U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3122

November 2012

Mercury and Halogens in Coal—Their Role in Determining 

Mercury Emissions From Coal Combustion

Introduction

Mercury (Hg) is a toxic pollutant. In its elemental form, 

gaseous mercury has a long residence time in the atmosphere, up 

to a year, allowing it to be transported long distances from emission sources. Mercury can be emitted from natural sources such 

as volcanoes, or from anthropogenic sources, such as coal-fired 

powerplants. In addition, all sources of Hg on the Earth’s surface 

can re-emit it from land and sea back to the atmosphere, from 

which it is then redeposited.

Mercury in the atmosphere is present in such low concentrations that it is not considered harmful. Once Hg enters 

the aquatic environment, however, it can undergo a series of 

biochemical transformations that convert a portion of the Hg 

originally present to methylmercury, a highly toxic organic form 

of mercury that accumulates in fish and birds. Many factors 

contribute to creation of methylmercury in aquatic ecosystems, 

including Hg availability, sediment and nutrient load, bacterial 

influence, and chemical conditions. In the United States, consumption of fish with high levels of methylmercury is the most 

common pathway for human exposure to Hg, leading the U.S. 

Environmental Protection Agency (EPA) to issue fish consumption advisories in every State (fig. 1).

The EPA estimates that 50 percent of the Hg entering the 

atmosphere in the United States is emitted from coal-burning 

utility powerplants (U.S. Environmental Protection Agency, 

2011a). An EPA rule, known as MATS (for Mercury and Air 

Toxics Standards), to reduce emissions of Hg and other toxic 

pollutants from powerplants, was signed in December 2011 

(U.S. Environmental Protection Agency, 2012). The rule, which 

is currently under review, specifies limits for Hg and other toxic 

elements, such as arsenic (As), chromium (Cr), and nickel (Ni). 

MATS also places limits on emission of harmful acid gases, such 

as hydrochloric acid (HCl) and hydrofluoric acid (HF). These 

standards are the result of a 2010 detailed nationwide program 

by the EPA to sample stack emissions and thousands of shipments of coal to coal-burning powerplants (U.S. Environmental 

Protection Agency, 2011b,c). The United States is the only 

nation to have collected such detailed information for Hg in both 

its coal and its utility emissions.

EXPLANATION

Mercury advisories

Statewide coastal

Statewide specific water bodies

Statewide freshwater (Maine) 

Statewide freshwater and other advisories

No advisory issued

Lakes only (Michigan and Minnesota) 

Figure 1. U.S. water bodies for which the Environmental Protection Agency issued fish consumption advisories in 2010. Advisories include statewide 

freshwater advisories (Maine), statewide freshwater advisories with additional advisories on specific water bodies, statewide advisories for specific 

water bodies, and statewide coastal advisories. In Michigan and Minnesota, statewide advisories are for lakes only. There are no fish consumption 

advisories for Hg in the District of Columbia and Puerto Rico; in Hawaii, there are only coastal advisories. Figure modified from U.S. Environmental 

Protection Agency. Alaska, Hawaii, and Puerto Rico not to common scale.
U.S. Geological Survey Coal Data

While U.S. Geological Survey (USGS) results were 

not used in developing the EPA MATS standards, the USGS 

domestic and international coal databases and research on trace 

elements in coal contain much information that is relevant to 

understanding how Hg in coal occurs and why some coals contain more Hg than others. Since the 1980s, the USGS has compiled data on the quality and chemical composition of U.S. coals 

by using samples that represent the entire thickness of a coal 

bed in the ground (Bragg and others, 1998). This Coal Quality 

Database, known as COALQUAL, contains data on the chemical 

composition of more than 7,000 U.S. coal samples and includes 

results for Hg, trace elements, and other important properties of 

coal, such as its ash content—the amount of inorganic material 

present—as well as its moisture and its sulfur content. Using 

COALQUAL, Tewalt and others (2001) determined a mean Hg 

concentration of 0.17 parts per million (ppm) for in-ground coal 

for the entire United States. 

