Methane (CH4) is a gas that forms naturally in the process of coal formation. It is also a potent greenhouse gas, with a global warming potential (GWP) over 20-30 times greater than CO2 over a 100 year period, despite the fact that it remains in the atmosphere for a shorter time (about 12 years vs. hundreds of years for CO2). When coal is mined, methane is released. In 2015, the U.S. EPA projected that 8% of of global anthropogenic methane emissions would come from coal mining. Steps to reduce methane emissions can have relatively near term effect. The Global Methane Initiative reports, "of all the short-lived climate forcers, methane has a large reduction potential and cost-effective mitigation technologies are available."
In addition to being a serious greenhouse gas, methane is highly combustible with serious implications for the safety of mine operations. Methane is highly explosive at concentrations of only 5 to 15%. You probably remember the Upper Big Branch mine disaster in West Virginia in 2010 that killed 29 people. This was a result of methane building up and exploding.
Coal Seam Methane
Methane is generated during the natural process of coalification (the transformation of plant material into coal) and is contained in the coal microstructure. Because natural gas is made up mostly of methane, coal bed methane can be seen as a useful "unconventional" source of natural gas. When concentration levels are high, methane recovered from coal mines can be fed into the existing gas pipeline network along with or in place of conventional natural gas. The gas can be be used for cooking and heating or for electricity generation with gas turbines and gas engine systems, among other things.
A range of technologies are used to recover methane from coal. They may be broken down into three categories.
- Coal Bed Methane: This is methane recovered from un-mined coal seams. It is done to reduce the risk of explosion when mining does take place or simply to use the methane as an energy source, whether the coal is extracted later or not. When the mine will remain unmined, the process is called Virgin Coal Bed Methane (VCBM).
- Coal Mine Methane: Methane recovered during mining activities as the coal is in the process of being extracted and thus emitting significant quantities of the gas. Methane recovery in this case is done to improve mine safety, to avoid emissions for environmental reasons and may be used as an energy source.
- Abandoned Mine Methane: Methane recovered from mines that have been abandoned following the completion of mining operations. In this case, the methane recovery is done for the energy value and to reduce atmospheric emissions, if significant amounts are occurring, of methane continuing to escape from the mine following the completion of mining activities. AMM is most effective when the mine has been sealed to trap the methane.
Underground mines account for the vast majority of global methane emissions from coal mines. Surface mines also emit, but less, because there is less pressure to trap methane in the coal. Methane emissions also occur during post-mining operations, including processing, storage, and transportation. Coal can continue to emit methane for months after mining, especially when it is crushed, sized, and dried. And, methane emissions from coal mines can continue after operations have ceased (Source: EPA).
How Much Methane is Emitted from Coal Mines?
According to the U.S. Environmental Protection Agency's Coalbed Methane Outreach Program's most recent assessment:
In 2012, U.S. coal mines emitted about 60 [million metric tons of carbon dioxide equivalent, MMTCO2e]. Between 1990 and 2012, U.S. emissions decreased by over 30%, in large part due to the coal mining industry's increased recovery and utilization of drained gas. Between 2005 and 2011, emissions and the amount of drained gas utilized both increased by around 10% due to increased production.
China leads the world in coal mine methane emissions with nearly 300 MMTCO2e in 2010 (over 20 billion cubic meters annually). Other leading emitters are the U.S., Ukraine, Australia, the Russian Federation, and India.
Methane Capture from Coal Mines
Methane is also the main component in natural gas, a valuable source of energy. Because of this, coal producers worldwide deploy technology to capture methane from coal mines. According to the EPA, there are 200 coal mine methane capture projects in 12 countries worldwide which will capture more than 16.8 billion cubic meters of methane annually, which is equivalent to over 239 MMTCO2e. In 2012 in the U.S., over 37 billion cubic feet of natural gas was recovered from coal mines. As a point of reference, the U.S. consumed approximately 27,000 billion cubic feet of natural gas in 2015, according to the EIA.