The key process of SCR denitrification
Publishdate：2020-03-09 Views：19
The commonly used and mature denitrification technologies in engineering mainly include low nitrogen combustion technology, SNCR flue gas denitrification technology, and SCR flue gas denitrification technology. This article only discusses the SCR flue gas denitrification technology, which is widely used in engineering and has high denitrification efficiency.
1. Flow field simulation test
The uniform distribution of the flue gas flow field entering the catalyst layer inlet of the reactor directly affects the various performance indicators of the denitrification system. If the flow field distribution is uneven, it will not only seriously affect the denitrification efficiency, increase ammonia escape, accelerate catalyst wear, but also block the catalyst or cause blockage and severe corrosion of the air preheater, thereby affecting the normal operation of the main engine. Therefore, The study of flow field simulation experiments is important in the design of denitrification systems. The research on flow field simulation experiments is mainly divided into two parts: computational fluid dynamics CFD calculation and physical model experimental verification.
The key to CFD calculation is the establishment of the calculation model and the setting of boundary conditions. When establishing the calculation model, it is necessary to determine whether the internal components of the flue gas system are simplified and the size of the calculation grid based on the actual design of the flue gas system, in order to achieve the goal of unified calculation speed and accuracy; In order to facilitate the setting of the inlet boundary conditions for the denitrification system, the outlet of the heat exchange tube bundle of the economizer is usually used as the inlet for CFD calculation of the denitrification system, and the inlet of the boiler air preheater is used as the outlet for CFD calculation of the denitrification system, making it easy to set CFD calculation conditions.
2.Design of 2 SCR reactors
As a crucial equipment for SCR denitrification, the design of the section of the SCR reactor should not only consider the optimal flue gas flow rate, but also the requirements for adapting to the layout and installation of different types of catalyst modules. Therefore, the design of the cross-section of the reactor and the support beam of the catalyst should be considered according to the universal design (meeting the requirements of honeycomb, flat, and corrugated plate catalyst modules), so that each type of catalyst module can be installed interchangeably.
In order to ensure the uniformity and incidence angle of flue gas in the catalyst layer, a flue gas rectification layer should be designed at the top of the reactor; In order to prevent the accumulation of ash from falling off the internal guide plate, support structure, and other components of the reactor, as well as the blockage of catalyst pores by flocculent debris in the flue, a ash crushing grid should be installed inside the reactor.
The lifting of catalysts is achieved through the installation of lifting rails and electric hoists outside the reactor. The design of the lifting rails should fully consider the weight of the catalyst module, lifting equipment, and electric hoists, as well as the inertia force caused by various swings during the lifting process. The design of the catalyst installation track inside the reactor should fully consider the ease of lifting the catalyst module and prevent the accumulation of dust.
In order to prevent the accumulation of ash in the catalyst layer from blocking the catalyst pores, a soot blower is usually installed at the upper part of each layer of catalyst. There are two types of soot blowers commonly used for SCR denitrification: sonic and steam. The selection and layout of these blowers should be determined based on the characteristics of the flue gas ash in the specific project and the cross-sectional size of the reactor.
3.ammonia spraying devices
The ammonia injection device, as one of the core components of the SCR method denitrification device, directly affects the denitrification efficiency and flue gas system resistance, thereby affecting the operating cost of the denitrification system.
At present, the ammonia injection devices used for SCR denitrification mainly include vortex mixers, static ammonia injection mixers, ammonia injection grilles, and rectangular tooth ammonia injection grilles. The ammonia spray grille independently developed by our company has low mixing resistance, good mixing effect, short mixing distance, convenient installation, and simple debugging. Compared with similar devices, it can reduce the resistance of the denitrification system by 50-100 Pa, save more than 4% of the induced draft fan power consumption, and achieve significant energy-saving effects.
4. Denitration flue ash hopper
The layout of SCR flue gas denitrification equipment can usually be divided into high dust layout and low dust layout according to the different reactor layout positions,. SCR reactors arranged with high dust are usually placed between the boiler economizer and the air preheater. Due to space limitations, SCR reactors cannot be directly placed under the boiler economizer (except for vertical boilers). Instead, the flue gas is led out through a horizontal flue and then connected to the SCR reactor through an upward flue. After undergoing denitrification reaction, it returns to the air preheater through the outlet flue of the SCR reactor.
