Exploring the regulation mechanism of Ca/Fe-based oxygen carrier in biomass chemical looping gasification

Biomass chemical looping gasification (CLG) is an efficient gasification technology, which enables the conversion of low-grade biomass into high-value hydrogen-rich syngas. Four Ca/Fe-based oxygen carriers with different molar ratios of Ca:Fe are synthesized using the sol-gel method, and the regulation mechanism of oxygen carriers is comprehensively investigated for the purpose of producing hydrogen-rich syngas during the biomass CLG process. First, these Ca/Fe-based oxygen carriers exhibit selective oxidation capabilities towards the pyrolysis and gasification products of biomass. The addition of Ca element significantly enhances the H-2 concentration, total yield, selectivity, and H-2/CO ratio of syngas. Second, the lattice oxygen and catalytic properties of oxygen carriers exert a significant influence on the conversion rate of biochar. In the later stages of CLG, the catalytic effect becomes prominent, meanwhile the contribution of active lattice oxygen diminishes. XRD analysis reveals that Ca2Fe2O5 exhibits the most effective catalytic ability for biochar conversion. Third, it is revealed that the catalytic effect of partially reduced oxygen carriers on the water-gas shift reaction, as well as the participation of deeply reduced oxygen carriers in the steam-iron reaction, together regulates the composition of syngas. The addition of Ca element significantly enhances their catalytic ability in the water-gas shift reaction. At equivalent temperatures, the Ca2Fe2O5 oxygen carrier achieves the highest CO conversion, demonstrating its advantages at producing hydrogen-rich syngas in CLG. Surprisingly, the Ca/Fe interaction significantly accelerates the steam-iron reaction rate. Additionally, the multiple cyclic redox experiments based on batch fluidized beds confirm the stable physicochemical characteristics of the Ca2Fe2O5 oxygen carrier, highlighting its potential for long-period operation and future industrial-scale implementation.