Page 424 - Advanced Mine Ventilation

Gas and Dust Explosions 393

3. All equipment used at the working face (an area inby of the last open crosscut) should be
either explosion proof or intrinsically safe.
4. A gas concentration check should be made prior to starting any mining equipment. Mining
machines should have methane detectors that can sound an alarm at 1.5% of methane and cut
off the power at 2%.
5. Continuous monitoring of areas where methane accumulation is likely should be required.
6. Routine and extensive inspection of all areas in a mine should be done to minimize methane
ignition risks.
7. Mined out areas should be isolated by explosion-proof stoppings.

The US Federal Regulations (and State regulations) [10] provide a comprehensive
list of all these precautions. Compliance with law is required. Citations and fines are
issued for any and all violations.

23.3.4 Prevention of Dust Explosions

The first step in this direction is the same as the control of respirable dust discussed
earlier in the book. Prevention of dust formation and suppression by water sprays
are strongly recommended. Prevention of methaneeair explosion is the second step
in the prevention of dust explosions. The third and the last step is to make the coal
dust inert by mixing some noncombustible dust with it. Gypsum powder, crushed
shale, and limestone powder are the most commonly used inerting material. In the
United States, limestone is mostly used because of its availability in the proximity
of coal mines.
The percentage of inert dust in the mixture with coal dust that will stop the propa-
gation of a coal dust explosion depends on a number of factors, such as
1 Volatile content of coal.
2 Moisture content.
3 Fineness of coal dust.
4 Strength of the initiator (explosive charge or a fixed volume methane explosion).
5 Concentration of any combustible gas, such as methane in air.
Excellent references on the subject, such as Cashdollar [8], Nagy [11], Cybulski [12],
and Richmond [13], are available. A summary of their findings will be presented here.

23.3.4.1 Effect of Volatile Matter in Coal

Fig. 23.8 summarizes the data by Cashdollar [8], Nagy [11], Cybulski [12], and Rich-
mond [13].
Percentage of inert dust increases with the weight percentage of volatiles in coal
from a low of 40% to a high of 80%. The US law now requires 80% noncombustible
in all airways. Legal requirements for inert dust are shown in Table 23.8 for some coal
mining countries. The higher the “s” value, the better is the level of protection.