Trace amounts of mercury can exist in coal and other fossil fuels. When these fuels burn, mercury vapor can be released to the atmosphere where it may drift for a year or more, spreading with air currents over vast regions of the globe. In 1995, an estimated 5,500 tons of mercury was emitted globally from both natural and human sources. Coal-fired power plants in the United States contributed less than 1 percent of the total.
High levels of mercury can have a toxic effect on the nervous systems of humans. The term "mad hatter" derives from the fact the mercury was used in leather tanning to make hats in previous centuries, and some people developed nervous disorders from continuous exposure to high mercury levels.
Emissions Levels Coming Down
The amount of mercury being deposited today on land and in water is actually much lower than in recent decades. Peat cores from Minnesota, for example, show that mercury deposition was highest in the 1950s, with levels about 10 times greater than those before 1900. By the 1980s, however, depositions had fallen to less than half of the 1950s. Emissions data from Sweden and measurements of mercury levels in birds and other animals in the United Kingdom also show a consistent pattern suggesting that mercury levels reached a peak around 1960.
Mercury emissions continued to fall in the decade of the 1990s. In 1993, U.S. yearly emissions totaled about 242 tons. By the end of the decade, emissions had declined to less than 160 tons per year.
The primary reason is that the use of mercury in batteries, fungicides and paints has been reduced. Also, municipal waste combustors, hazardous waste combustors, and medical waste incinerators have been regulated by the Environmental Protection Agency (EPA). The number of operating chlor-alkali plants has also declined from about 20 in 1990 to 12 in 2000, and those still operating have reduced their mercury use. Federal regulations reducing mercury emissions by 90 percent from municipal waste combustors and by 94 percent from medical waste incinerators were released in October 1995 and in August 1997. In 1998 mercury emissions from hazardous waste combustion facilities were also regulated.
Coal-fired power plants contribute only a small part of the total worldwide emissions of mercury. The estimated 48 tons of mercury they emit annually is about one-third of the total amount of mercury released annually by human activities in the United States.
On February 14, 2002, the Bush Administration announced its Clear Skies Initiative for multipollutant controls. The proposal required significant emission reductions of sulfur dioxide, nitrogen oxides, and mercury through an allowance-based cap-and-trade program. Specifically for mercury, the Clear Skies Initiative calls for a two-phase reduction in emissions below 1999 levels (48 tons) with an approximate 45 percent reduction beginning in 2010 and a 70 percent reduction beginning in 2018.
As part of the implementation of this initiative, on March 15, 2005, EPA issued the first-ever federal rule to permanently cap and reduce mercury emissions from coal-fired power plants. This rule makes the United States the first country in the world to regulate mercury emissions from coal-fired power plants.
- The Clean Air Mercury Rule will build on EPA's Clean Air Interstate Rule (CAIR) to significantly reduce emissions from coal-fired power plants--the largest remaining sources of mercury emissions in the country. When fully implemented, these rules will reduce utility emissions of mercury from 48 tons a year to 15 tons, a reduction of nearly 70 percent.
The Energy Department's Early Role - Assessing Emission Levels
When the Clean Air Act Amendments were enacted in 1990, mercury was singled out from the 189 substances designated "hazardous air pollutants" because of its potentially harmful effects on humans. Title III of the Act required EPA to determine whether utility boilers in the United States would have to be regulated to reduce emissions of hazardous air pollutants, especially mercury.
In 1993, the Department of Energy's Office of Fossil Energy gave five of the Nation's top environmental monitoring companies the task of assessing the release of trace impurities such as mercury from U.S. coal power plants. The work represented the most intensive effort to date to provide EPA with the critical data it needed on the release of mercury and other toxic impurities from the Nation's power plants.
The initial data collection effort included eight power plants, with measurements taken on the incoming coal and in the gas and wastewater effluents. In 1994, the Energy Department added a coal preparation plant to its study to assess the effectiveness of coal cleaning in removing trace impurities.
In a second phase of the project, eight additional power plants were studied from 1994 to 1996. The results showed air-toxic releases to be the very lowest limits of the most sophisticated detection equipment (approximately 100 parts per billion).
