| NOx is one of the main air pollutants and selective catalytic reduction (SCR) is an effective way to remove NOx in the flue gas from stationary sources. Recently, low-temperature SCR process has attracted more and more interests for its low energy consumption and operating cost. Based on the review of the current situation of SCR technology, it was pointed out that the following shortcomings are the main obstacle of the industrial application of the low-temperature SCR technology:low activity at low temperature, poor SO2 resistance of catalysts and uncertainty of reaction and SO2-poisoning mechanisms. In order to solve these problems, the low-temperature SCR reaction was systematically investigated by using MnOx based catalysts in this dissertation.Firstly, several metals were chosen to be doped into Mn/TiO2 catalyst. From activity tests and characterization results, it was found that Ce doping could greatly enhance the low-temperature SCR activity of the catalyst (the activity was enhanced from 62% to 95% at 100℃). The oxygen storage capacity and the surface acidity of the Mn/TiO2 catalyst were improved by the introduction of Ce, which would be beneficial to the adsoroption of NH3 and its activitation.Secondly, Mn-Ce oxides were supported on TiO2, Al2O3, ZSM5 and active carbon respectively. From the comparison results of these catalysts'activity, it was found that Mn-Ce/TiO2 and Mn-Ce/Al2O3 had relatively higher low-temperature SCR acivity than other supported samples. The SCR reaction mechanism of these two catalysts was then studied and the results indicated that there were several reaction paths for both of the samples and these reaction paths made different concentrations to the SCR activity of two catalysts because of the property differences between TiO2 and Al2O3. For Mn-Ce/TiO2 catalyst, the SCR reaction mainly took place between adsorbed NH3 species and gas-phase NO, while in the case of Mn-Ce/Al2O3, the SCR reaction commenced with NO oxidation, then its oxidation products reacted with NH3 to carry out SCR reaction.Fuethemore, the effects of SO2 on the SCR activity of Mn/TiO2 and Mn-Ce/TiO2 catalysts as well as its mechanism were investigated detailedly. A serious deactivation by SO2 was detected on Mn/TiO2 catalyst. The deposition of ammonium sulfate species on catalyst surface and the sulfation of catalyst active phase were proved to be the main reasons for the catalyst deactivation in the presence of SO2 during SCR reaction. Ce doping could effectively inhibit the sulfation of catalyst active phase and decrease the stability of the formed sulfate species. Finally, the relationship between SCR activity of Mn-Ce/TiO2 catalyst and operating parameters such as SO2 and H2O concentrations, reaction temperature, was investigated to provide possible guidances for industrial application. Especially the effects of reaction temperature on catalyst activity were studied in detail and the results suggested that low reaction temperature could relieve the sulfation of catalyst active phase and the deactivation by SO2. The deactivated samples were regenerated by different treatments and it was found that water-washing could effectively recover the most catalyst activity.
|
PDF Full Text Request