| Low rank coals(LRCs),which is easy to be mined,has large reserves and high volatile content,is the ideal raw material for coal gasification technology.However,high water content of LRCs will not only generate a large amount of waste water during gasification,but also reduce the service life of gasification equipment.Therefore,LRCs should be dehydrated and dried before gasification.Hydrothermal dehydration technology can achieve a higher dehydration rate and effectively inhibit the re-adsorption phenomenon of low-rank coal after drying.However,the current research on hydrothermal dehydration technology is mainly based on macroscopic experiments,and it is impossible to analyze the mechanism of hydrothermal dehydration from the microscopic level.In this paper,lignite was dried by hydrothermal dehydration to explore the relationship between dehydration effect and hydrothermal temperature.The experiment shows that high temperature hydrothermal dehydration can effectively remove the moisture of lignite,which can reduce the moisture content of lignite by 63.99%.In the process of hydrothermal treatment,capillary water,as the main removal object,was removed by 60.09% at 250℃,and free water and adsorbed water were removed by 43.56% and 52.94%,respectively.But at 300℃,the water removal rates of the three forms increased significantly,reaching 69.20%,57.78%,and 60.50%,respectively.This indicates that the lignite structure is further damaged during the hydrothermal process at 300°C,thereby releasing more water to participate in the dehydration process.Secondly,the average macromolecular model of lignite was constructed and optimized by using computational chemistry method.Based on this lignite model,a variety of coal-water monomolecular complex models and multi-molecular coal-water complex models were constructed.The adsorption energy and weak intermolecular interaction of monomolecular complexes were calculated by means of quantum chemistry.Molecular dynamics calculations were carried out on the decomposition temperature of oxygen functional groups contained in lignite molecules and the migration and removal law of water molecules in hydrothermal process.The results show that with the increase of hydrothermal temperature,the maximum probability of decomposition order of oxygen-containing functional groups is aromatic ether,aliphatic ether,carboxyl group,carbonyl group,alcohol hydroxyl group,phenolic hydroxyl group.The maximum probability of decomposition temperature of phenolic hydroxyl group is380℃,but phenolic hydroxyl group can decompose preliminarily at 300℃,which explains the reason why the dehydration rate of capillary water and free water of sample coal increases suddenly at 300℃.Finally,when exploring the migration and removal law of water molecules,it was found that the ambient water will move toward the lignite molecules and form a water network structure with the surface water molecules through hydrogen bonding,driving the surface water of lignite to move away from the lignite.As the temperature continues to rise,oxygencontaining functional groups in the lignite structure gradually decompose,leading to the loss of adsorption sites for adsorbed water,which is more likely to be bound by environmental water and move away from the lignite.At the high temperature stage,oxygen-containing functional groups decompose in large quantities,so that most of the adsorbed water can be detached from the lignite molecules.The adsorbed water was connected to the ambient water through hydrogen bonds,so that the structure of the water network evolves to the form of water molecular group and moves away from the lignite molecules by rolling motion.The water molecules in the water group were connected to each other through hydrogen bonds,which further hindrance the formation of hydrogen bonds between water molecules and oxygen-containing functional groups while limiting the movement of each other,and effectively promoting the improvement of dehydration effect. |
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