Investigating the Role of Methane on the Growth of Aromatic Hydrocarbons and Soot in Fundamental Combustion Processes

Combustion and Flame 134 (2003) 249Ð260

Investigating the role of methane on the growth of aromatic hydrocarbons and soot in fundamental combustion processes

J. F. Roeslera,*, S. Martinota, C.S. McEnallyb, L.D. Pfefferleb, J.-L. Delfauc, C. Vovellec

aIFP, 1 et 4 av. de Bois-Preáu, 92852 Rueil-Malmaison Cedex, France bDepartment of Chemical Engineering and Center for Combustion Studies, Yale University, New Haven, CT 06520-8286 USA cLaboratoire de Combustion et Syste`mes Reáctifs, CNRS, 45701 Orleáns Cedex 2, France Received 30 November 2002; received in revised form 24 March 2003; accepted 1 April 2003

Abstract Experimental results are presented on the effect of methane content in a non-aromatic fuel mixture on the formation of aromatic hydrocarbons and soot in various fundamental combustion configurations. The systems considered consist of a laminar flow reactor, a laminar co-flow diffusion flame burner, and a laminar, premixed flame burner, all of which operate at atmospheric pressure. In the flow reactor, the experiments are performed at 1430 K, constant C-atom flow rates, 98% nitrogen dilution, C/O ratio ϭ 2, and with fuel mixtures consisting of ethylene and methane. The diffusion flames are performed with fuel mixtures of methane and ethylene diluted in nitrogen to maintain a constant adiabatic flame temperature. The premixed flame experiments are performed with n-heptane and methane mixtures at a C/O ratio ϭ 0.67 with nitrogen-impoverished air. The results show the existence of synergistic chemical effects between methane and other alkanes in the production of aromatics, despite reduced acetylene concentrations. This effect is attributable to the ability of methane to enhance the production of methyl radicals that will then promote production channels of aromatics that rely on odd-carbon- numbered species. Benzene, naphthalene, and pyrene show the strongest sensitivity to the presence of added methane. This synergy on aromatics trickles down to soot via enhanced inception and surface growth rates by polycyclic aromatic hydrocarbon condensation, but the overall effects on soot volume-fraction are smaller due to a compensating reduction in surface growth from acetylene. These results are observed under the very fuel-rich environments existing in the flow reactor and diffusion flames. In the premixed flames, however, instabilities did not permit investigation of conditions with sufficiently high equivalence ratios to perturb the aromatic and soot-growth regions. © 2003 The Combustion Institute. All rights reserved.

Keywords: Soot; Aromatic hydrocarbons; PAH; Methane; Combustion; Flow reactor; Diffusion ﬂame; Premixed ﬂame

1. Introduction tion are complex. Early mechanistic models [1,2] suggested that the ﬁrst aromatic ring was formed by The molecular growth-processes leading to poly- the addition of acetylene to C radicals and that the cyclic aromatic hydrocarbons (PAH) and soot forma- 4 key growth process toward increasingly larger PAH

occurred by continuous addition of C2H2. Bi-aryl * Corresponding author. Tel: ϩ(33)1-47-52-5811; fax: formation by aromatic-ring addition was considered ϩ(33)1-47-52-7069. feasible only in the case of aromatic fuels. This type E-mail address: [email protected] (J.F. Roesler). of pioneering model was able to reproduce most of

0010-2180/03/$ Ð see front matter © 2003 The Combustion Institute. All rights reserved. doi:10.1016/S0010-2180(03)00093-2 250 J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260 the major PAH and soot trends observed experimen- The product samples are quenched and collected tally in combustion processes [1,3,4]. However, it along the center-line axis by means of an oil-cooled neglected other reaction pathways that are increas- probe at 180¡C. Only 20% of the total flow is sam- ingly shown to be of importance. pled; therefore, the measurements correspond to the In 1990, Miller and Mellius [5] published work products along the center-line and not to the bulk that suggested that benzene could be formed, in large mean. part, from the re-combination of the odd-carbon- At the probe exit, the soot is filtered at 180¡C, and numbered, resonantly-stabilized propargyl radicals. condensable species are then trapped. In a first pass, While this specific reaction is now well-recognized the trap is at ambient temperature, and the low-mo- and has been implemented in many recent kinetic lecular-weight gas-phase species, up to benzene, are models [6-9], there is growing evidence that many analyzed on-line with a Fourier-transform infrared other such radical species are involved in the produc- spectrometer (FTIR). The gas-phase species are then tion of at least the smaller PAH. This evidence is collected for subsequent analysis by gas chromatog- found in various theoretical analyses of reaction raphy (GC) with a flame ionization detector (FID) in channels and in experimental flame and modeling studies [10Ð14]. order to more accurately quantify the hydrocarbon If these reactions are indeed important, then add- gas-phase species with up to eight carbon atoms. The ing methane to the PAH and soot-forming regions in species quantified solely by FTIR are CO, CO2, and combustion systems should, under some conditions, H2O. The higher-molecular-weight species are col- promote their growth. The premixed flame results of lected on a second pass, with the trap at dry-ice Senkan and Castaldi [15] that show a methane, lam- temperature (Ϫ70¡C) that is then washed with inar, premixed, flat flame to produce more benzene CH2Cl2. The detailed speciation of the PAH from the and PAH than a similar ethane flame confirm this line trap is obtained by GC-FID and GC-mass spectrom- of thought. More direct evidence of this effect of etry (MS) analysis. All other wetted surfaces (probe, methane was obtained in flow-reactor studies [16,17]. Teflon liner, and soot filter) are washed ultrasonically

