Page 63 - Advanced Mine Ventilation

46 Advanced Mine Ventilation

Figure 4.2 Vertical extensions of gas emission space surrounding a longwall gob.

Fig. 4.2 [2] shows the vertical extent of the gas emission space created by longwall
mining and the percentage of gas content released by various coal seams contained in
the gas emission space as a function of its distance from the mined coal seam. The ver-
tical dimension of the gas emission space is highly dependent on the width of the long-
wall face. In general, the wider the longwall face, the greater the vertical dimension of
the gas emission space and, consequently, the higher the specific gob methane emis-
3
sion (ft of gas emitted per acre/day). In one study, the specific methane emission
increased by 50% when the face width increased from 630 to 700 ft [3]. Very wide
(1000 þ ft) longwall faces also exacerbate methane and respirable dust concentrations
at the tailgate and require a larger quantity of air at the intake end of the face to stay in
compliance with statutory requirements.
To properly plan for methane control, one must know the amount of ventilation air
that is available in strategic areas, e.g., longwall face and bleeders. Table 4.2 shows

Table 4.2 Desirable Ventilation Quantities for Longwall Faces

Category Face Intake (cfm) Tailgate (cfm) Bleeders (cfm)
Mildly gassy 30,000 25,000 100,000e150,000
Moderately gassy 50,000 40,000 150,000e250,000
Very gassy 80,000 60,000 250,000e350,000