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Show 5 is another way of increasing the amount of soot particles in the flame.) Addition of soot particles to the flame by using an external cracker was evaluated in exploratory tests on the IGT glass tank simulator. About 35% reduction in NOx was achieved, although the cracker produced only a fraction of the desired soot. A portion of the hot combustion air was mixed with a portion of the fuel in an insulated chamber under highly fuel-rich conditions to crack the natural gas. The resulting gas/soot mixture was reinjected into the furnace with the main portion of the natural gas. Fuel Staging: Burning part of the fuel with all the combustion air to lower peak flame temperature, and injecting the remaining fuel so that it mixes with the flame and burns downstream of this highly fuel-lean combustion zone. This technique was tested on the IGT glass tank simulator by introducing a portion of the natural gas (generally 500/0) through 90° side-ofport burners and the remaining natural gas through either an overport or two underport burners. Fuel staging was found to be effective in significantly reducing NOx when the secondary natural gas was injected through the overport burner to intersect with the port axis about 4 port widths downstream of the port exit. Decreasing this distance by varying the injection angle increased NOx. Underport secondary natural gas injection was not effective. A version of this technique has been tested by Sorg on commercial glass melters showing significant NOx reduction. Fish-Tail Flame Burner: Provide a thinner, wider, and longer flame to increase heat transfer and reduce peak flame temperatures. Internal Combustion Products Recirculation: Recycling the relatively cold combustion gases from around the flame back to the root of the flame to reduce the peak flame temperature. Firing With Oxygen Between Reversal: Firing natural gas with oxygen during the typical 40-second furnace reversal to increase production rate while generating less NOx. Lower Air Preheating: Heating the combustion air to a lower-than-normal preheat temperature to reduce the peak flame temperature. The impact of decreasing the level of air preheat was investigated on the IGT glass tank simulator, as well as on a full-scale sideport glass tank, by comparing NOx from different ports. Figure 2 il1ustrates the generalized correlation for the effect of combustion air temperature. The correlation indicates that each 100°C (180°F) change in combustion air temperature wil1 cause either a 27% decrease or a 370/0 increase in exhaust gas NOx level, depending on the direction of the change. This, however, may not be a viable approach, because reduction in combustion air temperature will have adverse effects on the production rate, as wel1 as the furnace thermal efficiency. External Flue-Gas Recirculation (Operation With Vitiated Combustion Air): Cooling and recirculating part of the flue gas to the burner to reduce the peak flame temperature and reduce the oxygen available in the flame zone. The impact of recirculating 70°C (l60°F) flue gases, containing 30/0 02, into the combustion air was investigated on the IGT glass tank simulator. As il1ustrated in Figure 3, flue-gas recirculation (FGR) was ineffective in that even 10% FGR only decreased NOx by 250/0 (9). The likely reasons are that 1) the flame temperature did not decrease in direct proportion to FGR because of reduced C02 and H20 dissociation with FGR and 2) the increase in port velocity with FGR would increase fuel/air mixing. The NOx (P-002\1194Maui.doc |