When coal beds are formed through the compression and heating of organic materials over geologic time spans the generation of methane entrapped in the coal is an integral part of the process. As the coal beds are mined, this entrapped, or adsorbed, methane is released. The U.S. Environmental Protection Agency estimates that methane emissions from coal mining in the U.S. will amount to 3.7 to 6.5 terragrams (185-325 billion cubic feet) per year in the year 2000, which constitutes a very significant source of greenhouse gas emissions. Methane emissions from coal mining operations in other parts of the world are estimated to be several times this amount.
Aside from environmental considerations, methane emissions from coal mining are a very serious safety hazard. When methane is mixed with air to where it consists of between 5% and 15% of the mixture, it is highly explosive and generally responsible for the all too familiar explosions in underground coal mines all over the world.
Coal mining companies employ different methods to reduce the possibilities for these explosions. The principal method is to force very large quantities of ventilation air into the underground workings to keep the methane content at the coal mining face below the lower explosive limit of 5% methane, actually in practice, below 2% methane. A secondary method that has been developed over the last few years is to drill wells into the coal seam in advance of mining in order to extract much of the methane before it is released into the mine, thereby reducing the load on the ventilation system. At the same time, methane produced from these wells may be of a sufficiently high quality that it can be sold to natural gas pipeline companies if there is an existing pipeline near the mine. This not only reduces methane emissions to the atmosphere but converts the methane into a useful resource.
When the mining operations reach the area of these gas extraction wells, the mine roof collapses after mining into what is called the gob. Methane continues to be produced in this gob area, but is then typically mixed with air from the mine ventilation system. Gas extracted from from the wells, referred to as gob gas, may contain only 30% to 95% methane. Though the wells are still useful in reducing the release of dangerous methane into the mine itself, the gas they produce is no longer suitable for injection into a natural gas pipeline unless it can be enriched by removing the air. GST's Air-X Process is expected to present a viable means of enriching low BTU gob gas into pipeline quality gas.
In order to encourage the recovery and use of methane emissions from coal mines, the U.S. Environmental Protection Agency has formed the Coalbed Methane Outreach Program to disseminate technical and financial information about the use of coal mine gas and help to remove barriers to such use. (www.epa.gov/outreach/)