Maintenance and upkeep of denitrification catalysts
Publishdate：2019-03-30 Views：15
Denitration is imperative with environmental requirements. The current difficulties are the large volume and low concentration of flue gas, but the total amount of NOX is relatively large. It is difficult to dispose of waste after absorption, adsorption, and denitrification, and the cost is high. Among numerous denitrification methods, SCR denitrification technology has been widely used commercially in Japan and Europe and America due to its simple denitrification device structure, no by-products, convenient operation, high reliability, high denitrification efficiency, and relatively low secondary investment. Under current technological conditions, SCR (Selective Catalytic Reduction) technology has been widely applied due to its stable operation, high denitrification efficiency, and low ammonia escape. It has become the mainstream flue gas desulfurization technology internationally for a considerable period of time. With the application and promotion of SCR flue gas denitrification technology in China, the importance of SCR catalysts is gradually being recognized. How to manage and maintain SCR catalysts, maintain their efficient activity, extend their lifespan, and reduce operating costs is an important issue that urgently needs to be addressed.
At present, all units with a capacity of over 300MW in China are equipped with denitrification devices. The reactor is equipped with three layers of catalyst, which can be divided into three types: plate type, honeycomb type, and corrugated plate type. All three catalysts have achieved performance in coal-fired SCR. The production and preparation of catalysts occupy the initial construction cost of the system, and the cost of catalysts is relatively high. Catalysts are an important component of the system. The typical lifespan of a catalyst is three years, and poor on-site maintenance cannot achieve its service life. Catalysts determine the operating cost of the system. Therefore, it is crucial to strengthen catalyst maintenance and repair, avoid replacing a large number of catalysts, save costs, and extend the service life of catalysts. Below, we will mainly discuss how to reduce catalyst costs from several aspects, including catalyst blockage and handling, operational precautions, catalyst regeneration, prevention of catalyst poisoning, catalyst transportation and storage.
1. Catalyst blockage and technical improvement plan
The main problem with the catalyst on-site is clogging and dust accumulation. During operation, it was found that the denitrification catalyst was severely clogged and the dust accumulation was difficult to clean. Steam soot blowers are installed on both sides of each catalyst layer on site, and acoustic soot blowers are arranged on one side. During the shutdown and maintenance of a honeycomb catalyst in a certain power plant, it was found that the upper catalyst had severe ash accumulation, with over 20 centimeters of ash accumulation in the corners, which required a large amount of cleaning work. After the surface ash cleaning is completed, the catalyst filter screen should be lifted to clean the dust accumulation between the filter screen and the catalyst, and then the dust accumulation in the honeycomb holes of the catalyst module should be checked and cleaned. It is difficult to clean the accumulated dust inside the small honeycomb holes. According to the on-site situation, a negative pressure vacuum pipeline can be used to clean it, and compressed air can be used to blow it through. Dust accumulation under the catalyst and steel beam should also be cleaned up to avoid secondary pollution and contamination of the lower air preheater equipment. Sealing and isolation measures should be taken. Even if welding wire is used to unblock the severely blocked honeycomb holes, a slight increase in force may damage the honeycomb holes. During maintenance, it is important to avoid damaging the module, and honeycomb holes that cannot be unblocked can only be left with defects. When the honeycomb holes are blocked or damaged in large areas, only the catalyst module can be replaced.
To avoid severe catalyst blockage, it is necessary to strengthen soot blowing during daily operation. The steam soot blower operates twice a day and is promptly purged based on the pressure difference between the inlet and outlet of the catalyst. The sonic soot blower operates continuously for 24 hours. The steam soot blower is composed of a gun barrel main pipe and a vertical branch pipe. During shutdown maintenance, check whether the position of the soot blowing hole on the soot blowing gun barrel is installed correctly. There have been instances of bending and breaking of the soot blowing pipeline on site, and it is necessary to reinforce the weld seam to ensure safe operation of the equipment. The diaphragm of the sonic soot blower needs to be replaced in a timely manner if there is any damage during inspection. Some sonic soot blowers have found traces of oil leakage. Check the quality of compressed air and pipeline filters.
