Proceedings Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

San Servolo, Venice, Italy; 17 - 20 November 2014

2014 by CISA Publisher, Italy

SUPERHEATER IN THE FURNACE FOR

INCREASING THE EFFICIENCY

T. NORMAN, L. MIKKELSEN AND O. HEDEGAARD MADSEN

Babcock & Wilcox Vølund A/S, Falkevej 2, DK-6705 Esbjerg Ø, Denmark

SUMMARY: Babcock & Wilcox Vølund have recently developed a new technology and obtained a

world patent for it. The basic idea is to improve the electrical efficiency by increasing the steam

parameters. The steam temperature in particular can be increased without increasing the superheater

corrosion. The new concept can be fully integrated into the boiler, and the waste-fired power plant

then has the same external layout as a classical waste-to-energy plant. The new superheater system

is currently being tested at a Danish power plant. To obtain high steam temperatures, superheaters

are fitted into the furnace above the last part of the grate where flue gas with very high energy and a

low content of corrosive elements is available. The heat rate is 20 - 30 kW/m2, which is five to ten

times the rates realised in standard convection superheaters. The results are a promising low

corrosion level and a material forming a thin layer of alumina (-Al2O3) show very promising

corrosion level at high material and flue gas temperature.

1. INTRODUCTION

The main problem associated with increasing the steam parameters are corrosion and fouling in the

boiler. The fouling can be handled by boiler cleaning equipment, whereas corrosion is destructive to

the boiler and plant operation and remains a difficult problem.

The current requirements for plant availability are up to 8300 hours per year. The operational

hours are one of the most important factors for the plant owner, because they are the basis of the

annual income and decide whether the plant can be run profitably. This results in a very

conservative business where investors tend to choose a well-proven technology in order to minimise

the financial risk.

The lifetime of the superheater tubes is a critical parameter. New materials, on-line boiler

cleaning and design tools such as CFD modelling have improved the steam data and thus boosted

the electrical efficiency.

A number of ways are available to increase the efficiency of a plant. This paper focuses on

increasing the steam parameters as cost-effectively as possible. Steam values of 55 bar and 425°C

are common and well-established inlet parameters for steam turbines in existing MSW plants.

Corrosion on heating surfaces in direct contact with flue gases is low in MSW boilers operated

with these steam parameters. Higher live steam temperature levels increase the corrosion risks in

the superheater sections. For higher steam temperatures, the superheater design is increasingly

constrained by the risk of corrosion. High live steam pressure levels lead to higher temperature

levels in the radiation part of the boiler, since at higher pressures the water evaporates at higher

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

temperatures. An increase of live steam pressure consequently leads to a higher risk of corrosion in

the radiation part of the boiler. Corrosion is mainly due to high temperature chlorine attack, either

through gaseous species or by chloride particles deposited on the boiler walls.

The increase in electrical efficiency achieved by increasing the steam temperature is almost

constant 0.5% / 25˚C. Consequently the aim should be to get the temperature as high as possible.

Nevertheless, the risk of corrosion in the superheaters is limiting the temperature and this limit is

440˚C for current technology in view of the European market standards for lifetime guarantees of

five years for the superheaters.

A new superheater system has consequently been developed. Flue gas measurements conducted

on a waste-to-energy plant in Denmark indicated that the flue gas is less corrosive over the last part

of the grate (Bøjer et al., 2008). This is in accordance with the results obtained from CFD

modelling. Superheaters may therefore be applied in the furnace above the last part of the grate to

obtain higher steam temperatures in the range of 480 – 520˚C. The results from a Danish test

facility are reported in this paper. They show a promising level of heat transfer and a low corrosion

level as expected from the theoretical analysis carried out prior to the test.

2. INCREASING THE EFFICIENCY OF A MSW POWER PLANT

2.1 Ways of increasing the efficiency

Babcock & Wilcox Vølund A/S trust that waste-to-energy is one of the best technologies for

converting non-recyclable waste into energy and new raw materials. The company wants to build

the best plants for this purpose, and Babcock & Wilcox Vølund A/S consequently aims to build

plants with the highest possible steam parameters.

