Chemical Adsorption Of Gaseous Elemental Mercury On Magnetic Fe Based Spinel

The emission of mercury from anthropogenic activities in China has become a serious concern. In China, approximately 40% of the emission of mercury comes from coal combustion. Recently, the revised Emission Standard of Air Pollutants for Thermal Power Plants of China (GB13223-2011) has been taken into effect, and the limited value of mercury emission has been capped. The control of elemental mercury emission studied to date mainly falls into one of two groups: powder activated carbon (PAC) injection, and the co-benefit of selective catalytic reduction (SCR) of NOx and flue gas desulfurization (FGD). However, PAC and SCR+FGD are currently extremely restricted in the control of elemental mercury emission in China due to the high sulfur content and the low chlorine content in feed-coal.In this work, magnetic Fe based spinel was developed to control the emission of elemental mercury from coal-fired flue gas and the chemical adsorption of gaseous elemental mercury on Fe based spinel was studied. After compared the physicochemical properties of Fe based spinel before and after elemental mercury adsorption, the mechanisms of the adsorption of elemental mercury on Fe based spinel and the interference of SO₂ with elemental mercury capture were studied. Then, the structure-activity relationship between the physicochemical properties of Fe based spinel and its capacity for elemental mercury capture was built according to the kinetic equation. Based on the structure-activity relationship, Fe based spinel with high capacity for elemental mercury capture, excellent SO₂ durability and low cost was devised.The novelties and important conclusions of this work are summarized as follows:(1) Ti4+ was incorporated intoγ-Fe₂O₃ to improve its ability for elemental mercury capture. The capacity of (Fe3-xTix)1-δO4 for elemental mercury capture was approximately proportional to the product of its BET surface area, the concentrations of reducible Fe3+ and cation vacancy on the surface. As Ti was incorporated intoγ-Fe₂O₃, the concentrations of reducible Fe3+ and cation vacancy on the surface, and the thermal stability all increased. As a result, the adsorption of elemental mercury onγ-Fe₂O₃ was promoted at 200-350 oC due to the incorporation of Ti. Because cation vacancy on (FeTi)0.8O4 can be destroyed by SO₂, the presence of SO₂ showed an obvious interference with elemental mercury capture by (Fe₂Ti)0.8O4.(2) MnOx was supported onγ-Fe₂O₃ to capture elemental mercury from the flue gas. Mn/γ-Fe₂O₃ had an excellent capacity for elemental mercury capture. The route of elemental mercury oxidization on Mn/γ-Fe₂O₃ was related to the ratio of Mn4+ to cation vacancy on the surface and reaction temperature. The presence of SO₂ showed a neglectable effect on elemental mercury capture by 15%-Mn/γ-Fe₂O₃-250 at 100-200 oC. The capacity of 15%-Mn/γ-Fe₂O₃-250 for elemental mercury capture at 150 oC was about 2.2 mg g-1 in the presence of 1000 ppmv of SO₂.(3) Mn cation was incorporated intoγ-Fe₂O₃ to improve its ability for elemental mercury capture. The capacity of (Fe_3-xMn_x)_1-δO₄ for elemental mercury capture was approximately proportional to the product of its BET surface area, the concentrations of Mn4+ and cation vacancy on the surface. Although the BET surface area of (Fe_3-xMn_x)_1-δO₄ decreased with the increase of Mn content, the concentrations of Mn4+ and cation vacancy on (Fe_3-xMn_x)_1-δO₄ both increased. As a result, the adsorption of elemental mercury onγ-Fe₂O₃ was promoted at 100-300 oC due to the incorporation of Mn. Chemical heterogeneity deriving from the oxidization kinetic difference between Fe₂+ and Mn2+/Mn3+ was observed in (Fe_3-xMn_x)_1-δO₄, which promoted the adsorption of elemental mercury on (Fe_3-xMn_x)_1-δO₄. The interference of SO₂ with elemental mercury capture by (Fe₂.2Mn0.8)1-δO4 was much less than that with (Fe₂Ti)0.8O4. The capacity of (Fe₂.2Mn0.8)1-δO4 for elemental mercury capture at 150 oC was about 1.9 mg g-1 in the presence of 1000 ppmv of SO₂.(4) Ti4+ was incorporated into (Fe_3-xMn_x)_1-δO₄ to further improve its ability for elemental mercury capture. BET surface area and the concentration of cation vacancy of/on (Fe_3-xMn_x)_1-δO₄ increased due to the incorporation of Ti. Furthermore, the probability of elemental mercury adsorbed on the cation vacancy resulting from the oxidization of Fe₂+ cation contacting Mn4+ cation on the surface obviously increased with the increase of Ti content in (Fe_3-xMn_x)_1-δO₄. As a result, elemental mercury capture by (Fe_3-xMn_x)_1-δO₄ was generally promoted due to the incorporation of Ti. Meanwhile, SO₂ durability of (Fe_3-xMn_x)_1-δO₄ for elemental mercury capture improved due to the incorporation of Ti. The capacity of (Fe₂Ti0.5Mn0.5)1-δO4 for elemental mercury capture at 150 oC was about 4.2 mg g-1 in the presence of 1000 ppmv of SO₂.(5) Ti and V were simultaneously incorporated intoγ-Fe₂O₃ to improve its ability for elemental mercury capture. Fe-Ti-V spinel had the novel property for oxidation storage and catalytic reduction of elemental mercury. Magnetic Fe-Ti-V spinel showed an excellent performance for elemental mercury capture at 100 oC, and the formed HgO can be catalytically decomposed by Fe-Ti-V spinel at 300 oC to reclaim elemental mercury and regenerate Fe-Ti-V spinel.(6) Fe-Ti spinel and Fe-Ti-V spinel showed excellent activity, N₂ selectivity and H2O/SO₂ durability for the selective catalytic reduction (SCR) of NO with NH3 at 300-400 oC. Although Mn-Fe spinel showed an excellent low temperature SCR activity, its activity was suppressed in the presence of H2O and SO₂. On Fe-Ti spinel and Mn-Fe spinel, chemical adsorption of elemental mercury may be compatible with the SCR of NO with NH₃.In conclusion, centralized control and resource utilization of elemental mercury in flue gas can be realized using magnetic Fe based spinel, which showed an excellent application prospect.