Maximizing fuel production rates in isothermal solar thermochemical fuel production

Abstract

Production of chemical fuels by isothermal pressure-swing cycles has recently 12 generated significant interest. In this process a reactive oxide is cyclically exposed to an inert gas, 13 which induces partial reduction of the oxide, and to an oxidizing gas of either H2O or CO2, which 14 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. KEYWORDS. Solar fuels, thermochemical cycle, kinetics, thermodynamics, ceria INTRODUCTION. The storage of solar energy in chemical fuels has been a major research goal of the past decade.[1] Solar-driven two-step thermochemical fuel production has emerged as an attractive route towards this goal by harnessing the high efficiencies, and potentially low costs,

Non-Stoichiometric Redox Active Perovskite Materials for Solar Thermochemical Fuel Production: A Review

“…Davenport et al [24,77,78] developed a thermo-kinetic model. Assuming that O 2 production takes place under quasi-equilibrium conditions, the O 2 production is entirely governed by the thermodynamic properties.…”

“…V ox corresponds to the volumetric flow rate of oxidant gas; χ H 2 O is the molar ratio of H 2 O in the oxidizing gas;P is P/P re f and K H 2 O,T is the equilibrium constant for water thermolysis. Details about the method for obtaining the Equations (24) and (25) can be found elsewhere [77]. In Equations (24) and (25), the only unknown remaining is P O 2 (δ, T) which can be computed thanks to Equation (26).…”

Impact of enhanced oxide reducibility on rates of solar-driven thermochemical fuel production

“…[20,21] Under conditions in which (i) the gas fully interacts with the solid, (ii) the solid has a spatially uniform nonstoichiometry at any given time, (iii) the gas has a spatially uniform composition within the reaction zone, immediately equilibrating with the solid, and (iv) the product gases are in equilibrium with respect to thermolysis, the hydrogen evolution profile can be obtained from simple mass balance considerations. The result is [22] y H2 = −ẏ ox (2…”

Thermodynamic Guiding Principles for Designing Nonstoichiometric Redox Materials for Solar Thermochemical Fuel Production: Ceria, Perovskites, and Beyond

“…The efficiency of a selected redox material critically relies on the specific reactor design that integrates solar, solid, and gas inputs. Several ideal reactor solid-gas flow configurations have been studied extensively in the literature, including fixed-bed flow (FBF), [7,8,13,[48][49][50] mixed flow (MF), [9] co-current or parallel flow (PF), [51,52] and countercurrent flow (CF) [12,51,53,54] for the conventional solid-gas reactors, as well as PF [55] and CF [56,57] membrane reactors. The FBF configuration is excluded from this study due to its transient nature; readers are directed to the excellent works by researchers from the University of Minnesota and the California Institute of Technology.…”