A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Is "clean coal" really clean?

July 9, 2010

Dear Cecil:

I've been reading about clean coal technology — in particular, coal gasification and its use in power production. Everyone touts the clean emissions consisting of CO2, 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?

Cecil replies:

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 their coal-fired plants came up with an improved technology called integrated gasification with combined cycle (IGCC). Coal is heated to produce gas, as with town gas, but here the stuff is burned on the spot in a turbine, which spins an electric generator. The hot exhaust from the turbine is then used to create steam, which spins another turbine, which makes more power (hence "combined cycle"). Currently two full-scale IGCC plants are running in the U.S.; cleaner and more efficient than previous systems, the process is seen as the new hope for coal.

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, Illinois, at a cost of nearly $2 billion. The project's backers claim they can deliver 99 percent sulfur and ash removal, 90 percent mercury removal, and low nitrogen oxide production. Plus — and this is what gets everybody's attention — the plant supposedly will capture carbon, too.

Controlling the toxic pollutants should be doable; the techniques involved are fairly well understood, if pricey. The coal's sulfur content will be converted to hydrogen sulfide and ultimately to marketable elemental sulfur. Mercury will be captured in a bed of activated carbon, which will then be landfilled or reprocessed to extract the mercury for storage or sale. Lead, arsenic, and other heavy metals will be removed by water scrubbing and captured in the plant's water-treatment system. Ash will be captured as molten slag or light fly ash and landfilled. Ideally the leftover chemicals you mention will be safely burned off in the combustion process.

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 all the underground storage beneath an area almost three and a half times the size of Texas. Should that be confirmed, our global-warming life rope just broke.

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References

“Carbon Sequestration” Science 325 (2009): 1644-1645.

Chu, Steven “Carbon Capture and Sequestration” Science 325 (2009): 1599.

Ehlig-Economides, Christine and Economides, Michael J. “Sequestering carbon dioxide in a closed underground volume” Journal of Petroleum Science and Engineering 70 (2010): 123–130.

Erora Group TEC IGCC Feasibility Analysis Report Prepared Pursuant to Agreement No. SIUC 04-15, Fixed Cost Contract Between The Board of Trustees of Southern Illinois University and the Erora Group, LLC, January, 2005.

FutureGen Alliance: Clean Energy for a Secure Future FutureGen Initial Conceptual Design Report PNWD-3760, May, 2007.

Holloway, Sam “Carbon dioxide capture and geological storage” Phil. Trans. R. Soc. 365 (2007): 1095-1107.

Jordan, Preston T. and Benson, S.M. “Well blowout rates and consequences in California Oil and Gas District 4 from 1991 to 2005: Implications for geological storage of carbon dioxide” Lawrence Berkeley National Laboratory, August 5, 2008.

Orr, Franklin M. et al. “Onshore Geologic Storage of CO2” Science 325 (2009): 1656-1658.

Testimony, Paul Thompson, Chairman of the Board FutureGen Alliance, Before the U.S. Senate Committee on Appropriations Subcommittee on Energy and Water Development, May 8, 2008.

United States Energy Information Administration, Office of Integrated Analysis and Forecasting, U.S. Department of Energy, Washington, DC 20585 Emissions of Greenhouse Gases in the United States 2008 DOE/EIA-0573 (2008), December, 2009.

United States Environmental Protection Agency Survey of Town Gas and By-Product Production and Locations in the United States (1880-1950) Air and Energy Engineering Research Laboratory, Research Triangle Park, NC. EPA-600/7-85-004, February, 1985.

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