Page 292 - Advanced Mine Ventilation

Postmining Degasification of Coal Mines 269

Figure 16.1 Vertical limits of the gas emission space.

Factors which govern the rate of gas emission in the longwall gobs can be summa-
rized as follows:
1. The density and proximity of gas-bearing zones in the gas emission space.
2. The gas content of coal seams and other gas-bearing horizons.
3. The extent to which overlying and underlying coal seams have been mined.
4. The rate of coal face advance. This is the most important factor.
5. The presence of geological disturbances and leftover coal pillars.
6. The subsidence characteristics of the area which determines the permeability of the gas emis-
sion space.
Fig. 16.1 showed only the vertical dimension of the gas emission space. The width
of the gas emission space is limited to the width of the longwall panel. In deep, gassy
mines, chain pillars get crushed, and there is little movement of gases from one long-
wall panel to the next.
The length of the effective gas emission space is limited by the subsidence of the
overlying strata. Fig. 16.2 shows that the gob is fully subsided (up to the full thickness
of the coal seam) when the face retreats from the setup entry by a distance equal to
1.3 depth of the coal seam [5].