Effect of radiation on nitrogen oxide emissions from nonsooty swirling flames of natural gas

Twenty-FifthSymposium(International)on CombustiondFheCombustionInstitute,1994/pp.235-242

EFFECT OF RADIATION ON N I T R O G E N OXIDE EMISSIONS FROM NONSOOTY SWIRLING FLAMES OF NATURAL GAS A. SAYRE, N. LALLEMANT,J. DUGUI~.ANDR. WEBER International Flame Research Foundation P.O. Box 10,000, 1970 CA IJMUIDEN, The Netherlands

The present study attempts to quantifythe effect of radiationon nonpremixed,turbulent diffusionflame temperature and NO~ emissionsfor flames subjected to different overall heat extraction. Comprehensive in-flamedata have been generated for two flames of similarstrain fieldand luminosityin order to facilitate flame comparison.The flame produced in the refractory-linedfurnace (flame 1) is shorter than that of the water-cooled confinement (flame 2) by around 30%, and the extent of combustionincreases by 20% at a distance 27-ram downstream of the burner outlet. The density-averagedmean flame temperatures were 1850 and 1675 K, and furnace NOt emissionswere 49 and 33 ppm for flames 1 and 2, respectively. The total radiative intensities calculatedspectrallyusing the ExponentialWide Band model are in very good agreement with the measurements and indicate that the amount of radiant energy emitted directly by both flames is similar.Radiationplays a crucial role in extractingthe energy from the postflame zone, and therefore, it affects the enthalpyof the products entrained from the externalrecirculationzone. Flame temperatures and NO. emissionsare stronglydependent on the energy of the flow entrained by the jets. It is concludedthat the radiant fraction cannot be used as an exclusiveNO, scalingparameter to describe the effect of radiationon emissionsof confinedturbulent diffusionflames. Proper scalingparameters should incorporate the energy entrained (from external recirculation zone) into the flame, and therefore, these parameters have to take into account the amount of energy extracted from the postflame zone.

Introduction It happens only too often that a low-NOx burner, when installed in a furnace different from which it was originally tested, produces unacceptable NOx emissions. This is predominantly due to unforeseen alterations to either in-furnace fluid flow or heat transfer. Therefore, the understanding of the effect of radiative heat transfer on the flame properties [1,2] and consequently NOr emissions [3] becomes of considerable technical importance. It is important to estabhsh how the NOx and CO emissions vary (scale) with burner inputs, burner size (thermal input), and thermal conditions in the furnace. For simple jet diffusion flames, there is a wealth of experimental information on how the emissions scale with Reynolds, Froude, and Damkt~hler numbers [4,5]. Recently, Turns and Myhr [3] provided some quantification of the effect of radiation on the flame temperature and NO emissions. They used four different fuels to generate nonswirlingjet flames of different luminosity. It was demonstrated that the radiant fraction (defined as the amount of energy radiated directly from the flame normalized by the total thermal input) should be taken into account while scaling NOx emissions of jet flames. In industrial situations, the scenario is often different. When the same burner is installed in furnaces of dif-

ferent heat extraction characteristics, the flame properties are altered mainly due to different overall heat transfer patterns in the furnace and not by alterations to the flame luminosity. The primary objective of this study is to quantify the effect of radiation on flame temperature, NO~ and CO emissions under conditions of high and low heat extraction without altering flame luminosity. Experimental Natural Gas Burner:

A swirl-stabilized natural gas burner, shown in Fig. 1, has been used. The burner design incorporates a movable block swirler for easy and continuous variation in inlet swirl (from 0 to 1.2) and for the possibility of handling ambient and preheated combustion air. Options for fuel staging and flue gas recirculation, two of the most effective methods of NO~ reduction with natural gas firing, are provided. In Fig. 1, the dimensions of the burner elements are normalized to the diameter of the combustion air duct (Do). The burner is available in 30 and 300 kW, 1.3, 4, and 12 MW versions and is being used in the SCALING 400 study [6]. For the experiments reported here, the 300-kW version (Do = 87 mm) is used. The central natural gas injector features

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Experimental Furnace: The experimental furnace used for the measurements is the vertically fired chamber of the Burner Engineering Research Laboratory. The confinement ratio, defined as the ratio of the furnace diameter to the burner quarl (tile) outlet diameter (1.66. Do), is 7.5 and 8.9 for the refractory and bare-metal walls, respectively. For the in-flame measurements, panels were constructed with ports to allow access at five traverses located at AD/D0 of 0.31, 1.25, 2.19, 3.94, and 4.96 (AD stands for axial distance measured from the quarl outlet).

