English
Flue gas recirculation refers to the part of flue gas produced by combustion, which isrecycled back to the combustion zone after cooling, or mixed with air and then sent into the combustion zone, thus reducing the oxygen concentration and the temperature of the combustion zone, so as to reduce the amount of NOx produced, which is called flue gas recirculation technology.
First, the idea of applying flue gas recycling technology to boilers was put forward in KenL. Maloney 1984, flue gas recycling technology of boiler can make the boiler run stably and increase the excess air coefficient, and the excess air reduction of the system can be as high as 50% or more. After scientific research, and based on thermal chemical analysis, it is found that the boiler adopts flue gas recycling technology which has a reduction reaction under low temperature, low oxygen and some of water vapor conditions, which can greatly reduce the discharge of NO, and make the flue gas in the tail of the boiler meet the requirements of environmental protection, while the flue gas recycling technology has been further developed.
At present, the boiler flue gas recycling technology is used to extract a part of the low temperature flue gas from the outlet of the boiler directly into the furnace, and after mixing with the combustion-supporting air, it is fed into the furnace, which reduces the combustion temperature and oxygen concentration, thus reducing the concentration of NOx from the combustion emission
The main reduction of flue gas recirculation technology is the thermal type of NOx, which has the most significant effect on the nitrogen reduction of gas boiler, which is due to the low nitrogen content of natural gas and the thermal type of NOx generation. Furthermore,the effect of this technology is also related to the amount of recirculating flue gas, and the flue gas recirculation rate is generally controlled at 10%～20%. If too high, the combustion instability will occur, and the incomplete combustion heat loss will also increase.Experience shows that NOx emission concentration of gas furnace can be reduced by more than 40% when the flue gas recycling rate is 10% ～15%.
Control method of flue gas recirculation
The fan inlet control baffle is used to adjust the amount of flue gas recovered by flue gas recycling. The baffle is controlled by the PLC through a 4mA～20mA signal. The optimal combustion curve is set by the amount of recovered flue gas and the combustion load to achieve automatic control. The operating conditions of the boiler automatically adjust the amount of flue gas recovered under different operating conditions to achieve the purpose of controlling the NOx concentration within a reasonable range under the operation of the boiler under different loads. Flue gas recycling technology will reduce the thermal efficiency of the boiler to a certain extent, and it needs to be minimized by reasonable recovery and precise control.