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.

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

- Total Emissions 6,000 to 8,000 metric tons
- Natural Sources of Hg 3,000 to 3,500 tons

> - Ocean may contribute as much as half
> - Erupting Volcanoes
> - Soil Vapor Flux
> - Geothermal Systems/Hot Springs
> - Degassing Volcanoes and Fumaroles
> - Vapor Flux from Mineralized Areas
> - Active Faults

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.

|  | 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. VIEW a large, higher quality PDF file

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

> - mercury is generally associated with pyrite, commonly secondary, arsenic-bearing pyrite
> - small proportions appear to be associated with clays and the organics
> - in coal with low iron content (no pyrite), mercury occurs as a selinide

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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