Carbon monoxide emissions from turbulent nonpremixed jet flames

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C O M B U S T I O N A N D F L A M E 94:462-468 (1993)

Carbon Monoxide Emissions from Turbulent Nonpremixed Jet Flames STEPHEN R. T U R N S and RAMARAO V. BANDARU Propulsion EngineeringResearch Center and Department of Mechanical Engineering The Pennsylvania State University University Park, PA 16802

Carbon monoxide emission indices were measured for turbulent jet flames produced by burning either methane, propane, ethylene, or a carbon monoxide-hydrogenmixture in air. Flame conditionswere varied by changing the initial jet velocityor diluting the fuel with N2. The CO emissions correlate stronglywith flame luminosity, and ostensibly, with in-flame soot, consistent with CO/soot measurements in laminar flames, low-Reynoldsnumber turbulent jet flames, pool fires, and other fire-related flame environments.The present measurements indicate that the physical and chemical processes controlling CO emissions from diffusion flames are similar over a wide range of flow conditions.

INTRODUCTION Carbon monoxide emissions from many combustion systems are regulated by law, and nationwide air quality standards for CO have been in place in the United States for several decades [1]. Carbon monoxide production is also exceedingly important in fire safety. Recent work [2-6], principally in the field of fire safety, has shown that CO and soot coexist in flame products in a strongly correlated manner. In a study of pool and crib fires, McCaffrey and Harkleroad [2] found a one-to-one correspondence between a fuel's radiative behavior and its soot and CO yield. Similarly, Fischer and Grosshandler [3] found a striking correlation between experimental measurements of soot absorption coefficients and CO concentrations in isopropanol pool fires. K6y15 and coworkers investigated CO and soot emissions from turbulent buoyant flames generated from low-velocity jets of gaseous fuels into air [4], and subsequently, from liquid-fuel pool fires [5]. Five gaseous and six liquid fuels having a wide range of sooting propensities were employed in their studies. Again a strong correlation was found between soot and CO emissions, with a linear log-log relationship relating soot and CO emission indices. Puri and Santoro [6], in their study of laminar diffusion flames, found evidence supporting their 0010-2180/93/$6.00

hypothesis that the strong correlation between soot and CO is a result of a competition between soot and CO for the oxidizing species OH. They further hypothesize than when high concentrations of soot exist, CO may be formed as an intermediate product, again contributing to the maintenance of simultaneously high levels of soot and CO. The present study was not formally designed to investigate the relationship between soot and CO, but rather is an offshoot of our investigations of the effects of fuel type and radiation on oxides of nitrogen emissions [7, 8]. In this article, we present results that indicate that the findings relating CO and soot, which were obtained from pool fires and other buoyant-flame environments, extend to test conditions far removed from those previously studied. Although the results from the highReynolds n u m b e r , h i g h - F r o u d e n u m b e r regimes studied here may not be pertinent to fire safety, they suggest that the processes controlling soot and CO emissions from diffusion flames are fundamentally the same over a wide range of flow conditions. EXPERIMENTAL M E T H O D S

A detailed description of the basic experimental facility is provided in Ref. 7, so only a brief summary is presented here. A 4.12-mm-i.d. Copyright © 1993by The Combustion Institute Published by Elsevier SciencePublishing Co., Inc.

C A R B O N M O N O X I D E EMISSIONS F R O M J E T F L A M E S straight-tube burner fired vertically upward into still air. The fuels employed, in order of decreasing sooting propensity, were C2H4, C3H 8, CH4, and a 95% C O / 5 % H 2 (by mass) mixture. The diluted products of combustion were collected in a forced draft hood, and the well-mixed product stream was sampled from the duct leading from the hood to the laboratory exhaust. To accurately measure CO emission indices, that is, grams of CO emitted per kilogram of fuel supplied to the burner, it was necessary to measure CO and CO 2 concentrations both in the sample stream and in the room air. CO concentrations were measured using a gas filter correlation analyzer ( T E C O model 48) and CO2 concentrations with either a long- or short-path nondispersive infrared analyzer (Horiba model PIR-2000), depending on the concentration level. A carbon balance was used to determine the CO emission indices, as outlined in the Appendix. Soot was not measured in this study and, hence, was not included in the carbon balance. When soot is included in the carbon balance, the percentage error in the CO emission indices is of the same order as the percentage of the fuel carbon escaping the flame as soot. For example, assuming that 2% of the fuel appears as soot, CO emission indices are overestimated by approximately 3%. Mean radiant heat fluxes were measured using a M e d t h e r m 64P-05-24 b r o a d b a n d (0.35-12 /zm), wide angle (150 °) radiometer positioned at axial and radial locations both equal to one half the visible flame length, These heat flux measurements were used to estimate the fraction of the fuel energy lost as radiation, that is, the radiant fraction, An. Global residence times r c, were calculated from visible flame dimensions, fuel properties, and estimated flame densities as follows [7]:

