Clean coal is a buzzword in political circles.

On the campaign trail, both presidential candidates tout the virtues of clean coal.

"And we must find a way to stop coal from polluting our atmosphere without pretending that our nation's most abundant energy source will just go away. It won't," Sen. Barack Obama said in Portsmouth, N.H., this month. "That's why we must invest in clean coal technologies that we can use at home and share with the world."

Democrat Obama has promised that clean coal will be the focus of his administration's $150 billion plan to develop zero-carbon technologies.

Said Republican Sen. John McCain in Missouri: "Perhaps no advancement in energy technology could mean more to America than the clean burning of coal and the capture and storage of carbon emissions.

"And to this end, as president, I will commit $2 billion each year on clean coal research and development. We will build the demonstration plants, refine the techniques and equipment and make clean coal a reality."

President Bush, lawmakers and policy wonks alike agree that clean coal is a viable solution to wean the nation from foreign oil without causing too much damage to the environment, what with the nation sitting on 250 years' worth of coal supply.

Yet clean coal production has been hit by project failures, irregular investment and technologies that have yet to be tested in the marketplace.

Earlier this year, the federal government canceled its support for a project in Illinois that would have demonstrated clean coal technology.

In Colorado, Xcel Energy has shelved its clean coal project, saying it doesn't need the power and citing regulatory concerns about carbon dioxide emissions.

That investors are skeptical is evident by their scant interest in Colorado's two clean coal companies, Rentech Inc. and Evergreen Energy Inc. - both trading well below $1 per share.

"I do think clean coal is a mess," said Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley. "The federal program that was canceled was problematic, and the private sector has not stepped up."

Today, a clean-coal power plant could cost up to 10 times the cost of a natural gas-fired power plant, and commercial deployment of the technology in a large plant could be years away, he added.

Speaking at a conference in Denver this week, renewable energy venture capitalist Vinod Khosla said policymakers and companies should be "pragmatically" and not "religiously" environmental.

"Coal with traditional carbon sequestration is extremely unlikely to work," Khosla, the co-founder of Sun Microsystems, said.