Study On Thiophene Sulfurs In Enrichment In Coal And Their Desulfurization Mechanism In Different Atmospheres

Pyrolysis is an important pre-combustion desulfurization technology.The atmosphere used for coal pyrolysis desulfurization is mainly air and steam,Ar,CO₂,etc..Treating coal with CO₂ and steam is a cheap and environmentally friendly desulfurization method.During the pyrolysis process,some inorganic sulfur and organic sulfur in coal are easily removed,but thiophene sulfur is relatively stable and not easy to decompose.So this paper calculates the removal mechanism of thiophene sulfur under CO₂ and water vapor atmosphere.In this dissertation,the molecular properties（Average Local Ionization Energies（ALIE）,Bond Dissocoation Energy（BDE）,Mayer Bond Order,Reduced Density Gradient（RDG））of thiophene sulfurs in coal and their conversion and migration mechanism are calculated and investigated through density functional theory（DFT）at the levels of M06-2X/6-311（g）and M06-2X/def2tzvp.ALIE,BDE and Mayer bond order analysis,and RDG were used to predict the reaction sites of sulfur-containing compounds,to determine the bond breaking sequence and bond strength,and to analyze the weak interaction between thiophene sulfurs and H₂O,respectively.Then the energy barriers（（35）G^?）and reaction energy（（35）G_P）of thiophene,2-methyl thiophene and benzo thiophene were calculated under both water vapor atmosphere and CO₂ atmosphere,and their thermodynamic and kinetic properties were analyzed by the reaction energy diagram.Based on the transition state theory（TST）,the maximum reaction energy barrier in the thermodynamic spontaneous reaction is selected as the speed-determined step to calculate the rate constant of the elementary reaction.The speed and spontaneity of the reaction are studied in terms of kinetics.The main research contents of this paper are as follows:（1）The conversion of sulfur compounds in coal to thiophene sulfurs is firstly calculated.The results show that aromatic sulfur compounds in coal can convert each other and generate phenylthio radicals or phenyl radicals through bond cleavage.Then,these unstable radicals undergo secondary reactions and migrate to thiophene sulfurs,such as benzothiophene,thiophene,and thioanthrene.Thiophene sulfur can generate larger molecular weight thiophene sulfur through its own condensation and enrichment,so it is necessary to remove thiophene sulfurs from coal.Then calculated the desulfurization mechanism of thiophene sulfur in water vapor and CO₂atmosphere.（2）In water vapor atmosphere,the mono-H₂O attacking the C-S bond of thiophene sulfur needs to overcome a higher reaction energy barrier,so it cannot kinetically and thermodynamically occur and form H₂S and COS.Compared with the mono-H₂O,the reaction energy can be significantly reduced for the reaction of bi-H₂O and thiophenes.Although the reactions of thiophene,2-methyl thiophene and benzo thiophene with bi-H₂O can occur thermodynamically,and generate H₂S at 500K,they need to overcome their high reaction energy barriers.Therefore,they are still impossibly occur in kinetic.At high temperatures（1200K）,water can decompose and generate free radicals（H·and OH·）.In addition,coal can also generate free radicals by breaking bonds.Thus,thiophene sulfurs can react with H·and OH·to form sulfur-containing gases,such as H₂S and COS.It shows that thiophene sulfurs and radicals can react spontaneously in thermodynamics,as their initial reaction energy barrier needing to overcome kinetically is rather low.Thus,the reaction of thiophene sulfur with water vapor is a radical-dominated reaction at high temperature.In addition,as water vapor is excessive in the reactor,COS can be easily converted to H₂S.Therefore,the main sulfur-containing gases are H₂S during the reactions of thiophene sulfurs and water vapor.（3）In CO₂ atmosphere,CO₂ or CO directly attacking the C-S bonds and C-C double bonds of thiophene sulfurs cannot form sulfur-containing gases,such as H₂S and COS,even at 1200K.However,free radicals（H·and·S·）with thiophene sulfurs can react and form volatile gases such as CS,CS₂,and then these sulfur-containing gases can further react with CO₂ to form COS.Therefore,in the CO₂ atmosphere,the detected COS are not generated by CO₂ directly attacking thiophene sulfurs rather than by the radical-dominated reaction,as free radicals can reduce the decomposition reaction barrier of thiophene sulfurs.