A typical high dust layout design method requires the design of ash hoppers in both the inlet and outlet flue of the SCR reactor (especially for high dust flue gas). This not only effectively reduces catalyst wear, but also effectively reduces blockage and wear of the air preheater. At the same time, it can also reduce the consumption of denitration and reducing agents, ensuring safe and stable operation of the system. In engineering construction, in order to reduce project costs, simplify systems, or be limited by space, the ash hoppers at the inlet and outlet of SCR reactors are usually eliminated. This will inevitably lead to unstable operation, increased catalyst wear, and accelerated catalyst deactivation. Based on the current diverse characteristics of coal combustion in domestic boilers and the layout characteristics of coal-fired power generation units in China, an ash hopper should be designed at the inlet flue of the SCR reactor to protect the catalyst, improve the reliability of system operation, reduce wear and tear in the flue, and reduce operation and maintenance costs.
5. Debugging of denitrification system
The debugging of the denitrification system is an important task to ensure the stability, reliability, and ability to achieve the design performance guarantee value of the system operation.
The debugging of the denitrification system can be divided into two main parts: debugging of the reducing agent supply system and debugging of the ammonia injection system. The reducing agent supply system (commonly used as liquid ammonia or urea solution. This article takes liquid ammonia as an example) mainly includes subsystems such as liquid ammonia unloading, liquid ammonia evaporation and supply, tank area water spraying, ammonia area fire protection, and waste gas collection and emission. The debugging of the reducing agent supply system is important for the airtightness inspection and nitrogen replacement of the ammonia pipeline before unloading ammonia. To ensure the airtightness of the ammonia pipeline and the thoroughness of nitrogen replacement, the key to debugging is the operation and control of the liquid ammonia evaporation system.
The debugging of the ammonia injection system is a crucial and important part of the denitrification system debugging, which not only affects whether the performance of the denitrification system meets the design requirements, but also affects whether the denitrification system can operate properly.
In engineering construction, due to the high activity of newly installed catalysts, even if the ammonia injection device is not adjusted during the initial operation of the denitrification system, it can usually meet the performance requirements. Due to the large workload of ammonia injection system debugging, denitrification projects often overlook the chemical debugging of the ammonia injection system, which will seriously affect the long-term operation of the denitrification system. Special attention should be paid in engineering construction.
6. Operation and maintenance management of denitrification system
The correct operation and regular maintenance of the denitrification system are the key to ensuring the normal operation of the denitrification device. Currently, the automation level of the constructed denitrification systems is relatively high. Except for the unloading of reducing agents, unmanned operation can be basically achieved. However, the correct operation, maintenance, and management of the system are very important. During the operation of the system, special attention should be paid to the changes in parameters such as dilution air volume, denitrification efficiency, ammonia escape rate, liquid ammonia consumption, catalyst layer resistance, air preheater resistance, etc. Regular inspections of analytical instruments, soot blowers, dilution fans, ammonia unloading compressors, catalyst activity, and ammonia pipeline leakage should be carried out as required.
7 Catalyst Selection
At present, the commonly used catalysts for SCR denitrification are mainly titanium oxide based catalysts, which are mainly composed of TiO2, V2O5, WO3, and MoO3, and mainly consist of honeycomb, flat, and corrugated plates.
Honeycomb catalysts are uniformly extruded and have a high specific surface area; The flat catalyst is made of stainless steel mesh as the substrate and coated with active materials, which has high resistance to fly ash wear and low pressure loss, and strong resistance to arsenic poisoning; The corrugated plate catalyst is made of reinforced glass fiber as the substrate and coated with active materials, with small mass, high activity, and strong resistance to CaO poisoning. In principle, different types of catalysts can meet the performance requirements of specific projects. However, the activity, pitch, effective specific surface area, catalyst volume, and resistance of different types of catalysts are not the same. Therefore, the best catalyst form and layout can be selected through comparison to improve the cost-effectiveness of the project and effectively reduce the initial construction and operation costs of the project.
