Homogeneous-heterogeneous Reaction Kinetics And Adsorption Mechanism Of Mercury Conversion In Coal-fired Flue Gas

Mercury pollution caused by coal combustion exhibits harmful effects on human health and ecosystem,and is regarded as a crucial problem associated with the sustainable development of human society.Mercury emission control has emerged as a research hotspot in both scientific and technical field,the fundamental understanding of mercury transformation mechanism and reaction kinetics is crucial for addressing this grand challenge.However,to date,the preliminary exploration of reaction mechanism and kinetics cannot quantitatively and accurately predict mercury transformation during coal combustion.Therefore,the development of comprehensive chemical kinetic model can provide deeper insight into atomic-level mercury transformation mechanism,and be essential for the development of effective mercury-capture technologies.Homogeneousreactionkineticsisregardedasthebasisof homogeneous-heterogeneous reaction kinetics.Homogeneous Hg/Br/Cl/C/H/O/N/S kinetic model was first developed,and validated by comparison with experimental data to estimate its accuracy and reliability.The dominant reaction pathway of homogeneous mercury transformation was revealed through sensitivity analysis.The transformation process of HBr and HCl in coal-fired flue gas was analyzed to investigate the mechanism of the effects of multi-species,multi-reactions and multi-parameters coupling on mercury oxidation.Model predictions are in good agreement with experimental data.High quench rate is much more favorable for mercury oxidation than low quench rate,because high quench rate results in a higher concentration of Cl or Br radicals which are responsible for mercury oxidation.HBr is much more effective than HCl in oxidizing mercury,because Br radical formation from HBr decomposition is much easier than Cl radical formation from HCl decomposition.The dominant reaction pathway of homogeneous mercury oxidation by HX?X=Cl or Br?is a two-step process controlling by Hg+X+M=HgX+M and HgX+X₂=HgX₂+X.In this dominant pathway,Hg⁰ is first oxidized by X radical into HgX,HgX is subsequently oxidized into HgX₂.The developed homogeneous kinetic model has a good performance to quantitatively predict the effects of HCl,HBr and quench rates on mercury oxidation.Fe₂O₃ within fly ash shows catalytic activity for mercury oxidation.However,no reaction kinetics of mercury oxidation over Fe₂O₃ surface was reported.A combined method of experiments,density functional theory and kinetic calculations was used to investigate the heterogeneous Hg/Cl oxidation mechanism over Fe₂O₃ surface.A heterogeneous kinetic model of mercury oxidation over Fe₂O₃ surface was developed based on the mechanistic studies.Moreover,a method was proposed to determine the surface active site density of Fe₂O₃.The results show that the chemisorption mechanism is responsible for Hg⁰ and HCl adsorption on Fe₂O₃ surface.A kinetic model including eight irreversible elementary surface reactions was developed and combined with homogeneous mechanism to predict heterogeneous mercury oxidation over Fe₂O₃ surface.Model predictions are in good agreement with experimental data obtained by three independent groups.The dominant reaction pathway of heterogeneous mercury oxidation over Fe₂O₃surface is a multi-step process.In this multi-step process,Hg⁰ is first adsorbed on Fe₂O₃surface to form Hg?s?,Hg?s?reacts with Cl?s?to generate HgCl?s?.Subsequently,HgCl?s?is further oxidized into HgCl₂?s?and finally desorbs from Fe₂O₃ surface.Bromine addition has been regarded as a kind of mercury emission control technologies.However,no attempts were made to investigate the heterogeneous Hg/Br reaction chemistry?especially the reaction kinetics?over Fe₂O₃ surface.Heterogeneous Hg/Br oxidation mechanism over Fe₂O₃ surface was investigated to develop the corresponding kinetic model.Temperature coefficient???was proposed to understand the interaction mechanism between temperature and reaction chemistry.HBr decomposes to form active bromine species over Fe₂O₃ surface,the active bromine species exists in the form of the complex Br-Fe compounds.HgBr and HgBr₂ adsorption on Fe₂O₃ surface are dominated by chemisorption mechanism,surface O and Fe atoms are identified as the active sites of HgBr and HgBr₂ adsorption,respectively.Heterogeneous mercury oxidation by HBr over Fe₂O₃ surface follows the Langmuir-Hinshelwood mechanism,where the adsorbed Hg⁰ reacts with the active surface bromine species produced from HBr decomposition.A new comprehensive heterogeneous reaction kinetic model including 17irreversible reactions was established to quantitatively describe the detailed reaction process of Hg/Br over Fe₂O₃ surface.The model predictions were found to be in good agreement with the experimental data.The dominant channel of Hg⁰ oxidation over Fe₂O₃surface in the presence of HBr is a four-step