The addition of methane was found to enhance the with CH2Cl2. Analysis of the collected solutions formation of aromatics and soot in n-heptane oxida- showed Ͻ 5% loss of any one of the reported PAH to tive pyrolysis. Detailed reaction mechanisms from these surfaces. The total masses of condensable spe- the literature were found to reproduce these effects, cies (hereafter referred to as “tar”) of molecular provided PAH formation was appropriately described weight, corresponding roughly to masses greater than with odd-carbon-numbered radical species. acenaphthylene, and the mass of soot (non-dichlo- The present work further investigates experimen- romethane-soluble, condensable species) are deter- tally the influence of methane as a parameter to be mined gravimetrically. considered in the formation of soot and PAH. Our In the experiments presented here, the fuel was goal was to further quantify the effect, under flow- initially C H that was gradually substituted with reactor conditions, and to determine whether or not 2 4 CH at a constant total-carbon molar content of 3%. these were applicable to classic, laminar, premixed 4 and diffusion flame conditions. A mixture parameter characterizing the initial fuel composition is defined as:

XCH 2. Experimental set-ups ␤ ϭ 4 (1) X ϩ 2X CH4 C2H4 2.1. The flow-reactor system and represents the fraction of fuel carbon injected as methane. This parameter was varied from 0 to 0.5. The flow-reactor device and the analytical tech- The oxygen content was also held constant to main- niques for quantifying the composition of the sam- ϳ pled products have been described in detail previ- tain a C/O ratio of 2.0, leading to an equivalence ously [17,18]. The reactor is made of quartz, with a ratio varying slightly between 6 and 7 due to added 30-mm inside diameter. It is inserted in a three-zone hydrogen in the presence of methane. The experi- furnace. The premixed reactants are rapidly heated in mental conditions investigated were for a tempera- the first zone through a coiled 1.5-m-long tube with ture of 1430 K, atmospheric pressure, and a total flow an inside diameter of 4 mm. The flow enters the rate of 0.54 slpm (standard liters per minute) with larger-diameter test section and is laminarized by nitrogen as the carrier gas. The product samples were passing through a porous quartz disk. The test section collected at a fixed position of 0.25 m downstream of is at atmospheric pressure and is isothermal within the porous quartz disk, corresponding to a bulk mean 5 K throughout a distance of 35 cm. residence time of 0.42 s. J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260 251

