Effects of phenolic hydroxyl and carboxyl groups on the concentration of different forms of water in brown coal and their dewatering energy

Abstract

Oxygen functional groups as primary adsorption site of water are one of key factors controlling water re-adsorption and drying behaviors of brown coal. The dewatering energy of brown coal because of different oxygen functional groups is still unclear. Thus, two kinds of oxidized brown coal samples carrying more phenolic hydroxyl or carboxyl groups with similar pore structures were chosen. The concentrations of different water forms of the samples were determined from the moisture holding capacity at constant humidity and temperature and from the results of dewatering experiments analyzed by thermogravimetry coupled with differential scanning calorimetry. The results showed that the concentrations of three forms of water, i.e. free water, capillary water, and molecular water, showed good correlations with concentration of carboxyl groups. Comparison of 1H NMR analysis of dried and moisturized samples showed that the types of oxygen functional groups predominantly affected the water environment in coal. IR spectra of dried and adsorbed D2O samples showed that OH–π hydrogen bond due to carboxyl groups was easier broken than that due to phenolic hydroxyl groups. The water desorption energy of primary site and dewatering energy of samples with more carboxyl groups were lower than that of samples with more phenolic hydroxyl groups, because hydrogen bonding energy of water bonded to hydroxyl oxygen of carboxyl groups was lower than that of water bonded to hydroxyl oxygen of phenolic hydroxyl groups.