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Comparison of non-fuel hydrocarbon concentrations measured in coflowing nonpremixed flames fueled with small hydrocarbons
Comparison of Non-Fuel Hydrocarbon Concentrations Measured in Coflowing Nonpremixed Flames Fueled With Small Hydrocarbons CHARLES S. MCENALLY* and LISA D. PFEFFERLE
Department of Chemical Engineering and Center for Combustion Studies, Yale University, New Haven, CT 06520-8286, Centerline concentrations of twenty-nine C2 to C12 hydrocarbons were measured by extractive gas sampling combined with on-line photoionization mass spectrometry in soot-containing methane and nitrogen-diluted ethane, ethylene, acetylene, propane, and n-butane coflowing nonpremixed flames. Additional measurements were made of temperature with thermocouples and of soot concentration with laser light scattering and thermocouple particle densitometry. The fuel hydrocarbon and nitrogen flowrates were chosen such that each flame had a visible flame length of approximately 40 mm and similar soot concentration profiles. Essentially the same species were detectable in each flame, and these species are generally those that are predicted to be most thermodynamically stable under rich flame conditions. The different fuels produce widely varying ratios of C2 to C5 non-aromatic species, enough to suggest that the primary pathways to aromatic species may be different among the flames. The fuels produce relatively similar ratios of aromatic species, but the concentrations of aromatics varies considerably among the fuels, even though the soot concentrations were very similar. However, the temperatures at the onset of soot formation were similar to within 100 K. © 1999 by The Combustion Institute
INTRODUCTION Although non-fuel hydrocarbon concentrations have been measured in laboratory-scale methane nonpremixed flames on many occasions [1–7], only a few corresponding measurements have been reported for other fuels: C1 to C3 hydrocarbons in propane/air counterflow flames [1]; C1 to C6 hydrocarbons in ethane, ethylene, and acetylene coflowing flames [8]; a similar range in methane/butane, methane/butene, and ethylene coflowing flames [5–9]; and C1 to C8 hydrocarbons in allene, 1,3-butadiene, and benzene coflowing flames [10]. Consequently, little is known about how fuel structure affects the identity and quantity of hydrocarbons formed in the pyrolysis zone of nonpremixed flames. This poses a significant gap in our understanding of soot formation since the specific pool of pyrolysis products formed in a given flame determines which of the many possible pathways are responsible for key hydrocarbon growth steps such as aromatic ring formation. To partially fill this gap, we have measured centerline concentration profiles of C2 to C12 * Corresponding author. E-mail: charles.mcenally@yale. edu 0010-2180/99/$–see front matter PII S0010-2180(98)00102-3
hydrocarbons in methane and nitrogen-diluted ethane, ethylene, acetylene, propane, and nbutane coflowing nonpremixed flames; this paper presents the results. The measured species range from the immediate pyrolysis products of the fuels to multi-ring aromatic species, so the resulting database is well suited to examining the formation and early growth of aromatic species. The measurements were performed by extracting gas samples from the flames with a quartz microprobe and analyzing them on-line with mass spectrometry. The fuels other than methane were diluted with nitrogen so that their centerline soot concentration profiles would be similar to those in the methane flame, which ensured that clogging of the microprobe orifice by soot particles would be tolerably slow. Furthermore, this procedure caused the total hydrocarbon concentrations in the flames to be roughly similar in magnitude. EXPERIMENTAL METHODS AND RESULTS Flame Descriptions The atmospheric-pressure, overventilated, axisymmetric, coflowing, nonpremixed laminar COMBUSTION AND FLAME 117:362–372 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
Fig. 1. Centerline profiles of soot concentration measured with elastic laser light scattering. The results have been normalized to the maximum value in the methane flame. The inset shows the flame geometry.
flames were generated with a burner in which the fuel flows from an uncooled 12 mm diameter vertical brass tube and the oxidizer flows from the annular region between this tube and a 102 mm diameter concentric tube (see the inset to Fig. 1) [7, 11]. The oxidizer was air (flowrate ' 40,000 cm3/min at STP), while the fuel was a mixture containing a hydrocarbon, argon, and, in some cases, nitrogen. Table 1 lists the hydrocarbon flowrates (Q HC), nitrogen flowrates (Q N2), and hydrocarbon mole fractions in the unreacted fuel (Y HC) for the six flames we studied. The methane flowrate was the same as that used in our previous experiments, while the nitrogen and hydrocarbon flowrates for the other cases were chosen to produce centerline soot concentration profiles that were similar to the one in the methane flame, as indicated by
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the laser scattering measurements discussed below. The argon flowrate was adjusted so that argon always had the same mole fraction in the fuel as in air (1%), and therefore could serve as an internal standard for calibrating the species measurements [7]. The fuel mixtures were examined for impurities with our mass spectrometers. The acetylene mixture contained roughly 800 ppm of acetone, due to the addition of this compound as an inhibitor to the acetylene cylinder by the supplier. Since the measurements in the acetylene flame gave no indication of being affected by this impurity (for example no other oxygenated species were detected), we did not attempt to remove it. No other impurities were detected in any of the fuel mixtures. The flames all had visible flame lengths of roughly 40 mm, but their morphologies differed considerably. For example, the propane and n-butane flames were significantly lifted from the burner, the acetylene flame was slightly lifted, and the other flames were anchored to the fuel tube lip. Table 1 lists the measured lift-off heights (H L) for each flame, where H L is defined as the lowest axial height above the fuel tube exit plane at which the thermocouple visibly glowed (temperature . 1100 K) during a radial traverse through the flame. Furthermore, while the centerline soot profiles were similar in each flame, soot first appeared in the annular region surrounding the centerline at different locations. This is indicated by the inception heights (H I) listed in Table 1, which are defined as the lowest axial height in each flame where
TABLE 1 Flame Characteristics Fuel Methane Ethane Propane n-butane Ethylene Acetylene
Q HC
Q N2
Q Ar
Y HC
HL
HI
HF
T max
240 108 63 44 115 114
0 180 300 300 410 810
2.4 2.9 3.7 3.4 5.3 9.3
0.99 0.37 0.17 0.13 0.22 0.12
0 0 2 4 0 1
17 24 28 29 20 19
40 38 37 37 38 38
1970 1930 1880 1890 1930 1850
Q HC, Q N2, and Q Ar, volumetric flowrates of the hydrocarbon, nitrogen, and argon portions of the fuel in cm3/min at STP (6 5%); Y HC, mole fraction of hydrocarbon in the unreacted mixture, including the argon (6 10%); H L, lift-off height of each flame in millimeters (6 1 mm); H I, lowest height where soot is detectable at any radial location in millimeters (6 1 mm); H F, flame height 5 height of maximum centerline temperature in millimeters (6 1 mm); T max, maximum centerline temperature in Kelvins (6 50 K).
364 soot scattering was detectable during a radial traverse through the flame. All of the measurements reported here are centerline profiles of flame properties as functions of the axial height above the fuel tube exit plane (Z; see Fig. 1). Miller et al. have shown that the primary hydrocarbon production zone in nonpremixed flames lies slightly inside the oxidizing flame front [12]; since our flames are overventilated coflowing flames, at some height this primary pyrolysis zone will coincide with the centerline. Thus centerline profiles must pass through this zone and suffice for characterizing the pool of hydrocarbons produced by fuel decomposition in these flames. Furthermore, for these lightly sooting flames the soot distribution is conical, with the region of maximum soot concentration overlapping the centerline, as opposed to the cylindrical distributions observed in highly sooting flames [11]. Indeed, in our propane and n-butane flames, virtually no off-axis soot exists. Therefore, the primary hydrocarbon production zone on the centerline just below the flame tip is more relevant to soot formation than the off-axis annular reaction zone near the base of the flame.