The commonly used and mature denitrification technologies in engineering mainly include low nitrogen combustion technology, SNCR flue gas denitrification technology, and SCR flue gas denitrification technology. This article only discusses the SCR flue gas denitrification technology, which is widely used in engineering and has high denitrification efficiency.
1. Flow field simulation test
The uniform distribution of the flue gas flow field entering the catalyst layer inlet of the reactor directly affects the various performance indicators of the denitrification system. If the flow field distribution is uneven, it will not only seriously affect the denitrification efficiency, increase ammonia escape, accelerate catalyst wear, but also block the catalyst or cause blockage and severe corrosion of the air preheater, thereby affecting the normal operation of the main engine. Therefore, The study of flow field simulation experiments is important in the design of denitrification systems. The research on flow field simulation experiments is mainly divided into two parts: computational fluid dynamics CFD calculation and physical model experimental verification.
The key to CFD calculation is the establishment of the calculation model and the setting of boundary conditions. When establishing the calculation model, it is necessary to determine whether the internal components of the flue gas system are simplified and the size of the calculation grid based on the actual design of the flue gas system, in order to achieve the goal of unified calculation speed and accuracy; In order to facilitate the setting of the inlet boundary conditions for the denitrification system, the outlet of the heat exchange tube bundle of the economizer is usually used as the inlet for CFD calculation of the denitrification system, and the inlet of the boiler air preheater is used as the outlet for CFD calculation of the denitrification system, making it easy to set CFD calculation conditions.
2.Design of 2 SCR reactors
As a crucial equipment for SCR denitrification, the design of the section of the SCR reactor should not only consider the optimal flue gas flow rate, but also the requirements for adapting to the layout and installation of different types of catalyst modules. Therefore, the design of the cross-section of the reactor and the support beam of the catalyst should be considered according to the universal design (meeting the requirements of honeycomb, flat, and corrugated plate catalyst modules), so that each type of catalyst module can be installed interchangeably.
In order to ensure the uniformity and incidence angle of flue gas in the catalyst layer, a flue gas rectification layer should be designed at the top of the reactor; In order to prevent the accumulation of ash from falling off the internal guide plate, support structure, and other components of the reactor, as well as the blockage of catalyst pores by flocculent debris in the flue, a ash crushing grid should be installed inside the reactor.
The lifting of catalysts is achieved through the installation of lifting rails and electric hoists outside the reactor. The design of the lifting rails should fully consider the weight of the catalyst module, lifting equipment, and electric hoists, as well as the inertia force caused by various swings during the lifting process. The design of the catalyst installation track inside the reactor should fully consider the ease of lifting the catalyst module and prevent the accumulation of dust.
In order to prevent the accumulation of ash in the catalyst layer from blocking the catalyst pores, a soot blower is usually installed at the upper part of each layer of catalyst. There are two types of soot blowers commonly used for SCR denitrification: sonic and steam. The selection and layout of these blowers should be determined based on the characteristics of the flue gas ash in the specific project and the cross-sectional size of the reactor.
3.ammonia spraying devices
The ammonia injection device, as one of the core components of the SCR method denitrification device, directly affects the denitrification efficiency and flue gas system resistance, thereby affecting the operating cost of the denitrification system.
At present, the ammonia injection devices used for SCR denitrification mainly include vortex mixers, static ammonia injection mixers, ammonia injection grilles, and rectangular tooth ammonia injection grilles. The ammonia spray grille independently developed by our company has low mixing resistance, good mixing effect, short mixing distance, convenient installation, and simple debugging. Compared with similar devices, it can reduce the resistance of the denitrification system by 50-100 Pa, save more than 4% of the induced draft fan power consumption, and achieve significant energy-saving effects.
4. Denitration flue ash hopper
The layout of SCR flue gas denitrification equipment can usually be divided into high dust layout and low dust layout according to the different reactor layout positions,. SCR reactors arranged with high dust are usually placed between the boiler economizer and the air preheater. Due to space limitations, SCR reactors cannot be directly placed under the boiler economizer (except for vertical boilers). Instead, the flue gas is led out through a horizontal flue and then connected to the SCR reactor through an upward flue. After undergoing denitrification reaction, it returns to the air preheater through the outlet flue of the SCR reactor.
