Apparent particle Sherwood numbers in the fluidized bed combustion of biomass and coal chars

Combustion of chars from a biomass fuel and from a bituminous coal has been carried out in a laboratory scale fluidized bed reactor with the aim of evaluating apparent particle Sherwood numbers. Comparison of the carbon conversion rate of the biomass with that of the bituminous coal indicated that extensive attrition of the biomass char particles and subsequent in-bed postcombustion of the fines generated occurred. This resulted in the enhancement of the particles apparent Sherwood numbers by a factor of about 2 in respect to the (true) Sherwood numbers evaluated for the bituminous coal char under equal combustion conditions. Half of the fixed carbon of the biomass char burnt along this pathway, the remainder being directly burnt as coarse char. True Sherwood numbers obtained from experimental data relative to the bituminous coal have been then further compared with those calculated according to available literature correlations. The comparison shows that some of the most widely used correlations in literature predict incorrect values and trends of Sh with the operating variables of the combustor. Experimental results point at some issues related to the determination of Sherwood numbers for application to fluidized bed combustion. Literature correlations give predicted Sherwood numbers that widely differ from each other. Furthermore apparent values of Sh can be dramatically affected by the occurrence of particle attrition phenomena during conversion.