Study On Migration Of Sulfur–containing Phases In High Sulfur Coal Under Microwave Irradiation

As an important fossil fuel,coal has made great contributions to economic growth and social development of China for decades.With excessive development of resources and consumption,the reserves of low-sulfur coal have been gradually depleted.The utilization of high-sulfur coal has become an inevitable choice for industrial process.However,it is accompanied by a large amounts of harmful gas released as a by-product,e.g.SO₂ and H₂S,causing severe environmental issues.Therefore,it is crucial to develop coal clean utilization.Microwave desulfurization is a relatively new sulfur removal method in which desulfurization is achieved by harnessing the differences in microwave response among various sulfur components in coal.To clarify the transition behavior of sulfur-containing phases in the process of microwave irradiation,the bond breaking rule of inorganic sulfur and organic sulfur functional groups in high-sulfur coal was studied.The main points of this study are following aspects:Taking two kinds of typical industrial high sulfur coal（NTC and LGC）as raw coal samples,and the sulfur form and functional group structure in the coal were analyzed by X-ray diffractometer and infrared spectrometer.The results show that the main forms of sulfur in coal are inorganic sulfur and organic sulfur,the inorganic sulfur mainly includes pyrite sulfur（FeS₂）and sulfate sulfur,and the content of FeS₂ is higher.Organic sulfur mainly contains mercaptan（-SH）,thioether（S-S）and sulfone（S=O）functional groups,and its content is S=O>-SH>-S-S-.The thermodynamic software Fact Sage was used to calculate the phase transformation mechanism of FeS₂ under different atmospheres.The results indicating that the transformation mechanism of FeS₂ under different conditions is similar.With the increase of temperature,part of FeS₂ is firstly transformed into meteorite pyrite（FeS）,and then the new phase of pyrrhotite（Fe₇S₈）is produced,and finally Fe₇S₈ is converted to FeS.This process is accompanied by the generation of sulfur-containing gases H₂S,COS and CS₂.The energy band structure,density of states and chemical bond length of different iron sulfide compounds were further calculated by Materials Studio software.The results show that FeS₂ has the widest electron distribution range,gentle state density peak,small bond length value and high stability,followed by FeS and Fe₇S₈.Then,microwave irradiation experiments were carried out on FeS₂ pure materials and high-sulfur coal samples to support the simulation results.Materials Studio software was used to simulate sulfur migration in benzenethiol（C₆H₆S）molecules before and after microwave and additives（2 mol/L Na OH、30%H₂O₂、v/v=1:4）treatment,and LST/QST tool was used to search the transition state of desulfurization reaction.According to the parameters of energy barrier,activation energy Ea and rate lnk,the addition of electric field and additives promotes the migration of sulfur in C₆H₆S.Then,the Wiser model was selected to study the bond order,bond length,Fukui index and electrostatic potential of typical chemical bonds in coal.The results show that the S-S bond has a large bond length and a small bond order,which is easy to break,while the S=O bond is difficult to break.Among them,-SH and S=O are electrophilic and nucleophilic reaction sites respectively,and the reactivity index of S atom is much greater than that of C atom.So,the microwave irradiation has little effect on the coal matrix in the process of removing organic sulfur.