Page 302 - Advanced Mine Ventilation

Postmining Degasification of Coal Mines 279

Table 16.6 Number of Gob Wells Versus Longwall Width

Width of Face (ft) Number of Gob Wells Spacing/Width (s/w)
450 24 þ 2 0.93
600 26 þ 2 0.64
750 28 þ 2 a 0.48
900 32 þ 2 a 0.35
1050 36 þ 2 a 0.26

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

Figure 16.9 Optimal spacing of gob wells on a longwall face.

gob wells are offset from the centerline by 100 ft, the optimum width where gob gas
drainage is most efficient is in the range 700e800 ft or 750 50 ft.

16.4 Gas Capture Ratios by Vertical Gob Wells

Vertical gob wells provide a great flexibility in coping with different sized longwall
panels mined at low, medium, or high rates. The high cost of longwall equipment re-
quires a high rate of mining (70e80 ft/day) for adequate return on the investment. The
capture ratio depends on both the number of gob wells and the rate of gob gas emis-
sions for the longwall gob. Table 16.7 shows typical capture ratios for some US long-
wall faces.
Number of gob wells per panel and hence the capture ratio goes down, if the long-
wall is mined at a very slow rate (10e20 ft/day).