Due to changes in coal type during on-site design, the catalyst cannot meet operational requirements, resulting in extensive blockage of the catalyst. When the SCR reactor experiences ash accumulation problems due to certain reasons, it has been proven that neither acoustic horn nor steam blowing can solve the problem. However, the disadvantages of steam soot blowers are even more prominent: high cost of supporting facilities, high failure rate, high operating cost, uneven soot blowing effect, steam wear of catalysts, intensified catalyst blockage, and shortened catalyst service life.
This plan adopts a comprehensive solution of enhancing the cleaning power of sound waves and increasing the use of soot blowers for technical renovation. Transform the 6 domestic sonic soot blowers on each side into imported MEGA soot blowers, and install 6 Ash Sweeper soot blowers on the opposite side. Adopting the famous new product MEGA Sootblowing Master from Martin Engineering Company in the United States, the cleaning intensity and cleaning area of each speaker can be improved. The cleaning diameter of a single speaker can reach 4-7 meters, and the vertical cleaning size can reach 18 meters or more. A comprehensive solution is creatively proposed to address the characteristics of large size and single-sided ash stacking in SCR reactors of power plants, which includes the installation of soot blowers and enhanced acoustic cleaning capabilities. Installed on the side with ash accumulation, effectively solving the problem of single-sided ash accumulation. Martin Corporation has successfully installed Ash Sweeper soot blowers on SCR reactors, solving the problem of ash accumulation in SCR reactors for more and more power plants.
The Martin sonic soot blower generates low-frequency and high-pressure sound waves, causing resonance and fluidization of dry particle aggregates, and removing materials from the container through airflow or gravity. The Martin 75Hz sonic soot blower produces 147 decibels of sound waves at a distance of 1 meter, with a base frequency of 75Hz. Point: Prevent the accumulation of dry particles, improve system efficiency and service life. Reduce system failure time, reduce maintenance costs, and be very effective around pipelines and obstacles; Sound can be blown 360 degrees to remove particles and clean parts that are not easily accessible. Will not cause fatigue or damage to the structure, improve catalyst efficiency, extend service life, improve equipment performance, extend service life, comply with emission regulations, lower operating costs, and lower procurement costs.
The operation of the sonic soot blower requires stable compressed air supply for the instrument, and the interior of the equipment is clean and free of dirt and dust. When the sonic soot blower is running normally, it emits a low and muffled sound. If it does not sound or if it does not sound properly, check the compressed air in the pipeline, check if the ball valve is fully open, the filter/pressure regulating valve is set to 0 at the factory, rotate clockwise to increase pressure, check the air flow direction in the pipeline (indicated by an arrow), and check if there is any air leakage. Check the direction of the pipeline layout, the circuit layout of the solenoid valve, ensure that the wires are connected properly, check if the voltage is correct, and check if the signal is correct. If the sonic soot blower emits a sharp and piercing sound, the opening degree of the manual valve in front of each sonic soot blower hose can be adjusted. Adjusting the opening degree of the manual valve cannot adjust the sonic soot blower to normal. You can open the cover plate on the sound head to check if the diaphragm is damaged or cracked. If it is found that the sound of the sonic soot blower is very low or only the sound of airflow is heard, observe whether the muffler on the sound head of the sonic soot blower is blocked. If the pipe is blocked and the horn emits a high sound, clean the blockage; The price of titanium alloy diaphragms for speaker heads is relatively high, with imported diaphragms costing over 8000 yuan. Pay attention to maintenance and upkeep, and replace the worn and damaged diaphragms in a timely manner. The principle of a soot blower is to fill a tank with compressed air for later use, and quickly release a certain amount of compressed air within a set time. Under the guidance of a specially designed nozzle (such as a flat rectangle), it "bombards" the accumulated ash in a specific direction or range, causing the accumulated ash to fly up and be carried away by the smoke, achieving the effect of dust cleaning. It can disperse biscuit ash and blow large particle ash to prevent fine ash from gathering around large particles. It has been widely accepted in various industries. Flexible configuration, with on-site installation of single tank without hose, as well as multi nozzle shared tank installation method. Select the appropriate model, nozzle, installation position, and blowing direction based on the specific situation of the project, in order to safely and effectively clean the catalyst. In some projects, the investment cost of the soot blower can be recovered after 3 months of installation (the above does not include the indirect benefits brought about by the increase in output due to the increase in boiler load and the extension of catalyst life). The soot blower is easy to install, easy to modify, requires less maintenance, has low operating costs, and extends the lifespan of the catalyst. The main air supply pipe of the on-site soot blower requires a stable pressure of 0.6MPa, which is led out from the compressed air main pipeline. Install accessories such as pressure gauges, unions, temperature gauges, pressure regulating valves, solenoid valves, isolation doors, etc. on the pipeline. 2. Precautions for catalyst operation