Fuel Gas: The volumetric composition of the natural gas used in the experiments is 0.965 CH4, 0.017 C2H6, 0.01 C3Hs, 0.01 C4+, 0.03 CO 2, and 0.013 N2. The following properties resulting from this composition are a density of 0.7 kg/Nm3, stoichiometric air requirement of 16.58 kga~y~/kgf~el, and a lower calorific value of 46.7 MJ/kg.

Measurement Techniques: Velocity measurements are performed nonintrnsively by applying the laser Doppler anemometry technique on the combusting flow seeded with finely sized zirconia particles in the combustion air. Miniaturized versions of standard semi-industrial International Flame Research Foundation (IFRF) probes are used to intrusively sample gas composition and measure gas temperature by suction pyrometry. The

permanent gases; CO, CO2, H2, 02, NO~, and total unburned hydrocarbons are measured using continuous gas analyzers. The flames were traversed with a narrow angle radiometer with a viewing angle of 1.42~ according to the technique described by Be6r and Claus [7]. As described in the following sections, two flames are traversed at the first two measurement locations, AD/D0 = 0.31 and 1.25 with the refractory-lined panel for the flame with hot wail conditions replaced with a cold target.

Experimental Results Description of the Flames: The experimental burner was used to generate stable and very symmetrical flames of 300-kW thermal input, two of which are described in this paper. The two flames differ only in furnace configuration with flame 1 being generated in the experimental furnace with refractory-line panels (hot wall condition), while flame 2 is generated in the same furnace assembled from water-cooled panels (cold wall condition). The two flames have identical burner input conditions, including inlet swirl (0.56), thermal input, and overall stoichiometry (15% excess air), as listed in Table 1. For both flames, all the fuel is supplied through the central gas injector (no fuel staging is applied). The flue NO~.emissions are 49 ppm (1.3 gyox/kgfuel) and 34 ppm (0.9 gNox/kgfuel)for flue gas temperatures of 1113 ~ and 935 ~ for flames 1 and 2, respectively. Around 29% of the total thermal input is extracted for flame 1, while for flame 2, a figure of 50% is applicable.

In-Flame Measurements: The in-flame measurements include temperature, CO, CO2, NO~, 02, and unburned hydrocarbons (UHC), as exemplified in Fig. 2. Time-mean axial velocities and root mean square (rms) values at the first traverse are shown in Fig, 3, demonstrating that an al-

NITROGEN OXIDE EMISSIONS FROM NONSOOTY SWIRLING FLAMES OF NATURAL GAS TABLE 1 Parameters of flames 1 and 2 Flame t 316 0.56 28 39 22.7 35 14 93 884

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most identical strain field prevails in both flames. The same observation is valid for all the measured traverses (see Reference 8). In both flames, a small internal recirculation zone (IRZ) is formed in the burner vicinity. At the first measurement traverse, around 5.7 and 6.9% of the inlet flow is recirculated back to the flame root. The reverse flow energy, defined as the recirculated mass flow rate times the sensible heat, is 12.6 and 15.2 kW for flames i mad 2, respectively. The measurements demonstrate that in flame 1, higher temperatures occur at the flame front (radial distance 0.8-1.0, see Fig. 3), than in flame 2. Large external recirculation zones (ERZ) are formed in the furnace. In flame 1, the temperature of recirculating gases is around 1350 K (see Fig. 3), and the NO~ concentration is around 37 ppm. In flame 2, the ERZ contains combustion products of 920 K (see Fig. 3) and NOx concentration of 25 ppm. An integration of the measured axial velocities shows that flame 1 entrains more than flame 2. At AD/D0 = 2.19, the entrainment is 125% for flame 1, while only 75% for flame 2; at AD/D 0 = 3.94, the entrainment figures are 200 and 110% for flames 1 and 2, respectively.

