Slow combustion of coal

The slow combustion of coal is, on account of its commercial importance, a subject of particular interest. Anthracite and anthracitic coals are but little affected by exposure to air, but the majority of bituminous coals undergo appreciable oxidation and deterioration. A photographic plate is affected by them in the dark in the presence of air or oxygen possibly owing to the production of traces of hydrogen peroxide. The rate of oxidation increases with the surface area and with the temperature; if the resulting heat is unable to escape sufficiently rapidly the temperature may rise to such a height as to cause spontaneous combustion. In coal mines the conditions are peculiarly favourable to this, and the resulting fires are known as gobfires. The same difficulties present themselves in connection with the storage and transport of coals. It was at one time believed that the spontaneous heating of coal was due solely to the oxidation of included pyrites or marcasite. Thus:

FeS₂ + H₂O + O₂ = FeSO₄ + H₂SO₄.

The researches of several investigators have shown fairly conclusively that this cannot be the main cause. Indeed, it is not difficult to show, from thermochemical considerations, that if the coal contained as much as 2 per cent, of pyrites, and the heat of the foregoing reaction were wholly accumulated in the mass of the coal, it would be totally inadequate to raise the temperature of the mass to ignition-point. On the other hand, a straightforward chemical reaction of this type is not the only manner in which a substance, such as pyrites, may possibly assist in the oxidation of coal. It is well known that iron compounds, by virtue of their power of alternate reduction and oxidation, frequently exert a marked catalytic action upon reactions of the above-mentioned type; and evidence is not altogether wanting that pyrites can and does exert a minor accelerating influence on the oxygen absorption of coal.

Apart from its thermochemical and catalytic activities, it seems very probable that pyrites can assist in the oxidation of coal in a purely mechanical manner as well, inasmuch as it tends to swell on oxidation and thus to increase the mechanical disintegration of the coal, thereby exposing larger surfaces to aerial attack.

When air is passed over coal free from occluded gases oxygen is absorbed, water and oxides of carbon being produced. This reaction proceeds even at temperatures as low as 25° C.; indeed, with freshly mined coal, oxidation proceeds at the ordinary temperature, although in this case it may not be attended with evolution of carbon dioxide. Up to about 80° C., however, the reaction is relatively slow, and even when coal is exposed to oxygen under high pressure the oxidation is attended by only relatively small amounts of the oxides of carbon. At about 80° C. the rate of oxidation undergoes a marked increase, and at 120° C. in oxygen and 135° C. in air oxidation proceeds freely.

Between 140° and 160° C. in oxygen and 200° to 270° C. in air oxidation becomes autogenous or self-propellant, and as soon as this point is reached the temperature rises rapidly to the ignition-point.

Moisture accelerates the oxidation of coal, but an excess of water retards it, partly becausc it enters the pores and thereby renders it less accessible to the atmosphere and partly also because its evaporation tends to keep down the temperature. Oxidation proceeds in the absence of bacteria.

It has been suggested that, just as in the combustion of carbon and the slow combustion of hydrocarbon gases, the first step in the slow combustion of coal consists in the formation of an additive compound or complex, consisting of oxygen and one or more of the substances present in coal.