Effect of Addition on Polycyclic Aromatic Formation in 1,3 Counter-Flow Diffusion Flames

NESRIN OLTEN and SELIM SENKAN* Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, CA 90095, USA

The effect of 3% O2 addition to the fuel on detailed chemical structure of a 1,3 butadiene counter-flow diffusion flame has been investigated by using heated microprobe sampling and online gas chromatography . Centerline gas temperature and species mole-fraction profiles were measured both in the absence and presence of oxygen on the fuel side. The rapid thermocouple insertion method was used to obtain the flame temperature profiles. Although the addition of oxygen to the fuel side did not significantly change the shape and peak values of the temperature profiles, species mole-fraction profiles were significantly altered. Flame reactions started earlier in the presence of oxygen in the fuel, resulting in the shift of the reaction zone toward the fuel burner port. The presence of oxygen in the fuel led to decreased peak mole fractions of the aromatic and two-ring polycyclic aromatic hydrocarbon species. In contrast, the peak mole fractions of polycyclic aromatic hydrocarbon having three or more rings significantly increased in the presence of oxygen in the fuel stream. An analysis of the data suggests the need to invoke new reaction mechanisms describing the formation and destruction of polycyclic aromatic hydrocarbon. © 2001 by The Combustion Institute

INTRODUCTION attributed to increased stoichiometric flame temperature [6]. The emission standards of combustion have PAH formation in low-pressure premixed been steadily reduced in recent years, and a flames of 1,3 butadiene were also studied by large research effort has been focused on low- using molecular beam sampling mass spectrom- ering the emission of nitrogen oxides, hydrocar- etry [7]. These investigators reported the spe- bon, and particulate matter. Addition of oxy- cies mole-fraction profiles up to two-ring PAH, genates to fuel reduces these pollutants by and provided kinetic arguments on PAH and reducing the flame temperatures and by altering soot formation based on species flux analysis. the combustion chemistry [1]. Oxygenate addi- Most recently, stack emissions of PAH from tion also reduces the level of precusors, specif- vapor and pool fires of 1,3 butadiene were ically , 1, 3 butadiene, propargyl, and determined using sorbent adsorption followed vinyl radicals in combustion process, which are by gas chromatography mass spectrometry [8]. responsible for the formation of aromatic and Accordingly, sooting tendency was found to polycylic aromatic hydrocarbon (PAH) species increase with increasing equivalence ratio and and soot in flames [2]. Consequently, the devel- decreased with increasing temperature. The ki- opment of a better understanding of its combus- netics of the elementary reactions of 1,3 buta- tion chemistry is of practical and scientific in- were also studied [9–14]. terest. Previously, the combustion of 1,3 However, despite the significance of 1,3 buta- butadiene has been studied with regard to soot diene in PAH and soot formation processes in formation in co-flowing diffusion flames [3–6]. hydrocarbon combustion, we are not aware of In some of these studies, it was advocated that any studies in which the detailed chemical struc- flame temperature should be the controlling tures of the flames of 1,3 butadiene have been parameter for soot formation due to its direct reported. In this paper, we focused on the effect impact on fuel , thus soot nucleation, of oxygen on PAH formation and reported rates [5]. The addition of oxygen to the fuel side detailed structure of opposed jet diffusion also increased soot formation, and this was also flames of 1,3 butadiene both in the absence and presence of oxygen added to the fuel side. Related research is also in progress in premixed *Corresponding author. E-mail: [email protected] flames, and will be reported later. COMBUSTION AND FLAME 125:1032–1039 (2001) 0010-2180/01/$–see front matter © 2001 by The Combustion Institute PII S0010-2180(01)00224-3 Published by Elsevier Science Inc. DIFFUSION FLAMES IN 1,3-BUTADIENE 1033


1,3 Butadiene Flame Conditions

Pure butadiene flame Oxygen-added butadiene flame

Fuel side Fuel side

C4H6 mole fraction 0.50 C4H6 mole fraction 0.50 Ar mole fraction 0.50 Ar mole fraction 0.47 Oxygen mole fraction 0.00 Oxygen mole fraction 0.03

Gas velocity, vf, cm/s 13.16 Gas velocity, Vf, cm/s 13.16 ␳ ␳ Fuel side density, f, g/L 1.92 Fuel side density, f, g/L 1.87 Oxidizer side Oxidizer side Oxygen mole fraction 0.22 Oxygen mole fration 0.22 AR mole fraction 0.78 Ar mole fraction 0.78

Gas velocity, Vo, cm/s 16.12 Gas velocity, Vo, cm/s 16.12 Burner separation, L, cm 1.65 Burner separation, L, cm 1.64 Ϫ Ϫ Strain ratea,K,s 1 37.6 Strain ratea,K,s 1 37.6

a ϭ ϩ ␳ ␳ 0.5 Ϫ1 Strain rate was calculated by using the equation: K (2Vo/L){1 (Vf/Vo)( f/ o) }, [s ].

