Page 72 - Advanced Mine Ventilation

Estimation of Ventilation Air Quantity 55

Table 4.3 Specific Gob Emissions on US Longwall Panels

Width of Longwall Face (ft) Specific Gob Emissions (MMCF/acre)
450 25
600 30
750 33
900 36
1050 40

Just as excessive methane emissions on a longwall face can limit the width of the
panel, very high specific gob emissions can also limit the panel width. The specific gob
emission increases with increasing width of longwall panels. Table 4.3 shows some
observed data [9] for the Central Appalachian Basin, United States.
The optimum width of a longwall panel is the width where gob wells are most effi-
cient in draining the gob gases, and the total number of gob wells for the panel is the
minimum. The most efficient gob drainage is reached when the spacing between the
two adjacent gob wells(s) is equal to half the width (w) of the longwall panel. To illus-
trate this point, spacing of gob wells for longwalls with different widths but the same
tonnage of extraction is calculated. The following assumptions are made for this
calculation:
1. The rate of extraction is the same for all longwall panels, 1 acre/day.
2. The longwall face is 1000 ft long and needs two gob wells near the setup entry.
3. A gob gas capture ratio of 70e80% will be achieved.
4. Specific gob emission is 30 MMCF/acre.

Table 4.4 shows pertinent data for various widths of longwall panels.
Fig. 4.10 shows a plot of s/w against longwall face widths. The most efficient cap-
ture of gob gas is obtained when s/w ¼ 0.5 or the longwall width is 700 ft. Because the
gob wells are offset from the center line by 100 ft, the optimum width where gob gas
drainage is most efficient is in the range 700e800 ft. A width of 750 ft is a good
average. Optimum width for a coal seam with a different specific job emission can
be similarly calculated.
Table 4.4 Number of Gob Wells Versus Longwall Width

Width of Face, (ft) Number of Gob Wells Spacing/Width, (s/w)
400 24 þ 2 0.93
600 26 þ 2 0.64
800 28 þ 2 a 0.48
900 32 þ 2 a 0.35
1000 36 þ 2 a 0.26

a
Total number of gob wells could be slightly higher because of declining capture efficiency.