Romney said “I like coal. I’m going to make sure we’re going to be able to burn clean coal.” Obama in the past has also used the phrase “clean coal.” 

There is no such thing as “clean coal.” There are ways to burn coal more efficiently, and there are scrubbers that can stop emissions of mercury and sulfur. And there are ways to capture carbon dioxide, the most common greenhouse gas, produced by burning coal and bury it, but all of those capture techniques have proven too expensive to be commercially viable. A project financed half by the Energy Department and half by the big utility AEP was abandoned.