Pilot-scale investigation and reactor modelling of chemical looping reverse water-gas shift reaction: co- versus countercurrent operation

Addressing climate change requires innovative approaches that can significantly reduce industrial carbon emissions while promoting the sustainable use of resources. This study investigates the chemical looping reverse water-gas shift reaction (CL-rWGS) in a kg-scale pilot reactor using iron oxide-based oxygen carriers, comparing cocurrent and countercurrent operational modes in terms of CO₂ conversion and reactor productivity, to reveal the extent of kinetic or thermodynamic limitations. Experimental results demonstrate that countercurrent operation consistently achieves higher average CO₂ conversions than cocurrent operation, reaching 46 % for a 20-min reduction duration, compared to 36 % in cocurrent mode. Notably, the countercurrent configuration temporarily surpasses the gas-phase equilibrium conversion of the rWGS reaction, while achieving a productivity rate of 2.9 mol_CO/kg_OC/h. The superior performance of countercurrent operation stems from the sequential interaction of CO₂ with Fe²⁺ before Fe⁰, thereby ensuring more efficient utilization of the oxygen carrier. Additionally, lowering the oxidation half-cycle duration enhances the average CO₂ conversion. The experimental pilot reactor data is best described by a model in which (1) equilibrium constraints are applied to CO₂ conversion profiles and oxidation behavior, yielding equilibrium plateaus which dictate the conversion levels, and (2) kinetic limitations merely influence the transitioning rate between these equilibrium plateaus. The reactor model exhibits strong agreement with countercurrent experimental data, while deviations are observed in the cocurrent mode, likely due to mass transfer limitations or unaccounted reaction dynamics. Overall, this study highlights countercurrent operation as a promising strategy for optimizing CL-rWGS.