Recovering Methane Emissions from Abandoned Coal Mines will protect our environment



What is Coal Mine Methane


Coal mine methane is a low-grade form of methane, which is the primary ingredient in natural gas. Coal mine methane is easily recovered and processed into clean burning natural gas and has the added benefit of limiting methane seepage into the atmosphere and damaging our environment.methane

“Methane is the primary constituent of natural gas. Thus, the collection and utilization of methane provides a valuable, clean-burning energy source that improves quality of life in local communities and can generate revenue and improve living standards. Producing energy from recovered methane can also replace higher-emitting energy resources such as wood, coal and oil…Capturing methane from coal mines can also improve safety conditions by reducing explosion hazards.” Source: US EPA

 

The United States Environmental Protection Agency describes Methane as the following:

      • A hydrocarbon that is a primary component of natural gas that can be used as energy.

      • A “greenhouse” gas that is 23 times as potent as carbon dioxide and a major contributor to global warming. Reducing emissions can lead to important energy, safety, economic and environmental benefits.

      • A gas emitted from a variety of both anthropogenic (human-influenced) and natural sources. Anthropogenic emission sources include agriculture, coal mines, landfills and natural gas and oil systems.


Abandoned coal mine methane

As mines mature and coal seams are mined out, mines are closed and eventually abandoned. The mining procedures of these mines can leave up to 60% of the coal in the ground. Often, mines may be sealed by filling shafts with gravel and capping them with a concrete seal. Vent pipes and bore holes may be plugged, as well.

As active mining stops, the mine’s gas production decreases, but the methane liberation does not stop completely. Following an initial decline, abandoned mines can liberate methane at a near-steady rate over an extended period of time. The gas migrates up through conduits, particularly if they have not been seal adequately. In addition, diffuse emissions can occur when methane migrates to the surface through cracks and fissures in the strata overlying the coal mine.

Methane is stored both as a free gas in coal’s pores and fractures, as well as on the coal surface through physical adsorption. As the partial pressure of methane in the fracture (cleat) system of the coal decreases, the methane desorbs and moves to free gas.

The rate of emissions is impacted most by:

  • The total gas (methane) content of the coal
  • Time since abandonment (decline curve)
  • Mine size
  • Flooding
  • Sealing

Active Vents provide a steady stream of methane

At some abandoned mines, vent pipes relieve buildup of pressure resulting from desorption and flow of methane into the mine void. These vents are installed to prevent methane from migrating into surrounding strata. An abandoned mine with an open (or “active”) vent will behave very much like a natural gas well. Methane emissions from vented mines are a function of the pressure differential between the vent and the gas in the coal bed.

Mine Seals often leak methane increasing green house gases

While many abandoned mines have open vents, some mines are sealed in an attempt to prevent escape of methane gas. Even during active mining, seals are placed in work-out areas of the mine to reduce fresh air ventilation requirements as a cost-saving measure. Old shafts are commonly plugged with cement.

It is common, however, for gas to leak out around these plugs or to make its way through fractures in the overlying strata. The seals are generally assumed to leak even at very low pressure differentials and they typically degrade over time. Although mine seals can impact the rate of flow, they are not considered to be effective at preventing atmospheric methane emissions over time.