process:Hg⁰?Hg?s??HgBr?s??HgBr₂?s??HgBr₂.HgBr?s??HgBr₂?s?is the rate-determining step of the whole mercury oxidation process.In the temperature range of 100-150oC,the promotional effect of mechanism M1is stronger than the inhibitory effect of mechanism M2,mercury oxidation efficiency increases with increasing reaction temperature.In the temperature range of 200-350oC,the inhibitory effect of mechanism M2 is much stronger than the promotional effect of mechanism M1,mercury oxidation efficiency decreases with increasing reaction temperature.Unburned carbon?UBC?within fly ash shows catalytic activity for mercury oxidation.However,the competitive adsorption mechanism among different flue gas species over UBC surface is still unknown.The competitive adsorption mechanism over UBC surface was investigated.A comprehensive chemical kinetic model consisting of a homogeneous Hg/Br/Cl/C/H/O/N/S mechanism with 352 elementary reactions and a new 38-step heterogeneous mechanism for the reactions on different catalytically active components surface was developed to quantitatively predict mercury oxidation in coal-fired flue gas.Model predictions are in good agreement with lab-,pilot-and full-scale experimental data.The effects of the content of UBC and Fe₂O₃ within fly ash,flue gas compositions,quench rate,fly ash loading,particle size and specific surface on mercury speciation distribution can be reproduced by this kinetic model.UBC shows a large chlorine storage capacity,and sustains mercury oxidation in a wide temperature range.At low UBC levels,the availability of UBC active sites determines the mercury oxidation efficiency,whereas at high UBC level,HCl availability becomes dominant for mercury oxidation.The dominant pathway of mercury transformation in coal-fired flue gas with bromine addition is a two-step process:Hg⁰?StHgBr?s??HgBr₂.MnFe₂O₄ has been regarded as a very promising sorbent for mercury emission control in coal-fired power plants because of its high adsorption capacity,recyclable and regenerable properties.However,the microcosmic reaction mechanism of mercury adsorption and oxidation over MnFe₂O₄ surface is still unclear.The adsorption mechanism of different mercury species over MnFe₂O₄ surface was investigated using density functional theory calculation.The interaction between mercury species and surface active sites was revealed by electronic structure analysis.The pathways of heterogeneous mercury oxidation by O₂ or HCl over MnFe₂O₄ surface were analyzed based on energy barrier and reaction heat.The DFT results indicate that Hg⁰,HgO,HgCl and HgCl₂ adsorption over MnFe₂O₄ surface are dominated by chemisorption mechanism.The orbital hybridization and overlap between Hg atom and surface Mn atom are closely associated with the higher mercury adsorption capacity of MnFe₂O₄ sorbent.HgO molecule adsorbed on MnFe₂O₄surface has a lower HOMO-LUMO energy gap,indicating that HgO molecule can stably exist on MnFe₂O₄ surface.It is found that Mn-terminated surface is much more favorable for Hg⁰ oxidation than Fe-terminated surface.Heterogeneous Hg⁰ oxidation by HCl occurs through a two-step reaction pathway?Hg⁰?HgCl?HgCl₂?,in which the first step?Hg⁰?HgCl?is the rate-determining step.Finally,a method based on HOMO-LUMO energy gap was proposed to screen mercury sorbents.This proposed screening method can lay the foundation for the rational design and directional control of mercury sorbents.Based on the computational screening method of HOMO-LUMO energy gap,a series of Cu_xMn_3-xO₄ sorbents were synthesized using low-temperature sol-gel auto-combustion synthesis method.Mercury removal efficiency and regenerable property of Cu_xMn_3-xO₄sorbents were evaluated in the fixed-bed reactor.The microcosmic mechanism of Hg⁰ adsorption on sorbent surface was investigated using density functional theory calculations.Cu_xMn_3-xO₄ sorbents show the excellent mercury removal efficiency and regenerability in the absence of HCl,which is closely associated with the mobile-electron environment caused by Jahn-Teller distortion.Mercury removal by CuMn₂O₄ sorbent is controlled by mass transfer process when the superficial velocity of flue gas is less than 16.9 cm/s.The surface reaction kinetics dominates mercury removal process when the superficial velocity is larger than 16.9 cm/s.During the regeneration process,mercury desorption from spent sorbents can be described as a first-order desorption reaction process.Hg⁰ adsorption on CuMn₂O₄ surface is dominated by chemisorption mechanism.O₂ dissociation over CuMn₂O₄ surface is an exothermic process,the activation energy barrier and reaction heat are 13.9 kJ/mol and-77.7 kJ/mol,respectively.Gas-phase HgO formation over CuMn₂O₄surface goes through three steps:Hg⁰ adsorption,Hg⁰ oxidation and HgO desorption.The bimolecular surface reaction between adsorbed mercury and chemisorbed oxygen is the rate-determining step of mercury adsorption-oxidation-desorption process.