The excavation of coal as part of the mining process causes gas to be released which passes into the mine ventilation air and is exhausted through the main fans on the surface. In some mines however, gas is captured by a gas drainage system either prior to mining (pre-drainage) or after mining (post drainage), with captured gas usually being piped to surface. Note that mines with gas drainage systems do not capture all the gas, and typically around 50% is still exhausted in the mine ventilation.

The majority of mines vent the captured gas to atmosphere, in some cases after flaring, although a few mines have, since the 1970's utilized the captured gas to generate power for both mine and community consumption. There is now a growing move to utilize gas from mines, both because of a desire to reduce atmospheric pollution and because it is a wasted resource.

In some cases the captured gas (at least a large proportion of it) is carbon dioxide and there is no means of utilizing this gas – it has limited commercial use, such as in soft drinks and fire extinguishers, which would be only a small fraction of the gas produced by mines, and it is easier to extract it from normal air.

Many mines however produce methane which can be burnt to utilise the heat either directly or, more commonly, to generate electricity. This process actually converts the methane to carbon dioxide and water so actually increases the carbon dioxide output, but the effect of methane on the atmosphere is far worse than carbon dioxide, by a factor of around 8.

It would be possible to burn the methane in a boiler to produce hot water for whatever purpose, but there are no examples of this in Australia. Two methods are currently used to generate electricity in Australian mines at present:

  • Burning the methane in a gas turbine which drives a generator.

  • Burning the methane in a "gas engine", a diesel generator set designed to run on methane gas as a fuel instead of diesel.

Both methods have proven successful although gas engines can be designed to run on very lean mixes and so are suited to mines which produce a mix of gases. Either option is suited to very rich mixes.

Another advantage of gas engines is that they are typically smaller capacity and are suited to building up capacity of a "power station" over time by the addition of other units, thereby allowing expenditure to be spread and for spare capacity to be provided to facilitate maintenance.

In some cases it is beneficial to allow a connection to an alternative gas supply, if available, to allow generation to continue in the event the gas supply from the mine becomes too lean. It would not be normal to continue generating if the gas supply ceases as the main purpose of such a plant is to utilize mine gas; it would probably be uneconomic to run on alternative supplies.

In most cases the power produced is utilized in the mine, feeding in with the external power supply. In the event that the power produced is greater than the mine usage, the excess if fed into the grid. Arrangements are made between the mine and the external supplier regarding payment for power exported. In some cases the gas utilization plant may be owned and operated by a company other than the mine on a toll basis.

In general, any utilization system should be designed to operate within the capacity of the gas capture system - any decision to change mining operations for the benefit of the utilization system would have to be based on financial or social considerations.

A process which is growing rapidly at present is referred to as "coal bed methane recovery". This is a process of draining methane from coal using surface boreholes, usually drilled at an angle other than vertical and curved to eventually follow a coal seam for some distance. This process is primarily aimed at recovering methane for commercial use in its own right and as such is not strictly part of coal mining. It is possible however, and in some cases is occurring, for a coal mining operation to carry out the same process primarily to remove the gas to assist later coal mining operations. The practicality of this process depends on the availability of surface drill sites and there being sufficiently long lead time available to drill and drain the gas before the coal is to be mined. Whatever the primary reason for the methane recovery, it would be utilized either as described above or would be fed into a gas supply grid. Gas mixtures with a high carbon dioxide content would not be suitable for the latter.

ACARP reports – various reports on research work related to use & collection of gases, in "Mine Site Greenhouse Gas section"

COAL SEAM METHANE - Sigra – a commercial site for work carried out by Sigra, but briefly describes equipment and contacts.