Raman spectroscopy characterization of structural evolution in middle-rank coals

The second coalification jump which occurred during the middle-rank led to abrupt changes of many physical and chemical properties of coal, and the change of the aggregate structure may be the fundamental reason. In order to investigate the structural evolution characteristics of middle-rank coal and its relation with the second coalification jump in detail, the structure characteristics of six middle-rank coals (Ro, max=1.10%−1.63%) that across the second coalification jump were studied by Raman spectroscopy, and the structural parameters were calculated by fitting the first-order and second-order Raman spectrum using the fitting software. The results indicated that the evolution of Raman structural parameters with Ro, max is not linear, reflecting the complexity of the structural evolution of coal. According to the evolution characteristics of Raman structural parameters, the coalification during the stage of Ro, max=1.10%−1.63% can be divided into three stages. The turning points are located near Ro, max=1.30% and Ro, max=1.50%, respectively, which are exactly equivalent to the positions of the second and the third coalification jump discovered in previous research. It indicated that the Raman structural parameters can reflect the occurrence of the coalification jump, moreover, Raman spectroscopy is an effective method to study the coal structure. The first stage is Ro, max=1.10%−1.30%, the long-chain aliphatic structures cracked and the remained shorter-chain aliphatic hydrocarbons and aliphatic substituted structures on the aromatic rings will form new alicyclic structures, which caused the branched degree increases and hindered the alignment of aromatic systems in coal. The order degree of aromatic system is thus reached the least near Ro, max=1.30%, with the smallest WG, the largest FG/D, the smallest AD/AG, the increase of AS/A1, and the significant decrease of A(2G)R/A2. In the second stage of Ro, max=1.30%−1.50%, the aromatization of the alicyclic structures formed in the previous stage resulted in an increase in the content of aromatic C-H structure and the least of amorphous carbon structure. Besides, the degree of aromatization and aromatic structural both increased, which showed that A(GR+VL+VR)/AD, A(GR+VL+VR)/AG and FG/D decreased significantly, AD/AG increased, WG and d(G-D) increased quickly. The last stage is Ro, max=1.50%−1.63%, the condensation reaction occurred between the aromatic rings formed in the second stage, leading to the reduction of A(2G)R/A2. Meanwhile, the various bridging bonds between aromatic ring systems continued to break, resulting in the formation of some small-scale aromatic structures, as evidenced by a decrease in A(2G)R/A2, a small decrease in WG, and an increase in A(GR+VL+VR)/AD and A(GR+VL+VR)/AG. These results are the basis for deeply understanding the mechanism of coalification jump and coalification. © 2023 China Coal Society. All rights reserved.