Macromolecular Structure Changes of Tectonically Deformed Coal: Evidence from Coal Pyrolysis, 13C NMR, and XRD Experiments

Research on tectonically deformed coal (TDC) is of significance to gas outburst prevention and coalbed methane development. Despite numerous works focusing on the structural characteristics of TDC, the influence of tectonic stress on coalification is a long-term controversial issue. In this study, the X-ray diffraction (XRD), C-13 solid-state nuclear magnetic resonance (NMR), and coal pyrolysis techniques were applied to compare the macromolecular structural characteristics and gas generation properties between weak TDC (WTDC) and strong TDC (STDC). The results show that the strong tectonic stress obviously promotes the evolution of coalification in advance. The basic structural unit (BSU) of the WTDC and STDC both increases with increasing coal rank, but the d((002)) values of the STDC are usually less than those of the WTDC, and the L-c, L-a, and N values of the STDC are larger than those of the WTDC. Furthermore, the impact of tectonic deformation on the BSU of coals becomes weaker as the coal rank increases. The aromaticity in coals is accelerating and increasing with the enhancement of deformation degree. The fractions of protonated aromatic carbon (f(H a)) and aromaticity (f(a)) in the STDC are higher than those in the WTDC of the same coal rank, while the relative contents of the oxygen-substituted aromatic carbon (f(a)(O)) aliphatic-substituted aromatic carbon (f(a)(S)), and aliphaticity (f(al)) in the STDC are lower than those in the WTDC. The gas generation potential of the STDC is lower than that of the WTDC of the same coal rank from the coal pyrolysis data. Furthermore, with an increase of temperature, the difference in the generation capacity of CH4 and H-2 between STDC and WTDC increases. The research results help us to illustrate the effect of tectonic stress on coalification and supply guidance on the prevention and management of gas outburst disasters.

Automated summary

Research on tectonically deformed coal (TDC) is important for gas outburst prevention and coalbed methane development. This study used XRD, C-13 solid-state NMR, and coal pyrolysis techniques to compare the macromolecular structural characteristics and gas generation properties between weak TDC (WTDC) and strong TDC (STDC). The results show that strong tectonic stress promotes coalification evolution in advance and has an impact on the structure and gas generation potential of coal.