Comparison of structural characteristics of high-organic-sulfur and low-organic-sulfur coal of various ranks based on FTIR and Raman spectroscopy

High-organic-sulfur coal has unique structural characteristics due to its high organic sulfur content. To study the influence of organic sulfur in coal on the molecular structure of coal, a series of high-organic-sulfur coals and low-organic-sulfur coals of different coal ranks were selected, and their structural parameters were compared and analyzed through FTIR and Raman spectroscopy. Through comparative analysis of the FTIR structural parameters of high-organic-sulfur coal and low-organic-sulfur coal, the aromatization parameters of coal do not increase linearly with increasing coal rank. And the aromaticity of low-organic-sulfur coals is higher than that of high-organic-sulfur coals at the same coal rank. The aromatization parameter DOC of high-organic-sulfur coal is relatively discrete with changing coal rank. By analyzing the FTIR maturity parameter ‘C’, it is found that high organic sulfur content in coal could diminish the maturity of the coal when the coal is in the overmature stage. And the A(CH2)/A(CH3) value of high-organic-sulfur coal is lower than that of low-organic-sulfur coal at the same coal rank, which is consistent with the analysis results of hydrocarbon-generating potentiality, when the coal rank is low, the hydrocarbon-generating potentiality of high-organic-sulfur coal is larger than low-organic-sulfur coal. Raman analysis shows that the degree of graphitization of high-organic-sulfur coals of the same coal rank is lower than that of low-organic-sulfur coals, and the graphitization degree of high-organic-sulfur coals first decreases and then increases as the coal rank increases. Sulfur acts as a heteroatom to hinder the graphitization of coal. These research results allow for us to better understand the influence of organic sulfur on the aromatic structure, aliphatic structure and the degree of graphitization of coal, which puts forward the guiding significance of scientific theory for the comprehensive utilization of coal with organic sulfur in different content.