zc;

psWI2L~L 3pod~u ° ,

(1)

where pf, Wf, and L f are the flame density, width, and length, respectively, P0 is the cold fuel density, fs is the mass fraction of the fuel in a stoichiometric mixture, and dj and u 0 are the jet exit diameter and velocity, respectively.

463

The utility of r c as a characteristic time in jet flames is discussed in Ref. 7. Two types of experiments were conducted. In the first, measurements were made in which initial jet velocity was parametrically varied. Tests were conducted for each of the four fuels. In the second experiment, various quantities of nitrogen were added to the fuel. Only C2H 4 and C O / H 2 fuels were employed in this experiment. These test conditions (Table 1) allowed a wide range of flame luminosities (degrees of soot incandescence) to be explored. RESULTS AND DISCUSSION Carbon monoxide emission indices are shown as functions of initial jet velocity in Fig. 1. There is a dramatic decrease in CO emission indices with velocity for the three hydrocarbon fuels. The CO emission indices were also ordered in the same manner as the fuel's sooting propensity and luminosity, with the most luminous flame, C2H4, producing the most carbon monoxide, and the least luminous, CH4, the least CO. Also shown in Fig. 1 are three data points from Ref. 4 for lowervelocity conditions corresponding rather closely to some of the present measurements. We see that the data follow the same trend a n d are in reasonable numerical agreement with our data. Differences are likely to be the result of differences in measurement and data reduction techniques, as well as burners (e.g., 5 mm versus 4.12 mm diameter). The degree of luminosity, or in-flame soot, can be inferred from the radiant fraction measurements also shown in Fig. 1, where we see that the C2H 4 had the largest radiant fractions. Radiant fractions, like the CO emission indices, decreased with initial jet velocity. It is also interesting to note that the CO emission indices for the hydrocarbon flames tend to asymptotically fall to a value near 0.2 g / k g at the highest velocities. The ranking of the CO emission indices according to fuel type and their convergence at high velocities is consistent with the idea that CO and soot emissions are highly correlated, as found by others for highly buoyant, generally larger and lowervelocity flames [2-6]. Although soot was not

464

S . R . T U R N S and R. V. B A N D A R U TABLE 1

Flame Parameters Fuel CH 4

C3Hs C 2H 4

95% CO/5% H2 C2H4 + N2 95% CO/5% H 2 + N2

Jet ReynoldsNo.a Range 3490-22,700 10,900-61,100 4210-29,000 6490-18,900 18,900-15,000 9930-12,150

FlameReynoldsNo? Range

Exit Froude No.c Range

760-3860 1280-4340 740-3710 1350-3020 2600-2150 1730-1990

5400-2.28-10s 3640-1.16-105 2150-1.02-105 4.37- 104-3.39 • 105 4.25. 104-4.95 • 104 9.22. 104-9.21 • 104

FlameFroude NO. d Range 0.074-1.48 0.047-0.49 0.043-0.76 12.9-67.1 0.05-3.11 13.5-74.8

