U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3084

September 2009

Emissions from Coal Fires and Their Impact on the 

Environment

Introduction—Coal Fires, Emissions, and Resource 

Implications

Self-ignited, naturally occurring coal fires and fires resulting from human activities persist for decades in underground 

coal mines, coal waste piles, and unmined coal beds. These 

uncontrolled coal fires occur in all coal-bearing parts of the 

world (Stracher, 2007) and pose multiple threats to the global 

environment because they emit greenhouse gases—carbon 

dioxide (CO2), and methane (CH4)—as well as mercury (Hg), 

carbon monoxide (CO), and other toxic substances (fig. 1). 

The contribution of coal fires to the global pool of atmospheric 

CO2

 is little known but potentially significant. For China, the 

world’s largest coal producer, it is estimated that anywhere 

between 10 million and 200 million metric tons (Mt) of coal 

reserves (about 0.5 to 10 percent of production) is consumed 

annually by coal fires or made inaccessible owing to fires that 

hinder mining operations (Rosema and others, 1999; Voigt and 

others, 2004). At this proportion of production, coal amounts 

lost to coal fires worldwide would be two to three times that for 

China. Assuming this coal has mercury concentrations similar to 

those in U.S. coals, a preliminary estimate of annual Hg emissions from coal fires worldwide is comparable in magnitude 

to the 48 tons of annual Hg emissions from all U.S. coal-fired 

power-generating stations combined (U.S. Environmental Protection Agency, 2002).

In the United States, the combined cost of coal-fire remediation projects, completed, budgeted, or projected by the U.S. 

Department of the Interior’s Office of Surface Mining Reclamation and Enforcement (OSM), exceeds $1 billion, with about 

90% of that in two States—Pennsylvania and West Virginia 

(Office of Surface Mining Enforcement and Reclamation, 2008; 

fig. 2). Altogether, 15 States have combined cumulative OSM 

coal-fire project costs exceeding $1 million, with the greatest 

overall expense occurring in States where underground coal 

fires are predominant over surface fires, reflecting the greater 

cost of extinguishing underground fires (fig. 2) (see “Controlling Coal Fires”).

In this fact sheet we review how coal fires occur, how 

they can be detected by airborne and remote surveys, and, most 

importantly, the impact coal-fire emissions may have on the 

environment and human health. In addition, we describe recent 

efforts by the U.S. Geological Survey (USGS) and collaborators 

to measure emissions of CO2

, CO, CH4, and Hg, using groundbased portable detectors, and combining these approaches with 

airborne thermal imaging and CO2 measurements. The goal of 

this research is to develop approaches that can be extrapolated 

to large fires and to extrapolate results for individual fires in 

order to estimate the contribution of coal fires as a category of 

global emissions.

Self-Heating Processes Leading to Spontaneous 

Combustion

Exposure of underground coal and (or) coal waste piles 

to atmospheric oxygen promotes spontaneous heat-generating 

reactions, primarily oxidation of the coal itself, and the oxidation of pyrite present in coal. In this process, carbon, the 

largest constituent of coal, combines with available oxygen to 

produce CO2

 and heat. Similarly, the sulfur present in pyrite 

combines with oxygen to produce sulfate, releasing heat, and, 

in the presence of water, forming sulfuric acid that can result in 

acidic drainage. Adsorption of water vapor onto coal surfaces 

and interaction of bacteria with coal are other heat-producing 

processes (Kim, 2007). Where oxygen is present and because 

of the decrease in the minimum required temperatures for coal 

combustion at depth and the insulating capacity of coal overburden, these heat-producing reactions can result in spontaneous 

combustion or extend the life of coal fires started by inadvertent 

combustion in mines. Where exposed at the surface, as in the 

Powder River Basin of Wyoming and Montana, coal beds can 

be ignited by wildfires, lightning, and human activities, as well 

as by spontaneous combustion (Heffern and Coates, 2004).

