Removal behaviors of moisture in raw lignite and moisturized coal and their dewatering kinetics analysis

ABSTRACT To investigate the dewatering kinetics and mechanism of low-rank coal, the dewatering behaviors of the Chinese lignite and its moisturized sample (prepared from dewatered coal moisturized under relative humidity of 75% at 303 K for 48 h) in nitrogen and the temperature range of 333–433 K were tested. Physical structure changes of raw coal and moisturized coal before and after drying were determined. The results indicate that drying process of lignite could be divided into four stages, which are increasing rate stage, constant rate stage, relatively fast decreasing rate stage, and relatively slow decreasing rate stage. Jander model and first-order kinetics model are favorite to describe the relatively fast decreasing rate stage and relatively slow decreasing rate stage, respectively, and the corresponding dewatering mechanism equations are and y = −ln(1 − α). The effective diffusion coefficients and diffusion activation energy were calculated by Fick’s second law. The diffusion activation energy of the dewatering stages, related to the relatively fast and slow decreasing rate stages, was 35.80, 40.75 kJ/mol for raw coal and 27.80, 37.34 kJ/mol for moisturized coal, respectively. The effective diffusion coefficient was significantly affected by drying temperature through the pore structure change of coal when other drying operation parameters were fixed. These prove that the forms of readsorbed water are not entirely the same as that in raw lignite, in which the former is relatively simple and the latter is more complex.