C. S. MCENALLY AND L. D. PFEFFERLE
Fig. 2. Centerline profiles of (radiation-corrected) gas temperature measured with a thermocouple.
show that, in accord with our procedure for choosing Q HC and Q N2, the soot profiles are very similar in each flame. In general scattering from soot depends on particle diameter as well as volume fraction; however, the TPD measurements verified that the onset of soot formation and the maximum soot concentration are indeed similar among the flames, and showed that the maximum centerline volume fraction is roughly 0.5 ppm.
Soot Concentrations Gas Temperatures Soot concentration profiles were measured with elastic laser light scattering and with thermocouple particle densitometry (TPD), a technique we have developed in which absolute soot volume fractions are inferred from measured rates of soot particle mass transfer to a thermocouple [13, 14]. The laser source for the scattering measurements was the second harmonic of a Nd:YAG laser at 532 nm, which had a pulse energy of approximately 1 mJ and was weakly focussed through the flame. Scattering was measured at a right angle to the laser beam with a monochromator/photomultiplier tube combination. The centerline laser scattering profiles, normalized to the maximum signal measured in the methane flame, are shown in Figure 1. In each flame the centerline soot concentration is negligible until Z ' 30 mm, peaks at 34 mm, and decreases to zero by 39 mm. (The background level of 0.05 is Rayleigh scattering from gasphase species.) Most importantly, the profiles
Figure 2 shows centerline profiles of gas temperature, which were measured with uncoated 75 mm wire-diameter type R thermocouples using procedures described previously [13]. The measurements are corrected for radiation heat transfer effects and a rapid insertion procedure was used to minimize the effects of soot deposition on the radiation correction. We estimate that the results are accurate to 650 K. Starting from the burner surface, the temperatures rise rapidly due to heat transfer from the annular flame front, peak at Z ' 40 mm where the flame closes on the centerline and then decrease as conduction spreads heat away from the centerline. All of the profiles contain a dip near 30 mm, which is an artifact caused by soot deposition onto the thermocouple junction [13]. We have shown elsewhere that the maximum centerline temperature is a good indicator of the location where the local equivalence ratio is equal to 1, and thus where Z is equal to the true
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES flame height (H F) [15]. Table 1 lists the values of H F determined in this manner, and the maximum temperatures themselves. Hydrocarbon Concentrations Species concentrations were determined by extracting gas samples from the flames with a narrow-tipped quartz microprobe and analyzing these samples on-line for acetylene with a variable-energy electron-impact/quadrupole mass spectrometer (VEQMS), and for C3 and larger hydrocarbons with a photoionization/time-offlight mass spectrometer (PTMS) [7]. In the latter instrument sample gases with an ionization energy less than 10.5 eV are ionized by absorption of single photons from a 118 nm laser beam. The quartz probe had an outer diameter of 9 mm but narrowed over its last 15 mm to a sub-millimeter tip, which contained a 240 mm orifice. Typical sample manifold pressures were 1 to 3 Torr. Centerline mole fraction profiles for several hydrocarbons are shown in Figs. 3 through 14. In general, all of the non-fuel hydrocarbons increase from low concentrations near the burner surface, peak somewhere between 17 and 28 mm, and then decrease to zero by 30 mm. The profiles are broad enough to be well resolved by the roughly 1 mm spatial resolution of the quartz probe. Measurements were not performed below 10 mm since probe disturbances there were significant, especially in the lifted propane and n-butane flames. Several sources of possible systematic error in these results have been examined. Separate profiles measured with the sample manifold pressures varied by a factor of 2 were identical, indicating that the concentrations were not affected by reactions in the probe [16, 17]. Variation of the VEQMS ionizing electron energy showed that the acetylene measurements were free from interferences from ion fragments, except possibly at low Z in the ethylene flame. Ion fragmentation is negligible in the PTMS for all flame conditions. Finally, measurements with varying sampling times indicate that negligible condensation/adsorption losses to the walls occur for C12 and smaller hydrocarbons. Overall, we estimate that the results have a relative uncertainty of 30%, due to random
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error, changes in the background composition and temperature, and the effects of soot clogging on the flowrate through the sample probe. The experiments were repeated and all of the trends identified below were reproducible. The benzene and acetylene concentrations were directly calibrated, and the benzene calibration was applied to all other species measured with the PTMS. Photoionization cross sections at 118 nm do not vary strongly for the hydrocarbons considered here [18]; we have compared the signals generated by the PTMS from equal concentrations of various hydrocarbons and they have always differed by less than a factor of 2. Thus we conservatively estimate that the uncertainty in the absolute concentration of all species is a factor of 3. These uncertainties are acceptable since our primary interest here is in relative concentrations. The calibrated benzene concentrations in the methane flame were about 70% larger than in our previous experiments in the same flame, most likely due to an error in applying our calibration procedure in the earlier work [19 – 22]. For the current results we have carefully checked all aspects of the calibration procedure (linearity of the instruments, the argon concentration in the fuel, and consistency of commercially supplied calibration gases) and everything is in order. The newer concentrations agree very closely to those measured by Saito et al. in a similar-sized methane/air flame [4] and with our earliest measurements in the methane flame [7]. We also obtained agreement to within 20% in a recent comparison of benzene measurements with detailed modeling in a lifted ethylene flame [23]. DISCUSSION Identities of the Detected Hydrocarbons One goal of this study was to determine which hydrocarbons are present in these flames, and how they compare with the compounds predicted to be most thermodynamically stable under rich flame conditions. Table 2 lists the masses and maximum concentration in each flame of acetylene and of the hydrocarbons that we were able to detect with the PTMS. (The
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C. S. MCENALLY AND L. D. PFEFFERLE TABLE 2 Maximum Measured Concentrations (PPM) Fuel
Mass (AMU) 26 40 42 50 52 54 56 64 66 68 70 74 76 78 80 90 92 94 102 104 106 116 126 128 130 140 142 152 154
Formula
Methane
Ethane
Propane
N-butane
Ethylene
Acetylene
C2H2 C3H4 C3H6 C4H2 C4H4 C4H6 C4H8 C5H4 C5H6 C5H8 C5H10 C6H2 C6H4 C6H6 C6H8 C7H6 C7H8 C8H2 C8H6 C8H8 C8H10 C9H8 C10H6 C10H8 C10H10 C11H8 C11H10 C12H8 C12H10
8100 280 36 180 100 60 2 16 9 2 ,2 18 6 410 5 2 18 2 90 10 ,2 10 8 100 5 2 4 37 4
20900 200 81 300 400 170 9 16 52 9 3 27 9 420 11 4 33 2 190 42 4 43 20 96 6 6 9 54 9
15200 450 820 250 310 220 80 16 83 18 3 27 8 570 30 5 55 2 230 62 12 52 26 125 10 7 10 75 12
15300 490 1100 220 360 280 140 18 98 27 10 27 9 580 36 7 57 2 230 69 13 50 26 132 12 8 11 70 14
20800 100 57 350 400 240 4 11 32 3 ,2 33 10 230 7 3 17 2 130 35 2 28 17 56 4 5 6 37 6
fuel 180 60 250 360 25 3 12 34 3 ,2 23 8 170 8 4 17 2 140 15 3 33 20 43 4 6 6 34 4
