Experimental measurements of combustion of simulated coal volatiles in a bubbling fluidized bed combustor

The combustion of volatiles has been studied experimentally in a laboratory-scale bubbling fluidized bed combustor, using a novel approach. To eliminate the complication of simultaneous char combustion, a ‘simulated’ coal particle was developed, which devolatilizes in a manner similar to coal but leaves behind an inert, non-combustible particle. When these particles were fired continuously in the combustor, in-bed volatiles combustion efficiencies did not vary greatly with changes in operating conditions, ranging from 45% to 66%. A unique combined oxygen/bubble probe was developed to provide in-bed measurements relating the volatiles combustion process to the hydrodynamics of the bed. The detailed in-bed data were particularly illuminating and showed that the previous interpretation of oxygen probe signals as differences in oxygen concentration between the bubble and particulate phases was incorrect. Volatiles were observed mainly in the particulate phase and gas mixing within bubbles was poor. A large number of separate volatile regions were observed in the bed, with higher frequency towards the top surface of the bed. The results provide a consistent set of both in-bed (i.e. from the combined probe) and overall (i.e. heat balance) data, against which models for volatiles combustion may be tested.