Figure 1. Emissions from the Centralia, Penn., underground coal 

fire; subsidence resulting from burning and subsequent collapse of 

coal pillars used as roof supports for mining tunnels under the town 

is also evident. Centralia, the largest and best known U.S. coal fire, 

has been burning since 1962 and has resulted in relocation of nearly 

all of the town’s residents. Photograph by Janet Stracher.
Impact of Emissions

Direct hazards to humans and the environment posed by 

coal fires include emission of pollutants, such as CO, CO2, nitrogen oxides, particulate matter, sulfur dioxide, toxic organic compounds, and potentially toxic trace elements, such as arsenic, Hg, 

and selenium (Finkelman, 2004). Mineral condensates formed 

from gaseous emissions around vents pose a potential indirect 

hazard by leaching metals from mineral-encrusted surfaces 

into nearby water bodies. Despite this combination of potential 

health hazards, few studies have concentrated on the immediate 

health impacts of coal fires (Finkelman, 2004). Some gaseous 

components from coal fires, such as Hg and CO2

, pose a globalscale threat. In the case of Hg, its addition to the atmosphere 

is thought to eventually contribute to high levels of the methyl 

form of Hg in some fish species, consumption of which is the 

primary human exposure pathway for Hg. In the case of CO2 and 

other greenhouse gases, emissions contribute to climate change, 

but data on these emissions are not sufficient for uncontrolled 

coal fires to be taken into account as a source category in current 

climate model projections.

USGS Coal Fire Investigations

In cooperation with non-USGS collaborators, USGS 

scientists have recently developed a ground-based approach for 

determining CO2 emissions from a coal fire. In this approach, 

measurements of CO2 emitted through vents that exhaust 

hot gasses from the fire (vent emissions) are combined with 

measurements of CO2 gradually diffused from an underground 

fire through the soil over the area of an underground coal fire 

(diffuse emissions). When combined, the vent and diffuse emissions give an estimate of the daily amount of CO2

 released by 

the coal fire. Vent emissions are also measured for gaseous CO, 

CH4, hydrogen sulfide, and Hg.

The ground-based approach was initially applied in a study 

of a burning coal waste pile near the town of Mulga, outside of 

Birmingham in northern Alabama. The Mulga fire was selected 

because it provided a simple case study to test procedures for 

sampling diffuse CO2 fluxes (the amount of CO2 emitted per 

area per day), along four transects of the fire area (fig. 3). Temperature measurements show localized hot spots in the Mulga 

coal fire, some of which exceed 300°C (572°F) (fig. 3). Results 

for the four transects indicate that diffuse CO2 flux varies by as 

much as three orders of magnitude (fig. 4). Comparison of diffuse CO2

 flux and surface soil temperature for the four transects 

shows some correlation between these parameters when all the 

results are combined. If such a relationship were found to be 

consistent, then it could be used to estimate diffuse CO2 emissions based only on temperature for portions of the fire lacking 

diffuse CO2 flux measurements. Using the same rationale, comparison of emissions from coal fires with similar attributes (for 

example, coal rank and type, caloric content, composition, size, 

and intensity) and weighting them according to the size and 

intensity of the fire may allow us to estimate areas or volumes 

of burning coal in order to calculate emissions on larger scales.

In May 2009, coal fires in the Powder River Basin of 

Wyoming and Montana were studied. The Powder River Basin 

was selected for study because of ongoing coal-fire activity and 

because ground-based and airborne sampling methods could 

be applied at the same time. The ground-based effort consisted 

of measuring diffuse CO2 fluxes at 116 points and emissions 

of multiple gases from 29 vents in three fires along the Tongue 

River near Sheridan, Wyo. At the same time, the airborne crew 

analyzed for tropospheric CO2 concentrations and collected 

airborne surface-temperature imaging throughout the northern half of the basin (fig. 5). The airborne results are currently 

being processed, and the airborne and ground datasets are being 

compared so that airborne and ground-based measurements of 

Figure 2. Histogram showing Office of Surface Mining 

Reclamation and Enforcement coal-fire-related project costs by 

State. Costs shown are for completed and future projects combined 

for 15 States where this amount exceeds $1 million. This figure 

shows that total costs are greatest in States where underground 

fires are predominant compared with States where costs for 

surface fires exceed those for underground fires. The bars for 

Pennsylvania and West Virginia are not shown to full extent in 

order to show more detailed perspective for other States; costs 

in Pennsylvania and West Virginia totaled $736 million and $238 

million, respectively. Source: Office of Surface Mining Reclamation 

and Enforcement (2008).

