| The mercury pollution in China has become a serious concern.Mercury species in flue gas of coal-fired plants mainly present as elemantal mercury(Hg0),oxidized mercury(Hg2+)and particulate-bound mercury(Hgp),of which elemental mercury is the most difficult to remove.The control of elimental mercury for coal-fired flue gas mainly includes SCR+FGD and powder activated carbon(PAC)injection.However,there is a major disadvantage for the two methods that elemental mercury can't be captured for the centralized control but converted to more toxic mercury species in fly ash or desulfurization gypsum.Therefore,these two methods are currently restricted in the control of elemental mercury emission in China.Recently,to meet the stricter emission standards for the ultra-low emission,Wet Electrostatic Precipitators(WESPs)are gradually installed downstream of the FGD in coal-fired plants of China.Magnetic sorbents may be injected into the flue gas downstream of the FGD to capture gasous elemental mercury.Then,the sorbents are removed from the flue gas by the WESP.The mixture collected by the WESP mainly contained spent magnetic sorbents,small amounts of ultrafine particulates and desulfurization gypsum.Therefore,it is practical to recover spent magnetic sorbents from the mixture collected by the WESP for the recycling and achieving the centralized control of elemental mercury pollution.To match the condition of WESP,the sorbent must have the following characteristics.Firstly,sorbent must has excellent superparamagnetism,which make it easy to be separated from the slurry.Secondly,the temperature window matches the temperature of flue gas downstream of the FGD(i.e.,40-80?).Thirdly,the mercury adsorbed on the sorbent should be stable,which is not easy to be leached during the spraying.Fourth,the mercury adsorbed on the sorbent can be quickly desorbed in appropriate temperature.Fifth,the cost of sorbent is low and the sorbent can be recycled.The Fe-based spinels could be used to capture elemental mercury but their temperature window didn't match the temperature range of the flue gas downstream of the FGD.Pyrrhotite showed excellent performance in removal of Hg0.However,pyrrhotite couldn't be easily separated from slurry due to week magnetism.If the advantages of excellent superparamagnetic properties of Fe-based spinels and excellent low-temperature adsorption activity of pyrrhotite were combined,the centralized control of Hg0 in coal-fired flue gas can be achieved.Therefore,based on early research,this paper innovatively used gaseous hydrogen sulfide to pretreate Fe-Ti spinel,developing an efficient magnetic sorbent matched with the WESP.In addition,according to the reaction mechanism and kinetics of the chemical adsorption of Hg0 on modified Fe-Ti spinel,the structure-activity relationship among H2S pretreatment,the physicochemical properties of modified Fe-Ti spinel and its ability for capturing gaseous elemental mercury from the flue gas at low temperatures was built.Meanwhile,the natural titanomagnetite was modified by calcination and H2S pretreatment to match the WESP,which further reduced the cost.The main results of this work are as follows:(1)Fe-Ti spinel was modified by H2S pretreatment and the ability for capturing gaseous elemental mercury was significantly increased,with average adsorption rate of 1.92?g g-1 min-1 and capacity of 0.69 mg g-1 at 60 ?.SO2 and water vapor in flue gas didn't affect the mercury adsorption capacity.Modified Fe-Ti spinel has superparamagnetism with the saturation magnetization of 24.6 emu g-1 and can be recovered by magnetic separation.The oxidant S22-was produced due to the reaction of spinel and H2S in the process of H2S pretreatment.The adsorption of gaseous elemental mercury on modified Fe-Ti spinel followed the Mars-Maessen mechanism.Gaseous elemental mercury adsorbed on the surface of the sorbent,then the adsorbed mercury was oxidized to the stable HgS by the oxidant S22-.The adsorbed mercury specie wasn't easy to be leached and can be quickly desorbed to high concentration gaseous elemental mercury in appropriate temperature.The sorbent could be recycled through H2S regeneration treatment.(2)To further reduce the cost of magnetic sorbent,natural titanomagnetite was used to capture gaseous elemental mercury.H2S pretreatment effectively improve the adsorption properties of natural titanomagnetite,and calcination further improve the capacity of modified natural titanomagnetite.The modified natural titanomagnetite showed excellent performance in removal of Hg0,with capacity of 8.1 ?g and capacity of 0.17 mg g-1 at 60 ?.The H2O and SO2 in the flue gas didn't affect the capacity of modified natural titanomagnetite.Modified natural titanomagnetite has superparamagnetism with the saturation magnetization of 18.8 emu g-1.The adsorption of gaseous elemental mercury on modified natural titanomagnetite also followed the Mars-Maessen mechanism.The Fe3+ content on the surface of natural titanomagnetite was increased by calcination,generating more oxidant(S22-)in the reaction with H2S.Therefore,the modified natural titanomagnetite obtained by H2S pretreatment after calcination has the most excellent adsorption ability.The mercury adsorbed on modified natural titanomagnetite was stable HgS,which couldn't be leached.The adsorbed mercury specie can be recovered by thermal desorption to high concentration gaseous elemental mercury in appropriate temperature,achieving the centralized control of mercury.The modified natural titanomagnetite could be recycled through calcination and H2S regeneration treatment.In conclusion,the modified Fe-Ti spinel obtained by H2S pretreatment showed excellent performance to capture Hg0 and resistance to water vapor and SO2,which can be used as a co-benefit of WESP for centralized control of Hg0 in coal fired flue gas.Modified natural titanomagnetite further reduce the cost of mercury pollution control,which showed important application prospects. |
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