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

Abstract

This article investigates the potential of biomass gasification to utilize CO2 as feed with the conventional air-gasification system for the conversion of carbondioxide to carbonmonoxide and thus enable the participation of CO2 in the fuel-making process. The effects air-CO2 gasification are examined in a reverse downdraft gasifier at varying total flux with constant CO2-N2 molar ratio (XCO2/XN2∼0.2) and at constant air flux with varying CO2 flux. The set of experiments show the suitability of CO2 reuse in biomass gasification process and identifies the maximum possible CO2 conversion for air gasification. A direct consequence of air-CO2 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-CO2 gasification at XCO2/XN2∼0.2, the maximum CO2 conversion is observed at a reactant flux of 0.134 kg m−2 s−1, above which the additional CO2 does not engage in the formation of CO. In this regard, the CO2 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. CO2 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-CO2 gasification, the specific energy output increases owing to lower biomass flux and higher carbon conversion. The observations for air-CO2 gasification at XCO2/XN2∼0.2are also supported by analyzing the yields and conversion at fixed air flux and varying CO2 flux. The choice of reactant fluxes used in this study enables the selection of appropriate fluxes in the scale-up process.