Page 234 - Advanced Mine Ventilation

214 Advanced Mine Ventilation

factors such as the rank of coal, the depth of burial, the immediate roof and floor to the
coal seam, geologic anomalies, tectonic pressures, and temperature prevailing at the
end of the coalification process. In general, the higher the rank of coal and the greater
the depth of coal seam, the higher is the gas content of coal. Gas content of coal seams
3
3
vary from a few ft /t to 800 ft /ton for depth up to 3000 ft. Coal seams are the source as
well as the reservoir for all gases.
Methane is the major component of gas in coal, comprising 80%e95% of the total
gas content. The balance is made up of ethane, propane, butane, carbon dioxide,
hydrogen, oxygen, and argon. All coal seams begin to oxidize when exposed to venti-
lation air in a mine and produce some carbon monoxide and hydrogen sulfide. Sulfur
dioxide and nitrogen oxides are produced by diesel equipment in coal mines. A brief
introduction to all these gases is presented below.

13.2 Properties of Gases in the Mine Atmosphere

13.2.1 Methane

It is a colorless, odorless, tasteless, nontoxic gas that is flammable in the range
5%e15% (by volume). It can be also fatal if a person walks into an atmosphere con-
taining very high concentrations of methane. It is lighter than air with a specific gravity
of 0.55 and, therefore, has a tendency to accumulate in cavities in the roof or layering
against the roof. Measurement of methane concentrations in such cavities should be
done remotely with extended probes.
Excess methane can slow down mining or even completely stop it creating great
economic losses. Methods to drain and control methane in mine airways will be dis-
cussed later in the book. Table 13.1 shows the main characteristics of methane and
other mine gases. The law requires that methane concentration in mine air be kept
below 1% (except bleeders can have 2%).

13.2.2 Ethane

It is very similar to methane except its concentration in coal seam gases is 0.25%e2%
only. Only deep, highly gassy coals have 2% ethane. Table 13.1 shows the main prop-
erties of ethane. It is usually not measured separately in mine air.

13.2.3 Propane and Butane
Coal seam gas has only a trace of these gases (a few ppm). Higher concentrations usu-
ally indicate a leakage from a natural gas well that may not have been plugged prop-
erly. The presence of helium also indicates a leakage of natural gas into the mine.

13.2.4 Carbon Monoxide
It is a colorless, odorless, tasteless, toxic gas. It is also flammable in the range
12.5%e74%. It is mostly created by spontaneous combustion of coal, explosions,
blasting, and internal combustion of diesel engines.