Experimental investigations of air-CO2 biomass gasification in reversed downdraft gasifier

Abstract This article investigates the potential of biomass gasification to utilize CO 2 as feed with the conventional air-gasification system for the conversion of carbondioxide to carbonmonoxide and thus enable the participation of CO 2 in the fuel-making process. The effects air-CO 2 gasification are examined in a reverse downdraft gasifier at varying total flux with constant CO 2 -N 2 molar ratio ( X CO 2 / X N 2 ∼ 0.2 ) and at constant air flux with varying CO 2 flux. The set of experiments show the suitability of CO 2 reuse in biomass gasification process and identifies the maximum possible CO 2 conversion for air gasification. A direct consequence of air-CO 2 gasification is observed on the biomass flux, with a 20–30% reduction in the biomass consumption rate compared to air-gasification mode. In the case of air-CO 2 gasification at X CO 2 / X N 2 ∼ 0.2 , the maximum CO 2 conversion is observed at a reactant flux of 0.134 kg m −2  s −1 , above which the additional CO 2 does not engage in the formation of CO. In this regard, the CO 2 conversion is limited by the char yield, which tends towards its complete utilization with an increasing flux and is designated as the zone of carbon boundary point. CO 2 injection results in char consumption by the diffusion dominated oxidation and the kinetic controlled Boudouard reaction, resulting in higher char conversion compared to air gasification. Moreover, though the gas calorific value decreases by 10–25% for air-CO 2 gasification, the specific energy output increases owing to lower biomass flux and higher carbon conversion. The observations for air-CO 2 gasification at X CO 2 / X N 2 ∼ 0.2 are also supported by analyzing the yields and conversion at fixed air flux and varying CO 2 flux. The choice of reactant fluxes used in this study enables the selection of appropriate fluxes in the scale-up process.