Understanding and controlling CO2 permeation across dual-phase membranes with tailored, multi- or single-pore microstructures

Abstract

The importance of finding new ways of CO2 separation or improving the existing ones, has increased significantly in recent years, because CO2 emissions have become a serious environmental concern. CO2 separation from different process streams, such as flue gases, has been researched extensively over the past few years. One way of separating CO2 is through molten carbonate dual-phase membranes, which consist of a porous ceramic support infiltrated with a molten salt. They can operate continuously at elevated temperatures (400- 900 °C) with high selectivity and low energetic penalties as opposed to other separation methods, such as absorption. One of the key challenges is understanding the contribution of various factors towards CO2 permeation, such as operating conditions, membrane structure and gas phase composition. In this thesis, dual-phase membrane systems consisting of a zirconia or alumina support with various pore geometries and an alkali metal carbonate eutectic mixture were investigated. It was found that below 600 °C, CO2 permeation is largely controlled by the geometry of the support material rather than its composition. Therefore, multi- or single-pore channels were laser drilled in dense polycrystalline and single crystal materials, and the geometry of the channels was tailored with high precision. By using an Al2O3 –carbonate multiple-pore system, it was found that at around 700 °C, CO2 permeation is generally limited by the diffusion in the melt, while at temperatures around 550 °C, the rate is limited by reactions at the gas-melt interface. In single-pore systems, an effect of permeation was visualised by equilibrating the internal gas phase (gas phase behind the meniscus) to the external gas phase and observing the displacement of the molten salt meniscus. Permeation rates were extracted at low driving forces, necessary for real applications. To enhance permeation, the use of humidified gas streams was investigated. It was found that above 550 °C, CO2 permeance was on the order of 10-7 mol m-2 s -1 Pa-1 compared to 10-9 mol m-2 s -1 Pa-1 under dry conditions. Furthermore, by coupling the permeation of CO2 with H2O, CO2 could be permeated against its own chemical potential difference. This work provides an understanding on membrane performance by unprecedented control over pore geometry and the effect of water with well-defined chemical potential gradients across the membrane.