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Locating fires in abandoned underground coal mines
International Journal of Coal Geology 59 (2004) 49 – 62 www.elsevier.com/locate/ijcoalgeo
Locating fires in abandoned underground coal mines Ann G. Kim * National Energy Technology Laboratory, U.S. Department of Energy, USA Received 7 April 2003; accepted 18 November 2003 Available online 6 March 2004
Abstract A Mine Fire Diagnostic (MFD) Methodology was developed to determine the location and extent of combustion zones in abandoned underground coal mines. In this method, a characteristic fire signature is based on the ratio of higher molecular weight hydrocarbon gases (C2 to C5) to total hydrocarbon gas. Initially, gas samples are obtained at the bottom of boreholes under baseline or static conditions. A second set of samples is obtained when a suction fan is used to influence the direction of gas movement. Pressure data define the degree of communication between boreholes. The value of the diagnostic ratio under communication conditions is taken as a measure of subsurface fire activity related to a particular flow direction. Using a Venn diagram technique, the results are mapped as quadrants on a borehole map of the site. Repetition of the communication tests provides overlapping quadrants that define hot, cold, and indeterminate areas. The MFD has been used to distinguish hearted and cold subsurface areas at four mine fire sites. At each of the sites, the extent of the fire could not be inferred from surface evidence, and the location of combustion zones had a significant impact on plans to control the fire. Although the method is labor intensive and requires drilling cased boreholes, it is relatively simple and provides information that cannot be obtained by other methods. D 2004 Elsevier B.V. All rights reserved. Keywords: Abandoned coal mines; Underground fires; Hydrocarbon desorption; Mine fires
1. Introduction Fires in abandoned mines and waste banks are a relatively common occurrence in coal-producing areas (Kim and Chaiken, 1993). Abandoned mine fires present serious health, safety, and environmental hazards due to the emission of toxic fumes, subsidence, and the deterioration in air quality. To locate this type of remote, subsurface fire, it is necessary that: (1) it have a measurable characteristic, (2) the characteristic
* Tel.: +1-412-386-6724; fax: +1-412-386-4579. E-mail address: [email protected] (A.G. Kim). 0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2003.11.003
be detectable through standardized sampling methods, and (3) the data are interpreted according to an appropriate algorithm. The emission of smoke and fumes at surface fractures and vents usually indicates a fire in an abandoned mine or waste bank. However, the surface evidence of fires may not be related by straight line paths to the source of combustion, since hot gases follow the path of least resistance. Therefore, the heated source of combustion products can be distant, laterally and vertically, from the surface expression. Also, fires in abandoned coal mines are usually confined to relatively small, discontinuous areas of smoldering combustion surrounded by large masses of insulating rock.
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Table 1 Interpretation of R1 values R1 value range
Relative coal temperature
0 to 50 50 to 100 >100
Normal Possible heated coal Heated coal
Methods using remote thermal characteristics or variations in the gross composition of the mine atmosphere have not been routinely successful at locating isolated combustion zones. Infrared photography discriminates temperature variations only within a few centimeters of the surface, usually indicating heated vents and fractures. Borehole temperatures measured in the mine void may be more accurate, although a thermocouple generally measures the max-
imum temperature in a volume of 0.03 to 0.06 m3 (1 to 2 ft3), within a radial distance of 0.25 m (10 in). Core drilling and near-surface geophysical imaging techniques will produce adequate information on structural features, but are less reliable for indicating combustion areas (Dalverny et al., 1996). Elevated temperatures can alter the mineralogy of iron bearing rocks, but magnetic anomalies are more likely to be associated with areas that have been heated and cooled than with active combustion. Electrical terrain conductivity surveys may indicate water flow, i.e., areas where combustion is unlikely (Dalverny and Kim, 1995). Near surface seismic surveys and ground penetrating radar can indicate subsidence areas and changes in subsurface structure, but these are not necessarily related to combustion (Cohen and Dalverny, 1995; Mowery, 1995).
Fig. 1. Artist’s depiction of the Mine Fire Diagnostic (MFD) Methodology and equipment.
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Subsurface changes in the concentration of O2, CO2, CO, and H2 have been used as the basis for geochemical combustion indicators (Jones and Trickett, 1955; Graham, 1920). Although changes in the concentration of these gases may be related to combustion, they may also be produced by processes other than combustion; and there may be a reservoir of such gases in which changes in concentration due to combustion are insignificant. In abandoned mines, submicron particulate (smoke) detectors have given ambiguous results (Litton and Kim, 1989). Using sealed boreholes as sampling points for gas composition, temperature, and pressure increases the amount of usable data, but these reflect a static environment in the mine and yield information about a relatively small volume around the borehole. However, the usefulness of boreholes in delineating fire zones can be improved if data are obtained under both ambient and controlled-flow conditions. The resulting data have both quantity and orientation components. A Mine Fire Diagnostic (MFD) methodology, developed by researchers at the former U.S. Bureau of Mines, is based on the temperature-dependent desorption of low molecular weight hydrocarbon gases from coal and on controlled underground airflow. At the request of the Pennsylvania Department of Environmental Protection (PA DEP) and the federal Office of Surface Mining, Reclamation and Enforcement (OSMRE) the method was used to define the subsurface location of fires in three abandoned bituminous coal mines and one abandoned anthracite mine.
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hydrocarbon gas desorbed from coal is over 90% CH4, probably because the smaller CH4 molecule more readily diffuses to the coal surface. At 150 jC, the concentration of CH4 in the desorbed gas drops to about 35% as the C2 – C5 hydrocarbons concentrations increase. Changes in the relative concentration of desorbed CH4 and the C2 – C5 alkanes can be detected at temperatures below 100 jC and can indicate the presence or absence of heated coal. Based on the observed changes in the relative hydrocarbon concentration, a hydrocarbon ratio (R1) has been defined as a fire signature (Kim, 1991):
R1 ¼
1:01½THC ½CH4 1000 ½THC ¼ c
where [THC] = concentration of total hydrocarbons, ppm; [CH4] = concentration of CH4, ppm; c = constant, 0.01 ppm (eliminates dividing by zero). As defined, the ratio is an increasing function of the percentage of higher hydrocarbons; it has a value of zero when no hydrocarbons are detected, and a value of 10 when CH4 is the only hydrocarbon. The limiting value is about 1000. The interpretation of values of the R1 ratio (Table 1) is based on data from controlled laboratory heating experiments.
2. Mine fire diagnostic methodology The Mine Fire Diagnostic Methodology consists of determining a unique signature for heated coal, a procedure for obtaining samples from abandoned mines, and an algorithm for interpreting the data and relating it to a subsurface location. The desorption of low molecular weight hydrocarbon gases from coal is strongly temperature dependent (Kim, 1974, 1978). Changes in the CH4 emission rate and the emission of other hydrocarbon gases at elevated temperatures are a function of the coal structure and the volume of gas adsorbed on the internal surface of the coal. At normal ground temperatures (f 13 jC),
Fig. 2. Equipment used in Mine Fire Diagnostic Methodology for temperature and pressure measurements and to obtain mine gas samples.
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Fig. 3. Portable generator and suction fan connected through steel pipe and flexible tubing to exhaust borehole for MFD communication test.
