04/00401 The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel: Scala. F. et al. Fuel Processinn Technology. 2003. 84. (1–3). 229–241

Abstract

Batch fluidized combustion of 5 mm particles of a petroleum coke has been carried out in 40 mm I.D. combustors to investigate the generation of fines due to the abrasion of bed solids on the surface of burning particles. The experimental technique used allowed the time resolution of attrited fines generation, providing detailed curves of attrition rates as a function of time for oxygen concentration in the fluidizing gas ranging from 21 to 0.75%. The influence on combustion has been shown, by comparison with results of purely mechanical attrition of particles in nitrogen-fluidized beds. The change of particle size as a function of burn-off and fractional time for complete conversion, the number of particles present in the bed throughout each run, and sizes of the attrited fines have been determined. The modifications produced in the texture of the cortical region of the coke particles by combustion have been examined under an optical microscope and related to the “activation” of the surface with respect to attrition.A descriptive model of combustion assisted attrition of particles of a petroleum coke burning in a fluidized bed has been proposed. It is based on the assumption that the coke particle is divided into elements burning at different rates, because of their different accessibility to oxygen. Elements reacting more slowly emerge as asperities on the external surface of the particle. They eventually burn or are abraded by bed solids forming, in this latter case, the attrited fines. The model provides a coherent framework for all the experimental data. In particular, it gives an explanation of the typical shape of attrition curves with a maximum somewhere between the beginning of the burning and burn-out, in terms of the competing effects of the “activation” of the particle surface to form detachable asperities and its reduction with burn-off.