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

Abstract

In this paper, experimental investigations on oxy-steam gasification of biomass in downdraft 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/m2s) covering ‘gasification’ and ‘combustion’ regimes with O2 fractions of 23, 30 and 40% by mass (rest steam). By restricting the upstream bed temperatures between 120 °C (to avoid steam condensation) and 150 °C (to prevent bulk devolatilization of bed), the intrinsic H2 yield from biomass is determined over an equivalence ratio (Φ) range of 3.5 to 1.2. This is shown to correspond to a ‘volatiles’ based equivalence ratio (ϕv) of 2.2 to 1. Interestingly, the H2 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 H2 source when ϕv > 2 and as ϕv → 1, ‘volatiles’ H2 drops close to zero and the major contribution is through the reaction, C + H2O → CO + H2. Increase in char conversion from about 20% at ϕv ∼2.1 to almost 100% as ϕv → 1, the corresponding increase in peak bed temperature and decrease in ‘higher hydrocarbons’ are consistent with the observed H2 yield. The important insight is, operating close to ϕv = 1, under slightly rich condition, leads to tar free exit gas with little or no compromise on H2 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 (∼800 °C), tolerating higher tar content.