Dear EarthTalk: as I understand it, coal that is used to fuel power plants and other industrial activity is a key culprit in pollution and climate change. So what is "clean coal" and is it really?

The term "clean coal" describes various processes that remove pollutants from coal, our cheapest, most abundant--and dirtiest--energy source. By reducing coal's environmental footprint through technological wizardry, the coal mining industry and the Bush administration hope to keep coal, which currently produces more than half of all U.S. electricity, a big part of our energy picture for many years to come.

Clean coal proponents also want to liquefy coal to turn it into a form of automotive fuel that, according to the industry-sponsored Coal-to-Liquids Coalition, costs less and burns cleaner in some ways than the traditional diesel fuel it could replace. Several members of Congress from coal states are keen on having the government subsidize the production of so-called liquid coal--which can be used anywhere diesel fuel currently goes--as a "homegrown" alternative to foreign oil. Industry analysts say there is enough coal in America to last hundreds of years, saving us untold expense and trouble obtaining regular petroleum from unfriendly foreign governments.

But major environmental groups, from the Sierra Club to the Natural Resources Defense Council, say that "clean coal" is anything but. The process involves heating coal to 1,000 degrees Fahrenheit and mixing it with water to produce a gas, then converting the gas into diesel fuel. Although the Coal-to-Liquids Coalition says that carbon dioxide emissions from the entire production cycle of liquid coal are "equal to, or slightly below, those of conventional petroleum-derived fuels," its claims are based on a single federal study, now six years old, that environmental leaders disagree with profoundly.

Jim Presswood, federal energy advocate of the Natural Resources Defense Council says, "Liquid CO2 emissions are twice as much as emissions from conventional petroleum-derived fuels." He says that even if CO2 emissions were captured as part of the process, at best liquid coal would be 12 percent worse than the gasoline equivalent. As some environmentalists have put it, liquid coal can turn any hybrid Prius into a Hummer.

The Washington Post editorialized, "To wean the U.S. off of just one million barrels of the 21 million barrels of crude oil consumed daily, an estimated 120 million tons of coal would need to be mined each year. The process requires vast amounts of water, particularly a concern in the parched West. And the price of a plant is estimated at $4 billion." Also, in recent years, particularly in Appalachia, mining companies have gone from simple excavation to blasting off the tops of mountains in an ecologically devastating process known as "mountain top removal."

For their part, greens acknowledge the importance of cleaning up coal and other dirty energy sources, but would rather see more funding devoted to researching, developing and implementing alternative and renewable energy sources that don't come with so much environmental baggage.

CONTACTS: Coal-to-Liquids Coalition, www.futurecoalfuels.org; Sierra Club's "Stopping the Coal Rush," www.sierraclub.org/environmentallaw/coal.

--Matthew Oliver, Minneapolis, MN