Page 307 - Advanced Mine Ventilation

284 Advanced Mine Ventilation

Secondly, as mining progresses to deeper levels, the gas content of coal, reservoir
pressures, and the diffusivity coefficient appear to increase [1]. Gas outbursts are vio-
lent expulsion of gas (and some coal fines) from the working face. These energy
release phenomena can have the same catastrophic consequences as the sudden floor
emissions. Estimation of dangers and methods of preventing gas outbursts will be also
discussed in this chapter.

17.1 Floor Gas Emissions

So far, most of the degasification effort has been confined to the working coal seam
and coal seams overlying the working coal seam. Very little attention has been paid
to the degasification of coal seams and other gas-bearing strata underlying the working
coal seam. Most of the underground mining is at present done by longwall mining. As
longwall mining progresses, the roof immediately behind the roof supports caves and
the overlying strata subside. Even the underlying strata heave and open up channels
that feed gas to the longwall gob. The gas pressure in overlying and underlying coal
seams is much higher than the ambient air pressure in the mine. Thus, the longwall
face and gobs become pressure sinks into which gas flows from the entire disturbed
zone. Chapter 16 summarizes the studies on the dimensions of the gas emission space
created by longwall mining and the percentages of gas contents released by various
coal seams contained in the gas emission space as a function of its vertical distance
from the mined coal seam. The vertical extent of the gas emission space is highly
dependent on the width of the longwall panel. In general, the wider the longwall panel
is, the larger is the vertical dimensions of the gas emission space and consequently, the
higher is the specific gob methane emission (volume of gas produced/acre of gob). In
the Appalachian Basin, the problem was not so acute, until the longwall face widths
increased to 1000 ft and beyond [2]. The volume of floor gas emissions on the face
is seldom large, but it creates local methane concentrations that exceed the statutory
limit of 1%. Such areas are also hard to ventilate. When floor emissions occur, the
ventilation air to the affected area is increased using curtains. Most emissions are
short-lived, but there are cases where the longwall faces are idled for 3e4 weeks. Eco-
nomic consequences of such production stoppage on modern longwall faces are very
serious. Solutions to this problem can be summarized as follows:
1. Premining drainage of the gas from underlying coal seams/other gas-bearing strata.
2. Postmining drainage of the gas emission space underlying mineable coal seams.
3. Drilling short vertical boreholes on the longwall face and pumping grouting material to seal
the leakage path.

17.1.1 Premining Methane Drainage
The area to be mined by longwall mining must be drilled from surface to a depth of
270 ft below the mineable coal seam to confirm the presence of other coal seams/
gas-bearing strata. Coal cores are carefully collected and gas contents and, sometimes,