Comparison of USGS and EPA Coal Databases

In 1999, and again in 2010, the EPA compiled data for 

coal shipments delivered to U.S. power stations. The 2010 data, 

which were used in developing the MATS standards, give a 

mean Hg concentration of about 0.12 ppm for delivered coal, 

much less than USGS results (0.17) for in-ground coal (fig. 2). 

The difference between the USGS and EPA results is explained 

by the fact that some coals, especially those high in sulfur, 

undergo preparation (cleaning) prior to delivery. The cleaning 

process reduces the amount of mineral matter present in the coal, 

such as pyrite (FeS2), and constituents that may be associated 

with mineral matter, such as sulfur. As one of the benefits of coal 

cleaning, Hg associated with mineral matter is reduced in many 

cases, thereby accounting for lower average Hg values in the 

EPA databases when compared to the USGS results.

The EPA databases also differ in the geographic distribution of samples when compared to that of COALQUAL. 

COALQUAL contains a greater proportion of samples from 

the Appalachian Basin, which in general have higher mercury 

contents than low-sulfur Western coals, such as those in the 

Powder River Basin (PRB) of Wyoming and Montana. Use of 

PRB coals has increased markedly during the last two decades, 

a trend that is reflected in the EPA databases for delivered coal 

when compared to values for in-ground coal as determined 

from COALQUAL. Nonetheless, COALQUAL is very useful in 

showing how U.S. coals differ by location and by rank, a classification based primarily on the heating value of coal; low-rank 

coals include lignite and subbituminous whereas bituminous and 

anthracite are higher rank. COALQUAL is also useful in comparing U.S. coal with international samples.

Figure 2. Graphs show a comparison of concentrations of mercury 

(Hg), in parts per million (ppm) on a dry basis, by percent of the 

number of samples determined for U.S. Geological Survey Coal Quality 

(COALQUAL) database and U.S. Environmental Protection Agency 

Information Collection Request (ICR) databases for 1999 and for 2010, 

Parts II and III. Rightmost column shows the percentage of samples in 

each database that exceeds 0.30 ppm Hg; all samples are included in the 

arithmetic means, median concentrations, and sample numbers shown. 

Figure modified from Kolker and Quick (in press).

1999 ICR 37,931 shipment samples

COALQUAL

A. U.S. Geological Survey database

B. U.S. Environmental Protection Agency databases

4,564 in-ground samples

2010 ICR part II 13,130 shipment samples

2010 ICR part III 889 feedstock samples 

>0.3

>0.3

>0.3

>0.3

20

10

0

0 0.1 0.2 0.3

20

10

0

0 0.1 0.2 0.3

20

10

0

Concentrations of dry mercury, in parts per million 

Concentrations of dry mercury, in parts per million 

0 0.1 0.2 0.3

20

10

0

0 0.1 0.2 0.3

Percentage of samples Percentage of samples

Average = 0.17

Median = 0.11

Average = 0.11

Median = 0.09

Average = 0.12

Median = 0.11

Average = 0.12

Median = 0.08
International Coal Data

Seven countries—China, the United States, India, Australia, 

South Africa, the Russian Federation, and Indonesia—listed in 

descending order of production, accounted for more than 90 

percent of the hard coal produced during 2010 (table 1). Of these 

countries, detailed nationwide surveys of Hg distribution in coal 

are available only for the United States and China. China is both 

the world’s largest producer and largest user of coal (International Energy Agency (IEA), 2011).

Recent compilations of average concentrations (means) 

for Hg in Chinese commercial coal range in value from 0.15 to 

0.19 ppm, based on analysis of thousands of samples. One such 

compilation, the USGS World Coal Quality Inventory (WoCQI), 

is a publicly available database that includes chemical analyses 

and other relevant information for 1,580 coal samples from 57 

countries (Tewalt and others, 2010). The USGS examined 305 

samples of Chinese coal in WoCQI (excluding 23 samples of 

cleaned coal that the WoCQI listed) and determined their mean 

value to be 0.16 ppm Hg (fig. 3A). A recent study by Dai and 

others (2012) gives a mean value of 0.163 ppm Hg for 1,666 

Table 1. Production data by country, listed in order of hard coal 

production. 

