Energy & Fuels 2000, 14, 765-780
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Deposit Formation in a 150 MWe Utility PF-Boiler during Co-combustion of Coal and Straw Karin H. Andersen,‡,§ Flemming J. Frandsen,*,† Peter F. B. Hansen,‡,| Kate Wieck-Hansen,⊥ Ingvard Rasmussen,⊥ Peter Overgaard,⊥ and Kim Dam-Johansen† Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark, and Midtkraft Energy Company A/S, Denmark Received July 20, 1999
A conventional pc-fired boiler at the Danish energy company I/S Midtkraft has been converted to coal-straw co-combustion, and a 2 year demonstration program was initiated in January 1996, addressing several aspects of coal-straw co-combustion. Deposition trials were performed as part of the demonstration program. A maximum straw share of approximately 20% (energy base) was used in the experiments. For the deposit samples collected, a visual analysis procedure was developed and each sample evaluated according to this. In addition, a number of samples were analyzed by scanning electron microscopy (SEM) combined with energy dispersive X-ray analyses (SEM-EDX) and bulk chemical analyses. In the visual analysis, a significant increase in the amount and tenacity of the upstream deposits was observed as a function of increased straw share, exposure time, and boiler load. The chemical analyses of the deposits show increased amounts of K and S during co-combustion, and the Fe-dominated upstream deposits formed during coal combustion are shifted toward more Ca- and Si-rich deposits during coal-straw cocombustion. However, the major part of K is observed to form K-Al silicates, which do not form problematic deposits. Co-firing straw also caused a change in the structure of the upstream deposits. During coal combustion an ordered, “finger” structure of the larger particles with small particles between was observed, whereas during co-combustion a more random deposition of the larger particles among the small ones was observed. No chlorine species was observed in the deposits collected, and selective chlorine corrosion is therefore not expected to constitute a problem in co-combustion of coal and straw up to 20% straw share, for the coal types utilized in the tests. However, deposition problems could arise when burning other coals, particularly coals with a high S or alkali metal content or a low content of ash. The behavior of K, Ca, S, and Cl was evaluated by use of thermodynamic calculations. The thermodynamically stable species agree with the observed behavior in the experiments, i.e. formation of stable K-Al silicate species as well as K2SO4 is predicted. The calculations also emphasize that the mixing between the coal and straw species is essential for the deposition behavior, primarily by affecting the split between K-Al silicates and K2SO4.
1. Introduction The Danish Government has committed the Danish power companies to burn 1-1.2 million tons of straw per year, by the year 2000. This is part of the national act to reduce the CO2 emissions to 80% of the 1988 level, in the year 2005. In addition, the Danish Government has recently imposed a ban on the erection of new utilities in Denmark relying partly or solely on coal as a fuel, i.e. in practice banning the use of coal in future utilities in Denmark. Biomass as an energy source for * Author to whom correspondence should be addressed at Department of Chemical Engineering, Building 229, Technical University of Denmark, DK-2800 Lyngby, Denmark. Phone: +45 45252883. Fax: +45 45882258. E-mail:
[email protected]. † Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark. ‡,§ Current address: Sun Chemical A/S, DK-4600 Ko ¨ ge, Denmark. ‡,| Current address: Rockwool International A/S, DK-2640 Hedehusene, Denmark. ⊥ Midtkraft Energy Company A/S, Studstrup Power Station, DK8541 Sko¨dstrup, Denmark.
the power industry has thus gained increased importance in Denmark. An overview of the Danish experiences with combustion of straw alone or in conjunction with coal is given by Frandsen et al.,1,2 and ash fusion and deposit formation in straw-fired boilers is examined by Hansen,3 whereas the corrosion behavior related to deposit for(1) Frandsen, F. J.; Nielsen, H. P.; Jensen, P. A.; Hansen, L. A.; Livbjerg, H. L.; Dam-Johansen, K.; Hansen, P. F. B.; Andersen, K. H.; Sørensen, H. S.; Larsen, O. H.; Sander, B.; Henriksen, N.; Simonsen, P. Deposition and Corrosion in Straw- and Coal-Straw Co-Fired Utility Boilers-Danish Experiences, Proceedings of the Engineering Foundation Conference. “The Impact of Mineral Impurities in Solid Fuel Combustion”, Kona, HI, Nov. 2-7, 1997; 1997. (2) Frandsen, F. J.; Nielsen, H. P.; Hansen, L. A.; Hansen, P. F. B.; Andersen, K. H.; Sørensen, H. S. Ash Chemistry Aspects of Straw and Coal-Straw Co-Firing in Utility Boilers. Proceedings of the 15th Annual International Pittsburgh Coal Conference, GreenTree Marriott Hotel, Pittsburgh, PA, Sept 14-18, 1998; University of Pittsburgh: Pittsburgh, PA, 1998. (3) Hansen, L. A. Melting and Sintering of Ashes. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, 1997 (ISBN-87-90142-31-4).
