Source: Boiler Circle
What is the role of flue gas oxygen content in guiding combustion adjustment?
The flue gas oxygen meter is installed after the low-temperature superheater and in front of the economizer, and it actually reflects the excess air coefficient. Within a certain range, the combustion efficiency can be improved by appropriately increasing the excess air coefficient of the circulating fluidized bed. It affects not only the smoke exhaust losses, but also the mechanical incomplete combustion heat losses, and to reduce these losses, the optimal combustion condition is the oxygen value corresponding to the lowest point of the sum of the two. This working condition is mainly related to the ratio of wind and coal, in normal operation, when adjusting the combustion, the first and second air volume is reasonably matched, and the coal feed is adjusted to meet the needs of the boiler load, at this time, there is an optimal oxygen content value, and we can make the flue gas oxygen content fluctuate around this optimal value by adjusting the coal feed. When the boiler is cut off or the coal quality changes, the oxygen content of the flue gas changes first, and the operator is reminded to adjust in time, therefore, the flue gas oxygen meter can not only intuitively help the operator accurately judge the abnormal situation, but also help adjust the combustion to the best working condition.
The oxygen content of flue gas is one of the important monitoring parameters of boiler operation, and it is an important basis for reflecting the perfection of combustion equipment and boiler operation, and its value is related to factors such as boiler structure, fuel type and nature, boiler load, and operating air distribution conditions. Controlling the oxygen content of flue gas is very important to control the combustion process and achieve safe, efficient and low pollution emissions.
Factors affecting the oxygen content of the boiler and measures to control the oxygen content:
Oxygen refers to the percentage of oxygen volume content in the flue gas used to guide the operation control of the boiler, and generally refers to the oxygen content of the flue gas at the inlet of the economizer (one stage through one stage). Under the condition that other conditions remain unchanged, if the air supply is insufficient, the oxygen gauge reading is small, and the combustion is incomplete, resulting in incomplete combustion heat loss; If the air supply is too much, the oxygen gauge reading is large, and the smoke exhaust volume increases, which increases the exhaust heat loss, and at the same time increases the power consumption of the supply and induced draft fan.
Factors influencing oxygen amount:
The oxygen content of the boiler is related to the boiler load, fuel properties, air distribution conditions and other factors.
1. Load rate. The higher the boiler load, the smaller the oxygen required, and generally the oxygen needs to be increased at low load to ensure good combustion conditions.
2. Fuel quality. When the fuel quality is poor, such as when the moisture or ash content is large, the fuel is difficult to ignite and burn out, and the oxygen amount should be increased appropriately to ensure the stability of combustion and improve the combustion efficiency.
3. Air leakage of the boiler body and feeding system. Because the boiler works under negative pressure, the cold air from the outside is easy to leak in through the manhole, inspection hole, water cooling jacket, etc., resulting in an increase in oxygen content, an increase in smoke exhaust loss and electric consumption of induced draft fans, and a reduction in the operation economy of the boiler.
4. Air supply volume. Too much air supply will increase the oxygen content and increase the power consumption of the induced draft fan.
5. Prevent or alleviate coking on the heating surface. Increasing the oxygen content can change the reducing atmosphere in the furnace and prevent or reduce coking on the heating surface.
Measures to control oxygen levels:
1. Through the boiler combustion adjustment test, the optimal ratio of boiler oxygen and primary and secondary air volume is determined, so that the sum of incomplete combustion heat loss and exhaust gas heat loss of the boiler is minimized.
2. Summarize the operation experience, combine the boiler combustion adjustment test and thermal performance test to determine the optimal oxygen content of the boiler under different conditions, and formulate the boiler oxygen control curve under different unit loads and fuel types.
3. Reduce the air leakage of the boiler body and the feeding system, strengthen the inspection of the water level of the slag fishing machine, each manhole, inspection hole and water cooling jacket during the inspection, and contact the maintenance and plugging in time if the parts with air leakage are found. When the feeding system equipment is adjusted, the buffer box of the feeder should maintain a certain material level, and ensure that the water-cooled jacket forms a material plug to prevent air leakage.
4. Regularly calibrate the oxygen meter.
What does it mean to control oxygen content, and what can you see from the change in oxygen content?
(1) Reasonable oxygen content combustion can prolong the service life of the water wall pipe wall, when the oxygen amount is too low, reducing gas and H2S gas with high content will be formed near the water wall, which is very corrosive to the water wall, breaking the Fe2O3 protective film, and reducing the life of the water wall for a long time.
(2) Reasonable oxygen content can make the boiler more stable combustion, when the oxygen content is too high, the furnace temperature will be significantly reduced, and when the oxygen content is too low, the furnace temperature will increase significantly, so the oxygen content is not controlled and the furnace combustion will be unstable.
(3) Reasonable oxygen content can improve the economy of the boiler, sufficient oxygen can make the combustion more sufficient, share boiler knowledge, pay attention to the WeChat public account boiler circle is conducive to complete combustion, too high oxygen content will increase the amount of flue gas, increase the heat loss of exhaust gas, excessive oxygen will also increase the power consumption of the fan, in addition, the oxygen amount increases, the flue gas flow and flue gas velocity increase, and the wear of the heating surface is also not good.
