The effect of temperature on the soot onset chemistry in one-dimensional, atmospheric-pressure, premixed ethylbenzene flames

Combustion and Flame 155 (2008) 232–246 www.elsevier.com/locate/combustflame

The effect of temperature on the soot onset chemistry in one-dimensional, atmospheric-pressure, premixed ethylbenzene flames Ali Ergut a , Rick J. Therrien a , Yiannis A. Levendis a,∗ , Henning Richter b , Jack B. Howard b , Joel B. Carlson c a Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115, USA b Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA c U.S. Army SBCCOM—Natick Soldier Center, Natick, MA 01760, USA

Received 14 November 2007; received in revised form 19 February 2008; accepted 10 March 2008 Available online 28 April 2008

Abstract This work was conducted to investigate the effects of temperature on soot formation/oxidation chemistry in the vicinity of the soot onset threshold (φ critical ) in one-dimensional, laminar, atmospheric-pressure premixed ethylbenzene flames. The effects of temperature on the evolution of soot precursors were observed just prior and subsequent to soot onset. Liquid ethylbenzene was prevaporized in nitrogen and blended with an oxygen–nitrogen mixture and, upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. Three flames at the same fuel-to-air equivalence ratio (φ = 1.74) but with different temperature profiles were obtained by regulating the total heat loss from the flame to the burner, as a result of altering the cold gas velocity of the reacting gases through the burner. A 100-K spread was detected among the three flame temperature profiles. The coolest flame was slightly sooting, the intermediate temperature flame was at the visible onset of sooting, and the hottest flame was not sooting. Combustion products were sampled at various heights in these flames. CO and CO2 mole fractions were found to increase with temperature, supporting the hypothesis that with increasing temperature the rate of oxidation reactions increases faster than the rate of soot formation reactions. Again supporting the same hypothesis, the mole fractions of at least some of the suspected soot precursor hydrocarbons decreased with increasing temperature. Similarly, both the number and the concentrations of detected polycyclic aromatic hydrocarbons (PAH) and oxygenated aromatic hydrocarbons were highest in the slightly sooting, i.e., the coolest flame. This flame also had the highest condensed phase/gaseous phase PAH ratio among the three flames. However, whereas in all three flames the mole fractions of PAH were disparate in the broad neighborhood of the flame zone, they converged to similar values in the postflame zone at 7 mm height from the surface of the burner. Experimentally obtained mole fractions of effluent species were compared with predictions from a detailed kinetic model. © 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved. * Corresponding author.

E-mail address: [email protected] (Y.A. Levendis). 0010-2180/$ – see front matter © 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2008.03.009

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Keywords: Premixed flames; Ethylbenzene; Soot onset; Flame temperature; PAH

1. Introduction The formation of soot in combustion processes affects the operation and design of combustion devices and has implications for both their effective operation and their pollutant emissions. To minimize emissions from combustion sources, a good understanding of the factors governing the propensity to form soot is required. Fuel composition, molecular structure, along with flame temperature, fuel/oxidizer ratio, gas dynamics of the system, and pressure are influential parameters that determine the threshold and quantity of soot formed in a given flame. There are two distinct facets to the effects of the fuel molecular structure on sooting propensity. First, as a given premixed fuel/oxidant combination is made increasingly fuel-rich, a rather sharp onset of sooting is observed (soot onset limit). The critical equivalence ratio (φ c ) is defined as the ratio of the actual fuel/air (or oxygen) mass ratio at which incipient soot is visually first detected as a faint orange glow in the flame to the stoichiometric fuel/air (oxygen) mass ratio, when the actual fuel/air (oxygen) mass ratio is increased from fuel-lean conditions. This facet of the tendency to soot is important in practical applications where total absence of sooting is desirable. The second facet is that further increasing the fuel concentration beyond the point of soot onset causes increasingly greater quantities of soot to form. It is important to note, though, that in burner-type combustion systems it is difficult to separate the effect of the equivalence ratio from that of the temperature, since temperature is a function of equivalence ratio, among other parameters. This work is the second of a series of investigations of the independent effects of equivalence ratio and temperature in atmospheric-pressure ethylbenzene flames. A recent study in this laboratory isolated the effect of equivalence ratio on the soot onset chemistry of a flame, keeping the temperature profiles nearly the same [1]. The results indicated that the soot onset limit is not a function of temperature alone; i.e., while the maximum measured flame temperatures were constant, a flame could be made to be either sooting, or at its sooting limit, or nonsooting, depending on the equivalence ratio. This manuscript aims at examining the effect of temperature on soot onset chemistry, this time keeping the equivalence ratio constant. The factors that affect the flame temperature are the equivalence ratio, the initial temperature of the reacting gases, the type of diluent, the diluent/oxygen

