2017-10-23_GENERAL DOCUMENTS - C1980007

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approximately 8.5-13 feet thick. The longwall removes a three (3) foot cut of material with each pass as it travels along the 1,000' wide cut, and can complete approximately 20 passes per day. The current panels take over a year to complete before needing to completely dismantle and move the longwall. As the wall slowly advances, large hydraulic panels hold the ceiling up above the mining face. As the longwall completes a pass and progresses forward, the remaining roof material collapses behind the active face. As material collapses, the ground and rock above are usually fractured at a thickness approximately 10 times that of the area to be filled. This fracturing results in additional release of methane into the mine, especially considering that the stratum directly above a coal seam often contains a large amount of trapped gas. The collapsed material consisting of rock and coal residue is together referred to as `gob'. Along with the longwall, mine sections performing `development work' also produce coal from the mine as they prepare the roads and access routes to future longwall panels. Development work uses a continuous miner machine, and leaves large pillars behind for structural support between roadways. MCC's conveyor system removing coal from the mine has a capacity of 3,000 tph. The longwall and mine development operations are capable of meeting that rate at best operating conditions, but do not operate near that rate on a continuous basis, as the breaker plant has a 1,650 tph maximum operational rate. For the safety of the workers, the atmosphere in the mine must be carefully maintained such that methane and coal dust are not present in explosive concentrations. In all finished areas of the mine, inert `rock dust' is used to coat walls, floor, and ceiling, to provide a non-flammable barrier between active work areas/equipment and flammable coal dust. Rock dust is applied wet such that it forms an inert cake once dried. The rock dust at the MCC mine is pulverized limestone, and is stored aboveground in one silo (AIRS Pt 022), which has a pipe that feeds directly into the underground mine. To prevent the buildup of methane gas to explosive concentrations (5-15% methane in air is highly explosive), MSHA requires that methane within the mine is kept below 1%. This is accomplished using several methods: Methane Drainage Wells (MDWs) or Mine Ventilation Boreholes (MVBs) are drilled from the surface, through 600-1,200 feet of cover, into the strata directly above the coal seam in front of the longwall. The stratum directly above the coal seam contains high amounts of gas. As this strata is high porosity, low permeability, MVBs at this mine are not used to pre -drain the gas, but instead control gas levels after the gob collapses and the strata is fractured. After drilling to within 20 feet above the coal seam, casing is added to the well, and a `floating' slotted pipe is inserted. Portable pumps, powered originally by propane but fired by mine gas after startup, are attached to the well and release most of the removed mine gas uncontrolled. Some gas is controlled through combustion in the pump (see Picture #1 at the end of this report for an example system). Once the longwall passes under the well and the gob collapses, the floating pipe is able to drop with the gob and will continue to pull methane from that region of the gob. MCC reported that their MVBs can pull 600-1000 cfm of gas, which will vary in methane concentration. For most current E seam panels, MCC will drill three (3) wells at the start of a new panel, and then drill the remaining MVBs with an approximately 700 foot spacing down the longwall face. During the 2012 inspection, MCC estimated that a maximum of 18 MVBs might be in operation at a time in the future, but to date, they had never utilized near that amount. At the time of the 2015 inspection, MCC reported operating four (4) or five (5) pumps at a time during the inspection period. In the collapsed gob sections behind the longwall, MVBs remain active for a short period of time (may run for 6-10 weeks) before being sealed. This mine did not begin to utilize MVBs until 2001, after high levels of methane were encountered while mining the B seam. There is a large amount of gas trapped in the strata between the B and E seams (E is located above the B seam). Therefore, during B seam mining there are much higher amounts of gas released as the above strata collapses. Wells are monitored weekly by taking two gas velocity readings to determine gas volume removed. Similarly, weekly bag samples are taken and analyzed in an onsite Gas Chromatograph to determine the methane concentration in the released gas. 2015 Inspection 0510015-INSP-2015 Page 2 of 37