acetone impurity in the acetylene flame is not listed). Since our analytical techniques are not isomer-selective, only elemental formulas are given for each mass. These elemental formulas were chosen by assuming that pure hydrocarbons dominated oxygenated hydrocarbons at each mass, and that formulas with a larger C/H ratio dominate when two pure hydrocarbon formulas are possible (i.e., 128 AMU is C10H8, not C9H20). These assumptions generate a set of formulas that agree well with gas chromatographic studies of hydrocarbons in nonpremixed flames [2, 4, 8]. The PTMS is able to simultaneously detect, with similar sensitivities, all species whose ionization energy is less than 10.5 eV, a category that includes all C3 and larger hydrocarbons except propane and butanes. Thus the C3 to C12 hydrocarbons not listed in
Table 2 are likely to be present, if at all, in concentrations of less than our detection limit, which is about 2 ppm. Ethylene has an ionization energy almost exactly equal to 10.5 eV, so it can be detected, but not quantified since the ionization efficiency is sensitive to the specific conditions in the molecular beam. It was detectable in all six of the flames studied here. Our sensitivity to C13 and larger hydrocarbons is relatively poor since these species adsorb onto the walls of the sampling system, and none of them were detectable in the present diluted flames. However, we have also performed measurements in undiluted flames of each fuel except acetylene; in those flames the aromatic species concentrations were much larger and we were able to qualitatively detect the hydrocarbons C12H12, C13H10, C13H12, C14H8, C14H10,
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
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TABLE 3 Comparison of Detected Hydrocarbons with Most Stable Classes of Stein and Fahr [24] Comparision of detected hydrocarbons with most Stable Classes C2H2, C2H4, C4H2, C4H4, C4H6, C6H2, C6H4, C6H6, C8H2, C8H6, C8H8, C10H6, C10H8, C12H8, C12H10, C14H8, C14H10, C16H10, C16H12, C18H10, and C18H12 Detected hydrocarbons that are not Most Stable Classes: C4H8, C6H8, C8H10, C10H10, C12H12, and C14H12 Detectable hydrocarbons that are Most Stable Classes and were not detected: C8H4, C10H2, C10H4, and C12H2
C14H12, C15H10, C15H12, C16H10, C16H12, C17H12, C18H10, and C18H12. The thermodynamic stability of hydrocarbons under rich flame conditions are indicated by the calculations of Stein and Fahr, who identified the most stable elemental formulas (“Most Stable Classes”; MSCs) and the most stable isomer for each of these formulas (“stabilomers”) [24]. These calculations are restricted to even carbon-number compounds. Generally the MSCs are condensed hydrocarbons with many higherorder carbon bonds and C/H ratios roughly equal to or greater than one; for C8 and larger species they are either aromatic molecules or polyacetylenes. Comparison of our measurements with the calculations of Stein and Fahr reveals a close correspondence between the hydrocarbons that are actually present in these flames and those hydrocarbons that are most thermodynamically stable. First, twenty-one of the detected hydrocarbons are MSCs, while only six of the detected hydrocarbons are non-MSCs, and only four C3 to C12 MSCs were not detected (see Table 3). Second, for any given carbon-number the concentrations of the MSCs are significantly greater than those of the non-MSCs. Third, Stein and Fahr neglected odd carbon-number hydrocarbons because they are generally less stable; aside from C3 species, the measured concentrations of odd carbon-number species are less than those of the immediately larger and smaller even carbon-number species. The most abundant non-MSC is butene (C4H8), but its concentrations are significant only in the n-butane and propane flames and they decay much more rapidly than the concentrations of the other C4 hydrocarbons (see Fig. 8). Butene can be formed directly by dehydro-
genation of the fuel in the n-butane flame and by reaction between allyl and methyl radicals in the propane flame [25, 26]. Presumably these routes are rapid enough to compensate for the unfavorable thermodynamics and thus allow large butene concentrations. Propene (C3H6) is another example of a thermodynamically unfavorable species that is present only in those flames where direct formation routes exist. The C3 to C12 MSCs that were not detected are either polyacetylenes or substituted polyacetylenes. The concentrations of C4H2 peak at Z ' 27 mm (Figure 5), significantly higher in the flame than most other species, and the concentrations of C6H2 peak at even higher Z. These results indicate that pyrolytic processes do tend to form polyacetylenes, but that the residence times in these flames, roughly 50 msec [19], are too short for significant amounts of the C8 and larger variants to accumulate. Other researchers have measured larger polycyclic hydrocarbons that are present in the
Fig. 3. Centerline profiles of acetylene mole fraction measured with the VEQMS.
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Fig. 4. Centerline profiles of C3H4 (allene, propyne) mole fraction measured with the PTMS.
Fig. 6. Centerline profiles of C4H4 (vinylacetylene) mole fraction measured with the PTMS.
gas-phase in methane flames [6], and in the particle-phase in ethylene flames [27], and the species identified were also generally MSCs. These results, combined with those presented here, indicate that the Stein and Fahr calculations are a good indicator of the hydrocarbons that will be present in the pyrolysis zones of nonpremixed flames. Indeed Dobbins et al. have suggested that hydrocarbon growth occurs among the MSCs in all flames, which explains the similarity of soot collected from different combustion systems [27].
The second goal of this study was to see how the concentrations of the pyrolysis products in nonpremixed combustion depend on fuel structure. This section discusses the C2 to C5 non-aro-
matic pyrolysis products, and the next section discusses the aromatic hydrocarbons. Comparison of any one species among the flames is misleading, since its concentrations can easily be changed by altering Q HC and Q N2. Of greater interest is the variation in the ratio between two species, which is less dependent on specific choices of flowrates. The species ratios we measured in undiluted flames of these same fuels are qualitatively similar to those discussed below for the nitrogen-doped flames. Figures 3 to 9 and Table 2 show that the ratios of most C2 to C5 hydrocarbons vary significantly among most of the flames. For example, the C4H4/C3H4 ratio peaks at roughly 4.5 in the ethylene flame, 2.5 in the ethane and acetylene flames, 1 in the n-butane and propane flames, and only 0.5 in the methane flame. While the C2 fuels tend to produce large con-
Fig. 5. Centerline profiles of C4H2 (diacetylene) mole fraction measured with the PTMS.
Fig. 7. Centerline profiles of C4H6 (butadienes, butynes) measured with the PTMS.
Quantities of Non-Aromatic Hydrocarbons
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
369 TABLE 4
Products of Hydrocarbon Mole Fractions Fuel Methane Ethane Propane n-butane Ethylene Acetylene a
Fig. 8. Centerline profiles of C4H8 (butenes) measured with the PTMS.
centrations of C4 species, the C4H6/C4H4 ratio is approximately 0.8 in the ethylene flame, 0.7 in the ethane flame, and only 0.1 in the acetylene flame. However, very little difference is observed between the propane and n-butane flames. Senkan and Castaldi have suggested that methane produces more methyl radicals than C2 fuels, and therefore relatively more odd carbon-number products [28]. Our results are partially consistent with this idea. The ratios of C3H4 and C5H4 to C4 products are higher for methane than for the C2 fuels, except C3H4/ C4H6 which is the same for methane and acetylene. However, methane produces very little C3H6, C5H6, or C5H8. The differences in the C3/C4 product ratios are potentially important because both C3 and
Fig. 9. Centerline profiles of C5H6 measured with the PTMS.
