There are primarily two methods to reduce NOx emissions: 1) denitrification during combustion, and 2) denitrification after combustion. Taking a 320MW coal-fired boiler as the research subject, the study discusses the characteristics of NOx formation during combustion, analyzes the main factors affecting NOx generation under different operating conditions, summarizes methods to control NOx during combustion, and aims to lower denitrification costs. The boiler employs technology imported from the American Combustion Engineering Company, utilizing balanced ventilation, a direct-impingement burner, and a four-corner circular combustion design. It features a medium-storage bin thermal wind powder feeding system, equipped with four steel ball mills. Denitrification is achieved through Selective Catalytic Reduction (SCR) and staged combustion technology. The unit control method is CCBF, with constant pressure operation above 230MW and sliding pressure operation below 230MW.
I. The Formation Mechanism of NOx
In NOx, NO accounts for over 90%, while NO2 makes up 5% to 10%. The NOx produced during coal combustion can be categorized into three types: thermal, fuel, and fast.
Thermal NOx is formed by the oxidation of nitrogen in the combustion air at high temperatures, increasing with the rise in reaction temperature. Key influencing factors include the temperature of the combustion reaction, oxygen concentration, and reaction time.
Fuel-type NOx is generated through the thermal decomposition of nitrogen compounds in the fuel during combustion, which is further oxidized. The thermal decomposition temperature of nitrogen is lower than the combustion temperature of coal powder, and fuel-type NOx is formed at temperatures between 600 to 800°C, accounting for 60 to 80% of the NOx generated during coal powder combustion. Additionally, there is a reduction reaction of NO, and the formation and reduction of NOx are related not only to combustion temperature and oxygen concentration but also to the characteristics of coal and the state of nitrogen compounds in coal.
The formation of rapid NOx is achieved through the collision of CH radicals produced by fuel with N2 molecules, resulting in the generation of HCN-type compounds, which are then further oxidized.
II. Characteristics of NOx Formation Under Different Operating Conditions
The generation of NOx during the combustion process of coal powder boilers shows significant differences under various operating conditions (with Shenhua coal and lean blending coal as the fuel types), such as load, excess air ratio, and the operation mode of the pulverizing system.
1. Load is divided into high load, low load, increasing load, and decreasing load, etc.
1) The research object is the operating condition with a high load of 320MW.
At a load of 320MW, the NOx level is higher than at 240MW; the main influencing factors include:
Furnace heat load
The level of furnace heat load affects both thermal and fuel-type NOx generation. As shown in Table 1, at high loads, the coal powder concentration is high, and the fuel air and auxiliary air temperatures are elevated, indicating a significantly higher furnace heat load compared to low loads.
The uniformity of furnace heat load also affects NOx generation. From the perspective of the steam pressure curve, instability in steam pressure during high load conditions increases the unevenness of local heat load in the furnace, thereby enhancing NOx formation.
b. Excess Air Ratio
Oxygen content is one of the main factors affecting NOx generation. At high loads, controlling the total airflow is essential, as a high furnace heat load and excessive NOx at the SCR inlet will increase ammonia dosing. Failing to adjust promptly can lead to excessive NOx emissions. The total airflow experiences a decline, resulting in a significant drop in NOx levels. However, the airflow cannot be too low, as low airflow not only increases heat loss from incomplete combustion and reduces boiler efficiency but also affects the stability of furnace combustion, potentially causing extinguishing. After the total airflow is decreased, it subsequently rises.
2) The low-load operating condition with 180MW as the research subject
From Table 1, it can be observed that the low load coal mill operates with fewer units, lower coal dust concentration, and lower wind temperature, resulting in a significantly lower furnace heat load compared to high load. Consequently, NOx levels are lower than during high load, but not by much, and even higher than when the load is at 240MW, with the main influencing factors being:
Excessive Air Ratio
To ensure stable combustion during low load conditions, the oxygen level must not be too low.
