Deactivation of denitrification catalysts
Publishdate：2018-06-27 Views：13
The deactivation of catalysts can be divided into physical deactivation and chemical deactivation. The typical chemical deactivation of SCR catalysts mainly refers to catalyst poisoning caused by alkali metals, alkaline earth metals, and As, while physical deactivation mainly refers to catalyst activity damage caused by high-temperature sintering, wear, and blockage.
(1) Taking titanium based catalysts as an example, prolonged exposure to high temperature environments above 450 ℃ can cause sintering of the active surface of the catalyst, microcrystalline aggregation, resulting in an increase in catalyst particles and a decrease in surface area, leading to a decrease in catalyst activity, as shown in Figure 2-24.
Figure 2-24 Sintering of catalysts
Adding tungsten to the commercial catalyst formula of titanium based vanadium will greatly reduce the sintering of the catalyst. The high operating temperature allowed for different tungsten contents is different, and the sintering phenomenon can be ignored when the SCR reactor operates at normal operating temperature. Therefore, the operating temperature of the SCR reactor must strictly comply with the manufacturer's guidance requirements.
(2) Among all the factors that cause the deactivation of SCR catalysts, dust accumulation in flue gas is a complex and influential factor. If the micropores of the catalyst are blocked by smoke particles, the surface active sites of the catalyst gradually lose their activity, leading to catalyst deactivation. Analysis shows that the fly ash deposited on the catalyst surface is mainly composed of particles with a diameter less than 5 μ Compared with fly ash in the flue gas, the number of sulfated particles in m particles significantly increases, and elements such as As and Na are more likely to accumulate on small particles, causing serious toxicity to the catalyst.
To reduce the impact of fly ash on catalysts, the following measures can be taken: ① In the SCR process, a pre dust removal device should be installed, and a large section ash hopper and ash removal grid should be installed at the outlet of the economizer; ② Reasonably blowing ash to reduce the deposition of fly ash on the catalyst surface; ③ Appropriate measures for uniform distribution of smoke; ④ Choose the appropriate catalyst type and performance parameters. To prevent honeycomb catalyst blockage, appropriate catalyst pitch and honeycomb size should be selected; ⑤ Select the appropriate amount of catalyst, increase the volume and surface area of the catalyst; ⑥ By using appropriate preparation techniques, increase the smoothness of the catalyst surface and slow down the deposition of fly ash on the catalyst surface.
For SCR denitrification systems, if the CaO in coal is too high, the catalyst activity will be weakened. The CaO content in coal in China is relatively high, such as the widely used Shenhua coal ash in power plants, which is 9% -24%, and the CaO content in ash is 13% -30% by mass. It is generally believed that the alkalinity of CaO reduces the acidity of the catalyst, but it does not cause a significant decrease in catalyst activity. The main reason for the decrease in catalyst performance is the reaction between CaO and SO3 in fly ash, which forms a layer of CaSO4 on the surface of the catalyst and covers the active sites of the catalyst, preventing the diffusion of reactants into the catalyst for denitrification reaction. Compared to plate catalysts, honeycomb catalysts are less affected by CaO and have stronger resistance to CaO poisoning.
Arsenic is a component present in most coal types, and arsenic poisoning in SCR catalysts is caused by the continuous accumulation of gaseous arsenic compounds, which block the channels entering the active sites of the catalyst. The main form of gaseous arsenic in flue gas is As2O3, which mainly deposits and blocks the mesopores of the catalyst, with a pore size of 0.1 μ M to 1 μ The holes between m. Regardless of the type of furnace used, the catalyst will exhibit significant arsenic poisoning. When there is a large amount of CaO in the flue gas, As2O3 will react with CaO and O2 in the flue gas to generate Ca3 (AsO4) 2. Ca3 (AsO4) 2 is a compound with very high thermal stability and will not cause catalyst deactivation. Therefore, when CaO and As2O3 are present simultaneously, the influence of the two substances on the catalyst will be greatly weakened. However, in general, the concentration of As2O3 emitted from coal-fired boilers will be much higher than that of CaO. By changing the microporous structure and distribution of catalysts, arsenic poisoning can be effectively prevented, which has been adopted by many catalyst manufacturers.
(4) During the combustion process of SO3 in flue gas, SO3 will be produced. Increasing the proportion of vanadium oxide in the catalyst can improve its denitrification activity, but it also increases the conversion of SO2 to SO3, thereby increasing the concentration of SO3 in the flue gas. Temperature plays a significant role in the conversion of SO2 to SO3, and even in catalysts with low or no vanadium oxide content, some SO2 is still converted to SO3.
When the temperature is low, SO3 reacts with NH3 in the flue gas to produce ammonium sulfate and ammonium bisulfate. Ammonium sulfate and ammonium bisulfate are small viscous particles, while ammonium sulfate is a white solid; Ammonium hydrogen sulfate is a viscous solid at 160-220 ℃. When the flue gas temperature is too low, it is easy to condense and adsorb on the surface of the catalyst and the air preheater, and then deposit, causing catalyst blockage and deactivation. In addition, ammonium bisulfate is corrosive and can cause corrosion of the air preheater.
