Page 419 - Advanced Mine Ventilation

388 Advanced Mine Ventilation

Figure 23.5 Pressureetime transient for detonation of a stoichiometric acetyleneeair mixture in
a 45.7 m (150 ft) long tunnel with initiation end closed. Instrument station is 18.3 m (60 ft)
from initiator point.

23.2.1 Explosive Limits of Dust in Air

The range of explosive dust concentration in air is large and is largely dependent on
particle size and the amount of volatile matter in coal. Tables 23.5 and 23.6 show perti-
nent data for Pittsburgh coal seam (HVB) and Pocahontas #3 coal seam (LVB) [8].
The minimum explosive concentration (MEC) decreases with particle size ranging
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from 130 to 85 g/m for Pittsburgh seam that has a higher fraction of volatiles. The
MEC is generally lower for a finer dust cloud as compared with a coarser dust cloud
for any coal.
The best way to make the coal dust nonexplosive or inert is to mix a noncombus-
tible dust, such as calcium carbonate, gypsum, or even shale dust, with it. The amount
of noncombustible dust needed to inert coal dust increases with decreasing coal dust
particle size reaching a value of 83%e87% when coal dust is 13e14 mm in size.
The upper limit of dust flammability is poorly defineddbecause it is difficult to
carry out such experiments. In an experimental mine, the flame was quenched at a
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coal dust concentration of 5 kg/m [9].