In order to meet the NOx emission requirements of environmental protection, the existing burner is modified to reduce the emission concentration of NOx, and to optimize the operation of the combustion process to control the emission of NOx at a reasonable level, and to reduce the product after combustion. Emission control to meet the emission standards required by the state.
The transformation of the low-nitrogen burner has a certain impact on the economy of the unit. Through the adjustment of combustion optimization operation, reasonable control of furnace oxygen and control of pulverized coal fineness, the problems existing in the operation of the unit can be solved. The main indicators of the unit after the transformation of the low-nitrogen burner can be controlled to a level equivalent to the design value through adjustment.
Key words: low nitrogen transformation; burner; combustion adjustment; NOx; optimal operation; high temperature corrosion
With the emergence of smog weather, air pollution control has been paid more and more attention. NOx is one of the main components of air pollution, and reducing NOx emissions has become the focus of attention. According to the provisions of GB13223-2011 "Emission Standards of Air Pollutants for Thermal Power Plants", from July 1, 2014, thermal power plants completed and put into operation on December 31, 2003 (except W-type flame boilers and circulating fluidized bed boilers) Implement the standard of NOx emission concentration limit of 100mg/Nm3.
Therefore, in order to achieve this goal, thermal power plants and universities are working hard to carry out experiments and renovations on reducing NOx concentration emissions. For coal-fired power plants, the control of NOx emission concentration is mainly achieved in two ways: one is to reduce the emission of NOx concentration during the combustion process, and adjust the combustion method to reduce the generation of NOx through low-nitrogen combustion technology; the other is After combustion, the control of the product NOx is further achieved through the denitrification device to meet the emission standards controlled by the state.
1 boiler combustion system
1.1 Boiler overview
A boiler in a power plant is a subcritical parameter, one-time intermediate reheating, natural circulation drum boiler, adopts balanced ventilation, four-corner tangential combustion, the design fuel is lignite, and the boiler is tightly closed. The boiler takes the maximum continuous load (i.e. BMCR working condition) as the design parameter. When the electrical load of the unit is 364.055MW, the maximum continuous evaporation of the boiler is 1125t/h; when the electrical load of the unit is 330MW (ECR working condition), the boiler’s The rated evaporation is 1070t/h. The main design parameters are shown in Table 1.
low nitrogen combustion
1.2 Design fuel characteristics
The arrangement of burners adopts tangential swing burners arranged in four corners. The design coal quality parameters are shown in Table 2. The burner can swing up and down, the maximum swing angle is ±20°. During the operation of the boiler, the NOx emission concentration is 550-610mg/Nm3, and the emission concentration after denitrification cannot meet the requirements of environmental protection.
Table 2 Design coal quality parameters
low nitrogen combustion
In order to control the emission of NOx, reduce the amount of NOx generated during the combustion process as much as possible, and ensure the concentration of nitrogen oxides at the SCR inlet at a reasonable level, the nitrogen oxides are mainly controlled by adjusting the combustion temperature, the excess air coefficient of each level and optimizing the operation mode. emission of matter.
The formation of nitrogen oxides mainly has three ways: fuel type, rapid type and thermal type. Among them, the generation of fuel-type nitrogen oxides accounts for more than 4/5, followed by thermal-type nitrogen oxides, and the formation of rapid-type nitrogen oxides is the smallest.
Reducing the generation of fuel-type nitrogen oxides is the main way to control nitrogen oxide emissions, mainly through the combination of low-nitrogen combustion technology in combustion and denitrification technology of flue gas after combustion, while the generation of other two types of nitrogen oxides accounts for Small portion.
2 Modification of low nitrogen burner
2.1 Modification scheme of low nitrogen burner
(1) Two imaginary cutting circles with slightly different diameters in the counterclockwise direction are formed in the center of the furnace, as shown in Figure 1. In order to weaken the rotation strength of the flue gas at the furnace outlet and reduce the deviation of the flue gas temperature at the furnace outlet caused by four-corner combustion, a SOFA burner is installed above the main burner, and the SOFA air chamber is designed to be anti-tangential so that the centerline of the nozzle outlet is the same as the center of the main nozzle The angle between the two lines is 12°, the purpose is to form a reverse momentum moment, balance the rotational momentum moment of the main burner, and achieve the purpose of reducing the deviation of the smoke temperature at the furnace outlet. rate to control NOx emissions.
low nitrogen combustion
(2) In the height direction of the burner, according to the swingable characteristics of the burner, when the burner swings down, it is ensured that the flame fills the space and the pulverized coal combustion space.