2.2. The laminar co-flow diffusion flame system Table 1 Volumetric flowrates for the laminar diffusion flames* The laminar co-flow diffusion flame burner and ␤ N CH C H Ar the analytical techniques have been described in de- 2 4 2 4 tail previously [19]. The flames are generated with an 0.00 500 0 220 7.27 atmospheric pressure, axisymmetric, co-flowing 0.03 484 11 213 7.15 burner. The fuel mixture flows from an un-cooled 0.05 468 23 206 7.04 12-mm-diameter vertical brass tube. The air flows 0.08 452 35 199 6.93 0.11 436 48 192 6.83 between this tube and a surrounding 108-mm inside- 0.14 417 61 183 6.68 diameter chimney. 0.18 397 75 175 6.53 Temperatures are measured with an un-coated 0.21 378 90 166 6.40 125-␮m wire-diameter/260-␮m junction-diameter 0.25 358 105 158 6.28 type R (Pt-Pt/13% Rh) thermo-couple. Further details 0.29 336 121 148 6.10 are provided elsewhere [20]. The relative and abso- 0.33 313 138 138 5.94 lute uncertainties in the results are estimated to be Ϯ5 0.38 290 156 128 5.79 and Ϯ50 K. The temperatures are corrected for radi- 0.43 267 176 118 5.66 ation heat-transfer effects using procedures described 0.48 239 195 105 5.44 in the paper cited above. 0.54 211 216 93 5.25 0.60 182 241 80 5.08 The reaction gases are sampled by means of a 0.67 154 271 68 4.98 quartz micro-probe. Analyses are then performed on- 0.74 124 309 54 4.92 line, with a commercial electron-impact/quadrupole 0.82 87 346 38 4.77 mass spectrometer (EQMS) and a custom-built pho- 0.90 46 384 20 4.54 to-ionization/time-of-flight mass spectrometer 1.00 0 421 0 4.25 (PTMS) [19]. The calibration mole fractions for * (units are cm3/min at STP). methane, acetylene, and benzene are determined by direct comparison with room temperature mixtures of known composition, while the rest are determined by assuming that their PTMS sensitivity is the same as N2 was adjusted to maintain a constant adiabatic flame temperature of 2230 K. Argon was added to the for benzene. The concentrations of C2H4, CO, and N2 cannot be measured due to mutual molecular-mass fuel in order to match its concentration in air. It was interference. The relative uncertainties in the hydro- also used as an internal standard to correct the con- carbon mole fractions are estimated to be Ϯ10%, and centration measurements for changes in the sample the absolute uncertainties are estimated to be Ϯ20% gas-flow rate generated by thermal effects and soot (methane, acetylene, and benzene) or ϩ100/Ϫ50% deposition. The air flow around the fuel jet was main- (all others). These uncertainties are acceptable, be- tained constant at 44.0 slpm. The total fuel jet flow cause we are primarily interested in how the hydro- rates were specifically chosen to make the maximum carbon mole fractions change as the fuel composition center-line temperature occur at a height of 71.4 mm. is altered. The temperature profiles along the center-line axis Soot-volume fractions are measured using laser- are then quite similar for the various conditions ob- induced incandescence (LII) The incandescence is served here. Since these are buoyancy-driven flames, excited at 1064 nm, as opposed to a visible wave- the center-line gas residence times are independent of length, in order to eliminate interferences from PAH this flow rate. Therefore, the time-temperature pro- fluorescence. The incandescent signal is collected files are nearly similar for all flames. orthogonal to the excitation beam and is monitored at Center-line profiles were measured for each spe- wavelengths from 400 to 450 nm. This detection cies in a subset of the flames in Table 1; then for each range discriminates against C2 fluorescence. The specific details are similar to those provided elsewhere species, a height above the burner was identified [21]. The relative uncertainty in the measurements is where its concentrations in each profile was within estimated to be Ϯ10%. 10% of the maximum; and then measurements of that The experimental conditions investigated are species were made at that height in all of the flames summarized in Table 1. In the present investigation, listed in Table 1. Thus, the final results are the max- the composition of the fuel consisted of mixtures of imum concentrations of each species as a function of ␤ CH4,C2H4, Ar, and N2. As with the flow-reactor . The exception is methane, for which the measure- ␤ experiments, the relative concentrations of CH4 and ment represents the maximum only for the lowest ␤ C2H4 were identified by the mixture parameter that values, whereas it represents the residual from the was varied over its full range. The concentration of fuel at the higher ␤ values. 252 J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260

Table 2 Initial conditions for the premixed ﬂames

Flame 1 2 n-C7H16 (mole %) 5.3 4.6 CH4 (mole %) 0.0 3.5 O2 (mole %) 27.5 26.7 N2 (mole %) 67.2 65.2 ␾ 2.12 2.16

T1(K) 450 450 C/O 0.67 0.67 P (atm) 1 1

U1 (cm/s at 450 K) 6.3 6.5

2.3. The laminar, premixed ﬂame system

The laminar, premixed flame burner and corresponding analytical techniques have been described Fig. 1. Flow-reactor concentrations of CO, CO2, and H2O ␤ in detail elsewhere [22]. The fuel is vaporized and measured as a function of the initial mixture parameter . Lines are curve fits to the data. mixed with oxygen and nitrogen in a pre-burner chamber. The burner surface consists of a porous plate made of 0.5-mm-diameter holes uniformly dis- 3. Results and discussion tributed over an area 40 mm in diameter at atmospheric pressure. The initial gas temperature is fixed 3.1. The flow-reactor system by heating the burner surface and up-stream feed line to 450 K, a temperature which prevents fuel conden- Figures 1 through 7 show the evolution of species sation. Gas samples are collected by means of a concentration as a function of the fuel-mixture pa- quartz micro-probe. The probe moves along the cen- rameter ␤, as measured at a point 25 cm down-stream ter-line axis of the flame. The sampling orifice of the from the porous quartz disk, corresponding to a bulk probe is 0.1 mm, and the down-stream pressure is mean residence time of 0.42 s. maintained at 1 kPa. The sampled gases are then The concentrations of CO, CO , and H O are stored in glass bottles for subsequent analysis by GC. 2 2 nearly constant (Fig. 1). These species are formed The hydrocarbon species were quantified with an predominantly before the porous disk in the pre-heat Al2O3/KCl column coupled to a FID. Quantification of N2,O2,H2, and CO was performed with a molecular-sieve column coupled to a thermal-conductivity detector, while for CO2 and H2O, a Porapak-Q was used. Aromatic species heavier than benzene were not measured with these techniques. Soot-volume fractions were measured by He/Ne laser-light atten- uation at 670 nm, with a soot refractive index of 1.57Ð0.46i [23]. The experimental conditions investigated are summarized Table 2. Two flames are compared, both at the same pressure and temperature. The difference is that in one case, the fuel is only n-heptane (␤ ϭ 0), whereas in the second, ϳ 10% of the total carbon is injected in the form of methane (␤ ϭ 0.1). Flames with larger fractions of methane were unstable with the given N2/O2 mixture composition. The C/O is maintained constant, which slightly raises the equivalence ratio from 2.12 to 2.16. Oxygen and nitrogen flow rates are also maintained. The effects on species Fig. 2. Flow-reactor concentrations of H2,C2H2,CH4, and concentration profiles and on soot production are pC3H4 measured as a function of the initial mixture param- then observed. eter ␤. Lines are curve fits to the data. J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260 253

