fluidisation velocity).20
Any further increase in gas velocity causes turbulence, with rapid mixing of the particles. A
particle bed in this state can be
described as ”fluidised”. (See
Figure 2.6)
25
Fluidised bed combustion or
gasification of coal or biomass
fuels uses a constant stream of
air in super- or sub-
stochiometric ratios, i.e.30
Chapter 2: Literature review
2-9
combustion or gasification conditions, which creates the necessary turbulence. The bed
particles are initially heated by a start-up fuel (mostly natural gas) after which the solid fuel
is fed to the bed continuously. The fuel ignites and releases heat allowing for the start-up fuel
to be shut down. The mixing of the particles encourages complete combustion or gasification
and allows a constant temperature to be maintained in the conversion zone. Part of the ash5
accumulates in the bed. In case of coal these ash particles together with sorbent material for
sulphur capture will form the bulk of the bed particles. In case of biomass fuels, which
contain much less ash the bed material consists mostly of sand. Ash from fuel conversion
and bed material are drawn from the bed at regular intervals and replaced when necessary
to maintain the bed at a correct level and to maintain bed properties. Advantages of fluidised10
bed conversion are:
• The bed can be operated at temperatures below 900°C. This temperature is low
enough to prevent defluidisation of the bed and unwanted shut-down of the furnace.
• The scrubbing action of moving particles on immersed water tubes increases the rate
of heat transfer.15
• The bed has a substantial thermal capacity. This allows a variety of fuels to be
burned, including those with a high moisture content.
• Adding crushed limestone ensures that more than 90% of any sulphur dioxide
released during combustion of sulphur bearing fuels, such as coal, peat and lignite,
can be retained in the bed.20
2.2.1 Combustion in atmospheric systems
From the two alternatives, FB combustion (FBC) and FB gasification (FBG), FBC is the most
developed and commercially available. Some hundred either bubbling (BFBC) or circulating25
FBC’s (CFBC) operate in the 50-300MWth size range worldwide.
Chapter 2: Literature review
2-10
Figure 2.7: BFBC process and residue sources (after Clarke,
1992)
Figure 2.8: CFBC process and residue sources (after Clarke,
1992)
The present status of BFBC in
the world has been reviewed by
Salmenoja (2000). The BFBC
(see Figure 2.7) is the oldest
application and is offered as a5
standard option by many
manufacturers for biomass. In
Finland, the first BFBC’s were
delivered to the Finnish pulp and
paper industry and were rather small in size, ranging from 15 to 50 MWth. Today the largest10
units are close to 300 MWth. The world’s largest BFBC for biofuels will have a capacity of
267 Mwth and will be built at Pietersaari, Finland. One important characteristic of the
bubbling bed is that it retains a more or less defined surface bed level. The velocity of the
primary air introduced at the bottom of the combustion chamber is limited to 1-2 m/s to
prevent excessive turbulence and loss of bed material due to bed entrainment. A typical bed15
temperature in BFBC boilers is in the range of 750-900°C. A major advantage of BFBC’s
is that a variety of fuels, many of them low-grade, can be used in a standard boiler.
However, the feed systems must be appropriate for the individual fuels. BFBC’s can be used
in both new and retrofit applications (Salmenoja, 2000).
20
The bed in a CFBC (see Figure 2.8) has no clearly definable bed surface. The fuel is injected
near the base of the combustion chamber. Together with the existing bed material, the fuel
is converted by the velocity of the
fluidising gas (typically around 8
m/s) into a turbulent cloud of25
solids which fills the whole
primary combustor. The most
dense particles remain near the
base of the furnace. The finer
particles occupy the upper part of30
Chapter 2: Literature review
2-11
Figure 2.9: PFBC process and residue sources (after Clarke,
1992)
the furnace and are entrained by the gas stream. Cyclones recover the major part of these
particles and return them to the bed. This ensures an effective high residence time of fuel
particles in the system, ensuring complete burn-out. The finest particles are not recirculated.
They escape through the cyclone and are extracted from the gas stream in a filter system
just before the gases are emitted into the atmosphere. CFBC’s for biomass such as peat,5
sludge, wood waste and coal have been built up to a range of 250 MWe. A basic difference
between BFB and CFB is the heat transfer from the bed material. In a bubbling bed most
of the heat released from the fuel to the bed material is kept in the lower part of the furnace,
while in CFB the heat is released from the circulating bed material to the furnace water
walls. This means that CFB’s can operate nearly isothermal, mostly at some 870°C10
(Salmenoja, 2000).
2.2.2 Combustion in pressurised systems
While pressurised FBC systems are already commercially applied for the conversion of coal15
and lignite, systems for biomass are still under development. Operating temperatures are
similar to those maintained in an atmospheric bubbling bed and the pressurised FBC
(PFBC) could be used for a wide range of fuels (See Figure 2.9).
Pressurised systems offer two distinct advantages compared with atmospheric systems:20
• The potential for
c o m b i n e d c y c l e
application
• A smaller unit size;
however, since the heat25
transfer tubes are
immersed within the bed,
the degree of size
reduc t i on w i l l be
determined by the total30
Chapter 2: Literature review
2-12
Figure 2.10: IGCC process showing residue sources (after
Clarke, 1992)
heat transfer requirements of the system.
PFBC is mainly demonstrated in coal fired power plants. Some coal-biomass mixtures have
been tested in a 1 MWth test facility in Sweden (Anderson et al., 1999).
5
2.2.3. Gasification
Nowadays, for gasification the
most important FBG concepts,
that are demonstrated in full scale10
are the airblown FBG’s, i.e. for
example an atmospheric pressure
FBG (Rensfelt, 1997), or an
atmospheric pressure circulating
FBG as demonstrated in Lahti,15
Finland. This is a gasifier (40
MWth) firing low calorific waste
derived and biofuels such as recycled fuel, peat, demolition wood waste and shredded tires
with moisture contents up to 60 wt%. The raw gas produced is directly fired in an existing
pulverised coal-fired boiler. With the gasification of biofuels up to 30% of the coal fed into20
the main boiler is replaced (Nieminen et al., 1999). A pressurised BFBG has been
demonstrated for biofuels (Salo et al., 1998). A pressurised CFBG was demonstrated in
Värnämo (Stahl et al., 1996). Although higher efficiencies and lower emissions have been
achieved than in conventional technologies, biomass integrated gasification combine cycle
(IGCC) concepts cannot compete at the present with natural gas fired combined cycles and25
low cost conventional CFBC. Furthermore a secured fuel supply for a large scale biomass
IGCC over its lifetime is questionable (Engström, 1999). Figure 2.10 shows a schematic of
an IGCC process. The gasifier product gases are burned in a gas turbine, producing
electricity. Excess heat is recovered in a steam cycle producing electricity with a classical
steam turbine.30
Chapter 2: Literature review
2-13
Figure 2.11: Schematic of ash-particle formation (partly after Flagan 1984)
2.3 Ash behaviour in fluidised beds
2.3.1 Conversion of fuel particles
When a fuel is burned in a fluidised bed, the ash-forming elements that are released from5
the fuel undergo different reaction paths. After entering the furnace fuel particles will heat
up rapidly and dry at first. After this the pyrolysis will start, i.e. organic volatile species will
be released from the fuel and the fuel particles will burn with a visible flame. During this
stage some reactive ash-forming elements will be released together with the gases. After the
volatile species left the particle char burning will start. A schematic of the fate of ash-forming10
matter in coal is shown in Figure 2.11. Most of the ash-forming elements will end up in the
residual ash in the case of burning coal by the coalescence of the fused included minerals
in coal during char burnout. The particle size of the residual ash depends on many factors.
A single fuel particle may fragment during combustion and each fragment may produce an
15
Chapter 2: Literature review
2-14
ash particle. The size of the ash particle will depend on the initial fuel diameter, its mineral
content, the uniformity of its distribution, and the number and size of fragments produced
during combustion. The fragmentation of a fuel particle is only partly understood. As
reaction occurs in depth within the porous char, a point is reached where the pores merge
and undermine segments of the partially burned char which are then released as fragments.5
(Flagan et al.1984) This description of the formation of ash is also valid for biomass fuels.
As described above, ash-forming matter in solid fuels can be divided in four fractions, three
that are inherent in the fuel (included minerals, organically associated, easily water soluble)
and one added to the fuel through geologic or processing steps (excluded minerals).10
According to Baxter (1993) a large fraction of the inherent ash-forming matter in lignites and
probably the dominant part in biomass fuels is associated with oxygen containing functional
groups. These functional groups provide sites for ash-forming matter to become
incorporated in the fuel matrix as, for example, chelates and cations. The release of this
atomically dispersed material from a fuel particle is influenced both by its volatility and the15
reactions of the organic portions of the fuel. Material that is volatile at combustion
temperatures includes derivates of the alkali and alkaline-earth metals, most probably Na
and K. Other non-volatile material can be released by convective transport during rapid
pyrolysis. The amount of fuel lost during the pyrolysis stage of combustion increases with
increasing hydrogen-to-carbon ratio, and to a lesser extend, with increasing oxygen-to-20
carbon ratio. Lignite, peat and biomass can lose more than 90% of their mass at this first
stage of combustion. Typically, the loss of volatiles during pyrolysis of biomass is some
75%. The large quantities of tars leaving the fuel can convectively carry ash-forming matter
out of the fuel, even if the inorganic material itself is non-volatile (Baxter, 1993).
25
The other class of ash-forming matter in solid fuels includes material that is added to the fuel
from extrageneous sources, the excluded minerals. In the case of coal, geological processes
and mining techniques contribute much of this material. In the case of biomass, fuel
processing in the field is likely to contribute the majority of it. This material is often
particulate by nature. Components of the minerals may be released from the fuel by either30
Chapter 2: Literature review
2-15
Figure 2.12: Fly ash particle mass size distribution downstream of
the process cyclone at flue gas temperatures 810-850/C (After
Valmari et al., 1999a)
thermal decomposition or vaporisation (Baxter, 1993).
A typical ash size distribution at the outlet of a fluidised bed boiler is bimodal. The vast
majority of the mineral inclusion, fuel contamination and entrained bed material are found
in the large particle mode of the size distribution. The smaller particle mode represents5
particles in sizes between 0.01 and 0.2 :m. These particles are smaller than can be
explained from the fragmentation of the fuel particles during conversion. The source of this
fume is volatilised ash-forming matter which nucleates homogeneously as the vapours
diffuse from the hot reducing atmosphere near the surface of burning char particles into the
cooler oxidising atmosphere (in case of combustion). The nuclei are initially very small but10
grow rapidly by condensation of additional ash-forming vapours and by coagulation (Flagan
et al., 1984).
Lind and Valmari studied the behaviour of the most important ash-forming elements firing
willow and forest residue in a CFBC (Lind 1999, Lind et al. 1999a, Valmari et al. 1998,15
1999a and b, Valmari 2000). During combustion of forest residue 30-40% of the ash was
retained in the bed, attached to quartz sand bed particles. Ca and P were believed to be
retained via particle collisions. K was believed to be retained due to a reaction between the
vapour phase potassium and the
sand bed particle. The fly ash20
size distributions consisted of two
distinct modes (see Figure 2.12)
The coarse mode contained
more than 90% of the mass in
both cases. Fine mode particles25
contributed 2% of the total fly
ash mass with forest residue and
8% with willow. When firing
forest residue potassium and
sodium were mainly present in a30
Chapter 2: Literature review
2-16
water insoluble form in de fly ash, indicating presence as silicates. Sulphur and chlorine were
volatilised. During combustion of a forest residue the sulphur had reacted to CaSO4. When
combusting willow 50% of the fuel sulphur remained as SO2. The particle sulphur was
found in the fine particle mode as alkali sulphates. These were not detected when firing a
forest residue. Half of the alkali chlorides was found to condense on coarser particles,5
whereas the other half condensed in the fine particle mode (See Figure 2.13).
