Introduction to the operating principle of selective catalytic reduction flue gas denitrification technology
Publishdate：2018-06-04 Views：15
Exhaust gas denitrification is currently a widely used method to reduce NOx emissions in developed countries. The commonly used methods include selective catalytic reduction (SCR) and selective non catalytic reduction (SNCR). The denitrification rate of SCR can reach over 90%. The invention right of SCR belongs to the United States, while Japan achieved its commercial application first in the 1970s. At present, this technology has been widely applied in developed countries. More than 93% of Japan's exhaust gas denitrification uses SCR, with over 300 operating devices. Germany introduced this technology in the 1980s and stipulated that power plants with a power generation capacity of over 50 MW must be equipped with SCR devices. There are over 100 SCR devices in operation in Taiwan. Daqing Petrochemical Plant Fertilizer Plant, Chuanhua Group Company, Beijing Yanshan Petrochemical Company Synthetic Rubber Plant, and Fujian Zhangzhou Power Plant have also introduced SCR devices from abroad.
1. Technical Principles
There are various reducing agents (CH4, H2, CO, and NH3) that can reduce NOx to N2, especially NH3 which can selectively react with NOx according to the following formula:
4NH3+4NO+O2=4N2+6H2O (1)
8NH3+6NO2=7N2+12H2O (2)
The above reaction can be effectively carried out within the range of 200~450 ℃ by using appropriate catalysts. Under the condition of NH3/NOx being 1 (molar ratio), a denitrification rate of 80% to 90% can be achieved. During the reaction process, NH3 selectively reacts with NOx to generate N2 and H2O, rather than being oxidized by O2.
2 Process flow
The process flow of SCR flue gas desulfurization device is shown in Figure 1, mainly composed of ammonia supply system, ammonia control system, catalyst, exhaust system, preheating system, and reactor. Liquid ammonia is transported by tank trucks to the liquid ammonia storage tank. The liquid ammonia output from the tank evaporates into ammonia gas in the evaporator, which is then heated to room temperature and sent to the ammonia buffer tank for backup. After the ammonia buffer tank is depressurized by the pressure regulating valve, it is sprayed into the exhaust gas through the nozzle of the ammonia spraying grid to mix with the exhaust gas, and then fully mixed by the static mixer before entering the catalytic reactor. When the exhaust gas temperature is low, the preheating system is used to heat the exhaust gas. When the exhaust gas that reaches the reaction temperature and is fully mixed with ammonia flows through the catalytic layer of the SCR reactor, ammonia undergoes a catalytic oxidation-reduction reaction with NOx, reducing NOx to harmless N2 and H2O.
3 Operation control
3.1 Activity of catalysts
Catalysts are the core of SCR technology, and their shapes are generally plate or honeycomb. Due to its excellent durability, corrosion resistance, high reliability, high reuse rate, and low pressure drop, honeycomb catalysts are widely used. The main components of commonly used catalysts are V2O5/TiO2. The cross-sectional size of honeycomb catalysts is generally 150 mm × 150 mm; Length from 400 mm to 1000 mm. The operating cost of an SCR device largely depends on the lifespan of the catalyst. Its service life depends on the rate of decay of catalyst activity. The deactivation of catalysts can be divided into physical deactivation and chemical deactivation. The typical chemical deactivation of SCR catalysts is mainly caused by catalyst poisoning caused by alkali metals (such as Na, K, Ca, etc.) and heavy metals (such as As, Pt, Pb, etc.). Alkali metals adsorb on the capillary surface of the catalyst, and metal oxides (such as MgO, KaO, etc.) neutralize SO3 on the catalyst surface to form sulfides, causing catalyst poisoning. Arsenic poisoning is caused by the combination of arsenic trioxide in exhaust gas and catalysts. The physical deactivation of catalysts mainly refers to the destruction of catalyst activity caused by high-temperature sintering, wear, and solid particle deposition blockage.
The wear and blockage of the SCR system can be alleviated through the chemical design of the reactor (setting up an automatic guide vane device to reverse the direction of ammonia injection from the flow direction). If there is dust in the exhaust gas, it is necessary to install a soot blower in the reactor to ensure the cleanliness of the catalyst surface.
If the exhaust gas contains solid particles that can poison the catalyst, the exhaust gas needs to be pretreated, such as using electrostatic dust removal, adding de arsenic agents, etc., to remove catalyst toxic solid particles and avoid catalyst poisoning.
