During boiler operation, exhaust temperature is a key indicator of boiler efficiency. Exhaust temperature directly reflects the boiler's heat utilization and has a crucial impact on its energy consumption, environmental performance, and operational safety.
However, in actual boiler operations, excessively high exhaust temperature is a common problem, not only troubling many boiler operators and maintenance personnel but also causing significant losses to the company. So, why does boiler exhaust temperature rise? And what adverse effects does excessively high exhaust temperature have on boiler operation?
This article focuses on the core issue of "excessively high boiler exhaust temperature," comprehensively analyzing the various common causes. Furthermore, based on practical considerations, it proposes a series of practical solutions, providing a professional and comprehensive reference for boiler operation and maintenance, helping companies reduce energy consumption and improve boiler efficiency and safety.
During long-term boiler operation, scaling and ash accumulation are very common on heating surfaces such as economizers, water walls, and air preheaters. Impurities in the water continuously deposit on the heating surface tube walls, forming hard scale. At the same time, fly ash from fuel combustion also adheres to the heating surface, forming an ash layer.
The presence of scale and ash accumulation significantly reduces the heat transfer efficiency of the heating surface, preventing the heat in the flue gas from being effectively transferred to the working fluid (water or steam) within the boiler. When the heat carried by the flue gas cannot be fully absorbed, the exhaust gas temperature rises.
The excess air coefficient refers to the ratio of the actual air consumption to the theoretical air consumption during boiler combustion. During boiler operation, an excessively high excess air coefficient indicates that too much air is being introduced into the furnace. This excess air significantly increases the flue gas volume, and after participating in combustion, this excess air carries a significant amount of heat with it and is discharged from the boiler.
This not only wastes heat and significantly increases exhaust heat loss, but also directly increases exhaust temperature, further reducing the boiler's overall operating efficiency.
Air leakage in the furnace and flue is also a common cause of excessively high boiler exhaust temperatures. Air leakage can occur to varying degrees in the furnace and flue due to aging seals, excessive installation gaps, and cracked welds.
When cold air from outside enters the furnace and flue through these leaks, it mixes with the flue gas, increasing the total flue gas volume and altering its temperature distribution, ultimately leading to increased exhaust temperature. Furthermore, air leakage can affect pressure stability within the furnace, disrupting normal combustion conditions and potentially posing a safety hazard.
Feedwater temperature is a critical parameter in boiler operation. Excessively high feedwater temperature can increase the initial temperature of the working fluid within the boiler. During the boiler's heat exchange process, the temperature difference between the flue gas and the working fluid is crucial for achieving heat transfer. When the feedwater temperature is too high, the temperature difference between the flue gas and the working fluid decreases, resulting in poor heat transfer and making it difficult for the working fluid to fully absorb the heat in the flue gas.
The heat that cannot be effectively absorbed is discharged with the flue gas, causing the exhaust gas temperature to rise.
Fuel quality has a direct impact on the boiler's combustion process and exhaust gas temperature. Fuels with high moisture content, low calorific value, or high ash content can seriously affect combustion efficiency. Fuels with high moisture content require more energy to evaporate the water during combustion, resulting in less heat actually used to heat the working fluid. Fuels with low calorific value cannot provide sufficient energy, resulting in unstable combustion conditions. Fuels with high ash content produce large amounts of fly ash after combustion, which not only easily causes dusting on the heated surface but also affects heat transfer.
These factors combined can lead to incomplete fuel combustion, increased flue gas volume, and ultimately high exhaust gas temperature.
For pulverized coal boilers, proper pulverized coal fineness is crucial. If the pulverized coal is too coarse, it will not burn fully in the furnace. Some of the unburned pulverized coal will be discharged with the flue gas, which not only wastes fuel but also increases the heat carried by the flue gas, leading to higher exhaust gas temperatures. If the pulverized coal is too fine, while it facilitates complete combustion, it will increase the energy consumption of the pulverizing system. Furthermore, when pulverized coal is burned in the furnace, it is prone to problems such as excessive combustion speed and unstable flames, which may also have an adverse effect on the exhaust gas temperature.
The combustion and air distribution methods of a boiler directly determine the combustion conditions in the furnace. Improper burner arrangement or improper air volume distribution can cause the flame center to shift, especially when the flame center shifts upward. This can cause the upper heating surface of the furnace to overheat, while the lower heating surface cannot fully absorb heat, resulting in uneven heat distribution throughout the furnace.
In addition, improper air distribution can lead to oxygen-deficient combustion or excess air in some areas, further reducing combustion efficiency and ultimately increasing the exhaust gas temperature.