Values of R1 z 100 indicate coal temperatures of at least 100 jC, but the absolute value of the R1 ratio is not directly correlated to coal temperature. Measured R1 values indicate an average temperature condition for a large volume of coal, and elevated values are due only to the presence of heated coal. Over a period of time, they will increase with increasing temperature
and decrease with decreasing temperature. Because hydrocarbon desorption occurs at relatively low temperatures, the mass of the coal from which the hydrocarbons are desorbed is much larger than that directly affected by combustion. The larger source increases signature sensitivity. The R1 ratio is independent of dilution by other mine gases, such as O2 and CO2. The only constraint on the use of R1 as an indicator of heated coal is that the concentration of total hydrocarbons in the gas samples exceeds 50 ppm. Although lower concentrations can be determined, apparent changes in the value of the ratio may be due to cumulative analytical error in low concentrations of C2 – C5 hydrocarbons. To determine changes in the combustion signature, access to the coal horizon and a method for remotely obtaining uncontaminated samples are required. In MFD, 20 cm (8 in.) ID boreholes are drilled to the base of the coal bearing strata and cased to within 0.9 m (3 ft) of the coal mine roof (Fig. 1). An instrumentation cap with ports for a thermocouple and sampling line is fastened to a flange welded to the borehole casing. The temperature probe is a 1.5 m stainless steel sheathed Chromel-Alum Type K thermocouple, suspended approximately 0.5 m below the casing. The
Fig. 4. Schematic diagram of gas flow during communication testing in the Mine Fire Diagnostic Methodology.
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Fig. 5. Results of one MFD communication test with suction at BH#26.
sampling line is a combination of polyethylene tubing almost the length of the casing attached to 1 cm (3/ 8 in.) stainless steel tubing near the mine void. This
minimizes weight and expense, while protecting the sample line from high temperatures. The sampling line is connected to a tee fitting for obtaining pressure
Fig. 6. Combined results of two MFD communication tests with suction at BH#26 and BH#3. Pressure contour line indicates limit of communication.
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measurements through a neoprene septum and gas samples through a compression plug. A battery-powered diaphragm pump and evacuated sample tubes are used to obtain samples of the mine gases. Pressure is measured with a Magnehelic pressure meter connected through a hollow needle in a Luer adapter (Fig. 2). An exhaust fan, powered by a portable generator, is used to control airflow during communication tests (Fig. 3). A section of 20 cm (8 in.) steel pipe between flexible duct connects a 2920 cfm fan to a 90j elbow at the borehole (Dalverny and Chaiken, 1991). In the MFD methodology, gas samples, temperature and pressure measurements are obtained from the mine before and during operation of a suction fan
attached to one of the network of boreholes cased to the depth of coal-bearing strata. In the representation of the concept (Fig. 4), the suction fan is attached to the left-most hole. If communication occurs between the suction hole and the neighboring holes, a decrease in static pressure is measured at the bottom of each borehole. If a fire exists at all points within a region, desorbed gases are detected in gas samples taken at each borehole (Case A). Similarly, a heating between the suction hole and its nearest neighbor (Case C) would be detected only at the suction hole. Iteration of the sampling using different boreholes in the grid pattern as the suction point provides data to determine the presence or absence of fire along various
Fig. 7. Site map of Renton Mine Fire Extinguishment Project. Borehole locations indicate the three discontinuous areas of combustion near the mine perimeter: Miller Farm, Plum Street and Danny Property.
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pathways between the sampling points. Using a Venn diagram technique, quadrants, with a radius equal to one-half the average distance between boreholes, are centered on a line connecting that borehole with the suction hole and coded for the R1 signature value (Fig. 5). Repeating the tests with different orientations allows spatial definition of both heated and cold subsurface zones (Fig. 6). A composite of the results of all tests bounds the probable fire zones and their cold boundaries through successive approximation.
3. Results The MFD methodology was demonstrated at four abandoned coal mine sites in cooperation with the Pennsylvania Department of Environmental Protection (PA DEP) and the federal Office of Surface Mining, Reclamation and Enforcement (OSMRE). The results of these tests are summarized below, and additional information on these case studies is given in the cited references.
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3.1. Renton The Renton abandoned mine fire site was a 24 ha (60 acre) region in Allegheny County, southwestern Pennsylvania (Dalverny and Chaiken, 1991). The mine is located in the Pittsburgh coal seam beneath a hill topped by a 4.4 million l (1 million gal) municipal water storage tank; the overburden ranged between 3 and 30 m (10 and 100 ft). The site had several large subsidence holes and venting areas. To extinguish the fire, a heat removal technique utilizing water injection and forced removal of steam was tried at the site. The MFD methodology was used to locate combustion zones and to monitor the progress of the extinguishment project. The MFD located three noncontiguous combustion zones, totaling approximately 4 ha (11 acres) near the perimeter of the site (Fig. 7). These coincided with three heated areas of indefinite extent that had been inferred from visual and historical evidence. Although combustion products had been detected in the central part of the mine, diagnostic tests indicated that heating
Fig. 8. Changes in R1 values at Renton site over a 3-year period indicating cooling in the area between Plum Street and the Danny Property (BH# 14), a long-term increase in combustion at the Miller Farm area (BH# 33), and a rapid increase in combustion followed by a constant level of combustion in the Plum Street area (BH# 57).
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was occurring only in areas closer to the buried outcrop. Over a period of 195 days, a total of 27 million l (7.1 million gal) of water was injected into the three mine fire areas. During and after the water injection phase of the project, MFD was used to follow the progress of the extinguishment effort (Fig. 8). Evidence of combustion activity decreased between the Plum Street area and the Danny property (Borehole 14). It apparently increased and then became static in the Plum Street area (Borehole 57), and increased in the Miller Farm area (Bore-
hole 33). At the Renton mine fire site, it was assumed that most of the injected water did not come in direct contact with heated coal or rock and was not converted to steam. Therefore, not enough heat was removed to extinguish the fire. MFD confirmed that the extinguishment method was not effective. 3.2. Large The mine fire near Large in Allegheny County, PA, is in the Pittsburgh coal seam, which, at this
Fig. 9. Composite results of MFD tests at the Large, PA mine fire site overlain on mine map. Discontinuous combustion areas were located along the buried outcrop and extending into the mine.
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location, outcrops at the base of a hill. Approximate depth of overburden ranges from 6 to 55 m (20 to180 ft). Small vents were visible on the hillside above the buried outcrop. The surface area affected by the mine fire is approximately 1 ha, and the surface slope averages 20j. Five natural gas pipelines and three sets of high-voltage power lines cross the property. Areas of venting were confined to a relatively small surface area and indicated probable propagation along the buried outcrop. The PA DEP was interested in trying foam injection as a heat removal method to extinguish the fire. To estimate the amount of foam and the injection points, it was necessary to determine the location and extent of the combustion area(s). Several 5 cm (2 in.) and 6.25 cm (2.5 in.) boreholes had been drilled and cased for temperature monitoring. The mine temperatures indicated that large areas of the mine were heated, but the data could not be used to define a combustion zone. (Kim and Dalverny, 1994). MFD testing, utilizing two 5 cm (2 in.), seven 6.25 cm (2.5 in.), and fifty 20 cm (8 in.) diameter cased
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boreholes, indicated an L-shaped combustion zone (Fig. 9). The base of the L (150 m) was near the outcrop; the leg of the L extended 180 m into the mine, probably along a set of main entries. Several small isolated combustion zones were located 300 – 1000 m from the primary combustion area. Noncontiguous heated areas were detected in the southwest quadrant of the project area. The direction of propagation into the mine appears to coincide with the location of a set of main entries. Heated areas have been detected to the north and south of this area. By overlaying the surface map on an old mine map, it appears that the fire may be in entries that intersect another set of entries extending north and south under a road adjacent to the site. Subsurface fires generally move toward a source of fresh air, which may have been a buried portal, that the old maps indicated existed in the adjoining valley approximately 1600 m (1 mile) from the site. Another potential source of air was permeable zones in the outcrop. The heated areas apparently extended beyond the site boundary, and a cold boundary for the fire area was not located.