Selective catalytic reduction (SCR) is the process of converting nitrogen oxides in flue gas into nitrogen and water with a catalyst, often using ammonia as the reducing agent. Ammonia is first mixed with the flue gas and then passed through the catalyst. The mixing process should ensure uniform distribution of flue gas temperature and reactants. By providing active area catalysts, the denitrification reaction can occur between 290 ℃ and 430 ℃. Ammonia diffuses into the microporous structure of the catalyst and is absorbed by the active region. NOx then undergoes a denitrification reaction with the absorbed NH3. The main factors affecting the reaction are the active region factor (a function of specific surface area, pore volume, and active ingredient concentration), flue gas temperature, and reactant concentration.
Smoke that meets the design specifications should be introduced to ensure that the catalyst achieves high catalytic activity. Otherwise, placing the catalyst in a reducing atmosphere will cause degradation of catalytic activity, which is particularly important during the boiler start-up phase. Because at this point, the boiler may change the composition of the flue gas due to incomplete combustion, and an oxidizing chemical atmosphere can better maintain catalyst activity. The temperature distribution of flue gas must comply with technical specifications and standards. Adjust the distribution of the flue gas flow field reaching the catalyst bed to ensure its uniformity meets the technical specification standards. Do not start the ammonia injection device before the catalyst temperature reaches the low continuous operating temperature to prevent sulfate and nitrate deposits on the catalyst surface. The temperature measurement point of the thermocouple should be located at the lower part of the catalyst as much as possible to ensure that the catalyst temperature is not lower than the low temperature. The technical specifications specify that the system operates at low temperatures. The stability performance of the SCR system can only be measured after it reaches equilibrium, and adjustments to the SCR process can only be made after reaching equilibrium. The equilibrium time of SCR chemical reactions varies with system design. It takes about three hours for the system to reach equilibrium when operating below 204 ℃, but it only takes 30 minutes for the system to reach equilibrium when operating above 316 ℃. Unless otherwise specified, the operating temperature should not exceed 430 ℃, as exceeding it may cause long-term catalyst failure.
When the reactor temperature drops to the low operating temperature specified in the technical specifications, ammonia injection must be stopped to prevent sulfate and nitrate deposits on the catalyst surface. Thermocouple temperature measurement points should be set in the downstream area as much as possible to ensure that the system temperature is not lower than the low continuous operating temperature. The suitable temperature for catalyst use is 320-400 ℃, and exceeding the suitable temperature range is an important reason for the degradation or even loss of catalyst performance. Please operate according to the technical specifications of the catalyst provided by the design manufacturer. During operation, it is advisable to gradually heat up or cool down as slowly as possible and strictly follow the rules below that of secondary ignition boilers. If possible, clean gas fuel should be used instead of oil fuel.
3 Catalyst regeneration
Although catalyst regeneration is currently a new business in China, it will be a matter of time before SCR denitrification units use regenerated catalysts. Due to the fact that ineffective denitrification catalysts are classified as hazardous waste (classified as solid waste), their collection, storage, transportation, and disposal must strictly follow the regulations for hazardous waste. Therefore, catalyst regeneration is the trend. At present, there are few companies internationally that can successfully promote professional denitrification catalyst regeneration technology to the market, among which the more famous are Coalogix in the United States and Ebinger Kat in Germany. It covers various forms of catalysts, including honeycomb, flat, and corrugated plates. Longjing Environmental Protection in China and Kejie Company in the United States have adopted a joint venture approach to become a domestic enterprise supported by advanced SCR catalyst regeneration technology from abroad, applying advanced catalyst regeneration technology and conducting related business in China. In addition, companies such as Chongqing Yuanda, Jiangsu Kenchuang, Jiangsu Wande, and Guangdong Qingyuan Hengde have publicly claimed to have mastered domestic technology for catalyst regeneration. Shandong Xige Meng International and Jiangsu China Building Materials Research Institute have signed cooperation plans with Kexi Environmental Protection of South Korea and Fulande Company of Austria, respectively, to prepare for entering the market of deactivated SCR catalyst regeneration.