Exhaust gas denitrification is currently a widely used method to reduce NOx emissions in developed countries. The commonly used methods include selective catalytic reduction (SCR) and selective non catalytic reduction (SNCR). The denitrification rate of SCR can reach over 90%. The invention right of SCR belongs to the United States, while Japan achieved its commercial application first in the 1970s. At present, this technology has been widely applied in developed countries. More than 93% of Japan's exhaust gas denitrification uses SCR, with over 300 operating devices. Germany introduced this technology in the 1980s and stipulated that power plants with a power generation capacity of over 50 MW must be equipped with SCR devices. There are over 100 SCR devices in operation in Taiwan. Daqing Petrochemical Plant Fertilizer Plant, Chuanhua Group Company, Beijing Yanshan Petrochemical Company Synthetic Rubber Plant, and Fujian Zhangzhou Power Plant have also introduced SCR devices from abroad.
1. Technical Principles
There are various reducing agents (CH4, H2, CO, and NH3) that can reduce NOx to N2, especially NH3 which can selectively react with NOx according to the following formula:
4NH3+4NO+O2=4N2+6H2O (1)
8NH3+6NO2=7N2+12H2O (2)
The above reaction can be effectively carried out within the range of 200~450 ℃ by using appropriate catalysts. Under the condition of NH3/NOx being 1 (molar ratio), a denitrification rate of 80% to 90% can be achieved. During the reaction process, NH3 selectively reacts with NOx to generate N2 and H2O, rather than being oxidized by O2.
2 Process flow
The process flow of SCR flue gas desulfurization device is shown in Figure 1, mainly composed of ammonia supply system, ammonia control system, catalyst, exhaust system, preheating system, and reactor. Liquid ammonia is transported by tank trucks to the liquid ammonia storage tank. The liquid ammonia output from the tank evaporates into ammonia gas in the evaporator, which is then heated to room temperature and sent to the ammonia buffer tank for backup. After the ammonia buffer tank is depressurized by the pressure regulating valve, it is sprayed into the exhaust gas through the nozzle of the ammonia spraying grid to mix with the exhaust gas, and then fully mixed by the static mixer before entering the catalytic reactor. When the exhaust gas temperature is low, the preheating system is used to heat the exhaust gas. When the exhaust gas that reaches the reaction temperature and is fully mixed with ammonia flows through the catalytic layer of the SCR reactor, ammonia undergoes a catalytic oxidation-reduction reaction with NOx, reducing NOx to harmless N2 and H2O.
3 Operation control
3.1 Activity of catalysts
Catalysts are the core of SCR technology, and their shapes are generally plate or honeycomb. Due to its excellent durability, corrosion resistance, high reliability, high reuse rate, and low pressure drop, honeycomb catalysts are widely used. The main components of commonly used catalysts are V2O5/TiO2. The cross-sectional size of honeycomb catalysts is generally 150 mm × 150 mm; Length from 400 mm to 1000 mm. The operating cost of an SCR device largely depends on the lifespan of the catalyst. Its service life depends on the rate of decay of catalyst activity. The deactivation of catalysts can be divided into physical deactivation and chemical deactivation. The typical chemical deactivation of SCR catalysts is mainly caused by catalyst poisoning caused by alkali metals (such as Na, K, Ca, etc.) and heavy metals (such as As, Pt, Pb, etc.). Alkali metals adsorb on the capillary surface of the catalyst, and metal oxides (such as MgO, KaO, etc.) neutralize SO3 on the catalyst surface to form sulfides, causing catalyst poisoning. Arsenic poisoning is caused by the combination of arsenic trioxide in exhaust gas and catalysts. The physical deactivation of catalysts mainly refers to the destruction of catalyst activity caused by high-temperature sintering, wear, and solid particle deposition blockage.
The wear and blockage of the SCR system can be alleviated through the chemical design of the reactor (setting up an automatic guide vane device to reverse the direction of ammonia injection from the flow direction). If there is dust in the exhaust gas, it is necessary to install a soot blower in the reactor to ensure the cleanliness of the catalyst surface.
If the exhaust gas contains solid particles that can poison the catalyst, the exhaust gas needs to be pretreated, such as using electrostatic dust removal, adding de arsenic agents, etc., to remove catalyst toxic solid particles and avoid catalyst poisoning.