Figure 2.13: Elemental mass size distributions downstream of the cyclone in CFBC (after Valm ari et al.,
1999a)
Chapter 2: Literature review
2-17
Figure 2.14: Typical slagging and fouling areas
as found in a boiler
2.3.2 Deposit formation
The mechanism
Deposit formation in a boiler can be divided
in slagging and fouling. Bryers (1996) defined5
slagging as deposition of fly ash on heat
transfer surface and refractory material in the
furnace volume primarily subjected to radiant
heat transfer. Fouling is defined as deposition
in the heat-recovery section of the boiler (See10
Figure 2.14).
Both the ash deposition rate and the
properties of the ash deposits are important
considerations in the operation in a15
combustor. The properties of ash deposits
most important to the successful operation of
a boiler include (see Baxter 1993):
1) The ease of removal from a combustor wall or heat exchanger surface
2) Viscosity20
3) Effective thermal conductivity
4) Effective emissivity
5) Deposit strength.
Additional deposit properties are:25
6) Elemental composition
7) Morphology
8) Porosity
9) Chemical species composition (Baxter 1993)
Chapter 2: Literature review
2-18
The formation of a hard deposit can be described by four most relevant steps:
1) Formation of an ash particle
2) Transport of the ash particle or ash-forming compound to a surface
3) Adhesion to the surface
4) Consolidation of the deposit5
After formation of an ash particle as described in 2.2.1 it is transported to a heat transfer or
boiler surface before deposition can take place. Typical transport processes are diffusion,
thermophoresis and inertial impaction. Diffusion and thermophoresis are processes of
particle transport in a gas due to local concentration and temperature gradients, respectively.10
In case of Fick diffusion molecules will move to a surface due to a concentration gradient
present. Brownian diffusion describes the random movement of small particles. Eddy
diffusion describes the diffusion in turbulent systems. In case of thermophoresis a particle
suspended in a fluid with a strong temperature gradient interacts with molecules that have
higher average kinetic energies on the side with the hot fluid than on the side with the cold15
fluid. The collisions of the high energy molecules on the hot side of the particle have more
impact than those on the colder side. This gives rise to a net force on the particle. In general,
these forces act in the direction opposite to that of the temperature gradient.
Inertial impaction is usually the process by which the bulk of the ash deposit is transported20
to a heat exchanger surface. The rate of inertial impaction depends on target geometry,
particle size distribution and density and gas flow properties. Inertial impaction is important
for large particles (10:m and larger).
After an ash particle hits the surface it may adhere to it. Adhesion can take place through25
glueing to the surface. This is possible when a partly molten phase is present that can act as
glue between a particle and a surface. When a gaseous ash-forming compound is formed,
this could diffuse to the surface and condensate directly.
30
Chapter 2: Literature review
2-19
Figure 2.15: A sche matic v iew of stic kyne ss criteria
The stickiness of ash particles is
strongly dependent on
temperature and physical state.
It has been shown that the
presence of a melt in an ash5
particle acts as a sticking agent
for the particle (Backman et al.,
1987). The physical state, i.e.
the share of melt vs. solid material in the particle is dependent on chemistry and elemental
composition of the particle and temperature. Inorganic compounds like those found in ash,10
do not melt at a certain temperature but have a temperature range, where both a melt and
a solid phase coexists. This temperature range between the first melting temperature (T0) and
the complete melting temperature (T100), also referred to as the liquidus temperature, may
be several 100 degrees of Celsius. From an ash stickiness point of view the temperature at
which enough melt is present is of major importance. Backman et al., (1987) defined the15
temperature at which 15% of the condensed phase, i.e. the sum of liquid and solid phases,
is molten as the critical stickiness temperature in recovery boiler deposits. This limit works
well for simple ionic salts, but for deposits containing silicon, leading to viscous melts,
another criterion is required. Here, the viscosity of a melt could be a criterion for stickyness
(See Figure 2.15). 20
When a deposit layer is formed, chemical sintering reactions may lead to consolidation of
the deposit. Condensation of ash compounds or ash-forming elements can influence the
efficiency of capture on a surface. The amount of condensate in a deposit depends strongly
on the mode of occurrence of the inorganic matter in the fuel. Low rank coals, lignites,25
biomass and similar fuels have the potential of producing large quantities of condensable
material. Furthermore the role of the condensate in determining deposit properties can be
substantially greater than the mass fraction of the condensate in the deposit might suggest.
Condensation increases the contacting area between an otherwise granular deposit
increasing the difficulty of removal by several orders of magnitude, influencing bulk strength,30
Chapter 2: Literature review
2-20
Figure 2.16: Mechanisms controlling the deposition and
maturation of ash deposits (after Laursen et al., 1998)
thermal conductivity mass
diffusivity etc. of the ash
deposit (Baxter, 1993).
L a u r s e n e t a l . ( 1 9 9 8)5
summarised the mechanisms
controlling deposition and
maturation of ash deposits as
summarised in Figure 2.16.
Apart f rom impact ion,10
thermophoresis, condensation
and chemical reaction, eddy
deposition is mentioned as
well. Small particles will be able
to follow small turbulent whirls,15
eddies, around the tube
surface. This might lead to
loose deposits on the lee side of
the heat exchanger tubes as well.
20
Figure 2.17 shows a schematic of the deposit present on a heat exchanger surface. Initially
condensation of volatile species on the heat exchanger surface will increase the inertial
impaction efficiency. Further nucleation of volatile ash components will increase the density
and strength of the deposit. The deposit thickness will be dependent on erosion and
shedding of the deposit.25
Chapter 2: Literature review
2-21
Figure 2.17: A schematic from deposit formation on a heat
exchanger pipe (after Frandsen 2000)
The role of compounds
formed during combustion
leading to deposit formation
Biomass contains potassium in
organic form, which will vaporise5
and decompose dur ing
combustion to form carbonates,
hydroxides, chlorides and
sulphates, depending on the local
composition and residence time10
of the products of combustion.
These compounds all exhibit very
low initial melting temperatures.
Their impact on fireside deposits
depends on their vapour pressure15
and whether they condense
homogeneously, on tube surfaces
or on other fly ash particles
making these sticky. When
potassium condenses on a fly ash particle, it forms a particle, which surface is enriched with20
potassium (Miles et al., 1996).
It was observed that most types of woods (containing high levels of calcium and low levels
of sulphur)hardly cause deposits when burned alone. When burned together with straw,
deposits become enriched with alkali sulphates and alkaline-earth sulphates. Calcium25
sulphate only partially prevented deposit build-up of potassium sulphates. Instead it was
found to act as a binder between particles on superheater tubes (Miles et al., 1996).
Stable chlorine compounds generated during combustion include alkali chlorides and
hydrogen chloride. Chlorine will increase the volatility of alkali compounds. The alkali30
Chapter 2: Literature review
2-22
chlorides formed tend to condense further downstream in the flue gas channel. The chloride
content is indicative for the volatility of the alkali compounds. Deposit formation tends to
increase with increasing degree of vaporisation of alkali compounds and thus with an
increase of the chlorine content in the fuel. Thus fuels high in alkali but low in chlorine show
less severe deposit formation than fuels with a higher amount of chlorine (Miles et al. 1996).5
As described above important compounds with respect to deposit formation often contain
potassium, chloride and sulphur. This might be the main reason why fundamental studies
have been carried out on the release of chlorine and potassium from the fuel. As stated
above already, during the pyrolysis stage potassium and chlorine containing compounds are10
released into the gas phase. Jensen et al. (2000) studied the release of potassium and
chlorine during pyrolysis of straw. They proposed a five-step release mechanism:
1) At a temperature of 200-400/C much of the organic matrix is destroyed, releasing
K and Cl to a liquid tar phase. Cl is further released as HCl or reacts with K.
Potassium could be present in de condensed phase as KCl or K2CO3. Some further15
release of HCl could take place caused to reaction of KCl with the char oxygen
containing functional groups, whereby K is bound to the char matrix.
2) At 400-700/C no significant amounts of K or Cl are released to the gas phase.
3) From 700-830/C all KCl evaporates. In this temperature range K also reacted with
silicon forming K-silicates.20
4) From 830-1000/C, K2CO3 decomposes and potassium is released as KOH or as free
K-atoms. Possibly potassium can also be released from the char matrix.
5) Above 1000/C potassium may be released to the gas phase from the char matrix and
from potassium silicates.
25
Yu et al. (2000) described the alkali release during pyrolysis of straw as taking place in two
steps, i.e. organic bonded release of potassium and inorganic potassium compound
vaporisation. These are in accordance with the extensive description of Jensen et al. (2000).
30
Chapter 2: Literature review
2-23
Corrosion
According to Salmenoja et al. (1996) chlorine induced corrosion is usually related to either
formation of gaseous hydrochloric acid or the deposition of alkali chloride on tube surfaces.
Corrosion from gaseous HCl is restricted mainly to high temperatures, high HCl
concentrations and reducing conditions. Alkali and metal chlorides reduce the melting5
temperature range and hence worsen corrosion. If the fuel contains also sulphur, and the
residence time is long enough and the temperature in the combustion zone is high enough,
alkali chlorides are converted to sulphates before being deposited on the tubes. Corrosion
is limited in this case because the formation of a melt is small at typical tube metal
temperatures due to the higher melting temperature of the alkali sulphates. The situation10
becomes more complicated when the alkali chlorides do not have enough time to become
sulphated completely before reaching the tubes. Sulphation of alkali chlorides deposited on
the tubes leads to the liberation of chlorine near the tube surface, thus making the chlorine
available for corrosion reactions.
15
Predicting ash deposition
There are many approaches to predicting ash deposition. One is the use of indices calculated
from the fuel composition. Indices are mainly based on the ash analysis presented as oxides
and were developed to predict deposit formation in coal fired boilers (Winegartner, 1974;
Skopurska et al., 1993; Reid 1984). An index based on alkaline and alkaline-earth metals20
was developed by Hupa et al. (1983) for boilers co-fired by bark, coal and oil. The portion
of sulphate-forming compounds (water soluble CaO+MgO+Na2O+K2O) is expressed as
a percentage of the total ash in the fuel fed to the boiler.
Another method is the determination of the fusibility of ash. With this technique a cone of25
(laboratory made) ash is heated and the temperature is recorded when the tip of the cone
is first deformed (IT); when the cone ash has molten into a spherical lump. With the height
equal to the width (ST); when the height is equal to half the diameter at the base (HT); and
when the cone is melted into a layer not more than 1.6 mm high (FT). The method is
completely empirical and interpretation of the values obtained is difficult (DIN 51730, 1978;30
Chapter 2: Literature review
2-24
ASTM D-1857-68, 1970). These predictors are the basis for prediction of deposit formation
and agglomeration in FBC today. It has, however, been reported to be poor indicators of
ash related operational problems with biomasses and energy crops (Juniper, 1995; Nordin
et al., 1995; Wall et al., 1995). Ash fusion temperatures as determined from the “whole” fuel
ash poorly predict ash deposit behaviour when assuming that reactive, mobile ash-forming5
elements form the initial deposits.
Another approach was introduced by Skrifvars et al., (1998). This approach was purely
based on chemistry and melting behaviour of ash-forming compounds. TPCE calculations
in combination with chemical fractionation were used to predict deposit formation. The10
same approach is used in this work. (See also 3.2)
2.3.3 Deposit formation: Field and laboratory experiences
Deposit formation in biomass fired FBC installations, i.e. in full-scale, pilot-scale and lab-15
scale equipment is studied all over the world. Only a few examples relevant for this work will
be reported here.
Wood derived fuels
According to Bryers (1996) pure wood does not contain a measurable amount of sulphur.20
Consequently pure wood is considered not to be a problematic fuel. However, sand
inclusions partitioned from the calcium-rich wood ash may react independently during
combustion with any potassium present as volatile species. The potassium absorbed on a
surface of the quartz surface produces low melting potassium silicates.
