- Email: [email protected]
Similarities and dissimilarities in n-hexane and benzene sooting premixed flames
Proceedings of the
Proceedings of the Combustion Institute 31 (2007) 585–591
Combustion Institute www.elsevier.com/locate/proci
Similarities and dissimilarities in n-hexane and benzene sooting premixed flames M. Alfe` a, B. Apicella b, R. Barbella b, A. Tregrossi b, A. Ciajolo a
b,*
Centro regionale di Competenza ‘‘Analisi e Monitoraggio del Rischio Ambientale,’’ CRdC-AMRA, Naples, Italy b Istituto di Ricerche sulla Combustione, C.N.R., Naples, Italy
Abstract The flame structure of atmospheric-pressure sooting premixed flames of aliphatic and aromatic hydrocarbons with the same carbon atom number (hexane and benzene) were studied at similar temperatures and C/O ratios by sampling and chemical and spectroscopic analysis. The differences in the oxidation mechanism of hexane and benzene in fuel-rich conditions were found to produce a different chemical environment in the yield of light hydrocarbons and their relative compositions where soot inception occurs. The predominance of acetylene and simple aromatic reactants in the oxidation region of the benzene flame favoured the early appearance and steep rise of soot particles. Large formations of saturated and unsaturated hydrocarbons were observed in the main oxidation region of the hexane flame whereas a delayed formation of aromatics (mainly PAH) was observed at soot inception only after complete oxygen consumption. There are differences in soot inception mechanisms reflected by the soot structure from UV–vis spectral shapes and mass specific absorption coefficients. In the benzene flame, they appeared to be more ordered and aromatic with a narrower size of aromatic systems and/or more curved aromatic structures. By contrast, less ordering with a more complex aliphatic/aromatic structure and a larger variety of aromatic systems were found to characterize soot formed in the hexane flame. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Soot; PAH; UV–vis spectroscopy
1. Introduction The overall effect of fuel aromaticity on soot yield is well recognized and attributed to the predominant role of steps preceding soot inception with respect to the surface and other further growth steps that would not be affected by the fuel structure [1,2]. Most of the experimental work has been carried out on low-pressure premixed flames of gaseous aliphatics (mainly ethylene and acetylene) *
Corresponding author. Fax: +390815936936. E-mail address: [email protected] (A. Ciajolo).
and compared with low-pressure premixed benzene flames [2–6]. The earlier formation of soot in benzene flames was found and correlated to the larger concentration of aromatics in the main oxidation region. In the present work, the effect of fuel aliphaticity and aromaticity on soot formation has been studied by comparing for the first time atmospheric-pressure premixed flames of pre-vaporized hydrocarbons having the same carbon atom number and different structure. Hexane and benzene at similar flame temperature and C/O ratio are considered since both these parameters strongly affect sooting propensity in premixed flames [7].
1540-7489/$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2006.07.187
586
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
Composition profiles of the major and minor products of fuel-rich combustion have been measured in both hexane and benzene flames to analyze the oxidative and pyrolytic environments in which soot inception and growth occur. Speculative deductions of the effect of fuel aromaticity on soot structure have also been made by UV–vis spectroscopic analysis of soot. 2. Experimental Benzene–air [8] and n-hexane–oxygen–60% nitrogen [9] sooting laminar premixed flames (C/O = O.77, C/O = 0.8, respectively, with a cold gas velocity of 4 cm/s) were stabilized on a water cooled sintered bronze burner (d = 60 mm). The different dilutions were used to obtain similar values of the flame temperatures (1830 K maximum) [8,9]. Temperature measurements were performed using a fast-response silica-coated fine wire (25 lm) Pt/Pt–13% Rh thermocouple with a bead size of about 50 lm. A fast-insertion procedure was used [10] to avoid massive soot deposition on the thermocouple bead. Temperatures were corrected for radiative losses. Soot, condensable species and gaseous combustion products were sampled by a stainless steel, water-cooled, isokinetic probe (i.d. 2 mm). Online gas chromatography with a thermal conductivity detector and a flame ionization detector was used to analyze stable gases (O2, N2, CO, CO2, H2) and light hydrocarbons (C1–C6). Soot and condensed species were collected in a cold trap and on a Teflon filter and then extracted by dichloromethane (DCM) to separate the soluble condensed species from soot. More detail on the experimental apparatus and procedure are given elsewhere [11,12]. Soot, insoluble in DCM, was dried and weighed and then suspended in N-methyl-2-pyrrolidinone (NMP) in order to perform UV–vis spectroscopy measurements. UV–vis absorption of the condensed species dissolved in DCM and of the soot suspended in NMP was made on an HP8452A spectrophotometer using a standard 1-cm path-length quartz cell.