Chamber heat load
The upper burner has been shut down, causing the flame center to shift downward, effectively extending the combustion time of coal powder within the furnace. As shown in Table 1, the wind speed is notably lower during low load conditions, which also increases the proportion of the burnout zone. The lower furnace temperature suppresses the formation of NOx.
c. Bellows Differential Pressure
At low loads, the total air volume is low. To prevent a decrease in boiler efficiency, it is necessary to ensure complete combustion air volume. The low differential pressure in the secondary air box requires reducing the lower auxiliary air and fuel air to enhance the rigidity of the secondary air, thereby reducing the oxygen concentration in the volatiles ignition zone.
It is evident that the generation of NOx during the combustion of coal powder at low loads is a complex process involving the combined effect of multiple factors.
3) The study object is the process of load change from 240MW to 320MW and from 320MW to 180MW. During the load addition process, it is a sequence of first adding wind and then coal. In load reduction, coal is reduced first followed by wind. Therefore, during the initial stage of the load change process, the oxygen content increases, leading to an upward trend in NOx.
During the load change process, in addition to the oxygen content, the furnace thermal inertia is a major factor affecting NOx formation, especially pronounced during load reduction. From the load reduction curve, the NOx curve rises rapidly, and the faster the load is reduced, the faster the NOx increases. This is because during load reduction, some burners are shut down, leading to a decrease in coal dust concentration. However, the furnace temperature does not drop significantly in a short period of time, resulting in a significant increase in NOx formation.
2. Impact of Flour System Operation on NOx
The research subject is the change in the operation mode of the pulverizing system, with the unit load remaining unchanged at 300MW.
All other air doors remained unchanged, except for the DD layer auxiliary wind, which was reduced from 67% to 59%.
The 300MW load remains stable; the ABC three sets of pulverizing systems are in operation. During the period from Point A to Point B, the B pulverizing system was shut down and then restarted after Point B.
During the ABC operation, the total airflow was significantly higher than during the AC operation, with values before point E exceeding those between E and F. After the B pulverizing system was shut down, the total airflow entering the furnace was reduced, resulting in a significant drop in SCR inlet NOx levels from above 400 mg/Nm3 to below 400 mg/Nm3.
Re-commissioning the B Grinding System, NOx levels have again surged above 400mg/Nm³. This is primarily due to the enhanced combustion of unburnt coal powder and fine coal powder in the third wind, as well as the increased generation of NOx from the oxygen-enriched combustion. Additionally, the remaining gaseous nitrogen-containing substances in the flue gas and the nitrogen in the coke being oxidized to produce NOx.
Conclusion
Through the study of NOx generation characteristics under different operating conditions (with the coal type unchanged), exploring and adopting technologies to reduce NOx generation during the boiler combustion process will save ammonia injection, ensure NOx compliance emissions, and achieve good safety and economic benefits. When controlling NOx generation during combustion, attention should be paid to the following points:
1. High excess air ratio leads to increased NOx formation. During operation, monitor the oxygen content at the SCR inlet, employ low-oxygen combustion, but ensure sufficient combustion air. Pay attention to carbon monoxide monitoring to prevent excessive oxygen deficiency, which can increase heat loss from incomplete combustion and reduce boiler efficiency. Be mindful of adjusting air flow during significant load changes and when starting or stopping the pulverizing system. Adjust secondary air in the main combustion zone according to load fluctuations to reduce oxygen concentration in the ignition zone. Utilizing staged air combustion can reduce NOx generation.
Higher furnace heat load results in more NOx generation. Appropriately lowering the hot air temperature and employing fuel分级 combustion techniques are beneficial in reducing NOx emissions.
3. Uneven furnace heat load leads to increased NOx generation. By employing equal distribution of powder, we maintain stable steam and furnace pressure, stabilize the flame center in the furnace, ensure the rigidity of the secondary air, and prevent flame deflection.
4. The more the number of flouring systems in operation, the greater the NOx generation. Enhance the operation and maintenance of the flouring systems in operation to ensure flouring output, and strengthen coal handling and warehouse management.
5. If there's a significant change in coal type, the combustion in the furnace becomes more complex, and the environmental and conditions affecting NOx generation become more variable. It is recommended that power plants try to burn the designed coal type or maintain coal type stability as much as possible.
