prevention and control of spontaneous combustion - Mining and

ventilation will remove heat, preventing a rise in temperature, while extremely poor ventilation
will not supply sufficient oxygen to support the process. On surface the major problems are
usually associated with the stockpiling of coal, or waste dumps containing rejected coal material,
in unconsolidated heaps where oxygen can come into contact with the coal and heat cannot
dissipate. The problem is compounded when rainfall causes erosion, thereby progressively
exposing more coal to the oxygen in the atmosphere. Very high ash carbonaceous shales will
also spontaneously combust under the right conditions, particularly if they contain high levels of
kerogen (a mixture of organic chemical compounds that make up a portion of the organic matter
in sedimentary rocks.) These shales provide a major source of additional fuel for coal induced
fires. Many strip mines have severe spontaneous combustion in the spoil heaps.
In addition to the known problems in stockpiles, waste dumps and spoil heaps, spontaneous
combustion may occur in the following situations:
• Coal outcrops
• Shallow workings exposed by subsidence, either directly or through fissures
• Tailings dams.
To sustain combustion, oxygen, fuel and heat must all be present. The absence of any one of
these elements will result in at least the temporary cessation of burning. Spontaneous
combustion in stockpiles and dumps can usually best be controlled by handling high risk
material in a way that limits its contact with oxygen, e.g. by compaction to minimise airflow, by
cladding with inert material to prevent the ingress of air, by limiting the height of the dump and
by orientation of the stockpile or dump with respect to the prevailing wind.
Fires in coal outcrops and in shallow underground workings present a special problem, both of
detection and control, because of access difficulties. Such fires tend to have the general
characteristics:
• Fires propagate in accordance with the topography and orientation of the mining activity,
exposed seam or stockpile, and the direction of prevailing winds, owing to “funnelling” of
air currents
• Venting fissures are present and extend to the surface, forming parallel to the ground
contours and developing in response to stress release along existing fracture systems.
Although there have been many cases of spontaneous combustion in underground coal mines in
South Africa, in surface dumps, in shallow workings in the vicinity of Witbank and even in ships
carrying export coal from South Africa, the biggest problem now is in surface mining. While it is
unusual for intact seams to burn in the highwall, the most common occurrence is when surface
mines extract seams previously partially mined by underground bord and pillar operations. Once
exposed to the air the pillars that were left over from the previous mining operations start to burn
within days and require special prevention and control techniques.
Factors affecting the spontaneous combustion of coal
All coal seams have some propensity to spontaneously combust and it is important that a mine
understands the risks associated with the coal it mines. The magnitude of the problem depends
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