Experimental And Mechanism Study On Mercury Catalytic Oxidation Promoted By Flue Gas Conditioning With Magnesium Chloride

Mercury is a global pollution which is volatility, persistence and bioaccumulation. Mercury from coal combustion is considered to be the main source of mercury pollution in the atmosphere. Developing technologies for mercury control in coal-fired boilers is becoming one of the most important environmental protection issues. Hg in coal combustion derived flue gas is present in three forms: elemental Hg (Hg0), oxidized Hg (Hg2+) and particulate-bound Hg (Hgp), Hg0 is the main form of mercury emissions from coal-fired power plants which is difficult to capture with typical air pollution control devices (APCD), while Hg2+ can be removed in the wet flue gas desulfurization (WFGD) facilities due to its high solubility in aqueous solutions and Hgp attached to fly ash can be captured in particulate control devices. A promising method for the removal of Hg0 from coal combustion flue gas is catalytic oxidation of Hg0 to Hg2+ which can be subsequently captured by WFGD systems. The key point of this method is how to improve the oxidation of Hg0 to Hg2+ by selective catalytic reduction (SCR) catalyst. As the oxidation of Hg0 by SCR catalyst is restricted by the content of chlorine in coal. The higher content of HCl in the exhaust gas which increases with the content of chlorine in the coal, the higher oxidation efficiency of mercury by SCR catalyst. Desulfurization wastewater is rich in magnesium chloride. Hydrogen chloride can be produced from the decomposition of magnesium chloride at high temperature. The method of injecting magnesium chloride to the gas flue in front of the SCR system for reduction of NOx in order to improve the oxidation of elemental mercury was proposed.Firstly, the performance and mechanism of mercury oxidation over a commercial SCR catalyst were studied in a fixed bed reactor experimentally. The results showed that the adsorption behavior of Hg0 on the surface of catalyst was mainly physical adsorption and O2 promoted the adsorption of mercury on the surface of catalyst; The presence of HCl in the reactant gases greatly increased the activity of SCR catalyst for the oxidation of Hg0 to oxidized mercury; The oxidation efficiency of Hg0 increased with the increase of temperature. Mercury oxidation followed the Eley-Rideal mechanism involving the adsorption state of mercury reacts with gas-phase (or weakly adsorbed) HCl.Secondly, The performance of mercury oxidation over a commercial SCR catalyst with magnesium chloride addition was studied in a fixed bed reactor experimentally. The influence of SCR catalyst containing different amount of magnesium chloride、temperature and gas on the oxidation of Hg0 were investigated. The research results show that hydrogen chloride and active chlorine which promote the homogeneous oxidation of elemental mercury could be generated from the decomposition of magnesium chloride. The magnesium chloride addition dramatically enhanced the Hg0 oxidation activity over the catalyst. In basic flue gas at 350℃, only 22.3% of Hg0 can be oxidized with SCR catalyst, while 83.4% can be oxidized with 1.0% magnesium chloride addition. The high temperature was conducive to oxidation of elemental mercury and 350℃ was the optimum temperature. O2 and NO promoted the oxidation of elemental mercury by SCR catalyst with magnesium chloride addition, however, SO2 and H2O inhibited it. Magnesium chloride addition had no obvious impacts on NOX conversion. Mechanism experiment indicated that the positive function of magnesium chloride addition on elemental mercury oxidation is due to HCl and active chlorine species generated from magnesium chloride.In the end, density functional theory of quantum chemistry is an accurate method for evaluating the mechanisms involved in elemental mercury adsorption on solid surfaces. The mechanism of active chlorine promoting the oxidation of elemental mercury by SCR catalytic was calculated based on the density functional theory to clarify the experimental phenomena. The periodic model was used to describe the surface of V2O5 (0 0 1). The simulation study shows that, the active chloride generated from magnesium chloride can combined with the oxygen on the surface of the V2O5 forming active sites, and Hg0 adsorbs on chlorine-modified V2O5 surface forming HgCl which escapes to the flue gas or strongly adsorbs on V2O5 surface, which helps the oxidation of Hg0 to Hg2+.