Fig. 10. Site map of Percy Mine Fire Project, showing the limit of excavation and double line of temperature monitoring boreholes.
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Several areas where hydrocarbon concentrations were below 20 ppm CH4 coincided with areas where water was observed in some boreholes. Water vapor condensed on the surface of the coal could block the desorption of hydrocarbons. However, if the temperature of the coal is cold enough to allow water condensation, it can be assumed to be below the combustion point.
The Large mine fire was typical of problems associated with determining underground fire locations from surface evidence. The observed venting above the buried outcrop gave no indication of the combustion areas in the interior portions of the mine. Also, borehole temperature data, when taken alone, would yield misleading results with regard to the location of the combustion zones. Because the com-
Fig. 11. Site map of MFD test at Percy Mine Fire site overlain on old mine map, showing location of 8 in. (P8xxx) communication boreholes and 3 in. (P3xxx) gas sampling boreholes in relation to temperature monitoring boreholes.
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bustion zones were much larger than originally estimated, the foam injection project was not attempted. 3.3. Percy Approximately 15 years before this MFD project, the Office of Surface Mining, Reclamation and Enforcement (OSMRE) extinguished part of a fire in the Percy mine, Fayette County, PA by excavation (Fig. 10). The abandoned mine was in the Pittsburgh seam on the flank of the Chestnut Ridge anticline. The approximate depth of excavation was 30.5 m (100 ft), and the remaining coal had a 10% dip away from the excavation boundary. A double line of temperature 2.5 cm (1 in.) monitoring boreholes on 7.5 m (25 ft) centers had been installed around the mine side of the excavated area. Increasing borehole temperatures and the emission of smoke from surface fractures indicated increased combustion in the unreclaimed portion of the mine. To develop a control plan, it was necessary to determine if combustion was occurring near a group of houses to the south of the reclaimed site. Ten 7.5 cm (3 in.) monitoring boreholes were placed along the section of the site nearest the houses. Thirteen 20 cm (8 in.) boreholes (monitoring and suction) were drilled on 30.5 m (100 ft) centers in a narrow area bounded by the houses (south), a water pipeline (west), and a local road (north) (Fig. 11). Based on previous experience, the atmosphere in underground mines is assumed to be in a steady state. Although this baseline condition will vary locally due to combustion, proximity to fresh air, microbial activity, etc., the composition of air within a given area is generally constant. When suction is applied to the underground system, changes in gas composition at a particular borehole are detected in air samples flowing from another area. Applied suction is usually a temporary perturbation, and the atmosphere is expected to revert to the original baseline composition. This condition did not hold for the Percy mine. There were significant changes in baseline gas composition over periods of as short as 1 day, but such changes might persist for as long as 3 weeks. The lack of a consistent baseline made the interpretation of the diagnostic results more difficult. Also, the layout of the site with essentially two parallel lines of boreholes usable as suction points, constrained the total angle which could
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be evaluated, particularly at the southern end of the site near the houses. Since the site conditions were not optimum, each borehole area was also evaluated with respect to temperature, total hydrocarbon concentration, the ratio N2/O2, and the presence of CO. For normal or uncontaminated air, the value of N2/O2 is 3.7. Higher values are related to O2 depletion. A decreasing value of N2/O2 during communication indicates that air is being drawn into the area, and this may coincide with a decrease in the total hydrocarbon concentration. Based on changes in N2/O2, the changes in baseline gas composition were attributed to an influx of normal air. Carbon monoxide was detected at several boreholes that also had R1 warm or hot values; the presence of CO indicates that combustion is occurring in a low O2 atmosphere. The absence of CO may indicate the absence of combustion, or there may be sufficient O2 to produce CO2 rather than CO. Evaluation of all the data supported the existence of a primary combustion zone near the road and a second area of combustion between the perimeter boreholes and the houses (Kim, 1997). The results of the diagnostic tests indicated that the primary combustion zone was located near the road. Combustion in this area affected boreholes within 60 m (195 ft). The highest average underground temperatures were also measured in the boreholes near the road. Combustion signatures were also detected at two of the 7.5 cm (3 in.) boreholes at the southern end of the site. To prevent the migration of toxic gases
Fig. 12. Anomalous snow melt at the Carbondale MFD site.
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toward the houses, OSM installed a cellular concrete barrier in the abandoned mine. 3.4. Carbondale The Carbondale mine fire site in Pennsylvania’s Northern Anthracite Field was located adjacent to an apartment complex. The complex had been built on an area excavated and backfilled during a previous fire control project. Anomalous snow melt (Fig. 12) indicated that combustion was occurring in the unreclaimed portion of the mine. The objective of the MFD project was to determine the proximity of heated zones to the apartment complex (Kim et al.,
1992). Drillers logs indicated that two anthracite seams were present in the area. Thirty-two 15 cm (6 in.) cased boreholes and two 10 cm (4 in.) cased boreholes had been installed at the 3.4 ha (8.5 acres) site. They extended from 16 m (52 ft) to 30.5 m (100 ft) beneath the surface through the coal seams; they were originally installed as monitoring holes and were subsequently used as suction and observation points for the MFD project. The depth of the boreholes and their diameter were sizing constraints that required two suction fans, each rated at 1.02 m (40 in.) vacuum w.c., connected in series to offset casing resistance and improve communication between boreholes.
Fig. 13. Composite results of MFD tests at the Carbondale site.
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The hydrocarbon ratio is strongly temperature dependent for bituminous samples, but anthracite has a lower concentration of higher hydrocarbons adsorbed on the coal surface. However, methane emission was found to be sufficiently temperature dependent to allow its use as a combustion indicator for anthracite. The use of MFD at Carbondale was considered a test under nonideal conditions because (1) the emission rate for low molecular weight hydrocarbons from anthracite is low and does not exhibit strong temperature dependence, (2) many of the boreholes were cased through the coal seams, (3) the borehole diameters and depths were such that half of the rated pressure drop of the suction fan would be used to overcome pipe resistance, and (4) a large rock fracture that bisected the region could limit the effectiveness of the communication tests. To overcome these conditions, several adaptations were made to the MFD method. Because laboratory tests indicated that the R1 ratio would not be a good indicator of heated anthracite, the change in the absolute concentration of CH4 in baseline and communication samples was used as the index of combustion. Pressure and gas composition data indicated that the casing length and the fault did not affect the diagnostic results. The tests conducted at Carbondale located two large and five small heated zones (Fig. 13). The noncontiguous combustion zones included one region lying adjacent to an apartment building that had not been detected using temperature and CO concentration measurements. The presence of hydrocarbon gas indicated that coal was heated above the normal underground temperature; time-dependent temperature monitoring indicated that the areas of combustion were moving uphill away from the apartment complex.
