| A large amount of conventional pollutants and hazardous trace elements are generated and discharged during coal combustion.Among the most toxic trace elements,arsenic is attracting more and more attention due to its toxicity,bioaccumulation,and carcinogenicity.The mode of occurrence and volatilization characteristics of arsenic were investigated in this work.Combustion experiments were carried out to study the effect of temperature,mode of occurrence of arsenic,and combustion atmospheres on the volatilization behavior of arsenic in a customized isothermal thermogravimetric reactor.In addition,density functional theory(DFT)in quantum chemistry was employed to explore the mode of occurrence of arsenic in coal,the detailed reaction pathways of arsenic oxide formation and the transformation mechanism of arsenic species in flue gas.The main content and conclusions are listed below:(1)Isothermal combustion experiments of six coals were conducted to study volatilization characteristics of arsenic at 600-1300?.The effect of temperature,mode of occurrence,and combustion atmospheres were considered.Experimental results show that the volatilization ratio of arsenic increases with temperature,and volatilization of arsenic increased significantly at 700??1000?.Volatile content in coal has a positive effect on the initial volatilization rate of arsenic,while ash content in coal has a tiny negative effect on the volatilization rate of arsenic.CO2 has a negative effect on the volatilization of arsenic and this inhibitory effect decreases with higher temperature.The volatilization of arsenic is promoted by H2O,and the positive effect is enhanced with more H2O.(2)DFT calculations were conducted to further investigate the structures of bound-sulfide arsenic.Different structures were obtained through optimizing the possible initial configurations,and the formation energy and geometric characteristics were summarized.Arsenic in pyrite occurs in two dominant forms:superficial arsenic and a substitute for sulfur.For superficial arsenic,there are four kinds of configurations,and the formation energy are in range of-234.14?-180.42 kJ/mol.There are two structures of substitutional arsenic with the formation energy of-619.34 and-722.11 kJ/mol,respectively.(3)DFT calculations were performed to investigate the mechanism for H2O influence on arsenic oxidation.All possible configurations of O2 and H2O adsorption on the FeS2(100)surface were taken into account,and the most stable structures for O2 and H2O molecule adsorption on the FeS2(100)surface were selected to investigate the effect of H2O on the arsenic oxide formation reaction.Calculation results show that the O2 molecule is likely to adsorb on a bridge site between As and Fe atoms and H2O molecule is likely to adsorb on a surface Fe site in the process of arsenic oxide formation on the FeS2(100)surface.O2 molecule is more likely to adsorb on the FeS2(100)surface with H2O,promoting arsenic oxide formation.From the kinetic analysis,the reaction rate constants increase with temperature,and the activation energy of arsenic oxide formation reaction with and with no H2O are 100.72 kJ/mol and 124.08 kJ/mol,respectively.It indicates that H2O has a positive effect on arsenic oxide formation during coal combustion.(4)Based on DFT calculations,the reactions between gaseous arsenic and gas components were studied to explore the conversion mechanism of arsenic.The reaction paths of arsenic species and gas components(NOx and CO)were searched,and the changes of geometric structure and energy were analyzed in the reaction process.Results show that different nitrogen oxides have a different oxidizing arsenic capabilities,and As reacts with NO2 easily to form AsO and NO.The reaction rate between As and NO2 is less affected by temperature,and the reaction occurs instantaneously.During the coal combustion,gaseous arsenic can be reduced by CO in flue gas,and the reaction of N2O gaseous arsenic and CO also can occur on char.The adsorption mode of AsO2 molecule on the char surface has a great influence on its reduction.Activation energy(109.7 kJ/mol)of the reaction that the two O atoms of AsO2 molecule were absorbed on the surface of char is higher than the homogeneous reduction(94.2 kJ/mol),while is only 0.8 kJ·mol-1 for the reaction that the As atom and O atom of AsO2 molecule were absorbed on the surface.Activation energy of As2O3 homogeneous reduction(75.9 kJ/mol)is lower than the heterogeneous reduction(94.2 kJ/mol),which suggests that As2O3 is more likely to react with CO in flue gas.(5)The adsorption process of arsenic on carbonaceous surface has been studied by DFT.By comparing the adsorption energy and bond change of different adsorption sites,the key step of arsenic adsorption on carbonaceous surface was obtained.In addition,the effect mechanism of CO2 on arsenic adsorption by CS was studied.The results indicate that there is a chemical adsorption between As and carbonaceous surface,while a physical adsorption between As2O3 and carbonaceous surface.With electrostatic potential analysis,it can be concluded that CO2 absorbed on carbonaceous surface enhances the active sites,and thus improves the adsorption capacity of the carbonaceous surface. |
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