An Experimental And Theoretical Research On Simultaneous Removal Of NO And Hg~0 From Simulated Coal-fired Flue Gas Over Modified Activated Coke(Carbon) At Low Temperatures

Coal combustion can not only supply the energy,but also engender various air pollutants including NO_x and Hg⁰,which threaten the ecological environment and people’s health.NO_x is considered as an incentive for numerous environmental problems,such as photochemical smog,regional acid rain and ozone depletion.Hg⁰ is volatility,toxicity,persistence and bioaccumulation,besides,its migration across regions with the atmospheric circulation might threaten the global human health and environmental safety.Traditional coal-fired flue gas pollution treatment technology that handles gas pollution one by one is complicated and uncontrollable,which suffers from some handicaps including large footprint requirement,huge investments of equipments and high operating costs.Considering that the SCR system has been widely employed in coal-fired power plants,how to make good use of SCR installation for simultaneous NO_x and Hg⁰ removal has aroused wide interests.Considering the SCR system is installed upstream of the particulate collector and flue gas desulfurization,and the particle collector can remove Hg ^P and the flue gas desulfurization by wet process can dislodge Hg²⁺,so the key is to control Hg⁰conversion in flue gas.However,traditional denitration catalysts didn’t effectively catalytic oxidation of Hg⁰ in the flue gas burnt the low chlorine coal,which cannot meet the requirements of mercury and mercury compounds.In addition,to reduce reaction costs and avoid deactivation,the SCR system was desired to be loaded downstream of the particle control devices and the desulfurization facilities where the flue gas temperature was lower than 200°C beyond the active temperature range of V₂O₅-WO₃（MoO₃）/TiO₂ catalyst.Therefore,it is of greatly practical significance to develop novel low temperature and high activity catalyst which can simultaneously remove NO and Hg⁰.Carbon based materials such as activated coke（carbon）have abundant pore structure,outstanding specific surface area,easy modification and regeneration,abundant sources and relatively cheap.Besides,they have the potentialities of controlling their surface functional groups and dealing with various pollutants in coal-fired flue gas at low temperatures（?200°C）.Based on this,activated coke（carbon）has been modified by loading active multiple metal oxides to develop novel denitration and demercuration catalysts（LaCe/AC,CoCe/BAC,LaMn/BAC,CoMn/BAC）.Combined with experimental investigation and theoretical study,this work is conducted to modify activated coke（carbon）with metal oxides for NO and Hg⁰ simultaneous removal at low temperatures.The primary coverage is as follows:A battery of La₂O₃ and CeO₂ modified activated cokes（LaCe/AC）were utilized for the simultaneous removal of NO and Hg⁰from simulated coal-fired flue gas.Various characterization methods including BET,SEM,XRD,TGA,NH₃-TPD and XPS were employed for characterizing samples’physicochemical characteristics.At180°C,25%LaCe/AC acquired 94.3%Hg⁰ removal efficiency and 91.3%NO removal efficiency.The influences of Hg⁰（NO）removal on NO（Hg⁰）removal were researched.The results exhibited that Hg⁰ removal had negligible effect on NO removal,inversely,NO removal imposed negative influence on Hg⁰ removal.The positive roles of NO and O₂ on Hg⁰removal could not overcome the inhibitory influence of NH₃ on Hg⁰removal.Both SO₂ and H₂O exerted negative impacts on NO and Hg⁰ removal.NO removal corresponded to the Langmuir-Hinshlwood mechanism,while catalytic oxidation and physisorption and chemisorption contributed to Hg⁰ removal.Based on mercury adsorption-desorption and mercury conversion tests,their contributions varied with time and chemisorption dominated others at first.Particularly,25%LaCe/AC eventually obtained 89.5%NO removal efficiency and 84.5%Hg⁰removal efficiency after the tests proceeding 27h,suggesting its the potential for low-temperature industrial application in the foreseeable future.A range of CoO_x and CeO₂ supported BACs originated from maize straw were prepared for simultaneously eliminating NO and Hg⁰.These samples’physicochemical characteristics