EXPERIMENTAL METHOD the flame by using a computer-controlled driver mechanism, and the data acquired automati- Details of the counter-flow diffusion burner cally after insertion. In order to minimize heat system that has been used in the experiments conduction along the wires, the thermocouple and the method of quantification in obtaining was positioned parallel to the burner surface. In the species mole-fraction profiles can be found the sooting parts of the flame, the soot deposits in our previous paper [15], thus will not be were burnt off with a torch between repeated. Flame samples were withdrawn by each measurement. After soot burn off, the using a heated (300°C) quartz microprobe thermocouple was also inspected by a micro- Ϫ (0.635-cm outer diameter) with a 200 ϫ 10 4 cm scope to insure that its surface was clean and diameter orifice at its tapered tip. The probe ready for the next insertion. Here we report the orifice diameter used was the smallest possible peak thermocouple readings without any radia- size to achieve sampling in such sooting flames. tion corrections. Soot particles in the sample were trapped by using a heated quartz filter before gas analysis. As discussed earlier, the system developed al- RESULTS AND DISCUSSION lows the quantitative determination of mole- fraction profiles for PAH up to five rings (252 Two flames of 1,3 butadiene were studied, and Daltons) [15]. The accuracies for the determi- the experimental conditions used are given in nation of species mole fraction were estimated Table 1. These conditions were selected after to be ϳ15%. considering a number of different conditions to Gas temperature measurements were accom- determine the most suitable sets for flame sam- plished by using a silica coated Pt/Pt ϩ 13%Rh pling and temperature measurement. As seen (R type, Omega) thermocouple with 0.075 mm from Table 1, the two flames studied essentially

wire and a bead diameter of 0.200 mm. Rapid were the same with the exception of 3% O2 that insertion technique was applied in order to was introduced into the fuel side of the second obtain the profiles. A data acquisition board flame. (5580Sci, American Data Acquisition Corpora- In order to obtain accurate temperature pro- tion, Los Angeles, CA) was used to digitize files, it is necessary to consider thermocouple analog inputs for computer data collection. disturbances and radiation effects. Unfortu- Stepper motors and the data-acquisition board nately, in fuel-rich flames, soot accumulation on were controlled via LABVIEW. In this method, the thermocouple significantly complicates the the thermocouple was quickly introduced into latter, rendering the acquisition of reliable tem- 1034 N. OLTEN AND S. SENKAN

as the visible blue-flame surface. As it can be

expected, the addition of O2 into the fuel stream caused a change in temperature distribution inside the flame. Neglecting the temperature change along radial direction within the flame, we can use the centerline temperature measure- ments of the flames to make a comparison. The

data lead to the conclusion that, addition of O2 into fuel stream causes only minor changes in the maximum flame temperature (27°C). The major impact of oxygen addition appears to be to lower the position of the flame front toward the fuel burner surface. This results in the

