Study On Homogeneous And Heterogeneous Oxidation Characteristics Of Hg~0 By Halogen In Simulated Flue Gas

China's energy structure of“rich coal,lean oil and less gas”determines that China's current energy consumption is still dominated by coal.The mercury pollution caused by coal combustion has attracted global attention,which is also known as the largest anthropogenic mercury emission sources.Hg⁰ is difficult to be captured by the conventional air pollutant control devices due to its high volatility and poor solubility.Halogen can effectively promote the conversion of Hg⁰ to Hg²⁺and Hg^p during flue gas cooling,and then they can be removed by desulfurization and dedusting devices.However,due to the complexity of coal-fired flue gas components and fly ash constituents,the mechanism of halogen on Hg⁰ oxidation is still unclear.Therefore,this paper focuses on Hg⁰ oxidation of both homogeneous and heterogeneous affected by halogen to explore the mechanism of halogen on Hg⁰ oxidation in the cooling process of coal-fired flue gas.The research and development of mercury migration and transformation,mercury pollution control technologies in coal-fired power stations was reviewed.The research progress of Hg⁰ homogeneous oxidation and heterogeneous oxidation mechanism was also summarized.On this basis,a new experimental test was desighed and fabricated to study the homogeneous/heterogeneous oxidation characteristics of Hg⁰ by halogen.The Hg Temperature Programmed Desorption?Hg-TPD?method was used to determine the mercury species adsorbed on the fly ash surface,which aimed to provide solid experimental support and theoretical basis for mercury oxidation technology.Firstly,the effects of halogen concentration,reaction temperature?100⁷00??and halogen species on Hg⁰ homogeneous oxidation were investigated in the homogeneous reaction experimental setup.The results show that the Hg⁰ oxidation rate under condition of the simulated flue gas atmosphere of N₂+O₂+CO₂+NO+SO₂ in the range of 100⁷00? is only a little.The halogen has a significant promoting effect,and the Hg⁰ oxidation rate is proportional to the halogen concentration.There are two reaction mechanisms for homogeneous oxidation between Hg⁰ and chlorine-containing components in different temperature ranges:Dickon reaction occurs at 300⁴00?,and Cl atoms play a major role at 400⁶00?.HCl exhibits a high oxidation effect at 300?,adding 100 ppmv HCl to achieve 60%Hg⁰ oxidation rate.HBr can promote the oxidation of Hg⁰ at 100?,and increament of temperature can promote the oxidation effect of Hg⁰.The Hg⁰ homogeneous oxidation by bromine-containing components reaches the highest value between 300? and 500?.The oxidation rate of HBr to Hg⁰ is always higher than that of HCl at the same concentration from 100? to 700?.Secondly,the effects of halogen-containing flue gas components on Hg⁰ heterogeneous oxidation with the presence of fly ash was studied on a fixed bed experimental device.The effects of reaction temperature,halogen concentration and halogen species on Hg⁰heterogeneous oxidation were also studied.The results show that under condition of the simulated flue gas atmosphere of N₂+O₂+CO₂+NO+SO₂,the existence of fly ash is beneficial for the Hg⁰ removal.In the presence of fly ash,the Hg⁰ removal rate is proportional to the HCl concentration.At 100⁵00?,2.5 ppmv HBr can reach Hg⁰ removal rate of 70%⁹5%,however,going on increasing HBr concentration is not significant.At 100?,fly ash has better chemosorption effect on Hg⁰ than other temperatures,mainly following Langmuir-Hinshelwood Mechanism.The Hg⁰ oxidation by fly ash at 300? and 500? is mainly catalytic oxidation,which mainly follows the Mars-Masessen mechanism.Therefore,the process of Hg⁰oxidation by fly ash is accompanied with a synergistic effect of various mechanisms.Hg⁰removal rate by HBr is much higher than HCl at low temperatures,and the gap of Hg⁰ removal rate between them decreases as the temperature increases.Finally,BET,SEM and XRF were used to analyze the physicochemical properties of the fly ash,and Hg-TPD experiments were carried out on the mixture of pure mercury compounds and SiO₂ as well as fly ash samples to study the mercury species adsorbed on their surfaces.The homogeneous/heterogeneous oxidation shares of Hg⁰ under various conditions were compared and analyzed.The results show that the specific surface area,pore structure,microstructure and chemical composition of fly ash change little at different temperatures.At100?,the addition of halogen promotes the chemical adsorption of fly ash to Hg⁰.The forms of mercury adsorbed onto the surface of fly ash were mainly HgCl₂ and HgBr₂.Organic mercury?Hg-OM?is formed by the combination of Hg⁰ in flue gas and organic matter on the surface of unburned carbon in fly ash.O₂ is oxidized on the fly ash surface,and Hg⁰ is oxidized into HgO.A small amount of Hg₂?NO₃?₂ may be generated due to the effect of NO and O₂ in flue gas.In the presence of HCl,the migration and transformation of mercury forms is that when the temperature is higher than 700?,the form of mercury in the flue gas is still Hg⁰.In the temperature range of 400⁶00?,Hg⁰ will firstly react homogeneously with the existing Cl atoms to generate HgCl.At 300⁴00?,HCl will be converted into Cl₂ by Deacon reaction,and Cl₂ can oxidize Hg⁰ to HgCl or HgCl₂.Chloride mercury compounds completely decompose when temperature is higher than 200?.During the oxidation and decomposition of Hg⁰ and Cl atoms,NO,O₂ and other components in the flue gas may form stable compounds of oxidized mercury(Hg⁺,Hg²⁺)with Hg⁺.At 300?,homogeneous oxidation palys a major role,and heterogeneous oxidation mainly comes from the catalytic oxidation of fly ash in flue gas.When the temperature decreases to 100?,heterogeneous oxidation plays main effect,which is mainly from the chemical adsorption and catalytic oxidation of fly ash.The mercury form adsorbed on the surface of fly ash is mainly HgCl₂.In the presence of HBr,the migration and transformation of mercury forms is that in the temperature range of 200⁵00?,Hg⁰ in flue gas reacts with Br to form HgBr,and HgBr decomposes rapidly at high temperature,then reacts with Br₂ to form HgBr₂ at moderate temperature.The generated HgBr₂ formed at 300⁵00? will be decomposed.During the formation and decomposition of bromo-mercury compounds,NO,O₂,SO₂ and other components in flue gas may react with Hg⁺or Hg²⁺to form stable Hg⁺and Hg²⁺compounds.With addition of HBr at 300⁵00?,the homogeneous oxidation of Hg⁰ is the main Hg⁰oxidation effect,and the Hg⁰ heterogeneous oxidation is mainly the catalytic oxidation of fly ash.When the temperature decreases to 100?,Hg⁰ heterogeneous oxidation palys the main role,which is mainly from the chemical adsorption and catalytic oxidation of fly ash.The mercury forms adsorbed on the fly ash surface is mainly HgBr₂.