A study of the low temperature oxidation of coal

The low-temperature (25–140°C) oxidation rate of a single coal was measured in a number of ways. The major technique used was to follow the temperature-time history of a sample in a well-insulated vessel, while at the same time performing gas analyses. It was found that oxygen absorption was the major exothermic step. The amount of product gases formed (carbon dioxide, carbon monoxide and water) was, especially at lower temperatures, very much smaller than the amount of oxygen absorbed and the reaction producing them was if anything endothermic. The presence of carbon dioxide and especially carbon monoxide inhibited the reaction, while the water appeared to act as a catalyst. As a result of this information an overall oxidation mechanism consistent with the experimental findings has been postulated. coal + oxygen ⇋; absorbed oxygen → “oxycoal” → CO, CO2 and H2O The effect of the coal particle size appears to be complex, but certainly the finer coal reacts faster. Surface area measurements (B.E.T.) show that these surface areas are proportional to the external surface area, but are many times larger. This suggests a very porous structure with blind pores. It has been established that the coal gradually loses its activity (“ageing”). It has been shown microscopically that the oxidised coal particles have fairly sharp regions of higher reflectivity on the edges which are known to petrographers as “oxidation rims”. It is postulated by the authors that the coal oxidises from the outside via a shrinking core model with diffusion resistance through the oxidised layer. This postulate is consistent with both the experimental “ageing” and the visual observations.