Energy & Fuels 2004, 18, 1187-1193
1187
Bed Agglomeration Characteristics of Biomass Fuels Using Blast-Furnace Slag as Bed Material Elisabet Brus,* Marcus O ¨ hman, Anders Nordin, and Dan Bostro¨m Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umea˚ , Sweden
Henry Hedman Energy Technology Centre, P.O. Box 726, SE-941 28 Pitea˚ , Sweden
Anders Eklund
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A° F-Energi & Miljo¨ AB, Box 8133, SE-104 20 Stockholm, Sweden Received December 4, 2003. Revised Manuscript Received April 13, 2004
Agglomeration of bed material may cause severe operating problems during fluidized bed combustion. The attack or coating layers that are formed on the bed particles during combustion play an important role in the agglomeration process. To reduce bed agglomeration tendencies, alternative bed materials may be used. In this paper, bed agglomeration characteristics during the combustion of biomass fuels using a relatively new bed material (iron blast-furnace slag) as well as ordinary quartz sand were determined. Controlled agglomeration tests lasting 40 h, using five representative biomass fuels (bark, olive residue, peat, straw, and reed canary grass) were conducted in a bench-scale fluidized bed. The bed materials and agglomerates were analyzed using SEM/EDS and X-ray diffraction. Chemical equilibrium calculations were performed to interpret the experimental findings. The results showed that blast-furnace slag had a lower tendency to agglomerate than quartz sand for most of the fuels. The quartz particles showed an inner attack layer more often than did the blast-furnace slag. The blast-furnace slag had a lower tendency to react with elements from the fuel. The outer coating layer had similar characteristics and thickness for both bed materials when the same fuel was combusted. However, the inner attack layer thickness was larger for quartz particles. SEM/EDS analyses of the agglomerates showed that the inner Ca-K-silicate-rich attack layer was responsible for the agglomeration of quartz sand. The composition of blast-furnace slag agglomerate was similar to the outer coating layer. Chemical equilibrium calculations showed that the original composition of the blast-furnace slag was close to the equilibrium composition, and hence there was no major driving force for reactions between that bed material and K and Ca from the fuel. The homogeneous silica-rich attack layer (with a low melting temperature) was not formed to the same extent for blast-furnace slag, thus explaining the lower bed agglomeration tendency.
Introduction Biomass is used increasingly for heat and power production since it is renewable and does not contribute significantly to the greenhouse effect. Fluidized bed boilers are particularly suitable for biomass combustion and gasification because they are insensitive to changes in fuel properties and operate at relatively low temperatures. However, biomass contains relatively large amounts of alkali metals and other ash-forming elements that can give rise to the formation of coatings around the bed particles inherent to a fluidized bed.1,2 * Corresponding author. E-mail:
[email protected]. (1) Latva-Somppi, J.; Kurkela, J.; Tapper, U.; Kauppinen, E. I.; Jokiniemi, J. K.; Johanson, B. Proceedings of the International Conference on Ash Behavior Control in Energy Conversion Systems, Pacifico Yokohama, Japan, 1998, 119-126. (2) Brus, E.; O ¨ hman, M.; Nordin, A.; Skrifvars, B-J.; Backman, R. IFRF Combustion Journal 2003, Article Number 200302, ISSN 1562479X.
These coatings may consist of compounds with low melting temperatures, and may result in agglomeration of the bed material3,4 and, in the worst case, total bed defluidization. Previous work has shown that the bed particle coatings from biomass combustion consist of multiple layers,2,5,6 and the composition of these layers depends on both the fuel and the bed material composition.6 The composition of the inner layer seems to depend more (3) Lin, W.; Krusholm, G.; Dam-Johansen, K.; Musahl, E.; Bank, L. Proceedings of the International Conference on Fluidized Bed Combustion 1997, 14 (2), 831-837. (4) O ¨ hman, M.; Nordin, A.; Skrifvars, B-J.; Backman, R.; Hupa, M. Energy Fuels 2000, 14, 169-178. (5) Berge, N.; Gamer, U.; Harnevi, H.; Larfeldt, J.; Ljungdahl, B.; Nyqvist, L.; Padban, N. 2003, Swedish report, SVF-803. (6) Nuttinen, L. H., et al. Coatings on bed particles from FBcombustion of different biomasses, Proceedings of the 17th International Conference on Fluidized Bed Combustion, 18-21 May, 2003, Jacksonville, Florida.
10.1021/ef034095c CCC: $27.50 © 2004 American Chemical Society Published on Web 07/03/2004
1188 Energy & Fuels, Vol. 18, No. 4, 2004
Brus et al.
Table 1. Bed Material Compositions
Table 2. Fuel Characteristics (elemental compositions)
chemical composition (wt %)
quartz sand
BFS bed material
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO2 P2O5 TiO2 S
98.9 0.181 0.123 0.123 0.129 0.0040 0.060 0.013
