Influence of biomass ashes on the structural evolution and oxygen-donating capacity of red mud during chemical looping gasification.

To efficiently convert the biomass wastes, chemical looping gasification has been proposed as a highly effective method to produce hydrogen-rich syngas. The lattice oxygens in oxygen carriers can supplant molecular oxygens, thereby improving syngas quality. However, the complex interactions between oxygen carrier and biomass ash are inevitable, obviously affecting reactivity of oxygen carrier. In this work, red mud, a solid waste rich in Fe2O3, was selected as oxygen carrier and biomass feedstocks with distinct differences in ash composition and content were used. The findings demonstrate a robust correlation between the fluctuations in gasification performance of biomass chars and the composition and quantity of ash. Specifically, the high concentration of alkali and alkaline earth metals in corn cob ash showed a substantial synergistic effect with red mud. Under the erosion of K+ and H2O, numerous cracks appeared on the surface of corn cob and enhanced reaction rate. In contrast, SiO2 in soybean straw ash reacted with Al2O3 in red mud to form aluminosilicate, exacerbating agglomeration and sintering of red mud. Based on the analyses of XRD and XPS, it is proved that the structural transformation of Fe2O3 follows the consecutive process of Fe2O3 → Fe3O4 → FeO → Fe → Fe3O4. Initially, Fe3+ interacted with biomass char and then was reduced to Fe2+ or Fe. This transformation occurred during the early stages of gasification, rapidly generating a significant volume of CO and H2. In the subsequent stages, water compensated for the depleted lattice oxygens, and iron elements eventually stabilized in their final form as Fe3O4.