Page 285 - Advanced Mine Ventilation

262 Advanced Mine Ventilation

Figure 15.11 Coal seam degasification with horizontal boreholes drilled from the surface.

coal fines, and gas production is maintained. Very expensive drill rigs and instruments
are needed to articulate a horizontal borehole assembly to stay in a thin coal seam and
reach a depth of 3000 to 5000 ft. The cost of drilling horizontal boreholes from surface
is, therefore, much higher ($100/ft) than the cost of drilling in-mine horizontal bore-
holes (usually less than $20/ft). The degree of degasification achieved is the same,
about 50% of gases in situ.
Premining degasification of all moderately and highly gassy coal seams is highly
recommended. Besides making mining safer, it improves coal productivity signifi-
cantly cutting down the cost of mining. In one very gassy mine, the coal productivity
jumped from 14 tons/man-day to 40 tons/man-day after about 70% of in situ gas was
drained. The produced gas can be processed to meet the pipeline gas specifics and mar-
keted to defray the cost of degasification [6].

15.6 Optimum Widths of Longwall Panels

3
Even after proper premining degasification, the coal seam contains 100e200 ft /t of
gas. This gas is released when the coal is mined by mining machines and broken
into small pieces. The total gas emissions reaching the tail end of the longwall face
increases with the width of the longwall panel. Statutory requirements limit methane
concentration at the tailgate to less than 1% (preferably 0.8%). It is also true that venti-
lation air reaching the tail end of the longwall face is dependent on the ventilation air
quantities at the head gate end of the face and air leak-off on the face. The wider the
longwall panel, the higher is the total leak-off as shown in Chapter 4, Figure 4.8. It is,
therefore, logical to conclude that for a given set of conditions, an optimum limit on the