Study On Transformation And Control Mechanism Of Mercury In The Wet Flue Gas Desulfurization And Wet Electrostatic Precipitators Systems

In recent years,the synergistic removal of mercury contaminants by WFGD and WESP has risen people's attention.The wet flue gas desulfurization?WFGD?can scrub the oxidized mercury from the flue gas by spraying the desulfurization slurry,and the wet electrostatic precipitators?WESP?can control the Hg⁰ re-emission from WFGD by applying the electric field and water film.By using the co-effect of WFGD and WESP,the the mercury in the flue gas can be controlled on the basis of low investment cost and operating cost,which has important guiding significance and practical value for the power plant.Since the oxidized mercury that absorbed in the desulfurization slurry could form various mercury compounds.The stability and the morphological transformation mechanism of the mercury compounds need further study.In order to control the released Hg⁰ downstream of WFGD,the removal mechanism of Hg⁰ by electric field and water film in WESP needs research.In this paper,the effects of Hg⁰ re-release are studied on the lab-scale experimental bench by simulating the operating conditions of the actual WFGD.The results have shown that the cumulative concentration of mercury in the desulfurization solution and the temperature of the solution are in proportional to the re-release of mercury in the desulfurization solution.Short residence time of flue gas can stimulate the Hg⁰ re-emission.The precipitation of Ca²⁺and Mg²⁺can reduce the concentration of S?IV?in the desulfurization solution and break the ion balance of the solution.Therefore,the re-emission of Hg⁰ in the calcium-based desulfurization solution at pH 8 is 30.2?g/m³,which is greater than that in the non-calcium-based desulfurization solution,19.2?g/m³.The pH of desulfurization solution drop and the concentration of S?IV?rise,because of 300 ppm SO₂ is added in the flue gas.Due to the stable mercury compounds are formed,the re-emission of Hg⁰ is inhibited at 0.2?g/m³.O₂ reduces the concentration of SO₃^2-by oxidation,which can promote the decomposition of mercury compounds.Therefore,when the concentration of O₂in the flue gas is only 3%,the concentration of Hg⁰ can reach 81.3?g/m³.The NO,because of its insolubility,has no significant effect on the re-release of Hg⁰.The main mercury compounds and their decomposition pathways in Hg²⁺-SO₃^2-and Hg²⁺-Cl^--SO₃^2-systems are determined by UV-visible spectrophotometry and thermodynamic principles.The reaction kinetics are also applied to calculate the decomposition activation energies of Hg?SO₃?₂^2-and ClHgSO₃^–are 43.49 J/moL and 24.08 J/moL,respectively.The effects of temperature,pH,SO₃^2-,SO₄^2-,Ca²⁺,Mg²⁺and heavy metal ions on the stability of mercury compounds in Hg²⁺-SO₃^2-and Hg²⁺-Cl^--SO₃^2-systems are also studied.The increase of temperature accelerates the movement of activated molecules in the solution and promotes the decomposition of mercury compounds.When the pH of the solution decrease from 8 to4,the residual rate of mercury in the Hg²⁺-SO₃^2-and Hg²⁺-Cl^--SO₃^2-systems are 57.32%and85.2%,respectively,because H⁺is the decomposition product of mercury compounds.SO₃^2-tends to react with Hg²⁺to form stable mercury compounds,and the concentration of SO₄^2-is inversely proportional to the Hg⁰ re-emission.Ca²⁺and Mg²⁺not only form insoluble suspensions to reduce SO₃^2-in the solution,but also provides active surfaces for the reduction reaction of mercury,which may stimulate the decomposition of mercury compounds.Fe²⁺can reduce Hg²⁺to form Hg⁰,Fe³⁺and Cu²⁺can oxidize SO₃^2-to form SO₄^2-.The presence of heavy metals promotes the decomposition and release of mercury compounds,and the decomposition reactions of mercury compounds accord with the pseudo first-order reaction kinetics.The results of analysis show that the reactions of ionization,dissociation and the oxidation of Hg⁰ occur around the spur end of the discharge electrode in the spur-plate electric field of WESP.As accelerate the airflow velocity to 11.2 cm/s,the position of the turbulent flow region on the plate is advanced to 53 cm,the thickness of the mass transfer boundary layer is compressed to 5 cm and increased the gas concentration gradient.All these effects promote the mass transfer of the system.The corona wind can also affect the flow state and mass transfer of gas in the electric field.The voltage should be raised to produce corona effect as the increase of flow velocity increased.Increasing the WESP operating voltage,adjusting the pole spacing,and enhancing the average electric field strength can improve the driving speed and ensure the treatment efficiency of the electrostatic precipitator system.Hg⁰,which re-release from WFGD,is the main source of Hg⁰ entering WESP flue gas.The study on the distribution of Hg⁰ in WESP showed that after treating by WESP the Hg⁰ in the flue gas decrease by 25.36%,and the Hg²⁺in the flue gas and circulating water increase,and some Hg⁰ enter the water film.When the flue gas does not contain easily ionized gas,even if the electric energy density is increased to 650 J/L,the removal rate of Hg⁰ is only 6.5%.It is difficult to ionize Hg⁰ directly in the WESP,therefore,the removal of Hg⁰ in WESP is mainly Through the oxidation of free radicals.Under the condition of 15 J/L power density,the removal rate of HgO is about 17%when H₂O and O₂ coexist,which is lower than the effect of O₂ alone and higher than the effect of water vapor alone.It shows that H₂O and O₂ compete to consume high-energy electrons in the electric field.Further,comparing the oxidation of O₂ ionized products·O and O₃ to Hg⁰,it is found that the oxidation of Hg⁰ by·O is the main pathway for Hg⁰ control in WESP.In addition,SO₂ and NO can also generate·O by oxidative removal of Hg⁰ in the WESP electric field.The flow rate of water film is proportional to the removal rate of Hg⁰ in the flue gas,when the electric field is off.The water film flow rate is increased from 0 L/h to 150 L/h,the concentration of Hg⁰ is reduced from 28%to 19.5%,after the electric field is turned on.Water vapor can inhibit O₂ from generating free radicals in electric field,thus reducing the oxidation efficiency of Hg⁰.Hence,the overall performance of water film inhibits mercury removal in the WESP.The study also indicates that,the Hg⁰ re-emission affects the removal of Hg⁰ in WESP.Since the acid gas is absorbed by water film with the operation of WESP,the dropping pH and increasing Cl^-,SO₃^2-and NO₃^-in circulating water are beneficial for Hg⁰ removal.The ways of adding the oxidant in water film to promote the reduction of Hg⁰ in the WESP are proposed finally.NaClO₂ can not only inhibit the Hg²⁺reduction,but also oxidize Hg⁰ in circulating water,which promotes the Hg⁰ removal in the WESP.