Sulfur dioxide and coal

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This article is part of the Coal Issues portal on SourceWatch, a project of CoalSwarm and the Center for Media and Democracy. See here for help on adding material to CoalSwarm.

Coal-fired power plants are the largest human-caused source of sulfur dioxide, a pollutant gas that contributes to the production of acid rain and causes significant health problems, particularly through its role in forming particulates. Coal naturally contains sulfur, and when coal is burned, the sulfur combines with oxygen to form sulfur oxides. Although natural processes like volcanic eruptions, biological decomposition, and forest fires also produce sulfur dioxide, emissions caused by human activity far exceed natural production in developed countries.[1]

The EPA estimates that more than 65%, or over 13 million tons per year, of SO2 production in the U.S. comes from electric utilities,[2] 93 percent of which is produced by coal power plants.[3] In China, the world’s largest consumer of coal, approximately 22.5 million tons of sulfur were released in 2004, and over 30% of the country now experiences acid rainfall.[4]

Acid rain is rain saturated with chemicals like sulfur dioxide and nitrogen oxide, which causes acidification of lakes and streams and contributes to the damage of trees at high elevations and many sensitive forest soils. In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation's cultural heritage. Prior to falling to the earth, sulfur dioxide and nitrogen oxide gases and their particulate matter derivatives—sulfates and nitrates—contribute to visibility degradation and harm public health.[5]

Environmental impacts of SO2

Sulfur dioxide contributes to the formation of acid rain, which damages forests, crops, and buildings, and acidifies lakes, streams, and rivers, making them unsuitable for aquatic plant and animal life. Continued exposure to SO2 over an extended period of time can permanently alter the natural variety of plants and animals in an ecosystem.[6][7] Sulfate particles are also the major cause of smog in many parts of the U.S.[6]

In 2010, plant experts, scientists, environmentalists and ranchers said they believe sulfur dioxide pollution from coal-fired power plants is slowly killing vegetation across Texas. Sulfur dioxide has been known to kill vegetation in other parts of the country. In Texas, the deaths of pecan trees, oaks, elms and willows have been documented. In Central Texas, not far from a power plant operated by the Lower Colorado River Authority, pecan growers say thousands of trees have died and nut production has steadily decreased since the plant began operating in 1979. After consulting experts, the growers believe the plant's sulfur dioxide emissions are to blame. The U.S. Environmental Protection Agency has toured some of the farms and is reviewing the data.[8]

Health impacts of SO2

The main health effect of SO2 is to impair the function of the upper respiratory system. High concentrations of sulfur dioxide can affect breathing, cause respiratory illnesses, and aggravate existing heart and lung diseases. Exposure at very low concentrations can irritate the lungs and throat and cause bronchitis. Exposure to low levels of SO2 over a long period depletes the respiratory system's ability to defend against bacteria and foreign particles. [7] Particularly sensitive groups include children, the elderly, people with asthma, and those with heart or lung disease.[6]

U.S. legislative and regulatory efforts to minimize SO2 emissions

Clean Air Act

In 1963, the U.S. Congress passed the Clean Air Act, establishing standards for harmful pollutants including lead, carbon monoxide, ozone, sulfur dioxide, nitrogen oxide, and particulate matter. The legislation allowed older power plants to be exempt from the new standards, because Congress assumed these plants would be phased out of service. By 1977, many states had failed to meet the new targets, and the older plants continued to operate and release high levels of pollution. To address these problems, Congress passed a series of new amendments to the Clean Air Act, including the New Source Review (NSR). This new legislation required the older industrial facilities to submit to an EPA assessment and install modern pollution control technologies before any expansion.[9]

Congress passed more amendments to the Clean Air Act in 1990, including Title IV, which addressed the problem of acid rain by curbing sulfur dioxide and nitrogen oxide emissions. The acid rain controls were implemented in two phases and placed a decreasing cap on emissions over several years.[10] Included in the 1990 amendments was a controversial provision allowing companies to buy, sell and trade pollution credits, provided they cut half of their overall emissions.[11] In 2005 the U.S. EPA approved the Clean Air Interstate Rule, requiring a 57 percent cut in sulfur dioxide emissions and a 61 percent cut in nitrogen oxide emissions from power plants by 2015.[12]


