13 C NMR spectra of tectonic coals and the effects of stress on structural components

Abstract

High-resolution 13C Nuclear Magnetic Resonance (NMR) spectra of different kinds of tectonic coals were obtained using the NMR (CP/MAS+TOSS) method. On the basis of this, after simulation synthesis and division of spectra, the relative contents of carbon functional groups were calculated. Combined with results of R o, max , XRD testing and element analysis, stress effects on the composition of macromolecular structures in tectonic coals were studied further. The results showed that R o, max was not only the important index for describing coal rank, but was also effective for estimating the stress effect of tectonic coals. Under tectonic stress action, R o, max was the most direct indicator of the coal structure and chemical components. Changes in the stacking L c of the coal basic structure unit (BSU) and L a / L c parameters could distinguish the temperature and stress effects on metamorphic-deformed environments, and reflected the degree of structural deformation. Therefore, on the whole, L c and L a / L c can be used to index of the degree of structural deformation of tectonic coals. In different metamorphic and deformed environments, different kinds of tectonic coals are formed under structural stress. The changes in characteristics of the macromolecular structure and chemical composition are such that as the increase in structural deformation becomes stronger, from the brittle deformation coal to ductile deformation coal, the ratio of width at the half height of the aromatic carbon and aliphatic carbon peaks (H fa /H fal ) was increased. As carbon aromaticity was raised further, carbon aliphaticity reduced obviously and different compositions of macromolecular structure appeared as a jump and wave pattern except for in wrinkle structure coal, which might result chiefly from stress effects on the macromolecular structure of different kinds of tectonic coals. The macromoecular changes of wrinkle structure coal are reflected mainly on physical structure. In the metamorphic and deformed environments of the middle and high rank coals, different kinds of tectonic coals, when compared with the metamorphic and deformed environments of low rank coal, have a greater change in H fa /H fal , carbon aromaticity, carbon aliphaticity and a different composition of macromolecular structure. So these parameters can be used to estimate increases of coal rank, and to a certain degree, the stress effects on structural components of tectonic coals.