Page 70 - Advanced Mine Ventilation

Estimation of Ventilation Air Quantity 53

the tail end of the longwall face as a function of degree of degasification is shown in
Fig. 4.9. For high coal productivity, highly gassy coal seams should be degassed
3
before mining to reduce the gas content of coal to at least below 100 ft /t. It is also
prudent to assume a peak emission rate that is 25% higher than the average emission
rate. The minimum ventilation air quantity should be able to dilute the peak methane
emissions to below statutory requirements, i.e., 1% in the United States.

4.6.2 Gas Layering on Longwall Faces
The second criterion for the adequacy of ventilation on longwall faces is the prevention
of gas layering near the roof or floor. Gas layering in any mining roadway including
the longwall face is governed by (a) the methane emission rate, (b) ventilation air ve-
locity, and (c) the effective width of the airway. The gas layering number (GLN) is
mathematically expressed as follows [6]:

1
V D 3
GLN ¼ (4.2)
41 Q
where Q is the methane emission rate in cfm; V is the air velocity in ft/min; D is the
effective width of the airway (longwall face), ft.
A minimum value of 5 for GLN is considered necessary to prevent layering. The
higher the value of GLN, the less likely it is that gas layering will occur.
A typical calculation is shown here.
Assume, Q ¼ 500 cfm; D ¼ 10 ft
And a GLN of 6 for safety
The necessary air velocity to prevent gas layering from Eq. (4.2) is 906 ft/min.
Assuming a mining height of 6 ft, the minimum ventilation air required at the tail
end of a longwall face is 54,317 cfm to prevent gas layering.

4.6.3 Mathematical Modeling of Methane Flow

The flow of methane and air on longwall faces can be modeled mathematically. Funda-
mental basis of such models are already developed [7]. Main assumptions made are as
follows:
1. Symmetry in the directions (y-z) perpendicular to the longwall face (x direction).
2. The density effect of a lighter gas such as methane is neglected.
3. When mining is in progress, a steady-state situation is likely to prevail, i.e., time dependence
of methane concentration is discarded.
Taking a mass balance over a small element of the longwall face and applying the
above assumptions, the turbulent dispersion of methane on longwall faces can be rep-
resented by the following mathematical model:

2
d c dc
Ex 2 uðxÞ þ qðxÞ pðxÞ¼ 0 (4.3)
d x dx