Clean Coal Technology

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This article is part of the CoalSwarm coverage of "clean coal."
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As described in the article Clean coal, the meaning of the term "clean coal" has evolved over time, with two main current uses:

  • Incrementally cleaner coal: One usage generally refers to efforts to reduce the amount of air pollution created by coal-based power generation. As used in government programs, the term typically refers to measures to reduce air pollution through practices such as chemical processes for washing coal of minerals or impurities, smokestack scrubbers, or coal gasification. It should be noted that most such processes create new waste pollution streams.
  • Futuristic promises of "zero-emissions coal": The other usage of the term is typically found in coal industry advertising and messaging, where a future in which coal can be used without environmental damage is implied, often indirectly rather than explicitly.

When used to describe currently available practices that reduce air pollution the name is attributed to coal chemically washed of minerals and impurities, sometimes gasified, burned and the resulting flue gases treated with steam, with the purpose of removing sulfur dioxide, and reburned so as to make the carbon dioxide in the flue gas economically recoverable. The coal industry uses the term "clean coal" to describe technologies designed "to enhance both the efficiency and the environmental acceptability of coal extraction, preparation, and use"[1], with no specific quantitative limits on any emissions, particularly carbon dioxide.

The burning of coal has been shown to be one of the principal causes of anthropogenic climate change and global warming, according to the United Nations Intergovernmental Panel on Climate Change.[2] The concept of clean coal is said to be a solution to climate change and global warming by coal industry groups, while environmental groups believe the claim is misleading and inaccurate. Greenpeace is a major opponent of the concept because emissions and wastes are not avoided, but are transferred from one waste stream to another.[3] The 2007 Australian of the Year, paleontologist and environmental activist Tim Flannery made the assertion that "Coal can't be clean."[4]

There are no coal-fired power plants in commercial production or construction which capture all carbon dioxide emissions. It is has been estimated that it will be at least fifteen to twenty years before any commercial-scale clean coal power stations (coal-burning power stations with carbon capture and sequestration) are commercially viable and widely adopted.[5] This time frame is of concern to environmentalists because of the belief that there is an urgent need to mitigate greenhouse gas emissions and climate change to protect the world economy.[6] Even when CO2 emissions can be caught, there is considerable debate over the necessary carbon capture and storage that must follow.


The byproducts of coal combustion are very hazardous to the environment if not properly contained. This is seen to be the technology's largest challenge, both from the practical and public relations perspectives. As mentioned in the article Environmental impacts of coal, a typical 500MW coal-fired power plant generates an enormous amount of air pollutants each year, including 10,000 tons of sulfur dioxide (SO2), 10,200 tons of nitrogen oxide (NOx), 500 tons of particulate matter, and 3.7 million tons of carbon dioxide (CO2), along with many other toxins.[7]

While it is possible to remove most of the sulfur dioxide, nitrogen oxides, and particulate (PM) emissions from the coal-burning process, carbon dioxide (CO2) emissions and radionuclides will be more difficult to address.[8] Technologies do exist to capture and store CO2, but they have not been made available on a large-scale commercial basis due to the high economic costs.[9]

Sulfur dioxide

The sulfur gas produced by burning coal can be partially removed with scrubbers or filters. In conventional coal plants, the most common form of sulfur dioxide control is through the use of scrubbers. To remove the SO2, the exhaust from a coal-fired power plant is passed through a mixture of lime or limestone and water, which absorbs the SO2 before the exhaust gas is released through the smokestack. Scrubbers can reduce sulfur emissions by up to 90 percent, but smaller particulates are less likely to be absorbed by the limestone and can pass out the smokestack into the atmostphere.[10][11] In addition, scrubbers require more energy to operate, thus increasing the amount of coal that must be burned to power their operation.[12][11]

Other coal plants use "fluidized bed combustion" instead of a standard furnace. Fluidized bed technology was developed in an effort to find a combustion process that could limit emissions without the need for external emission controls such as scrubbers. A fluidized bed consists of small particles of ash, limestone and other non-flammable materials, which are suspended in an upward flow of hot air. Powderized coal and limestone are blown into the bed at high temperature to create a tumbling action, which spurs more effective chemical reactions and heat transfer. During this burning process, the limestone binds with sulfur released from the coal and prevents it from being released into the atmosphere. Fluidized bed combustion plants generate lower sulfur emissions than standard coal plants, but they are also more complex and expensive to maintain.[13][11] According to the Union of Concerned Scientists, sulfur emissions decreased by 33 percent between 1975 and 1990 through the use of scrubbers and fluidized bed combustors, as well as switching to low-sulfur coal.[11]

