Effect Of The Bonding Structure Of Low And Medium Rank Coals On Direct Liquefaction And Pyrolysis

As a result of China’s current energy status,characterized by an abundance of coal but a scarcity of petroleum and gas,coupled with the rising demand for oil and increasing foreign dependency,the direct conversion of coal into high-quality oil and gas is of great significance in ensuring China’s energy security.Additionally,leveraging the abundant polycyclic aromatic hydrocarbon structure of coal to produce high value-added chemicals and coal-based materials is crucial for achieving the"carbon peaking and carbon neutrality"goal.Among the various coal-to-oil technological routes,direct coal liquefaction and coal pyrolysis not only facilitate the conversion of coal into oil and gas but also enable the preparation of high-value chemicals and carbon-based material precursors.This dual capability underscores their vital and irreplaceable development potential.In recent years,a variety of high-grade coals with very low inertinite contents have been mined in the Hami region of Xinjiang,China.The construction of new liquefaction and pyrolysis bases has been initiated,tailored to the reactivity of different coal types.The exinite-rich coals are considered excellent feedstocks for direct liquefaction,and the investigation of the structure characteristics of different maceral contents in coal and their interactions during direct liquefaction is essential for overcoming the choke points in new direct liquefaction technologies.Coal pyrolysis for tar production is crucial in the strategic layout of national coal-to-oil production.Despite the early initiation of coal pyrolysis base construction,several issues persist,including low tar yield,poor quality,and significant coking.Addressing these problems requires effective regulation of the coking reaction of volatiles during coal pyrolysis.To achieve these objectives,this paper examines the direct liquefaction and pyrolysis of coal.The research focuses on two main areas:"maceral contents activities and their interactions in the direct liquefaction of coal"and"volatiles’reaction during coal pyrolysis."Firstly,an oil-rich coal with exinite contents of 17.1%from Hami,Xinjiang,was used as the research object,and the isopycnic density gradient centrifugation（DGC）technique was used to obtain four coal samples with different maceral contents,with those of lower density exhibiting a higher content of the exinite content.Eight covalent bonds concentrations of the raw coal and four components were calculated based on the results of ¹³C NMR,elemental analysis,and FTIR,and a matrix computation was proposed to represent the bonding structure model of oil-rich coal.Based on the typical bonding structure model of oil-rich coal,the liquefaction of exinite and vitrinite components was studied in a hydrogen donor solvent medium,and the interaction between exinite and vitrinite in direct liquefaction was also explored.In addition,the coking reaction of volatiles during coal pyrolysis of three low and medium rank coals were studied in a two-stage reactor by using Naomaohu coal from Xinjiang with excellent pyrolysis characteristics and selecting Zhundong coal and Zichang coal,which have been applied in industrial operation,as a comparison.The variation of tar properties and coke structure is analyzed by Fourier transform infrared spectroscopy（FTIR）,electron spin resonance（ESR）,Gel permeation chromatography（GPC）,and near-edge X-ray absorption fine structure（NEXAFS）,and the kinetic model of the coking reaction was established and the reaction mechanism was discussed.Thirdly,the mechanism of the volatile reactions on the formation of the products during coal pyrolysis was explored from the perspective of the radical reactions based on the molecular structures of three low and medium rank coals.Finally,model compounds were built based on the organic structure of coal and its pyrolysis characteristics,and ab initio methods were employed to investigate the homolytic cleavage of covalent bonds,the free radicals’generation,the free radicals’stability,and the free radicals’coupling process during coal pyrolysis,and the following main conclusions were obtained:（1）The average aromatic ring of oil-rich coal is two to three rings with approximately a quarter of substitutive degree,and the oxygen-containing functional groups in substituents are more than carbon-containing functional groups.The content of weak bonds in oil-rich coal is 44.5%,and with an increase in the exinite content,the alkyl side-chain and methylene bridge-chain become longer.The bonding structure model for the oil-rich coal obtained based on theoretical matrix calculations shows a predominance of aliphatic carbon structures in the pure exinite and the absence of aromatic structures.There is a synergistic effect between the exinite and vitrinite during the liquefaction process.When the liquefaction temperature is no more than 420℃,the interaction of exinite and vitrinite promotes radicals coupling,reduces hydrogen consumption,and facilitates the hydrocarbon gases generation,while the liquefaction temperature higher than 440℃,this interaction intensifies the secondary cleavage of radical fragments,increases hydrogen consumption,and promotes condensation and coking.（2）The temperature of tar condensation（540 ℃）is 60 ℃ lower than that of coke formation（600 ℃）.The oxygen-containing functional groups in tar can inhibit the coking of volatiles,although oxygen content is higher in the initial coking.The coking behavior of volatiles can be expressed by first-order+autocatalytic kinetics with activation energy lower than 40k J/mol when the high temperature volatiles produced by the first-stage reactor directly enter the second-stage reactor to generate coke.The activation energy of volatile coking is significantly lower than that of coal tar decomposition to coking.The oxygenated structure of the Coke_-tarserves to inhibit the formation of further coke,which condenses at higher temperatures to form Coke_wall with a higher number of aromatic rings.（3）NMH coal contains more H·in the volatiles compared to ZD and ZC coals,so regulating the volatile reactions of NMH coal is more effective in improving the tar quality.The contents of PAHs in the three coal tars increased as the temperature of the volatile reaction raised from 440 to 700℃,and the breaking of bridge bonds in the volatile reaction is the dominant process for the increase of PAHs,with fewer PAHs formed by the condensation of volatiles.The NEXAFS was used to analyze the carbon and oxygen structure in raw coal and coke,it was found that the aliphatic carbon in coke is significantly enhanced and the R-C could be converted to Ar-C·with increasing temperature of the volatile reaction,and the O in coke exists mainly as C=O.（4）Model compounds were created based on the organic structure of coal and combined with chemical thermodynamics,kinetics,and the formation of free radicals from coal pyrolysis.Free radicals are formed from readily breakable groups in the organic structure of coal at lower temperatures of pyrolysis.These radicals not only stabilize other radicals but also couple with each other,leading to the formation of coke,which includes:oxygen-centred（R?）,aromatic methyl（Ar?H₂）,tertiary carbon（R₃?）,and oxygen-containing carbon-centred（O?R）radicals.From the kinetic perspective,extending the residence time of volatiles is conducive to the formation of coke.Stable radicals couple to create heavier components during the transition state and energy barriers.Increasing the temperature enhances the energy of the coupled reactants（stable radicals）making it possible to cross the transition state of the energy barriers and coupling reactions,resulting in higher temperatures conducive to the formation of coke.