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 (R_o,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 R_o,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 R_o,max=1.10%−1.63% can be divided into three stages. The turning points are located near R_o,max=1.30% and R_o,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 R_o,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 R_o,max=1.30%, with the smallest W_G, the largest F_G/D, the smallest A_D/A_G, the increase of A_S/A₁, and the significant decrease of A_(2G)R/A₂. In the second stage of R_o,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)/A_D, A_(GR+VL+VR)/A_G and F_G/D decreased significantly, A_D/A_G increased, W_G and d_(G-D) increased quickly. The last stage is R_o,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 W_G, and an increase in A_(GR+VL+VR)/A_D and A_(GR+VL+VR)/A_G. These results are the basis for deeply understanding the mechanism of coalification jump and coalification.