Posted 11 February 2012 - 06:06 PM
Exposure to air causes slow oxidation of coal and heat is generated in the storage pile. If the heat is not dissipated this can reach even ignition. Spontaneous coal heating depends on:
1. The character of coal. Oxidation rate is usually higher for coals with high natural or bed moisture, oxygen, volatile matter, finely divided pyrites. High rank coals generally present low oxidation rate.
2. The extent of coal surface exposed. Finer sizes are expected to rapidly oxidized. But when stored alone, they tend to form a fairly tight pile resisting air intrusion. Big sizes offer passageways to air, but their exposed surface is small and heat is rapidly removed by the air. The greater risk of fire concerns piles of mixed coal sizes, such as run of mine. Such a pipe tends to arrange big sizes at base, small sizes at summit. Air tends to enter the pipe at bottom and flow upward with a chimney effect, while the center of the cone has minimum air circulation. Oxidation rate (OR) is approximately proportional to the cubic root of surface area (m2/kg).
3. The coal temperature. OR increases by a factor of about 2.2 for each 10 oC (for 30 - 100 oC). If coal gets stored in a hot day, all the mass of pile is at high temperature to accelerate oxidation. If coal has already formed a pile, increase of ambient temperature is less important. Due to low conductivity of coal, region of increased temperature is largely at the outer layers, not going deep. Same for sun radiation.
4. Moisture. Wetting and drying may increase OR, but this has not been clearly determined.
Recommended precautions in coal storage.
α. The coal should be piled in a long bank of height not more than 20 ft (not pyramid or ridge). Coal is best built up in layers and compressed by a roller as laid down. Rolling has been successful in preventing heating, forcing the fines into the spaces around the large sizes. Bulk density increases by rolling from about 50 lb/ft3 to 65 lb/ft3.
β. Coal from different sources should be piled separately.
γ. Minimize further handling of coal to prevent breakage and fines. Avoid any cause of coal segregation.
δ. Do not store coal in contact with any source of heat, or anything that might generate heat. Same for steam lines, even if buried or insulated. Small quantities of heat losses accumulate in the coal and speed up oxidation.
ε. Storage area should be level, firm, well drained, without fences or other obstructions that may furnish a channel of air to the pile.
στ. Storage during hot, bright days should be avoided.
ζ. Alternate wetting and drying of coal is undesirable.
η. Do not use pipes or air ducts to ventilate coal.
θ. Regularly measure temperature by thermometers, deep in the pile. In case of temperatures higher than 66 oC, the coal should be dug out and used immediately, or wet down thoroughly.
Source: Ph Wilson, J. Wells, "Coal, Coke, and Coal Chemicals", McGraw-Hill, 1950, Chapter 5 (Preparation and cleaning of coal for coking: storage of coal).
Coal supplier should be advised for specific practical recommendations in addition to above.
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