Page 153 - Synthetic Fuels Handbook

FUELS FROM COAL 139

On the other hand, contour mining prevails in mountainous and hilly terrain taking its
name from the method in which the equipment follows the contours of the earth.
Auger mining is frequently employed in open-pit mines where the thickness of the overbur-
den at the high-wall section of the mine is too great for further economic mining. This, however,
should not detract from the overall concept and utility of auger mining as it is also applicable
to underground operations. As the coal is discharged from the auger spiral, it is collected for
transportation to the coal preparation plant or to the market. Additional auger lengths are added
as the cutting head of the auger penetrates further under the high wall into the coal. Penetration
continues until the cutting head drifts into the top or bottom, as determined by the cuttings
returned, into a previous hole, or until the maximum torque or the auger is reached.

5.3.2 Underground Mining

The second method for the recovery of coal is underground (or deep) mining. This is a method
in which the coal is extracted from a seam by means of a shaft mine enters earth by a vertical
opening from the surface and descends to the coal seam and there overlying strata are not
removed. In the mine, the coal is extracted from the seam by conventional mining, or by con-
tinuous mining, or by longwall mining, or by shortwall mining, or by room and pillar mining.
Conventional mining (also called cyclic mining) involves a sequence of operations in
the order (a) supporting the roof, (b) cutting, (c) drilling, (d) blasting, (e) coal removal,
and (f ) loading. After the roof above the seam has been made safe by timbering or by roof
bolting, one or more slots (a few inches wide and extending for several feet into the coal)
are cut along the length of the coal face by a large, mobile cutting machine. The cut, or
slot, provides a free face and facilitates the breaking up of the coal, which is usually blasted
from the seam by explosives. These explosives (permissible explosives) produce an almost
flame-free explosion and markedly reduce the amount of noxious fumes relative to the
more conventional explosives. The coal may then be transported by rubber-tired electric
vehicles (shuttle cars) or by chain (or belt) conveyor systems.
Continuous mining involved the use of a single machine (continuous miner) that breaks the
coal mechanically and loads it for transport. Roof support is then installed, ventilation is advanced,
and the coal face is ready for the next cycle. The method of secondary transportation is located
immediately behind the continuous miner and requires installation of mobile belt conveyors.
The longwall mining system involves the use of a mechanical self-advancing roof in
which large blocks of coal are completely extracted in a continuous operation. Hydraulic or
self-advancing jacks (chocks) support the roof at the immediate face as the coal is removed.
As the face advances, the strata are allowed to collapse behind the support units. Coal
recovery is near that attainable with the conventional or continuous systems as well as effi-
cient mining under extremely deep cover or overburden or when the roof is weak.
The shortwall mining system is a combination of the continuous mining and longwall
mining concepts and offers good recovery of the in-place coal with a marked decrease in
the costs for roof support.
Room and pillar mining is a means of developing a coal face and, at the same time, retaining
supports for the roof. Thus, by means of this technique, rooms are developed from large tunnels
driven into the solid coal with the intervening pillars of coal supporting the roof. The percent-
age of coal recovered from a seam depends on the number and size of protective pillars of coal
thought necessary to support the roof safely and of the percentage of pillar recovery.

5.3.3 Mine Safety and Environment Effects
Mining operations are hazardous and each year a number of coal miners lose their lives or
are seriously injured through the occurrence of roof falls, rock bursts, fires, and explosions.