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Show levels with an increase in furnace temperature only slightly greater over the temperature range than the increase predicted by Figure VII for the internal recirculation burner with low primary fuel flow. However, it should be remembered that the curves presented for the internal recirculation burner are for a specific set of furnace operating conditions. Should another set of operating conditions be chosen and the same curves generated for the increase in N O x levels with respect to furnace temperature for varying primary gas flow rates, a different set of correction curves will be obtained. In fact, for fuels with higher heating values, the slope of the curves increase and in most cases, the curve shown in Figure VIII actually predicts an increase in the N O x levels over the range that is less than the increase given by the prediction curve for internal recirculation burner with low primary fuel rates. 1.60 1.60 1.40 1.30 1.20 1.10 1.00 13 NOx vs FURNACE TEMPERATURE CONVENTIONAL BURNER RELATIVE NOx LEVELS ^ y y 00 MOO 1600 1600 1700 1600 1900 20 FURNACE TEMPERATURE (DEG F) 00 Figure Vffl Figure IX is a correction curve for combustion air preheat for various primary fuel gas flow rates. Note the increased air preheat temperature in this case has a greater impact on N O x production when the primary gas flow rate is low. The increased mass in both the primary and secondary combustion zones tempers the increase in combustion temperature that is normally associated with preheated air. With the high primary gas flow rates, more recirculated gas is present in the combustion zones providing a greater tempering effect which in turn limits the effect of preheated air on the formation of N O x. NOx vs COMBUSTION AIR TEMPERATURE INTERNAL RECIRCULATION BURNER RELATIVE NOx LEVEL *-* ^ J 1 1 1 1 1 PRIMARY FUEL - 1i% -*-30% -*-M% / / ^ .f A^ ^ P5 : 0 60 100 160 200 260 300 360 400 460 600 660 600 660 700 AIR TEMPERATURE (DEG F) Figure IX Figure X3- shows the traditional curve that is used for correcting N O x levels for preheated air. Note that this curve indicates a doubling of N O x levels when the preheated air temperature is approximately 500°F. However, the tempering effect of the recirculated gas in the internal recirculation burner results in correction curves with less slope. For this particular case, both the correction curves for the high primary gas flow rate and low primary gas flow rate are below the curve for conventional burners. 10 rv-2i |