Combustion and Attrition of Biomass Chars in a Fluidized Bed

The fluidized bed combustion of char from three different biomass fuels, pine seed shells, olive husk, and wood chips, was investigated in a bench scale combustor. A combination of experimental techniques was used to characterize the relevance of attrition phenomena during the combustion of the chars and their impact on the fuel particle size distribution and overall carbon conversion. Results showed that, depending upon the biomass, extensive primary and secondary fragmentation could be experienced by the char particles, significantly influencing the particle size distribution of the fuel in the bed. This is the result of the mechanical properties of the raw fuel particles and the large porosity of the char after devolatilization. Char conversion closely followed the shrinking-particle constant-density model and occurred to a large extent via the generation of carbon fines by percolative fragmentation followed by postcombustion during their residence time in the bed. Approximately 25-45% of the initial fixed carbon followed this pathway, with the remainder being directly burnt as coarse char. This resulted in an enhancement of the combustion rate of the char particles. The significance of the particle shape was also highlighted in the evaluation of the particle-exposed surface. Finally, indications were given on how to account for attrition phenomena in a simplified population balance on biomass char.