Air-steam gasification of biomass in a fluidized bed under simulated autothermal and adiabatic conditions

Biomass gasification tests with air-steam mixtures were conducted in a bubbling fluidized-bed (FB) gasification pilot plant. The effect of steam addition on the performance of gasification was studied for different air-to-biomass ratios and biomass throughputs. The reactor was operated near adiabatic conditions, with the aim of reproducing the behavior of full-scale FB units. The temperature of the air-steam mixture was fixed at 400 'C for all tests, a value that can be achieved by energy recovery from the off-gas without tar-condensing problems. The stoichiometric ratio (ratio of actual to stoichiometric air flow rates) and steam-to-biomass flow rate ratio were varied from 0.19 to 0.35 and from 0 to 0.45, respectively. It is shown that the proper selection of operating conditions makes it possible to increase the gasification efficiency from 40% to 60%, while maintaining the low heating value of the gas at around 5 MJ/Nm(3). In addition, an appreciable increase of char and tar conversion is achieved. The air-steam gasification concept explored in this work is an interesting option for the improvement of direct air-blown biomass gasification because it can improve the process efficiency considerably without significantly increasing capital costs.