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Discussion

Rate of Combustion: The conical flame shape derived using the in-flame measurements agrees very well with the visible flame boundaries taken from the photography: For both flames, a contour of 5000 ppm of carbon monoxide

is found to match the qualitative description of the boundary from the photography. It is calculated that for flame 1 (hot walls), the flame length, volume, and surface area are 19.6 cm (2.25" Do), 4185 cm a, and 1332 cm 2, respectively. For flame 2 (cold wails), the figures of 25.6 cm (2.95"D0), 5863 cm 3, and

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1640 cm 2 are applicable. Another way of estimating the flame length is by calculating the extent of methane combustion (ZcH4) according to rncn 4 = 2~

FIG. 3. Axial velocities and temperatures at AD/D0 0.31. The Q designates flame 1 temperatures, and the 9 represents flame 2. The open symbols refer to the velocities with Z~ and [] representing flame 1 and flame 2 mean axialvelocities and O and V representing flame 1 and flame 2 rms a;dal velocities, respectively. RD stands for radial distance, DO 87 ram.

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where rhcH4 is the methane mean-mass flow rate, YCH4is the mean-mass fraction of methane,/3 is the mean density, and tJ is the mean-axial velocity. The integration is performed from the flame center to the boundary of the jet (forward flow boundary). Figure 4 shows the extent of reaction confirming that

at AD/D0 = 2,25, methane combustion is completed for flame 1, while in flame 2, the combustion proceeds until a distance of AD/D 0 = 2.95. This is in agreement with the flame boundary determined visnally.

The measured peak temperatures are 1945 and 1864 K, while the mass-mean-averaged values within the fame volumes defined above are 1850 and 1675 K for flames 1 and 2, respectively. It is estimated that the residence time is in the range 6--10 ms for flame i and around 40% longer for flame 2. For both flames, the NOx concentrations increase with the distance from the burner tip until the end of the flame. The measurements show that NOx concentrations remain practically unaltered in the postflame part of the furnace. In Fig. 4, the mass-averaged values of NOx volume fraction within the forward flow

NITROGEN OXIDE EMISSIONS FROM NONSOOTY SWIRLING FLAMES OF NATURAL GAS

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RadiativeHeat Transfer: The radiative transfer equation (RTE) reads as follows:

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where the first term is the gradient of intensity and the second and third terms represent the absorption and emission of energ'y, respectively. The total intensity leaving a gas layer of length L can be obtained by integrating Eq. (3) over wave number v in em -1 and path length. This results in the following expression: I(L)=

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FIG. 5. Comparison between measured and predicted total radiance for flame 2 with the cold target at AD/D0 = 0.31. The 9 designates the measured total radiance. The solid line shows the spectral calculation using the exponential wide band model. The dotted lines represent the contributions of H~O, CO~, CO, and CH~ to the predicted total radiance. RD/Do= 0.0 corresponds to the axis of the burner.

where I (L) is the total intensity leaving the gas layer in W/(m ~" sr), Iv(0) is the spectral intensity incident upon the gas layer, I ~ is the black-body spectral intensity in W/(m z" sr' cm- i), and k~ is the spectral absorption coefficient of the gas in m- 1. The spectral transmissivity of the gas is given by rv(s', L) =