25
Miles et al. (1995a) found the same trend. When firing wood in an FBC with potentially
large amounts of contaminations (soil), the role of potassium may be reduced when
compared to agricultural wastes high in potassium. The same was the case when co-firing
wood-derived fuels with coal. (Skrifvars et al., 1997c). The role of alkaline-earth metals, i.e.
calcium was more pronounced. Miles believed initial deposition takes place by deposition30
Chapter 2: Literature review
2-25
of hydroxides, followed by sulphation of alkaline and alkaline-earth elements. If large
amounts of soil contamination or clay are present, the role of silicon may still be quite
pronounced in secondary deposit growth by particle impaction following the initial formation
of condensed layers on surfaces. Complex alkali-alkaline earth-alumino silicates form or are
incorporated into superheater deposits in this manner (Miles et al. 1995a).5
Skrifvars et al. (1997c) found that alkali sulphates dominate the deposit composition in the
hotter part of a CFBC firing forest residue. Alkali sulphates and chlorides were the two major
components in the deposits collected in the colder region in the flue gas channel. The same
trend was found in a wood fired CFBC. Experiments with a semi-full scale CFBC firing coal,10
peat or wood showed the same trends (Skrifvars et al., 1998a). Deposits from wood were
enriched in potassium chlorine and sulphur when compared to coal and peat. These
deposits were predicted to be sticky at temperatures around 730-750/C. Even if coal or peat
contained an order of magnitude higher amount of ash-forming elements in the fuel the
resulting compounds were well-behaving and fairly non-sticky.15
As mentioned by Baxter et al. (1998) deposits from agricultural residues showed differences
in composition when comparing the wind and lee side of a deposit probe. Alkali compounds
deposited through condensation or thermophoresis were found equally distributed around
the probes, whereas silicate, i.e. the larger particles were enriched at the wind side, indicating20
inertial impaction as deposit mechanism. At the same time Skrifvars et al. (1998) presented
the approach used in this work. A combination of chemical fractionation and
thermodynamic multi-phase multi-component equilibrium calculations was used to predict
deposit formation of coal, forest residue and wood chips. The possibility of formation of
silicates was omitted from the calculations. Skrifvars’ prediction suggests that a mix of wood25
chips fired together with construction residue could cause more problematic deposit
formation than when firing a forest residue.
Skrifvars et al. (1999b) summarised the measurements as carried out and described by
Skrifvars et al. (1997b, 1998a) and Hansen et al. (1997, 1998). After a comparison between30
Chapter 2: Literature review
2-26
lee and wind side deposits it is concluded that chlorine and sulphur play a dominant role in
deposit formation in CFBC firing biomass fuels, i.e. wood, bark, forest residue and wheat
straw with or without co-firing with coal. An indication of sulphation of the deposits
containing alkali chlorides was shown in deposits from a 18 MWth CFBC. Sulphation could
not inhibit chlorine to reach deposits. Only switching to pure coal or peat firing could5
achieve this.
Steenari et al. (1999) studied the composition of fly ash deposits from a CFB firing a mixture
of 70 wt% wood and 30 wt% coal or 30-40 wt% wood and 60-70 wt% peat. Compared to
fly ash from combustion of wood and bark, the co-combustion ashes studied had lower10
contents of calcium, potassium, manganese and chlorine. The admixture of coal or peat to
the wood fuels added aluminium, iron, magnesium and sulphur to the ash. The same was
shown by Dayton et al. (1999) studying the formation of chlorides and alkali metals in case
of co-firing wood (oak) and straw with coal. The amount of HCl detected during the
combustion of a coal/wheat straw sample was higher than expected based on the15
combustion results for the pure fuels. The amounts of KCl(g) and NaCl(g) detected during
the combustion were lower than expected. Chemical equilibrium analysis indicated that the
amount of condensed alkali was enhanced, due to the formation of potassium silicates.
Energy crops20
Skrifvars et al., (1997b) studied the behaviour of Salix (moisture content up to 50 wt%) and
a mixture of forest residue with Salix in a semi full-scale CFBC. Deposit samples were taken
at the cyclone inlet (850/C) and at two different locations in the convective part
(temperature 680 and 250/C, respectively). Sulphur and chlorine were found in all samples
locations and sampled deposits. Skrifvars states that even if no severe deposit build-up was25
noticed in the test run, a molten phase may be present in the fly ash, that eventually can be
responsible for the formation of a deposit. Calcium oxide in the fly ash is supposed to have
recarbonised at lower temperatures causing deposits in the sampling location downstream
of heat-exchangers in the convective part of the flue gas channel.
30
Chapter 2: Literature review
2-27
Agricultural waste
A wide range of agricultural wastes fired as such or together with wood in full-scale and
laboratory-scale equipment were studied by Miles et al. (1995a through c) and Baxter et al.
(1998). The results of the full-scale experiments showed the influences of fuel composition
on the deposits formed. A bubbling fluidised bed unit burning wood and almond shells5
developed superheater deposits enriched in potassium and sulphate, chlorine and
carbonates, indicating that mechanisms of condensation and chemical reaction were
significant in the deposit formation. The mechanisms of condensation and sulphation of the
deposit, depend on mass transfer rates. This means that at the front side of the tubes
sulphation and alkali enrichment was high. The concentrations of sulphates, chlorides and10
carbonates along the convective pass varied with the stability of the compounds. As the
temperature decreased less sulphates and carbonates were found. Many deposits in the
superheater region were enriched in calcium as well due to lime stone addition to the
fluidised bed. Superheater deposits from the bubbling fluidised bed had a higher
concentration of chlorides than those from the circulating fluidised beds, which may be15
related to differences in fuel composition but also indicative of the differences due to
recirculation. The composition of the fire wall deposits reflected the composition from
impacting particles as expected.
Laboratory experiments showed that wheat straw deposits were in many respects similar to20
rice straw deposits and illustrated many of the mechanisms of ash deposition. Probe deposits
from straw firing were enriched in potassium and chlorine directly at the probe surface,
developing outwards into a matrix of sintered silicate-rich flyash particles. Phosphorous was
enriched in the outer layers as well. Switch grass deposits also showed potassium enrichment
and a greater enrichment of sulphate when compared to straw. Initial deposits containing25
alkali chloride were found (Baxter et al., 1998).
The almond shells and hulls, high in potassium, formed fine-textured deposits, rich in
potassium, more than could be accounted for by sulphates and chlorides. Baxter suggests
this is due to the deposition of hydroxides or carbonates. The deposits from the almond30
Chapter 2: Literature review
2-28
shells-wood blend were potassium and sulphate enriched, as in the deposits from full-scale
facilities, but contained more silicon and less calcium. These differences probably accounted
for the addition of limestone in the full-scale unit (Baxter et al., 1998).
The fouling tendency of straw and bagasse was found to decrease when the fuel had been5
exposed to rain in the field. The high alkali content was thereby lowered reducing the
amount of deposit formed. Also rice straw is known for its high fouling tendency and might
be only acceptable for existing boilers, when rain-leached and spring-harvested, at a low
concentration with more conventional fuels (Miles et al., 1995a).
10
Jensen et al. (1997) measured deposit formation in a wheat and barley straw fired grate
boiler. As found by Miles et al. deposits were enriched in potassium. Jensen finds an
enrichment of chlorine as well. Submicron aerosol particles with a mean particle diameter
of approximately 0.3:m were generated. Vaporised potassium compounds are supposed
to condense on the boiler walls and superheater surfaces acting as glue for impacting silicon-15
, calcium-rich particles. The surfaces are probably dominated by elements from the aerosols
or from condensation of vapours, while larger particles are covered with a layer of KCl
(Jensen et al., 1997). An increase in the local gas temperature and straw potassium content
gives an increase in the amount of hard deposit formed which has a higher chlorine- to
potassium ratio than the loose deposits found.20
Hansen et al. (1998) described similar studies in co-combustion of coal and straw in full
scale power plants in Denmark. In an 80MWth CFBC quartz was used as bed material and
limestone added for sulphur capture. The boiler was designed for firing 50-50 wt% coal and
straw. Mature deposits from the superheater tubes situated in the convective pass contained25
mainly potassium, sulphur and some SiO2, Al2O3, and CaO. The chlorine content was very
low. It was suggested that KCl was deposited initially on the tubes after which sulphation
took place with subsequent release of gas phase chlorine. Corrosion was found to be 5-25
times higher than when firing coal alone, probably due to the role of chlorine released after
sulphation of the initial deposit.30
Chapter 2: Literature review
2-29
2.3.4 Bed agglomeration: the mechanism
In literature terms such as agglomeration, sintering and defluidisation are used freely in
different ways to describe the unwanted collapse of a fluidised bed. In this work
agglomeration is defined as the phenomenon where particles gather into clusters of larger5
size than the original particles. Sintering is defined as the process in which fine particles
become chemically bonded at a temperature that is sufficient for atomic diffusion. Since in
fluidised bed conversion particles can be held together by a molten phase both the terms
agglomeration and sintering can be used to describe the same phenomena. The initial
agglomeration temperature is defined as the temperature where the first molten phases10
appear that are able to “glue” bed particles together into agglomerates. Defluidisation is
defined as the total collapse of the fluidised bed resulting in a rapidly decreasing pressure
drop or erratic behaviour of the bed, which results in substantial temperature changes. The
presence of agglomerates does not, by definition cause total defluidisation of the bed.
Instead the defluidisation temperature will still be dependent on boiler-specific conditions as15
well.
Bed agglomeration is a quite complex phenomenon that can take place in fluidised bed
boilers under certain circumstances. Agglomeration cannot only be explained by looking at
physical phenomena, such as temperature, particle size distribution, mixing processes with20
resulting shear stresses between particles and particle attrition, abrasion, fragementation and
cleavage, but also by chemical phenomena. An example is the reaction between ash-forming
components and bed particles, leading to coating build-up and possible agglomeration when
molten phases are formed. These molten phases could glue particles together. In practice
both physical and chemical phenomena will play together leading to bed agglomeration or25
not.
Bed agglomeration is tightly connected to the release of ash-forming matter. It is possible
that easily released ash-forming elements would rather condense on bed particle surfaces
than form a submicron fume and hence be transported to the flue gas channel. This kind of30
Chapter 2: Literature review
2-30
Figure 2.18:Schematic of bed agglomeration as described by
Öhman (2000)
coating formation has been
detected when firing biomass in
a quartz bed. It was found that
the coating usually does not
exceed a thickness of some 10-5
50 :m for a mean bed particle
diameter of 350-500 :m
(Skrifvars et al., 1997d).
Ö h m a n d e f i n e d t h e10
agglomeration sub-processes as
follows (See Figure 2.18):
1) Ash deposition on the bed material is probably dominated by i) an attachment of
small particles to the bed particle surface ii) condensation of gaseous alkali species
on bed particles and iii) chemical reaction of the gaseous alkali on the particle15
surfaces.
2) As the continuous deposition on the bed particles proceeds, the inner layer of the
coating is homogenised and strengthened via sintering.
3) The melting behaviour of the homogeneous silicate layer controls the adhesive
forces, which are responsible for the final temperature-controlled agglomeration20
process (Öhman 1999, Öhman et al. 2000).
The formation of a coating may initiate:
1) The formation of agglomerates when sticky, i.e. partially molten. In this way two
different particles can become glued together. The molten phase could be either a25
viscous glassy silicate melt or a non-viscous melt. The former has been found to be
the common reason for bed agglomeration in FBC’s using quartz as bed material
(Skrifvars et al., 1997a, Nordin 1993). A non viscous melt has been identified
occasionally when high sulphur lignites were combusted (Manzoori, 1990).