4. Discussion and conclusions The mine fire diagnostic methodology is based on the assumptions: (1) changes in the relative concentrations of hydrocarbon gases underground are due only to the presence or absence of heated coal, (2) a sufficiently large vacuum applied to underground regions will cause the gases desorbed from heated coal to flow toward the point of suction, and (3) chemical and physical analyses of underground air flowing in a
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controlled manner between borehole sampling points can locate the source of the hydrocarbons. Corollary assumptions are: the measured pressure gradient can characterize the communication effect at each borehole; underground gas flows follow a straight line path toward the suction point; the depth of the borehole and the length of the casing have no effect on the measured pressures or on the gas concentration; ambient surface conditions have no effect; and changes in gas concentration are measurable. In reality, these corollary assumptions do not always hold. However, with both measurable pressure gradients and measurable changes in gas concentrations, repetition of the tests minimizes the effects of nonideal subsurface conditions. Timedependent monitoring can differentiate heating and cooling periods resulting from combustion front movement and/or fire control activities (Justin and Kim, 1988; Dalverny et al., 1990). Coal mine entries can be considered two-dimensional fracture systems constrained by rock strata with much lower permeability. Controlling the movement of underground gases overcomes the limitations of a point source measurement by allowing gases from a large region to be sampled through one borehole. The MFD method increases the detection zone of normal point source measurements through controlled gas movement. Measuring changes in hydrocarbon concentration of the moving gas, and plotting the results as vectors (magnitude and direction) rather that point source (magnitude) measurements, bounds the area(s) affected by combustion. Although independent confirmation of the results of these studies were not possible, in the three bituminous mine fires investigated, the R1 fire signature was consistent with other indicators. The technique of borehole suction and sampling detected thermally separate heated zones. At all four field sites, the data obtained clearly indicated heated areas, areas where data are insufficient to make a judgment, and areas in which combustion was not occurring. The MFD methodology represents a significant improvement in locating and monitoring abandoned mine fires. Its advantages are: (1) changes in characteristic hydrocarbon emission are caused only by changes in temperature, (2) changes in hydrocarbon emission can be detected for temperatures below the ignition point of coal, increasing the effective amount of coal serving as a source, (3) hydrocarbons can be
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detected routinely at concentrations as low as 1 ppm, (4) the use of an appropriate ratio can usually eliminate the effects of sample dilution with air, (5) induced pressure effects can be detected as much as several hundred meters from the suction point, and (6) the system, while somewhat labor intensive, is simple to operate and requires only readily available equipment. The severity of the problem and the need to more accurately determine the location of a mine fire will determine if the Mine Fire Diagnostic Methodology should be used.
Acknowledgements The MFD methodology was developed and implemented under the auspices of the former U.S. Bureau of Mines with funding from and in cooperation with both the U.S. Office of Surface Mining Reclamation and Enforcement and the Pennsylvania Department of Environmental Protection. Robert F. Chaiken and L.E. Dalverny, formerly with the U.S. Bureau of Mines, made significant contributions to the development of the Mine Fire Diagnostic Methodology. Ethel Burse, Andrew Kociban, and Christina Manns, all currently with the U.S. Department of Energy, performed much of the MFD fieldwork, as did Thomas Justin and Joseph P. Slivon, both retired. I appreciate the very helpful comments provided by Robert Chaiken and William Ehler of OSMRE in their reviews of the manuscript.
References Cohen, K.K., Dalverny, L.E., 1995. Seismic characterization of a coal waste bank fire in Eastern Ohio to aid reclamation. Proc. 17th Annual Abandoned Mine Land Conf., National Association of Abandoned Mine Land Programs, French Lick, Indiana, October 15 – 18, 1995, pp. 70 – 86. Dalverny, L.E., Chaiken, R.F., 1991. Mine Fire Diagnostics and Implementation of Water Injection with Fume Exhaustion at Renton, PA. US Bureau of Mines, Washington, DC, RI 9363, 42 pp. NTIS: PB91230243. Dalverny, L.E., Kim, A.G., 1995. Magnetometer and terrain conductivity surveys at a burning coal waste bank. Proc. 17th Annual Abandoned Mine Land Conf., National Association of
Abandoned Mine Land Programs, French Lick, Indiana, October 15 – 18, 1995, pp. 49 – 69. Dalverny, L.E., Chaiken, R.F., Kim, A.G., 1990. Mine fire diagnostics in abandoned bituminous coal mines. Proc. Mining and Reclamation Conference, Charleston, WV, April 23 – 26, 1990. West Virginia University, Morgantown, WV, pp. 527 – 534. Dalverny, L.E., Cohen, K.K., Mowery, G.L., Kim, A.G., 1996. The use of multiple geophysical techniques for site assessment in mine reclamation. Proc. Symposium on Application of Geophysics to Engineering and Environmental Problems (SAGEEP), April 28 – May 2, 1996. Keystone, CO, pp. 887 – 896. Graham, J.I., 1920. The normal production of carbon monoxide in coal mines. Trans. Inst. Min. Eng. 60, 222 – 231. Jones, J.H., Trickett, J.C., 1955. Some observations on the examination of gases resulting from explosions in colleries. Trans. Inst. Min. Eng. 114, 768 – 791. Justin, T.R., Kim, A.G., 1988. Mine fire diagnostics to locate and monitor abandoned mine fires. Proc. Mine Drainage and Reclamation Conference, Pittsburgh, PA, April 17 – 22, 1988. US Bureau of Mines, Washington, DC. IC 9184, pp. 348 – 355. Kim, A.G., 1974. Low Temperature Evolution of Hydrocarbon Gases from Coal. US Bureau of Mines, Washington, DC. RI 7965, 23 pp. Kim, A.G., 1978. Experimental Studies on the Origin and Accumulation of Coalbed Gas. US Bureau of Mines, Washington, DC. RI 8317, 18 pp. Kim, A.G., 1991. Laboratory Determination of Signature Criteria for Locating and Monitoring Abandoned Mine Fires. US Bureau of Mines, Washington, DC. RI 9348, 19 pp. Kim, A.G., 1997. Results of diagnostic tests at the Percy Mine fire site. Interagency report to the U.S. Office of Surface Mining Reclamation and Enforcement, Pittsburgh, PA, February, 1997, 8 pp. Kim, A.G., Chaiken, R.F., 1993. Fires in Abandoned Coal Mines and Waste Banks. US Bureau of Mines, Washington, DC. IC 9352, 58 pp. NTIS: PB93232551. Kim, A.G., Dalverny, L.E., 1994. Mine fire diagnostics at the large mine site. Proc. International Land Reclamation and Mine Drainage Conference and the Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, April 24 – 29, 1994. US Bureau of Mines, Washington, DC, Spec. Publ. 06A-94, pp. 139 – 147. Kim, A.G., Justin, T.R., Miller, J.F., 1992. Mine Fire Diagnostics Applied to the Carbondale, PA, Mine Fire Site. US Bureau of Mines, Washington, DC, RI 9421, 16 pp. NTIS: PB93114676. Litton, C.D., Kim, A.G., 1989. Improved Mine Fire Diagnostic Techniques. Society for Mining, Metallurgy and Exploration Annual Mtg., SME Preprint No. 89-182, 7 pp. Mowery, G.L., 1995. Ground penetrating radar and infrared thermography tests at a burning coal waste bank. Proc. 17th Annual Abandoned Mine Land Conf., National Association of Abandoned Mine Land Programs, French Lick, Indiana, October 15 – 18, 1995, pp. 87 – 105.
