Page 422 - Advanced Mine Ventilation

Gas and Dust Explosions 391

t ¼ 326:3 95:5 log l (23.13)

where t is in centigrade and l in mm.
The coal dust explosion temperature is difficult to calculate but measured values

range from 2500 to 2600 C.

23.2.4 Dust Explosion Pressure
Maximum and near maximum explosion pressure and pressure rise rates for coal dust
3
occur within a concentration range of 200e1000 g/m . Thus, there is a broad range of
dust concentrations exhibiting the most damaging effects. Fig. 23.7(A) shows a typical
pressure rise curve for Pittsburgh coal seam (HVA) in a 20 L chamber [8]. The
3
maximum pressure of almost 7 atm is achieved at around 600 g/m , a concentration
10 times the MEC.
dp
It is also clear from Fig. 23.7(B) that the rate of pressure rise, , generally increases
dt
3
with dust concentration reaching a limit at concentration of 600 g/m . The rate of pres-
sure rise also increases with a decrease in particle size [5].

23.3 Prevention of Gas Explosions

Both gas and dust explosions can be practically eliminated if proper actions are taken.
Two major sources of ignition, namely, spontaneous combustion and frictional igni-
tions, are already discussed in Chapters 21 and 22, respectively. Some additional pre-
cautionary measures are discussed here.

23.3.1 Methane Drainage

As discussed in Chapters 13e16, draining methane from coal prior to mining and post-
mining can be a big help in minimizing the risk of mine explosions. All coal seams
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with a gas content of above 100 ft /t must be degassed prior to mining. All longwall
panels should have some vertical gob wells to prevent excessive gas pressures in
the gobs. Coal seams underlying the working coal seam must be degassed to prevent
methane outburst and consequent methane inundation.

23.3.2 Ventilation
The second line of defense against mine explosion is good ventilation. The air quantity
and its velocity should be large enough to dilute methane below the statutory limit of
1% and eliminate the risk of gas layering near the roof or floor. The subject is fully
discussed in Chapters 1e6 in this book.