Design of a Remote Methane Detector (RMD) for the Coal Industry

Currently, the coal industry has no way of measuring the overall emissions from open-cut mines. At best, they rely on spot measurements and a general knowledge of:

• The gas content of the mined coal
• The gas content of unmined coal and discarded low grade coal
• The mining methods
• The time history of mining operations in the area.

There are instruments based on measuring attenuation of tuned lasers or infrared beams, which is proportional to the concentration of the target gas. However they rely on reflecting the beam off a target and hence give an integrated measurement of concentration along the path with no range resolution. These instruments are useful for measuring methane in confined spaces (explosion risk) or detecting leaks in pipes, but cannot be used to define a plume in free space.

Methane plumes from coalmines are relatively dilute, with expected concentrations of only 20% above the background concentration of about 1.2 ppmv. The aim, therefore, is to design an instrument that can reliably measure concentrations of methane in a plume that is of order 0.2 ppmv above background levels, with a spatial resolution of order 100-300 metres, at a range of between 1 and 10 kilometres (the scale length of a typical mine site), along an open path. The concept is to make a number of linear measurements of range-resolved concentrations through a plume to determine the total flux from a mine site, and in addition, provide the capability for deriving the distribution of individual fluxes from complex of sources in a typical mine area. It is envisaged that a survey of a coal basin would need to be repeated on at least a quarterly basis to provide meaningful time resolution.

The proposed instrument is based on the Differential Absorption Lidar (DIAL) concept. In broad terms, the instrument will send intense laser pulses through the target area, and capture light scattered back from aerosol particles in the air. The laser pulses will be at two wavelengths, one with no absorption and one that is absorbed strongly by methane. The differences between the intensities of the return pulses at the two wavelengths will provide measures of the concentration of methane in the atmosphere. The time between sending the outwards pulse and receiving the return pulse provides a measure of the path distance. Each set of measurements along a single line will provide scores of sample points, providing a leap in accuracy compared to current practice, which at best utilises just one or two measurement points.

Rain and dust should not be a major problem unless the visibility is reduced significantly in the infrared region. It is worth noting that the DIAL can work equally well during day and night periods, thus diurnal variations in the release of CH₄ could be measured. The performance of a DIAL system under large thermal gradients is unknown at this stage, but would be a subject for future study.

The proposed technique for measuring flux crossing a plane is to measure range-resolved concentrations of gas along several lines across a horizontal plane on a still day, or a vertical plane on a windy day. However, the useful number of lines will be constrained by the rate at which the meteorological conditions change.

In order to turn range-resolved concentration measurements into flux measurements we plan to run the Gaussian modelling programmes ISC3 or AUSPLUME to back-calculate the emission rates that would be required to produce the measured methane concentrations. ISC3 allows input of pollutant retention in mining-pits, but is usually applied to particulate matter; whereas AUSPLUME applies to gases, but has no facility to input mine-pit retention.