The oxidation of coal

The paper draws attention to the importance of the part played by peroxidic bodies or active oxygen in the weathering of coal; comparative investigations have been made with both coal and carbon, which are shown to behave similarly. The initial reaction appears to involve chemisorption, which, in the presence of water, is followed by the formation of a coal-or carbon-oxygen-water complex. The normal inherent wafer content of coals is a function of their internal surface; this is a minimum with coals containing c. 89 per cent carbon. The peroxygen content of coals shows a similar variation with carbon content. The peroxygen complex is markedly unstable above approximately 70° c., a temperature long known to be critical in regard to spontaneous combustion of coal. Below it the complex is formed easily and remains relatively stable, little oxygen entering the coal substance; also the amount of ulmification remains very small. The small quantity of gaseous oxidation products consists chiefly of carbon monoxide with a less amount of CO2. Above 7°D c. the complex starts to break down, although with carbon it does not appear to be completely destroyed until 350°c. is reached. Oxidation above 70°C. is also marked by a big increase in the oxygen content of a coal, its general ulmification and in the yield of gaseous oxidation products; also carbon dioxide then preponderates over the monoxide. These observations were substantiated by means of gravimetric balances for the coal-oxygen reaction at 50° and 90° c. The pressure changes at these two temperatures were also different; whereas a gradual fall in pressure was recorded at 50° c., at 90° c. a temporary rise in pressure occurred. The remarkable difference in the behaviour above and below this critical temperature is common to both coal and carbon, and in view of the ability of carbon to form a carbon-oxygen-water complex, it is evident that with both materials the overall oxidation is controlled by the rate of breakdown of the complex. Of paramount importance is the necessity for the presence of water; indeed, an explanation is now forthcoming of the classical experiments of C. J. Baker and H. B. Baker in 1887-88, which demonstrated that dry carbon cannot be readily oxidized even at red heat.