(3) In order to prevent coking in the furnace, a relatively small thermal power of a single nozzle is used to prevent coking in the burner area. The arrangement of burners in groups and reasonable air distribution can effectively control NOx emissions.
(4) The burner adopts the combustion technology of horizontal thick and light pulverized coal to improve the ability of the boiler to operate at low load. The flow is separated into thick and thin parts; the two parts are separated by a vertical partition, and there is a corrugated combustion-stabilizing blunt body at the outlet of the burner. The high concentration of pulverized coal in the dense-phase airflow has good ignition characteristics. Even under low load conditions, the air-to-coal ratio of the dense-phase airflow can still be maintained in a more appropriate range, so that the ignition characteristics will not deteriorate significantly. The high-temperature flue gas recirculation zone formed by the blunt body can fully provide a heat source for the ignition of pulverized coal, and the combination of the two provides a guarantee for stable combustion at low loads.
3 NOx emission and combustion adjustment after transformation
After the transformation of the low-nitrogen burner, it can be seen from Table 3 and Table 4 that under the load conditions of 300MW and 220MW, the NOx emission levels are different under different operating oxygen levels and different openings of the SOFA damper. Under 220MW load, when the operating oxygen amount is 3.8%, and the SOFA damper opening is 50%, the NOx emission concentration is controlled at 248.5mg/Nm3; under the 300MW load, the operating oxygen amount is 3.1%, and the SOFA damper opening is 90% , the NOx emission concentration was controlled at 207.1mg/Nm3. Under these two operating conditions, the NOx emission concentration is controlled at a relatively low level.
Table 3 Comparison of NOx emissions under different operating parameters
low nitrogen combustion
4 Analysis of problems after transformation
(1) The steam parameters deviate from the design value
The combustion zone of the boiler adopts air staged combustion technology, which makes the temperature of the main combustion zone drop, and the temperature distribution in the furnace is more uniform. The serious contamination and coking of the water wall is improved, the heat absorption of the water wall increases, and the smoke temperature at the furnace outlet The temperature rise of the superheater and reheater decreases, making the temperature of the superheater, especially the reheater, lower than the design value.
(2) The carbon content of fly ash and slag increases
The low-nitrogen combustion technology mainly adopts low-temperature and oxygen-deficient combustion technology, which makes the temperature of the main combustion area drop more, delays the ignition of coal powder, and burns in this area with oxygen deficiency, controls the excess air coefficient, and the combustion of coal powder in this area As far as possible, the carbon content of fly ash and slag increased. The increase of q4 will inevitably lead to the reduction of boiler thermal efficiency.
(3) Intensified high temperature corrosion phenomenon
Pulverized coal is anoxic-combusted in the main burner area, and insufficient combustion will produce a large amount of CO and reducing gas H2S, which will intensify the high-temperature corrosion in the water-cooled wall area.
Utilizing the staged combustion technology of air, the overburning air is increased in the upper layer, and the total air volume remains the same, the increase of the secondary air in the upper layer will inevitably reduce the secondary air in the lower layer, so that the mixing process of the primary and secondary air in the lower layer will be delayed, and the furnace will be more stable. As the center of the flame moves upward, the temperature of the flue gas at the corresponding furnace outlet rises, which is likely to cause coking and ash accumulation.
(4) The combustion situation in the furnace is complex
Since the modification of the low-nitrogen burner changes the dynamic field in the entire furnace, the combustion in the furnace is also adversely affected. The combustion operation optimization method designed before the transformation of the low-nitrogen burner is no longer suitable, which will cause large fluctuations in furnace negative pressure, partial combustion, high exhaust gas temperature, high carbon content in fly ash, and low reheater temperature, etc., making the It is more difficult to adjust under the low-temperature and low-oxygen combustion operating conditions, resulting in a decrease in the ability to stabilize combustion at low loads.
