Analysis of the Reaction Activity of SCR Denitration Catalysts
Publishdate：2019-03-22 Views：15
Keywords: SCR denitrification; Catalysts; Activity; Measures
Chinese Library Classification Number: X701 Literature Identification Code: A
The nitrogen oxides produced by coal combustion can induce luminescent chemical smoke, form acid rain, and cause greenhouse effect. According to statistics, the nitrogen oxides produced by coal-fired power plant boilers account for over 40% of the total nitrogen oxide production from coal combustion.
In June 2010, the Ministry of Environmental Protection issued the "Guidelines for the Compilation of the Twelfth Five Year Plan for the Total Control of Major Pollutants", which clearly included nitrogen oxides in the indicator system of total control and made the power industry a key focus of emission control.
On July 29, 2011, the "Emission Standards for Air Pollutants from Thermal Power Plants" adjusted the emission standards for nitrogen oxides to 100mg/m3 (standard state), and required active and newly built thermal power units to meet the upper limit of nitrogen oxide mass concentration emissions of 100mg/m3 (standard state) by July 1, 2014 and 2012, respectively.
On September 12, 2014, the Development and Reform Commission, the Ministry of Environmental Protection, and the Energy Administration jointly issued the Action Plan for Energy Conservation, Emission Reduction, Upgrading, and Renovation of Coal Power (2014-2020), which clearly stated that under the condition of a benchmark oxygen content of 6%, the emission concentrations of smoke, sulfur dioxide, and nitrogen oxides should not exceed 10mg/m3, 35mg/m3, and 50mg/m3, respectively.
The increasingly strict nitrogen oxide emission standards for thermal power units have made selective catalytic reduction (SCR) full flue gas denitrification technology, which has high denitrification efficiency, low NH3/NOx molar ratio, NH3 escape, and low SO2/SO3 conversion rate, the choice for flue gas denitrification in China's thermal power enterprises.
Overview of SCR denitrification technology
In 1959, Engelhard Corporation of the United States applied for the invention of SCR technology; Japan officially began researching and developing this technology in 1972, and achieved its industrial application in 1978; The research on SCR technology in China began as early as the 1990s. Currently, over 80% of flue gas denitrification methods use SCR technology.
At present, domestic manufacturing enterprises have fully mastered SCR denitrification technology, especially catalysts that can be fully produced domestically. Domestic manufacturing enterprises have the ability to supply power plant flue gas SCR denitrification complete sets with different performance. At present, the SCR denitrification device for million kilowatt units in China has been in operation for many years.
The principle of SCR denitrification process is as follows: ammonia/air mixture is injected into the flue gas channel at a constant temperature, fully mixed with boiler flue gas at a constant temperature, and then passed through the catalyst layer of the SCR reactor. Under the action of the catalyst, NOx in the flue gas undergoes sufficient chemical reduction reaction with ammonia, generating N2 and H2O. The main chemical reactions are as follows:
4NO+4NH3+O2 → 4N2+6H2O (enriched with oxygen)
6NO2+8NH3 → 7N2+12H2O
NO+NO2+2NH3 → 2N2+3H2O (hypoxia)
The first reaction mentioned above is the main one. According to statistics, NO accounts for over 90% of the NOx products produced during coal combustion. This reaction can only occur within a narrow temperature range of around 980 ℃ when the catalyst is not involved. It can react within the actual operating temperature range of thermal power plants from 290 ℃ to 430 ℃ with the participation of suitable catalysts.
During the SCR denitrification process, there are also side reactions where SO2 is oxidized to SO3 and SO3 reacts with escaping NH3 below 320 ℃ to generate NH4HSO4. Liquid NH4HSO4 will adsorb on the surface of the catalyst, causing the SCR denitrification catalyst to lose its activity. Therefore, the reaction temperature during the operation of the SCR reactor should generally be controlled above 330 ℃.
2、 Problems in the operation of SCR denitrification catalysts
In November 2004, the start-up of the flue gas denitrification project for the 600MW units of Guohua Ninghai Power Plant and Taishan Power Plant marked the official start of flue gas denitrification work in China. After nearly a decade of development, China's coal-fired SCR flue gas denitrification work in thermal power generation has made rapid progress. However, in recent years, some practical problems have also been exposed during the operation of SCR flue gas denitrification projects put into operation in China.
