Page 298 - Advanced Mine Ventilation

Postmining Degasification of Coal Mines 275

are drilled to intersect other gassy coal beds. If mining scheme proceeds from the top to
the bottom seams in a basin, the entries in a working mine can be used to drain coal
seams at lower levels. Methane flow from these entries is high, averaging
700e1000 CFM for highly gassy seams. Nearly 50% of total emission at the longwall
face has been captured [7].

16.3 US Gob Degasification Method

US longwall panels are much larger than European longwall panels and are mined at a
faster rate. Cross-measure boreholes simply cannot drain enough methane to keep the
bleeder entries at less than 2% methane as required by Federal regulations [8]. Thakur
[9] estimated the feasible ventilation air quantities in modern longwall mines as shown
in Table 16.4.
The bleeder air, therefore, can only capture a small fraction (15%e30%) of total
gob emissions. Gob drainage technique must capture 60%e80% of the total gob
gas emissions. The only technique that can do this is the use of vertical gob wells.
Their size and number will vary with the rate of gob gas emissions and will be dis-
cussed later in the chapter.

16.3.1 Construction of a Vertical Gob Well

Fig. 16.7 shows the vertical section of a typical gob well.
A12e15 in. surface casing is set at the bedrock for a depth of about 100 ft. Next, a
1
1
5
5
9 / 8 e12 / 4 in. borehole is drilled to a depth below all water aquifers and a 7 / 2 e9 / 8 in.
1
7
casing is set. Next a 6 / 4 e7 / 8 in. borehole is drilled to the top of the coal seam being
1
1
mined stopping 30e90 ft above the coal seam. A 4 / 2 e6 / 2 in. diameter casing is low-
ered in the borehole, and it is anchored in a strong stratum about 300e1000 ft above
the coal seam depending on the depth of the coal seam. The casing below this point is
not cemented but slotted to allow gob gases to enter the gob well. The slots are typi-
cally 1 in. wide 2 ft high and are cut at different locations on the perimeter of the
casing like a spiral. This preserves the integrity of the casing. For shallow coal seams,
old casings or pipes can be used but for deeper coal seams, new J55 or K55 casings are
1 1
recommended. The size of the gob well casing varies from 4 / 2 to 12 / 4 in. outside
diameter depending on the volume of gas to be drained.
Table 16.4 Feasible Ventilation Quantities for US Longwall Faces
Category of Coal Seam Face Intake (CFM) Tailgate (CFM) Bleeders (CFM)
Mildly gassy 30,000 25,000 100,000e150,000
Moderately gassy 50,000 40,000 150,000e250,000
Highly gassy 80,000 60,000 250,000e350,000