Perseverance pays off

DESPITE having in place industry accepted best practice to prevent and manage frictional ignition (FI), the Oaky No1 longwall mine in central Queensland was, until recently, plagued by FIs. The last FI event took place in July 2003 and challenged virtually everything the industry knew about frictional ignition.
Perseverance pays off Perseverance pays off Perseverance pays off Perseverance pays off Perseverance pays off

CAR drum as used at Oaky No 1

Staff Reporter

Published in Australia's Longwalls

A year later and the Xstrata Coal-owned mine has in place an extraordinary array of approved systems, technologies and innovations that will undoubtedly redefine frictional ignition management practice in future. And, according to mine operations manager Simon Burnett, the process is far from over.

Finding solutions for the FI problem at Oaky 1 has been a long, iterative process that began with previous mine owners MIM, continued under new owners Xstrata Coal, and has involved many thousands of man-hours, a lot of lateral thinking and the input of the mine’s workforce.

After two earlier, minor FIs in development workings, deeper problems began to emerge in 2002 when the mine kicked off its new LW21 panel.

The mine had just moved to a thinner seam area and bought new longwall equipment consisting of a 3000 tonne per hour, 300m wide panline, 1040t DBT shields and an Eickhoff SL300 shearer equipped with 1550mm drums and cowls.

Three FIs occurred within the space of three months, the first on May 10, 2002, within the first 200m of retreat, as the tailgate drum was cutting roof into the tailgate roadway.

The investigation concluded that the shearer drum cowls created eddies leading to ventilation dead spots that allowed a flammable mixture of methane to accumulate. A stone band containing pyrites within the seam was thought to be the source of the ignitions.

A number of practices were then put in place including the removal of the tailgate cowl, the formation of a site-based Frictional Ignition Management Team (FIMT), and the reduction of the methane trip level from 1.25% to 1%.

Just six days later a second FI occurred when the maingate drum was cutting into the floor to correct a problem with the horizon.

The Mines Inspectorate immediately issued a directive for production to stop until the company could show it had taken steps to reduce the likelihood of a repeat incident.

Again, a host of management actions were introduced. Stone cutting was minimised; the minimum quantity of ventilation was raised to 30m3 per second; a water venturi was installed in the shearer; and the methane alarm level was further reduced to 0.75% and trip level to 0.9%.

The third FI occurred on July 4, 2002 as the shearer was cutting toward the maingate with the maingate drum cutting to the roof. The ignition occurred on the maingate drum and burned for around 30 seconds. At the time the shearer speed was restricted at 6-8 metres per minute due to the methane alarms that had been reasonably constant at around 0.69%.

Tighter controls were yet again introduced. Maximum shearer speed was reduced to 5m/min; methane trip level reduced to 0.7%; more venturi sprays were fitted and in October 2002 rotating air curtain (RAC) drums were fitted with no cowls.

Expert reports were also obtained from the CSIRO on face airflow patterns and Simtars provided a report into methane gas sources and values on LW21.

The controls following the LW21 frictional ignitions were numerous and also included changes to the configuration of the equipment, gas monitoring, pick back flushing sprays, minimum face ventilation requirements, FI procedures and TARP’s.

Another outcome was to introduce in-seam gas drainage, (being carried out by Valley Longwall) primarily to reduce methane around the cutter drums; typically the gas that is released most easily.

With all this in place, the mine then put forward a plan to increase the production rate by removing the restrictions. Risk assessments were backed up by an intense monitoring program with changes reviewed by FIMT. The Inspectorate lifted its directive in December 2002.

By the completion of LW21 in May 2003, the mine had successfully ramped up to a shearer speed of 7.5m/min.

The fourth ignition occurred in July 2003, after less than 100m retreat in longwall 22.

The shearer at chock 87 was cutting to the maingate with the maingate drum cutting to the roof. The shearer was fitted with RAC drums and the speed was 7.5m/min. Methane sensor 1 was reading 0.85% and methane sensor 2 (the one that trips the shearer) was reading 0.54%.

“That was really difficult because we had just done 2.5 million tonnes since the previous FI and pretty much everything was in place that the industry considered were adequate FI controls. It pushed the bounds of what everyone knew,” recalled consultant Greg Burgess.

The investigation concluded that mining conditions at the inbye end of the longwall panels have a higher risk of frictional ignition, which reduces as the panel retreats.

“The fourth FI resulted in the intense scrutiny of all the controls that were in place and the reassessment of many industry held beliefs by the FIMT.”

Investigations and risk assessments were conducted into face ventilation, face geology, pick trials and cutting cycles. Many external experts assisted with the risk assessments and an intense monitoring program was implemented to determine the gas environment throughout the panel and around the shearer during production.

This work had several innovative outcomes, one of which was the decision to reverse the rotation of the RAC drum. It was found that coal being crushed behind the shearer was introducing a lot of fines into the coal clearance system and liberating higher volumes of gas.

Once the drum rotation was implemented coal loading onto the AFC pans immediately improved, particle size increased and the amount of gas being liberated diminished. Other downstream spill-off benefits were less ‘slurry’ going to the coal preparation plant. The modified cutting method and manual horizon control, managed by the face operators, as well as the reverse drum rotation has contributed to a 7% overall improvement in yield for the year to date.

By far the most revolutionary change was the decision to reverse the ventilation flow through the RAC drum, introducing fresh air from the goaf to the face.

“From our testing we recognized we needed to here to read on.



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