1. Catalyst blockage
In the production process, a large amount of fly ash generated from the combustion of middling coal and the particles of ammonia salt formed in the denitrification process are deposited on the surface or in the pores of the catalyst, causing the blockage of SCR denitrification catalyst, seriously preventing nitrogen oxides, ammonia and oxygen from reaching the active surface of the catalyst, resulting in passivation and reduction of the activity of the catalyst. Moreover, localized blockage of the catalyst can also cause wear and tear on the catalyst, seriously affecting the normal production and operation of the denitrification system.
2. Catalyst wear
Due to the collision between the fly ash generated by the boiler system and the surface of the catalyst under high temperature and high flow rate conditions, as well as the unreasonable design of the SCR reaction chamber, about 30% of the catalyst surface has accumulated dust for a long time, causing severe local blockage, resulting in a 30% to 50% increase in the flow rate of the flue gas passing through the remaining unobstructed holes of the catalyst. The increase in smoke incidence angle caused by excessive ash accumulation area also exacerbates the wear of the catalyst, making the overall structure of the catalyst gradually become loose.
Catalyst poisoning
Gaseous arsenic compounds, heavy metals such as Pt and Pb, as well as water-soluble alkali metals such as Na, K, Ca, etc. in the flue gas enter the interior of the catalyst and pile up, reacting with other substances at the active site of the catalyst, resulting in a decrease in its activity.
The gaseous arsenide molecules contained in the system flue gas first easily react with O2 and V2O5 on the catalyst surface, forming a saturated layer of arsenic, and then penetrate into the small pores inside the catalyst. The solidification of As2O3 in the active region destroys the capillary of the catalyst, limits the diffusion of reaction gases such as NH3 within the catalyst, and seriously affects the activity of the catalyst.
As the concentration of alkali metals covering the surface of the catalyst continues to increase, the activity of the catalyst also weakens. Especially in the presence of liquid water, its activity rapidly decreases as the moisture content of the flue gas increases. High fluidity alkali metals have strong activity and are easy to enter the interior of the catalyst, which will have long-lasting toxicity to the catalyst. At the same time, the condensed water in the pores of the catalyst will expand and vaporize due to system heating, which will damage the structure of the catalyst.
The toxicity of alkali metals such as Li2O, Na2O, K2O, Rb2O, and Cs2O, as well as salt substances of alkali metals, to the catalyst increases sequentially.
3、 Activity measures for SCR denitrification catalyst
Considering the above problems that often occur in the actual production and operation of SCR flue gas denitrification projects, the following perspectives can be considered for handling in practical operation: 1. Chemical system design
Designing key systems such as flue gas channels, ammonia injection and mixing systems, and SCR reaction chambers to reduce the resistance of the SCR system and ensure uniform distribution of temperature and flow fields in the reactor is essential for achieving optimal catalyst process performance. It eliminates the formation of high ash, high velocity, and biased flow areas on the inlet section of the SCR reaction chamber, avoiding catalyst blockage and wear.
Increase the size of the ash hopper at the outlet of the economizer upstream of the catalyst, or install a guide baffle above the ash hopper at the outlet of the economizer; At the same time, pre dust removal equipment such as large ash filters can be combined to improve and enhance interception capabilities, prevent large particle fly ash in flue gas from entering the denitrification system, and maintain the safe and stable operation of the system.
Install a suitable ash hopper at the outlet of the SCR device flue, and adjust the catalyst layer blowing method reasonably based on the ash content of the coal entering the furnace, the temperature inside the reactor, and the boiler blowing method and frequency of use.
Design a grid with metal wire mesh on the catalyst bed of the SCR device layer, and ensure that the inter node distance of the wire mesh is smaller than the pore size of the selected catalyst.
Based on the actual space and system resistance requirements of the denitrification reactor site, a reasonable layout of the catalyst bed is designed to effectively improve the utilization rate of the catalyst.
Hardening design measures are adopted at the edge of the catalyst inlet to improve edge hardness and resist the impact and erosion of dust particles.
For the old power plant of the denitrification renovation project, the impact of installing a new denitrification device on existing equipment should be considered, and those that need to be renovated should be considered comprehensively to ensure the overall system design is perfect.
2. Strengthen process operation management
Strengthen the knowledge training for SCR equipment process personnel, and proficiently master relevant operational skills in the system. Strictly follow the requirements of the operation manual, closely monitor changes in resistance, temperature, denitrification efficiency, and NH3 escape indicators of the SCR system during operation, establish an operational database for the SCR system, and continuously accumulate experience in the operation management and system maintenance of the SCR denitrification device.