At present, there are two schemes for regeneration treatment of denitrification catalysts after deactivation in China: on-site regeneration and factory regeneration, which are the same process as the initial experiences in Europe and the United States. However, after 2005, the United States no longer uses on-site regeneration methods. In response to the high cost, labor-intensive and labor-intensive offline activation of existing SCR flue gas denitrification toxic catalysts in thermal power plants, a toxic catalyst online activation experimental process was designed. Based on the offline activation technology of previous catalysts and combined with practical engineering applications, a new online activation process for catalysts has been developed. Through experimental analysis and comparison, compare the catalyst deactivation efficiency and desulfurization efficiency after activation of various activation solutions, while considering the specific conditions of the power plant and the cost of activation solutions, and find a better activation solution formula and its supporting process. The activity of the deactivated catalyst activated by this activation solution reaches 82% of that of the fresh catalyst, and the conversion rates of SO2/SO3 are also close, meeting the engineering needs.
4. Prevent catalyst poisoning
The catalyst for denitrification is an important component of the denitrification system, and its performance directly affects the overall denitrification effect of the system. Studying the causes of catalyst poisoning and extending the lifespan of catalysts is of great significance in reducing the operating costs of the system. The method of using catalysts to immerse in alkaline solutions and then conducting denitrification experiments has proven the poisoning of catalysts caused by alkali metals. It has been found that both increased alkali metal concentration and humidity can cause accelerated degradation of catalysts. Therefore, this situation should be avoided in actual working conditions. Low nitrogen combustion technology has been widely adopted in the construction of new thermal power units. It is believed that with the increasingly strict environmental protection laws, regulations, and standards in China, as well as increased enforcement efforts, mature technology, high denitrification rate, and no secondary pollution, technology will gradually become the mainstream technology in China's flue gas denitrification market.
5. Precautions for catalyst storage and handling
Normally, the catalyst unit is installed in the carbon steel module, and then these modules are installed in the SCR reactor and react with the flue gas. Usually, only professionals from catalyst manufacturers can operate individual catalyst units. The catalyst unit needs to be placed in a special transportation container for future use when returning for transportation. When receiving the catalyst unit, an inspection should be conducted first. Any problems found should be reported to the catalyst manufacturer first. Catalysts must be stored and transported in a well packaged condition to avoid any form of impact, vibration, and mechanical collision. The modules transported to the site should be installed and used as soon as possible to reduce indirect damage to the catalyst. The module packaging should be removed before being loaded into the reactor on site. To prevent accidental damage to the catalyst module, please do not work within or near the storage module area. Protective equipment such as masks and gloves should be worn when handling and disposing of catalysts. After handling, rinse your mouth, face, and hands with clean water to avoid inhaling catalyst dust. Catalysts should be stored in ventilated and dry indoor warehouses to avoid contact with rainwater, seawater, heavy humidity, or other moisture, oil, and solvents. Moisture can not only poison the surface of the catalyst and make it inactive, but also easily cause the catalyst to rupture when it dries. Avoid prolonged exposure of the catalyst to sunlight. At the same time, catalysts should avoid contact with volatile organic gases and be kept away from reducing atmospheres to prevent the loss of activity caused by the reduction of active components in the catalyst. Stay away from alkali metals, alkaline earth metals such as potassium and sodium, calcium and magnesium, and other substances. Halogens can easily deactivate catalysts. Doing a good job in the operation and maintenance of catalysts can not only prolong the service life of equipment, but also reduce the cost of denitrification. It also reduces the huge workload of maintenance, shutdown, ash cleaning, replacement and transportation of denitration catalysts, and shortens the maintenance period.