[C3H4][C3H4]a [C2H2][C4H4]a [C2H2][C4H6]a 1.0 0.51 3.1 2.9 0.14 0.30
1.0 12 6.1 6.0 12 23
1.0 6.8 7.2 8.8 8.9 2.2
Values normalized to the methane flame.
C4 radicals have been suggested as possible aromatic ring precursors in aliphatic-fueled flames. The specific C3 route is the recombination of C3H3 to produce phenyl radical [29, 30], C3H3 1 C3H3 5 C6H5 1 H,
(R1)
and the C4 routes are the reactions of n-C4H3 or n-C4H5 with acetylene to form phenyl radical or benzene [31, 32], n-C4H3 1 C2H2 5 C6H5,
(R2)
n-C4H5 1 C2H2 5 C6H6 1 H.
(R3)
If we make the rather bold assumption that the C3H3/C3H4, n-C4H5/C4H6, and n-C4H3/C4H4 ratios are the same in each flame at Z 5 20 mm (but not necessarily equal to each other), and ignore the temperature differences among the flames, then the rate of (R1) is proportional to the square of the C3H4 mole fractions, the rate of (R2) is proportional to the product of the acetylene and C4H4 mole fractions, and (R3) is proportional to the product of the acetylene and
Fig. 10. Centerline profiles of C6H6 (benzene) measured with the PTMS.
370 C4H6 mole fractions. Values of these products are listed in Table 4, where they have been normalized to the values in the methane flame. The results indicate that the rates of (R1) to (R3) all vary by roughly an order of magnitude or more among the flames, which, given that the benzene concentrations vary by less than a factor of four (Table 1), suggests that the relative importance of (R1) through (R3) may differ among the flames. If the effects of temperature on the rate constant are accounted for with typical literature rate constants for these reactions [30, 32], then the ranges for (R1) and (R3) are essentially unchanged, and that of (R2) is significantly widened. The C5H6 concentrations are also interesting since the reactions of cyclopentadienyl (C5H5) radical with methyl radical and with itself have been suggested as important pathways to benzene and naphthalene [30, 33, 34]. In premixed flames oxidation of benzene provides a significant source for C5H5 [30], but this mechanism does not apply to nonpremixed flames where oxidation occurs after hydrocarbon growth. In an earlier study we added small quantities of C5-ring containing hydrocarbons (cyclopentane, cyclopentene, methylcyclopentene, and indene) to the fuel in a methane co-flowing flame, and observed increases in benzene and naphthalene that were consistent with these reactions of cyclopentadienyl [20]. However, the C5H6 concentrations in the undoped methane flame were so much smaller than in the doped flames that we deemed it unlikely that enough C5H5 was present in the undoped flame to significantly
Fig. 12. Centerline profiles of C8H6 (phenylacetylene) measured with the PTMS.
C. S. MCENALLY AND L. D. PFEFFERLE
Fig. 13. Centerline profiles of C8H8 (styrene) measured with the PTMS.
contribute to benzene and naphthalene formation. The current results show that the other fuels produce much larger C5H6 concentrations than methane (see Fig. 9), which raises the possibility that cyclopentadienyl reactions are an important source of aromatics for these other fuels. Quantities of Aromatic Hydrocarbons Figures 10 to 14 and Table 2 show that the ratios of aromatic species concentrations vary less among the flames than do those of nonaromatic species. The concentrations of most aromatic species vary in roughly the same order with respect to fuel type: n-butane, propane . ethane . methane, ethylene, and acetylene. Exceptions to this are that methane produces
Fig. 11. Centerline profiles of C7H8 (toluene) measured with the PTMS.
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
Fig. 14. Centerline profiles of C10H8 (naphthalene) measured with the PTMS.
relatively large concentrations of benzene and naphthalene, ethylene relatively large concentrations of styrene, and acetylene relatively large concentrations of phenylacetylene, indene, and diethynylbenzene. The enhanced concentration of styrene (vinylbenzene) in the ethylene flame is reasonable since this flame likely contains the largest concentrations of ethylene and vinyl radical. Similarly, the enhanced concentrations of acetylene-substituted species in the acetylene flame is consistent with the high acetylene concentrations in this flame. The enhanced concentrations of benzene and naphthalene in the methane flame are more surprising. They may imply that the H-atom concentrations in the methane flame are lower, and thus more of the one-ring and two-ring species are tied up in the relatively unreactive form of benzene and naphthalene. Another interesting point is that the aromatic species concentrations vary widely while the concentrations of soot are very similar. In our earlier experiments with doped methane flames, we found that soot qualitatively correlated with benzene (for non-aromatic additives) or naphthalene (for aromatic additives) [19 –22]. These results showed that formation of the first and second aromatic rings are key steps towards soot formation. Apparently though conditions vary too widely among the current flames for such simple relationships to work well. For example, the large acetylene concentrations in the acetylene and ethylene flames may mean that less aromatics are required to generate a
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given volume fraction of soot. The higher temperatures in the methane flame may have a similar effect. Thus we conclude that in general concentrations of one or two-ring aromatics are not a reliable surrogate for soot concentrations. While benzene does not correlate well with the soot concentrations, the temperatures at the onset of soot formation on the centerline (Z 5 30 mm) are the same to within 100 K (ranging from 1750 K for methane to 1650 K for acetylene), even though the temperatures vary by more than 600 K lower in the flames. This is consistent with the observation of Glassman and co-workers that soot inception occurs at a nearly uniform temperature [35]. Furthermore, like them we observe that, to the extent that the inception temperatures do vary, they are systematically lower in the flames with more nitrogen dilution. Conclusions 1. Essentially the same C2 to C12 hydrocarbons are present in the hydrocarbon growth regions of methane, ethane, ethylene, acetylene, propane, and n-butane coflowing nonpremixed flames. These species are generally those that are predicted to be most thermodynamically stable under rich flame conditions. 2. These fuels produce widely varying ratios of C2 to C5 species, enough to suggest that the primary pathways to aromatic species may differ among their flames. The ratios between aromatic products are much more uniform. 3. The concentrations of any given aromatic species may vary for different fuels, even though the soot concentrations are similar. However, the temperatures at the onset of soot formation agreed to within 100 K in these flames. We appreciate assistance in conducting the experiments from Roman Caudillo and Elanor Williams, and partial financial support from the United States Air Force (Grant F49620-94-10085) and the United States Environmental Protection Agency (R821206-01-0).
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McEnally, C. S., and Pfefferle, L. D., Combust. Sci. Technol. 128:257–278 (1997). McEnally, C. S., and Pfefferle, L. D., Combust. Flame 112:545–558 (1998). McEnally, C. S., and Pfefferle, L. D., Combust. Sci. Technol. 131:323–344 (1998). McEnally, C. S., Schaffer, A. M., Long, M. B., Pfefferle, L. D., Smooke, M. D., Colket, M. B., and Hall, R. J. Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh (in press). Stein, S. E., and Fahr, A., J. Phys. Chem. 89:3714 –3725 (1985). Pitz, W. J., Westbrook, C. K., Proscia, W. M., and Dryer, F. L., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 831– 843. Westbrook, C. K., and Pitz, W. J., Combust. Sci. Technol. 37:117–152 (1984). Dobbins, R. A., Fletcher, R. A., and Chang, H.-C., Combust. Flame 115:285–298 (1998). Senkan, S. M., and Castaldi, M., Combust. Flame, 107:141–150 (1996). Miller, J. A., and Melius, C. F., Combust. Flame 91:21–39 (1992). Marinov, N. M., Pitz, W. J., Westbrook, C. K., Castaldi, M. J., and Senkan, S. M., Combust. Sci. Tech. 116 –117:211–287 (1996). Westmoreland, P. R., Dean, A. M., Howard, J. B., Longwell, J. P., J. Phys. Chem. 93:8171– 8180 (1989). Wang, H., and Frenklach, M., Combust. Flame 110: 173–221 (1997). Melius, C. F., Colvin, M. E., Marinov, N. M., Pitz, W. M., and Senkan, S. M., Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 685– 692. Moskaleva, L. V., Mebel, A. M., and Lin, M. C., Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 521–526. Glassman, I., Nishida, O., and Sidebotham, G., in Soot Formation in Combustion. H. Bockhorn, ed. SpringerVerlag, Berlin, 1994, pp. 316 –324.