were characterized.At 230°C,15%Co_0.4Ce_0.6/BAC exhibited 84.7%NO removal efficiency and 96.8%Hg⁰ removal efficiency.The negative effects of 5%H₂O or 400ppm SO₂ were observed.The interrelationship between Hg⁰ removal and NO removal was researched,the results indicated that abundant Hg⁰had inappreciable negative influence on NO removal,and NH₃obviously restrained Hg⁰ removal,while NO could react with O₂ to produce NO₂,which mildly promoted Hg⁰ removal.The characterizations suggested15%Co_0.4Ce_0.6/BAC owned better texture properties,stronger redox ability and lower crystallinity,which contributed to its excellent performance.Besides,cerium oxide and cobalt oxide emerged a synergetic effect,which could induce more Co³⁺and Ce³⁺,resulting in more active oxygen and oxygen vacancies as well as more anionic defects.The removal mechanisms of NO and Hg⁰ were investigated,both adsorption reactions and catalytic oxidation reactions were responsible for Hg⁰removal,Meanwhile,the catalytic oxidation reactions for Hg⁰ removal dominated gradually with the Hg⁰removal reactions proceeding,which followed the Mars-Maessen mechanism.While the main NO removal reactions could be explained by the Langmuir-Hinshlwood mechanism.A plenty of LaO_x-MnO_x supported on biomass activated carbons derived maize straw（LaMn/BAC）samples were used to simultaneously remove NO and Hg⁰ from simulated coal-fired flue gas.15%LaMn/BAC yielded prominent Hg⁰ removal efficiency（91%）and superior NO removal efficiency（82%）at 230°C.Combined with the results of BET and SEM,the introduction of LaO_x and MnO_x could supply some catalytic sites,but the metal oxides would block or cover the surface of carriers,decreasing BET areas and pore volumes.Besides,a synergetic effect appeared between LaO_x and MnO_x,resulting in bigger BET areas and total pore volumes and better dispersion of metal oxides,which are propitious to the simultaneous NO and Hg⁰ removal.The results revealed that Hg⁰ concentration had inappreciable influence on NO removal,while NO had evidently positive effect on Hg⁰removal.Meanwhile,NH₃ had an adverse effect on Hg⁰ removal.The NO and Hg⁰ removal mechanism were researched,the main NO removal reactions could be explained by the Langmuir-Hinshlwood mechanism,whereas both adsorption reactions and catalytic oxidation reactions were responsible for Hg⁰ removal,Meanwhile,the catalytic oxidation reactions for Hg⁰removal dominated gradually with the Hg⁰removal reactions proceeding,which followed the Mars-Maessen mechanism.In addition,the slightly inhibitive effect of 8%H₂O or 500ppmSO₂ was also observed,indicating the development of low temperature simultaneously denitration and demercuration catalyst with good resistance to SO₂ and H₂O is of significantly practical interest.A series of CoO_x modified MnO_x/biomass activated carbons（CoMn/BACs）prepared by the ultrasound-assisted impregnation method were employed for the simultaneous removal of NO and Hg⁰ from simulated coal-fired flue gas for the first time.The physicochemical properties of such samples were characterized by XRD,BET,SEM,NH₃-TPD,H₂-TPR,FTIR and XPS.15%CoMn/BAC exhibited preferable performance for NO and Hg⁰removal in a wide temperature range from 160 to 280°C,and it yielded prominent NO removal efficiency（86.5%）and superior Hg⁰ removal efficiency（98.5%）at 240°C.The interaction between NO removal and Hg⁰ removal lessened corresponding separate efficiency,the adverse effect of NH₃ on Hg⁰ removal could not be offset by promotional influences of NO and O₂.Compared with15%Mn/BAC,the addition of CoO_x with suitable amount into 15%CoMn/BAC could contribute to the synergistic effect between MnO_x and CoO_x,resulting in the increase of BET surface area and surface active oxygen species as well as Mn⁴⁺concentration,the enhancement of redox ability and the strength or amount of surface acid sites,restraining the crystallization of MnO_x,which might be responsible for the improvement of catalytic performance and resistance to SO₂ and H₂O.Additionally,the hydrophobic property of BAC further strengthened H₂O tolerance.The results of stability and recyclability tests indicated that 15%CoMn/BAC possessed a promising application potential for NO and Hg⁰simultaneous removal at low temperature.