This paper describes experimental results of water vapor adsorption of coal and numerical simulations relating to the spontaneous combustion of coal in a low temperature range. The calculation model using a thermal model as the physical effect of water vapor adsorption in addition to the oxidation due to the equilibrium exposure time concept is proposed.
Coal specimens supplied by the Akabira Coal Mine, the Taiheiyo Coal Mine and the Genesee Coal Mine (Alberta, Canada) were used for the water vapor adsorption experiments. The equilibrium moisture content of coal against relative humidity and unsteady water vapor adsorption rate caused by the step change of relative humidity were measured at several temperatures. The new numerical model to express the water vapor adsorption rate is proposed.
The calculation models were successfully applied to the numerical simulations of spontaneous combustion in the spherical deposit of coal. The numerical model includes heat conduction, oxygen and water vapor diffusions in the coal deposit. Numerical results show that the water adsorption has an effect to accelerate rising coal temperature at the early stage (<200 hours), but also has an effect to restrain the rise of temperature over the long period.