to use different conditions of dilution (hexane–oxygen–60% N2) and slightly different C/O ratios (C/O = 0.8 and 0.77 for hexane and benzene, respectively). In these conditions a comparison of concentration profiles could be misleading due to the different carbon concentration. In order to allow a direct comparison of flame structures the composition profiles have been reported in terms of carbon molar yields calculated on the total molar carbon. To compensate for the perturbation of the probe the carbon yield profiles are shifted upstream by few millimetres on the basis of the temperature profiles. 3.1. Carbon yields of the main products An overall description of the structures of hexane and benzene flames is given in Figs. 1 and 2 which report the axial temperature and carbon yield profiles calculated from the feed carbon, Coxy, that is CO + CO2 carbon yield and HC + Cpyr, that is light C1–C5 hydrocarbons carbon yield (HC) and heavy hydrocarbons + soot carbon yield (Cpyr). From the fuel carbon conversion and total reacted carbon, (Figs. 1 and 2) the extension of the main oxidation region in the benzene flame (up to 5 mm) compared with the hexane flame (up to 3 mm) due to the higher resistance of the benzene aromatic ring to oxidation can be noted. The profiles of CO and HC carbon yields, also reported in Figs. 1 and 2, show that they are the main contributors to Coxy and HC + Cpyr as typically found in fuel-rich flames. The lower formation of CO and HC i.e. the larger oxidation to CO2, in the benzene flame can be just due to the smaller C/O ratio. However, the complete oxidation of benzene to CO2 is also favoured because it already forms CO first [13], immediately avail-
3. Results and discussion In this work, the effect of fuel structure in terms of aliphaticity and aromaticity on soot formation has been studied for the first time by comparing atmospheric-pressure premixed flames of pre-vaporized aliphatic (hexane) and aromatic (benzene) hydrocarbons. To get a stable flame of hexane with a maximum flame temperature similar to that of a benzene–air flame (Tmax 1830 K) it was necessary
Fig. 1. Temperature profiles, conversion profiles of fuel carbon, and carbon yield profiles of total reacted carbon, Coxy, CO carbon and HC + Cpyr in the n-hexane flame.
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
587
undergo oxidative and thermal pyrolysis with formation of many smaller hydrocarbons to finally form ethylene and then acetylene. Having shown the overall structures of the hexane and benzene flames, it is convenient to confine the discussion to comparisons of HC, heavy hydrocarbons and soot (Cpyr) reporting them as carbon mole yields of each component evaluated from HC + Cpyr carbon. 3.2. Distribution of HC
Fig. 2. Temperature profiles, conversion profiles of fuel carbon, and carbon yields profiles of total reacted carbon, Coxy, CO carbon and HC + Cpyr in the benzene flame.
able for oxidation to CO2, and forms unsaturated C4–C5 hydrocarbons available for polymerization [6]. The HC + Cpyr, is largely formed in the main oxidation region of the hexane flame where it reaches a maximum followed by a significant decrease downstream of the flame (Fig. 1) in contrast with the relatively lower and later production of HC + Cpyr observed in the benzene flame (Fig. 2). This effect is due to the significant differences in the oxidation/pyrolysis paths of hexane and benzene, which shows different distributions of the HC + Cpyr components in Fig. 3 where carbon yields of HC and Cpyr are shown. Light hydrocarbons are obviously the main constituents of HC + Cpyr in both hexane and benzene flames. The carbon yield of HC is much higher in the hexane flame due to both the higher C/O ratio of the hexane flame and, more importantly, to the oxidation mechanism of aliphatic hydrocarbons that in the initial reacting steps
Fig. 3. Carbon yield profiles of total HC species carbon and total particulate carbon (Cpyr) in the n-hexane flame (open symbols) and in the benzene flame (black symbols).
The different origin, oxidative and/or pyrolytic, of HC can be inferred from the carbon contributions of the main HC components of HC + Cpyr here described. 3.2.1. Hexane flame The carbon distribution of total HC and main hydrocarbon components evaluated from HC + Cpyr carbon in the hexane flame is shown in Fig. 4. The total HC accounts for all the HC + Cpyr (100%) in the main oxidation region of the hexane flame after decreasing from the oxidation to the pyrolytic zone up to about 75% of HC + Cpyr. As the contribution of HC to HC + Cpyr decreases, the hydrocarbon distribution changes. Methane and ethylene are the primary products in the main oxidation region; ethylene is completely consumed as a typical intermediate oxidation product, whereas methane shows a slower decrease reaching a quite constant final value due to its lower reactivity. Acetylene in this flame follows the typical trend of pyrolytic species forming between oxidation and pyrolysis zones where acetylene becomes the main HC product. The later formation of diacetylene demonstrates that it is a pyrolysis product derived from acetylene dehydrogenation. 3.2.2. Benzene flame The carbon contribution of HC to HC + Cpyr remains quite constant along the flame accounting
Fig. 4. Distribution in the n-hexane flame of the total HC species, methane, ethylene, acetylene and diacetylene on the basis of the total HC + Cpyr.