Chapter 2: Literature review
2-31
2) Chemical reaction sintering the bed particles together forming an agglomerate. This
mechanism has been identified in fluidised bed boilers using limestone as bed
material. The agglomeration had mainly occurred as a pluggage of the cyclone return
leg in an CBC (Skrifvars, 1997a).
5
2.3.5 Bed agglomeration: Field and laboratory experiences
Fluidised bed combustion power plants often employ limestone injection for control of
sulphur emissions, with additional benefits of reduced bed agglomeration. Other additives
such as, kaolin, dolomite or magnesium oxide, have been used to reduce bed agglomeration10
as well (Miles et al. 1995a). Miles et al. (1996) stated that bed agglomeration in fluidised bed
combustors at temperatures above some 760/C is dominated by silica. Examples of
experimental studies as shown below show the same trend. Either silica-rich bed material
or silica-rich fuels are involved in bed agglomeration processes due to formation of low
melting potassium silicates (See also paper IV and V).15
Agglomeration under reducing conditions
Maniatis et al. (1993) studied the FBG of wood and bark. They showed that an operation
temperature above 900/C resulted in sand agglomeration due to formation of low-melting
alkali silicates. 20
Ergudenler et al. (1993) found that a fluidised bed agglomerated under reducing conditions
when firing wheat straw. The sand bed agglomerated at some 800/C in the presence of
straw ash, resulting in channelling and defluidisation. Changes in air velocity and/or fuel
feeding rate did not improve agglomeration characteristics. Potassium was believed to be25
the major contributor to the agglomeration process. Ergudenler speculates that potassium-
oxide forms, melts and penetrates through the voids of the silica sand and then forms low
melting potassium silicates. However, in practice it is believed that volatile reactive
compounds such as alkali chlorides, sulphates and carbonates will react with the bed
material forming low-melting alkali silicates (See also 3.3)30
Chapter 2: Literature review
2-32
Four biomasses gasified in a pilot scale FBG with a dolomite bed, being pine sawdust, bark,
straw and pine bark were studied by Moilanen et al. (1995). It was shown that in the
gasification of pine sawdust, very high carbon conversions could be achieved already at
relatively low temperatures, whilst with bark and straw gasification high conversion
efficiencies could be achieved at above 850/C, but sintering of the straw ash caused severe5
operational problems. This could be accounted for by the potassium and silicon present in
the fuel, forming a silica-containing glassy melt, that caused bed agglomeration at these
temperatures (See also IV).
Liliendahl et al. (1999) studied defluidisation through multivariate methods. The fuels were10
Miscanthus, Reed Canary Grass, Salix, and olive waste in a lab-scale (P)FBG with sand as
bed material (See also V). Evaluation indicated that the temperature and the potassium and
sodium contents, and also to some degree the pressure seems to enhance defluidisation,
whereas the presence of calcium seems to have the opposite effect.
15
Bed agglomeration under oxidising conditions
Öhman et al. (1998) studied the behaviour of olive flesh, Lucerne, wheat, wood, Reed
Canary Grass, wood residue, bark, RDF and cane trash in a laboratory scale FBC using
quartz as bed material. With this FBC onset of defluidisation is measured with an accuracy
of ±5/C. (See also VI).20
Lin et al. (1997) found that during particle sampling agglomerates in a straw fired FBC had
been formed before the pressure drop could start to decline. This means that the time at
which the pressure drop begins to fall is not the time corresponding to the initial
agglomeration temperature (as described by Öhman). 25
Lin stated that small agglomerates will segregate to the lower part of an FBC. When the
number of agglomerates exceeds a critical value, a layer of defluidised agglomerates will be
formed. The height of this defluidised layer increases with an increase in number and size
of the agglomerates. This mechanism could explain the defluidisation at rather low30
Chapter 2: Literature review
2-33
Figure 2.18: Defluid isation te mpe rature s of som e biom ass fue ls
as determined by Öhman et al.(1998) and Skrifvars et al.(1999a)
temperatures, i.e. some 800/C.
At h igher tempera tu res
defluidisation is believed to be
caused by molten phases glueing
particles together. 5
A qualitative comparison
between the defluidisation
temperatures of different biomass
fuels is possible with Öhmans10
standardised method (see Figure
2.18). Skrifvars et al. (1999)
described the same experiments and compared the defluidisation temperatures as
determined by Öhman with the ASTM ash fusion initial deformation temperature (Tint) and
sintering temperature (Tsint) as determined from compression strength experiments. It was15
shown that in all cases the fusion temperature fails to predict bed agglomeration. The
sintering test predicted problematic behaviour for Lucerne, wheat straw and Reed Canary
Grass. Moderate behaviour is expected for olive flesh, cane trash and RDF. A bark and forest
residue were considered to be non-problematic fuels.
20
The compression strength tests seemed to be able to predict defluidisation fairly well when
compared to the lab-scale FBC. However, no data for comparison with full-scale equipment
were available.
Turn et al. (1998) carried out similar tests under gasification conditions and using alumina25
silicate as bed material firing bagasse and bananagrass as fuels. The experiments showed
as before that the silica in the bed captures potassium released from the fuel. Chlorine was
not detected in the bed material samples. Ca, K and P together with Si were the main
elements found in the coating on bed particles. Turn set up an element balance and found
that most of the potassium and roughly half of the calcium was retained in the bed.30
Chapter 2: Literature review
2-34
Werther et al. (2000) mentioned in an overview on combustion of agricultural residues that
an FBC firing various coals, sewage sludge and wood chips could be operated without
agglomeration problems. Firing coffee husk, however, led to agglomeration after six hours
when fed above the bed, when feeding within the bed agglomeration occurred after four
hours. Similar observations were reported by others who fired sunflower husk, cotton husk,5
cotton stalk, coffee husk, palm fibre, soy husk, pepper waste, groundnut shells and coconut
shell (Babat et al., 1997).
The role of additives for prevention of bed agglomeration
Moilanen et al. (1996) studied the agglomeration in FBC of among others rape and wheat10
straw, Reed Canary Grass, pine saw dust, pine bark, willow, and Miscanthus. As bed
material Al2O3 was used. Also, laboratory ash was made at different temperatures. The ashes
of wheat straw, willow and Miscanthus prepared at 500/C showed much higher sintering
than when heat treated at 850/C. In the fluidised bed tests the bed material was not totally
agglomerated in any of the cases in spite of the clear sintering observed in the bed samples.15
Öhman et al. (2000a) showed that addition of kaolin can shift agglomeration of bed material
to higher temperatures when firing biomass in a lab-scale FBC. The results showed that
kaolin was transformed to meta-kaolinite particles, which adsorbed potassium species. The
increased agglomeration temperature was explained by the decreased fraction of melt in the20
bed particle coatings.
Steenari et al. (1998) studied the role of additives such as kaolin and dolomite in sintering
of straw ash. As Öhman she found that potassium was captured by kaolin leading to a
higher first melting point. Dolomite added to wheat and barley ash reacted with silica to form25
calcium and magnesium silicates. No reactions between potassium and dolomite could be
detected.
Chapter 3: Results
3-1
Figure 3.1:The a sh com position of the stu died fu els
3 RESULTS
3.1 Biomass fuel characterisation
3.1.1 Traditional fuel analysis5
The three main ash-forming elements of the 30 biomass fuels as described in this thesis are
K, Ca and Si. The different biomasses could be presented in the triangular composition
diagram as shown above in Figure 3.1. In this diagram the main ash-forming components
are normalised to 100% as if they were the only ones present in the fuel. Fuel #1 through10
#23 represent the fuels that were fractionated. Fuel #1 through #16 are discussed in papers
I through III and VI; Fuel #17 through #23 represent the fuels as discussed in report VII and
by Miles et al.(1995a through c). Fuel #24 through #30 represent the fuels for which only
Chapter 3: Results
3-2
TPCE calculations were carried out (see chapter 3.2.2)[IV-V]. The Figure shows the wide
range of fuels studied. The amount of Si , presented as SiO2, in the fuels varied from 1 to 90
%wt, Ca, presented as CaO, varied between 5 and 77 %wt and K, presented as K2O, varied
between 2 and 72 %wt. Fuels low in Si, i.e. containing less than 10 %wt SiO2 are Wood
chips, Eucalyptus, Forest residue, Salix (#1), Lucerne, Almond hulls and Almond shells.5
Fuels high in Si are Reed Canary Grass, Rice straw and Peat (#3). Fuels low in Ca, but high
in K are Almond hulls and Almond shells. Eucalytus and Wood chips are the fuels highest
in Ca.
3.1.2 Chemical fractionation10
Ash-forming elements can be present in a reactive form or a less-reactive form. Reactive ash-
forming elements can be present as easily soluble salts or organically associated compounds.
These can volatilise, leading to deposit formation in the flue gas channel after condensation,
or interact with bed material in a FB causing agglomeration. Included minerals could be15
released from the fuel during the char burning stage and be entrained from the bed. Less-
reactive ash-forming elements can be present either as external minerals in the fuel. Thus,
knowledge of the structure of the fuel, i.e. the way ash-forming elements are bound is
important for understanding how fuels behave in boilers and furnaces. For this purpose the
chemical fractionation technique or SEM/EDX analysis can be used. Papers I, II, III, VI and20
VII show results for the fractionation of a bituminous coal, two types of peat, Salix (willow),
several types of Scandinavian forest residue, wood and wood chips, two types of pine bark,
construction residue wood, two wheat straws, almond hulls, almond shells, olive pits, paper,
rice straw and switch grass, respectively.
25
In all fuels the major part of Si, Al, and Fe was found in the residual fraction; only a minor
part is leached by the three leaching agents used. The alkali and alkaline-earth materials (K,
Na, Ca and Mg) occur in organic structures or reactive mobile inorganic components. The
fractionation results seem to be consistent with this biological function. The major part of the
alkali materials was found in the water-soluble fraction. Hence, they are believed to be easily30
Chapter 3: Results
3-3
Figure 3.2: Fractionation result of Bark #6 Figure 3.3: Fractionation result of Wood #12
volatilised, making them easily available for chemical reaction with other components
leading to possible deposition, corrosion and agglomeration. This means that fuels with
lower alkali content should be less problematic when fired in a boiler. Potassium and sodium
were also found in the residual fraction as components of mineral soil contamination of the
fuels. Ca was supposed to be present organically bonded in the cell wall as easily ion-5
exchangeable material, thus found in the acetate fraction. Alkaline-earth materials are less
volatile. Non-metallic materials (S, P, Cl) occur as plant nutrients. Chlorine plays an
important part in the transformation of inorganic alkali compounds during combustion and
gasification. The fractionation results suggest that all chlorine is present in an easy
vaporisable form.10
A large portion of the ash-forming elements in the studied coal and peat were found in the
acidic and rest fraction, indicating that most of the ash forming elements were associated
with either the earth alkali carbonates and -sulphates or silicates and clay minerals.
15
The results for the biomass fuels showed a different trend. In biomass fuels a major part of
the inorganic matter was associated with either water-soluble compounds or with the organic
matter. The relatively large amount of inorganic compounds leached out by the acid or
remaining in the rest fraction was probably originating from mineral inclusions or from
impurities included as a result of fuel sampling by foresting and handling the biomass fuels.20
In case of Bark #6 [I] the high amount of Ca leached out by the HCl could be accounted
for by calcium containing internal minerals, most probably calcite or calcium oxalate.
Chapter 3: Results
3-4
Figure 3.4a: Amount of element leached after
leaching Bark #6 with water
Figure 3.4c: Amount of element leached after
leachin g Bark #6 w ith HC l.
Figure 3.4b: Amount of element leached after
leaching Bark #6 with NH4AC
The sum of all fractions, i.e. H2O, NH4Ac, HCl and rest fraction was not always the same as
the amount as determined in the untreated solid. Especially for Cl a large difference was
noticed.
