Nitrogen-selective membrane technology may be applied for post-combustion capture of CO2 from coal- or natural gas-fired power plants, where CO2 concentrations are potentially too dilute for efficient separation via traditional solvent-based capture processes. Consisting of more than 70% of the flue gas mixture, nitrogen may be separated in an early stage in power plants to render subsequent CO2 capture more efficient. There might be benefits associated with the reduced gas volume sent to pollutant control devices in conventional power plants, once nitrogen is separated beforehand. Another co-benefit of a nitrogen-selective membrane is the potential for ammonia synthesis by sweeping hydrogen on the permeate side of the membrane, since nitrogen atoms diffused through the membrane may be reactive, thereby decreasing an activation barrier for ammonia formation.
It has been known that diffusion of nitrogen through the bulk metal is limited due to a large activation barrier, resulting in a low nitrogen flux through the metals. A thin metal-alloy composite membrane is desired to enhance permeation properties. Therefore, the aim of this study is to produce thin metal and metal alloy films on porous ceramic substrates that are stable under thermal and flue gas conditions. Metals chosen are Nb and Mo, which are in Group V and VI, respectively, and anodized alumina was used as porous substrates.
Thin films of Nb and Mo 100 nm-thick have been deposited via e-beam evaporation. The presence of dense grain boundaries with random hillocks were confirmed by high-resolutoin scanning electron microscope (HR-SEM). Due to a large number of local pinhole defects, the membrane does not exhibit an appreciable selectivity toward nitrogen. While improving the film quality by modifying the support surface structure and varying deposition parameters, we first have investigated the impact of thermal (20, 150, 300, 450, 500, 550, and 600 ˚C) and gas conditions (Ar, N2, CO2) on Mo-alumina composite membrane samples. Grain growth was predominant at and above 450 ˚C in the presence of all gases. The continuity of the Mo film became completely destroyed at 600 ˚C upon nitrogen exposure maybe due to further phase transformation. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have shown various extents of Mo oxidation states and potential nitride formation on the surface upon gas exposure at activated temperatures. While Mo is chosen as a reference case, Nb thin films are expected to be more reactive toward nitrogen. The tests with Nb films are in progress.
In future studies, pinhole-free thin films will be subsequently developed and nitrogen permeation will be measured. Successful development of this composite membrane for nitrogen separation may have a significant impact on effective post-combustion CO2 capture.