Scale effects on the agglomeration behavior during FB combustion of a biomass

The fluidized bed combustion of a biomass residue (pine seed shells) was investigated both in a bench-scale and in a pilot-scale reactor. Extensive and systematic bed agglomeration problems were experienced during combustion of this fuel, as a consequence of the high potassium content in the ash. The focus of the study was the effect of the combustor scale on the characteristic time and the extent of bed agglomeration during combustion. Bed agglomeration times as well as temperature and pressure gradients were measured at different operating conditions on both reactors. An already developed diagnostic tool based on the measurement of the dynamic pressure signal inside the bed was successfully tested at the larger scale for its capability to predict the bed agglomeration onset. SEM/EDX analysis on agglomerate samples discharged from the bed confirm that bed agglomeration/defluidization is a consequence of a potassium enrichment on the sand particle surface. This enrichment, in conjunction with high temperature spots, leads to local melting and particle aggregation.