Received 18 March 1998; revised 15 June 1998; accepted 18 June 1998 AQ1: Au: Please update.
Department of Chemical Engineering and Center for Combustion Studies, Yale University, New Haven, CT 06520-8286, Centerline concentrations of twenty-nine C2 to C12 hydrocarbons were measured by extractive gas sampling combined with on-line photoionization mass spectrometry in soot-containing methane and nitrogen-diluted ethane, ethylene, acetylene, propane, and n-butane coflowing nonpremixed flames. Additional measurements were made of temperature with thermocouples and of soot concentration with laser light scattering and thermocouple particle densitometry. The fuel hydrocarbon and nitrogen flowrates were chosen such that each flame had a visible flame length of approximately 40 mm and similar soot concentration profiles. Essentially the same species were detectable in each flame, and these species are generally those that are predicted to be most thermodynamically stable under rich flame conditions. The different fuels produce widely varying ratios of C2 to C5 non-aromatic species, enough to suggest that the primary pathways to aromatic species may be different among the flames. The fuels produce relatively similar ratios of aromatic species, but the concentrations of aromatics varies considerably among the fuels, even though the soot concentrations were very similar. However, the temperatures at the onset of soot formation were similar to within 100 K. © 1999 by The Combustion Institute
INTRODUCTION Although non-fuel hydrocarbon concentrations have been measured in laboratory-scale methane nonpremixed flames on many occasions [1–7], only a few corresponding measurements have been reported for other fuels: C1 to C3 hydrocarbons in propane/air counterflow flames [1]; C1 to C6 hydrocarbons in ethane, ethylene, and acetylene coflowing flames [8]; a similar range in methane/butane, methane/butene, and ethylene coflowing flames [5–9]; and C1 to C8 hydrocarbons in allene, 1,3-butadiene, and benzene coflowing flames [10]. Consequently, little is known about how fuel structure affects the identity and quantity of hydrocarbons formed in the pyrolysis zone of nonpremixed flames. This poses a significant gap in our understanding of soot formation since the specific pool of pyrolysis products formed in a given flame determines which of the many possible pathways are responsible for key hydrocarbon growth steps such as aromatic ring formation. To partially fill this gap, we have measured centerline concentration profiles of C2 to C12 * Corresponding author. E-mail: charles.mcenally@yale. edu 0010-2180/99/$–see front matter PII S0010-2180(98)00102-3
hydrocarbons in methane and nitrogen-diluted ethane, ethylene, acetylene, propane, and nbutane coflowing nonpremixed flames; this paper presents the results. The measured species range from the immediate pyrolysis products of the fuels to multi-ring aromatic species, so the resulting database is well suited to examining the formation and early growth of aromatic species. The measurements were performed by extracting gas samples from the flames with a quartz microprobe and analyzing them on-line with mass spectrometry. The fuels other than methane were diluted with nitrogen so that their centerline soot concentration profiles would be similar to those in the methane flame, which ensured that clogging of the microprobe orifice by soot particles would be tolerably slow. Furthermore, this procedure caused the total hydrocarbon concentrations in the flames to be roughly similar in magnitude. EXPERIMENTAL METHODS AND RESULTS Flame Descriptions The atmospheric-pressure, overventilated, axisymmetric, coflowing, nonpremixed laminar COMBUSTION AND FLAME 117:362–372 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
Fig. 1. Centerline profiles of soot concentration measured with elastic laser light scattering. The results have been normalized to the maximum value in the methane flame. The inset shows the flame geometry.
flames were generated with a burner in which the fuel flows from an uncooled 12 mm diameter vertical brass tube and the oxidizer flows from the annular region between this tube and a 102 mm diameter concentric tube (see the inset to Fig. 1) [7, 11]. The oxidizer was air (flowrate ' 40,000 cm3/min at STP), while the fuel was a mixture containing a hydrocarbon, argon, and, in some cases, nitrogen. Table 1 lists the hydrocarbon flowrates (Q HC), nitrogen flowrates (Q N2), and hydrocarbon mole fractions in the unreacted fuel (Y HC) for the six flames we studied. The methane flowrate was the same as that used in our previous experiments, while the nitrogen and hydrocarbon flowrates for the other cases were chosen to produce centerline soot concentration profiles that were similar to the one in the methane flame, as indicated by
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the laser scattering measurements discussed below. The argon flowrate was adjusted so that argon always had the same mole fraction in the fuel as in air (1%), and therefore could serve as an internal standard for calibrating the species measurements [7]. The fuel mixtures were examined for impurities with our mass spectrometers. The acetylene mixture contained roughly 800 ppm of acetone, due to the addition of this compound as an inhibitor to the acetylene cylinder by the supplier. Since the measurements in the acetylene flame gave no indication of being affected by this impurity (for example no other oxygenated species were detected), we did not attempt to remove it. No other impurities were detected in any of the fuel mixtures. The flames all had visible flame lengths of roughly 40 mm, but their morphologies differed considerably. For example, the propane and n-butane flames were significantly lifted from the burner, the acetylene flame was slightly lifted, and the other flames were anchored to the fuel tube lip. Table 1 lists the measured lift-off heights (H L) for each flame, where H L is defined as the lowest axial height above the fuel tube exit plane at which the thermocouple visibly glowed (temperature . 1100 K) during a radial traverse through the flame. Furthermore, while the centerline soot profiles were similar in each flame, soot first appeared in the annular region surrounding the centerline at different locations. This is indicated by the inception heights (H I) listed in Table 1, which are defined as the lowest axial height in each flame where
TABLE 1 Flame Characteristics Fuel Methane Ethane Propane n-butane Ethylene Acetylene
Q HC
Q N2
Q Ar
Y HC
HL
HI
HF
T max
240 108 63 44 115 114
0 180 300 300 410 810
2.4 2.9 3.7 3.4 5.3 9.3
0.99 0.37 0.17 0.13 0.22 0.12
0 0 2 4 0 1
17 24 28 29 20 19
40 38 37 37 38 38
1970 1930 1880 1890 1930 1850
Q HC, Q N2, and Q Ar, volumetric flowrates of the hydrocarbon, nitrogen, and argon portions of the fuel in cm3/min at STP (6 5%); Y HC, mole fraction of hydrocarbon in the unreacted mixture, including the argon (6 10%); H L, lift-off height of each flame in millimeters (6 1 mm); H I, lowest height where soot is detectable at any radial location in millimeters (6 1 mm); H F, flame height 5 height of maximum centerline temperature in millimeters (6 1 mm); T max, maximum centerline temperature in Kelvins (6 50 K).
