Fluidized-bed combustion of a biomass char: The influence of carbon attrition and fines postcombustion on fixed carbon conversion

The fluidized-bed combustion of char from a biomass fuel (Robinia Pseudoacacia) has been studied with a focus on the fate of fixed carbon under the combined effects of coarse char combustion and attrition, of attrited carbon fines postcombustion and elutriation. Extensive postcombustion under oxidizing conditions prevents the measurement of the rates of carbon fines generation by attrition from collection of elutriated carbon. A novel technique is hereby presented to overcome this problem: the rates of coarse char attrition are evaluated from the comparison of the apparent carbon conversion rate of the actual test fuel, with that of a reference fuel, selected among those which have a negligible propensity to attrition. Results from the application of the proposed technique indicate that extensive fines generation by attrition, followed by their almost complete postcombustion, occurs during fluidized-bed combustion of Robinia char. About half of the fixed carbon burns along this pathway, the remainder being directly burnt as coarse char carbon. The occurrence of extensive attrition-postcombustion leads to enhancement of the coarse char particles' apparent Sherwood numbers by a factor of about two. Analysis of the dependence of the ratio between the fines generation rate and the parallel coarse char combustion rate on operating variables of the reactor is directed to shed light on the prevailing mechanism of carbon fines generation by attrition. It appears that, differently from low-volatile solid fuels, the percolative fragmentation mechanism plays a leading role on carbon attrition.