Comprehensive Investigation of Various Structural Features of Bituminous Coals Using Advanced Analytical Techniques

An in-depth investigation was carried out on five Chinese coals using a range of advanced analytical techniques focused specifically on extracting structural parameters. Detailed investigations were carried out using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray diffraction followed by peak deconvolution and data analysis. Correlations were established for parameters determined from different techniques. The FTIR data showed good linear relationships between the apparent aromaticity (f a(FTIR)) and (R/C)u with the H/C atomic ratio for all coals under investigation. These results indicate that FTIR spectroscopy coupled with appropriate data analysis can be successfully used to determine aromaticity and the coal rank. Raman spectroscopy data showed a negative linear relationship between the GL fraction and H/C ratio; no well-defined relationship was observed between other band fractions and the H/C ratio. The decrease of A D/A G with increasing H/C ratio indicates the growth of aromatic rings; i.e., the structure of the sample was closer to that of graphite. This result is in good agreement with the decrease of apparent aromaticity (f a(FTIR)) as determined by the FTIR spectroscopy. A good linear relationship was observed between the structural parameters (f a(X‑ray) and R X‑ray) determined with X-ray and coal rank (represented by the H/C ratio). Even though the correlations among parameters derived from three techniques showed a similar trend and were consistent with each other, FTIR, and X-ray diffraction techniques were found to be better than Raman spectra to characterize coal maturity. These findings have led to a simplified coal model based on the complementary information from different techniques on various aspects of the coal structure.