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Coal Fires:
A Synopsis of Their Origin, Remote Sensing Detection, and Thermodynamics of Sublimation

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Technology / EnviroMine
Environmental Technology for Mining

Editor: Shannon Shaw, RGC
Co-Editor: Sebastien Fortin, RGC
Sponsored by: Robertson GeoConsultants Inc.

Glenn B. Stracher
University System of Georgia, Division of Science and Mathematics,
East Georgia College, Swainsboro, Georgia 30401

Tammy P. Taylor
Chemistry Division, Mail Stop J514, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545

Anupma Prakash
Geological Survey Division, International Institute for Aerospace Survey and Earth Sciences,
P.O. Box 6, 7500 AA Enschede, The Netherlands

Although geologic evidence suggests that coal fires are a natural event, large-scale coal mining since the industrial revolution has led to the proliferation of these fires in major coal producing countries. Most coal mine-related fires are ignited by spontaneous combustion, surface fires, and mine-related activities. These fires cause a local rise in surface temperature that can be detected using remotely sensed data acquired in the thermal infrared and short wave infrared portions of the electromagnetic spectrum. P-T stability diagrams for condensates from coal gas may be derived using thermodynamic loop analysis. These diagrams predict conditions that tend to favor the condensation of gaseous exhalations as opposed to their absorption into the atmosphere.

Coal has been mined for over a thousand years as a heating and cooking fuel, but large-scale mining did not begin until the 19th century, in support of the industrial revolution (World Coal Institute, 2000). Although evidence of coal fires from the Pleistocene (Zhang Xiangmin and Kroonenberg, 1996) suggests that such fires are a natural event, strip (opencast) and deep (underground) coal mining by humans has promoted the proliferation and severity of these fires. Such fires burn today in coal seams and culm banks of major coal producing countries including China, the United States, India (Figure 1), and Indonesia.

Figure 1. Photograph of a shallow coal fire in an Indian coalfield. The coal seam on the left is on fire. Excavated material on the right is being backfilled to seal the goafs (openings) left after mining. Smoke emitting from the fire is high in particulate matter, oxides and dioxides of carbon, nitrogen, and sulfur.

In this paper we provide a brief synopsis of the origin of coal mine-related fires, the state of remote sensing technology used in fire detection and prevention, and the thermodynamics of sublimation products from coal fires.

The majority of coal mine-related fires are started by spontaneous combustion, surface fires, and mine-related activities. Spontaneous combustion may be induced by coal fines, oil-soaked rags, hay, manure, and lumber in culm banks (Jones and Scott, 1939; Anthony et al., 1977, p. 29; U.S. Department of Energy, 1993) or by exothermic oxidation reactions catalyzed by oxygen and moisture circulating through coal seam joints. One such reaction is (Limacher, 1963):

Surface fires ignited by burning trash, lightning, and forest or bush fires may spread to culm banks or coal seams (DeKok, 1986, p. 20; Discover, 1999). The Centralia, Pennsylvania mine fire (Figure 2), one of the worst underground fires in the United States, has been burning since May 1962 (Gessinger, 1990; Memmi, 2000). The fire began as a surface fire when the Centralia Borough Council decided to ignite trash to reduce the volume of and control rodents in an abandoned strip-mining cut used as an unregulated dump at the edge of town. Burning trash ignited the Buck anthracite seam concealed behind the refuse, and the fire spread along the seam to tunnels beneath Centralia (DeKok, 1986, p. 20; Geissinger, 1990; Memmi, 2000). Today, only about 20 people in Centralia remain out of approximately 1100 former residents (Schogol, 2001). Youngstown, Pennsylvania appears to be destined for the "ghost town" fate of Centralia because of the Percy mine fire burning underground there for over 30 years. The Percy mine fire began when burning trash at the surface ignited a coal seam beneath Youngstown (Glover, 1998).

Figure 2. Northern, eastern middle, western middle, and southern anthracite fields of Pennsylvania. The Centralia underground coal fire (red box), burning since 1962, is in the western middle field (Columbia County). Courtesy of Timothy Altares, Pennsylvania Dept. of Environmental Protection, Harrisburg, PA.

Mine-related activities responsible for starting coal fires include cutting and welding, explosives and electrical work, and smoking which may ignite gases such as methane and hydrogen (Mine Safety and Health Administration, 1996; Pennsylvania Department of Environmental Protection, 2001). Coal fires raging across the Xinjiang autonomous region of northern China, for example, are thought by Chinese officials to have started accidentally in small illegal mineshafts excavated by local farmers (Wingfield-Hayes, 2000). Noxious gases and particulates released by combustion associated with one such fire, the Liu Huangou coalfield fire, are being blown by winds over the region's capital city, Urumqi, one of the 10 worst polluted cities in the world (Wingfield-Hayes, 2000).

