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Energy & Fuels 2007, 21, 136-140
Distribution of Soot Molecular Weight/Size along Premixed Flames as Inferred by Size Exclusion Chromatography M. Alfe`,† B. Apicella,‡ R. Barbella,‡ A. Tregrossi,‡ and A. Ciajolo*,‡ Istituto di Ricerche sulla CombustionesCNR, P.le V. Tecchio, 80, 80125 Napoli, Italy, and Dipartimento di Ingegneria Chimica, UniVersita` “Federico II”, P.le V. Tecchio, 80, 80125 Napoli, Italy ReceiVed July 14, 2006. ReVised Manuscript ReceiVed October 3, 2006
The evolution of size/molecular weight (MW) of soot from inception to mature soot was studied by means of size exclusion chromatography (SEC) coupled with on-line UV-visible spectroscopy of soot sampled along the flame axis of ethylene, hexane, and benzene premixed flames. Polystyrene calibration and microfiltration were used for the evaluation of the soot MW and size, respectively. Overall, soot exhibited a wide MW/size distribution peaking in two main regions, well detached from each other. The first corresponds with particles/ aggregates having MW > 20 000 u, here called the “particle-size region”, and the second one corresponds with molecules in the 100-5000 u MW range called the “molecular-size region”. The components in the particle-size regions dominated, particularly downstream of the flames and everywhere in the benzene flame. Two classes of components were evident in the particle-size region: one with d > 20 nm (MW > 59 500 u) and the second class well below 20 nm discriminated by SEC, coupled with microfiltration. Meaningful differences in the relative contributions of these two classes were found to occur along the flames, affected by the fuel aromaticity, and they were signatures of the soot growth process and of the different soot inception mechanisms in aliphatic and benzene flames respectively.
Introduction The physical and chemical characterization of combustionformed particles in the size range extending down to nucleation (diameter < 0.01 µm) is important from a practical point of view because of the relationship of particle size to health effects and to global climate change.1,2 Also, the study of soot inception and growth processes is important to understand the formation mechanism, the subject of much experimental and modeling work leading to a rich body of literature on soot formation3,4 collected in contributions to round-table discussions on soot formation.5-8 The most intriguing and complex phase of the soot formation process is the inception of the first nuclei which, within milliseconds, coagulate and coalesce forming aggregates of thousands of atoms. Experimental methods have been applied to detect the molecular weight and size of soot precursors and * Corresponding author. Tel.: +39 081 7682254. Fax: +39 081 5936936. E-mail:
[email protected]. † Universita ` “Federico II”. ‡ Istituto di Ricerche sulla CombustionesCNR. (1) Obrdorster, G. EnViron. Health Perspect. 2002, 110 (8), 440. (2) Jacobson, M. Z. Nature 2001, 409, 695. (3) Haynes, B. S.; Wagner, H. Gg. Prog. Energy Combust. Sci. 1981, 7, 229. (4) Haynes, B. S. Soot and Hydrocarbons in Combustion. In Fossil Fuel Combustion; Bartok, W., Sarofim, A. F., Eds.; John Wiley & Sons, Inc.: New York, 1991: pp 261-326. (5) Siegla, D. C.; Smith, G. W. Particulate carbon formation during combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum Press: New York and London, 1981. (6) Lahaye, J.; Prado, G. Soot in combustion systems and its toxic properties; Plenum Press: New York and London, 1997. (7) (7) Soot formation in Combustion. An International Round Table Discussion; Jander, H., Wagner, H. G., Eds.; Vandenhoech & Ruprecht: Gottingen, Germany, 1990. (8) Bockhorn, H. Soot formation in Combustion; Bockhorn, H., Ed.; Springer-Verlag: New York, 1994.
