Residence time of fine particles in the fluidized bed combustion of a biomass

The ordinary and sound-assisted steady fluidized bed combustion of a biomass powder (Robinia Pseudoacacia) has been studied. The parameters monitored were: the carbon elutriation rate, the carbon loading in the bed and the efficiency of fixed carbon conversion. The application of sound (150dB and 120Hz) was found to reduce carbon elutriation rate by a factor of about 1.5 and to increase fixed carbon conversion efficiency by 2-3%. An extremely small apparent fines elutriation constant was found, consistent with an average fines residence time in the bed of tens of minutes. Results have been analyzed within a theoretical framework based on the hypothesis that fine particles are present in the bed either as freely moving fines or as fines attached onto inert bed particles. Accordingly the fate of fixed carbon in the bed depends on the competition between: elutriation of free fines, adhesion and of elutriable fines onto bed material, generation of elutriable fines by attrition of fine-coarse aggregates and combustion of fines. Experimental and theoretical results converge in indicating that Robinia fines are characterized by extensive adhesion onto bed particles, probably due to stickiness of the ash at temperatures relevant to fines burning.