When people talk about clean coal it's more of a verb than a noun.

In a sense, it’s an attempt to bring this storied energy source into the 21st century with a somewhat greener profile.

Coal mineOver the centuries, coal’s dominance has been undisputed: the Industrial Revolution was powered by coal and so is the digital age of the Internet and flat-screen televisions. Coal-fired power plants generate nearly 45 percent of America’s electricity, compared to 23.8 percent from natural gas and 19.6 percent from nuclear power plants. Coal as a source of energy is time-tested, comparatively cheap, abundant within the security of our own boarders and...dirty.

But, modern technology should help mitigate coal’s effects on the environment.

What is clean coal? Clean coal technology encompasses a variety of technologies and techniques to reduce harmful emissions and improve the efficiency of coal-burning power plants.

Evolving clean coal technology has been undeniable effective: emissions of sulfur dioxide, nitrogen oxides, and particulate matter per kilowatt-hour have been reduced by more than 80 percent since 1970. That means nitrogen oxide emissions have been reduced by more than one-third and sulfur dioxide emissions have dropped by more than 56 percent even as the use of coal to make electricity has nearly tripled.

One method used to reduce sulfur dioxide emissions is to simply wash away the sulfur before burning the coal. Coal chunks are fed into large water-filled tanks where the coal floats to the surface and the sulfur impurities sink.

Most of the sulfur, however, must be removed using flue gas desulfurization units, or smoke stack scrubbers, that spray a mix of limestone and water into the flue gases and captures the sulfur.

Coal factory in ChicagoNitrogen oxide emissions, a byproduct of burning, are reduced by carefully calibration of the burners. Fluidized bed boilers — a technology that is about 30 years old — burn coal particles suspended on upward-blowing jets of air. The burning coal looks like volcano lava — fluidized. Fluidized bed boilers burn at 1,400 F — much cooler than traditional boilers. While hot enough to make steam, that’s not hot enough to make nitrogen oxide.

Add some limestone to the coal in the fluidized bed boilers and sulfur emissions are reduced. Fluidized bed boilers remove 90 percent of pollutants while the coal is burning.

The Clean Coal Technology Program of the United States Department of Energy has sponsored tests of such boilers in Colorado, Ohio and Florida.

Coal gasification — converting coal into synthetic gas by a process using incomplete combustion to create carbon monoxide and then breaking the carbon monoxide down into a substitute natural gas — is cleaner still.

Critics of clean coal technology — including DeSmog Project, Greenpeace USA and Rainforest Action Network— argue that the environmental costs of burning coal are still too high. Burning coal contributes 40 percent of U.S. CO2 emissions, which contribute global climate change, the groups say.

The mining practice known as mountaintop removal — which involves clear cutting and blasting away as much as 1,000 feet of mountaintop and pushing the debris downhill — destroys streams and wildlife habitat, the groups argue.

And while use of clean coal technology has reduced some pollutants, the groups note, mercury pollution from coal plants continue to climb. Carbon dioxide emissions — the prime driver of global warming — have increased 27 percent since 1990.

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Photos: Wikimedia Commons; swanksalot/Flickr

See also:

What is carbon capture?