Effect Of Low-temperature Pyrolysis And Liquid Phase Dissociation On Structural Transformation Characteristics Of Low-rank Coal

China has abundant coal reserves.Because of the characteristics of high moisture content and low coalification degree,the utilization of low-rank coal,mainly lignite,has been greatly limited.In order to promote the utilization of the coal,the researchers from the various countries have proposed many methods for the coal upgrading.Among them,pyrolysis and liquid phase treatment are the most widely used and most basic upgrading techniques.For the more efficient use of lignite,the study on its physical and chemical structure during the upgrading process is very important.Therefore,in this work,the transformation of the microstructure of the lignite during the upgrading process were carried out using muffle furnace and high pressure reactor,respectively.At the same time,the structural analysis techniques were used to explore the transformation of the lignite microstructure during the different treatment processes.The main work includes the following three parts.Yimin lignite(YM)was chosen as the experimental samples.The low-temperature pyrolysis was carried out in the muffle furnace,and FTIR and Raman spectroscopy were also used for the coal structure analysis.The results showed that YM lignite had complex structure,including hydroxyl,aliphatic group,aromatic group and C=O,C-O functional groups.The low-temperature pyrolysis could improve the rank of the lignite.During the pyrolysis process,the contents of hydroxyl,carboxyl groups and the conjugated C=O continuously decreased,with the increase of C-O functional groups.The aromatic hydrocarbons in the lignite existed in the form of di-,tri-,tetra-and penta-substitutions.It can be found based on Raman analyses that the aromatic hydrocarbons in the lignite macromolecular had the highest relative content.During the low-temperature pyrolysis process,the macromolecular structure of the lignite hardly changed,but the content of the small-molecule aromatic hydrocarbons and amorphous carbon increased.The hydrothermal treatment(HT)of Yimin lignite was conducted using an autoclave and the structure of the samples before and after the treatment were analyzed.The results showed that HT had a more efficient upgrading effect on the lignite than low-temperature pyrolysis,and HT could achieve the best upgrading effect at 200?.Under the low temperature treatment(?150?),the unstable macromolecules and aliphatic groups in the lignite were destroyed,and C=O were decomposed to form the various ethers and hydroxyl groups,meanwhile,the small molecular aromatic rings and cross-linked structures increased.With the increase of HT temperature(150?200?),the removal of the heterocyclic ring and the defect structure led to the decrease in the cross-linked structure and the increase in the macromolecular aromatic ring,thus,the resulted coal quality increased.In the high temperature stage(200?300?),the aromatic structure was destroyed together with the increase of carboxyl group,conjugate C=O,small molecule aromatic structure and amorphous carbon.During the subsequent hydrothermal experiment,hydrogen peroxide was added to conduct the wet oxidation treatment of the coal and the samples were analyzed by the same method and standard as the pyrolysis and hydrothermal treatment process.The results showed that the oxidation treatment had a significant destructive effect on the lignite and its strength was much higher than that of HT and low-temperature pyrolysis.At the low temperature stage(?100?),aliphatic group and some macromolecular aromatic hydrocarbons were destroyed,and carboxyl and some aromatic ethers and hydroxyl groups were also removed,forming aliphatic ethers and hydroxyl groups,meanwhile,the formation of small molecular aromatic structures and amorphous carbon occurred.As the oxidation temperature increased(100?150?),small molecular aromatic structures and aliphatic ethers,hydroxyl groups were broken to form amorphous carbon and a large number of cross-linked structures,and the formation of carboxyl and conjugated C=O occurred.With the continuous increase in the oxidation degree(?150?),C=O was reduced,and the macromolecular structures were oxidized to form aliphatic structures.For the stable components in the mixed products,the substitution reaction was strengthened,resulting in the stabilization of macromolecular aromatic structure,and the aromaticity was also improved.Under these conditions,the solid products were mainly composed of macromolecular aromatic hydrocarbon structure with higher degree of consistency and graphite carbon matrix.