Study On Molecular Structure Of Vitrinite In Coking Coals And The Mechanism Of Coking During Pyrolysis

An accurate understanding of the structure and the coking mechanism during pyrolysis of coking coal is the basis for expanding coking coal resources and stabilizing coke quality.However,due to the complex composition and non-homogeneous amorphous structure of coal,its pyrolysis process contains a reaction network involving various kinds of free radicals.Current experimental methods and instrumentation are unable to accurately identify the categories of reactions involved,resulting in the inability to clearly understand the structure and the coking mechanism of coking coal.The molecular dynamics simulation based on reactive force fields provides a new way to study the radical reactions and establish the relevant coking mechanism at the molecular level.Vitrinite is the most abundant microscopic component in coking coal,which can produce a large number of non-volatile liquid phase products to bond other components during the coking process.Herein,vitrinite directly determines the caking properties of coking coal and the quality of the coke.In order to understand the structure and pyrolysis mechanism of coking coal,this work presents a theoretical study on the molecular structure and the coking mechanism during pyrolysis with vitrinite as the sample.Four coking coals with various ranks,namely Yanzhou gas coal,Awirgol fat coal,Fangshan coking coal,and Lu’an lean coal,are separated by the Zn Cl₂ density gradient centrifugation,producing vitrinite samples with>90%contents（abbreviated as YZV,AWV,FSV,and LAV）.Their caking characteristics are determined using the Gieseler plastometry and the caking index.Solid-state carbon-13 nuclear magnetic resonance spectroscopy,Fourier transform infrared spectroscopy,X-ray diffraction analysis,and Raman spectroscopy are used to characterize vitrinite samples.The molecular structure models for vitrinite samples are also established by combining the above characterization and density functional theory.AWV has the highest fluidity（MF=17675 ddpm）,the widest plasticity range（ΔT=105.1℃）,and the best caking properties（G=99.32）among the four samples.FSV and LAV with higher degree of metamorphism have larger aromatic nucleus.YZV and AWV with lower degree of metamorphism contain more aliphatic structure,while the aliphatic structure in AWV exists mainly in the form of cyclic aliphatic structures.The pyrolysis characteristics of vitrinite samples are studied using thermogravimetric-mass spectrometry and in-situ diffuse reflectance infrared Fourier transform spectroscopy.Solid products at different coking stages are prepared in the crucible furnace and characterized using Solid-state carbon-13 nuclear magnetic resonance spectroscopy,X-ray diffraction analysis,and Raman spectroscopy.With the vitrinite models as initial structures,molecular dynamics simulations of the vitrinite coking process are performed using the reactive force fields.Based on the results of the elemental analysis and thermogravimetric experiments,a simulation strategy is proposed for open systems and the simulation results consistent with the experiments were obtained.To explain the origin,composition and generation process of the mobile phase,volatile generation and carbon structure evolution mechanisms during this process are described from a microscopic perspective.From the comprehensive experimental and simulation results,it is found that the categories,amount,and temperature of volatile evolution are related to the cyclic aliphatic structure in vitrinite samples.Volatile gases mainly include H₂ and hydrocarbons,in which the gases with fewer carbon atoms account for a relatively large proportion.Hydrocarbon gases have similar generation mechanisms,that is,all of them mainly originate from alkyl carbocycles and side chains.They need to take hydrogen atoms from the vitrinite structure during the generation process.In addition,the secondary reaction of alkene gases can accelerate the hydrogen transfer in the condensed phase.Compared to the other samples,the temperature of the volatile fraction of AWV is concentrated in the plastic range,providing a large amount of transferable hydrogen for the production of the mobile phase.In the coking process during pyrolysis,the carbon structure gradually becomes ordered with increasing temperature.The expansion of the carbon layer size originates from the aromatization of the cyclic aliphatic structure in the thermoplastic stage and the cross-linking reaction between the carbon layers during the solidfication stage.The growth and stacking of the carbon layers undergo a"vertical-horizontal-vertical"trend.Cyclic aliphatic structure contributes to the formation of more mobile phases in the thermoplastic stage,which is the essential reason for the high fluidity of AWV.In the formation process of the mobile phase,the cyclic aliphatic structure in vitrinite also can act as a precursor for the aromatic ring,expanding the scale of the aromatic nucleus.Meanwhile,it also stabilizes the aromatic radicals generated by the bond-breaking reactions by providing transferable hydrogen atoms,avoiding their solidification reaction.In addition,it avoids the formation of volatile molecules that escape the vitrinite by bridge-bond breaking before the plastic range,contributing to the formation of the mobile-phase molecule with a certain scale in the plastic range.The transferable hydrogen generated from the alkyl side chains in the YZV sample escaped as volatile before the range of mobile phase,and the less content of saturated carbon structure in the FSV sample resulted in less amount of transferable hydrogen that generated by the pyrolysis reaction,so both of the two samples relatively had low mobility.While the LAV sample had the least amount of transferable hydrogen that generated from the saturated carbon structure,which made it difficult to form mobile phase.