Fluidized bed combustion of a biomass char (Robinia pseudoacacia)

Fluidized bed combustion of char from a biomass, Robinia pseudoacacia, was investigated in a bench scale combustor. Different experimental techniques have been adopted to characterize the combined role of combustion and comminution phenomena (primary, secondary, and percolative fragmentations, attrition by abrasion) in determining fixed carbon conversion and the rate of carbon elutriation. Comparison of experimental results obtained under steadily oxidizing conditions and under alternating oxidizing/inert conditions suggested mechanistic aspects of the fluidized bed combustion of biomass char. Fixed carbon combustion was almost always complete. Conversion occurred to a large extent via the generation of carbon fines followed by postcombustion during their residence time in the bed. Approximately half of the initial fixed carbon followed this pathway, the remainder being directly burnt as coarse char. The prevailing mechanism of carbon fines generation in the bed was percolative fragmentation rather than attrition by abrasion. In spite of the extensive generation of elutriable carbon fines, the combined effect of high fuel reactivity and of relatively long fines residence times in the reactor determined the large combustion efficiency. It is inferred from experimental results that char fines adhesion onto bed solids might be relevant to the observed phenomenology.