Strengthen the operation, monitoring, and management of soot blowing. Especially for layer catalysts, a combined operation of acoustic soot blower and steam soot blower should be adopted, and the soot blowing plan should be adjusted in a timely manner according to process requirements and actual operating conditions to avoid catalyst blockage.
When burning coal with high arsenic content, adding Mo as an auxiliary agent to the catalyst can change the adsorption position of arsenic and weaken the adverse effect of arsenic on catalyst activity; It is also possible to lower the reaction temperature as much as possible while ensuring the activity of the SCR denitrification catalyst, and promote the natural nucleation of gaseous arsenic elements. To reduce the volatilization of arsenic during the combustion process, high arsenic coal and high calcium ash coal can be appropriately mixed for combustion, or 1% to 2% limestone can be added to the furnace. Arsenic reacts with CaO in the limestone to solidify gaseous arsenic into solid CaAsO4 that is non-toxic to the catalyst. But when the CaO concentration is too high, the amount of CaSO4 formed will also increase, leading to blockage of the catalyst CaSO4. Therefore, at a fixed arsenic concentration, the service life of the catalyst increases first and then gradually decreases with the increase of CaO content in coal.
3. Strictly control system water condensation
During boiler ignition start-up and SCR denitrification system shutdown, the temperature at the catalyst is relatively low, and the water vapor contained in the flue gas is prone to condensation and condensation on the surface of the catalyst at the reactor, which will seriously affect the activity and lifespan of the catalyst. During this period, the denitrification catalyst can be preheated and protected by an air heating system to maintain a low humidity level in the denitrification reactor and extend the service life of the catalyst.
During the storage and transportation of catalysts, necessary measures should also be taken to ensure their dryness and avoid a decrease in their mechanical properties.
Conclusion
The performance of catalysts directly affects the operational efficiency of SCR flue gas denitrification systems. Strengthening the maintenance of catalysts and maintaining their long-term high activity are key issues in the operation of SCR flue gas denitrification. Ash blockage, wear, and poisoning can all promote catalyst deactivation. Analyzing the causes of catalyst deactivation, targeted design of SCR denitrification system, and formulation of appropriate catalyst deactivation prevention measures are of great significance for improving catalyst service life, reducing operation and maintenance costs, and achieving significant social and economic benefits.
Keywords: SCR denitrification; Catalysts; Activity; Measures
Chinese Library Classification Number: X701 Literature Identification Code: A
The nitrogen oxides produced by coal combustion can induce luminescent chemical smoke, form acid rain, and cause greenhouse effect. According to statistics, the nitrogen oxides produced by coal-fired power plant boilers account for over 40% of the total nitrogen oxide production from coal combustion.
In June 2010, the Ministry of Environmental Protection issued the "Guidelines for the Compilation of the Twelfth Five Year Plan for the Total Control of Major Pollutants", which clearly included nitrogen oxides in the indicator system of total control and made the power industry a key focus of emission control.
On July 29, 2011, the "Emission Standards for Air Pollutants from Thermal Power Plants" adjusted the emission standards for nitrogen oxides to 100mg/m3 (standard state), and required active and newly built thermal power units to meet the upper limit of nitrogen oxide mass concentration emissions of 100mg/m3 (standard state) by July 1, 2014 and 2012, respectively.
On September 12, 2014, the Development and Reform Commission, the Ministry of Environmental Protection, and the Energy Administration jointly issued the Action Plan for Energy Conservation, Emission Reduction, Upgrading, and Renovation of Coal Power (2014-2020), which clearly stated that under the condition of a benchmark oxygen content of 6%, the emission concentrations of smoke, sulfur dioxide, and nitrogen oxides should not exceed 10mg/m3, 35mg/m3, and 50mg/m3, respectively.
The increasingly strict nitrogen oxide emission standards for thermal power units have made selective catalytic reduction (SCR) full flue gas denitrification technology, which has high denitrification efficiency, low NH3/NOx molar ratio, NH3 escape, and low SO2/SO3 conversion rate, the choice for flue gas denitrification in China's thermal power enterprises.
Overview of SCR denitrification technology
In 1959, Engelhard Corporation of the United States applied for the invention of SCR technology; Japan officially began researching and developing this technology in 1972, and achieved its industrial application in 1978; The research on SCR technology in China began as early as the 1990s. Currently, over 80% of flue gas denitrification methods use SCR technology.
