Page 310 - Advanced Mine Ventilation

Floor Gas Emissions and Gas Outbursts 287

Gas production can be enhanced by installing an exhausting fan with a negative pres-
sure of 3e5 psi. These boreholes provide a bypass for floor gases, and thus, gas emis-
sion on longwall floor is minimized.

17.1.2 Postmining Methane Drainage

Postmining methane drainage from the longwall gobs is perhaps the most effective
technique for preventing floor gas emissions on the longwall panels. There are several
techniques currently in practice for gob drainage, but the two most suitable for US coal
mines are the following:

1. Cross-measure boreholes drilled in the floor and
2. Vertical gob wells completed in the coal seams below the mineable coal seam.

17.1.2.1 Cross-Measure Boreholes in the Floor
This is by far the most popular method of methane control on European longwall faces.
Fig. 17.3 shows a typical layout for a retreat longwall face.
Boreholes 2e4 in. in diameter are drilled from the tailgate to a depth of 100e500 ft
in the floor. The inclination of these boreholes varies from 20 to 50 degrees, and the
axis of the boreholes is inclined to the longwall axis at 15e30 degrees. Sometimes,
more than one borehole is drilled from the same drill site. The drill sites are typically
100 ft apart. These boreholes are manifolded to a larger pipeline system, and gas
is withdrawn using a vacuum pump. Vacuum pressures vary from 4 to 120 in. of
water depending on the local conditions. The amount of methane captured by the

Figure 17.3 Methane drainage with cross-measure boreholes.