Cause analysis of problems in application and transformation of low nitrogen combustion technology
In recent years, China's environmental pollution is serious. Due to the strict requirements of the national environmental protection policy, electric power enterprises attach great importance to energy conservation and emission reduction, and have formulated and implemented reasonable and effective measures. The use of low-nitrogen combustion technology to retrofit boilers can not only significantly reduce the emission of NO, but also improve the stability and safety of boiler work and save costs. However, there will be some problems after boiler operation. Analyze the causes and find scientific solutions to improve the stability of boiler combustion and ensure the economic operation of the boiler.
1、 The current situation of NO governance.
At present, the harm of NO, the mechanism of NO generation and the technology of reducing NO in the combustion process of coal-fired power generation have been comprehensively studied at home and abroad. There are three main types of NO: thermal NO, fuel NO and fast NO; Among them, fuel type NO accounts for about 80-90%, which is the main object of various low NO process control; The second is thermal type, which is mainly caused by local high temperature in the furnace. The quantity of fast type NO is very small. NO control methods can be divided into pre-combustion treatment, in-combustion treatment and post-combustion treatment. Pre-burning denitrification refers to the conversion of fuel into low-nitrogen fuel, which is still in the research stage due to its complex process and high cost; Combustion denitrification methods mainly include: first, inhibiting the generation of NO; second, reducing the generated NO; the nitrogen after combustion is mainly flue gas denitrification, including selective catalytic reduction method, selective non-catalytic reduction method, etc.
At present, the NO reduction method is widely recognized as a method to reduce NO, mainly including low nitrogen combustion technology for nitrogen removal during combustion and flue gas denitration technology for nitrogen removal after combustion; According to the generation mechanism of NO, low nitrogen combustion technology is adopted for nitrogen removal in combustion, mainly including low oxygen combustion, air staged combustion, air staged combustion, flue gas recirculation, etc. Its main mechanism is to form three zones of oxidation reduction, main reduction and burnout through longitudinal arrangement. For tangentially fired boilers, two zones can also be formed, namely, near wall zone and central zone, through transverse double-zone arrangement, so as to realize the division of fuel and air distribution in the furnace Staged, low temperature and low oxygen combustion can reduce the production of NO during pulverized coal combustion.
2、 Problems and cause analysis after application and transformation of low nitrogen combustion technology.
Low-nitrogen combustion technology is used in large thermal power boilers. Practice has proved that this technology is very effective in reducing the production of nitric oxide. However, in practical work, due to the different types of coal and boiler models used in the boiler, the amount of NO production is different due to the different types of coal used, and the resulting problems are also different.
1. The increase of ash and combustibles led to the decrease of furnace efficiency.
After the transformation of the low nitrogen burner, the production of NO will be greatly reduced, but when the same coal is used, the combustible content of fly ash will also increase significantly. The main reason is that the low temperature and low oxygen combustion mode is adopted, which makes the temperature of the main combustion zone drop more, whether the pulverized coal ignites is controlled and delayed, and reduces the oxygen content in the ignition zone, which reduces the combustibility of pulverized coal, prolongs the combustion process, and increases fly ash and slag combustibles. Some modifications to the burners have changed the area of the primary and secondary air nozzles, delayed the mixing of primary and secondary air, and are not conducive to the ignition and combustion of pulverized coal airflow.
2. The steam parameters deviate from the design value, the cooling water volume of superheater increases, or the reheater temperature is too high.
On the one hand, due to the combustion delay, the flame center moves upward, the flue gas temperature at the furnace outlet increases, the superheated steam temperature of the boiler increases, and the reheated steam temperature increases. The problem of the existing superheated steam temperature and reheated steam temperature exceeding the design value is aggravated, and the amount of superheated and reheated desuperheating water increases; In addition, as the temperature of the main combustion zone decreases and the furnace temperature distribution is more uniform, the fouling and slagging condition of the original furnace water wall can be improved, the heat absorption of the water wall increases, the flue gas temperature at the outlet of the furnace decreases, the superheater temperature rises, and the reheater temperature rises. For the existing problem of superheated steam temperature, the reheated steam temperature is low, and the design value cannot be reached.
After the transformation of low nitrogen combustion technology, there are many problems with the increase of desuperheating water volume of boiler superheater. Due to the prolongation of pulverized coal combustion process and the use of burnout air, the flue gas temperature at the furnace outlet increases. At this time, the temperature in the furnace decreases, and the radiation absorption heat of water wall in the furnace decreases. On the heating surface that forms convection, the absorption heat increases, and the desuperheating water volume of superheater increases.
3. The combustion environment in the boiler is deteriorated, and the coal blending, air distribution and combustion stability are reduced.
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 the boiler transformation are basically not applicable, which not only affects the various indicators of the boiler, but also reduces the ability of the boiler to stabilize combustion under low load.
4. The adaptability of the boiler to the coal becomes poor.
Through the transformation of low nitrogen type burner, a large number of combustion optimization and adjustment have been carried out to make the boiler adapt to the NO emission level to a large extent. After the change of coal burning in boiler, the balance between economic indicators and environmental indicators at the beginning of boiler will be broken. When using high calorific value and high volatile coal, although the NO emission concentration rises slightly, it is easier to adjust; If the coal used is poor or has a high water content, the NO emission can be slightly reduced, but it is difficult to control.