5 Improvement measures
(1) Reasonably control the amount of oxygen in operation
The control of oxygen amount during operation not only affects the economy of the unit, but also has a certain inhibitory effect on NOx emissions. Therefore, the fine adjustment of oxygen amount is very important. The excess air ratios in the combustion zone and reduction zone are controlled separately. Through the adjustment and control of different oxygen levels, the main operating parameters such as fly ash carbon content, reheater temperature and boiler efficiency were tested to find out the boiler parameter changes under different operating conditions, and the optimal operating oxygen of the unit was obtained. quantity. Under the load of 300MW, the relationship curve between the furnace oxygen and NOx can be obtained through the oxygen variation test.
low nitrogen combustion
(2) Optimization and adjustment of combustion operation
After the transformation of the low-nitrogen burner, the emission of NOx is controlled within the standard of environmental protection requirements, and the emission of NOx is reduced as much as possible during the operation process to reduce the operating cost of denitrification, but it is also necessary to take into account the impact on the boiler efficiency, high-temperature corrosion, etc. impact on safety and security of operations.
Orthogonal tests on main indicators such as different coal mill operation combinations, different distribution methods, and different operating oxygen levels are carried out to explore the optimal operation optimization and adjustment mode of the unit. Under the load of 300MW, the relationship curve between SOFA opening and NOx can be obtained as shown in Figure 3 through the experiment of changing SOFA opening.
low nitrogen combustion
(3) The water wall is sprayed with anti-coking and corrosion materials
The boiler with serious coking before the burner transformation will be improved, the temperature in the main combustion zone will drop, and the temperature distribution in the furnace will be more uniform, so that the contamination and coking of the water wall will be reduced, and the coking will be eliminated by spraying the water wall phenomenon, and the situation of high temperature corrosion has also been resolved.
6 Conclusion
By optimizing the operation during the combustion process, the emission of NOx is controlled at a reasonable level, and through the control of product emissions after combustion, the emission standards required by the country are met. The transformation of the low-nitrogen burner has a certain impact on the economy of the unit. Through the adjustment of combustion optimization operation, reasonable control of furnace oxygen and control of pulverized coal fineness, the problems existing in the operation of the unit can be solved. The main indicators of the unit after the transformation of the low-nitrogen burner can be controlled to the highest standard of similar unit design.
In order to meet the NOx emission requirements of environmental protection, the existing burner is modified to reduce the emission concentration of NOx, and to optimize the operation of the combustion process to control the emission of NOx at a reasonable level, and to reduce the product after combustion. Emission control to meet the emission standards required by the state.
The transformation of the low-nitrogen burner has a certain impact on the economy of the unit. Through the adjustment of combustion optimization operation, reasonable control of furnace oxygen and control of pulverized coal fineness, the problems existing in the operation of the unit can be solved. The main indicators of the unit after the transformation of the low-nitrogen burner can be controlled to a level equivalent to the design value through adjustment.
Key words: low nitrogen transformation; burner; combustion adjustment; NOx; optimal operation; high temperature corrosion
With the emergence of smog weather, air pollution control has been paid more and more attention. NOx is one of the main components of air pollution, and reducing NOx emissions has become the focus of attention. According to the provisions of GB13223-2011 "Emission Standards of Air Pollutants for Thermal Power Plants", from July 1, 2014, thermal power plants completed and put into operation on December 31, 2003 (except W-type flame boilers and circulating fluidized bed boilers) Implement the standard of NOx emission concentration limit of 100mg/Nm3.
Therefore, in order to achieve this goal, thermal power plants and universities are working hard to carry out experiments and renovations on reducing NOx concentration emissions. For coal-fired power plants, the control of NOx emission concentration is mainly achieved in two ways: one is to reduce the emission of NOx concentration during the combustion process, and adjust the combustion method to reduce the generation of NOx through low-nitrogen combustion technology; the other is After combustion, the control of the product NOx is further achieved through the denitrification device to meet the emission standards controlled by the state.