588
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
for 80% of HC + Cpyr. In this case, methane remains quite constant along the whole flame accounting for less than 20% of HC + Cpyr, whereas acetylene is the main component already from the beginning of the flame since it derives from the unique oxidation mechanism of benzene [13]. Thus, the different origin of acetylene, as a typical oxidation product of the fuel rather than pyrolytic product is clearly demonstrated in the benzene flame. Also diacetylene is shown to have a different origin in the benzene flame where it is an oxidation product of benzene rather than a pyrolytic product, having a maximum near the end of the oxidation region of the benzene flame just before the acetylene peak. 3.3. Comparison of sooting behaviour of hexane and benzene flames In summary, it has been shown that both the trends and the contributions of HC to the total feed carbon (Fig. 3) as well as the relative HC distribution (Figs. 4 and 5) are different in the benzene and hexane flames since the hydrocarbons can have oxidative and/or pyrolytic origins depending on the fuel structure (aliphatic or aromatic) and on the flame region where they are formed. The different oxidation paths followed by hexane and benzene strongly affect the chemical gaseous environment in which molecular growth and homogeneous soot inception occur. The contributions of heavy hydrocarbons and soot to HC + Cpyr are reported in Fig. 6 for both flames. Soot is formed early in the main oxidation region of the benzene flame as already found in previous work [3–6,8] and the soot yield evaluated on the basis of HC + Cpyr carbon is much higher in the benzene than in the hexane flame. The more dramatic effect of the fuel on sooting behaviour in hexane and benzene flames can be noted by the relative trends of soot and heavy hydrocarbons, traditionally called condensed species from sampling [8,11,12].
Fig. 6. Distribution of condensed species and soot in the n-hexane flame (open symbols) and in the benzene flame (black symbols) on the basis of the total HC + Cpyr. The dashed lines with blank and black arrows indicate the end of the main oxidation zone of the n-hexane and benzene flames, respectively.
In the hexane flame, the formation of condensed species occurs at the end of the oxidation region and precedes soot inception. As soot contribution increases the condensed species continues to rise but with a slower rate. Thereafter, condensed species and soot yield profiles run approximately parallel and only downstream of the flame does soot formation prevail slightly over the condensed species. By contrast, the maximum formation of condensed species occurs early in the benzene flame. Thereafter a rapid decrease occurs just before soot inception reaching a very low value downstream of the flame where the soot formation process is completed. The large contribution of condensed species, mainly containing mono-ring-aromatic and polycyclic aromatic hydrocarbons, testifies to the large presence of aromatic radicals and aromatic molecules in the oxidation region of the benzene flame which contains a pool of aromatic building bricks prone to rapid growth leading, already in the oxidation zone, finally to spherical soot particles. 3.4. Soot structure as inferred by UV–vis spectroscopy
Fig. 5. Distribution in the benzene flame of the total HC species, methane, ethylene, acetylene and diacetylene on the basis of the total HC + Cpyr.
In view of the deep dissimilarities in the chemical environment which precede or accompany soot inception, UV–vis spectroscopy of hexane and benzene soot suspended in N-methyl pyrrolidinone has been carried to search for the effect of fuel on soot structure. The spectra of soot sampled at inception (young soot) and downstream of the flame (mature soot) are shown in Figs. 7 and 8 together with the spectra of condensed species for the hexane and benzene flames. The condensed species spectra exhibit the typical fine structure of two to seven-ring PAH as already
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
Fig. 7. UV–vis spectra of condensed species and soot at the inception and at the end of soot formation region in the hexane flame.
Fig. 8. UV–vis spectra of condensed species and soot at the inception and at the end of soot formation region in the benzene flame.
