Clean coal technology is an umbrella term used to describe technologies being developed that aim to reduce the environmental impact of coal energy generation. These include chemically washing minerals and impurities from the coal, gasification, treating the flue gases with steam to remove sulfur dioxide, carbon capture and storage technologies to capture the carbon dioxide from the flue gas and dewatering lower rank coals (brown coals) to improve the calorific quality, and thus the efficiency of the conversion into electricity.
Clean coal technology is an attempt to reduce or hide the impact of burning coal. Burning coal has hugely harmful effects, and most of the clean coal technologies attempt to capture the harmful chemicals preventing them from reaching the ecosphere. Historically, the primary focus was on sulfur dioxide and particulates, due to the fact that it is the most important gas which leads to acid rain. More recent focus has been on carbon dioxide (due to its likely impact on global warming) as well as other pollutants.
Coal, which is primarily used for the generation of electricity, is the second largest domestic contributor to carbon dioxide emissions in the USA.
The term ‘clean coal' is often stated in quotation marks by its critics due to claims that it is a misnomer and a public relations term. However, the U.S. government employs the term in its research, as demonstrated by the U.S. Department of Energy's Clean Coal Technology Program. The DOE defines clean coal as "a new generation of energy processes that sharply reduce air emissions and other pollutants from coal-burning power plants."
The Clean Coal Power Initiative is providing government co-financing for new coal technologies that can help utilities cut sulfur, nitrogen and mercury pollutants from power plants. Also, some of the early projects are showing ways to reduce greenhouse emissions by boosting the efficiency by which coal plants convert coal to electricity or other energy forms.
According to the United Nations Intergovernmental Panel on Climate Change, the burning of coal, a fossil fuel, is blamed for climate change and global warming. As 25.5% of the world's electrical generation in 2004 was from coal-fired generation, reaching the carbon dioxide reduction targets of the Kyoto Protocol will require modifications to how coal is utilized. That is, changing power plants etc to be cleaner at a retooling or replacement cost.
The byproducts of coal combustion are considerably hazardous to the environment if not properly contained. This is clean coal's largest challenge, both from the practical and public relations perspectives.
In addition to the direct pollutants from burning coal, power plants produce large amounts of solid and liquid waste products, mostly in the form of fly-ash and bottom ash. These waste products are stored in landfills and large ponds. Increasing emission controls at the plants results in increased waste products. It is estimated that coal burning power plants produce over 100 million tons of waste per year and that well over 2 billion tons of waste is stored at plants in the USA. While the hazard content of ash by percentage is very low, the concentration of millions of tons at plant sites creates the danger of significant pollution in the event of containment failures. A small fraction of coal ash is beneficially used in the manufacture of concrete and other construction materials. The use of ash in construction materials sequesters the hazardous ingredients and prevents their release in quantities large enough to be hazardous. Unfortunately the economics of beneficial use are such that some subsidy would be required from the power plant for widespread use.