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I’ve been reading about clean coal technology—inparticular, coal gasification and its use in power production. Everyone touts the clean emissions consisting of carbon dioxide, which gets sequestered underground, and water vapor. But what about the other by-products, such as sulfur, nitrogen oxide, and ash? What about trace metals such as mercury, lead, and uranium? Remember, coal gasification isn’t new technology—gas companies are still cleaning up the sites of old “town gas” plants contaminated with dangerous chemicals such as xylene, toluene and benzene. What becomes of all this stuff? —Paul

You seem to know a lot about this, Paul, but I fear a salient point has eluded you. The big problem with clean coal isn’t dealing with all those dirty by-products. Rather, it’s getting rid of the stuff you’re calling clean.

If you’re a coal exec, promoting your industry’s product is no simple proposition. On the one hand, coal is a cheap, locally abundant energy source. On the other, it’s filthy. (It’s also dangerous and environmentally destructive to mine, but one thing at a time.) In part because of the filth factor, coal has never been high on anybody’s list of cool energy technologies.

Witness town gas. To make it, coal was converted via heat into hydrogen and carbon monoxide, which was then piped to homes and businesses to power lamps, stoves, and furnaces. A major drawback: the process left vast amounts of coal tar and other crud to dispose of.

Much of this could be sold, but a lot was buried in hopes that someone else would deal with it later. Between 1889 and 1950 more than 1,500 U.S town-gas plants produced 11 billion gallons of tar, about a quarter of which wound up as waste. When natural gas became widely available after World War II, it chased town gas out of the market.

Coal gasification got another chance when electric utilities seeking to comply with environmental regulations at coal-fired plants came up with a technology called integrated gasification with combined cycle (IGCC).

The latest iteration of IGCC is the FutureGen project, proposed by a public-private consortium. Touted as “the world’s first near-zero emission coal-fueled power plant,” it’s set to be built (pending federal approval) at Mattoon, Ill., at a cost of nearly $2 billion. Controlling the toxic pollutants should be doable; the techniques are fairly well understood, if pricey.

Unfortunately, all that’s the easy part. More challenging will be the stated goal of capturing 90 percent of FutureGen’s carbon dioxide, which is a clean emission only if we can figure out what to do with what we capture. Some carbon compounds can be sold for industrial use, but the main idea is to inject CO2 into subterranean oil or gas reservoirs or porous rock formations. This technology is still in its infancy—as of late 2009 not even a dozen significant projects were under way worldwide. Risks include slow leakage of stored gas plus the occasional full-on blowout.

The most serious problem, though, may be running out of planet. Opinions vary on how much CO2 we can stuff underneath the U.S.: while some say as much as 14 trillion metric tons, Department of Energy researchers estimate it’s more like 3.5 trillion. Even the lower figure should be plenty. If we were somehow able to capture 90 percent of all the CO2 produced by U.S. coal power plants annually, theoretically we’d have enough room to store 2,000 years’ worth.

But that could be optimistic. A recent paper (Ehlig-Economides and Economides, 2010) claims actual underground storage capacity could be less than a hundredth of what scientists now assume. If so, U.S. coal power plants over a 30-year period would require underground storage almost three and a half times the size of Texas. Should this be confirmed, our global-warming life rope just broke. —Cecil Adams

Is there something you need to get straight? Take it up with Cecil at straightdope.com.