| The mercury removal from coal-fired flue gas is the main approach to achieve mercury emission control,and related research has been extensively studied.The mercury removal efficiency of conventional pollutant control units is unstable and is easily affected by the change of boiler operation.The high operating cost of single mercury removal unit also greatly limits its industrial application.It is essential to develop the source control of mercury,which can effectively reduce the pressure and cost of coal-fired flue gas pollutant control.Coal combustion can be decoupled into two sub-processes:pyrolysis and combustion.Capturing mercury in the pyrolysis gas purification stage can greatly improve the mercury removal efficiency and reduce the end treatment cost.Mercury mainly exists in the form of elemental mercury(Hg0)in coal pyrolysis gas.Using adsorbents to capture Hg0 is the most common method to control mercury emissions from pyrolysis gas.Metal oxide adsorbents have been widely used due to their wide sources,low cost and no secondary pollution.However,the mercury removal efficiency of metal oxides in coal pyrolysis gas with strong reducibility and high temperature(>200℃)is very low,and the mechanism of mercury removal is still unclear,which limits the research and development of mercury removal in coal pyrolysis gas.Therefore,it is of great significance to develop metal oxide adsorbents that can efficiently remove Hg0 from coal pyrolysis gas and explore the mechanism of mercury removal.In this paper,a new idea of using Fe-Ti spinel with excellent thermal stability and redox ability as adsorbent to remove mercury from coal pyrolysis gas is proposed,and the mercury removal efficiency of Fe-Ti spinel adsorbent at high temperature is improved by loading precious metal Ag onto Fe-Ti spinel.Firstly,aiming at the defect of low mercury removal efficiency of metal oxides in reducing atmosphere,Fe-Ti spinel adsorbent was prepared by co-precipitation method.The effects of reaction temperature and gas compositions on Hg0 removal performance were investigated in a fixed-bed reactor.The physicochemical properties of the adsorbents before and after the reaction were characterized and analyzed by XRD,H2-TPR,BET,SEM,VSM and XPS.Furthermore,density functional theory(DFT)calculation was also used to reveal the adsorption mechanism of mercury over Fe-Ti spinel adsorbent.The results showed that Fe-Ti spinel adsorbent had good mercury removal performance in the temperature range of 120～180℃.The average mercury removal efficiency could reach 95%at 150℃.Among the components of pyrolysis gas,H2S played an important role for Hg0 capture;while CO,H2 and H2O in pyrolysis gas had negative effects on the removal of Hg0.The active storage oxygen(lattice oxygen and adsorbed oxygen)and Fe3+species on Fe-Ti spinel adsorbent surface were involved in the transformation of H2S and the capture of Hg0.In the presence of H2S,the Hg0 removal over FeTi spinel adsorbent followed the Eley-Rideal mechanism,in which H2S firstly adsorbed on the Fe-Ti spinel surface and dissociated to form active sulfur species,then active sulfur reacted with gas phase Hg0 to form black HgS,thus capturing Hg0.In order to further improve the high temperature mercury removal efficiency of Fe-Ti spinel,Ag/Fe-Ti spinel adsorbents were prepared by impregnation method,and used for the removal of Hg0 from high temperature pyrolysis gas.The effects of reaction temperature,Ag loading amount and H2S on mercury removal efficiency were investigated.The removal mechanism of Hg0 also was clarified by designing H2S cut-off and H2S-pretreatment experiments and combing with DFT simulation calculation.The results showed that lower reaction temperature and higher Ag loading amount were beneficial to the mercury removal performance of Ag/Fe-Ti spinel adsorbents in the range of 200～350℃.In the 200～250℃temperature range,the mercury removal efficiency of Ag/Fe-Ti spinel adsorbent was more than 90%.H2S was a key gas component in the process of mercury removal,which could be adsorbed on the surface of Ag/Fe-Ti spinel adsorbent to form active sulfur species.The adsorption mechanism of mercury over Ag/Fe-Ti spinel followed the Langmuir-Hinshelwood mechanism,in which H2S was adsorbed on adsorbent surface and generated active sulfur species to react with adsorbed Hg0.Fe-Ti spinel adsorbent showed a high ability to remove mercury from coal pyrolysis gas,and its mercury removal performance at high temperature could be improved by loading noble metal silver,which had strong industrial promotion and application prospects. |
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