Primary and secondary fragmentation of coals in a circulating fluidized bed combustor

Primary and secondary fragmentation of two Kentucky No. 9 coals, having similar proximate and ultimate analyses but different swelling indexes (2.5 and 9, respectively), were studied in a laboratory scale circulating fluidized bed combustor (CFBC). The apparatus, having a 41-mm i.d. and 1.92-m-high riser, was operated keeping fixed the gas velocity and the size of inert bed material at values of practical interest. Two experimental procedures were used to separately investigate primary and secondary fragmentation effects taking place during fluidized bed combustion of coals. Particle multiplication factor, i.e., the number of particles generated per one mother particle, was used to quantify these effects. Statistical functions of fragmentation (the probability of breakage by primary fragmentation, the probability density that a shrinking particle of a given size breaks into fragments, and the size distribution of subparticles produced by secondary fragmentation) were also determined and embodied into an available model for circulating fluidized bed combustion of coals. On the basis of this mathematical model, the relevance of primary fragmentation on some output variables chosen to characterize the performance of a circulating fluidized bed combustor was quantified. © 1994 Combustion Institute.