What is Methane?
Chemical Compound: CH4
Methane, or methyl hydride, is a colourless, odourless gas which is lighter than air. In the atmosphere, the gas is transformed into water and carbon dioxide; it is also one of the most potent greenhouse gases.
Methane is naturally emitted from the decomposition of organic matter and digestive process of ruminant animals; other sources include fossil fuel extraction, landfill, industrial process, food production and mining. It is commonly referred to as marsh gas, as it occurs abundantly in wetlands.
Safety Hazards of Methane
Methane occurs naturally in the atmosphere; however, at these concentration levels, it has not been shown to have adverse effects on human life.
If levels rise, the risk also increases. Initially, individuals experience fatigue, dizziness and headaches, progressing to more severe symptoms of nausea, agitation and displaced speech. In high concentrations, methane deposes oxygen causing asphyxiation.
Currently, there are no specified occupational expose limits for methane gas. The National Institute for Occupational Safety and Health’s (NIOSH) in the United States recommends a maximum of 1000 ppm (0.1%) during an eight hour work period.
Exposure Standard Details
|Exposure level (ppm)||Effect or Symptom|
|1000||NIOSH 8-hours TLV*|
|50,000 to 150,000||Potentially explosive|
*TLV = Threshold Limit Value
Methane is a highly flammable and explosive gas, quickly ignited when exposed to heat, sparks or flames. When sufficient quantities of CH4 accumulate, 5 – 15% by volume, within a mixture of air, it forms a highly explosive gas. Above 15% (150,000 ppm) insufficient amounts of oxygen is present in the air, and the gas burns without explosive elements.
The hazard and associated risks of the effect of ignition and explosion of methane is one of the most severe natural hazards. The volatile nature is one of the reasons methane poses such a high risk in the mining industry.
Methane in Mining
Methane is recovered from mines due to two main reasons, safety and energy production. Coal reserves are available in almost every country worldwide; the most significant reserves are in the USA, Russia, China and India.
Coal Mine Methane (CMM) is a blend of air and methane released from rock strata as a direct result of mining activities. Methane trapped within a coal seam emerges in a mine after the removal of layers of a coal face. Additionally, the gas can seep out of vents from abandoned mines, or appear after a rock collapse from a post-mining area.
Due to its volatile nature, methane poses an extreme working hazard, requiring effective removal from the ventilation system. In some cases, further subtraction occurs through a series of wells, boreholes and pipelines, forming a degasification system (drainage system). Ventilation systems move the Methane contaminated air away from areas where personnel are working and into the shafts directed towards the surface. This air is captured and utilised in gas engines rather than pumped into the atmosphere due to its high greenhouses gas effect.
Monitoring CH4 Levels
Methane gas monitors feature a non-dispersive infrared (NDIR) sensor. Measurement is based on the physical property that CH4 molecules absorb infrared light or particular wavelengths. By shining light through the target gas and using suitable optical filters, the light detector will give an output that can be converted into a CH4 concentration value.
NIOSH states, methane monitors are required by law to be mounted on machinery and throughout a mine to alert personnel of any increase in the gases levels while extracting coal.
Routine monitoring of methane is required at the working face of mine, due to the higher propensity for ignition. Monitors are needed to provide alerts when levels exceed 1%. As levels can rise and fall rapidly, and the reaction times of personnel are critical, there is a warning at 1% rather than the dangerous level of 5% to ensure an adequate and timely response.
The concentration of methane can occur away from the working face, which is often monitored less frequently. To ensure workers are continually protected, portable gas monitors need to be carried, reducing the risk to personnel moving across the site. The DMIRS specifically outlines maximum atmospheric levels of methane in the code of practice:
9.29. Monitoring of toxic, asphyxiant and explosive gasesMines Safety and Inspection Regulations 1995
(3) The manager of an underground mine must ensure that, in any workplace in that mine, the atmosphere does not contain more than 12500 ppm, or 1.25% by volume, of methane.
The Aura-FX Methane Monitor measures CH4 levels; ensuring it remains within a safe range. The sensor emits an initial warning signal at 1.1%, with an alarm sounding when levels reach 2.2%.
Numerous gases associated with mining, tunnelling and underground construction are generalised into combustible, toxic and asphyxiate types. Because of the hazardous nature of these gases and the unique and restrictive structure of underground environments, these gases must be continuously monitored to mitigate risk.
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