Page 110 - Advances In Productive, Safe, and Responsible Coal Mining

94 Advances in Productive, Safe, and Responsible Coal Mining

in place a maintenance program that ensures that all water sprays are functioning prop-
erly and that clogged or damaged sprays are replaced immediately.

6.7.2 Prevention of gob gas explosions
Fig. 6.5 shows a frame from an illustration video published by MSHA [14] to visualize
the gob gas accumulations that may have led to the UBB explosion. In MSHA’s UBB
investigation report [12], it is suggested that an explosive gas zone (EGZ), shown in
green in the figure, migrated from the gob area into the active longwall face area
where it was ignited, likely by the shearer cutting into sandstone roof in the
tailgate area.
This explosion and numerous other incidents, including fires and gas ignitions at
the Willow Creek (1998 and 2000) and the Buchanan (2005 and 2007) mines, as dis-
cussed in detail by the author [4], demonstrate that EGZs can be present in longwall
gobs and that they can cause fatal mine explosions and fires. The safety hazards posed
by EGZs can be characterized as follows:
If the EGZ lies closely behind the longwall face, flames can penetrate shield supports and
l
reach the active face area creating blast trauma and burn hazards.
l EGZs can be pushed around inside the gob by roof collapses and cave-ins. If they get pushed
out into the face area, sudden methane inundations of the face area can result.
l Fresh air flowing into the gob can create an explosion hazard as the gas composition in the
gob moves from fuel-rich inert to explosive, thereby creating an EGZ.
l Ignition sources inside the gob can be frictional ignitions or spontaneous combustion.
The best method to prevent the formation of EGZs in longwall gobs is progressive
sealing of the gob inby the face, along with injecting nitrogen from the headgate side
through headgate seals in combination with a back return ventilation arrangement on
the tailgate side. Marts et al. [8] demonstrated from CFD modeling calculations that
the injection of nitrogen forms an inert barrier between the methane-rich atmosphere
deep inside the gob and the face area. This effect is shown in Fig. 6.6.
Color coding is again based on Coward’s triangle. Both gobs are progressively
sealed. Gob A shows the typical formation of an explosive fringe zone between the fresh
air region along and behind the face and inside of the gob. Gob B shows the effect of
nitrogen injection that forms a dynamic seal between the fresh air and the fuel-rich zone.
Methane injection points on the headgate side are also indicated in Fig. 6.4.

Green represents methane

Fig. 6.5 An EGZ in the longwall gob at the upper big branch mine [14].