Figures 3-2 through 3-4c show the fractionation results for Bark #6 [I, III] and Wood #125
[I]. The results for the ash analysis of the untreated fuels were obtained from triple
measurements. In the figures the 95% confidentiality limits are also shown (Davies et al.,
1988). The largest interval was found for the elements which can be associated with soil
contamination of the fuel, such as Si, Al and Fe. This could be related to fuel sampling
before analysis10
The fractionation of Bark #6 �I, III� has been carried out three times. The bark was analysed
Chapter 3: Results
3-5
Figure 3.5a: Wood #12 (I)”forest residue” Figure 3.5b: Forest residue #14 (VII) “brun
grot”
Figure 3.5c: Fores t residu e #1 5(I) “grö n grot” Figure 3.5d: Forest resid ue #7 (II,III)
at three different laboratories, i.e. in triple (obtained from KCL), in quadruple (obtained from
Fortum Power and Heat oy) and two analysed once, (obtained from the University of Oulu)
respectively. The Figures 3.4a through 3.4c show the amount of leached elements as
obtained from the analysis of the liquid samples. Where possible the 95% confidentiality
limits are shown as well. The two first laboratories obtain results which are comparable. The5
last mentioned laboratory used partially non-standardised techniques, which led to
uncomparable results, such as in the case of K and Ca analyses.
Differences in fractionation results in one fuel group
Apart from analysis problems, the fractionation results for one fuel group such as “Forest10
residue” can be very different. Soil contamination, presence or absence of roots, leaves and
branches and seasonal effects greatly influence the outcome of the traditional fuel analysis
and the chemical fractionation. This indicates that for every fuel to be fired a representative
sample should be taken and analysed which makes the chemical fractionation as used today,
very time-consuming and expensive. However, an extensive database of fractionation results15
Chapter 3: Results
3-6
for different classes of fuels could give some insight of what to expect when planning to fire
a certain type of fuel.
Wrongly categorising a fuel can lead to misunderstanding of the fractionation results. Figure
3-5 gives an example of different fractionation results of samples of “forest residue”. Wood5
#12 is a “forest residue” low in ash, i.e. some 0.5 %wt and should be called “wood”. The
other three forest residues, #7,14 and 15 are better comparable.
3.1.3 SEM/EDX analysis biomass fuels
10
SEM/EDX analyses of biofuels before and after fractionation and after gentle ashing made
understanding and better interpretation of the fractionation results and fuel behaviour under
combustion conditions possible. Here only the results for Bark #6 will be presented as an
example. In paper I also SEM/EDX analysis of Wood #12 is presented.
15
Solid samples were taken from Bark #6 after each fractionation step. In addition 0.5 g of
Bark I, and Wood I, was ashed in a laboratory furnace during 15 minutes at a temperature
of 450, 650 or 850/C. All these samples were subjected to SEM/EDX analysis in an attempt
to verify the fractionation results and to distinguish between internal and external minerals
in the fuel. Hereto they were immobilized on a metal plate and coated with carbon, after20
which direct analysis of the samples took place. The samples were analysed for Si, Al, Fe,
Ti, Mn, Mg, Ca, Na, K, S, P, and Cl by point analysis and by use of X-ray maps.
The structure of Bark #6 after fractionation
The SEM/EDX images from samples of Bark #6 retrieved before and after each leaching25
step are shown in Figures 3.6a through 3.6d. The Figures a through c show the embedded
minerals which were rich in Ca (white). These do not leach out in the water or the acetate
step. Only after the total break up from the organic structure, which happens during leaching
with HCl the calcium containing minerals dissolve.
30
Chapter 3: Results
3-7
Figure 3.6c:The structure of Bark leached
with Ac etate
Figure 3.6d: The structure of Bark leached
with HCl
Figure 3.6a:The structure of untreated Bark Figure 3.6b: The structure of Bark leached
with water
The structure of Bark after laboratory ashing
Figures 3.7a and b show the structure of Bark #6 after laboratory ashing. Figure 3.7a shows
that at low temperatures the structure of the fuel remains intact. The white arrows point at
the calcium containing minerals still embedded in the fuel.
Chapter 3: Results
3-8
Figure 3.7a: The structure of Bark after
ashing at 450/C during 15 minutes; the bar
indicates 200:m; The arrows point at calcium
contain ing m inerals
Figure 3.7b: The structure of Bark after
ashing at 850/C during 15 minutes; The bar
indicates 200:m; The arrows point at calcium
containing minerals released from the fuel
At 650/C the fuels start to disintegrate and part of the calcium-containing minerals is
released from the fuel and could be pointed out by SEM/EDX. At even higher temperatures
the fuel structure disintegrates into small particles and the calcium-containing minerals are
completely released from the organic matter. They can be seen as small white “needles” on
the SEM/EDX picture as shown in Figure 3.7b. Some are pointed out by the white arrows.5
3.2. Ash behaviour in (P)FBG and FBC
3.2.1 Prediction of deposit formation
The fuel-specific characterisation method presented here combines advanced fuel10
characterisation tests with thermodynamic multi-component, multi-phase equilibrium
calculations (TPCE), which describe the chemical conversions of the ash components in an
FBC.
A principle sketch of the method is presented in Figure 3.8. The method is divided into three15
separate steps being:
Chapter 3: Results
3-9
Figure 3.8: Simplified schedule of the deposit predictor
1) chemical fractionation,
2) TPCE calculations, and
3) melting range estimations.
It was assumed that the ash-forming elements leached by water and ammonium acetate5
represent the more reactive species, many of which are volatile could form fine reactive
components. Therefore, the results from the analysis of the water and acetate fractions were
combined. This combined fraction was assumed to form one fraction of reactive
components. These could form deposits in the superheater and economiser surfaces of an
FBC/FBG (Skrifvars et al., 1998) or Stoker fired combustor or they could interact with less10
reactive components in the fuel or react with the bed particles, forming a coating. This last
event could lead to bed agglomeration under certain process conditions (see 3.2.2).
Ash-forming elements leached by hydrochloric acid and those present in the residual fraction
were assumed to represent the less reactive ash-forming elements that could remain in the
bed and form the more coarse ash particles. Part of the less-reactive components could be15
entrained from the bed together with bed material.
Figure 3.9 shows how the combined fractions, i.e. the reactive fraction and the less-reactive
fractions were distributed in the fuels. The vast majority of the ash-forming elements in coal
Chapter 3: Results
3-10
Figure 3.9: The distribution of leached elements over the reactive and less-reactive fractions
and peat are found in the fraction assumed to form the bottom ash, while the share of the
ash-forming matter forming the fine ash fraction is much higher in the biomasses. Grasses
such as wheat straw, switch grass and rice straw have a skeleton rich in silicon which will be
found in the less-reactive fraction. Scandinavian bark contains calcium-rich minerals which
were also found in the less-reactive fraction (See 3.1.3).5
The analysis results from the reactive fractions of the fuels were used as input in the
thermodynamic calculations. With this method, the chemical interaction of the two ash
fractions with the combustion or gasification gases were modelled. The interaction between
the reactive and less-reactive fraction has not been considered. In the calculations the fuel10
composition was taken from the ultimate fuel analysis to make the formation of a realistic
gas phase possible. An air factor of 1.2 �II, III �for the combustion cases and 0.3 �VII� for the
gasification cases and atmospheric pressure were assumed. The calculations were carried
out for a temperature range 500-1200/C. The presence of aluminium in the calculations
could lower the first melting point of silicates to below 500/C due to uncertainties in the15
Chapter 3: Results
3-11
calculations, introduced by extrapolation of the thermodynamic data. In the calculations the
presence of aluminum was therefore omitted to obtain a realistic first melting temperature.
When interpreting the results of the analysis, its limitations should be recognised. The
equilibrium approach implies that no reaction kinetics, transport limitations (diffusion etc.)5
or fluid dynamic effects are taken into account. Also many deposit formation mechanisms
are highly species-specific, resulting in deposit compositions that are not easily related to the
fuel ash composition. Nevertheless, a careful interpretation of the results gave useful
information about the systems studied. The assumption that all reactive ash forming
elements leave the bed could lead to an overestimation of the amount of deposit formation10
due to neglected interaction of reactive species with bed material. However, a qualitative
estimation of how a deposit is expected to behave can be made.
For ash-related problems in an FBC it was found that the amount of melt present in the
condensed phases was of major importance. In order to deposit on a surface or agglomerate15
ash particles should contain a certain amount of liquid melt. Based on experience with black
liquor recovery boilers it was assumed that an amount of 15% wt of the condensed phases
molten at a certain temperature (T15) enables deposit formation in the flue gas channel
(Backman et al., 1987)(see Figure 2.15). This limit was used in this investigation as well.
Whenever the amount of melt, from the reactive fraction, exceeded 15%, fly ash deposition20
in the flue gas channel was predicted at a certain temperature. Thus from the equilibrium
analysis, the amount of melt (as %wt) was calculated as a function of temperature.
Table 3.1 summarises the sticky temperatures, T15, as predicted by the above-mentioned
method for the fractionated fuels for combustion. In this same table the percentage of25
“reactives” compared to the total ash, the highest amount of a liquid phase predicted, and
the range at which the melt is predicted to appear are summarised. With the percentage of
“reactives” compared to the total ash the ability of the less-reactive coarse ash to function
as a cleaning agent in the flue-gas channel is characterised. Certain coals, for example, are
known not only to keep heat exchanger surfaces clean but even to cause erosion of tubes.30
Chapter 3: Results
3-12
Coal and peat as well as additives are also known to decrease fouling tendencies in co-
combustion with problematic fuels (Skrifvars et al.,1999; Hupa et al., 1983).
In Table 3.3 the fuels are ranked according to selected data in Table 3.1. A qualitative “star
ranking” is used where one star means a bad ash behaviour and three stars a good ash5
behaviour. In Table 3.2 the ranges for the rankings are identified. Table 3.3 shows that the
coal could be the less problematic fuel to be combusted. Peats, barks, and woods are also
fuels in the higher “star ranking”. Forest residues can be found both in a higher and lower-
ranking range indicating there are forest residues that cause little problems and forest
residues that form deposits when combusted, depending on fuel composition and10
distribution of ash-forming elements in the fuel. Salix, straws, grasses, almond shells and
hulls are in the lower range and should be expected to be rather troublesome to fire causing
deposit formation.
When looking at measured deposit formation (See also 2.3)as taken from some different15
sources, the same trends can be seen. Coal (Lind 1999) and peat (Skrifvars et al., 1998) are
found to be less problematic when combusted, whereas Salix (Lind et al., 1999a)and forest
residues (Skrifvars et al., 1997, Valmari et al., 1999) can form deposits (II).
In paper III the amount of collected deposits on hotter short-term probes at a flue gas20
temperature of 820-880/C is presented. The measurements confirm the ranking as presented
above as well. Adding a forest residue to a wood type fuel decreased the amount of deposit
on the probes (Peltola et al., 1999). Adding coal or forest residue to a wood chips fired
boiler decreased the deposit formation (Skrifvars et al., 1999) and firing straw or adding
straw to a coal fired boiler increased the deposit formation on the probe dramatically25
(Hansen, 1997).