364 soot scattering was detectable during a radial traverse through the flame. All of the measurements reported here are centerline profiles of flame properties as functions of the axial height above the fuel tube exit plane (Z; see Fig. 1). Miller et al. have shown that the primary hydrocarbon production zone in nonpremixed flames lies slightly inside the oxidizing flame front [12]; since our flames are overventilated coflowing flames, at some height this primary pyrolysis zone will coincide with the centerline. Thus centerline profiles must pass through this zone and suffice for characterizing the pool of hydrocarbons produced by fuel decomposition in these flames. Furthermore, for these lightly sooting flames the soot distribution is conical, with the region of maximum soot concentration overlapping the centerline, as opposed to the cylindrical distributions observed in highly sooting flames [11]. Indeed, in our propane and n-butane flames, virtually no off-axis soot exists. Therefore, the primary hydrocarbon production zone on the centerline just below the flame tip is more relevant to soot formation than the off-axis annular reaction zone near the base of the flame.
C. S. MCENALLY AND L. D. PFEFFERLE
Fig. 2. Centerline profiles of (radiation-corrected) gas temperature measured with a thermocouple.
show that, in accord with our procedure for choosing Q HC and Q N2, the soot profiles are very similar in each flame. In general scattering from soot depends on particle diameter as well as volume fraction; however, the TPD measurements verified that the onset of soot formation and the maximum soot concentration are indeed similar among the flames, and showed that the maximum centerline volume fraction is roughly 0.5 ppm.
Soot Concentrations Gas Temperatures Soot concentration profiles were measured with elastic laser light scattering and with thermocouple particle densitometry (TPD), a technique we have developed in which absolute soot volume fractions are inferred from measured rates of soot particle mass transfer to a thermocouple [13, 14]. The laser source for the scattering measurements was the second harmonic of a Nd:YAG laser at 532 nm, which had a pulse energy of approximately 1 mJ and was weakly focussed through the flame. Scattering was measured at a right angle to the laser beam with a monochromator/photomultiplier tube combination. The centerline laser scattering profiles, normalized to the maximum signal measured in the methane flame, are shown in Figure 1. In each flame the centerline soot concentration is negligible until Z ' 30 mm, peaks at 34 mm, and decreases to zero by 39 mm. (The background level of 0.05 is Rayleigh scattering from gasphase species.) Most importantly, the profiles
Figure 2 shows centerline profiles of gas temperature, which were measured with uncoated 75 mm wire-diameter type R thermocouples using procedures described previously [13]. The measurements are corrected for radiation heat transfer effects and a rapid insertion procedure was used to minimize the effects of soot deposition on the radiation correction. We estimate that the results are accurate to 650 K. Starting from the burner surface, the temperatures rise rapidly due to heat transfer from the annular flame front, peak at Z ' 40 mm where the flame closes on the centerline and then decrease as conduction spreads heat away from the centerline. All of the profiles contain a dip near 30 mm, which is an artifact caused by soot deposition onto the thermocouple junction [13]. We have shown elsewhere that the maximum centerline temperature is a good indicator of the location where the local equivalence ratio is equal to 1, and thus where Z is equal to the true
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES flame height (H F) [15]. Table 1 lists the values of H F determined in this manner, and the maximum temperatures themselves. Hydrocarbon Concentrations Species concentrations were determined by extracting gas samples from the flames with a narrow-tipped quartz microprobe and analyzing these samples on-line for acetylene with a variable-energy electron-impact/quadrupole mass spectrometer (VEQMS), and for C3 and larger hydrocarbons with a photoionization/time-offlight mass spectrometer (PTMS) [7]. In the latter instrument sample gases with an ionization energy less than 10.5 eV are ionized by absorption of single photons from a 118 nm laser beam. The quartz probe had an outer diameter of 9 mm but narrowed over its last 15 mm to a sub-millimeter tip, which contained a 240 mm orifice. Typical sample manifold pressures were 1 to 3 Torr. Centerline mole fraction profiles for several hydrocarbons are shown in Figs. 3 through 14. In general, all of the non-fuel hydrocarbons increase from low concentrations near the burner surface, peak somewhere between 17 and 28 mm, and then decrease to zero by 30 mm. The profiles are broad enough to be well resolved by the roughly 1 mm spatial resolution of the quartz probe. Measurements were not performed below 10 mm since probe disturbances there were significant, especially in the lifted propane and n-butane flames. Several sources of possible systematic error in these results have been examined. Separate profiles measured with the sample manifold pressures varied by a factor of 2 were identical, indicating that the concentrations were not affected by reactions in the probe [16, 17]. Variation of the VEQMS ionizing electron energy showed that the acetylene measurements were free from interferences from ion fragments, except possibly at low Z in the ethylene flame. Ion fragmentation is negligible in the PTMS for all flame conditions. Finally, measurements with varying sampling times indicate that negligible condensation/adsorption losses to the walls occur for C12 and smaller hydrocarbons. Overall, we estimate that the results have a relative uncertainty of 30%, due to random
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error, changes in the background composition and temperature, and the effects of soot clogging on the flowrate through the sample probe. The experiments were repeated and all of the trends identified below were reproducible. The benzene and acetylene concentrations were directly calibrated, and the benzene calibration was applied to all other species measured with the PTMS. Photoionization cross sections at 118 nm do not vary strongly for the hydrocarbons considered here [18]; we have compared the signals generated by the PTMS from equal concentrations of various hydrocarbons and they have always differed by less than a factor of 2. Thus we conservatively estimate that the uncertainty in the absolute concentration of all species is a factor of 3. These uncertainties are acceptable since our primary interest here is in relative concentrations. The calibrated benzene concentrations in the methane flame were about 70% larger than in our previous experiments in the same flame, most likely due to an error in applying our calibration procedure in the earlier work [19 – 22]. For the current results we have carefully checked all aspects of the calibration procedure (linearity of the instruments, the argon concentration in the fuel, and consistency of commercially supplied calibration gases) and everything is in order. The newer concentrations agree very closely to those measured by Saito et al. in a similar-sized methane/air flame [4] and with our earliest measurements in the methane flame [7]. We also obtained agreement to within 20% in a recent comparison of benzene measurements with detailed modeling in a lifted ethylene flame [23]. DISCUSSION Identities of the Detected Hydrocarbons One goal of this study was to determine which hydrocarbons are present in these flames, and how they compare with the compounds predicted to be most thermodynamically stable under rich flame conditions. Table 2 lists the masses and maximum concentration in each flame of acetylene and of the hydrocarbons that we were able to detect with the PTMS. (The
