Preparation And Performance Of Renewable Transition Metal Sulfide Adsorbent For Adsorbing Gaseous Mercury

Mercury pollution has increasingly become a global environmental hot issue.As a signatory to the Minamata Convention and the country with the biggest mercury emission in the world,China faces great pressure to reduce mercury emission.Coal-fired flue gas and smelting flue gas are two important parts of anthropogenic mercury emission sources in our country,so it is imperative to reduce it.Mercury usually exists in flue gas in three forms:particle-bound mercury（Hg^p）,elemental mercury（Hg⁰）and divalent mercury(Hg²⁺),while Hg⁰ is the most difficult to remove.At present,the control of Hg⁰ pollution in coal-fired flue gas and smelting flue gas is an urgent problem to be solved.Transition metal sulfide has a large number of active sites,the adsorption rate is fast,and can be used as a Hg⁰ adsorbent.In this paper,a new idea of using renewable transition metal sulfide to control Hg⁰ in coal-fired flue gas and smelting flue gas is proposed.A variety of process routes are designed,a variety of transition metal sulfide adsorbents are synthesized,and the reaction of Hg⁰ on transition metal sulfide surface is systematically studied.Based on physicochemical properties of transition metal sulfides,adsorption properties of Hg⁰ and changes in chemical properties of transition metal sulfides before and after the reaction,the reaction mechanism of Hg⁰ on transition metal sulfides were revealed.Then,the effect of flue gas components on removal performance of Hg⁰ from transition metal sulfides was clarified.The structure-activity relationship between physicochemical properties of transition metal sulfide and Hg⁰adsorption performance,and a cheap,efficient and recyclable transition metal sulfide adsorbent was designed.The main research achievements of this work are summarized as follows:（1）According to the current situation of mercury pollution emission control in smelting flue gas,sulfureted deactivated V₂O₅-Mo O₃/Ti O₂,Fe Mo S_x/Ti O₂ and sulfureted Mo O₃/Fe-Ti spinel were developed based on the technology of centralized control high concentration of Hg⁰ from smelting flue gas by adsorption.H₂S modification significantly improves the Hg⁰adsorption performance of deactivated V₂O₅-Mo O₃/Ti O₂.Sulfureted deactivated V₂O₅-Mo O₃/Ti O₂ has resistance to H₂O and SO₂ when adsorbing high concentration of Hg⁰,and still maintains about 100%removal rate of Hg⁰ after multiple cycles.The adsorption of Hg⁰ on the surface of metal sulfide follows the Mars-Maessen mechanism.Fe S₂ and Mo S₃showed a synergistic effect on Fe Mo S_x/Ti O₂ adsorption of high concentration Hg⁰,which significantly improved the adsorption rate and adsorption capacity of Fe Mo S_x/Ti O₂.Sulfureted Mo O₃/Fe-Ti spinel has strong saturation magnetization and superparamagnetism,and can be injected into the smelting flue gas downstream of the condenser to achieve Hg⁰removal.After loading Mo O₃,the non-selective adsorption sites the sulfureted Fe-Ti spinel increases significantly,which promotes the adsorption of Hg⁰.（2）Aiming at the control of mercury pollution from coal-fired flue gas,sulfureted HPW/γ-Fe₂O₃ and sulfureted Mo O₃/Fe-Ti spinel were synthesized based on the process of recovering Hg⁰ mercury from coal-fired flue gas with magnetic adsorbent and WESP.Sulfureted HPW/γ-Fe₂O₃ has excellent adsorption properties,magnetization,reproducibility and thermal stability,and can still maintain about 100%removal rate of Hg⁰ and high magnetization after multiple cycles.HPW grafting significantly improved the thermal stability and Hg⁰ adsorption capacity of sulfuretedγ-Fe₂O₃.Although HPW grafting provides a large number of non-selective adsorption sites for sulfuretedγ-Fe₂O₃,the affinity of these non-selective adsorption sites to Hg⁰ is smaller than that of the original non-selective adsorption sites on sulfuretedγ-Fe₂O₃.The adsorption characteristics of sulfureted Mo O₃/Fe-Ti spinel are different in the adsorption of high and low concentration of Hg⁰.The selective adsorption sites on the surface of transition metal sulfides have strong affinity for Hg⁰,and the adsorption rate is fast,but the number is small,while the number of non-selective adsorption sites with low adsorption rate is large.The loading of Mo O₃ brings about a large number of non-selective adsorption sites for sulfureted Fe-Ti spinel,which results in the adsorption of high concentration Hg⁰ is much more obvious than that of low concentration Hg⁰.The design strategies of high and low concentration Hg⁰ adsorbents are different.Increasing the initial adsorption rate and the amount of S⁰ is more beneficial to the adsorption of low concentration Hg⁰.（3）Based on the existing SCR process in coal-fired power plants,a new process for removing Hg⁰ from coal-fired flue gas at low temperature is proposed.A variety of transition metal sulfide adsorbents,such as sulfureted V₂O₅-Mo O₃/Ti O₂,sulfureted HPW/γ-Fe₂O₃ and sulfureted HPMo/γ-Fe₂O₃ have been synthesized to remove Hg⁰ at low temperature in complex flue gas containing HCl.While Fe Mo S_x/Ti O₂ is developed to remove Hg⁰ and Hg²⁺at the same time.The catalytic oxidation of Hg⁰ follows the Langmuir-Hinshelwood mechanism,and the reaction rate is related to the concentration of Hg⁰ in physical adsorption and the concentration of active Cl*.Although sulfuration inhibited the formation of active Cl*on V₂O₅-Mo O₃/Ti O₂,it significantly promoted the physical adsorption of Hg⁰,which significantly improved the removal performance of Hg⁰.The active Cl*oxidizes S_x on the surface of sulfureted HPW/γ-Fe₂O₃ to SO₄^2-,resulting in a significant decrease in its chemisorption of Hg⁰.SO₂ and H₂O protect S_x by consuming active Cl*,and counteract the effect of HCl on Hg⁰ conversion on the surface of sulfureted HPW/γ-Fe₂O₃.The adsorption of Hg²⁺on the surface of transition metal sulfide follows the Mars-Maessen mechanism,and can only be adsorbed at the selective adsorption site,while there is competition between Hg⁰ and Hg²⁺adsorption.At low space speed,Fe Mo S_x/Ti O₂ can achieve complete removal of mercury species from flue gas with different proportions of Hg²⁺.The oxidation and adsorption of Hg⁰on the surface of sulfureted HPMo/γ-Fe₂O₃ exist simultaneously.Sulfureted HPMo/γ-Fe₂O₃has the ability to adsorb Hg⁰ completely at relatively low space velocities in coal-fird flue gas regardless of whether Hg⁰ oxidation occurs,while the transfer of mercury pollution to wet desulfurization does not occur.The relevant research in this paper can realize the centralized control of flue gaseous mercury pollution and recovery and utilization of mercury resources,which effectively promotes the transformation and upgrading of Hg⁰ pollution emission control from morphological control to centralized control in China.