ON THE RELEVANCE OF SURFACE GROWTH IN SOOT FORMATION IN PREMIXED FLAMES

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1 Proceedings of the Combustion Institute, Volume 28, 2000/pp ON THE RELEVANCE OF SURFACE GROWTH IN SOOT FORMATION IN PREMIXED FLAMES A. D ALESSIO, 1 A. D ANNA, 1 P. MINUTOLO, 2 L. A. SGRO 1 and A. VIOLI 1 1 Dipartimento di Ingegneria Chimica Università degli Studi di Napoli Federico II 2 Istituto di Ricerche sulla Combustione CNR Piazzale Tecchio Napoli, Italy The role of surface growth mechanisms in particle mass accumulation was investigated in rich, premixed, ethylene/air flames from non-sooting to moderately sooting conditions using in situ optical diagnostics and predictions from a detailed chemical kinetic model. Particles formed just after the flame front, which are transparent to the visible light but absorb in the UV range, have been detected in large amounts in nonsooting flames and earlier in the flame than soot particles in sooting-flames, using UV-visible light optical diagnostics. For C/O 0.8, the amount of visible-transparent particles accounts for the total mass of soot detected later in the flame, indicating that surface growth processes are negligible and that soot formation is a rearrangement of the carbonaceous material already present in the form of smaller particles. Furthermore, predictions from the kinetic model, which does not include surface growth reactions, agree well with experiments for C/O 0.8. The model is able to predict the total carbon contained in particles observed in the non-sooting flames and in slightly sooting flames, as well as that observed up to the onset of soot formation in richer flames. For C/O 0.8, additional soot growth mechanisms need to be included in the mechanism to account for the amount of soot observed in the later part of the flames. Interestingly enough, this late growth mechanism occurs almost simultaneously with a strong increase in the coagulation rate of the particles, thus indicating that both effects are related and are probably due to a major change in the chemical nature of the particle surface. Introduction The currently accepted picture of soot formation consists of different chemical and physical steps: the formation and growth of large aromatic hydrocarbons and their transition to particles, the growth of solid particles by addition of components from the gas phase, and the coagulation of primary particles to large aggregates [1 4]. Wagner [5] described the carbon deposition process in laminar, premixed flames through first-order kinetics, df v K sg ( fv f v ) (1) dt where f v is the total amount of soot formed, and typical values of K sg range between 30 and 500 s 1 [6], thus indicating that the process of soot loading is much slower than that of oxidation in the main reaction zone. This slow process was attributed to surface growth (i.e., a gas-to-particle transfer of hydrocarbon fragments) whose rate should be proportional to the soot surface area and typical hydrocarbon concentration [6]. However, experiments have shown that soot growth does not depend either upon surface area, as determined by scattering/extinction measurements, or upon acetylene concentration [7]. In order to eliminate these incongruities, it has been proposed that soot growth takes place at active sites, and thus the problem shifts to evaluating the properties of active sites and the role of H atoms in their activation. In addition to acetylene, large polycyclic aromatic hydrocarbons (PAHs) have been proposed as species which react with the surface of soot particles, contributing to rapid mass growth [8,9]. These experimental observations have been taken into account by a detailed mechanism and model for soot formation [10] which assumes that particle inception is primarily determined by PAH coagulation. Surface growth is determined mainly by the hydrogen abstraction, acetylene addition (HACA) mechanism; PAH condensation on the surface is also considered and modeled in terms of elementary chemical reactions on surface active sites. Nucleation kinetics control the number of nascent particles, while the carbon mass accumulated in soot is determined primarily by surface reactions [1]. A different point of view on soot formation in premixed flames was taken by this group some years ago [11]. By combining in situ optical techniques (light 2547

