Construction Of Structural Model And Molecular Simulation Of Methane Formation Mechanism During Coal Pyrolysis For Shendong Vitrinite

With the development of sustainable development strategy in China, west China playing a role of the strategic depth has been increasingly concerned. Coal reserve of the west China accounts for 40% of it in China. As a result, taking the Western coal as material, the product with diversification, high technological content and high added value will be a reasonable way to make use of the Western coal. Basic research on coal chemistry and thermal conversion technology of Western coal with weak reduction properties have important significance for high efficient and clean burning at present and in the future.Taking Shendong coal from west China as the research object, sink-float centrifugation was used to get the vitrinite. ¹³C CP/MAS NMR, computer assistant molecular design (CAMD), thermal gravimetry–mass spectrum (TG-MS) and molecular simulation were used to study the structural characteristics of SV and formation mechanism of methane in the pyrolysis products, main content as follow:1. ¹³C CP/MAS NMR was used to study the structural characteristics of SV. The result showed that aromatic structure units were dominated by naphthalene including two rings. Furthermore, benzene rings and heterocyclic aromatic containing heteroatom were the predominant in the other aromatic structure units.2. Based on the test results of ¹³C CP/MAS NMR and ultimate analysis of SV combined with XPS analysis results, initial chemical structure model of SV was constructed. ACD/CNMR preditor software was used to simulate the ¹³C NMR spectra of initial chemical structure model. According to the comparison results of simulation spectrum with experimental spectra, initial chemical structure model of SV was modified. Furthermore, chemical structure model of SV was in good agreement with the results of experiments.3. Molecular Simulation (MM and MD) was adopted to simulate the minimization geometry of SV model, relaxation process and reversibility of model structure changing after heat- treatment. It is showed that structural parameters (d002= 3.83 (?), La=12.6 (?) and Lc =8.09 (?)) of SV structural model was basically identical with experimental data (d002=3.85 (?), La=12.3 (?) and Lc = 8.52) of XRD. Force condensing the whole macromolecule framework in the coal macromolecule is mainly from non-bonding energy, while the non-bonding energy is dominated by van der Waals energy. Hydrogen bond in the non-bonding energy was mainly caused by molecular interaction. Aromatic lamella of structure model were almost in parallel permutation, and simulation value of SV model density was 1.13 g·cm-3. In the simulation results of relaxation process, the structure changed distinctly when the temperature reached 623 K. SV model structure caused from temperature changing is irreversible.4. The model structure for SV calculated by semi empirical method(AM1) suggested that C-O bond between ether bond and aromatic carbon, C-C with highcrosslinking degree and C-C bond linked with carbonyl carbon are highly active, which break easily in the pyrolysis process. In SV structure model, there were more negative charge in the N, O and edge C, while there were less charge in structural aromatic carbon.5. Based on the semi empirical method and reactivity analysis for SV and SI , in the SV pyrolysis process, primary cracking located in high crosslinking degree of coal structure and C-C bond linked with carbonyl carbon; in the SI pyrolysis process, primary cracking located inβsite C-C bond linked with aromatic carbon and C-C bond linked with carbonyl carbon.6. By means of quantum chemistry calculation combined with TG-MS, systematical analysis on formation mechanism of methane in the SV pyrolysis products indicate that there are 8 reaction types during methane generation. (1) At about 200℃. Physical desorption of adsorptive methane in coal; (2) At about 300℃. Deprotective reactions for methoxy and partial adeps- methyl ofβsite; (3) At about 400℃. Removal adeps- methyl for free radical intermediates which formed during depolymerization and decomposition of coal macromolecular structure byβsite cracking; (4) At about 460℃. Removal from methyl ofβsite in coal structure; (5) and (6) Between 500-700℃, on hydrogen free-radical. In front half part, removal for aryl-methyl in the coal structure of their own and formed at 400℃, and removal ofβsite adeps- methyl similar with 1-hexamethylindane structure; in bottom half, removal of aryl-methyl formed in the aromatization; (7) At 800℃. Direct removal of residual aryl-methyl after previous coal pyrolysis; (8) Between 900-1000℃. Reactions between H2 from pyrolysis and active sites of graphite-like structure from coal condensation reaction.