Chapter 3: Results
3-13
Table 3.1: Summarised results from the TPCE for the deposit prediction (8=1.2)
Fuel* corrected values
Reactive
amount
(g/kg)
Reactive
% of total
ash
Total
fractions
(g/kg)
T15 (/C) Melt
range
(/C)
Max m elt
(%wt)
#4: Coal* 0,9 2,9 30,0 >1200 >1200 -
#3: Peat*5 3,4 13,6 25,0 >1200 550-750 5
#10 : Peat 6,9 11,0 63,2 >1200 550-600 ,04
#8: Wood 2,2 29,6 7,3 620 575-750 27
#9,#12: Wood 1,9 79,6 2,4 >1200 775-850 3
#13 : Wood 2,5 54,8 4,6 >1200 750- 22
#6: Bark10 7,3 55,9 13,1 >1200 >1200 -
#11 : Bark 6,6 27,1 33,7 >1200 600-650
850-900
3
#7: For.res.* 8,3 67,2 12,3 >1200 650-700 10
#14 : For.res 7,8 71,5 10,9 780 600-650
800-950
19
#15 : For.res 6,6 59,0 11,2 >1200 >1200 -
#16: Cons.res.15 6,5 16,7 39,0 675 650-725 15
#1: Salix* 5,8 62,2 9,3 860 825-1000 18
#2: Salix* 9,0 57,4 15,7 880 825-1000 24
#5: Wheat Straw 13,0 66,4 19,7 880 875-1080 48
#23: Wheat Straw 6,2 34,0 18,1 1000 980-1180 58
#22: Switch grass 20 3,6 22,3 15,9 1060 1050-1190 31
#17: Almo nd hulls 28,4 81,3 34,9 870 750-1200 63
#18: Almo nd shells 40,7 7,5 91,0 950 680-1160 27
#21: Rice straw 23,4 24,9 93,9 600 >600 92
Chapter 3: Results
3-14
Table 3.2: The used “star ranking” [III]
Fuel Reactive
amount
(g/kg)
Reactive
% of total
ash
Total ash
(g/kg db)
T15
(/C)
Melt range
(/C)
Max m elt
(%wt)
q >9 >60 >90 <700 >150 >15
qq5 2-9 30-60 20-90 700-900 50-150 5-15
qqq <2 <30 <90 >900 <50 <15
q=bad, qqq=good
The fuels as described in Paper VII were also fired in full-scale equipment and a laboratory
scale multi-fuel combustor (Miles et al., 1995a through c). It was found that silicon-rich10
deposits were formed firing rice straw and wheat straw and switch grass. Deposit
characteristics for almond shells and hulls were similar. Potassium compounds from almond
hulls were supposed to be typical bonding agents between silica or media particles in
superheater deposits. In some cases a glass was formed.
15
The composition of deposits found in full-scale or laboratory scale boilers will be different
from the composition as calculated with the TPCE calculations. The thermodynamics
assumes equilibrium, which will not be reached in practise. This means that in deposits often
different layers can be detected, each with its own composition and its own melting
behaviour. In the calculations it is assumed that these layers have interacted and reacted20
with each other, leading to an overall mean composition and subsequently melting
behaviour. However, the calculations as carried out in this work have shown to give useful
results in predicting deposit formation independent from boiler geometry
25
Chapter 3: Results
3-15
Table 3.3: Summarised results from the TPCE for the deposit prediction (8=1.2)
Fuel Reactive
amount
(g/kg)
Reactive
% of total
ash
Total
fractions
(g/kg)
T15 (/C) Melt
range
(/C)
Max
melt
(%wt)
Total
q
#4: Coal qqq qqq qq qqq qqq qqq 17
#3: Peat qq qqq qq qqq q qq 13
#10: Peat5 qq qqq qq qqq qqq qqq 16
#8: Wood qq qqq qqq q qq q 12
#9,1 2: Wood qqq q qqq qqq qq qqq 15
#13: Wood qq qq qqq qqq q q 12
#6: Bark qq qq qqq qqq qqq qqq 16
#11: Bark10 qq qqq qq qqq qqq qqq 16
#7: For.res. qq q qqq qqq qqq qq 14
#14: For.res qq q qqq qq qqq q 12
#15: For.res qq qq qqq qqq qqq qqq 16
#16 : Cons.res. qq qqq qq q qq qq 12
#1: Salix15 qq q qqq qq q q 10
#2: Salix qq qq qqq qq q q 11
#5: Wheat Straw q q qqq qq q q 9
#23: Wheat straw qq qq qqq qqq q q 12
#22: Switch grass qq q qqq qqq qq q 12
#17: Almo nd hulls20 q q qq qq q q 8
#18: Almo nd shells q q q qqq q q 8
#21: Rice straw q q q q q q 6
Chapter 3: Results
3-16
Figure 3.10. SEM/EDX image from sand bed material taken from
a PFBG firing Lucerne at 15 bar and 800/C
Figure 3.11. SEM/EDX image from sand bed material taken
from a PFBG firing Miscanthus at 5 bar and 900/C
3.2.2 Prediction of bed agglomeration
Bed agglomeration is a quite complex phenomenon that can take place in a fluidised bed
boiler under certain circumstances. It cannot be stressed enough that agglomeration cannot
be explained by looking only at physical phenomena, such as temperature, particle size5
distribution, mixing processes, shear stresses between particles and attrition of particles. Also,
chemical phenomena should
be taken into account.
In case of predicting bed
agglomeration behaviour of the10
sole fuels and the interaction of
the sole fuels with bed material
are of major importance. When
considering bed agglomeration
all ash-forming elements15
present in the fuel should be
taken into account. The
reactive elements could play a
role in the formation of a
coating on the bed material20
(VI), whereas less-reactive
material could become trapped
into a sticky coating. Thus,
w h e n m o d e l l i n g b ed
agglomeration the entire fuel25
should be taken into account
and not only a reactive or less-
reactive fraction as used for
predicting deposit formation. In
this way the thermodynamical30
calculations could simulate the
Chapter 3: Results
3-17
interaction of all ash-forming elements with the bed material.
The composition of “bridges” and “coatings” of the agglomerates as found with SEM/EDX
analysis showed that the main elements involved were the elements Si, K and Ca when a
silica bed was considered. Figures 3.10 and 3.11 show some examples of SEM images of5
agglomerates found. This is in agreement with other studies (see 2.2.4)
Papers IV, V and report VII describe the modelling of the agglomeration tendency of biomass
fuels under reducing conditions (IV, V, VII) and oxidising conditions(VII).
Large differences between combustion and gasification cases could not be noted. In case of10
gasification reduced species are formed which could have a higher volatility and lower first
melting point when compared to the combustion cases (VII). However the trends as
described here count for both combustion and gasification conditions.
The role of reactive ash forming elements in bed agglomeration(VI)15
Neither the chemical interaction between bed material and reactive fine particles nor the less-
reactive coarse fraction alone can explain the formation of agglomerates. Physical
phenomena will take their role as well. However, the formation of a coating on bed material
is a prerequisite for agglomeration and defluidisation. The SEM/EDX analyses such as shown
above can show which elements are involved, but not where they come from.20
In case of a sand bed, silicon found in the coating on the bed particles could originate both
from the fuel and the bed particles. Calcium and potassium could originate from the fuels
reactive or less-reactive particle fractions. The thickness of the coating formed is a function
of chemistry and erosion of the coating due to physical processes. There are two possible25
ways to obtain a typical 10:m thick coating:
1) The coating grows outwards onto the particle, assuming the bed particle act as an
inert carrier for the coating material. In this case all elements in the coating destine
from the fuel and coating formation should occur independent of bed material
2) The coating grows inwards into the particle. In this case reactive elements could react30
with the bed particle.
Chapter 3: Results
3-18
In both cases growth is limited due to either erosion of the coating, diffusion limitation or
both.
By comparing the amount of bed material after controlled combustion experiments as carried
out at ETC, (i.e. with the amount of bed material before the experiment started and the5
amount of fuel fed to the combustor) the total weight of coating present can be calculated
and distributed over the three main coating-forming elements. After this, the distribution of
these elements over the reactive and less-reactive fraction of the ash forming elements could
be combined with the quantity of the elements needed to build up the coating.
10
Up to 60 %wt of the potassium fed to the bed was found in the coatings. This could be
introduced by the potassium present in the reactive fraction alone. The rest of the potassium
could escape the bed as gaseous KCl.
Some 8-30 %wt of the calcium present in the fuel ended in the coatings. This means that up15
to 92 % of the calcium might react to form other components such as CaSO4. Calcium is
divided evenly over the reactive and less-reactive fractions in most biomass fuels. This makes
it difficult to determine which fraction is responsible for the calcium found in the coatings.
As described in section 3.1 calcium is possibly present as included calcite or as oxalate
minerals which are only leached by HCl. Even these calcium minerals could be considered20
reactive when released from the fuel. The calcium leached by water and acetate is supposed
to form submicron particles of for example CaSO4 or Ca3(PO4)2. Such components were
identified in coatings by SEM/EDX analysis. Even the included “reactive” minerals could be
involved in coating formation.
25
The amount of reactive silicon entering the boiler with the fuel is in most cases insufficient
to form a coating. The less-reactive particle fraction represents in most cases a contamination
of sand particles that enter the combustor together with the biomass fuel (or are present in
the skeleton of the fuel) The fuel contaminations could act as alternative bed material.
30
Chapter 3: Results
3-19
Coating formation could take place by reaction between Si, Ca and K. In practice
components containing these three elements will not coincide. There are two possibilities:
1) Silicon reacts with gaseous potassium forming potassium silicates with a first melting
point as low as 750/C. This could be the first sticky layer formed on bed particles,
which catches other small particles released from the fuel such as solid calcium5
components. After this first capture all components present in the coating could
interact forming a sticky coating raising the first melting point to some 800/C.
2) Silicon reacts with calcium first forming calcium silicates with a first melting point of
above 1500/C. This second route is considered unlikely due to the first melting point,
making capture of potassium components by glueing impossible at the low10
temperatures found in FBC. It is found that the first agglomerates occur at some
800/C in all cases studied (VI).
The modelling of bed agglomeration in (P)FBG (IV,V)
Agglomeration tendencies for four fuels, i.e. Salix #24, Miscanthus #26, Reed Canary Grass15
#27 and Lucerne #29 in (P)FBG could be compared to the calculations. Thirteen bed
samples from these tests were embedded in epoxy, cross-sectioned and polished for
SEM/EDX analysis. Experiences from the experiments were compared with the results
obtained from the SEM/EDX analysis and with the TPCE calculations.
20
Table 3.4 gives a summary of the results obtained. In this table the experiments are sorted
fuel wise. The column “presence of molten phases” summarises the results as obtained from
the TPCE calculations. Here the calculated amount of molten phases as %wt of total ash is
presented. As can be seen the amount of molten phases could exceed 100% as a result of
interaction of the fuel with bed material and subsequent melting of bed material. In the25
column “process experiences” experiences from the experiments are divided into three
categories, i.e.:
I) No signs of defluidisation or agglomeration of the bed;
II) Bed disturbances or presence of agglomerates after visual inspection of the bed;
III) Clear defluidisation of the bed. 30
The last column shows whether agglomerates were found with SEM/EDX analysis or not.
