Aluminum-doped strontium ferrites for a two-step solar thermochemical air separation cycle: Thermodynamic characterization and cycle analysis

Abstract

Separation of O2 from air to produce a high-purity stream of N2 is considered via a two-step solar thermochemical cycle based on aluminum-doped strontium ferrite reduction/oxidation reactions. The cycle steps encompass the (1) thermal reduction, driven by concentrated solar irradiation; and (2) re-oxidation with atmospheric air to produce a N2 stream. Samples were synthesized via the sol-gel method, and crystalline structures were confirmed with x-ray powder diffractometry. Thermogravimetry was used to measure the nonstoichiometry at chemical equilibrium for molar B-site Al dopant fractions between 0 and 0.20, temperatures between 673 and 1373 K, and volumetric gas flow compositions between 1 and 90% O2–Ar. The compound energy formalism was applied to thermogravimetric measurements to predict equilibrium nonstoichiometry as a function of dopant fraction, temperature, and O2 partial pressure. The results were used to predict partial reaction enthalpies and entropies. Aluminum doping increased the strontium ferrite oxygen affinity but reduced the range of nonstoichiometries. A strontium ferrite sample underwent 10 thermal reduction-oxidation steps in an upward flow reactor coupled to a high-flux solar simulator to study cyclability. A thermodynamic cycle analysis was performed, and cycle efficiencies were determined relative to an ideal separation process. For a solar reactor at 1073 K, 0.59 mol N2 per mol strontium ferrite could be produced at a purity of nearly 99% with a cycle efficiency of 4.0%.