Modern Gasworks Practice

372 MODERN GASWORKS PRACTICE damp and moisture, the pyrites being oxidized by the oxygen contained in the water. This idea probably originated in the faet that if a quantity of pyrites is picked from the coal, made into a heap, and then exposed to air and moisture, the heat of the pile rapidly rises, o wing to the oxidization of the sulphur due to the action of the air and moisture. In many cases a temperature will be reached at which combustion in air beging, and the mass will then burst into flame. In contradiction of this theory, however, it must be pointed out that wliile some coals containing a very high percentage of pyrites are quite safe, those containing a small proportion often give great tr puble when stacked. The storage of coal presents several disadvantages, the chief of which are:—■ (a) Possibility of spontaneous ignition. (b) Loss due to weathering. (c) Loss due to cost and effect of handling. There seems to be little donbt that spontaneous combustion is the outcome of a combination of several causes, and that while pyrites itself is not alone respon-sible, it may play some subsidiary part in assisting the action. Factors such as the size, dampness, proportion of volatile matter, and general composition, all play their part in bringing about a rise in temperature. It is, however, interesting to note that conclusions arrived at by the United States Bureau of Mines were that moisture plays no part in promoting spontaneous Hearing, and that high volatile matter does not of itself increase the liability to fire. It is common knowledge that coal as soon as it leaves the seam has a great avidity for oxygen, and, remarkable as it may seem, the greater the proportion of oxygen in combination with. the coal the greater appears to be its proclivities for absorbing an additional quantity from the air. Lewes pointed out that the “ resin bodies are the compounds present in the coal most likely to possess this property. It is the Chemical reactions so caused which leadto slow combustion, and, when accelerated by any rise in the surrounding temperature, this is capable of generating sufficient heat to lead to the spontaneous ignition of masses of broken coal large enough to prevent the escape of heat as it is developed.” Many bitunainous coals will absorb more than three times their volume of oxygen; but atter a time they get coated with an oxidized surface, and their absorptive properties become less vigorous until new surfaces are exposed by the breakage of lumps, etc. It is probable, then, that the spontaneous heating in coal heaps is due to the rise of temperature occasioned by the oxidation of part of the carbon together with the sulphur in the coal; and in cases where actual firing takes place, it may be assumed that the heat generated in the stack is greater than that dispersed by radiation. Thus the temperature rises till the ignition point is reached. The greater liability of some coals to fire is due, therefore, not to the sulphur they contain, but to their proclivities for absorbing oxygen ; and the greater the quantity of oxygen absorbed, the greater will be the tendency to fire. The heating effect, too, is probably cumulative; for a rise in temperature increases the capacity of the coal for taking up oxygen.