Page 106 - Advances In Productive, Safe, and Responsible Coal Mining

90 Advances in Productive, Safe, and Responsible Coal Mining

Bleeder exhaust
Tailgate

Face

Headgate
(Not to scale)
Oxygen in original atmosphere (%) 16 8 6 Fuel rich inert
20
Fresh
18
air
Explosive
14
12
10

4
2
0
6
4
2
0 Fuel lean inert
8 10 12 14 16 18 20
Methane (%)
Fig. 6.2 Computational fluid dynamics model image (top) showing the formation of an
explosive methane-air fringe (red) surrounding the methane-rich area in the center of the gob.
Color coding is based on Coward’s triangle of methane explosibility (bottom) with explosive
mixtures in red, fuel-rich mixtures in yellow, fresh air in blue, and inert atmospheres in green.
longwall and pose a fire and explosion hazard for the mine. An example of the for-
mation of such an explosive fringe is shown in Fig. 6.2.
A fuel-rich zone of methane-air forms in the center of the gob, surrounded by a
fringe of explosive methane-air mixtures along bleeder entries in both gate roads.
Since bleeder entries must be traveled regularly for inspections, mine examiners
may be exposed to explosion and fire hazards. Also, depending on ventilation pres-
sures, the location of explosive zones may vary within the gob and explosive mixtures
may form in close proximity to the active longwall face. This will be discussed further
in Section 6.7.2.

6.6.2 Progressively sealed gobs
Mine operators may petition MSHA to operate a progressively sealed gob instead of a
bleeder system. In conjunction with progressive sealing, injections of nitrogen can be
used to effectively control the explosion hazard resulting from methane accumulations
in the gob. Progressive sealing is especially important if the coal tends to spontane-
ously combust, which is discussed further in Section 6.7.3. Progressive sealing is a
ventilation technique successfully applied in European and Australian longwall coal
mines.
For progressively sealed gobs with multiple gate road entries, Fig. 6.3 shows loca-
tions of seals built to close off crosscuts between gate road entries on the headgate side