Locating fires in abandoned underground coal mines Ann G. Kim * National Energy Technology Laboratory, U.S. Department of Energy, USA Received 7 April 2003; accepted 18 November 2003 Available online 6 March 2004
Abstract A Mine Fire Diagnostic (MFD) Methodology was developed to determine the location and extent of combustion zones in abandoned underground coal mines. In this method, a characteristic fire signature is based on the ratio of higher molecular weight hydrocarbon gases (C2 to C5) to total hydrocarbon gas. Initially, gas samples are obtained at the bottom of boreholes under baseline or static conditions. A second set of samples is obtained when a suction fan is used to influence the direction of gas movement. Pressure data define the degree of communication between boreholes. The value of the diagnostic ratio under communication conditions is taken as a measure of subsurface fire activity related to a particular flow direction. Using a Venn diagram technique, the results are mapped as quadrants on a borehole map of the site. Repetition of the communication tests provides overlapping quadrants that define hot, cold, and indeterminate areas. The MFD has been used to distinguish hearted and cold subsurface areas at four mine fire sites. At each of the sites, the extent of the fire could not be inferred from surface evidence, and the location of combustion zones had a significant impact on plans to control the fire. Although the method is labor intensive and requires drilling cased boreholes, it is relatively simple and provides information that cannot be obtained by other methods. D 2004 Elsevier B.V. All rights reserved. Keywords: Abandoned coal mines; Underground fires; Hydrocarbon desorption; Mine fires
1. Introduction Fires in abandoned mines and waste banks are a relatively common occurrence in coal-producing areas (Kim and Chaiken, 1993). Abandoned mine fires present serious health, safety, and environmental hazards due to the emission of toxic fumes, subsidence, and the deterioration in air quality. To locate this type of remote, subsurface fire, it is necessary that: (1) it have a measurable characteristic, (2) the characteristic
* Tel.: +1-412-386-6724; fax: +1-412-386-4579. E-mail address: [email protected] (A.G. Kim). 0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2003.11.003
be detectable through standardized sampling methods, and (3) the data are interpreted according to an appropriate algorithm. The emission of smoke and fumes at surface fractures and vents usually indicates a fire in an abandoned mine or waste bank. However, the surface evidence of fires may not be related by straight line paths to the source of combustion, since hot gases follow the path of least resistance. Therefore, the heated source of combustion products can be distant, laterally and vertically, from the surface expression. Also, fires in abandoned coal mines are usually confined to relatively small, discontinuous areas of smoldering combustion surrounded by large masses of insulating rock.
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Table 1 Interpretation of R1 values R1 value range
Relative coal temperature
0 to 50 50 to 100 >100
Normal Possible heated coal Heated coal
Methods using remote thermal characteristics or variations in the gross composition of the mine atmosphere have not been routinely successful at locating isolated combustion zones. Infrared photography discriminates temperature variations only within a few centimeters of the surface, usually indicating heated vents and fractures. Borehole temperatures measured in the mine void may be more accurate, although a thermocouple generally measures the max-
imum temperature in a volume of 0.03 to 0.06 m3 (1 to 2 ft3), within a radial distance of 0.25 m (10 in). Core drilling and near-surface geophysical imaging techniques will produce adequate information on structural features, but are less reliable for indicating combustion areas (Dalverny et al., 1996). Elevated temperatures can alter the mineralogy of iron bearing rocks, but magnetic anomalies are more likely to be associated with areas that have been heated and cooled than with active combustion. Electrical terrain conductivity surveys may indicate water flow, i.e., areas where combustion is unlikely (Dalverny and Kim, 1995). Near surface seismic surveys and ground penetrating radar can indicate subsidence areas and changes in subsurface structure, but these are not necessarily related to combustion (Cohen and Dalverny, 1995; Mowery, 1995).
Fig. 1. Artist’s depiction of the Mine Fire Diagnostic (MFD) Methodology and equipment.
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Subsurface changes in the concentration of O2, CO2, CO, and H2 have been used as the basis for geochemical combustion indicators (Jones and Trickett, 1955; Graham, 1920). Although changes in the concentration of these gases may be related to combustion, they may also be produced by processes other than combustion; and there may be a reservoir of such gases in which changes in concentration due to combustion are insignificant. In abandoned mines, submicron particulate (smoke) detectors have given ambiguous results (Litton and Kim, 1989). Using sealed boreholes as sampling points for gas composition, temperature, and pressure increases the amount of usable data, but these reflect a static environment in the mine and yield information about a relatively small volume around the borehole. However, the usefulness of boreholes in delineating fire zones can be improved if data are obtained under both ambient and controlled-flow conditions. The resulting data have both quantity and orientation components. A Mine Fire Diagnostic (MFD) methodology, developed by researchers at the former U.S. Bureau of Mines, is based on the temperature-dependent desorption of low molecular weight hydrocarbon gases from coal and on controlled underground airflow. At the request of the Pennsylvania Department of Environmental Protection (PA DEP) and the federal Office of Surface Mining, Reclamation and Enforcement (OSMRE) the method was used to define the subsurface location of fires in three abandoned bituminous coal mines and one abandoned anthracite mine.
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hydrocarbon gas desorbed from coal is over 90% CH4, probably because the smaller CH4 molecule more readily diffuses to the coal surface. At 150 jC, the concentration of CH4 in the desorbed gas drops to about 35% as the C2 – C5 hydrocarbons concentrations increase. Changes in the relative concentration of desorbed CH4 and the C2 – C5 alkanes can be detected at temperatures below 100 jC and can indicate the presence or absence of heated coal. Based on the observed changes in the relative hydrocarbon concentration, a hydrocarbon ratio (R1) has been defined as a fire signature (Kim, 1991):
R1 ¼
1:01½THC ½CH4 1000 ½THC ¼ c
where [THC] = concentration of total hydrocarbons, ppm; [CH4] = concentration of CH4, ppm; c = constant, 0.01 ppm (eliminates dividing by zero). As defined, the ratio is an increasing function of the percentage of higher hydrocarbons; it has a value of zero when no hydrocarbons are detected, and a value of 10 when CH4 is the only hydrocarbon. The limiting value is about 1000. The interpretation of values of the R1 ratio (Table 1) is based on data from controlled laboratory heating experiments.
2. Mine fire diagnostic methodology The Mine Fire Diagnostic Methodology consists of determining a unique signature for heated coal, a procedure for obtaining samples from abandoned mines, and an algorithm for interpreting the data and relating it to a subsurface location. The desorption of low molecular weight hydrocarbon gases from coal is strongly temperature dependent (Kim, 1974, 1978). Changes in the CH4 emission rate and the emission of other hydrocarbon gases at elevated temperatures are a function of the coal structure and the volume of gas adsorbed on the internal surface of the coal. At normal ground temperatures (f 13 jC),
Fig. 2. Equipment used in Mine Fire Diagnostic Methodology for temperature and pressure measurements and to obtain mine gas samples.
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Fig. 3. Portable generator and suction fan connected through steel pipe and flexible tubing to exhaust borehole for MFD communication test.
