Long-term flue gas exposure effects of silica membranes on porous steel substrate

In this work we investigate the long-term effects of exposing an inorganic membrane for 1100 h in a flue gas stream of a coal power plant. Of particular importance, from an industrial testing perspective, was the effect of fly ash deposition, water vapour and acid gases on the integrity of the membrane made of cobalt silica coated on a substrate of 316L steel, with interlayers of 310S steel, yttria-stabilized zirconia and γ-alumina. Subsequent to the flue gas testing, the membrane was characterized for single gas permeance, SEM and EDX spectroscopy. Diffusion of nickel and chromium during sintering was observed at the interface of the 316L/310S steels, resulting in a reduced capacity to withstand corrosion in this area. Single gas permeation testing following flue gas exposure revealed a maximum permeation of 1.85 × 10−8 mol m−2 s−1 Pa−1 and 2.13 × 10−8 mol m−2 s−1 Pa−1 for helium and hydrogen respectively, and selectivity of 5.1 and 5.2 for He/N2 and H2/CO2 respectively, was achieved at a pressure difference of 2 × 105 Pa (2 bar) at 200 °C. The permeation behavior of the membrane appeared to be altered as a result of flue gas exposure with the membrane displaying a reduced H2 flux in contrast to an unexposed but otherwise identical membrane which displayed fluxes an order of magnitude higher than the membrane used in the power plant. This change in permeation behavior was thought to be the result of densification of the silica matrix following long-term exposure to flue gas containing water vapour. Micro-fractures in the surface of the cobalt silica gas separation layer were also observed, possibly the result of expansion due to corrosion. However, bulk diffusion was not observed suggesting that the layer was not completely compromised.