| Because of the advantages of high volatile content and high activity,lignite was regarded as a high-quality chemical raw material for pyrolysis,gasification and liquefaction to produce high value-added products.However,as a typical low-rank coal,lignite had high moisture content,which restricted the efficient conversion and utilization of lignite.Although hydrothermal treatment（HT）could remove the moisture irreversibly and improve the quality of lignite effectively,the reaction environment with high temperature and high pressure（>300°C,8.5MPa）not only placed more stringent requirements on the energy consumption and equipment,but also enhanced the carbon loss of lignite,resulting in the reduction of solid yield and the difficulty of waste liquid treatment.As deep eutectic solvents（DES,synthesized by ChCl and ZnCl2）was high efficiency and rich in hydrogen bond acceptors and donors,a novel process for the efficient dehydration and upgrading of lignite via DES assisted HT（DES-HT）was proposed.The objectives of this thesis were to reveal the effect of DES on physicochemical properties,moisture occurrence form and combustion characteristics,and clarify the mechanism of how DES promoted the dehydration and upgrading of lignite under hydrothermal conditions.Firstly,the effects of DES on the elemental composition,industrial composition,pore structure and oxygen functional groups（OFG）of lignite were revealed by studying the physicochemical properties of lignite upgraded by DES-HT.Compared with HT method,DES-HT method further improved the quality of lignite.Compared with HT lignite,the removal efficiency of moisture,volatile matter and O element of lignite upgraded with 3g ChCl:ZnCl2 were increased by 6.69%,4.45%and 6.21%at 300°C,respectively.The release of gases（such as CO2,H2 and CO）produced by the removal of volatile and OFG enriched the pore structure of DES-HT lignite.When the addition amount of DES or the upgrading temperature increased,the developed pores would gradually collapse along with the melting deformation and shrinkage of coal particles.After upgrading with DES,the hydrophilic groups of lignite decomposed in large quantities.At 300℃,the removal efficiency of carboxyl group increases from 24.66%（HT）to 44.90%（3g ChCl:ZnCl2）.Meanwhile,the removal efficiency of hydroxyl group increased from13.45%to 24.80%.The hydrophobicity of lignite was further enhanced.Combined with the physicochemical properties of DES-HT lignite,this thesis revealed the effect of DES on the moisture occurrence form and combustion characteristics by thermogravimetric analysis.The results of drying experiments showed that due to the destruction of hydrophilic OFG and the hydrogen bond between OFG and moisture,the DES-HT method could further remove the bound moisture which were difficult to be removed by HT.At 300℃,the lignite upgraded with 3g ChCl:ZnCl2 increased the removal efficiency of capillary water from 66.43%（HT）to83.22%.Meanwhile,the removal efficiency of molecular water was increased from58.61%（HT）to 68.54%（DES-HT）.The results of combustion experiments showed that the combustion curves of coal samples gradually delayed to high-temperature zone because of the quality improvement.Compared with HT method,the DES-HT method not only reduced the spontaneous combustion tendency,but also improved the combustion reactivity of lignite at the combustion stage,which was conducive to subsequent transportation,storage and utilization.Density functional theory calculations were employed to investigate the mechanism of how DES promoted the removal of moisture in lignite under hydrothermal conditions.The analysis of molecular interaction showed that the hydrogen bond strength between OFG and water molecules was greatly reduced via forming new intermolecular interaction between DES and water molecules.The hydrogen bond strength formed between moisture molecules and benzyl alcohol significant decreased from-34.12 k J/mol to-24.33 k J/mol after DES adding.The repulsion strength between methyl benzoate and moisture molecules was reduced from13.00 k J/mol to 7.23 k J/mol,which greatly reduces the energy consumption required for the removal of capillary water.At the same time,the analysis of Mayer bond order and Laplacian bond order showed that the bond order of C-O bond in benzyl alcohol and methyl benzoate was higher than the original state under HT conditions,indicating that the HT method improved the stability of groups.However,the stability of C-O bond was significantly reduced under DES-HT conditions,which illustrated that the functional groups were more easily broken.In addition,the stability of O-H bonds in benzoic acid and methyl formate was also further reduced under DES-HT conditions.As a result,both total moisture content and molecular water content of lignite could be removed more effectively by DES-HT. |
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