1. Scale and dust accumulation on hot surface
Whether it's the slag and ash accumulation on the water-cooled wall of the furnace, or the ash buildup on the superheater, convective tubes, economizer, and preheater, all these issues will lead to an increased heat transfer resistance on the flue gas side, resulting in poor heat transfer and reduced cooling efficiency of the flue gas, ultimately causing the exhaust gas temperature to rise.
2. Excessive air coefficient
Under normal circumstances, as the excess air coefficient at the furnace outlet increases, the exhaust gas temperature rises. After the excess air coefficient increases, although the volume of flue gas and the velocity of smoke increase, and the convective heat transfer is enhanced, the increase in heat transfer is not as significant as the increase in flue gas volume. This can be understood as the increased smoke velocity causing the flue gas to leave the heating surface before it has sufficient time to transfer heat to the working substance.
The wind leakage coefficient in the furnace and tail smoke channel of a negative pressure boiler is inevitable, and a permissible wind leakage coefficient for a certain heating surface is specified. As the wind leakage coefficient increases, its impact on the exhaust gas temperature is similar to that of an increased excess air coefficient. Moreover, the closer the wind leakage point is to the furnace, the greater its influence on the rise in exhaust gas temperature.
4. Water Temperature
When the steam turbine load is too low or the high-pressure heater is disconnected, the boiler feedwater temperature will decrease. Generally speaking, when the feedwater temperature rises, if the fuel quantity is maintained constant, the heat transfer temperature difference of the economizer decreases, resulting in a lower heat absorption by the economizer, which in turn raises the exhaust gas temperature.
5. Water content in fuel
Increased moisture in the fuel leads to a higher volume of smoke, thereby raising the exhaust gas temperature.
