Clean coal is a concept for processes or approaches that mitigate emissions of carbon dioxide (CO2) and other greenhouse gases, and radioactive materials, that arise from the use of coal, mainly for electrical power generation, using clean coal technology. Currently, the term clean coal is used in the coal industry primarily in reference to carbon capture and storage, which pumps and stores CO2 emissions underground. Plants using integrated gasification combined cycle (IGCC) involve coal gasification, which provides a basis for increased efficiency and lower cost in capturing CO2 emissions. Prior to the current focus on carbon capture and storage, the term clean coal had been used to refer to technologies for reducing emissions of NOx, sulfur, and heavy metals from coal.
There are seven clean coal technologies currently deployed: carbon capture and storage, flue-gas desulfurization, fluidized-bed combustion, integrated gasification combined cycle (IGCC), low nitrogen oxide burners, selective catalytic reduction (SCR), and electrostatic precipitators.
The widespread deployment of pollution-control equipment to reduce sulphur dioxide, NOx and dust emissions is just one example that brought cleaner air to many countries. The need to tackle rising CO2 emissions to address climate change later introduced Carbon Capture and Storage (CCS).
Within the United States, Carbon Capture and Storage technologies are mainly being developed in response to regulations by the Environmental Protection Agency—most notably the Clean Air Act—and in anticipation of legislation that seeks to mitigate climate change.
According to the United Nations Intergovernmental Panel on Climate Change, the burning of coal, a fossil fuel, is a significant contributor to global warming. (See the UN IPCC Fourth Assessment Report). For 1 ton of coal burned, 2.86 tons of carbon dioxide is created.
Sequestration technology[clarification needed] has yet to be tested on a large scale and may not be safe or successful. Sequestered CO2 may eventually leak up through the ground, may lead to unexpected geological instability or may cause contamination of aquifers used for drinking water supplies.
As 25.5% of the world's electrical generation in 2004 was from coal-fired generation (see World energy resources and consumption), reaching the carbon dioxide reduction targets of the Kyoto Protocol will require modifications to how coal is utilized.
The byproducts of coal combustion are hazardous to the environment in large quantities, but are present in extremely trace amounts in coal byproducts. According to the United States Geological Survey there is no cause for societal concerns or alarm regarding hazardous composites of coal byproducts.
It is possible to remove most of the sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter (PM) emissions from the coal-burning process, but carbon dioxide (CO2) emissions and the trace amounts of radionuclides  are more difficult to remove.
Coal-fired power plants are the largest aggregate source of mercury: 50 tons per year come from coal power plants out of 150 tons emitted nationally in the USA and 5000 tons globally. However, according to the United States Geological Survey, the trace amounts of mercury in coal byproducts do not pose a threat to public health.
Potential financial cost of clean coal
Whether carbon capture and storage technology is adopted worldwide will “…depend less on science than on economics. Cleaning coal is very expensive.” 
Cost of converting a single coal-fired power plant
According to an article in NCG magazine, April 2014, the converting of a conventional coal-fired power plant is done by injecting the CO2 into ammonium carbonate after which it is then transported and deposited underground (preferably in soil beneath the sea). This injection process however is by far the most expensive. Besides the cost of the equipment and the ammonium carbonate, the coal-fired power plant also needs to use 30% of its generated heat to do the injection (parasitic load). A test-setup has been done in the American Electric Power Mountaineer coal-burning power plant.
Costs for new coal-fired power plants
Newly built coal-fired power plants can be made to immediately use gasification of the coal prior to combustion. This makes it much easier to separate off the CO2 from the exhaust fumes, making the process cheaper. This gasification process is done in new coal-burning power plants such as the coal-burning power plant at Tianjin, called "GreenGen".
On a country-wide scale
The projected nationwide costs for the implementing of CCS in coal-fired power plants in the USA (presumably using a conventional tactic, see above) can be found in the Wall Street Journal article. Credit Suisse Group says $15 billion needs to be invested in CCS over the next 10 years for it to play an important role in climate change. The International Energy Agency says $20 billion is needed. The Pew Center on Global Climate Change says the number is as high as $30 billion. Those figures dwarf the actual investments to date.
In the US, the Bush administration spent about $2.5 billion on clean coal technology — a large amount, but far less than the amounts previously suggested. CCS proponents say both the government and the private sector need to step up their investments.
