Study on the Relationship Between Pore Structure and Water Forms in Pore Using Partially Gasified Lignite Char

The oxygen-containing functional groups and pore structure of lignite are the main factors controlling its moisture readsorption and drying behaviors. However, the effects of these two factors were not separated in former studies. To minimize the effect of differences in surface chemical properties, several partially gasified lignite char samples with different pore structure but similar surface functional groups were used. The surface areas and volumes of micro-, meso-, and macro-pores of samples were determined by CO2 adsorption, helium pycnometry, and mercury intrusion porosimetry. The concentrations of different forms of water in moisturized char samples were determined from the equilibrium moisture contents at constant humidity and temperature and from the results of dewatering experiments analyzed by thermogravimetry coupled with differential scanning calorimetry (TGA-DSC). The congelation characteristics of water in moisturized char samples were studied by low-temperature DSC. The relationship between pore structure and the content of different forms of water was analyzed by individual linear regression. The results showed that three forms of water were distinguished, free water, capillary water, and molecular water. The free water and capillary water were frozen at −12 °C and −40 °C, respectively. The dewatering heat per gram of water was in the order: molecular water > capillary water > free water. The concentrations of free-, capillary-, and molecular water were strongly correlated with the macropore volume, mesopore volume, and surface area, respectively.