Fuel Staging Burners for NOx Control

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Figures 9 and 10 compare the conventional burner a~d staged fuel burner at various air preheat levels. FIgure 11 is a plot of the data for 2% oxygen and. s.hows that. the staged fuel burner is much less sensItIve to aIr preheat levels than conventional burners. 350 300 S 250 tl- C') 0 ~ 200 ~ CJ I0II : 150 g I .:c 100 0z 50 1 00 M Btu/hr. o 100 200 300 400 500 eoo AIR PREHEAT TEMPERATURE,oF Figure 11 Comparison of NOx vs. Air Preheat for Various Firing Configurations at 2% Excess Oxygen NEW SOURCE PERFORMANCE STANDARD The proposed New Source Performance Standard (NSPS) for industrial boilers requires that NOx emission levels not exceed 0.1 pounds of NOx per million Btu fired. For natural gas this level corre­sponds to about 75 ppm of NOx at 3% oxygen. The data presented show that staged fuel burners can meet this level provided that air preheat levels are moderate. This is generally the case with industrial boilers. Being able to meet this level with low NOx design burners is beneficial in that it eliminates the need to employ flue gas recirculation (FGR). Using flue gas recirculation can increase the boiler capital and operating costs in several ways. First, a hot fan and duct work is required to convey and inject the flue gas into the combustion air. Second, the boiler must either be designed with larger convective flow area or the boiler fan horsepower must be increased to handle the added mass flow due to the recirculated flue gases. COMBINED STAGED FUEL AND FGR Although flue gas rec~rculati?n can be exp.ensive to employ it can be combIned wIth fuel stagIng to achieve extremely low emission levels. These very low emission levels are sometimes required in are~s wh~re very restrictive emission rules apply. Fuel stagIng wIth FGR would be a much less expensive NOx control 349 DUCT UW'LI! TO ..... 'fIIU .. EHT AOOW NO.. CO, 0, [ EXTERNAL MANFOLDS PA.WARY AND IeGQNOAAY FUEL ITATIC TC BURNER PLENUM ------COMBUSTION AIR BLOWER Figure 12 Test Installation for Staged Fuel Boiler Burner with Flue Gas Recirculation method than selective catalytic reduction or Thermal DeNOx. In order to test the combination of fuel staging and flue gas recirculation components of the indirect air preheat system were rearranged to provide vitiated combustion air that simulated combustion air mixed with flue gases. The firing rate of the duct burner and the cooling air to the heat exchanger were adjusted to provide various levels of FGR at various temperatures. Figure 12 shows the setup for these tests. Figure 13 shows the NOx emission rates that resulted when different levels of FGR were combined wi th fuel staging. These data were collected using a 50 million Btu per hour burner with 500 deg F combustion air. The data show that the addition of low levels of 120 110 100 90 N 80 0 ~ CO) 70 0 I- 0 w 60 I- (J W a:: a:: 50 0 ~ :E 40 Q. Q. ;C 0 30 z 20 10 50 M Btu/hr. 0 5 10 15 20 25 % FLl£ GAS RECflCULA TION Figure 13 Staged Fuel Burner: NO x VS. FGR (2% O2- 500°F Air)