Denitration is imperative with environmental requirements. The current difficulties are the large volume and low concentration of flue gas, but the total amount of NOX is relatively large. It is difficult to dispose of waste after absorption, adsorption, and denitrification, and the cost is high. Among numerous denitrification methods, SCR denitrification technology has been widely used commercially in Japan and Europe and America due to its simple denitrification device structure, no by-products, convenient operation, high reliability, high denitrification efficiency, and relatively low secondary investment. Under current technological conditions, SCR (Selective Catalytic Reduction) technology has been widely applied due to its stable operation, high denitrification efficiency, and low ammonia escape. It has become the mainstream flue gas desulfurization technology internationally for a considerable period of time. With the application and promotion of SCR flue gas denitrification technology in China, the importance of SCR catalysts is gradually being recognized. How to manage and maintain SCR catalysts, maintain their efficient activity, extend their lifespan, and reduce operating costs is an important issue that urgently needs to be addressed.
At present, all units with a capacity of over 300MW in China are equipped with denitrification devices. The reactor is equipped with three layers of catalyst, which can be divided into three types: plate type, honeycomb type, and corrugated plate type. All three catalysts have achieved performance in coal-fired SCR. The production and preparation of catalysts occupy the initial construction cost of the system, and the cost of catalysts is relatively high. Catalysts are an important component of the system. The typical lifespan of a catalyst is three years, and poor on-site maintenance cannot achieve its service life. Catalysts determine the operating cost of the system. Therefore, it is crucial to strengthen catalyst maintenance and repair, avoid replacing a large number of catalysts, save costs, and extend the service life of catalysts. Below, we will mainly discuss how to reduce catalyst costs from several aspects, including catalyst blockage and handling, operational precautions, catalyst regeneration, prevention of catalyst poisoning, catalyst transportation and storage.
1. Catalyst blockage and technical improvement plan
The main problem with the catalyst on-site is clogging and dust accumulation. During operation, it was found that the denitrification catalyst was severely clogged and the dust accumulation was difficult to clean. Steam soot blowers are installed on both sides of each catalyst layer on site, and acoustic soot blowers are arranged on one side. During the shutdown and maintenance of a honeycomb catalyst in a certain power plant, it was found that the upper catalyst had severe ash accumulation, with over 20 centimeters of ash accumulation in the corners, which required a large amount of cleaning work. After the surface ash cleaning is completed, the catalyst filter screen should be lifted to clean the dust accumulation between the filter screen and the catalyst, and then the dust accumulation in the honeycomb holes of the catalyst module should be checked and cleaned. It is difficult to clean the accumulated dust inside the small honeycomb holes. According to the on-site situation, a negative pressure vacuum pipeline can be used to clean it, and compressed air can be used to blow it through. Dust accumulation under the catalyst and steel beam should also be cleaned up to avoid secondary pollution and contamination of the lower air preheater equipment. Sealing and isolation measures should be taken. Even if welding wire is used to unblock the severely blocked honeycomb holes, a slight increase in force may damage the honeycomb holes. During maintenance, it is important to avoid damaging the module, and honeycomb holes that cannot be unblocked can only be left with defects. When the honeycomb holes are blocked or damaged in large areas, only the catalyst module can be replaced.
To avoid severe catalyst blockage, it is necessary to strengthen soot blowing during daily operation. The steam soot blower operates twice a day and is promptly purged based on the pressure difference between the inlet and outlet of the catalyst. The sonic soot blower operates continuously for 24 hours. The steam soot blower is composed of a gun barrel main pipe and a vertical branch pipe. During shutdown maintenance, check whether the position of the soot blowing hole on the soot blowing gun barrel is installed correctly. There have been instances of bending and breaking of the soot blowing pipeline on site, and it is necessary to reinforce the weld seam to ensure safe operation of the equipment. The diaphragm of the sonic soot blower needs to be replaced in a timely manner if there is any damage during inspection. Some sonic soot blowers have found traces of oil leakage. Check the quality of compressed air and pipeline filters.
