Cause Analysis Of Application And Transformation Of Low NOx Combustion Technology
Cause Analysis Of Application And Transformation Of Low NOx Combustion Technology
In recent years, China’s environmental pollution has been serious. Due to the strict requirements of the national environmental protection policy, the power enterprises attach great importance to energy conservation and emission reduction and have formulated and implemented reasonable and effective measures. Adopting low nitrogen combustion technology to transform the boiler, can not only greatly reduce no emissions, but also improve the stability and safety of the boiler work, and save costs. However, there will be some problems after the boiler operation. Analyze the causes, find out scientific solutions, improve the stability of the boiler combustion, and ensure the economic operation of the boiler.
1、 Current situation of no treatment
At present, the harm of no, the generation mechanism of no, and the technology of reducing NO in the combustion process of coal-fired power generation have been comprehensively studied at home and abroad, mainly including 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 various low no process control; The second is thermal type, which is mainly caused by local high temperature in the furnace, and the amount of rapid type no is 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. Its process is complex and its cost is high. At present, it is still in the research stage; Combustion denitrification methods mainly include: first, inhibiting the generation of no; second, reducing the generated no; nitrogen after combustion is mainly flue gas denitrification: including selective catalytic reduction method and selective noncatalytic reduction method.
No reduction method is a widely recognized method to reduce no, mainly including low nitrogen combustion technology for denitrification during combustion and flue gas denitrification technology for denitrification after combustion; According to the generation mechanism of no, low nitrogen combustion technology is adopted 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 the vertical arrangement. For four corner tangential combustion boilers, two zones of near wall zone and central zone can also be formed through horizontal double zone arrangement, so as to realize the division Staged, low temperature and low oxygen combustion to reduce no generation during pulverized coal combustion.
2、 Analysis of problems and causes after application and transformation of low nitrogen combustion technology
Low nitrogen combustion technology is adopted in large-scale thermal power boilers. The practice shows that this technology is very effective to reduce the production of nitric oxide. However, in actual work, due to the different coal types and boiler models used in the boiler, there are differences in no production due to the different types of coal used, and the resulting problems are also different.
1. The increase of ash and combustibles leads to a decrease in furnace efficiency
After the transformation of a low nitrogen burner, the production of no will be greatly reduced, but when the same coal is used, the increase of fly ash combustibles is also large. The main reason is that the low temperature and low oxygen combustion mode is adopted, which makes the temperature in the main combustion area drop more, and whether the ignition of the pulverized coal is controlled and delayed, and reduces the oxygen content in the ignition area so that the combustibility of the pulverized coal is decreased, the combustion process is prolonged, and the combustibles of fly ash and slag are increased. Part of the transformation of the burner changes the area of the primary and secondary air nozzles, which delays the mixing of the primary air and the secondary air, and is not conducive to the ignition and combustion of the pulverized coal flow.
2. The steam parameters deviate from the design value, the cooling water volume of the superheater increases or the temperature of the reheater is too high
On the one hand, due to the delay of combustion, the flame center moves upward, the flue gas temperature at the furnace outlet rises, the superheated steam temperature of the boiler rises and the reheated steam temperature rises. On the other hand, the problem that the original superheated steam temperature and reheated steam temperature exceed the design value is aggravated, and the desuperheating water volume of the superheated and reheated steam increases; In addition, as the temperature in the main combustion zone decreases and the furnace temperature distribution is more uniform, the serious fouling and slagging of the original furnace water-cooled wall can be improved, 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 original problem of superheated 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 of desuperheating water volume of the boiler superheater. Due to the extension of the 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 absorbed by the heat of the water-cooled wall in the furnace decreases. On the heating surface forming convection, the absorbed heat increases, and the desuperheating water volume of the superheater increases.
3. The combustion environment in the boiler deteriorates, and the coal and air distribution reduce the combustion stability
Due to the low temperature and low oxygen combustion, the furnace temperature is reduced. In the low temperature and low oxygen environment, the combustion of pulverized coal is delayed, and the ability of combustion to form ash is also weakened. The combustion environment in the furnace is worse than that before the transformation.
The coal and air distribution methods used before the boiler transformation are basically not applicable, which not only affects the boiler’s various indicators but also reduces the boiler’s ability to stable combustion under low load.
4. The adaptability of the boiler to coal is poor
Through the transformation of the low nitrogen type burner, a large number of optimized combustion adjustments have been carried out to make the boiler adapt to the no emission level to a great extent. After the change of the coal-fired boiler, the balance between the economic indicators and environmental protection indicators at the beginning of the boiler will be broken. When using high calorific value and high volatile coal, although the no emission concentration slightly increases, it is easier to adjust; If the coal type used is poor or the water content is high, the no emission can be slightly reduced, but it is difficult to control.