found in previous work [11] and do not show significant differences along the flames and between the flames. This is a signature that almost the same PAH distribution is found independently of the flame heights and of the fuel. The spectra of hexane and benzene soot exhibit an unstructured broad shape with a large absorption in the UV that mildly decreases toward the visible up to 700 nm [14]. The relative increase of the visible absorption with respect to the uv absorption, i.e. the broadening of the band width, is found in the ‘‘maturation’’ process of both hexane and benzene soot which can be associated with the rise of the aromatic system size [15] and in turn to the increase
589
in particle size. The ratio of visible to UV absorption for young soot starts at a lower value in the hexane flame relative to the benzene flame. This is interpreted as being due to the smaller size of young hexane soot relative to young benzene soot. This effect can be related to the initial phase of aromatic formation necessary for soot inception in aliphatic flames that is sudden in the benzene flame. Here, the build-up of aromatics is very fast leading due to the larger availability in the main oxidation region of the benzene flame of reactive aromatic building bricks which fall to very low values in the soot inception region (Fig. 6). The final formation of shell soot particles of relatively larger sizes and higher sphericity in the inception phase of benzene soot is foreseen. By comparing the soot spectra of hexane and benzene soot, the narrower bandwidth of benzene soot can also be observed which for planar aromatic systems corresponds to a smaller size of aromatic clusters [16]. Soot structure is generally considered to be unaffected by the fuel structure, however there have been very few, but intriguing observations of ‘‘anomalies’’ in the properties of soot produced from aliphatic and aromatic hydrocarbons. In a pioneering work of Haynes et al. [1], the lower depolarization ratio measured in benzene flames in comparison with ethylene flames has been attributed to the ‘‘intrinsic anisotropy,’’ i.e. a higher internal order of the benzene soot particles. However, in the same work the behavior of soot particles, once formed is indistinguishable in terms of number density, etc. A larger reactivity toward oxidation of soot produced from benzene pyrolysis has also been found and ascribed to the different nanostructure of benzene soot which showed a larger degree of curvature [17]. The difference in structure of soot as well as the aromatization process occurring in the hexane and benzene flames is demonstrated by the difference in the profiles of the mass specific absorption coefficients of hexane and benzene soot reported in Fig. 9. Indeed, the measurement of the specific absorption coefficient can give further information about the evolution of the aromatization process occurring along the flame by assuming that the absorption coefficient is an indicator of soot aromaticity. The absorption coefficients are usually expressed as molar specific absorption coefficients for pure compounds, but for complex mixtures of unknown molecular weight the specific absorption coefficients can be evaluated only on mass basis (m2/g). The mass absorption coefficients at 300 nm of hexane soot varies slightly from about 4 to 5 m2/ g, whereas the mass absorption coefficients of benzene soot extend in a larger range steeply increasing from 2 to 10 m2/g (upper part of Fig. 9). A similar trend is also exhibited in the visible as shown in the lower part of Fig. 9, where the
590
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
Fig. 9. Axial profiles of mass specific absorption coefficients of soot sampled in the hexane (open symbols) and benzene (black symbols) flames at 300 nm (upper part) and 500 nm (lower part).
absorption coefficients of soot at 500 nm are reported. It is worth noting that the absorption coefficients at 500 nm of soot range from 1 to 5 m2/g similar to those measured for carbon black [18], but generally lower than the values reported in literature for particulates produced from different fuels and combustion systems (7–10 m2/g) [19– 21]. The much slower and reduced increase of the absorption coefficients of hexane soot is likely due to a more complex soot formation mechanism. Indeed, soot inception and growth in the hexane flame occurs in a chemical environment rich in hydrogen, saturated and unsaturated pyrolytic hydrocarbons (Figs. 3 and 4) and large PAH (Fig. 6) which presumably together form soot particles with a relatively low aromatic content and a more disordered structure and with a larger variety of aromatic systems linked with aliphatic bridges. By contrast, the steep rise of the absorption coefficient of benzene soot up to very high values appears to reflect the more pure aromatization process occurring as soot nucleates and progressively ages in a very reactive (high oxygen and high temperature) environment poorer in hydrogen and hydrocarbon species (mainly acetylene) (Figs. 3 and 5) and richer in small aromatic reactants that produce particles with a narrower size distribution of aromatic systems. As mentioned, the narrowing of the spectral band width corresponds to a decrease of the size of aromatic clus-
ters as postulated by Robertson and Reilly [16] for graphitic planar systems. However, preliminary high resolution transmission microscopy (HRTEM) measurements of soot have shown the appearance of both bent subunits and graphene layers [22]. In the case of bent aromatic structures, the deviation from planar structures results in a lower electron density of the graphitic bond, thus the curvature increase has the same narrowing effect on the band widths observable due to the decrease of aromatic size for planar aromatic systems. Consequently, the higher absorption coefficient (Fig. 9) and the narrowing of the band width in the case of benzene soot (Fig. 8) can be attributed both to the decrease of the aromatic size unit of planar aromatic systems and to the increase of radius of curvature of bent substructures. HRTEM has also shown that benzene soot exhibits a much lower size [23] and a higher proportion of curved structures [22] compared with soot coming from aliphatic fuel flames. Hence, it can be concluded that the spectral features of the benzene soot have to be due to a different internal structure of primary particles in terms of both size and shape which in turn derives from a different soot inception mechanism. Further verification of the differences in soot structure is needed to yield valuable information about the soot inception process. 4. Conclusions The flame structures of atmospheric-pressure sooting premixed flames of aliphatic and aromatic hydrocarbons with the same carbon atom number (hexane and benzene) were studied at similar temperature and C/O ratio conditions by sampling and chemical and spectroscopic analysis. The larger formation of CO2 along with the lower formation of hydrocarbons (mainly acetylene) characterized the benzene oxidation in contrast with the large abundance of saturated and unsaturated hydrocarbons characterizing hexane oxidation. The prevalence of acetylene and simple aromatic reactants in the oxidation region of the benzene flame favours the early appearance of soot particles with respect to hexane where more time is necessary for the formation of the right mixture for soot inception (significant pool of aromatics, mainly PAH, and methane and acetylene) after oxygen consumption. The differences in soot inception mechanism is reflected in differences in the soot structure which, from UV–vis spectroscopic analysis, is much more ordered and aromatic with a narrower size of aromatic systems in the benzene flame. By contrast, there is a lower degree of ordering corresponding to more complex aliphatic/aromatic structures with a larger variety of aromatic systems in the hexane flame.
