Flue gas recirculation is a method to increase the absorption of convective superheaters and reheaters. Its working principle is similar to increasing the excess air volume in the furnace, reducing the radiant heat absorption in the furnace, thereby increasing the heat absorption of the rear convective heating surface. It uses a recirculation fan to extract part of the flue gas from the tail flue and reintroduce it into the furnace. During operation, the flue gas flow through the superheater and reheater can be changed by adjusting the recirculated flue gas volume, which in turn alters the steam temperature.
The thermodynamic characteristics of flue gas recirculation depend on the location where the recirculated flue gas enters the furnace. When the recirculated flue gas is introduced below the cold ash hopper, as the volume of recirculated flue gas increases, the relative absorption of the radiant heating surface in the furnace decreases while the relative absorption of the convective heating surface increases. Therefore, the further along the flue gas process the heating surface is located, the greater the increase in absorption, meaning a larger temperature adjustment range.
When the flue gas recirculation is introduced at the furnace outlet, the change in the furnace heat absorption is very small, but the flue gas temperature at the furnace outlet drops significantly. The adjustment range for the superheated steam temperature and reheated steam temperature is also quite small.
Therefore, the main purpose of this method is not for temperature adjustment, but to lower the flue gas temperature at the furnace outlet, preventing overheating of the screen superheater and slagging of the convective superheater.
The advantage of using flue gas recirculation for temperature adjustment is that it can increase the amplitude of flue gas volume adjustment within the furnace without increasing the excess air coefficient, saving heating surfaces, with relatively quick response times. It can also even out the thermal load in the furnace. The disadvantage is the use of a recirculation fan, which increases plant electricity consumption. Additionally, due to the high-temperature operation, maintenance costs are higher, especially for pulverized coal boilers where wear on the recirculation fan is quite severe.
Summary report from Beijing Weima Wall-Hung Boiler Repair: Advantages and disadvantages of flue gas recirculation.
Source: http://blog.sina.com.cn/weimabigualu