aRej = u o d j / v o based on cold fuel properties. bRef 14.2 rhf/(/~fLf) based on flame properties at characteristic temperature Tp thf is flowrate of stoichiometric mixture. CFre = u2/gdj. dFrf 12 rhouo/[wgWf2Lf( go - pf)]. See Ref. 7. measured in the present study, measurements of soot volume fractions by Kent and Bastin [9], in a comprehensive parametric study covering conditions similar to those of the present study, support our contention that in the more luminous flames more soot is present within both the flame proper and the postflame region. Postflame soot volume-fraction data from the Kent and Bastin study of acetylene flames [9], as shown in Fig. 1, follow a trend similar to our CO emission index measurements. In particular, we note that the soot yield reaches a near-zero asymptote as velocity increases. The convergence of the CO emission indices at the higher velocities for the various flames suggests that very little or no soot is present in the postflame gases at these conditions, and thus, the CO yield is based on chemical kinetic limitations apart from soot. The fuel-dilution experiments discussed below support this hypothesis. Since radiant losses affect flame temperatures, one might argue that the differing CO yields among fuel types is related to temperature effects and not solely a result of CO-soot interactions. Characteristic flame temperatures [7, 8] were calculated for each flame, taking into account the measured radiant losses. For the ethylene and propane flames, the characteristic temperatures between the fuels differed by only approximately 10 K at corresponding velocities. These small temperature differences result from the higher adiabatic flame temperature of ethylene compensating for its greater heat loss. This com-

parison suggests that temperature differences did not play a major role in determining the relative CO yields among the hydrocarbon fuels. Unlike the hydrocarbon flames, no soot is produced in the C O / H 2 flames. The CO emission trend of these flames also markedly departs from those of the hydrocarbon flames, with CO emission indices increasing, rather than decreasing, with initial velocity (Fig. 1). Several factors may account for this behavior. First, with no soot in the flames, there is no competition for hydroxyl radicals; thus, with regard to chemical effects, the trend is influenced primarily by the CO oxidation step, CO + O H ---, CO 2 + H, and the shifting composition of the radical pool. Residence times, both in flamelets and on a global scale, are decreased as initial velocity is increased. Shortened residence times may prevent complete CO burnout via the relatively slow CO + O H oxidation step. Global residence times in the C O / H 2 flames are much less than in the hydrocarbon flames; for example, with an initial velocity of about 65 m / s , the global residence time (~'c) for the C O / H 2 flame is 7.1 ms, while the corresponding zo for the C3H 8 flame is 129 ms. The difference in residence times is principally related to the greatly different stoichiometries and densities of the C O / H 2 and hydrocarbon flames. A second consideration that may be a factor in determining the CO emission index trends for the C O / H z flames is turbulent mixing. Unlike in the hydrocarbon flames, CO in the

CARBON MONOXIDE EMISSIONS FROM JET FLAMES C O / H 2 flames is not an intermediate product of combustion, but rather originates as a cold reactant. It may be possible for a small parcel of CO to pass unreacted through the highly turbulent environment of the flame. This argu-

0.4

I

I

465

ment is also consistent with the higher CO levels emitted by the C O / H 2 flame in comparison to the hydrocarbon flames. Dilution of the fuel by an inert is another means to alter the soot production in hydro-

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Fig. 1. Carbon monoxide emission indices and radiant fractions versus initial jet velocity for ethylene, propane, methane, and 95% carbon monoxide/5% hydrogen (by mass)jet diffusion flames shown as solid lines (dj = 4.12 ram). Numbers in parentheses are carbon monoxide concentrations (ppm) corrected to stoichiometric conditions. Solid symbols for ethylene flames (dj = 5 mm) from Ref. 4. Also shown (dashed line) are soot volume fractions measured in acetylene flames (dj = 3 mm) from Ref. 9.

466

S . R . TURNS and R. V. BANDARU

carbon flames, decreasing the flame luminosity and soot volume fractions within the flame [8, 9]. Figure 2 shows CO emission indices for nitrogen-diluted ethylene and C O / H 2 flames.

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Once again we see a dramatic drop in CO emissions as flame luminosity and soot content diminish for t h e ethylene. The CO emission index drops by more than a factor of four as

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C A R B O N M O N O X I D E EMISSIONS F R O M J E T F L A M E S the radiant fraction falls from about 25% to 7%. We also observe that for the C 2 H 4 flame the CO index asymptotically falls to a value nearly the same as for the velocity experiments. This suggests that the final kinetic limitations are similar in all of the hydrocarbon flames, which is plausible, since in the absence of soot, the final CO oxidation proceeds by the same pathways and in nominally similar chemical environments. For the N2-diluted C O / H 2 flames, we observe again the opposite CO emission trend as for the ethylene flame. With dilution, global residence times are shorter (r a = 13.2 ms undiluted versus r c = 4.3 ms with 50% dilution) and flamelet strain rates are higher. Thus, the increasing CO emissions with dilution is consistent with either chemical kinetic or mixing limitations.

