Page 293 - Advanced Mine Ventilation

270 Advanced Mine Ventilation

Figure 16.2 Subsidence over a longwall gob as a function of face retreat/depth.

Thus, in a mine with a depth of 2000 ft, the length of the “effective” gas emission
space is 2600 ft. This “effective” length is very useful in deciding the spacing of gob
wells on longwall panel as discussed later in this chapter.
It is desirable to use this “effective length” for design purposes because it slightly
overestimates the number of gob wells per panel. In reality, the gob areas beyond
this “effective length” continue to produce methane for a long time, albeit at a lower
rate.
Obviously, there can be considerable variations in the gas flows into the longwall
gobs because of varying combination of above factors. Experience and data obtained
in one geographical area, therefore, can be transferred only qualitatively to other areas.
A summary of field data is provided in Table 16.1.

Table 16.1 Typical Longwall Gob Gas Emissions

Total Gob
Rate of Mining Emissions
a
Coal Seam Gassiness (ft/day) (MMCFD)
1. Pittsburgh (PAeOH) Mildly gassy 70e100 1e3
2. Pittsburgh (WV) Moderately gassy 70e80 3e8
3. Pocahontas #3 (WV) Moderately gassy 70e80 8e10
4. Pocahontas #3 (VA) Highly gassy 60e70 25e30
5. Blue Creek/Mary Lee (AL) Highly gassy 60e70 15e20

a
Millions of cubic feet per day.