Presenting at this year's Coal Operators' Conference in Wollongong, New South Wales, Simtars principal scientific advisor Darren Brady spoke on the importance of recognising the shortcomings and advantages of each monitor and how to use the three techniques together to get the best monitoring results.
While there is no requirement under legislation to use all three methods, the Queensland coal industry has adopted the trio as standard, giving the state's mines arguably the best gas monitoring in the world.
Real-time systems are used for real-time warning and are essential for incidents such as belt fires. Tube bundle systems suit long-term trending that can spot the onset of spontaneous combustion and the gas chromatograph provides a complete analysis providing results for key gases in the assessment of spon com.
Brady said while real-time monitoring had its strengths in the immediacy of results, its shortcomings lay in its limited measuring ranges. While its range can detect the onset of a problem, if an incident such as a fire occurred which generated gas, it could quickly go out of the range of the monitor, rendering it useless to get a true indication of concentrations.
The real-time monitor also should not be used for long-term trending. It needs to be calibrated regularly, so is not suitable for inaccessible areas such as the goaf.
The tube bundle system measures carbon monoxide, carbon dioxide, methane and oxygen. Unlike real-time monitoring, tube bundles can be located in the goaf as only the surface analysers need to be accessed.
“Generally systems are set up to measure oxygen, carbon monoxide, carbon dioxide and methane,” Brady said.
“Given their ability to measure carbon monoxide down to 1ppm, the long-term stability of these analysers and the frequent sampling, this technique is best for long-term trending of carbon monoxide, and carbon monoxide make, to identify a spontaneous combustion event."
He said the technique was best for monitoring explosibility during a routine sealing operation and for the early onset of any spontaneous combustion event, adding "to get this improved stability and analytical capability, the immediate availability of the results is sacrificed".
In the case of an explosion often it appears the tube bundle monitoring is still working but Brady warned the area the tubes are sampling may not be the same due to damage.
“A good tube bundle system will include monitoring of the vacuum pressure in each of the tubes, so following an explosion this data can be used to determine whether a tube has been compromised or not," he said.
The Gas Chromatography (GC) system is the only one of the three techniques to be able to measure hydrogen, nitrogen, ethylene and methane.
“Determination of nitrogen is particularly important for determining oxygen deficiency in some spontaneous combustion indicating ratios," Brady said.
While the GC does give a complete analysis of all the gases, and only takes a few minutes to undertake the analysis, time delays are still prevalent with the time it takes to get the samples to the GC.
Despite this, in the case of a significant spontaneous combustion event, fire or following an explosion, it is the only technique that will allow us to make an accurate determination of the explosibility of the underground environment, Brady said.
It is also an ideal tool for assessing the safety of the underground atmosphere for re-entry by mines rescue teams following an emergency.
Brady concluded that each technique had strengths and weaknesses which must be known by those both operating them and using the generated results for interpretation.
“As beneficial as they are, it must be remembered that monitoring systems on their own are not going to provide a successful solution to gas monitoring,” Brady said.
“Success depends on systems, processes and training built around the hardware and the way these systems are used."
