Studies of a fixed bed chemical looping reactor for H2 production by in-situ and operando x-ray and neutron diffraction.

Abstract

The purpose of this thesis focused on the investigation of the use of non-stoichiometric oxides as oxygen carrier materials for chemical looping H₂ production. It concentrated on the use of strontium doped lanthanum ferrite (La0.6Sr0.4FeO3-δ, LSF641) for the chemical looping water-gas shift reactor. Chemical looping consists of the cyclic oxidation and then reduction of a solid oxygen carrier material. This process is used to split a redox reaction into its two half reactions using the solid as an intermediate. In a water-gas shift reactor this consist of the oxidation of CO to CO2 and the reduction of H2O to H2. It was shown that theoretically such a system would achieve higher conversions than a conventional reactor; 50% when operated at 1093 K for the conventional reactor and complete conversion when operated using a suitable non-stoichiometric oxide in a chemical looping system. When studied experimentally it was found that such a system could achieve conversions of 88% for both the reduction and oxidation half cycles. The system was also shown to continue to achieve these high conversions for over 1000 cycles without noticeable degradation in product quality or oxygen carrier material. By using the thermal and chemical expansivities of LSF641 it was possible to determine its oxidation state of the LSF641 in operando. This was carried out through the use of synchrotron x-ray diffraction, this showed that how the gradient in chemical potential changed along the bed as a function of time. This was compared with the results of an equilibrium limited thermodynamic model of the system. Both the outlet gas composition and the internal chemical potential of the system were compared to experimental results and showed good agreement. This indicates that the system operates close to its thermodynamic limitations making it a worthy system for further study.