Production of power from fossil fuel combustion inevitably generates carbon dioxide (CO_2) as a by-product. To mitigate global climate change, this CO_2 must be captured and sequestered. Currently, the capture (separation) step accounts for the majority of the total cost. As a result, cost-effective methods of CO_2 capture need to be developed to improve the process economics, and membranes - an energy-efficient and environmentally benign separation technology - may play a role. Working with the U.S. DOE, MTR is developing membrane-based technology for post-combustion capture of CO_2 from coal-fired flue gas. The low partial pressure of CO_2 in flue gas is the major challenge for all capture technologies. To overcome this issue, MTR has proposed using a countercurrent/sweep process to recycle a portion of the CO_2 in flue gas back to the boiler, by using a process air stream to sweep the permeate side of the membrane. In this way, the partial pressure of the carbon dioxide on the permeate side is maintained lower than that on the feed side. Carbon dioxide then continually passes from the flue gas into the sweep air stream. Using the novel membranes developed by MTR that are highly permeable to carbon dioxide, but relatively impermeable to oxygen and nitrogen, very little of the nitrogen and oxygen pass from the air sweep into the flue gas. Because the feed and permeate total pressures are very nearly the same, the only compression equipment needed are fans to circulate the gases across the membrane. A useful separation can be performed at minimal energy cost.
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