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The intensity distributions showaa in Fig. 5 are eaIculated spectrally using the exponential wide band model (EWBM) [9] 'along with the Chan and Tien scaling method [10,11]. The total pressure of the gas was taken as one atmosphere. The spectral calculations, which take into account the nonhomogeneity of the temperature and composition fields, are performed over wave number bands, assuming that the spectral overlap is not significant. In view of the temperature and concentrations prevailing in the flames, the following bands are included: 10-20 pm rotational; 6.3, 2.7, 1.87, and 1.38/zm for H20; 10.4, 9.4, and 4.3/tm for CO2; 2.35 and 4.7/tin for CO; 7.66, 3.31, and 2.37pm for CH4. The 10-20r H20 band is included in view of the fairly low temperatures in the ERZ (around 900-I350 K) and the high concentrations of water vapor [12]. The 15 pm CO 2 band, though of some importance at these temperatures, has been neglected since it requires special procedures to account for the overlapping. Moreover, it is expected that the band from water vapor in the 1020 a m region is stronger since the HzO/CO 2 partial pressure ratio is around 2. Following James and Edwards [1], the 2.7-pro band of CO2 is neglected due to a maximum underprediction of total intensity by 8% when this band is excluded. The importance of the 2-pm band of CO2 is estimated to be less than 2% of the total emissions [13] and, hence, is not included in the present calculatious. All important

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FIG. 6. Comparison of the radiative intensity absorbed for flames 1 and 2 at AD/D0 = 0.31. The 9 and 9 designate the measured radiance for flame 1 with and without the contribution of the walls, respectively. The 9 designates the measured radiance for flame 2; (3 shows the calculated total intensity from Eqs. (6) and (7). RD/Do = 0.0 corresponds to the axis of the burner. 21.9 kW/(m~. sr) is the contribution to the total radiance from the wall for flame 1; 12.9 kW/(mL sr) is the contribution due to the temperature difference in the ERZ of flames 1 and 2. bands of CO [9] and CH4 [9,14] are incorporated. As shown in Fig. 5, the calculated radiance agrees well with the measured values. The figure shows that the contribution from water vapor is about twice that of CO2. The contribution of CO and CH 4 is seen to be unimportant. Since the total radiance calculations are uniquely based on the in-flame measurements, the good agreement between the measured and predicted total radiance provides a strong confidence in both the measurements and the calculation procedure. Figure 6 shows the measured radiance of flames 1 and 2 with the cold target positioned on the other side of the furnace. For flame i, the measured radiance with the cold target replaced with a refractory panel (T = 1157 K) is also shown. In all three cases, the difference between the radiance leaving the flame region (RD/D0 = + 1) and that entering (RD/D0 = - 1 ) is around 10 kW/(m2-sr) [for flame 1 without the cold target, it is (47.6-37.3) kW/(mz" sr); for flame 1 with the cold target, it equals (24.5-15.4) kW/(m 2" sr); and for flame 2 with bare metal wails, it is (11.4-2.5) kW/(m2" sr)]. The total radiance incoming at the edge of the flame is certainly affected by both the wall emission and the net

emission of the ERZ (see Fig. 6). However, the 10 kW/(m 2" sr) difference in the flame region is not affected by either. Equation (3) can be used to estimate how much of the intensity has been absorbed within both flames. To this end, only the emission term of the right-hand side is considered, and the solution is as follows:

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(6)

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NITROGEN OXIDE EMISSIONS FROM NONSOOTY SWIRLING FLAMES OF NATURAL GAS as I (0) the values measured at the flame boundaries, are shown in Fig. 6. All the bands of H20, CO2, CO, and CH4 were accounted for in the calculation of the Planek mean emission coefficient Eq. (7). For both flames, the absorbed intensity is about the same at 15 kW/(m2" sr). Similarly, at the second measurement traverse, despite the differences in the flame temperatures and alterations to the gas composition, the absorption terms of Eq. (3) are very similar, Thus, the difference in the amount of energy absorbed by the two flames is proportional to the flame volume ratio (5863/4185 = 1.4), with more energy absorbed by flame 2 than flame 1. The amount of energy emitted fi'om both flames can be calculated as follows: Eemitted = 4"O"q'4"kp(~, T)'Vfl ....

(8)

where Vflame is the flame volume; T and ~ are the mean flame temperature and partial pressures, calculated using density averaging; and cr is the StefanBoltzmann constant. The Planek mean coefficient has been calculated using the EWBM to be 0.85 m - t (hot walls) and 1.0 m -~ (cold walls). The radiant emissions calculated using the mean flame temperature, concentrations, and volume, as specified above, are 9.4 and 10.5 kW, which corresponds to radiant fractions of 3.0 and 3.3% for flames 1 and 2, respectively. If the nonuniformity of the in-flame temperature and chemical composition are accounted for, the radiant flame emissions are 21.8 and 18.8 kW, using density-averaged properties to calculate the local coefficient.

