GASIFICATION OF TWO BIOMASS FUELS IN BUBBLING FLUIDIZED BED

The gasification of two biomass fuels, beech wood and a dry granular sewage sludge from the Swiss Combi? process, has been experimentally studied. The gasification was carried out in a laboratory scale ABFB facility, operated at steady state. The fuels both have a high content of volatile matters but differ principally for their ash content, which is approximately 1% and 56% in beech and sewage sludge, respectively. The attention was focused on the presence of particulate and tar in the producer gas which affect the process efficiency and give negative drawbacks in the utilization in motors or turbines for power generation. The influence of operating variables (i.e. process temperature and equivalence ratio) on the gasification performances was explored. Results show that the composition of the producer gas is quite independent of whatever fuel is gasified. As far as sewage sludge is concerned, process performances are poorer and steady state operation is difficult, because of the high elutriation rate of fines, the continuous increasing of the bed height and the interaction between ash and bed materials. The tar yield was always high for both fuels. Unexpectedly, the gasification of a blend of two biomasses gave a minimum tar yield, probably ascribed to a catalytic effect. A difference was found comparing the comminution behavior of beech wood and dry sewage sludge. The former undergoes fragmentation with the generation of relatively large, elutriable fragments; the latter is principally subjected to mechanical abrasion, leading to a finer elutriable particulate, since the residual fuel particle after the devolatilization conserves a good mechanical resistance.