Due to changes in coal type during on-site design, the catalyst cannot meet operational requirements, resulting in extensive blockage of the catalyst. When the SCR reactor experiences ash accumulation problems due to certain reasons, it has been proven that neither acoustic horn nor steam blowing can solve the problem. However, the disadvantages of steam soot blowers are even more prominent: high cost of supporting facilities, high failure rate, high operating cost, uneven soot blowing effect, steam wear of catalysts, intensified catalyst blockage, and shortened catalyst service life.
This plan adopts a comprehensive solution of enhancing the cleaning power of sound waves and increasing the use of soot blowers for technical renovation. Transform the 6 domestic sonic soot blowers on each side into imported MEGA soot blowers, and install 6 Ash Sweeper soot blowers on the opposite side. Adopting the famous new product MEGA Sootblowing Master from Martin Engineering Company in the United States, the cleaning intensity and cleaning area of each speaker can be improved. The cleaning diameter of a single speaker can reach 4-7 meters, and the vertical cleaning size can reach 18 meters or more. A comprehensive solution is creatively proposed to address the characteristics of large size and single-sided ash stacking in SCR reactors of power plants, which includes the installation of soot blowers and enhanced acoustic cleaning capabilities. Installed on the side with ash accumulation, effectively solving the problem of single-sided ash accumulation. Martin Corporation has successfully installed Ash Sweeper soot blowers on SCR reactors, solving the problem of ash accumulation in SCR reactors for more and more power plants.
The Martin sonic soot blower generates low-frequency and high-pressure sound waves, causing resonance and fluidization of dry particle aggregates, and removing materials from the container through airflow or gravity. The Martin 75Hz sonic soot blower produces 147 decibels of sound waves at a distance of 1 meter, with a base frequency of 75Hz. Point: Prevent the accumulation of dry particles, improve system efficiency and service life. Reduce system failure time, reduce maintenance costs, and be very effective around pipelines and obstacles; Sound can be blown 360 degrees to remove particles and clean parts that are not easily accessible. Will not cause fatigue or damage to the structure, improve catalyst efficiency, extend service life, improve equipment performance, extend service life, comply with emission regulations, lower operating costs, and lower procurement costs.
The operation of the sonic soot blower requires stable compressed air supply for the instrument, and the interior of the equipment is clean and free of dirt and dust. When the sonic soot blower is running normally, it emits a low and muffled sound. If it does not sound or if it does not sound properly, check the compressed air in the pipeline, check if the ball valve is fully open, the filter/pressure regulating valve is set to 0 at the factory, rotate clockwise to increase pressure, check the air flow direction in the pipeline (indicated by an arrow), and check if there is any air leakage. Check the direction of the pipeline layout, the circuit layout of the solenoid valve, ensure that the wires are connected properly, check if the voltage is correct, and check if the signal is correct. If the sonic soot blower emits a sharp and piercing sound, the opening degree of the manual valve in front of each sonic soot blower hose can be adjusted. Adjusting the opening degree of the manual valve cannot adjust the sonic soot blower to normal. You can open the cover plate on the sound head to check if the diaphragm is damaged or cracked. If it is found that the sound of the sonic soot blower is very low or only the sound of airflow is heard, observe whether the muffler on the sound head of the sonic soot blower is blocked. If the pipe is blocked and the horn emits a high sound, clean the blockage; The price of titanium alloy diaphragms for speaker heads is relatively high, with imported diaphragms costing over 8000 yuan. Pay attention to maintenance and upkeep, and replace the worn and damaged diaphragms in a timely manner. The principle of a soot blower is to fill a tank with compressed air for later use, and quickly release a certain amount of compressed air within a set time. Under the guidance of a specially designed nozzle (such as a flat rectangle), it "bombards" the accumulated ash in a specific direction or range, causing the accumulated ash to fly up and be carried away by the smoke, achieving the effect of dust cleaning. It can disperse biscuit ash and blow large particle ash to prevent fine ash from gathering around large particles. It has been widely accepted in various industries. Flexible configuration, with on-site installation of single tank without hose, as well as multi nozzle shared tank installation method. Select the appropriate model, nozzle, installation position, and blowing direction based on the specific situation of the project, in order to safely and effectively clean the catalyst. In some projects, the investment cost of the soot blower can be recovered after 3 months of installation (the above does not include the indirect benefits brought about by the increase in output due to the increase in boiler load and the extension of catalyst life). The soot blower is easy to install, easy to modify, requires less maintenance, has low operating costs, and extends the lifespan of the catalyst. The main air supply pipe of the on-site soot blower requires a stable pressure of 0.6MPa, which is led out from the compressed air main pipeline. Install accessories such as pressure gauges, unions, temperature gauges, pressure regulating valves, solenoid valves, isolation doors, etc. on the pipeline. 2. Precautions for catalyst operation