241

and Eout is the energy contained in the jet at the end of the flame region. The first term is precisely known (see Table 1), while Eout and E~,t~ can be calculated using the in-flame measurements. For jet-flame 1, the calculated values are Eo~t = 517 kW and Eentr = 183 kW, while for jet-flame 2, values of Eout = 375 kW and Eentr = 118 kW are applicable. It is important to realize that the energy values cannot be calculated with an accuracy better than perhaps 1020% due to the necessity of interpolating between the measurement traverses. However, the figures clearly demonstrate that the jet-flame 1 entrains twice as much energy as jet-flame 2 and, therefore, is "hotter." The amount of energy emitted (radiant fraction) and absorbed by both flames is substantially lower than the energy entrained from the ERZ. The latter is strongly dependent on the temperature of the combustion products of the ERZ and, therefore, related to the amount of energy extracted from the postflame zone by radiation (mainly) and convection.

Increase of the Methane Combustion Rate: Figure 4 shows that just downstream of the burner quarl, the extent of methane combustion is about 65% for flame 1, while only 40% for flame 2. The combustion products entrained from the ERZ into the jets are at a higher temperature for flame 1 (1350 K) than for flame 2 (920 K). This results in a higher temperature of the gases recirculated via the IRZ back into the fuel injector (around 70 to 90 K hotter for flame 1) and, therefore, in the higher combustion rate inside the burner guard for flame 1.

Energy Balance of the Expanding Jets: The considerations on radiative transfer indicate that the flames emit an almost identical amount of energy. Also, flame 2 may absorb around 40% more energy than flame 1. However, the analysis so far does not provide a reason for the substantially higher temperatures of flame 1. It is thus essential to compare the energy absorbed by the flames with that entrained from the ERZ into the jet flames. Consider the expanding, combusting jets with the outer boundaries determined by the contour of zero axial velocity (where the jets interact with the ERZ). The first jet, corresponding to flame 1, should have the length of 2.25" Do, while the second jet (corresponding to flame 2) should be 2.95" Do long. The flame volumes defined earlier are included within the expanding jets. The energy balance for the jets is as follows: Ein + Eentr + Eabs = Eout + E ....

(9)

where Ein is the thermal input, Eentr is the energy entrained into the expanding jet, E~bs and E .... are the radiant energy absorbed and emitted by the jets,

Conclusions Detailed in-flame measurements of velocity, turbulence, gas composition, temperature, and radiation were taken in two nonsooty, swirling fames of natural gas. The flames are of almost identical velocity (strain) fields with very rapid mixing prevailing. Flame 1 was positioned in a refractory-lined furnace with approximately 29% of the total thermal input extracted. Flame 2 was located in a furnace configured with water-cooled walls in which 50% of the thermal input was extracted. The NOx flue emissions were 49 ppm (1.3 gNoJkgf, el) and 34 ppm (0.9 gyoJkgf.el) for flames 1 and 2, respectively, with almost all the NOx formed within the flame volumes. It has been shown that the energy entrained from the ERZ into the swirling jets has strongly affected the flame temperatures. The lower NO~ emissions of flame 2 have been attributed to entraining the strongly cooled recirculating gases. The following has been concluded: 1. When the same burner is positioned in the refractory-lined rather than water-cooled furnace,

242

PRACTICAL ASPECTS OF COMBUSTION

the flame has shortened from 2.95"D0 to 2.25" Do. The extent of (methane) combustion reaction increased from 45 to 65% at the first measurement traverse 0.31"D0 dowalstream of the burner quark A mechanism for the faster methane combustion rate inside the burner guard for flame 1 is proposed. 2. The amounts of radiant energy emitted and absorbed directly by both 300-kW flames (radiant fractions) are very similar. This implies that the radiant fraction cannot be used as an exclusive NO~ scaling parameter to describe the effect of radiation on emissions of confined turbulent diffusion flames. Radiation plays a crucial role in extracting the energy from the postflame zone and, therefore, controls the ERZ temperature (which corresponds to the furnace outlet temperature). The flame temperature and consequently NO~ emissions are strongly dependent on the latter through the jet entrainment. Proper NO~ scaling parameters should incorporate the energy entrained (from the ERZ) into the flame. Therefore, these parameters must account for the amount of energy extracted from the postflame zone.

