| Coal resources are rich while gas and oil resources are deficienct in China.The conflict between the growth of energy consumption and the insufficiency of traditional fossil energy resources and the protection of ecological environment has becoming more and more violently,which puts forwards the demand of utilizing the fossil energy resources more reasonably.Dehydrating and upgrading of low-quality lignite is an important way to obtain cleaner coal for better utilization of coal resources.Restraining the water re-absorption may greatly improve the utilization efficiency of lignite.As hydrothermal treatment technology(HTC)matured,it has become an effective and promising way of lignite upgrading.The Lonchang lignite in Guizhou Province was chosen as the experimental samples to study the effects of hydrothermal upgrading process on the water re-absorption characteristics and the evolution of sulfur,oxygen and nitrogen functional groups of coal samples.The macromolecular structure model of coal had been constructed,and the mechanisms on how hydrothermal upgrading effected on fuel characteristics,physicochemical structure change characteristics and water re-absorption characteristics of coal samples at micro-level were revealed.Longchang coal samples were upgraded by hydrothermal method to improve the hydrophobic property.The hydrothermal upgrading temperatures were set at 200,250,300,350 and 400 ? respectively,with each hydrothermal time for 1 hour.The moisture and volatile content of coal sample could be reduced effectively while the fixed carbon content increased.The volatile content of HTC-400 was only 14.92%,and the reduction range was 55.99% comparing with that of the raw coal.Coal rank was increased significantly by the decrease of O/C and H/C ratio.In the process of hydrothermal upgrading,decarboxylation,dehydration and demethanization existed at the same time,but decarboxylation and dehydration were the main reactions.The contribution of decarboxylation and dehydration and demethanization to the migration of functional groups was different at different hydrothermal temperatures.The decarboxylation reaction was more violent with the increase of hydrothermal temperature,with the ratio of hydrogen to carbon atoms decreased.The effect of ambient temperature and humidity on water re-absorption process was studied in temperature and humidity chamber,while research on the laws of water re-absorption process and equilibrium moisture content of hydrothermal upgraded lignite were based on the variable control method.When the humidity was 80%,the ambient temperature was set at 10,15,20,25 and 30?,the maximum equilibrium adsorption capacity increased firstly and then decreased with the increase of ambient temperature.For all samples,the maximum adsorption capacity reached at 15?.According to the relationship between the amount of water re-absorption and time,the process of water re-absorption was divided into two processes: the physical adsorption process of rapid water absorption and the chemical adsorption process of slow water absorption.Among all hydrothermal upgraded samples,the adsorption capacity of HTC-300 was the highest in all relapse environments,and the physical adsorption speed was the fastest,which was related to the physical and chemical structure characteristics of coal samples at this temperature.Based on methods such as BET,FTIR,XPS and 13 C NMR,the effects of hydrothermal upgrading on the morphology,pore size and functional group structure of coal samples were studied,and the regularity and internal mechanism of hydrothermal upgrading on water re-absorption characteristics of low-quality coal were revealed.The total pore volume of hydrothermal upgraded lignite was much larger than that of raw coal,which led to a larger mass diffusion coefficient of hydrothermal upgraded lignite than that of raw coal.The change of total pore volume was very small within 350?.When the temperature reached 400?,the total pore volume decreased rapidly,even lower than that of raw coal.Therefore,in the initial stage of water re-absorption,especially at low temperature and humidity,the absorption rate of raw coal was higher slightly than that of hydrothermal upgraded lignite.As the ambient temperature and humidity increased,the absorption rate of hydrothermal upgraded lignite was higher than that of raw coal in initial stage.This was mainly due to the enhanced physical adsorption caused by developed surface pore structure,regardless of the weaker chemical adsorption ability caused by that the oxygen-containing functional group structure was changed or decreased leading,and the higher content of hydrophilic groups in raw coal.So the water re-absorption rate of hydrothermal upgraded coal was obviously higher than that of raw coal in the initial stage.With the increase of hydrothermal upgrading temperature,the content of hydroxyl,carboxyl and carbonyl groups in HTC-200,HTC-250 and HTC-400 decreases and was lower than that of raw coal,while the content of hydroxyl,carboxyl and carbonyl groups in HTC-300 and HTC-350 were higher than that of raw coal,but also decreased with the increase of hydrothermal temperature.In addition,the hydroxyl signals of HTC-350 and HTC-300 were very obvious.The water re-absorption of HTC-300 was higher than that of other samples may be caused by the higher content relatively of hydroxyl group.Although the change of hydroxyl group was consistent with that of HTC-350,the C-O content of HTC-300 decreased sharply when the temperature was raised to 350 ?,offsetting the effect of hydroxyl group on water adsorption partially.Moreover,there was a turning point in the total of thiophene and mercaptan in HTC-300,which may also lead to differences in water re-absorption characteristics.The macromolecule structure model of samples was constructed,and the effect of hydrothermal upgrading reaction process on the water re-absorption process and the equilibrium moisture content of absorption was discussed from the micro-structure.The relationship between the molecular structure characteristics and functional group properties of coal samples and water re-absorption was revealed.According to the specific number of constituent units in the calculated chemical structure,the molecular formula of raw coal was C181H144O16N2S3 with molecular weight of 2699,while the molecular formula of HTC-300 was C181H144O16N2S3 with molecular weight of 2701,which were basically consistent with the experimental results.The model image was drawn by software of ACD/NMR Chemsketch.The model image was drawn by software of ACD/NMR Chemsketch.There are 60 figures,35 tables and 147 references in this dissertation. |
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