Effect of sequential thermal dissolution on the structure and pyrolysis characteristics of Naomaohu lignite

Abstract

Cyclohexane-insoluble portion (CHISP), ethanol-insoluble portion (EISP) and isopropanol-insoluble portion (IPISP) were obtained by sequential thermal dissolution (STD) from Naomaohu lignite (NL), with cyclohexane (CH), ethanol (E) and isopropanol (IP) as the solvent. The insoluble portions were characterized by Ultimate analysis, Scanning electron microscope (SEM), X-ray photoelectron spectrometer (XPS), Fourier transform infrared spectrometer (FTIR), Thermogravimeter (TG-DTG) and Pyrolysis-chromatography/mass spectrometry (Py-GC/MS). As a result, compared to NL, the gradually increased of C element and decreased of O, N and S in the insoluble portion (ISP) demonstrated that STD is a carbon-rich and heteroatom removal process. The yield of CHISP reached 98.64 % and exhibited smooth surface. The existence of –COO– species in EISP and IPISP indicated that E and IP might be esterified or ester-exchanged with carboxylic acid, while the reduced >CO species illustrated that the nucleophilic oxygen atom in E and IP attacked the carbon atom on the carbonyl group in NL. The comprehensive structural parameter (CSPM) and comprehensive volatile release characteristics index (CVRCI) of NL, CHISP, EISP and IPISP were calculated by FTIR and TG-DTG, respectively. Results showed that STD process improves pyrolysis reactivity, and the CSPM was directly related to CVRCI, presenting a linear relationship (R2=0.952). That is, CSPM can be used as an important parameter to predict the pyrolysis reactivity (CVRCI) of coal. Py-GC/MS showed that, compared to NL, the number of group component was increased in CHISP and decreased in EISP and IPISP during the sequential thermal dissolution process, indicating that there might be non-covalent bond break and formation during the process