Applicability of an Equilibrium Model to Predict the Conversion of CO₂ to CO via the Reduction and Oxidation of a Fixed Bed of Cerium Dioxide

Production of hydrogen and synthesis gas via solar thermochemical partial redox cycles is one route to renewable fuels and storage of solar energy. The efficiency at which these cycles produce fuel for candidate non-stoichiometric metal oxides and reactor concepts is normally evaluated from thermodynamic models that implicitly assume that the transport processes and reaction kinetics are rapid enough that the gas and solid attain chemical equilibrium. In this paper, we develop an equilibrium model of a fixed-bed reactor and demonstrate its applicability for reduction and oxidation of porous ceria (CeO₂) particles with a volume-specific surface area of ∼10⁶ m² m^-3 over a wide range of gas flow rates and reaction temperatures. The model predicts the measured rate of O₂ production during reduction for mass-specific flow rates up to 900 mL min^-1 g_ceria^-1 and for temperatures from 740 to 1500 °C, and it predicts the measured rate of CO production during oxidation for flow rates up to 50 mL min^-1 g_ceria^-1 at 1500 °C. It does not apply for oxidation below 930 °C. We compare the equilibrium model developed for the fixed-bed reactor to the models for the mixed flow and countercurrent flow reactors. In comparison to the mixed flow reactor, the fixed-bed reactor reduces the sweep gas and excess oxidizer required for fuel production, an important step toward increasing efficiency closer to the theoretical limit established by the countercurrent flow reactor.