Theoretical and experimental investigations of metallic membranes for CO2 capture [electronic resource]
Keywords: Energy Resources Engineering

Approximately 85% of the global energy demand for electricity generation is supplied from fossil fuel resources, mainly from coal-fired power plants, which accounts for approximately 40% of the total CO2 emissions. Due to the environmental impact and recent proposed EPA regulations on CO2 emissions, it becomes clear that carbon capture technologies must be advanced for carbon capture and storage (or utilization) to be an economically viable strategy for mitigating CO2. Carbon capture may be applied through two different approaches, i.e., precombustion or postcombustion depending on whether a given fossil fuel resource is gasified or combusted, respectively. This dissertation is dedicated to focus on existing precombustion and novel postcombustion membrane technologies. In the studies presented within this dissertation, a high concentration of H2 in synthesis gas for precombustion capture of CO2 and a high concentration of N2 in the flue gas for the postcombustion capture of CO2 are the main driving forces for the use of the metallic membrane technology described within this work. The main scope of this dissertation is showing the value of integration of atomic-scale modeling with experimental testing in improving the material properties of traditional metallic membranes, subsequently leading to the design of new membranes and addressing the challenges that arise from material selection and process scale-up during technology deployment. Palladium-based metallic membranes have been the primary focus in the membrane community as a potential precombustion CO2 capture technology from gasification systems because of their high hydrogen permeability, surface catalytic activity and thermal stability at high temperatures. However, palladium is not only an expensive material to apply at large scale, but it loses its activity and mechanical integrity from sulfur poisoning due to the H2S content of the synthesis gas produced from the coal gasification process. Alloying palladium may be an option for overcoming sulfur poisoning. In this study, the effect of alloying palladium with copper or niobium on the H2 and H2S interactions with the metal membrane surface has been investigated using plane-wave density functional theory-based electronic structure calculations using the Vienna ab initio Simulation Package. The overall d-band centers calculated for H2 adsorption at the fcc-fcc site for the pure and alloyed-Pd surfaces indicate that the H2 adsorption strength trend is palladium > copper > niobium. In addition, the H2S binding trend is found to be copper < palladium < niobium, with the copper-alloyed surfaces exhibiting the weakest binding and niobium-alloyed surfaces the strongest binding. The second part of this dissertation is focused on the design and testing of a new N2-selective membrane technology using metallic membranes for the postcombustion CO2 capture application. This technology benefits from the high concentration of N2 present in the flue gas stream (73 wt.%) to separate N2 and concentrate CO2 in the retentate stream of the membrane for CO2 capture as it provides atomic nitrogen on the permeate side of the membrane during separation. These properties along with H2 provided as a sweep gas ammonia synthesis as a byproduct of the separation process may be possible. Within this study, N2 in particular, has been investigated to determine the mechanism of subsequent atomic diffusion into the bulk crystal structure of vanadium and its alloys using density functional theory. It was determined that the octahedral site is the most favorable binding site for N within bulk vanadium, with a binding energy of -2.132 eV. Nitrogen binding in vanadium is nearly twenty orders of magnitude stronger compared to the well-known H binding case (-0.076 eV). These strong N binding energies indicate that N may have difficulty diffusing through the metal. Alloying vanadium with ruthenium may aid in tuning the N binding energy so that it approaches that of H; alloying vanadium with 6.25 at.% ruthenium yields a weaker N binding energy of -0.889 eV. It is anticipated that increasing the content of ruthenium in the alloy will lead to a further reduction in the stability of N within the bulk crystal lattice. Experimental efforts have involved the design and construction of a gas permeation test system and execution of pure gas permeation tests to determine the N2 transport mechanism through the membrane foils comprised of vanadium at temperatures ranging from 600 to 900 °C. The overall experimental findings emphasize the possibility of a solution-diffusion transport mechanism of N2 through vanadium at 700 °C, 800 °C and 900 °C. Characterization of the foils before and after the permeation tests using XRD, XPS and SEM techniques indicate nitride formation on the surface and in the bulk of the vanadium, thereby providing further evidence that N2 transport through the vanadium membranes takes place via a solution-diffusion mechanism. In total, characterization and flux measurements were compared alongside electronic structure predictions from density functional theory and demonstrate the viability of using Group V metals and their alloys for N2-selective membrane applications.