Molecular Simulation On Oil Shale Kerogen Molecular Structure During Pyrolysis Reaction

The traditional thermal analysis method can give the thermal transformation characteristics or laws of kerogen on the macro level,but it is still very difficult to reveal the pyrolysis mechanism and infer the pyrolysis process only through the pyrolysis curve and the corresponding products.The function of the molecular model of kerogen is to combine the analytical data obtained by various methods to form a theory that can be used to judge and predict the chemical reactivity of kerogen.Using molecular simulation technology to study the molecular structure of kerogen and correlate its pyrolysis reactivity will greatly deepen the understanding of the changes in the macromolecular structure of kerogen as well as the generation and distribution of products during the thermal transformation of oil shale.Therefore,it is necessary to understand the pyrolysis process of kerogen at the molecular level,understand the evolution mechanism of its molecular structure in the thermal transformation process,and obtain the chemical reaction details of main products and intermediate products,so as to reveal the relationship between the molecular structure of kerogen and its reactivity.The oil shale samples in this paper were obtained from Huadian of Jilin Province,Fushun of Liaoning Province,Longkou of Shandong Province,Maoming of Guangdong Province,and Yaojie of Gansu Province,respectively.By the molecular simulation and experimental research on the molecular structure of kerogen and its pyrolysis reaction process were carried out.By means of rapid pyrolysis gas chromatography-mass spectrometry,solid nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy,the organic structure of the samples was studied in detail.Based on the analysis of the detection spectrum,the carbon skeleton composition,the type and content of heteroatoms,the composition and distribution of pyrolysis products and the type of characteristic functional groups of kerogen were obtained,and the microstructure characteristics of kerogen were analyzed qualitatively and quantitatively.Using statistical structure analysis method,the molecular structure parameters of kerogen were obtained,and then theoretical calculation and analysis were carried out,which provided accurate and detailed experimental data and theoretical parameters for the construction of the molecular structure model of kerogen.Based on the idea of "number of grids",energy evaluation,density evaluation,chemical bond concentration evaluation and other methods and theories were used to construct,optimize and verify the molecular structure models of kerogen with different maturity.By designing different molecular structure models of kerogen,the interference of isomerism to energy optimization process is dealt with reasonably and effectively.This method can significantly improve the accuracy of molecular modeling.By constructing the relationship between the energy and density of the multi-molecule model of kerogen,the three-dimensional molecular structure model of kerogen was finally determined,and the energy composition was analyzed.This relationship can be used to screen the reasonable molecular configuration.and greatly reduce the interference of isomerism in modeling.The results show that in the process of energy optimization,the molecular models of kerogen with different maturity show different degrees of torsion and compression.In order to meet the requirements of energy reduction process,the chain structure in each model,especially the fat chain,is highly twisted and curled to enclose the aromatic structure,while the external carbon chain tends to form a ring structure during energy minimization.The chain structure in kerogen helps to stabilize the spatial structure of the molecule and transform it into a ring or aromatic structure.In the initial stage of kerogen pyrolysis,a certain amount of energy needs to be absorbed to disassemble the molecular structure and stretch it into a chain structure before further pyrolysis reaction can occur.Based on the above kerogen three-dimensional molecular structure model,combined with quantum chemical,molecular dynamics,density functional theory,such as molecular simulation method,in-depth systematically carried out the molecular structure of kerogen of quantum chemistry and molecular dynamics simulation study,illustrates the model structure into,break key rules,implement the product distribution prediction,thus reveals the microscopic mechanism of chemical evolution during pyrolysis process,and the reactivity of organic molecular structure from an electronic point of view.The electron density distribution of carbon skeleton and heteroatomic compounds as well as the front orbital characteristics of the molecular model were obtained by studying the electronic characteristics of the molecular structure model of kerogen,and the chemical reactivity of kerogen was predicted.Based on Mayer bond level analysis,the thermal decomposition mechanism model of kerogen is proposed,which can predict the pyrolysis bond breaking behavior of kerogen quickly and accurately.In view of the thermal transformation mechanism of sulfur compounds,the migration path of sulfide was systematically simulated and experimentally studied.Through the design of the reaction path of sulfur-containing compounds,the chemical processes such as breaking of chemical bonds,dehydrogenation and free radical reaction during the pyrolysis of kerogen were studied.Based on the six molecular structure models of mother material.19 models of sulfur-containing compounds were selected,and 34 chemical reaction paths were designed.Based on the density functional theory and transition state theory,different types of sulfide in the process of conversion of reactants,intermediates and products between the evolution of the mechanism were revealed and explored.The reaction path of sulfide during the pyrolysis process was proposed and optimized,while the experiment methods are unable to provide the chemical reaction of sulfur compound in details.