[Values are in millions of metric tons (Mt) from 2010 data from International Energy Agency (2011)]

Country Hard coal Brown coal

China 3,162 *

United States 932 65

India 538 33

Australia 353 67

South Africa 255 0

Russian Federation 248 76

Indonesia 173 163

Others 525 639

 World total 6,186 1,043

*

Brown coal is included in hard coal data. The seven countries listed 

accounted for more than 90 percent of hard coal production.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Anthracite

n = 59

Bituminous

n = 218

Subbituminous

n = 23

Lignite

n = 5

Outliers greater than

 UQ +1.5 × IQD

A. People’s Republic of China 

Concentration of dry mercury, in parts per millionConcentration of dry mercury, in parts per million

EXPLANATION

Number of WoCQI samples examined 

per coal rank (total 305)

0.01

0.1

1

10

100

Outliers greater than

UQ +1.5 × IQD

Coals mined for power

and industrial use

n = 16

Mean = 0.20

Median = 0.21

Mineralized coals

n = 35

Mean = 7.5

Median = 1.4

B. Guizhou Province

Number of WoCQI samples examined (total 51)

Median

Outlier greater than 

UQ + 1.5 × IQD

Upper quartile

+1.5 interquartile distance

Number of samples

-1.5 interquartile distance

Lower quartile

n = 59

Figure 3. Compilation of U.S. Geological Survey results for mercury (Hg) in Chinese coal, expressed in parts 

per million (ppm) on a dry basis. Data are shown as boxplots where each box encloses 50 percent of the 

data range and the median value is shown as a horizontal line. The top and bottom of the box, defined as the 

upper and lower quartiles, indicate ±25 percent of the sample population. The interquartile distance (IQD) 

is shown by the box height. Outliers are defined as either greater than the upper quartile (UQ) plus 1.5 times 

the IQD or less than the lower quartile minus 1.5 times the IQD. A, Comparison of commercial coals mined 

throughout China by coal rank, including lignite, subbituminous, bituminous, and anthracite coals, listed in 

order of increasing rank. Plot includes 305 samples from 25 provinces and autonomous regions. Results give 

an overall mean mercury content of 0.16 ppm. From Belkin and others (2006) and Tewalt and others (2010). 

B, Comparison of commercial coals mined in Guizhou Province with Guizhou Province mineralized coals that 

are enriched in trace elements such as Hg. For Guizhou Province, the mean Hg content of mineralized coals 

is nearly 40 times that of coals mined for power generation and industrial use. Inclusion of such mineralized 

coals in database compilations has led to past overestimates of the Hg content of Chinese coals. Data from 

Belkin and others (2006) and Tewalt and others (2010). WoCQI, World Coal Quality Inventory.
samples of Chinese coal, which is virtually identical to that 

determined by the USGS from WoCQI data. 

These recent results for China contradict previous estimates 

that had implied that the Hg content of Chinese coal was much 

higher than that of U.S. coal. This impression probably resulted 

because the earlier compilations of the Hg content of coal used 

for the Chinese utility sector had included mercury- (and arsenic-) enriched mineralized coals, which are present locally in 

some parts of China—in Guizhou Province, for example—and 

were used primarily for domestic (as opposed to industrial and 

utility) purposes (fig. 3B).

Information for the next five largest producers of hard 

coal during 2010—India, Australia, South Africa, the Russian Federation, and Indonesia—is more limited, and, with the 

exception of Australia, and possibly of the Russian Federation, 

does not allow nationwide calculations of means for Hg in coal 

in these countries. For India, Australia, and the Russian Federation, USGS coal sampling is limited to specific study sites that 

exclude important coal-producing regions. In South Africa, 

USGS sampling is more extensive, but the number of samples 

is still too few to allow determination of a nationwide mean for 

Hg. New studies that improve sampling of coal and of emissions 

in major coal-using countries other than China and the United 

States could provide better estimates of the extent to which these 

countries contribute to global emissions of Hg.

