Insight into the macromolecular structural differences between hard coal and deformed soft coal

Coalification and tectonic deformation have significant influence on the macromolecular structure of coal. In this paper, Fourier transform infrared (FTIR) spectral and X-ray photoelectron spectrometer (XPS) measurements were conducted on soft and hard coals with metamorphism varying from bituminous coal to anthracite. Structural parameters were estimated from curve-fitting analysis, and the correlations were established for these parameters. Comparison of microstructural differences was also made between the studied soft and hard coals. The results indicate that both the apparent aromaticity, f(a), and the aliphatic structural parameter, A(CH2)/A(CH3), are positively correlated with increasing vitrinite reflectance, R-o. The oxygen-containing groups including carbonyl/carboxyl groups and hydroxy groups are gradually removed with increasing coalification. The enhancement of coalification degree is capable of elevating the atomic ratio of aromatic carbon. The overall evolutionary paths of microstructures in soft and hard coals are briefly described as the increase of aromatic rings, the decline of aliphatic methyl groups and the removal of C] O and OH groups, leading to the higher apparent aromaticity (f(a)) and maturity with coal rank increasing from bituminous coal to anthracite. Comparison of microstructural differences between soft and hard coals suggests that coal macromolecular structure is obviously affected by metamorphism degree and tectonic deformation. For bituminous coal, more C] O, OH groups and methyl groups are lost in soft bituminous coal, and tectonic deformation can promote the coalification progress to some degree. As a result, soft bituminous coal contains greater aromatic rings and higher maturity than corresponding hard coal, demonstrating the precocious evolutionary characteristic. However, it is only suitable for bituminous coal and could not be applicable to anthracite based on this study. At the stage of high-rank anthracite, the removal of methyl groups and oxygen-containing groups (C=O, OH) is faster in hard anthracites, due to the conversion of more hydroaromatic methyl structures to aromatic rings. Meanwhile, coal macromolecular structure is less affected by tectonic deformation for high-rank coals. Consequently, as compared with soft anthracite, hard anthracite contains greater aromatic rings and less C=O and OH groups. Analysis of the microstructural differences between soft and hard coals can be conducive to studying the construction of coal molecular model and the gas transportation within coal.