Values of R1 z 100 indicate coal temperatures of at least 100 jC, but the absolute value of the R1 ratio is not directly correlated to coal temperature. Measured R1 values indicate an average temperature condition for a large volume of coal, and elevated values are due only to the presence of heated coal. Over a period of time, they will increase with increasing temperature
and decrease with decreasing temperature. Because hydrocarbon desorption occurs at relatively low temperatures, the mass of the coal from which the hydrocarbons are desorbed is much larger than that directly affected by combustion. The larger source increases signature sensitivity. The R1 ratio is independent of dilution by other mine gases, such as O2 and CO2. The only constraint on the use of R1 as an indicator of heated coal is that the concentration of total hydrocarbons in the gas samples exceeds 50 ppm. Although lower concentrations can be determined, apparent changes in the value of the ratio may be due to cumulative analytical error in low concentrations of C2 – C5 hydrocarbons. To determine changes in the combustion signature, access to the coal horizon and a method for remotely obtaining uncontaminated samples are required. In MFD, 20 cm (8 in.) ID boreholes are drilled to the base of the coal bearing strata and cased to within 0.9 m (3 ft) of the coal mine roof (Fig. 1). An instrumentation cap with ports for a thermocouple and sampling line is fastened to a flange welded to the borehole casing. The temperature probe is a 1.5 m stainless steel sheathed Chromel-Alum Type K thermocouple, suspended approximately 0.5 m below the casing. The
Fig. 4. Schematic diagram of gas flow during communication testing in the Mine Fire Diagnostic Methodology.
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Fig. 5. Results of one MFD communication test with suction at BH#26.
sampling line is a combination of polyethylene tubing almost the length of the casing attached to 1 cm (3/ 8 in.) stainless steel tubing near the mine void. This
minimizes weight and expense, while protecting the sample line from high temperatures. The sampling line is connected to a tee fitting for obtaining pressure
Fig. 6. Combined results of two MFD communication tests with suction at BH#26 and BH#3. Pressure contour line indicates limit of communication.
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measurements through a neoprene septum and gas samples through a compression plug. A battery-powered diaphragm pump and evacuated sample tubes are used to obtain samples of the mine gases. Pressure is measured with a Magnehelic pressure meter connected through a hollow needle in a Luer adapter (Fig. 2). An exhaust fan, powered by a portable generator, is used to control airflow during communication tests (Fig. 3). A section of 20 cm (8 in.) steel pipe between flexible duct connects a 2920 cfm fan to a 90j elbow at the borehole (Dalverny and Chaiken, 1991). In the MFD methodology, gas samples, temperature and pressure measurements are obtained from the mine before and during operation of a suction fan
attached to one of the network of boreholes cased to the depth of coal-bearing strata. In the representation of the concept (Fig. 4), the suction fan is attached to the left-most hole. If communication occurs between the suction hole and the neighboring holes, a decrease in static pressure is measured at the bottom of each borehole. If a fire exists at all points within a region, desorbed gases are detected in gas samples taken at each borehole (Case A). Similarly, a heating between the suction hole and its nearest neighbor (Case C) would be detected only at the suction hole. Iteration of the sampling using different boreholes in the grid pattern as the suction point provides data to determine the presence or absence of fire along various
Fig. 7. Site map of Renton Mine Fire Extinguishment Project. Borehole locations indicate the three discontinuous areas of combustion near the mine perimeter: Miller Farm, Plum Street and Danny Property.
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pathways between the sampling points. Using a Venn diagram technique, quadrants, with a radius equal to one-half the average distance between boreholes, are centered on a line connecting that borehole with the suction hole and coded for the R1 signature value (Fig. 5). Repeating the tests with different orientations allows spatial definition of both heated and cold subsurface zones (Fig. 6). A composite of the results of all tests bounds the probable fire zones and their cold boundaries through successive approximation.
3. Results The MFD methodology was demonstrated at four abandoned coal mine sites in cooperation with the Pennsylvania Department of Environmental Protection (PA DEP) and the federal Office of Surface Mining, Reclamation and Enforcement (OSMRE). The results of these tests are summarized below, and additional information on these case studies is given in the cited references.
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3.1. Renton The Renton abandoned mine fire site was a 24 ha (60 acre) region in Allegheny County, southwestern Pennsylvania (Dalverny and Chaiken, 1991). The mine is located in the Pittsburgh coal seam beneath a hill topped by a 4.4 million l (1 million gal) municipal water storage tank; the overburden ranged between 3 and 30 m (10 and 100 ft). The site had several large subsidence holes and venting areas. To extinguish the fire, a heat removal technique utilizing water injection and forced removal of steam was tried at the site. The MFD methodology was used to locate combustion zones and to monitor the progress of the extinguishment project. The MFD located three noncontiguous combustion zones, totaling approximately 4 ha (11 acres) near the perimeter of the site (Fig. 7). These coincided with three heated areas of indefinite extent that had been inferred from visual and historical evidence. Although combustion products had been detected in the central part of the mine, diagnostic tests indicated that heating
Fig. 8. Changes in R1 values at Renton site over a 3-year period indicating cooling in the area between Plum Street and the Danny Property (BH# 14), a long-term increase in combustion at the Miller Farm area (BH# 33), and a rapid increase in combustion followed by a constant level of combustion in the Plum Street area (BH# 57).
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was occurring only in areas closer to the buried outcrop. Over a period of 195 days, a total of 27 million l (7.1 million gal) of water was injected into the three mine fire areas. During and after the water injection phase of the project, MFD was used to follow the progress of the extinguishment effort (Fig. 8). Evidence of combustion activity decreased between the Plum Street area and the Danny property (Borehole 14). It apparently increased and then became static in the Plum Street area (Borehole 57), and increased in the Miller Farm area (Bore-
hole 33). At the Renton mine fire site, it was assumed that most of the injected water did not come in direct contact with heated coal or rock and was not converted to steam. Therefore, not enough heat was removed to extinguish the fire. MFD confirmed that the extinguishment method was not effective. 3.2. Large The mine fire near Large in Allegheny County, PA, is in the Pittsburgh coal seam, which, at this
Fig. 9. Composite results of MFD tests at the Large, PA mine fire site overlain on mine map. Discontinuous combustion areas were located along the buried outcrop and extending into the mine.
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location, outcrops at the base of a hill. Approximate depth of overburden ranges from 6 to 55 m (20 to180 ft). Small vents were visible on the hillside above the buried outcrop. The surface area affected by the mine fire is approximately 1 ha, and the surface slope averages 20j. Five natural gas pipelines and three sets of high-voltage power lines cross the property. Areas of venting were confined to a relatively small surface area and indicated probable propagation along the buried outcrop. The PA DEP was interested in trying foam injection as a heat removal method to extinguish the fire. To estimate the amount of foam and the injection points, it was necessary to determine the location and extent of the combustion area(s). Several 5 cm (2 in.) and 6.25 cm (2.5 in.) boreholes had been drilled and cased for temperature monitoring. The mine temperatures indicated that large areas of the mine were heated, but the data could not be used to define a combustion zone. (Kim and Dalverny, 1994). MFD testing, utilizing two 5 cm (2 in.), seven 6.25 cm (2.5 in.), and fifty 20 cm (8 in.) diameter cased
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boreholes, indicated an L-shaped combustion zone (Fig. 9). The base of the L (150 m) was near the outcrop; the leg of the L extended 180 m into the mine, probably along a set of main entries. Several small isolated combustion zones were located 300 – 1000 m from the primary combustion area. Noncontiguous heated areas were detected in the southwest quadrant of the project area. The direction of propagation into the mine appears to coincide with the location of a set of main entries. Heated areas have been detected to the north and south of this area. By overlaying the surface map on an old mine map, it appears that the fire may be in entries that intersect another set of entries extending north and south under a road adjacent to the site. Subsurface fires generally move toward a source of fresh air, which may have been a buried portal, that the old maps indicated existed in the adjoining valley approximately 1600 m (1 mile) from the site. Another potential source of air was permeable zones in the outcrop. The heated areas apparently extended beyond the site boundary, and a cold boundary for the fire area was not located.
