A lignite molecular model was constructed based on experimental results of multiple techniques, which was then adjusted by concentrations of covalent bonds.

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Following the low-temperature preoxidation of Zhundong coal, the relationship between oxygen-containing functional groups and the release of Na and Cl during pyrolysis was systematically investigated using a one-stage quartz fluidized-bed/fixed-bed experimental system. The different forms of Na were quantified by sequential extraction and inductively coupled plasma optical emission spectrometry (ICP-OES), while the different forms of Cl were quantified using the Eschka method and X-ray photoelectron spectroscopy (XPS). Fourier transform infrared (FTIR) spectroscopy was also employed to characterize the functional groups. During oxidation at 200 °C, large quantities of Cl were released, and oxygen-containing functional groups were produced. In addition, inorganic Na was converted into organic Na through ion exchange between Na and the oxygen-containing groups. In this study, NaCl was loaded onto the acid-washed coal sample, but it was found that the release of Na and Cl was not coordinated during pyrolysis. Indeed, the preoxidation of coal effectively inhibited the release of Na and promoted the release of Cl at 500 and 600 °C, whereas at 700, 800, and 900 °C, Cl release was inhibited. On the basis of the results obtained from sequential extraction and XPS analysis, it was apparent that in preoxidized coal samples, the conversion of inorganic Na/Cl to organic Na and organic Cl was promoted during pyrolysis. In addition, the decrease in Cl content in the gas phase may account for the fixation of Na.