Figure 3. Map showing the distribution of surface temperature (at 

10 centimeters depth), measured in degrees Celsius, in the Mulga, 

Ala., coal pile in December 2008 over the extent of the coal fire, an 

area of about 9.1 hectares (22.6 acres). Results are extrapolated 

from measurement points shown. Locations of transects 1 to 4 (T1, 

T2, T3, and T4) are also shown.
coal fires can be integrated into a combined sampling approach 

in future studies.

Airborne and Satellite Remote Sensing Techniques

Early detection and routine monitoring of coal fires are two 

critical components in strategizing fire fighting efforts and controlling these fires. Hot spots (local areas of elevated temperatures) are a precursor to impending coal mine fires (Prakash and 

others, 1997). Temperature profiles across developing hot spots, 

when used as input in numerical models, help to estimate depth 

of underground fires (Prakash and Berthelote, 2007). Airborne 

thermal infrared (TIR) sensors, such as those operating in the 

broad 8- to 14-micrometer wavelength, are very effective in 

identifying subtle hot spots and in quantifying the temperature 

distribution of active coal fires (fig. 5).

Satellite remote sensing in the TIR wavelength offers a 

simple, cost-effective tool for detecting, mapping, monitoring, 

and characterizing coal fire areas. Currently available technology with spatial resolution ranging from 60 to 120 meters (m) 

can detect hot spots associated with individual large coal fires. 

With advancements in sensor technology, future satellite missions will likely have TIR sensors with higher spatial resolution, 

on the order of 10 to 20 m, and a shorter revisit cycle of about 

4 to 5 days. Such a configuration will be better suited for early 

detection and monitoring of coal fires.

Controlling Coal Fires

While remote sensing techniques are helpful in pinpointing 

the locations of existing and potential underground coal fires, 

extinguishing burning coal beds can be difficult and costly. As 

a result, in many parts of the world, coal-bed or underground 

mine fires have burned uncontrollably for decades. There is no 

one best method to control coal fires, but several approaches 

have been tried. For near-surface fires, combustion may be 

contained by excavating the fire, but this can be expensive and 

disruptive to the environment, especially in the case of large 

fires. Another approach is to inject a flowable mixture of materials underground to help extinguish the fire (Colaizzi, 2004). 

Products engineered to control underground coal fires generally consist of a solid noncombustible material, such as mud, 

fly ash, or cement, combined with an inert gaseous component, 

such as nitrogen or CO2

, and, in some cases, water as a third 

component. Some combination of these materials is applied or 

injected to contain the fire and cut off the supply of oxygen. The 

approach used and the sequence of steps needed to extinguish 

coal fires depend very much on the geometry, depth, and geologic setting of the fire.

Figure 4. Plot of diffuse carbon dioxide (CO2) flux against measured temperature determined for the Mulga, Ala., coal fire for (A) transect 1, 

(B) transect 2, (C) transect 3, and (D) transect 4.

Transect 2

10-CENTIMETER SOIL 

TEMPERATURE, IN °C

Transect 1

10-CENTIMETER SOIL 

TEMPERATURE, IN °C

Transect 4

10-CENTIMETER SOIL 

TEMPERATURE, IN °C

Transect 3

10-CENTIMETER SOIL 

TEMPERATURE, IN °C

Figure 5. A, Predawn quantitative airborne thermal infrared image 

(from May 2009) of the Welch Ranch coal fire, along the Tongue 

River (at left margin of image), near Sheridan, Wyo., measuring 

surface temperature, in degrees Celsius. Blue-green-border box 

indicates main active fire area. B, Plot of temperature distribution 

(in degrees Celsius) along an analysis line through the fire (shown 

in fig. 5A as a blue-green line). Image is courtesy of Airborne 

Research Consultants, LLC. Image orientation is not at N–S.

A

B
Summary and Conclusions

Uncontrolled coal fires occur in the United States and virtually all coal-bearing parts of the world. One of the deleterious 

effects of these fires is the emission of greenhouse gases and 

potentially toxic substances harmful to humans and the environment on global to local scales. Additionally, these fires limit 

access to coal resources and inadvertently consume a portion of 

these resources that may have otherwise been available for use. 

Initial work by USGS researchers and non-USGS collaborators has concentrated on quantifying emissions for fires in the 

United States selected to develop ground-based and combined 

ground-based/airborne measurement approaches. The goal of 

this research is to develop approaches that can be extended to 

larger fires and, ultimately, to begin to quantify the impact of 

coal fires on global emissions of substances such as Hg and 

CO2. The magnitude of this impact is little known. The nature 

and extent of coal-fire emissions must be quantified so that they 

can be taken into account in global atmospheric models and so 

their potential impact on people residing in proximity to fires 

and on the environment can be better assessed.

