Page 215 - Advances In Productive, Safe, and Responsible Coal Mining

194 Advances in Productive, Safe, and Responsible Coal Mining

Methane accounts for 20% of global anthropogenic GHG emissions, and coal
mines constitute 8% of methane emissions or about 400 MMTCO 2 E annually [1].
The capture of CMM provides major benefits while mitigating environmental risks.
CMM drainage at first began as a technology for improving the safety and productivity
of underground coal mining by preventing explosions. Not only does it provide the
same service now, but it also decreases GHG emissions from coal mines and mitigates
air pollution because it is a clean-burning fuel.
Coal mine gases that affect air quality when naturally liberated are CH 4 ,CO 2 , CO,
and H 2 S. Added to these is coal dust, nitrogen oxides (NO x ), sulfur oxides (SO x ), and
diesel particulate matter [21]. Capturing and recovering CMM using drainage systems
when methane content is above 25% by volume is normally feasible; however,
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feasible or not, degasification is necessary in sections with over 8m /min (283ft /min)
of CMM emissions.
A major concern is that even after coal mines are shut down, CMM continues to be
released. It is worth mentioning that CMM is the riskiest gas released from abandoned
underground coal mines because each tonne of coal removed can result in about 15t of
CMM emissions. Therefore, it is important to control AMM emissions to less than 1%
by volume in order to minimize the risk of GHG emissions and explosions in aban-
doned mines. The methane content of AMM ranges from 30% to 80%; however, it
typically contains no oxygen and its composition changes slowly. The main hazard
in abandoned coal mines is accumulation of AMM in discarded underground mining
structures. Falling barometric air pressure causes expansion of methane, which can
result in AMM overflow. Therefore, applying suitable mine reclamation strategies
is essential to abate any destructive events. Filling of near-surface mine openings
and sealing by injections along with degasification systems can mitigate the risk of
AMM explosions or emissions to the atmosphere. Actually, many factors can affect
potential AMM volumes, including time since abandonment, gas content and adsorp-
tion characteristics of the coal seam, methane emission rates during active mining,
mine flooding, presence of vent holes, and mine seals.
Flaring of CMM or VAM is an abatement option that may be considered if it is not
feasible to utilize recovered gas, mainly due to low concentrations of methane or unat-
tractive markets. With the proper equipment and procedures, unused drained CMM
can be safely flared to minimize GHG emissions, because converting CH 4 to CO 2
lowers the greenhouse effect. Based on the overall oxidation reaction of CMM when
CH 4 is fully oxidized, burning 1kg (2.2lb) of CH 4 produces 2.75kg (6lb) of CO 2 .In
other words, when 1kg (2.2lbs) of CH 4 is mitigated, 20.25kg (45lbs) of CO 2 emis-
sions are reduced in terms of the GHG impact. In addition, flaring converts methane
with an average GWP of 25 to CO 2 with a GWP of one.
It is important to dilute emitted CMM with low methane content through mixing
with the ventilation air. Despite low methane concentrations (1% or less), collectively
VAM is the single largest source of CMM emissions globally, so there is concern with
just venting it to the atmosphere. When methane content is high, CMM should be cap-
tured by drilled boreholes to enhance the ventilation system and secure miners’ health.
In fact, there are many varieties of respiratory disorders that can result from the inha-
lation of CMM and coal dusts in underground coal mines. The more common