Robust Microporous Cobalt Oxide Doped Silica Membranes for High Temperature Industrial Coal Gasification Syngas Separation

With global demand for energy increasing alongside public concern over humanity’s potential contribution to climate change, there is a growing demand for scientific progress in cleaner power generation technologies. As wholly renewable energy technologies remain uneconomical for large scale energy delivery, this encouraged the development of more environmentally and economically sustainable processes using carbon fuels. Such technologies being pursued include advanced coal gasification power plants and emission cleaning processes for carbon dioxide capture and storage. Efficient and economical gas separation technologies are one of the major bottlenecks in the development of cleaner power production and silica membranes are currently being investigated as an alternative to the well established and energy intensive methods such as cryogenic distillation and adsorption-based systems. Silica membranes are porous, amorphous, ceramic materials that separate gases based on size exclusion and/or molecular weight difference. Within this field, several types of silica membranes have been investigated including Vycor glass membranes, chemical vapour deposition membranes, and sol-gel membranes. This thesis focuses on sol-gel silica membranes as sol-gel techniques have the advantage of being a simple and low-cost method of producing silica membranes. It is well known that pure silica membranes are hydrothermally unstable, leading to degradation of membranes in wet gas streams similar to those expected in industry. Various methods to mitigate such degradation have been investigated including the use of carbon templating and metal doping. In this thesis, cobalt was selected for incorporation into the silica matrix, both as a metal (CoSi) and as a metallic oxide (CoOxSi) and its effect on the structure, chemistry, stability and performance of resulting membranes was investigated. CoSi membranes were fabricated successfully and achieved good membrane performance characteristics as well as demonstrating the enhanced hydrothermal stability of the doped material achieving H2/N2 selectivity of 1100 at a H2 permeance of 9.5×10-8 mol•s-1•m-2•Pa-1 at 190°C under wet and re-dried conditions. Further investigation was carried out to determine the mechanism of protection provided by the cobalt dopant. The hydrothermal stability of both CoSi and CoOxSi materials were compared and it was found that CoOxSi is significantly more stable than CoSi. Study of a CoOxSi membrane under extreme hydrothermal conditions revealed that degradation still occurred, and was more pronounced at higher water feeds and temperatures, but the membrane performance stabilised over time and was still capable of significant selectivity after more than 200 hours of testing. Furthermore no significant degradation was observed with thermal cycling under hydrothermal conditions. Continuing the investigation of the stability of the material, the performance CoOxSi membranes were evaluated during exposure to trace amounts of hydrogen sulfide (H2S). H2S stability is important as it is present as a contaminant within coal gasification streams, and it is known to poison the surface of metallic Pd membranes, and many metal oxide catalysts including cobalt oxide. This investigation found no degradation attributable to exposure to H2S (at levels of up to 70ppm) at 200°C for a membrane, or at 500°C for CoOxSi xerogels. Crucially, no cobalt sulfide compounds were detected, suggesting that the cobalt oxide itself is protected by the microporous structure of the silica. In order to improve the robustness of the CoOxSi membranes as a whole, CoOxSi membranes were formed on porous steel substrates. Porous steel substrates are preferable for the commercialisation of silica membranes as they should provide the advantages of greater shock resistances and durability, in addition to reduced complexity and cost for the sealing of membranes and membrane modules into industrial unit operations. Thus an optimisation of the production process for CoOxSi membranes on porous steel substrates was undertaken for both platelet and tubular supports. It was shown that the superior chemical stability of Y2O3 stabilised ZrO2 over TiO2 as an interlayer material enabled higher quality membranes to be produced. The best results achieved were on a tubular substrate with a H2/N2 selectivity of 16.0 at a H2 permeance of 5.6×10-7 mol•s-1•m-2•Pa-1 at 450°C. Mixed gas testing at 450°C demonstrated that the membrane as a whole was stable and effective under both wet and dry gas conditions, concentrating a ~50:50 H2:N2 mixture up to 80% H2 purity. Finally to demonstrate the overall robustness of the developed CoOxSi membrane in an industrial setting, a CoOxSi membrane was tested in an operational coal fired power plant flue gas stream for 1100 hours. Post-exposure tests showed that the membrane retained an activated diffusion mechanism, and analysis of developed defects showed the presence of steel interlayer interface corrosion and fly-ash contamination, pointing to potential challenges that will need to be overcome with further research and industrial testing. Overall, this thesis explores the continual improvement and investigation of CoOxSi gas separation membrane performance in pure gases, hydrothermal conditions and trace levels of H2S contamination, and the subsequent optimisation on steel substrates, including a rare real-world stability test using an operational coal-fired power plant