Using Rust to Capture CO2 from Coal Plants
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.
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.
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.
In the next step in the chemical loop, the iron is moved to another chamber. It’s exposed to the oxygen in air, forming iron oxide in a chemical reaction that generates heat, which is used to generate electricity. (Alternatively, the iron can be exposed to steam to produce hydrogen for fuel cells or to be made into liquid fuel at a refinery.) The iron oxide then returns to the first chamber to react with more syngas, closing the loop.
Implementing such a system at a full-scale power plant has two main challenges, says David Thimsen, a senior project manager for advanced coal generation at the Electric Power Research Institute. The first challenge is designing mechanisms for moving the iron and iron oxide around inside the plant. The second is ensuring that the materials aren’t too expensive. Thimsen says the approach being taken by the Ohio State researchers may not prove to be the best version of chemical looping. The metal oxides can be expensive, for one thing. And gasifying the coal prior to reacting it with the oxides would incur an energy penalty, especially since it involves a process of separating oxygen from air.
Another chemical looping approach is being developed by Alstom Power, under another $5 million DOE project. In that system, Thimsen says, the oxygen-carrying material is derived from limestone, which is cheap. That system has been successful in a small pilot plant, and will be tested in a larger 3,000-kilowatt prototype plant. The Ohio State researchers are also in the early stages of developing an approach that doesn’t involve a separate gasification step. That approach could be 10 to 20 percent more efficient than the version for the pilot plant, Li says.
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