Construction Of Molecular Model Of Huolinhe Lignite And Study On The Pyrolysis Reactions By Molecular Dynamics Simulations

Pyrolysis upgrading can effectively solve the problems of high moisture content and easy spontaneous combustion of lignite,and the pyrolysis products are energy materials with high added value,which is an effective way for the efficient and clean utilization of lignite.Coal pyrolysis process is very complicated,involving many coupled chemical reaction pathways,which makes it difficult to understand the pyrolysis reaction mechanism deeply by using experimental methods alone.Reactive molecular dynamics(Reax FF-MD)simulations can provide information on the breaking and recombination of chemical bonds in complex macromolecular systems.With the help of Reax FF-MD simulations,the relationship between coal structure and reactivity can be explored at the molecular level,and the microscopic reaction mechanism of the pyrolysis process can be revealed.In this dissertation,Huolinhe lignite was taken as the research object,and three key issues were studied,including the construction of molecular structure model of lignite,molecular dynamics simulations of isothermal pyrolysis and molecular dynamics simulations of non-isothermal pyrolysis.Firstly,the chemical characteristics of Huolinhe lignite organic structure were comprehensively studied by using ultimate analysis,solid carbon nuclear magnetic resonance spectroscopy(¹³C NMR),Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS).The results show that the carbon skeleton of Huolinhe lignite organic structure was mainly composed of small ring aromatic structures such as benzene and naphthalene,and the main connection types of the benzene ring were three-substituted and four-substituted.The aliphatic structure mostly existed in the form of short-branched methylene chains.The main forms of oxygen,nitrogen and sulfur were phenolic hydroxyl,pyrrole and thiophene.The typical structural parameters were analyzed and compared by FTIR and ¹³C NMR techniques,and the results were in good agreement.Based on the characteristic parameters of organic structure of coal,a two-dimensional(2D)molecular model of Huolinhe lignite was preliminarily constructed by the method of“average structural unit”.The molecular model was modified by the concentration of the main covalent bonds in coal.The optimal three-dimensional(3D)molecular model of Huolinhe lignite was obtained by geometric optimization and simulated annealing.The rationality of the model was evaluated by physical density and radial distribution function.The results show that the relative error of the concentration of the main covalent bonds between Huolinhe lignite molecular model and coal was less than 5%,indicating that the model can well reflect the chemical properties of coal.Model optimization significantly reduced the total energy of the system,and the change of van der Waals energy was the most obvious,which showed that van der Waals energy played an important role in the stability of the model.By comparing the physical density and radial distribution function of coal obtained by simulation and experiment,it is confirmed that the molecular model of Huolinhe lignite was reasonable.Reax FF-MD simulations were used to study the pyrolysis products distribution,product evolution tendency,migration mechanism of main elements and microscopic reaction mechanism of isothermal pyrolysis in the temperature range of1600-3000K.The results show that,at lower temperatures(below 2200K),the primary pyrolysis reactions were dominant.Lignite mainly underwent decomposition of macromolecular structure and breakage of bridge bonds.Moreover,increasing the temperature was conducive to the migration of carbon,hydrogen,and oxygen in char to tar and pyrolysis gas.At relatively high temperatures(above2200K),the secondary reactions played an important role.Increasing the temperature was conducive to the migration of carbon in coal into char through condensation.At the same time,most of hydrogen entered into pyrolysis gas by decomposition.The influence of heating rate on the product distribution,product evolution and potential energy of the system during non-isothermal pyrolysis of Huolinhe lignite were researched by Reax FF-MD simulations.According to the simulation trajectory and products analysis,the reaction pathways of typical products and non-isothermal pyrolysis reaction mechanism were explored.The results show that,at low heating rates(2K/ps and 10K/ps),increasing the heating rate had a significant effect on the pyrolysis products,and the initial temperature of primary decomposition reaction and secondary reaction were obviously delayed.Moreover,the lower the heating rate was,the stronger the secondary reaction of tar was.At relatively high heating rates(20K/ps,50K/ps and 100K/ps),the inhibition effect of increasing heating rate on the secondary reaction of tar was weakened,and the influence of changing heating rate on pyrolysis products was small.The optimal yield of tar was affected by both pyrolysis temperature and heating rate.At lower temperatures,thermal hysteresis played a leading role.The lower the heating rate,the higher the tar yield.At higher temperatures,the inhibition effect of high heating rate on the secondary reaction of tar became prominent.The influence of heating rate on tar yield should take into account the thermal hysteresis and the secondary reaction of tar.