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:
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.
