Page 291 - Advanced Mine Ventilation

268 Advanced Mine Ventilation

With the onset of longwall mining in Europe in 1940s, the initial work on methane
control on longwall was naturally done there. The basic principle of methane control
has always been, and still remains, some means of “bypassing methane from the gas
emission space without letting it mix with the mine air”. In most cases, the bypassing
mechanism is either a strategically located borehole or a roadway. A successful
methane control program depends on the following basic premises:
1. Determination of the geometry of the gas emission space and location of gas-bearing hori-
zons therein.
2. Estimation of the rate of methane influx into the longwall gob.
3. A scheme to bypass the gas in the most economic and efficient manner and thus prevent it
from entering the mine atmosphere.
In successful methane drainage programs a high proportion, usually between 50%
and 70%, of the total gas emissions in a working district is removed before it can enter
the mine airways. Advantages of postmining methane drainage can be summarized as
1. Generally reduced gas delays in the mine leading to increased safety and higher productivity.
2. Reduced air requirements and corresponding savings in ventilation horsepower.
3. Possible use of mine gas as an additional source of fuel.

16.1 The Gas Emission Space

Fig. 16.1 shows the vertical extents of gas emission space with respect to the mine
working according to various authors [1].
Calculations of gas influx are based on the concept that there are finite limits for the
gas emission zones above and below the mine workings. The smallest range of gas
emission space is given by Gunther [2] in the roof at 300 ft and by Lidin [3] in the floor
at 70 ft. The greatest range of gas emission space is given in the roof at 1000 ft and in
the floor at 300 ft by Thakur [1]. Winter [4] also observed heavings of 4e7 in. at
depths of 400e500 ft below the mined face. This amount of heaving will create suf-
ficient improvement in permeability for a substantial gas influx. In general, larger long-
wall panels create larger gas emission space. By far, the largest amount of gas
encountered in longwall gobs originates from coal seams overlying the mined area.
Fig. 16.1 also shows various estimates of the amount of gas emitted by underlying
and overlying gas-bearing strata. Gunther [2] assumes that overlying seams lose all gas
to the gob, whereas the loss of gas from underlying seams decreases linearly with dis-
tance from the mined seam. Lidin [3] assumes linear relationship between the amount
of gas emitted and distance for both overlying and underlying strata. Other authors
show exponential and power law relationships between the percentage of total gas
emitted and the distance between the gas-bearing zones and mine workings. In general,
the percentage of gas lost by the gas-bearing zones reduces with increasing distance
from mine workings. The rate of methane flow in the gob is also proportional to the
rate of face advance. Hence, methane emission from longwall gob is much higher
today than it was in past.