Transformation of aromatic structure of vitrinite with different coal ranks by HRTEM in situ heating

Three vitrinite samples of different rank, denoted as V-YJL, V-AW, and V-J15, were investigated by in situ heating while in a High resolution transmission electron microscope (HRTEM) to study the aromatic structural transformations from 200 to 800 degrees C. Thermogravimetry coupled with Fourier transform infrared spectroscopy and mass spectrometry (TG-FTIR-MS) was employed to measure gaseous release from 30 to 900 degrees C. The relationships between aromatic structural transformations and gaseous release were also discussed in this work. The results showed that the rank of coal influenced the changes of aromatic layers. The change of naphthalene of low-rank coal (V-YJL, R = 0.61%) had four stages. For higher rank coals with R = 1.09% (V-AW) and R = 2.22% (V-J15), the changes of naphthalene had three and two stages, respectively. The increase in naphthalene content occurred at the same temperature with the release of CO2 and H2O, suggesting the increase of naphthalene probably caused by the decomposing of oxygen-containing functional groups. The noticeable change in 4 x 4 aromatic layers is from 650 to 750 degrees C, accompanying CH4 release, suggesting the increase of 4 x 4 aromatic layers may result from the break of bonds like aryl-methyl bonds or aryl-alkyl bonds.