The role of moisture in the self-heating of low-rank coals

The self-heating of coal mainly involves exothermic reactions of oxygen at reactive radical sites within the coal and the enhancing or moderating effect that water has on these reactions. The thermal response of samples of low-rank coals, dried by heating under nitrogen flow at 105°C and exposed to dry oxygen, is similar to or slightly less than that observed when they are flow-dried at 30°C and tightly bound moisture remains. The most likely reason is that moisture affects the nature of the radical sites where oxidation occurs. By hindering the formation of stabilized radicals, it encourages faster oxidation which may lead to enhanced thermal response, although some of the extra heat may be taken up by the residual moisture. When loosely bound moisture is allowed to remain in the coal, the thermal response on exposure to dry oxygen decreases very quickly, due mainly to hindered access to reactive sites and dissipation of heat generated by any oxidation that does occur. The effect of desorption is comparatively minor and the course of the oxidation reaction responsible for generating heat does not appear to be changed by the presence of small quantities of loosely bound moisture.