2 2548 SOOT FORMATION AND DESTRUCTION scattering, fluorescence, and extinction measurements) in the UV and visible ranges, it was found that particles with typical sizes around 2 3 nm are formed quite early in the soot pre-inception zone and in rich flames below the soot threshold [12 14]. This particulate material is initially transparent to visible radiation and shows spectroscopic properties typical of two- and three-ring aromatics. Initially, the particles have a much lower coagulation rate than the gas kinetic limit, and their coagulation rate increases above the gas kinetic limit only after they become light-absorbent in the visible range [14]. The importance of a precursor inception step in soot formation was underlined also by Dobbins and Subramaniasivam [15] in their analysis of soot formation in coflowing diffusion flames. More recently, D Anna and Violi [16] proposed a model for the formation of high-molecular-mass structures in rich premixed flames, starting from the observation that resonantly stabilized radicals react at a much faster rate than other radicals to produce benzene and PAHs. They proposed that all of the carbonaceous material formed in rich flames, including PAHs, tar, and soot, is created by the fast reactions of resonantly stabilized radicals, thus neglecting the surface growth mechanism. The model was able to predict with remarkable success the total carbonaceous material formed in flames of ethylene and benzene in a range of C/O feed ratios from stoichiometric to moderately sooting regimes [17]. Their model does not yet describe the detailed formation routes for soot and tar but assumes that the formation of PAH is followed by a fast polymerization process, leading to the formation of high-molecular-mass structures having two- and three-ring subunits. Soot formation occurs as a consequence of the aromatization/graphitization of these high-molecular-mass structures. This approach explicitly neglects the importance of surface growth processes in the accumulation of carbon mass and, instead, assumes all carbonaceous condensed matter, including PAHs, soot, and tar, are formed through reactions involving small PAHs. In this paper we assess the relevance of surface growth mechanisms in the formation of soot and high-molecular-mass structures in premixed flames. We have evaluated carbon concentrations from UV and visible extinction measurements along the axis of ethylene/air flames with C/O ratios ranging from the stoichiometric to a moderately sooting regime and compared the results with those obtained by the kinetic model. The quantity of carbon, which is in excess with respect to that predicted by the model, is attributed to surface growth, and it is correlated with the coagulation rate of the particles. Experimental Methods Extinction and scattering measurements were performed in premixed, flat ethylene/air flames stabilized on a water-cooled capillary burner. The cold gas velocity was 10 cm/s for all the flames, and the C/O ratio was changed in the range. A Xe lamp was used as the light source for extinction measurements. The light beam was first passed through a predispersing monochromator to avoid stray light interferences caused by the strong emission of the lamp in the visible range when measuring absorption in the UV range. Two lenses focused the beam on the center of the burner and then on the entrance slit of a spectrometer. The fourth harmonic of a pulsed Nd:YAG laser (k o 266 nm) was the light source for laser light scattering (LLS), and the signals were analyzed by an ICCD camera. Further details on scattering measurements were reported in a previous paper [18]. The flame temperature was measured in the nonsooting region by the Na-D line reversal method using a calibrated tungsten strip lamp. The flame temperature was employed in the evaluation of gas contribution to light scattering and absorption, and also as input data to the kinetic model. The position of the flame front was measured by the OH emission at nm (A 2 R X 2 P). Numerical Modeling We used a new kinetic mechanism which was developed to predict the formation of high-molecularmass aromatic compounds in a wide range of operating conditions [16]. The model was validated in different combustion systems (premixed laminar flames, counterflow diffusion flames, plug-flow and perfectly stirred reactors) for aromatic and aliphatic fuels [16,19,20]. The detailed reaction mechanism contains 340 reactions and 90 chemical species. The mechanism includes the pyrolysis and oxidation of C 1 and C 2 species, the formation of benzene, and further reactions leading to aromatic compounds with up to three rings. The formation and growth of aromatic compounds, as identified by detailed chemical kinetic modeling, occurs more through the combination of resonantly stabilized radicals than through a multistep process of acetylene addition [16]. Major routes of the first aromatic ring formation are propargyl self-combination and the addition of propargyl to 1-methylallenyl with the formation of benzyl radicals and their decomposition to benzene. These two routes account for approximately 90% of benzene production; the remainder is formed by C 2 H 2 addition to n-c 4 H 5 and n-c 4 H 3 radicals. The combination of cyclopentadienyl-like radicals and the propargyl addition to benzyl radicals are the controlling steps of aromatic growth, whereas the mechanism which involves replicating H-atom abstraction from an aromatic molecule followed by acetylene addition (HACA mechanism) is of minor