Fig. 3. Flow-reactor concentrations of C H ,CH ,CH , 2 4 4 4 4 2 Fig. 5. Flow-reactor concentrations of the poly-aromatic and aC3H4 measured as a function of the initial mixture parameter ␤. Lines are curve ﬁts to the data. species that show the strongest rise when the initial mixture parameter ␤ is increased. Lines are curve ﬁts to the data. regions on a very short time scale compared to that of The concentrations of CH4,C2H2,pC3H4, and H2 PAH growth. The slightly varying concentrations ex- are displayed in Fig. 2. As would be expected, when press changes in the chemistry in this region as the the relative amount of methane in the fuel increases, fuel mixture composition changes. These variations so do the amounts of H2 and CH4 during the reaction. are not very important, as these species have little In contrast, the amount of acetylene decreases. As interaction with the PAH and soot-growth processes. previously noted [17], the propyne in this system For detailed information regarding the evolution of varies proportionally with the three former species species concentrations along the reactor axis, the due to a near partial equilibrium of the global reac- reader is referred to reference [17]. tion

Fig. 4. Flow-reactor concentrations of single-ring aromatic Fig. 6. Flow-reactor concentrations of the poly-aromatic species as a function of the initial mixture parameter ␤. species that show a mild rise when the initial mixture Lines are curve ﬁts to the data. parameter ␤ is increased. Lines are curve ﬁts to the data. 254 J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260

diacetylene, its concentration decreases dramatically, much more so than acetylene, a trend that is coherent

with partial equilibrium of the global reaction 2C2H2 º ϩ C4H2 H2. The evolution of single-aromatic-ring hydrocarbons is shown in Fig. 4. Here, we observe a strong increase in benzene as ␤ increases. The maximum of this species was not reached in the present set of experiments. This increase is attributed to the enhanced production of propyne and allene, which would lead to larger quantities of propargyl radicals and accelerate the production of benzene via: ϩ º C3H3 C3H3 C6H6 (4) In contrast, phenylacetylene increases much less and attains a maximum at ␤ ϭ 0.25. This is easily