As a result of these mandates, many coal power plants have installed smokestack “scrubbers,” which trap sulfur emissions before they are released into the air and turn them into sludge and solid waste. Not only is scrubber technology expensive, but the waste byproducts require some means of disposal. The EPA estimates that only 30% of these materials are recycled, most often for commercial building products, while the other 70% is deposited in landfills.[13] In Ohio, for example, the state's EPA recently approved a landfill for a FirstEnergy plant in Jefferson County. The landfill is expected to accumulate between 1.2 million and 1.6 million tons of sludge per year. Another landfill for AEP’s Gallia County plant took on 3 million tons of scrubber sludge and coal ash in 2007.[12]

According to data from the Energy Information Administration, the following proportions of coal-fired power plants with capacity over 100 MW had SO2 scrubbers in 2005:[14]

SO2 Removal Rate # of Plants Total Capacity
Over 90% 94 46,734 MW
80-89% 49 21,613 MW
70-79% 52 20,950 MW
16-69% 11 3,825 MW
None 628 220,664 MW

It is possible that some coal-fired plants with SO2 scrubbers did not report their scrubbers to the EIA, and thus that the above table overstates the number of plants without SO2 controls. However, out of 257 U.S. coal-fired power plants which produced more than 2,000 GWh of power in 2006, 86 had SO2 emissions that were higher than 10 lb/MWh – compared with an average of 1 lb/MWh for coal plants with state-of-the-art SO2 scrubbers.[15][16] We can surmise that these 86 plants almost certainly have zero or extremely minimal SO2 scrubbers, or have SO2 scrubbers that were not functioning in 2006.

Low-sulfur coal

An alternative to scrubber technology is using lower sulfur coal to fuel power plants. The Department of Energy estimates that the use of the lowest sulfur coal can result in up to 85 percent lower SO2 emissions than the use of many types of higher sulfur coal.[17] In the U.S., coal from eastern states including Ohio, Pennsylvania, and West Virginia has higher sulfur content, accounting for 3 to 10 percent of the coal's weight; coal from western states such as Wyoming, Montana, Utah, and Colorado can have sulfur contents that make up less than 1 percent of its weight.[18] However, low-sulfur coal is significantly more expensive than higher sulfur coal, and often incurs additional transportation costs. [19][20] At a plant in Virginia, Dominion decided against both domestic low-sulfur coal and scrubber technology, and instead chose to import lower sulfur coal from Indonesia and Columbia as a less expensive way to meet new emission standards.[21]

The U.S. EPA approximates that these measures to comply with SO2 emission standards cost power companies and consumers about $1 billion to $2 billion a year, and that the changes have resulted in a 40% decrease in SO2 emissions from 1990 levels.[11]

National Ambient Air Quality Standards

In November 2009, the EPA proposed new National Ambient Air Quality Standards for sulfur dioxide, the first time since 1971 that the agency has recommended tightening controls on SO2 to protect public health. The old limits measured sulfur dioxide averages over 24-hour and one-year periods. The new rule would require one-hour measurements, such that a spike of emissions above a new limit — between 50 and 100 parts per billion in one hour — would no longer be acceptable. The EPA estimates that if the rule is enacted with the strongest limits the agency is recommending, the benefits by 2020 would include 4,700 to 12,000 fewer premature deaths per year and 3.6 million fewer cases of worsened asthma. The agency also calculated that the health benefits of the new regulations would greatly outweigh the $1.8 billion to $6.8 billion costs of the new rules. A public hearing is scheduled in Atlanta in January 2010, with the new rules scheduled to become final by June 2010.[22]

The new standard was issued on June 3, 2010.[23]

