Research On The Removal Of Elemental Mercury In Flue Gas In Pre-SCO Process

The sustained high-speed national economic development makes the demand of energy supplies increasingly urgent.Coal combustion,as a mature approach,will not be replaced in the short term,resulting in increasingly serious environmental pollution caused by nitrogen oxides,sulfur oxides,heavy metals,etc in flue gas.Through the coal combustion,most of the mercury in coal would exist in the form of elemental mercury(Hg0)in flue gas.Hg0 could not be effectively removed by currently available air pollution control devices.How to transform elemental mercury into easily captured oxidized mercury(Hg2+)becomes the focus of study.A fraction of Hg0 might be oxidized into Hg2+ in SCR process.However,the Hg0 oxidation is handicapped by the low content of HCl in flue gas and the NH3-SCR reaction.Based on the theory of Fast-SCR,in this study,a catalytic oxidation unit at the upstream of SCR unit is introduced to obtain the efficient oxidation of mercury and simultaneously achieve a certain NO to NO2 conversion ratio in order to improve the DeNOx efficiency of the following SCR process,which can effectively resolve the problems mentioned above.It is of great significance that the enhanced removal of Hg0 and NO in Pre-SCO conditions provides the technique storage for control of mercury pollution from coal-fired flue gas.CoOx-MnOx/Ti-Ce catalyst was firstly developed in thi study for the simultaneous oxidation of Hg0 and NO.The results indicated that Co3+/Co2+,Mn4+/Mn3+,Ce4+/Ce3+ redox couples and surface oxygen species had been involved in the catalytic oxidation of Hg0 and NO.A possible synergetic effect Among Co,in and Ce species might improve the activity further.O2 could promote the oxidation reaction by replenishing the consumed chemisorbed oxygen and the lattice oxygen.SO2 showed an inhibitory effect on Hg0 oxidation.NO somewhat promoted Hg0 oxidation.H20 could adsorb onto the active sites over catalyst surface,thus obstructing the imteraction of Hg0 with catalyst and resulting in the decrease of Hg0 oxidation efficiency.NO oxidation was more susceptible to SO2 than Hg0 oxidation over the catalyst.In the absence of HCl,the optimum reaction temperature for the simultaneous oxidation of Hg0 and NO over Co12Mn5/TiCe20 catalyst was about 300 ℃.Addition of some HCl or NH4Cl into the flue gas might be necessary for the promotion of Hg0 oxidation at higher temperatures,if using C012Mn5/TiCe20 as the pre-sco catalyst.To achieve the oxidation of mercury over catalyst in the absence of HC1 and presence of HCl with low concentration,CuCl2-CoOx/Ti-Ce catalyst was developed by using CuCl2 to modify CoOx/Ti-Ce catalyst.The Hg0 oxidation efficiency was above 95%over this catalyst in the temperature range of 300-380 ℃.CuCl2 promoted the oxidation of Hg0.The oxidation process occurred at CuCl2 sites and CoOx sites separately for Hg0 and NO over the catalyst.Well dispersion of CuCl2 and CoOx species over catalyst surface was beneficial to the oxidation of Hg0 and NO.7 Wt%was the optimal loading of CuCl2.Hg0 reacted with Cl in CuCl2,leading to the consumption of active Cl.A persistent Hg0 oxidation activity could be achieved over CuCl2-CoOx/Ti-Ce catalyst with the aid of HCl of low concentration.500 ppm SO2 did not show a significant negative effect on Hg0 oxidation but suppressed NO oxidation.The activity for mercury oxidation would be suppressed when SO2 concentration further increased to 2000 ppm.At high SO2 concentrations,more SO2 molecules entered the catalyst bulk and more and more active sites became occupied and completely covered by the sulphates,leading to the loss of activity.The sulfur tolerance of CuCl2-CoOx/Ti-Ce catalyst was further investigated and Mo was adopted to modify this catalyst.The active Cl in catalyst would not be rapidly lost in the presence of low and moderate concentration of SO2,which retained the ability to oxidize Hg0 effectively.SO2 interacted with catalyst to generate sulfate species on its surface and thus increased the amount of Lewis acid sites of catalyst,which promoted the adsorption and oxidation of Hg0 to some extent.Mo addition could restrain the crystallinity of cobalt oxides on the surface of CoOx/Ti-Ce catalyst.An interaction of Mo with Co increased the Co2+ and Oα concentration and enhanced the reducibility of catalyst.The addition of Mo suppressed the reaction between the catalyst and SO2 and inhibited the formation of sulfate species over the catalyst and consequently increased the SO2 resistance of catalyst.The reaction mechanisms and kinetics of Hg0 oxidation over CoOx-MnOx/Ti-Ce and CuCl2-CoOx/Ti-Ce catalysts were studied pertinently.The Hg0 oxidation over CoOx-MnOx/Ti-Ce followed Langmuir-Hinsnelwood mechanism.HCl and Hg0 in flue gas adsorbed onto the surface of catalyst to form HCl(ads)and Hg0(ads),respectively.Then,HCl(ads)reacted with Hg0(ads)to produce HgCl2.The Hg0 oxidation efficiency increased with the concentration of HCl in flue gas.The Hg0 oxidation over CuCl2-CoOx/Ti-Ce followed Mars-Maessen mechanism.Elemental mercury at gaseous state collided with the CuCl2 sites of catalyst and reacted with the active Cl to form HgCl2.The consumption of active Cl species could be compensated by the gas phase HCl with the aid of O2.The Hg0 oxidation reaction rate over CUCl2-CoOx/Ti-Ce was positively correlated with the concentration of Hg0 in flue gas,while independent of the concentration of HCl and O2.The Hg0 oxidation efficiency did not rely on the HCl concentration.Therefore,CuCl2-CoOx/Ti-Ce was superior to CoOx-MnOx/Ti-Ce for the oxidation of Hg0 in the presence of HCl with low concentration.