|The entrance to the Pike River Coal mine. Photo/NZPA|
published by the nzherald.co.nz, Nov. 19, 2010
|From this Wiki site|
The combustion of methane, CH4, is exothermic. When combined with two O2 molecules, it decomposes to form CO2 (gas) and 2 H2O (liquid) molecules, releasing 891 kJ of energy. It is both flammable and explosive. When mixed with oxygen and nitrogen in air in certain mixtures (orange region on the ternary diagram to the right), it will explode. The blue line represents air. If there is less than about 5% methane in the mixture it is too lean to explode (the LEL point on the diagram), and if there is more than about 15%, it is too rich to explode (the UEL point).
The process of mining coal generates an enormous amount of coal dust. When high concentrations of this are suspended in air, and there is even a small ignition source, the nearly instantaneous reaction of the fine coal particles with oxygen can produce an explosion. The ignition source does not need to be a flame; it can be an electrostatic discharge, friction, or sparks from machinery. Dust explosions are not restricted to coal mines, but have occurred around grain silos, flour mills, and metal works (aluminum, titanium). They are intentionally part of thermobaric weapons; see my older post here.