Page 398 - Advanced Mine Ventilation

Prevention of Frictional Ignitions 367

Gas layering in a mine airway is governed by methane emission rate, air velocity,
and the width of the mine entry. The gradient of the airway has a significant impact on
gas layering, but it can be ignored for most US coal mines because the coal seams
are relatively flat. The gas layering number (GLN) is mathematically expressed as
follows [5,6]:

6V
GLN ¼ (22.1)
1=3
ðQ=DÞ
where Q is the emission rate of methane (L/s); V is the velocity of air (m/s); D is the
width of mine airway (m).
0:0258 V
In the FPS system, GLN ¼ 1=3
ðQ=DÞ
Q is the emission rate of methane (CFM); V is the velocity of air (ft/s); D is the
width of mine airway (feet).
A minimum value of five for GLN is considered necessary to prevent layering. The
higher the value of GLN, the less likely it is that gas layering will occur. However, the
formula provides only an empirical guideline, and it must be verified by physical
measurements of methane in mine entries, and ventilation air quantities should be
increased to avoid gas layering.
To illustrate this point further, two cases are discussed here.

22.2.1 Development Heading in a Highly Gassy Mine
Assuming a methane emission rate in a 20-ft-wide heading to be 50 CFM and using the
abovementioned formula, a minimum velocity of 263 ft/min is needed. Assuming a
seam height of 6 ft, the required ventilation air to prevent gas layering is 31,560
CFM. This quantity is almost 10 times the minimum quantity required by law at the
face and illustrates the need for premining degasification and adequate ventilation
when the gas contents of coal seams are high.

22.2.2 Longwall Face in a Highly Gassy Mine
The typical value of methane emissions from a longwall face in degassed but highly
gassy coal seams is 500 CFM. Assuming a width of 10 ft and a height of 6 ft, the
minimum air quantity needed to prevent layering is 42,782 CFM. Ventilation air
quantities on longwall faces should be further increased to provide a safety factor of
at least 1.5.
An innovation by the erstwhile US Bureau of Mines known as “shearer-clearer
spray system” has also proved very successful in providing increased ventilation to
minimize methane concentrations in areas difficult to ventilate and thereby minimize
frictional ignitions [7]. In this system, a number of water sprays are so mounted on and
around a longwall shearer as to create additional flow of air at the face and dilute
methane concentrations (as well as respirable dust) to safe levels (Fig. 22.3).