Transport Rule

The Clean Air Transport Rule is a proposed rule of the EPA requiring significant reductions in sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions that cross state lines. By 2014, the rule and other state and EPA actions would reduce power plant SO2 emissions by 71 percent over 2005 levels and NOx emissions by 52 percent, according to EPA estimates. The proposed rules will replace the EPA's 2005 Clean Air Interstate Rule, which was vacated in July 2008 by the D.C. Circuit Court. Acting under federal court order, the Obama administration proposed the new air-quality rules on July 6, 2010, for coal-burning power plants. The proposed rule would require NOx and SO2 emission reductions in 2012 and additional SO2 emission reductions in 2014.[24]

On August 21, 2012, a federal appeals court in Washington, D.C. ruled 2-1 against the Transport Rule. The court determined that the rule forced states to be accountable for pollution that was not theirs, potentially making some states responsible for reducing pollution in other states that may be primarily caused by third party states. Second, the court found that the way in which the rule mandates obligations on polluting states is in conflict with existing mandates under the Clean Air Act, as the EPA did not allow states the initial opportunity to implement the required reductions within their borders. The two judges in the majority were appointed by George W. Bush; the dissenter by Bill Clinton.[25]

New Source Review standards

On March 16, 2011, the U.S. Environmental Protection Agency (EPA) announced its proposed emissions standards to limit mercury, acid gases and other toxic pollution from power plants.[26]

EPA is also proposing to revise the New Source Review performance standards (NSPS) for fossil-fuel-fired plants. This NSPS would revise the standards new coal- and oil-fired power plants must meet for particulate matter (PM), sulfur dioxide (SO2), and nitrogen oxides (NOx). The proposed standards should reduce mercury emissions from power plants burning coal and oil by 91 percent, acid gas pollution by 91 percent, direct particulate matter emissions by 30 percent, and sulfur dioxide (SO2) emissions by 53 percent, down to 2.1 million tons of annual SO2 emissions.[26]

The monetized benefits from the new standards are estimated to be $59 billion to $140 billion annually, compared to annual compliance costs of approximately $10.9 billion. The EPA also projects that the proposed standards will create up to 31,000 short-term construction jobs and 9,000 long-term utility jobs.[27]

Requirements of the new standards include:[26]

  • For all existing and new coal- and oil-fired electric utility steam generating units (EGUs), the proposed standards would establish numerical emission limits for mercury, PM, and HCl.
  • For all existing and new oil-fired EGUs, the proposed toxics rule would establish numerical emission limits for total metals, HCl, and HF.
  • Actions available to power plants to meet the emission limits include wet and dry scrubbers, dry sorbent injection systems, activated carbon injection systems, and baghouses, all part of Maximum Achievable Control Technology (MACT).
  • The proposed standards would establish work practices, instead of numerical emission limits, to limit emissions of organic air toxics, including dioxin/furan, from existing and new coal- and oil-fired power plants.
  • The proposed revisions to the NSPS would include revised numerical EGU emission limits for PM, SO2, and NOX.


A 2011 report to Congress by the National Acid Precipitation Assessment Program - part of the U.S. Department of the Interior, U.S. Geological Survey - states that, as of 2009, emissions of SO2 and NOx declined by about two-thirds relative to levels in the 1990s. Despite these emission reductions, the report also indicates that "full recovery from the effects of acid rain is not likely for many sensitive forests and aquatic ecosystems. For example, in the Adirondack Mountains of New York, an especially sensitive region, 30 percent of the lakes were receiving acid rain during 2006-08 in excess of the level needed to prevent harm. Based on models which analyze various emission scenarios, the report concludes that beyond current SO2 and NOx emission levels, future emission reductions would likely promote additional and more widespread recovery as well as to prevent further acidification in some U.S. regions."[28]

Citizen Action

Farmers, pecan growers say coal plant's sulfur dioxide kills plants

In December 2010, plant experts, scientists, environmentalists and ranchers stated that they believe sulfur dioxide pollution from coal-fired power plants is slowly killing vegetation across Texas, in particular pollution from the Fayette Power Project in Texas.