A number of methods can be used to increase the efficiency of a plant. The general parameters

influencing the boiler and electrical efficiencies are listed in Table 1 below:

Table 1. Ways of increasing the efficiency

Boiler efficiency Electrical efficiency

Reduction of excess air High steam parameters (p, T)

Low flue-gas temperature at boiler outlet Steam reheating

Low moisture content in fuel High feedwater temperature

Oxy-fuel combustion Stepwise preheating of condensate and feedwater

External economiser after filter Preheating combustion air with steam bleed from

the turbine

Integration with other power generators such as a

gas turbine

Air preheating with a flue-gas heat exchanger

(enamel-coated)

Reduction of auxiliary consumption

Stability of steam production

Every effort is made to pursue all these methods wherever possible. In regions where only power

production is relevant, high steam parameters are very important. However, increasing the steam

parameters also increases the risk of corrosion. Waste is one of the most challenging fuels to burn,

giving rise to many different problems, of which corrosion is one. One of the most important factors

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

here is the composition of the waste. Waste from a particular geographical region or a specific type

of waste will lead to different corrosion problems. Nevertheless, the two main parameters affecting

high-temperature corrosion in waste incinerators are the temperatures of the metal tubes and of the

flue gas.

Ways of increasing the steam temperature:

Refractory-covered platen superheater in second pass (monolithic SiC)

Hanging superheater in first pass

Superheater in furnace (SteamBoost™)

Superheater panel in furnace

Final superheater in front of the convection pass (extra evaporator to even out the flow)

External superheater (fired with low corrosive fuel).

2.2 Hypotheses underlying a superheater in the furnace

Waste incineration is among the most complex of combustion processes. The processes

occurring in a burning fuel bed include drying, ignition, pyrolysis and gasification as well as solid

and gas combustion. The release of the volatile elements Cl, Na, K, Pb, Zn and S to the flue gas and

the aerosol formation from them is of special interest. These elements are present at high

concentrations in ash deposit layers, where they may form various sulphate and chloride salts

capable of inducing high corrosion rates. Deposits with a high Cl content in particular lead to high

corrosion of boiler tubes.

Figure 1 is a simplified illustration of the various processes occurring in a fuel bed on a grate.

Pyrolysis gases are released in the ignition zone due to rapid heating of the top waste layer before

combustion starts. A large zone of the grate is sub-stoichiometric and results in the formation of

gasification products. Thus, pure gas-phase combustion is achieved directly above the waste layer

where a relatively large part of the total energy is released from the waste. The burnout of these

gases, soot and particles leaving the bed forms the radiant flames above the grate.

The release of chlorides from the fuel into the flue gas depends on the properties of the chlorine-

bearing components and on the firing conditions. The overall distribution is illustrated in

Figure 1, where the ignition, pyrolysis/devolatilisation and burning zones are located in the first

part of the grate and fuel bed. The major parts of the corrosive species are released in the first part

of the combustion grate and hence in the front of the furnace. The rear parts of the grate are

characterised by burnout of a relatively clean char, thereby releasing relatively clean combustion

products which are much less corrosive. This phenomenon can be utilised to split up the flue gases

from the grate into two or more fractions, one of which exhibits a high heat flux and a low chlorine

concentration. This fraction could then be used in a high-temperature superheater to increase the

steam temperature and thereby the electrical efficiency of waste-fired power plants. The concept is

named SteamBoost™ (Bøjer et al., 2008).

The history behind this concept is summarised below:

2006 Concept developed in cooperation between BWV and DTU.

2007 Patent filed in cooperation between BWV and DTU.

2007 Patent transferred to sole ownership of BWV.

2007 Flue-gas measurements on Danish plant to test the concept. Concept development with

CFD.

2012 Test facility established in a Danish plant (AffaldPlus).

2013 Test upgraded to a steam temperature of 440°C.

From September 2014, it is planned to operate the test facility at 480°C.

