Based on the Chinese national standard GB13813, a rotating friction test device was set up for investigating the influence of coal dust on the ignition of methane/air mixtures by friction sparks from rubbing of titanium against steel. By using a high speed video camera and an infrared thermal imager, it was observed that it was the flying titanium particle sparks that ignited the methane/air mixtures during the process of rubbing a titanium rod against a rotating A3 steel disk, rather than the hot surface of the titanium rod. A set of experiments, using three different particle size fractions of lignite, bituminous coal and anthracite, were conducted to test the influence of coal dust on the process of ignition of methane/air mixture by titanium friction sparks. It was found that the ignition delay time generally increased when introducing coal dust. Furthermore, the smaller the coal particle size the longer the ignition delay time. A possible reason could be that smaller coal particles absorb a greater fraction of the energy of the titanium sparks, which reduces the ability of the sparks to ignite the methane/air mixture. The shortest ignition delay times were observed with bituminous coal and the longest with lignite, with the anthracite in between. This is in accordance with the increasing water content of the coals in the same order, but not with the variation of the contents of volatiles and sulfur. Presumably the emission of volatile and sulfur compounds is not as important as the heat loss by water evaporation. (C) 2013 Elsevier Ltd. All rights reserved.