Underground » Detection and Prevention of Fires and Explosions
Spontaneous combustion has long been a problem in the mining industry. Suspected or actual cases of spontaneous combustion can cause production losses of the order of $1million per day. Research overseas and in Australia has shown that a range of volatile organic compounds are present from low temperature heating of coals. The development of a method for the early detection of the presence of these volatile organic compounds would allow the mines to identify the onset of a heating and monitor its progress. Remedial steps to control the situation could then be undertaken while there is sufficient latitude in the progression of the heating.
Another important component of this work sought to determine the volatile organic profile for Australian coals both at low and high temperatures recognising that the early detection of the presence of these compounds would allow mines to better identify the onset of a heating and monitor its progress.
STAGE 1 REPORT – PUBLISHED JULY 2004
Electronic noses are being used to detect a wide variety of problems from contaminated foods in the processing industries to occupational hygiene and environmental problems such as toxic chemicals in soils. A pilot study conducted at Simtars indicated that electronic noses could have the potential to detect the presence of volatile organic compounds from a low temperature heating.
Coal samples were obtained from both New South Wales and Queensland mines for small scale testing of the gases given off as the coals were heated. Gas samples were also collected from both Simtars' 16m3 large scale spontaneous combustion reactor and the 2m column at the University of Queensland. The samples were analysed using an electronic nose. Gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC) were used to identify some of the components present in the gases given off by the large scale spontaneous combustion coal sample.
A number of issues both from a practical mine situation and a scientific standpoint have been identified that need to be addressed before these devices can be used to detect a developing heating. Aluminised bags routinely used in the mining industry to collect gas samples for analysis of the permanent gases are not suitable for use with electronic nose devices as they have a "fingerprint" from the polymer lining that interferes with the coal heating fingerprint. Tedlar bags used for environmental gas sampling are also unsuitable due to their fingerprint. Gas samples can be collected in glass gas bulbs with aluminium seals and successfully analysed. However the aluminium foil used to seal the bulbs is a prohibited item in underground mines.
It was also found that the volatile organic fingerprint of the coal heating is extremely weak at temperatures below 100?C and easily compromised by contaminants in the sampling system. The electronic nose is however able to discriminate as to whether a heating is at an advanced stage or not. The instrument is also capable of providing detailed fingerprints of gases evolved during coal heating once an advanced pyrolysis stage is reached.
Before a useful application can be developed, a better understanding of the range of volatile compounds that are or are not present from low temperature coal heating is required. This would involve further investigation into the chemical species present in the off gases from Australian coals.
STAGE 2 REPORT – PUBLISHED DECEMBER 2007
The project identified a possible range of aromatic and branched chain hydrocarbons and oxygenated species that might be present in coal undergoing potential spontaneous combustion. Analytical techniques able to analyse for these compounds were also identified. A series of small, medium and large scale test were undertaken to confirm the presence of these compounds in high to low rank Australian coals.
It was found that there were three series of compounds that could be used as indicators of coal temperature for both Bowen Basin and Upper Hunter coals.
From the BTEX/trimethyl benzene based profile, toluene in combination with benzene and xylene would indicate a minimum coal temperature of 60ºC for a Bowen Basin coal. If either ethyl benzene or trimethyl benzene were present then the coal temperature would be expected to be at least 80-100ºC. In the case of an Upper Hunter coal, the presence of toluene and benzene would indicate a minimum coal temperature of 60ºC. If either xylene, ethyl benzene and or trimethyl benzene were present then the coal temperature would be expected to be at least 80-100ºC.
At ≥60ºC, octane and the higher alkanes are progressively observed in the Australian coals investigated.
The carbonyl profile indicates that while acetaldehyde and acrolein are found in both Bowen Basin and Upper Hunter coal below 60ºC, the Bowen Basin coals do not exhibit a more complex fingerprint until 80ºC when propionaldehyde is among the additional components identified. The complex fingerprint Upper Hunter coal exhibits at 60°C includes acetaldehyde, acrolein, propionaldehyde, formaldehyde and butyraldehyde. The presence of formaldehyde in the off-gases from Australian coals was found to be dependent on the coal rank.
It is also possible to derive a series of mine specific aromatic hydrocarbon, alkane and acetaldehyde based ratios for use in identifying the onset of a heating.
A methodology for sampling at a mine and manipulating the data to identify the initiation of a heating is proposed. However the current availability of suitable detection instruments and the associated lengthy analysis turnaround times mean future developments in instrumentation are needed before such analysis could be undertaken routinely to allow for timely intervention in the event that a potential heating is identified.