Air separation via two-step solar thermochemical cycles based on SrFeO3−δ and (Ba,La)0.15Sr0.85FeO3−δ perovskite reduction/oxidation reactions to produce N2: rate limiting mechanism(s) determination

Abstract

Two-step solar thermochemical cycles based on reversible reactions of SrFeO_3−δ and (Ba,La)_0.15Sr_0.85FeO_3−δ perovskites were considered for air separation. The cycle steps encompass (1) the thermal reduction of SrFeO_3−δ or (Ba,La)_0.15Sr_0.85FeO_3−δ perovskites driven by concentrated solar irradiation and (2) oxidation in air to remove O₂ and produce N₂. Rate limiting mechanisms were examined for both reactions using a combination of isothermal and non-isothermal thermogravimetry for temperature-swings between 673 and 1373 K, heating rates of 10, 20, and 50 K min⁻¹, and O₂ pressure-swings between 20% O₂/Ar and 100% Ar at atmospheric pressure. Evolved O₂ and associated lag due to transport behavior were measured with gas chromatography and used with measured sample temperatures to predict equilibrium compositions from a compound energy formalism thermodynamic model. Measured and predicted chemical equilibrium changes in deviation from stoichiometry were compared. Rapid chemical kinetics were observed as the samples equilibrated rapidly for all conditions, indicative that heat and mass transfer were the rate limiting mechanisms. The effects of bulk diffusion (or gas diffusion through the bed or pellet) were examined using pelletized and loose powdered samples and determined to have no discernable impact.