Structural Characteristics of Coal Vitrinite during Pyrolysis

Abstract

Vitrinite samples inserted into a furnace at room temperature and heated at a rate of 10 °C/h were subjected to X-ray diffraction analysis, Fourier transform infrared spectroscopy (FTIR), and solid-state 13C nuclear magnetic resonance spectroscopy (13C NMR) to characterize the macromolecular structure of vitrinite of different ranks of coal, which was discussed with respect to changes in its chemical structure. The results demonstrate that the relationship between average reflectance of the vitrinite and temperature is linear. The structural parameters of vitrinite separated from the coking coal sample collected from the Lujiatuo mine (LJTV) (d002 = 3.56 Å, Lc = 11.62 Å, La = 10.99 Å) were obtained. The FTIR spectra include bands characteristic of aliphatic C–H stretching, with the ratio of aliphatic oxygen-containing compounds decreasing with increasing rank of the vitrinite samples. The C═O stretching contribution is lower than the aliphatic C–H stretching contribution, whereas the aromatic carbon contribution is high in all of the samples. The vitrinite structural parameters, e.g., the A factor, C factor, CH2/CH3, Aar/Aal, Al/OX, Al/C═C, and C═O/C═C, were calculated. The intensity of the aromatic carbon peak is considerably greater than that of the aliphatic carbon peak. The 13C NMR spectra reveal that the aliphatic carbon content decreases progressively with increasing thermal maturity for the replacement of aromatic hydrogens by condensation. The CCH3 groups are removed more slowly than are the C(CH2)C groups. The coalification progress was divided into two stages based on the CH2/CH3 ratio, which first decreases and later increases. As revealed by the structural parameter fa, the aromaticity of vitrinite increases during pyrolysis.

Structural Characteristics of Coal Vitrinite during Pyrolysis

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