The core logic of low-NOx emission burners, which reduce NOx emissions by adjusting airflow, lies in precisely controlling the oxygen concentration and temperature distribution in the combustion zone, thereby suppressing the formation of thermal and fuel-based NOxes. This process requires combining technologies such as staged air combustion, flue gas recirculation, and rich-lean combustion. Through dynamic optimization of airflow distribution, the combustion process alternates between oxygen-deficient and oxygen-rich phases, ultimately achieving the goal of reducing emissions.
Staged air combustion is one of the fundamental technologies for adjusting airflow in low-NOx burners. Its principle is to divide the air required for combustion into primary and secondary air. Primary air is supplied in a lower proportion at the beginning of combustion, creating an oxygen-deficient combustion environment. At this time, due to insufficient oxygen concentration, the nitrogen in the fuel is mainly converted into harmless nitrogen gas, rather than NOxes. Secondary air is supplied in the middle and later stages of combustion to ensure complete combustion. By adjusting the ratio of primary to secondary air, the oxygen concentration at the beginning of combustion can be controlled, avoiding a surge in thermal NOxes caused by localized high temperatures. For example, after adjusting the primary air ratio, a waste incineration plant experienced a significant reduction in furnace outlet temperature, resulting in a decrease in NOx emissions.
Flue gas recirculation (FGR) technology further dilutes the oxygen concentration and lowers the flame temperature by reintroducing a portion of the low-temperature flue gas into the combustion zone. After mixing with fresh air, the recirculated flue gas is sent into the furnace as secondary or primary air. Its high heat capacity and low oxygen content effectively suppress peak combustion temperatures. For example, after adopting FGR, a coal-fired power plant experienced a significant decrease in nitrogen oxide emissions, while boiler efficiency improved due to reduced heat loss. The key to this technology lies in controlling the recirculation rate, which needs to be dynamically adjusted according to fuel characteristics and combustion conditions to avoid unstable combustion or excessive carbon monoxide emissions due to excessively low oxygen concentration.
Rich-lean combustion technology achieves non-uniform mixing of fuel and air by adjusting the airflow. Some fuel burns richly under oxygen-deficient conditions, while another part burns leanly under oxygen-rich conditions, with the overall airflow remaining constant. In the richer region, insufficient oxygen leads to a lower combustion temperature and reduced nitrogen oxide formation; in the leaner region, although there is excess oxygen, the temperature is diluted by the richer region, limiting the formation of thermal nitrogen oxides. After applying this technology, a power plant experienced a significant reduction in nitrogen oxide emissions, and combustion efficiency improved due to fuel stratification. This technology requires a specially designed burner, such as a dual-channel nozzle or swirl vanes, to precisely control the fuel-air mixing ratio.
Airflow regulation also needs to work in conjunction with burner structural optimization. For example, a burner with a swirl vane design can change the air-fuel mixing path by adjusting the vane angle, shortening mixing time and improving temperature uniformity, reducing localized high-temperature areas. After applying this type of burner, a waste incineration plant saw a reduction in temperature fluctuations in the flame core area and a significant decrease in the initial nitrogen oxide concentration. Furthermore, an intelligent fuel identification module can analyze the fuel calorific value in real time and dynamically adjust the airflow and fuel ratio to ensure a smaller excess air coefficient deviation, further reducing nitrogen oxide generation caused by excess air.
The "3T+E" principle (temperature, time, turbulence, excess air) in combustion process control also relies on airflow regulation. Infrared thermometers and intelligent control systems allow for real-time adjustment of feeding speed and airflow, keeping furnace temperature within a safe range and preventing a surge in thermal nitrogen oxides caused by high temperatures. Simultaneously, adjusting secondary air velocity optimizes flue gas residence time in the combustion chamber, ensuring complete combustion of unburned products and reducing the formation of fuel-based nitrogen oxide precursors. For example, after a plant adopted laser Doppler velocimeters to monitor the furnace flow field, the flue gas residence time deviation decreased, further reducing nitrogen oxide emission concentrations.
Airflow regulation in low-NOx burners also needs to be coordinated with end-of-pipe treatment technologies. For instance, selective non-catalytic reduction (SNCR) technology injects a reducing agent into the high-temperature zone of the furnace, and its efficiency is significantly affected by the amount of ammonia injected and the temperature distribution. Optimizing the oxygen concentration and temperature field in the combustion zone through airflow regulation can improve the denitrification efficiency of SNCR and reduce the risk of ammonia escape. Similarly, selective catalytic reduction (SCR) technology requires operation within a specific temperature window; airflow regulation can assist in controlling flue gas temperature to ensure catalyst activity and denitrification efficiency.
Low-NOx emission burners achieve NOx reduction through airflow regulation, requiring the comprehensive application of technologies such as air staging, flue gas recirculation, and rich-lean combustion, combined with burner structural optimization and intelligent control systems. This process not only demands precise control of oxygen concentration and temperature distribution but also requires consideration of combustion efficiency and stability, ultimately achieving clean emission targets through the synergy of multiple technologies.
