The nitrogen content in the combustion gas is relatively low, therefore, fuel-type NOx is not its main control type.
Thermal NOx refers to the oxidation of N2 in the combustion air at high temperatures to generate NOx. The generation mechanism of thermal NOx is generally described by the Zeldovich mechanism: when the temperature is below 1500°C, the amount of thermal NOx generated is very small; above 1500°C, for every 100°C increase in temperature, the reaction rate increases by 6 to 7 times. In actual combustion processes, due to the uneven temperature distribution in the combustion chamber, if there are local high-temperature areas, more NOx will be generated in these regions, which may play a key role in the overall NOx generation in the combustion chamber.
Prompt NOx is generated rapidly in the reaction zone when hydrocarbon fuels are burned in a fuel-rich condition. In actual combustion processes, various factors change independently, and many parameters are in constant flux. Even the combustion of the simplest gaseous fuels must go through the mixing of fuel and air, the combustion producing flue gas, until it finally exits the furnace. Parameters such as the temperature in the furnace, the degree of mixing of fuel and air, and the residence time of flue gas in the furnace, all of which significantly affect NOx emissions, are continuously changing.
After the fuel and air mixture enters the furnace, the temperature of the mixture gas flow quickly rises due to convection and radiation heating from the surrounding high-temperature flue gas. When the ignition temperature is reached, the fuel begins to burn, and the temperature sharply rises to near adiabatic levels. At the same time, due to the convective and radiative heat exchange between the flue gas and the surrounding medium, the temperature gradually decreases until it matches the temperature of the surrounding medium, meaning the flue gas cools while flowing through the entire furnace. Thus, it can be seen that the flame temperature distribution in the furnace is actually uneven. Typically, the highest temperature is found at a certain distance from the burner outlet, with lower temperatures before and after it, indicating the presence of local high-temperature areas. Since the temperature in this area is much higher than the average temperature in the furnace, it has a significant impact on the amount of NOx generated: the higher the temperature, the more NOx is produced. Therefore, in the furnace, to suppress NOx generation, it is necessary not only to lower the average temperature in the furnace but also to ensure a uniform temperature distribution to avoid local high temperatures.
