Identification of potential for methane ignition by rock friction in Australian coal mines

The relative propensity for the different rock types in Australian undergro und coal mines to ignite methane-bearing atmospheres by frictional contact, either with mining equipment or with other rock materials, has been evalua ted by laboratory testing using an instrumented rotating-wheel apparatus. A five-point incendivity categorisation has been developed for the rocks tes ted, and the results compared to petrographic composition, mineralogy and r elated chemical properties. Recent experience with ignitions in New South W ales collieries has also been taken into account. With the progressive decl ine in ignitions from other sources, frictional effects are now the most co mmon source of methane ignition in modern underground coal mines. Conglomerates, lithic or quartzose sandstones with low clay and carbonate c ontents, and siliceous cherty tuffs appear to represent the most potentiall y incendive materials in roof and floor strata, along with siliceous or pyr itic impregnations that may occur in the coal seam itself. Temperatures in excess of 1500 degreesC may be developed within less than 1 s during rock-o n-rock contacts, with ignitions emanating from the contact point or related heat trail rather than from incandescent particles ("sparks") ejected duri ng the friction process. The average temperature and the coefficient of dyn amic friction developed during rock-on-rock testing also appear to be relat ed to frictional incendivity characteristics. (C) 2001 Elsevier Science B.V . All rights reserved.