Evolution of Pore Structure during Pressurized Dewatering and Effects on Moisture Readsorption of Lignite

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.