Capturing Carbon Dioxide With A Small pH Change

Coal-fired power plants produce one-third of the carbon dioxide emissions in the U.S. Scientists have been developing strategies to capture this carbon and prevent its release into the atmosphere, in hopes of curbing global climate change. But so far, those methods are too expensive to implement because of their high energy requirements. Now, researchers report a novel electrochemical method that could capture CO₂ using less energy (Ind. Eng. Chem. Res. 2013, DOI: 10.1021/ie402538d).

The flue gas that wafts from the smokestacks of coal plants contains around 15% CO₂. Carbon-capture methods separate out the CO₂, purify it, and collect it for sequestration or for use in industrial applications. Currently, the most efficient carbon-capture method, according to Seth W. Snyder of Argonne National Laboratory, is to bubble the flue gas through an aqueous solution of amines. The amines react with the CO₂, trapping it in solution. The method captures about 95% of the CO₂ from the flue gas, but to recover the CO₂, the amine solution must then be heated up under vacuum, which requires a hefty energy input. Using this technology in coal plants would double the cost of electricity, Snyder says.

To help bring costs down, the researchers sought a method that doesn’t need additional energy to release the captured CO₂. Snyder came up with a method that exploits the chemistry of CO₂ in water under changing pHs. In a sodium phosphate buffer, CO₂ can exist either as a bicarbonate ion when the solution is basic, or in its gaseous form when the solution is acidic. So a device could capture CO₂ by passing flue gas through a basic solution, forcing CO₂ to dissolve as bicarbonate. Then the device could release the pure CO₂ by bringing down the solution’s pH to reform the gas, allowing it to bubble out of the solution.

Snyder and his colleagues built a carbon-capture device that continuously pumps a phosphate buffer between two chambers separated by a cation-exchange membrane. By running a current through the device, the researchers use electrochemical reactions to maintain a pH difference between the two chambers, so that one has a pH of 8 and the other had a pH of 6. Flue gas enters through the basic chamber, and then the resulting bicarbonate solution is pumped to the acidic chamber, where purified CO₂ exits. To speed up the bicarbonate-to-CO₂ conversion, the researchers added the enzyme carbonic anhydrase to the acidic chamber, which catalyzes the transformation.

To test the device, the team used a simulated flue gas containing 15% CO₂. By measuring the CO₂ in the flue gas after it went through the device, the researchers determined that their method captures 80% of incoming CO₂. The gas that escaped from the acidic liquid was more than 98% pure CO₂.

This study was a proof-of-principle test, Snyder says, so his team hasn’t determined how much energy the method would require to run on large scale. They next plan to look at the method’s efficiency and possibly increase it by using a more stable version of carbonic anhydrase.

“I think this is a great study,” says Jamie A. Hestekin of the University of Arkansas. “To attack such a challenging problem, we definitely need different ways of doing things, and for that I applaud this paper.” In future experiments, Hestekin thinks the team should test the device with real flue gas, which is filled with impurities that could interfere with the process. For example, sulfur-based compounds could knock out the enzyme, he says.