What are the common causes of an increase in the boiler’s exhaust gas temperature?

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Why should the change in exhaust gas temperature be monitored frequently while the boiler is in operation? What are the common causes of an increase in the boiler’s exhaust gas temperature? Because the heat loss due to smoke exhaust is the largest among all the heat losses in a boiler, accounting for approximately 6% of the heat input ； For every increase of 12–15°C in the flue gas temperature, the heat loss due to flue gases increases by 1% ； At the same time, the flue gas temperature can reflect the operating condition of the boiler; therefore, it is one of the most important indicators during boiler operation and must be closely monitored. The factors that increase the flue gas temperature are as follows: 1) Scaling, soot accumulation, and slag formation on the heated surfaces. 2) The excess air coefficient is too high. 3) The air leakage coefficient is too high. 4) An increase in moisture content in the fuel. 5) Increase in boiler load. 6) The quality of fuel deteriorates. 7) The operating mode of the coal feeding system is unreasonable. 8) Secondary combustion in the tail flue. 1. Air leakage analysis: Air leakage refers to leakage in the furnace and flue ducts, and it is one of the main reasons for an increase in exhaust gas temperature. It is a problem related to operation management, maintenance, and equipment design. Furnace air leakage mainly refers to air leakage at the furnace roof seal, viewing ports, manway doors, and the seal water tank at the furnace bottom ； Flue gas leakage refers to the leakage of flue gas at the rear section before the oxygen meter. 2. Analysis of the operation status of the coal grinding system: During operation, increasing the temperature at the outlet of the coal grinder is certainly beneficial for reducing the flue gas temperature. To increase the outlet temperature of the coal grinder, the cold air dampers are generally kept fully closed. However, during operation, a certain amount of cold air must be supplied to maintain the seals of the coal feeder and the coal grinder, which in turn displaces some of the hot primary air. As a result, the amount of air passing through the preheater decreases, leading to an increase in the flue gas temperature. Another factor contributing to a higher wind rate, as well as coarser coal powder, can also cause an increase in the flue gas temperature. 3. Analysis of ash accumulation on the heating surfaces Ash accumulation on the boiler’s heating surfaces refers to the buildup of ash and slag there, as well as ash accumulation on the heat transfer elements of the air preheater. Ash accumulation on these heating surfaces reduces their heat transfer efficiency, leading to a decrease in the boiler’s heat absorption, a reduction in the heat released by the flue gases, and an increase in the smoke temperature at the inlet of the air preheater. This, in turn, results in an increase in the exhaust gas temperature ； Dust blockage in the air preheater reduces its heat transfer area, which in turn decreases the amount of heat released by the flue gases and raises the exhaust gas temperature. (Extreme measures such as controlling the flue gas flow rate and maintaining positive pressure in the boiler, as well as the problem of ash accumulation on the heating surfaces at the rear of the boiler that arises during operation) It is important to strengthen boiler soot blowing during operation; when the load permits, the boiler should be thoroughly soot-blown at least once a day. During periods of low load, efforts should be made to increase the frequency of soot blowing to ensure that the boiler’s heating surfaces remain clean. 4. Analysis of ambient air temperature: During actual operation, the ambient wind temperature is higher than the designed value; as a result, the wind temperature at the inlet of the air preheater is high, the heat transfer temperature difference across the air preheater is small, which leads to less heat being released by the flue gases. Consequently, the exhaust gas temperature rises. Learn about boiler knowledge every day, follow the WeChat official account Boiler Circle. Meanwhile, the amount of hot air required by the coal grinding system decreases, the amount of primary air flowing through the air preheater reduces, and the flue gas temperature rises – these are all objective environmental factors. 5. A high feedwater temperature results in a small heat transfer difference between the flue gas temperature and the water temperature, which in turn raises the exhaust gas temperature. (Activation and deactivation of the turbine high and low pressure heaters! 6. Analysis of the arrangement of heating surfaces: If the fouling coefficient of the furnace is not estimated accurately during boiler design, resulting in an unreasonable arrangement of the heating surfaces, or if the poor structure leads to insufficient heat absorption by these surfaces, it will cause the flue gas temperature at the inlet of the air preheater to be high, thereby increasing the exhaust gas temperature. In such cases, redesign and recalculation are necessary; measures such as increasing the number of tubes in the economizer can be taken to reduce the flue gas temperature at the inlet of the air preheater. 7. Analysis of changes in coal quality fed into the furnace: An increase in moisture content in the fuel, as well as a decrease in the lower heating value of the coal supplied to the boiler, both lead to an increase in exhaust gas temperature. These changes will increase the flue gas volume and specific heat of the flue gas; consequently, the temperature drop of the flue gas in the convective zone decreases, and the flue gas exhaust temperature rises. Therefore, coal with low moisture content and high calorific value should be used as much as possible. 8. Analysis of flue gas temperature measurement: Due to the non-uniformity of the temperature and velocity fields of the flue gas at the exit of the air preheater, if the temperature measurement points are not positioned appropriately, the resulting temperature values will contain certain errors, and the displayed values may be on the high side. 9. The operators should, during low-load operation, strive to keep the total air volume to the boiler and the oxygen level in the furnace at low values. Ash cleaning of the air preheater should be carried out carefully during each shift, in order to control and reduce the flue gas temperature as much as possible.