Page 286 - Advanced Mine Ventilation

Premining Degasification of Coal Seams 263

width of the longwall panel is reached when it will not be possible to keep the methane
concentrations below the statutory limits.
Various methods to estimate the total methane emission rate at the tailgate have
been discussed by Thakur [7]. The optimum width of a longwall panel actually
depends on the following three variables:

1. The rate of mining. For a 1,000 ft wide face, a 70e100 ft/day rate of mining is needed for
economic reasons.
2. The total gas emissions on the longwall face.
3. The specific gob gas emission.
The rate of mining controls the other two variables to a large extent. Only longwall
face methane emissions will be discussed here. The impact of specific gob emissions
will be discussed in Chapter 16.

15.6.1 Estimation of Total Methane Emissions at the Longwall
Tailgate by Direct Measurements

An estimate of methane emissions at the tail end of a longwall face can be derived by
the following equation:

Q ¼ Qo þ VðA BÞ CðXÞ (15.6)
3
where Q ¼ total methane emissions at the tail end of a longwall face, ft /min;
3
Qo ¼ total methane emissions when no mining is being done, ft /min; V ¼ the rate of
mining in tons/min; A and B are the gas contents of coal prior to mining and after
mining respectively; C(X) is the methane lost in the gob with air leakage.
An example:
In a properly degassed, moderately gassy coal seam, the following data were
measured.
Qo ¼ 100 CFM
V ¼ 8 t/min
3
3
A ¼ 120 ft /ton; B ¼ 40 ft /ton
C(X) ¼ averaged for the entire face at 250 CFM (50,000 CFM lost in gob containing an
average of 0.5% methane).
Hence,
Q ¼ 100 þ 8(120e40) 250
¼ 490 CFM.
To dilute it to 1%, the ventilation air should be 49,000 CFM with a gas layering
index greater than five.