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.