Mercury in U.S. Coal — Abundance, Distribution, and Modes of Occurrence
Poster

By Susan J. Tewalt, Linda J. Bragg, and Robert B. Finkelman

U.S. Geological Survey, 956 National Center, Reston, VA 20192¹

INTRODUCTION

In February 1998, the U.S. Environmental Protection Agency (EPA, 1998a,b) issued a report citing mercury emissions from electrical utilities as the largest existing anthropogenic source of mercury (Hg) released to the air. EPA officials estimated that about 50 tons of elemental mercury are emitted each year from U.S. coal-burning powerplants, with lesser amounts coming from oil- and gas-burning units. According to EPA estimates, emissions from coal-fired utilities account for 13 to 26 percent of the total (natural plus anthropogenic) airborne emissions of mercury in the United States. On December 14, 2000, the EPA announced that it would require a reduction in mercury emissions from coal-fired powerplants; regulations would be proposed by 2003 and final rules for implementation completed by 2004 (EPA, 2000).

ENVIRONMENTAL SIGNIFICANCE OF MERCURY

In the natural environment, mercury goes through a series of chemical transformations that convert elemental mercury to a highly toxic form that is concentrated in fish and birds (fig. 1). The most toxic form of mercury is methylmercury, an organic form created by a complex bacterial conversion of inorganic mercury. Methylmercury enters the food chain and bioaccumulates (that is, it becomes enriched from additional uptake via the food chain). Many States in the United States have fishing advisories that restrict fishing because of high levels of mercury detected in the fish.

Figure 1. Simplified geochemical cycle of mercury.

ABUNDANCE AND DISTRIBUTION OF MERCURY IN COAL

The U.S. Geological Survey (USGS) has compiled a nationwide coal information database over the last 25 years. A subset of these data, called COALQUAL (Bragg and others, 1998) contains analyses of over 7,000 coal samples that have been collected or calculated to represent the entire thickness of a coal bed in the ground.

Figure 2 is a histogram of the mercury concentrations for samples of conterminous U.S. coal from the COALQUAL database. The mean (average) value for all analyses is 0.17 part per million (ppm). The standard deviation for the mean is also 0.17, and the median value is 0.11 ppm. About 80 percent of the mercury concentrations in the database are less than 0.25 ppm.

Figure 2. Histogram of mercury concentrations for conterminous U.S. coal samples from the COALQUAL database.

Coal in the United States is distributed across the country, ranging in both geologic age and rank. Table 1 lists the median and mean values for mercury concentrations (in ppm) and gross calorific value (in Btu/lb, or British thermal units per pound) as well as the number of analyses in selected coal regions of the United States. The mercury data in table 1 have been calculated back from the remnant moisture whole-coal analytical basis in COALQUAL to an as-received whole-coal analytical basis, similar to the analytical basis for calorific values in the COALQUAL database.

[To calculate Btu/lb to Kcal/kg, multiply by 0.556. ppm, parts per million; Btu, British thermal units;
Kcal, Kilocalories; kg, kilograms; N, northern; C, central; S, southern; no., number.]

The concentration of mercury can also be presented on an equal-energy basis (input load) in pounds of Hg per trillion (1012) Btu to provide a convenient unit of comparison between coals from different areas (fig. 3). All the data from table 1 used in this analysis yield a mean U.S. input load of 14 lbs Hg/1012 Btu (with a median of 9.7 lbs Hg/1012 Btu and a standard deviation of 15). A mean was calculated from individual sample input loads for each selected U.S. coal region. The mean input loads were arbitrarily grouped into 5-lb intervals and are color coded in figure 3.

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Figure 3. Map showing mercury input loadings of in-ground coal for selected U.S. coal-producing regions in 5-lb intervals. x, mean in pounds of mercury per 1012 British thermal unit (lbs of Hg/1012 Btu); n, number of samples.]

MODES OF OCCURRENCE AND REDUCTION OF MERCURY

The COALQUAL dataset does not take into account the potentially substantial reduction of mercury by physical coal cleaning because the analyses represent coal as it exists in the ground. The modes of occurrence of an element can affect the way it behaves in coal cleaning, combustion, and leaching.

Because of the low concentrations and its volatility, it is difficult to determine the modes of occurrence of mercury in coal. USGS research indicates that much of the mercury is associated with pyrite, although it can be organically bound, elemental, and in sulfide and selenide minerals. Figure 4, which shows leaching results on individual coal samples, indicates that most of the mercury is removed with nitric acid (HNO₃) and is likely to be associated with pyrite.

Figure 4. Bar chart showing the removal of mercury with various leaching acids: ammonium acetate, hydrochloric acid, hydrofluoric acid, and nitric acid (from Palmer and others, 1998). Arrows indicate that the amount of mercury removed could be greater than what is shown based on detection limits of the analytical method. CH3COONH4, ammonium acetate; HCl, hydrochloric acid; HF, hydrofluoric acid, HNO3, nitric acid.]

The USGS, in collaboration with industry, has researched the removability of mercury from coal by conventional physical coal-cleaning techniques. These studies indicate that an average 37 percent of mercury is removed by coal cleaning (Toole-O'Neil and others, 1999). Mercury emissions from coal usage can also be reduced by fuel switching, selective mining, chemical coal cleaning, and possible flue gas controls that are currently being developed.

REFERENCES

Bragg, L.J., Oman, J.K., Tewalt, S.J., Oman, C.L., Rega, N.H., Washington, P.M., and Finkelman, R.B., 1998, The U.S. Geological Survey Coal Quality (COALQUAL) Database -- v. 2.0: U.S. Geological Survey Open-File Report 97-134, 1 CD-ROM.

Palmer, C.A., Kolker, A., Finkelman, R.B., Mroczkowski, S.J., Willett, J.C., Taylor, K.C., and Bullock, J.H., Jr., 1998, Arsenic and mercury modes of occurrence in coal -- Implications for reducing emissions of these elements from coal-fired power plants, in Proceedings of the Air Quality Conference, McLean, Va.: Grand Forks, University of North Dakota Energy and Environmental Research Center (unpaginated).

Toole-O'Neil, B., Tewalt, S.J., Finkelman, R.B., and Akers, D.J., 1999, Mercury concentration in coal -- Unraveling the puzzle: Fuel, v. 78, p. 47-54.

U.S. Environmental Protection Agency, 1998a, Mercury emission and electric utilities: Mercury Emissions and Electric Utilities Fact Sheet, http://www.epa.gov/ttncaaa1/t3/fact_sheets/hg17th.html.

_________, 1998b, Mercury study report to Congress: Washington, D.C., http://www.epa.gov/air/mercury.html.

_________, 2000, EPA to regulate mercury and other air toxics emissions from coal- and oil-fired powerplants: Washington, D.C., http://www.epa.gov/ttncaaa1/t3/fact_sheets/fs_util.pdf.