mole ratio, the heat losses to the surrounding environment, and, to lesser extent, the pressure. Kaskan [2] showed that for a given equivalence ratio, the flame temperature can be changed by varying the total cold gas velocity through the burner because the heat lost to the burner decreases as the gases flow faster and the flame temperature remains high. This effect can be used to generate flames that have different temperatures on the same burner even if all the other parameters listed above are the same. There are many publications regarding the effect of temperature on φ c : Street and Thomas [3] found that φ c increased with higher unburned (i.e., presumably also with higher burned) gas temperatures. Millikan [4] observed that increasing the (burned gas) temperature of a given premixed flame, by increasing the cold gas velocity, extends the φ c to higher values. He concluded that temperature has a dominant role in two competing processes occurring in premixed flames: (a) the pyrolysis rate of the fuel intermediate (acetylene) leading to the precursors, and (b) the rate of oxidative (hydroxyl radical) attack on these precursors. Both rates increase with temperature, but the oxidative rate increases faster [4,5]. Thus, the higher the temperature the lower the tendency of premixed flames to soot. Takahashi and Glassman [5] correlated the sooting limits of premixed flames for a wide range of fuels by accounting for C–C bonds and flame temperatures and reported that the structure of the fuel molecule did not seem to be important, as the sooting tendency of decane was the same as that of benzene. They postulated that all fuels break down to the same essential species, such as acetylene, which build into soot. Macfarlane et al. [6] investigated the effect of temperature and pressure on soot formation and observed that hotter flames had a higher φ c compared to the cooler flames, with all other parameters kept constant. Olson and Madronich [7] contrasted the effects of calculated and measured temperatures on soot threshold in premixed flames and concluded that the soot yield for each fuel at and above the soot threshold limit is uniquely determined by the measured flame temperature in the sooting region (their measurements were at 20 mm from the surface of the burner), regardless of how this temperature was achieved. The most detailed study so far on the effect of temperature on sooting limit in premixed flat flames was done by Böhm et al. [8]. They changed the temperature of the flame by controlling the heat loss, which in turn was accomplished by varying total cold gas flow through the burner at a constant fuel/air ratio. They reported that with increasing temperature, the

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critical C/O ratio, and hence φ c , increased. They investigated φ c at temperatures between ∼1500 and 2000 K. Below 1600 K, flames became rather unstable, but the presence of a “shield flame” kept the flames on the burner. They concluded that the critical C/O ratio, and hence φ c , reached a minimum at temperatures slightly lower than 1600 K for ethylene and benzene flames. For still lower temperatures the threshold curve bent back toward higher φ c , forming an inverted bell shaped curve. The soot volume fraction reached a maximum at a temperature between the low and the high temperature limits for a specific equivalence ratio. Harris et al. [9] measured φ c as a function of temperature (1600–1880 K) for premixed flames of methane, ethane, propane, ethylene and acetylene at atmospheric pressure. They found a linear relationship between ln(φ c ) and 1/T . They also developed a predictive correlation that shows the influence of temperature, OH concentration and C/H ratio on sooting tendency. Soot formation mechanisms are also discussed extensively in the literature (e.g., Refs. [10–13]) and a review was provided in our previous paper [1]. Previous findings on the effects of temperature on soot chemistry in flat flames are briefly discussed below. It has been reported that in premixed flames high temperatures favor oxidation of light pre-aromaticring species over formation of soot precursors and, thus, suppress soot formation [10]. Markatou et al. [14] investigated the onset of sooting in laminar premixed flames of ethane, ethylene and acetylene computationally and predicted φ c in reasonable agreement with the experiments of Harris et al. [9]. The analysis of their computational results suggested that the appearance of soot is controlled by two factors, concentration of acetylene and growth of polycyclic aromatics and the latter is limited by the rise in flame temperature toward the end of the main reaction zone. They also suggested that soot particle inception and hence the sooting limit are determined by particle nucleation, not by either particle growth or oxidation reactions. They also claimed that the dependence of the critical equivalence ratio on temperature may be partly explained by the reversibility of reaction steps leading to the formation and growth of PAH. Ciajolo et al. [15] investigated the effect of temperature on soot chemistry in sooting ethylene/O2 flames, for flames with temperature levels between the low- and high-temperature soot thresholds of the bell-shaped domain of soot formation reported by Böhm et al. [8]. They varied the temperatures of their flat flames by changing the cold gas velocity while keeping the mixture composition constant. They observed the profiles of temperature, fuel, acetylene, benzene, soot, total PAH, and condensed species con-

centration. They confirmed that the soot concentration passes through a maximum at a temperature between the low- and high-temperature soot threshold, which agreed with the results of Böhm et al. [8] for premixed flat flames, as well as with Graham [16] and Frenklach et al. [17] in shock tube experiments. They concluded that the intermediate temperature flame conditions were ideal for the formation and coagulation of soot, whereas in the low- and high-temperature flames either PAH coagulation or oxidation effects hindered soot growth. The present study investigates, both experimentally and theoretically, the changes in the concentrations of fixed gases, light hydrocarbons, individual polycyclic aromatic hydrocarbons, and oxygenated aromatic hydrocarbon species at, or near, the hightemperature sooting limit of premixed flames. Experiments were conducted with three ethylbenzene flames operated at a fixed equivalence ratio but different temperatures; direct sampling in both the flame and postflame regions was conducted. In recent prior work [1], flames with similar temperature profiles but different equivalence ratios had been investigated. One of those flames (φ = 1.74) was at the visible sooting limit. This flame became the starting point of the present study. Then the temperature of this flame was decreased, at constant equivalence ratio, mostly by reducing its cold gas velocity through the burner until streaks of soot were clearly present inside the flame. This procedure resulted in the second flame of this study, which had the same equivalence ratio, but was ∼50 ◦ C cooler than the original flame. A third flame was then obtained by increasing the temperature of the original flame, again by ∼50 ◦ C, this time by simply increasing its cold gas velocity until no soot was visually present in the flame, i.e., until the orange glow completely disappeared, while the equivalence ratio was again kept constant at φ = 1.74. The mole fractions of fixed gases, light hydrocarbons, PAHs, and oxygenated aromatic hydrocarbons in these two flames, one slightly sooting, the other nonsooting, were compared to those in the aforementioned flame at the soot onset limit. In summary, the premixed ethylbenzene flames examined in this work were all at the same equivalence ratio (φ = 1.74), but they all experienced different temperature profiles and had dissimilar visual appearances.