their further growth to soot particles. Molecular beam/mass spectrometry and chromatographic techniques (gas chromatography (GC) and liquid chromatography) equipped with mass spectrometry have been shown to detect polycyclic aromatic hydrocarbons (PAH) up to 300-400 of mass units. Only in early work, very small solid particles of 1.5 nm, corresponding to a molecular weight of about 2000 u, have been identified by electron microscopy.9 Laser microprobe mass spectrometry (LMMS)10 and real-time aerosol mass spectrometry11 have shown particles of 10 nm in diameter identifying low molecular mass fragments (up to 400 u) of soot precursors and particles. The gap between the last GC-analyzable PAH (300-400 u) in the molecular size range ( 20 nm) followed by a second prevalent peak. As mentioned before, the latter corresponds to particles with a diameter above 5 nm and below 20 nm, corresponding to the average diameter of elementary soot particles.3 (21) Ciajolo, A.; Barbella, R.; Tregrossi, A.; Bonfanti, L. Proceedings of the Twenty-seVen Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 1481-1487. (22) Clar, E. Polycyclic Hydrocarbons; Academic Press: London, 1974.
Distribution of Soot MW/Size by SEC
Figure 3. Height normalized chromatograms of soot sampled in the ethylene flame at different heights above the burner at 350 nm of absorbance wavelength.
The first peak rapidly rises at the beginning of the soot formation region (Figure 3a) and becomes dominant at z ) 8 mm (Figure 3b), corresponding with the maximum soot formation rate (Table 1). Up to the end of the flame, (z ) 14 mm), the first peak due to large size soot particles/aggregates remains predominant, whereas contributions from particles of sizes between 5 and 20 nm and of species in the molecular-size region are quite negligible. Overall, from the inception up to the end of soot formation, the reduction of soot size from a multimodal to a monomodal dispersion testifies to the gradual occurrence along the ethylene flame of particle growth/coagulation from elementary young soot particles (g5 nm) toward the largest soot particles/aggregates (>20 nm) of mature soot. A similar evolution of soot MW/size distribution has been found along the n-hexane flame as can be observed in Figure 4 where the normalized SEC chromatograms of n-hexane soot are reported. It appears that the first peak is more relevant at soot inception (z ) 7.5 mm) (Figure 4a) with respect to the ethylene flame and becomes prevalent slightly before the maximum of soot formation rate (z ) 9.5 mm) (Figure 4a). However, the general similarity of the soot MW/size distribution along the ethylene and hexane flames suggests a similarity in the soot formation process in aliphatic flames. By contrast, the axial trend of the MW/size distribution of soot in the benzene flame appears to be different from the aliphatic flames as can be seen in Figure 5 where the normalized SEC chromatograms of soot sampled along the benzene flame are reported. In the soot inception region of the benzene flame, soot particles appear to be already very large (>20 nm), as demonstrated by the earlier relevant presence of the first peak in the SEC chromatogram of soot sampled at inception (z ) 6 mm). The first peak, comparable with the second peak at soot inception, immediately becomes the predominant peak (z ) 7
Energy & Fuels, Vol. 21, No. 1, 2007 139
Figure 4. Height normalized chromatograms of soot sampled in the hexane flame at different heights above the burner at 350 nm of absorbance wavelength.
Figure 5. Height normalized chromatograms of soot sampled in the benzene flame at different heights above the burner at 350 nm of absorbance wavelength.
