Study On SO₂ Resistance Of Mn-Ce/Al₂O₃ For Low-Temperature Selective Catalytic Reduction Of NO_x

Selective catalytic reduction (SCR) is an effective way to remove NOx from flue gas. In order to make use of the temperature of flue gas, commercial SCR reactor is located upstream of desulfurizer and electrostatic precipitator which causes serious problems of dust erosion and SO2 poisoning. Low-temperature SCR reactor is located downstream of desulfurizer and the electrostatic precipitator which may solve these problems and arose researchers’attention. However, there are a few amount of SO2 in flue gas after the desulfurizer and catalysts are generally very sensitive to SO2 at low temperature. Thus, it is necessary to research catalysts that have high SO2 resistance at low temperature. In this paper, based on Mn-Ce/Al2O3 catalyst, the sulfur resistance and poisoning mechanism was researched.Firstly, the catalytic performance of Mn/Al2O3 modified with Ce, Co and Zr was researched. Experiment results indicate that the catalyst modified with Ce shows the best performance of NO removal. When the react temperature is between 120-240 ℃, NO removal efficiency is 100%. After the injection of H2O and SO2, NO removal efficiency is maintained above 72%. When H2O and SO2 is cut off, reaction activity of the catalyst totally recovers. BET and XRD analyses shows that the catalyst modified with Ce has larger specific surface area. It can improve the dispersion of active components and provide more available surface active sites. Moreover, the adding of Ce increases the mean pore size which can decrease resistance to mass transfer. Those characters improve the catalytic performance.Secondly, the performance of catalysts doped with different amount of Ce was researched. The results show that catalyst which molar ratio of Ce:Al was 0.12 has the best catalytic activity and best resistance to H2O and SO2. At 100 ℃, the NO removal efficiency is 100%. After H2O and SO2 injection, NO removal efficiency is maintained at about 75% within 6 h test and the activity returns to 100% after H2O and SO2 removal. TG analyses determines that less ammonium sulfate salt generated at the catalyst surface after Ce doping which reduces the poisoning of SO2. The results of quantum chemistry calculation based on density functional theory shows that compared with MnOx, SO2 is more easily adsorbed on CeO2, which can protect the active site of MnO2.Finally, effects of the reaction conditions such as temperature, SO2 concentration, gas hourly space velocity and intermittent injection of SO2 on the catalytic performance of Mn-Ce(0.12)/Al2O3 were researched. Results show that Mn-Ce(0.12)/Al2O3 has good resistance to H2O and SO2 when the reaction temperature is between 140-180 ℃, GHSV is between 45000-60000 h-1, and SO2 concentration is between 100-200 ppm. And higher temperature, lower SO2 concentration, lower gas hourly space velocity can improve the resistance to SO2 and H2O of the catalyst. Compared to single injection, intermittent injection of SO2 undermine the SO2 resistance of the catalyst.In conclusion, compared to Co and Zr, Ce is a better modifier and Mn-Ce(0.12)/Al2O3 has a better performance of NO removal. The results of characterization and quantum chemistry calculation shows that Mn-Ce(0.12)/Al2O3 has a better performance of NO removal because Ce adding increases specific surface area, improves porous structure and adsorbs SO2 to protect MnOx.