Enhanced Performance of Alkali Metal Doped Fe2O3 and Fe2O3/Al2O3 Composites As Oxygen Carrier Material in Chemical Looping Combustion

In this study alkali metal doped Fe2O3 and Fe2O3/Al2O3 composite oxygen carriers were synthesized from spray pyrolysis, and the reactivity and stability as oxygen carrier materials were evaluated in a fixed bed reactor for 50 isothermal redox cycles using CH4 as the fuel. We find that both Fe2O3 and Fe2O3/Al2O3 composite showed reactivity degradation over multiple cycles, with a clear phase separation between Fe and Al, in the composites. In contrast, alkali metal doping (similar to 5 mol %) with Na, K, and Cs was found to stabilize the reactivity of the Fe2O3 over the 50 redox cycles and prevent phase Fe-Al separation in the composite. Methane to CO2 selectivity was found to be relevant to the dopant type, which decreased in the order of dopant type K, Cs, and Na. The best performing alkali metal, K, enhanced long-term stability significantly, with no observed degradation in kinetics and total conversion performance in the methane step as well as reduced coke formation. Adding an alumina matrix to K doped Fe2O3 helped promote CO2 generation as well as minimize coking and was found to be the best performing material.