Gas Control (Underground Coal Mining)

Some mines do not have a gas problem of any type, the quantity of gas in the coal being so low that only small percentages (or even zero) are ever detected in the air. In such mines, other than continuously or regularly checking gas levels to ensure conditions have not changed, no special processes are required.
Note that this comment refers to naturally occurring gas coming from the seam or surrounding strata. The ventilation system still has to deal with introduced gases such as diesel exhaust, shotfiring fumes and blackdamp (an atmosphere deficient in oxygen).
There are two ways of controlling gas levels in the atmosphere within a mine:

• Provide sufficient air such that gas concentrations in the air are always below levels which would create safety issues (usually including a generous factor of safety).
  
• Capture the gas and contain it until removed to a location where it is not a danger to the mine (usually in pipe ranges or in boreholes).
  

Operationally, the first option is simplest as it does not involve any additional mining procedure. Practically and financially there are limits however, as an increase in the quantity of diluting air requires an increased fan performance and/or additional roadways to reduce resistance. Both of these come at a cost. Additional roadways may also require additional mining units to maintain continuity of production (with longwalls particularly) and this may then require additional ventilation of itself, compounding the problem.
There are 4 ways of capturing/ containing gas to keep it out of airways:

• Drilling boreholes into the coal seam (possibly also in adjacent seams) prior to mining and connecting these to a pipe range to remove any gas to a place where it can be dealt with, most commonly the surface. Usually a vacuum is applied to the pipe range to enhance gas capture. This process is known as "pre-drainage".
  
• Drilling boreholes in roof or floor strata to capture gas released from the strata and flowing through it towards mine airways. Such holes would normally be attached to a pipe range as for pre-drainage. This process is known as "post drainage" as it captures gas released by the mining process or "cross measure drainage" as the boreholes are drilled across the strata beds.
  
• Removing gas from live goaf areas, either using pipes installed through goaf seals and attached to a pipe range as above or by using boreholes drilled from surface down to strata just above the goaf. Vacuum may be applied to such boreholes to assist in gas removal. This process is known as "goaf drainage". The use of boreholes to surface may be limited by surface land use. It should also be noted that even though reference is made to goaf "seals", it is not possible to seal a live goaf as the face area must remain open until mining is completed and the equipment removed.
  
• Sealing off worked out areas of the mine, including standing goaf areas, to contain gas still being made within those workings or goaf areas. Whilst there is always likely to be some leakage, high quality seals may limit contamination of the mine ventilation. Flooding of worked out areas is another effective way of controlling gas liberation, however it may introduce other risks of inrush to future mine workings.
  

Note that these comments relate to controlling gas levels in the airways. Gas drainage for the purpose of eliminating outbursts may involve the same pre-drainage process as above but the target for gas removal is only the area in which development roadways are to be driven. It may also be the case that sufficient gas is pre-drained to avoid outbursts but the remaining gas can still cause problems if released into the atmosphere.