| As the largest source of anthropogenic mercury emissions, mercury emitted from coal-fired power plants has been identified as a hazardous pollutant to both human health and environment. Therefore, the control of mercury emission from coal-fired utilities is vital to mitigate global mercury pollution. In recent years, many studies have been conducted to evaluate the effectiveness of Hg0 adsorption by solid sorbents. However, little theoretical calculations have been carried out to gain an increased understanding of the details of interactions between Hg0 and solid sorbents. In this paper, Density Functional Theory (DFT) was employed to examine the binding mechanism of elemental mercury (Hg0) on the simulated CaO and activated carbon (AC) surface. Furthermore, modified calcium-based rice husk ash sorbents and novle silver nano-particles sorbnts were synthesized and evaluated their effectiveness of Hg0 adsorption. These investigations will provide useful information for developing mercury emission control technologies.Adsorption of mercury and mercury chloride on a CaO surface has been investigated by means of DFT cluster model calculations. The systematic comparison of different embedding clusters (shell models, point charges cluster and bare cluster) and calculation functions and basis used for mercury are presented. The results clearly indicate that the SM (shell model) cluster is better than PC embedding clusters and bare clusters in the aspects of computational accuracy in the gas-solid adsorption system. The computations also show that the elemental mercury can only coordinate to the O2- anion and has a very weak binding energy, which infers mercury is weakly physisorbed at the surface of the CaO. When the mercury chloride molecular axis is parallel to the surface, the Hg atom coordinate to the O2- anion and has a large binding energy, which means that mercury chloride adsorbed on surface of CaO is mainly chemisorption. The present calculations indicate that CaO injection could substantially reducing gaseous mercury chloride, but have no apparent effect on the mercury, which is compatible with the available experimental results. Besides obtaining structures and energies, the temperature effect on adsorption are also studied. The results suggest that the high temperature has different effect on the adsorption because of the different nature of adsorption, which accord with the EPA experimental conclusion. The research method provides the valuable information for understanding the mechanism of the mercury and mercury chloride adsorption on the surface of CaO and developed one potential method to optimize and select sorbents for removing the trace elements in flue gas.To get a clear understanding of the impacts of acid gases (NO2, HCl and SO2) on mercury species (Hg0 and HgCl2) capture by Calcium (Ca)-based sorbents and elucidate the mechanisms in mercury adsorption process, DFT-based quantum mechanical calculations were conducted. Shell-model (SM) embedded clusters were employed to simulate CaO surface and the adsorption of Hg0, HgCl2, HCl, NO2 and SO2 on the CaO surface was investigated theoretically so the fundamental interactions between Hg species and acid gases could be explored. Results showed that HgCl2, HCl, NO2 and SO2 were strongly adsorbed on the CaO surface with chemisorption as the likely adsorption mechanism, while binding energy of Hg0 was minimal indicating a physisorption mechanism. For HgCl2 removal, the acid gases (NO2, HCl and SO2) and HgCl2 competed for the same basic (O2-) sites of CaO, thus inhibiting HgCl2 sequestering by Ca-based sorbents. For Hg0 capture, the presence of SO2 likely promoted the formation of Hg2O other than HgO, while simultaneously apparently intercepted pre-adsorbed oxygen (Oads) on CaO surface, thus inhibiting Hg0 oxidation.Potassium permanganate-modified and Iodine-modified calcium-based rice husk ash sorbents (KMnO4CaO/RHA and I2/CaO/RHA) were synthesized and characterized by X-ray diffraction, X-ray fluorescence, and N2 isotherm adsorption/desorption. Adsorption experiments of vapor-phase elemental mercury (Hg0) were performed in a laboratory-scale fixed-bed reactor. The KMnO4/CaO/RHA and I2/Ca0/RHA performances were compared with that of modified Ca-based with fly ash sorbent (CaO/FA). Effects of oxidant loading, supports, pore size distribution, iodine impregnation modes, and temperature were investigated as well to understand the mechanism in capturing Hg0. For KMnO4CaO/RHA, the effect of various wt%KMnO4 (2-10) loading indicated that CaO/RHA comprising of 5 wt%KMnO4 showed higher performance; Increasing the temperature and decreasing the Hg0 concentration caused an increase in the Hg0 capture by modified CaO/RHA sorbent (CaO/RHA/KMnO4); Under the optimized operation parameters, the CaO/RHA/KMnO4 sorbent could remove over 95%of Hg0. However, the surface area, pore size distribution, and iodine impregnation modes of the sorbents did not produce a strong effect on Hg0 capture efficiency, while fair correlation was observed between Hg0 uptake capacity and iodine concentration; Increasing temperature in the range of 80-140℃caused a rise in Hg0 removal. Additionally, the reaction mechanisms that could explain the experimental results were presumed based on the characterizations and adsorption study.DFT calculations were performed on the possible pathways of Hg0 on virgin activated carbon (AC), halogen(X)-embedded AC and sulfur-impregnated AC. The activated carbon surface was modeled by a fused-benzene ring cluster in which the edge atoms on the upper side were unsaturated in order to simulate the active sites. It was concluded that Hg0 adsorbed on surface of AC was mainly physisorption. When Hg0 adsorbed on halogen-embedded AC surface, HgX was found to be stable on the surface, which indicated that embedding X into AC matrix could possibly promote Hg0 binding. For sulfur deposited on AC surface, sulfur embedded edges of graphene, especially zigzag edges, were viable active sites, while sulfur-embedded basal planes and organic sulfur (in the form of thiophene) were not viable sites. Short-chain sulfur allotropes were more effective in Hg0 uptake than long-chain sulfur species. The short-chain sulfur species could more easily result in the single sulfur embedded form. The resulting form was most likely responsible for Hg0 capture. The Hg0 reacting with sulfur sites of AC, thus yielding the bond of Hg-S, was the dominant pathway. The sulfur atom embedding into the edge plane of AC can possibly promote Hg0 binding in comparison with the case of the bare AC surface.Three methods (direct calcinations, water boil and acid wash) were adopted to prepare the rice husk ash sorbents and binary liquids technology was employed to synthesize silver nano-particles on the supports and their performance of removal Hg0 was tested in a fixed bed reactor. The characterizations of the sorbents were analyzed by using method of X-ray fluorescence (XRF), nitrogen (N2) adsorption/desorption, X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), and Transmission electron microscope (TEM). It was observed that acid-washing sorbent exhibited highest contents of amorphous SiO2, highest activity, highest special surface area, and highest hydroxyl. Therefore, this sorbent was the optimum support for silver particles. TEM results displayed that silver nano-particles (-8nm) were covered the support uniformly using binary liquids technology. Fixed-bed absorbers showed that the nano-silver-loaded sorbent had a mercury removal efficiency of 90%at 150℃in a stimulated flue gas. The mercury capture was via an amalgamation mechanism. Moreover, this sorbent could be potentially regenerated above 350℃, and thus would be promising in the industrial applications.
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