Page 69 - Advanced Mine Ventilation

52 Advanced Mine Ventilation

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Figure 4.9 Methane emissions on longwall faces as a function of gas content of coal ft /t.

It is clear from this observation that the wider the longwall panel, the smaller are the
ventilation air quantities reaching the tail end of the face. It is also true, as discussed
later in the book, that the wider the longwall panel, the higher is the methane emissions
at the tail end of the longwall face. It is, therefore, easy to conclude that for a given set
of conditions, a limit on the longwall face width will be reached when it will not be
possible to dilute the methane emissions enough to meet the statutory requirements.

4.6.1 Gas Emissions on Longwall Faces

Gas emissions on the longwall face is dependent on (a) the residual gas content of
degassed coal seam or the degree of degasification, (b) the rate of coal extraction,
and (c) the diffusivity of coal [4]. The latter is a measure of the rate of gas emission
from mined coal. Higher rank coal, e.g., low volatile bituminous coals, have a much
higher diffusivity than high volatile bituminous coals [5], and as such, they release
a higher fraction of their original gas content on mining.
An approximate estimate of methane emissions at the tail end of a longwall face can
be derived from the following equation:

Q ¼ Q þ WðA BÞ CðxÞ (4.1)
0
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where: Q ¼ total methane emissions at the tail end of longwall, ft /min; Q 0 ¼ total
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methane emitted when no mining is done, ft /min; W ¼ average rate of mining in
t/min; A and B ¼ gas contents of coal before and after mining, respectively; C(x) ¼
methane lost to gob areas by air leak-off and is a function of the distance from the head
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gate, m /min.
Assuming a 1000-ft-wide longwall face has a daily advance of 50 ft and a ventila-
tion leak-off of 50e70%, average methane emissions from highly gassy coal seams at