Mechanism Study On Regeneration And Cyclic Mercury Removal Of Manganese Based Sorbent

The adverse impact of mercury on human health and environmental system has attracted wide attention due to its toxicity,accumulation and global transportation.Coal combustion is a significant source of anthropogenic mercury emissions to the atmosphere.Elimination of mercury emissions from coal combustion contributes to international effort to reduce mercury pollution.The most applicable mercury control tecnology is the activated carbon injection.However,the spent activated carbon sorbent has not been regenerated.The retained mercury in activated carbon would release under certain conditions,resulting in secondary mercury emission pollution.Regenerable mercury removal sorbents have the potential to be the cost-effective alternatives to activated carbon,since it makes the reuse of sorbent and mercury recovery possible.In this study,manganese oxides were acted as mercury removal active phase to form regenerable adsorbent.The intrinsic correlation between thermal regeneration performance and the crystallographic structure were investigated.Mercury adsorption and oxidation behaviors,regeneration methods and the effects of SO₂ and O₂ on mercury removal,as well as the involved mechanism have been interpreted comprehensively.Manganese oxides modified?-Al₂O₃ sorbents were prepared by incipient wet impregnation.The mercury removal performance of prepared sorbents with manganese nitrate and manganese acetate were compared on fixed bed continuous mercury exposure set up.The micro morphology,structure,elementary composition and the chemical state of the manganese modified alumina were characterized by nitrogen adsorption,FESEM-EDS and XPS.The mercury removal mechanism and the thermal regeneration performance of the sorbents were studied in the fixed bed continuous mercury exposure test,mercury pulse injection test,and mercury TPD experiments.The analysis of surface manganese chemical state was explored by XPS.The results showed that mercury removal capacity of the alumina modified by manganese acetate is 1.3?1.4 times of that modified by manganese nitrate.The mixed oxidation state of II,III,IV valence manganese on the alumina surface serves the active mercury removal sites.More than 80%of mercury removal efficiency were obtained with the reaction temperature of 125°C or more.Mercury is retained by adsorption and oxidation reaction,the corresponding efficiencies are around 50%and 40%,respectively.The cyclic mercury removal performance are inhibited by the transformation of higher chemical valence of manganese to the lower.While the manganese modified alumina shows considerable regeneration potential when used in the mercury pulse injection test.For the purpose of identifying the inherent correlation between thermal regeneration performance and the crystallographic structure,mercury removal efficiency and the thermal regeneration characteristics were investigated on the fixed bed continuous mercury exposure set up.?-MnO₂ and?-MnO₂ were found to possess a mercury removal efficiency of higher than 96%after 120 min mercury exposure,which is much higher than the other manganese oxides with different crystallographic structures.The thermal effects of regeneration treatment on the physical structures and the chemical activity of?-MnO₂ and?-MnO₂ were revealed based on the experimental results of thermal regeneration cyclic demercuration and the TGA,XRD,TEM and XPS analyses.?-MnO₂ showed a perfect mercury capacity of its initial 128?g/g over 5 regeneration cycles.While that of?-MnO₂on the fifth cycle was reduced to 68.74?g/g compared with its initial 131.42?g/g for the first cycle.The microstructure of?-MnO₂was destroyed throughout regeneration cycles due to its worse capability to retain lattice oxygen than?-MnO₂.The reconstruction and phase transformation of?-MnO₂ result in the collapse of crystallographic structure and degradation of oxidization for mercury.Thus,the?-MnO₂ have a higher recyclability of mercury capture and better durability for regeneration than?-MnO₂.?-MnO₂ oriented magnetic composite was synthesized through in-situ growth of?-MnO₂onto the Fe-Si core-shell particles.Mercury removal characteristics were studied on the fixed bed continuous mercury exposure set up.The outlet SO₂/SO₃ were sampled based on the IPA method to provide an in-depth understanding of SO₂ adsorption and oxidation behaviors over the composite and the effect on mercury removal.The regeneration performance of the composite was tested in the simulated flue gas and the combination of thermal regeneration and water wash regeneration was evaluated.The adsorption and oxidation of SO₂ would result in deposited sulfates and free SO₃,which are favored by the elevated temperatures.The sulfation of the composite surely consumes the active sites for mercury removal.While the addition of O₂ relieves the inhibitory effect of SO₂ on mercury removal through competitive adsorption.It is summarized that the oxygen and the lower reaction temperature inhibit the surface sulfation and active sites consumption by SO₂.Combination of water wash and thermal regeneration treatment gives stable cyclic mercury removal efficiency for the composite.The quantitative analysis of sulfates and manganese in the washing liquid suggests the presence of Mn₂(SO₄)₃ and Mn(SO₄)₂.Based on the density functional theory,?-MnO₂(100)slab surface was cleaved and geometry optimized.Hirshfeld atomic charge analysis,difference of electron density and partial density of states were adopted to interpret the mercury adsorption mechanism.The competitive adsorption of Hg⁰,O₂,SO₂ were figured out according to the ground state calculation of slab surface with the individual molecules anchored.Mercury adsorption mechanism on SO₂/O₂ modified slab surface was studied by the band structure analysis,electron transfer from mercury atom to slab and the potential adsorption/desorption energy configuration.Electrons from mercury was injected into the slab surface via chemisorption of mercury and redistributed among the surface Mnand O atoms,causing obvious electron density changes.O₂ and SO₂ would adsorb on the surface of two-coordinated O and five-coordinated Mn.SO₂ shows stronger competitive adsorption over the five-coordinated Mnthan mercury and O₂,as indicated by the much higher adsorption energy.Mercury adsorption over the slab surface is inhibited and enhanced by SO₂ and O₂,respectively,which is verified by the experimental results.The nature of the surface property modification by the adsorption of SO₂ and O₂ were attributed to the changes in band gap,surface electron mobility and the number of electron holes of the slab surface.Adsorbed mercury tends to bond with the dissociated oxygen and produce stable Hg O with high desorption energy in absence of SO₂,instead of desorption as the free elemental mercury.The desorption energy of the adsorbed mercury to be dissociated into free elemental mercury and detached Hg O would be lower in presence of SO₂,indicating that SO₂ is capable to promote desorption of the adsorbed mercury.