The measures taken to prevent ammonium salt deposition include: ① designing a reasonable catalyst formula to reduce the conversion rate of SO2; ② Reduce the amount of ammonia escaping. If selecting the appropriate NH3/NOx molar ratio, appropriate catalyst volume, and reasonable system design, especially the design of the mixing device, to achieve uniform distribution of flue gas concentration on the catalyst surface; ③ Under low load conditions, stop spraying ammonia when the temperature does not meet the requirements. The deposition of ammonium salts can only occur when the boiler is operating at low load and the temperature is lower than the condensation temperature of ammonium salts.
The catalyst blockage caused by ammonium salt deposition can be decomposed into ammonium sulfate by heating, restoring partial activity of the catalyst. However, long-term below allowable temperature can cause irreversible changes in catalyst activity. Flushing the air preheater can remove ammonium salt deposits.
(5) The wear and tear of catalysts is mainly caused by the impact of fly ash on the surface of the catalyst. The wear of catalysts is a function of gas velocity, fly ash characteristics, impact angle, and catalyst characteristics, therefore high flue gas flow rate and particle concentration will accelerate this wear. In addition to the erosion of high-temperature flue gas, the operation of the soot blower in the SCR system will also cause significant wear and tear. In addition, for honeycomb catalysts, the worn pores will reduce the flow resistance and pressure drop when passing through the flue gas. In contrast, more flue gas will flow through, thereby exacerbating this wear effect. Catalysts with treated surfaces and edges will have higher resistance to wear.
The measures taken to prevent catalyst wear include: designing catalysts reasonably; Select appropriate flue gas velocity; Large particle fly ash with strong wear and tear should be removed as much as possible from the flue gas. The main measures taken in catalyst design include: ① top hardening. Increase the hardness of the honeycomb catalyst end to resist wear on the ash facing surface. For flat plate catalysts, due to their support frame being a metal mesh, the metal substrate is exposed to the windward surface after wear, which can prevent the progressive wear of flue gas. It is generally believed that the anti wear performance of flat plate catalysts is good Thickening. Increase the wall thickness of the overall catalyst and increase the wear allowance to extend the mechanical life of the catalyst. This is also beneficial for the cleaning and regeneration of the catalyst The use of a homogeneous catalyst structure results in a certain degree of abrasion on the ash facing surface and inner wall of the catalyst under high ash content. After the surface coating of the catalyst undergoes wear, the activity of the catalyst will be significantly reduced.
Sintering, wear, and ash accumulation can all cause catalyst deactivation, among which ash accumulation has a serious impact on SCR catalysts.
The deactivation of catalysts can be divided into physical deactivation and chemical deactivation. The typical chemical deactivation of SCR catalysts mainly refers to catalyst poisoning caused by alkali metals, alkaline earth metals, and As, while physical deactivation mainly refers to catalyst activity damage caused by high-temperature sintering, wear, and blockage.
(1) Taking titanium based catalysts as an example, prolonged exposure to high temperature environments above 450 ℃ can cause sintering of the active surface of the catalyst, microcrystalline aggregation, resulting in an increase in catalyst particles and a decrease in surface area, leading to a decrease in catalyst activity, as shown in Figure 2-24.
Figure 2-24 Sintering of catalysts
Adding tungsten to the commercial catalyst formula of titanium based vanadium will greatly reduce the sintering of the catalyst. The high operating temperature allowed for different tungsten contents is different, and the sintering phenomenon can be ignored when the SCR reactor operates at normal operating temperature. Therefore, the operating temperature of the SCR reactor must strictly comply with the manufacturer's guidance requirements.
(2) Among all the factors that cause the deactivation of SCR catalysts, dust accumulation in flue gas is a complex and influential factor. If the micropores of the catalyst are blocked by smoke particles, the surface active sites of the catalyst gradually lose their activity, leading to catalyst deactivation. Analysis shows that the fly ash deposited on the catalyst surface is mainly composed of particles with a diameter less than 5 μ Compared with fly ash in the flue gas, the number of sulfated particles in m particles significantly increases, and elements such as As and Na are more likely to accumulate on small particles, causing serious toxicity to the catalyst.
To reduce the impact of fly ash on catalysts, the following measures can be taken: ① In the SCR process, a pre dust removal device should be installed, and a large section ash hopper and ash removal grid should be installed at the outlet of the economizer; ② Reasonably blowing ash to reduce the deposition of fly ash on the catalyst surface; ③ Appropriate measures for uniform distribution of smoke; ④ Choose the appropriate catalyst type and performance parameters. To prevent honeycomb catalyst blockage, appropriate catalyst pitch and honeycomb size should be selected; ⑤ Select the appropriate amount of catalyst, increase the volume and surface area of the catalyst; ⑥ By using appropriate preparation techniques, increase the smoothness of the catalyst surface and slow down the deposition of fly ash on the catalyst surface.
