Mechanism Study On Reduction Conversion Of Oxidized Mercury In Coal-fired Flue Gas For Hg-CEMS

Mercury and its compounds are globally transportable,bioaccumulative,and highly toxic.As the main source of anthropogenic mercury emissions to the atmosphere,the development of mercury control technology and mercury online monitoring devices in coal-fired power plants is an important issue for environmental and ecological protection.Mercury continuous emission monitoring system（Hg-CEMS）is an advanced online monitoring technology for flue gas mercury with the advantages of high accuracy and continuous detection of valence mercury.However,the development of this technology in China seriously lags behind the developed countries,and domestic Hg-CEMS devices basically depend on imports at present.To this end,this work targets one of the core technologies of converting flue gas oxidized mercury(Hg²⁺)into Hg⁰ efficiently in Hg-CEMS by conducting basic experimental and theoretical research.This research will develop the Hg-CEMS Hg²⁺conversion technology and equipment with its own independent intellectual property rights,fill the gaps in this field and promote the localization process of online mercury monitoring technology in China.In this paper,two technical routes were proposed,one is the high-temperature decomposition（HTD）reduction of Hg²⁺that has the ability to resist the interference of acid gases in the flue gas,the other is the low-medium-temperature catalytic（LMC）reduction of Hg²⁺.The fixed-bed experimental investigation,physical and chemical characterization of materials,thermodynamic analysis and quantum chemical calculations were used to study the characteristics and influential factors of the HTD and LMC process systematically.And the intrinsic mechanisms of Hg²⁺HTD,Hg⁰ reoxidation,and Hg²⁺LMC reduction were deeply explained.In this work,the HgCl₂ homogeneous HTD reaction was investigated on the influencing mechanism by the thermodynamic analysis.The factors affecting the decomposition reduction efficiency were carefully conducted in fixed-bed experiments to reveal the intrinsic mechanisms of the HTD reaction of HgCl₂ and the reoxidation reaction of Hg⁰ under different conditions.The reaction path was theoretically studied by density functional theory（DFT）calculations.The results showed that there exists a significant thermodynamic barrier for the HTD reaction of HgCl₂.The reactive Cl generated during decomposition has a strong reoxidation effect on the product Hg⁰.The filler such as appropriately sized quartz particles can significantly improve the temperature inhomogeneity of the reaction region.The increase of reaction sites and the promotion of reactive Cl adsorption lead to a certain extent speeds up the positive reaction rate and conversion of the HTD reaction.It was found that the presence of HCl and O₂ in flue gas alone has almost no effect on the conversion rate of HTD reduction reaction because HCl or O₂ alone has a higher energy barrier to react with Hg⁰.But the simultaneous presence of both of them can seriously reduce the conversion rate.The reactive Cl atoms produced at high temperature spontaneously oxide with Hg⁰by its high thermodynamic autonomy and low kinetic energy barrier to form easily oxidized intermediates and result in re-oxidation of Hg⁰.After recognizing the essence of those phenomena,an idea was proposed of adding a quench at the reaction end and filling acid remover that was verified to effectively inhibit the generation of reactive Cl and significantly improve the efficiency of the HTD reduction of HgCl₂.The key preparation parameters such as active components,precursors,and support materials ratios of the efficient acid remover were investigated by combining fixed-bed experiments and thermodynamic calculations.The mechanisms of the microscopic physicochemical properties of different materials on the acid removal efficiency were explored to obtain guidelines for the design of efficient acid remover by various characterization techniques such as X-ray diffraction,nitrogen adsorption specific surface area,and porosity analysis,scanning electron microscopy,programmed temperature desorption,and Fourier infrared spectroscopy.The DFT calculation revealed the intrinsic mechanism of active Cl removal by acid remover at the electronic level.The poisoning mechanism of acid remover under high SO₂ concentration was discussed and the anti-poisoning mechanism of active components was obtained.The