At present, domestic manufacturing enterprises have fully mastered SCR denitrification technology, especially catalysts that can be fully produced domestically. Domestic manufacturing enterprises have the ability to supply power plant flue gas SCR denitrification complete sets with different performance. At present, the SCR denitrification device for million kilowatt units in China has been in operation for many years.
The principle of SCR denitrification process is as follows: ammonia/air mixture is injected into the flue gas channel at a constant temperature, fully mixed with boiler flue gas at a constant temperature, and then passed through the catalyst layer of the SCR reactor. Under the action of the catalyst, NOx in the flue gas undergoes sufficient chemical reduction reaction with ammonia, generating N2 and H2O. The main chemical reactions are as follows:
4NO+4NH3+O2 → 4N2+6H2O (enriched with oxygen)
6NO2+8NH3 → 7N2+12H2O
NO+NO2+2NH3 → 2N2+3H2O (hypoxia)
The first reaction mentioned above is the main one. According to statistics, NO accounts for over 90% of the NOx products produced during coal combustion. This reaction can only occur within a narrow temperature range of around 980 ℃ when the catalyst is not involved. It can react within the actual operating temperature range of thermal power plants from 290 ℃ to 430 ℃ with the participation of suitable catalysts.
During the SCR denitrification process, there are also side reactions where SO2 is oxidized to SO3 and SO3 reacts with escaping NH3 below 320 ℃ to generate NH4HSO4. Liquid NH4HSO4 will adsorb on the surface of the catalyst, causing the SCR denitrification catalyst to lose its activity. Therefore, the reaction temperature during the operation of the SCR reactor should generally be controlled above 330 ℃.
2、 Problems in the operation of SCR denitrification catalysts
In November 2004, the start-up of the flue gas denitrification project for the 600MW units of Guohua Ninghai Power Plant and Taishan Power Plant marked the official start of flue gas denitrification work in China. After nearly a decade of development, China's coal-fired SCR flue gas denitrification work in thermal power generation has made rapid progress. However, in recent years, some practical problems have also been exposed during the operation of SCR flue gas denitrification projects put into operation in China.
1. Catalyst blockage
In the production process, a large amount of fly ash generated from the combustion of middling coal and the particles of ammonia salt formed in the denitrification process are deposited on the surface or in the pores of the catalyst, causing the blockage of SCR denitrification catalyst, seriously preventing nitrogen oxides, ammonia and oxygen from reaching the active surface of the catalyst, resulting in passivation and reduction of the activity of the catalyst. Moreover, localized blockage of the catalyst can also cause wear and tear on the catalyst, seriously affecting the normal production and operation of the denitrification system.
2. Catalyst wear
Due to the collision between the fly ash generated by the boiler system and the surface of the catalyst under high temperature and high flow rate conditions, as well as the unreasonable design of the SCR reaction chamber, about 30% of the catalyst surface has accumulated dust for a long time, causing severe local blockage, resulting in a 30% to 50% increase in the flow rate of the flue gas passing through the remaining unobstructed holes of the catalyst. The increase in smoke incidence angle caused by excessive ash accumulation area also exacerbates the wear of the catalyst, making the overall structure of the catalyst gradually become loose.
Catalyst poisoning
Gaseous arsenic compounds, heavy metals such as Pt and Pb, as well as water-soluble alkali metals such as Na, K, Ca, etc. in the flue gas enter the interior of the catalyst and pile up, reacting with other substances at the active site of the catalyst, resulting in a decrease in its activity.
The gaseous arsenide molecules contained in the system flue gas first easily react with O2 and V2O5 on the catalyst surface, forming a saturated layer of arsenic, and then penetrate into the small pores inside the catalyst. The solidification of As2O3 in the active region destroys the capillary of the catalyst, limits the diffusion of reaction gases such as NH3 within the catalyst, and seriously affects the activity of the catalyst.
As the concentration of alkali metals covering the surface of the catalyst continues to increase, the activity of the catalyst also weakens. Especially in the presence of liquid water, its activity rapidly decreases as the moisture content of the flue gas increases. High fluidity alkali metals have strong activity and are easy to enter the interior of the catalyst, which will have long-lasting toxicity to the catalyst. At the same time, the condensed water in the pores of the catalyst will expand and vaporize due to system heating, which will damage the structure of the catalyst.
The toxicity of alkali metals such as Li2O, Na2O, K2O, Rb2O, and Cs2O, as well as salt substances of alkali metals, to the catalyst increases sequentially.
