Coal is abundant and cheap, but burning it is a dirty business. This week researchers at Ohio State University announced a milestone in the development of a far cleaner way to use the energy in coal—a process called chemical looping that has the potential to reduce or eliminate a wide range of pollutants, including carbon dioxide and smog-forming nitrogen oxides.
One version of the technology ran continuously for over a week in a 25-kilowatt test facility, the researchers reported, the longest any such process has run. The successful test clears the way to ramp up the technology in a one-megawatt demonstration plant that’s being planned in collaboration with the energy company Babcock and Wilcox.
In ordinary coal plants, coal is pulverized to make a fine powder and then burned in air to produce steam to drive turbines. This process makes very hot flames that can create the pollutant nitrogen oxide, and the carbon dioxide generated is difficult to isolate and capture because it makes up only a small fraction of the exhaust gases.
In chemical looping, coal doesn’t react with air. Instead, it’s exposed to oxygen-bearing materials such as iron oxide. The coal reacts with these materials, and the energy bound up in coal breaks the bond between the oxygen and the iron. The reaction produces nearly pure carbon dioxide gas and iron metal (along with the mineral wüstite). Electricity is generated when the iron is moved out of the reaction chamber and is essentially burned—that is, allowed to react with oxygen in air. This releases heat to produce steam.
This rather convoluted process has at least two advantages. It produces a pure stream of carbon dioxide that’s easy to capture and ready to be stored underground. And the burning of iron in air also takes place at lower temperatures that don’t produce nitrogen oxide.
The process was originally suggested as a way to improve the efficiency of power plants. Now the hope is that it could, if deployed at a large scale, be one of the cheapest ways to reduce carbon dioxide emissions, leading to only a small increase in the cost of electricity.
An alternative approach to producing high-purity carbon dioxide streams is to burn coal in pure oxygen. But the equipment for producing pure oxygen is expensive.
Liang-Shih Fan, a professor of chemical and biomolecular engineering at Ohio State, says that his process could prove inexpensive. He says others have had difficulty using iron oxide because of its limited ability to carry oxygen. But he’s developed a new type of reactor that enhances the practical oxygen-carrying capacity of the iron oxide, decreasing the amount of material needed and improving the economics.
The Ohio State researchers are also pursuing another version of chemical looping, in which coal is gasified and the resulting combination of hydrogen and carbon monoxide gases, called syngas, reacts with the iron oxide (see “Using Rust to Capture CO2 from Coal Plants”). Construction has started on a 250-kilowatt pilot plant to test that approach. The recent test eliminates the gasification step by using pulverized coal instead of syngas, so it could potentially be cheaper.
The work is funded by the Department of Energy’s National Energy Technology Laboratory, which helped develop the fracking technology behind the current natural gas boom in the United States. NETL is also funding another approach to chemical looping, which uses calcium sulfate rather than iron oxide. That technology, which is being developed by Alstom, was demonstrated in a three-megawatt facility last year. The Ohio State work is at an earlier stage, but Fan says his approach could use far less material, making it more practical.