Ignitions in Mixtures of Coal Dust, Air, and Methane from Abrasive Impacts of Hard Minerals With Pneumatic Pipeline Steel.
Laboratory equipment, which simulates abrasive impacts between steel and minerals that might be present during pneumatic transport of coal, was used by the Bureau of Mines to characterize the potential explosion hazard of such collisions in an atmosphere of fine coal dust, air, and methane. A variety of coal mine rock materials, including sandstone, limestone, and pyrite-bearing limestone, were impacted with specimens of pipeline steel. Tests were conducted in atmospheres containing zero to 6.4 Vol-pct methane mixed with zero to 300 mg/l coal dust. Coal dust in air alone was not ignited by abrasive impacts, but additions of as little as 1 vol/pct methane to the coal dust and air mixture resulted in ignitions. Steel impacting against sandstone caused ignitions in mixtures of coal dust, air, and methane, and the probability for ignitions increased with an increase in methane. No ignitions occurred with impacts of steel on limestone or pyrite-bearing limestone, even when large showers of sparks were produced. Studies of the effect of tangential collision velocity showed that no ignitions of coal dust, air and methane occurred below 900 fpm (4.57 M/sec). Ignitions in coal-air-methane mixtures were found to be caused by a hot friction- induced smear on the impacted rock at the impact site rather than by sparks. High-speed photography was used to verify this observation.