Structural characterization of high fidelity for bituminous and semi-anthracite: Insights from spectral analysis and modeling

Abstract

Although molecular structure evolution in response to coalification process has been reported to be prevalent recently, their characterization accuracy for coals of different ranks still needs to be enhanced. In this paper, the occurrence information of the carbon skeleton structure, functional groups, and nitrogen and sulfur heteroatoms in coals of different ranks (Ro, max = 0.99% for XZ-02, 1.39% for YT-09, 1.63% for YT-11, 2.29% for ZC-15) were extracted via ultimate and spectrum analysis. Then four molecular structure models were created using computer-aided molecular design (CAMD). Results show that the oxygen atoms in coal primarily exhibit phenolic hydroxyl groups, ether groups, and carbonyl groups. The nitrogen heteroatoms are present as pyrrole and protonated pyridine, and the sulfur heteroatoms mainly served as thiophene. The content of aliphatic structures gradually decreased as coalification continued. Their length first decreased (Ro, max ≤ 1.9%) and subsequently increased (Ro, max > 1.9%). The content of aromatic structures gradually increased accompanied by the gradually cleaving of oxygen functional groups and alkyl side chain from the aromatic clusters. Hence, the degree of aromatization gradually increased, and condensation reactions dominated in the late stage of coalification. The initial molecular formulas and structural models were determined for the four coals. Also, the bonding modes among the aromatic clusters, aliphatic side chains, and oxygen functional groups within the molecular structure were adjusted based on the comparison between experiment and calculation spectra, finally creating the following molecular representations: C174H128O8N2 (XZ-02), C167H114O5N2S2 (YT-09), C151H96O7N2 (YT-11), and C146H80O4N2 (ZC-15). The fitting carbon spectra of these molecular representations were consistent with the experimental carbon spectra whose error is within the allowable range (relative error < 15%), verifying the applicability of the molecular structure model and the above method. Furthermore, the mechanism of coalification was discussed based on the variations of the side chains and chemical bonds. The above research can provide a theoretical reference for the molecular structural evolution for coals of different ranks and a high fidelity method to create molecular structure representation.