The currently used flue gas recirculation method extracts a portion of low-temperature flue gas before the air preheater of the boiler and mixes it directly with the supply air before sending it into the furnace. This not only reduces the combustion temperature, but also lowers the oxygen concentration, thereby reducing the NOx emission concentration.
The effect of reducing NOx emissions by flue gas recirculation method is related to the fuel type and flue gas recirculation. Experience has shown that when the flue gas recirculation rate is 10-15%, the NOx emission concentration of gas stoves can be reduced by more than 40%. The reduction rate of NOx increases with the increase of flue gas recirculation rate. And it is related to the type of fuel and combustion temperature. The higher the combustion temperature, the greater the impact of flue gas recirculation rate on NOx reduction rate.
The ratio of the amount of recycled flue gas to the amount of flue gas without flue gas recirculation is called the flue gas recirculation rate.
The flue gas recirculation rate of gas boilers is generally controlled at 10-20%. When using a higher flue gas recirculation rate, combustion will be unstable and incomplete combustion heat loss will increase.
The flue gas recirculation method can be used alone on a boiler or in combination with other low NOx combustion technologies to reduce NOx emissions. According to actual measurements in 2015, when the flue gas circulation rate is 15%, the NOx emissions from the NTFB burner are within 30mg/m ³.
Control method for flue gas recirculation: The amount of flue gas recovered by flue gas recirculation is adjusted through the inlet control baffle of the fan, and the PLC controls the baffle through a 4-20 mA signal. The combustion curve is set based on the amount of recovered flue gas and the combustion load to achieve automatic control. The recovery amount of flue gas is automatically adjusted according to the operating conditions of the boiler under different working conditions to meet the requirement of controlling the NOx concentration within a reasonable range under different loads of the boiler. Reasonable recovery and control have an almost zero impact on the thermal efficiency of the boiler
Flue gas recovery requires the installation of FGR fans to avoid negative pressure caused by pipeline resistance and chimney altitude. Therefore, FGR fans need to be installed to ensure sufficient amount of recovered flue gas is sent into the furnace. The FGR fans adopt frequency conversion control, which can meet the needs of different loads of the boiler for flue gas recovery and greatly reduce power consumption. At the same time, the flue gas can be controlled within a reasonable flow rate, resulting in a high utilization rate and effectively reducing nitrogen oxides.