2. Experimental apparatus and procedure Atmospheric-pressure, premixed, laminar ethylbenzene flat flames were stabilized over a 50.8-mmdiameter sintered bronze burner, Fig. 1. The burner temperature was controlled with air flowing through embedded copper tubing. Flames were isolated from

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Fig. 1. Burner and sampling set up. Table 1 The compositions of the atmospheric-pressure premixed ethylbenzene flames at the same equivalence ratio (φ = 1.74; C/O = 0.664) but different temperatures Visual appearance

Temperature (relative to each other)

Ethylbenzene (mole fraction)

O2 (mole fraction)

N2 (mole fraction)

Cold gas velocity (cm/s)

Slightly sooting Sooting limit Nonsooting

Low Intermediate High

0.043 0.047 0.047

0.260 0.284 0.284

0.697 0.669 0.669

7.44 7.81 10.09

the ambient air by a concentric sheath flow of N2 . This nitrogen flow was introduced into the burner at room temperature and at a cold gas velocity in the vicinity of 2 cm/s. Flames were stabilized with a perforated plate positioned 30 mm from the burner surface and water-cooled with a copper coil. The plate had 52 drilled holes, the biggest of which had a diameter of 12.7 mm and was positioned along the axis of the flame. Liquid ethylbenzene (from Aldrich), placed in 50-ml glass syringes (Hamilton) and driven by a dual infusion/withdrawal syringe pump (World Precision Instruments Inc.), was introduced into a stainless steel vaporizer (2 liters in volume) equipped with a nitrogen-flow-assisted atomizer. The vaporized fuel–nitrogen mixture was then introduced into the bottom of the burner, through heated tubing, and it was mixed with a preheated oxidizing gas mixture (50 vol% O2 –50 vol% N2 ). The vaporizer and the tubing were heated to a temperature, 20–50 ◦ C higher than the boiling point of ethylbenzene, which is 137 ◦ C. The ensuing mixtures are presented in Table 1. To warrant effective and continuous vaporization of ethylbenzene, the nitrogen flow into the vaporizer could not be dropped below a certain flow rate threshold; therefore the relative amount of nitrogen is slightly higher in the coolest flame, where the total cold gas velocity is lowest. Fuel/oxygen ratio (and

hence equivalence ratio) was kept constant. The nitrogen mole fraction was varied only to obtain stable flat flames. Attempts to generate additional flames, either cooler or hotter, with the same equivalence ratio but different temperatures resulted in instabilities. Product gases were withdrawn from different heights in the flames by a quartz probe, cooled with water flowing through a copper cooling jacket around the probe. A 1-cm-i.d. quartz probe was used, with a tip diameter of 4.4 mm. The quartz probe was kept in the flame before sampling started for a duration that was sufficiently long to allow its tip to equilibrate with the flame temperature, in order to minimize disturbance of the temperature field in the flame. Sampling was performed isoaxially and isokinetically to minimize the disturbance of the flow field. Flow rates corresponding to isokinetic conditions were determined based on the experimentally obtained temperature profiles. An error analysis that related the uncertainties in the reactants to those in the products was conducted in Ref. [1]. 2.1. Temperature measurements Temperature measurements were conducted along the centerline of the flame with four Pt/Pt–10% Rh

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(type-S) Omega thermocouples. The bead diameters were 76, 178, 356, and 762 µm and measurements were performed individually for each thermocouple in order to minimize disturbances in the flame. Each measurement was repeated at least three times, and in some cases up to seven times, in order to monitor the repeatability of the temperature measurements. In most cases the repeatability was good, with standard deviations being within 30 K, especially in the postcombustion region, above 1.5 mm from the surface of the burner. There were steep temperature gradients between the surface of the burner and the flame front; therefore the standard deviations of the measurements in this region were higher, reaching 150 K. Upon completion of the temperature measurements, radiative heat losses from the thermocouple beads needed to be accounted for to determine the true

flame temperature. To be consistent with the previous study [1], the temperatures reported in Fig. 2 were obtained with the 76-µm-diameter bead thermocouple and were corrected for radiative losses using the quasi-steady energy balance outlined by McEnally et al. [18]. It is important to note that there were large deviations (up to 100 K) of the temperatures obtained, applying the same quasi-steady energy balance to the beads of different-sized thermocouples. Since the thinnest thermocouple disturbs the flame less and since the spatial resolution of such a thermocouple is better than that of the thicker ones, temperatures obtained with this thermocouple were deemed to be most accurate. It should be mentioned herein that temperatures measured with the 76-µm-diameter thermocouple, with corrections for radiating losses based on the quasi-steady energy balance were within 40 K of those obtained using four thermocouples in a modified Nichol’s extrapolation method, explained in detail in previous publications [1,19]. It is thus shown that a flame at a given equivalence ratio may be slightly sooting, may be at the onset of sooting, or may be nonsooting, depending on the temperature of the flame (Fig. 3). The streaks that are evident in Fig. 3a are at the edges of the burner plug. Lower temperatures encountered at the periphery are reported to favor soot formation [20]. Small impurities and/or imperfections in the silicone sealant on that side of the burner may have induced localized gas flow distractions, causing faster cooling compared to other sections of the periphery. Sampling and temperature measurements were performed at the centerline of the flame; hence the results reported should not be affected by these small perturbations.

Fig. 2. Temperature profiles of atmospheric-pressure ethylbenzene flames that have the same equivalence ratio but different cold gas velocities. Temperature measurements were conducted with a 76-µm-diameter thermocouple. Correction for radiative losses was performed using a quasi-steady energy balance at the bead.

2.2. Sampling procedure Sampling measurements commenced at 1, 3, and 7 mm above the burner surface for each flame. Hence they took place both inside the flame and in the postflame zone. Quartz wool (obtained from Ohio Valley

Fig. 3. Pictures of atmospheric-pressure premixed ethylbenzene flames at the same equivalence ratio (φ = 1.74; C/O = 0.664) but different temperatures: (a) Tmax = 1826 K, Vcold gas = 7.44 cm/s; (b) Tmax = 1892 K, Vcold gas = 7.81 cm/s; (c) Tmax = 1921 K, Vcold gas = 10.09 cm/s.

