Xin Li

Date of Award

Spring 2022

Document Type

Open Access Dissertation


Mechanical Engineering

First Advisor

Kevin Huang


Due to the burning of fossil fuels and human activities, many greenhouse gases such as carbon dioxide are released into the atmosphere, which causes global warming and has a significant negative impact on the human living environment. Therefore, it is imperative to develop carbon capture technologies that can prevent carbon dioxide emission and reduce carbon dioxide concentration in the air. Further, if the captured carbon dioxide can be converted back to fossil fuels, the emission of carbon dioxide is delayed, and more time is gained to develop new and advanced carbon free and clean energy technologies. Motivated by this thought, a multiphase carbonate membrane that can capture carbon dioxide from the point source and convert a fossil fuel such as methane into valuable chemicals with the captured carbon dioxide as a soft oxidizer has been proposed.

In this thesis, mathematical models of four types of solid/molten-carbonate CO2 transport membranes were first developed with analytical and numerical approaches. These four types of membranes are mixed oxygen-vacancy and carbonate-ion conducting membrane (MOCC), mixed electron and carbonate-ion conducting membrane (MECC), mixed electron and oxygen-vacancy conducting/ molten carbonate dual-phase membrane (MOECC), and mixed electron and oxygen-vacancy conducting /metal/molten carbonate triple-phase membrane (MOEECC). The analytical solutions can be derived for two basic types of membranes, MOCC and MECC, and they agree well with numerical solutions. For membranes with more charge carriers, numerical solutions are the only way to calculate flux since the analytical solutions cannot solve the nonlinear concentrations of charge carriers across the membrane. The models developed are the foundations for improving the membrane performance and designing next-generation high-performance CO2 transport membranes. After validated by experimental data, a new type of plug flow chemical potential driven reactor based on MOEECC membrane was modeled and the results were compared with the co-fed fixed-bed reactor in terms of C2 yield, selectivity, and CH4 conversion rate of the oxidative coupling of methane reaction. The overall results indicated the plug flow membrane reactors have much improved OCM performance over the co-fed fixed-bed reactor in terms of C2 yield and coking resistance.

A new kind of MOEECC membrane was also experimentally studied in this thesis. The oxygen flux was improved by Mn-doping CeO2 (MDC) as a replacement of benchmark Sm-doped CeO2. The upper limit of Mn-doping was determined by XRD to be 6mol%. The co-precipitation method was used to synthesize the nanosized power mixture of MDC and NiO, from which a porous MDC-NiO matrix was obtained after sintering at high temperatures. A eutectic molten carbonate (MC) was then allowed to fill the pores in the porous matrix and in-situ react with NiO to form electronically conducting LNO phase. The best MC loading was determined to be 0.1g for a matrix with 16% porosity, 0.916cm2 surface area and 0.75 mm thickness. With this membrane, the flux densities of CO2 and O2 were obtained and the CO2 and O2 pathways were confirmed by specially designed experiments.