Low NOx technology overview, Tempest® 4441
CONVENTIONAL FIRING
LOW NOX INJECTION (LNI)
When a 4441 is in conventional mode, the gas enters the burner at the base of the stabilizer cup along the burner centerline. The flame starts burning inside this cup when an ignition spark and a small amount of air is added to the gas. Because the flame is burning in the stabilizer cup and the tile it can be monitored with a flame supervision system. Additional air is added to the fuel in multiple stages as the flame moves through the burner. This keeps the flames oxygen content low, and the outer air helps cool the stabilizer and the tile. Just before the flame leaves the tile, it tapers down to a small exit hole which greatly increases the flames velocity as it exits the tile. The final air/fuel reaction happens in the furnace where the high velocity flame jet exiting the burner entrains the furnace atmosphere and mixes it into the burning flame envelope. As long as the furnace atmosphere has a low oxygen content (2-3% O2) the entrained furnace gases (P.O.C) will further dilute the oxygen in the flame and lower peak flame temperature which helps minimize the amount of NOx created. Unfortunately, there are many conditions that can increase the amount of NOx created in flames, including: — Preheated air — High furnace temperatures — High amounts of oxygen in the furnace — Fuels like Propane — Furnace geometry that restricts atmosphere recirculation
4441 LNI burners are equipped with a strategically placed gas injector, outboard of the tile exit. Once the furnace is above 1450°F (790°C), the burner can be switched into the LNI firing mode by moving the gas from the central burner location to the outboard injector. The flame will then relight in the furnace, and because all the gas enters the furnace through the injectors, the flame can’t re-enter the burner tile. When operating in LNI mode, air continues to flow through the burner and tile, while gas is switched to the outboard injector. This arrangement keeps the air and gas streams physically separated and allows both air and fuel jets to pull the furnace gases (P.O.C.) into each flow stream individually before they mix together and start combusting. Because LNI allows both air and gas streams to be vitiated (made less reactive) with inert poc and separated longer, an increase of furnace temperature has less impact on the production of NOx than burners using conventional or DMC NOx control. LNI is also better at suppressing NOx when using preheated combustion air compared to other NOx reduction techniques. It is important that the thermocouple used to bypass the flame supervision is installed in the coolest section of the furnace to insure the temperature is high enough everywhere in the furnace before switching. If the furnace temperature drops below the transition temperature, the burner must be put back in conventional mode and the flame supervision system returned to conventional firing operation.
Bulletin 4441 LNI Page 4
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