What is Coal Seam Methane?
Methane (CH4) is a gas formed as part of the process of coal formation – coalification. When coal is mined methane is released from the coal seam and the surrounding disturbed rock strata. Methane can also be released as a result of natural erosion or faulting.
The methane content in coal seams generally increases the deeper the seam, and also with age. As the depth of the coal seam increases, so does the pressure level. This in turn reduces the level of permeability, causing the methane to be much more tightly bound to the coal and surrounding rock strata. Underground mining can therefore produce substantially greater levels of methane than surface mining. In fact, underground mines account for the overwhelming majority (up to 90%) of all methane emissions from the coal sector.
|Depth Interval (metres)||Mean methane content (cubic metres per tonne of coal)|
Source: IEA CCC 2005
Methane is highly combustible – its release can have serious implications for the safety of mine operations. It is also a potent greenhouse gas (GHG) – 23 times more harmful than carbon dioxide (CO2).
Tackling methane emissions is therefore an important step in meeting the challenge of climate change and in ensuring the safety of mining operations. Methane can also act as a valuable source of energy- it is the principal constituent of natural gas - allowing countries to further diversify their energy supplies.
Why Recover Methane from Coal Seams?
There are three main drivers for the recovery of methane from coal seams.
Methane is released during the process of extracting coal in both surface and underground mining. The released methane then mixes with air, which becomes highly explosive if methane concentration levels reach 5-15%. Methane explosions are devastating, causing significant loss of life and damage to property. There is significant industry effort to prevent these accidents occurring.
The risk of explosion is a particular problem in underground mines, where providing a sufficient level of ventilation air is essential. Failure to provide enough air to dilute the methane below the 5-15% range can put miners at risk due to the threat of explosion. In surface mining, the released methane is heavily diluted by its immediate exposure to air and therefore the risk of explosion is minimal.
Methane explosions in underground mines cause a large number of deaths around the world each year. While these numbers are still too high, they have been falling year on year in recent times as awareness of the hazards of methane has increased and methane recovery technologies have been improved and deployed on a wider scale.
(2) Reducing Greenhouse Gas Emissions
The recovery of methane released during coal mining plays an important role in global efforts to reduce GHG emissions and mitigate climate change. Methane is one of five greenhouse gases covered by the Kyoto Protocol and this provides opportunities for countries to meet their obligations under Kyoto by undertaking projects that reduce methane emissions from coal mining activities. These projects can be developed domestically or in host countries under the Protocol’s Clean Development Mechanism (CDM) and Joint Implementation (JI) scheme.
The global warming potential (GWP) of methane is 23 times greater than that of CO2, which means that for a given volume of methane emitted, the resulting global warming effect will be 23 times stronger over one hundred years compared to the same volume of CO2. Methane remains in the atmosphere for a period of approximately 12 years after it has been emitted. In comparison, CO2 is estimated to have an atmospheric lifetime of 50-200 years.
The differences in the GWP and atmospheric lifetimes of methane and CO2 means that methane has a relatively large global warming effect over a short period of time, whereas CO2 has a relatively small global warming effect but over a much longer period of time.
Coal mining is an important anthropogenic source of methane emissions. Although agriculture accounts for by far the largest proportion of methane emissions from human activities, emissions from all coal mining related activities - extraction, transport and storage - accounted for around 8% of total global anthropogenic methane emissions in 2006.
Global Methane Emissions from Human Activities (2006)
Source: M2M 2006
(3) Energy Generation
Methane is a valuable energy resource; 70-90% of natural gas is methane (with the rest being made up of ethane, propane, butanes, pentanes and higher molecular weight hydrocarbons, elemental sulphur, and sometimes helium and nitrogen). Coal seam methane therefore provides a useful ‘unconventional’ source of natural gas.
Coal seam methane resources are distributed differently to the ‘conventional’ natural gas found in natural gas fields. This allows countries with restricted access to natural gas but plentiful coal supplies to utilise alternative sources of natural gas.
Coal seam methane of a high quality (typically with a concentration level of over 93%) can be fed into the existing gas pipeline network to supplement or replace conventional natural gas. The United States has been utilising vast quantities of coal seam methane in its natural gas supply since the early 1990s. This can be supplied directly to homes and businesses for use in cooking and heating. Coal seam methane can also be utilised to replace or supplement conventional natural gas in electricity generation systems such as gas turbines and gas engine systems. These systems are often deployed directly on mine sites to provide auxiliary power to the mines themselves.
How is Coal Seam Methane recovered?
A range of technologies are available to recover methane from coal and these can be broken down into three categories.
(1) Coal Bed Methane (CBM)
Methane recovered from un-mined coal seams. The coal seams may be mined in the future but this is largely dependent upon geological factors such as coal depth and quality.
(2) Coal Mine Methane (CMM)
Methane recovered during mining activities as the coal is in the process of being extracted and thus emitting significant quantities of the gas.
(3) Abandoned Mine Methane (AMM)
Methane recovered from mines that have been abandoned following the completion of mining operations. Significant amounts of methane may remain trapped in the mine or may continue to be emitted from openings.