| The mercury emission during the process of coal combustion has attracted increasing attentions in recent years. Therefore, China government has updated “Emission Standard of Air Pollutants for Thermal Power Plants”(GB13223-2011) to cap mercury emissions. The method of oxidizing elemental(Hg~0) to its oxide, and then removed through the existing air pollution control devices, has been proved a promising choice. Therefore, developing an efficient catalyst with wide active temperature window is of significance, aiming at the low chlorine content in feed-coal of China and relatively narrow active temperature window of catalyst.In this work, a series of catalysts modified by silver were prepared, and evaluated for the removal Hg~0. In addition, the effects of temperature and flue gas composition were investigated, and the Hg~0 oxidation mechanisms of catalyst at different temperature were discussed through the characterization of physical and chemical and the activity analysis of mercury and chlorine, which provided theoretical basis for the practical application of catalysts. The novelties and important conclusions of this work are summarized as follows:1. To improve the catalytic oxidation ability for gaseous elemental mercury, silver was introduced to V2O5-TiO2 catalyst, and various dispersants were added to further improve the performance of catalyst. It was found that the addition of polyvinylpyrrolidone(PVP) during the preparation of the catalysts can improve the dispersion of silver nanoparticles more effectively, resulting in a higher Hg~0 oxidation efficiency up to 90%; However, the oxidation of Hg~0 on the catalyst was slightly inhibited due to the larger silver nanoparticles with smaller specific surface area when the ionic liquid [bmim][BF4](ILs) was used as the additive; Silver species mainly existed in the form of metallic silver, and the oxidation performance of itself was not high, which was as a cocatalyst to increase the ability of active component vanadium oxide.2. A series of selective catalytic reduction(SCR) catalysts modified by silver were prepared, and Hg~0 oxidation performance of catalysts was investigated. The characterization results and Hg~0 oxidation experiment showed that the performance of V2O5-MoO3-TiO2(V-Mo-Ti) catalyst for the oxidation of Hg~0 was higher than that of V2O5-WO3-TiO2(V-W-Ti), and Mo was resistant toward sulfur dioxide; Ag was employed to further enhance the oxidation of V-Mo-Ti, and adding only 0.5% Ag can keep the Hg~0 oxidation efficiency to approximately 90% with 5 ppmv HCl, which has an increase of 20-40% compared to the V-Mo-Ti catalyst; Besides, the active temperature window of catalyst was enlarged from 150 to 400 ℃; Ag and Mo enhanced the Hg~0 adsorption ability of catalyst; Ag-Mo/V-Ti could also oxidize Hg~0 when only O2 and N2 existed, and the performance was better at low temperature; HCl was the main catalytic reaction components; Semi-Deacon reaction was proceed at low temperature, and Full-Deacon reaction was carried out at high temperature for Hg~0 oxidation reaction; In addition, the adsorbed oxygen and reactive oxygen species could also oxidize Hg~0 to mercury oxide, which continued to react with HCl to generate HgCl2.3. MoO3-TiO2(Mo-Ti) catalysts modified by silver were prepared, and Hg~0 oxidation performance was also investigated. The performance of Ag(2%)-Mo-Ti catalyst combining Ag and Mo for Hg~0 oxidation was improved obviously regardless of at high or low temperatures, while the performance of catalyst was higher at low temperature, and was suitable for operation under low temperature; The gas components analysis showed that HCl was the main catalytic component, and the catalyst still had high oxidation efficiency in the absence of O2; In addition, Ag(2%)-Mo-Ti had a superior resistance toward SO2; Adsorbed active Cl reacted with adsorbed Hg~0 followed by Langmuir Hinshelwood mechanism at low temperature; Moreover, silver and molybdenum to generate silver molybdate, beneficial for transforming lattice oxygen into the chemical adsorption to participate in the reaction,which can also improve the oxidation performance of catalyst.4. CeO2-TiO2(Ce-Ti) catalysts modified by silver were prepared, and Hg~0 oxidation performance was investigated. The performance of Ag-Ce-Ti was higher at high temperature, and HCl was the main catalytic component, HCl participated in the reaction in the form of active chlorine at low temperature, chlorine was generated at around 400 ℃ and then taken part in the reaction; Ag-Ce-Ti had a superior resistance toward SO2 at low temperature, while SO2 greatly inhibited the reaction; Combined with Ag, Mo and Ce, the Hg~0 oxidation performance of catalyst was better whether at low temperature or at high temperature, which also had a superior resistance toward SO2.5. AgI-TiO2 was introduced for the removal of Hg~0 in flue gas, and Ag was added to enhance the catalytic activity and stability. AgI-TiO2 had an excellent effect for Hg~0 catalytic oxidation, and the catalytic efficiency can reach 100% with 5 ppmv HCl at 350 ℃, which was better than that of KI-Ti; Adding silver to Ag I-Ti O2 can prolong effectively the reactive time with high efficiency; And 2% Ag content was suitable, for which the Hg~0 oxidation efficiency was still above 90% after 10 h; The gas components analysis showed that HCl was the main catalytic component, and O2 was necessary; HCl was adsorbed and activated by silver to generate active Cl, which then oxidize iodine to active I; I in Ag(2%)-Ag I-Ti mainly acted as an accelerant for Hg~0 oxidation by facilitating formation of the intermediates Hg-I*, and then, chlorine can further convert the intermediate to the final product of HgCl2; Active I would reacted with Ag to generate AgI, maintaining the content of iodine for Ag(2%)-AgI-Ti catalyst; In addition, Ag(2%)-AgI-Ti manifested a good thermostability and chemical stability.6. The NH3 oxidation and DeNOX performance of the catalysts were analyzed, which found that Ag(5%)-Ti had high NH3 oxidation ability at various temperatures, and the mechanisms of the NH3 oxidation were different at different temperatures; The N2 O was generated at low temperature, while NO production at high temperature was more; The DeNOX performance of catalyst firstly increased and then decreased with the increasing temperature, and the De NOX efficiency was highest at 250 ℃, while it was adverse for DeNOX at high temperature, overall, Ag was applicable to the removal of NH3; Ag can improve the NH3 oxidation ability of V-Mo-Ti, Mo(5%)-Ti and Ce(5%)-Ti, and increase the NH3 catalytic oxidation choice of Mo(5%)-Ti and Ce(5%)-Ti at high temperature; When silver was doped, the DeNOX performance of V-Mo-Ti was improved at low temperature, which was reduced above 300 ℃; When silver was doped into Mo(5%)-Ti and Ce(5%)-Ti, the NO concentration was increased, indicated that silver was bad for the DeNOX reaction; The NH3 oxidation ability of Ag(2%)-AgI-Ti increased with the rise of temperature, and catalyst oxidized NH3 into NO mainly, so Ag(2%)-Ag I-Ti was not suitable for the DeNOX reaction.In this work, the catalysts prepared had higher Hg~0 oxidation ability at low concentration of HCl and the active temperature window was wide, which showed an excellent application prospect. |
PDF Full Text Request