In order to investigate the dewatering kinetics and mechanism of low rank coal, the dewatering behaviors of a 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, 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 y= 1/3 2 [1-(1-α)] 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, were 35.80 kJ/mol, 40.75 kJ/mol for raw coal and 27.80 kJ/mol, 37.34 kJ/mol for moisturized coal, respectively. The effective Downloaded by [University of Cambridge] at 17:16 04 June 20162 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 re-adsorbed water are not entirely the same as that in raw lignite, in which the former is relatively simple and the latter is more complex.

In this study, pressurized method was used to dry lignite at moderate temperature to change its pore structure but preserve its oxygen-containing functional groups. The effects of drying conditions (time, pressure, and temperature) on equilibrium moisture content (EMC) and pore structure of dewatered coals were investigated, and the correlations between pore structure and EMC were also evaluated. The pore structure parameters of raw coal and dewatered coals were measured by nitrogen adsorption experiments. The EMC of dewatered coals was obtained by gravimetric method. The results indicated that the porous structure of dewatered coal was jointly affected by three factors (drying time, pressure, and temperature) in the initial pressurized drying stage. The drying pressure exhibited obvious effect in the initial stage of drying lignite. Destruction of pores under pressure was prevented due to the water present in these pores. To further improve the pore structures of dehydrated coals obtained by high-pressure treatment, the temperature was increased to above 140 °C under 3 MPa; thus, a large number of macropores were evolved into mesopores. Furthermore, the experiments on water reabsorption by dewatered coals indicated that the EMC (0.15–0.18) of dehydrated coal was the lowest when the pressure was 3 MPa, temperature was 140–160 °C, and the time required was 30 min. The moisture readsorption contents of dehydrated coals were found to be positively correlated with its pore volume at high relative humidity. When the relative humidity was below 20%, they were related to specific surface areas or oxygen-containing functional groups. Therefore, pressure in the process of drying lignite was the main factor influencing the pore structure and the water reabsorption of dewatered coals, and the drying temperature was dominant under the pressurizing conditions.

No abstract