467

APPENDIX The calculation of the CO emission indices is based on conservation of mass, atoms and species. Conservation of mass in the system yields (A1)

/'h T = Fh S q- Fh F ~- ?'hA,

where rh r is the unknown total mass flow into the exhaust duct, Fh F is the measured fuel f l o w r a t e , i'h A is the flowrate of air required to stoichiometrically burn the fuel, and rh s is flowrate of ambient air diluting the stoichiometric combustion products in the duct. Conservation of the mass of carbon yields

YC, F -+- Fh/;n Yco,~ Mc° + Yc'o2.~ Mco2

CONCLUSIONS Carbon monoxide emissions in high-velocity, high-Reynolds number jet flames correlate strongly with flame luminosity, and ostensibly, with in-flame soot. These results are consistent with C O / s o o t measurements in laminar flames, low-Reynolds number turbulent jet flames, pool fires, and other fire-related flame environments. This indicates a similarity in the physical and chemical processes that extends over a wide range of flow conditions. The CO yields for conditions in which little soot is present (high velocity or high N 2 dilution) asymptotically fall to values near 0.2 g/kg. This convergent behavior for all three hydrocarbon fuels investigated is further evidence of the linkage between CO and soot. In C O / H : flames, which are devoid of soot, CO emissions increased with decreasing residence times (increasing velocity o r N 2 dilution). These results likely reflect chemical kinetic limitations or CO leakage through the flame as a result of incomplete mixing.

This work was supported by the Gas Research Institute under Contract No. 5086-260-1308 with T. R. Roose and J. A. Kezerle serving as technical monitors. The authors would also like to thank Dr. R. Puri and Prof R. J. Santoro for their helpful comments on a draft of this paper.

Mc

Mc

× Yco,i Mc---S+ Yco, M o2 (A2) where Y/,F, Y/,~, and Y/,/ refer to the mass fractions of i in the fuel, ambient air, and flame, respectively, and the M i are the molecular weights. Mass fractions of CO and CO 2 in the duct are related to the flame mass fractions by

mF

+

'

mm.E,o for

i = CO, COe,

(A3)

where Y~,D are the mass fractions in the exhaust duct. The mass fractions of CO and CO 2 in the exhaust duct are determined from the measured molar concentrations, Xi. . . . . (measured in a dry sample of the duct mixture). A hydrogen balance is required to convert the measured concentrations to a wet basis: Y~,O,D = -

1--

. mf

+ rh--T

YH20,~

~

Yu2,F"

(A4)

468

S.R.

T U R N S and R. V. B A N D A R U

T h e C O emission index is calculated using

bustion Institute, Pittsburgh, 1988, pp. 1241-1249. 4. K6ylii, U. O., Sivathanu, Y. R., and Faeth, G. M., Fire

Elc° =

Safety Science--Proceedings of the Third International Symposium, 1991, pp. 625-634. 5. KSylii, O. 6 , and Faeth, G. M., Combust. Flame

rh--~ + 1 Yco,f × 1000.

(A5)

w h e r e Yco, f is d e t e r m i n e d f r o m the simultaneous solution o f Eqs. A 1 - A 4 .

REFERENCES 1. Seinfeld, J. H., Atmospheric Chemistry and Physics of Air Pollution, Wiley, New York, 1986. 2. McCaffrey, B. J., and Harkleroad, M., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 1251-1261. 3. Fischer, S. J., and Grosshandler, W. L, Twenty-Second Symposium (International) on Combustion, The Corn-

87:61-76 (1991). 6. Puri, R., and Santoro, R. J., Fire Safety Science-Proceedings of the Third InternationalSymposium, 1991, pp. 595-604. 7. Turns, S. R., and Myhr, F. H., Combust. Flame 87:319-335 (1991). 8. Turns, S, R., Myhr, F. H., Bandaru, R. V., and Maund, E. R., Combust. Flame 93:255-269 (1993). 9. Kent, J. H., and Bastin, S. J., Combust. Flame 56:29-42 (1984). Received 11 December 1992; revised 12 March 1993