attributable to the reduced C2H2 and enhanced H2 Fig. 7. Flow-reactor concentrations of tar, soot, and total concentrations that eventually off set the effect of the PAH measured by GC analysis as a function of the initial increased benzene in the growth steps. Toluene, on mixture parameter ␤. The concentrations are expressed in the other hand, increases by a factor of 5, due to the terms of their carbon-atom equivalent. Lines are curve fits to fact that it further feeds on methyl radicals in its the data. formation process from benzene. The PAH shown in Fig. 5 and Fig. 6 display profiles of two types. The examples of the first type, CH ϩ C H ºH ϩ pC H (2) 4 2 2 2 3 4 naphthalene and pyrene, show trends similar to ben- The relation yields: zene, with an increased growth of about a factor of 2.5. The examples of the second type, acenaphthyl- ͑CH ͒͑C H ͒ ͑ ͒ ϭ 4 2 2 ene, fluoranthene, phenanthrene, and C H (this pC3H4 ͑ ͒ Keq (3) 18 10 H2 curve most likely represents cyclopenta(c,d)pyrene concentrations but possibly contains other indistin- The present results confirm again that Keq has a value of 20 at 1430 K. In general, acetylene and guishable minor isomers), show trends more like that hydrogen are present in larger proportions than meth- of phenylacetylene. This suggests that there are those ane in fuel-rich combustion products. Therefore, ad- PAH whose growth depends more directly on the dition of methane will generate a larger relative con- presence of odd-carbon-numbered species and those PAH whose growth depends more on even-carbon- centration change of itself than of H2 and C2H2. Furthermore, at iso-carbon content, a change in meth- numbered hydrocarbons. Obviously, the cyclopenta ane is twice that of acetylene. These effects will aromatics and acetylenated aromatics require the ad- promote propyne formation. dition of acetylene, and these are found in the PAH The concentrations of ethylene, allene, vinyl- category showing lesser growth. acetylene, and diacetylene are displayed in Fig. 3. For It is difficult to assess what the dominant reaction ethylene, its concentration increases by a factor of 2, pathways are exactly; however, some conclusions despite the reduced amount injected. This result is may be reached as to what some pathways are not, due to the fact that, at the extent of reaction observed, particularly regarding acetylene addition. The main ethylene is being produced primarily as a product in path to naphthalene is not likely to be from succes- the decomposition of the larger quantities of meth- sive C2H2 addition reactions to phenyl, as in the ane. However, at an earlier reaction time, for a posi- hydrogen-abstraction-carbon-addition (HACA) tion only 1 cm down-stream from the quartz disk, the mechanism [1]. The concentration of acetylene de- ␤ concentration of C2H4 rises only by 10%, to a value creases as increases, and, assuming that the con- near 1,000 ppm. Thus, in the earlier oxidation region, centration of phenyl radicals varies proportionally to methane acts more as an inhibitor in the decomposi- benzene, it is not possible for such reactions to induce tion of ethylene. The allene profile follows the trend the observed 2.5-fold increase in C10H8. Similar rea- of its propyne isomer. Vinylacetylene has a profile soning holds for a pathway involving C4H2 addition that remains rather flat (only a 20% increase) and to phenyl. This leaves many other possible pathways, follows closely a partial-equilibrium tendency with such as the addition of C4H4 to phenyl [6,24], the º ϩ the global reaction C2H4 C2H2 H2.Asfor recombination of cyclopentadienyl radicals [10,12], J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260 255 the addition of propargyl to toluene [11,19], or the addition of C2H2 to an o-xylyl radical, followed by dehydrogenation. This last pathway is the main source of naphthalene during the oxidation of o- xylene [25]. It was found to dominate in methane oxidative pyrolysis at an equivalence ratio of 12 [26], based on a modeling study using the detailed reaction mechanism of Marinov et al. [12]. This pathway possibly becomes important at the higher values of ␤. As with naphthalene, it seems very unlikely for the production pathways of pyrene to involve exclu- sively HACA reaction steps that describe PAH growth by successive C2H2-addition reactions [1]. This holds, whether the HACA sequence is initiated from benzene, naphthalene, or phenanthrene. Given the decrease in acetylene content, it is difficult to Fig. 8. Laminar, co-flow diffusion-flame maximum temper- expect the successive HACA steps to yield the ob- ature and soot-volume fraction along the center-line axis as served 3-fold increase. In order to account for the a function of the mixture parameter ␤. strong increase in pyrene, other reaction pathways thus need to be considered, such as biaryl addition with isomerization [11,27Ð29] or addition reactions are not well-controlled, as it takes place before en- involving the cyclopentadiene moiety [10]. trance of the gases into the test section. A more Regarding the formation of tar (Fig. 7), there thorough investigation would require gathering soot- appears to be either large uncertainty in the data or concentration profiles along the reactor axes under all some contrasting trends as methane is added. The conditions. While the present data show no increase global trend is nonetheless an increase, consistent in soot, they show no decrease either, a fact that with the chromatographable PAH. For means of com- would be anticipated by mere observation of acety- parison, we have included the trend of the estimated lene and hydrogen concentration changes. This result total measured PAH mass obtained by chromatogra- suggests that soot growth under the observed condi- phy and by assuming a response factor of unity for all tions occurs from the combination of enhanced in- FID unidentified peaks. A similar curve, but with the ception and PAH surface addition, compensated by

