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 or thousands of years for CO2). When coal is mined, methane is released. According to the EPA, about 9% of global anthropogenic methane emissions comes from coal mining, most of which is purposefully vented to maintain safe mining conditions. Steps to reduce methane emissions can have relatively near term effect. The Global Methane Initiative reports, "of all the short-lived climate forces, 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 may 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 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:
U.S. coal mines emitted nearly four billion cubic meters or 61 million metric tons of carbon dioxide equivalent (MMTC02E) in 2015. Between 1990 and 2015, U.S. emissions decreased by 40 percent, in large part due to the coal mining industry's increased recovery and utilization of drained gas and decrease in ventilation air methane emissions.
By 2020, global methane emissions from coal mines are estimated to reach nearly 800 MMTCO2E, accounting for 9 percent of total global methane emissions. China leads the world in estimated coal mine methane (CMM) emissions with more than 420 MMTCO2E in 2020 (more than 27 billion cubic meters annually). Other leading global emitters are the United States, Russia, Australia, Ukraine, Kazakhstan, 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 more than 200 coal mine methane capture projects in 15 countries worldwide which will capture more than 4 billion cubic meters of methane annually, which is equivalent to over 60 MMTCO2e. In 2015 in the U.S., over 33 billion cubic feet of natural gas were recovered from coal mines. As a point of reference, the U.S. consumed approximately 27,500 billion cubic feet of natural gas in 2015, according to the EIA.