CO2 Membrane Separation for High Temperatures

Worldwide concern over greenhouse gas emissions and global warming is rising. To address this environmental issue, numerous solutions are being suggested from various kinds of fields. Because of the high level of global concern, the Carbon Electrochemistry research team in LCSE has focused on the carbon capture and sequestration. As a way to separate and capture carbon dioxide directly from the emission sources, we have developed a ceramic membrane which operates at high temperature.

The idea of this membrane is to separate carbon dioxide from highly concentrated sources such as flue gas at the temperature one might expect in a power plant gasifier or other reaction chamber. The separation membrane can operate at high temperatures ranging from 600°C to 800°C. Currently available polymeric membranes would melt at this temperature range, and thus would make it necessary to cool down the gas before separation and heat it up again afterwards. The new membranes can work at the high temperature of the power plant and promise greatly improved efficiency. So far the concept of the membrane has been proven to be feasible. Now our project is to better characterize the membranes and to improve their performance. Different sets of experiments are occurring to find ways to improve the design of ceramic membranes and ultimately to make them commercially available.

Figure 1. Depiction of the transport occurring across the membrane thickness. In the upstream gas mixture (feed), the partial pressure of CO2 is high and combines with an oxide anion to become CO32-. At the opposite face where PCO2 is low (permeate), CO32- decomposes releasing gas phase CO2 and an oxide back into the solid oxide phase.

The membrane being investigated is a composite material derived from a combination of molten carbonate and solid oxide electrolyte technologies. CO2 is transported across the membrane as a carbonate ion in a molten carbonate phase. CO2 permeation occurs through a facilitated transport mechanism, driven by passive chemical potential gradients. On the surface of high CO2 pressure side of the membrane, conversion between gaseous CO2 and CO32- occurs through the donation of an oxide ion, O2-, from a conductive solid oxide phase. The reverse is true where the partial pressure of CO2 is low. Carbonate ions convert back into CO2, releasing the oxide ion back into the solid oxide phase to complete the circuit (fig. 1). Thus the driving force is the partial pressure gradient of CO2 established across the membrane. It is also important to note that we are investigating membranes with a micro-tubular structure, which has many advantages over membrane designs with a planar structure.

Figure 2. SEM images of membrane cross sections within the bulk. The image on the left is of the porous solid oxide phase. On the right is the solid oxide infiltrated with a tertiary alkali carbonate mixture.

As we have already successfully developed high-temperature membranes that can separate carbon dioxide from other gases, our current research goals are to control the membrane’s structure and other properties and gain an understanding of the limitations of such a system. We are building upon the research performed by Jennifer Wade and Catherine Lee (along with Klaus Lackner and Alan West) on high temperature membranes, the abstract of which can be found here.

Klaus Lackner, Ewing-Worzel Professor of Geophysics, (PI)

Alan West, Professor and Chair of Chemical Engineering, (PI)

Tao Wang, Assistant Professor at Zhejiang University

Jennifer Wade, PhD 2008