Underground » Ventilation, Gas Drainage and Monitoring
The spontaneous combustion of coal remains a hazard in underground coal mines. Previous research has indicated that alkanes larger than ethane, propane, volatile organics and aldehydes may be present when coal self-heats. The presence of these compounds in operating underground coal mines in Queensland and New South Wales has not previously been confirmed or quantified. The potential to use these compounds in new ratios and as indicators of the state of a coal heating is unknown, and is the subject of this project.
A survey was conducted of the goafs and roadways of four Queensland and two New South Wales mines, some of which were over 18 years old. Gas samples were collected in Tedlar and aluminium gas sampling bags. The samples were analysed by gas chromatography, gas chromatography/mass spectrometry and high performance liquid chromatography to determine the alkane, volatile organic and aldehyde components.
Modifying the standard configuration used by Queensland and New South Wales mines micro gas chromatographs, by incorporating a Silica 5CB column, allows the C4 to C8 hydrocarbons and BTEX components to be analysed. The analysis indicated that mines with a carbon dioxide seam gas were characterised by an absence of hydrocarbons larger than ethane and no detectable benzene, toluene, ethyl benzene and xylene (BTEX). Mines with methane or methane-ethane seam gas content were found to have hydrocarbons in the form of alkanes from propane to n-heptane existing in the mine goafs under certain conditions. In addition, cyclic alkanes including methylcyclopentane, cyclohexane and methylcyclohexane were also identified in the goafs. The concentration of the alkane components identified in the goafs ranges from 0.5ppm to less than 100ppm.
Benzene and toluene exist in a limited set of circumstances in some goafs.
The aldehydes identified in the goafs and working areas comprises mainly acetaldehyde, acetone, butyraldehyde, methyl ethyl ketone and in some instances propionaldehyde.
There is potential to use alkanes such as iso-butane, n-butane, iso-pentane and n-pentane to develop new alkane/ethane, alkane/iso-butane, alkane/n-butane, alkane/iso-pentane and alkane/n-pentane ratios to determine the state of coal oxidation in a longwall.
There is also potential to use n-hexane, n-heptane, toluene and benzene as indicators of the coal temperature as the coal progress from an oxidation state to self-heating state.