Coal Mine Explosion in New Zealand

The entrance to the Pike River Coal mine.  Photo/NZPA
published by the nzherald.co.nz, Nov. 19, 2010

On November 19, 1 man was killed and 27 miners are missing due to an explosion in the Pike River coal mine on the south island of New Zealand.  The mine has been under development since the 1970's, and is excavating from the largest hard-coking coal deposit in New Zealand. It is in an environmentally sensitive area, and has been lauded as an exemplary development in a sensitive area.  The tunnel, seen at the left, goes nearly 2.5 km nearly horizontally into the mountain.  The trapped miners may only be a few hundred feet underground.

From this Wiki site

Dangers in coal mines arise from at least three factors: the presence of both methane and carbon dioxide gases, and coal dust.  In this particular area of New Zealand, active faults produce a seismic risk as well.  Other causes of accidents are rock bursts, collapse of natural or artificial pillars, flooding, malfunctioning of equipment, and improper use of explosives in the mining process.  Methane gas is nasty stuff: an asphyxiant, flammable and potentially explosive.

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.