Can meet the current national environmental protection requirements. Therefore, coal power generation enterprises must carry out boiler optimization and transformation in accordance with the pollutant emission limits required by the state, and actively adopt a series of environmental protection measures. Boiler renovation adopts low-nitrogen combustion technology, which can not only greatly reduce NO emissions, but also improve the stability and safety of boiler work, while saving costs.
1. Current status of NO governance
The harm of NO, the formation mechanism of NO in the combustion process of coal-fired power generation and the technology of reducing NO have been fully researched at home and abroad, which can be divided into three types: thermal NO, fuel NO and rapid NO; among them, fuel NO Type NO accounts for about 80-90%, and is the main object of various low NO technology control; followed by thermal type, mainly due to local high temperature in the furnace, and rapid NO generation is very small. The control methods of NO can be divided into treatment before combustion, treatment during combustion and treatment after combustion. Denitrification before combustion refers to converting fuel into low-nitrogen fuel. The technology is complicated, difficult and costly, so it is currently in the research stage; denitrification during combustion mainly includes: one is to inhibit the formation of NO in combustion, and the other is to reduce the formation of NO NO; denitrification after combustion mainly refers to flue gas denitrification: including selective catalytic reduction method, selective non-catalytic reduction method, etc.
At present, it is recognized by everyone and has been widely used in various coal-fired unit boilers. The method of reducing NO is mainly the low-nitrogen combustion technology for denitrification during combustion and the flue gas denitrification technology for denitrification after combustion; denitrification during combustion is based on The low-nitrogen combustion technologies adopted for the formation mechanism of NO mainly include: low-oxygen combustion, air staged combustion, fuel staged combustion, flue gas recirculation, etc. The main mechanism of this technology is to arrange the burners vertically to form redox, main reduction, Burn out three areas, for the four-corner tangential combustion boiler, two areas near the wall area and the central area can also be formed by horizontal double-area arrangement, so as to realize the division, classification, low-temperature and low-oxygen combustion of fuel and air distribution in the furnace, and reduce coal consumption. NO production during powder combustion.
2. Existing problems and cause analysis after the application and transformation of low-nitrogen combustion technology
The application of low-nitrogen combustion technology in a large number of coal-fired boilers in power stations has proved that this technology is very effective in reducing the amount of NO produced. However, in actual work, due to the different types of coal used in boilers and the different models of boilers, the amount of NO produced is also different, and the problems that arise are also different.
2.1 Increase in ash and slag combustibles, resulting in reduced furnace efficiency
After retrofitting the low-nitrogen burner, the production of NO decreased a lot, but when the same type of coal was used, the fly ash combustibles also increased significantly. The main reason is that the low-nitrogen combustion technology uses low-temperature and low-oxygen combustion methods, the temperature of the main combustion zone will drop more, and whether the pulverized coal catches fire will be controlled and delayed, and the oxygen content in the fire zone will be reduced, so that the pulverized coal will burn. The capacity of embers decreases, the burning process is lengthened, and the combustibles of fly ash and slag increase. Some boilers changed the area of the primary and secondary air nozzles of the burner and the area of the burner air nozzle during the transformation, resulting in a delay in the mixing of the primary air and the secondary air, which is not conducive to the ignition and combustion of the pulverized coal airflow.
2.2 The steam parameters deviate from the design value, the desuperheating water volume of the superheater increases or the reheater overheats
After the boiler adopts air-staged low-nitrogen combustion technology transformation, on the one hand, the combustion delays, the flame center moves up, the smoke temperature at the furnace outlet rises, and the superheated steam temperature and reheated steam temperature of the boiler rise. The problem of temperature exceeding the design value is exacerbated, and the amount of desuperheating water for overheating and reheating increases. On the other hand, the temperature in the main combustion zone is reduced, and the temperature distribution in the furnace is more uniform. For the serious contamination and slagging of the water wall of the original furnace, it will be improved. The temperature rise of the reheater decreases, and the original problem of superheated steam temperature and low reheated steam temperature cannot reach the super design value.
