Bed agglomeration problems in fluidized-bed biomass combustion

Bed material agglomeration was studied experimentally in a fluidized-bed biomass combustor, Four biomass fuels, four bed materials, three bed additives, and three NOx additives were tested in a temperature range of 670-870 degreesC and at two pressure levels of 1.0 and 1.5 MPa. Two types of agglomeration were observed, a homogeneous and a heterogeneous type. The first occurred at low temperature and could be partly compensated for by erosion of the bed. The second took place at high temperatures and often involved interaction between the fuel ash and the bed material. The immobility of the bed particles made the heterogeneous agglomeration a self-accelerated process. The occurrence of hot spots in the bed was the precondition for heterogeneous agglomeration being induced. When silicon was present, alkali metals were the main contributors to heterogeneous agglomeration. Aluminum and iron compounds were able to suppress agglomeration through the high melting point of the eutectics that were formed.