The Transformation And Capture Mechanism Of As And Se In High Temperature Flue Gas

In the application of solid fuels,in addition to traditional SO2,NOx,and inhalable particulate matter,a large number of heavy metals emitted have also received widespread attention.The heavy metals arsenic(As)and selenium(Se)are almost completely volatilized in gaseous form during the combustion process.As and Se in the flue gas is easily enriched on submicron particles or released into the atmosphere.As and Se not only have serious impacts on human health and the environment but also cause poisoning when passing through the selective catalytic reduction(SCR)at the tail of the boiler,thereby increasing the operating cost of coal-fired power plants.Therefore,analyzing the transformation and capture mechanisms of As and Se in flue gas is of great significance for reducing the operating costs,risks,and industrial environmental safety of coal-fired power plants.Combining adsorption experiments,thermodynamic calculations,kinetic calculations,and density functional theory calculations,this paper investigates the transformation and capture mechanism of As and Se in combustion flue gas and fuel gasification,providing a theoretical basis for As and Se emission control.By the kinetic and adsorption calculation in flue gas,the transformation and capture mechanism of heavy metal selenium in sludge pyrolysis/gasification was analyzed.Based on the quantum chemistry software Gaussian09 and the B3LYP-D3(BJ)/def2-TZVP theory,a homogeneous selenium transformation model of H/C/N/O/Cl/Se was constructed,including 15 selenide and 51 selenium transformation reaction mechanisms.The kinetic transformation of selenium was investigated at the limiting temperature of gasification(700 and 1200℃).According to thermodynamic calculations,almost all selenium existed in the form of H2Se at 700℃,and the Se0 content significantly increased above 900℃.At 1200℃,the system contained 35%Se0 and 65%H2Se.However,at 700 ℃,limited by kinetic transformation,only SeO2 was present in the system.The selenium system at 1200 ℃ completed the transformation process within 0.4 seconds.The reduction effect of CO on SeO2 was much stronger than that of H2.Due to the strong electronegativity of selenium atoms in H2Se molecules,H2Se dissociated after being adsorbed on CaO and MgO surfaces,generating HSe that was not directly adsorbed.Al2O3 and Fe2O3 could directly bond with selenium atoms through metal atoms.The order of adsorption strength of metal oxides on H2Se was:CaO>Al2O3>MgO>Fe2O3.The paper revealed the SeO2 capture mechanism by four metal oxides,CaO,Fe2O3,Al2O3,and MgO.MgO and CaO showed the highest capture efficiency at 550℃ while Al2O3 and Fe2O3 showed a significant decrease in adsorption capacity above 400℃.Overall,MgO had the best adsorption effect;Fe2O3 had the worst enrichment rate for SeO2.CaO,MgO,and Al2O3 adsorbed SeO2 and generated CaSeO3,MgSeO3,and Al2(SeO3)3,respectively,while the selenium forms on the Fe2O3 surface were Se4+,Se0 and Se2-.The decomposition temperatures of the adsorbed products of CaO,MgO,Al2O3,and Fe2O3 were 750℃,540℃,455 ℃,and 420℃,respectively.The density functional theory calculation results showed that the order of the intensity of the metal oxide adsorption of SeO2 was:CaO>MgO>Fe2O3>Al2O3.The active site of CaO,MgO,Al2O3 and Fe2O3 capture SeO2 are O top,O top,Al top,O top and Fe top,respectively.Finally,the decomposition path of SeO2 on the Fe2O3 surface was calculated,including two reaction paths,with activation energy barriers of 80.56 kJ/mol and 76.58 kJ/mol,respectively.Combined with adsorption experiments and density functional theory calculations,the mechanism of SO2 on CaO capture selenium was revealed.The adsorption experiment showed SO2 had a significant inhibitory effect at a concentration of 0～1000ppm,with the inhibitory effect alleviated at 1000～1500ppm SO2 and intensified at 1500～2000ppm.SO2 and SeO2 competed for the O top position on the CaO surface,and SO2 adsorbed on the CaO surface inhibited the adsorption of SeO2.Meanwhile,SO2 in the flue gas was reduced to SeO and Se0 through homogeneous reactions,alleviating the inhibitory effect of SO2 on the CaO surface.The adsorption strength was significantly reduced after sulfation,and only Se0 adsorption was chemical adsorption.The isothermal adsorption of SeO2 on CaSO4 conforms to the Frundlish adsorption equation,and SeO2 exhibited multi-layer adsorption on the CaSO4 surface,mainly through physical adsorption.The SO2 in the flue gas inhibits the capture of selenium by CaO.The transformation and capture by CaO and Fe2O3 mechanism of arsenic in flue gas were analyzed.Based on the quantum chemistry software Gaussian09 and the B3LYP-D3(BJ)/def2TZVP theory,a reaction kinetics model including 24 kinds of arsenide and 54 arsenic transformation elementary reactions was constructed.The arsenic in the flue gas existed in the form of As3+.Arsenic mainly existed in the form of As4O6 below 627℃.When the temperature was above 627℃,the wet flue gas mainly existed in the form of AsOOH;In dry flue gas above 927℃,there were chain-AsS2O3 and AsO2,and as the temperature continued to rise to 1127℃,chain-As2O3 spontaneously transformed into AsO2.Finally,based on the quantum chemistry calculation module CASTEP,the arsenic capture by CaO and Fe2O3 mechanism was calculated.The O top position on the CaO surface and the Fe top position and O top position on the Fe2O3 surface were active adsorption sites for arsenic adsorption,and the order of adsorption strength was:chain As2O3>AsO2>AsOOH>As4O6.