Acknowledgnwnts We would like to thank N. Fornaeiari, R. Sanford, and L. Clayton for their assistance during the measurements. The BERL is funded by the Gas Research Institute and the U.S. Department of Energy, Office of Industrial Processes, Advanced Industrial Concepts Division. The project was financed by the Gas Research Institute with the supervision of Dr. R. V. Gemmer. REFERENCES 1. James, R. K., and Edwards, D. K., J. Heat Trans. 99:221 (1977).

2. Gore, ]. P., and Faeth, G. M., Twenty-First Symposium (International) on Con{bustion, The Combustion Institute, Pittsburgh, 1986, p. 1521. 3. Turns, R. S., and Myhr, F. H., Combust. Flame 87:319, (1991). 4. Bilger, R. W., and Beck, R. E., Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1975, p. 541. 5. Chen, R.-H., and Driscoll, J. F., Twenty-Third Symposium (International) on Combustion, 1990, p. 281. 6. Weber, R., Driscoll, J. F., Dahm, W. J. A., and Waibel, R. T., "Scaling Characteristics of the Aerodynamics and Low NO~ Properties of Industrial Natural Gas Burners," The SCALING 400 Study, GRI-93/0227, (1993); also avaiable as IFRF Doc No. F 40/y/9

(1993), 7. Be~r, J. M., and Claus, J., "The Traversing Method of Radiation Measurement in Luminous Flames," IFRF Doe. C 72/a/6 (1967). 8. Sayre, A. N., Lallemant, N., Dugu6, J., and Weber, R., "Scaling Characteristics of the Aerodynamics and Low NO~ Properties of Industrial Natural Gas Burners," The SCALING 400 Study, Part IV: The 300 kW BERL Test Results, Report submitted to the Gas Research Institute, 1994. 9. Edwards, D. K., in Advances in Heat Transfer (T. F. Irvine, Jr. and J. P. Harnett, eds.), Academic Press, New York, 1976, vol. 12, p. 115. 10. Chan, S. H., and Tien, C. L., J. Quant. Spectrosc. Radiat. Trans. 9:1291 (1969). 11. Tien, C. L., and Lee, S. C., Prog. Energy Combust. Sei. 8:41 (1982). 12. Leckner, B., Combust. Flame 19:33, (1972). 13. Leckner, B., Condgus.t. Flame 17:37, (1971). 14. Brosmer, M. A., and Tien, C. L., J. @rant. Spectrose. Radiat. Trans. 33:521 (1985).

COMMENTS Ruey-Hung Chen, University of Central Florida, USA. As previously demonstrated in a smaller scale swirl burner, the internal recirculation zone properties, such as its residence time and the ratio of fuel jet momentum to recirculation zone velocity, determine the fuel-air mLxing and flame behavior [1,2]. They should similarly determine NOx (at least thermal NOx) emission. Would you comment on the role of ERZ as it may affect the internal recireulation, as implied in your paper? REFERENCES 1. Chen, R.-H., and Driscoll, J. F., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1989, p. 535.

2. Chen, R.-H., et al., Combust. Sci. Technol. 71:197 (1990).

Author's Reply. The burner design utilizes both bluffbody and swirl stabilization in concert with radial injection of fuel into the combustion air stream to achieve stability over a wide range of operating conditions. The flames that are compared were produced under similar burner inlet conditions; hence, they possess a similar strain field and internal recirculation zone. The lower NO~ emissions for Flame 2 (cold walls) than for Flame 1 (hot walls) are therefore concluded to be solely a consequence of the lowering of the thermal NO~ production rate due to the entrainment of the colder combustion products into the reacting shear layer.