Quantitative investigation on the structural characteristics and evolution of high-rank coals from Xinhua, Hunan Province, China

Abstract

The structural features and evolution from anthracite to meta-anthracite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). All the high-rank coals (4.23% < Rr < 8.23%) show a broad (002) peak in their XRD patterns, and the relative intensity of (002) and (10) diffraction peaks increase slightly with coal rank. The crystallite parameters Lc and La derived from XRD data vary around 2 nm and 5.67–9.14 nm, respectively, but do not change significantly with rank. Five types of carbon bonds were peak-fitted from the original XPS spectral profiles, including sp2 hybridized CC (64%–74%), followed by C–C/C–H, carbon–oxygen bonds, and π plasma. The lattice-fringe lengths of anthracites are mainly in the range of 0.6–1.5 nm examined by HRTEM, and trace amount of short aliphatic chains were detected in anthracite samples from micro-FTIR, whereas more aromatic rings condensed into larger polyaromatic clusters in the meta-anthracites and the residual aliphatic carbon atoms are in the form of quaternary aliphatic carbon forming Y-type dislocation. The Y-type dislocation and screw dislocation between edges of lattice-fringes are common in the meta-anthracite compared to anthracite; in addition, bent carbon layers are more frequent. Quantitative information of lattice-fringes was used to investigate the structural evolution of coal. The average diameter of local molecular-oriented domains increases with rank for the meta-anthracite samples, as does the degree of preferential orientation, but these local oriented domains are still distributed randomly at sub-micron scale and without stiff stacked graphene layers.