Carbon structure evolution of coal slime under different pyrolysis methods: Quantitative analysis of carbon structure and finite-element simulation calculations

Microwave pyrolysis (MWP) has exhibited several advantages compared to conventional pyrolysis (CP). However, comprehensive analysis of the changes in the carbon structure of coal-char from various aspects has not been systematically reported. This paper investigated the carbon structure properties of coal slime (CS), and the temperature distribution within MWP-char was analyzed using finite-element simulation. The results demonstrated that MWP-char displays lower H/C and O/C molar ratios than CP-char as the pyrolysis temperature increased. This finding suggests that microwaves contributed considerably to the deoxygenation and dehydrogenation reactions of CS and was verified by Fourier Transform Infrared (FTIR) spectroscopy results. Owing to the selective excitation of polar functional groups by microwave energy, a "local hot spot" was generated inside the sample, and finite-element simulation verified this finding. The "local hot spot" in the sample promoted the formation of carbon defects in char, stimulated the transformation of small aromatic rings to large ones, and enhanced the condensation reaction of CS. The use of microwaves reduced the thickness and increased the diameter of the aromatic layer of CS-char, which increased the aromaticity, dielectric constant, and dielectric loss of MWP-char. A correlation was observed among the aromaticity of char, degree of carbon defects, and dielectric constant. This study provides effective theoretical and experimental basis for the efficient conversion of microwave-enhanced CS into carbon-based materials and high-value fuels.