Chapter 3: Results
3-20
In six out of 13 cases formation of agglomerates was predicted and found with SEM/ED
analysis. In two cases no or little agglomeration was predicted, nor were agglomerates found
with SEM/EDX analysis. In two cases out of 13 the modelling predicted some degree of
agglomeration while no agglomerates could be detected with SEM/EDX analysis. However,
in these cases agglomerates were detected after visual inspection of the bed.5
Table 3.4: Summarised results from the TPCE calculations, experiments and SEM/EDX
analysis (IV,V)
Fuel P
(bar)
T (/C) bed
material
Presence of
molten
phases
(%wt of ash)
Process
experience
Agglomerates
found w ith
SEM
Salix #2410 5 892 sand 64 II Yes
R.C.G . #27 15 830 sand 11 III Yes
R.C.G . #27 1 900 dolom ite 0,3 I No
Miscanthus #26 15 807 sand 26 III No
Miscanthus #26 5 877 sand 26 III Yes
Miscanthus #2615 1 900 dolom ite - II No
Miscanthus #26 1 850 dolom ite - II Yes
Miscanthus #26 1 900 olivine - II Yes
Lucerne #29 5 875 sand 75 III Yes
Lucerne #29 15 797 sand 105 III Yes
Lucerne #2920 1 850 dolom ite 39 II No
Lucerne #29 1 900 dolom ite 36 II Yes
Lucerne #29 1 900 olivine - II Yes
R.C.G.= Reed Canary Grass
Chapter 3: Results
3-21
Figure 3.12: The formation of alkali components dependent on
the presence of Chlorine, Pottasium ans silicon in the gasifier
The first melting point of a fuel
interacting with bed material
and the amount of melt formed
will together determine whether
bed agglomeration will take5
place or not. TPCE calculations
carried out for the same process
conditions as the experiments
predict a substantial amount of
melt present in almost all cases.10
Exceptions were atmospheric
gasification of Miscanthus in a dolomite or olivine bed at around 900/C, atmospheric
gasification of Reed Canary Grass in a dolomite bed, and gasification of Lucerne in an
olivine bed. In case of gasification of the four fuels in a sand bed a molten silicon-rich melt
will determine the occurrence of bed agglomeration. The sand bed present may interact with15
the potassium present in the fuels, thus forming mainly potassium silicates melting below
800/C. In case of the atmospheric gasification of Miscanthus and Reed Canary Grass in a
dolomite bed the prediction indicates the occurrence of a melt free gap in a temperature
range between the molten salt phase, which occurs due to the presence of dolomite, and the
silicate molten phase which occurs due to the presence of the silica in the fuels. In case of the20
atmospheric gasification of Lucerne and Miscanthus in a magnesium olivine bed in the
prediction indicated the same, i.e. the presence of a melt free gap between the presence of
a molten salt phase due to the presence of an excess of magnesium, and a molten silicate
phase.
25
The modelling results showed that the amounts and types of alkali components formed are
dependent on the presence of components such as silica, calcium and chloride, as
summarized in Figure 3.12. The diagram represents the components, which could be
formed from different fuel types as a function of the chloride/potassium and potassium,
calcium/silica ratios. In cases of low silica but high potassium K2CO3 formed. If chloride was30
abundant as well, KCl formed. High silica contents in the fuel yielded potassium silicates,
Chapter 3: Results
3-22
Figure 3.14: The phases containing potassium under gasification
in presence of an excess of dolomite at 10 bar
Figure 3.13 The phases containing potassium under gasification
cond itions at 1 0 bar (n o bed mate rial pre sent)
whereas high silica and high chloride gave potassium silicate formation combined with the
release of HCl.
Figure 3.13 shows the phases
containing potassium for these
four fuels at 10 bar 700/C and5
900/C, respectively. It should be
noted that the fuels were very
different. Salix and Miscanthus
contained 1-2 %wt ash, whereas
the other fuels contain up to10
8.6%wt ash. At 700/C the major
part of potassium was present as
solid potassium salts or silicates.
When considering Salix or
Lucerne approximately 15% of15
the potassium was present as a salt melt. At 900/C all ash was molten. For the other fuels a
potassium silicate melt was found. Figure 3.13 shows that an increase in temperature when
firing Salix caused the alkali melt that was present at 700/C to volatilise. The rest of the alkali
was present in a molten silicate phase at 900/C. In case of Lucerne, which was low in silicon,
the solid alkali salt phase that was present at 700/C was molten at 900/C.20
Figure 3.14 shows the results for
these fuels when an excess of
dolomite was present as bed
material. The presence of25
dolomite decreased the relative
amount of silicate present
shifting even fuels with a high
silicon content, such as Reed
canary Grass, to the right side of30
Chapter 3: Results
3-23
Figure 3.16: The phases containing potassium under gasification
conditions in presence of an excess of sand at 10 bar
the diagram (Figure 3.12).
Instead of a potassium silicate
melt the major contribution to
the molten phases was now
accounted for by a molten5
potassium salt phase.
Figure 3.15 shows the results
for the fuels when an excess of
sand was present as bed10
material. In this case the
amount of silica available for
reaction with potassium is increased drastically, thereby shifting the fuels to the left side of
the diagram in Figure 3.12. Thus, potassium silicates were preferably formed with a first
melting point below 800/C.15
20
Chapter 4: Conclusions
4-1
4 CONCLUSIONS
4.1 Fuel characterisation
The extended fuel characterisation as used in this work provided useful information on the5
distribution of ash-forming elements in different fuels.
In this study clear differences have been shown in the distribution of the ash-forming
elements in the different fuels. In the geologically older fuels more ash-forming elements
were present as excluded and/or included minerals. In relatively young fuels up to half of the10
amount of ash-forming elements was present in the soluble fraction after leaching with water
and ammonium acetate.
At this stage, although informative, the chemical fractionation method in combination with
SEM/EDX analysis, is very time consuming and expensive. This means that it is an excellent15
research tool, but not available yet as a standard analysis. The method should be simplified
and standardised for biomass fuels.
4.2 Deposit formation
20
The combination of fractionation with TPCE calculation has shown to be more useful than
other traditional methods based on fuel ash analysis in the laboratory for predicting ash
behaviour in FBC.
The definition of reactive components as the sum of elements leached with water and25
ammonium acetate should be used with care when used as a basis for ash deposit
prediction. It was shown in this work that SEM/EDX could give valuable additional
information about the way ash-forming elements are present in the fuel. Hence, a smart use
of the combination of traditional fuel analysis, chemical fractionation and SEM/EDX analysis
could give a solid base for prediction of deposit formation with, for example, TPCE30
calculations.
Chapter 4: Conclusions
4-2
This work shows the easily leached elements form the main constituents in the fine fly ash,
and are consequently a reasonable approximation of the fly ash compounds.
Prediction of the presence of a molten phase in the fly ash, together with the reactive
amount, total ash composition, melt range, T15, and maximum amount of melt, gives the5
possibility to rank fuels in order of deposit formation tendencies.
The ranking of the fuels as studied in this work was presented as less-problematic <
problematic was as follows: coal< peat < wood derived fuels < annual crops < agricultural
waste, which very well corresponds to the general practical experiences with these fuels.10
In the nearby future weighing factors should be used to express the relative importance of
the above-mentioned parameters on deposit formation.
A direct comparison between prediction models based on physical phenomena and the15
method as described in this work is impossible.
It can be assumed that thermodynamic equilibrium calculations can be used for reactive ash-
forming compounds such as easily soluble alkali salts. When composing the reactive fraction
containing calcium only from the water and acetate fraction, the amount of reactive calcium20
could be underestimated. Included calcium containing minerals should be included as well
in the reactive fraction and, thus, in the prediction for deposit formation. The same accounts
for silicon. In case silicon is present as soil contamination it should be omitted from the
reactive fraction. However, when present as included minerals, part of the silicon as leached
in the HCl fraction could be accounted for as reactive.25
Chapter 4: Conclusions
4-3
4.3 Agglomeration
It is shown in this work that TPCE calculations increase our understanding of alkali
behaviour in (P)FBG/C. Interaction of bed material with alkali components released by the
fuels determines wether a fuel volatilises and interacts with the bed material.5
The calculations as presented in this work can be used as guidelines for predicting bed
agglomeration. A comparison between the TPCE calculations, SEM/EDX analysis of bed
material, lab- and bench-scale experiments showed good agreement.
10
Calculations showed that the presence of an excess of dolomite/calcite decreases the amount
of alkali components in the bed due to an increase in the amount volatilises. An excess of
amount of silicates increases the amount of alkali retained in the bed. This leads to formation
of low melting alkali silicates and subsequent bed agglomeration. At atmospheric pressure
the amount of melt formed could be smaller, when compared to high pressures, thereby15
decreasing the risk for bed agglomeration.
The mechanism leading to bed agglomeration as studied in this work is in agreement with
Öhman (1999) and Öhman et al. (2000). Chemical fractionation results revealed that when
firing woody biomass fuels potassium and calcium present in a bed coating are originating20
from the reactive fraction in the fuel, i.e. leachable with water or ammonium acetate and/
or present as included small minerals as pointed out by SEM/EDX analysis.
25
Chapter 5 References
5-1
5 REFERENCES
1. Anderson J., Anderson L., Power Gen. Europe, Frankfurt, Germany,http://www10.abb.se/carbon/pfbc7.html (1999).
2. ASTM D-1857-68: The fusibility of coal and coke ash, Annual book of ASTMstandards, part 19 (1970)
3. ASTM Standards D3172-89, Standard practice for proximate analysis of coal andcoke, Annual book of ASTM standards (1984).
4. ASTM standards D 3176-89e1 Standard practise for ultimate analysis of coal andcoke ash, Annual book of ASTM standards (1984).
5. ASTM D 271-68 : Laboratory sampling and analysis of coke and coal, Annualbook of ASTM standards (1984)
6. ASTM standard E1755-95 Standard test method for ash in biomass, Annual bookof ASTM standards (1984).
7. Babat D.W., Kulkunari S.V., Bhandarkar V.P., in proceedings of the 14th
International Conference on Fluidised bed Combustion, Preto A. eds.,Vancouver, ASME, New York, (1997), pp 165-174
8. Backman R., Hupa M., Uppstu E., Tappi J., (1987), 70(6), pp 123-
9. Backman R., Sodium and sulphur chemistry in combustion gases, Academicdissertation, Åbo Akademi university, Turku, Finland (1989)
10. Baxter L.L., Biomass and Bioenergy, (1993), 4(2), pp 85-102
11. Baxter L.L., A Task 2 . Pollutant emission and deposit formation duringcombustion of biomass fuels, Livermore, (CA), (1994)
12. Baxter L., Miles T., Miles jr. T., Jenkins B., Milne T. Dayton D., bryers R., OdenL., Fuel Processing and Technology (1998), 54, pp 47-48
13. Benson S.A., Holm P.L., Ind. Chem. Eng.Prod. Res.Dev., 24, (1985), pp 145-149.
14. Borman G.L., Lagland K.W., “Combustion Engineering”, WCB McGrawhill,(1998), §2.3.
Chapter 5 References
5-2
15. Bryers R., Prog. Energy Combust. Sci, (1996), 22, pp 29-120
16. Clarke L.B., “Application for coal-use residues”, IEA Coal Research, UnitedKingdom, (1992), Chapter 2
17. Clean Coal Technology, “Options for the future”, OECD/IEA (1993).
18. Communication from the Commision, “ Energy for the future: renewable energy
sources of energy”, COM(97)599, (1997).
19. Davies O.L., Goldsmith P.L, Statistical methods in research and production” , 4th
edition, Longman Scientific and Science, Essex, England, (1988), Chapter 4
20. Dayton D., Belle-Oudry D., Energy and Fuels, (1999), 13, pp 1203-1211
21. DIN 51718 Bestimmung des wassergehaltes, Deutsche Normen, (1978).
22. DIN 51719 Bestimmung des Aschegehaltes, Deutsche Normen, (1978).
23. DIN 51730: Testing of solid fuels, determination of ash melting, Deutsche
Normen (1984)
24. Engström F., Gasification Technology Conference, San Fransisco, CA, (1999).
25. Ergudenler A., Ghaly A., Biomass and Bioenergy, (1993), 4(2), pp 135-147
26. Faaij A., Energy from biomass and waste, Academic dissertation, University of
Utrecht, Utrecht, The Netherlands, (1997).
27. Flagan R.C., Sarofim A.F., Prog. Energy Combust. Sci., (1984), 10, pp 171-175.
28. Frandsen F., in Chemistry in combustion processes, Part IIb, Turku, Finland
Chapter 5 References
5-3
(2000)
29. Hansen P., Andersen K., Wieck-Hansen K., Overgaard P., Rasmussen I.,
Frandsen F., Hansen L., Dam-Johansen K., Fuel Processing Technology, (1998),
54, pp 207-225
30. Hansen P., “Deposit formation in coal/biomass fired CFB under load variations”,
ELSAM R&D project no 348, (1997)
31. Hannes J., “Mathematical modelling of circulating fluidised beds”, Academic
dissertation Delft University of Technology, Delft,(1996) Introduction Ch.5.