1 boiler combustion system
1.1 Boiler overview
A boiler in a power plant is a subcritical parameter, one-time intermediate reheating, natural circulation drum boiler, adopts balanced ventilation, four-corner tangential combustion, the design fuel is lignite, and the boiler is tightly closed. The boiler takes the maximum continuous load (i.e. BMCR working condition) as the design parameter. When the electrical load of the unit is 364.055MW, the maximum continuous evaporation of the boiler is 1125t/h; when the electrical load of the unit is 330MW (ECR working condition), the boiler’s The rated evaporation is 1070t/h. The main design parameters are shown in Table 1.
low nitrogen combustion
1.2 Design fuel characteristics
The arrangement of burners adopts tangential swing burners arranged in four corners. The design coal quality parameters are shown in Table 2. The burner can swing up and down, the maximum swing angle is ±20°. During the operation of the boiler, the NOx emission concentration is 550-610mg/Nm3, and the emission concentration after denitrification cannot meet the requirements of environmental protection.
Table 2 Design coal quality parameters
low nitrogen combustion
In order to control the emission of NOx, reduce the amount of NOx generated during the combustion process as much as possible, and ensure the concentration of nitrogen oxides at the SCR inlet at a reasonable level, the nitrogen oxides are mainly controlled by adjusting the combustion temperature, the excess air coefficient of each level and optimizing the operation mode. emission of matter.
The formation of nitrogen oxides mainly has three ways: fuel type, rapid type and thermal type. Among them, the generation of fuel-type nitrogen oxides accounts for more than 4/5, followed by thermal-type nitrogen oxides, and the formation of rapid-type nitrogen oxides is the smallest.
Reducing the generation of fuel-type nitrogen oxides is the main way to control nitrogen oxide emissions, mainly through the combination of low-nitrogen combustion technology in combustion and denitrification technology of flue gas after combustion, while the generation of other two types of nitrogen oxides accounts for Small portion.
2 Modification of low nitrogen burner
2.1 Modification scheme of low nitrogen burner
(1) Two imaginary cutting circles with slightly different diameters in the counterclockwise direction are formed in the center of the furnace, as shown in Figure 1. In order to weaken the rotation strength of the flue gas at the furnace outlet and reduce the deviation of the flue gas temperature at the furnace outlet caused by four-corner combustion, a SOFA burner is installed above the main burner, and the SOFA air chamber is designed to be anti-tangential so that the centerline of the nozzle outlet is the same as the center of the main nozzle The angle between the two lines is 12°, the purpose is to form a reverse momentum moment, balance the rotational momentum moment of the main burner, and achieve the purpose of reducing the deviation of the smoke temperature at the furnace outlet. rate to control NOx emissions.
low nitrogen combustion
(2) In the height direction of the burner, according to the swingable characteristics of the burner, when the burner swings down, it is ensured that the flame fills the space and the pulverized coal combustion space.
(3) In order to prevent coking in the furnace, a relatively small thermal power of a single nozzle is used to prevent coking in the burner area. The arrangement of burners in groups and reasonable air distribution can effectively control NOx emissions.
(4) The burner adopts the combustion technology of horizontal thick and light pulverized coal to improve the ability of the boiler to operate at low load. The flow is separated into thick and thin parts; the two parts are separated by a vertical partition, and there is a corrugated combustion-stabilizing blunt body at the outlet of the burner. The high concentration of pulverized coal in the dense-phase airflow has good ignition characteristics. Even under low load conditions, the air-to-coal ratio of the dense-phase airflow can still be maintained in a more appropriate range, so that the ignition characteristics will not deteriorate significantly. The high-temperature flue gas recirculation zone formed by the blunt body can fully provide a heat source for the ignition of pulverized coal, and the combination of the two provides a guarantee for stable combustion at low loads.
3 NOx emission and combustion adjustment after transformation
After the transformation of the low-nitrogen burner, it can be seen from Table 3 and Table 4 that under the load conditions of 300MW and 220MW, the NOx emission levels are different under different operating oxygen levels and different openings of the SOFA damper. Under 220MW load, when the operating oxygen amount is 3.8%, and the SOFA damper opening is 50%, the NOx emission concentration is controlled at 248.5mg/Nm3; under the 300MW load, the operating oxygen amount is 3.1%, and the SOFA damper opening is 90% , the NOx emission concentration was controlled at 207.1mg/Nm3. Under these two operating conditions, the NOx emission concentration is controlled at a relatively low level.
Table 3 Comparison of NOx emissions under different operating parameters
low nitrogen combustion
4 Analysis of problems after transformation
(1) The steam parameters deviate from the design value
The combustion zone of the boiler adopts air staged combustion technology, which makes the temperature of the main combustion zone drop, and the temperature distribution in the furnace is more uniform. The serious contamination and coking of the water wall is improved, the heat absorption of the water wall increases, and the smoke temperature at the furnace outlet The temperature rise of the superheater and reheater decreases, making the temperature of the superheater, especially the reheater, lower than the design value.
