2015 Volume 94 Issue 10 Pages 1169-1172
Low rank coals are susceptible to spontaneous combustion, because oxidation rate of low rank coals are higher than that of high rank coals. Low rank coals, however, must be heated over a critical temperature of around 70 °C before spontaneous combustion starts. The mechanism by which the coals are heated over the critical temperature has not been completely elucidated. This note focused on the adsorption of water vapor as a possible mechanism of the temperature increase. When three kinds of brown coals/lignites pre-dried at 80 °C in an inert atmosphere and cooled to 28 °C were exposed to an ambient and stagnant atmosphere of 28 °C with 70 % of relative humidity, the coal temperatures increased up to 40 to 43 °C in a minute or so. When the coals predried and cooled to 38 °C were exposed to an stagnant atmosphere saturated with water vapor at 38 °C , the coal temperatures increased up to over 60 °C in a minute or so. These results showed that the rapid adsorption of water vapor will be a cause of temperature increase leading to the spontaneous combustion of low rank coals.
