A differential scanning calorimetric (DSC) study on the characteristics and behavior of water in low-rank coals

High moisture content in low-rank coals significantly limits their application and there is a R&D need for advanced drying technologies and upgrading of lignite. Fundamental understanding of coal-water interactions is a critical part of the development of drying technologies. In this study, forms of water in low-rank coals were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses based on their freezing characteristics. Two types of free and freezable bound water were detected during freezing process in DSC experiments. The amount of free and freezable bound water was experimentally determined from their heat of freezing. These two types of water accounted for 18.22-78.25% of total water in coal, which indicated the presence of the third type of water, namely "non-freezable water". The presence of the third type of water was confirmed in DSC heating process experiments. Moisture reabsorption experiments on heat treated samples showed that the physical and chemical structures of coal have a significant effect on its interaction with water. When coal samples with 14.75% water contents were subjected to low temperature XRD analysis, water in frozen state was detected at -80 °C indicating that part of "freezable bound water" that was not detected by DSC was in frozen state at low temperatures. Amount of heat generated by phase transition of the third type of water for coal samples with water contents less than 18% was very small and therefore was not detected by the DSC.