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Hollow Fibre La0.6Sr0.4Co0.2Fe0.8O3-Δ Mixed Conducting Membranes for Oxygen Separation and Methane Combustion

Alan Thursfield and Ian S. Metcalfe. Chemical Engineering and Advanced Materials, University of Newcastle, Merz Court, newcastle-upon-tyne, United Kingdom

Global warming caused by green-house gases such as carbon dioxide has become an issue of importance for governments worldwide as the body of evidence grows that indicate it is a reality. The combustion of fossil fuels such by industry, automobiles and domestic heating and power requirements produces carbon dioxide in ever increasing amounts. One of the strategies currently under investigation to stem the release of carbon dioxide into the atmosphere focuses on capturing it from conventional power stations. The advanced zero emissions power plant (AZEP) utilises metal oxide membrane technology to selectively provide oxygen to the combustion process in modified conventional power stations. This has the benefit of avoiding expensive cryogenic oxygen production equipment and excludes nitrogen from the reaction mix, thereby avoiding the difficult nitrogen separation. The carbon dioxide produced will then be a pure stream that can be sequestered or provided to industry for uses such as oil recovery.

The metal oxide of choice for such membranes is based on the perovskite phase of general formula ABO3. This versatile crystal structure can be chemically designed to possess useful defect chemistry properties such as oxygen ion vacancies and electronic conductivity i.e., mixed ionic-electronic conductivity (MIEC). The MIEC membranes have the ability to incorporate and transport oxygen when subjected to high temperatures and an oxygen chemical activity gradient. The required transmembrane oxygen fluxes are demanding but can be overcome by using the latest membrane fabrication technology. Membranes in the form of hollow fibres provide a high surface area to reactor volume ratio. To test the concept of carbon dioxide capture using this new technology MIEC La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes of length 25cm and inner and outer diameters of 1mm and 1.5mm have been fabricated and tested for oxygen separation over the temperature range 500-1000oC. The hollow fibres were used in small bundles in shell and shell-less modules to compare the oxygen separation performances using oxygen supplied to the shell-side at different flow rates and when non-flowing oxygen from the atmosphere is used. During operation, in situ leak monitoring was undertaken and it was apparent that the module could be adequately sealed and that the membranes remained intact on the timescale of the study.

At higher temperatures of operation it appeared that the oxygen flux was mass transfer limited as this flux was dependent upon the gas flow rates employed on either side of the membrane. In this way flow rates could be used to manipulate the transmembrane oxygen flux. Initial experiments to increase the transmembrane flux showed that at low temperatures, where slow surface processes are the rate determining step, the application of a porous catalysts film on the membrane outer-surface could improve the flux markedly.

In experiments in which the modules were used for methane oxidation, it was found that the product distribution was dependent upon the oxygen flux with carbon monoxide and hydrogen production being favoured at low fluxes and carbon dioxide and water production being favoured at higher fluxes. It was observed that at low oxygen flowrates periodic increases in the transmembrane oxygen flux were observed. The cause of this behaviour is unclear but may be as a result of phase/stoichiometric changes associated with the membrane material or oxygen storage-release processes.