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Show Experimental Facilities: Two main experimental facilities are being used to perfonn thepresent research study. A 5 mmBtulhr upfired test facility is used for experimental testing of the baseline and prototype burners. The upfired facility has a 4 foot square cross section and is 12 feet high. The facility has a combination of refractory and water cooled walls allowing the modification of wall heat transfer rates. The facility is equipped with numerous sampling locations for extractive emissions and temperature probing. A second test facility is used allowing much higher firing rates. This facility is rated at 50 mmBtulhr and also is equipped with various sample ports for in-situ emissions and temperature measurements. The 50 mmBtulhr facility is horizontally fired into an 8 foot diameter, 40 foot long pipe which has water sprayed on its external shell to prevent overheating and simulate cool boiler heat transfer surfaces. The front floor of the facility is lined with refractory brick to simulate the refractory floors present in most package boilers. Numerical Approach: The numerical approach taken during this study is to use chemical kinetics codes, advanced computational fluid dynamic codes and the experimental data in concert and develop a reliable burner perfonnance predictive capability. While not presented in this current paper, the chemical kinetics codes CHEMKIN II and GRIMech 2.0 are used to evaluate the effects of operating conditions on the kinetic fonnation and destruction of NOx within the flame. The computational fluid dynamics are evaluated using the commercial CFD code FLUENT. Two versions of the code have been evaluated in this research program, using structured and unstructured grid systems. The NOx models from the CFD code are used to evaluate the relative fonnation of prompt, thennal and third-body NOx from the burner. The effects of air preheat, flue gas recirculation, burner mixing rates, and heat release patterns are investigated with the use of CFD. RESULTS Experimental: The emissions results for the baseline burner configuration are listed in Figure 2. The 5 mmBtulhr NOx emissions had a baseline of 18 ppm without flue gas recirculation. The data in this figure were all obtained at the high fire condition. No attempts were made to optimize emissions perfonnance at turn down conditions. This NOx emissions level for the baseline burner is decreased to less than 5 ppm NOx with the addition of flue gas recirculation. The ability to decrease NOx emissions below 5 ppm is limited by the stability of the burner. At high FGR rates, the burner no longer has enough flame stability to permit stable combustion. When the baseline burner design is scaled up to higher firing rates, the baseline NOx emissions level increases due to the increase in firing intensity. The baseline burner at 45 mmBtulhr emits 26 ppm without the use of flue gas recirculation. When FGR is applied, the emissions level decreases to less than 5 ppm at approximately 300/0 FGR. The same flame stability limit exhibited with the 5 mmBtulhr size prevents NOx emissions levels below 5 ppm with the addition of more FGR. |
