Why do some circulating fluidized bed boilers need flue gas recirculation?

　　The use of flue gas recirculation in small circulating fluidized bed boilers has a certain environmental protection effect. By extracting a part of low-temperature flue gas from the economizer flue or other flues and sending it into the furnace, the combustion temperature can be reduced, and NO production can be reduced. However, for large circulating fluidized bed boilers, the effect of flue gas recirculation is limited, and it will cause new problems such as increased wear and tear, so it should be used with caution.