The incident occurred at Anglo Coal’s Dartbrook operation 140km north-west of Newcastle in the Hunter Valley. In March, the mine moved longwall operations from the Wynn to Kayuga Seam to begin mining the first block.
The first panel proved to be the start of the problem. The longwall had experienced some gas issues which were initially overcome by localised ventilation. The longwall had to then mine through a pre-mined dyke, slowing the advancing longwall. This then culminated in a breakdown around the Christmas period. According to Dartbrook general manager Tim Hobson, “Put these things together and you can start to see some indicators”
In the days preceding the heating, three ventilation fans were stopped for statutory checks. Unusual gas trends were noted, including a sharp rise in carbon monoxide at the longwall tailgate and slightly elevated levels of hydrogen. The initial action was to move the MG Nitrogen injection point forward from 11 cut through to 9 cut through.
At 10am on April 1 the heating was confirmed when ethylene was found with high levels of hydrogen and carbon monoxide.
An evacuation was triggered at 10.15am, with all sections notified by phone, DAC or PED Trakka system. Measures were taken during the evacuation to monitor 8 cut through K100 in the tailgate by tube bundle, check the MG nitrogen injection line and shut off the power.
The Incident Management Team (IMT) – including the mine manager, technical manager, ventilation officer and local check inspector – was formed and the authorities were alerted. Dartbrook also bought in outside specialist assistance in the form of CSIRO’s Rao Balusu and University of Queensland’s David Cliff.
According to Hobson, several factors contributed to the heating. Horizon control difficulties meant a lot of coal was being left in the goaf. There was a prolonged period of slow retreat caused by soft floor and roof conditions, which allowed the coal to dry and oxidise in the goaf. “The slow rate of retreat was the final straw that broke the camel’s back,” Hobson said.
The heating was identified to be located within the vicinity of 7 cut through and 8 cut through on the tailgate side of the longwall panel.
Dartbrook opted for several measures to control the heating. Firstly, the MG Nitrogen injection point was moved forward, and the main mine fans were slowed from 510rpm to 410rpm to reduce face quantity from about 80 cubic metres per second to 60cu.m/s. The mine shield liquid nitrogen system was then mobilised within 36 hours and 2500-4000cu.m of nitrogen per hour were injected down a goaf drainage borehole adjacent to K100 8 cut through.
The mine was re-entered on April 4; a pre-shift inspection was carried out the following day and normal mining activities recommenced on April 5.
Dartbrook is currently carrying out on-going goaf atmosphere control with two Floxal units.