Halogens in Coal and Their Influence on Mercury 

Capture

Halogens are elements in a group that includes fluorine (F), 

chlorine (Cl), bromine (Br), and iodine (I); they have similar 

chemical properties, and they form negatively charged ionic species. At high temperatures in the boilers of coal-fired powerplants, 

Hg from coal exists primarily in the form of gaseous elemental 

mercury (Hg0). Air-pollution control devices (APCDs) present in 

modern coal-fired powerplants have little capacity to capture Hg0

because it is extremely nonreactive and insoluble. Halogens are 

important because they can convert Hg0 to oxidized forms of Hg 

at temperatures present in the flue gas of powerplants (fig. 4). This 

oxidized Hg combines with halogens, rendering these Hg-halogen 

HgCl2

Hgp

HgBr2

HgCl2

Gas-phase

reactions

Gas-particulate

reactions 

Hgp

Hg0 Hg–

A. Gaseous Hg B. Particulate Hg

HgBr2

Hg0

 Figure 4. Diagram showing the pathways for mercury (Hg) during coal 

combustion, and the pathways for mercury oxidation as the combustion 

gas cools. In reactions taking place in the flame (shown in red), all of the 

mercury from coal is released into the flue gas as elemental mercury 

(Hg0

); in oxidation reactions, which occur primarily as temperatures 

decrease (shown in blue), some of the Hg0

 in the boiler becomes 

oxidized mercury (Hg2+) by reacting with halogens (primarily bromine 

and chlorine), forming gaseous mercury-halogen complexes such as 

mercuric chloride (also “mercury II chloride”) (HgCl2

), or mercuric 

bromide (also “mercury II bromide”) (HgBr2

). Mercury oxidation can 

also take place by reactions between Hg0

 and the surfaces of particles, 

especially if any unburned carbon is present suspended in the flue gas. 

Oxidized mercury formed by either of these two processes can become 

trapped on particle surfaces, especially on unburned carbon, thereby 

forming particulate mercury (Hgp

). Gaseous elemental mercury thus 

oxidized is water soluble and can be removed by existing scrubbers 

that were initially designed to remove sulfur dioxide (SO2

). Particulate 

mercury (mercury that is bound to particles) can be captured by airpollution control devices designed to limit particle emissions. Halogens 

such as bromine and chlorine promote mercury oxidation, thereby 

allowing a portion of the mercury to be captured and prevented from 

entering the atmosphere.
Concentration of dry chlorine, in parts per million 

Concentration of dry bromine, in parts per million 

0 1,000 2,000 3,000 4,000

0

25

50

75

100

Br/Cl = 0.02

EXPLANATION

Deep-basin Illinois coals

Eastern Kentucky Southwestern Reserve

Lignite and subbituminous 

Medium volatile bituminous and higher

Coals

Bituminous

Figure 5. Scatter-plot showing average chlorine (Cl) and bromine 

(Br) contents, in parts per million (ppm), for various ranks (grades) of 

coal produced in 110 U.S. counties. Those with lower rank coal (lignite, 

subbituminous) are green, bituminous coal are blue, and higher rank 

coal (medium volatile bituminous and higher) are red. Contents of Br in 

coal are commonly about 2 percent of the Cl content (Br/Cl = 0.02), the 

ratio generally ranging from 1 to 4 percent (dashed lines indicate this 

complexes much more readily captured by APCDs than is Hg0 by 

itself. Chlorine is the most abundant halogen in coal, and it has the 

largest influence on Hg oxidation and capture. Bromine, which 

is heavier than Cl, is much less abundant in most coals than Cl 

(fig. 5), but heavy halogens such as Br are proportionally more 

effective as Hg oxidants than lighter halogens such as Cl. The 

actual proportion of Hg that conventional APCDs capture depends 

on the type of coal that is burned, the type and set-up of APCDs, 

the halogen content of that coal, and the flue gas temperature. 

Mercury-specific approaches, such as adding sorbents (materials 

that have the capacity to adsorb) containing halogens to the boiler, 

may be used to achieve greater reductions of Hg under MATS 

(Granite and others, in press).

