The Use of Strontium Ferrite in Chemical Looping Systems

This work reports a detailed chemical looping investigation of strontium ferrite (SrFeO_3−δ), a material with the perovskite structure type able to donate oxygen and stay in a nonstoichiometric form over a broad range of oxygen partial pressures, starting at temperatures as low as 250°C (reduction in CO, measured in TGA). SrFeO_3−δ is an economically attractive, simple, but remarkably stable material that can withstand repeated phase transitions during redox cycling. Mechanical mixing and calcination of iron oxide and strontium carbonate was evaluated as an effective way to obtain pure SrFeO_3−δ. In–situ XRD was performed to analyse structure transformations during reduction and reoxidation. Our work reports that much deeper reduction, from SrFeO_3−δ to SrO and Fe, is reversible and results in oxygen release at a chemical potential suitable for hydrogen production. Thermogravimetric experiments with different gas compositions were applied to characterize the material and evaluate its available oxygen capacity. In both TGA and in-situ XRD experiments the material was reduced below δ=0.5 followed by reoxidation either with CO₂ or air, to study phase segregation and reversibility of crystal structure transitions. As revealed by in-situ XRD, even deeply reduced material regenerates at 900°C to SrFeO_3−δ with a cubic structure. To investigate the catalytic behaviour of SrFeO_3−δ in methane combustion, experiments were performed in a fluidized bed rig. These showed SrFeO_3−δ donates O₂ into the gas phase but also assists with CH4 combustion by supplying lattice oxygen. To test the material for combustion and hydrogen production, long cycling experiments in a fluidized bed rig were also performed. SrFeO3−δ showed stability over 30 redox cycles, both in experiments with a 2-step oxidation performed in CO₂ followed by air, as well as a single step oxidation in CO₂ alone. Finally, the influence of CO/CO₂ mixtures on material performance was tested; a fast and deep reduction in elevated pCO₂ makes the material susceptible to carbonation, but the process can be reversed by increasing the temperature or lowering pCO₂.