NSF/EPA Partnership for Environmental Research: New Methods for Carbon Dioxide Separation and Recovery [TSE01-F]

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CTS-0124445New Methods for Carbon Dioxide Separation and Recovery M. Rakowski DuBois and R. D. NobleUniversity of Colorado, Boulder, CO AbstractThe broad objective of this project is to develop an efficient electrochemical pumping process for separating an acid gas from emission sources and concentrating it for recovery and/or disposal. The electrochemical pumping approach is being applied to the specific problem of removing and recovering CO2 from dilute gas streams. The research activities include the identification and synthesis of appropriate molecular carriers for CO2. Three different types of molecular-based carriers for CO2, which differ in their advantages and disadvantages, are being evaluated. In one approach, a redox-active quinone is reduced to produce a base of sufficient strength to bind CO2 directly in absorption from the waste-stream source. After CO2 is captured in this manner, the quinone-CO2 complex is oxidized electrochemically to release concentrated CO2 and to regenerate the oxidized form of the carrier. A second approach develops redox-active metal complexes that function in a similar cycle with bicarbonate ion. These have the advantage of being air-stable and water-soluble. A third approach investigates quinone carriers that produce an electrochemically controlled pH swing. CO2 is complexed at high pH by hydroxide ions generated by quinone reduction and released at low pH produced by quinone oxidation. Work on each carrier system includes electrochemical studies of the reaction-absorption rates and binding constants of CO2 as functions of carrier oxidation state and studies of the electrochemical-pumping systems to determine appropriate operating parameters, characteristics of reproducibility and stability, and energy efficiencies. In addition, process-modeling studies and investigations of absorption/desorption and related mass transport phenomena are being carried out. Current CO2 concentrations in the atmosphere have increased significantly from pre-industrial levels, and as the use of fossil fuels continues, CO2 levels will continue to rise. In this scenario, climate models predict a significant global warming with serious environmental impacts. Efficient methods must be developed in which CO2 is captured and stored for future use or sequestered in a stable form. The easiest point to capture CO2 is from the waste streams of large producers such as power plants, chemical plants, etc. before the CO2 is diluted by mixing with the atmosphere. It is estimated, however, that with today's best technology the recovery of CO2 from the exhaust of power plants will result in a 20-30% reduction in their overall efficiency. Estimates based on prior research and modeling studies suggest that an electrochemical pumping process with appropriate molecular carriers can be much more efficient than current conventional approaches for CO2 recovery. As appropriate carriers are developed, this process should find application in the efficient removal of CO2 as well as other acid gases from waste streams of industrial plants.