Mn-doping Ca2Fe2O5-based oxygen carriers in chemical looping gasification of biogas residue: A comprehensive study on the reaction mechanisms

Abstract

The disposal of solid residues from the anaerobic digestion to generate biogas remains a serious challenge, which can be beneficially solved to generate the syngas based on gasification technologies. Among of them, chemical looping gasification (CLG) of biogas residues likely achieves the high-quality syngas production using the oxygen carriers (OCs). Since the nature of the solid–solid contact in CLG reactions resulting in greatly worsening reaction rates and syngas product quality, the development an oxygen-uncoupling type OCs is highly demanding. This study introduces the preparation, characterizations and performance evaluation on Bi-functional Mn/Ca2Fe2O5 oxygen carriers (OCs) achieving the oxygen-uncoupling and catalytic CO2 conversion to CO. The Mn-doping Ca2Fe2O5-based OCs with Mn in 20 mol.% (20MnCaFe) achieves an optimal gasification efficiency, carbon conversion efficiency in 72.31% and 95.72% respectively with lower CO2 yield. The intensive characterization of used OCs revealed that the proper Mn-doping helped on the increase of d-spacing, corresponding to more high-charge Mn ions and improved oxygen-ion migration and oxygen activation, and thus greater oxygen evolution reaction performance, and the incorporation of Mn in Mn/Ca2Fe2O5, leading the structural distortion of octahedral Fe-O bond and the stronger oxycarbide adsorption ability. Studies confirmed the correspondence of 20MnCaFe to its capabilities of optimal oxygen uncoupling, catalytic conversion of CO2 to CO and the coordinated oxidability, resulting in the improvement of syngas quality. A clear reaction pathway involving Mn/Ca2Fe2O5 in CLG is exhibit, that is oxygen uncoupling, CO2 catalytic conversion, the regeneration of oxygen uncoupling vacancy and finally recoverable oxygen uncoupling.