Optical-fibre technology provides a means of fast, remote sensing without the requirement for electrical equipment in the mine, while spectroscopic detection in the near-infrared offers sensitive, accurate and selective detection of methane.
Despite this, there are currently no commercially available spectroscopic fibre optic methane detection systems available for use in underground coal mines, according to ACARP.
In the first stage of the ACARP project, the spectroscopic fibre optic sensing technique was developed using pre-filled open path gas cells with known concentrations of methane.
The measurement of average methane concentration using a number of gas cells connected via a single optical fibre was also investigated. The results showed that open path gas cells had sufficient accuracy, fast response time and excellent sensitivity and robustness to have the potential as the future underground mine gas sensing systems.
The second stage of the project involved the development and experimental analysis of three microstructure fibre optic sensor heads: tapered fibre, drilled single mode fibre and drilled hollow core fibre.
The results showed that the tapered fibre and the drilled single mode fibre were unable to detect a signal corresponding to methane due to limited interaction volume between the gas and the laser light.
The drilled hollow core fibre was able to detect methane with an excellent accuracy and a reasonable response time even though the methane gas was diffused into the core of the fibre through only one hole drilled in the hollow core fibre.
The presence of methane gas in mine environment poses some safety and health concerns and requires constant monitoring by means of an intrinsically safe, fast, reliable and accurate gas monitoring system.
In addition, recent carbon tax legislation requires that gases ventilated from mines be accurately measured.