Intrinsic hydrogen yield from gasification of biomass with oxy-steam mixtures

In this paper, experimental investigations on oxy-steam gasification of biomass in down-draft packed bed reactor are presented; propagation regimes and the associated hydrogen yield will be the focus of the current work. Steady flame propagation is shown to be established for a range of oxygen mass flux (16-120 g/m(2)s) covering 'gasification' and 'combustion' regimes with O-2 fractions of 23, 30 and 40% by mass (rest steam). By restricting the upstream bed temperatures between 120 degrees C (to avoid steam condensation) and 150 degrees C (to prevent bulk devolatilization of bed), the intrinsic H-2 yield from biomass is determined over an equivalence ratio (Phi) range of 3.5 to 1.2. This is shown to correspond to a 'volatiles' based equivalence ratio (phi(v)) of 2.2 to 1. Interestingly, the H-2 yield over this entire range is within 30-40 g/kg of biomass. Using equilibrium calculations, it is shown that the 'unburnt volatiles' is the major H-2 source when phi(v) > 2 and as phi(v) -> 1, 'volatiles' H-2 drops close to zero and the major contribution is through the reaction, C + H2O -> CO + H-2. Increase in char conversion from about 20% at phi(v) similar to 2.1 to almost 100% as phi(v) -> 1, the corresponding increase in peak bed temperature and decrease in 'higher hydrocarbons' are consistent with the observed H-2 yield. The important insight is, operating close to phi(v) = 1, under slightly rich condition, leads to tar free exit gas with little or no compromise on H-2 yield. This hitherto unknown result is perhaps the reason why all earlier works focused only on highly fuel rich conditions and/or very high steam temperatures (similar to 800 degrees C), tolerating higher tar content. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.