Study On Mercury Removal From Coal-fired Flue Gas By Metal Oxides/Halides Modified Bio-chars

Mercury in coal-fired flue gas,with strong toxicity and bioaccumulation,has caused great harm to human health and environment and has become one of the global environmental issues of widespread concern.Among element mercury?Hg⁰?removal adsorbents,activated carbon is one of the most widely studied adsorbents due to its mature technology and its industrial application.However,the high cost and loss have hindered the further development of this technology.As one of the six major development priorities in the"Medium and Long-term Development Plan for Renewable Energy",biomass has very broad space for development and utilization.However,the utilization of biomass has yet to be further improved.The continuous development and application of its technology have important practical and strategic significance to promote China's economic and social long-term sustainable development,and improve the ecological environment.Biomass char is produced as a by-product during the production of biomass oil and biomass gas from biomass resources,resulting in a solid waste pollution.Therefore,with the purpose of waste recycling and resource utilization,two representative biomass resources on land and in the sea,rice straw and Enteromorpha,are chosen to prepare adsorbent carriers by pyrolysis.Then metal oxides,ammonium halides and metal chlorides are used to modify these prepared adsorbent carriers to prepare new modified adsorbents.Three modified biomass chars adsorbents are used for Hg⁰ removal in the fixed bed reactor experimental system.The main contents and results are as follows:Rice straw chars are prepared by pyrolysis and modified with copper-cerium metal oxides by ultrasonic-assisted impregnation.The preparation parameters of the modified adsorbent,the reaction temperature and the concentration of flue gas components on Hg⁰removal are investigated in a fixed-bed reactor.A series of characterization methods are used to characterize and analyze the adsorbents.The Hg⁰ removal mechanisms are also discussed.The results show that the copper-cerium metal oxides modification with ultrasound-assisted impregnation enhances the BET specific surface area of rice straw chars,improves the pore structure of chars and enhances the Hg⁰ removal capability of the adsorbent.The best Cu/Ce molar ratio is 1/5,and the best loading is 0.18 mol/L.The best calcination temperature and reaction temperature are 260?and 150?,respectively.Both O₂ and NO promote the Hg⁰removal,while SO₂ and H₂O inhibit the Hg⁰ removal.The adsorbed Hg⁰ is oxidized through the active sites on the surface of the adsorbent.The chemisorption plays a key role in the Hg⁰removal process.CeO₂ has strong oxygen storage capacity and Ce³⁺/Ce⁴⁺conversion capability,thus can continuously provide active oxygen.Hence,the presence of CeO₂contributes to the continuous Cu/Ce redox cycle.Enteromorpha chars are prepared by pyrolysis,and modified by ammonium chloride?NH₄Cl?and ammonium bromide?NH₄Br?.The effects of preparation parameters,reaction temperature and concentration of flue gas components on Hg⁰ removal are investigated in a fixed-bed reactor.A series of characterization methods are used to characterize and analyze the adsorbents.The Hg⁰ removal mechanisms are also discussed.The results show that ammonium halides modifications block the pore structure,and the best loading value of NH₄Cl/NH₄Br is 5 wt.%.The optimum pyrolysis temperature and reaction temperature are800?and 130?,respectively.Both O₂ and NO promote the Hg⁰ removal,while SO₂ has an inhibitory effect on Hg⁰ removal.Low concentration of H₂O contributes to Hg⁰ removal while high concentration of H₂O suppresses Hg⁰ removal.The adsorbed Hg⁰ is oxidized through the active sites on the surface of the adsorbent,and the halide covalent functional group plays a key role in the Hg⁰ removal.Metal chlorides have synthetic advantages of non-metal chlorides and metal oxides,therefore are widely used as adsorbent modification reagents.Enteromorpha chars are modified by metal chlorides,and the effects of preparation parameters,reaction temperature and concentration of flue gas components on Hg⁰ removal are investigated in a fixed-bed reactor.A series of characterization methods are used to characterize and analyze the adsorbents.The Hg⁰ removal mechanisms are also discussed.The results show that metal chlorides modification significantly improves the Hg⁰ removal performance of the adsorbents.Fe Cl₃ and CeCl₃ modifications show better Hg⁰ removal performances than CoCl₂,CuCl₂ and ZnCl₂ modifications.The FeCl₃ modification breaks the surface of E8 and increases the BET specific surface area,while the CeCl₃ modification causes the blockage of pore structure,resulting in a decrease of the BET specific surface area.Optimal loading values of FeCl₃ and CeCl₃ are both 3 mol/L,and the optimal reaction temperature is 130?.Both O₂ and NO promote the Hg⁰ removal,while H₂O inhibits the Hg⁰ removal.Low concentrations of SO₂contribute to Hg⁰ removal while high concentrations of SO₂ inhibit Hg⁰ removal.The removal of Hg⁰ is the result of the synergistic effect of chlorine-containing functional groups and metal oxides.