Application of Low Nitrogen Combustion Technology and Problems After Transformation

Application of Low Nitrogen Combustion Technology and Problems After Transformation
The extensive application of low nitrogen combustion technology in coal-fired boilers in power plants has proven that NOx reduction is effective and obvious. However, due to the different types of coal used in boilers, the furnace type and NOx emission levels also vary. The emission reduction effect and problems caused by application are also different; Among them, the NOx emission level of the tangential combustion boiler itself is relatively low, and the NOx emission reduction effect after transformation is good, while other impacts are also relatively small. Secondly, the "W" type flame combustion boiler is relatively poor. The specific problems and reasons are as follows:
1. The increase of combustible materials in ash and slag leads to a decrease in furnace efficiency:
After the transformation of the low nitrogen burner, although NOx has been greatly reduced, even with the use of the same coal, the combustible matter of fly ash has greatly increased. The main reason is that low nitrogen combustion technology uses low-temperature and low oxygen combustion, resulting in a significant decrease in temperature in the main combustion zone. It controls and delays the ignition of coal powder and reduces the amount of oxygen in the ignition area, thereby reducing the combustion ability of coal powder and prolonging the combustion process. Fly ash and slag combustibles increase. In some modifications, changes have occurred in the primary and secondary air nozzles of the burner, as well as in the area of the burnt out air nozzle, resulting in delayed mixing of secondary air and primary air, which is not conducive to the ignition and combustion of the coal powder airflow. According to the experimental data of the boiler renovation, the combustible content of fly ash in tangential combustion boilers increased by 0.5-1%, the combustible content of fly ash in counter combustion boilers increased by 1-1.5%, and the combustible content of fly ash in "W" flame combustion boilers increased by 2-4%, affecting a decrease in boiler efficiency of 0.4-1.6%.
2. After the transformation of low nitrogen combustion technology, the amount of water for reducing temperature in the boiler superheater has increased, and there are many problems. Due to the prolonged combustion process of coal powder and the use of overheated air, the temperature of flue gas at the furnace mouth increases; At the same time, the furnace temperature decreases. The radiation heat absorption of the water wall inside the furnace decreases, and the heat absorption share of the convective heating surface increases, resulting in an increase in the water volume of the desuperheater.
3. The steam parameters deviate from the design value, the superheated water consumption for superheated reheating increases, the screen screen overheats or the reheater overheats
After the boiler burner adopts the air staged low nitrogen combustion technology, on the one hand, the combustion is delayed, the flame center moves upward, the flue gas temperature at the furnace mouth rises, the boiler Superheated steam temperature rises, and the reheated steam temperature rises. The problem of hot steam temperature exceeding the design value has worsened, and the amount of water for overheating and reheating has increased. On the other hand, the temperature in the main combustion zone decreases, and the temperature distribution in the furnace is more uniform. If the pollution and slagging of the original furnace water wall are severe, it will be improved. The heat absorption of the water-cooled wall increases, the temperature of the flue gas at the furnace inlet decreases, and the temperature rise of the superheater, reheater, and reheater decreases. The original problems such as overheating of the steam and low temperature of the reheated steam cannot meet the design value.
4. The performance of thermal automatic control decreases, the steam parameters fluctuate greatly, and the AGC response speed of the unit is slow
After the transformation of low nitrogen burners, under the same coal type and load, the combustion reaction of fuel in the furnace slows down, and the flue gas temperature distribution and heat absorption of heating surfaces at all levels change. The specific manifestation is slow and moderate thermal automatic control. The obvious increase in overshoot phenomenon leads to significant fluctuations in steam parameters; For some regions, the AGC Responsivity of the units is required to be high, and the AGC response speed is often slow, which cannot meet the requirements of the power grid. The main reason is that the control system settings and control curves of thermal engineering have not been optimized and adjusted accordingly, such as the original static and dynamic load coal volume control curves, the decoupling control system for the cold and hot air doors of the crushing system, and the automatic control system for superheated water; Boiler main control feedforward system with primary frequency regulation.