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

Figure 1. The SteamboostTM concept

2.2.1 Testing the concept

The CHEC Research Centre at the Department of Chemical and Biochemical Engineering at the

Technical University of Denmark carried out a series of full-scale experiments in order to verify the

Cl release profile in a typical waste-fired power plant.

The objective of the study was to measure a concentration profile of the elements Cl, Na, K, Zn,

Pb and S as a function of the location on the grate in a waste-to-energy boiler. A heat-flux and

chlorine-release profile along the grate provides information on the location at which heat is

released with the lowest concentrations of corrosion-promoting species in the flue gas. This tests the

basic idea of separating the flue gas from the grate into two or more fractions, with one fraction

having a relatively high heat flux and low chlorine concentration.

Measurement of the concentration and location of the release of Cl, Na, K, Pb, Zn and S from the

grate, combined with information on the magnitude and location of the heat flux from the grate, can

reveal the region of the flue gas with a high heat flux but low concentrations of corrosive elements.

Measurements were conducted at Vestforbrænding Unit 5 - a waste-fired CHP plant in

Copenhagen, Denmark. The plant was commissioned in 1998 and can process up to 30 tonnes per

hour. The waste is burned on a forward-acting grate with a width of 9.75 m and a length of 13.1 m,

comprising a total of 18 zones. Each zone has individually controlled primary-air and grate speeds.

The location of the measuring positions (1-6) relative to the grate is shown in Figure 2. The

measurements were performed by inserting a suction probe into the designated ports 1, 2, 3, 4 and 5

(Bøjer et al., 2007 and Bøjer et al., 2008).

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

Figure 2. The furnace, grate and position of measuring ports at Vestforbrænding, Unit 5

Figure 3. Measured chlorine release (Bøjer et al., 2007)

Figure 4. Measured alkali release (Bøjer et al., 2007)

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

3. TEST FACILITY WITH SUPERHEATER IN THE FURNACE

The objective of the test facility is to determine the heat transfer rates that can be used for design

purposes and to implement corrosion-testing of various materials in the superheater at high

temperatures. The test should clarify which materials are best suited for a SteamBoost™

superheater. The project will also clarify the lifetime of this superheater.

3.1 Description of the installation

The steam is taken as a partial stream of 1-2% from the inlet flow to the last superheater (SH3)

which increases the temperature of the normal steam flow of the plant from 370 to 400°C.

However, the temperature of the steam at the inlet to the test superheater is only approximately

340°C because of an unexpected high heat loss from the pipeline from SH3 down to the test loop

located at the back wall of the furnace.

The first phase of the test was consequently carried out at a relatively low steam temperature in

order to minimise corrosion. Nevertheless, the performance of the superheater could still be tested

and analysed. After one year’s operation at this low temperature, the test superheater was upgraded

to the solution shown in Figure 6 with four tube loops instead of only one. When the steam flows

through the three red tubes shown in Figure 6, it is heated from 340 to 420°C, resulting in an outlet

temperature from the last loop in the test superheater of 440°C.

The installation of the test facility and the tests carried out so far were fully financed by Babcock

& Wilcox Vølund A/S. This was possible thanks to a very close cooperation with AffaldPlus. The

plant staff has been very flexible with respect to problems relating to how this kind of test facility

can be integrated into a commercially operated plant.

Figure 5. Pipes and valves at the test facility

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

Figure 6. CAD drawing of the superheater

Figure 7. Overview of superheater installation

To ensure that the rear end of the furnace is “clean” and not contaminated by recirculation zones

drawing corrosive gases from the front, a flow of air in the opposite direction is introduced through

a large number of small nozzles in the back wall as shown in Figure 8.

The flow creates an air barrier which eliminates the recirculation zone. The CFD results in

Figure 7 show that the concentration of corrosive species in the area where the superheater is placed

is very low. The calculation shows that the concentration of the corrosive species can be reduced by

75% with this new SteamBoost™ concept (Madsen and Sødring, 2011).