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
References [1] B.S. Haynes, H. Jander, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 84 (1980) 585–592. [2] S.J. Harris, A.M. Weiner, Combust. Sci. Technol. 38 (1984) 75–87. [3] K. Homann, H.Gg. Wagner, Proc. Combust. Inst. 11 (1967) 371–379. [4] J. Howard, J. Bittner, Structure of Sooting Flames, in: J.G. Lahaye (Ed.), Soot in combustion systems and its toxic properties, Prado Plenum Press, New York, London, 1983, pp. 57–93. [5] H. Bockhorn, F. Fetting, H.W. Wenz, Ber. Bunsenges. Phys. Chem. 87 (1983) 1067– 1073. [6] M. Brei, H. Jander, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 91 (1987) 30–36. [7] F. Takahashi, I. Glassman, Combust. Sci. Technol. 37 (1984) 1–19. [8] A. Tregrossi, A. Ciajolo, R. Barbella, Combust. Flame 117 (1999) 553–561. [9] A. Tregrossi, V. Raimo, R. Barbella, M. Alfe`, B. Apicella, in: Proceedings of the Joint Meeting of The Scandinavian-Nordic and Italian Sections of The Combustion Institute, Ischia September 18–21, Associazione Sezione Italiana del Combustion Institute, 2003, vol. I, 5.8.1. [10] J.H. Kent, H.Gg. Wagner, Combust. Sci. Technol. 41 (1984) 245–269. [11] A. Ciajolo, R. Barbella, A. Tregrossi, L. Bonfanti, Proc. Combust. Inst. 27 (1998) 1481– 1487.
591
[12] A. Ciajolo, A. D’Anna, R. Barbella, A. Tregrossi, A. Violi, Proc. Combust. Inst. 25 (1996) 2327–2333. [13] K. Brezinsky, Progress Energy Combust Sci 12 (1986) 1–24. [14] B. Apicella, M. Alfe`, R. Barbella, A. Tregrossi, A. Ciajolo, Carbon 42 (2004) 1583–1589. [15] E. Clar, Polycyclic Hydrocarbons, Academic Press, London, 1974. [16] J. Robertson, E.P. O’Reilly, Phys. Rev. B 35 (6) (1987) 2946–2957. [17] R.L. Vander Wal, A. Tomasek, Combust. Flame 136 (1–2) (2004) 129–140. [18] C. Jager, Th. Henning, R. Shlo¨gl, O. Spillecke, J. Non-Crystalline Solids 258 (1999) 161–179. [19] D.M. Roessler, F.R. Faxvog, R. Stevenson, G.W. Smith, in: D.C. Siegla, G.W. Smith (Eds.), Particulate Carbon: Formation During Combustion, Plenum Press, New York, 1981, pp. 57–85. [20] I. Colbeck, B. Atkinson, Y. Yohar, J. Aerosol Sci. 28 (5) (1987) 715–723. [21] M. Schnaiter, H. Horvath, O. Mo¨lher, K-H. Naumann, H. Saathoff, O.W. Scho¨ck, J. Aerosol Sci. 34 (2003) 1421–1444. [22] M. Alfe`, B. Apicella, R. Barbella, A. Ciajolo, P. Minutolo, L. Kennedy, W. Merchan–Merchan, A. Ciajolo, in: Proceedings of the GRICU Congress 2004, Ischia (Napoli) September 12–15, 2004. [23] M. Alfe`, B. Apicella, R. Barbella, A. Ciajolo, L. Kennedy, W. Merchan-Merchan, J.N. Rouzaud, A. Tregrossi, in: Proceedings of the 28th Meeting of The Italian Sections of The Combustion Institute, Napoli, July 4–6, 2005.