Nitrogen oxides

NOx pollutants do not form in significant amounts at temperatures below 2800 degrees Fahrenheit. Initially the focus of NOx controls was on finding ways to burn fuel in stages. "Low-NOx" burners use a staged combustion process, which uses a lower flame temperature during some phases of combustion to reduce the amount of NOx that forms. These burners also limit the amount of air in the initial stages of combustion, when the nitrogen naturally occurring in the coal is released, so that there is less oxygen present to bond with the nitrogen. As of 2006, the Department of Energy estimated that approximately low-NOx burners were installed on conventional coal plants as of 2006. These burners can reduce nitrogen oxide emissions by 40 percent or more. In the case of fluidized bed technology, combustion occurs at temperatures of 1,400 to 1,700 degrees Fahrenheit, lower than the threshold at which nitrogen oxide pollutants form.[13]

In 2005, the EPA passed the Clean Air Interstate Rule, which requires a 61 percent cut in nitrogen oxide emissions from power plants by 2015.[14] This level of emissions reduction requires a different technology. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) both convert NOx into water (H2O) and nitrogen (N2). SCR is capable of reducing NOx emissions by approximately 90 percent. SNCR is a simpler and less expensive technology than SCR, but it also provides a lower level of NOx reduction.[15][16][17]

Particulate matter

There are several methods of removing fine particulate matter before it can be released from the smokestack. Wet scrubbers remove dust pollutants by capturing them in liquid droplets and then collecting the liquid for disposal.[18] Electrostatic precipitators electrically charge particles in the flue gas and collect the particles on plates to remove them from the air stream. Wet electrostatic precipitators combine the functions of a standard dry electrostatic precipitator with a wet scrubber and spray moisture to the air flow to help collect extremely fine particulate matter (PM2.5), making the process more effective.[19][20] Fabric filter baghouses are another means of controlling particulate matter emissions. As dust enters the baghouse compartment, larger particles fall out of the system, while smaller dust particles are collected onto cloth filters.[21][22]

Coal waste

While it is possible to control some of the toxic emissions released by coal-fired power plants, the fesulting waste creates more problems for the environment. The pollution controls used to capture harmful emissions concentrate toxins and heavy metals such as mercury into coal ash and sludge. According to the Union of Concerned Scientists, a typical 500MW coal plant produces 120,000 tons of ash and 193,000 tons of sludge per year from the smokestack scrubber, of which more than 75% nationally is disposed of in unlined, unmonitored onsite landfills and surface impoundments. Toxic substances in ash and sludge include arsenic, mercury, chromium, and cadmium.[23] In 2006, coal plants in the United States produced almost 72 million tons of fly ash, up 50 percent since 1993.[24] A 2007 EPA report cited 67 cases in the United States of damage to ground or surface water from coal combustion products.[25]

Carbon dioxide

Carbon Capture and Storage

The processes described above for controlling the emissions of some pollutants cannot remove CO2 from plant emissions and as such are not a means of combatting climate change. The technology behind the development of zero-carbon emissions coal is Carbon Capture and Storage (CCS), which is a means of separating out carbon dioxide when burning fossil fuels, collecting it and subsequently “dumping” it underground or in the sea. CCS is an integrated concept consisting of three distinct components: CO2 capture, transport and storage (including measurement, monitoring and verification). All three components are currently found in industrial operation today, although mostly not for the purpose of CO2 storage.[26] Some capture technologies are economically feasible under specific conditions, while others remain in the research stages. To date, there has not been a single application of CCS to large scale (> 500 MW) power stations.[27]

Concerns about the potential use of CCS technology include the projected high costs of the process; the human health dangers of large, rapid releases of carbon dioxide; the global warming risk posed by small levels leakage over long periods; and the increases in coal mining that would be necessary to run scrubbers as well as carbon capture and storage systems. Critics of CCS contend that there is no such thing as "clean coal," since even technology projected by 2020 will still release large amounts of pollutants compared to renewable energy sources such as wind, concentrated solar power, photovoltaic power, hydropower, and geothermal power. 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. Opponents note that carbon capture and storage technology has yet to be used or proven on such a large scale and that it may not be successful. There are also concerns that pumping sequestered CO2 into oil and gas wells to help make the fuels easier to pump out of the ground will lead to further consumption of fossil fuels, and CO2 emissions, thus adding to global warming.[28]

