Discharge and ignition characteristics from indentation fracture of coal mine roof

Abstract Electric effects are generated during the deformation and fracture of coal mine roof sandstones. Previous research has mostly focused on the mechanism of electric effect generation and the electromagnetic radiation released. However, few studies have investigated the hazard characteristics of the high-energy electrons released. This study explores the discharge and ignition characteristics of roof indentation fracture to identify the real ignition source of a gas explosion in a coal mine goaf. Theoretical and experimental analyses conclude that the charge of the coal mine roof sandstone will gradually escape from the surface during the loading process. The charge release process of the rock before fracture is related to the loading rate, and an instantaneous high voltage will be formed during fracture of the rock. The compressive strength of the rock plays a key role in this type of instantaneous discharge process. An electric spark can be generated at the moment of rock fracture under loading. The high-energy electrons released by rock deformation and rupture can ionise the air. Moreover, the electric spark can persist for more than 20 ms, which considerably exceeds the induction period for gas ignition. When the quartz content of the roof rock is high and reaches a certain compressive strength, stress mutations can cause the roof to undergo instantaneous rupture and generate electric sparks. The premixed gas air gathered in the area is thus broken down and ionised, igniting the gas and producing a gas explosion.