Not long ago, the phrase “clean coal” seemed like an oxymoron. Coal-fired power plants emit a witch’s brew of air pollutants that, unless it is removed with scrubbers, tarnishes the air, creates acid rain and can cause asthma or heart attacks. And coal plants emit twice the planet-heating carbon dioxide of natural-gas-fired power plants.
A new type of reactor, however, one that captures more than 99 percent of the carbon dioxide generated by burning coal, could make “clean coal” feasible. Carbon dioxide can be stowed safely beneath the earth’s surface where it can’t contribute to climate change.
This reactor would capture carbon without jacking up the price of electricity, and this could make it commercially viable. “It’s an entirely new way to generate power from coal that’s low-carbon,” says Karma Sawyer who directs the clean-coal research program at the U.S. Department of Energy’s (DOE) research agency ARPA-E, which funded the work.
Burning coal is responsible for producing about 40 percent of the world’s electricity, but it produces three-fourths of the more than 12 billion tons of carbon dioxide emitted during electricity and heat generation. To make coal nonpolluting, that carbon dioxide would have to be captured before it’s emitted and permanently locked away under the earth. But despite years of research, not one of the coal-fired power plants in the United States does this.
Nevertheless, coal-fired power plants still supply much of the world’s electricity, and coal reserves in the U.S. and elsewhere remain plentiful and affordable. For these reasons--and because of the coal industry’s political clout--the DOE has invested more than $3.4 billion toward carbon-capture and storage technologies.
Today’s most advanced carbon-capture technology, called amine scrubbing, is effective and mature, but it’s too expensive. In amine scrubbing, named after the alkylamines used in the process, coal is first burned the usual way, with air, and the resulting flue gas bubbles through a liquid that traps the carbon dioxide. Then the liquid is heated to release the carbon dioxide, which escapes much as a cool can of soda emits carbon dioxide bubbles as it warms to room temperature. This process sucks up almost one-third of the energy produced by the entire power plant--enough to warrant an 80 percent price hike for consumers. Such a spike in cost is untenable, so utilities have shied away from installing such scrubbers.
A few years ago, the DOE challenged researchers to devise a technology that could remove more than 90 percent of the carbon dioxide emitted by a plant, while keeping the price of coal-powered electricity from a conventional plant from rising more than 35 percent to date. So far the DOE has invested in research on more than a dozen experimental carbon-removal technologies. “There’s no silver bullet yet, which is why we have a big program,” says Lynn Brickett, division director of the Existing Plants Division of the DOE’s National Energy Technology Laboratory in Pittsburgh, Pennsylvania.
One of the most promising new technologies starts with pulverized coal, a dry mix the consistency of talcum powder that’s already burned in many coal-fired power plants. The pulverized coal is mixed with partially rusted iron particles the size of ice cream sprinkles inside a hot reactor at 1,650 degrees Fahrenheit. The coal-iron mixture undergoes a chemical reaction that removes the rust and produces carbon dioxide and steam, which is then cooled and liquid water condenses out, leaving a highly purified stream of carbon dioxide.
The rust-free iron particles then move to a second reactor, where they are burned under air, causing them to rust again. This rusting reaction produces enough heat to boil water, and the resulting steam drives an electricity-producing turbine.
The carbon-capturing material does not need to be separately heated to liberate pure carbon dioxide, as it does in amine scrubbing, and for that reason “the capture energy requirements are almost negligible,” explains Liang-Shih Fan, the Ohio State University chemical engineer who spearheaded this research.