TG-DSC Study To Measure Heat of Desorption of Water during the Thermal Drying of Coal and To Examine the Role of Adsorption of Water Vapor for Examining Spontaneous Heating of Coal over 100 °C

A sensitive thermogravimetric analysis and differential scanning calorimetry (TG-DSC) apparatus enabled accurate measurement of the heat of adsorption/desorption of water on/from coal at temperatures over 100 °C. This technique was first used to measure the heat of desorption of water during the thermal drying of as-received coal. For three low-rank coals, the heats of desorption were almost constant at 2300–2350 kJ/kg until the water content decreased to 0.35 kg/kg-dried coal, which is close to the heat of vaporization, and gradually increased to reach ∼5000 kJ/kg at the completion of drying at 107 °C. The role of adsorption of water vapor on the dried coal in relation to the coal–oxygen interaction was then examined to evaluate the propensity to spontaneous heating of dried coal. For the three coals tested, the heat generated by the adsorption of water vapor from moist air with a relative humidity of 0.15 was 90–130 kJ/kg-dried coal in 30 min; these values are 1.2–4 times larger than the heats generated by the coal–oxygen interaction, even at 107 °C. This observation suggests that the adsorption of water vapor plays an important role in the spontaneous heating of coal, even over 100 °C. The transient changes in the heats of adsorption/desorption of water on the approach to equilibrium were also successfully estimated and discussed in relation to the isosteric heat of adsorption/desorption and heat of vaporization of water.