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Title	NOx Reduction by Fuel Injection Recirculation: Insights from Laminar Flame Studies
Creator	Feese, J. J.; Turns, S. R.
Publisher	Digitized by J. Willard Marriott Library, University of Utah
Date	1996
Spatial Coverage	presented at Baltimore, Maryland
Abstract	Flue-gas recirculation (FGR) is a well-known method used to control oxides of nitrogen (NOx) in industrial burner applications. Recent small- and large-scale experiments have shown that introducing the recirculated flue gases with the fuel results in a much greater reduction in NOx, per unit mass of gas recirculated, in comparison to introducing the flue gases with the combustion air. That fuel injection recirculation (FIR) is more effective than windbox FGR is quite remarkable. At present, however, there is no definitive understanding of why FIR is more effective than conventional FGR. The objective of the present investigation is to ascertain whether or not chemical and/or molecular transport effects alone can explain the differences in NOx reduction observed between FIR and FGR by studying laminar diffusion flames. Numerical simulations of counterflow diffusion flames using full kinetics were performed and NOx emission indices calculated for various conditions. Studies were conducted in which a N2 diluent was added either on the fuel- or air-side of the flame for conditions of either fixed initial velocities or fixed fuel mass flux. Results from these simulation studies indicate that a major factor in diluent effectiveness is the differential effect on flame zone residence times associated with fuel-side versus air-side dilution. Simulations in which flow velocities were fixed as diluent was added either to the air or fuel stream showed lower NOx emissions for air-side dilution; however, if instead, fuel mass fluxes were fixed as diluent was added, which results in an increase in the velocity of the streams, fuel-side dilution was more effective. Experiments using laminar jet flames were conducted in which either the air or fuel stream was diluted with N2. The experiments showed that fuel-side dilution results in somewhat greater NOx emission indices than air-side dilution. The higher flame temperatures measured with fuel dilution appear to be the principal cause of the higher emissions. Since fuel dilution is more effective than air dilution in suppressing in-flame soot formation, radiant heat losses with fuel dilution may be less causing temperatures to be somewhat higher. The results of both the numerical simulations and the experiments suggest that, although molecular transport and chemical kinetic phenomena are affected by the location of diluent addition depending on flow conditions, the greater effectiveness of FIR over FGR in practical applications more likely results from differences in turbulent mixing and heat transfer.
Type	Text
Format	application/pdf
Language	eng
Rights	This material may be protected by copyright. Permission required for use in any form. For further information please contact the American Flame Research Committee.
Conversion Specifications	Original scanned with Canon EOS-1Ds Mark II, 16.7 megapixel digital camera and saved as 400 ppi uncompressed TIFF, 16 bit depth.
Scanning Technician	Cliodhna Davis
ARK	ark:/87278/s6qf8wgk
Setname	uu_afrc
ID	10894
Reference URL	https://collections.lib.utah.edu/ark:/87278/s6qf8wgk

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Title	Page 12
Format	application/pdf
OCR Text	Show (Table IV), by adding N2 diluent to the fuel stream or to the air stream while maintaining the fuel and air flowrates fixed. Emissions measurements and flame photographs were obtained for various levels of dilution. TABLE IV. Baseline Experimental Flame Conditions (No Diluent) Fuel Temperature (K) Air Temperature (K) Fuel Flowrate (mg/s) Air Flowrate (mg/s) Mean Fuel Velocity (cm/s) Mean Air Velocity (cm/s) Unheated Reactants 293 293 3.0 243 5.0 2.4 Heated Reactants 413 398 3.3 243 7.8 3.5 NO, NOx and CO emission indices were calculated from measured mole fractions of NO, NOx, CO, and C02 in the product gases [11]. Visual flame length measurements were obtained from color flame photographs. Although all flames were nonsooting, i.e., no soot escaped the flame, the appearance of a yellow luminosity indicated the presence of in-flame soot. Typically, the region closest to the fuel tube exit was blue (soot free), while the remainder of the flame exhibited a yellow luminosity. This soot-free length of the flame at each condition also was measured. Results and Discussion NOx emission indices and visible flame lengths are shown as functions of diluent fraction in Fig. 9 for the flames with heated reactants. Here we see that air dilution was more effective than fuel dilution in reducing NOx emissions. We also see that visible flame lengths increased somewhat as diluent was added to the air stream, while no effect on flame length was observed for fuel dilution. Similar trends were found for both NOx and flame lengths for the flames with unheated reactants; the effects, however, were less pronounced. Flame length trends with diluent addition are nominally consistent with the theoretical predictions of Roper [13,14]. Using Roper's analysis, we predict that the addition of diluent to Z = 0.15 should result in an increase in flame length of 15% for air dilution and an increase of 8% for fuel dilution. The experimental visible flame lengths showed increases of 18% and 0%, respectively, while flame lengths based 11
Setname	uu_afrc
ID	10882
Reference URL	https://collections.lib.utah.edu/ark:/87278/s6qf8wgk/10882