Page 428 - Advanced Mine Ventilation

Gas and Dust Explosions 397

smaller loading of the dust. In heavy loading, the total weight on a plank is about
8e9 lbs per square feet of the airway cross section. The lightly loaded barriers may
contain only half as much. The typical length of the barriers is 100e200 ft. The volu-
3
metric stone dust concentration to quench a gas/dust explosion is 7e12 kg/m . This is
150e200 times the lower explosion concentration limit for coal dust. A typical set of
stone dust barriers may have a total stone dust load of 5e7 tons.
The fineness of the dust is the same as that of the rock dust used to inert the coal
dust. In the United States, 100% of the dust must be smaller than 850 microns, 70%
finer than 75 microns with less than 5% combustibles and less than 4% silica. Most
rock dusts tested by the author were finer than 37 microns (400 mesh) and had no silica
in it. Finer the dust, the better is its ability to disperse in air and higher its residence
time in air. Some chemicals, such as stearic acid, are mixed with dust to prevent
caking.
Stone dust barriers appear to work better in test galleries but do not do as well in
actual mines. It would be, therefore, prudent to (1) degas the coal seam, (2) use
only permissible equipment in the working face, and (3) inert the coal dust with
noncombustible dust. US coal mine regulations do not require stone dust barriers.

Problems

The following gas analysis was received after a mine fire. Calculate the composition
point and determine if the atmosphere is explosive.

Gas Concentration (by Volume)
CO 2.5
CO 2 0.2
CH 4 2.3
H 2 2.0
O 2 10
N 2 (100 for all other gases)

References

[1] Byrer C, et al. Coalbed methane: a miner’s curse and a valuable resource. In: Thakur P,
et al., editors. Coalbed methane from prospect to pipeline. Elsevier Publishing; 2014.
p. 1e6.
[2] Bodurtha FT. Industrial explosions prevention and protection. McGraw-Hill Book Com-
pany; 1980. p. 157.
[3] Coward HF, Jones GW. Limits of flammability of gases and vapors. USBM Bulletin #503.
1952.