Towards chemical equilibrium in thermochemical water splitting. Part 1: Thermal reduction

Abstract

The efficiency of many processes strongly depends on their thermodynamic reversibility, i.e., proximity to equilibrium throughout the process. In thermochemical cycles for water and/or carbon dioxide splitting, thermochemical air separation, and thermochemical energy storage, operating near equilibrium means that the oxygen chemical potential of the solid and gas phases must not differ significantly. We show that approaching this ideal is possible in thermal reduction only if the reaction step occurs at a specific, reaction coordinate- and material-dependent temperature. The resulting thermal reduction temperature profile also depends on the ratio of gas and solid flows. • Method to achieve reversible/equilibrium reduction in water splitting cycles. • Reversible isothermal reduction is not feasible in practice. • Strong technical and physical limitations on the amount of sweep gas. • Limited impact of gas purity on reduction extent at fixed flow/maximum temperature. • High reduction temperatures expand the solution space for high cycle efficiency.