Effect of equivalence ratio on the CO selectivity of Fe/Ca-based oxygen carriers in biomass char chemical looping gasification

Abstract

Equivalence ratio is one of the important operating parameters for biomass gasification. The oxygen carrier possesses high producer gas production in a wide range of equivalence ratio (oxygen carrier/biomass ratio), which is exceedingly desirable for biomass chemical looping gasification (CLG). In this study, the effect of different equivalence ratios on the CO selectivity (the evolved gas composition) of Fe2O3, Fe2O3/CaO and CaFe2O4 oxygen carriers in biomass char CLG was investigated in a fixed bed reactor. The results showed that the CO selectivity of CaFe2O4 had little changes and decreased from 70.39% to 59.79% and 73.40% to 63.98% with increasing equivalence ratio from 0.5 to 3.0 at 900 °C and 1000 °C, respectively. In sharp contrast, the CO selectivity of Fe2O3 changed markedly, which declined respectively from 65.86% to 31.13% (900 °C) and 69.68% to 47.27% (1000 °C). Fe2O3/CaO illustrated the similar tendency as Fe2O3 at 900 °C but as CaFe2O4 at 1000 ℃. The three oxygen carries exhibited a high CO production at lower equivalence ratio, and CaFe2O4 displayed the best performance of CO selectivity in the given equivalence ratio range. X-ray diffraction (XRD) spectrums showed that the oxygen carriers were transformed to different iron phases with difference equivalent ratios. The Fe (III) in the three oxygen carries was reduced completely to Fe (0) at the equivalence ratios of 0.5 and 1.0, while FeO and Ca2Fe2O5 were the dominant species in the spent oxygen carriers as equivalence ratio was further increased. The trends of CO selectivity obtained by thermodynamic analysis for three oxygen carries were consistent with the experimental results.