3 RELEVANCE OF SOOT SURFACE GROWTH 2549 importance. According to these mechanisms, aromatics are rapidly formed in the main flame region, and they grow to 2- or 3-ring PAHs. The reactive coagulation of small PAHs is responsible for the formation of high-molecular-mass aromatics having two- to three-ring subunits. Soot formation is a consequence of the aromatization/graphitization of these high-molecular-mass structures. Thermochemical information was obtained, when available, from the CHEMKIN thermodynamic database [21]. Thermodynamic properties for other species were estimated by using Benson s group additivity method [22]. The transport parameters were obtained from the CHEMKIN database [23] and from Wang and Frenklach [24]. Unless specifically mentioned, each elementary reaction in the mechanism is reversible, and the rate coefficients of the forward reactions were either taken from literature or estimated on the basis of analogous reactions. The reverse reaction rates were calculated using equilibrium constants. For most of the recombination and decomposition reactions, the pressure dependence in the Troe format and third-body efficiencies were taken into account [25]. Results and Discussion Light absorption was measured at two wavelengths, in the UV region at 266 nm and in the visible region at 532 nm. Aromatic chromophores absorb radiation at a wavelength of 266 nm, where totally aliphatic compounds are generally not light-absorbent. Nevertheless, at flame temperatures, CO 2 absorptivity becomes much stronger than that at ambient temperature, and it also extends above 266 nm. Therefore, in order to estimate the light absorption due to aromatic chromophores produced by the flame, we subtract the contribution due to CO 2 from the flame light absorption. CO 2 absorption at 266 nm was estimated from the cross section for CO 2 at the flame temperature [26] and from the CO 2 mole fraction predicted by the kinetic model. CO 2 contribution to the flame absorption is 100% in the stoichiometric flame and ranges from about 50% to 0.2% from the leanest to the richest flame examined in this paper. We attribute absorption in the visible region (532 nm) exclusively to soot particles. However, both small aromatic chromophores and sootlike chromophores absorb in the UV region at 266 nm. Therefore, in this work, we attribute absorption in the UV partly to the visible-transparent and partly to sootlike chromophores. From the absorption measurements at the two wavelengths, we estimate the volume fraction of visible-transparent and sootlike structures. For this calculation, the optical properties of visible-transparent structures in the UV region and those of sootlike particles in both the visible and the UV regions were required. The optical properties of soot have been well studied, and we used values reported in the literature [27,28]: k 266 nm m 1.4 i0.75 sootlike k 532 nm m 1.5 i0.6 sootlike For the visible-transparent particles, we used the value k 266 nm m 1.4 i0.08 vis-trans In a previous paper [13], we considered a variable refractive index for visible-transparent particles, which was determined from their absorption and fluorescence spectral behaviors. In this paper, we simplified the analysis by using a constant mean value for m. For a chromophore with mass M 200 amu, an imaginary part of the refractive index at 266 nm equal to 0.08 corresponds to an absorptivity of the order of cm 1 L/mol, which is a typical value for 2-3 ring PAH chromophores. We chose this value for the imaginary part of the refractive index of the visible-transparent material based on our previous work [11 14], which showed that this material contains two- to three-ring chromophores that do not interact with each other. Thus, the total absorptivity is given by the sum of the absorptivities from single functionalities. The real part of the refractive index for the particles is assumed to be the same as that of soot. From the absorption measurements in the visible, we estimated the volume fraction of sootlike material from the equation 2 2 6p m 1 kext Im f v (2) k m 2 Using this equation and the refractive index of soot at the two wavelength, in the visible and UV regions, the amount of UV absorption due to sootlike material, K soot(266 nm), was obtained. Then, the absorpabs tion due to the visible-transparent material, abs, was calculated by subtracting K vis-trans(266 nm) abs K soot(266 nm) from the total absorption measured at 266 nm. Finally, their volume fraction was determined from equation 2. The accuracy of the results depends mainly on the uncertainties of the experimental measurements of the absorption coefficient, which was estimated to be about , and on the uncertainties on the values assumed for the refractive index, which were estimated considering the range of data reported in the literature. Figure 1 reports the total volume fraction (i.e., the sum of visible-transparent and sootlike material) and the volume fraction of the sootlike component for three flames. The first flame, with C/O 0.51, is a non-sooting flame. Only visible-transparent material is formed very rapidly and remains constant along