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

Figure 8. Flow pattern and air nozzles in back wall

3.2 Operation

The basic operation of the test facility is to extract 1-2% of the steam produced from the plant and

heat it further in the superheater in the furnace to 440 – 480°C. A number of pipes and valves are

needed to make this possible, as can be seen in Figure 5 and Figure 9. A safety cooling system

based on compressed air also had to be implemented: when this system is in operation, the steam

flow is cut off from the test superheater, which is then cooled by a large air flow instead.

The safety cooling system is used when the heat transfer to the test superheater is too large and

consequently generating too high material temperatures. Secondly, maintenance work can then be

carried out on the test superheater system without affecting normal operation of the plant.

Figure 9. Screen dump from PLC during operation

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

4. RESULTS AND DISCUSSION

4.1 Performance

A test campaign was run to determine the performance of the test superheater with respect to heat

transfer. A large number of different settings of the combustion system were tested. The campaign

showed that the distribution of the primary air under the grate and the ratio of the primary to

secondary air has a decisive effect on the performance. The final setting produces a suitable flame

position and a highly intense flame.

Analysis of the operating data showed that the heat input to the superheater could be kept above

30 kW/m2 by maintaining a high velocity through the nozzle in the front ceiling combined with a

reduced amount of primary air, so that the excess air was held at a level of 0.5 – 0.6.

Figure 10. Heat input to superheater combined with velocity of secondary air. The red curve shows

the velocity of the air entering the furnace through the front ceiling of the furnace. The

blue curve shows the difference in the enthalpy of the steam at the inlet and outlet of the

test superheater.

During the next test phase, a number of tests will be carried out with corrosion and fouling

probes to obtain detailed information about the corrosion potential of the flue gas while optimising

the combustion air supply and the control system. The aim is to minimise corrosion and maximise

the heat transfer.

4.2 Corrosion

The test superheater was installed in 2012 during the revision of the plant and was tested for a year

with steam temperatures in the range of 350-370°C. The corrosion test showed interesting results

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

despite the first phase of the test being carried out at a relatively low steam temperature with respect

to corrosion. In the second test phase, the last superheater loop was operated with an inlet

temperature of about 420°C while the outlet temperature from the loop was about 440°C. These

conditions prevailed for the past nine months during which the corrosion results were reported.

The measured corrosion of the various materials is listed in Table 2 in terms of material loss. The

tube thicknesses were measured mechanically with an accuracy of approximately ±0.1 mm.

Table 2. Measured corrosion after nine months of operation

Test phase 1 (350 °C) Test phase 2 (440 °C)

Material Average loss (mm/year) Average loss (mm/year)

16Mo3 0.8

13CrMo4-5 2.3

Haynes 214 (alumina) 0.0 0.3

Esshete 1250 1.1 1.6

Sanicro 25 0.9 1.5

Inconel 625 overlay 0.5 1.3

Inconel 686 overlay 0.6 1.1

Haynes 214 overlay 0.8 0.9

Haynes G35 overlay 0.4 1.6 (greatly increased corrosion)

Figure 11. Haynes 214 and 13CrMo4-5. Corrosion after 9 months exposure at approx. 440°C.

The measured corrosion rates at 440°C show that 13CrMo4-5 exhibits the highest corrosion rate,

while the tubes of the overlay welding, the Esshete 1250 and the Sanicro 25, exhibit a medium

corrosion rate, and the Haynes 214 tube has a very low corrosion rate. A general observation is that

the corrosion rate increases with increasing steam temperature.

Low alloyed materials (e.g. 16Mo3 and 13Mo4-5) form an iron oxide during corrosion, whereas

Haynes 214 13CrMo4-5

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

higher alloyed steels (e.g. Sanicro 25, Esshete 1250, Inconel 686, Inconel 625 and Haynes G35)

form a chromium oxide scale during corrosion. Aluminium-containing alloys (e.g. Haynes 214)

may form a dense protective layer of -Al2O3 (alpha-alumina) during corrosion. Experience has

shown that this layer forms at very high temperatures, and that the oxides offer corrosion protection

in the following order: alpha-alumina > chromium oxide > iron oxide.

