Analysis of the Reasons for the Application and Transformation of Low Nitrogen Combustion Technology

Analysis of the Reasons for the Application and Transformation of Low Nitrogen Combustion Technology
In recent years, China has suffered from severe environmental pollution. Due to the strict requirements of national environmental protection policies, power companies attach great importance to energy conservation and emission reduction, and have formulated and implemented reasonable and effective measures. By using low nitrogen combustion technology to transform boilers, not only can NO emissions be significantly reduced, but also the stability and safety of boiler operation can be improved, and costs can be saved. However, there may be some problems after the operation of the boiler. Analyze the causes, find scientific solutions, improve the stability of boiler combustion, and ensure the economic operation of the boiler.
1、 The current situation of NO governance.
At present, comprehensive research has been conducted both domestically and internationally on the hazards of NO, the mechanism of NO generation during coal-fired power generation combustion, and the technology for reducing NO. There are mainly three types: thermal NO, fuel NO, and rapid NO; Among them, fuel type NO accounts for about 80-90% and is the main object of control for various low NO processes; The second type is thermal type, mainly caused by local high temperature inside the furnace, and the quantity of rapid NO production is very small. NO control methods can be divided into pre combustion treatment, in combustion treatment, and post combustion treatment. Pre combustion denitrification refers to the conversion of fuel into low nitrogen fuel, which has a complex process and high cost, and is currently in the research stage; The combustion denitrification method mainly includes: firstly, inhibiting the generation of NO, secondly, reducing the generated NO. The nitrogen after combustion is mainly used for flue gas denitrification, including selective catalytic reduction method, selective non catalytic reduction method, etc.
The NO reduction method is currently widely recognized as a way to reduce NO, mainly through low nitrogen combustion technology for denitrification during combustion and flue gas denitrification technology for denitrification after combustion; According to the mechanism of NO generation, low nitrogen combustion technology is used for denitrification during combustion, mainly including low oxygen combustion, air staged combustion, air staged combustion, flue gas recirculation, etc. The main mechanism is to form three zones of oxidation-reduction, main reduction, and burnout through vertical arrangement. For a four corner tangential combustion boiler, two zones can also be formed by horizontal double zone arrangement, namely the near wall zone and the center zone, to achieve fuel and air distribution zoning in the furnace Graded, low-temperature, and low oxygen combustion to reduce the production of NO during coal powder combustion.
2、 Analysis of the problems and causes that arise after the application and transformation of low nitrogen combustion technology.
The use of low nitrogen combustion technology in large-scale thermal power boilers has been proven to be highly effective in reducing the production of nitric oxide. However, in practical work, due to the differences in coal types and boiler models used in boilers, the amount of NO produced varies depending on the type of coal used, and the resulting problems also vary.
1. The increase in ash and combustibles has led to a decrease in furnace efficiency.
After retrofitting the low nitrogen burner, the production of NO will significantly decrease, but when using the same coal type, the combustible content of fly ash will also increase significantly. The main reason for this is the use of low-temperature and low oxygen combustion, which reduces the temperature of the main combustion zone significantly, controls and delays the ignition of coal powder, and reduces the oxygen content in the ignition zone, resulting in a decrease in the combustibility of coal powder, a prolonged combustion process, and an increase in combustible materials such as fly ash and slag. Partial modifications to the burner have changed the area of the primary and secondary air nozzles, causing a delay in the mixing of primary and secondary air, which is not conducive to the ignition and combustion of coal powder airflow.
2. The steam parameters deviate from the design values, the cooling water volume of the superheater increases, or the temperature of the reheater is too high.
The transformation of the boiler with air classification and low nitrogen combustion technology is carried out. On the one hand, due to combustion delay, the flame center moves upward, and the flue gas temperature at the furnace outlet increases, resulting in an increase in the superheated steam temperature and reheated steam temperature of the boiler. This exacerbates the problem of the original superheated steam temperature and reheated steam temperature exceeding the design value, and increases the amount of water for superheated and reheated cooling; In addition, due to the decrease in temperature in the main combustion zone, the furnace temperature distribution is more uniform, which can improve the serious fouling and slagging situation of the original furnace water-cooled wall. The heat absorption of the water-cooled wall increases, the flue gas temperature at the furnace outlet decreases, the superheater temperature rises, and the reheater temperature rises. For the existing problem of overheated steam temperature, the problem of low reheated steam temperature cannot reach the design value.
After the transformation of low nitrogen combustion technology, there are many problems with the increase in the amount of water for the boiler superheater to reduce temperature. Due to the prolongation of coal powder combustion process and the use of exhaust air, the outlet flue gas temperature of the furnace increases. At this time, the temperature inside the furnace decreases, and the radiation absorption of heat by the water-cooled wall inside the furnace decreases. On the heating surface that forms convection, the absorption of heat increases, resulting in an increase in the amount of water for the superheater to reduce temperature.
3. The combustion environment inside the boiler deteriorates, leading to a decrease in coal blending, air distribution, and combustion stability.
Due to the low temperature and low oxygen combustion, the furnace temperature will be reduced. In the low temperature and oxygen deficient environment, the pulverized coal combustion will be delayed, and the ability to burn ash will also be weakened. The combustion environment in the furnace will become worse than before.
The coal and air distribution methods used before boiler renovation are basically not applicable, which not only affects the various indicators of the boiler, but also reduces the ability of the boiler to maintain stable combustion at low loads.
4. The adaptability of boilers to coal types has deteriorated.
Through the transformation of low nitrogen type burners, a large amount of optimization and adjustment of combustion have been carried out, making the boiler largely adapt to the level of NO emissions. After the change, the use of coal in boilers will break the balance between the initial economic and environmental indicators of the boiler. When using high calorific value and high volatility coal, although the NO emission concentration slightly increases, it is easier to adjust; If the coal used is of poor quality or has a high moisture content, the NO emissions can be slightly reduced, but it is difficult to control.