Evolutionary Analysis of Coal Molecular Structure under Different Metamorphism: An Attempt to Use Electron Paramagnetic Resonance

[Image: see text] The metamorphic process of coal is often affected by magma intrusion and stratigraphic tectonic movement. It is of great significance to analyze the evolution characteristics of coal structure under different metamorphisms in the study of coalification. Nineteen coal samples collected from Huainan coalfield (Zhangji coal mine, Panji-4 coal mine, and Xinji-2 coal mine) were tested by electron paramagnetic resonance (EPR) to clarify the average evolution information on organic molecular structure during coal metamorphism. The results showed that compared with deep burial metamorphism, magma intrusion and stratigraphic tectonic stress could deepen the metamorphic degree of coal. For coal samples under three kinds of metamorphism, the relationship between three EPR parameters (free radical concentration N(g), line width ΔH, and Lande factor g) and vitrinite reflectance R(max)(0) approximately satisfied a quartic polynomial distribution. At the same time, with the increase of R(max)(0), the changes of EPR parameters fluctuated, showing a phased and stepwise trend of change. Compared with related studies, the results showed that there were great differences in the N(g) and ΔH distribution between high-rank and medium-rank coals during organic evolution. In this paper, the functional relationship between EPR parameters of coal and R(max)(0) under different metamorphisms was constructed, and the internal relationship between them and its influencing factors was analyzed, which provides a reference for the study of molecular structure evolution characteristics of coal in Huainan coalfield.