Selective catalytic reduction (SCR) is the process of converting nitrogen oxides in flue gas into nitrogen and water with a catalyst, often using ammonia as the reducing agent. Ammonia is first mixed with the flue gas and then passed through the catalyst. The mixing process should ensure uniform distribution of flue gas temperature and reactants. By providing active area catalysts, the denitrification reaction can occur between 290 ℃ and 430 ℃. Ammonia diffuses into the microporous structure of the catalyst and is absorbed by the active region. NOx then undergoes a denitrification reaction with the absorbed NH3. The main factors affecting the reaction are the active region factor (a function of specific surface area, pore volume, and active ingredient concentration), flue gas temperature, and reactant concentration.
Smoke that meets the design specifications should be introduced to ensure that the catalyst achieves high catalytic activity. Otherwise, placing the catalyst in a reducing atmosphere will cause degradation of catalytic activity, which is particularly important during the boiler start-up phase. Because at this point, the boiler may change the composition of the flue gas due to incomplete combustion, and an oxidizing chemical atmosphere can better maintain catalyst activity. The temperature distribution of flue gas must comply with technical specifications and standards. Adjust the distribution of the flue gas flow field reaching the catalyst bed to ensure its uniformity meets the technical specification standards. Do not start the ammonia injection device before the catalyst temperature reaches the low continuous operating temperature to prevent sulfate and nitrate deposits on the catalyst surface. The temperature measurement point of the thermocouple should be located at the lower part of the catalyst as much as possible to ensure that the catalyst temperature is not lower than the low temperature. The technical specifications specify that the system operates at low temperatures. The stability performance of the SCR system can only be measured after it reaches equilibrium, and adjustments to the SCR process can only be made after reaching equilibrium. The equilibrium time of SCR chemical reactions varies with system design. It takes about three hours for the system to reach equilibrium when operating below 204 ℃, but it only takes 30 minutes for the system to reach equilibrium when operating above 316 ℃. Unless otherwise specified, the operating temperature should not exceed 430 ℃, as exceeding it may cause long-term catalyst failure.
When the reactor temperature drops to the low operating temperature specified in the technical specifications, ammonia injection must be stopped to prevent sulfate and nitrate deposits on the catalyst surface. Thermocouple temperature measurement points should be set in the downstream area as much as possible to ensure that the system temperature is not lower than the low continuous operating temperature. The suitable temperature for catalyst use is 320-400 ℃, and exceeding the suitable temperature range is an important reason for the degradation or even loss of catalyst performance. Please operate according to the technical specifications of the catalyst provided by the design manufacturer. During operation, it is advisable to gradually heat up or cool down as slowly as possible and strictly follow the rules below that of secondary ignition boilers. If possible, clean gas fuel should be used instead of oil fuel.
3 Catalyst regeneration
Although catalyst regeneration is currently a new business in China, it will be a matter of time before SCR denitrification units use regenerated catalysts. Due to the fact that ineffective denitrification catalysts are classified as hazardous waste (classified as solid waste), their collection, storage, transportation, and disposal must strictly follow the regulations for hazardous waste. Therefore, catalyst regeneration is the trend. At present, there are few companies internationally that can successfully promote professional denitrification catalyst regeneration technology to the market, among which the more famous are Coalogix in the United States and Ebinger Kat in Germany. It covers various forms of catalysts, including honeycomb, flat, and corrugated plates. Longjing Environmental Protection in China and Kejie Company in the United States have adopted a joint venture approach to become a domestic enterprise supported by advanced SCR catalyst regeneration technology from abroad, applying advanced catalyst regeneration technology and conducting related business in China. In addition, companies such as Chongqing Yuanda, Jiangsu Kenchuang, Jiangsu Wande, and Guangdong Qingyuan Hengde have publicly claimed to have mastered domestic technology for catalyst regeneration. Shandong Xige Meng International and Jiangsu China Building Materials Research Institute have signed cooperation plans with Kexi Environmental Protection of South Korea and Fulande Company of Austria, respectively, to prepare for entering the market of deactivated SCR catalyst regeneration.
