||Mercury is a volatile and persistent heavy metal which causes serious damages to human health. The monitoring data of the atmospheric, hydrosphere, lithospheric mercury revealed that the anthropogenic activities are the main sources of mercury. Among these sources, coal-fired power plants contributed the most part of the mercury emission. The most widest mercury international convention - Minamoto Convention has already announced to regulate mercury emissions from coal-fired power plants since October 9, 2013. Due to its excellent photocatalytic performance, applying TiO2 to Hg0 removal in pilot scale has been investigated in recent years. However, it is difficult to elevate it in full scales due to a lot of problems being solved. One difficult problem is that the high temperatures inhibit the photocatalytic reactivity of TiO2 and thus reduce the Hg0 adsorption on the surface of TiO2. |
This study aims to investigate the enhancement of photo-reactivity of TiO2 for Hg0 removal at high temperatures and the transformation efficiency of element mercury to oxidized mercury at high temperatures by oxidation process occurred on its surface. The research can be divided into four parts: (1) The effects of operating parameters on the removal efficiency of element mercury by commercial TiO2 at high temperatures, such as calcination temperatures for preparation of TiO2 crystal, wave length of irradiation, the irradiation intensity and oxygen concentration in the flue gas, and etc. (2) The investigation of WO3/TiO2 prepared by sol-gel method on the Hg0 removal at high temperatures. The surface characteristics of WO3/TiO2 were analyzed by BET specific surface analyzer, XRD, SEM, TEM, HRTEM, UV-visible spectrum, XPS, Raman spectrum, photoluminescence spectrum, and etc. The influences of reaction temperatures, influent Hg0 concentrations, gases components, and etc on Hg0 removal were investigated and the Langmuir-Hinshelwood thermodynamic model were further established. (3) The effects of influent concentrations of HCl, O2, NO and SO2 on the transformation efficiency from Hg0 to HgCl2 catalyzed by TiO2 and WO3/TiO2. (4) The investigation on V2O5/TiO2 prepared by sol-gel method for the Hg0 removal at high temperatures. The influences of V2O5 doping amount, reaction temperatures, influent Hg0 concentrations on Hg0 removal were investigated and the dynamic model was also established to investigate the mechanisms on Hg0 removal.
Experimental results indicated that increasing reaction temperatures from 100°C to 160oC would decrease the photo-oxidation efficiency of Hg0 by TiO2, particularly at 140oC, the removal efficiencies of Hg0 dropped significantly. The calcination temperatures of 400oC and 500oC brought higher photo-oxidation efficiencies than those at 300oC and 600oC. Irradiation of 254 nm could effectively raise the photo-oxidation efficiency of Hg0 at 140°C and 160oC. The increase of irradiation strength from 15 W to 20 W was verified to significantly enhance the photo-oxidation efficiency of Hg0 at 140oC and 160oC. However, the effect of 5% O2 present in the flue gas on the photo-oxidation capacity of Hg0 at 140°C and 160oC was limited. The photo-oxidation of Hg0 at 120-160oC was enhanced by WO3/TiO2 using sol-gel method. From the XPS spectra of W4f7/2 electrons in various WO3/TiO2 the tungsten atom existed as WO3 distributing on the surface of TiO2. The differences of PL and UV-visible spectra between WO3/TiO2 and TiO2 illustrated the main peak of WO3/TiO2 shifted toward the short wavelength, illustrating that the band gap between the excited molecular level and the basic molecular level was enlarged. Moreover, the intensity of WO3/TiO2 dropped sharply as compared to TiO2(sol-gel) because the addition of WO3 precursor led to the prolonged separation time of the photo-induced electron and hole, which was considered to be beneficial to improve the photocatalytic reactivity of WO3/TiO2. The removal efficiencies of Hg0 were different from various WO3 doped TiO2. Among these, the highest removal efficiency of Hg0 was reached by 3%WO3/TiO2. The removal efficiency was promoted from 20% by TiO2(sol-gel) to 68% by 3%WO3/TiO2. The L-H model simulating the Hg0 removal by TiO2 and WO3/TiO2 could be used to determine the equilibrium constants KHg0 of various WO3/TiO2. The change of Gibb’s free energy △G between Hg0 and WO3/TiO2 at 160oC ranged from -47 to -57 KJ/mol and decreased as the temperatures increased.
With HCl and incident near-UV irradiation at 160oC, one part of Hg0 was believed to be adsorbed on the surface of TiO2 and WO3/TiO2s as adsorption, while the other was desorbed from the surface as HgCl2 entering the flue gas as an oxidant. The enhancements of adsorption efficiency and the oxidation efficiency were achieved by the addition of O2 and HCl. The increase of HCl concentration and the near-UV irradiation promoted the adsorption efficiency of Hg0 while decreased the oxidation efficiency of Hg0. The total removal efficiency of Hg0 was not affected by adding NO into the HCl and O2 atmosphere, while for the adsorption and oxidation efficiency, the former was enhanced and the later was reduced by NO. An inhibition of the removal efficiency of Hg0 was caused by the addition of SO2 into HCl and O2 atmosphere without the irradiation of near-UV light, while under near-UV irradiation, the removal efficiency of Hg0 was enhanced as the inhibition of SO2 was diminished.
The thermo-catalytic removal efficiencies of Hg0 by V2O5/TiO2 were higher than their photocatalytic removal efficiencies. From the investigation of various removal efficiencies of Hg0 by different V2O5/TiO2 and V2O5, the enhancements were attributed to the synergic effect of structure combination of V2O5 and TiO2. The activated sites for Hg0 adsorption were identified as V=O bond of V2O5. These dynamic models were used to fit the adsorption traces including Fick’s diffusion model, pseudo-first order kinetic model and pseudo-second order kinetic model, respectively. The first order kinetic model was fitted best among these three models which illustrated that the driving force resulted from the concentration derivation of Hg0 in the gaseous phase and on the surface of TiO2.