mm) much earlier in the benzene flame relative to the ethylene and n-hexane flames (Figures 3 and 4). In addition, it is noteworthy that the peaks in the molecular-size region are almost
140 Energy & Fuels, Vol. 21, No. 1, 2007
negligible, already in the soot inception region of the benzene flame (Figure 5a). Overall, the MW/size distribution of benzene soot appears simply as large soot particles/aggregates already in the soot inception region. This is different than the aliphatic flames where the MW/size distribution is initially wider and richer in small soot particles which slowly evolve toward large size. However, it cannot be excluded that a structural/shape effect could be responsible for the explanation of the earlier appearance and predominance of the first peaks in the particle size distribution of benzene soot. Indeed, as mentioned in the Introduction, the anomalous earlier elution of fullerenes (Figure 1), previously attributed by Herod et al.18 just to the sphericity of fullerenes, could indicate a shape effect; in particular, a more spherical structure of benzene soot particles rather than a larger size can be partially responsible for the earlier SEC elution of young benzene soot. Even in this situation, it has been shown that the fuel aromaticity strongly affects the structure and/or the size of soot particles. The differences in the axial evolution of the soot size distribution along aliphatic and benzene flames can be interpreted by considering that soot formation occurs much earlier in the benzene flame where the overlapping of the oxidation and pyrolytic regions is known to occur3,23 (Table 1). The overlapping of the oxidation and pyrolytic regions derives in turn from the massive presence of benzene and consequently from aromatic precursors in the oxidation region of benzene flames.17,23 The formation of young soot in a more oxidative and aromatic-rich environment of the benzene flame compared with the pyrolytic environment rich in hydrogen and light hydrocarbons for aliphatic flames is intuitively expected to cause a difference in relative contributions by inception and growth processes to the total soot mass. A higher particle generation rate and a higher fraction of the final soot mass attributable to nucleation, the residual part being due to growth, have been observed in benzene flames.3 By contrast, the distribution of PAH, considered to be the main soot precursors, and soot particle sizes have been generally found to be very similar in aliphatic and aromatic flames.17,23-27 (23) Bittner, J. D.; Howard, J. B. Pre-particle chemistry in soot formation. In Particulate carbon formation during combustion; Siegla, D. C., Smith, G. W. Eds.; Plenum Press: New York, 1981; pp 109-142. (24) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1983, 87, 1067. (25) McKinnon, J. T.; Howard, J. B. Proceedings of the Twenty-fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 965-971. (26) Alfe`, M.; Apicella, B.; Barbella, R.; Tregrossi, A.; Ciajolo, A. accepted for presentation. Proceedings of the Thirty-one Symposium (International) on Combustion, The Combustion Institute: Pittsburgh, PA, 2006.
Alfe` et al.
In a pioneering paper regarding the optical study of soot in benzene and ethylene flames,27 benzene soot showed a much lower degree of depolarization with respect to that of ethylene soot, tentatively attributed to a more ordered structure of benzene soot particles rather than to a different agglomerate size. Furthermore, high-resolution transmission electron microscopy (HRTEM) images of soot formed in benzene flames exhibited a larger presence of fullerenic carbon with respect to ethylene soot, consistent with the larger presence of fullerenes detected in benzene flames.28 Another difference in the structure of aliphatic and benzene soot regards the absorption coefficient of soot which has been found to increase much more slowly along the hexane flame than in the benzene flame, reaching lower final absorption values than benzene soot.26 In agreement with these observations, the difference in the soot inception process between aromatic and aliphatic flames has been found to also cause a difference in the size and structure of soot and in its evolution in the soot formation region. The SEC technique appears to be a relative easy and fast way to detect differences in soot formation processes and soot characteristics, provided that an accurate evaluation of the effects of soot structure in terms of morphology and intrinsic aromaticity be carried out. The study of these effects, along with the extension of the MW/size detection range by using new SEC columns and possibly new solvents, is underway. Conclusions Insights into the differences in soot particle inception and growth along flames of aliphatic and aromatic fuels were obtained by means of a comparative SEC analysis of soot sampled along the axis of ethylene, n-hexane, and benzene flames. In the soot inception region, soot exhibited a wide MW/size distribution being composed of particles of large size (diameter > 20 nm) and smaller size (diameter ) 5-20 nm). Also, species in the molecular-size region (20 nm) along the flame. Soot growth was found to occur gradually along both ethylene and n-hexane flames, whereas the growth process was found to be much faster in the benzene flame where the nascent soot particles immediately exhibited a larger presence of large soot particles/aggregates. EF060320P (27) Haynes, B. S.; Jander, H.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 585. (28) Grieco, W. J.; Howard, J. B.; Rainey, L. C.; Vander Sande, J. B. Carbon 2000, 38, 597.
