Capturing carbon dioxide from smokestacks and then storing it underground could make it possible to continue using fossil fuels without making such a large contribution to global warming. But the current method of capturing the carbon dioxide requires a lot of energy—it can lower the output of a power plant by a third and nearly double the cost of electricity.
Two novel approaches—one developed at MIT and the other at the Pacific Northwest National Laboratory—could lower these costs by up to half. Both provide a low-energy way to trigger the material that captures the carbon dioxide to release the gas so that it can be stored. Then the material can be reused. The MIT process uses electrochemical reactions, instead of the steam used now, to trigger the carbon dioxide release. The other uses a solvent that can be triggered to release carbon dioxide by mixing in certain chemicals. Papers describing the approaches have just been posted online by the journal Energy and Environmental Science.
Existing carbon capture technology has not been widely deployed because it is expensive (see “Grasping for Ways to Capture Carbon Dioxide on the Cheap” and “Will Carbon Capture Be Ready on Time?”). In the conventional approach, the gases in power plant exhaust are separated using a solution containing amines that selectively bind carbon dioxide. The amines will release the carbon dioxide if they’re heated up, but this requires a large amount of energy, which would come from steam that could otherwise be used to generate power.
Researchers at MIT developed a way to get the amine solution to release carbon dioxide without heating it. They run it through a device that resembles a battery—it contains positive and negative electrodes made of copper. But instead of producing power, it uses electricity to regenerate amines (see “Fuel Cells Could Offer Cheap Carbon Dioxide Storage”).
Once a solution of amines has absorbed carbon dioxide, it’s pumped to one electrode. Electricity applied to that electrode produces copper ions. The copper ions bind more strongly to the amines than the carbon dioxide does, displacing the carbon dioxide and causing it to bubble off. The copper-amine solution is then pumped to the opposite electrode, where the copper is removed. The amines can then be used to capture more carbon dioxide.
After small-scale tests, the researchers calculated that the process would use far less energy than a conventional system added to an existing power plant—45 kilojoules per mole, compared with 77 kilojoules per mole for the conventional system. The process would also be far easier to implement in existing power plants. The conventional process would require a plant to be extensively restructured with new steam handling equipment and steam turbines, since it gets its heat from the steam. The new system doesn’t require this equipment because it gets its energy from electricity, reducing capital costs.
The system developed at the Pacific Northwest National Laboratory uses an organic solvent to absorb carbon dioxide. To release it, the solvent is mixed with a hydrocarbon, such as hexane, at slightly elevated temperatures. The amount of heat needed is so small that it wouldn’t require diverting steam within a power plant, which makes the process attractive for retrofits. Cooling down the mixture of hydrocarbon and solvent a little causes them to separate, allowing the solvent to be used to capture more carbon dioxide.
Both approaches are at an early stage, having only been demonstrated at a small scale in the lab. Howard Herzog, one of the MIT researchers, says that while the MIT system could be a significant improvement over conventional approaches, “further development is needed to scale up the technology and integrate it with real industrial processes.” PNNL is starting a project to build a larger system in the lab for testing.
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