Model of transport and chemical kinetics in a solar thermochemical reactor to split carbon dioxide

Solar thermochemical reactors to carry out the nonstoichiometric reduction and oxidation of cerium dioxide (ceria) to split water and carbon dioxide provide a pathway to store sunlight in a chemical fuel. One of the challenges in the design of these reactors is understanding the complex coupling of heat and mass transfer and redox chemistry. To elucidate this coupling, we present a three-dimensional, transient model of a recently developed prototype solar reactor that implements an isothermal, pressure-swing ceria redox cycle. Radiative transport is modeled by a hybrid Monte Carlo/finite volume approach and paired with the transport and chemical processes within a fixed bed of porous ceria particles. Morphology specific reaction rate coefficients for the gas-solid reactions in ceria are extracted for the first time from global rate measurements in a bench-top reactor at 1773 K. Results demonstrate the interdependent spatial and temporal variations in temperature, species concentration and reaction rates, and provide insight on the effects of optical properties on reactor performance. For a solar input of 4.2 kW, the reactor achieves nearly isothermal cycling at 1791 K with carbon monoxide produced continuously at 3.6×10−4 mol s−1. At this temperature, global reaction rates are driven by advective mass transport rates and the intrinsic material thermodynamics. Predicted surface temperatures and fuel production rates compare favorably to measured data.