Generation and Extraction of CMM in general
CMM is generated during extration of coal. The methane is a product from the coalification of the biological material started 300 million years ago. A part of this methane is still connected to the coal layers and the surrounding. This methane is a risk for mining, because of possible explosive gas-air-mitures. For safe mining it is necessary to capture the methane.
- pre degassing
- Boreholes underground
- Boreholes from surface
The pictures above show different kind of boreholes for extraction of CMM. The type of gas extraction vary depending on the coalfield and geological conditions.
Possibilities for the CMM-extraction of abandoned coal mines
- Using historical boreholes
- Drilling of new boreholes
- Using existing infrastructure
In abandoned mines methane is still set free. In many area the overburden is fraced and the gas escapes to the atmosphare. To improve the safety old connection between the underground tunnels and the surface are used to vent the gas. In most cases the underground areas of the abandoned mine are connected. So the gas from a big area can exhaust by one borehole. Today with the closure of the mines old pipes remain preserved in the shaft even when the shaft is filled. The pipes are used to vent the gas from the old mine. Only if there is no existing pipe or borehole a new one has to be drilled to capture the methane.
Many information are given in http://www.unece.org/publications/energy/welcome5.html