Coal Size and Stockpile Ventilation

Oxidation increases with increasing fineness of coal and the rate of oxidation of coal with oxygen of air is proportional to the specific internal surface. The proportional coefficient at low temperatures is the cube root but analysis also shows that both rate and extent of oxidation increase with the decrease in particle size, until a critical particle diameter is reached, below which the rate remains fairly constant. 

The natural ventilation in coal storage piles is generally adequate to remove sensible heat as fast as it is liberated in the oxidation process. However, in situations where the ventilation is adequate to maintain oxidation but not adequate to dissipate the heat produced, the coal absorbs the heat causing a rise in the internal temperature. A chain reaction follows in that the oxidation rate increases with increasing temperature which, if allowed to proceed unchecked in a storage pile, the ignition temperature of the coal will eventually be reached and the pile will begin to smolder. 

If coal is to be stored for prolonged periods of time, the pile should be constructed so that air (in the case of fine coal or mixed sizes such as ROM coal) is excluded. On the other hand, if the coal is to be stored as lump coal, air should be allowed to circulate freely through the pile. 

When coal stockpiles are constructed by allowing mixed varied size coal to fall, roll, or slide, the larger pieces tend to collect at the bottom outside of the pile and the fines will collect at the top and inside of the stockpile. As a result, air will move easily through the outer parts of the stockpile but with much led freedom in the interior of the stockpile. Such a pile will allow the development of hot spots which can lead to spontaneous ignition of the coal with subsequent combustion of stockpile. 

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