Research On The Removal Of Elemental Mercury In Coal-fired Flue Gas By Electrolytic Treatment Of Desulfurization Wastewater

In recent years,with the improvement of people's environmental awareness,the problems of SO₂ pollution and mercury pollution caused by coal combustion have received extensive attention.Currently,most domestic and foreign coal-fired power plants use limestone-gypsum wet flue-gas desulfurization?WFGD?to remove SO₂ from coal-fired flue gas.The desulfurization wastewater generated during the operation contains high concentration of Cl^-,which will lead to a reduction in desulfurization efficiency and equipment corrosion,and it is difficult to remove the highly concentrated chloride ions from the wastewater using the traditional triple box method,and the wastewater cannot be reused after treatment,the goal of zero discharge of power plant wastewater cannot be achieved.On the other hand,the mercury in the coal mainly exists in the form of elemental mercury after combustion,which cannot be directly removed by the existing pollutant control devices.The selective catalystic reduction catalyst can oxidize Hg⁰ to water-solube Hg²⁺,unfortunately,the Hg⁰ oxidation efficiency of SCR catalyst is very low under the low chlorine conditions.In view of the above problems,an electrochemical method was proposed to remove the Cl^-in desulfurized wastewater,and the electrolytic products were used for the removal of Hg⁰ in the SCR process,which provides a new solution for the zero discharge of desulfurized wastewater and the control of mercury pollution from coal-burning flue gas.The idea has important industrial application value.The conventional two-compartment electrolyzer was firstly used to treat the desulfurization wastewater,and factors affecting the electrolysis effect,such as current density,electrode spacing and electrolyte temperature,were investigated based on the chloride ion removal rate,current efficiency and energy consumption.The results indicated that when the chloride ion concentration in desulfurization wastewater was10000 mg/L,the optimal electrolysis conditions were as folllows:the current density was 20 m A/cm²,electrode spacing was 1cm,electrolyte initial temperature was 30?.under the conditions,the chloride ion removal rate reached 54.34%,the current efficiency was 55.02%,and the energy consumption was 6.78 k Wh/kg Cl^-.Further research in this paper found that the residual Ca²⁺and Mg²⁺deposited on the cathode after softening treatment of desulfurization wastewater,resulting in a 210 mg weight gain of the cathode,and the turbidity of the cathode chamber solution increased to 19NTU,which may cause the ion exchange membrane to be blocked.In order to solve the problem of calcium and magnesium ions scaling in the cathode,the double membrane and three compartment electrolysis method was introduced to remove Cl^-from desulfurization wastewater.By utilizing the selective permeability of anion and cation exchange membranes,the scale-forming ion and OH^-were blocked in the anode and cathode chambers respectively to avoid the formation of precipitates.And the influence of the electrolysis parameters on the treatment effect was investigated,the results showed that the increase of current density could improve the rate of electrolysis reaction,thus the removal rate of chloride ion increased,and the energy consumption was positively correlated to the current density;the current efficiency and chloride ion removal rate increaseed with the decrease of the anodic p H,energy consumption was relatively low;the higher concentration of chloride ion in the anode chamber improved current efficiency,and it was beneficial to reduce the chlorine evolution potential of the Ru O₂-Ir O₂/Ti electrode,which was conducive to the occurrence of chlorine evolution reaction;the influence of Na NO₃ concentration in the middle chamber on the electrolysis effect was relatively small.When the current density was 20 m A/cm²,anodic p H was 4,Cl^-concentration was 20000 mg/L and the concentration of Na NO₃ in the middle chamber was 0.5 mol/L,the chloride ion removal amount was 7487 mg/L,current efficiency was 75.81%,and the energy consumption for electrolysis reached 3.72 k Wh/kg Cl^-.Finally,the oxidation performance of Hg⁰ by Cl₂ produced by the desulfurization wastewater electrolysis was investigated on the simulated flue gas platform.The results indicated that the Hg⁰ oxidation rate increased with the increase of Cl₂ concentration,and the removal rate was the highest at 350?;compared with HCl,Cl₂ could maintain a high Hg⁰ oxidation activity in the temperature range of 200?350?.The addition amount of Cl₂ was only 25%of that of HCl when the same Hg⁰ oxidation rate was reached.