Experimental Study Of NH₃-SCR Characteristics And Kinetics On γ-Fe₂O₃-based Catalyst

NO_x is the main air pollutants in China. The excessive emissions will cause photochemical smog, acid rain, ozone hole, gray haze and a series of environmental problems, so it is uttermost to control the emission from coal-fired power plant. Now, NH₃-SCR is widely used in deNOx technology. The typical commercial catalysts is V2O5-WO3（MoO3）/TiO2,which has good NOx conversion between 340～400℃.For its active ingredient is expensive, poisonous vanadium element cannot be discarded at random and harmless treatments of waste catalyst cost is higher, it is urgent to develop suitable NH₃-SCR technology with high efficiency, cheap and pollution-free. Compared with vanadium titanium base catalyst, ordinary iron oxide catalysts is cheaper, better resistance to H2O and SO2, non-toxic and its active ingredient has a wealth of sources. But its denitration temperature is high. As a result, this paper researched new iron oxide catalysts by precipitation process. XRD and activity test showed γ-Fe₂O₃ catalysts is the ideal new NH₃-SCR catalysts with good activity in medium temperature, which could replace V2O5-WO3（MoO3）/TiO2 in the future. Furthermore, the dynamics study of catalysts is great important in providing experimental data and theoretical basis for later process simulation of NH₃-SCR, denitration reactor design and optimization and industry enlargement. So this paper carried on dynamics research of y-Fe2O3 catalysts and the effect on dynamics when manganese doped. Eventually,the denitration mechanism and optimizing mechanism were analyzed according to the result of kinetics study.The factors such as reaction temperature, NH3/NO,O2 concerntration,H2O and SO2 concentration were researched respectively so as to know its effect on denitration character of γ-Fe₂O₃ catalyst. The research showed that NOx conversion exceeds 90% when the temperature is above 325℃.The NOx conversion of γ-Fe₂O₃ is higher than commercial vanadium titanium base catalyst when temperature below 270℃. When NH3/NO is greater than 1 the NOx conversion won’t increase accordingly and when O2 concentration exceeds 3.5% NOx conversion remain the same. There is severe competitive adsorption between H2O and NH3 that cause the drop of NOx conversion. And H2O helps to produce trace amounts of hydroxyl groups which give rise to irreversible chemical inactivation.SO2 won’t inhibit SCR activity when below 0.04% while NOx conversion dropped a little when 0.08% SO2 added. There’s no metal sulfation happened and γ-Fe₂O₃ performed fine resistance to SO2.The transient dynamics study indicated that NH3 could quickly adsorbed and activated on surface active site and O2 could quickly participated in SCR reaction. Then steady-state kinetics study was carried on and power-law kinetics model was built and got the reaction order with respect to NO, NH3, O2 is 0.41,0 and 0.27 respectively. Finally, the study combined kinetics model with Arrhenius equation showed activation energy of magnetic γ-Fe₂O₃ catalysts is 28.77 KJ/mol. According to kinetics results, NO could adsorb on γ-Fe₂O₃ and gas NO involved E-R mechanism and adsorbed NO involved L-H mechanism both exist on γ-Fe₂O₃ catalysts.The effect to NH₃-SCR activity and kinetics were studied when manganese doped in γ-Fe₂O₃ catalysts. And the optimizing mechanism was analyzed accordingly. The research indicated that manganese embedded in y-Fe2O3 crystal structure or highly dispersed on the surface. Fe0.7Mn0.3Oz has excellent low temperature denitration activity and NOx conversion exceeds 90% after 70℃.The steady-state kinetics study shows that the reaction order with respect to NO, NH3, O2 is 0.46,0 and 0.44 respectively. Mn doping help to increase the reaction order of NO and the adsorption between NO and catalyst was weaken. Both E-R and L-H reaction path exist on Fe0.7Mn0.3Oz catalyst. And the step of nitrite being oxidized to nitrate was suppressed so that the amount of nitrite on catalyst was increased accordingly. The reaction path of L-H was optimized and the activation energy was reduced to 25.43KJ/mol.