(2) The carbon content of fly ash and slag increases
The low-nitrogen combustion technology mainly adopts low-temperature and oxygen-deficient combustion technology, which makes the temperature of the main combustion area drop more, delays the ignition of coal powder, and burns in this area with oxygen deficiency, controls the excess air coefficient, and the combustion of coal powder in this area As far as possible, the carbon content of fly ash and slag increased. The increase of q4 will inevitably lead to the reduction of boiler thermal efficiency.
(3) Intensified high temperature corrosion phenomenon
Pulverized coal is anoxic-combusted in the main burner area, and insufficient combustion will produce a large amount of CO and reducing gas H2S, which will intensify the high-temperature corrosion in the water-cooled wall area.
Utilizing the staged combustion technology of air, the overburning air is increased in the upper layer, and the total air volume remains the same, the increase of the secondary air in the upper layer will inevitably reduce the secondary air in the lower layer, so that the mixing process of the primary and secondary air in the lower layer will be delayed, and the furnace will be more stable. As the center of the flame moves upward, the temperature of the flue gas at the corresponding furnace outlet rises, which is likely to cause coking and ash accumulation.
(4) The combustion situation in the furnace is complex
Since the modification of the low-nitrogen burner changes the dynamic field in the entire furnace, the combustion in the furnace is also adversely affected. The combustion operation optimization method designed before the transformation of the low-nitrogen burner is no longer suitable, which will cause large fluctuations in furnace negative pressure, partial combustion, high exhaust gas temperature, high carbon content in fly ash, and low reheater temperature, etc., making the It is more difficult to adjust under the low-temperature and low-oxygen combustion operating conditions, resulting in a decrease in the ability to stabilize combustion at low loads.
5 Improvement measures
(1) Reasonably control the amount of oxygen in operation
The control of oxygen amount during operation not only affects the economy of the unit, but also has a certain inhibitory effect on NOx emissions. Therefore, the fine adjustment of oxygen amount is very important. The excess air ratios in the combustion zone and reduction zone are controlled separately. Through the adjustment and control of different oxygen levels, the main operating parameters such as fly ash carbon content, reheater temperature and boiler efficiency were tested to find out the boiler parameter changes under different operating conditions, and the optimal operating oxygen of the unit was obtained. quantity. Under the load of 300MW, the relationship curve between the furnace oxygen and NOx can be obtained through the oxygen variation test.
low nitrogen combustion
(2) Optimization and adjustment of combustion operation
After the transformation of the low-nitrogen burner, the emission of NOx is controlled within the standard of environmental protection requirements, and the emission of NOx is reduced as much as possible during the operation process to reduce the operating cost of denitrification, but it is also necessary to take into account the impact on the boiler efficiency, high-temperature corrosion, etc. impact on safety and security of operations.
Orthogonal tests on main indicators such as different coal mill operation combinations, different distribution methods, and different operating oxygen levels are carried out to explore the optimal operation optimization and adjustment mode of the unit. Under the load of 300MW, the relationship curve between SOFA opening and NOx can be obtained as shown in Figure 3 through the experiment of changing SOFA opening.
low nitrogen combustion
(3) The water wall is sprayed with anti-coking and corrosion materials
The boiler with serious coking before the burner transformation will be improved, the temperature in the main combustion zone will drop, and the temperature distribution in the furnace will be more uniform, so that the contamination and coking of the water wall will be reduced, and the coking will be eliminated by spraying the water wall phenomenon, and the situation of high temperature corrosion has also been resolved.
6 Conclusion
By optimizing the operation during the combustion process, the emission of NOx is controlled at a reasonable level, and through the control of product emissions after combustion, the emission standards required by the country are met. The transformation of the low-nitrogen burner has a certain impact on the economy of the unit. Through the adjustment of combustion optimization operation, reasonable control of furnace oxygen and control of pulverized coal fineness, the problems existing in the operation of the unit can be solved. The main indicators of the unit after the transformation of the low-nitrogen burner can be controlled to the highest standard of similar unit design.