results showed that CaO is a highly promising active component of the acid remover.CaO obtained by the precursor Ca（CH₃COO）₂ possesses higher acid removal efficiency due to its developed surface structure and excellent internal mass transfer capability.The addition of support materialγ-Al₂O₃ enhances the microstructural stability of the acid remover and increases dispersion of the active CaO.The sample of acid remover CaO/_30%γ-Al₂O₃reaches an HTD reduction efficiency of HgCl₂ up to 97.2%under the condition of the coexistence of HCl and O₂.The mechanism is that the abundant alkaline sites on the surface of acid remover strengthen the adsorption effect of weakly acidic HgCl₂ molecules and other acidic components,which promotes the occurrence of the forward reaction.The active Cl atoms undergo strong chemisorption on the CaO surface that impels the Cl atoms to interact with O atoms on the CaO（100）surface.SO₂ can cause a sulfation reaction on the material surface and generates a dense inert product layer of sulfate.It greatly deteriorated the adsorption and mass transfer capability of the material surface.However,the excellent microstructural stability and high dispersion of the active component effectively mitigate the effect of the sulfation process.The mechanism of the low-medium-temperature catalytic（LMC）reduction of Hg²⁺was explored for the first time.The influence laws of active substance type,loading rate,reactant concentration,and reaction temperature on the catalytic reduction efficiency were investigated in a fixed bed.Three types of catalysts,CuO-based,Fe₂O₃-based,and Pt-based,were tested and characterized as mutual references by X-ray diffraction,nitrogen adsorption specific surface area and porosity analysis,scanning electron microscopy,X-ray photoelectron spectroscopy,and programmed temperature rise desorption/reduction,etc.,to investigate the multi-faceted factors that control the LMC reaction of HgCl₂.The results showed that the homogeneous reaction of NH₃ reduction of HgCl₂ cannot occur without any catalyst due to the presence of kinetic barriers,and a suitable catalyst needs to be selected to reduce the reaction energy barriers.Compared with Fe₂O₃-based and Pt-based catalysts,CuO-based catalysts exhibit outstanding catalytic activity and reach a maximum efficiency of 89.8%due to the well-developed surface physical characteristics,high dispersion,good interaction with the support material,and excellent surface acid-base and chemical environment.The weakly basic adsorption sites on the catalyst surface facilitate the chemisorption and activation of HgCl₂,while the abundant Br（?）nsted acidic sites on the surface are more favorable for the adsorption and activation of NH₃ molecules.The oxygen defects/vacancies and adsorbed state oxygen on the catalyst surface are also important factors to promote the LMC reduction of HgCl₂.The geometric optimization and energy calculation of the molecular reaction system of HgCl₂adsorption by the active components of the three types of catalysts were carried out based on DFT.The conformational relationship between the electronic structure and the catalytic activity was deeply explored.The molecular orbital（MO）theory was used to investigate the electron transfer law during the activation of the system,and the energy path of the catalytic reduction reaction was studied by searching the transition state and confirming the rate-determining step.The results showed that the adsorption energy of HgCl₂ on the surface of the three types of catalysts reflects the strength of catalytic activity within a certain range,however,too large adsorption energy causes difficulty in the desorption of the product Hg⁰ from the material surface,and hinders the catalytic complete cycle,which is consistent with the interpretation of Sabatier’s principle.The MO of HgCl₂ molecules undergoes obvious hybridization overlap after adsorption on the CuO（111）surface,which exhibits strong chemisorption and activation.The distribution characteristics of HOMO and LUMO fit the adsorption energy and electron gain/loss patterns of different CuO（111）adsorption configurations.The catalytic reduction reaction process of HgCl₂ on the CuO（111）surface follows the Langmuir-Hinshelwood mechanism.The dissociation of Cl atoms in HgCl₂ molecules and HgCl molecules is the rate-determining step in the two-step reaction.The whole reduction of HgCl₂ to Hg⁰ requires overcoming a reaction energy barrier of 169.63 kJ/mol.