Prohibitive cost of retrofitting existing coal plants for carbon capture

According to the U.S. Department of Energy, it is not economical to retrofit existing coal plants with carbon capture technology:

Existing CO2 capture technologies are not cost-effective when considered in the context of large power plants. Economic studies indicate that carbon capture will add over 30 percent to the cost of electricity for new integrated gasification combined cycle (IGCC) units and over 80 percent to the cost of electricity if retrofitted to existing pulverized coal (PC) units. A recent study from the National Energy Technology Laboratory (NETL) confirms that additional alternatives need to be pursued to bring the cost of carbon capture down. In addition, the net electricity produced from existing plants would be significantly reduced - often referred to as parasitic loss - since 20 to 30 percent of the power generated by the plant would have to be used to capture and compress the CO2.[29]

Cost of carbon capture and storage for new plants

Adding carbon and capture technology to new coal plants makes electricity from coal more expensive than energy from solar thermal and wind power, even when "firming costs" are included for alternatives (see table).

Capturing and compressing CO2 requires much energy, significantly raising the running costs of CCS-equipped power plants. In addition there are added investment or capital costs. The process would increase the energy needs of a plant with CCS by about 10 to 40 percent. The costs of storage and other system costs are estimated to increase the costs of energy from a power plant with CCS by 30 to 60 percent, depending on the specific circumstances.

The following table shows cost comparisons published by the California Energy Commission in 2008 for various power-generation technologies, including integrated gasification combined cycle (IGCC) with CCS.

Costs of energy with and without CCS (US cents per kWh - 2008)

Pulverized coal IGCC Nuclear Solar thermal Wind
Without capture 10.6 11.5 15.3 12.7 8.9
With capture and storage N/A 17.3 15.3 12.7 8.9
Figures prepared for California Energy Commission by Energy and Environmental Economics, Inc., a consulting firm that prepares studies for utilities, governmental regulators, law firms, and non-profit agencies.[30] These estimates include firming resource costs. Further information can be found at Comparative electrical generation costs.

The cost of CCS depends on the cost of capture and storage which vary according to the method used. Geological storage in saline formations or depleted oil or gas fields typically cost US$0.50–8.00 per tonne of CO2 injected, plus an additional US$0.10–0.30 for monitoring costs. When storage is combined with enhanced oil recovery to extract extra oil from an oil field, the storage could yield net benefits of US$10–16 per tonne of CO2 injected (based on 2003 oil prices). However, as the table above shows, the benefits do not outweigh the extra costs of capture.

Schwarzenegger clause

In October 2008, the European Parliament's Environment Committee voted to support a limit on CO2 emissions for all new coal plants built in the EU after 2015. The so-called "Schwarzenegger clause" applies to all plants with a capacity over 300MW, and limits their annual CO2 emissions to a maximum of 500 grammes per kilowatt hour. The new emissions standard essentially rules out traditional coal plant technologies and mandates the use of Carbon Capture and Storage technologies. The Committee also adopted an amendment to support the financing of 12 large-scale commercial CCS demonstration projects, at a cost that could exceed €10 billion.[31][32]

U.S. Clean Coal Power Initiative (CCPI)

According to the U.S. Department of Energy:

"The Clean Coal Power Initiative (CCPI) is a 10-year, $2 billion program designed to support the Clean Coal Technology Roadmap milestones with the government providing up to 50 percent of the cost of demonstrating a range of promising technologies. CCPI is implemented through a series of five solicitations over the 10-year period, two of which have already been issued and selections made. CCPI provides the means to demonstrate those technologies proven through R&D to have commercial potential. Demonstrations are at a commercial scale in actual operating environments, which is essential to moving them to the threshold of commercialization."[33]

As of April, 2008, 8 projects were active and 4 had been withdrawn.[33] According Department of Energy Fact Sheet, the multi-year Clean Coal Power Initiative (CCPI), "is driven by private-sector-proposed projects in response to a government solicitation. Potential applicants include technology developers, service corporations, R&D firms, energy producers, software developers, academia, and other interested parties. The private sector cost share must be at least 50 percent. Funding is awarded to applicants, selected as a result of these open competitions, who can rapidly move promising new concepts to a point where private-sector decisions on deployment can be made." [34]

Round I participants:[34]

