Study On Characteristics Of Hg²⁺ Solid Catalytic Reductant In Coal-fired Flue Gas

In view of the severe coal-fired mercury pollution status and the increasingly strict emission standards for coal-fired flue gas,achieving accurate online monitoring of mercury concentrations in different forms of coal-fired flue gas has become a major issue in the development of mercury emission control technology and environmental supervision.The mercury continuous emission measurement system（Hg-CEMS）has become a necessary environmental monitoring instrument for coal-fired power plants because it can monitor the mercury emission concentration of flue gas in real time.However,the current mercury analysis and detection technology can only measure the concentration of Hg⁰.The detection of Hg²⁺concentration needs to be measured by a mercury form separation and conversion device to reduce Hg²⁺to Hg⁰.At present,domestic Hg²⁺solid-state catalytic reducing agent technology is still blank.Therefore,this paper proposes a new mercury form separation and conversion device based on Hg²⁺catalytic reducing agent,and systematically studies the preparation and reduction characteristics of Hg²⁺catalytic reducing agent.First,the SnCl₂solution was used for wet reduction and the 30B carbon adsorption tube method for sampling,and the initial Hg²⁺concentration was calibrated.In the equilibrium phase calculation module of HSC thermochemical simulation software,the possibility and direction of the reduction reaction between the active component to be studied and Hg²⁺were calculated and analyzed.Using physical adhesion method,withγ-Al₂O₃as the supporting framework material,Li₂Si O₃aqueous solution as the binder,sulfite K₂SO₃,Na₂SO₃and metal elemental Fe,Zn,Ni as active components,a batch of catalytic reducing agent was prepared.In order to meet the requirements of the characteristics of Hg²⁺catalytic reducing agent,exploratory experiments of Hg⁰adsorption oxidation and Hg²⁺adsorption reduction were carried out on the fixed bed test bench and the catalytic reduction test bench respectively,with a view to preliminarily screening out more potential catalytic reducing agents.The results show:30B carbon adsorption tube method can more accurately calibrate the initial Hg²⁺concentration.Fe,Zn,Ni and K₂SO₃,Na₂SO₃have the possibility of reacting with Hg²⁺;from the difficulty analysis,Zn and K₂SO₃are more likely to react with Hg²⁺.The results of Hg⁰adsorption oxidation experiments show that the prepared catalytic reducing agents basically maintain a penetration rate of about 95%for Hg⁰,which meets the necessary conditions for the selective adsorption of Hg⁰by catalytic reducing agents.The Hg²⁺adsorption reduction experiment shows that Zn is in the range of 200℃-400℃,compared with Fe and Ni,it has a significantly higher Hg²⁺reduction efficiency.K₂SO₃has better Hg²⁺reduction performance than Na₂SO₃.When the temperature is above 300℃,the Hg²⁺reduction efficiency of the catalytic reducing agent prepared with Zn and K₂SO₃as active components is above 60%.Then,by changing the loading of Li₂Si O₃aqueous solution,a batch of precursors were prepared,combined with SEM and XRD,the surface characteristics were analyzed,and then the optimal loading of Li₂Si O₃aqueous solution was determined.It is clear that Zn and K₂SO₃are used as the reducing active components of Hg²⁺.By changing the loading of Zn and K₂SO₃on the precursor and the particle size ofγ-Al₂O₃,a batch of different catalytic reducing agents Zn-Li-Al and KS-Li-Al were prepared.Through SEM and XRD characterization analysis,the surface characteristics were studied.The Hg²⁺reduction performance of Zn-Li-Al and KS-Li-Al was carried out on the catalytic reduction test bench.Using ICS characterization,Zn²⁺and Cl^-concentrations of Zn-Li-Al with reaction time of 2h,4h,8h and16h were analyzed,and the mechanism of catalytic reduction reaction was discussed.The results show:it is appropriate to selectγ-Al₂O₃particles and Li₂Si O₃aqueous solution solid-liquid ratio of 1g/ml to prepare the precursor.The Hg²⁺reduction performance of Zn-Li-Al and KS-Li-Al is continuously improved with the increase of Zn and K₂SO₃loading,and it is continuously improved with the increase of the particle size ofγ-Al₂O₃.The best preparation molar ratio ofγ-Al₂O₃and Zn、γ-Al₂O₃and K₂SO₃is 1:2,and the best use size ofγ-Al₂O₃particle is 10-16 mesh.The Hg²⁺reduction mechanism of KS-Li-Al can be expressed as the reaction between K₂SO₃and Hg²⁺to produce more stable KCl,Hg⁰,SO₂and O₂,and in view of the change in the valence state of the S element,it is speculated that there is an intermediate product containing S element.Zn-Li-AL,after assuming that the reduction reaction product is Zn Cl₂,the Zn²⁺and Cl^-concentrations were measured by ICS,and the results show a good linear fit with the theoretical Zn²⁺and Cl^-concentration values.Therefore,it is speculated that the Hg²⁺reduction mechanism of Zn-Li-Al is that Zn reacts with Hg Cl₂,and Zn Cl₂and Hg⁰are generated on the surface.After clarifying the best preparation conditions of Zn-Li-Al and KS-Li-Al,in order to explore the influence of atmosphere and operating parameters on the Hg²⁺reduction performance,the effect of HCl and SO₂concentrations,initial Hg²⁺concentration and reaction space velocity on the Hg²⁺reduction performance of Zn-Li-Al and KS-Li-Al were also researched.The results show:SO₂has no toxic effect on both.HCl owns toxic effect on KS-Li-Al,because HCl will form a competitive relationship with Hg²⁺,preferentially reacts with K₂SO₃,causing K₂SO₃inactivation.With the increase of the initial Hg²⁺concentration,the Hg²⁺reduction efficiency of Zn-Li-Al and KS-Li-Al decreased slightly.The practical application space velocity of Zn-Li-Al and KS-Li-Al is 47750h^-1and 95500h^-1,respectively.