Proceedings of the Combustion Institute 31 (2007) 585–591
Combustion Institute www.elsevier.com/locate/proci
Similarities and dissimilarities in n-hexane and benzene sooting premixed flames M. Alfe` a, B. Apicella b, R. Barbella b, A. Tregrossi b, A. Ciajolo a
b,*
Centro regionale di Competenza ‘‘Analisi e Monitoraggio del Rischio Ambientale,’’ CRdC-AMRA, Naples, Italy b Istituto di Ricerche sulla Combustione, C.N.R., Naples, Italy
Abstract The flame structure of atmospheric-pressure sooting premixed flames of aliphatic and aromatic hydrocarbons with the same carbon atom number (hexane and benzene) were studied at similar temperatures and C/O ratios by sampling and chemical and spectroscopic analysis. The differences in the oxidation mechanism of hexane and benzene in fuel-rich conditions were found to produce a different chemical environment in the yield of light hydrocarbons and their relative compositions where soot inception occurs. The predominance of acetylene and simple aromatic reactants in the oxidation region of the benzene flame favoured the early appearance and steep rise of soot particles. Large formations of saturated and unsaturated hydrocarbons were observed in the main oxidation region of the hexane flame whereas a delayed formation of aromatics (mainly PAH) was observed at soot inception only after complete oxygen consumption. There are differences in soot inception mechanisms reflected by the soot structure from UV–vis spectral shapes and mass specific absorption coefficients. In the benzene flame, they appeared to be more ordered and aromatic with a narrower size of aromatic systems and/or more curved aromatic structures. By contrast, less ordering with a more complex aliphatic/aromatic structure and a larger variety of aromatic systems were found to characterize soot formed in the hexane flame. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Soot; PAH; UV–vis spectroscopy
1. Introduction The overall effect of fuel aromaticity on soot yield is well recognized and attributed to the predominant role of steps preceding soot inception with respect to the surface and other further growth steps that would not be affected by the fuel structure [1,2]. Most of the experimental work has been carried out on low-pressure premixed flames of gaseous aliphatics (mainly ethylene and acetylene) *
Corresponding author. Fax: +390815936936. E-mail address: [email protected] (A. Ciajolo).
and compared with low-pressure premixed benzene flames [2–6]. The earlier formation of soot in benzene flames was found and correlated to the larger concentration of aromatics in the main oxidation region. In the present work, the effect of fuel aliphaticity and aromaticity on soot formation has been studied by comparing for the first time atmospheric-pressure premixed flames of pre-vaporized hydrocarbons having the same carbon atom number and different structure. Hexane and benzene at similar flame temperature and C/O ratio are considered since both these parameters strongly affect sooting propensity in premixed flames [7].
1540-7489/$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2006.07.187
586
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
Composition profiles of the major and minor products of fuel-rich combustion have been measured in both hexane and benzene flames to analyze the oxidative and pyrolytic environments in which soot inception and growth occur. Speculative deductions of the effect of fuel aromaticity on soot structure have also been made by UV–vis spectroscopic analysis of soot. 2. Experimental Benzene–air [8] and n-hexane–oxygen–60% nitrogen [9] sooting laminar premixed flames (C/O = O.77, C/O = 0.8, respectively, with a cold gas velocity of 4 cm/s) were stabilized on a water cooled sintered bronze burner (d = 60 mm). The different dilutions were used to obtain similar values of the flame temperatures (1830 K maximum) [8,9]. Temperature measurements were performed using a fast-response silica-coated fine wire (25 lm) Pt/Pt–13% Rh thermocouple with a bead size of about 50 lm. A fast-insertion procedure was used [10] to avoid massive soot deposition on the thermocouple bead. Temperatures were corrected for radiative losses. Soot, condensable species and gaseous combustion products were sampled by a stainless steel, water-cooled, isokinetic probe (i.d. 2 mm). Online gas chromatography with a thermal conductivity detector and a flame ionization detector was used to analyze stable gases (O2, N2, CO, CO2, H2) and light hydrocarbons (C1–C6). Soot and condensed species were collected in a cold trap and on a Teflon filter and then extracted by dichloromethane (DCM) to separate the soluble condensed species from soot. More detail on the experimental apparatus and procedure are given elsewhere [11,12]. Soot, insoluble in DCM, was dried and weighed and then suspended in N-methyl-2-pyrrolidinone (NMP) in order to perform UV–vis spectroscopy measurements. UV–vis absorption of the condensed species dissolved in DCM and of the soot suspended in NMP was made on an HP8452A spectrophotometer using a standard 1-cm path-length quartz cell.
to use different conditions of dilution (hexane–oxygen–60% N2) and slightly different C/O ratios (C/O = 0.8 and 0.77 for hexane and benzene, respectively). In these conditions a comparison of concentration profiles could be misleading due to the different carbon concentration. In order to allow a direct comparison of flame structures the composition profiles have been reported in terms of carbon molar yields calculated on the total molar carbon. To compensate for the perturbation of the probe the carbon yield profiles are shifted upstream by few millimetres on the basis of the temperature profiles. 3.1. Carbon yields of the main products An overall description of the structures of hexane and benzene flames is given in Figs. 1 and 2 which report the axial temperature and carbon yield profiles calculated from the feed carbon, Coxy, that is CO + CO2 carbon yield and HC + Cpyr, that is light C1–C5 hydrocarbons carbon yield (HC) and heavy hydrocarbons + soot carbon yield (Cpyr). From the fuel carbon conversion and total reacted carbon, (Figs. 1 and 2) the extension of the main oxidation region in the benzene flame (up to 5 mm) compared with the hexane flame (up to 3 mm) due to the higher resistance of the benzene aromatic ring to oxidation can be noted. The profiles of CO and HC carbon yields, also reported in Figs. 1 and 2, show that they are the main contributors to Coxy and HC + Cpyr as typically found in fuel-rich flames. The lower formation of CO and HC i.e. the larger oxidation to CO2, in the benzene flame can be just due to the smaller C/O ratio. However, the complete oxidation of benzene to CO2 is also favoured because it already forms CO first [13], immediately avail-
3. Results and discussion In this work, the effect of fuel structure in terms of aliphaticity and aromaticity on soot formation has been studied for the first time by comparing atmospheric-pressure premixed flames of pre-vaporized aliphatic (hexane) and aromatic (benzene) hydrocarbons. To get a stable flame of hexane with a maximum flame temperature similar to that of a benzene–air flame (Tmax 1830 K) it was necessary
Fig. 1. Temperature profiles, conversion profiles of fuel carbon, and carbon yield profiles of total reacted carbon, Coxy, CO carbon and HC + Cpyr in the n-hexane flame.