In the United States, clean coal was mentioned by former President George W. Bush on several occasions, including his 2007 State of the Union Address. Bush's position was that carbon capture and storage technologies should be encouraged as one means to reduce the country's dependence on foreign oil.
During the 2008 US Presidential campaign, both candidates John McCain and Barack Obama expressed interest in the development of CCS technologies as part of an overall comprehensive energy plan. The development of clean coal technologies could also create export business for the United States or any other country working on it.
The American Reinvestment and Recovery Act, signed in 2009 by President Obama, allocated $3.4 billion for advanced carbon capture and storage technologies, including CCS demonstration projects.
Former Secretary of State Hillary Clinton has said that "we should strive to have new electricity generation come from other sources, such as clean coal and renewables,” and former Energy Secretary Dr. Steven Chu has said that “It is absolutely worthwhile to invest in carbon capture and storage," noting that even if the U.S. and Europe turned their backs on coal, developing nations like India and China would not.
In Australia, carbon capture and storage was often referred to by former Prime Minister Kevin Rudd as a possible way to reduce greenhouse gas emissions. (The previous Prime Minister John Howard has stated that nuclear power is a better alternative, as CCS technology may not prove to be economically favorable.)
Environmentalists such as Dan Becker, director of the Sierra Club's Global Warming and Energy Program, believes that the term clean coal is misleading: "There is no such thing as clean coal and there never will be. It's an oxymoron." The Sierra Club's Coal Campaign has launched a site refuting the clean coal statements and advertising of the coal industry.
Complaints focus on the environmental impacts of coal extraction, high costs to sequester carbon, and uncertainty of how to manage end result pollutants and radionuclides. In reference to sequestration of carbon, concerns exist about whether geologic storage of CO2 in reservoirs, aquifers, etc., is indefinite/permanent.
Critics also believe that the continuing construction of coal-powered plants (whether or not they use carbon sequestration techniques) encourages unsustainable mining practices for coal, which can strip away mountains, hillsides, and natural areas. They also point out that there can be a large amount of energy required and pollution emitted in transporting the coal to the power plants.
The Reality Coalition, a nonprofit organization composed of Alliance for Climate Protection, Sierra Club, National Wildlife Federation, the Natural Resources Defense Council and the League of Conservation Voters, ran a series of television commercials in 2008 and 2009. The commercials were highly critical of clean coal, stating that without capturing CO2 emissions and storing it safely that it cannot be called clean coal.
Greenpeace is a major opponent of the concept because they view emissions and wastes as not being avoided but instead transferred from one waste stream to another. According to Greenpeace USA Executive Director Phil Radford, "even the industry figures it will take 10 or 20 years to arrive, and we need solutions sooner than that. We need to scale up renewable energy; “clean coal” is a distraction from that."
The term "clean coal" is increasingly used in reference to carbon capture and storage, an advanced process that eliminates or significantly reduces carbon dioxide emissions from coal-based plants and permanently sequesters them. More generally, the term has been found in modern usage to describe technologies designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation, and use.
U.S. Senate Bill 911 in April, 1987, defined clean coal technology as follows:
"The term clean coal technology means any technology...deployed at a new or existing facility which will achieve significant reductions in air emissions of sulfur dioxide or oxides of nitrogen associated with the utilization of coal in the generation of electricity."
In historical usage, "clean coal" has had quite different meanings:
- In the early 20th century, prior to World War II, clean coal (also called "smokeless coal") referred to anthracite and high-grade bituminous coal, used for cooking and home heating.
- The term also appeared in a speech to mine workers in 1918, when clean coal meant coal that was "free of dirt and impurities".
- Asia-Pacific Partnership for Clean Development and Climate
- Bio-energy with carbon capture and storage
- Clean coal technology
- Carbon Capture and Storage
- Carbon sink
- Coal in the United States
- Fossil fuel phase out
- Energy development
- Energy Policy Act of 2005
- Fluidized bed combustion
- James E. Hansen
- JEA Northside Generating Station (Jacksonville)
- Low carbon power generation
- Mitigation of global warming
- Mountaintop removal mining
- Pleasant Prairie Power Plant
- Refined coal
- Waste management
- World Coal Institute
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- The definition of clean coal
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- "Clean coal" history lesson
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