Fig. 10. Site map of Percy Mine Fire Project, showing the limit of excavation and double line of temperature monitoring boreholes.
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Several areas where hydrocarbon concentrations were below 20 ppm CH4 coincided with areas where water was observed in some boreholes. Water vapor condensed on the surface of the coal could block the desorption of hydrocarbons. However, if the temperature of the coal is cold enough to allow water condensation, it can be assumed to be below the combustion point.
The Large mine fire was typical of problems associated with determining underground fire locations from surface evidence. The observed venting above the buried outcrop gave no indication of the combustion areas in the interior portions of the mine. Also, borehole temperature data, when taken alone, would yield misleading results with regard to the location of the combustion zones. Because the com-
Fig. 11. Site map of MFD test at Percy Mine Fire site overlain on old mine map, showing location of 8 in. (P8xxx) communication boreholes and 3 in. (P3xxx) gas sampling boreholes in relation to temperature monitoring boreholes.
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bustion zones were much larger than originally estimated, the foam injection project was not attempted. 3.3. Percy Approximately 15 years before this MFD project, the Office of Surface Mining, Reclamation and Enforcement (OSMRE) extinguished part of a fire in the Percy mine, Fayette County, PA by excavation (Fig. 10). The abandoned mine was in the Pittsburgh seam on the flank of the Chestnut Ridge anticline. The approximate depth of excavation was 30.5 m (100 ft), and the remaining coal had a 10% dip away from the excavation boundary. A double line of temperature 2.5 cm (1 in.) monitoring boreholes on 7.5 m (25 ft) centers had been installed around the mine side of the excavated area. Increasing borehole temperatures and the emission of smoke from surface fractures indicated increased combustion in the unreclaimed portion of the mine. To develop a control plan, it was necessary to determine if combustion was occurring near a group of houses to the south of the reclaimed site. Ten 7.5 cm (3 in.) monitoring boreholes were placed along the section of the site nearest the houses. Thirteen 20 cm (8 in.) boreholes (monitoring and suction) were drilled on 30.5 m (100 ft) centers in a narrow area bounded by the houses (south), a water pipeline (west), and a local road (north) (Fig. 11). Based on previous experience, the atmosphere in underground mines is assumed to be in a steady state. Although this baseline condition will vary locally due to combustion, proximity to fresh air, microbial activity, etc., the composition of air within a given area is generally constant. When suction is applied to the underground system, changes in gas composition at a particular borehole are detected in air samples flowing from another area. Applied suction is usually a temporary perturbation, and the atmosphere is expected to revert to the original baseline composition. This condition did not hold for the Percy mine. There were significant changes in baseline gas composition over periods of as short as 1 day, but such changes might persist for as long as 3 weeks. The lack of a consistent baseline made the interpretation of the diagnostic results more difficult. Also, the layout of the site with essentially two parallel lines of boreholes usable as suction points, constrained the total angle which could
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be evaluated, particularly at the southern end of the site near the houses. Since the site conditions were not optimum, each borehole area was also evaluated with respect to temperature, total hydrocarbon concentration, the ratio N2/O2, and the presence of CO. For normal or uncontaminated air, the value of N2/O2 is 3.7. Higher values are related to O2 depletion. A decreasing value of N2/O2 during communication indicates that air is being drawn into the area, and this may coincide with a decrease in the total hydrocarbon concentration. Based on changes in N2/O2, the changes in baseline gas composition were attributed to an influx of normal air. Carbon monoxide was detected at several boreholes that also had R1 warm or hot values; the presence of CO indicates that combustion is occurring in a low O2 atmosphere. The absence of CO may indicate the absence of combustion, or there may be sufficient O2 to produce CO2 rather than CO. Evaluation of all the data supported the existence of a primary combustion zone near the road and a second area of combustion between the perimeter boreholes and the houses (Kim, 1997). The results of the diagnostic tests indicated that the primary combustion zone was located near the road. Combustion in this area affected boreholes within 60 m (195 ft). The highest average underground temperatures were also measured in the boreholes near the road. Combustion signatures were also detected at two of the 7.5 cm (3 in.) boreholes at the southern end of the site. To prevent the migration of toxic gases
Fig. 12. Anomalous snow melt at the Carbondale MFD site.
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toward the houses, OSM installed a cellular concrete barrier in the abandoned mine. 3.4. Carbondale The Carbondale mine fire site in Pennsylvania’s Northern Anthracite Field was located adjacent to an apartment complex. The complex had been built on an area excavated and backfilled during a previous fire control project. Anomalous snow melt (Fig. 12) indicated that combustion was occurring in the unreclaimed portion of the mine. The objective of the MFD project was to determine the proximity of heated zones to the apartment complex (Kim et al.,
1992). Drillers logs indicated that two anthracite seams were present in the area. Thirty-two 15 cm (6 in.) cased boreholes and two 10 cm (4 in.) cased boreholes had been installed at the 3.4 ha (8.5 acres) site. They extended from 16 m (52 ft) to 30.5 m (100 ft) beneath the surface through the coal seams; they were originally installed as monitoring holes and were subsequently used as suction and observation points for the MFD project. The depth of the boreholes and their diameter were sizing constraints that required two suction fans, each rated at 1.02 m (40 in.) vacuum w.c., connected in series to offset casing resistance and improve communication between boreholes.
Fig. 13. Composite results of MFD tests at the Carbondale site.
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The hydrocarbon ratio is strongly temperature dependent for bituminous samples, but anthracite has a lower concentration of higher hydrocarbons adsorbed on the coal surface. However, methane emission was found to be sufficiently temperature dependent to allow its use as a combustion indicator for anthracite. The use of MFD at Carbondale was considered a test under nonideal conditions because (1) the emission rate for low molecular weight hydrocarbons from anthracite is low and does not exhibit strong temperature dependence, (2) many of the boreholes were cased through the coal seams, (3) the borehole diameters and depths were such that half of the rated pressure drop of the suction fan would be used to overcome pipe resistance, and (4) a large rock fracture that bisected the region could limit the effectiveness of the communication tests. To overcome these conditions, several adaptations were made to the MFD method. Because laboratory tests indicated that the R1 ratio would not be a good indicator of heated anthracite, the change in the absolute concentration of CH4 in baseline and communication samples was used as the index of combustion. Pressure and gas composition data indicated that the casing length and the fault did not affect the diagnostic results. The tests conducted at Carbondale located two large and five small heated zones (Fig. 13). The noncontiguous combustion zones included one region lying adjacent to an apartment building that had not been detected using temperature and CO concentration measurements. The presence of hydrocarbon gas indicated that coal was heated above the normal underground temperature; time-dependent temperature monitoring indicated that the areas of combustion were moving uphill away from the apartment complex.
