Agglomeration and Defluidization in FBC of Biomass Fuels — Mechanisms and Measures for Prevention
The use of biomass fuels in fluidized bed combustion (FBC) and gasification (FBG) is becoming more important because of the environmental benefits associated with these fuels and processes. However, severe bed agglomeration and defluidization have been reported due to the special ash forming constituents of some biomass fuels. Previous results have indicated that this could possibly be prevented by intelligent fuel mixing. In the present work the mechanisms of bed agglomeration using two different biomass fuels as well as the mechanism of the prevention of agglomeration by co-combustion with coal (50/50%w) were studied. Several repeated combustion tests with the two biomass fuels, alone (Lucerne and olive flesh), all resulted in agglomeration and defluidization of the bed within less than 30 minutes. By controlled defluidization experiments the initial cohesion temperatures for the two fuels were determined to be as low as 670°C and 940°C, respectively. However, by fuel mixing the initial agglomeration temperature increased to 950°C and more than 1050°C, respectively. When co-combusted with coal during ten hour extended runs, no agglomeration was observed for either of the two fuel mixtures. The agglomeration temperatures were compared with results from a laboratory method, based on compression strength measurements of ash pellets, and results from chemical equilibrium calculations. Samples of bed materials, collected throughout the experimental runs, as well as the produced agglomerated beds, were analysed using SEM EDS and X-ray diffraction. The results showed that loss of fluidization resulted from formation of molten phases coating the bed materials; a salt melt in the case of Lucerne and a silicate melt in the case of the olive fuel. By fuel mixing, the in-bed ash composition is altered, conferring higher melting temperatures, and thereby agglomeration and defluidization can be prevented.
KeywordsHigh Melting Temperature Biomass Fuel Normal Combustion Compression Strength Measurement Scientific Group Thermodata Europe
Unable to display preview. Download preview PDF.
- Bitowft, B. K. and Bjerle, I. (1986). “A generic study of the sintering aspects of biomass in a fluid-bed gasifier.” Energy from Biomass and Waste XI, 511–529Google Scholar
- Gulyurtlu, I., Reforco, A. and Cabrita, I. (1991). “Fluidised bed combustion of corkwaste.” Fluidized Bed Combustion ASME, 1421–1424Google Scholar
- Nordin, A. (1993). “On the chemistry of combustion and gasification of biomass fuels, peat and waste - environmental aspects.” Thesis, Dept. of Inorganic Chemistry, Umeä UniversityGoogle Scholar
- Nordin, A. (1994). “Chemical elemental characteristics of biomass fuels.” Biomass and Bioenergy, 6(5), 339–347.Google Scholar
- Nordin, A., Marklund, S., Wikström, E. (1995). “Construction of a pilot-scale fluidized bed combustor for combustion chemistry applications.” Submitted for publication.Google Scholar
- Höglund, K., Wikström, A. Hägerstedt, L-E., Medin, K., Hagström, U., Nordin, A. (1995). “Illustration of slagging and fouling problems during combustion of biomass fuels”, Poster presentation at the Int. Conti on Application of Advanced Technology to Ash-related Problems in Boilers, Waterville Valley. July 16–21.Google Scholar
- Roscoe, J. C., Witowski, A.R. and Harrison, D. (1980). Trans. I. Chem. Engrs., 58, 69.Google Scholar
- Skrifvars, B-J., Hupa, M. and Hiltunen, M. (1992). “Sintering of ash during fluidized bed combustion.”, MEC Research, 31, 1026–1030Google Scholar
- Skrifvars, B-J. (1994). “Sintering tendency of different fuel ashes in combustion and gasification conditions.” Thesis, Dept. of Chemical Engineering, Abo Akademi University.Google Scholar
- Skrifvars, B-J. and Hupa, M. (1995) “Characterization of biomass ashes.” Proc. of the Int. Conf. on Application of Advanced Technology to Ash-related Problems in Boilers, Waterville Valley, July 16–21.Google Scholar
- Viktorén, A. (1991). “Combustion of salix in a CM-boiler.” Report no. 416, Thermal Engineering Research Foundation, Stockholm, 36 p.Google Scholar
- Yates, J. G. (1983). Fundamentals of fluidized-bed chemical processes. Butterworths, LondonGoogle Scholar