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Specialty Chemical) was positioned inside the probe, 10 mm above its tip, to capture condensed species. In the upper part of the probe, precleaned XAD-4 resin adsorbed semivolatile PAH not captured by the quartz wool. Samples of light gaseous hydrocarbons and fixed gases were collected by gas-tight 1-ml glass syringes downstream from the probe, see Fig. 1, and were injected into an HP 6890 Series GC, equipped with flame ionization and thermal conductivity detectors (GC-FID/TCD). O2 , CO, CO2 , C1 –C4 , and single-ring aromatic hydrocarbons were quantified with two parallel columns (100/120 Carbosieve S-II and HP-5 capillary column). The instrument was calibrated regularly with CO–CO2 mixtures and with Scotty IV analyzed gas mixtures containing known concentrations of C1 to C4 aliphatic hydrocarbons and single-ring aromatics. The XAD and quartz wool samples were spiked with deuterated standards of naphthalene-d8, acenaphthene-d10, anthracene-d10, chrysene-d12, and perylene-d12. After extraction with methylene chloride in a Dionex ASE 200 accelerated solvent extractor, analysis was conducted by gas chromatography coupled to mass spectrometry (GC-MS) using a Hewlett–Packard (HP) Model 6890 GC with an HP Model 5973 mass selective detector. The details of the sampling/analysis method and a detailed experimental error analysis can be found elsewhere [1]. The error analysis took into account the uncertainties in (i) the flame composition due to limitations in the precision of flow metering devices, (ii) the sampling process, as during sampling and transfer of samples, some—especially the most volatile—PAH may be lost, and (iii) the extraction and quantification.

3. Results and discussion 3.1. Temperature profiles The temperature profiles of the flames examined in this study are presented in Fig. 2. The visibility of soot in a premixed ethylbenzene flame diminished as the maximum measured temperature increased at a given equivalence ratio, φ. The temperature profiles were different for each condition as a result of the combination of the effects of the fraction of the diluent (in this case N2 ) in the flame and the total cold gas velocity. These parameters are listed for each flame in Table 1. Decreasing the amount of diluent in the flame decreases the amount of heat transferred to this inert gas, and the higher cold gas velocity causes the flame to lose less heat to the burner. These two effects together cause the temperature of the flame to rise, as presented in Fig. 2.

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3.2. Concentrations of combustion species 3.2.1. Fixed gases and light hydrocarbon concentrations The light volatile hydrocarbons and fixed gases, which were not adsorbed by XAD-4 resin, were collected with gas-tight syringes at a location further downstream in the sampling line (Fig. 1). Then they were injected into the GC and were analyzed as described above. Oxygen was consumed from 26–28% initial concentration to 1% within the first millimeter in all of the flames. There was slightly more oxygen present in the cooler flame in the early part of combustion suggesting that oxygen is consumed faster in higher temperature flames. Oxygen mole fractions detected in the postflame region (7 mm away from the surface of the burner) were similar in each flame, i.e., ∼0.7% (Fig. 4). CO mole fractions increased along the axis of each flame, rising fast in the initial stages of the flame, followed by a mild increase in the postflame region; see Fig. 4. CO mole fractions were recorded to be 18.6, 18.8, and 19.6% at 7 mm above the burner surface in the cases of Tmax = 1826 K, Tmax = 1892 K, and Tmax = 1921 K flames, respectively. CO2 mole fractions also experienced a similar trend; they increased asymptotically to 7.1, 7.5, and 8.4% in the aforementioned flames, respectively. Since CO2 mole fractions also increase with temperature, the increase in CO mole fraction with temperature is not directly related to CO2 conversion. The increase of concentrations of both species indicates that the oxidation reactions are a preferred route at higher temperatures. This argument is also supported by the abundances of light hydrocarbons that are suspected to be soot precursors (ethylene, acetylene, and benzene); they ranked inversely proportional to temperature. See Fig. 4. Generally, the detected mole fractions of light hydrocarbons were found to be an order of magnitude higher in the cooler slightly sooting flame than in the hotter nonsooting flame, with those in the intermediate temperature “soot-onset” flame being in between. Mole fractions of most light hydrocarbon species initially increased along the axis of the flame, experienced a peak in the primary reaction zone (detected at 3 mm from the burner surface in the reported tests), and thereafter decreased as they were presumably consumed in the postflame zone. An exception was benzene, which experienced a somewhat monotonically decreasing trend in all flames, apart from the cooler slightly sooting one, where it increased gradually. Styrene mole fractions experienced mixed behavior as the axial profile of styrene in the cooler slightly sooting flame experienced a peak, see Fig. 4, whereas its axial profiles in the other two flames experienced monotonic decreases.

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Fig. 4. Mole fractions of fixed gases, light hydrocarbons, and one-ring aromatic hydrocarbons in premixed ethylbenzene flames at the same equivalence ratio (φ = 1.74; C/O = 0.664) but different temperatures. Points represent experimental measurements and lines represent predictions of the kinetic model.