The data also showed that various types of boilers and pollution control devices could have widely varying effectiveness in limiting the release of mercury and other air toxic impurities. DOE's analyses found that about 37 percent of the mercury in coal is removed during coal cleaning processes. Of the approximately 100 tons of mercury still contained in the coal feedstocks entering power plant boilers, about half is captured by existing air pollution control equipment.
But the effectiveness of mercury capture at individual plants can vary, a problem made especially difficult because mercury can be released in both elemental and oxidized forms. Oxidized mercury typically ranges from 30 to 70 percent of the total mercury in flue gas, depending upon the amount of mercury in coal and the way the coal is burned.
Existing pollution control devices such as electrostatic precipitators (which remove solid particles) can be effective in removing elemental and in some cases, oxidized mercury. Typically, removals range between zero and 30 percent, but can be as high as 60 percent for elemental mercury. Wet scrubbers are effective in removing oxidized mercury ranging from 75 to 99 percent, with overall total mercury removals of 55 percent. Dry flue gas desulfurization scrubbers can remove both oxidized and elemental with total mercury removals as high as 90 percent when coupled with a baghouse. Baghouses also remove both forms of mercury, but their effectiveness depends on the type of filter and other power plant specifics (mainly fly ash properties and temperature).
In short, pollution control systems can be effective in removing as high as 90 percent of the incoming coal's mercury levels in a few cases, but very little mercury is removed in others.
The Energy Department's Mercury Control Technology Program
The Energy Department recognized in the early 1990s that while other industries have cost-effective mercury control technologies, no single technology can cost-effectively provide add-on mercury control for all power plant configurations or fuel types.
IF DOE IS SUCCESSFUL...
The estimated market penetration for mercury control technologies is 320,000 megawatts under President Bush's proposed Clear Skies Initiative. If DOE can help produce lower cost and more effective mercury control technologies, U.S. ratepayers could save $550-$800 million per year by 2010.
Injecting activated carbon as a sorbent to capture flue gas mercury has shown the most promise as a near-term mercury control technology, but the process applied to coal-fired boilers is still in its early stages and its effectiveness under varied conditions (e.g., fuel properties, flue gas temperatures, and trace-gas constituents) is still being investigated.
Moreover, without better technologies, significant costs could be added to the generation of the Nation's electric power. For a 90 percent control level, DOE estimated in the mid-1990s that annual costs could range between $2 and $6 billion per year (based primarily on tests using activated carbon as the standard process for removing mercury from the flue gas of coal combustion.) The DOE cost analysis became the basis for system-wide estimates of control costs ultimately used by EPA in the 1997 Mercury Study Report to Congress.
The Energy Department's Office of Fossil Energy has set two goals for developing improved mercury control technologies:
A near-term goal to develop mercury controls that can achieve 50 to 70 percent mercury capture at 50 to 75 percent of the cost of current powdered activated carbon injection (current cost estimates for activated carbon technology are in the range $50,000 to $70,000 per pound of mercury removal). These technologies would be available for commercial demonstration by 2007.
A longer-term goal is to develop advanced mercury control technologies that can achieve 90 percent or greater capture at 50 to 75 percent of the cost of existing technologies and be available for commercial demonstration by 2010.
The Office of Fossil Energy (FE) has been sponsoring studies on mercury emissions from coal based power generators to identify effective and economical control options for the past decade. These studies include mercury removal from flue gas by enhancing conventional pollution controls, identifying combustion modifications and developing advanced control methods. However, technology to cost-effectively reduce mercury emissions from coal-fired power plants is not yet commercially available.
The new program builds on past DOE and other R&D organizations' mercury measurement and control efforts by conducting tests of the most promising methods at large scale at utility sites. Large-scale tests will produce valid information and data to evaluate mercury removal effectiveness, the economic impact of the control method, and the potential impact on plant operations which will lead to economical and effective mercury control technology being commercially available in the short term by 2007. In addition, research on advanced and improved mercury control technology for the long term (by 2010) is ongoing.
FE's mercury control research effort is intended to ensure timely development of the most cost-effective pollution control technologies that could be installed once final emission limits for mercury and other hazardous air pollutants from coal-burning facilities are established. Today, FE manages the largest funded program in the country for developing (1) an understanding of mercury emissions; (2) measurement of these emissions; and (3) control technology(s) for these emissions for the U.S. coal-fired electric generating industry.