Density-functional Study On The Desulfurization Mechanism Of Organic Sulfur In Coal

The sulfur in coal have great influence on coal utilization. The quantum chemistry calculation method has been gradually used to study the correlation between coal structure and reactivity in coal chemistry, and has been gradually formed a new research direction.Taking XingYu clean coal from Shanxi as the research object, we determine the occurrence and distribution of organic sulfur using the whole group separation experiment.(1)The main sulfur in XingYu clean coal is organic sulfur, the organic sulfur content accounts for more than83%, about90%of organic sulfur distribute in residue coal of the macromolecular structure, and lightweight components contain almost no organic sulfur.(2)The relative contents of organic sulfur in all groups decrease according to asphaltene component>refined coal groups>residue coal component>light components.The coal XPS peaks were assigned and we get the relative content of organic sulfur in coal, the conclusions are as follow: the relative content of three kinds of main forms of organic sulfur in coal from high to low is athiol(ether),thiophene,(sub)sulfone. It can get eight small molecular compounds in XingYu clean coal and the total coal components detected by GC/MS through the total groups separation experiments. It contains five kinds thiophene sulfur and three kinds mercaptan sulfur ether.We build the new partial model and carry out the optimization calculation of the eight sulfur model compounds (Dibenxothiophene,3-methyl-dibenzothiophene et al)and XingYu coal. The relationship between the highest occupied orbital and the lowest unoccupied orbital of the activity of compounds are discussed. For dibenzothiophene, the highest occupied molecular orbitals are mainly distributed on S7,C3,C5,C9,C11,C12,C2,C13; For the eight model sulfur compounds in Xingyu clean coal, the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals are mainly distributed on S, it illustrates that the sulfur atom is electrophilic activity or nucleophilic activity, these are the main groups which influence the compounds activity. It shows that the negative charge of the model sulfur compounds mainly distributed on the sulfur atoms through the charge population analysis, and illustrate that the sulfur atom in sulfur model compounds may be the possibility offensive point affected by the electrophilic reagents. It can predict that the atom prefer to coordinate in the formation of complexes. The bond strength of C-S single bond in the region of the crosslinking degree in XingYu coal is little weak, these bond are more easy to break during the coal fracturing, and the activity is higher.The ground state properties of the sulfur model compounds under the external electric fields are studied through the theoretical calculation, such as molecular equilibrium geometries, energy, dipole moment, electron transferring and the change of thermodynamic properties. When the external electric field is applied, the molecular activity is enhanced and the molecules are more and more unstable. In addition, with the external electric field, the resonant frequency of groups in molecule changed and moved to the low frequency. When temperature change in the range of300K-800K, the total energy, binding energy, the energy of the highest occupied molecular orbital(HOMO), the energy of the lowest unoccupied molecular orbital(LUMO), the energy gap, the total potential energy, spin polarization energy change with the temperature slightly. The degree of difficulty and easy of microwave desulfurization have certain relation with the large and small of the molecular polarity. The change of thiophene, thiol ether and similar molecular under the external energy field are compared. The polar molecules containing S,O,N are sensitive to the external electric field, and1,2dibenzene ethane molecules only constituting with C,H atom has no reaction to the external electric field. And it can get the conclusion from the theoretical calculation of the molecular electric dipole moment changing with the external electric field and comparison of experimental results of microwave desulfurization.Studying on the character of the sulfur model compounds in coal and the process of the sulfur degradation, it can provide reliable theoretical basis for researching on the structure and reactivity of coal under the function of the external electric field. Comparing with the path analysis of degrading to being H2S under the external electric field0.0005au and without the external electric field, it can get that there have the same degradation path with or without the external electric field, but the structure and performance of all the intermediates and transition states in degradation process have changed. The degree of difficulty of each path are different from the potential energy profile of the thiophene degradation. When the external electric field is0.001au, there only have the first path, the second path might not exist when the optimization structure was not found in somewhere progress. It illustrate indirectly from the theory that the microwave have effect on the path of generating H2S, the intermediates and transition states through pyrolysis. The activation energy of Dimethyl disulfide and Dibenzothiophene through pyrolysis is reduced with the increase of the external electric field. It illustrate that the microwave not only have the heating effect but also have non-thermal effect which is not caused by temperature in theory. The organic reactions under the action of microwave changed the reaction kinetics and reduced the reaction activation energy.It can explain from energy mechanism why the removal effect is better for the mercaptan sulfur and is poor effect for the thiophenic sulfur in the microwave desulfurization experiment of XingYu clean coal.