After the transformation of low-nitrogen combustion technology, there are many problems caused by the increase of the amount of desuperheating water in the boiler superheater. Due to the longer process of pulverized coal combustion and the use of exhaust air, the temperature of the flue gas at the furnace outlet becomes higher. At this time As the temperature of the furnace decreases, the radiation heat absorption of the furnace water wall will decrease, and the heat absorption of the convective heating surface will increase, which will increase the amount of desuperheating water in the superheater.
2.3 The internal combustion environment of the boiler deteriorates, and the coal blending, air distribution, and combustion stability become lower
Due to the low-temperature and low-oxygen combustion, the temperature of the furnace decreases, and the pulverized coal will delay ignition in the low-temperature and oxygen-deficient environment, and the ability to burn to ashes will also be weakened, and the combustion environment in the boiler will become worse than before the transformation.
The coal blending and air distribution methods used before the boiler transformation are largely inapplicable, which will not only affect various indicators of the boiler, but also reduce the ability of the boiler to burn stably at low load.
2.4 The adaptability of the boiler to the type of coal becomes poor
After the transformation of the low-nitrogen burner, the combustion is optimized and adjusted to a large extent, which can well match the NO emission level and the boiler economy. However, when the type of coal used by the boiler changes, it will break the balance between the boiler's economic indicators and environmental indicators at the beginning. If the coal with high calorific value and high volatility is used, the NO emission concentration will increase slightly, but it is easier to adjust and control; if the coal used is inferior or contains more moisture, the NO emission will be slightly reduced, but compared difficult to control.
3 Countermeasures for existing problems after boiler low-nitrogen burner transformation
At present, the transformation of boiler low-nitrogen burners in coal-fired power plants has not been fully completed, and at the same time, the problems in the application of this technology are gradually exposed. In view of the existing problems, the following solutions are proposed:
3.1 Adequate assessment before renovation
Various emission indicators of boilers are very important, especially NO emission concentration is closely related to coal type, boiler type selection, and burner type. For boilers in operation, the type of furnace has been determined. Benefits, the quality of coal used by boilers is mostly blended and inferior to the original design coal types. Therefore, before using low-nitrogen combustion technology for transformation, it is necessary to fully evaluate the existing main types of coal and commonly used coal types in the boiler. In the feasibility demonstration, due to the improper selection of coal types, there are many cases where the NO emission reduction effect after transformation is not obvious and new problems arise. The second is to conduct a thorough test on the boilers in operation to fully evaluate the combustion performance and steam in the operation of the boilers. Parameters, heating surface wall temperature, coking and slagging, operation adjustment, thermal automation, etc., put forward scientific and reasonable transformation expectations, balance boiler economic indicators and environmental indicators, gradually solve existing problems, and prevent new problems from appearing.
3.2 Optimal adjustment, using scientific combustion methods
After the boiler low-nitrogen burner has been modified, the type of the burner has been determined, but under different conditions of the boiler, the amount of NO produced by burning different coal types will also be different. It can be seen that the operating mode of the boiler plays a leading role . Therefore, in order to reduce the emission of NO, people must optimize and adjust the combustion method, and take into account the operating economy to the greatest extent on the premise of meeting the environmental protection emission requirements.
3.2.1 Layered coal blending and mixed combustion in the boiler
In the case of ensuring that the concentration of the exhaust gas meets the environmental protection requirements and the combustion is stable, the most economical type of coal should be used, and the layered combustion in the furnace can not only ensure the stability of the boiler but also control the generation of NO.
3.2.2 Automatic control of optimized thermal engineering
After the low-nitrogen technology is used to transform, the fuel combustion time in the boiler becomes longer, so the thermal control system and control curve should be optimized and adjusted. According to the problems in the actual work of the boiler, the required control curve and control system should be optimized to improve its response ability under load.
3.2.3 Continuous combustion optimization adjustment
After the boiler low-nitrogen combustion technology transformation, in addition to the coal type, it is mainly related to the operation mode of the boiler. To explore and optimize, according to the boiler combustion optimization and adjustment test, when the coal quality has changed greatly, it usually takes nearly two months of adjustment to find out the law that both environmental protection emission indicators and operating economic indicators are taken into account. Therefore, continuous combustion optimization and adjustment is essential.
