Page 109 - Advances In Productive, Safe, and Responsible Coal Mining

Mine ventilation networks optimized for safety and productivity 93

effect on mine ventilation. Due to the heat and smoke generated, the air over the fire
expands and makes the fire act as a regulator. The energy added to the air by the fire
makes the fire act as a fan at the same time. This effect is stronger if the fire is on a
ramp ventilated uphill. If the ramp is ventilated downhill, a fire can cause air reversal.
There are several ventilation programs that allow examination of such air reversal
effects caused by fires. Model calculations are also important to determine an appro-
priate sealing sequence for portals and shafts so that air reversals are prevented, espe-
cially in the fire area.
After sealing the mine, the atmosphere can be inertized by pumping in nitrogen,
boiler gas, or, if the topography is suitable, flooding the mine with water. Boreholes
should be drilled to access mine workings for air quality and temperature sampling to
verify that the fire has been extinguished. Coal mine fires often require several months
of wait time before the mine can be reopened.

6.7.1 Prevention of face ignitions

Face ignitions are frequently caused by longwall shearers or continuous miners cutting
abrasive rocks such as quartzitic sandstone with worn cutter bits [10]. According to
one study of statistics published by MSHA [11], US coal mines have experienced
between 30 and 60 frictional ignitions annually. Since 2010, the number of reported
ignitions has gone down to below 20 per year. Still, any frictional ignition has the
potential to start a major mine explosion or fire. A frictional ignition is suspected
as the primary cause of the 2010 explosion at the Upper Big Branch (UBB) mine
in the US State of West Virginia [12]. This explosion fatally injured 29 miners.
Contrary to common belief, it is not the sparks that ignite methane but a smear of
white-hot metal abraded from the cutter and left on the rock [13]. Such hot smears are
often caused when the softer steel shank of a worn or missing cutter bit rubs against the
rock. Therefore, prevention of frictional ignitions begins with regular inspections of
cutter drums and replacing all worn or missing bits immediately. The mine operator
should have a program in place to ensure that cutting is always done with sharp bits.
This also reduces dust formation, as dull bits grind coal and rock and produce signif-
icantly more dust.
A well-designed, maintained, and functioning water spray system is equally impor-
tant. Wet-head cutting drums on continuous miners and longwall shearers are
designed to have sprays directly in front of or behind the cutting bit. Typically, water
sprays are more effective if the water is finely dispersed. Smaller water droplets evap-
orate more easily and thereby take away heat from the cutting area. Water sprayed
behind the bit cools off any hot smears. A simple way to ensure the effectiveness
of water sprays is to continuously monitor both the water flow and the pressure of
the bit spray system. European mining laws require an interlock that shuts down
the mining equipment if either the flow rate or the water pressure deviate from design
values by more than 5%. Such interlock systems are not required in US mines. In
order to keep water nozzles from clogging, the supply water must be thoroughly fil-
tered and filters and pumps maintained regularly. Again, mine operators should have