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
587
undergo oxidative and thermal pyrolysis with formation of many smaller hydrocarbons to finally form ethylene and then acetylene. Having shown the overall structures of the hexane and benzene flames, it is convenient to confine the discussion to comparisons of HC, heavy hydrocarbons and soot (Cpyr) reporting them as carbon mole yields of each component evaluated from HC + Cpyr carbon. 3.2. Distribution of HC
Fig. 2. Temperature profiles, conversion profiles of fuel carbon, and carbon yields profiles of total reacted carbon, Coxy, CO carbon and HC + Cpyr in the benzene flame.
able for oxidation to CO2, and forms unsaturated C4–C5 hydrocarbons available for polymerization [6]. The HC + Cpyr, is largely formed in the main oxidation region of the hexane flame where it reaches a maximum followed by a significant decrease downstream of the flame (Fig. 1) in contrast with the relatively lower and later production of HC + Cpyr observed in the benzene flame (Fig. 2). This effect is due to the significant differences in the oxidation/pyrolysis paths of hexane and benzene, which shows different distributions of the HC + Cpyr components in Fig. 3 where carbon yields of HC and Cpyr are shown. Light hydrocarbons are obviously the main constituents of HC + Cpyr in both hexane and benzene flames. The carbon yield of HC is much higher in the hexane flame due to both the higher C/O ratio of the hexane flame and, more importantly, to the oxidation mechanism of aliphatic hydrocarbons that in the initial reacting steps
Fig. 3. Carbon yield profiles of total HC species carbon and total particulate carbon (Cpyr) in the n-hexane flame (open symbols) and in the benzene flame (black symbols).
The different origin, oxidative and/or pyrolytic, of HC can be inferred from the carbon contributions of the main HC components of HC + Cpyr here described. 3.2.1. Hexane flame The carbon distribution of total HC and main hydrocarbon components evaluated from HC + Cpyr carbon in the hexane flame is shown in Fig. 4. The total HC accounts for all the HC + Cpyr (100%) in the main oxidation region of the hexane flame after decreasing from the oxidation to the pyrolytic zone up to about 75% of HC + Cpyr. As the contribution of HC to HC + Cpyr decreases, the hydrocarbon distribution changes. Methane and ethylene are the primary products in the main oxidation region; ethylene is completely consumed as a typical intermediate oxidation product, whereas methane shows a slower decrease reaching a quite constant final value due to its lower reactivity. Acetylene in this flame follows the typical trend of pyrolytic species forming between oxidation and pyrolysis zones where acetylene becomes the main HC product. The later formation of diacetylene demonstrates that it is a pyrolysis product derived from acetylene dehydrogenation. 3.2.2. Benzene flame The carbon contribution of HC to HC + Cpyr remains quite constant along the flame accounting
Fig. 4. Distribution in the n-hexane flame of the total HC species, methane, ethylene, acetylene and diacetylene on the basis of the total HC + Cpyr.
588
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
for 80% of HC + Cpyr. In this case, methane remains quite constant along the whole flame accounting for less than 20% of HC + Cpyr, whereas acetylene is the main component already from the beginning of the flame since it derives from the unique oxidation mechanism of benzene [13]. Thus, the different origin of acetylene, as a typical oxidation product of the fuel rather than pyrolytic product is clearly demonstrated in the benzene flame. Also diacetylene is shown to have a different origin in the benzene flame where it is an oxidation product of benzene rather than a pyrolytic product, having a maximum near the end of the oxidation region of the benzene flame just before the acetylene peak. 3.3. Comparison of sooting behaviour of hexane and benzene flames In summary, it has been shown that both the trends and the contributions of HC to the total feed carbon (Fig. 3) as well as the relative HC distribution (Figs. 4 and 5) are different in the benzene and hexane flames since the hydrocarbons can have oxidative and/or pyrolytic origins depending on the fuel structure (aliphatic or aromatic) and on the flame region where they are formed. The different oxidation paths followed by hexane and benzene strongly affect the chemical gaseous environment in which molecular growth and homogeneous soot inception occur. The contributions of heavy hydrocarbons and soot to HC + Cpyr are reported in Fig. 6 for both flames. Soot is formed early in the main oxidation region of the benzene flame as already found in previous work [3–6,8] and the soot yield evaluated on the basis of HC + Cpyr carbon is much higher in the benzene than in the hexane flame. The more dramatic effect of the fuel on sooting behaviour in hexane and benzene flames can be noted by the relative trends of soot and heavy hydrocarbons, traditionally called condensed species from sampling [8,11,12].
