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MERCURY IN U.S. COALS

INTRODUCTION

Mercury is the only element for which legislation is being considered to reduce emissions from coal combustion in the US. The mercury emitted from the power plants is not harmful; however, in the natural environment the mercury can go through a series of chemical transformations that convert the mercury to a highly toxic form that is concentrated in fish and birds (Figure 23). The most toxic form of mercury is methylmercury, which is an organic form created by bacterial conversion of inorganic mercury. Methylation rates (creation of methylmercury) in ecosystems are a function of mercury availability, bacterial population, nutrient loads, pH and Eh, sediment load, and sedimentation rates (National Research Council, 1978). Methylmercury enters the food chain, particularly in aquatic organisms and bioaccumulates. Cases of mercury poisoning have occurred from eating contaminated fish for prolonged periods, both in the U.S. and abroad. Pregnant woman and subsistence fisherman are particularly vulnerable. Because high levels of mercury have been detected in fish, many U.S. States have issued temporary, regional fishing advisories that restrict fishing.

Diagram showing geochemical cycle of mercury in the environment

Figure 23. Geochemical cycle of mercury in the environment. Methylation occurs via aquatic microorganisms.

Table 10. Global Emissions of Hg to the Atmosphere per Year

About 50 tons of mercury are emitted each year from U.S. coal-burning power plants. Coal burning is the largest uncontrolled anthropogenic source of mercury.

One issue under consideration is how much mercury is deposited in the U.S. from coal combustion in Asia?

Table 11. Mercury values in selected U.S. coal areas from the COALQUAL1 database

[ppm, parts per million. Data from Bragg and others (1998)]
Coal area
Mean
(ppm)
Maximum
(ppm)
Number
of samples
Appalachian
0.20
2.9
4,399
Eastern interior
0.10
0.4
301
Fort Union
0.13
1.2
300
Green River
0.09
1.0
418
Gulf Coast
0.22
0.6
29
Hams Fork
0.09
1.0
142
Pennsylvania anthracite
0.18
1.3
52
Powder River
0.10
1.4
616
Raton Mesa
0.09
0.5
40
San Juan River
0.08
0.9
194
Southwest Utah
0.10
0.5
42
Uinta
0.08
0.6
271
Western interior
0.18
1.6
311
Wind River
0.18
0.8
42
1COALQUAL = U.S. Geological Survey Coal Quality (COALQUAL) Database: Version 2.0
(http://energy.er.usgs.gov/products/databases/CoalQual/index.htm)
Mercury concentration in coal. This is the way that mercury data are presented in most publications. This may be misleading because, in order to obtain similar energy outputs, more low-rank coal has to be burned than a higher-ranked coal. This can result in a net mobilization of more total mercury to the environment. A better way to compare mercury data for coal is on an equal energy basis.

Table 12. Mercury on equal energy basis, mean values for samples in selected U.S. coal areas.

[BTU, British thermal units; ppm, parts per million. Data from Bragg and others (1998)]
Coal area
Mercury
(pounds / 1012 BTU)
Mean
(ppm)
Appalachian
15.4
0.20
Eastern interior
8.2
0.10
Fort Union
21.8
0.13
Green River
6.6
0.09
Gulf Coast
36.4
0.22
Hams Fork
4.8
0.09
Pennsylvania anthracite
15.4
0.18
Powder River
12.6
0.10
Raton Mesa
6.6
0.09
San Juan River
7.7
0.08
Southwest Utah
11.0
0.10
Uinta
7.3
0.08
Western interior
16.1
0.18
Wind River
18.7
0.18
Mercury contents in coal vary between coal basins.

Map showing mercury input loadings of in-ground coal for selected U.S. coal-producing regions   Figure 24. Mercury input loadings (in pounds of Mercury per 1012 British thermal units (lbs Hg/1012 Btu) of in-ground coal for selected U.S. coal-producing regions. From Tewalt and others, 2001. See footnote to table 11 for COALQUAL reference.

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Table 13. Mercury input loads for top-producing U.S. coal beds

[BTU, British thermal units; GC, Gulf Coast; APP, Appalachian; PRB, Powder River Basin; EINT, Eastern Interior. Data from Bragg and others (1998).]
Coal bed(s)

Mean
(pounds / 1012 BTU)

Maximum
(pounds / 1012 BTU*)

 Number
of samples
Wilcox Group (GC)
26.4
79.4
34
Upper Freeport (APP)
25.1
32.0
226
Lower Freeport (APP)
24.5
120.0
100
Lower Kittanning (APP)
18.5
70.1
182
Middle Kittanning (APP)
17.8
115.0
231
Blue Creek (APP)
15.2
55.3
62
Pittsburgh (APP)
13.9
84.6
128
Alma (APP)
11.9
55.8
18
Stockton-Lewiston (APP)
10.6
51.6
20
Cedar Grove (APP)
9.0
32.0
15
Number 2 Gas Lower Elkhorn (APP)
8.6
32.8
35
Chilton (APP)
7.0
12.6
2
Pocahontas Number 3 (APP)
6.8
47.0
50
Winifrede (APP)
6.8
35.0
20
Coalburg (APP)
4.6
19.4
25
Wyodak, Wyodak-Anderson (PRB)
19.0
126.0
36
Rosebud, Rosebud-McKay (PRB)
8.6
28.4
10
Number 12 (EINT)
10.4
24.9
7
Number 6 (EINT)
8.4
2.38
23
Number 9 (EINT)
6.2
20.7
16
Beulah-Zap (FU5)
8.6
10
Mercury contents vary within coal basins, between coal beds in each basin and within each bed.

MODES OF OCCURRENCE OF MERCURY IN COAL

It is the mode of occurrence of an element that will determine how it will behave during coal cleaning.

MERCURY EMISSION -- REDUCTION

  • Physical coal cleaning may be effective in removing mercury. Conventional coal cleaning removes about 37% of the mercury, on average.
  • Selective mining may also be a practical option for reducing mercury emissions.
  • Modifying combustion conditions, such as using fluidized-bed combustion which operates at lower temperatures than do conventional power plants and as a limestone "bed" that may capture pollutants.
  • Post combustion pollution control -- use of electrostatic precipitators or baghouses to capture particulates, or flue-gas desulfurization systems to remove pollutants from the gaseous effluents. New pollution control systems (such as carbon injection) designed specifically for mercury capture are being developed and tested.
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