No. 530 March 2008 

Frictional Ignition of Methane-Air in the Presence 

of Liquid Hydrocarbons
 

Background 

Frictional ignitions continue to be a problem facing the 

U.S. underground coal mining industry. Many methane ignitions are associated with the impact of mining machine cutter 

bits on quartzitic material (e.g., sandstone) during the coalcutting process. Most ignitions result in small methane-air 

fireballs often only a few feet in size and limited to the cutter 

head location. However, under certain conditions, frictional 

ignitions can lead to larger methane explosions and/or fires 

with the potential for causing serious injury or death to the 

mining machine operator and other nearby miners. The 

requirements for a frictional ignition are a flammable volume 

of methane-air and an ignition source. This source is usually 

the hot streak caused by the frictional impact of the mining bit 

on hard rock, and the frequency of occurrence increases with 

increased bit wear. Another possible ignition source is the frictional impact of rock on rock during a roof fall. 

In addition to flammable methane gas, some underground 

coal mines encounter heavier molecular weight liquid hydrocarbons that can generate flammable vapors. For example, 

a coal mine in Utah experienced significant inflows of liquid 

hydrocarbons, with approximately 1,200 gal being pumped 

from the mine per day. The Mine Safety and Health Administration (MSHA) reported that this mine experienced several 

ignitions, including one in 1998 that led to a significant fire. 

MSHA listed liquid hydrocarbons as contributing to this fire 

[Elkins et al. 2001]. On July 31-August 1, 2000, there were 

multiple explosions and a fire at the same mine, resulting in 

2 fatalities and 12 injuries. The MSHA investigation report 

listed methane and liquid hydrocarbons as contributing to the 

explosions and fire [McKinney et al. 2001]. 

Approach 

The National Institute for Occupational Safety and 

Health’s (NIOSH) Pittsburgh Research Laboratory conducted 

a series of experiments to investigate the effect of added 

hydrocarbon liquids on the frictional ignition potential from

 Figure 1.—Frictional ignition of methane in experimental test 

apparatus. 

the impact of mining bits on sandstone. The experiments were conducted in a laboratory test apparatus (Figure 1) where a single 

miner bit on a cutter drum struck a sandstone block. The bit was 

worn so that the shank steel was exposed. A 100-ft3

 mixture of 

flammable methane-air was enclosed by a plastic diaphragm 

that broke and vented the explosion after ignition. The relative ease 

of ignition was measured by the number of strikes necessary to 

cause ignition. Tests were conducted with flammable methane-air 

mixtures and with added liquid hydrocarbons on the sandstone 

block. 

The first series of tests was conducted to determine the number 

of strikes of the bit on the sandstone block necessary to ignite a 

flammable mixture of methane-air. With the cutter drum rotating, 

the sandstone block was moved horizontally in front of the cutter 

bit so that there would be a fresh surface for each strike of the bit. 

Additional tests were done with hydrocarbon liquid sprayed on the 

sandstone just before each test. Since hydrocarbon liquid samples 
 

 

 

 

 

 

 

 

 

from actual mines were not available for this study, a liquid 

hydrocarbon simulant was mixed based on the MSHA 

analyses of the hydrocarbons at the Utah mine [Elkins et al. 

2001]. MSHA reported that “the hydrocarbons were tested and 

were determined to be chemically similar to a mixture of 

15% gasoline, 35% diesel fuel, and 50% motor oil” [Elkins 

et al. 2001]. 

Results 

Using a worn bit, five tests were conducted with only 

methane-air in the test chamber. The mixture did not ignite in 

any of the five tests, with an average of greater than 90 

strikes per test. An additional five tests were conducted with 

added liquid hydrocarbon simulant on the sandstone. The 

methane-air mixture ignited in each of these five tests, with an 

average of about 30 strikes to ignition. The explanation may 

be that the liquid hydrocarbon ignites more easily than 

methane, and then it forms a pilot flame that ignites the 

methane. The hydrocarbon vapors can ignite at significantly 

lower temperatures than the methane. 

Based on these results, NIOSH researchers determined that 

the presence of a hydrocarbon liquid in the mining environment 

has the potential to increase the ignition hazard associated with 

the frictional heating from impact of steel on sandstone. 

Recommendations 

Improving the ventilation airflow should be the main 

method for reducing any flammable gas volume, thereby limiting the extent of flame propagation if a frictional ignition 

should occur. The frictional ignition probability decreases 

with new sharp bits. Replacing worn bits on a regular basis 

and using adequate back-mounted water sprays (that cool the 

frictional hot spots) have been shown to reduce the frictional 

ignition probability of methane when used on longwall and 

continuous mining machines. 

The increased fire and explosion hazard associated with the 

presence of heavy hydrocarbons can be estimated by collecting 

and measuring the flash point of the fresh hydrocarbon liquid. 

If the flash point is at or below the ambient mine temperature, 

the liquid can ignite and burn even without methane. If the flash 

point is above ambient temperature, the liquid hydrocarbon can 

enhance the ignition probability of methane. 

When using a handheld methanometer in the presence of 

heavier hydrocarbons, the instrument needs to be calibrated 

with gas mixtures that reflect the presence of the heavier hydrocarbon vapors. The instrument manufacturer should be contacted 

for advice on calibration procedures that will ensure accurate 

measurement of potentially flammable mixtures. 

In summary, extra precautions to prevent fires and explosions 

must be taken when heavier molecular weight liquid hydrocarbons 

are present in addition to methane in an underground coal mine. 

For More Information 

For more information on these findings, contact Kenneth L. 

Cashdollar, NIOSH Pittsburgh Research Laboratory, P.O. Box 

18070, Pittsburgh, PA 15236–0070; phone: (412) 386–6753; 

e-mail: KCashdollar@cdc.gov. 

References 

Elkins DW, Luzik SJ, Lemon JOD [2001]. Report of investigation, underground coal mine fire, November 25, 1998. Willow 

Creek mine, Cyprus Plateau Mining Corporation, Helper, Carbon 

County, Utah, ID. No. 42–02113. Arlington, VA: U.S. Department 

of Labor, Mine Safety and Health Administration. 

McKinney R, Crocco W, Tortorea JS, Wirth GJ, Weaver CA, 

Urosek JE, Beiter DA, Stephan CR [2001]. Report of investigation, 

underground coal mine explosions, July 31-August 1, 2000. 

Willow Creek mine, MSHA ID. No. 42–02113, Plateau Mining 

Corporation, Helper, Carbon County, Utah. Arlington, VA: U.S. 

Department of Labor, Mine Safety and Health Administration. 

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DHHS (NIOSH) Publication No. 2008–124