Low-rank coals such as lignite generally have lower Cl contents than higher rank coals such as bituminous coal (fig. 5). The 

halogen content of coal is also controlled to a large degree when 

coal interacts with groundwater before being mined. It is normal 

for the salinity of groundwater to increase with depth, due to 

chemical exchange with geologic formations. For example, in 

the Illinois Basin, where coal has been sampled together with 

such formation water, there is an excellent correspondence 

between Cl in the coal and that in the water (Gluskoter and 

Rees, 1964). Similarly, in the Appalachian Basin, Cl content of 

coal increases with the age of coal-bearing geologic formations, 

which in general increase with depth in the basin (Bragg and 

others, 1991; table 2). Halogens are desirable for their capacity 

range). In some deep-basin Illinois coals (open circles), and in coals 

from the Southwestern Reserve District of eastern Kentucky (open 

squares), the interaction of coal with saline brines elevated Cl contents. 

Br/Cl ratios are less consistent for the higher rank coals (medium 

volatile bituminous and higher). Data from COALQUAL, Bragg and others 

(1998), and Illinois State Geological Survey (2007).

to promote Hg capture, but Cl contents in the deepest coals, in 

Illinois and elsewhere, may be so high that they cause corrosion 

of utility boilers, and therefore are not usable. If these Cl-rich 

coals could potentially be used to blend with halogen-poor coal, 

their capacity to promote Hg capture could be increased. 

Table 2. Mean chlorine content of coal by formation in order of 

increasing age in the central Appalachian Basin, Pennsylvania, 

and West Virginia, United States. 

[From Bragg and others (1991). ppm, parts per million]

1

Detection limit is 100 ppm. Samples in which chlorine was not 

detected are assumed to contain 70 percent of the detection limit. 

2

Samples are from southern West Virginia; all other samples are from 

Pennsylvania.

Age Formation Number of 

samples

Mean1

,

in ppm

Lower Permian(?) Dunkard Group 44 162

Upper Pennsylvanian Monongahela 73 477

Upper Pennsylvanian Conemaugh 41 828

Middle Pennsylvanian Allegheny 709 1,097

Middle Pennsylvanian Kanawha2 36 1,408

Lower Pennsylvanian New River2 56 1,503
Summary

The distribution of Hg and of halogens in coal is among the 

most important factors in determining the amount of Hg emitted to the atmosphere by coal combustion. New EPA rules are 

expected to limit Hg emissions from coal-fired utility power stations in the United States. These rules are the result of detailed 

sampling of U.S. coal and powerplant emissions. The EPA databases show trends in U.S. coal use, such as coal preparation and 

the shift to greater use of low-sulfur Western coals, which are not 

evident in COALQUAL, the USGS compilation for in-ground 

coal data. Nonetheless, COALQUAL remains useful in showing 

compositional differences among U.S. coals and in comparing international coal data. For China, recent compilations are 

sufficient to estimate mean Hg contents on a nationwide basis, 

and these results indicate that the Hg content of Chinese coals 

is comparable on average to that of in-ground coal in the United 

States as determined by the USGS. This finding contradicts 

earlier estimates of Hg in Chinese coal, which had suggested 

that Chinese coals contained more Hg than those in the United 

States. Less is known about the Hg content of coal in some of 

the most important coal-producing countries. New studies that 

assess the Hg and halogen contents of commercial coals, such as 

those conducted on U.S. coals by the EPA and the USGS, would 

allow countries elsewhere in the world to compile reliable data 

that would enable them to limit the proportion of Hg emitted by 

powerplants during coal combustion.

References Cited

Belkin, H.E., Zheng, B., and Finkelman, R.B., 2006, Mercury in 

coal from the People’s Republic of China [abs.]: International 

Conference on Mercury as a Global Pollutant, 8th, Madison, 

Wis., 2006, p. 83–84.

Bragg, L.J., Finkelman, R.B., and Tewalt, S.J., 1991, Distribution of chlorine in United States coal, in Stringer, J., and 

Banerjee, D.D., eds., Chlorine in coal: Amsterdam, Elsevier, 

p. 3–10.