References Cited

Colaizzi, G.J., 2004, Prevention, control and/or extinguishment 

of coal seam fires using cellular grout: International Journal 

of Coal Geology, v. 59, p. 75–81.

Finkelman, R.B., 2004, Potential health impacts of burning coal 

beds and waste banks: International Journal of Coal Geology, 

v. 59, p. 19–24.

Heffern, E.L., and Coates, D.A., 2004, Geologic history of natural coal-bed fires, Powder River basin, USA: International 

Journal of Coal Geology, v. 59, p. 25–47.

Kim, A.G., 2007, Greenhouse gases generated in underground 

coal-mine fires, in Stracher, G.B., ed., Geology of coal fires—

Case studies from around the world: [Geological Society of 

America] Reviews in Engineering Geology, v. 18, p. 1–13.

Office of Surface Mining Reclamation and Enforcement, 2008, 

Abandoned mine land inventory system (AMLIS): Office of 

Surface Mining Reclamation and Enforcement Web site at 

http://www.osmre.gov/aml/AMLIS/AMLIS.shtm. (Accessed 

August 25, 2009.)

Prakash, Anupma, and Berthelote, A.R., 2007, Subsurface coal 

mine fires—Laboratory simulation, numerical modeling and 

depth estimation, in Stracher, G.B., ed., Geology of coal fires—

Case studies from around the world: [Geological Society of 

America] Reviews in Engineering Geology, v. 18, p. 211–218.

Prakash. A., Gupta, R.P., and Saraf, A.K., 1997, A LandsatTM 

based comparative study of surface and subsurface fires in the 

Jharia coalfield, India: International Journal of Remote Sensing, v. 18, no. 11, p. 2463–2469.

Rosema, A., Guan, H., Veld, H., Vekerdy, Z., Ten Katen, A.M., 

Prakash, A., and Genderen, J.V., 1999, Manual of coal fire 

detection and monitoring: Delft, Netherlands Institute of 

Applied Geosciences Report 99–221–C, 245 p.

Stracher, G.B., ed., 2007, Geology of coal fires—Case studies from around the world: [Geological Society of America] 

Reviews in Engineering Geology, v. 18, 283 p.

U.S. Environmental Protection Agency, 2002, Results of 1999 

information collection request: U.S. Environmental Protection Agency Web site at http://www.epa.gov/ttn/atw/combust/

utiltox/icrdata.xls. (Accessed August 25, 2009.)

Voigt, S., Tetzlaff, A., Zhang, J., Kunzer, C., Zhukov, B., Strunz, 

G., Oertel, D., Roth, A., van Dijk, P., and Mehl, H., 2004, 

Integrating satellite remote sensing techniques for detection 

and analysis of uncontrolled coal seam fires in north China, 

in Stracher, G.B., ed., Coal fires burning around the world—

A global catastrophe: International Journal of Coal Geology, 

v. 59, no. 1–2, p. 121–136.

For more information, contact

Allan Kolker Mark Engle

U.S. Geological Survey U.S. Geological Survey

12201 Sunrise Valley Drive 12201 Sunrise Valley Drive

Mail Stop 956 Mail Stop 956

Reston, VA 20192 Reston, VA 20192

Telephone: (703) 648–6418 Telephone: (703) 648–6454

E-mail: akolker@usgs.gov E-mail: engle@usgs.gov

By Allan Kolker,1 Mark Engle,1 Glenn Stracher,2

James Hower,3

 Anupma Prakash,4 Lawrence Radke,5

Arnout ter Schure,6

 and Ed Heffern7

1

U.S. Geological Survey Eastern Energy Resources Science Center, Reston, VA.

2

East Georgia College, Swainsboro, GA.

3

University of Kentucky, Center for Applied Energy Research, Lexington, KY.

4

Geophysical Institute, University of Alaska-Fairbanks, Fairbanks, AK.

5

Airborne Research Consultants LLC, Saunderstown, RI.

6

Electric Power Research Institute, Palo Alto, CA.

7

U.S. Bureau of Land Management, Branch of Solid Minerals, Cheyenne, WY.