Performance Of The Low-temperature Selective Catalytic Reduction Of No_x With NH₃ Over Mn Doped γ-Fe₂O₃ Catalysts

Selective catalytic reduction of NOx with NH3(NH3-SCR) is widely used to remove nitrogen oxides(NOx) in the flue gas from stationary sources like coal-fired power plants, which has high selectivity and high efficiency. As for low-temperature NH3-SCR technology, the SCR reactor is placed downstream of the electrostatic precipitator and desulfurizer, which is different from traditional NH3-SCR technology. Low-temperature NH3-SCR has attracted widespread attention from scholars at home and abroad for many advantages, such as avoiding the negative impact of dust and trace elements on catalysts, slowing the SO2 poisoning process of catalysts and matching well with the existing boiler system. However, the SCR performance of catalysts at low temperatures(<200℃) is still a stringent challenge, including the low SCR activity and narrow activity temperature window. Hence, there is an urgent need for the research and development of catalysts with high efficiency for low-temperature SCR technology. Considering the low SCR activity of magnetic γ-Fe2O3 catalysts at low temperatures(50-200℃), modification with additives was used to develop novel iron-based low-temperature SCR catalysts with high efficiency in this work. Then the influence of titration method and calcination temperature on the SCR performance of catalysts at low temperatures was revealed, and the optimization mechanism of microwave drying over the catalysts was also investigated.1. Different additives were utilized to modify the low-temperature SCR activity of γ-Fe2O3 catalysts. The results indicated Mn was the best additive to improve the low-temperature SCR activity of γ-Fe2O3 catalysts and its optimum doping ratio was 0.3. It showed that the doping of Mn significantly increased the low-temperature SCR activity and widened the activity temperature window of γ-Fe2O3 catalysts. The highest NOx conversion of Fe0.7Mn0.3Oz catalysts was 100%, which was 75 percentage points higher than that of γ-Fe2O3 catalysts. When Mn was doped into γ-Fe2O3, the ability to oxidize NO to NO2 of catalysts was enhanced. And the doping of Mn could optimize the pore structure as well as the pore size distribution of γ-Fe2O3 catalysts, and increase its BET surface area and pore volume. In addition, Mn could react with iron oxide to form the solid solution. Therefore, the doping of Mn enhanced the low-temperature SCR activity of γ-Fe2O3 catalysts.2. Based on the above study, effects of titration methods and calcination temperature on the low-temperature SCR activity of modified iron-based catalysts were systematically investigated. The results showed that compared with positive titration, the reverse titration method increased the low-temperature SCR activity and widened the activity temperature window of Fe0.7Mn0.3O2 catalysts. The catalysts calcined at 350℃ exhibited the best low-temperature SCR activity, whose NOx conversion was above 90% at 70℃ and 100% at 100 ～200℃. However, when the calcination temperature was higher than 350℃, sintering took place and α-Fe2O3 appeared among Fe0.7Mn0.3O2 catalysts, which was unfavorable to the low-temperature SCR reaction. Higher specific surface area and pore volume, flourishing pore structure, broad pore size distribution and γ-Fe2O3 of appropriate crystallinity contributed to the excellent low-temperature SCR activity of Fe0.7Mn0.30z-R and Fe0.7Mn0.3Oz-350 catalysts.3. Influence of different drying parameters and microwave parameters on the low-temperature SCR performance of the novel iron-based catalysts was studied and the reason for which microwave drying improved the low-temperature SCR performance of catalysts was analyzed. The mechanism of microwave radiation modifying the low-temperature SCR activity of Fe0.7Mn0.3Oz catalysts was as follows. When Fe0.7Mn0.3Oz catalysts were prepared with microwave drying, CO32-could stabilize the existence of γ-Fe2O3 phase and guarantee its purity in the catalysts. The results of activity tests indicated that microwave parameters had an obvious effect on the low-temperature SCR activity of Fe0.7Mn0.3O3 catalysts. Catalysts treated by microwave at P30 for 25 minutes showed the best low-temperature SCR activity, and its NOx conversion was up to 98% at 60℃ and maintained 100% at 70～200 ℃. However, higher microwave power could reduce the dispersion of Mn oxide, cause the collapse of pore structure and decrease the specific surface area of the catalysts.In addition, the resistance to SO2 and H2O poisoning of Fe0.7Mn0.3Oz catalysts was studied. It was found that SO2 had an irreversibly adverse impact on the low-temperature SCR activity of the catalyst, and the NOx conversion on Fe0.7Mn0.3Oz catalysts decreased from 100% to 72.8% at 125℃ with 0.03% SO2 in the feed gas. H2O exhibited an reversibly inhibiting effect on the low-temperature SCR activity of the catalysts, and the activity of the catalysts decreased obviously at 100℃ with the presence of 5% H2O. However, this inhibiting effect would be significantly weakened as the reaction temperature rose to 125℃.