North American TwinBed ® II LNI Flames and NOx Emissions
Sheet 4343-8
LNI (Low NOx Injection) Flame and NOx Emissions with TwinBed II
combustion zone
TwinBed II is operated as a conventional high velocity burner when furnace temperature is below 1450°F (790°C). When the furnace is above that temperature, the TwinBed II burners can be automatically switched to LNI firing to inhibit formation of NOx. Air continues to flow through the center port of the burner, but gas is switched to strategically placed outboard injectors. (Figure 1.) Mixing of gas and air is radically changed. Whereas before mixing and combustion was concentrated primarily within the burner tile, now the furnace space in front of the burner is used for mixing and combustion. Each jet of gas creates a venturi effect (suction) which pulls the furnace gases (poc) into the gas stream. The air stream also creates a venturi effect to mix air with furnace gases. Both the air and gas streams are thus vitiated (made less reactive) with inert poc. Dilution of the air/gas mixture with inert gases slows the combustion process, reducing hot spots in the flame and thus reducing NOx formation. LNI mixes more poc into the air/ gas mixture before combustion than is possible with conventional firing techniques. The effect of multiple gas streams entering the air stream at different angles and locations affects the speed of air/gas mixing and combustion reaction to reduce NOx emissions. This is accomplished without complex refractory tiles used in staged air burners. Flame appearance changes with the switch from conventional firing to LNI. The traditional feather shaped flame is gone and in its place is an almost invisible cloud of combustion. Although flame appearance is very different, there is no significant change in heating pattern or heat transfer. Laboratory tests and actual
POC recirculation
combustion zone
POC recirculation
hot air
gas
air
Figure 1: Injectors promote mixing of poc with air/gas mixture.
U. S. Patent 4,945,841
experience confirm the quality of heating is maintained when firing LNI. Graph 1 shows a laboratory furnace temperature profile measured by a series of thermocouples positioned on the furnace wall along the furnace length. The temperature profiles are virtually the same for both burners. An additional benefit results from mixing air and gas outside of the burner tile area. The tile temperature is at or below furnace temperature since it carries only hot air when firing or poc when exhausting gases. Unlike conventional firing, there is no combustion within the burner tile which decreases thermal wear on the burner.
LNI vs. NOZZLE MIX
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NOZZLE MIX LNI
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Graph 1: LNI and nozzle mix firing display similar heat release profiles operating in test combustion chamber.
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Distance from burner, ft
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