Hydrogen production via chemical looping with La0.7Sr0.3FeO3-d [delta] and iron oxides :a kinetic and thermodynamic study

Abstract

The purpose of this thesis is to investigate the kinetic and thermodynamic performance of perovskite-type material La0.7Sr0.3FeO3-δ (LSF731) and iron oxides for use as oxygen carrier materials (OCMs) in a chemical looping water splitting processes. Chemical looping water splitting is a gas-solid reaction where the OCM is cyclically reduced and oxidised in separate steps. Typical reducing gases include carbon monoxide, methane or syngas, while for hydrogen production the oxidising gas must be water. As the oxidising and reducing gases are kept separate, where is no need to separate carbon-containing contaminants from the hydrogen product. An equilibrium limited thermodynamic model for LSF731 was created. LSF731 is able to continually change its oxygen content depending on the oxygen chemical potential of a gas mixture to which it is exposed. Wave theory was used to create expressions for reaction front velocities that would occur with mixtures of different gas (carbon monoxide and carbon dioxide or water and hydrogen) at varying ratios. Results showed that reaction front velocities were higher for carbon monoxide and carbon dioxide mixtures and both mixtures achieved a maximum reaction front velocity at a δ (oxygen non-stoichiometry) of 0.25. A series of kinetic experiments were carried out in a differential microreactor and it was found that the rate of reduction with carbon monoxide was significantly lower than the rate of oxidation with water, suggesting that although thermodynamically carbon monoxide and carbon dioxide mixtures should have higher reaction front velocities, they are in fact strongly kinetically limited. Further experiments were carried out to compare the performance of LSF731 and iron oxide in a more practical way. A reverse flow integral reactor was used with a 6 cm bed of either fresh or prereduced OCM. 100 redox cycles of 5 mol% carbon monoxide in helium and 5 mol% water in helium were performed. It was found that LSF731, when operated in a reverse flow reactor, is able to overcome equilibrium limitations which would restrict any material with a discontinuity in oxygen content versus oxygen chemical potential, such as iron oxide.