Before the Haynes 214 material was installed in the superheater, the material was heat-treated at

1095°C for 45 minutes. This was done in order to initiate the formation of a thin layer of alumina

(-Al2O3). Haynes 214 was also tested as an overlay welding. In this case, no heat treatment was

performed before the installation of the superheater. Nevertheless, it seems to perform well at high

temperatures compared to the other overlay welds. A difference is that Haynes 214 shows a low

relative increase in corrosion when the steam temperature is increased.

No protective alumina layer forms during normal operation, which explains the large differences

in corrosion rate observed for the Haynes 214 tube and the Haynes 214 overlay welding.

As mentioned earlier, a number of tests will be carried out with corrosion and fouling probes in

order to obtain detailed information about the corrosion during the next test phase. The steam

temperature will be increased by an additional 40°C to obtain a temperature level which would

allow an interesting increase in electrical efficiency for a power plant. The next test phase is

planned to run for 18 months.

When the test superheater is upgraded from 440°C to 480°C, the superheater loops will be

replaced by new ones. This will make it possible carry out a more detailed analysis of the corrosion

results going beyond the basic loss measurements reported in this paper.

5. CONCLUSIONS

A new innovative superheater system is currently being developed and tested at a Danish power

plant. To obtain high steam temperatures, superheaters were applied in the furnace above the last

part of the grate where flue gas with very high energy level and low content of corrosive elements is

available. In the first test phase, the steam temperature was relatively low with respect to the

corrosion test. Nevertheless, the performance of the superheater could still be tested and analysed,

and the heat rate was found to be 20 - 30 kW/m2, which is five to ten times the rates realised in

standard convection superheaters. After one year of low-temperature operation, the test superheater

was upgraded to a solution using four tube loops instead of only one, resulting in an outlet

temperature from the test superheater of 440°C. In September 2014, when the test will have been

running for a year at 440°C, it will be upgraded to 480°C. The results show a promising low

corrosion level, as expected from the theoretical analysis carried out prior to the test, and a material

forming a thin layer of alumina (-Al2O3) show very promising corrosion levels at high material

and flue gas temperature.

During the next test phase, a number of tests will be carried out with corrosion and fouling

probes to obtain detailed information about the corrosion potential of the flue gas while optimising

the air supply and combustion control. This will be carried out in order to minimise the corrosion

and maximise the heat transfer.

ACKNOWLEDGEMENTS

The project was originally funded by the PSO Project 59281 and STVF (The Danish Technical

Research Council) and was run in cooperation with the CHEC Research Group at the Department of

Chemical and Biochemical Engineering, The Technical University of Denmark.

Venice 2014, Fifth International Symposium on Energy from Biomass and Waste

AffaldPlus in Næstved, Denmark, gave us access to the plant and permitted us to implement the

superheater system in the furnace. This was crucial for ensuring the feasibility of the project.

REFERENCES

Hedegaard Madsen O.; Next Generation of Waste Fired Power Plants, NAWTEC 15, Miami, USA,

21 – 23 May 2007.

Bøjer M., Arendt Jensen P., Frandsen F., Dam-Johansen K., Hedegaard Madsen O. and Lundtorp

K.; Release of Potentially Corrosive Constituents from the Grate of a Waste-to-Energy Boiler,

IT3’07, May 14–18, 2007, Phoenix, AZ, USA.

PCT/IB2006/053560; BOILER PRODUCING STEAM FROM FLUE GASES UNDER

OPTIMISED CONDITIONS, 2006.

Bøjer M., Arendt Jensen P., Frandsen F., Dam-Johansen K., Hedegaard Madsen O., Lundtorp K.;

Alkali/Chloride release during refuse incineration on a grate: Full-scale experimental findings,

Fuel Processing Technology, vol: 89, issue: 5, pages: 528-539, 2008, Elsevier Science.

Hedegaard Madsen O., Wagner Sødring T.; A New Concept To Improve the Electrical Efficiency

Based on the Combustion Process in the Waste Fuel Bed on a Grate, ISWA 2011, Korea