What are the factors that affect the passivation of SCR catalysts?

The factors affecting the passivation of SCR catalysts include: 1. Sintering. Prolonged exposure to high temperatures above 450°C can cause sintering of the catalyst, leading to changes in the crystal structure of TiO2 within it, an increase in particle size, a decrease in surface area, and reduced activity. Adding WO3 can minimize the sintering of the catalyst. 2. Alkali metal poisoning: If alkali metal ions (Na+, K+, etc.) come into direct contact with the catalyst, it will cause the catalyst’s activity to gradually decrease. The mechanism is that alkali metal ions adsorbed on the active sites of the catalyst occupy the acidic sites on the catalyst surface, thereby reducing its activity. Therefore, in catalyst design, the impact of alkali metals on the catalyst should be considered to provide a greater design margin. 3. Arsenic poisoning: As poisoning is mainly caused by gaseous As2O3 in smoke. As2O3 diffuses into the pores of the catalyst, undergoes capillary condensation within those pores, or reacts with the catalyst’s active sites, thereby reducing the catalyst’s activity. Generally speaking, in dry slag discharge boilers, catalyst arsenic poisoning is not severe. However, in liquid slag discharge boilers, catalyst arsenic poisoning is a serious problem due to the recirculation of fly ash after the electrostatic precipitator. Therefore, during the catalyst preparation process, methods for controlling the pore distribution of the catalyst should be employed to ensure a uniform pore distribution within it; by regulating the number of capillary pores, \"capillary condensation\" can be reduced. Additionally, MoO3 can be added to the catalyst; the reaction between MoO3 and gaseous As2O3 is used to reduce As poisoning. 4. Effect of calcium: CaSO4, formed by the reaction between free CaO in fly ash and SO3, can adsorb on the surface of the catalyst, thereby preventing the reactants from diffusing to the surface of the catalyst and entering its interior. Catalyst manufacturers generally reduce the impact of CaSO4 on catalysts by controlling the pore size distribution within the catalyst and using appropriate spacing. 5. Catalyst clogging: Catalyst clogging is mainly caused by the deposition of ammonium salts and fine particles of fly ash in the pores of the catalyst, which prevents NOx, NH3, and O2 from reaching the active surface of the catalyst, thereby leading to catalyst deactivation. Catalyst clogging can be prevented by adjusting the gas flow distribution, selecting an appropriate catalyst spacing and unit volume, and maintaining the temperature of the flue gas entering the SCR reactor above the temperature at which ammonium salts precipitate. For the high-ash SCR process, so as to ensure unobstructed catalyst channels, soot blowers should be installed. 6. Fly ash erosion: The erosion and wear of catalysts are mainly caused by the impact of fly ash on the catalyst surface. Abrasion strength is related to gas flow velocity, fly ash properties, impact angle, and the properties of the catalyst itself. Measures to reduce abrasion: First, use corrosion-resistant catalyst materials and treat the top of the catalyst to increase the hardness at its edges; second, optimize the air flow distribution using computational fluid dynamics flow models; third, employ methods such as installing air flow regulation devices vertically within the catalyst bed to address this issue.