To assess the repeatability of our measurements, a statistical analysis was conducted on one of the major species, the CO, using a 90% confidence interval. This provided a range for which the true CO mole fraction might fall for each flame at a given sampling point, based on the calculated standard deviations. What was discovered was that at the 1-mm sampling point the ranges for each flame sampled were very broad and overlapped for every point, showing that there is a lot of uncertainty about where exactly these points lie in relation to one another. This is not surprising, due to the proximity of the sampling point to the primary reaction zone. However, for the 3- and 7mm points the ranges were much narrower, and led to much smaller portions of each range overlapping, though overlap still occurs. Based on the data collected, there is a high confidence in the accuracy of the suggested tend (especially regarding the 3- and 7mm cases). However, to prove this definitively, more samples would need to be taken. 3.2.2. Polycyclic aromatic hydrocarbon (PAH) and oxygenated species concentrations Mole fractions of the total light hydrocarbons and the total polycyclic aromatic species are shown in Fig. 5; those of the major polycyclic aromatic hydrocarbons (PAH) and oxygenated aromatic hydro-

carbons are shown in Figs. 6 and 7, respectively. Naphthalene was the most abundant PAH species, followed by acenaphthylene and phenanthrene. Benzaldehyde, phenol, and benzofuran were found to be the most abundant semivolatile oxygenated species, in mole fractions comparable to those of naphthalene, the most abundant PAH. A striking observation is that the mole fractions of most PAHs in the cooler slightly sooting flame and in the intermediate-temperature soot-onset flame were typically of the same order of magnitude, whereas those in the hotter nonsooting flame were 1–3 orders of magnitude lower at the first sampling point for species that have three rings or fewer in their structure (1 mm above the surface of the burner); see Figs. 5, 6, and 7. Thereafter, the profiles of PAHs and oxygenated compounds in the cooler slightly sooting flame experienced peaks, in the manner of those of light hydrocarbon species, followed by precipitous consumption in the postflame zone. Most of the PAHs and oxygenated compounds in the intermediate-temperature soot-onset flame were consumed partially or nearly totally along its axis. In the hotter nonsooting flame, concentrations of most PAH and oxygenated species decreased again along the axis, with the exception of naphthalene, biphenyl, anthracene, benz[a]anthracene, benzaldehyde, phenol,

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Fig. 5. Mole fractions of total light hydrocarbons and total polycyclic aromatic species in premixed ethylbenzene flames at the same equivalence ratio (φ = 1.74; C/O = 0.664) but different temperatures.

and benzofuran, the mole fractions of which stayed almost constant in the sampling region. In the cooler slightly sooting flame, 49 polycyclic aromatic compounds were quantified at the lower sampling locations inside the flame, whereas in the postflame region, only 20 species could be detected. The remaining species were below the detection limit of the instruments. In this flame most of the species were found both in the condensed and gaseous phase; i.e., they were present both in XAD and in quartz wool samples. Most species were more abundant in the gaseous phase, with the exception of 4H-cyclopenta[def]phenanthrene, pyrene, benz[a]anthracene, chrysene, phenanthrene, and methylphenanthrenes. The mole fraction of phenanthrene in the condensed phase was six times higher than in the gaseous phase at 7 mm from the surface of the burner. A total of 30 polycyclic aromatic compounds were quantified in the intermediate-temperature flame. Again, most of the detected species were more abundant in the gaseous phase, except phenanthrene, methylphenanthrenes, cyclopenta[cd]pyrene, and benz[a]anthracene. At the highest sampling point (7 mm above the surface of the burner), again only 20 compounds were observed; most of the remain-

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ing compounds were presumably consumed to levels below the detection limit. The smallest number of PAH compounds, a total of 22, were detected and quantified in the hottest nonsooting flame, at the lowest sampling point (1 mm from the surface); most of these were also quantified at other sampling locations. Most of these 22 species were in the gaseous phase. However, styrene, phenanthrene, anthracene, and cyclopenta[cd]pyrene were consistently detected in the condensed phase. Phenanthrene and cyclopenta[cd]pyrene were mostly in the condensed phase, except at the lowest sampling point. Most of the semivolatile species (PAHs and oxygenates) were present in comparable total mole fractions (gaseous + condensed phase) in the postflame regions of all three flames (7 mm above the surface of the burner). Since sampling along the axis of each flame was performed at three points only, it is difficult to suggest accurate trends for the species profiles. Assuming that the species profiles experience the most drastic changes around the primary reaction zone and also assuming that far from this region changes are more gradual, it may be argued that the differences among the 1- and 3-mm sampling locations were due to their relative distance from the primary reaction zone. The sampling point 3 mm above the surface of the burner was observed to be in the postflame zone of all flames (see Fig. 2). Clearly, at this location there were very pronounced differences among the mole fractions of semivolatile species in the three different temperature flames; see Fig. 8a. However, further downstream, in the postflame zone (7 mm from the surface of the burner), the mole fractions of many major semivolatile hydrocarbon species, such as naphthalene, biphenyl, anthracene, phenanthrene, cyclopenta[cd]pyrene, and benz[a]anthracene, converged to similar values. This was also the case for most oxygenated semivolatile aromatic hydrocarbons. At this postflame location, where changes were expected to be more gradual, the fact that concentrations of species converged to similar values seems to suggest that the observed temperature difference of ∼90 K did not have a significant effect on species profiles. In fact, when concentrations of individual species 7 mm from the burner surface are plotted against the temperature of each flame at the same location, an almost horizontal plot is obtained for most of the species, with little decrease at higher temperatures (Fig. 8b). Another argument for this behavior may be that PAHs and other semivolatile hydrocarbons could be more prone to consumption by oxidation in the hot primary reaction zone, whereas they could be amenable to consumption by conversion to soot in the postflame zone, where temperatures are declining.

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Fig. 6. Mole fractions of polycyclic aromatic hydrocarbons and soot concentration in premixed ethylbenzene flames at the same equivalence ratio (φ = 1.74; C/O = 0.664) but different temperatures. Points represent experimental measurements and lines represent predictions of the kinetic model.

