Calera's process has a side benefit that could make it particularly attractive to the owners of existing coal- and gas-fired power plants: It traps the so-called criteria pollutants — sulfur dioxide, nitrogen oxides, particulates, heavy metals — that the Clean Air Act requires power plants to "scrub" from their smokestacks by 2012. Roughly half the plants in the U.S. haven't complied with the law, because of the expense and the fact that 20 percent of the electricity a plant produces would have to be used for scrubbing. Add a scrubber to separate out carbon — the most conventional route to clean coal — and you eat up another 20 percent. Total cost: $1.7 billion for a 500-megawatt plant. "If you own an old coal plant that's already at 35 percent efficiency, you're pretty much out of business," says Calera CEO Brent Constantz. In contrast, he estimates, it would cost $400 million for a 500-megawatt plant to install his company's technology.