Influence of the Stoichiometric Ratio on Tar Composition during Fluidized Bed Gasification

Abstract

Fluidized bed gasification is a promising technology for the energetic valorization of biomass and wastes. However the presence of heavy tars limits the use of the gas to applications where the gas is not cooled. Several parameters such as the temperature profile inside the gasifier and the gas residence time influence the concentration and composition of the tar mixture and thus the gas dew point. In autothermal air-blown fluidized bed gasifiers the stoichiometric ratio (fed air/stoichiometric air) determines the reactor temperature but the influence over the tar composition depends on the competition of the different fractions (light gas, char and tar) for the oxygen available. This paper analyzes the tar composition in the gas produced during the conversion of biomass pellets in a fluidized bed reactor when the stoichiometric ratio is raised from 0 (pyrolysis) to 0.21 (standard autothermal gasification with air). The tests were conducted at 800°C and 900°C and constant gas residence time and volatiles/carrier gas ratio. The results are discussed and compare with previous literature works, explaining some aspects of existing tar conversion models for biomass gasification.