4. Discussion and conclusions The mine fire diagnostic methodology is based on the assumptions: (1) changes in the relative concentrations of hydrocarbon gases underground are due only to the presence or absence of heated coal, (2) a sufficiently large vacuum applied to underground regions will cause the gases desorbed from heated coal to flow toward the point of suction, and (3) chemical and physical analyses of underground air flowing in a
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controlled manner between borehole sampling points can locate the source of the hydrocarbons. Corollary assumptions are: the measured pressure gradient can characterize the communication effect at each borehole; underground gas flows follow a straight line path toward the suction point; the depth of the borehole and the length of the casing have no effect on the measured pressures or on the gas concentration; ambient surface conditions have no effect; and changes in gas concentration are measurable. In reality, these corollary assumptions do not always hold. However, with both measurable pressure gradients and measurable changes in gas concentrations, repetition of the tests minimizes the effects of nonideal subsurface conditions. Timedependent monitoring can differentiate heating and cooling periods resulting from combustion front movement and/or fire control activities (Justin and Kim, 1988; Dalverny et al., 1990). Coal mine entries can be considered two-dimensional fracture systems constrained by rock strata with much lower permeability. Controlling the movement of underground gases overcomes the limitations of a point source measurement by allowing gases from a large region to be sampled through one borehole. The MFD method increases the detection zone of normal point source measurements through controlled gas movement. Measuring changes in hydrocarbon concentration of the moving gas, and plotting the results as vectors (magnitude and direction) rather that point source (magnitude) measurements, bounds the area(s) affected by combustion. Although independent confirmation of the results of these studies were not possible, in the three bituminous mine fires investigated, the R1 fire signature was consistent with other indicators. The technique of borehole suction and sampling detected thermally separate heated zones. At all four field sites, the data obtained clearly indicated heated areas, areas where data are insufficient to make a judgment, and areas in which combustion was not occurring. The MFD methodology represents a significant improvement in locating and monitoring abandoned mine fires. Its advantages are: (1) changes in characteristic hydrocarbon emission are caused only by changes in temperature, (2) changes in hydrocarbon emission can be detected for temperatures below the ignition point of coal, increasing the effective amount of coal serving as a source, (3) hydrocarbons can be
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detected routinely at concentrations as low as 1 ppm, (4) the use of an appropriate ratio can usually eliminate the effects of sample dilution with air, (5) induced pressure effects can be detected as much as several hundred meters from the suction point, and (6) the system, while somewhat labor intensive, is simple to operate and requires only readily available equipment. The severity of the problem and the need to more accurately determine the location of a mine fire will determine if the Mine Fire Diagnostic Methodology should be used.
Acknowledgements The MFD methodology was developed and implemented under the auspices of the former U.S. Bureau of Mines with funding from and in cooperation with both the U.S. Office of Surface Mining Reclamation and Enforcement and the Pennsylvania Department of Environmental Protection. Robert F. Chaiken and L.E. Dalverny, formerly with the U.S. Bureau of Mines, made significant contributions to the development of the Mine Fire Diagnostic Methodology. Ethel Burse, Andrew Kociban, and Christina Manns, all currently with the U.S. Department of Energy, performed much of the MFD fieldwork, as did Thomas Justin and Joseph P. Slivon, both retired. I appreciate the very helpful comments provided by Robert Chaiken and William Ehler of OSMRE in their reviews of the manuscript.
References Cohen, K.K., Dalverny, L.E., 1995. Seismic characterization of a coal waste bank fire in Eastern Ohio to aid reclamation. Proc. 17th Annual Abandoned Mine Land Conf., National Association of Abandoned Mine Land Programs, French Lick, Indiana, October 15 – 18, 1995, pp. 70 – 86. Dalverny, L.E., Chaiken, R.F., 1991. Mine Fire Diagnostics and Implementation of Water Injection with Fume Exhaustion at Renton, PA. US Bureau of Mines, Washington, DC, RI 9363, 42 pp. NTIS: PB91230243. Dalverny, L.E., Kim, A.G., 1995. Magnetometer and terrain conductivity surveys at a burning coal waste bank. Proc. 17th Annual Abandoned Mine Land Conf., National Association of
Abandoned Mine Land Programs, French Lick, Indiana, October 15 – 18, 1995, pp. 49 – 69. Dalverny, L.E., Chaiken, R.F., Kim, A.G., 1990. Mine fire diagnostics in abandoned bituminous coal mines. Proc. Mining and Reclamation Conference, Charleston, WV, April 23 – 26, 1990. West Virginia University, Morgantown, WV, pp. 527 – 534. Dalverny, L.E., Cohen, K.K., Mowery, G.L., Kim, A.G., 1996. The use of multiple geophysical techniques for site assessment in mine reclamation. Proc. Symposium on Application of Geophysics to Engineering and Environmental Problems (SAGEEP), April 28 – May 2, 1996. Keystone, CO, pp. 887 – 896. Graham, J.I., 1920. The normal production of carbon monoxide in coal mines. Trans. Inst. Min. Eng. 60, 222 – 231. Jones, J.H., Trickett, J.C., 1955. Some observations on the examination of gases resulting from explosions in colleries. Trans. Inst. Min. Eng. 114, 768 – 791. Justin, T.R., Kim, A.G., 1988. Mine fire diagnostics to locate and monitor abandoned mine fires. Proc. Mine Drainage and Reclamation Conference, Pittsburgh, PA, April 17 – 22, 1988. US Bureau of Mines, Washington, DC. IC 9184, pp. 348 – 355. Kim, A.G., 1974. Low Temperature Evolution of Hydrocarbon Gases from Coal. US Bureau of Mines, Washington, DC. RI 7965, 23 pp. Kim, A.G., 1978. Experimental Studies on the Origin and Accumulation of Coalbed Gas. US Bureau of Mines, Washington, DC. RI 8317, 18 pp. Kim, A.G., 1991. Laboratory Determination of Signature Criteria for Locating and Monitoring Abandoned Mine Fires. US Bureau of Mines, Washington, DC. RI 9348, 19 pp. Kim, A.G., 1997. Results of diagnostic tests at the Percy Mine fire site. Interagency report to the U.S. Office of Surface Mining Reclamation and Enforcement, Pittsburgh, PA, February, 1997, 8 pp. Kim, A.G., Chaiken, R.F., 1993. Fires in Abandoned Coal Mines and Waste Banks. US Bureau of Mines, Washington, DC. IC 9352, 58 pp. NTIS: PB93232551. Kim, A.G., Dalverny, L.E., 1994. Mine fire diagnostics at the large mine site. Proc. International Land Reclamation and Mine Drainage Conference and the Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, April 24 – 29, 1994. US Bureau of Mines, Washington, DC, Spec. Publ. 06A-94, pp. 139 – 147. Kim, A.G., Justin, T.R., Miller, J.F., 1992. Mine Fire Diagnostics Applied to the Carbondale, PA, Mine Fire Site. US Bureau of Mines, Washington, DC, RI 9421, 16 pp. NTIS: PB93114676. Litton, C.D., Kim, A.G., 1989. Improved Mine Fire Diagnostic Techniques. Society for Mining, Metallurgy and Exploration Annual Mtg., SME Preprint No. 89-182, 7 pp. Mowery, G.L., 1995. Ground penetrating radar and infrared thermography tests at a burning coal waste bank. Proc. 17th Annual Abandoned Mine Land Conf., National Association of Abandoned Mine Land Programs, French Lick, Indiana, October 15 – 18, 1995, pp. 87 – 105.