Bragg, L.J., Oman, J.K., Tewalt, S.J., Oman, C.L., Rega, N.H., 

Washington, P.M., and Finkelman, R.B., 1998, Coal quality 

(COALQUAL) database, version 2.0: U.S. Geological Survey 

Open-File Report 97–134 CD-ROM.

Dai, S., Ren, D., Chou, C.-L., Finkelman, R.B., Seredin, V.V., 

and Zhou, Y., 2012, Geochemistry of trace elements in 

Chinese coals—A review of abundances, genetic types, and 

impacts on human health and industrial utilization: International Journal of Coal Geology, v. 94, p. 3–21.

Gluskoter, H.J., and Rees, O.W., 1964, Chlorine in Illinois coal: 

Illinois State Geological Survey Circular 372, 23 p.

Granite, E.J., Pennline, H.W., and Senior, C.L., eds., in press, 

Mercury emissions control for coal-derived gas streams: Berlin, Wiley-VCH.

Illinois State Geological Survey (ISGS), 2007, Coal mines, 

coal geology and resource data online; Illinois coal quality 

data: Illinois Geological Survey, accessed April 19, 2012, 

at http://www.isgs.uiuc.edu/maps-data-pub/coal-maps/stratdatabase/coal-quality-nonconf.xls.

International Energy Agency (IEA), 2011, Key world energy 

statistics: International Energy Agency, 80 p., accessed February 27, 2012, at http://www.iea.org/textbase/nppdf/free/2011/

key_world_energy_stats.pdf.

Kolker, Allan, and Quick, J.C., in press, Mercury and halogens 

in coal, in Granite, E.J., Pennline, H.W., and Senior, C.L., 

eds., Mercury emissions control for coal-derived gas steams: 

Berlin, Wiley-VCH.

Tewalt, S.J., Bragg, L.J., and Finkelman, R.B., 2001, Mercury 

in U.S. coal—Abundance, distribution, and modes of occurrence: U.S. Geological Survey Fact Sheet 095–01, 4 p.

Tewalt, S.J., Belkin, H.E., SanFilipo, J.R., Merrill, M.D., Palmer, 

C.A., Warwick, P.D., Karlsen, A.W., Finkelman, R.B., and 

Park, A.J., comps., 2010, Chemical analyses in the world coal 

quality inventory, version 1: U.S. Geological Survey OpenFile Report 2010–1196, 4 p.

U.S. Environmental Protection Agency (EPA), 2011a, Mercury 

and air toxics standards, cleaner power plants, accessed April 

19, 2012, at http://www.epa.gov/mats/powerplants.html. 

U.S. Environmental Protection Agency (EPA), 2011b, Air 

toxics standards for utilities—MATS ICR data (EGU_

ICR_part I_and_part II), accessed March 23, 2012, at 

http://www.epa.gov/ttn/atw/utility/utilitypg.html.

U.S. Environmental Protection Agency (EPA), 2011c, Air 

toxics standards for utilities—MATS ICR data (EGU ICR 

part III) ACCESS data file, accessed March 23, 2012, at 

http://www.epa.gov/ttn/atw/utility/utilitypg.html.

U.S. Environmental Protection Agency (EPA), 2012, National 

emission standards for hazardous air pollutants from coal-and 

oil-fired electric utility steam generating units and standards 

of performance for fossil-fuel-fired electric utility, industrialcommercial-institutional, and small industrial-commercialinstitutional steam generating units: Federal Register, v. 77, 

no. 32, February 16, p. 9304–9513, accessed April 19, 

2012, at http://www.gpo.gov/fdsys/pkg/FR-2012-02-16/

pdf/2012-806.pdf.

For more information, please contact

Allan Kolker

U.S. Geological Survey

12201 Sunrise Valley Drive

Mail Stop 956

Reston, VA 20192

Telephone: (703) 648-6418

Email: akolker@usgs.gov

By Allan Kolker,1 Jeffrey C. Quick,2 Connie L. Senior,3

and Harvey E. Belkin1

1

U.S. Geological Survey.

2

Utah Geological Survey.

3

ADA Environmental Solutions, Inc.