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Energy & Fuels 2000, 14, 618-624
The Role of Kaolin in Prevention of Bed Agglomeration during Fluidized Bed Combustion of Biomass Fuels Marcus O ¨ hman* and Anders Nordin Energy Technology Centre, Department of Inorganic Chemistry, Umeå University, P.O. Box 726, S-941 28 Piteå, Sweden Received September 10, 1999. Revised Manuscript Received December 23, 1999
Agglomeration of bed material and fuel ash may cause problems during fluidized bed combustion of biomass fuels. Previous results have shown that a “sticky” coating, which covered the original bed material and consisted of Ca-K-silicates, was directly responsible for the bed agglomeration during biomass combustion. The melting behavior (stickiness) of these coatings was very sensitive to the potassium content. Prior studies have also indicated that bed agglomeration could possibly be prevented by introducing low-cost additives such as kaolinite. The objectives of the present work were, therefore, to illustrate the effect of kaolin addition on the actual agglomeration temperature of two troublesome biomass fuels, and to contribute to the understanding of the role of kaolin in prevention of bed agglomeration. By controlled agglomeration experiments in a 5 kW bench scale fluidized bed reactor, the critical temperatures for agglomeration in a normal quartz bed when firing wheat straw or bark were determined to be 739 and 988 °C, respectively. By adding kaolin, 10% w/w of the total amount of the bed, the initial bed agglomeration temperatures increased to 886 and 1000 °C, respectively. Samples of bed materials, collected throughout the experimental runs, as well as final agglomerates were analyzed using SEM/EDS and X-ray diffraction. These results showed that kaolin was transformed to meta-kaolinite particles, which adsorbed potassium species. The increased agglomeration temperature was explained by the decreased fraction of melt in the bed particle coatings, i.e., coatings were somewhat depleted in the potassium content by the corresponding potassium-enrichment in the kaolin-derived aggregates.
Introduction The most promising energy conversion technologies for solid fuels, and biomass in particular, are based on fluidized bed combustion (FBC) or gasification (FBG). Owing to the relatively low temperatures in FBC and FBG, the extent of slagging and deposit formation can be kept to a minimum. However, bed agglomeration could be a potential problem, which can decrease both the heat-transfer in the bed and the fluidization quality, resulting in poor conversion efficiencies and loss of control of bed operational parameters. In the most severe cases, bed agglomeration can lead to total defluidization, resulting in unscheduled plant shutdowns. In a recent study,1 the “mechanism” of the chemical processes during bed agglomeration of eight different biomass reference fuels were determined by extensive SEM/EDS analysis of bed material samples as well as of agglomerates. The results from the study showed that (i) the bed particles were coated with a relatively homogeneous ash layer, most commonly consisting of potassium-calcium-silicates; (ii) melting of the coating was directly responsible for the bed agglomeration; and (iii) the melting behavior of the coatings was very sensitive to the relative amount of potassium present. Thus, if the potassium content in the coating could be * Corresponding author. (1) O ¨ hman, M.; Skrifvars, B. J.; Nordin, A.; Backman, R.; Hupa, M. Energy Fuels, accepted for publication.
kept to a minimum, the agglomeration temperature could potentially be increased significantly. Several authors have previously proposed the use of various kinds of mineral additives for alkali sorption2-4 or for increasing the melting temperatures of a system of interest.5-7 In a comparative study between kaolinite, bauxite, and emalthite, kaolinite proved to be the most efficient mineral for alkali sorption.8 In addition, cocombustion of troublesome biomass fuels with coal has been proposed as a simple measure to introduce “lowcost” additives in the form of the coal mineral matter.9,10 Bed agglomeration was, for example, effectively prevented by mixing problematic biomass fuels with coal.11 The in-bed ash compositions were altered toward higher (2) Luthra, K. L.; Leblanc, O. H. J. Phys. Chem. 1984, 88, 18961901. (3) Lee, S. H. D.; Johnson, I. J. Eng. Power 1980, 102, 397-402. (4) Punjak, W. A.; Uberoi, M.; Shadman, F. Energy Fuels 1988, 2, 702-708. (5) Ivarsson; E.; Nilsson, C. Special Report 153, Swedish University of Agricultural Sciences, Department of Farm Buildings, 1998. (6) Steenari, B. M., Lindqvist, O. Biomass Bioenergy 1998, 14, 6776. (7) Wile´n, C.; Staahlberg, P.; Sipila, K.; Ahokas, J. Energy Biomass Wastes 1987, 10, 469-484. (8) Uberoi, M.; W. A.; Shadman, F. Prog. Energy Combust. Sci. 1990, 16, 205-211. (9) Andries, J.; Vegelin, R. J.; Klaus, R. G. H. Proceedings of the 8th European Conference on Biomass for Energy, Environment, Agriculture and Industry, 1994, 3-5 October, Vienna. (10) Belle-Oudry, D. A.; Dayton, D.; Nordin, A. Energy Fuels 1999, 13, 1203-1211.
10.1021/ef990198c CCC: $19.00 © 2000 American Chemical Society Published on Web 03/23/2000
Kaolin in Prevention of Bed Agglomeration
Energy & Fuels, Vol. 14, No. 3, 2000 619
Table 1. Fuel and Bed Material Characteristics wheat straw
bark
90.3 5.9 0.596 20.7 6.90 1.73 0.385 0.361 18.2 2.88 4.92 2.52
90.6 3.0 1.16 6.36 27.6 2.35 1.18 2.73 6.84 1 0.333 1.24
dry substance asha Nab Kb Cab Mgb Alb Feb Sib Sb Clb Pb a
bed material
0.0297 0.0497 0.0879 0.0778 0.0957 0.0860 46.3