In China, local miners often use abandoned underground mines for shelter and burn coal to keep warm during the winter. In India, such abandoned mines have been reported to be hideouts for the illegal distillation of alcohol. These kinds of activities are also reported to be responsible for initiating coal mine fires.

Remote sensing technology has made it possible to detect coal fires and study their effects. Thermal and optical images along with field-based measurements (Figure 3) are used to determine the location, size, depth, propagation direction, burning intensity, temperature, and coal consumption of a fire (Zhang et al., 2002, in review; Vekerdy et al., 1999; Prakash and Gupta, 1999). This information has been useful for fighting fires in northern China. Figure 4 shows a processed remote sensing image (Landsat Thematic Mapper (TM) image ) of the Ningxia coal mining area in northwestern China (Prakash et al, 2001). The coal fire areas coded in yellow and red were derived from thermal images. The gray background image is a higher resolution optical image acquired in the near infrared region of the electromagnetic spectrum. The two images have been fused to show the exact locations of the coal fires. The ground control points (pink crosses), measured using the high precision kinematic global positioning system (GPS), were used to transform local coordinates to standard international coordinates. Coal mining and coal dump areas are outlined on this image in green.

Figure 3. Field-based temperature measurements recorded over fissures associated with underground fires. The fissures form when land subsides due to a reduction in coal volume during subsurface burning.

Figure 4. Processed Landsat TM image of the Ningxia coal mining area in northwestern China. See text for description. (From Prakash et al., 2001).

Subsidence due to underground fires in northwestern China has been identified with thermal, microwave, and optical satellite data (Prakash et al., 2001). Research using synthetic aperture radar (SAR) to identify subsidence is currently being conducted at the International Institute for Aerospace Survey and Earth Sciences (2001; Prakash, 2000).

Pollutants associated with coal fires include condensates associated with subsurface combustion. The condensates form when gas exhaled through surficial vents or cracks first cools and then condenses (Lapham, 1980; Stracher 1995). This process is analogous to the exhalation-condensation mechanism by which minerals form in fumarolic or solfatara environments (Stoiber and Rose, 1974). Condensates from anthracite fires in eastern Pennsylvania include (Lapham et al., 1980 and references therein): selenium (Se), sulfur (S) (Figure 5), galena (PbS), orpiment (As2S3), gypsum (CaSO4.2H2O), mullite (Al6Si2O13), downeyite (SeO2), salammoniac (NH4Cl), and laphamite (As2(Se,S)3). Stracher (1995) derived a P-T stability diagram for the condensation of orthorhombic sulfur (Figure 6) from anthracite gas associated with the Centralia, Pennsylvania mine fire using an analytical technique called thermodynamic loop (TL) analysis.

Figure 5. Anthracite smoker (gas vent) from underground Centralia mine fire. Note sulfur (yellow) condensate around the gas vent. Tip of Glenn Stracher's right shoe (size 7) is in the lower left hand corner of the photo (From Stracher, 1995).

Figure 6. P-T stability diagram for the condensation of orthorhombic sulfur from anthracite gas associated with the Centralia, PA mine fire (From Stracher, 1995).

This and any such stability diagram serves as an environmental indicator of conditions that tend to favor the condensation of gaseous exhalations as opposed to their absorption into the atmosphere. Stracher and Taylor are currently working on P-T stability diagrams for Selenium (Se) and Galena (PbS).

Spontaneous combustion, surface fires, and mine-related accidents most frequently start coal mine-related fires. Remote sensing technology is useful in detecting and investigating fires, observing and quantifying their effects, and in providing information critical for fighting fires. P-T stability diagrams can be derived for the condensation products of such fires provided thermodynamic data is available. These diagrams are useful in predicting condensation conditions.

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Prakash, A. and Gupta, R.P., 1999, Surface fires in Jharia Coalfield, India - their distribution and estimation of area and temperature from TM data: International Journal of Remote Sensing, v. 20, no. 10, p. 935-1946.

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World Coal Institute, 2000, Coal-power for progress, 4th edition: London, England, 32 p. See also (August 2001).

Zhang J., Wagner W., Prakash A., Mehl H., and Voigt S., 2002, Detecting coal fires using remote sensing techniques (in review).

Zhang Xiangmin and Kroonenberg, S.B., 1996, Pleistocene coal fires in Xinjiang, northwest China: International Geologic Congress, 30th, Beijing, China, Abstracts, v. 1, p. 457.

The authors thank Shannon Shaw of Robertson GeoConsultants Inc. for her assistance in facilitating the publication of this manuscript.

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