Pressure Swing Redox Processing to Split H2O/CO2

Abstract

Thermochemical gas splitting of water and/or carbon dioxide via redox is a promising technology to renewably produce syngas for the diversification of gas-to-liquid (GTL) fuels. Here, a metal oxide intermediate undergoes a two-step redox cycle, where either oxygen or hydrogen and/or carbon monoxide is produced. To date, experimental demonstrations have yet to report efficiencies necessary for economic feasibility, primarily due to the thermodynamic properties of the benchmark ceria material considered and the requirement for high reduction temperatures (Tred > 1400°C) and large temperature swings between redox regimes. As a result, recent research has largely focused on discovering new materials with thermodynamic properties that permit lower temperature operation without sacrificing product yields. However, such efforts have proven difficult, as materials that are reducible at low temperatures often become harder to oxidize (and produce fuel).

In terms of efficiency, performing each reaction at the same temperature (i.e., isothermal operation) is appealing due to the elimination of the significant sensible heating penalties that are associated with conventional temperature swing strategies. Although isothermal operation restricts fuel capacities due to operating the exothermic oxidation reaction under unfavorable conditions (i.e., higher temperatures), proper material selection is crucial for efficient operation. Material selection for isothermal focuses on materials that undergo large extents of reaction within the attainable range of oxygen chemical potential (i.e., approximately 10^-5 bar < pO₂ < 10^-3 bar).

Here, iron aluminate class of materials and the effect of pressure on the oxidation reaction are investigated for efficient fuel production via an isothermal thermochemical cycle. Iron aluminates are comprised of solely iron and aluminum, making them a cheap alternative to complex perovskites that contain rare earth metals. However, literature is limited in describing the defect and kinetic mechanism of iron aluminate for thermochemical applications. Furthermore, operating the oxidation pressure is beneficial to removing costly downstream compression costs, but pressure has yet to be explored due to the belief that pressure has no impact on the equimolar reaction. This work clarifies conflicting work on the mechanism of iron aluminates with phase equilibria data and multiphase Rietveld Refinement; consequently, a solid solution of magnetite Fe₃O₄ and hercynite FeAl₂O₄ exists under thermochemical conditions instead of FeAl₂O₄ solely existing. In accordance with established defect structure for iron oxides, cation vacancies are assigned as the predominant point defect responsible for iron aluminates superior water splitting capability and used to couple a defect model with equilibrium data to establish equilibrium maps for making thermodynamic predictions. Additionally, elevated pressures during oxidation were demonstrated to have thermodynamic and kinetic benefits, opening the door for more efficient fuel production via a pressure-swing, isothermal redox cycle. Furthermore, the kinetics of the iron aluminate (Fe_1/3Al_2/3)₃-δO₄ were investigated to understand this pressure benefit, while co-splitting capability was demonstrated for the first time with (Fe_1/3Al_2/3)₃-δO₄. Lastly, a continuous pressure-swing, isothermal redox cycle using iron aluminates in dual fluidized bed reactors was performed under practical conditions to demonstrate large scale feasibility.