Handbook for Methane Control in Mining - AMMSA

82Figure 6–3.—Simplified illustration of a typical longwall panellayout in a U.S. coal mine. Ventilation controls are not shown.The second phase of undergroundmining involves complete or partialextraction of the coal pillars.Smaller pillars are extracted by continuousmining machines by splittingthem into even smaller pillars.Larger panels of coal (up to 1,000 ftby 10,000 ft or more) are extractedby the longwall method of mining.In either case, the mined coalproduces methane. In addition,extracting these pillars or longwallpanels causes the overlying strata tosubside 4 and the underlying strata toheave. The ventilated mine workingsconstitute a natural pressuresink, into which methane flowsfrom the entire disturbed area, orwhat is known as the gas emissionspace. Figure 6–4 shows the limitsof the gas emission space as suggestedby four different authors[Lidin 1961; Thakur 1981; Winter1975; Gunther and Bélin 1967].The gas emission space may extendto 270 ft below the coal seam beingmined and approximately 1,000 ftabove it.The gob methane emission ratemainly depends on the rate of longwalladvance, the geology, the sizeof the longwall panel, and the gascontent and thickness of any coalseams in the gas emission space.Figure 6–4.—Limits of the gas emission space.4 In the United States, the subsided region is called a gob.