| As a harmful element in coal,sulfur element polluted the environment due to the formation of sulfur dioxide in the process of coal utilization.It also affected the quality of coal chemical industry and metallurgical products.Organic sulfur,in the form of organic compounds,is one main sulfur form of coal.It was difficult to remove from coal due to its stable chemical composition,limiting the utilization of high organic sulfur coal resources.Inorganic sulfur and organic sulfur of coal could be removed by microwave combined with additives.There were many studies on the mechanism of inorganic sulfur removal,while the microwave removal mechanism of organic sulfur was not yet clear.In view of the above problems,some representative coal-like organic sulfur-containing model compounds were selected as the research objects instead of the organic sulfur-containing groups of coal after analyzing the occurrence form of sulfurcontaining groups in coal.Changes of sulfur forms before and after desulfurization were analyzed by XANES.Then the sulfur form migration of organic sulfur-containing groups was clarified during microwave desulfurization.Based on the transmission reflection method,the dielectric properties of sulfur-containing model compounds and minerals in coal were analyzed using a test system of coal dielectric properties.A theoretical evaluation model for the breaking of sulfur-containing bonds in the microwave field was built combining the dielectric properties with the bond dissociation energy of sulfur-containing bonds.The synergistic desulfurization effect of microwave field and oxidation was revealed.Using quantum chemical calculations,the activation effect of sulfur-containing groups and the breaking mechanism of sulfurcontaining bonds in the microwave field were studied at the molecular level.The microwave non-thermal effect in the microwave desulfurization process of coal was investigated from the aspects of reaction thermodynamics and kinetics.Combining the test analysis at the macro level and the theoretical calculation at the micro level,the basic principles of the organic sulfur removal from coal by microwave with additives were analyzed,which could provide theoretical guidance for exploring efficient coal desulfurization methods.Two kind of cleaned coals collected from Xinyu(XY)and Guxian(GX)coal mines were used to analyze the sulfur occurrence patterns in coal.Their total sulfur contents and organic sulfur contents accounted for 2.16% and 2.12%,89.35% and 75.00%,respectively.Analysis results of XPS and XANES showed that the forms of sulfur-containing groups in the two coals included pyrite,thiol,thioether / disulfide,thiophene,sulfoxide,sulfone,and sulfonate / sulfuric acid.Besides,the thiophene content was the most,accounting for more than 50% of the total sulfur content.Therefore,ten kinds of organic sulfur-containing model compounds were selected to replace the organic sulfur-containing groups in coal as the research objects.At the same time,kaolinite,montmorillonite,quartz and pyrite were determined as the research objects of minerals in coal after XRD analysis.In order to simulate the environment of coal matrix,sulfur-containing model compounds were loaded onto the activated carbon by in-situ loading method.The XANES analysis results of XY coal and model objects after microwave irradiation for different times showed that the low-valent organic sulfur-containing groups in coal were easily oxidized into the corresponding sulfoxides and sulfones under microwave and peroxyacetic acid conditions.Besudes,thioethers were more easily oxidized than thiophenes.The desulfurization effect of the model materials were diphenyl disulfide > dibenzyl sulfide > phenyl sulfide > dibenzothiophene.Because the C-S bonds in the sulfide and thiophene were difficult to break during the desulfurization process,the sulfur forms mainly stayed in sulfoxide and sulfone.However,the S-S bond in disulfide was easier to break than C-S,and generatd sulfur-containing free radicals,which were then converted into soluble sulfonates / sulfates and removed from coal.Based on the transmission reflection method,a dielectric property testing system coal was established.The dielectric property test results of the selected sulfurcontaining model compounds and minerals were as follows: the microwave heating efficiency of thiophenes was lower than other sulfur forms,while the dielectric properties of thiol / disulfide were higher than other low-valent sulfur-containing groups.In addition,the dielectric constants of sulfoxides and sulfones in the oxidation valence state were significantly higher than those in the low-valence thiols / thioethers and thiophenes,indicating that the oxidation treatment was beneficial to enhance the microwave response ability of low-valence sulfur-containing groups in coal.The dielectric constants of sulfur-free model compounds were lower than that of sulfurcontaining model compounds,and the dielectric characteristic curves of each sulfurcontaining model compounds owned their obvious characteristic peaks.It suggested that the selective heating of organic sulfur-containing groups could be achieved at a specific frequency,which could reduce the damage of desulfurization to the coal matrix.In addition,the microwave response capabilities of pyrite and montmorillonite in coal were much higher than those of organic sulfur-containing groups,so the two minerals could significantly increase the heating rate of the desulfurization system in the microwave field,resulting in the combined effect of desulfurization and ash reduction in the desulfurization process.Triphenyl methyl mercaptan,phenyl sulfide and dibenzothiophene were selected to represent the sulfur-containing groups of thiols,thio ethers and thiophenes in coal,respectively.The desulfurization reaction of coal was theoretically calculated from the molecular level using quantum chemical calculation.The calculation results showed that: the sulfur-containing groups could be activate by the extra electric field and oxidation,reducing the strength of their sulfur-containing bonds and increasing their molecular polarity.The C-S bond strength in thiophenes was high due to the existence of multicentric bond in its sulfur-containing ring.In order to quantitatively analyze the fracture law of sulfur-containing bonds,a theoretical bond dissociation time evaluation model for sulfur-containing bonds in microwave field was bulit on the basis of the bond dissociation energy and the dielectric constants.As the low-valent thiol thioethers and thiophenes were oxidized into the corresponding sulfoxides and sulfones,the theoretical breaking times of their sulfur-containing bonds were significantly shortened.Therefore,there was synergy action between microwave field and oxidation assistant in the process of removing sulfur from coal by microwave with oxidation assistants,which promoted the breaking of sulfur-containing bonds.Reaction paths of sulfur-containing model compounds and peroxyacetic acid were calculated.Results showed that the C-S bond of triphenyl methyl mercaptan tended to break,while the C-S bonds phenyl sulfide and dibenzothiophene were difficult to appear fracture due to their high bond dissociation energy.The stable chemical structure of thiophene groups and their weak microwave response ability made them difficult to remove from coal.The extra electric field could significantly reduce the strength of the C-S bonds in the molecules near the reaction transition state,which was conducive to the breakage of C-S during the reaction and improving the removal effect of organic sulfur-containing groups in coal.Reaction thermodynamics and kinetic analysis showed that the desulfurization reactions had strong spontaneous in thermodynamics,but they were easier to achieve in kinetics.The effect of the extra electric field on the thermodynamic and kinetic quantities depends on the direction of the applied electric field,demonstrating the duality of the microwave field non-thermal effect on coal microwave desulfurization. |
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