(511c) Non-Isothermal Conditions in Countercurrent Thermochemical Reactors for Fuel Production

The efficiency of two-step thermochemical water and carbon dioxide splitting cycles based on non-stoichiometric, redox active, metal oxides is critically dependent on the selection of the operating conditions and the design of thermochemical reactors. Thermodynamically, the reduction step is favored at high temperature and low oxygen partial pressure, while the re-oxidation step is favored at low temperatures and excess oxidant (water or carbon dioxide). Countercurrent reactors can maximize the chemical potential utilization of the material offering high efficiencies. However, as several authors have highlighted, an oversimplified thermodynamic analysis of these reactors could lead to a violation of the second law of thermodynamics. Some previous works have analyzed these reactors based on isothermal reduction or oxidation conditions, where chemical equilibrium (ΔG = 0) is reached only at one point along the flow path. The present work proposes a method to determine the chemical equilibrium at each infinitesimal point along the flow path, leading to non-isothermal conditions based on both mass conservation and Gibbs chemical equilibrium criteria. Our results provide higher flexibility on the design and use of countercurrent reactors in thermochemical cycles being especially relevant to solar driven reactors where the temperature distribution is not homogeneous in the reduction or oxidation reactors. Additionally, material heating may be more rapid than the reduction reaction contributing to the inhomogeneity at least for reduction.