366
C. S. MCENALLY AND L. D. PFEFFERLE TABLE 2 Maximum Measured Concentrations (PPM) Fuel
Mass (AMU) 26 40 42 50 52 54 56 64 66 68 70 74 76 78 80 90 92 94 102 104 106 116 126 128 130 140 142 152 154
Formula
Methane
Ethane
Propane
N-butane
Ethylene
Acetylene
C2H2 C3H4 C3H6 C4H2 C4H4 C4H6 C4H8 C5H4 C5H6 C5H8 C5H10 C6H2 C6H4 C6H6 C6H8 C7H6 C7H8 C8H2 C8H6 C8H8 C8H10 C9H8 C10H6 C10H8 C10H10 C11H8 C11H10 C12H8 C12H10
8100 280 36 180 100 60 2 16 9 2 ,2 18 6 410 5 2 18 2 90 10 ,2 10 8 100 5 2 4 37 4
20900 200 81 300 400 170 9 16 52 9 3 27 9 420 11 4 33 2 190 42 4 43 20 96 6 6 9 54 9
15200 450 820 250 310 220 80 16 83 18 3 27 8 570 30 5 55 2 230 62 12 52 26 125 10 7 10 75 12
15300 490 1100 220 360 280 140 18 98 27 10 27 9 580 36 7 57 2 230 69 13 50 26 132 12 8 11 70 14
20800 100 57 350 400 240 4 11 32 3 ,2 33 10 230 7 3 17 2 130 35 2 28 17 56 4 5 6 37 6
fuel 180 60 250 360 25 3 12 34 3 ,2 23 8 170 8 4 17 2 140 15 3 33 20 43 4 6 6 34 4
acetone impurity in the acetylene flame is not listed). Since our analytical techniques are not isomer-selective, only elemental formulas are given for each mass. These elemental formulas were chosen by assuming that pure hydrocarbons dominated oxygenated hydrocarbons at each mass, and that formulas with a larger C/H ratio dominate when two pure hydrocarbon formulas are possible (i.e., 128 AMU is C10H8, not C9H20). These assumptions generate a set of formulas that agree well with gas chromatographic studies of hydrocarbons in nonpremixed flames [2, 4, 8]. The PTMS is able to simultaneously detect, with similar sensitivities, all species whose ionization energy is less than 10.5 eV, a category that includes all C3 and larger hydrocarbons except propane and butanes. Thus the C3 to C12 hydrocarbons not listed in
Table 2 are likely to be present, if at all, in concentrations of less than our detection limit, which is about 2 ppm. Ethylene has an ionization energy almost exactly equal to 10.5 eV, so it can be detected, but not quantified since the ionization efficiency is sensitive to the specific conditions in the molecular beam. It was detectable in all six of the flames studied here. Our sensitivity to C13 and larger hydrocarbons is relatively poor since these species adsorb onto the walls of the sampling system, and none of them were detectable in the present diluted flames. However, we have also performed measurements in undiluted flames of each fuel except acetylene; in those flames the aromatic species concentrations were much larger and we were able to qualitatively detect the hydrocarbons C12H12, C13H10, C13H12, C14H8, C14H10,
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
367
TABLE 3 Comparison of Detected Hydrocarbons with Most Stable Classes of Stein and Fahr [24] Comparision of detected hydrocarbons with most Stable Classes C2H2, C2H4, C4H2, C4H4, C4H6, C6H2, C6H4, C6H6, C8H2, C8H6, C8H8, C10H6, C10H8, C12H8, C12H10, C14H8, C14H10, C16H10, C16H12, C18H10, and C18H12 Detected hydrocarbons that are not Most Stable Classes: C4H8, C6H8, C8H10, C10H10, C12H12, and C14H12 Detectable hydrocarbons that are Most Stable Classes and were not detected: C8H4, C10H2, C10H4, and C12H2
C14H12, C15H10, C15H12, C16H10, C16H12, C17H12, C18H10, and C18H12. The thermodynamic stability of hydrocarbons under rich flame conditions are indicated by the calculations of Stein and Fahr, who identified the most stable elemental formulas (“Most Stable Classes”; MSCs) and the most stable isomer for each of these formulas (“stabilomers”) [24]. These calculations are restricted to even carbon-number compounds. Generally the MSCs are condensed hydrocarbons with many higherorder carbon bonds and C/H ratios roughly equal to or greater than one; for C8 and larger species they are either aromatic molecules or polyacetylenes. Comparison of our measurements with the calculations of Stein and Fahr reveals a close correspondence between the hydrocarbons that are actually present in these flames and those hydrocarbons that are most thermodynamically stable. First, twenty-one of the detected hydrocarbons are MSCs, while only six of the detected hydrocarbons are non-MSCs, and only four C3 to C12 MSCs were not detected (see Table 3). Second, for any given carbon-number the concentrations of the MSCs are significantly greater than those of the non-MSCs. Third, Stein and Fahr neglected odd carbon-number hydrocarbons because they are generally less stable; aside from C3 species, the measured concentrations of odd carbon-number species are less than those of the immediately larger and smaller even carbon-number species. The most abundant non-MSC is butene (C4H8), but its concentrations are significant only in the n-butane and propane flames and they decay much more rapidly than the concentrations of the other C4 hydrocarbons (see Fig. 8). Butene can be formed directly by dehydro-
genation of the fuel in the n-butane flame and by reaction between allyl and methyl radicals in the propane flame [25, 26]. Presumably these routes are rapid enough to compensate for the unfavorable thermodynamics and thus allow large butene concentrations. Propene (C3H6) is another example of a thermodynamically unfavorable species that is present only in those flames where direct formation routes exist. The C3 to C12 MSCs that were not detected are either polyacetylenes or substituted polyacetylenes. The concentrations of C4H2 peak at Z ' 27 mm (Figure 5), significantly higher in the flame than most other species, and the concentrations of C6H2 peak at even higher Z. These results indicate that pyrolytic processes do tend to form polyacetylenes, but that the residence times in these flames, roughly 50 msec [19], are too short for significant amounts of the C8 and larger variants to accumulate. Other researchers have measured larger polycyclic hydrocarbons that are present in the
Fig. 3. Centerline profiles of acetylene mole fraction measured with the VEQMS.
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C. S. MCENALLY AND L. D. PFEFFERLE
Fig. 4. Centerline profiles of C3H4 (allene, propyne) mole fraction measured with the PTMS.
Fig. 6. Centerline profiles of C4H4 (vinylacetylene) mole fraction measured with the PTMS.
gas-phase in methane flames [6], and in the particle-phase in ethylene flames [27], and the species identified were also generally MSCs. These results, combined with those presented here, indicate that the Stein and Fahr calculations are a good indicator of the hydrocarbons that will be present in the pyrolysis zones of nonpremixed flames. Indeed Dobbins et al. have suggested that hydrocarbon growth occurs among the MSCs in all flames, which explains the similarity of soot collected from different combustion systems [27].
The second goal of this study was to see how the concentrations of the pyrolysis products in nonpremixed combustion depend on fuel structure. This section discusses the C2 to C5 non-aro-
matic pyrolysis products, and the next section discusses the aromatic hydrocarbons. Comparison of any one species among the flames is misleading, since its concentrations can easily be changed by altering Q HC and Q N2. Of greater interest is the variation in the ratio between two species, which is less dependent on specific choices of flowrates. The species ratios we measured in undiluted flames of these same fuels are qualitatively similar to those discussed below for the nitrogen-doped flames. Figures 3 to 9 and Table 2 show that the ratios of most C2 to C5 hydrocarbons vary significantly among most of the flames. For example, the C4H4/C3H4 ratio peaks at roughly 4.5 in the ethylene flame, 2.5 in the ethane and acetylene flames, 1 in the n-butane and propane flames, and only 0.5 in the methane flame. While the C2 fuels tend to produce large con-
Fig. 5. Centerline profiles of C4H2 (diacetylene) mole fraction measured with the PTMS.
Fig. 7. Centerline profiles of C4H6 (butadienes, butynes) measured with the PTMS.
