Study On Catalytic Oxidation Of Elemental Mercury With The Modified Mn/Al2O3 Catalyst

Mercury pollution has become one of the major environmental issues in the air pollution control because of its volatility, persistence, and bioaccumulation. Coal-fired utility is believed to be the largest anthropogenic sources in China because of the huge comsumption and high mercury content of coal. Mercury capture from flue gas is the main way to reducing the mercury emissions. Mercury in coal combustion flue gas includes three forms: elemental mercury (Hg⁰), oxidized mercury (Hg²⁺), and particle-bounded mercury (Hg^p). Hg²⁺ is water-soluble and therefore can be effectively captured by the wet flue gas desulfurization (FGD) systems as a co-benefit. Most Hgp can be collected by electrostatics precipitators (ESPs) or fabric filters (FF) together with flyash. However, Hg⁰ is the most difficult to be removed because of its high volatility and low solubility in water. So the removal of Hg⁰ is the key point of flue gas mercury controlling.Using the existing flue gas pollution control equipments to achieve the purpose of simultaneous removal of mercury is a preferred alternative for mercury controlling. Therefore, how to improve the conversion from Hg⁰ the the other two forms is the main research direction. This paper aims at developing catalytic oxidation technology combined with existing equipment: develop the modified metal oxide as catalyst to improve the oxidation efficiency of Hg⁰ by HCl after the dust removal devices, then the oxided mercury can be effectively captured by the FGD systems.Based on the previous study on catalysts, MnOx was employed as the basic catalyst component. The effect of temperature and flue gas was firstly studied, the results showed that the highest catalytic activity was achieved at about 300℃, which was much higher than at low temperature; SO₂ showed significant negative effect on the catalytic activity. In order to improve the catalytic activity and sulfur tolerance of Mn/Al₂O₃ catalyst at lower temperatures, this study put emphasis on the modification method: several transition metal elements and rare earth metal elements were employed as dopants to modify the catalyst of Mn/Al₂O₃. After the catalytic activity investigations at different temperature, HCl concentration, SO₂ concentration and dopping content, it was found that the activity of the modified catalysts was apparently improved, especially the catalytic activity and sulfur tolerance at lower temperatures. The activity of catalyst modified with transition metal elements were in the trends below: Mo>W>Cu>Sr, the activity of catalyst modified with rare earth metal elements were in the trends below: Ce>La>Sm>Y.At the low temperature of 150℃, the Mn/Al₂O₃ catalyst doped by Mo and Ce showed higher activity than the other elements. For the adsorption ability of Hg⁰, the t1/2 was prolonged from 34min to 2000min and 1000min, respectively; for the catalytic oxidation activity, the efficiency was increased by 35% and 26%, respectively; for the sulfur tolerance, the catalytic oxidation efficency of Hg⁰ only decreased by 11% and 2%,respectively. So the activity of Mn/Al₂O₃ catalyst modified with Mo and Ce was close to noble catalyst Pd / Al₂O₃.The mechanism of negative effect of SO₂ was speculated through some experiments: SO₂ was readily absorbed on the catalyst, so it would compete against Hg⁰ and HCl for the active sites on the catalyst and led to the immediately reduction of adsorption efficiency; on the other hand, the presistence of SO₂ may lead to sulphation caused by the long-term adsorption in the catalyst surface, then reduce the catalytic activity. The mechanism of the modified catalysts was also speculated through somecharacterization methods. It has been proved by the results of TEM and XRD that the dispersion of Mn oxides became more even and the crystallization became smaller after the doping of Mo, which was favorable to the catalytic activity. Also, Mo in the catalyst might preferentially adsorb sulfur species and it can minimize the poison of sulfur to Mn activity sites effectively. In addition, TPR showed the likely presence of the bimetallic Mo-Mn complex was more stable to sulfur dioxide.Through the experiments and the characterization methods in this study, it was found that the modified catalyst Mn/Al₂O₃ catalyst could be used in the low temperature atmosphere after the dust removal devices. This can aviod the interference of dust, and also keep a high catalytic activity and sulfur tolerance, so it has good prospects for industrial use. And the research results of this paper were helpful to the control of mercury pollution in the coal-fired flue gas.