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Causes of boiler flue gas temperature rise

Time :2024-07-27 2

Exhaust heat loss is the largest heat loss in thermal power plant boilers, generally 6% of the heat sent to the furnace. For every 12–15℃ increase in flue gas temperature, the exhaust heat loss increases by 0.5%. Therefore, flue gas temperature rise is one of the important indicators of boiler operation.

Reasons for flue gas temperature rise:

1. Slag and ash accumulation on the heating surface. Whether the slag and ash of the water-cooled wall are accumulated, or the superheater, convection tube bundle, economizer and preheater with volcanic ash accumulation will increase the thermal resistance measurement of the flue gas, the heat transfer deterioration will make the cooling effect of the flue gas poor, and lead to an increase in exhaust temperature.

2. The excess air coefficient is too high. In general, the exhaust temperature increases with the increase of the excess air coefficient at the furnace outlet. With the increase of the excess air coefficient, although the smoke volume increases, the smoke velocity increases, and the heat transfer to Liu Fang is enhanced, the increase in heat exchange is not as much as the increase in the smoke volume. It can be understood that when the smoke velocity increases, the smoke does not have enough time to transfer heat to the working medium when leaving the heating surface.

3. The air leakage coefficient is too high. Air leakage in the furnace and tail shaft flue of negative pressure boilers is inevitable, and the allowable air leakage coefficient for a certain heating surface is specified. When the air leakage coefficient increases, the effect on the exhaust temperature is similar to that of the superheated air coefficient. The closer the air leakage is to the furnace, the greater the effect on the flue gas temperature rise.

4. Feed water temperature. When the turbine load is too low or the high-pressure heater is disconnected, the boiler feed water temperature will decrease. Generally speaking, when the feed water temperature increases, if the fuel oil amount remains unchanged, the heat transfer temperature difference of the economizer decreases, the heat absorption of the economizer decreases, and the flue gas temperature rises.

5. Water in the fuel. The water in the fuel increases the volume of the smoke, and therefore also increases the exhaust temperature.

6. Boiler load. Although the boiler load increases, the exhaust volume, steam, feed water, and air volume also increase proportionally, but the exhaust temperature increases due to the increase in the temperature of the flue gas at the furnace outlet. When the load increases, the furnace outlet temperature increases, and the temperature difference between the convection heating surface and the heat absorption surface increases. Therefore, the more convection heating surfaces there are, the smaller the impact of boiler load changes on the exhaust temperature.

7. Fuel type. When the calorific value of the gas is reduced, the temperature of the furnace is reduced, the radiation heat transfer in the furnace is reduced, and the non-combustible components of the low calorific value gas are mainly nitrogen, carbon dioxide, and water, so the smoke volume increases and the exhaust temperature rises. After the pulverized coal furnace is changed to burn oil, although the outlet furnace of the excess air coefficient is lower than that of the fuel oil, when burning coal, because the ash content of the fuel oil is very small, there are no large volcanic ash particles, and there are no large volcanic ash particles to clean the flue gas on the heating surface, the convection heating surface pollution is more serious. Therefore, the exhaust temperature of the boiler that burns poorly and often produces black smoke increases. When there is a tail ball ash removal device, the exhaust temperature is slightly lower than that of burning coal because the tail is cleaner.

8. Operation mode of the pulverizing system. For the closed powder storage silo pulverizing system, when the pulverizing system is running, due to some water in the fuel entering the furnace, the furnace temperature decreases and the smoke volume increases. The cold air leaking into the pulverizing system enters the furnace as the primary air, and the air flowing through the air preheater is reduced, causing the flue gas to heat up. On the contrary, when the pulverizing system is not running, the exhaust temperature decreases.