The phenomenon of exhaust flue gas temperature being higher than the designed value exists in operation of some boilers for supercritical units put into service in recent years

the designed parameters of economizer and air preheater for some boilers of supercritical units have been analysed

it shows that the recoverable heat at the end of boiler and the heat required for drying in the coal pulverizing system were limited with increasing of the steam pressure and the feed-water temperature.By using the heat and mass balancing method

the recoverable heat at the end of boiler and the heat required for drying in coal pulverizing system have been calculated respectively for two boilers burning bituminous coal and lean coal with different water content and volatile matter content.The results of calculation show that the main causes resulting in high exhaust flue gas temperature are as follows:the controlled outlet temperature of the coal pulverizer is lower

the temperature difference between outlet flue gas and feed-water of the economizer is large etc

under condition of increasing the feed-water temperature

whereas further increasing the outlet water temperature of econimizer may intensify the deterioration of heat transfer.