Can meet the current national environmental protection requirements. Therefore, coal power generation enterprises must carry out boiler optimization and transformation in accordance with the pollutant emission limits required by the state, and actively adopt a series of environmental protection measures. Boiler renovation adopts low-nitrogen combustion technology, which can not only greatly reduce NO emissions, but also improve the stability and safety of boiler work, while saving costs.
1. Current status of NO governance
The harm of NO, the formation mechanism of NO in the combustion process of coal-fired power generation and the technology of reducing NO have been fully researched at home and abroad, which can be divided into three types: thermal NO, fuel NO and rapid NO; among them, fuel NO Type NO accounts for about 80-90%, and is the main object of various low NO technology control; followed by thermal type, mainly due to local high temperature in the furnace, and rapid NO generation is very small. The control methods of NO can be divided into treatment before combustion, treatment during combustion and treatment after combustion. Denitrification before combustion refers to converting fuel into low-nitrogen fuel. The technology is complicated, difficult and costly, so it is currently in the research stage; denitrification during combustion mainly includes: one is to inhibit the formation of NO in combustion, and the other is to reduce the formation of NO NO; denitrification after combustion mainly refers to flue gas denitrification: including selective catalytic reduction method, selective non-catalytic reduction method, etc.
At present, it is recognized by everyone and has been widely used in various coal-fired unit boilers. The method of reducing NO is mainly the low-nitrogen combustion technology for denitrification during combustion and the flue gas denitrification technology for denitrification after combustion; denitrification during combustion is based on The low-nitrogen combustion technologies adopted for the formation mechanism of NO mainly include: low-oxygen combustion, air staged combustion, fuel staged combustion, flue gas recirculation, etc. The main mechanism of this technology is to arrange the burners vertically to form redox, main reduction, Burn out three areas, for the four-corner tangential combustion boiler, two areas near the wall area and the central area can also be formed by horizontal double-area arrangement, so as to realize the division, classification, low-temperature and low-oxygen combustion of fuel and air distribution in the furnace, and reduce coal consumption. NO production during powder combustion.
2. Existing problems and cause analysis after the application and transformation of low-nitrogen combustion technology
The application of low-nitrogen combustion technology in a large number of coal-fired boilers in power stations has proved that this technology is very effective in reducing the amount of NO produced. However, in actual work, due to the different types of coal used in boilers and the different models of boilers, the amount of NO produced is also different, and the problems that arise are also different.
2.1 Increase in ash and slag combustibles, resulting in reduced furnace efficiency
After retrofitting the low-nitrogen burner, the production of NO decreased a lot, but when the same type of coal was used, the fly ash combustibles also increased significantly. The main reason is that the low-nitrogen combustion technology uses low-temperature and low-oxygen combustion methods, the temperature of the main combustion zone will drop more, and whether the pulverized coal catches fire will be controlled and delayed, and the oxygen content in the fire zone will be reduced, so that the pulverized coal will burn. The capacity of embers decreases, the burning process is lengthened, and the combustibles of fly ash and slag increase. Some boilers changed the area of the primary and secondary air nozzles of the burner and the area of the burner air nozzle during the transformation, resulting in a delay in the mixing of the primary air and the secondary air, which is not conducive to the ignition and combustion of the pulverized coal airflow.
2.2 The steam parameters deviate from the design value, the desuperheating water volume of the superheater increases or the reheater overheats
After the boiler adopts air-staged low-nitrogen combustion technology transformation, on the one hand, the combustion delays, the flame center moves up, the smoke temperature at the furnace outlet rises, and the superheated steam temperature and reheated steam temperature of the boiler rise. The problem of temperature exceeding the design value is exacerbated, and the amount of desuperheating water for overheating and reheating increases. On the other hand, the temperature in the main combustion zone is reduced, and the temperature distribution in the furnace is more uniform. For the serious contamination and slagging of the water wall of the original furnace, it will be improved. The temperature rise of the reheater decreases, and the original problem of superheated steam temperature and low reheated steam temperature cannot reach the super design value.
