Study On The Mechanism Of Sulfur Removal From Coal By Microwave Assisted Oxidation

Coal is widely used in coking,oil refining,fuel and other fields,but sulfur in coal not only affects the quality of coking,but also generates harmful gases that adversely affect human health and environmental safety when burned,so coal desulfurization has become the focus of coal-related research.In this paper,the mechanism of sulfur removal from coal by microwave-assisted oxidation system is experimentally investigated.Proximate analysis,ultimate analysis,morphological sulfur determination,Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,X-ray diffractometer（XRD）,and scanning electron microscopy（SEM）were selected to characterize the coal samples before and after desulfurization to investigate the desulfurization effect of microwave-assisted oxidation system.In addition,the desulfurization mechanism of microwave-assisted oxidation system was explored through the desulfurization effect of model compounds and quantum chemical calculation.The results of the study are as follows.（1）The desulfurization effects of microwave,ultrasonic and UV light-assisted acetic acid-hydrogen peroxide systems on Ningdong Shuangma coal were investigated separately.Comparing the maximum desulfurization rates of the three auxiliary means,the microwave-assisted desulfurization was the best,with the desulfurization rate reaching46.28%at a microwave power of 900 W and an irradiation time of 5 min.The organic sulfur removal rate was improved by 49.26%compared with the unassisted means.It was found that the microwave assistance exposed more sulfur-containing groups,which enhanced the desulfurization effect.Calculation simulations by Gaussian09 software package found that the applied electric field can reduce the reaction energy barrier of the model compound,which facilitates the reaction.（2）To study the effect of microwave-assisted citric acid-hydrogen peroxide system and sodium hydroxide-hydrogen peroxide system on sulfur removal from Ningxia Yuanyanghu coal,it was found that the microwave-assisted citric acid-hydrogen peroxide system and sodium hydroxide-hydrogen peroxide system could increase the desulfurization rate to43.67%and 45.31%,respectively.The sulfur in the lower valence state in the treated coal,including inorganic sulfur and thiophene sulfur,is oxidized to sulfoxide,sulfone or sulfate sulfur in the higher valence state,and the microwave assisted can enhance the oxidation effect.Also,the two systems can increase the contact area between chemical reagents and coal samples,which increases the possibility of sulfur-containing components being oxidized,thus enhancing the sulfur removal effect on coal samples.（3）The microwave-assisted citric acid-hydrogen peroxide removal method for sulfur removal from coal was optimized by response surface method.The results showed that the citric acid dose was the main factor affecting the desulfurization rate,and the optimized process parameters were microwave power 900 W,microwave irradiation time 6.8 min,and citric acid dose 0.11 g.The desulfurization rate could reach 72.23%at this time.After the treatment,the external surface of the coal sample was smoother and the internal structure became loose,resulting in easier contact between the sulfur-containing groups and the reagents.The Fe²⁺generated under the action of microwave and the Fe³⁺generated by further oxidation of hydrogen peroxide react with citric acid to generate complexes,respectively,and the addition of microwave will reduce the energy required for the reaction,making the reaction more likely to occur.（4）The microwave-Fenton system reached the best desulfurization effect at a microwave power of 800 W and irradiation time of 5 min,with a desulfurization rate of 77.52%.The minerals in the coal sample were significantly reduced after desulfurization.The microwave-Fenton system can generate a large number of hydroxyl radicals to break the C-S bond,while exposing more sulfur-containing groups to the reagent,resulting in more low-valent sulfur oxidized to high-valent sulfone and sulfate sulfur.Calculation simulations with the Gaussian09 software package were chosen to find that with the enhancement of the applied electric field,the reaction energy barrier of the oxidation reaction decreases and the C-S bond is more easily broken,which is favorable for the reaction to proceed.