Energy & Fuels 2000, 14, 169-178
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Bed Agglomeration Characteristics during Fluidized Bed Combustion of Biomass Fuels Marcus O ¨ hman* and Anders Nordin Energy Technology Centre, Department of Chemistry, Inorganic Chemistry, Umeå University, P.O. Box 726, S-941 28 Piteå, Sweden
Bengt-Johan Skrifvars, Rainer Backman, and Mikko Hupa Process Chemistry Group, University, FIN-20500 Turku, Finland Received June 2, 1999
The in-bed behavior of ash-forming elements in fluidized bed combustion (FBC) of different biomass fuels was examined by SEM/EDS analysis of samples collected during controlled agglomeration test runs. Eight fuels were chosen for the test. To cover the variations in biomass characteristics and to represent as many combinations of ash-forming elements in biomass fuels as possible, the selection was based on a principal-component analysis of some 300 biomass fuels, with respect to ash-forming elements. The fuels were then combusted in a bench-scale fluidized bed reactor (5 kW), and their specific agglomeration temperatures were determined. Bed samples were collected throughout the tests, and coatings and necks formed were characterized by SEM/ EDS analyses. On the basis of their compositions, the corresponding melting behaviors were determined, using data extracted from phase diagrams. The bench-scale reactor bed samples were finally compared with bed samples collected from biomass-fired full-scale fluidized bed boilers. In all the analyzed samples, the bed particles were coated with a relatively homogeneous ash layer. The compositions of these coatings were most commonly constricted to the ternary system K2O-CaO-SiO2. Sulfur and chlorine were further found not to “participate” in the agglomeration mechanism. The estimated melting behavior of the bed coating generally correlated well with the measured agglomeration temperature, determined in the 5 kW bench-scale fluidized bed reactor. Thus, the results indicate that partial melting of the coating of the bed particles would be directly responsible for the agglomeration.
Introduction The fate of ash-forming elements during combustion of biomass fuels is important for the efficiency and availability of biomass-fired boilers. Ash-related operating problems, such as slagging and fouling, have been reported extensively in the literature for most conventional combustion technologies. Due to the inherent advantages of low process temperatures, isothermal operating conditions, and fuel flexibility, fluidized bed technology has been found to be the most suitable approach to converting a wide range of biomass fuels into energy. With increasing operational experience of fluidized bed boilers, however, bed agglomeration has, more than occasionally, been reported as a major problem.1-3 Bed agglomeration may, in the worst cases, result in total defluidization of the bed, resulting in an unscheduled plant shut down. Recent studies4-7 have further indicated that certain “new” biomass fuels, such * Corresponding author. (1) Skrifvars, B.-J.; Hupa, M.; Hiltunen, M. Ind. Eng. Chem. Res. 1992, 31, 1026-1030. (2) Dawson, M. R.; Brown, R. C. Fuel 1992, 71, 585-592. (3) Salour, D.; Jenkins, B. M.; Vafei, M.; Kayhaian, M. Biomass Bioenergy 1993, 4, 117-133. (4) Nordin, A. Fuel 1995, 74, 615-622. (5) Viktoren, A. Thermal Engineering Research Foundation, Report no. 416, 1991.
Figure 1. Scores (fuel abbreviations) and loading elements (within rectangles) for the two significant principal components for 300 solid fuels and the studied fuel samples (within circles). Plot marks (abbreviations) for the fuel types are as follows: wood (W); bark (B); wood residue/logging debris (Wr); barley (b); reed canary grass (c1); reed canary grass - delayed harvest (c2); Lucerne (l); rape (r); wheat straw (we); peat (P); coal (C); bagasse (ba); cane trash (le); timothy (t); municipal solid waste (M); refuse-derived fuels (RDF); salix (s).
as different types of energy crop, would be especially problematic. Several authors8-11 have identified differ-
10.1021/ef990107b CCC: $19.00 © 2000 American Chemical Society Published on Web 01/17/2000
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Energy & Fuels, Vol. 14, No. 1, 2000 Table 1. Fuel and Bed Material Characteristics
dry substance asha Nab Kb Cab Mgb Alb Feb Sib Sb Clb Pb a
wheat straw
wood
peat
90.3 5.9 0.596 20.7 6.90 1.73 0.385 0.361 18.2 2.88 4.92 2.52
92.0 0.45 2.80 3.91 15.3 3.03 1.82 0.964 6.63 2.22 2.22 1.65
46.8 5.4 0.629 1.41 14.8 0.435 2.58 17.1 16.4 7.41 0.370 0.946
cane trash
wood residue
reed canary grass
bark
RDF
93.5 5.5 0.222 13.3 4.79 2.17 2.70 1.61 20.6 1.8
53.3 3.2 0.603 5.89 21.7 1.79 1.77 1.54 10.3 1.25 0.313 1.35
90.5 5.7 0.907 2.96 3.92 0.763 0.761 0.600 36.0 1.75 0.526 0.911
90.6 3.0 1.16 6.36 27.6 2.35 1.18 2.73 6.84 1 0.333 1.24
61.3 16.4 3.04 2.53 12.4 1.10 10.4 1.24 17.3 0.976 2.32
1.00
bed material
0.0297 0.0497 0.0879 0.0778 0.0957 0.0860 46.3
