The greenhouse effect is the natural process of the sun warming the Earth’s surface. Greenhouse gases in the atmosphere (primarily carbon dioxide, methane, nitrous oxide, ozone, and water vapor) trap the sun’s heat and cause the global temperature to rise, otherwise known as global warming.
Methane poses a unique challenge in the coal industry as risks associated with it are not only a safety hazard, but a significant contributor of greenhouse gas (GHG) into the atmosphere. It is the second most abundant greenhouse gas following carbon dioxide, and 28-times more potent than carbon dioxide per molecule in trapping heat in the atmosphere.
In Australia fugitive methane emissions from coal mining and handling and decommissioned mines represented 5.3 per cent of Australia’s total net GHG emissions in 2017.
Australia’s resources sector is at the forefront of developing and deploying low emissions technologies, which are central to Australian and global efforts to address climate change.
Technology is also a key focus of the MCA climate action plan released earlier this year.
Critically, the industry will develop the minerals required for a low emissions future
Coal mine methane (CMM) refers to methane released from the coal and surrounding rock strata due to mining activities. In underground mines, it can create an explosive hazard to coal miners, so it is removed through ventilation systems. In some instances, it is necessary to supplement the ventilation with a degasification system consisting of a network of boreholes and gas pipelines. In abandoned mines and surface mines, methane might also escape to the atmosphere through natural fissures or other diffuse sources.
Coal mine methane is emitted from five sources:
- De-gasification systems at underground coal mines (also commonly referred to as drainage systems). These systems may employ vertical and/or horizontal wells to recover methane in advance of mining (known as “pre-mine drainage”) or after mining (called “gob” or “goaf” wells).
- Ventilation air from underground mines, which contains dilute concentrations of methane.
- Abandoned or closed mines, from which methane may seep out through vent holes or through fissures or cracks in the ground
- Surface mines, from which methane in the coal seams is directly exposed to the atmosphere.
- Fugitive emissions from post-mining operations, in which coal continues to emit methane as it is stored in piles and transported.
Methane is an odorless, tasteless, colorless, inflammable, lighter-than-air gas formed by the decomposition of organic and other carbonaceous materials. Methane’s low density (approximately 50% of the density of air) causes it to concentrate in the higher parts of underground mine environments. If ventilation is insufficient to properly mix the air with the mine air, methane levels between 5% and 15% can form an explosive mixture which, if ignited, can have catastrophic consequences. In this range, methane can be ignited easily, resulting in a violent explosion that may result in further explosions in the presence of combustible coal dust.
Drainage of methane from coal seams and from overlying formations is an effective technique to alleviate methane emission problems in coal mines. However, in order for methane drainage to be successful and produce the expected results, methane drainage systems must be designed according to specific geologic conditions, such as gassiness of the coal seam and overlying strata, specific gas emissions of the mine, and coal seam thickness and continuity.
What is the extent of the problem?
Gas drainage practices should be designed based on mine-specific conditions and needs using engineering control approaches. Currently, most gas drainage applications in individual underground coal mines are based on trial and error and by copying the practices used by other mines located in the same coal basin.
The gas emission zone liberates and accumulates significant amounts of coal mine methane as a by-product of active mining. In most active mines, coal mine methane is controlled by well bores, called gob gas vent holes. Despite the presence of these well bores, it is not possible to capture all of the methane generated within the gas emission zone. As a consequence, a large amount of gas migrates into the mine and is exhausted to the atmosphere with ventilation air. Eventually, part of the gas generated during mining stays in the emission zone, and with additional emissions, becomes abandoned mine methane after the panels are sealed or the mines are abandoned completely. Abandoned mines that are sealed properly may become gas reservoirs from which methane can be extracted and utilised by available technologies. Capturing and utilising coal mine and abandoned mine methane can alleviate the harmful effects of methane emissions to the atmosphere and can provide an energy resource through use of the recovered gas. Assessment of coal mine emissions and capturing methane also help to ensure the safety of coal mines.
Contact us today to discuss your gas & methane drainage requirements.