4 2550 SOOT FORMATION AND DESTRUCTION Fig. 2. Percentage of carbon contained in aromatic compounds supplied by the fuel predicted by the model (lines) and that contained in the total particulate matter estimated from absorption measurements (solid circles) for the C/O 0.51 flame below the soot threshold. Fig. 1. Particle volume fractions of total material (solid squares) and soot (open squares) at different heights above the burner in ethylene air flames: C/O 0.51 (below the soot threshold), C/O 0.77, C/O the flame axis. The second flame is slightly sooting (C/O 0.77). In the region between the flame front and soot inception, a large amount of visible-transparent material is already formed, and soot inception can be observed by the onset of detectable visible absorption at z 4 mm. Although soot concentration increases almost linearly in the flame, the total volume fraction remains constant. In more rich conditions, C/O 0.92 (Fig. 1c), even though a large amount of material is already formed before soot inception, the total volume fraction is not constant along the flame but, instead, increases slightly with z. From the volume fractions of visible-transparent and sootlike components, assuming their densities to be 1 g/cm 3 and 1.8 g/cm 3, respectively, we evaluated the percent of the carbon originally supplied by the fuel that is contained in the particles. The percent carbon contained in the total material was compared to that contained in the aromatic compounds predicted by the model. This comparison shows good agreement for the C/O 0.51 flame (Fig. 2). Figure 3 reports the percentage of carbon from the experiments and the model at two fixed heights above the burner, z 6mmandz 10 mm, for flames ranging from the stoichiometric value up to moderately sooting conditions. The two heights represent approximately the region of the flame just at soot inception (z 6 mm) and the final postoxidation zone (z 10 mm). The part of the plot related to non-sooting and slightly sooting flames can be seen more clearly in Fig. 3c, which is in semilog scale. In this range (C/O ), the total amount of material produced in flames, estimated from absorption measurements, is well predicted by the model. The rate of increase of the total mass with C/O, which is different below and above the soot threshold (C/O 0.65), is also well predicted. The strong increase in the total mass of material with C/ O above the soot threshold is not due to the formation and growth of soot particles because it is observed also at z 6 mm, just above soot inception. Instead, this increase comes from a corresponding increase in the total amount of visible-transparent material. The detailed analyses of the slightly sooting flame with C/O 0.77 (Fig. 4) reveals that the total amount of material is already formed at 4 mm, the height above the burner corresponding to soot inception. Above soot inception, the total amount of material remains almost constant, and it is well predicted by the model.

5 RELEVANCE OF SOOT SURFACE GROWTH 2551 Fig. 4. Percentage of carbon contained in aromatic compounds supplied by the fuel predicted by the model (lines) and that contained in the total particulate matter estimated from absorption measurements (solid circles) for the C/O 0.77 flame. Also plotted on the figure is the percentage of carbon contained in soot (open circles). Fig. 3. Percentage of carbon contained in aromatic compounds supplied by the fuel predicted by the model (lines) and that contained in the total particulate matter estimated from absorption measurements (circles) for a range of C/ O ratios from non-sooting to moderately sooting conditions. Two flame heights are presented (A) z 6 mm (solid lines and dark circles) and (B) z 10 (dashed lines and open circles). In (C), the points at z 6 and z 10 are reported in semilog scale for clarity; note that many points overlap. This result indicates that, at least in slightly sooting flame conditions, the processes of soot inception and mass growth do not involve surface growth by acetylene addition via the HACA mechanism. Instead, soot formation occurs through the rearrangement of a large amount of visible-transparent particulate material, which is formed in the flame before soot inception from fast reactions among small PAHs. According to this point of view the soot threshold itself is not related to a surface growth mechanism, as is commonly thought, but is instead dependent on the Fig. 5. Percentage of carbon contained in aromatic compounds supplied by the fuel predicted by the model (lines) and that contained in the total particulate matter estimated from absorption measurements (solid circles) for the C/O 0.92 flame. Also plotted on the figure is the percentage of carbon contained in soot (open circles). strong increase in the PAH budget. This material undergoes a chemical transformation, aromatizes by internal rearrangement, and transforms from visibletransparent to visible-absorbing, or sootlike, material without the addition of material from the gas phase. The same conclusion cannot be completely extended to richer flames. In C/O 0.8 flames, the model predicts well the total material formed in the inception region (Fig. 3A) but underestimates the material detected later in the final soot-forming region (Fig. 3B). The analysis of the C/O 0.92 flame reported in Fig. 5 shows that for the first few millimeters, just above the inception of soot, the model predicts well the total mass of material detected in the flame, and

