Synergy of red mud oxygen carrier with MgO and NiO for enhanced chemical-looping combustion

Low-cost oxygen carriers prepared by natural ores or solid wastes have been attracted much attention for large-scale application in chemical looping combustion (CLC) technology. However, these oxygen carriers usually exhibit low activity and poor stability because of their unsatisfied microstructure and/or components. In this study, red mud as oxygen carrier was modified by active (NiO or MnOx) and inert oxides (MgO, Al2O3 or ZrO2) to optimize its structure and composition for CLC of methane. The synergy among different components in the oxygen carriers for methane oxidation was discussed in detail. The results showed that the effect of MnOx on the reducibility of red mud was relatively weak, while the presence of a suitable amount of NiO significantly promoted the reactivity for methane complete oxidation. Addition of 15 wt% NiO could increase the average CH4 conversion and CO2 selectivity from 53% to 65%–75% and 99%, respectively, which is beneficial for the following capture of CO2 after combustion. Moreover, the addition of MgO into the NiO-RM carrier could improve the stability of oxygen carrier for successive chemical looping process. The formation of NiFe2O4 and MgNiO2 in the oxygen carrier contributed to the enhanced reducibility, and the multiple interactions among Fe, Ni and Mg oxides promoted the overall performance of oxygen carriers. During the reaction with methane, Ni oxides were firstly reduced to metal Ni, which acted as a catalyst to activate methane, improving the oxidation rate of methane by oxygen carriers.