What is the impact of increasing feedwater temperature on boiler efficiency?

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The first scenario assumes that the boiler’s evaporation rate remains constant. As the feedwater temperature increases, the heat transfer temperature difference in the economizer decreases, resulting in a reduced amount of heat absorbed. This leads to an increase in the flue gas temperature downstream of the economizer, as well as an increase in the temperature and pressure in the air preheater; consequently, the heat transfer rate increases, the temperature of the hot air rises slightly, and the flue gas temperature increases, all of which reduce the efficiency of the boiler. However, when the feed water temperature increases, the heat available for evaporation rises, resulting in an increased amount of evaporation. To maintain a constant evaporation rate, it is necessary to reduce the amount of fuel used, which lowers the flue gas temperature and improves the thermal efficiency of the boiler. Since the impact of these two factors on the thermal efficiency of the boiler is roughly equal, keeping the boiler’s evaporation rate constant results in little change in the boiler’s thermal efficiency when the feedwater temperature is increased. The second scenario assumes that the fuel amount remains constant; when the feedwater temperature increases, the temperature and pressure in the economizer drop, the temperature of the flue gas after the economizer rises, the heat absorbed by the air preheater increases, the exhaust gas temperature goes up, and the thermal efficiency of the boiler decreases. As the temperature of the hot air increases, the furnace temperature rises, resulting in an increased heat absorption by the water wall. When the feedwater temperature increases, the heat used to raise the water temperature decreases, while the heat used for evaporation increases. Therefore, as the feedwater temperature increases, if the fuel amount remains unchanged, the evaporation rate rises, but the thermal efficiency of the boiler decreases. In summary, increasing the feedwater temperature does not improve boiler efficiency, whether the evaporation rate remains constant or the fuel amount remains constant. However, increasing the feedwater temperature can indeed improve the cycle thermal efficiency of power plants, thereby reducing coal consumption for power generation.