6 2552 SOOT FORMATION AND DESTRUCTION Fig. 6. The experimentally derived coagulation rate of the total particles (triangles) and the percentage of carbon observed in excess to that predicted by the model for the C/O 0.92 flame. soot inception can be completely justified by the rearrangement of visible-transparent particles. At z 8 mm, the total particulate matter comprises about 7% of the feed carbon. For z 8 mm, the amount of aromatics predicted by the model remains almost constant, whereas the total mass measured experimentally increases. This increase in carbon content, observed in the total amount of material from z 8mmtoz 16 mm, is the same as that observed in soot. This result suggests that for this flame, a process of surface growth for soot by means of gasphase compounds such as acetylene should be considered. At the end of the flame, surface growth accounts for about 60% of the mass of carbon in soot, and the remaining fraction comes from transformations of the visible-transparent particles. It is important to note that even in this rich flame, soot particles account for less than one-half of the total particulate matter observed at all heights above the burner. From these results, it appears that both the first soot nuclei and the visible-transparent particles are inactive with regard to heterogeneous surface reactions. It is reasonable to hypothesize that the activation of surface growth after 8 mm is related to a change of the surface properties of the particles because, at this height, the coagulation rate of the particles also shows a sharp increase. This is evident in Fig. 6, where the coagulation rate and the percentage of C attributed to surface growth by acetylene are reported versus height above the burner for the C/O 0.92 flame. The coagulation rate was evaluated from the Smoluchowsky equation for a monodispersed aerosol, dn 2 KN (3) dt where K is the coagulation rate and N is the number concentration of the particles. N was estimated by measuring light scattering and extinction at 266 nm and by solving the system of equations for these quantities in the Rayleigh approximation [14]. Even with the limitations of the Rayleigh equation, which provides the average number concentration, and of equation 3, a change in the particle coagulation rate of several orders of magnitude was calculated that cannot be explained by using a more complex coagulation rate model. Figure 6 shows that below z 8 mm, when surface growth is almost zero, the coagulation rate for the particles is also very low, several orders of magnitude lower than that expected from the gas kinetic theory. When the particles become active with regard to acetylene addition, the coagulation rate increases and approaches the value predicted by the gas kinetic limit at approximately z 10 mm. This result suggests that both effects are strongly correlated and can be ascribed to changes in the chemical properties of the particle surface. Final Remarks Several interesting observations can be deduced from the results in this paper. First, the excellent agreement between the results obtained by UV absorption measurements and PAH formation modeling based on the chemistry involving resonantly stabilized radicals validates the experimental and numerical methods. The results indicate a significant formation of small PAHs well below the soot threshold conditions where their formation is generally neglected. The soot formation threshold occurs because of an increase in the amount of PAH predicted by model, which corresponds to the visible-transparent material observed in the flame. This regime, in which all particulate matter formed in flames can be predicted by the early formation of two- to three-ring PAHs, extends to slightly sooting conditions, up to C/O 0.8 for the ethylene/air flames examined in this work. For higher C/O ratios, the carbon balance between the total particulate formed and the formation of small PAHs is not respected, and other mechanisms should be invoked. Surface growth due to gasphase compounds is the most obvious candidate, and attention should be focused on the chemical and physical nature of the particle surface. A fundamental study of this problem is difficult at this stage, due to the lack of reliable data on the surface reactivity of flame-generated particles. However, the correlation between the increase of the coagulation rate and the increase in surface growth processes indicated by our results may furnish an interesting starting point. Also, a comparison of the chemical and physical characteristics of the particles formed in slightly