This is supported by observations of Wang et al. [21], who reported the global rate of conversion of PAHs to soot to be orders of magnitude higher than the global rate of PAH oxidation in postflame zones of furnaces, albeit experimenting with lower temperatures than those found herein. To investigate whether such behavior is coincidental, additional results were drawn from other two flames, currently under investigation in the laboratory. These two flames both have the same equivalence ratio (φ = 1.83), which is higher than that of the flames discussed herein (φ = 1.74). They have different temperature profiles from each other; one is at the soot onset limit (Tmax = 1918 K) and the other one is slightly sooting (Tmax = 1888 K). Similar convergences of species mole fractions were also observed for those flames in the postflame regions.

It was also investigated whether such convergence occurred because of reaching equilibrium concentrations. Under conditions similar to those of the flames reported herein, at equilibrium, fluoranthene concentrations were reported to be 1.5 to 3 times higher than acephenanthrylene concentrations [22]. In the present work, the fluoranthene and acephenanthrylene concentrations in the slightly sooting coolest flame were of the same order of magnitude, whereas in the other two flames there was at least one order of magnitude difference between the concentrations of these two species. Hence those flames most likely did not reach their equilibrium concentrations. Also, the temperature difference between the hottest and the coolest flames at the 7-mm sampling location (∼90 K) is too broad to have the same equilibrium concentration for each of the species in each flame. Hence, the convergence in the postflame region most probably is due

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Fig. 7. Mole fractions of oxygenated polycyclic aromatic species in premixed ethylbenzene flames at the same equivalence ratio (φ = 1.74; C/O = 0.664) but different temperatures. Points represent experimental measurements and lines represent predictions of the kinetic model.

Fig. 8. The change of mole fractions of polycyclic aromatic species with temperature at (a) 3 mm and (b) 7 mm above the surface of the burner in premixed ethylbenzene flames at different temperatures but all at the same equivalence ratio (φ = 1.74; C/O = 0.664).

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to chemical kinetics rather than thermodynamics; oxidation reactions are likely to consume PAHs in the hotter flame, whereas in the cooler flames soot formation reactions are likely to consume PAHs, bringing their concentrations to values similar to those in the hotter flame coincidentally at the 7-mm axial location. The difference in hydrocarbon concentrations among these three flames, especially in the initial stages of the flames, was more pronounced than in flames of the same fuel that had similar temperature profiles but different equivalence ratios, examined in a previous study [1]. Thus, for equivalence ratios (φ = 1.68–1.83) and temperatures (Tmax = 1826–1921 K) investigated in both studies, temperature appears to be a more important parameter than equivalence ratio in influencing sooting tendency.

4. Kinetic model A kinetic model describing the formation and consumption of PAHs and carbonaceous material (soot) has been developed and successfully tested [23]. Modeling calculations were conducted for all investigated conditions using experimental temperature profiles. Numerical predictions were compared to data and observations were qualitatively assessed. Model calculations for sooting [24] and nearly sooting [25] low-pressure flames showed the predictive capability of the model. All model computations were conducted with the Premix code of the Chemkin software package [26] using the experimental temperature profile. Differently from earlier versions of this software, the unmodified gas phase Interpreter now allows the handling of molecules with numbers of carbon and hydrogen atoms sufficiently large to correspond to soot particles of increasing size. In the previous work, the model was refined for the first decomposition steps of ethylbenzene; pertinent reactions were taken from the literature or estimated [27]. Further details of the model can be found elsewhere [1,27]. The resulting reaction mechanism used, consisted of 335 species and 8086 reactions. Oxidation of intermediates of ethylbenzene depletion by O, OH, and O2 is also included in the model. This model has been previously used for the description of fuel-rich ethyl alcohol flames [27] and highly sooting [27] and nearly sooting [1] ethylbenzene flames. The predictive capability of the model for PAHs was found to be satisfactory in both cases. Results of the kinetic model are included in Figs. 4, 6, and 7, where experimentally detected mole fractions of selected combustion byproducts are compared with model predictions. The model predicted declining mole fractions of oxygen and rapidly increasing CO and CO2 throughout the flame region

1–2 mm from the surface of the burner, increasing mildly thereafter. It predicted that the mole fractions of the hydrocarbon species increased rapidly in the flame region until maxima were reached and thereafter first decreased precipitously at the downstream side of the flame to low mole fractions, followed by mild decreases further in the postflame zone. This bell-shaped curve in the flame was absent in the case of acetylene, four-ring species (such as pyrene and cyclopenta(cd)pyrene), and soot. In fact, for each flame, the shape of the profile of acetylene and the shape of the profile of soot were closely matched. The model predicted the CO and CO2 mole fractions fairly well in the flames (including the postflame regions), but somewhat underpredicted the O2 mole fractions in the postcombustion zones of the flames. It overpredicted the methane and especially the acetylene mole fractions. The model was reasonably successful in its predictions of ethylene, ethane, and benzene in these flames. However, it underpredicted toluene (not shown in Fig. 4), styrene, and phenol. Again, it was overall successful in its predictions of naphthalene, biphenyl, and phenanthrene. It overpredicted most cases of indene, acenaphthylene, fluorene, fluoranthene, pyrene, and cyclopenta[cd]pyrene. A reaction flux analysis was conducted in recent work [1] for the case of a slightly sooting ethylbenzene flame (albeit at a somewhat higher equivalence ratio, φ = 1.83) in order to gain an understanding of major pathways involved in the consumption of the fuel and its conversion to PAHs and finally to soot. The interested reader is referred to that work. In the case of the hotter nonsooting flame as well as the flame at the sooting limit, the model also predicted that the species mole fraction profiles converged and/or crossed each other at 5–7 mm from the surface of the burner. This observation agrees with the argument that the convergence in the postflame region is caused by kinetic effects. The last plot in Fig. 6 depicts the predicted soot concentrations in the flame. Little differentiation in the soot profiles of the three flames is evident. The model predictions for soot concentrations were between 300 and 800 ng/cm3 . Based on a soot density of 2 g/cm3 [28], this corresponds to soot volume fractions in the range of 1.5 × 10−7 –4 × 10−7 for the ethylbenzene flames. Böhm et al. [8] reported that the soot in their ethylene/oxygen flame at ∼1800 K became visible at volume fractions somewhere in the range of 10−9 –10−8 . Based on these numbers, the model predicts that perhaps the ethylbenzene flames should have been visible, however, the reader should keep in mind that the oxidation of some PAH and nascent soot particles is missing in the model. Therefore, the predicted soot profiles are most likely overestimated, which calls for additional work in this area.