Sulfur dioxide has been known to kill vegetation in other parts of the country. In Texas, the deaths of pecan trees, oaks, elms and willows have been documented.

In Central Texas, not far from the Fayette plant, pecan growers claimed thousands of trees have died and nut production has steadily decreased since the plant began operating in 1979. After consulting science experts, the growers contended that sulfur dioxide emissions from the plant were the cause.

The U.S. Environmental Protection Agency is currently reviewing the data and is expected to respond in early 2011 on their findings.[29]



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  2. Sulfur dioxide - What is it? Where does it come from?, EPA, accessed April 2008.
  3. Electricity from coal, Power Scorecard, accessed April 2008.
  4. “Pollution From Chinese Coal Casts a Global Shadow”, New York Times, June 11, 2006.
  5. "Effects of Acid Rain" EPA, accessed April 2010.
  6. 6.0 6.1 6.2 Sulfur dioxide: Health and environmental impacts of SO2,EPA, accessed April 2008.
  7. 7.0 7.1 Sulfur Dioxide (SO2) in Minnesota], Minnesota Pollution Control Agency, December 1997.
  8. Ramit Plushnick-Masti, "Farmers, pecan growers say coal plant kills plants" Bloomberg, Dec. 28, 2010.
  9. A Timeline of the Clean Air Act, Environmental Defense Fund, accessed April 2008.
  10. The Effects of Title IV of the Clean Air Act Amendments of 1990 on Electric Utilities, Energy Information Administration, March 1997.
  11. 11.0 11.1 “Cap-and-Trade Model Eyed for Cutting Greenhouse Gases”, San Francisco Chronicle, December 3, 2007.
  12. 12.0 12.1 “Utilities amassing landfills: Tougher air standards send tons of plants' sludge, coal ash into ground”, Columbus Dispatch, April 14, 2008.
  13. Coal Combustion Residues and Mercury Control, EPA Interim Report on the Control of Mercury Admissions from Coal-Fired Electric Boilers, April 2002.
  14. Form EIA-767 Database, Energy Information Administration website, 2005.
  15. Dirty Kilowatts 2007 Report Database, Environmental Integrity Project, accessed May 2008.
  16. Environmental Integrity Project, "Dirty Kilowatts: America’s Most Polluting Power Plants", July 2007, p. 8.
  17. Anuual Energy Outlook 2002 with Projections to 2020, US Department of Energy, January 2002.
  18. Cleaning up Coal, U.S. Department of Energy, accessed April 2008.
  19. "Tennessee: Low-Sulfur Coal Price Doubles, Following Gas & Oil", Chattanooga Times, November 30, 2004.
  20. Energy Policy Act Transportation Rate Study: Final Report on Coal Transportation, U.S. Energy Information Administration, October 2000.
  21. Dominion Chooses Coal Imports Over Scrubbers for Chesapeake Plant, Power Engineering, December 8, 2006.
  22. Renee Schoof, "EPA proposes sulfur dioxide limits for first time since 1971," McClatchy Newspapers, November 29, 2009.
  23. John Broder, "E.P.A. Tightens Its Sulfur-Dioxide Limits" NY Times, June 3, 2010.
  24. "Air Transport" EPA, accessed Feb. 2011.
  25. "EPA "Transport Rule" Struck Down by Court," American Spectator, August 21, 2012.
  26. 26.0 26.1 26.2 "Fact Sheet: Proposed Mercury and Air Toxics Standards" EPA, accessed March 2011.
  27. John Walke, "A little background on the EPA’s new mercury and air toxics rule" Grist, March 16, 2011.
  28. "Acid Rain Study Show Substantial Decreases, But More Progress Is Needed" USGS Press Release, Jan. 19, 2012.
  29. "Farmers, pecan growers say coal plant kills plants" Ramit Plushnick-Matsi, Business Week, December 28, 2010.

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