Furnace negative pressure is a critical control parameter during boiler operation. A normal furnace negative pressure helps maintain stable combustion conditions and flue gas flow. Excessive furnace negative pressure can cause a large amount of cold air to be drawn in through leaks in the furnace, increasing flue gas volume and disrupting the normal flue gas flow path, affecting heat exchange. Excessive furnace negative pressure, or even positive pressure, can prevent flue gas from being discharged smoothly, causing some high-temperature flue gas to accumulate inside the furnace, also leading to increased exhaust gas temperature.
Both excessive and insufficient furnace negative pressure can disrupt flue gas flow, reduce heat exchange efficiency, and ultimately increase exhaust gas temperature.
For boilers with a water-sealed furnace bottom, a regular supply of furnace bottom sealing water is crucial. A failure of this water supply can degrade the furnace bottom's sealing performance, allowing large amounts of cold air to leak into the furnace. The intruding cold air mixes with the flue gas in the furnace, increasing the flue gas volume while also lowering the furnace temperature, affecting combustion efficiency.
Ultimately, these factors lead to an increase in exhaust gas temperature, potentially threatening the safe operation of the boiler.
Changes in boiler load also affect exhaust gas temperature. When the boiler is operating at high load, more fuel is consumed to meet the required output power. This increased fuel consumption also increases the flue gas volume. Furthermore, during high-load operation, the furnace temperature is higher, and the flue gas's residence time within the furnace is relatively shortened. This prevents sufficient heat transfer to the working fluid, resulting in more heat being discharged with the flue gas, thus increasing the exhaust gas temperature.
In addition to the common causes listed above, there are other factors that can also cause excessively high boiler exhaust gas temperatures. For example, poor heat transfer performance in the air preheater itself, such as damage or blockage of heat exchange elements, can prevent the air preheater from effectively recovering heat from the flue gas, leading to higher exhaust temperatures. Unstable pulverizing system operation, such as uneven pulverized coal supply and fluctuating mill output, can affect combustion stability and, in turn, exhaust temperatures. Poor steam-water quality can cause impurities in the water to easily form scale on the heating surfaces, reducing heat transfer efficiency and also causing higher exhaust temperatures.
The most direct impact of excessively high exhaust temperatures is reduced boiler efficiency. According to relevant statistics, for every 10°C increase in exhaust temperature, boiler efficiency decreases by approximately 0.5% to 1%. This is because increased exhaust temperature means that the heat carried by the flue gas cannot be effectively utilized, and a large amount of heat is lost through the exhaust, reducing the boiler's utilization of the fuel's heat.
As boiler efficiency decreases, more fuel is consumed to maintain normal boiler output. Increased fuel consumption directly leads to higher energy costs for businesses, which in the long term will impose a significant financial burden and reduce their economic benefits.
High-temperature flue gas can cause severe corrosion and wear on equipment components such as boiler heating surfaces, air preheaters, and economizers. High temperatures accelerate oxidative corrosion of metal materials. Furthermore, fly ash and other particles carried in the flue gas at high speeds can cause erosion and wear on equipment surfaces.
The higher the exhaust gas temperature, the more severe this corrosion and wear, shortening the lifespan of equipment and increasing repair and replacement costs.
Excessively high exhaust gas temperatures are often associated with incomplete fuel combustion, which in turn leads to increased emissions of pollutants such as carbon monoxide (CO), black smoke, and particulate matter. Emissions of these pollutants can exceed environmental standards, not only polluting the environment but also potentially leading to penalties from environmental protection authorities and negatively impacting the company's public image.
Excessively high exhaust gas temperatures can increase the temperature of the boiler's heating surface. If the heating surface is severely ash- and coke-prone, the high temperature can easily lead to localized overheating of the heating surface. Localized overheating can weaken the strength of the heating surface tube walls. When the tube wall temperature exceeds its tolerance limit, a tube burst can occur, seriously threatening boiler operational safety and potentially causing casualties and property damage.
Proper combustion adjustment is one of the key measures to reduce boiler exhaust gas temperatures. First, the air volume and air-to-pulverized gas ratio should be properly controlled based on the fuel characteristics and boiler operating conditions to ensure complete combustion of the fuel within the furnace and minimize incomplete combustion losses. Second, the burner's operating status should be regularly inspected, ash and coke deposits should be promptly cleaned from the burner nozzles, and the burner's angle and position should be adjusted to prevent flame drift and ensure even flame distribution within the furnace, thereby improving heat utilization efficiency.