32. Hupa, M., Backman, R: Slagging and fouling during combined burning of bark
with oil, coal, gas, or peat, in Fouling of heat exchanger surfaces (Ed: R Bryers),
United Engineering Trustees, New York, NY, (1983), pp 419-432.
33. Jensen P.A., Stenholm M., Hald P., Energy and Fuels, (1997), 11, pp 1048-1055
34. Jensen P., Frandsen F., Dam-Johansen K., Sander B., Energy and Fuels, (2000),
14, pp 1280-1285
35. Juniper, L. Combustion News, Australian Combustion Technology Centre, (1995)
pp 1-4.
36. Kendall A., McDonald A, Williams A., “The power of biomass”,
http://ci.mond.org/9709/970913.html, (1997).
37. Laursen K., Frandsen F., Larsen O., Energy and Fuels, (1998), 12, pp 429-442
38. Liliedahl T., Kusar H., Rosén E., Sjöström K., in the proceedings of the 2nd Ole
Chapter 5 References
5-4
Lindström Symposium, Royal Institute of Technology, Sweden (1999), pp 88-91
39. Lin W., Gitte K., kim D.J., Esther M., Bank L., in the proceedings of the 14th
International conference on Fluidised bed Combustion, Preto A. eds, Vancouver,
ASME, New York, (1997), pp 831-837
40. Lind T., “ Ash formation in circulating fluidised bed combustion of coal and solid
biomass”, Espoo, Finland, VTT Chemical technology, 1999, Academic
dissertation
41. Lind T., Kauppinen E., Sfiris g., Nilsson K., Maenhaut W., Energy &Fuels,
(1999a), 13(2), pp 379-89
42. Maniatis k., Bridgewater A., Buekens A., in the proceedings of an international
conference on pyrolysis and gasification, Ferrero G., Maniatis K., Bueskes A.,
Bridgewater A., (1993), pp 274-281
43. Manzoori A.R., “Role of inorganic matter in agglomeration and defluidisation
during the circulating fluidised bed combustion”, PhD thesis, University of
Adelaide, Australia, (1990)
44. Miles T.R., Miles T.R., Baxter L.L., Bryers R.W., Jenkins B.M., Oden L.L.,“Alkali
deposits, Found in biomass power plants”, summary report, NREL, (1995a).
45. Miles T.R., Miles T.R., Baxter L.L., Bryers R.W., Jenkins B.M., Oden L.L.,“Alkali
deposits, Found in biomass power plants”,a preliminary investigation of their
extend and nature, NREL, (1995b).
46. Miles T.R., Miles T.R., Baxter L.L., Bryers R.W., Jenkins B.M., Oden L.L.,“Alkali
deposits, Found in biomass power plants”, the behaviour of inorganic material in
Chapter 5 References
5-5
biomass- fired power boilers- fiels and laboratory experiences, NREL, (1995c).
47. Miles T, Miles T., Baxter L., Bryers R., Jenkins B., Oden L., Biomass and
Bioenergy (1996), 10(2-3), pp 125-138
48. Moilanen A., Nieminen M., Sipilä K., Kurkela E., in the proceedings of the 9th
Europan Bioenergy Conference & 1st European Energy from Biomass technology
Exhibition, Denmark (1996).
49. Moilanen A., Kurkela E., Am. Chem. Soc. Div. Of Fuel Chem., (1995), 40(3), pp
668-693
50. Nieminen J., Palonen J., Kivelä M., VGB Towertech., (1999), 10, 69-74.
51. Nordin, A., Öhman, M., Skrifvars, B. J., and Hupa, M. Proc. of the Eng. Found.
Ash Conf., Waterville Valley, NH, (16-22, July 1995).
52. Phyllis, Netherlands Energy Research Foundation, ECN, “Database for biomass
and waste”, http://www.ecn.nl/phyllis/, (Dec 2000)
53. Reid W.T., Prog. Energy Comb. Sci., (1984), 10, pp 159-175
54. Rensfelt E.K.W., Int. Conf. on Gasification and Pyrolysis of Biomass, Stuttgart,
Germany, (1997).
55. Salmenoja K., Mäkelä K., Hupa M., Backman R., Journal of the Institute of
energy, (1996), 69, pp 155-162
56. Salmenoja K., Field and Laboratory studies on Chlorine induced superheater
corrosion in boilers fired with biofuels, Academic dissertation, Åbo Akademi
Chapter 5 References
5-6
University, Turku Finland, (2000).
57. Salo K., Horvath A., Patel J., Pressurised Gasification of Biomass, Int. Gas
Turbine and Aeroengine Congress and Exhibition, Stockholm, Sweden, June
(1998).
58. Skrifvars B-J., Sintering tendency of different fuel ashes in combustion and
gasification conditions, Academic dissertation, Åbo Akdemi University, Turku
Finland (1994).
59. Skrifvars B-J., “Ash Chemistry and sintering-verification of the mechanisms, in
LIEKKI 2 annual Book, eds. Hupa M, Mattinlinna J., (1997a), Åbo Akademi
University, Turku, Finland,
60. Skrifvars B-J., Sfiris G., Backman R., Widegren K., Hupa M., Energy and Fuels
(1997b), 11, pp 843-848
61. Skrifvars B-J., Lauren T., Backman R., Hupa M., Binderup-Hansen P., in the
proceedings of The Engineering foundation Conference on the impact of mineral
impurities in solid fuel combustion, November HI, (1997c)
62. Skrifvars B-J., Hupa M., Backman R., in the proceedings of the 1st south east
European symposium on Fluidised beds in energy production, chemical and
process engineering and ecology, Macedonia, (1997d), pp 65-84
63. Skrifvars B-J., Blomquist J-P., Hupa M., Backman R., in the proceedings of the
15th Annual International Pittsburgh Coal Conference,, Pittsburgh, (PA), USA,
(1998)
64. Skrifvars B-J., Backman R., Hupa M., Sfiris G., Albyhammer T., Lyngfelt A., Fuel
Chapter 5 References
5-7
(1998a),77(1/2),pp 65-70
65. Skrifvars, B-J., Backman, R., Hupa, M: Ash chemistry and behaviour in advanced
co-combustion, in Final report of the EU/JOULE 3 project Operational problems,
trace element emissions and by-product management for industrial biomass co-
combustion (JOF3-CT95-0010), (Ed:s H. Spliethoff, K. R G Hein). Brussels
(1999).
66. Skrifvars B-J., Öhman M., Nordin A., Hupa M., Energy and Fuels, (1999a),
13(2), pp 359-363
67. Skrifvars B-J., Laurén T., Backman R., Hupa M., in Impacts of mineral impurities
in solid fuel combustion, Gupta R., eds., Kluwer, New York, (1999), pp 525-539
68. Skopurska N., Couch N.,Coal characterization for predicting ash deposition; an
international perspective, presented at the Engineering Foundation Conference on
the impact of ash deposition in coal fired plants, Birmingham, UK, (1993)
69. Ståhl K., Neergaard M., VGB Kraftwerk, (1996), 4, pp 327-330.
70. Steenari B., Lindqvist O., Biomass and Bioenergy (1998), 14(1), pp 67-76
71. Steenari B., Lindqvist O., Fuel, (1999), 78, pp 479-488
72. Turn S., Kinoshita C., Ishimura D., Zhou J., Fuel (1998), 77(3), pp 135-146
73. Valmari T., Kauppinen E., Kurkela J., Jokiniemi J., Sfiris G., Revitzer H., J.
Aerosol. Sci., (1998), 9(4), pp 445-459
74. Valmari T, Lind T., Kauppinen E., Energy Fuels, (1999a), 13(2),pp 379-389
Chapter 5 References
5-8
75. Valmari T, Lind T., Kauppinen E., Energy Fuels, (1999b), 13(2),pp 390-395
76. Valmari T., “Potassium behaviour during combustion of wood in circulating
fluidised bed power plants”, Academic dissertation, VTT Publications, Finland
(2000)
77. Wall, T. F., Creelman, R. A., Gupta, S., Coin, C., and Lowe, Proc. of the Eng.
Found. Ash Conf., Waterville Valley, NH, (July 1995).
78. Werther J., Saenger M., Hartge E-U., Ogada T., Siagi Z., Progress in Energy and
Combustion Science, (2000), 26, pp 1-27
79. Winegartner E.C., Coal fouling and slagging parameters, ASME Special
Publications, (1974).
80. Yu C., Zhang W., in the Proceedings of the International Conference of Progress
in Thermochemical Biomass Conversion, (2000), Austria
81. Öhman M., Nordin A., Energy and Fuels, (1998), 12, pp 90-94
82. Öhman M., “Experimental studies on bed agglomeration during fluidised bed
combustion of biomass fuels”, Academic Dissertation, Umeå, Sweden (1999)
83. Öhman M., Nordin A., Skrifvars B-J., Backman R., Hupa M., Energy and Fuels,
(2000), 14, pp 169-178
84. Öhman M., Nordin A.,, Energy and Fuels, (2000a), 14, pp 618-624
RECENT REPORTS FROM THE ÅBO AKADEMI PROCESS CHEMISTRY GROUP, COMBUSTION AND MATERIALS CHEMISTRY:
00-1 K. Salmenoja Field and Laboratory Studies on Chlorine-induced Superheater Corrosion in Boilers Fired with Biofuels
00-2 T. Norström Approaches for Prediction of NOx Emissions Using Computational Fluid Dynamics
00-3 L. Fröberg (Ed.) Proceedings of the 40th IEA FBC Meeting
00-4 T. Bergenwall, J. Konttinen, S. Kallio, P. Kilpinen
Oxidation of a Single Char Particle – Development and Testing of a Robust Numerical Solution Procedure
00-5 H. Ylänen Bone Ingrowth into Porous Bodies Made by Sintering Bioactive Glass Microspheres
00-6 B. O. Skrifvars Chemical Equilibrium Analysis in the Study of Corrosion
00-7 A. Brink, P. Kilpinen Modeling gas phase nitrogen chemistry at fuel rich conditions — An extension to the BKH-model for the range 0.1<λ<o.5 and 1200 K<T<1350 K
00-8 K. Sandelin, R. Backman Equilibrium Distribution of Arsenic, Chromium, and Copper when Burning Impregnated Wood
00-9 A. Brink, J. Keihäs, M. Hupa Specifying boundary conditons for CFD modeling of a slab reheating furnace
00-10 A. Brink, P. Kilpinen A simplified kinetic rate expression for describing the oxidation of fuel-N in biomass combustion
00-11 E. Nordström. H. Ylänen, M. Hupa, A. Itälä, H. Aro
Porous, Surface Pre-Treated Bioactive Glass for Initial Fixation of Hip Joint Implant, Final Report of the Tekes Project 40296/99
00-12 N. DeMartini Ammonia Formation Behavior in Green Liquor at 90°C
00-13 N. DeMartini The Effect of Oxidation on Black Liquor Nitrogen
RECENT REPORTS FROM THE ÅBO AKADEMI PROCESS CHEMISTRY GROUP,
COMBUSTION AND MATERIALS CHEMISTRY:
01-01 M. Zevenhoven, T. Laurén, B-J. Skrifvars, R. Backman
The Chemistry and Melting Behavior of Fly Ash Deposits in Co-combustion of Bark, Peat and Forest Residue
01-02 M. Zevenhoven The Prediction of Deposit Formation in Combustion and Gasification of Biomass Fuels Fractionation and Thermodynamic Multi-phase Multi-component Equilibrium (TCPE) Calculations
01-03 Maria F.J. Zevenhoven-Onderwater
Ash Forming Matter in Biomass Fuels
ISSN 1457-7895 ISBN 952-12-0813-9
Åbo Akademis tryckeri Åbo, Finland, 2001