Page 210 - Advances In Productive, Safe, and Responsible Coal Mining

Coal mine methane: Control, utilization, and abatement 189

Fig. 10.4 Schematic of postdrainage system using (A) postdrainage crossmeasure along with
predrainage horizontal boreholes, and (B) postdrainage vertical gob wells (not to scale) [12].

may be drilled. The boreholes are connected to an in-mine vacuum piping system,
which transports released CMM out of the mine.
When a coal seam is fully extracted by longwall mining or after pillar extraction in
room-and-pillar mining, the immediate roof strata tend to fracture due to stress con-
centration and coal extraction resulting in caving of the overlying rock strata [13]. This
caving releases CMM into the gob area, which can be extracted through gob wells as
shown in Fig. 10.4B. Fractured roof strata in the gob area are a significant source of
CMM; and in deep, gassy mines, the ventilation system is unable to sufficiently dilute
CMM emitted from the gob area into mine workings. Actually, the CMM that orig-
inates and accumulates in the gob area above the mined-out panel is the main source of
methane emissions during longwall mining. In these situations, vertical wells are
drilled from the surface to drain CMM from the gob area. As shown in Fig. 10.5, these
wells are generally drilled to a point 2–15m (7–50ft) above the coal seam prior to
the mining. As the mining face advances under these wells, methane-charged coal
and strata around the well will fracture, which increases permeability of any gas-
bearing strata. Then, using a vacuum system, CMM emitted from the fractured strata
flows into these gob wells and to the surface. The rate of CMM emission in the gob
area mainly depends on the rate of advancement, geological conditions, panel size, gas