Analysis of Water Forms in Lignite and Pore Size Distribution Measurement Utilizing Bound Water as a Molecular Probe

Based on the congelation characteristics, different types of water in lignite and their drying behavior were investigated by using low-temperature differential scanning calorimetry over a temperature range from 25 to −80 °C. The water in lignite was classified into three types: free water, bound water, and nonfreezable water. It was shown that during drying, the free water in lignite was removed first, and then the bound water began to be evaporated. With the moisture decreasing, the congelation peak of free water moves toward lower temperature, which may be explained by the formed capillary force distribution in the pores driving the free water to flow toward the smaller pores and increasing the confinement effect of the pores. However, for bound water, this mobility was restricted by the stronger coal–water interaction; thus, its congelation peak remained at around −42 °C. The amounts of three types of water were determined by a new method. It shown that all of the bound water and more than 30% of nonfreezable water needs to be removed in real drying cases. Based on the melting point depression of bound water in lignite, using bound water as a molecular probe, the pore size distribution of lignite was also determined. The maximum and minimum measured pore diameters were 86.47 and 7 nm, respectively.