Methane ignition by frictional impact heating

Abstract

As part of a program conducted by the Bureau of Mines to reduce the explosion hazard in coal mines, a study was made of the frictional ignition source that can develop to cause the ignition of methane when a coal mining machine encounters rock strata or hard inclusions. Such impacts between metal and rock reproduced by a laboratory machine caused a number of ignitions. A model is proposed that considers the mechanisms of energy dissipation during tangential impact. The total energy calculated from the model agrees well with the impact energy measured with the laboratory machine. Calculations show that during severe tangential impact in which a smear of molten metal is deposited on the rock, most of the impact energy is converted into heat at the interface while only about 1% of the energy goes into rock removal. The hot smear on the rock has the potential for igniting methane. The variables that affect the temperature, lifetime, and area of the hot smear are considered from the model. The cooling curve of a typical impact zone is calculated from the model and compares favorably with experimental findings. Calculations from the model show the hot area to increase linearly with impact speed; this agrees with the experimental results of the increasing ignition probability with increasing speed. The model is used to predict the effect of the heat of oxidation, heat of formation, dimensions of the tool, and dimensions of the rock on the ignition of methane.