C12 and lighter species removed, is also shown. reduced acetylene surface addition. These lighter species evaporate during the weighting To conclude this section, these flow-reactor data and are not accounted for in the tar [17]. The large have confirmed earlier results, in which methane was difference between the tar and GC-PAH curves mixed with n-heptane [17] at a fixed ␤ of 0.1. Here, shows that there remains a large portion of unre- with mixtures of C2H4 and CH4, we observe again solved soot-precursor matter, much as observed in that increasing the proportion of methane, with 0 Ͻ ␤ premixed flame [30] and flow-reactor studies [31]. Ͻ 0.5 at constant carbon content, generates a pro- Optical analysis [32] reveals this matter to contain a nounced increase in the formation of aromatic spe- significant fraction of aliphatic structures. This matter cies. The data are rich in information for testing is therefore not presently taken into account in cur- PAH-formation pathways. They show that the forma- rent soot-precursor models that consider only con- tion of PAH up to at least 4 aromatic rings is not densed PAH. always dominated by acetylene addition. Despite the large increase in tar and PAH, there is Having assessed these effects in ideal flow-reactor no observed enhanced soot formation. This result conditions, we next investigate their relevance to contrasts with previous experiments that used n-hep- practical flame conditions. tane instead of ethylene [17], which gave a 2-fold increase in soot production at ␤ ϭ 0.1. While the 3.2. Diffusion-flame results exact reason for these differences has not yet been assessed with certainty, we speculate that they orig- The laminar, co-flow, diffusion-flame system rep- inate from changes in soot induction times that are resents a system of more practical relevance than the affected by residual oxygen from the oxidative re- flow reactor. The parameter of prime interest is the gion. The amount of this oxygen (under 500 ppmv) maximum soot-volume fraction, which is shown in and the impact of methane on this quantity vary with Fig. 8 as a function of ␤. An overall maximum is the initial fuel type. In addition, the oxidative chem- reached at ␤ ϭ 0.4. This variation in soot loading is istry is fast, and the conditions under which it occurs observed to have the reverse effect on the maximum 256 J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260

Fig. 9. Laminar, co-flow diffusion-flame maximum species Fig. 10. Laminar, co-flow diffusion-flame species maximum concentrations along the center-line axis as a function of the concentrations along the center-line axis as a function of the mixture parameter ␤:C H ,CH , and C H . Lines are curve ␤ 2 2 4 3 4 mixture parameter :C6H6,C7H8, and C8H6. Lines are fits of data. curve fits of data. measured temperatures, also shown in Fig. 8. As the species and soot are then also promoted (see Fig. 8, soot loading increases, the temperature decreases due Fig. 10, and Fig. 11). to thermal radiation losses. The results further suggest that acetylene addition The soot profile shows the existence of a syner- to phenylacetylene is neither the only nor the domi- ␤ Ͻ gistic effect of methane, in the sense that for 0.6, nant pathway for the production of naphthalene. As ␤ the values reached exceed those of either the diluted increases, we see that all even-carbon-numbered spe- ethylene flame or the methane flame. As discussed cies have decreasing concentrations, and that species below, this synergistic effect is at least partly attrib- requiring an obviously direct addition of acetylene utable to chemical factors that are similar to those for their production have decreasing concentrations observed in the flow reactor. Measurements were also made in similar flames without any nitrogen dilution, and in that case, the soot-volume fractions decreased monotonically from the neat ethylene flame to the neat methane flame. However, the soot-volume fractions in the intermediate flames were still larger than the weighted contribution of the neat flames. This result suggests that the same synergistic effects were present but were outweighed by: 1) the decrease in flame temperature as methane was added; and 2) the very large difference in the sooting tendency of the two fuels. We now look at species concentration variations with ␤. The maximum center-line acetylene concentrations decrease as methane is added, as shown in Fig. 9, because the ethylene is being replaced by a fuel that forms acetylene less readily. In contrast, the

C3H4 concentrations increase, because the increase in methane is proportionately larger than the decrease in acetylene. The benzene concentrations also increase Fig. 11. Laminar, co-flow diffusion-flame species maximum (see Fig. 10), which shows the importance of propar- concentrations along the center-line axis as a function of the ␤ gyl radicals as benzene precursors, just as in the mixture parameter :C10H8 and C12H8. Lines are curve fits flow-reactor results. The growth of larger aromatic of data. J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260 257

In an earlier study, some of the current authors have shown that propargyl addition to benzyl is a viable channel toward naphthalene production [19]. Here, this channel could well explain the observed growth in naphthalene, since toluene concentrations

