Mn-doped Ca2Fe2O5 oxygen carrier for chemical looping gasification of biogas residue: Effect of oxygen uncoupling

Abstract

Anaerobic digestion is an important approach to treat and utilize kitchen waste, while kitchen waste derived biogas residue (BR) disposal remains a serious problem in this process. This study investigated the feasibility of chemical looping gasification (CLG) as an efficient way to treat and generate syngas from BR. Besides, Mn-doped Ca2Fe2O5 (MCF) as a novel oxygen carrier with mild oxygen uncoupling capability was developed, and its performance against traditional Ca2Fe2O5 (CF) oxygen carrier was compared. Results indicated that, significant oxygen release capability was observed on the prepared MCF oxygen carrier. In the gasification tests, CLG with MCF at 800 ℃ showed optimal total gas yield, carbon conversion, and cold gas efficiency, which increased 0.08 Nm3/kg, 17.22%, and 13.37% compared to CF, respectively. According to the XPS, H2-TPR and XRD characterization results, the optimal performance of MCF could be attributed to the oxygen uncoupling characteristic brought by Mn addition. The formation of the oxygen uncoupling capability of MCF could be explained by three reasons: (1) Mn in MCF mainly existed in form of Mn3+ and Mn4+, which resulted in the extremely strong oxidability at a temperature as low as 250 ℃. (2) The higher Fe3+/Fe2+ ratio of 1.55 in MCF than CF caused by Mn addition led to greater oxidability of Fe ions. (3) MCF had a higher lattice oxygen amount than CF at about 3.86%, and the lattice oxygen in its crystal structure had been transformed into active lattice oxygen species, which were between lattice oxygen (O2–) and atomic adsorbed oxygen (O−). As a result, this paper investigated the potential of CLG to treat and utilized BR, and the superiority of CLG with oxygen uncoupling (CLGOU) on MCF was highlighted. It’s hoped that this study could provide a fundamental knowledge on CLG and CLGOU of BR, and benefit to better downstream process design of anaerobic digestion industry.