When burned, sulfur is mainly converted to sulfur dioxide, which causes lung irritation and long-term health problems. Sulfur dioxide can be transported in the atmosphere over long distances. Gradually it further oxidizes to sulfur trioxide, which then readily combines with water droplets to make sulfuric acid, the main component of acid rain. In that form, sulfur oxides have poisoned thousands of United States water bodies and forest areas.
Burning of fossil fuels for energy produces more than 80 percent of the atmospheric sulfur dioxide in the United States. U.S. coals contain about 0.5 to 6 weight percent sulfur, with concentrations highest in the Midwest and lowest in the far West. Crude petroleum typically contains 0.2 to 3 weight percent sulfur. Average sulfur content of crude petroleum refined in the United States has been gradually increasing and is now about 1.5 percent.
Before the 1980s U.S. coal was burned as mined except for washing to remove stones, with nearly all its sulfur released to the atmosphere. Some of the sulfur in "sour" crude petroleum was removed by refineries, but light refined fuel grades with up to one percent sulfur were common, and some heavy residual fuel grades contained up to three percent. A history of sulfur dioxide emissions is largely a history of the use of coal, influenced somewhat by the use of petroleum and by industrial processes, notably metal smelting.*
Historical growth in U.S. sulfur dioxide emissions began during the 1870s when industries increased use of coal power. Emissions fell when the Great Depression of the 1930s cut back industrial activity, but they rose rapidly during World War II. During the decade after the war, 1945 to 1955, many residential and industrial coal furnaces were converted to use oil or gas, and the railroads replaced their remaining coal-steam locomotives with diesel-electric locomotives, causing sulfur dioxide emissions to fall. After that pause, emissions again rose rapidly as post-war prosperity stimulated the use of electricity, still largely fueled by coal but also, from the 1950s until the oil-price shocks of the 1970s, by high-sulfur residual oil.
The key change in reversing a long-term U.S. trend of sulfur dioxide emissions was the federal Clean Air Act of 1970 (PL 91-604), setting goals for stationary source performance and for motor vehicle pollution reduction and establishing the Environmental Protection Agency. Chief architect of the 1970 act was Sen. Edmund S. Muskie (1914-1996), Democrat of Maine, then Chair of the Senate Public Works Committee. Predecessor legislation included the Clean Air Act of 1955 (PL 84-159), the Clean Air Act of 1963 (PL 88-206) and the Air Quality Act of 1967 (PL 90-148). National Ambient Air Quality Standards set by the Environmental Protection Agency under the 1970 Clear Air Act in turn provoked state efforts that began a reduction in sulfur dioxide emissions.
As with older legislation, the Clean Air Act of 1970 left many responsibilities for achieving air quality to the states, and the effectiveness of state programs varied. Sulfur dioxide reductions of the 1970s and 1980s were mostly achieved by fuel substitution, particularly the use of low-sulfur coal. Regulations limiting sulfur content of fuels for highway uses were also left to state programs, most requiring 0.5 percent sulfur or less, but no regulations limited off-road, railroad or marine fuels. New power-plants were required to meet emission standards, but existing plants were not.
Amendments to the Clean Air Act in 1977 (PL 95-95) relaxed and stretched out some requirements for automobiles but also introduced "Prevention of Significant Deterioration" requirements for areas where air quality meets standards and "New Source Review" requirements for upgrades to power-plants. Improvements beyond routine maintenance were to include upgrades to meet emission standards. Some plant operators began making improvements in increments, each described as routine maintenance. By 1990 federal and state efforts had reduced sulfur dioxide emissions to about 75 percent of the historical peak in 1970, but they were failing to achieve much continued progress.
The federal Clean Air Act of 1990 (PL 101-549) introduced more stringent standards and new goals. Acid rain was recognized as a multistate issue, beyond the reach of a single state, and regional control efforts were authorized. A "cap and trade" program required monitoring of emissions and allowed power-plant operators to choose between reducing emissions and purchasing unused emission allocations from other power-plants.
During five years of a so-called "Phase I" 110 large emission sources were targeted by the 1990 law, and during a subsequent "Phase II" all stationary sources with 25 MW or larger electrical capacity were affected. The new law required implementation of "Best Available Control Technology" and set a national goal of 40 percent reduction below 1980 levels for stationary source emissions, to be achieved within ten years. The law also authorized new standards for mobile sources, eventually affecting all highway petroleum fuels as well as off-road, railroad and marine fuels.
The 1990 act stimulated installation of flue gas scrubbers, the best available technology developed since the 1970 act. The most effective units use water-activated lime, magnesia or soda -- usually lime -- that combines with sulfur dioxide and removes it. Current state-of-the-art scrubbers capture 90 to 95 percent of sulfur dioxide emissions, and some produce recyclable byproducts, typically gypsum for wallboard and other building products. Advanced dry-process technologies have approached wet-process effectiveness for sulfur dioxide capture and may have advantages for removal of other pollutants, but they do not produce recyclable byproducts.
The substantial costs of installing and operating scrubbers were resisted by most power-plant operators. By the mid-1990s they were competing in a deregulated economy and could no longer count on electricity rates that would compensate for their costs. Recognizing the patterns of resistance, including incremental plant improvements to skirt New Source Review, starting in 1999 the Clinton administration filed legal complaints against several major power-plant operators, seeking compliance with standards.
As of fall, 2007, we are still in the downdraft of changes inspired by the 1990 Clean Air Act. The law's sulfur dioxide emissions goal for the year 2000 was achieved by 2003. Most of the Clinton administration lawsuits have been settled with major emission reductions, although some remain open. The lawsuits prolonged and deepened trends in this stage of reductions. A look at the trends shows that U.S. sulfur dioxide emissions have reached levels of about 100 years earlier and may go below 10 million tons of sulfur dioxide per year some time between 2015 and 2020, last seen at the end of the 1900s.
Should there be a new effort? Should we try to extend progress and achieve emission levels found only before the development of heavy industries? While further power-plant upgrades might take us that far, we are engaged with another long-term concern: control of greenhouse gas emissions, principally carbon dioxide and methane. Mining and burning coal emits more greenhouse gases than any other technology for generating electricity. As we replace coal with solar, wind or nuclear technology, lower sulfur dioxide emissions will follow.
* Sources for U.S. sulfur dioxide emissions
1990-2005: U.S. Environmental Protection Agency, U.S. Greenhouse Gas Emissions and Sinks: 1990-2005, April 15, 2007, Annex Table A-236, pp. A-290 and A-291, www.epa.gov/climatechange/emissions/downloads06/07AllAnnexes.pdf.
1970-1985: U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1999, Data Tables, Table A-8, pp. 153-155, www.epa.gov/air/airtrends/aqtrnd99/pdfs/Appena.pdf.
1900-1965: Rudolf B. Husar, Sulfur and Nitrogen Emission Trends for the U.S., Center for Air Pollution Impact and Trend Analysis, Washington University, St. Louis, 1992, http://capita.wustl.edu/capita/CapitaReports/EmisTrends/soxnem.html.
Also see: S.J. Smith, E. Conception, R. Andres and J. Lurz, Historical Sulfur Dioxide Emissions 1850-2000: Methods and Results, Pacific Northwest National Laboratory Report PNNL-14537, January, 2004, www.pnl.gov/main/publications/external/technical_reports/PNNL-14537.pdf.
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