(Fig. 10) remain relatively flat, while C3H4 concentrations increase up to values of ␤ ϭ 0.6. These two species should be indicative of the relative amounts of benzyl and propargyl. It is not clear, however, why the toluene profile is so flat. This contrasts strongly from the flow-reactor data, in which toluene displays the strongest increase of all aromatic species and illustrates that kinetic interpretation from these diffusion flames is rather speculative and needs the support of detailed, kinetic modeling or of other re- active systems. Finally, while the cyclopentadienyl pathway [10,12] may exist, the concentration of cyclopenta- Fig. 12. Laminar, co-flow diffusion-flame species maximum diene decreases monotonically by a factor 4 as ␤ concentrations along the center-line axis as a function of the increases. Such a trend tends to discredit the rele- ␤ mixture parameter :C4H4,C4H2, and C10H6. Lines are vance of the above pathway in these co-flow diffu- curve fits of data. sion flames and corroborates the observations of McEnally and Pfefferle [19]. In summary, the results show that adding methane relative to their parent aromatic. Indeed, phenylac- to ethylene diffusion flames promotes soot formation etylene and phenyldiacetylene (C10H6 in Fig. 13) via a synergistic, chemical mechanism that is similar decrease relative to benzene, and acenaphthylene de- to the one observed in the flow-reactor experiments. creases relative to naphthalene. Thus, the HACA The results also confirm the need to consider PAH mechanism may not be able to fully explain the growth pathways that include odd-carbon-numbered increasing naphthalene. Analogously, neither can the species. reaction of vinylacetylene addition to phenyl that was recently suggested and incorporated in a detailed 3.3. Premixed-flame results reaction mechanism by Appel et al. [6]. The premixed-flame experiments investigated two mixture conditions, one with pure n-heptane as fuel (␤ ϭ 0), and the other consisting of a mixture of n-heptane and methane, with ␤ ϭ 0.1. The choice of these conditions was motivated by earlier flow-reactor experiments with n-heptane [17]. The fact that ethylene is not used is a secondary consideration, as the present study aims at identifying the role of methane in various combustion configurations. Further- more, the chemistry in premixed flames is such that n-heptane is rapidly consumed in the low-temperature region to form predominantly ethylene. This is observed in Fig. 13, which shows the rapid conver- sion of n-heptane into ethylene under both flame conditions. Although not measured specifically for these flames, previous studies [22] indicate that the temperature profile should reach a maximum at the point of ethylene disappearance, i.e., at 4 mm down- stream from the burner. The soot measurements, extending to 20 mm Fig. 13. Laminar, premixed flame species-concentration down-stream from the burner, are given in Fig. 14. profiles: C7H16 and C2H4. Lines are curve fits of data. Flame Overall, we see that there is a negligible effect of the 1: without CH4; flame 2: with CH4. presence of methane on the production of soot. Ob- 258 J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260

methane concentration in flame 2 is considerably enhanced in the early flame region, the concentration of propyne is actually slightly reduced. This is explained by modeling analysis that shows propyne to derive predominantly from the decomposition of n- heptane in this region. As a consequence, contrary to the diffusion-flame and flow-reactor results, the addition of methane slightly reduces the concentration of benzene. Since the post-flame reaction paths are similar, no large change in other aromatic species or soot concentrations are expected. The results relate well to experimental observations discussed by Glassman [33], which showed sooting limits in premixed flames at a fixed adiabatic temperature to be more related to the number of C-C bonds than to the fuel structure. This conclusion was explained by the fact that much of the fuel charac- teristics are destroyed in the oxidizing region before Fig. 14. Laminar, premixed flame soot-volume fractions. the sooting region. Harris and Weiner [34] also Lines are curve fits of data. Flame 1: without CH4; flame 2: reached a similar conclusion regarding the minor role with CH . 4 of fuel structure in premixed flames, provided the C/H ratio does not change significantly. For the present data, where the C/O ratios remain close to servation of the measured intermediate gas-phase those of the sooting limits and the C/H ratio changes species shows that the concentration profiles of H 2 little, the same reasoning can be applied to explain and C H are nearly unchanged by the presence of 2 2 the weak effect of methane substitution. A more methane. The final concentration of H is, however, 2 distinct effect may possibly have been observed for slightly higher in flame 2 as would be expected, since mixtures at higher values of ␤, in which case, less- the C/O is preserved but the C/H ratio decreases. diluted flames or the use of Ar for dilution would be Figure 15 shows the profiles of methane and benzene. required for stabilization purposes. Here, we focused The methane in flame 2 generates a perturbation only on realistic fuel/air flames for practical consider- in the low-temperature regions of the flame. Eventu- ations. Within stability limits of these flames, no ally, oxidation reduces the methane concentration to enhanced sooting or aromatic-species production was nearly the same value as in flame 1. Although the observed.

