Experimental study on molecular structure differences between the tectonic coal and primary coal in Pingdingshan coalfield

In order to investigate the microcosmic differences between tectonic coal and primary coal, the fault coal sample D, soft stratified coal sample R and primary coal sample P were obtained from the same geological unit of Pingdingshan coalfield. The microstructures of those three coal samples were tested by adopting the Fourier transform infrared spectroscopy (FT-IR) and Nuclear magnetic resonance spectrum of carbon (C-13-NMR). Results show that: the peaks of FT-IR and C-13-NMR testing curves of those three coal samples are similar, which reveals that coal mainly contains aliphatic groups (e.g. methyl groups), small amount of oxygen-containing functional groups and more aromatics groups. However, the peak fitting of those testing results and the calculation of relevant parameters indicate that, the hydrogen enrichment degree of coal samples D and R are 41.07% and 34.75% higher than that of P coal sample, respectively. Meanwhile, the oxygen-rich degree of those two tectonic coal samples are larger than that of the primary coal. The coal aromaticity D-fa is 0.78, R-fa is 0.80, P-fa is 0.86, thus the aromaticity of tectonic coal is less than that of primary coal. Moreover, according to the calculation results of XBP (the ratio of aromatic bridge carbon to peripheral carbon), it could be concluded that the degree of aromatic structural condensation of tectonic coal is obviously smaller than that of primary coal. Compared with primary coal, the tectonic coals generally have the structural characteristics of retrograde metamorphism on the micro level. This could be a potential microcosmic index for predicting the tectonic regions in coal seam and could provide a microcosmic guidance for controlling gas outburst.