Creation and Generation Mechanism of Macromolecular Representation for Dongsheng Coal Vitrinite

Abstract

Insights into macromolecules of coal were critical for improving the understanding of the coal upgrading and coalification process. Here, the creation and generation of macromolecular representation for Dongsheng coal vitrinite was clarified using industry analysis, elemental analysis, and the peak fitting technology of 13 C nuclear magnetic resonance ( 13 C NMR), Fourier transform infrared spectrum, and X-ray diffraction. The structural parameters and macromolecular representation (C 167 H 148 N 2 O 27 ) were innovatively calculated and created based on these characterization results and chemical shift correction, finally obtaining the plane macromolecular models whose 13 C NMR spectrum was close to the experimental spectrum. The property parameters of basic structural units were L a (average lateral sizes) = 19.917 Å, L c (stacking heights) = 24.776 Å, d 002 (interlayer spacing) = 3.488 Å, N (number of stacking layers) = 5.6213, and L a / L c < 1. Suffering from the dynamic metamorphism effects, the length of intermolecular aromatic lamellae for Dongsheng coal vitrinite was 7–8 aromatic rings in size. The aromatic clusters were dominated by benzene, naphthalene, and anthracene, and their numbers were 2, 4, and 2 per vitrinite model, respectively. Hydrogenated aromatic rings, ether bonds, and oxygen-methylene serve as the main bridge bonds to connect the aromatic clusters, where the short aliphatic chains were distributed around the edge of aromatic rings. Oxygen atoms exist in the form of hydroxyl, ether bond, carbonyl, and carboxyl groups, and their numbers were 2, 7, 4, and 8 per vitrinite model, respectively. The nitrogen atoms exist in the form of pyridine and pyrrole. The entropy weighting method was used to estimate the rationality of the macromolecular representation of long frame coal vitrinite, providing a new mathematical evaluation method for molecular simulation. Comparison of various macromolecular models from different geological conditions indicates that tectonic stress can promote the degree of aromatization and ring condensation. The thermal history and tectonic stress have a compensation effect for promoting the aromatization process. Aliphatic carbons were the most unstable units under thermal history and tectonic stress, and they are more easily removed from the aliphatic structure, followed by methyl. This finding of this paper can provide significance for coal liquefaction engineering in Dongsheng coalfield.

Carbon Structure of Coal from the CP/MAS 13C NMR Spectra: Effect of Contact Time and Potential Quantitative Modification

“…The magnetization intensity in a 13 C NMR spectrum is related to the quantity of different types of carbon atoms, and therefore peak-fitting or interval integration from a single CP/MAS 13 C NMR spectrum has been utilized to determine the carbon distribution in many studies. − However, there has been much debate on the quantitative reliability of the CP/MAS experiments when using coals with complex and heterogeneous structures as samples. ,− Some studies ,, have reported that the apparent intensity of 13 C NMR spectra is merely qualitative, and the derived structural parameters deviate from other analytical approaches to some extent. Values for the carbon aromaticity of coals obtained from the CP experiments are generally lower than those from the Bloch-decay experiments. , In the CP/MAS 13 C NMR experiments, aromatic and aliphatic carbons have different cross-polarization efficiencies; thus, a single spectrum at insufficient contact time may not entirely represent the magnetization of all carbon types. , Furthermore, even for aliphatic carbon alone, the rate of polarization is affected by the number of attached protons and the distance from the surrounding protons .…”