Removal of tar by secondary air in fluidised bed gasification of residual biomass and coal

This study has shown that injecting secondary air to a fluidised-bed at temperatures over 830-850 degrees C decreases the tar contents of the gasifier off gas. Tests carried out at a laboratory scale show that the formation of tars strongly depends on the raw material type and the operating conditions, especially the gasification temperature. It is also shown that an optimum ratio of secondary to primary air is about 20% for forest residue and about 10% for lignite, obtaining a tar removal efficiency of about 90 and 80%, respectively. The offgas composition and its low heating value were also analysed with and without secondary air injection for forest residue gasification. CO, H-2, gaseous hydrocarbon contents and the low heating value of the gas decreased and N-2 increased with secondary air. However, the CO2 contents kept at a close level, 16.5-18.5% when increasing the secondary air ratio from 0 to 50 vol.%. At the optimum ratio for the secondary to primary air, a stoichiometric secondary air gasification is proposed to estimate the mixed gas composition. The expression developed in this study is independent of the intermediate stages of the process and it is applicable to forest residue gasification. (C) 1999 Elsevier Science Ltd. All rights reserved.