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.

Over 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 fired power plant in Ohio A coal power plant in Ohio churns out a lot of emissions. (Photo: peggydavis66/flickr)

Nitrogen 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.

The first clean coal plant

Petra Nova clean coal plant near Houston Ductwork takes flue gas from the coal plant to the carbon caption facility at the Petra Nova plant. (Photo: NRG Energy)

The first large-scale clean coal facility in the U.S. opened in early January 2017. The Petra Nova Project is a joint effort from NRG Energy and JX Nippon Oil & Gas Exploration Corporation. Located outside of Houston, the plant first captured carbon dioxide in September, and has since piped more than 100,000 tons of it 80 miles away. It is used at the West Ranch field, where it is used to force more oil from the ground, says the Washington Post.

A release from the companies calls it "the world's largest post-combustion capture system." The companies say the system captures more than 90 percent of carbon dioxide released from the equivalent of a 240-megawatt coal-fueled unit. They say the plant can capture more than 5,000 tons of carbon dioxide every day, which is equal to taking more than 350,000 cars off the road.

Critics weigh in

mountaintop removal mining Mountaintop removal mining in Kentucky as seen from an airplane. (Photo: Doc Searls/flickr)

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. Fossil fuels, including coal, accounts for about 37 percent of U.S. CO2 emissions, according to the Environmental Protection Agency, which contribute global climate change.

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.

Editor's note: The story was originally published in June 2011 and has been updated with new information.