A typical high dust layout design method requires the design of ash hoppers in both the inlet and outlet flue of the SCR reactor (especially for high dust flue gas). This not only effectively reduces catalyst wear, but also effectively reduces blockage and wear of the air preheater. At the same time, it can also reduce the consumption of denitration and reducing agents, ensuring safe and stable operation of the system. In engineering construction, in order to reduce project costs, simplify systems, or be limited by space, the ash hoppers at the inlet and outlet of SCR reactors are usually eliminated. This will inevitably lead to unstable operation, increased catalyst wear, and accelerated catalyst deactivation. Based on the current diverse characteristics of coal combustion in domestic boilers and the layout characteristics of coal-fired power generation units in China, an ash hopper should be designed at the inlet flue of the SCR reactor to protect the catalyst, improve the reliability of system operation, reduce wear and tear in the flue, and reduce operation and maintenance costs.
5. Debugging of denitrification system
The debugging of the denitrification system is an important task to ensure the stability, reliability, and ability to achieve the design performance guarantee value of the system operation.
The debugging of the denitrification system can be divided into two main parts: debugging of the reducing agent supply system and debugging of the ammonia injection system. The reducing agent supply system (commonly used as liquid ammonia or urea solution. This article takes liquid ammonia as an example) mainly includes subsystems such as liquid ammonia unloading, liquid ammonia evaporation and supply, tank area water spraying, ammonia area fire protection, and waste gas collection and emission. The debugging of the reducing agent supply system is important for the airtightness inspection and nitrogen replacement of the ammonia pipeline before unloading ammonia. To ensure the airtightness of the ammonia pipeline and the thoroughness of nitrogen replacement, the key to debugging is the operation and control of the liquid ammonia evaporation system.
The debugging of the ammonia injection system is a crucial and important part of the denitrification system debugging, which not only affects whether the performance of the denitrification system meets the design requirements, but also affects whether the denitrification system can operate properly.
In engineering construction, due to the high activity of newly installed catalysts, even if the ammonia injection device is not adjusted during the initial operation of the denitrification system, it can usually meet the performance requirements. Due to the large workload of ammonia injection system debugging, denitrification projects often overlook the chemical debugging of the ammonia injection system, which will seriously affect the long-term operation of the denitrification system. Special attention should be paid in engineering construction.
6. Operation and maintenance management of denitrification system
The correct operation and regular maintenance of the denitrification system are the key to ensuring the normal operation of the denitrification device. Currently, the automation level of the constructed denitrification systems is relatively high. Except for the unloading of reducing agents, unmanned operation can be basically achieved. However, the correct operation, maintenance, and management of the system are very important. During the operation of the system, special attention should be paid to the changes in parameters such as dilution air volume, denitrification efficiency, ammonia escape rate, liquid ammonia consumption, catalyst layer resistance, air preheater resistance, etc. Regular inspections of analytical instruments, soot blowers, dilution fans, ammonia unloading compressors, catalyst activity, and ammonia pipeline leakage should be carried out as required.
7 Catalyst Selection
At present, the commonly used catalysts for SCR denitrification are mainly titanium oxide based catalysts, which are mainly composed of TiO2, V2O5, WO3, and MoO3, and mainly consist of honeycomb, flat, and corrugated plates.
Honeycomb catalysts are uniformly extruded and have a high specific surface area; The flat catalyst is made of stainless steel mesh as the substrate and coated with active materials, which has high resistance to fly ash wear and low pressure loss, and strong resistance to arsenic poisoning; The corrugated plate catalyst is made of reinforced glass fiber as the substrate and coated with active materials, with small mass, high activity, and strong resistance to CaO poisoning. In principle, different types of catalysts can meet the performance requirements of specific projects. However, the activity, pitch, effective specific surface area, catalyst volume, and resistance of different types of catalysts are not the same. Therefore, the best catalyst form and layout can be selected through comparison to improve the cost-effectiveness of the project and effectively reduce the initial construction and operation costs of the project.
