Experimental Research And Reaction Mechanism On The Elemental Mercury Removal With Iron Oxide

In recent years,mercury emission has caused persistently environmental contamination and received a global attention.The strategies for better control of mercury release are highly demanded because of high consumption of fossil fuels in China.The control of elemental mercury,because of its volatility,persistence and bioaccumulation of methyl mercury,are complicated and difficult in atmospheres.Although various sorbents have been researched,and a preliminary business application have been provided,a more mature and effective technology is still necessary.Metal oxide,which is of scientific and technological importance because it is easily accessed and separated,is nonsecondary pollution,and is less expensive than other sorbents,is currently the focus of our concern and proved to be a promising technology.It's complicated and important for the mercury removal in the presence of real flue gas because of the interactions between different flue gas compositions and elemental mercury,including homogeneous and heterogeneous reactions.It would be feasible to adopt a comprehensive utilization of flue gas components.By combining with the density functional theory calculations and designed experiments,a systematic research on mercury removal with iron oxides was performed by the way of separation and then synthesis.Based on the coal-fired flue gas(HCl,HBr,O2,SO2)and reducing atmosphere(H2S)in gasification,the role of flue gas and gas-solid reaction were analyzed by considering the homogeneous reaction,solid surface properties,etc.and designing a series of contrasted experiments.Furthermore,the DFT study in quantum mechanics was applied in exploring the stable configurations and pathways between mercury and iron oxide surface from the molecular level,and the possible reaction pathways were thoroughly discussed to receive detailed reaction steps and summarize the behaviors and characteristics of elemental mercury removal on iron oxides.Halogen gases from coal-fired flue gas are considered the important factors that effected elemental mercury oxidation.Considering the heterogeneous reaction process,mercury speciation on iron oxide surface,solid surface characteristics and the important active ingredients in heterogeneous reaction process,an laboratory scale experimental scheme was designed to clarify the mechanism of mercury oxidation on Fe2O3 surface with HCl in a wide temperature range(80-780?),and the pathways of mercury transition and mechanisms were synthetically discussed combined with the DFT calculations: At low temperature,the mercury removal mechanism followed Eley-Rideal mechanism,in which HCl overcame low energy barrier to preferentially dissociate and adsorb on the Fe2O3 surface,and then reacted with Hg0;and at medium temperature it followed Langmuir-Hinshelwood,in which HCl and Hg0 could be simultaneously adsorbed on the Fe2O3 surface and then reacted.At high temperature above 580 ?,both the homogeneous reaction between Hg0 and HCl and Langmuir-Hinshelwood mechanism were favorable to the transformation of Hg0.In the presence of HBr,the elemental mercury removal efficiency with ?-Fe2O3 could amost reach about 100%,which is much better than HCl.Because of the complexity in complex flue gas constituents during mercury transformation,the role of competitive inhibition and synergic promotion on capturing mercury with mixed gases,including SO2,HCl/O2,HCl/ SO2,etc.,was explored.In the coexistence of HCl and O2,the Hg0 conversion pathways and mechanisms on ?-Fe2O3 were researched at different temperatures combining with the conclusions on HCl and different designed experimental conditions.At 80?,the existing O2 competed and hindered the formation of Cl active sites,resulting in a negative effect in Hg0 removal,however,the role of O2 was opposite at 380?,HCl and O2 on ?-Fe2O3 formed activated iron substrate or gaseous intermediate products,capturing more elemental mercury.A further comparation on mercury removal experiments at different SO2 concentrations and mixed HCl and SO2 gases on ?-Fe2O3 were also carried on.The mercury removal performance differences on iron oxides in various conditions were contrasted at low temperature,and the interactions between SO2 and HCl as well as the mechanisms were discussed comparing with the theoretical results.The influence of different SO2 concentrations on mercury conversion varied largely,low SO2 concentration could synergistically facilitate the Hg0 oxidation in the presence of HCl while the dissociation between SO2 and HCl competed against each other and shorten the reaction time.At the higher SO2 concentration the Hg0 removal efficiency was largely decreased,and it might be that in SO2 dissociation the large amount of O atoms competed with the active sites with Hg0.Considering the reducing atmosphere of H2 S in gasification and the similarities with HCl,the role of H2 S on mercury oxidation was considered and a series of experiments were performed.A clear elaboration of the effective active substance,migration mechanism and key transformation point,was performed by combining with the designed experimental scheme,re-release of Hg0-absorbed samples,TG-DSC as well as the TPD technologies.In calculations,the Hg0 reaction followed the Eley–Rideal mechanism,in which H2 S firstly dissociated on the substrate surface with two steps,then gaseous mercury bonded to the preadsorbed sulphur atom on the surface,forming HgS with the adsorption energy of-136.73 kJ/mol and belonging to chemisorption.According to the above,it can be pointed out that the experimental and theoretical results are in good agreement,and the reaction temperature around 230? was a critical turning point in capturing Hg0.In order to explore the characteristics and principles of elemental mercury removal on iron oxide under actual application conditions,including injection mass,elemental mercury concentration,flue gas temperature and residence time,we established the test platform of duct injection for removal of Hg0 with simulated flue gas,performed a basic experimental study on demercuration,and preliminarily revealed the features and rules.It lay a foundation for exploring the better performance of mercury removal,forming manageable oxidation state mercury,and filtrating efficient adsorbents.The elemental mercury removal experiments in simulated flue gas implied that the metal oxide was beneficial to capture mercury,and HCl was important for transforming Hg0.