7 RELEVANCE OF SOOT SURFACE GROWTH 2553 sooting conditions by PAH coagulation and aromatization without surface growth, and those particles whose mass is largely due to acetylene addition may shed some light on this issue. High-pressure flames also show a correlation between low coagulation rate and low surface growth [29]. It may also be of some interest to investigate high-pressure combustion. As a final remark, we emphasize that the conditions studied in this paper are particularly relevant for practical systems. In these conditions, while the amount of visible-transparent particles is low with respect to particle loading in more rich conditions, their number concentration is significant. The small size, large number density, and partial solubility in water of these flame-generated particles render them important for their possible effects on human health and climate [18]. Extinction at 266 nm can be used to track the presence of these visible-transparent particles in combustion applications and study their behavior as emissions in the atmosphere. REFERENCES 1. Haynes, B. S., and Wagner, H. G., Prog. Energy Combust. Sci. 7: (1981). 2. Glassman, I., Proc. Combust. Inst. 22: (1988). 3. Homann, K. H., Proc. Combust. Inst. 20: (1984). 4. Bockhorn, H., Schëfer, T., Growth of Soot Particles in Premixed Flames by Surface Reactions, in Soot Formation in Combustion, (H. Bockhorn, ed.), Springer-Verlag, Heidelberg, Germany, Wagner, H. G., Soot Formation: An Overview, in Particulate Carbon Formation During Combustion (D. C. Siegla and G. W. Smith, eds.) Plenum Press, New York, Harris, S. J., and Weiner, A. M., Combust. Sci. Technol. 31: (1983). 7. Bockhorn, H., Detailed Mechanism and Modeling of Soot Particle Formation, in Soot Formation in Combustion, Springer-Verlag, Heidelberg, Germany, Marr, J. A., Allison, D. M., Giovane, L. M., Yerkey, L. A., Monchamp, P., Longwell, J. P., and Howard, J. B., Combust. Sci. Technol. 85:65 76 (1992). 9. Lam, F. W., Howard, J. B., and Longwell, J. P., Proc. Combust. Inst. 22: (1989). 10. Frenklach, M., and Wang, H., Detailed Mechanism and Modeling of Soot Particle Formation, in Soot Formation in Combustion, Springer-Verlag, Heidelberg, Germany, D Alessio, A., D Anna, A., D Orsi, A., Minutolo, P., Barbella, R., and Ciajolo, A., Proc. Combust. Inst. 24: (1992). 12. Minutolo, P., Gambi, G., D Alessio, A., and D Anna, A., Combust. Sci. Technol. 101: (1994). 13. Minutolo, P., Gambi, G., and D Alessio, A., Proc. Combust. Inst. 27: (1998). 14. Minutolo, P., Gambi, G., D Alessio, A., and Carlucci, S., Atmos. Environ. 33: (1999). 15. Dobbins, R. A., and Subramaniasivam, H., Soot Precursor Particles in Flames, in Soot Formation in Combustion (H. Bockhorn, ed.), Springer-Verlag, Heidelberg, Germany, D Anna, A., and Violi, A., Proc. Combust. Inst. 27: (1998). 17. Violi, A., D Anna, A., and D Alessio, A., A Kinetic Model of Particulate Carbon Formation in Rich Premixed Flames of Ethylene and Benzene, paper 47.2, in Fifth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal, in press (2000). 18. Sgro, L. A., Minutolo, P., Basile, G., and D Alessio, A., Chemosphere 42(5 7): (2000). 19. Violi, A., D Anna, A., and D Alessio, A., Chem. Eng. Sci. 54: (1999). 20. Violi, A., D Anna, A., and D Alessio, A., Chemical Kinetic Modeling of Opposed Flow Difference Flames, paper v. 8 in Italian Section Meeting of the Combustion Institute, Firenze, Italy, May 2 5, 1999, pp Kee, R. J., Rupley, F. M., Meeks, E., and Miller, J. A., CHEMKIN III: A FORTRAN Chemical Kinetics Package for the Analysis of Gas Phase Chemical Kinetics and Plasma Kinetics, Sandia report SAND Benson, S. W., Thermochemical Kinetics, John Wiley & Sons, New York, Kee, R. J., Dixon-Lewis, G., Warnatz, J., Coltrin, M. E., and Miller, J. A., The CHEMKIN Transport Database, Sandia report SAND Wang, H., and Frenklach, M., Combust. Flame 110: (1997). 25. Troe, J., Ber Bunsen-Ges. Phys. Chem. 87:161 (1983). 26. Jensen, R. J., Guettler, R. D., and Lyman, J. L., Chem. Phys. Lett. 277: (1997). 27. Dalzell, W. H., and Sarofim, A. F., ASME J. Heat Transfer 91: (1969). 28. Chang, B. H., and Charalampopoulos, T. T., Proc. R. Soc. London A 430: (1990). 29. Bohm, H., Feldermann, C., Heidermann, T., Jander, H., Luers, B., and Wagner, H. G., Proc. Combust. Inst. 24: (1992). COMMENTS Lisa Pfefferle, Yale University, USA. In our non-premixed, coflowing methane and ethylene flames, TEM analysis of transparent precursor and mature soot suggest that for the low-sooting regime most of the carbon present in the mature soot was to a large extent present in the initial precursor particles (the particle number density stage constant across the transition and the primary particle site decreased presumably due to carbonization). Can you comment on the implications of your work in rich premixed flames for plausible processes in coflowing non-premixed

