Evolution of structural and surface chemistry during pyrolysis of Zhundong coal in an entrained-flow bed reactor

In this study, a typical Zhundong coal sample was examined, and a series of step-wise pyrolysis experiments, where the temperature was gradually elevated in small increments, were carried out by using an entrained-flow bed reactor. In addition, chars from pyrolysis were investigated by X-ray diffraction and Raman spectroscopy with the aim of revealing the microcrystalline parameters and degree of crystallinity, defects, and disorder. Then, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were employed to observe the changes in the chemical surface properties of Zhundong coal before and after pyrolysis. The XRD and Raman spectra indicate that the transition from small aromatic ring systems to large aromatic ring systems in coal with the increase in pyrolysis temperature results in the increase in the lateral size of the basic lattice unit of carbon. Furthermore, condensation and the combination of aromatic rings cause the stacking height of the basic lattice units to first decrease and then increase. When the pyrolysis temperature is lower than 1000 °C, the content of graphite decreases, and the inter-layer spacing of the lattice changes slightly with an increase in temperature. As the pyrolysis temperature continues to rise, the content of graphite increases, the inter-layer spacing of the lattice decreases sharply, reaching a minimum of 0.347 nm at 1500 °C, which represents the degree of increase in the graphitisation of coal char and the transformation of the microcrystalline structure of coal char to that of graphite. FT-IR and XPS analyses show that exchangeable sodium is primarily connected with oxygen functional groups in coal char, and the oxygen functional groups are easily decomposed during coal pyrolysis. Meanwhile, the content of CH/CC/CC groups increases with increasing pyrolysis temperature, while that of CO decreases. As a result, there are more hydrocarbon groups on the surface of the coal char, whereas less oxygen functional groups are exposed to the coal char surface.