Study On Mercury Removal From Flue Gas By Mn-Co-Fe Spinel And Its Capture In High Gradient Magnetic Field

At present,the most mainstream mercury removal technology from coal-fired flue gas is still activated carbon injection（ACI）mercury removal technology,but this technology has the disadvantages of high price,poor recoverability and reducing the quality of fly ash,which limits its large-scale popularization and application.Therefore,in recent years,it has become a promising research topic to replace activated carbon with magnetic renewable materials to remove mercury.In this paper,the static mercury removal performance of magnetic Mn-Co-Fe spinel,the key parameters affecting the static mercury removal efficiency and its influence law,the removal process of zero-valent mercury,the cyclic regeneration performance,the mechanism of adsorption and oxidation to remove Hg⁰,the dynamic mercury removal performance,the key parameters affecting the dynamic mercury removal efficiency and its influence law,the magnetic capture performance,the key parameters affecting the magnetic capture rate and its influence law are studied.A variety of Mn-Co-Fe spinel were prepared,and the influences of Mn doping amount,fuel addition amount,ignition temperature,calcination temperature,reaction temperature and flue gas composition on the static mercury removal performance of spinel were investigated on the fixed-bed mercury adsorption system.Combined with the research of physical and chemical properties,Mn_0.5Co_0.5Fe₂O₄ spinel with excellent static mercury removal performance and magnetic properties was screened.The Hg⁰ removal process of the material was explored by using temperatures programmed desorption study and X-ray photoelectron spectroscopy analysis.The regeneration temperature of the material was determined according to the temperatures programmed desorption peak temperature,and its regeneration performance were investigated by five times of regeneration experiments.The results show that the optimum synthesis conditions of Mn-Co-Fe spinel are as follows:Mn doping ratio of 0.5,O/F ratio（Ratio of total oxidation price of oxidant to total reduction price of fuel）of 1,ignition temperature of 400℃,calcination temperature of 500℃,corresponding static mercury removal efficiency of 86.4%,specific saturation magnetization of 41emu/g.The static mercury removal efficiency of Mn_0.5Co_0.5Fe₂O₄ spinel increased with the increase of reaction temperature.O₂ and HCl can promote the mercury removal of Mn_0.5Co_0.5Fe₂O₄ spinel,while SO₂ can inhibit it.The removal process of Hg⁰ on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel is that Hg⁰（g）is first adsorbed to the spinel surface to form Hg⁰（ads）,and then Hg⁰（ads）is oxidized to Hg O by active components of the spinel surface such as high-valence manganese ions(Mn^4（10）and Mn^3（10）),high-valence cobalt ions(Co^3（10）)and high-valence iron ions(Fe^3（10）).The Mn_0.5Co_0.5Fe₂O₄ spinel can maintain the static mercury removal efficiency of more than 85%in 5 cycles of regeneration experiments.The adsorption and oxidative removal mechanisms of Hg⁰ by Mn_0.5Co_0.5Fe₂O₄ spinel were investigated by adsorption kinetics and density functional theory（DFT）.The results show that the adsorption process of Hg⁰ on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel can be divided into two stages:surface adsorption and internal diffusion adsorption,and the adsorption process is controlled by the mass transfer of the external membrane and the adsorption of mercury on the active sites,followed by the internal diffusion.The adsorption of Hg⁰ on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel includes physical adsorption and chemical adsorption.The adsorption of Hg O on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel belongs to chemical adsorption.The Hg/O reaction on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel is a two-step process,and the Hg O formation step is the rate control step of the Hg/O reaction.Hg Cl can stably exist on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel,and Hg Cl₂dissociates on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel.The Hg/Cl reaction on the surface of Mn_0.5Co_0.5Fe₂O₄ spinel is mainly carried out in two steps:Hg⁰→Hg Cl→Hg Cl₂.Among them,Hg⁰→Hg Cl is the rate control step of the whole Hg/Cl reaction process.A simulated flue gas injection mercury removal experimental system was established,and the dynamic mercury removal performance of Mn_0.5Co_0.5Fe₂O₄ spinel were studied.The effects of inlet Hg⁰ concentration,residence time,spinel spray amount and flue gas temperature on the dynamic mercury removal performance were investigated.The results show that the dynamic mercury removal efficiency and unit mercury adsorption capacity of Mn_0.5Co_0.5Fe₂O₄ spinel can be increased to 85.2%and 88.6?g/g by increasing the inlet Hg⁰ concentration to 10.4?g/m³ and the residence time of spinel to 2.0s;Increasing the spraying amount of spinel to 0.20g/m³ can increase the dynamic mercury removal efficiency of Mn_0.5Co_0.5Fe₂O₄ spinel to 90.1%,but reduce the unit mercury adsorption capacity to 46.9?g/g;The dynamic mercury removal efficiency of Mn_0.5Co_0.5Fe₂O₄ spinel increased with the increase of flue gas temperature.A high-gradient magnetic field trapping spinel experimental system was established,and the magnetic trapping performance of Mn_0.5Co_0.5Fe₂O₄ spinel were studied.The effects of particle mass concentration,external magnetic field intensity,residence time in the action period of magnetic field,magnetic field form,wire diameter and wire filling rate on the magnetic capture performance were investigated.The results show that,under the same trapping conditions,the magnetic trapping rate of Mn_0.5Co_0.5Fe₂O₄ spinel can be improved by arranging the iron wire grid to construct high-gradient magnetic field.Under high-gradient magnetic field,increasing particle mass concentration,increasing external magnetic field intensity,increasing residence time in the action period of magnetic field,reducing wire diameter,and increasing wire filling rate can all improve the magnetic trapping rate of Mn_0.5Co_0.5Fe₂O₄ spinel;When the mass concentration of particles is 0.74g/m³,the external magnetic field intensity is 900m T,the residence time in the action period of magnetic field is 1.07s,the wire diameter is 0.7mm,and the wire filling rat is 12%,the magnetic capture rate can reach 96.1%.