For SCR denitrification systems, if the CaO in coal is too high, the catalyst activity will be weakened. The CaO content in coal in China is relatively high, such as the widely used Shenhua coal ash in power plants, which is 9% -24%, and the CaO content in ash is 13% -30% by mass. It is generally believed that the alkalinity of CaO reduces the acidity of the catalyst, but it does not cause a significant decrease in catalyst activity. The main reason for the decrease in catalyst performance is the reaction between CaO and SO3 in fly ash, which forms a layer of CaSO4 on the surface of the catalyst and covers the active sites of the catalyst, preventing the diffusion of reactants into the catalyst for denitrification reaction. Compared to plate catalysts, honeycomb catalysts are less affected by CaO and have stronger resistance to CaO poisoning.
Arsenic is a component present in most coal types, and arsenic poisoning in SCR catalysts is caused by the continuous accumulation of gaseous arsenic compounds, which block the channels entering the active sites of the catalyst. The main form of gaseous arsenic in flue gas is As2O3, which mainly deposits and blocks the mesopores of the catalyst, with a pore size of 0.1 μ M to 1 μ The holes between m. Regardless of the type of furnace used, the catalyst will exhibit significant arsenic poisoning. When there is a large amount of CaO in the flue gas, As2O3 will react with CaO and O2 in the flue gas to generate Ca3 (AsO4) 2. Ca3 (AsO4) 2 is a compound with very high thermal stability and will not cause catalyst deactivation. Therefore, when CaO and As2O3 are present simultaneously, the influence of the two substances on the catalyst will be greatly weakened. However, in general, the concentration of As2O3 emitted from coal-fired boilers will be much higher than that of CaO. By changing the microporous structure and distribution of catalysts, arsenic poisoning can be effectively prevented, which has been adopted by many catalyst manufacturers.
(4) During the combustion process of SO3 in flue gas, SO3 will be produced. Increasing the proportion of vanadium oxide in the catalyst can improve its denitrification activity, but it also increases the conversion of SO2 to SO3, thereby increasing the concentration of SO3 in the flue gas. Temperature plays a significant role in the conversion of SO2 to SO3, and even in catalysts with low or no vanadium oxide content, some SO2 is still converted to SO3.
When the temperature is low, SO3 reacts with NH3 in the flue gas to produce ammonium sulfate and ammonium bisulfate. Ammonium sulfate and ammonium bisulfate are small viscous particles, while ammonium sulfate is a white solid; Ammonium hydrogen sulfate is a viscous solid at 160-220 ℃. When the flue gas temperature is too low, it is easy to condense and adsorb on the surface of the catalyst and the air preheater, and then deposit, causing catalyst blockage and deactivation. In addition, ammonium bisulfate is corrosive and can cause corrosion of the air preheater.
The measures taken to prevent ammonium salt deposition include: ① designing a reasonable catalyst formula to reduce the conversion rate of SO2; ② Reduce the amount of ammonia escaping. If selecting the appropriate NH3/NOx molar ratio, appropriate catalyst volume, and reasonable system design, especially the design of the mixing device, to achieve uniform distribution of flue gas concentration on the catalyst surface; ③ Under low load conditions, stop spraying ammonia when the temperature does not meet the requirements. The deposition of ammonium salts can only occur when the boiler is operating at low load and the temperature is lower than the condensation temperature of ammonium salts.
The catalyst blockage caused by ammonium salt deposition can be decomposed into ammonium sulfate by heating, restoring partial activity of the catalyst. However, long-term below allowable temperature can cause irreversible changes in catalyst activity. Flushing the air preheater can remove ammonium salt deposits.
(5) The wear and tear of catalysts is mainly caused by the impact of fly ash on the surface of the catalyst. The wear of catalysts is a function of gas velocity, fly ash characteristics, impact angle, and catalyst characteristics, therefore high flue gas flow rate and particle concentration will accelerate this wear. In addition to the erosion of high-temperature flue gas, the operation of the soot blower in the SCR system will also cause significant wear and tear. In addition, for honeycomb catalysts, the worn pores will reduce the flow resistance and pressure drop when passing through the flue gas. In contrast, more flue gas will flow through, thereby exacerbating this wear effect. Catalysts with treated surfaces and edges will have higher resistance to wear.
The measures taken to prevent catalyst wear include: designing catalysts reasonably; Select appropriate flue gas velocity; Large particle fly ash with strong wear and tear should be removed as much as possible from the flue gas. The main measures taken in catalyst design include: ① top hardening. Increase the hardness of the honeycomb catalyst end to resist wear on the ash facing surface. For flat plate catalysts, due to their support frame being a metal mesh, the metal substrate is exposed to the windward surface after wear, which can prevent the progressive wear of flue gas. It is generally believed that the anti wear performance of flat plate catalysts is good Thickening. Increase the wall thickness of the overall catalyst and increase the wear allowance to extend the mechanical life of the catalyst. This is also beneficial for the cleaning and regeneration of the catalyst The use of a homogeneous catalyst structure results in a certain degree of abrasion on the ash facing surface and inner wall of the catalyst under high ash content. After the surface coating of the catalyst undergoes wear, the activity of the catalyst will be significantly reduced.
Sintering, wear, and ash accumulation can all cause catalyst deactivation, among which ash accumulation has a serious impact on SCR catalysts.
