About Coal Mine Methane
- What is coal mine methane (CMM)?
- Why is EPA concerned about CMM?
- How is methane emitted from coal mines?
- Do closed coal mines emit methane?
- How is CMM utilized?
- What is the difference between coalbed methane (CBM) and CMM?
What is coal mine methane (CMM)?
CMM refers to the methane released from coal and the surrounding rock strata from mining activities. This methane in mines poses a safety risk due to its explosiveness when mixed with air. Methane is also a greenhouse gas that is more than 25 times more potent than carbon dioxide on a mass basis over a 100-year time period (learn more about methane here).
Methane gas in coal formations was generated at the same time the coal was formed, when plant debris (such as that found in swamps) slowly changed into coal after being buried and covered for a long time through a process known as coalification.
CMM is released by different types of mines:
- Active underground mines, which release methane through degasification systems (drainage system methane) and ventilation systems (ventilation air methane or VAM).
- Abandoned or closed mines release abandoned mine methane (AMM) from diffuse vents, ventilation pipes, boreholes, or fissures in the ground.
- Surface mines emit less methane than underground mines, but because surface mines produce large volumes of coal, some surface mines can also emit methane in large quantities.
Underground coal mines account for the vast majority of methane emissions from coal mining in the United States and globally. Most methane from coal mining is released in the form of diluted VAM. Less significant sources of CMM come from surface mines and post-mining activities, such as storage and transportation.
Technology is readily available to recover and use CMM (learn about uses of CMM).
Why is EPA concerned about CMM?
Methane (CH4) emissions from coal mining and abandoned coal mines accounted for about 8% of total U.S. methane emissions in 2019. It was the fifth-largest methane-emitting sector, based on the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2019.
Methane is the second most important greenhouse gas after carbon dioxide. In fact, methane is 28–36 times more potent than carbon dioxide on a mass basis over a 100-year time period. For more information about methane »
CMM emissions represent a wasted potential source of energy (when not captured) and a safety hazard. The recovery and use of CMM emissions have benefits for local and global environments.
How is methane emitted from coal mines?
There are three primary ways methane is released because of mining activities:
- Degasification systems at active underground mines. Also commonly referred to as drainage systems, these systems employ vertical and/or horizontal wells to recover methane before (pre-mine drainage) or during (post-mine drainage) mining activities to help the ventilation system keep in-mine methane concentrations sufficiently low (i.e., well below the explosive limit) to protect miners.
- Ventilation systems release VAM. VAM refers to the very dilute methane that is released from underground mine ventilation shafts.
- Closed or abandoned mines emit abandoned mine methane (AMM) from diffuse vents, ventilation pipes, boreholes, or fissures in the ground.
Other minor sources of methane from coal mines include surface mines and post-mining activities, as coal continues to emit methane as it is stored in piles and transported.
Do closed coal mines emit methane?
Yes! When coal mines are no longer operated to produce coal, they are known as closed (or "abandoned") mines. Even though active mining no longer occurs, these abandoned mines can still produce significant methane emissions from diffuse vents, fissures, or boreholes. This methane can be deliberately extracted and used to generate power or for other end uses. There are several thousand abandoned coal mines in the United States. Of these, EPA has identified some 400 abandoned mines that are considered "gassy" and has developed profiles of successful projects at abandoned mines and mines that might be good candidates for project development. See opportunities for project development at abandoned mines (Abandoned Coal Mine Methane Opportunities Database [July 2017]).
How is CMM utilized?
Before CMM can be used, it must be captured by drainage and/or ventilation systems and brought to the surface. Project developers review available data on the coal mine such as drainage efficiency, gas quality and quantity, potential markets, and other considerations about potential projects. Specific gas end uses depend on the gas quality, the methane concentration, and the presence of other contaminants. In the United States, most recovered CMM is sold to natural gas pipeline systems. Worldwide, CMM is often used for:
- Power generation
- District heating
- Boiler fuel
- Sales to natural gas pipeline systems
CMM can also be used for coal drying, a heat source for mine ventilation air, vehicle fuel (as compressed or liquefied natural gas), manufacturing feedstock, or a fuel source for fuel cells. In 2019, U.S. coal mines recovered and destroyed or utilized more than 37 billion cubic feet of CMM. Projects in the United States utilize drainage system methane, VAM, and AMM.
What is the difference between coalbed methane (CBM) and CMM?
CBM refers to methane that is found in and extracted from coal seams. It is formed during the coalification process, which is the transformation of organic plant material into coal. CBM is also known as virgin coal seam methane or coal seam gas, and is widely considered an "unconventional" source of natural gas. In the United States, CBM is a valuable resource that accounts for about 5% of total U.S. natural gas production annually.
CMM refers to methane released from the coal and the surrounding rock strata due to mining activities. In underground mines, since CMM can create an explosive hazard to mine workers, it is removed through ventilation and degasification systems. In abandoned mines and surface mines, methane can also escape to the atmosphere through natural fissures or other diffuse sources. Although CMM is similar to CBM in that it is a subset of the methane found in coal seams, it refers specifically to the methane found within mining areas (e.g., within a mining plan), while CBM refers to methane in coal seams that will never be mined. Because CMM would be released through mining activities, the recovery and use of CMM are considered emissions avoidance.