At present, there are two schemes for regeneration treatment of denitrification catalysts after deactivation in China: on-site regeneration and factory regeneration, which are the same process as the initial experiences in Europe and the United States. However, after 2005, the United States no longer uses on-site regeneration methods. In response to the high cost, labor-intensive and labor-intensive offline activation of existing SCR flue gas denitrification toxic catalysts in thermal power plants, a toxic catalyst online activation experimental process was designed. Based on the offline activation technology of previous catalysts and combined with practical engineering applications, a new online activation process for catalysts has been developed. Through experimental analysis and comparison, compare the catalyst deactivation efficiency and desulfurization efficiency after activation of various activation solutions, while considering the specific conditions of the power plant and the cost of activation solutions, and find a better activation solution formula and its supporting process. The activity of the deactivated catalyst activated by this activation solution reaches 82% of that of the fresh catalyst, and the conversion rates of SO2/SO3 are also close, meeting the engineering needs.
4. Prevent catalyst poisoning
The catalyst for denitrification is an important component of the denitrification system, and its performance directly affects the overall denitrification effect of the system. Studying the causes of catalyst poisoning and extending the lifespan of catalysts is of great significance in reducing the operating costs of the system. The method of using catalysts to immerse in alkaline solutions and then conducting denitrification experiments has proven the poisoning of catalysts caused by alkali metals. It has been found that both increased alkali metal concentration and humidity can cause accelerated degradation of catalysts. Therefore, this situation should be avoided in actual working conditions. Low nitrogen combustion technology has been widely adopted in the construction of new thermal power units. It is believed that with the increasingly strict environmental protection laws, regulations, and standards in China, as well as increased enforcement efforts, mature technology, high denitrification rate, and no secondary pollution, technology will gradually become the mainstream technology in China's flue gas denitrification market.
5. Precautions for catalyst storage and handling
Normally, the catalyst unit is installed in the carbon steel module, and then these modules are installed in the SCR reactor and react with the flue gas. Usually, only professionals from catalyst manufacturers can operate individual catalyst units. The catalyst unit needs to be placed in a special transportation container for future use when returning for transportation. When receiving the catalyst unit, an inspection should be conducted first. Any problems found should be reported to the catalyst manufacturer first. Catalysts must be stored and transported in a well packaged condition to avoid any form of impact, vibration, and mechanical collision. The modules transported to the site should be installed and used as soon as possible to reduce indirect damage to the catalyst. The module packaging should be removed before being loaded into the reactor on site. To prevent accidental damage to the catalyst module, please do not work within or near the storage module area. Protective equipment such as masks and gloves should be worn when handling and disposing of catalysts. After handling, rinse your mouth, face, and hands with clean water to avoid inhaling catalyst dust. Catalysts should be stored in ventilated and dry indoor warehouses to avoid contact with rainwater, seawater, heavy humidity, or other moisture, oil, and solvents. Moisture can not only poison the surface of the catalyst and make it inactive, but also easily cause the catalyst to rupture when it dries. Avoid prolonged exposure of the catalyst to sunlight. At the same time, catalysts should avoid contact with volatile organic gases and be kept away from reducing atmospheres to prevent the loss of activity caused by the reduction of active components in the catalyst. Stay away from alkali metals, alkaline earth metals such as potassium and sodium, calcium and magnesium, and other substances. Halogens can easily deactivate catalysts. Doing a good job in the operation and maintenance of catalysts can not only prolong the service life of equipment, but also reduce the cost of denitrification. It also reduces the huge workload of maintenance, shutdown, ash cleaning, replacement and transportation of denitration catalysts, and shortens the maintenance period.
