Adsorption Mechanism Of Mercury,Arsenic And Lead In Flue Gas By Defective Unburned Carbon

There are lots of kinds of pollutants such as dust,NO_x,SO_x are emited from coal fired power plant every year,and the heavy metals such as Hg,As,Pb in flue gas has been regarded as the main source of heavy metals in the environment.At present,the main removal technologies and equipment of heavy metal pollutants in coal-fired power plants include activated carbon injection technology,selective catalytic reduction equipment,electrostatic precipitator and bag filter.However,these methods have the disadvantages of high cost,difficult maintenance and low efficiency.Therefore,it is of great significance to research a kind of material with low price and high removal efficiency for the removal of heavy metals in coal-fired power plants.Fly ash is a kind of carbon material rich in many mineral elements produced by pulverized coal combustion.Unburned carbon in fly ash has a carbon structure similar to that of activated carbon and is considered as a cheap and efficient adsorbent.Although some theoretical studies have explored the adsorption process of unburned carbon on its surface,the internal adsorption mechanism between unburned carbon and trace elements is still unknown,and the current research lacks the analysis of the effect of defective structure on the structure and adsorption performance of unburned carbon.Therefore,based on zigzag and armchair carbon models,we build 12 kinds of defective unburned carbon structures,and these structures are analyzed by atomic dipole moment corrected Hirshfeld population（ADCH）,electron localization function diagram（ELF）,density deformation diagram and electrostatic of stationary potential diagram.The electronic density difference diagram,electrostatic of stationary potential diagram,density of state diagram（DOS）and electronic localization function of As₂O₃ and PbCl₂ were analyzed;the adsorption of Hg⁰,As₂O₃ and PbCl₂ on 12 kinds of defective models were studied systematically,and 18,12 and 12 stable adsorption configurations were obtained respectively.Finally,the differences of Hg⁰,As₂O₃ and PbCl₂ in the complete zigzag and armchair models were compared by using the parameters of adsorption energy,bond length and electron density to further understand the adsorption mechanism.The results show that the free electron number near the active sites of defective zigzag and armchair models are enhanced significantly,and the intensity of the electron localization at the active sites in the electron localization function diagram of the defective armhair models are also significantly enhanced.Therefore,it can be concluded that the defective structure makes the intensity of active sites enhanced in the unburned carbon model.The results of ADCH charge analysis show that the non-metallic properties of the sites in zigzag model are stronger than those in armhair model.There are both physical and chemical adsorption when Hg⁰ exists in the model of defective unburned carbon.On the defective sites,Hg⁰ is concentrated on the unburned carbon surface in the form of chemisorption,which indicates that the defective position is the real adsorption site of Hg⁰on the unburned carbon.The electron density difference diagram shows that the large amount of electron transfer between C-Hg makes Hg⁰ adsorbed on the defect position stably.The presence of lactone（-COO）,carboxyl（-OH）,carbonyl（-CO）,and semiquinone（-O）functional groups did not strengthen the adsorption of Hg⁰ on the defective structure.In addition,the oxygen atoms in the semiquinone functional group act as the active sites to bond with Hg⁰.The defect improves the enrichment capacity of As₂O₃ on the unburned carbon model,among which the armhair model is more obvious for the enhancement of enrichment capacity.Mayer bond analysis showed that the binding strength of solid surface and gas molecules is related to the adsorption energy of the system.PbCl₂ exists in the form of dissociation adsorption in the unburned carbon.The defective armchair model has a more obvious improvement on the enrichment of PbCl₂ on the surface of unburned carbon.