10.1021/ef9901589 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/17/2000
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mation in straw-fired boilers has been examined by Nielsen.4 As part of the ELSAM (The Jutland-Funen Electricity Consortium) biomass strategy, a conventional 150 MWe pc-fired boiler at the Danish energy company I/S Midtkraft was converted to coal-straw co-combustion. A 2 year demonstration program was initiated in January 1996, and concluded in 1998. The main purpose of the program was to evaluate the economical and technical feasibility of this coal-straw co-combustion technology compared to other technologies. The demonstration program included experiments to demonstrate the plant operation and operating costs during co-combustion of coal and straw, as well as experiments to clarify the fuel and process chemistry. These included analyses of fuels, bottom and fly ash and flue gas, deposits, and high-temperature (HT) corrosion. Also a number of in situ gas, temperature, and aerosol measurements were performed to establish the chemical environment in the boiler during the deposition and corrosion experiments. For further details on the demonstration program as a whole, please refer to Hansen et al.5 and Overgaard and Hansen.6 This paper provides an overview of the results from the deposition trials, but also includes an introduction to the inorganic thermodynamics of coal-straw cocombustion. More information can be found in Andersen.7 2. Background The ash-forming elements occur in solid fuels mainly as internal or external mineral grains, as simple salts, e.g. NaCl and KCl, or associated with the organic matrix of the fuel. In coal, a large fraction of the inorganics is present as minerals, whereas in most biomasses the major part of the inorganics is present as simple salts or is organically associated. Approximately 0.5-4 wt % 8 of the inorganics in coal vaporize during combustion, whereas between 30 and 75 wt % of the inorganics in straw9-14 is vaporized at 1200 °C. (4) Nielsen, L. B. Combustion Aerosols from Potassium-Containing Fuels. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, 1998. (5) Hansen, P. F. B.; Andersen, K. H.; Wieck-Hansen, K.; Overgaard, P.; Rasmussen, I.; Frandsen, F. J.; Hansen, L. A.; Dam-Johansen, K. Co-Firing Straw and Coal in a 150 MWe Utility Boiler: In Situ Measurements. Proceedings of the Engineering Foundation Conference. “Biomass Usage for Utility and Industrial Power”, Snowbird, UT, Apr 28th-May 3rd, 1996; 1996. (6) Overgaard, P.; Hansen, P. B. H. Co-Firing Coal and Straw in a 150 MWe Power Boiler. Conference Proceedings of POWER-GEN EUROPE ‘97; PennWell: Utrecht, The Netherlands, 1997. (7) Andersen, K. H. Deposit formation during coal-straw co-combustion in a utility PF-boiler, Industrial Ph.D. Thesis, I/S Midtkraft and Department of Chemical Engineering, Technical University of Denmark, 1998. (8) Flagan, R. C.; Friedlander, S. K. Particle Formation in Pulverized Coal Combustion-A Review. In Recent Developments in Aerosol Science; Shaw, D. T., Ed.; Wiley; New York, 1978. (9) Livingston, W. R. Straw ash characteristics; Babcock Report Number 34/91/08, Contractor Report, Department of Energy, ETSU B 1242; Babcock Energy Ltd.: 1991. (10) Baxter, L. L. Ash deposition during biomass and coal combustion: A mechanistic approach. Biomass Bioenergy 1993, 4 (2), 85102. (11) Christensen, K. A. The Formation of Submicron Particles from the Combustion of Straw. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, 1995 (ISBN 87-9014204-7). (12) Dayton, D. C.; Milne, T. A. Mechanisms of Alkali Metal Release During Biomass Combustion. Prepr. Symp.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (3), 758-762.
Andersen et al.
During combustion, the inorganics in the fuel are transformed into ash through a complex combination of chemical and physical processes. The minerals may undergo fragmentation, shedding, and coalescence during char burnout. Similarly, the vaporized inorganic species may take part in several transformations during combustion. These include heterogeneous condensation on heat-transfer surfaces or fly ash particles or homogeneous nucleation due to a local supersaturation of salts, e.g. Na2SO4, K2SO4, or KCl,8,11 thereby forming a condensation aerosol which can subsequently coagulate to form larger particles. Thus, the resulting ash can be divided into two fractions: (1) the residual ash which mainly consist of the transformed minerals (>1 µm) and (2) the submicron ash (
