Methane sensors play a critical role in coal mining

Around 60 per cent of the coal seams currently being worked worldwide are too deep for open cast mining, so underground mining techniques need to be used. Miners therefore have to enter and work in confined spaces, which exposes them to the dangers posed by methane.

Mine gas is a natural product, produced during the geochemical conversion of organic substances to coal (during carbonisation). The resultant gas is present both as liberated gas, in fissures, and as absorbed gas on the inner surface of the coal. During mining operations the mine gas is released, causing a risk of fire and explosion. The main constituents of mine gas are oxygen (02), nitrogen (N2), carbon dioxide (CO2) and methane (CH4). If blasting operations are used in the mine, then carbon monoxide (CO) can also occur in large quantities. In addition, hydrogen sulphide (H2S) can be present. The concentration of CH4 depends on the quality and depth of the coal seam: in general, the higher the energy value of the coal and the deeper the coal bed, the higher the concentration of CH4.

The dangers of CH4 (historically called Firedamp) have been known for centuries. Records date back to 1812 in the United Kingdom, where, after an explosion in a mine in Newcastle upon Tyne that killed 92 people, Sir Humphrey Davy experimented with metal gauze to prevent a naked flame from igniting firedamp. This simple form of flame arrester initiated the development of the Davy lamp that enabled the concentration of CH4 to be determined by the height of a flame. In recent years, the traditional shielded lamps have been replaced with methane detectors based on infrared absorption sensors.

To make coal mines safer, ventilation air systems are now installed, which ventilate the mines at high flow rates, resulting in a Ventilated Air Methane (VAM) stream, with CH4 in concentrations of 0.2-1.2 per cent by volume. This air is then typically vented to the atmosphere.

However, VAM poses an environmental threat, as CH4 is 21 times more harmful to the atmosphere than CO2, so regulations limit the level of discharge. Edinburgh Sensors' accurate and stable infrared CH4 detection products enable mine operators to measure the CH4 concentration and comply with the regulations.

In more modern ventilated methane systems, the low concentrations of CH4 are used as a fuel to generate heat and electricity, and in these systems the input level of methane is measured in order to:

1. generate Carbon Credits (as a result of burning CH4)
2. allow additional fuel to be provided to the oxidizer if the CH4 level falls below the minimum operational level; and
3. ensure total CH4 emissions are within acceptable limits.

Edinburgh Sensors has actively commercialised infrared absorption gas detection technology over the last 30 years; this has resulted in a reputation for reliable, accurate, long-term stability and low-maintenance gas-sensing products that have been used extensively in VAM systems.

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