Page 68 - Advanced Mine Ventilation

Estimation of Ventilation Air Quantity 51

4.5.2 Postmining Degasification

Because of very high gas emissions in the gob area, the first gob well must be installed
within 50e100 ft from the setup entry. Subsequent gob wells may be drilled at 6e15
acre spacing, depending on the rate of mining and gas emissions experienced. In Vir-
ginia and Alabama, the gob wells are generally 9e12 in. in diameter. Powerful
blowers with 75e125 Hp motors creating a suction of 5e10 in. of mercury are needed
to capture up to 70% of gas emissions.

4.5.3 Ventilation Layout and Quantities

Ventilation layout for a longwall panel in very gassy mines, as shown in Fig. 4.6,is
similar to the layout for moderately gassy coal seams, but ventilation air quantities
are higher, as shown in Table 4.2. The bleeder shafts are bigger in diameter to handle
ventilation air quantities of 250,000 to 350,000 cfm. A well-placed bleeder shaft can
serve four to six longwall panels depending on the size of the panels.

4.6 Limitations on the Longwall Face Width Owing to
Face Methane Emissions

Ventilation air quantities reaching the tail end of the longwall face is dependent on the
ventilation air quantities (and therefore, the air velocity) at the head gate end of the
longwall face and the air leak-off on the face. The wider the longwall panel, the higher
is the leak-off factor. Fig. 4.8 shows a plot of some actually observed data in coal
seams with a thickness of 5e6 ft.

Figure 4.8 Air leak-off on longwall faces as a function of width, feet.