Ventilation/Gas Control

The ability to provide adequate ventilation to a longwall panel can be a major factor in the success of an installation, especially in gassy conditions. Most gassy mines now use some degree of gas pre-drainage of the seam being worked which greatly reduces the gas to be dealt with during longwall extraction. However in many cases much of the gas make from a longwall panel comes from sources in the roof and/or floor and requires some form of post drainage of strata or goaf drainage, or otherwise must be handled by the mine ventilation network.
The tendency for longwall panels to become longer and wider and for machinery to become larger increases panel resistance making the ventilation task more challenging, particularly when development constraints from time or cost considerations lead to the need to minimize the amount of development which is acceptable. Generally, the thinner the seam, the more difficult the challenge becomes.
The provision of high ventilating pressures by the use of large surface fans and/or underground booster fans is becoming more frequent, but this entails high capital and operating costs and is not always an option in seams liable to spontaneous combustion. The fact that all longwalls have some airflow through goaves means that particular attention is required to methods and standards where spontaneous combustion is a risk. All longwalls, especially those which do not extract the full seam, leave some coal in the goaf as well as chain pillars each side, and this remaining coal in a poorly ventilated goaf can form ideal situations for spontaneous combustion to occur.
There are 3 basic ways of ventilating a longwall panel:

• "U Ventilation" where intake air comes in the maingate and returns in the tailgate; there is no connection to any airways behind the face line. If a longwall advancing face is ever used, this is the only means of ventilation available.
  
  

  U Ventilation

  
  
  
• "R Ventilation" in which the main ventilation circuit is as for U ventilation, but there is another connection (a "bleed") maintained to return airways behind the face.
  

  R Ventilation

• "Z or Y Ventilation" where both main and tailgates carry intake air and all the return air is carried to main returns behind the face. With this system a variable regulator on the maingate intake is often used to adjust the balance between face flow and maingate intake flow.
  
  

  Z or Y Ventilation

There are other possible variations, especially if more roadways are available at each end of the face, and there can be special cases where layouts are not quite normal, however the majority of faces use one of the above arrangements.
For U and R ventilation, the air flow across the face itself is from main to tail (in the opposite direction to the coal flow and sometimes referred to as "antitropal" ventilation). For Z ventilation the flow is from tail to main (or "homotropal"). If Uni-di cutting is used the air flow direction will govern the direction of cutting.
With regard to controlling gas concentrations in the airways (and face temperatures if these are a problem), the greater the air flow the better in general. However on a longwall face where there is invariably a lot of dust and fine coal, air velocities of more than 3-4 metres/second become very uncomfortable and this may limit the quantity of air which can reasonably be supplied. This is one reason that U ventilation may be unsuccessful.
With R ventilation, the gas make from the face and goaf is split in 2 directions, some diluted by the face air and some by air which passes the face and becomes a bleed to other main returns. With the Z system all the gas passes to the returns behind the face, but extra intake air is added to the flow from the face adjacent to the maingate.
There can be two problems with returning face air through the tailgate end of the face:

• There will be a tendency for air to sweep the goaf edge behind the supports and bring gas over the tailgate end of the face and the tailgate drive. To avoid gas problems in this area it is often necessary to erect a brattice wing from the goaf edge past the tail gate drive and allow air from the goaf to mix and be diluted further outbye. This entails erecting and maintaining brattice in often poor working conditions.
  
• Access to any part of the tailgate may be difficult (or impossible) while the face is operating because of dust or gas levels, so that any tailgate work can only be carried out during scheduled longwall outages or downtimes.
  

With an R or Z system, an airway can be maintained along the goaf edge behind the face to the first cut through so that the draw at the maingate end of the face is away from personnel and equipment. Maintaining this airway is important; if it closes too tightly for the R system the air flow balance would be lost resulting in increased face gas levels, and for the Z system face return air would have to pass outbye over the maingate work area. In this case brattice may be required to separate the return air from the equipment and personnel which as well as being inconvenient, may limit the face air flow. The use of an R or Z system may also limit the length of chain pillars which can be used to whatever distance a goaf airway can be maintained.
With the R and Z systems, local control of the ventilation is often carried out using pressure measurements at regulators rather than measuring flows, the pressure values for ideal conditions being determined by experience. At times reference is made to "holding gas in the goaf" by adjusting the pressure across it, so the gas does not contaminate the face area. In fact the process is actually holding the gas front in the goaf away from the face. Because the goaf is open ended any gas make must be removed somewhere and it is prevented from coming onto the face by being dragged towards some other location - it is not actually "held" in the goaf.
If it is not possible to control any gas make successfully by diluting it with ventilation, there are three possibilities, either as alternatives or in conjunction:

• The use of "sewer" airways where higher gas levels than that normally permitted by statutory limits are allowed and personnel access is prevented unless production is stopped and gas levels reduced.
  
• Cross-measure post drainage where boreholes are drilled through the strata above and/or below the seam and connected to a gas drainage range to capture some of the gas before it can reach the mine airways.
  
• Goaf drainage where gas is drawn from the goaf cavity either underground by pipes which are open to the goaf area and connected to a gas drainage range or direct to surface through boreholes drilled to (or close to) the goaf cavity from the surface.
  

For the latter two options, suction may be applied to the gas drainage range or surface boreholes to assist gas capture levels.
It may be possible to use the longwall direction of mining in relation to seam dip to assist in gas control; for a goaf where methane predominates, buoyancy effects of the gas will assist in keeping gas away from the face if it is worked down dip; if carbon dioxide predominates working up-dip will assist. It is more likely for this benefit to be obtained as a bonus as other factors are more likely to determine mining layouts and mining directions.