earlier pyrolysis of the fuel in the presence of O2 causing a shift in the position of the flame front. Fig. 1. Major species and temperature profiles for both Additives can effect the behavior of diffusion pure butadiene and oxygen-doped butadiene flame. In this flames in several ways. They can undergo chem- figure and all the following, symbols are experimental data points (filled symbols butadiene, and empty symbols oxygen- ical reactions with the fuel or fuel pyrolysis doped butadiene flames), and lines show the trend lines. products and initiate the combustion process earlier. The additives can also act as a partici- pating species in heat transfer by altering the perature measurements very difficult. With the mixture thermal diffusivity, or they can serve as implementation of the rapid insertion method a heat sink via their heat capacity [4]. As can be and computerized data acquisition, we believe seen in Fig. 1, the temperature increases earlier we generated a practical optimum set of condi- in the O2 added flame. Because the maximum tions for temperature measurement. The tem- flame temperatures were very close to one perature profiles shown in Fig. 1 show that the another, and fact that the molecular diffusivity maximum centerline flame temperatures mea- of oxygen is only slightly larger than butadiene, sured at 8.5 mm for the pure butadiene flame the primary effect of added O2 to the fuel side and at 7.5.mm for the O2 added butadiene appears to be chemical in nature. This is also flame, respectively. In Fig. 1, as well as in supported by the rapid decrease in the fuel mole subsequent figures, the symbols represent the fraction and the earlier formation of H2O (Fig. experimental data (hollow symbols for the oxy- 1). In fact, it is well known from fuel pyrolysis gen-doped fuel flame), whereas the lines indi- experiments that the addition of small amount cate trends. As seen in Fig. 1, for the pure of oxygen can accelerate the pyrolysis process butadiene flame, the maximum temperature [18]. Therefore, it is not surprising to see a more reached was 1713°C, which is well below the rapid decline in the fuel profile in the fuel-rich stoichiometric flame temperature (2000°C) of side of the flame in the present experiments. butadiene in air [16]. The difference is most In Figs. 2 and 3 the mole-fraction profiles for likely due to radiation losses from the thermo- H2, CO, CH4,C2H6,C2H4,C2H2, and C3H4 are couple and the flame. Another reason may be presented, in which the latter two are believed that, a diffusion flame may not reach stoichio- to be important PAH precursors [19–21]. As metric flame temperatures due to incomplete seen in these figures, the peak C3H4 mole conversion of CO to CO2 (nonequilibrium ef- fractions decreased by the addition of O2 to the fects) [17]. It is interesting to note that the fuel side in the order of 4%, where the peak oxygenated butadiene flame exhibited only a C2H2 mole fraction decreased 11%. Although slightly higher peak temperature of 1740°C. these changes are within the limits of accuracy Within spatial resolution of the thermocouple of our measurements, systematic variations measurements, the maximum flame tempera- were clear, especially when the data are plotted ture, which correspond to the main reaction on a linear basis. Consistent with earlier pyrol- zone, is generally situated at the same position ysis, levels in the oxygenated buta- DIFFUSION FLAMES IN 1,3-BUTADIENE 1035

Fig. 2. CO, H2,CH4, and C3H4 mole-fraction profiles. Fig. 4. containing aliphatic pyrolysis product mole- fraction profiles. diene flame were higher. Another interesting observation is the reduction of the CO/CO2 to formation, whereas vinylacety- ratio from 0.88 in the 1,3 butadiene flame to lene is precursor to diacetylene formation [20]. 0.55 in the oxygen-doped flame. Another way of forming diacetylene was pro-

In Fig. 4, the mole-fraction profiles of C4 posed by Miller and Melius, involving the reac- products are presented. As seen in this figure, tion between acetylene and C2H. This mecha- vinylacetylene and diacetylene maximum mole nism implies that the decreasing C2H2 levels fractions did not change much by the addition of should also decrease the diacetylene level,

O2 to the fuel side. However, significant differ- which is consistent with our experiments [22]. ences were present closer to the fuel burner The mole-fraction profiles for C5 products surface, in accordance with the earlier start on are presented in Fig. 5. In this figure, it is seen the pyrolysis reactions in the presence of oxy- that levels are higher in the gen. Butadiene is proposed to be the precursor oxygen-doped flame than pure butadiene flame

Fig. 3. Carbon containing aliphatic pyrolysis product mole- Fig. 5. Carbon containing aliphatic pyrolysis product mole- fraction profiles. fraction profiles. 1036 N. OLTEN AND S. SENKAN

Fig. 6. Carbon containing aliphatic pyrolysis product mole- Fig. 7. and alkyl group substituted one-ring aro- fraction profiles. matic species mole-fraction profiles. over the entire fuel-rich zone, i.e., until the peak decrease in the peak benzene level is closer in is reached. As noted earlier, the general behav- magnitude to the decrease in C2H2 levels in the ior of the oxygen-doped flame is the early oxygen-doped flame, suggesting that the major- formation of pyrolysis and oxidative pyrolysis ity of benzene formation should be via reactions products. The results presented in Fig. 5 are [1] and [2] [19]. consistent with this picture. The effect of oxygen presence in the fuel side Recent attempts to understand the detailed had a similar impact on the higher aromatic kinetic mechanism of PAH formation in flames hydrocarbon products as shown in Figs. 8 and 9. relied on the use of data , acetylene, or The maximum level of phenyl acetylene and small flames [19–23]. These investiga- mole fractions also decreased 15%, sim- tions let to the following reaction as the domi- ilar to benzene. The growth from benzene to nant route for benzene formation: larger aromatics has been attributed to succes- ϩ ϭ ϩ C4H5 C2H2 C6H6 H● (1) as well as the reaction of the n-C4H3 radical with acetylene to form phenyl radical, ϩ ϭ C4H3 C2H2 C6H5● (2) Alternately, benzene formation was also sug- gested to proceed via the recombination of the stabilized propargyl radicals [24,25]: ϩ ϭ C3H3● C3H3● C6H6 (3) ϩ ϭ ϩ C3H3● C3H3● C6H5● H● (4) If any of these reactions were to dominate benzene formation process, one would expect a decrease in benzene levels on the oxygenated butadiene flame because the peak mole frac- tions of both the acetylene and C3H4 decreased relative to the pure butadiene flame. The 15% Fig. 8. Ring aromatic species mole-fraction profiles. DIFFUSION FLAMES IN 1,3-BUTADIENE 1037

