Published in the May 2008 Coal USA Magazine

The explosions were to claim the lives of two workers and injure 12 others. But this wasn't the first time that Willow Creek had recorded an incident, with the mine experiencing several ignitions, one of which led to a significant fire in 1998.

Unlike many of the explosions in underground coal mines, methane wasn't the only factor to blame. It had a co-conspirator – liquid hydrocarbons.

The Mine Safety and Health Administration investigation report into both the 1998 and 2000 fires listed methane and liquid hydrocarbons as contributing to the explosions and fire. Willow Creek had been pumping 1200 gallons per day from the mine, as liquid hydrocarbons can generate flammable vapors.

The findings led the National Institute of Occupations Safety and Health Pittsburgh Research Laboratory to conduct a series of experiments. Generally, methane ignitions are caused by the impact of bits, especially worn bits, on quartzitic material, like sandstone, during cutting. These ignitions can sometimes lead to explosions and/or fires.

What NIOSH wanted to test was the effect of added hydrocarbon liquids on the frictional ignition potential from the impact of mining bits on sandstone. Researchers performed a laboratory test where a single worn miner bit (with shank steel exposed) on a cutter drum struck a sandstone block. Every strike of the bit contacted new surface of the sandstone.

Tests were conducted with just methane and with methane with added liquid hydrocarbons matching those of measurements taken by the Mine Safety and Health Administration from Willow Creek.

According to NIOSH, five tests, with an average of more than 90 strikes per test, with only methane turned up no ignition. Another five tests with the liquid hydrocarbon stimulant added resulted in the methane-air mixture igniting every time, with an average of 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," NIOSH said in its Technology News.

The researchers determined that the presence of hydrocarbon liquid in the mining environment has the potential to increase the ignition hazard associated with frictional heating from the impact of steel on sandstone.

NIOSH said the usual means of avoiding flame propagation of a frictional ignition should be carried out, including improving ventilation airflow, replacing worn bits on a regular basis and using water sprays.

It said 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 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," NIOSH said.

NIOSH said handheld methanometers needed to be calibrated with gas mixtures so they reflected the presence of hydrocarbon vapors, and mines should contact the instrument manufacture for instructions to do this.

Summing up, NIOSH said 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.