Effects of microwave/HAc–H2O2 desulfurization on properties of Gedui high-sulfur coal

• Microwave irradiation treatment facilitates the desulfurization reaction in high-sulfur coal.
• Effects of microwave desulfurization on the quality of high-sulfur coal are explored.
• Possible chemical reactions in microwave/HAc-H2O2 desulfurization are listed.
• Influence of microwave/HAc-H2O2 on caking property of caking coal is studied in theory for the first time.
• The variation of organic oxygen functionalities in the coal is determined via chemical titrimetry and XPS.

A high-sulfur coking coal obtained from Shanxi province in northern China was desulfurized by microwave/HAc–H2O2 treatment. The main properties of coal samples before and after desulfurization treatment, including the elemental composition, caking index, organic constituents and sulfur content were analyzed. Experimental results showed that microwaving treatment at 500 W for 40 s combined with HAc–H2O2 (1:5 v/v) resulted in a yield of 99.27% clean coal and a desulfurization rate of 22.62%. Proximate and ultimate analyses showed that the ash content in coal was decreased from 9.09% to 7.90%, while the volatile matter significantly increased by 0.75%, and that the oxygen concentration in the coal increased from 4.15% to 7.08% following treatment. After desulfurization, the decreases in the caking index (from 72.4 to 45.42) and the impairment of calorific value was also observed. The negative impacts on the technological properties of coking coal may be attributed to oxidization caused by a HAc–H2O2 addition with the acceleration of microwaving. Results from FT-IR and XPS analyses depicted a considerable increase in oxygen-bearing functional groups, in accordance with results obtained by an ion-exchange titration method. These findings confirmed that the oxidization plays an important role in the present desulfurization process, and takes a dominant responsible for the changes of various properties of coal.