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The chemical kinetics model was used to obtain reaction flux analyses for key species. Results were reported in Ref. [1]. Monomolecular decomposition of ethylbenzene (C8 H10 ) to benzyl (C6 H5 CH− 2 ) and methyl (CH− ) radicals was found to be the dominant 3 fuel consumption pathway. Interestingly, decomposition of ethylbenzene by reaction with hydrogen radicals to form benzene (C8 H10 + H  C6 H6 + C2 H5 ) was found to play only a secondary role. Recombination of propargyl radicals (C3 H3 ) was found to play a major role in benzene formation, but toluene (C7 H8 ) was identified as an equally important precursor. The first two-ring PAH compound, naphthalene, was found to be nearly exclusively formed by reaction of benzyl with propargyl radicals. Subsequently, further PAH growth occurs by either a sequence of hydrogen-abstraction/acetylene-addition (HACA) steps or addition of small aromatic hydrocarbons after hydrogen abstractions and followed by dehydrogenation and ring closure. A detailed investigation of PAH growth in a nearly sooting benzene flame has been published previously [29]. The PAH formation mechanism, as described in the current model, was developed and model predictions of PAHs and their radicals were compared successfully with experimental data. While the pathway to naphthalene in an ethylbenzene flame differs significantly from that in a benzene flame, further PAH growth should be very similar. For instance, no strikingly high concentrations of methyl-substituted PAHs have been observed in the ethylbenzene flame; i.e., propargyl is not expected to form major growth species beyond naphthalene. Soot inception, i.e., the fast transformation of PAHs to small particles, was found to be largely controlled by reactions between PAH radicals and of parent PAHs. Details of the sectional model used for the description of soot formation have been discussed elsewhere [23]. An important difference between the ethylbenzene flames around the sooting threshold discussed here and the strongly sooting ethylbenzene flame reported previously [27] is the significantly higher level of PAH concentrations in the latter case. According to the understanding of soot formation on which the present model is built, differences in final soot concentrations will be much larger than those of its precursors. Higher concentrations of small nuclei result in faster surface growth rates, while increasing molecular masses lead to increased rate constants. A positive feedback loop is therefore established. The reaction pathway toward soot formation can be understood as a sequence of tight equilibria between formation, oxidation, and consumption by reactions leading to larger species. The results of the present work indicate the need for further refinements of the description of the competition between these three processes. Although, consistent with the picture of soot formation

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Fig. 9. Conceptual illustration of the conditions of the flames of the present work, in view of the findings of Böhm et al. [8] and Ciajolo et al. [15]. Additional work is needed to better define the PAH trend depicted in (c) in the vicinity of the low-temperature sooting limit.

outlined above, PAH concentrations increase corresponding to the sooting tendency, the model predicts similar soot concentrations for all three flames in the vicinity of the soot onset threshold. Missing oxidation pathways for many PAHs included in the model as soot precursors may contribute significantly to this lack of sensitivity of the model.

5. Comments on the relationship between soot precursors and soot onset Unraveling of the etiology for the phenomenon of the soot onset is challenging. Based on Millikan’s [4] and Takahashi and Glassman’s [5] results, soot onset is perceived as an inflection point in the competition between fuel pyrolysis to form soot precursors and oxidation of those precursors. Böhm et al. [8] discovered that if the critical C/O ratio of three hydrocarbon fuels (they examined ethylene, acetylene, and benzene) is plotted as a function of temperature, it forms an inverted bell shaped curve, as discussed in the Introduction. They also discovered that for a fixed C/O ratio there is a low-temperature and a high-temperature soot onset limit—see a conceptual illustration in Fig. 9a—and that the highest soot volume fraction was found somewhere in between these two values. Ciajolo et al. [15] took Böhm’s findings a step further and investigated three points within the inverted bell shaped curve of Fig. 9a

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for the case of sooting fuel-rich ethylene flames. For a fixed C/O ratio, and thus a fixed φ (in the vicinity of 2.4), they investigated a point near the low-temperature sooting limit, a point near the hightemperature sooting limit, and one in between these two points. Ciajolo et al. confirmed Böhm’s findings for premixed flat flames as well as Frenklach et al.’s [17] observations in shock tube experiments, i.e., that the highest soot volume fraction was observed at a temperature intermediate between the high- and low-temperature sooting limits, as illustrated in Fig. 9b. In Ciajolo’s case, their intermediate temperature flame experienced Tmax = 1720 K. However, they also observed that cumulative PAH concentrations increased with decreasing temperature in their examined range of 1520 K < Tmax < 1820 K; see a conceptual illustration in Fig. 9c. Ciajolo et al. suggested that in their high-temperature flame (Tmax = 1820 K), oxidation reactions were very active in consuming PAH and, thus, prevented further formation of tarlike species, which they postulated to be the soot precursors. Therefore, there were smaller amounts of PAHs and, also, smaller amounts of soot in their high-temperature flame. In their coolest flame (Tmax = 1520 K), the oxidation process was deemed to be less effective, thereby allowing the buildup of higher concentrations of PAH. However, such low temperatures hindered the reactive coagulation of PAH molecules and aliphatic radicals to tar-like material and, eventually, to soot. Finally, Ciajolo et al. concluded that in their intermediatetemperature flame (Tmax = 1720 K), the temperature level was ideal for the formation of both PAHs and tar as well as their subsequent conversion to soot. Based on results from burning ethylbenzene in shock tube experiments [16], the assumption that ethylbenzene flames follow the bell-shaped trends observed by Böhm et al. [8] can be made. Thus, the results observed herein can be better explained. The flames investigated in the present work were all in the vicinity of the high-temperature sooting limit, with one point just within the inverted bell-shaped curve, one point on the curve, and one point just outside of the curve, as illustrated in Fig. 9a. Therefore, the flames herein may be directly compared only with Ciajolo et al.’s high-temperature flame and, perhaps, with their intermediate-temperature flame. In the present work the hottest flame observed (Tmax = 1920 K) was hotter than all flames of Ciajolo, it was nonsooting and had the lowest concentrations of PAH. Ciajolo et al. also observed the lowest concentration of PAH in their hottest flame (Tmax = 1820 K) and explained it based on enhanced oxidation reactions. This seems to be confirmed in the present work by the fact that measured O2 concentrations were indeed the lowest and the concentrations of the major oxidation

