Investigation On Selective Adsorption Mechanism Of Gaseous Oxidized Mercury In Coal-fired Flue Gas

Under the background of Minamata convention on mercury and Ultra-low emissions requirement on coal-fired power plants,the reduction policy of atmospheric mercury pollution is also increasingly stricter in China.The research and development on mercury speciation monitoring facilities have become an important topic for coal-fired boilers,which is the largest anthropogenic mission source of atmospheric mercury pollution.To date,the investigation on the mechanism of effective separation of gaseous elemental mercury（Hg⁰）and oxidized mercury(Hg²⁺)selective adsorption in coal-fired flue gas is the most fundamental content to study.Therefore,this thesis took the above research contents as the starting point,proposed two strategies to separate HgCl₂ from Hg⁰ in coal-fired flue gas:（1）separation by physical adsorption;（2）separation by physical adsorption.Methods like adsorption evaluations,material characterization,Density Functional Theory（DFT）computational studies,and molecular simulations were combined to systematically investigate the feasibility of separating HgCl₂ from Hg⁰ in flue gas by using CaO/SiO₂ sorbent or the UiO-66 porous framework.Firstly,disparities between Hg⁰ and HgCl₂ were investigated by literature review and frontier molecular orbital analysis on chemical reactivity,molecular size and polarity.It is found that Hg⁰is a nonpolar monatomic molecule,and its stable chemical property is attributed to the inert electron pair effect on its 6s orbital.On the contrary,HgCl₂ is a linear molecule with quadrupole,which has a lower LUMO level than that of Hg⁰ to accept electrons easier.That is,HgCl₂ is a also typical Lewis acid.Two separation strategies can be ascertained based on these disparities:（1）we can chemically separate HgCl₂ from Hg⁰ by means of Lewis acid-base interactions,or（2）physically separate HgCl₂ from Hg⁰ via the molecular sieving effect as well.Secondly,Adsorption behaviors of Hg⁰ and HgCl₂ on surfaces of alkaline sorbents were evaluated.CaO,MgO,KCl,and NaCl are found to be invalid for Hg⁰ adsorption,while robust Lewis acid-base interactions are responsible for the chemical adsorption and binding on those surfaces.CaO has the relatively higher HOMO level and weaker Mulliken electronegativity than others,which result in the strongest E_ads of HgCl₂ on its（001）surface.Besides,Hg⁰ only shows an E_ads of-14.51 kJ/mol on the same surface.Therefore,CaO is identified as a promising candidate for chemical separation of gaseous HgCl₂ and Hg⁰.Thirdly,A combination of fixed-bed adsorption test,sorbent characterization analysis and theoretical calculations is applied to study the dispersion effect of porous support.It is found that CaO constituents are evenly dispersed by Mesoporous SiO₂ support.The adsorption capacity of CaO/SiO₂ for HgCl₂ depends on both the available pore volume and basic sites.The CaO/SiO₂sorbent with monolayer dispersion of basic CaO constituents shows the optimal adsorption capacity for HgCl₂.HgCl₂ was bonded with the CaO/SiO₂ sorbent into four different manners with intensity in increasing order:Monodentate<Tridentate<Bidentate<Bridging.The Tridentate adsorption is the primary adsorption configuration on CaO/SiO₂.The different adsorption behaviors of HgCl₂ are ascribed to the unsaturated O sites with different coordination numbers.O sites with lower coordination show stronger basicity(O_5C<O_4C<O_3C).Furthermore,competitive effect of SO₂ on HgCl₂ adsorption by CaO/SiO₂ is fully evaluated.Both the fixed-bed adsorption test and characterization analysis suggest that SO₂ will compete with HgCl₂ for basic sites of CaO surfaces.Competitive adsorption can also lead to the porous channel plugging and basic sites deactivation of the CaO/SiO₂ sorbent.The adsorption capacity for HgCl₂ was extremely diminished,which is further certified by DFT calculations.The competitive effect of SO₂ to HgCl₂ is essentially caused by its strong Lewis acid nature.SO₂shows an E_adsds of-176.27 kJ/mol on the CaO（001）facet,which is significantly higher than that of HgCl₂.If the basic sites was preoccupied by SO₂,HgCl₂ adsorption intensity was severely weaken to-18.56kJ/mol.That is the significant shielding effect of CaSO₃ product layer.In addition,the competition from SO₂ can destabilize the bonding of HgCl₂.The strong exothermic process of SO₂ adsorption would promote the transformation from adsorbed HgCl₂ to HgO phase.Besides,the release of Hg⁰ from CaO/SiO₂ sorbent is because of heterogeneous reduction of the adsorbed HgCl₂ by SO₂.Additionally,sulfur resistance performance of low-valence doped CaO/SiO₂ was investigated.DFT calculations indicate that after being doped by low-valence dopants like Na and K cations,the formation energy of O vacancy（OV）is significantly lowered.OV is also a strong Lewis base on CaO surface.The E_ads of HgCl₂ will gain more than that of SO₂ with the introduction of OVs on CaO surface,which would be conducive to the simultaneously stabilize the bonding of HgCl₂and diminish the competition from SO₂.The existence of OV can decrease the energy barrier of O^2-diffusion on both the surface and bulk of CaO.As thus,the migration rate on the gas-solid interface will be enhanced,which can further mitigate the shielding effect by CaSO₃ layer.Finally,force fields of are originally identified in this study that are able to describe the intramolecular and intermolecular interactions of Hg⁰,HgCl₂,other flue gas components and UiO-66 frameworks.Our study suggests that the secondary porous chancel UiO-66 has mean pore size of 8.5?,which is slightly wider than the molecular size of HgCl₂（8.16?）,and spacious enough for Hg⁰（3.14?）to breakthrough.As such,UiO-66 is an ideal porous material for HgCl₂selective adsorption.Henry’s constant of HgCl₂(6.71×10^-2 mol/kg/Pa)in UiO-66 is significantly higher than that of other gaseous components like Hg⁰(7.72×10^-5 mol/kg/Pa)and SO₂(4.01×10^-5mol/kg/Pa).This quality can not only guarantee the efficient separation of HgCl₂ from Hg⁰,but also avoid the competitive adsorption by SO₂ and other flue gas components.The Van de Waals interaction is the primary factor for HgCl₂ selective adsorption in UiO-66,while the coulombic interaction can additionally improve the selectivity of HgCl₂.UiO-66 modified by polar groups（-NH₂ and-OH）shows a increasing electrostatic attraction to gaseous HgCl₂.The force field model developed in this study has stopped gaps of molecular simulations on Hg⁰ and HgCl₂ in porous materials.Moreover,all the thermodynamics parameters like Henry’constant,heat of adsorption,adsorption isotherms,etc.,proposed by our research are significant basis to mercury adsorption,separation,removal and oriented design of mercury sorbents for the industrial applications.