Using Rust to Capture CO₂ from Coal Plants

Process could capture carbon more cheaply.

Researchers at Ohio State University are developing a novel process for generating electricity from coal that also promises to make capturing carbon-dioxide emissions cheaper. The work is being done with the help of a $5 million grant from the U.S. Department of Energy's new Advanced Research Projects Agency-Energy. The technology has been proven in laboratories; researchers will use the new funds to demonstrate it in a 250-kilowatt, pilot-scale power plant.

Carbon trap: This laboratory device extracts energy from fossil fuels and produces an easy-to-capture stream of carbon dioxide. A larger version will be tested in a new 250-kilowatt power plant. Credit: Fanxing Li

A coal-fired plant based on the process, which is called chemical looping, would produce a highly concentrated stream of carbon dioxide. Such a stream would be easier to capture and store underground than the standard method of capturing diluted carbon dioxide in the exhaust gas of conventional coal-fired power plants. The new method could make it less expensive for coal plants to meet pending regulations on CO2 emissions.

Chemical looping could be a big improvement over systems for capturing carbon dioxide from conventional power plants. The typical systems reduce the power output of coal plants by as much as 30 percent and, because of the reduced power output and the cost for additional equipment, increase the cost of electricity by 85 percent. With chemical looping, say Fanxing Li, a research scientist at Ohio State, "you don't see that energy penalty," and as a result, "hopefully we can prove that it's cheaper."

Most coal-fired power plants burn pulverized coal in air, and since air is mostly nitrogen, so is the exhaust emissions--only about 14 percent is carbon dioxide. "You have to waste a lot of energy to separate the nitrogen from the carbon dioxide," Li says. With chemical looping, the coal isn't exposed directly to air. Instead, looping involves a series of chemical reactions in which a solid material, acting as an intermediate, first captures oxygen from the air and then transfers it to the fuel--without the nitrogen or other gases in air. The reactions produce heat, which can be used to generate electricity, along with a stream of concentrated CO2 that can easily be captured.

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In the version of chemical looping the researchers will use in the pilot plant, the coal is first gasified, a common process that involves converting coal into syngas-- a combination of carbon monoxide and hydrogen gas. The syngas is exposed to particles of iron oxide--that is, rust--which act as an oxygen carrier. As it reacts with the syngas, the iron oxide releases its oxygen, forming metallic iron. The oxygen oxidizes the carbon monoxide, forming carbon dioxide, and the hydrogen, forming steam. At this stage, the steam and carbon dioxide leave the system. The steam can easily be removed by condensing it, leaving behind highly concentrated carbon dioxide that can be captured and stored.