An interesting subject! Some seams are more liable to SC than others, Low
Rank coal is more prone than higher rank coal, and other factors include,
weak roof, floor, thick coal, inferior coal and/or carbonaceous shale in
close contact with each other and/or the seam worked. Depth of working is
also a prime factor, since the strata is effected as also the air temp. -
deep workings = higher rate of oxidation.
Pillars present a problem, in as much that the area surrounding them does not
consolidate as much as areas that can crush tight. It is obviously important
that no combustible material is left within 'deads'. The sealing up of 'dead
areas' is likewise very important, since the least amount of draft will
assist in SC - this also applies to air doors, where if they are not properly
installed, strata leakage will cause heating. In fact, really, any coal left
in the mine is a risk: coal in the floor will break when it lifts, thus
allowing air to enter, and this applies indeed to roof and cheeks. Roadway
junctions are also a hazard, in that should air may be passing/meeting with
different temps. and pressures. Even the 'simple' practice of methane
drainage must be carried out with very great care, since too much drawing
will result in too much air going back in. Newly broken coal is more of a risk
than old coal believe it or not - due to the fact that it oxidises faster.
Areas that have stopped work temp. or otherwise are thus a risk due to
breathing, since the waste will not settle near the face and new coal is
standing.
It is easy to see amongst all this why coal tips can be a big problem
sometimes. There are extremely extensive u/g coal fires burning in old
workings in India, but there is simply not the money to put them all out, and
the process at the moment is very slow indeed. These fires have been burning
for more than forty years now, and have consumed enough coal to run the UK
for a considerable period - unfortunately India is much more interested in
spending it's money on nuclear projects (and space project amazingly), than
protecting it's natural resources and providing employment for it's people.
However...
In agriculture hay and straw is prone to SC, and this is due to two prime
reasons alone: 1), damp material, and/or, 2). bad/loose stacking. You only
have to feel the heat from a manure heap or compost heap to see how fire can
break out with the right materials present.
I hope the above might be of use. Regards, Bernard
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