Page 207 - Advances In Productive, Safe, and Responsible Coal Mining

186 Advances in Productive, Safe, and Responsible Coal Mining

gob wells, and vertical degasification wells have also been developed and are cur-
rently used in mines with high methane emissions. In underground coal mining, as
CMM emits into the mine area from the cracked coal or rock strata, timely monitoring
of hazardous gases and proper implementation of well-designed ventilation and meth-
ane drainage systems must go hand in hand with taking steps to reduce frictional
ignitions and educating all miners regarding safety regulations. When this is done,
even highly gassy mines can be safely and successfully operated.

10.3.1 Ventilation control measures
CMM emissions can adversely affect both safety and productivity of underground
coal mines. Ventilation is the universal methane control technique used in under-
ground coal mines where it is essential to circulate fresh air across active faces using
mechanical fans. Therefore, underground coal mining requires carefully designed and
well-maintained ventilation systems that can control large amounts of methane and
dust produced during coal extraction. In this respect, it is necessary to implement sys-
tems that closely monitor CMM levels to ensure that methane concentrations are kept
at low levels.
According to figures obtained by the National Renewable Energy Laboratory [9],
the average amount of CMM released from surface and underground coal mines in the
Illinois Basin of the United States are 1.91 and 4.23g of methane per kilogram of
mined coal, respectively. This region is not known for being gassy. Therefore, face
ventilation systems at underground coal mines must be able to safely handle gas flows
by focusing on the peak CMM levels, not overall levels.
To reduce CMM concentrations within the mine, methane is diluted by fresh air
that is circulated in sufficient quantity by the ventilation system. By mixing CMM
with ventilation air, VAM results. The maximum concentration of methane in mine
air is restricted to 1% (or at maximum up to 1.25%) in roadways where personnel
travel, and less than 2% in the face area. In order to keep CMM concentrations below
these acceptable levels, usually more than 10t of fresh air have to be circulated
through the mine for every tonne of coal mined. This, in turn, results in venting large
quantities of VAM with methane concentrations typically under 1% that are currently
exhausted to the atmosphere at most mine shafts worldwide. An alternative is to use
VAM by feeding it into a boiler and generating heat or power. Such an option might be
attractive at mines that generate their own power.
As mentioned, the density of CMM is roughly half that of air, which results in a
buoyant high-concentration layer at the mine roof that does not readily mix into
the ventilation air stream. This phenomenon is largely a result of inadequate ventila-
tion. Depending on the emission rate, entry width, seam slope, etc., an air velocity of
0.4–0.8m/s (1.3–2.6ft./s) measured at the mine roof was enough to prevent CMM lay-
ering at that point. In the absence of other means to promote mixing, raising air veloc-
ity is a highly effective way to reduce CMM ignition risk. Higher air velocity promotes
better mixing in addition to lowering the average CMM concentration. CMM layering
is not as critical at the mine floor or rib since the gas readily mixes into the ventilation
air stream, losing its buoyancy [4]. However, it is important to manage velocity at