Utilising O2 chemical potential difference to drive CO2 separation by ceramic-carbonate dual phase membranes

Abstract

Carbon dioxide (CO2) separation using selective membranes is a promising technology for continuous CO2 capture. However, driving force for CO2 permeation is often limited, resulting large energy input to sustain CO2 permeation flux. In this work, ceramic-carbonate dual phase membranes were fabricated for high temperature CO2 separation at 600-850°C. These membranes consist of molten tertiary carbonate mixture (Li/Na/K, melting point 397°C) infiltrated into porous network of ceramic solid oxide support. When mixed ionic and electronic conductive La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF6428) is used for membrane support, O2 can co-permeate with CO2 electrochemically via the molten salt (CO32-) based on the principle of molten-carbonate fuel cells (MCFCs). Chemical potential gradient of O2 can be exploited to drive CO2 permeation. It overcomes the driving force limitation and potentially reduces the energy requirement for CO2 capture, even allowing CO2 to permeate against its own chemical potential gradient (uphill permeation). This project aimed to investigate a novel approach on promoting CO2 permeation flux by enhancing the thermodynamic driving force of O2. Firstly, LSCF6428 membrane has been selected to investigate CO2-O2 co-permeation from a mechanistic viewpoint. Temperature dependence of CO2 flux in presence of O2 in feeding gas has been studied in order to find out the apparent activation energy for CO2 permeation. Several CO2 flux-driving force models have been established; each model was associated with a rate-determining scenario incorporating the driving force contribution from O2. Experimentally observed CO2 flux-driving force relationship revealed that the CO2-O2 co-permeation was likely limited by global interfacial reaction (CO2 + 1/2O2 + 2e- ↔ CO32-). It was further proposed that permeate side O2 removal may enhance the overall driving force and promote uphill CO2 permeation flux. A system combining uphill CO2 permeation with downstream O2 removal has been developed, utilising LSCF6428 membrane to separate out CO2 and O2 from feeding gas (1:1:20 ratio of CO2, N2 and O2), and Cu-based oxygen carrier for O2 removal. The Cu successfully removed O2 on membrane permeate side from 400 ppm to 30ppm at 600°C and from 3000ppm to 200ppm at 800°C. Uphill CO2 flux at 800°C reached 7.62 × 10-4 mol·m-2·s-1 with O2 removal, a 30% enhancement to the flux without O2 removal. This result was consistent with the prediction from suitable flux-driving force model.