Non-thermal effect of microwave on organic sulfur removal from coal by microwave with peroxyacetic acid

This work investigated the non-thermal microwave effect during organic sulfur removal from coal by microwave irradiation combined with oxidation assistants. XANES was used to analyze the change of sulfur forms in the desulfurization process of high organic sulfur coal under microwave heating and conventional heating conditions. The effects of microwave fields on the molecular properties, bond strength and reaction kinetic of organic sulfur-containing groups were studied by quantum chemical calculation. Results showed that the desulfurization rate with microwave heating improved by 26% at the same temperature compared with conventional heating. The XANES analysis indicated that microwave radiation promoted the transformation of low-valence sulfur into high-valence sulfur, which resulted in the sulfur in the treated coal mainly existing as heterocyclic sulfur and oxidized sulfur. For the theoretical calculation, it is observed that the nucleophilic reaction activity of sulfur-containing groups could be improved and the strength of CS bands could be reduced under the extra electric fields due to the non-thermal microwave effect. Besides, the reaction activation energy was reduced and the reaction rate was increased under the same extra electric fields, promoting the formation of oxidized sulfur in coal. Therefore, there was a competitive relationship between sulfur-containing bond dissociation and group oxidation under the action of microwave non-thermal effect, which provided a reference for coal desulfurization research.