Fluidized bed combustion of biomass and waste-derived pelletized fuels

The fluidized bed combustion of three biogenous fuels (two pelletized biomasses and one mechanically dewatered sludge) was investigated in two bench scale reactors. A combination of experimental techniques was used to characterize the patterns of fuel devolatilization and char conversion, the relevance of attrition and fragmentation and their impact on the fuel particle size distribution, as well as the primary ash particle size distribution. Results showed that all the three fuels were characterized by a relatively long devolatilization time and by limited primary fragmentation. Therefore, the use of pelletized fuels can provide a means to ensure effective lateral distribution of volatile matter across the combustor cross-section. On the other hand, the three fuels exhibited very different char conversion patterns. The high-ash sludge burned according to a shrinking core conversion pattern with negligible secondary fragmentation. The low-ash pelletized wood burned according to the shrinking particle conversion pattern with significant secondary fragmentation. The medium-ash pelletized straw exhibited a unique combustion pattern, that we referred to as "cenospheric combustion". The ash particles exhibited a hollow structure, resembling cenospheres, with a coherent inorganic outer layer strong enough to prevent particle fragmentation. Inert bed particles were found to be attached to the ash pellets, because of significant melting or softening of the ash. The carbon elutriation rate was very low for all the three fuels, reflecting the large intrinsic reactivity of char fines. As a consequence, the impact of carbon attrition on combustion efficiency was negligible. Consistently with the combustion patterns reported before, the primary ash particle size distribution of the pelletized sludge reflected the original size of the parent fuel, that of the pelletized wood showed a large fraction of fine particles, while that of the pelletized straw showed the occurrence of ash agglomeration phenomena.