A Chemical Approach to Biomass Gasification

This paper describes GASDS, a comprehensive model of a biomass gasifier on a travelling grate. This general mathematical model deals with the complex multiphase gas¬solid problem of biomass gasification both at the reactor and the particle scale. Mass and energy balances plus proper closure equations describe the behavior of both the phases. Key elements of the model are the detailed kinetic schemes describing the biomass volatilization, including the formation of TAR species and char, and the secondary gas phase as well as the heterogeneous reactions. The model has been already tested and validated on semi¬quantitative basis in comparisons with a 12 MW biomass combustor operating in Belgium. In this paper, the model is extended and compared to recent experimental data obtained in a biomass regenerative gasifier. The comparison with experimental data highlights the role of devolatilization, mass transfer phenomena, and mainly the kinetics of char gasification in controlling system's reactivity and syngas composition. An alternative configuration for biomass gasification is also presented and discussed. It is based on three different steps, including biomass pyrolysis through the convective heating of a syngas recycle stream, followed by the oxidation of volatile products (gas and tar compounds) with an air/steam mixture and a final char gasification. The advantage of the proposed configuration lies in the direct oxidation of the released tars to form the hot stream used for char gasification.