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Energy & Fuels 1997, 11, 1048-1055
Deposition Investigation in Straw-Fired Boilers P. A. Jensen,*,† M. Stenholm,‡ and P. Hald† Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark, and dk-TEKNIK, Gladsaxse Møllevej, DK-2860 Søborg, Denmark Received September 13, 1996X
Applying annual crops as fuel in power plants may give rise to severe deposition in the boilers. In Denmark, straw is applied as fuel in some small combined power and district heating boiler plants. Large differences have been observed in the amount of deposition in the furnace chamber and on superheater tubes when different straw fuel parcels are applied. We have performed 12 experiments at two straw-fired power plants with a fuel input of 23 and 31 MW. In all the experiments the boilers were fed by one type of reasonably uniform straw during an 8 h period. Deposition probes simulating superheater coils were inserted near the superheaters and in the boiler chamber. A long range of parameters was measured, including local temperature, exit flue gas composition, and aerosol particles in the flue gas. Detailed chemical analyses have been performed on the straw, bottom and flue ash, and deposition samples. On the basis of the experimental data the transport of the minerals through the boilers has been followed. For wheat and barley straws the extent of deposition could be correlated with the content of potassium in the straws.
Introduction Biomass fuels have increasingly been applied as renewable solid fuels for heat and power generation during recent years. Mainly wood has been applied, but agricultural waste like straw can also be used for power production. In Denmark, straw has been applied as fuel in small combined power and district heating plants since the late 1980s. New equipment has been developed to handle and burn straw. Straw is by no means a trivial fuel; problems like combustion fluctuations, deposition in the boiler, corrosion of superheaters, and a poor burnout have been observed at the boiler plants. However, most of the problems can be handled by an appropriate design of the boiler.1,2 Compared with wood and coal, annual crops like cereal straws have a high content of potassium that, together with other mineral components of the straw, gives rise to severe deposition on boiler surfaces. The deposition reduces the heat transfer, and if severe deposits build up on the convective heat transfer surfaces, the result can be that the induced draft fan cannot sustain a subpressure in the boiler chamber. In that case the boiler has to be stopped and the deposits must be removed manually. Several measures have been taken in the design of straw-fired boilers to reduce the amount of deposits. Convective heat transfer surfaces are placed at a long distance from the fuel inlet, whereby a relatively low * Author to whom correspondence should be addressed. Present address: Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark. † Risø National Laboratory. ‡ dk-TEKNIK. X Abstract published in Advance ACS Abstracts, June 15, 1997. (1) Nielsen, C. Utilisation of Straw and Similar Agricultural Residues. Biomass Bioenergy 1995, 9, 315-323. (2) Jensen, P. A.; Stenholm, M.; Jørgensen, K. H. Interview and Literature Study of Combustion and Deposition Characteristics of Straw (In Danish). Risø-R-786(DA); Risø National Laboratory: Denmark, 1994.
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local gas temperature is obtained. Soot blowing and an ironball cleaning system are applied on the convective heat transfer surfaces, and no refractory is found in the boiler chamber. Still, in some cases severe deposition takes place near the straw fuel inlet, on the grate at the bottom of the boiler, and on superheaters. Generally, the hardest deposit, which is the most difficult to remove, is seen on the superheater coils that are first in contact with the flue gas. Large differences in the amount of deposition are seen when different fuel parcels are applied. Therefore, knowledge of how to characterize the straw for predicting the deposition properties in the boiler is needed. Cereal straw typically has a high content of potassium (0.5-1.3%), chlorine (0.2-0.7%), and silicon (0.3-1.0%) as well as minor amounts of Ca, Mg, Al, Na, Fe, S, and P.2 The mineral elements to some extent leave the combusting fuel by evaporation, chemical decomposition, and entrainment of particles. Significant amounts of potassium, chlorine, and sulfur are released to the gas phase at combustion conditions. Equilibrium calculations3 indicate that the released potassium is likely to appear in the gas phase as mainly KCl and KOH and the chlorine as KCl and HCl. These theoretical results have been verified experimentally by Dayton et al.4 When the gas is cooled later in the boiler, the potassium will condense mainly as potassium sulfate and potassium chloride and large amounts of aerosols are formed.5 Depositions on surfaces are generated by condensation, impaction of large particles (>10 µ), and thermophoreses (3) Hald, P. Alkali Metals at Combustion and Gasification. Equilibrium Calculations and Gas Phase Measurings. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, 1994. (4) Dayton, D. C.; French, R. J.; Milne, T. A. The Direct Observation of Alkali Vapor Release During Biomass Combustion and Gasification. 1. The Application of Molecular Beam/Mass Spectrometry to Switchgrass Combustion. Energy Fuels. 1995, 9, 855-865. (5) Christensen, K. A.; Livbjerg, H. A Field Study of Submicron Particles From the Combustion of Straw. Aerosol Sci. Technol. 1996, 25, 185-199.
© 1997 American Chemical Society
Deposition Investigation in Straw-Fired Boilers
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Figure 2. Sketch of the Slagelse boiler. Figure 1. Sketch of the Haslev boiler.
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of small particles (