4. Summary and conclusions

The effect of methane content on the formation of PAH and soot production in a non-aromatic fuel mixture was investigated in three fundamental combustion processes, including a laminar flow reactor, a laminar co-flow diffusion-flame burner, and a laminar, premixed flame burner. The results varied significantly with the combustion system used. In the flow reactor, increasing the content of methane at constant carbon content showed PAH concentrations to increase. This increase reached a factor 2.5 within the range of conditions observed and could have possibly been larger at still higher methane contents. The total mass of heavy hydrocarbons, or tar, also rose, while the soot concentration stayed constant. Fig. 15. Laminar, premixed flame species-concentration In the diffusion flames, increasing the methane profiles: CH4 and C6H6. Lines are curve fits of data. Flame fraction of fuel carbon while decreasing that of eth- 1: without CH4; flame 2: with CH4. ylene also generated increases in PAH concentration J.F. Roesler et al. / Combustion and Flame 134 (2003) 249Ð260 259 and soot, if the ethylene was nitrogen-diluted, that Acknowledgments could be attributed to synergistic chemical effects. If the ethylene was not diluted with nitrogen, then the Co-authors C.S. McEnally and L.D. Pfefferle soot concentrations decreased monotonically, but gratefully acknowledge financial support from the synergistic effects were still evident. National Science Foundation and the US Environ- In contrast to the other two systems, the pre- mental Protection Agency. mixed flame results showed no synergy in the presence of methane. The maximum amount of methane that could be used and the maximum attainable References equivalence ratio were limited, due to flame instabilities. For the conditions attained, the detailed flame [1] M. Frenklach, D.W. Clary, W.C. Gardiner, Jr, S.E. structures suggested that the methane contained in Stein, Twentieth Symposium (International) on Com- bustion, The Combustion Institute, Pittsburgh, 1994, the fuel was primarily consumed by the oxygen, p. 887. leading to a virtually unperturbed flame in the PAH [2] M. Frenklach, Twenty-Second Symposium (Interna- and soot-growth regions. tional) on Combustion, The Combustion Institute, The synergy of methane with other alkanes to Pittsburgh, 1998, p. 1075. produce PAH is attributable to the ability of methane [3] M. Frenklach, J. Warnatz, Combust. Sci. Technol. 51 to produce methyl radicals that will then promote (1987) 265. production channels of aromatics that rely on odd- [4] H. Bockhorn, Soot Formation in Combustion—Mech- carbon-numbered species. This is clearly the case for anisms and Models. Springer Series in Chemical Phys- benzene that is formed from the propargyl recombi- ics 59, Springer-Verlag, Heidelberg, 1994. [5] J.A. Miller, C.F. Mellius, Combust. Flame 91 (1992) nation reaction. As for further aromatic growth, the 21. HACA mechanism could merely be enhanced by the [6] J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame increased formation of benzene. However, the data 1221 (2000) 122. strongly suggest otherwise. Indeed, in conjunction [7] N.M. Marinov, W.J. Pitz, C.K. Westbrook, A.E. Lutz, with decreasing acetylene concentrations, the acety- A.M. Vincitore, S.M. Senkan, Twenty-Seventh Sym- lenated or cyclopenta fused aromatics, which require posium (International) on Combustion, The Combus- direct addition of acetylene, show lesser growth than tion Institute, Pittsburgh, 1998, p. 881. aromatics for which various growth channels involv- [8] H. Richter, T.G. Benish, O.A. Mazyar, W.H. Green, ing odd-carbon-numbered species have been specu- J.B. Howard, Proceedings of the Combustion Institute, Vol. 28, The Combustion Institute, Pittsburgh, 2000, lated in the literature. This is the case for naphtha- p. 2609. lene, for which proposed production channels have [9] A. D’Anna, A. Violi, A. D’Alessio, Combust. Flame been the recombination of cyclopentadienyl radicals 121 (2000) 418. and the addition of benzyl and propargyl. Regarding [10] C.F. Melius, M.E. Colvin, N.M. Marinov, W.J. Pitz, soot, the synergy must arise from the enhanced PAH S.M. Senkan, Twenty-Sixth Symposium (Internation- that act either as precursors or surface growth species al) on Combustion, The Combustion Institute, Pitts- in reactions that compensate for the lesser growth by burgh, 1996, p. 685. acetylene addition. [11] M.B. Colket, D.J. Seery, Twenty-Fifth Symposium These results show the complexity of PAH chem- (International) on Combustion, The Combustion Insti- istry and the important role of methyl radicals and of tute, Pittsburgh, 1994, p. 883. [12] N.M. Marinov, W.J. Pitz, C.K. Westbrook, M.J. odd-carbon-numbered species in the growth process. Castaldi, S.M. Senkan, Combust. Sci. Technol. 211 These data should provide a good basis for testing (1996) 116Ð117. detailed reaction mechanisms by specifically sensi- [13] N.M. Marinov, W.J. Pitz, C.K. Westbrook, A.E. Lutz, tizing the reactions with odd-carbon-numbered spe- A.M. Vincitore, S.M. Senkan, Twenty-Seventh Sym- cies. Clearly, the HACA mechanism does not fully posium (International) on Combustion, The Combus- describe the molecular growth process in many ap- tion Institute, Pittsburgh, 1998, p. 605. plications. [14] A. D’Anna, A. Violi, Twenty-Seventh Symposium From a practical point, the results suggest that (International) on Combustion, The Combustion Insti- methane is not an actual soot promoter in flame tute, Pittsburgh, 1998, p. 425. [15] S.M. Senkan, M. Castaldi, Combust. Flame 107 situations. However, it does interact synergistically (1996) 141. with alkane fuels to produce more PAH and soot than [16] J.F. Roesler, X. Montagne, M. Auphan de Tessan, would otherwise have been expected. 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