Page 203 - Advances In Productive, Safe, and Responsible Coal Mining

182 Advances in Productive, Safe, and Responsible Coal Mining

In general, coal can store about six to seven times more methane than the equivalent
volume of rock in a conventional reservoir.
From the author’s viewpoint, it is possible to categorize gases derived from coal
mines into four key forms: (i) coal mine methane (CMM) or working mine methane
(WMM), (ii) coal seam methane (CSM) or coal bed methane (CBM) collected from
unmined coal beds, (iii) abandoned mine methane (AMM) drained from depleted or
inactive mines, and (iv) syngas from underground coal gasification (UCG) by which
coal is gasified in situ.
As mentioned, CMM is a general term for all methane released mainly during and
after coal-mining operations. Therefore, CMM is a type of coal gas present in active
working mine sites, and may be captured through a drainage system or vented from
coal mines by a ventilation system; i.e., ventilation air methane (VAM). In drainage
systems, CMM may be captured via surface vertical premining boreholes, horizontal
premining boreholes ahead of the coal face, or postmining wells drilled into the
gob area.

10.2.1 Methane chemistry
Methane is a colorless, odorless, tasteless, and flammable chemical compound with
the chemical formula CH 4 . Methane is the second most abundant GHG accounting
for about 15% of global GHG emissions produced by human activities and is respon-
sible for more than a third of total anthropogenic radiative forcing. Methane is more
than 20 times more effective at trapping heat than CO 2 , making it one of the most
potent GHGs.
Methane is lighter than air, having a specific gravity of 0.554. It is only slightly
soluble in water. It burns readily in air, forming CO 2 and water vapor; the flame is
pale, slightly luminous, and very hot. The boiling point of methane is 161.5°C
(258.7°F) and the melting point is 182.5°C (296.5°F) [5]. Methane is not toxic when
inhaled, but it can produce suffocation by reducing the concentration of oxygen
inhaled. In general, methane is very stable, but mixtures of methane and air, with
the methane content between 5% and 15% by volume, are explosive. Explosions of
such mixtures have been frequent in coal mines, causing many disasters worldwide.
It is worth mentioning that methane content more than 15% may not necessarily be
explosive.
Even though methane-air mixtures under 5% are not explosive, a considerable mar-
gin of safety must be provided (usually to less than 2%) due to the compounding effect
of coal dust. An effective mine ventilation system will therefore ensure that the vol-
ume of gas mixture is minimized below the explosive range in a safe level (usually less
than 1% or up to 1.25%). In addition, due to the fact that inert gases such as nitrogen or
CO 2 cannot chemically react with methane, they can be added to an explosive
methane-air mixture to make it nonexplosive [4].
Methane, which is also known as methyl hydride, is a group 14 hydride and the
simplest alkane, a series of hydrocarbons. It is the main constituent of natural gas.
In coal mining, methane is the main constituent of marsh gas (swamp gas) and fire-
damp (flammable gas), and may be captured commercially from gaseous coal seams