(4) Reasonable oxygen content can control NOx emissions, and oxygen amount control can be used to burn boilers.
The generation of NOX has a great impact, and the NOX content increases with the increase of oxygen, so the operation of high oxygen is not good for the NOX control of the boiler.
High oxygen content:
The coal quality may deteriorate, the amount of coal supply may be reduced, the bed temperature may decrease, the main steam pressure and temperature may decrease, the NOX may increase, the air may be added, and the coal may be blocked or shed coal.
Low oxygen content:
The coal quality may improve, the coal feed may increase, the bed temperature may rise, the main steam pressure and temperature may rise, the wind may be reduced, coking may occur, and the fan may be abnormal.
Effect of oxygen control on boiler high temperature corrosion and slagging
When the oxygen content is too low, a reducing atmosphere and a high content of H2S gas will be formed near the boiler water wall, and the corrosion of H2S gas on the water wall is very strong, which will destroy the protective film of Fe2O3 and make the pipe wall suffer from continuous corrosion. The ash melting temperature of the ash in the reducing gas will be greatly reduced, which is easy to cause slag formation in the furnace. Therefore, too low oxygen control will cause the risk of high temperature corrosion and slagging formation, which will affect the safety of boiler operation. In order to prevent slagging and high-temperature corrosion of the water wall, the carbon monoxide content in the flue gas should be controlled below 120PPm. The carbon monoxide content in flue gas is closely related to the amount of oxygen, the type of fuel and the mode of milling operation.
Effect of oxygen control on boiler combustion stability:
The stability of boiler combustion is the foundation of boiler combustion, the pulverized coal jet is mainly through the high temperature flue gas in the coil suction furnace to make its own temperature reach the ignition temperature and then start the combustion reaction, the oxygen content in the operation is too high, the furnace temperature in the combustion area is significantly reduced, whether it is the radiation heat transfer of pulverized coal gas flow or the heat exchange of coil suction will be weakened, the ignition conditions of the pulverized coal gas flow will deteriorate, the combustion stability will decrease, and even the risk of boiler fire extinguishing will occur. This is especially true at low loads.
Effect of oxygen control on boiler operating economy:
Reasonable combination of air and powder is an important measure to improve the economy of boiler operation. Within a certain range, the increase of operating oxygen can improve the contact and mixing between fuel and air, which is conducive to complete combustion, and reduces the heat loss of incomplete combustion of combustible gas and the incomplete combustion of solids. However, the increase of oxygen will increase the flue gas volume of the boiler and increase the exhaust gas heat loss. Excessive oxygen consumption will also increase the power consumption of the fan. Therefore, oxygen control has a great impact on the operation economy of the boiler, and the reasonable operating oxygen amount should minimize the sum of the heat losses and the highest thermal efficiency of the boiler. The amount of oxygen should be controlled to the right of the inflection point where the carbon monoxide level rises abruptly, i.e., the area where the boiler heat loss is minimal. In addition, the increase of oxygen content, flue gas flow and flue gas velocity also have an adverse effect on the wear of the heating surface.
Impact of oxygen control on NOx emissions:
The oxygen control has a great influence on the NOX generated by boiler combustion, and the total NOX content generated by boiler combustion increases with the increase of oxygen, so the operation of high oxygen is unfavorable to the NOX control of the boiler. Our company's boiler adopts staged combustion technology, which reduces the NOX emission in the flue gas through the setting of the ember air, and the size of the ember gas input will have an impact on the combustion economy of the boiler. In the case of a certain amount of oxygen, excessive input of ember air will cause a lack of air in the main combustion area, affect the combustion of pulverized coal, and reduce the thermal efficiency of the boiler. The input of ember wind is too small to achieve the purpose of reducing NOx emissions in flue gas. The goal of improving boiler efficiency and controlling NOx emissions should be achieved through the integrated control of oxygen and ember air.
The oxygen content of flue gas is an extremely important economic operation assessment index in the monitoring of boiler air pollutants, and to a certain extent, the size of its value is the key to determine whether the pollutant emission concentration meets the standard. The oxygen content of flue gas is a comprehensive and systematic problem. In actual production, the boiler should install the necessary flue gas monitoring instruments to provide the operator with a reference basis for operation adjustment; At the same time, it is necessary to strengthen operation management and eliminate non-standard operation behaviors. Many factors affecting the oxygen content of flue gas can be effectively controlled, and the concentration of pollution emission can also be controlled according to the requirements. The operator should strictly control the air volume ratio to make the fuel fully combusted, and at the same time pay attention to avoid or eliminate air leakage, and control the oxygen content of the flue gas as much as possible within the range of economic indicators. In this way, it can not only reduce the heat loss of exhaust smoke, improve the thermal efficiency of the boiler, but also better control the emission of pollutants, and play a positive role in protecting and improving the regional ambient air quality.