High moisture content is a major factor degrading coal quality. Although steam instantaneous dynamic dewatering can effectively overcome the drawbacks of conventional drying, such as high energy consumption and unstable temperature control, how it removes the different forms of moisture in coal remains poorly understood. In this study, high-moisture coal was investigated. By analyzing the occurrence forms of moisture, the moisture distribution before and after steam instantaneous dynamic dewatering was quantified and spatially resolved, and the changes in the low-temperature crystallization behavior of moisture and in its pore-scale location under different dewatering conditions were clarified. Moisture in coal occurs as free water, bound water and non-freezable water, and its binding strength increases with decreasing pore size. Analysis by low-temperature differential scanning calorimetry and low-field nuclear magnetic resonance revealed that bound water and non-freezable water coexist in low-rank coal, whereas high-moisture lignite additionally contains free water. Dewatering experiments showed that a critical pressure of 3.0 MPa removes more than 60% of the total moisture, overcoming the barrier to bound-water removal in micropores and enabling the simultaneous removal of free water and bound water. Above the critical pressure, the removal efficiency of non-freezable water levels off, and the pore moisture distribution approaches a dynamic equilibrium. These findings clarify the moisture-form transition mechanism during steam instantaneous dynamic dewatering and provide a basis for optimizing dewatering parameters for high-moisture coal.

high-moisture coal; steam instantaneous dynamic dewatering; moisture occurrence form; dewatering efficiency