Regular cleaning and maintenance of boiler heating surfaces is an important means of maintaining good heat transfer performance. First, a reasonable cleaning plan should be developed based on the scale and ash accumulation on the heating surfaces. Use methods such as high-pressure water jets, compressed air soot blowing, and chemical cleaning to regularly remove scale and ash from the heating surfaces. Second, routine inspection and maintenance of the heating surfaces should be strengthened to promptly detect and repair any damage, leaks, and other problems, ensuring their integrity and heat transfer performance.
Reducing air leakage in the furnace and flue is an effective way to lower exhaust gas temperatures. First, strengthen the inspection and maintenance of the furnace and flue sealing systems, regularly replace aging seals, and repair cracked welds and excessive installation gaps to improve sealing performance. Second, air leakage monitoring devices can be installed in key locations of the furnace and flue to monitor air leakage in real time, promptly identify and address leaks, and control air leakage within a reasonable range.
Selecting high-quality fuels with low moisture content, high calorific value, and low ash content can improve combustion conditions and reduce exhaust gas temperatures at the source. Furthermore, lower-quality fuels can be pre-treated before use. For example, crushing and screening can be used to remove impurities and large lumps, and drying can be used to reduce the fuel's moisture content and improve its combustion performance.
For pulverized coal boilers, the parameters of the pulverizing system should be appropriately adjusted based on the fuel's characteristics and combustion requirements. First, the fineness of the pulverized coal should be controlled to ensure it meets combustion requirements, neither too coarse nor too fine. Second, the outlet temperature of the pulverizing system should be controlled to avoid excessively high or low temperatures, ensuring the dryness and uniformity of the pulverized coal and improving combustion stability and efficiency.
Periodically test and analyze the steam and water quality of the boiler to ensure it meets relevant standards. If substandard steam or water quality is found, prompt measures should be taken to address it, such as adjusting the operating parameters of the water treatment equipment and replacing water treatment chemicals. Furthermore, heating surfaces should be cleaned promptly based on the steam or water quality to prevent scale buildup and ensure heat transfer efficiency.
For newly built boilers or those undergoing technical renovations, optimizing the layout of the heating surfaces can improve heat exchange efficiency and reduce exhaust gas temperatures. For example, the number of tube banks in the economizer can be appropriately increased to expand the heat exchange area; the design of the air preheater can be improved by employing high-efficiency heat exchange components and structures to improve heat transfer efficiency; and if the heating surface layout within the furnace is unreasonable, redesign and modification should be carried out to ensure adequate flame contact with the heating surface, improving heat transfer efficiency.
Strengthening the monitoring and management of boiler operation will enable timely detection of problems such as abnormal exhaust gas temperatures and the implementation of appropriate measures to address them. First, high-precision temperature monitoring devices should be installed on the boiler's exhaust duct to monitor exhaust temperature changes in real time. If the exhaust temperature rises abnormally, an alarm signal should be issued promptly. Second, a comprehensive operation management system should be established, and operator training should be strengthened to enhance their professional skills and sense of responsibility. This will enable them to master the boiler's operating characteristics and adjustment methods, promptly identify and address various operational issues, and ensure that the boiler is always in optimal operating condition.
In summary, excessively high boiler exhaust temperatures are not caused by a single factor but rather result from a combination of factors, including scaling and ash accumulation on the heating surfaces, an excessively high excess air coefficient, air leakage, and poor fuel quality. This problem not only reduces boiler efficiency and increases energy consumption, but also exacerbates equipment corrosion and wear, deteriorates environmental performance, and even poses serious operational safety risks, posing a significant threat to the company's economic benefits and production safety.
Therefore, the key to effectively addressing the problem of excessively high boiler exhaust temperatures lies in practicing the core principles of "rational combustion, optimized operation, and regular maintenance." By optimizing combustion adjustments to ensure sufficient fuel combustion, strengthening operational monitoring and management to achieve efficient boiler operation, and regularly performing maintenance tasks such as cleaning the heating surfaces and checking for air leaks to ensure equipment performance, a multi-pronged, comprehensive approach can fundamentally reduce exhaust gas temperatures and significantly improve boiler operating efficiency.
Ultimately, these systematic, comprehensive measures will not only provide a solid guarantee for the safe, stable, and long-term operation of boilers and effectively mitigate various safety risks, but will also effectively reduce energy waste and pollutant emissions, helping enterprises achieve their key energy conservation and emission reduction goals and promoting green and sustainable development.
