Characterisation and application of low temperature YBaCo4O7+δ for oxygen enrichment processes by redox cycling

Abstract

Oxygen enrichment is of growing importance in industry, with processes ranging from oxyfuel combustion to the Claus process relying upon this technology. The major limiting factor of the technology is the cost of oxygen. Oxygen production is most often performed by cryogenic distillation, however redox cycling of oxygen non-stoichiometric materials is an attractive and potentially cheaper alternative. Metal oxides such as copper or cobalt oxide, can be used to produce oxygen by redox cycling however, low stability over redox cycling make them unattractive. Non-stoichiometric materials, such as perovskites, have greater cycling stability, however, three main issues prevent widespread implementation, namely: Issues with characterisation techniques – oxygen diffusion rates are often so high that mass transfer limitations mask the true rates of oxygen diffusion in a material using traditional diffusion characterisation techniques such as electrochemical relaxation making accurate modelling of the materials difficult High temperature operation – The materials often studied operate at temperatures of approximately 800°C. For oxygen enrichment processes, air is typically the feed gas for the oxidation step, requiring air to be heated by approximately 750°C. Some of this heat can be recovered, however it is more efficient to have a material that does not require such a large temperature increase. Low oxygen capacity – Many perovskite and non-stoichiometric materials have a cyclable oxygen capacity two orders of magnitude lower than that of the metal oxides. This requires fast cycling which can lead to particle degradation as well as large pressure drops and unwanted gas mixing, depending upon the size and type of reactor bed. In this work, a new characterisation technique to avoid mass transfer limitations is explored. Additionally, the thermodynamics and crystal structure of a low temperature and high oxygen capacity non-stoichiometric material, namely YBaCo4O7+δ, is studied, before a feasibility study of the material for oxygen enrichment is performed.