Page 61 - Advanced Mine Ventilation

44 Advanced Mine Ventilation

4.1 The Modern Mine Layout

Underground coal mining today is done by both (a) room and pillar and (b) longwall
mining. Generally, shallow coal seams are mined by the room and pillar technique, but
deeper seams are mined by longwall methods. The latter method is gaining ground as
coal seams are getting deeper. In the United States, slightly more than 50% of coal is
mined by longwall faces. The trend in longwall mining is for larger panels, bigger
longwall equipment, and higher production capacity and productivity. Many longwall
panels today are 1000 to 1500 ft wide and 10,000 to 12,000 ft long containing 2.5 to 4
million tons of coal.
The main benefits are
1. Improved safety and reduced injury rates because of improved longwall/development coal
ratios and fewer longwall moves
2. Improved recovery of in situ coal
3. Improved productivity and reduced cost per ton
On the other hand, the main disadvantages are
1. Ventilation and methane control on the face and in the gob (mined out areas)
2. Respirable dust control
3. Ground control
4. Unknown geologic anomalies, such as faults, wash outs, sand channels, etc
5. Escape from longwall face in case of an emergency, such as fire
Figure 4.1 shows a typical layout of a modern coal mine. A set of mains are driven
from the ventilation shafts that are sunk first of all. Branching from the mains, sub-
mains are driven on either side. Typically from a submain (of fiveeseven headings),
two sets of gate roads, (three headings in 6e7 ft thick seams) and (four headings in
5 ft or less thick coal seams), are driven. A typical mine will have one longwall panel
and three continuous miner sections. One drives the submains, and the other two drive
the gate roads.
High emissions of methane can be encountered in development headings, at the
longwall face and in the longwall gob areas. Although the gas emitted in development
headings and the longwall face comes from the coal seam being mined, the longwall
gob acts as a pressure sink and draws gas from several coal seams overlying and un-
derlying the coal seam being mined. Beside the width of the longwall panel, the other
factor that has great influence on gas emissions in the gob is the rate of face advance.
Daily methane production from the gob is linearly proportional to the rate of face
advance. Hence, the gob emission/acre of mining must be determined for a given width
and the rate of advance of the longwall face for proper placement of gob wells on the
panel and estimation of bleeder air quantities. All mining sections, development or
longwall, produce gas. Some premining and postmining methane drainage is generally
needed because ventilation alone cannot meet the legal requirements. It will be dis-
cussed in detail in Section 4.3 of the chapter.
A mine like this can produce 5 to 7 million tons per year of clean coal. Very large
mines may have two such smaller mines in one producing 10 to 12 million tons per