Maximizing fuel production rates in isothermal solar thermochemical fuel production

Production of chemical fuels by isothermal pressure-swing cycles has recently generated significant interest. In this process a reactive oxide is cyclically exposed to an inert gas, which induces partial reduction of the oxide, and to an oxidizing gas of either H₂O or CO₂, which reoxidizes the oxide, releasing H₂ or CO. At sufficiently high temperatures and sufficiently low gas flow rates, both the reduction and oxidation steps become limited only by the flow of gas across the material and not by material kinetic factors. In this contribution, we develop a numerical model describing fuel production rates in this gas-phase limited regime. The implications of this behavior are explored under all possible isothermal pressure-swing cycling conditions, and the outcome is optimized in terms of fuel production rate as well as fuel conversion and utilization of input gas of all types. Fuel production rate is maximized at infinitesimally small cycle times and attains a value that is independent of material thermodynamics. Gas utilization is maximized at infinitesimally small gas inputs, but the values can be made independent of cycle time, depending on manipulation of flow conditions. Gas-phase conditions (temperature, oxidant and reductant gas partial pressures, and CO₂ vs H₂O as oxidant) have a strong impact on fuel production metrics. Under realistic, finite cycle times, material thermodynamics play a measurable role in establishing fuel production rates.