The desulfurization efficiency, combustion properties, combustion kinetics, and structural changes of Kentucky coal under microwave irradiation (MI) combined with hydrochloric acid solution pickling were investigated and compared using a thermogravimetric analyzer coupled to a Fourier transform infrared spectrometer. The results show that the desulfurization rate increases with the concentration of hydrochloric acid solution. The introduction of MI can effectively improve the desulfurization efficiency. After desulfurization, the ignition temperature and burnout temperature of coal samples increase, the combustion index decreases, and the combustion performance deteriorates. The maximum weight loss rate decreases with the increase of the concentration of hydrochloric acid solution, and the activation energy of the coal sample after microwave treatment increases, and the reaction difficulty increases. The infrared analysis results show that MI and hydrochloric acid pickling treatment have little effect on the functional group structure of coal samples.

According to the analysis of the characteristics of coal samples, organic sulfur compounds with different structures were selected and their microwave absorption performance was observed in the range of 2.0 GHz to 8.0 GHz. The relationship between absorption performance and desulfurization efficiency was studied by the microwave desulfurization experiment. Results showed that microwave frequency change slightly affects the real part of the complex dielectric constants of organic sulfur compounds. In contrast, it had a significant effect on the imaginary part of complex dielectric constants, dielectric loss tangent, microwave penetration depth and the absorption coefficient, except for the imaginary part of complex dielectric constants and dielectric loss tangent of thiophene compounds. The polarization ability of compounds in the microwave field followed the order: thiophene > mercaptan > thioether, while the efficiencies of mercaptan and thioether for absorbing electromagnetic energy and transforming it into thermal energy were more substantial than that of thiophene. The stronger microwave absorbing performance, the higher the desulfurization rate. Mercaptan and thioether compounds were more accessible to remove than thiophene from coal by microwave radiation and chemical additives. GRAPHICAL ABSTRACT

In the present paper, X-ray photoelectron spectrum (XPS) is used to reveal the surface texture, main elements and occurrence mode of different-density-level sample coal of Fenxi County. The experimental results demonstrate that: (1) there is an extremely low content of nitrogen in the sample coal; (2) carbon mainly exists in the form of C-C and the contents of carbonyl (C=O) and carboxyl is still relatively low; (3) with the increasing of density, such content has a growing trend; (4) the organic sulfur of the coking coal in Fenxi County has the three major forms, i.e., （sub）sulfone, thiophene and thiol (ether), with their respective content in a low-to-high order.