Approaching the design of novel oxygen carrier materials for hydrogen production via the chemical looping water-gas shift process in a packed bed reactor

Abstract

The work presented in this thesis focused on the use of a chemical looping process that uses novel oxygen carrier materials (OCMs) for the production of hydrogen. A methodology was presented for the selection of suitable OCMs based on the possible conversions and outlet product quality, with a focus on the non-stoichiometric materials La0.4Sr0.6Fe0.67Mn0.33O3-δ (LSFM6473) and Ce0.8Zr0.2O2-δ (CZ82), using La0.4Sr0.6FeO3-δ (LSF641) as a comparative benchmark. The chemical looping process used in this work was via the chemical looping water-gas shift process (CLWGS). In this process a solid OCM is repeatedly oxidised and reduced in two half cycles in a packed bed reactor flowing consecutive counter-current streams of carbon monoxide and water vapour as the reducing agent and the oxidising agent. Such a system can overcome the thermodynamic limitations of conventional mixed feed reactors with no need to separate the outlet hydrogen and carbon dioxide streams, thereby reducing plant costs. The use of non-stoichiometric materials instead of phase changing materials allows for high conversions in both half cycles to be achieved during steady cycling. It was shown that the case for an optimal oxygen non-stoichiometry vs. chemical potential relationship exists for non-stoichiometric OCMs, and this was verified by use of a reactor model. This hypothesis was used to select LSFM6473 and CZ82 as OCMs, and during experimental studies in a packed bed reactor both achieved conversion and outlet product qualities in excess of OCMs with less optimal oxygen non-stoichiometry vs. chemical potential relationships such as LSF641, further verifying the hypothesis. The oxygen non-stoichiometry and thermodynamics of the perovskite LSFM6473 was investigated using thermo-gravimetric analysis and iodometric titration. The substitution of 33% Mn onto the B-site of LSFM6473, when compared with no doping, resulted in an increase in the change in oxygen non-stoichiometry in the oxygen partial pressure range that sees the greatest extent of reaction of the water-gas shift reaction at constant temperature. The high oxygen capacity and stability of the material LSFM6473 was also verified by in-situ x-ray diffraction analysis of the reactor bed. Stable hydrogen production was seen for over 450 cycles with approximately 80% conversion of water vapour using 3 minute feed durations, indicating the viability of this OCM for industrial scale use.