Handbook for Methane Control in Mining - AMMSA

6reduced pressure, except at very lowpressures, such as below ¼ atmosphere.At elevated pressures, thelower limits of hydrocarbon-airmixtures generally decrease slightly,but the upper limits increase greatly.Figure 1–3 shows the variation inmethane lower explosive limit andupper explosive limit with elevatedpressure.Effect of temperature on explosibilitylimits. The effect oftemperature on the explosibilitylimits of methane is modest. Forexample, the LEL of methane-airmixtures at -100 °C is 5.6% methane,and at +100 °C it is 4.8% methane.The UEL of methane-air mixturesat +100 °C is 16.3% methane[Zabetakis 1965].Figure 1–3.—Effect of elevated pressure on methane explosibilitylimits.LESS COMMON SOURCES OF METHANE IGNITIONSThere are many well-known methane ignition sources in mines, ranging from frictional ignitionscaused by cutting bits (Chapter 3) to open flames, explosives, and electrical sparking. However,there are other less recognized ignition sources. A review of these is worthwhile here.Hot solids. The temperature at which a hot solid can ignite methane is quite high. Coward andRamsay [1965] report that the minimum ignition temperature in a closed vessel is about 675 °C,but when the hot surface is exposed to convection currents, the minimum temperature is higher.For example, ignition from a hot steel bar requires 990 °C. Kuchta [1985] found that ignitionsby any heated surface depend on the dimensions of the surface. He reports methane ignitiontemperatures ranging from 630 to 1,220 °C.However, when an ignitable dust is present on the hot surface, this dust is more readily ignitedthan methane. The burning dust can then ignite the methane. Kim [1977b] reported laboratorystudies in which the spontaneous ignition temperature of coal dust layers was as low as 160 °C.As a result, Mine Safety and Health Administration (MSHA) regulations require that the surface