Why is methane a problem?
Methane (CH4) is the third most abundant greenhouse gas in the atmosphere (after water vapour and carbon dioxide (CO2)). Any methane emitted today lasts only about 10 years in the atmosphere, but has a Global Warming Potential (GWP), according to the US Environmental Protection Agency, of 28-36 over 100 years
. In contrast, CO2 has a GWP of 1 but lasts thousands of years. GWP is the “measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2).” The higher the GWP, the more energy is absorbed, the less energy escapes into space, and thus the more the earth is warmed.
The UNFCCC (United Nations Climate Change) reports similar figures:
An added danger is that “Oxidation of methane is responsible for the majority of the ozone formation
in the troposphere”, and this ozone (O3) is, with methane, one of the most important gases involved in global warming.
Where does the methane in the atmosphere come from?
In 2012, the latest year for which global figures are available, human activity caused 60% of the estimated 570 million tonnes (Mt) of global methane emissions. Although agriculture accounts for the greatest share of anthropogenic sources (about 25%) the energy industries, coal, gas and oil
, and biofuels, account for almost the same amount.
Why are methane emissions from coal such a problem?
Over millions of years the pressure of layers of soil and rock converted buried plant material into coal (whose main constituent is 60 to 95% carbon) and methane. The deeper the coal seam, the higher the pressure, and the more tightly methane is bound within the coal
Methane is not the only gas found in coal - other gases present can include hydrogen, oxygen, and nitrogen. Mining operations can release these and the mining process can add to these, and can also result in the Presence of othr gases such as carbon dioxide; carbon monoxide; nitrogen and nitrogen dioxide; hydrogen sulfide – which, along with sulfur dioxide, gives the gas its smell of rotten eggs; and hydrogen. Many of these gases are poisonous or potentially explosive.
When a coal seam is mined, the pressure is reduced and methane is released to the atmosphere.
The World Coal Association (WCA) states on its website that underground mines account for up to 90% of all methane emissions from the coal sector
– and that tackling emissions is important in both meeting the challenge of climate change and ensuring the safety of mining operations.
In March 2020 the IEA estimated that coal-fired power plants accounted for about266 billion tonnes of CO2.
How is the coal methane problem being tackled?
The WCA signed a Memorandum of Understanding
with UNECE (United Nations Economic Commission for Europe) Methane to Markets Partnership in January 2017 stating that, since coal would continue to be part of the global energy mix for the foreseeable future, cooperation was needed to support technology that reduces emissions.
UNECE has also adopted The 1999 Gothenburg Protocol
to Abate Acidification, Eutrophication and Ground-level Ozone – “Aware that methane and carbon monoxide emitted by human activities contribute, in the presence of nitrogen oxides and volatile organic compounds, to the formation of tropospheric ozone.”
The GMI website has extensive links to tools and resources concerning the mitigation of coal mine methane (CMM), such as the Coal Mine Methane Technology Database
detailing technologies utilizing methane from coal mining activities, and other projects around the world.
The coal sub-committee of the GMI
exists to foster “international collaboration to advance methane capture and use projects that bring more gas to market” and also “facilitates investment and financing opportunities and other cooperative activities.”
The United States Environmental Protection Agency (EPA) has a Coalbed methane outreach program – Active Underground mines
– giving information about coal mine methane recovery from abandoned as well as active mines.
However in 2018, under the Trump administration, the EPA coal rules were revised
reducing Federal pollution controls on coal-burning power stations.
Other international initiatives include The Climate and Clean Air Coalition (CCAC)
which “is a voluntary partnership of governments, intergovernmental organizations, businesses, scientific institutions and civil society organizations committed to protecting the climate and improving air quality through actions to reduce short-lived climate pollutants.” In their 2017-2018 annual report the CCAC Scientific Advisory Panel states that emissions from coal mines can be reduced by over 50%. They also report that they have financed Mongolia to assist domestic users replacing inefficient solid fuel (wood and coal) heating systems with energy efficient technology.
What technologies are available for recovering methane from coal?
These are also known as gas drainage methods.
Coal Bed Methane (CBM)
- is recovered from unmined coal seams – wells are drilled into the coal seam and then ground water is pumped out from the formation. This reduces the pressure holding the methane within the coal seam, and natural gas can flow to the surface. The obtained gas is sent to a compressor station and can then be sold and injected into natural gas pipelines
Coal Mine Methane (CMM)
– is recovered during mining activities while the coal is being extracted, thus releasing the gas.
One method of CMM is Ventilation Air Methane (VAM
) , a technique which utilises ventilation systems to dilute methane with air to below the explosive range of 5-15% methane and remove it from working areas of a mine through ventilation shafts. The methane can then be captured and used.
Methane in the strata surrounding the works can also be recovered by drilling vertical gob wells
“directly into the coal seam’s surrounding strata” before mining activities start in a section of the mine. Once that section has been mined, the props and artificial roof are removed, the surrounding rock collapses, and any gob gas can be collected via the previously drilled gob wells.
Abandoned Mine Methane (AMM)
– is recovered from mines no longer being worked. Every country with coal resources also has abandoned mines. It is estimated that the UK has 900 such mines, 400 of which leak methane. The Shanxi Province in China has over 4,700 abandoned mines.
Methane can either be trapped in the remaining coal or can continue to leak through openings. Sealed abandoned mines
can afford an excellent opportunity for methane extraction, which can then be used as an energy source.
What are the advantage and disadvantages of pre-mining and post-mining drainage of methane?
Pre-drainage methods can generally produce higher quality gas but are costly, especially when achieved using vertical wells drilled into deep coal seams.
Post-drainage methods generally produce lower quality gas as methane becomes diluted by air, however these methods are cheaper.
International Energy Agency. Statistics and reports about coal.
Methane National Atmospheric Emissions Inventory. UK data only, from 1990 to 2016 - for agriculture and downstream energy.
Update of Estimated Methane Emissions from UK Abandoned Coal Mines Department of Energy and Climate Change 25th May 2011
This Energy insight was first published 7 June 2019. Links checked and statistics updated, where found, 30 April 2020.