Page 290 - Advanced Mine Ventilation

Postmining Degasification of 16

Coal Mines

Chapter Outline

16.1 The Gas Emission Space 268
16.2 European Gob Degasification Methods 271
16.2.1 Packed Cavity Method and Its Variants 271
16.2.2 Cross-Measure Borehole Method 272
16.2.3 The Superjacent (or Hirschback) Method 274
16.3 US Gob Degasification Method 275
16.3.1 Construction of a Vertical Gob Well 275
16.3.2 Location of Gob Wells on the Longwall Panel 276
16.3.2.1 The Size of Gob Well and Gas Production Capacity 277
16.3.2.2 Distance of Gob Wells From the Tailgate 277
16.3.3 Gob Well Spacing on the Longwall Face 278
16.4 Gas Capture Ratios by Vertical Gob Wells 279
16.5 Gob Well Production Decline 280
Problem 281
References 281

When coal is extracted either by the room and pillar technique or the longwall method,
it causes the overlying strata (containing several coal seams) to subside and the under-
lying strata to heave and release gas. The mine atmosphere acts as a pressure sink
drawing all gases to the mine workings. Thus, in the postmining phase, the gas emis-
sion reservoir is considerably expanded. The coal seam that is being mined does not
make significant contributions to the gob gas emissions. The postmining gas emissions
can overwhelm mine ventilation if it is not captured and controlled.
Depending on the number of gas-bearing zones in the gas emission space and their
gas contents, the total methane emission from longwall gobs could vary from a few
hundred to more than several thousand cubic feet of gas per ton of mined coal. Hence,
the ventilation of longwall faces demands a large quantity of air. This ventilation need
is further enhanced by high air losses specially on caving faces (with no stowing of the
gobs). The old system of longwall ventilation, the “U” pattern, where all the air was
brought down one gate road and exited through the other gate road, loses large quan-
tities of air through gobs giving a high methane concentration at the return end of the
face. The “Y” ventilation pattern where intake air is brought down the headgate and
return air leaves via two or more tailgate roads relieves the situation to some extent
but, on most longwalls, some kind of methane control is still needed. (Refer to Chapter
4 in the book.)

Advanced Mine Ventilation. https://doi.org/10.1016/B978-0-08-100457-9.00016-X
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