Quantities of Non-Aromatic Hydrocarbons
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
369 TABLE 4
Products of Hydrocarbon Mole Fractions Fuel Methane Ethane Propane n-butane Ethylene Acetylene a
Fig. 8. Centerline profiles of C4H8 (butenes) measured with the PTMS.
centrations of C4 species, the C4H6/C4H4 ratio is approximately 0.8 in the ethylene flame, 0.7 in the ethane flame, and only 0.1 in the acetylene flame. However, very little difference is observed between the propane and n-butane flames. Senkan and Castaldi have suggested that methane produces more methyl radicals than C2 fuels, and therefore relatively more odd carbon-number products [28]. Our results are partially consistent with this idea. The ratios of C3H4 and C5H4 to C4 products are higher for methane than for the C2 fuels, except C3H4/ C4H6 which is the same for methane and acetylene. However, methane produces very little C3H6, C5H6, or C5H8. The differences in the C3/C4 product ratios are potentially important because both C3 and
Fig. 9. Centerline profiles of C5H6 measured with the PTMS.
[C3H4][C3H4]a [C2H2][C4H4]a [C2H2][C4H6]a 1.0 0.51 3.1 2.9 0.14 0.30
1.0 12 6.1 6.0 12 23
1.0 6.8 7.2 8.8 8.9 2.2
Values normalized to the methane flame.
C4 radicals have been suggested as possible aromatic ring precursors in aliphatic-fueled flames. The specific C3 route is the recombination of C3H3 to produce phenyl radical [29, 30], C3H3 1 C3H3 5 C6H5 1 H,
(R1)
and the C4 routes are the reactions of n-C4H3 or n-C4H5 with acetylene to form phenyl radical or benzene [31, 32], n-C4H3 1 C2H2 5 C6H5,
(R2)
n-C4H5 1 C2H2 5 C6H6 1 H.
(R3)
If we make the rather bold assumption that the C3H3/C3H4, n-C4H5/C4H6, and n-C4H3/C4H4 ratios are the same in each flame at Z 5 20 mm (but not necessarily equal to each other), and ignore the temperature differences among the flames, then the rate of (R1) is proportional to the square of the C3H4 mole fractions, the rate of (R2) is proportional to the product of the acetylene and C4H4 mole fractions, and (R3) is proportional to the product of the acetylene and
Fig. 10. Centerline profiles of C6H6 (benzene) measured with the PTMS.
370 C4H6 mole fractions. Values of these products are listed in Table 4, where they have been normalized to the values in the methane flame. The results indicate that the rates of (R1) to (R3) all vary by roughly an order of magnitude or more among the flames, which, given that the benzene concentrations vary by less than a factor of four (Table 1), suggests that the relative importance of (R1) through (R3) may differ among the flames. If the effects of temperature on the rate constant are accounted for with typical literature rate constants for these reactions [30, 32], then the ranges for (R1) and (R3) are essentially unchanged, and that of (R2) is significantly widened. The C5H6 concentrations are also interesting since the reactions of cyclopentadienyl (C5H5) radical with methyl radical and with itself have been suggested as important pathways to benzene and naphthalene [30, 33, 34]. In premixed flames oxidation of benzene provides a significant source for C5H5 [30], but this mechanism does not apply to nonpremixed flames where oxidation occurs after hydrocarbon growth. In an earlier study we added small quantities of C5-ring containing hydrocarbons (cyclopentane, cyclopentene, methylcyclopentene, and indene) to the fuel in a methane co-flowing flame, and observed increases in benzene and naphthalene that were consistent with these reactions of cyclopentadienyl [20]. However, the C5H6 concentrations in the undoped methane flame were so much smaller than in the doped flames that we deemed it unlikely that enough C5H5 was present in the undoped flame to significantly
Fig. 12. Centerline profiles of C8H6 (phenylacetylene) measured with the PTMS.
C. S. MCENALLY AND L. D. PFEFFERLE
Fig. 13. Centerline profiles of C8H8 (styrene) measured with the PTMS.
contribute to benzene and naphthalene formation. The current results show that the other fuels produce much larger C5H6 concentrations than methane (see Fig. 9), which raises the possibility that cyclopentadienyl reactions are an important source of aromatics for these other fuels. Quantities of Aromatic Hydrocarbons Figures 10 to 14 and Table 2 show that the ratios of aromatic species concentrations vary less among the flames than do those of nonaromatic species. The concentrations of most aromatic species vary in roughly the same order with respect to fuel type: n-butane, propane . ethane . methane, ethylene, and acetylene. Exceptions to this are that methane produces
Fig. 11. Centerline profiles of C7H8 (toluene) measured with the PTMS.
NON-FUEL HYDROCARBONS IN NONPREMIXED FLAMES
Fig. 14. Centerline profiles of C10H8 (naphthalene) measured with the PTMS.
relatively large concentrations of benzene and naphthalene, ethylene relatively large concentrations of styrene, and acetylene relatively large concentrations of phenylacetylene, indene, and diethynylbenzene. The enhanced concentration of styrene (vinylbenzene) in the ethylene flame is reasonable since this flame likely contains the largest concentrations of ethylene and vinyl radical. Similarly, the enhanced concentrations of acetylene-substituted species in the acetylene flame is consistent with the high acetylene concentrations in this flame. The enhanced concentrations of benzene and naphthalene in the methane flame are more surprising. They may imply that the H-atom concentrations in the methane flame are lower, and thus more of the one-ring and two-ring species are tied up in the relatively unreactive form of benzene and naphthalene. Another interesting point is that the aromatic species concentrations vary widely while the concentrations of soot are very similar. In our earlier experiments with doped methane flames, we found that soot qualitatively correlated with benzene (for non-aromatic additives) or naphthalene (for aromatic additives) [19 –22]. These results showed that formation of the first and second aromatic rings are key steps towards soot formation. Apparently though conditions vary too widely among the current flames for such simple relationships to work well. For example, the large acetylene concentrations in the acetylene and ethylene flames may mean that less aromatics are required to generate a
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given volume fraction of soot. The higher temperatures in the methane flame may have a similar effect. Thus we conclude that in general concentrations of one or two-ring aromatics are not a reliable surrogate for soot concentrations. While benzene does not correlate well with the soot concentrations, the temperatures at the onset of soot formation on the centerline (Z 5 30 mm) are the same to within 100 K (ranging from 1750 K for methane to 1650 K for acetylene), even though the temperatures vary by more than 600 K lower in the flames. This is consistent with the observation of Glassman and co-workers that soot inception occurs at a nearly uniform temperature [35]. Furthermore, like them we observe that, to the extent that the inception temperatures do vary, they are systematically lower in the flames with more nitrogen dilution. Conclusions 1. Essentially the same C2 to C12 hydrocarbons are present in the hydrocarbon growth regions of methane, ethane, ethylene, acetylene, propane, and n-butane coflowing nonpremixed flames. These species are generally those that are predicted to be most thermodynamically stable under rich flame conditions. 2. These fuels produce widely varying ratios of C2 to C5 species, enough to suggest that the primary pathways to aromatic species may differ among their flames. The ratios between aromatic products are much more uniform. 3. The concentrations of any given aromatic species may vary for different fuels, even though the soot concentrations are similar. However, the temperatures at the onset of soot formation agreed to within 100 K in these flames. We appreciate assistance in conducting the experiments from Roman Caudillo and Elanor Williams, and partial financial support from the United States Air Force (Grant F49620-94-10085) and the United States Environmental Protection Agency (R821206-01-0).
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Received 18 March 1998; revised 15 June 1998; accepted 18 June 1998 AQ1: Au: Please update.