AMM - Emissions Analysis



Abandoned Mine Emissions are difficult to calculate due to lack of reliable data and the varying ages of abandoned mines and subsequent procedures for closing the mines. It has been suggested that AMM emissions could range up to 34 Bcf per year.

ABANDONED COAL MINE EMISSIONS


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.

CMM   CMM Pipeline Construction

FACTORS INFLUENCING AMM EMISSIONS


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 pressure differential between the cleat system and the open mine void provides the energy to move the methane into the mine. Driven by this pressure differential between the gas in the mine and atmospheric pressure, the methane will eventually flow through existing conduits and will be emitted to the atmosphere.
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


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. Mines with open vents are known to “breathe” with atmospheric changes. The mines emit methane during low atmospheric pressure and pull air in during high atmospheric pressure.

MINE SEALS


While many abandoned mines have open vents, some mines are sealed in an attempt to prevent unauthorized access or 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.


Facts and Figures



The Northern Appalachian Basin liberates over 3.25 Bcf/yr of Abandoned Coal Mine Emissions. There is about 1.0 Bcf/yr of avoided emissions through recovery efforts. The net emissions in this area remain about 2.28 Bcf/yr.

The Nelms 1 abandoned mine in Harrison County, Ohio is a sealed mine abandoned 06/10/77. Its active mine emissions were 1.9mmcfd.

The Nelms 2 abandoned mine in Harrison County, Ohio is a vented mine abandoned in 02/29/96. Its active mine emissions were 1.5mmcfd.

The Oak Park No 7 abandoned mine in Harrison County, Ohio is a sealed mine abandoned in 05/13/88. Its active mine emissions were 0.75mmcfd.

The Rose Valley No 6 abandoned mine in Harrison County, Ohio is uncategorized and abandoned in 08/28/80. Its active mine emissions were 0.5mmcfd.

The Vail abandoned mine in Harrison County, Ohio is a vented mine abandoned in 01/04/84. Its active mine emissions were 0.35mmcfd

The Cadiz Portal mine was not abandoned at the time of this EPA documentation and report.