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The New Zealand Railways Magazine, Volume 3, Issue 3 (July 2, 1928)

[section]

The subject of weathering cannot be gone into without involving the subject of spontaneous combustion.

Owing to market conditions and the crowding of transport facilities the storing of coal has become more and more of a necessity in recent years. Users of coal are compelled to have on hand a reserve supply that will enable them to carry on without interruption in case of truck shortage, or other unforseen circumstances. For this purpose the amount of coal stored will vary, from a few hundred, to several thousand tons.

Most bituminous coal will ignite spontaneously if placed in large heaps. Moreover, it suffers disintegration (more or less) during storing and handling.

At ordinary temperature, the atmosphere oxidation of the coal is going on slowly all the time, and continues with increasing rapidity at higher temperatures until, in many cases, it results in ignition. Any heating of the coal in the stock represents so much heat loss which will not be available for the boiler. Most of the atmosphere oxidation does not produce any sensible heating, but it results, all the same, in an appreciable loss of heat units in the coal.

The changes taking place in stored coal may be divided into two classes: (1) oxidation of pyrite, marcasite and other inorganic constituents; and (2) the direct oxidation of the organic matter of the actual coal. To the change in the inorganic matter most of the visible changes are due. The iron sulphide changes into sulphate of iron and sulphuric acid, and the latter in its turn unites with the calcium and magnesium carbonates, almost invariably present in coal, to form gypsum and magnesium sulphate. All such changes result in a large increase in volume and a marked disintegration of the coal. They will also, in many instances, bring about a considerable increase in the weight of the coal unless removed by the leaching action of water.

“Right Away!” A daily scene on the N.Z.R.

“Right Away!”
A daily scene on the N.Z.R.

The conclusion is that stored coal may increase in weight, but that its heating value decreases most rapidly during the first week after mining, and continues to decrease more and more slowly for an indefinite time. The losses due to disingration of the coal and to spontaneous ignition are of greater importance than any change of weight or heating value. The larger the size of coal stored the less liable is it to fire spontaneously.

Storage of coal under water will prevent disintegration of the coal to a very large extent, and will absolutely prevent spontaneous ignition. Apart, however, from the advantage mentioned of storing coal under water, there seems to be little in favour of any particular method of storing coal.

I do not know of any case during the last 27 years on the South African Railways where we have had a case of any of our stored coal firing, although some time ago there was a pretty bad case of firing on the Cornelia Mines at Viljoen's Drift. The firing was due possibly to spontaneous combustion of the fine coal dust in the mine chambers.