Fig. 9. Ring aromatic species mole-fraction profiles. Fig. 11. Ring aromatic species mole-fraction profiles.

sive addition of C2H2 [26]. This mechanism im- tive under the experimental conditions. Indeed, plies that the effect of oxygen doping on larger such an alternative pathway, involving the reac- aromatics should be similar to those on benzene, a tion of two cyclopentadienyl radicals has been result consistent with our measurements. suggested [27]. However, because the cyclopen- These trends were also observed for the tadiene maximum levels were nearly the same in mole-fraction profiles of two-ring polycyclic ar- both flames, the decrease in level omatic presented in Figs. 10, 11, in the oxygenated butadiene cannot be ex- and 12. Naphthalene formation is the first step plained by this mechanism either. It appears in the growth process to larger PAH molecules. that naphthalene undergoes subsequent reac- The decrease in the maximum level of naphtha- tions in the presence of oxygen. This is sup- lene was about 20% in the case of oxygen added ported by the presence of a large number of flame. This suggests that another mechanism, products containing the naphthalene backbone other than C2H2 addition, must also be opera- (Figs. 10 and 11).

Fig. 10. Ring aromatic species mole-fraction profiles. Fig. 12. Ring aromatic species mole-fraction profiles. 1038 N. OLTEN AND S. SENKAN

Fig. 13. Ring aromatic species mole-fraction profiles. Fig. 14. Ring aromatic species mole-fraction profiles.

PAH with five-membered rings are believed levels in the oxygen-doped flame. to have important role in molecular weight Alternately, although the peak biphenyl levels growth processes in the gas phase because of were ϳ20% higher in the oxygenated butadiene the reactivity of the [28]. Acenaph- flame, at 3.5 ppm it cannot account for the thylene, which can be produced by the addition formation of 20 ppm phenanthrene. These re- of acetylene to naphthalene, was also detected sults clearly suggest that other pathways for the in both flames and as in the case with other formation of larger PAH must be put forward to hydrocarbons, its peak level was lower in the account for the experimental results. presence of added oxygen (Fig. 12). On the The PAH growth progressed further in the other hand, the drop in acenaphthylene was not case of oxygen-doped flame, forming cylopen- as large as drop in C H level, which again 2 2 ta(cd) and its benzo(ghi)fluoran- suggests that C H addition is not the only route 2 2 thene as the largest detectable PAH (Fig. 15). in PAH growth process. These PAH were not detected in the pure The mole-fraction profiles of PAH possessing butadiene flame. As noted earlier, some oxygen- three and four aromatic rings are presented in ated aromatic species, such as , benzalde- Figs. 13 and 14. It is interesting to note that hyde, and , were also produced in the species with three or more rings were more oxygen-doped flame and these are presented in abundant in the oxygen-doped flame than the Fig. 15. With the exception of furan, all the pure butadiene flame. Evidently, oxygen addi- other oxygenates reached their peak levels be- tion promotes the formation of higher molecu- fore the PAH. For the case of furan, its mole- lar weight PAH, and not the smaller aromatics. fraction profile exhibited its maximum later, at Phenanthrene is another key species for PAH about the same flame height as the PAH. growth and has been suggested to form by the sequential addition of C2H2 to naphthalene [19], and the reaction of C2H2 with biphenyl CONCLUSIONS radicals [29]. Phenanthrene mole fractions reached about 20 ppm and 15 ppm in the In summary, although the addition of oxygen to oxygen-doped and the pure butadiene flame, the fuel side of a 1,3 butadiene counter-flow respectively. As shown earlier in Fig. 10, naph- diffusion flame did not significantly change the thalene levels were lower in the oxygenated shape and peak values of the temperature pro- butadiene flames. This combined with lower file, species mole-fraction profiles were signifi-