products, CO and CO2 , were the highest in the hottest flame herein (Tmax = 1920 K). To the contrary, the O2 concentrations were the highest, whereas the CO and CO2 concentrations were the lowest in the coolest slightly sooting flame herein (Tmax = 1826 K). As flame temperature was decreased, increases in total and in individual PAH concentrations, as well as in the visual luminescence of soot, were typically observed. This trend also compares well with Ciajolo’s flames; their intermediate flame (Tmax = 1720 K) experienced higher PAH and soot concentrations than their high-temperature flame (Tmax = 1820 K), and this fact may be explained by the diminished effect of oxidation reactions. Thus, as the temperature of the flames in the present work approaches the point of maximum soot concentration, from the side of the high-temperature sooting limit in Fig. 9b, it was expected that both PAH and soot concentrations would increase. This was indeed observed by an increase in soot luminosity and confirmed by an increase in the measured PAH mole fractions.

6. Conclusions One-dimensional, atmospheric-pressure, premixed ethylbenzene flames at a fixed equivalence ratio (φ = 1.74; C/O = 0.664) but at different temperatures were stabilized over a flat-flame burner. The temperature profiles of the flames were mostly altered by changing the cold gas velocity of the total gas flow through the burner; this changed the heat loss from the flame to the burner, while keeping the molar ratio of reactants constant within the limits of flame stability. Axial temperature profiles were determined via thermocouple measurements, concentrations of fixed gases and light hydrocarbons were determined via probe sampling followed by GC analysis, and concentrations of PAHs and oxygenated semivolatile species were detected via probe sampling followed by GC/MS analysis. The maximum temperatures of the flames at constant equivalence ratio were varied by up to 100 K. Similarly, the entire axial temperature profiles also varied by as much as 100 K. Within such a temperature variation (T = 100 K), three flames with different visual appearance were obtained: (i) a slightly sooting flame (Tmax = 1826 K), (ii) a flame at the sooting limit (Tmax = 1892 K), and (iii) a nonsooting flame (Tmax = 1921 K). The results of probe sampling followed by GC analysis in these flames indicated that CO and CO2 mole fractions were higher in the hotter nonsooting flame and they decreased with decreasing flame temperature. Both CO and CO2 mole fraction profiles along the axis of each flame experienced increasing

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trends, initially steep and then mild, as expected. The mole fractions of most of the light hydrocarbons increased in the initial stages of combustion (within the first 3 mm above the burner surface), reached peak values, and then decreased. The order of abundance of light volatile hydrocarbon species was opposite to that of CO and CO2 : they increased as flame temperatures decreased. The hottest flame had the lowest concentration of light hydrocarbons, some of which are suspected soot precursors, and, accordingly, soot was not detected in this flame, either visually or gravimetrically. The same trend was also observed in the case of the semivolatile hydrocarbons (PAHs and oxygenated aromatic hydrocarbons). The hotter nonsooting flame had consistently much lower numbers and concentrations of PAHs and the number as well as the abundance of PAHs increased with decreasing temperature. Most of these species were detected in both gaseous and condensed phases, while in the hotter flame most of the species were present only in the gaseous phase, especially in proximity to the primary reaction zone (lowest sampling point). In the postflame zones of the three flames the mole fractions of most individual semivolatile PAH and oxygenated hydrocarbon species converged to similar values. The high temperatures promote oxidation reactions, and at lower temperatures PAHs are presumably consumed primarily via soot formation reactions. The convergence of species mole fractions in the postflame region is arguably due to the effects of these two competing reaction pathways in different flames. Model predictions of the species mole fractions ranged from satisfactory in ethane, ethylene, benzene, naphthalene, phenanthrene, and anthracene to fair in fluorene to inconsistent in pyrene and cyclopenta[cd]pyrene. The change in concentrations of soot precursors just prior to and after the soot onset at various critical equivalence ratios may shed light on details of soot inception. The first phase of such an investigation in this laboratory addressed the effect of equivalence ratio on soot onset chemistry across the sooting limit at constant maximum flame temperature [1]. The second phase was presented herein and addressed the effect of temperature at constant equivalence ratio. It appears that within the investigated range of equivalence ratios [1] and temperatures (present study), temperature is the most effective parameter in affecting the chemical compositions of species. Ongoing work [30] concentrates on flames that have the same equivalence ratio (φ) as the flames in the previous study [1] but are all at the sooting limit, i.e., have different temperature profiles. Such flames will be controlled so that the temperature profiles will be in the same range as those investigated in this study.

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