After the transformation of low-nitrogen combustion technology, there are many problems caused by the increase of the amount of desuperheating water in the boiler superheater. Due to the longer process of pulverized coal combustion and the use of exhaust air, the temperature of the flue gas at the furnace outlet becomes higher. At this time As the temperature of the furnace decreases, the radiation heat absorption of the furnace water wall will decrease, and the heat absorption of the convective heating surface will increase, which will increase the amount of desuperheating water in the superheater.
2.3 The internal combustion environment of the boiler deteriorates, and the coal blending, air distribution, and combustion stability become lower
Due to the low-temperature and low-oxygen combustion, the temperature of the furnace decreases, and the pulverized coal will delay ignition in the low-temperature and oxygen-deficient environment, and the ability to burn to ashes will also be weakened, and the combustion environment in the boiler will become worse than before the transformation.
The coal blending and air distribution methods used before the boiler transformation are largely inapplicable, which will not only affect various indicators of the boiler, but also reduce the ability of the boiler to burn stably at low load.
2.4 The adaptability of the boiler to the type of coal becomes poor
After the transformation of the low-nitrogen burner, the combustion is optimized and adjusted to a large extent, which can well match the NO emission level and the boiler economy. However, when the type of coal used by the boiler changes, it will break the balance between the boiler's economic indicators and environmental indicators at the beginning. If the coal with high calorific value and high volatility is used, the NO emission concentration will increase slightly, but it is easier to adjust and control; if the coal used is inferior or contains more moisture, the NO emission will be slightly reduced, but compared difficult to control.
3 Countermeasures for existing problems after boiler low-nitrogen burner transformation
At present, the transformation of boiler low-nitrogen burners in coal-fired power plants has not been fully completed, and at the same time, the problems in the application of this technology are gradually exposed. In view of the existing problems, the following solutions are proposed:
3.1 Adequate assessment before renovation
Various emission indicators of boilers are very important, especially NO emission concentration is closely related to coal type, boiler type selection, and burner type. For boilers in operation, the type of furnace has been determined. Benefits, the quality of coal used by boilers is mostly blended and inferior to the original design coal types. Therefore, before using low-nitrogen combustion technology for transformation, it is necessary to fully evaluate the existing main types of coal and commonly used coal types in the boiler. In the feasibility demonstration, due to the improper selection of coal types, there are many cases where the NO emission reduction effect after transformation is not obvious and new problems arise. The second is to conduct a thorough test on the boilers in operation to fully evaluate the combustion performance and steam in the operation of the boilers. Parameters, heating surface wall temperature, coking and slagging, operation adjustment, thermal automation, etc., put forward scientific and reasonable transformation expectations, balance boiler economic indicators and environmental indicators, gradually solve existing problems, and prevent new problems from appearing.
3.2 Optimal adjustment, using scientific combustion methods
After the boiler low-nitrogen burner has been modified, the type of the burner has been determined, but under different conditions of the boiler, the amount of NO produced by burning different coal types will also be different. It can be seen that the operating mode of the boiler plays a leading role . Therefore, in order to reduce the emission of NO, people must optimize and adjust the combustion method, and take into account the operating economy to the greatest extent on the premise of meeting the environmental protection emission requirements.
3.2.1 Layered coal blending and mixed combustion in the boiler
In the case of ensuring that the concentration of the exhaust gas meets the environmental protection requirements and the combustion is stable, the most economical type of coal should be used, and the layered combustion in the furnace can not only ensure the stability of the boiler but also control the generation of NO.
3.2.2 Automatic control of optimized thermal engineering
After the low-nitrogen technology is used to transform, the fuel combustion time in the boiler becomes longer, so the thermal control system and control curve should be optimized and adjusted. According to the problems in the actual work of the boiler, the required control curve and control system should be optimized to improve its response ability under load.
3.2.3 Continuous combustion optimization adjustment
After the boiler low-nitrogen combustion technology transformation, in addition to the coal type, it is mainly related to the operation mode of the boiler. To explore and optimize, according to the boiler combustion optimization and adjustment test, when the coal quality has changed greatly, it usually takes nearly two months of adjustment to find out the law that both environmental protection emission indicators and operating economic indicators are taken into account. Therefore, continuous combustion optimization and adjustment is essential.