Weight loss behavior and pyrolysis products distribution of the insoluble portion from Zhundong sub-bituminous coal

With Zhundong sub-bituminous coal (ZSBC) and its extraction residue (ER) as the research objects, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) were used to systematically investigate the thermogravimetric behavior and pyrolysis products distribution of ER. SEM analysis showed that the surface morphology of ER was smoother than that of ZSBC, and the particle distribution was uniform, which confirmed that the extraction process mainly removed the free soluble organic matter from the coal. FTIR spectra showed that the intensity of the aliphatic —CH₂ stretching vibration peak of ER was weakened, while the characteristic peaks of aromatic ring and oxygen-containing functional groups were intact, indicating that the macromolecular skeleton structure of the coal was not damaged by ultrasonic extraction at room temperature. TG-DTG analysis showed that ER presented similar three-stage pyrolysis behavior to raw coal, while with the increase of heating rate, the pyrolysis characteristic temperature of ER shifted to high temperature region, and the coke yield decreased from 66.63% to 63.11%, indicating that the increase of heating rate promoted the formation of volatiles, which was conducive to the pyrolysis process of ER. The pyrolysis activation energy (E_a) of ER was calculated by Friedmann, OFW and KAS kinetic methods, and it was found that, with the conversion rate (α) increased from 0.2 to 0.8, the value of E_a was 39.07−654.82, 29.60−662.67, and 34.76−675.73 kJ/mol, respectively. The results show that the pyrolysis process gradually transitions from weak bonds that are easy to break to polycondensation of aromatic rings that require high energy. Thermodynamic parameters (ΔH, ΔG, and ΔS) confirmed that the pyrolysis of ER was a non-spontaneous reaction with endothermy and entropy reduction. Py-GC/MS analysis showed that the aromatic hydrocarbon content in ER pyrolysis products increased significantly at 700 ℃ compared with 450 ℃, while phenols decreased, which was mainly attributed to the aromatization reaction and secondary cleavage caused by ether bond cleavage. Based on the product distribution, a multipath pyrolysis mechanism including primary pyrolysis and secondary reaction was proposed. In summary, the research reveals the characteristics of the structure and pyrolysis kinetics of ER, and the relationship between the characteristics and the pyrolysis products, and thus provides a theoretical basis for the sequential conversion of ZSBC.