3、 Activity measures for SCR denitrification catalyst
Considering the above problems that often occur in the actual production and operation of SCR flue gas denitrification projects, the following perspectives can be considered for handling in practical operation: 1. Chemical system design
Designing key systems such as flue gas channels, ammonia injection and mixing systems, and SCR reaction chambers to reduce the resistance of the SCR system and ensure uniform distribution of temperature and flow fields in the reactor is essential for achieving optimal catalyst process performance. It eliminates the formation of high ash, high velocity, and biased flow areas on the inlet section of the SCR reaction chamber, avoiding catalyst blockage and wear.
Increase the size of the ash hopper at the outlet of the economizer upstream of the catalyst, or install a guide baffle above the ash hopper at the outlet of the economizer; At the same time, pre dust removal equipment such as large ash filters can be combined to improve and enhance interception capabilities, prevent large particle fly ash in flue gas from entering the denitrification system, and maintain the safe and stable operation of the system.
Install a suitable ash hopper at the outlet of the SCR device flue, and adjust the catalyst layer blowing method reasonably based on the ash content of the coal entering the furnace, the temperature inside the reactor, and the boiler blowing method and frequency of use.
Design a grid with metal wire mesh on the catalyst bed of the SCR device layer, and ensure that the inter node distance of the wire mesh is smaller than the pore size of the selected catalyst.
Based on the actual space and system resistance requirements of the denitrification reactor site, a reasonable layout of the catalyst bed is designed to effectively improve the utilization rate of the catalyst.
Hardening design measures are adopted at the edge of the catalyst inlet to improve edge hardness and resist the impact and erosion of dust particles.
For the old power plant of the denitrification renovation project, the impact of installing a new denitrification device on existing equipment should be considered, and those that need to be renovated should be considered comprehensively to ensure the overall system design is perfect.
2. Strengthen process operation management
Strengthen the knowledge training for SCR equipment process personnel, and proficiently master relevant operational skills in the system. Strictly follow the requirements of the operation manual, closely monitor changes in resistance, temperature, denitrification efficiency, and NH3 escape indicators of the SCR system during operation, establish an operational database for the SCR system, and continuously accumulate experience in the operation management and system maintenance of the SCR denitrification device.
Strengthen the operation, monitoring, and management of soot blowing. Especially for layer catalysts, a combined operation of acoustic soot blower and steam soot blower should be adopted, and the soot blowing plan should be adjusted in a timely manner according to process requirements and actual operating conditions to avoid catalyst blockage.
When burning coal with high arsenic content, adding Mo as an auxiliary agent to the catalyst can change the adsorption position of arsenic and weaken the adverse effect of arsenic on catalyst activity; It is also possible to lower the reaction temperature as much as possible while ensuring the activity of the SCR denitrification catalyst, and promote the natural nucleation of gaseous arsenic elements. To reduce the volatilization of arsenic during the combustion process, high arsenic coal and high calcium ash coal can be appropriately mixed for combustion, or 1% to 2% limestone can be added to the furnace. Arsenic reacts with CaO in the limestone to solidify gaseous arsenic into solid CaAsO4 that is non-toxic to the catalyst. But when the CaO concentration is too high, the amount of CaSO4 formed will also increase, leading to blockage of the catalyst CaSO4. Therefore, at a fixed arsenic concentration, the service life of the catalyst increases first and then gradually decreases with the increase of CaO content in coal.
3. Strictly control system water condensation
During boiler ignition start-up and SCR denitrification system shutdown, the temperature at the catalyst is relatively low, and the water vapor contained in the flue gas is prone to condensation and condensation on the surface of the catalyst at the reactor, which will seriously affect the activity and lifespan of the catalyst. During this period, the denitrification catalyst can be preheated and protected by an air heating system to maintain a low humidity level in the denitrification reactor and extend the service life of the catalyst.
During the storage and transportation of catalysts, necessary measures should also be taken to ensure their dryness and avoid a decrease in their mechanical properties.
Conclusion
The performance of catalysts directly affects the operational efficiency of SCR flue gas denitrification systems. Strengthening the maintenance of catalysts and maintaining their long-term high activity are key issues in the operation of SCR flue gas denitrification. Ash blockage, wear, and poisoning can all promote catalyst deactivation. Analyzing the causes of catalyst deactivation, targeted design of SCR denitrification system, and formulation of appropriate catalyst deactivation prevention measures are of great significance for improving catalyst service life, reducing operation and maintenance costs, and achieving significant social and economic benefits.