8 2554 SOOT FORMATION AND DESTRUCTION flames. Do you believe that the soot inception mechanism is the same in both types of flames? Author s Reply. Our experimental results in the inception region of premixed flames are quite consistent with your observations. In this paper we show that fast PAH formation can justify the total amount and time dependence of carbonaceous material in slightly sooting flames. We have applied the same kinetic model to coflowing diffusion flames and compared model results with data of the Yale group [1]. It appears that the mechanism of fast PAH formation justifies the amount of carbon present in the transparent precursors on the flame axis. However, the mechanism of surface growth by acetylene is necessary to explain the experimental determined amount of soot found in the outer anulus region of the flames. REFERENCE 1. D Anna, A., D Alessio, A., and Kent, J., Combust. Flame (2000). M. J. Wornat, Princeton University, USA. You attribute the increase in coagulation and soot surface growth to a change in the nature of the particle surface, perhaps related to an increase in aromatization of the particle, you postulated. Could you comment on why an increase in particle aromatization would bring about the observed increase in soot surface growth and coagulation? Also, what methods might be employed to characterize postulated changes in particle surface properties? Author s Reply. Our experimental results have shown a simultaneous increase of the scattering coefficients and of the difference between the measured and predicted amounts of carbonaceous species in sooting conditions. We have attributed the increase of scattering to an increase of the coagulation rate and the increase of the difference between measured and predicted particulate to the contribution of acetylene from the gas phase. Both processes should be related to a change of the reactivity of the particle surface. This argument is post hoc ergo propter hoc. Detailed mechanism of the increase of reactivity toward molecules or particles is really a subject of investigation. We may speculate on the delocalization of radical sites on the particle surface resulting in the formation of long-lived reactive species. We may propose different in situ spectroscopic techniques like UV-IR absorption, fluorescence, spontaneous Raman, and CARS. We are anxious to see other methods to be employed for the characterization of this fast process. Hai Wang, University of Delaware, USA. If we define soot particles having sizes larger than 1 nm in diameter, we would arrive at the conclusion that surface reactions with acetylene and/or PAHs are significant in their contributions to final soot mass. On the other hand, if soot is defined as particles that can be seen by visible-light extinction ( 10 nm) we would conclude that very little soot mass comes from surface processes. Am I correct by stating that the difference between your conclusion and previous soot models is a consequence of the difference in the definition of soot? Author s Reply. The HACA mechanism has been developed for modeling mostly the formation of PAH and mature soot particles. Soot formation process is a relatively slow process, and therefore a surface-growth mechanism based on acetylene addition has been invoked. We are dealing with the formation of smaller nanoparticles which have a much faster formation rate and for which in some regimes surface growth is not important. We would be very interested in a modeling of our experimental data by other groups, but so far soot models appear to be different. A. Williams, Leeds University, UK. Flameless combustion furnaces operated in near-sooting conditions have enhanced radiative characteristics. Is it possible that this is due to the PAH species outlined in this paper? Author s Reply. This is a very interesting question. In fact, it has been observed by different research groups that heat flux from flameless combustion furnaces is very high. This effect has been attributed to the extended reaction volume and to the high CO 2 concentration in the reactive volume. The attribution of this effect to IR absorbing PAHs should consider the effect of the temperature on the formation of these structures. In fact, it has been shown that the yield of PAH species follows a bell-shaped curve increasing the temperature with a maximum at around 1400 K and a dramatic decrease for temperatures below 1300 K [1]. A spectroscopic characterization of the flameless combustion, particularly in the UV range, should give stronger elements about the presence of these structures in flameless combustion. REFERENCE 1. Violi, A., D Anna, A., and D Alessio, A., Chem. Eng. Sci. 54, n15 16: (1999).