4 Gas Detection
Figure 1: Chromatogram for helium, hydrogen, oxygen and nitrogen separation on the first column channel. Experimental setup and conditions
The analysis described below uses a 490 Micro GC (Agilent Technologies, Inc.), which consists of a quad cabinet and is equipped with four independent column channels. Each column channel is a complete, miniaturised gas chromatography (GC) with electronic carrier gas control, micro-machined injector, narrow- bore analytical column, and micro-thermal conductivity detector (µTCD).
The first channel installed is equipped with a 10 meter CP- MolSieve 5Å column, running on argon as carrier gas, for the analysis of helium, hydrogen, oxygen, and nitrogen. Channels 2 and 3 are identical and, like the first channel, are equipped with a 10 meter CP-MolSieve 5Å column. However, these channels have the optional backflush functionality and run on helium carrier gas, for the analysis of methane and carbon monoxide. Ethane and ethylene are analysed on a fourth channel using a 10 meter a PoraPLOT U column. For full details of the analytical conditions for all channels, see the methodology application note, available online [2].
EZChrom Chromatography Data Software (Agilent Technologies, Inc.) is used for data acquisition, and EZGas Professional software (SIMTARS), specifically written for the mining industry, is used for calibration and result generating. The analysis results are exported to Segas Professional (SIMTARS) for additional combustibility calculations, combustion ratios and trend analysis.
Fast mine safety analysis in less than 100 seconds
The first column channel, equipped with a CP-Molsieve 5Å column, is used to analyse permanent gases, including helium, hydrogen, oxygen and nitrogen. Figure 1 shows a chromatogram where the compounds of interest are well separated.
The molecular sieve channel runs on argon as the carrier gas, which enables the determination of low concentrations of helium and hydrogen. When argon is used as a carrier gas compared to helium, all other compounds will have an increased detection limit by approximately a factor of 10. However, oxygen and nitrogen are present at percentage levels in the mine atmosphere, which allows the use of argon carrier gas for detection of these gases. Concentration results for hydrogen, oxygen, and nitrogen are used for combustibility calculations. Helium, naturally available in the atmosphere at low ppm concentrations, is analysed on this channel as well. On a molecular sieve column, helium and hydrogen elute close together. Analysis of helium prevents it from being incorrectly reported as hydrogen, which can result in the erroneous conclusion that spontaneous combustion is occurring. Helium may also be used as a tracer gas to determine gas movements in the underground mine. Channel two also includes a 10 meter MolSieve 5Å, this time with helium as the carrier gas. This channel is used for the analysis of methane and carbon monoxide. Figure 2 shows a chromatogram for two different samples, one containing a medium level for carbon monoxide (~200 ppm) and the other with a very low level of carbon monoxide. In this
chromatogram, excellent separation and analysis of methane and carbon monoxide is obtained in less than 100 seconds.
The typical limit of detection for
the µTCD is 1 ppm for early eluting components on a Wall Coated Open Tubular (WCOT) column and 10 ppm on Porous Layer Open Tubular (PLOT) and micro-packed column types. The CP-MolSieve 5Å column is a PLOT-type column; however when it comes to carbon monoxide at low levels, the exact concentration is of less importance than the trend. Even a slight increasing trend of the chromatogram’s base line at the carbon monoxide retention time is monitored for early indications of spontaneous combustion in the mine. This MolSieve 5Å channel is equipped with backflush functionality to ensure moisture, carbon dioxide, and the C2 hydrocarbons are backflushed to vent, to maintain the separation efficiency of the molecular sieve column. Moisture and carbon dioxide tend to adsorb quickly to the Molsieve 5Å stationary phase changing its chromatographic properties. This could result, over time, in retention shifts and loss of separation. For SIMTARS, the analysis of methane for explosion risk reasons and carbon monoxide for combustion identification are of high importance, especially when the Micro GC is taken into the field after a mine disaster. Therefore, this column channel is duplicated to the third position of the instrument to allow optimised operation for the analysis of each, and to have a backup column available at all times. When one column is reconditioned, the other column can still be used for analysis.
The fourth channel, equipped with a 10 meter PoraPLOT U column and helium as the carrier gas, is used to analyse carbon dioxide, ethane, and ethylene. Figure 3 shows an example for baseline separation of these three components. The right carbon dioxide level is of importance for the safety of the mine workers and rescue personnel. Moreover, the results for carbon dioxide and ethane are also used in the combustibility calculations by SIMTARS. As mentioned above, ethylene and hydrogen are formed when coal temperatures rise above 100°C, and are therefore used as an early warning for spontaneous combustion or a fire.
Excellent repeatability for quantity and retention time
Repeatability, reported as relative standard deviation, shows excellent results for both concentration and retention time. For full details, see the methodology application note, available online [2]. Typical values, based on quantity, are determined around 0.05% RSD for components that are present in the sample at percentage
Figure 2: Chromatogram for methane and carbon monoxide on the second column channel.
Figure 3: Chromatogram for carbon dioxide, ethane and ethylene on the fourth column channel.
levels and between 0.1 to 0.6% for ppm level components. Retention time repeatability, for all components of interest, is calculated at 0.015% or lower.
Conclusion
The 490 Micro GC is a powerful tool for accurate mine safety analysis, and it provides a complete, fast and on-site analysis of the mine gases collected from underground. Moreover, the system detects compounds that are not covered by the mine’s continuous monitoring system. Using an analysis system that determines mine environment samples in less than 100 seconds resulting in multiple results per hour for accurate trend analysis allows better informed decision-making for the prevention of mine disasters. In addition, such analysis provides rapid and reliable results to determine, after a mine disaster, the status of the underground environment before deciding to send in a rescue teams.
References
1. Mines Sector Health and Safety Strategy 2011 to 2013. Health and Safety Executive. March 2011. Available at:
http://www.hse.gov.uk/mining/strategy-2011-2013.pdf.
Accessed July 2012.
2. Brady D, van Loon R. Fast On-Site Mine Safety Analysis by the Agilent 490 Micro GC. Agilent Technologies Inc., application note 5991-0438EN, 2012. Available at:
http://www.chem.agilent.com/Library/applications/5991- 0438EN.pdf. Accessed July 2012.
IET September / October 2012
www.envirotech-online.com
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