  • Great River Energy, Underwood, ND - Increasing Power Plant Efficiency–Lignite Fuel Enhancement
  • NeuCo, Inc., Boston, MA - Demonstration of Integrated Optimization Software at the Baldwin Energy Complex
  • University of Kentucky Research Foundation, Lexington, KY - Advanced Multi-Product Coal Utilization By-Product Processing Plant
  • WMPI PTY., LLC, Gilberton, PA - Gilberton Coal-to-Clean Fuels and Power Co-Production Project
  • Western Greenbrier Co-Generation, LLC, Lewisburg, WV - Western Greenbrier Co-Production Demonstration Project
  • Wisconsin Electric Power Co., Milwaukee, WI - TOXECON Retrofit for Mercury and Multi-Pollutant Control on Three 90 MW Coal-Fired Boilers

Round II participants:[34]

  • Excelsior Energy, Inc., Minnetonka, MN - Mesaba Energy Project
  • Pegasus Technologies, Incorporated, Chardon, OH - Mercury Specie and Multi-Pollutant Control
  • Southern Company Services, Birmingham, AL - Demonstration of a 285-MW Coal-Based Transport Gasifier



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  2. IPCC Fourth Assessment Report, Intergovernmental Panel on Climate Change, 2007.
  3. Clean Coal Myths and Facts,, accessed April 2008.
  4. "Coal Can't Be Clean", Herald Sun, February 14, 2007.
  5. "What is 'clean' coal and can it really save Australia's environment?",, February 20, 2007.
  6. Stern Review: The Economics of Climate Change, BBC News, October 30, 2006.
  7. "Coal Power: Air Pollution," Union of Concerned Scientists, accessed August 2008
  8. Coal Combustion: Nuclear Resource or Danger, ORNL Review Vol. 26, No. 3&4, 2003.
  9. Clean coal technology: How it works, BBC News, November 28, 2005.
  10. "Sulfur Dioxide Scrubbers," Duke Energy, accessed November 2008.
  11. 11.0 11.1 11.2 11.3 How Coal Works, Union of Concerned scientists, Aug 23, 2008.
  12. "Sulfur Dioxide Scrubbers," Duke Energy, accessed November 2008.
  13. 13.0 13.1 "Fluidized bed technology - an overview," Department of Energy, April 17, 2007.
  14. “Utilities amassing landfills: Tougher air standards send tons of plants' sludge, coal ash into ground”, Columbus Dispatch, April 14, 2008.]
  15. Selective catalytic reduction, De-NOx Technologies, accessed November 2008.
  16. Selective non-catalytic reduction, De-NOx Technologies, accessed November 2008.
  17. Nitrogen Oxides, Tennessee Valley Authority, accessed November 2008.
  18. Particulate Matter Controls: Chapter 2, EPA, December 1998.
  19. Particulate Matter Controls: Chapter 3, EPA, December 1998.
  20. Environmental Equipment: Particulate Control, Babcock & Wilcox, accessed November 2008.
  21. Particulate Matter Controls: Chapter 1, EPA, December 1998.
  22. Baghouse/Fabric Filter Knowledgebase, Neundorfer, accessed November 2008.
  23. "Coal Power: Wastes Generated," Union of Concerned Scientists, accessed August 2008
  24. "Fly ash: Culprit at Lafarge? Residue of coal-burning is being examined as possible source of mercury pollution," Times Union, October 26, 2008.
  25. "Indiana town to Chesapeake: Fly-ash battle won’t be easy," Virginia Pilot, October 17, 2008.
  26. Carbon Capture and Storage
  27. Carbon Dioxide Capture and Storage: Special Report of the Intergovernmental Panel on Climate Change, pg 107
  28. ‘Clean coal’ push concerns environmental activists, Ohio Valley Environmental Coalition, October 16, 2005.
  29. "Retrofitting the Existing Coal Fleet with Carbon Capture Technology," U.S. Department of Energy, accessed December 2008
  30. E3 Selected Client List accessed 6/08
  31. "EU vote makes CCS ‘mandatory’ for coal power plants," Carbon Capture Journal, October 8, 2008. (Subscription required.)
  32. "Equipping power plants to store CO2 underground," European Parliament press release, October 7, 2008.
  33. 33.0 33.1 "Clean Coal Power Initiative," National Energy Technology Laboratory website, accessed April 2008
  34. 34.0 34.1 34.2 "Program Facts," Department of Energy fact sheet, accessed April 2008 (PDF File)

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Wikipedia also has an article on Clean Coal Technology. This article may use content from the Wikipedia article under the terms of the GFDL.