Published in June 2008 Australian Longwall Magazine

The idea has already attracted immense interest from ventilation professionals, academics and mine personnel from underground coal mines in Australia, USA, China and Europe who have visited the New South Wales operation to examine the system.

The system has also received industry accolades, winning a highly commended award at the New South Wales Minerals Council OHS Innovations Awards this year.

While spontaneous combustion is a worry and an issue for all underground coal mines, it was a particularly prevalent one for Austar. The mine has a catastrophic past.

Formerly called Southland, the operation suffered a major spontaneous combustion incident in a sealed goaf on Christmas Eve, 2003, which resulted in the loss of a longwall and the sealing of the mine. The mine was picked up from the administrator by Yancoal Australia and renamed Austar.

The Austar team inherited a mine with a fire affected sealed area, residual heatings, combustible and noxious gas and oxygen ingress due to ventilation and barometric pressure differentials.

While the team was able to recover the mine it still faced the challenges of managing the fire affect sealed area with its gas and excessive oxygen levels and ensuring that mining operations could recommence without the risk of a spontaneous combustion event.

Like seals in any other coal mine, a flow path exists across the seals which allows them to “breathe in and out”. This is caused by pressure differentials across the seals due to mine ventilation pressures, diurnal and synoptic barometric variations. Alternately air will flow through the coal around the seals.

"The risk is that oxygen is able to enter the sealed off area, thus providing conditions suitable for spontaneous combustion, explosive mixture of gases and the potential for an explosion," Austar mine manager Sharif Burra said in the mine's entry into the OHS Innovation Awards.

The quantity of air flow through the seals is a relationship between the pressure across the seal and the resistance that it provides. Conventional mine ventilation strategy has been to reduce the flow by erecting seals with high resistances to try and keep the amount of oxygen entering the goaf as small as possible.

"The Austar Coal Mine team deduced that if there was no pressure difference between the mine and the goaf (across the seal), then it would be impossible for there to be any flow," Burra said.

A pressure source equal or slightly higher in magnitude to that caused by the mine induced and barometric pressure variations was required. "Although the solution was very simple it was a world first," Burra said.

The Austar team decided if a pressure source could be put into a ventilation chamber and maintained greater than that caused by mine ventilation and barometric pressure differential, then it would be impossible for flow to occur from the mine, through the chamber and then into the goaf.

An Air Liquide Australia 70m3/hr nitrogen plant was sourced which had the capacity to generate up to 7bar pressure at the plant discharge point and could be reticulated up to 8km from the unit.

The chamber seals were designed to promote the retention of positive pressure and provide a preferred leakage path into the goaf.

Nitrogen was then injected into this chamber under pressure such that a positive pressure was maintained within the chamber, and was maintained and checked with pressure and atmosphere monitoring within the chamber and the goaf. There was also a provision to inject directly into the goaf at all locations if required.

"The results of pressurizing the chambers with the nitrogen were immediate and so successful that it was recognised that a system could be developed where a chamber would be installed at all seal sites and pressurised during the mining process, thus eliminating the ingress of oxygen into the goaf," Burra said.

A higher capacity 1936m3/hr Air Liquide Floxal Unit was acquired to pressurise the chambers installed in the maingate behind the face of the adjacent longwall block and in the seals.

Not only has the positive pressure chamber system eliminated the reliance on explosion resistant seals as the primary control for an explosion, but effectively eliminated the risk of spontaneous combustion within a sealed goaf.

"An unanticipated but very significant benefit of the positive pressure chamber system has been the effective management of the active goaf during longwall recovery and seal up. Whilst Austar Coal Mine does not have large quantities of methane gas that would lead to an explosive atmosphere, a quick and effective means of sealing a completed longwall panel following equipment recovery is critical in reducing the risk of an accelerated oxidization / spontaneous combustion event. This also has the potential to eliminate explosive atmospheres at other mine sites during longwall seal-up," Burra said.

He said mines throughout the world could find similar benefits as Austar with the implementation of Positive Pressure Chambers.