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.

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We have proposed a degradative solvent extraction method which upgrades as well as dewaters low rank coals and biomass wastes at 350 °C using 1-methlynaphthalene as a model solvent. The proposed solvent treatment is an effective method to produce high quality extracts having similar physical and chemical properties from several kinds of low rank coals and biomasses. Three solid fractions (Residue, Deposit, and Soluble) were fractionated and recovered after the solvent extraction. Soluble and Deposit are expected to be precursors for producing value added products. In this work the effects of solvent on the degradative solvent extraction of two low-rank coals, Loy Yang (LY) and Pendopo (PD), were examined by using four solvents: 1-methylnaphthalene (1-MN), kerosene, a 1 to 1 mixture of 1-MN and kerosene, and a solvent rich in alkyl benzenes, A150. It was judged that the solvent does not affect the degradation reaction at 350 °C, and hence the performance of this degradative solvent extraction method such as selective deoxygenation and effective dewatering is realized by all of the solvents used. The yield distributions of extracted products were dominated by the solubility of solvent used as expected. The Hildebrand regular solution theory seems to represent the differences in the yields and elemental compositions of Soluble fractions. 1-MN, having 21.3 (J/cm 3 ) 1/2 of solubility parameter δ, gave the largest yield of Soluble followed by A150 (δ ≅ 18.8 (J/cm 3 ) 1/2 ), the mixed solvent (δ ≅ 19.1 (J/cm 3 ) 1/2 ), and kerosene (δ ≅ 16.7 (J/cm 3 ) 1/2 ). Preparation of solvent treated coals (STCs) from different solvents gave the yields close to the sum of the yields of Soluble, Deposit, and Residue for all solvents used. Most of the heating values of solid products were over 29 MJ/kg and rather close to subbituminous coal. All Solubles were found to melt completely at rather low temperature. The properties of Solubles can be changed by solvents used. It was found that A150 may be utilized as a practical solvent when Soluble is the target product and that kerosene is expected to be a practical solvent for preparing STC from low-rank coal.

We have recently presented an extraction method, which we call degradative solvent extraction, for producing high quality extracts having similar physical and chemical properties from several types of low rank coals and biomasses. The degradative solvent extraction is an effective method to remove oxygen from low grade carbonaceous resources by dehydration and decarboxylation reactions, which minimize the loss of heating value of starting material. Three solid fractions, Residue, Deposit, and Soluble, were obtained from the extraction method when the product was fractionated. Soluble and Deposit are expected to be precursors for producing value-added products. Residue is expected to be utilized as a high quality solid fuel because of its high heating value comparable to subbituminous coal. When the extraction product was not fractionated, the product is the mixture of the three fractions just upgraded. We called the upgraded product “solvent-treated coal” (STC) when it was prepared from coal. STC is also expected to be a high quality solid fuel. To evaluate the potential of Residue and STC as high quality solid fuels, estimation of their propensity to spontaneous heating is essential in addition to their heating values and rates of combustion and/or gasification. In a previous work the authors have proposed that the weight change and the heat generation rate measured at 107 °C in both dry air and wet air can be an index to estimate the propensity to spontaneous heating. This work applied the method to estimate propensity to spontaneous heating of Residue and STC prepared from two low-rank coals. The propensity to spontaneous heating of the parent coals and chars prepared in an inert atmosphere at 350 °C is also examined for comparison purposes. It was found that the degree of propensity to spontaneous heating of Residue and STC is much less than that of the corresponding parent coal and chars.