Influence of coal particles on methane/air mixture ignition in a heated environment

The temperature at which coal dust glows is normally much lower than the auto-ignition temperature (AIT) of methane/air mixtures, and thus a better understanding is needed regarding methane/air ignition in a heated environment in the presence of coal particles. A horizontal tube apparatus was used to test the effect of brown coal and two kinds of bituminous and anthracite on methane/air combustibility. For the four coal samples tested, the presence of coal particles significantly reduced the minimum temperature for ignition of methane/air mixtures in a heated environment. No. 1 bituminous coal with 12 mm diameter decreased the ignition temperature value from 595 to 500 °C. It is thought that pre-ignition of low-AIT volatiles emitted from the heated coal particles ignited the methane/air mixtures. Volatiles, sulfur content, and large porosity of piled coal particles all enhanced ignition of methane/air mixtures in a hot environment, while water content and small particle size reduced ignition. For anthracite, no ignition occurred when temperatures of the heated environment were lower than the AIT of methane (595 °C), except for the 12-mm-diameter sample. Anthracite did not readily ignite methane/air mixtures and the ignition mechanism was somewhat similar to that of a burning cigarette.