Molecular structure characterization of middle-high rank coal via XRD, Raman and FTIR spectroscopy: Implications for coalification

In recent years, the major coal producing areas in China have entered the period of deep mining, with coal rank changing from low-rank coal (LRC) to middle-high rank coal (MHRC). Even though material properties of MHRC, such as pore size and shape, have attracted increasing attentions, its molecular structures still remains unclear. This study probes molecular structural properties of five MHRC samples using X-ray diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopy. Test results show that with the deepening of coalification, the aromatic rings of MHRC increase gradually. The aliphatic side chains between the aromatic rings continuously fall off. The ordered structure of coal continues to enhance, resulting in an increase in the degree of graphitization of coal. The average lateral sizes (L-a), stacking heights (L-c) and interlayer spacing (d(002)) of the crystallite structures of MHRC samples derived from the XRD range from 20.65 to 31.68, 10.62 to 19.21 and 3.42 to 3.66 angstrom, respectively. The L-a values derived from the Raman spectra using the classical linear relationship between 1/L-a and the I-D1/I-G band ratio are higher (36.36-59.91 A) than the values obtained from XRD. The FTIR spectra reveals that MHRC samples contain aliphatic functional groups such as -CH2 and -CH3, aromatic functional groups such as C=C and aromatic ring -CH, and oxygen-containing functional groups such as -OH, C=O and C-O. The aromaticity (f(a) ) of the coal samples determined by FTIR ranges from 0.66 to 0.98, and compared with the f(a) valued from XRD (0.63 to 0.93), which shows that the MHRC contains abundant of aliphatic components. On the basis of these results, the structural parameters obtained by different spectral techniques are also used to comparatively study the effects of coalification on the molecular structure of coals. The structural parameters of coal derived by these methods reflect the coalification jumps at R-o = 0.5, 0.7-0.9, 1.2, 2.6, 3.3, 3.5-4.0 and 4.2-4.8%. Among them, the jump at R-o = 4.2-4.8% is unreported in previous studies and should be further studied in subsequent research. The application range of structure parameters used to evaluate the coal rank such as d(002), L-a (XRD), f(a) and G-D-1 should be considered because of coalification jumps. The parameters such as 'C', I-D1/I-G and L-a (Raman) are better choices for evaluating coal rank for their better correlation with R-o. It is worth pointing out that these parameters have implications for coalification and should be concerned in further study.