C2H2 levels cannot account for the increased cantly altered. Flame reactions started earlier in DIFFUSION FLAMES IN 1,3-BUTADIENE 1039

and Glassman, I., Combust. Sci. Tech. 22:235–250 (1980). 5. Gomez, A., Sidebotham, G., and Glassman, I., Com- bust. Flame 58:45–57 (1984). 6. Glassman, I., and Yaccarino, P., Combust. Flame 24: 107–114 (1980). 7. Cole, J. A. (1982). M. S. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA. 8. Catallo, W. J., Chemosphere, 143–147 (1998). 9. Benson, S., J. Chem. Physics 46:4920–4926 (1967). 10. Harkness, J. B., Kistiakowsky, G. B., and Mears, W. H., J. Chem. Physics 5:682–694 (1937). 11. Kistiakowsky, G. B., and Ransom, W. W., J. Chem. Physics 7:725–735 (1939). 12. Benson, S. W., and Haugen, G. R., J. Phys. Chem. 71:1735–1746 (1967). 13. Brezinsky, K., Burke, E. J., and Glassman, I., Twentieth International Symposium on Combustion, The Combus- tion Institute, Pittsburg, 1984, pp. 613–622. Fig. 15. Cyclopenta(cd)pyrene, benzo(ghi)fluoranthene, 14. Dagaut, P., and Cathonnet, M., Combust. Flame 113: and oxygenated species mole-fraction profiles formed only 620, 1998. in the oxygen-doped butadiene flame. 15. Olten, N., and Senkan, S. M., Combust. Flame 118: 500–507 (1999). 16. Glassman, I., Combustion (2nd ed.) Academic Press, the presence of oxygen in the fuel, resulting in San Diego, 1997, p. 23. 17. Macek, A., Flammability Limits: Thermodynamics And the shift of the reaction zone toward the fuel Kinetics, National Bureau of Standards Internal Re- burner port. The presence of oxygen in the fuel port 76-1076 (1976). led to decreased peak mole fractions of the 18. Smith, S. R., and Gordon, A. S., J. Phys. Chem. 60:759 aromatic and two-ring PAH species. In contrast, (1956). the peak mole fractions of PAH having three or 19. Frencklach, M., and Warnatz, J., Combust. Sci. Tech. 51:265–283 (1987). more rings significantly increased in the pres- 20. Markatau, P., Wang, H., and Frencklach, M., Combust. ence of oxygen in the fuel stream. Analysis of Flame 93:467–482 (1993). the data suggests the need to invoke new reac- 21. Kazakov, A., Wang, H., and Frencklach, M., Combust. tion mechanisms describing the formation and Flame 100:111–120 (1995). destruction of PAH. 22. Miller, J. A., and Melius, C. F., Combust. Flame 91:21–39 (1992). 23. Seshadri, K., Mauss, F., Peters, N., and Warnatz, J., This work was supported, in part, by the Na- Twenty-Third International Symposium On Combustion, tional Science Foundation and the U.S. Environ- The Combustion Institute, Pittsburg, 1990, pp. 559– mental Protection Agency. The authors also thank 566. Kitty Lee and Manabu Yamomoto for their help 24. Marinov, N. M., Pitz, W. J., Westbrook, C. K., Castaldi, M. J., and Senkan, S. M., Combust. Sci. Tech. with the experiments. 116–117:211–287 (1996). 25. Leung, K. M., and Lindstedt, R. P., Combust. Flame 102:129–160 (1995). REFERENCES 26. Wang, H., and Frencklach, M., Combust. Flame 110: 1. Curran, H. J., Glaude, P. A., Marinov, N. M., Pitz, 173–221 (1997). W. J., and Westbrook, C. K., Fall Meeting of the 27. Dean, A. M., J. Phys. Chem. 94:1432 (1990). Western States Section of the Combustion Institute, 28. Benish, T. G., Lafleur, A. L., Taghizadeh, K., and October 25–26, 1999. Howard, J. B., Twenty-Sixth International Symposium 2. Flynn, P. F., Durret, R. P., Hunter, G. L., zur Loye, On Combustion, The Combustion Institute, Pittsburg, A. O., Akinyemi, O. C., Dec, J. E., and Westbrook, 1996, pp. 2319–2326. C. K., Society Of Automotive Engineers Publication 29. Richter, H., Grieco, W. J., and Howard, J. B., Com- SAE-01-0509, 1999. bust. Flame 119:1–22 (1999). 3. Wegert, R., Wiese, W., and Homann, K. H., Combust. Flame 95:61–75 (1993). Received 24 April 2000; revised 5 December 2000; accepted 20 4. Schug, K. P., Manheimer–Timnat, Y., Yaccarino, P., December 2000