Study On Titania-based SCR Catalysts And Their Poisoning Mechanism Of Potassium And Lead

Nitrogen oxides (NOx) are recognized as a major source of air pollution in the world. The selective catalytic reduction of NOx with NH3 as reductant is most efficient among the technologies abating NOx in flue gas at present. Catalysts are critical to selective catalytic reduction of NOx. They are the main factor affecting the NOx removal and economical efficiency of the whole SCR system. Poisoning is inevitable for SCR catalysts in their using process and relates with the fuel characteristics intimately. The fuel composition is complex in our country. The content of sulfur and ash in coal as well as the water content in waste is high. Therefore, the study on the adaptability of SCR catalysts to the firing flue gas in our country is important and meaningful. With the financial support from the National High Technology Research and Development of China (863 Program) (No. 2007AA061802) and National Science Foundation of China (No.50776079), the systemic studies were carrird out on the effect of preparation and operating parameters on the NO reduction of titania-based catalysts, as well as the V2O5/TiO2 catalysts'poisoning mechanism by potassium and lead. The main results are shown as following:1. The correlation between preparation parameters and the DeNOx performance of titania-based SCR catalysts was studies. The effect of the TiO2 property, calcination condition and V2O5 loadings on the DeNOx performance of V2O5/TiO2 catalysts were analyzed in detail. Large surface area and weak TiO2 crystallinity are advantageous to SCR reaction over the catalysts. When the 1 wt.% V2O5/TiO2 catalysts were calcinated at 500℃for 5 h, their DeNOx activity was highest and the NO conversion reached 92.7～98.8% at the typical SCR reaction temperature (300～400℃). The increase in orderly V2O5 crystallites and reaction temperature would lead to the N2O formation, thereby decreasing the selectivity of the catalysts. The CeO2/TiO2 cayalysts prepared by a single sol-gel mothod have the best SCR activity. The most active catalysts were obtained with a Ce loading of 0.6. The optimal calcination temperature was 500℃. At the typical SCR reaction temperature,98.6% of NO conversion and 100% of N2 selectivity were obtained at the gas hourly space velocity (GHSV) of 50,000 h-1 over the catalysts.2. The effect of operating parameters on the DeNOx performance of titania-based SCR catalysts was studied. In the actual operating condition, the SCR reaction over V2O5/TiO2 catalysts is zero-order for O2, first-order for NO and zero-order for NH3 when the molar ratio of NH3 to NO is higher than or equal to 1. H2O has an inhibiting effect on SCR reaction, while reduce the formation of N2O over V2O5/TiO2 catalysts.The presence of SO2 has no large impact on NO reduction activity over V2O5/TiO2 catalysts in dry or wet simulated flue gas. The best CeO2/TiO2 catalysts were effective for NO removal within a wide range of GHSV from 12,000 h-1 to 100,000 h-1. The presence of oxygen played an essential role in NO reduction, and the activity of the Ce(0.6)Ti catalyst was not depressed when oxygen concentration was higher than 1%. The effect of SO2 and H2O on the activity of the Ce(0.6)Ti catalyst was bound up with the reaction temperature.3. Action mechanism of different species of potassium or lead on V2O5/TiO2 catalysts was clarified. Different species of potassium or lead resulted in the change of specific surface area and porosity of the catalysts, the change of V valence and the decrease in the numbers of active sites at the surface of the catalysts. The main reason of the catalyst deactivation by K and Pb is that K or Pb coordinates to Br(?)nsted acid sites on the catalysts'surfaces and neutralizes the acidity of them. However, Br(?)nsted acid sites are important for the absorption and activation of NH3 in SCR reaction. According to the catalyst characterization results, the possible path in which the species of potassium or lead cause the deactivation of V2O5/TiO2 catalysts was proposed.4. Based on a model for the "shell-progressive" poisioning of a single catalyst particle in a catalytic reactor, the poisonsing kinetics was studied and the poisonsing kinetic equation was established. Combining the activity retention of the model simulation with that of the experimental values, the relation of the K or Pb loadings over the catalysts with the reaction time was established. The GHSV (15,000～60,000 h-1), mean catalyst particle diameter (137～335μm) and reactor diameter (4-20 mm) have almost no effect on the activity retention. The combined effect of the poison concentration and poisoning reaction rate constant on the activity retention was simulated based on the established model. The simulated results accord with the catalyst operating results reported in a published paper.