Organic Matter Structure Of Naomaohu Coal And Its Evolution During Direct Liquefaction Process

Direct coal liquefaction is an important way of coal conversion.With the rapid development of China's economy and the increasing demand for petroleum resources,the development of direct coal liquefaction technology will have important strategic and practical significance.For the severe reaction conditions of direct coal liquefaction,it is very important to select the coal with good liquefaction performance and understand its structural changes in the process of direct coal liquefaction,which can give guidance in realizing the mild liquefaction of coal.In this paper,the organic matter structural characteristic of a typical coal for liquefaction-Naomaohu coal(NMHC),and its structural evolution in the process of liquefaction are studied.The study contents and conclusions are shown as follows:(1)The structure of NMHC was characterized by 13C NMR,FTIR,XPS,and XRD.It is found that the aromatic carbon and aliphatic carbon ratio of NMHC are 50.2%and 42.6%,respectively,and the average aromatic ring structure is double ring.The oxygen-containing functional groups in NMHC mainly include C-O and C=O functional groups.The main form of nitrogen in NMHC is pyrrole nitrogen,and the main forms of sulfur are aromatic sulfur,sulfone,and sulfate.The aromatic clusters in NMHC are mostly disordered and separated by various bridge bonds.The results of ruthenium ion catalytic oxidation(RICO)show that NMHC has relatively abundant-CH2CH2-,-CH2CH2CH2-,and-CH2CH2CH2CH2-alkyl bridge bonds structures.There are also a small number of alkyl bridge bonds connecting three aromatic cluster structures and aromatic cluster structures containing long chain substituents with more than 20 carbon atoms in NMHC.The yield of benzene carboxylic acids(BCAs)by alkali/oxygen oxidation of NMHC is 14.6%.According to the yield distribution of 12 kinds of BCAs and the results of physical analysis and RICO,the structure characteristics of NMHC organic matter are further elaborated.The molecular formula of its structural model is C7888H7007O1473N82S32 and average molecular weight is 127397.There are few aromatic clusters with four rings or more.The ratio of single ring,double rings,and three rings is 38:195:127.There are 76 aromatic clusters which can be converted to BCAs,and 284 aromatic clusters which cannot be converted to BCAs in NMHC.High H/C ratio and aliphatic carbon ratio,abundant 1-3 rings of aromatic cluster structures,abundant ether bond and ester bond structures,and alkyl bridge bonds with 2-4 carbon atoms are beneficial to improve the liquefaction performance of NMHC.(2)The weak bonds such as ether bonds and ester bonds in NMHC were selectively broken by supercritical ethanolysis.It was found that the ethanolysis conversion of NMHC could reach 70.3%at 370?.The results of thermogravimetric analysis(TGA),13C NMR,and FTIR of ethanolysis residue showed that almost all weak bonds in NMHC were broken at 370?.Due to the break of weak bonds,NMHC was depolymerized into small molecular compounds which can be dissolved in ethanol.The results of gas chromatography-mass spectrometry(GC/MS)analysis showed that the ethanolysis products included alcohols,esters,aldehydes,ketones,ethers,acids,aliphatic hydrocarbons,aromatic hydrocarbons,phenols,and heteroatom compounds.Esters,alcohols,and aldehydes are three kinds of compounds with high relative contents,which are 28.6%,27.6%,and 19.3%,respectively.These small molecular compounds can reflect the structural characteristics of NMHC organic matter and the distribution of the primary reaction products of NMHC pyrolysis.Therefore,supercritical ethanolysis can be used as an effective method to obtain the primary reaction products of coal pyrolysis.(3)NMHC was liquefied under mild conditions(400?,3 MPa H2)and a high conversion and a high oil+gas yield could be obtained.In the process of coal liquefaction,the solid particles of coal and residues are not only reactants but also carriers of catalysts,so it is of great significance to study the structural characteristics of residues in coal liquefaction process.The reaction mixture was extracted with n-hexane and tetrahydrofuran(THF),respectively,and hexane insoluble residues(ReHEIS)and THF insoluble residues(ReTHFIS)were obtained.The structures of these two kinds of residues at different reaction times were studied.It is found that the particle sizes of ReHEIS and ReTHFIS have a decreasing trend with the extension of reaction time,and the particle surface becomes loose and porous while raw coal has a relatively flat surface.There exist peak value of specific surface area and pore volume,but the specific surface area and pore volume of ReHEIS reach the maximum at 0 min,while the specific surface area and pore volume of ReTHFIS reach the maximum at 15 min.These results indicate that there are two reaction forms of coal particles:one is "peeling" reaction,that is,organic matter is falling off from the outer layer during liquefaction process,and the other is constantly dissolving from the main structure of coal during liquefaction process with the formation of pore structure.These dissolved substances and the condensation products formed in the later period have impact on the surface space structure of coal.In the coal liquefaction process,most of the aliphatic structure and some small aromatic compounds can gradually split off from coal skeleton structure.The condensation degree of aromatic clusters structure increases with the extension of reaction time.The carbonyl carbon and oxygen-attached carbon are decreased.Methylene chain and side chain of aromatic clusters structure are broken.From the comparison results of these two kinds of residues,it can be seen that the aromatic condensation degree of ReHEIS is relatively small,and its pyrolysis rate is relatively large.In the early stage of the reaction(before 15 min),the hydrogenation reaction is relatively intense,and the H/C of ReHEIS is high.With the extension of the reaction time,the H/C begins to decrease.The presence of hydrogenated aromatic rings in ReHEIS at 60 min and 90 min resulted in the abnormal changes of aliphatic carbon content and H/C of the two kinds of ReHFIS.The aromatic condensation degree of ReTHFIS is higher than that of ReHEIS.With the extension of reaction time,the aromatic structure of ReTHFIS becomes regular,and the graphitization degree is increased,while the pyrolysis rate is slowed down,and H/C is slightly increased in the heating stage and then gradually decreased.