Ceramic-Carbonate Dual-Phase Membrane for High Temperature Carbon Dioxide Separation From Nitrogen

The combustion of coal for electricity generation accounts for roughly 40% of carbon dioxide emissions in the United States. Flue gas at these plants typically contains 15-16% CO(2) and 70-75% N(2) at temperatures ranging from a few hundred degrees to one thousand degrees Celsius depending on its location within the plant. It is desirable to separate CO(2) from flue gas using a CO(2) selective membrane at these conditions in an effort to conserve energy as conventional separation techniques require cooling flue gas to room temperature or below. In addition, the use of membranes to separate CO(2) is an attractive option due to its continuous nature that can handle large flow rates.

Carbon dioxide transports through a dense ceramic-carbonate membrane relies on the ability of the ceramic support to conduct oxygen ions and the molten carbonate to conduct carbonate ions. Carbon dioxide transports through the dense ceramic-carbonate membrane by reacting with oxygen ions in the ceramic phase on the membrane surface forming a carbonate ion, which then transports through the molten carbonate phase to the  permeate side where the reverse reaction takes place, releasing oxygen ions and CO(2) to the ceramic phase and the permeate stream, respectively. The high temperature carbon dioxide permeaton and stability behavior of a ceramic-carbonate dual-phase membrane was performed for one month at 900 degrees Celsius. The dual-phase membrane is composed of a fluorite-type ceramic samarium doped ceria oxide (SDC) support of composition Ce(0.8)Sm(0.2)O(1.9-delta) infiltrated with a eutectic molten carbonate mixture composed of Li(2)CO(3), Na(2)CO(3), and K(2)CO(3) in a 0.425, 0.325, and 0.25 molar ratio, respectively.

The carbon dioxide permeation properties of SDC-carbonate membranes synthesized from the citrate method were studied in a CO(2)/Ar gradient at 900 degrees Celsius in a 50/50 CO(2)/N(2) feed environment for one month, maintaing a CO(2) permeation flux of approximately 0.6 mL.min(^-1).cm(^-2) after 30 days. The membrane showed CO(2) permeation flux has an exponential dependence to temperature in the range of 700-900 degrees Celsius with a measured activation energy of 63 kJ^.mol(^-1). CO(2) permeation also shows a significant dependence to feed  CO(2) partial pressure with approximately a 100% increase in CO(2) permeation when increasing the feed partial pressure from 0.1 to 0.9 atm CO(2). XRD analysis shows the membrane maintains a fluorite structure after exposure to experimental conditions for one month