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Show REDUCING NOx EMISSIONS USING DILUTE GAS INJECTION Equilibrium NOx Production vs. Oxygen Concentration and Temperature Perhaps the most widespread and commonly used N O x reduction method is the addition of flue gas recirculation or FGR. By recirculating furnace gases back into the combustion zone, peak flame temperatures and local oxygen concentrations are reduced. F G R is most effective in combustion systems that have both a low flue gas exit temperature and oxygen level, as both N O x reduction mechanisms are optimized. Unfortunately the N O x reductions achieved with F G R are not without cost. Quite often the addition of an F G R system will require larger burners, inlet air piping, and combustion air blowers to accommodate the increase in oxidant volume. Operating and maintenance costs typically increase when F G R is added to a combustion system. Changes to the combustion air blower, or the use of a separate F G R fan, increase power consumption and flue gas exit temperatures, reducing the system's thermal efficiency and increasing operating costs. Water condensation in the recirculation system can be corrosive, reducing the operating life of piping, valves, and burner internals. Many high temperature applications cannot practically recirculate combustion gases due to process constraints, such as the addition of fluxing agents, particulate carryover, or air infiltration. Also, operation of the combustion system with high levels of F G R tends to reduce the stability of the burner, potentially resulting in control difficulty and increasing frequency of burner relights. Many hidden costs of F G R can be avoided by using the largest "free" source of F G R available, the furnace itself. Many types of burners develop in-furnace recirculation zones that bring low oxygen, reduced temperature gases directly into the flame envelope. These gases again reduce peak flame temperature and reduce local oxygen levels. A plot of equilibrium N O x levels vs. oxidant vitiation is shown in Figure 1. Traditional burner systems are extinguished by these high recirculation rates. Dilute gas injection combustion systems such as LNI/FDI are designed to take advantage of the dilution to provide ultra low N O x emission levels. LNI/FDI COMBUSTION SYSTEMS 12000 10000 8000 6000 4000 2000 0 A- •v N •N*V> V: •- 1144 K Oxidant -•- 1366 K Oxidant -A~ 1589 K Oxidant Cv ^ ^ ^ £ ^ = » - 25 20 15 10 Oxidant Oxygen. % Figure 1 Schematic of LNI Burner An LNI/FDI burner operates as a high velocity nozzle mixing burner when the furnace temperature is below 1030 K. Above 1030 K, fuel is switched to strategically positioned nozzles adjacent to the burner tile port. These injectors are displaced from the burner tile exit inside the furnace [Figure 2]. Fuel and oxidant streams mix thoroughly with furnace gases, becoming extremely dilute before combining in front of the burner tile. Oxygen concentrations can be reduced to less than 5 % in the oxidant stream. The dilute gas streams autoignite and achieve complete combustion within the furnace environment. In the flame envelope, entrained gases limit maximum in-flame combustion temperatures that generate high N O x emissions. All combustion takes place within the furnace, not inside the tile port, providing short high temperature residence times that further inhibit N O x production. After combustion, the gases lose their heat through radiation and convective heat transfer to the work. These cooled gases travel throughout the furnace and are again entrained by the burner oxidant and fuel jets, sustaining the N O x inhibiting process. Separated high velocity oxidant and gas jets can reduce the combustion system N O x emissions by as much as 9 0 % . Figure 2 2 |