Fig. 6. Distribution of condensed species and soot in the n-hexane flame (open symbols) and in the benzene flame (black symbols) on the basis of the total HC + Cpyr. The dashed lines with blank and black arrows indicate the end of the main oxidation zone of the n-hexane and benzene flames, respectively.
In the hexane flame, the formation of condensed species occurs at the end of the oxidation region and precedes soot inception. As soot contribution increases the condensed species continues to rise but with a slower rate. Thereafter, condensed species and soot yield profiles run approximately parallel and only downstream of the flame does soot formation prevail slightly over the condensed species. By contrast, the maximum formation of condensed species occurs early in the benzene flame. Thereafter a rapid decrease occurs just before soot inception reaching a very low value downstream of the flame where the soot formation process is completed. The large contribution of condensed species, mainly containing mono-ring-aromatic and polycyclic aromatic hydrocarbons, testifies to the large presence of aromatic radicals and aromatic molecules in the oxidation region of the benzene flame which contains a pool of aromatic building bricks prone to rapid growth leading, already in the oxidation zone, finally to spherical soot particles. 3.4. Soot structure as inferred by UV–vis spectroscopy
Fig. 5. Distribution in the benzene flame of the total HC species, methane, ethylene, acetylene and diacetylene on the basis of the total HC + Cpyr.
In view of the deep dissimilarities in the chemical environment which precede or accompany soot inception, UV–vis spectroscopy of hexane and benzene soot suspended in N-methyl pyrrolidinone has been carried to search for the effect of fuel on soot structure. The spectra of soot sampled at inception (young soot) and downstream of the flame (mature soot) are shown in Figs. 7 and 8 together with the spectra of condensed species for the hexane and benzene flames. The condensed species spectra exhibit the typical fine structure of two to seven-ring PAH as already
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
Fig. 7. UV–vis spectra of condensed species and soot at the inception and at the end of soot formation region in the hexane flame.
Fig. 8. UV–vis spectra of condensed species and soot at the inception and at the end of soot formation region in the benzene flame.
found in previous work [11] and do not show significant differences along the flames and between the flames. This is a signature that almost the same PAH distribution is found independently of the flame heights and of the fuel. The spectra of hexane and benzene soot exhibit an unstructured broad shape with a large absorption in the UV that mildly decreases toward the visible up to 700 nm [14]. The relative increase of the visible absorption with respect to the uv absorption, i.e. the broadening of the band width, is found in the ‘‘maturation’’ process of both hexane and benzene soot which can be associated with the rise of the aromatic system size [15] and in turn to the increase
589
in particle size. The ratio of visible to UV absorption for young soot starts at a lower value in the hexane flame relative to the benzene flame. This is interpreted as being due to the smaller size of young hexane soot relative to young benzene soot. This effect can be related to the initial phase of aromatic formation necessary for soot inception in aliphatic flames that is sudden in the benzene flame. Here, the build-up of aromatics is very fast leading due to the larger availability in the main oxidation region of the benzene flame of reactive aromatic building bricks which fall to very low values in the soot inception region (Fig. 6). The final formation of shell soot particles of relatively larger sizes and higher sphericity in the inception phase of benzene soot is foreseen. By comparing the soot spectra of hexane and benzene soot, the narrower bandwidth of benzene soot can also be observed which for planar aromatic systems corresponds to a smaller size of aromatic clusters [16]. Soot structure is generally considered to be unaffected by the fuel structure, however there have been very few, but intriguing observations of ‘‘anomalies’’ in the properties of soot produced from aliphatic and aromatic hydrocarbons. In a pioneering work of Haynes et al. [1], the lower depolarization ratio measured in benzene flames in comparison with ethylene flames has been attributed to the ‘‘intrinsic anisotropy,’’ i.e. a higher internal order of the benzene soot particles. However, in the same work the behavior of soot particles, once formed is indistinguishable in terms of number density, etc. A larger reactivity toward oxidation of soot produced from benzene pyrolysis has also been found and ascribed to the different nanostructure of benzene soot which showed a larger degree of curvature [17]. The difference in structure of soot as well as the aromatization process occurring in the hexane and benzene flames is demonstrated by the difference in the profiles of the mass specific absorption coefficients of hexane and benzene soot reported in Fig. 9. Indeed, the measurement of the specific absorption coefficient can give further information about the evolution of the aromatization process occurring along the flame by assuming that the absorption coefficient is an indicator of soot aromaticity. The absorption coefficients are usually expressed as molar specific absorption coefficients for pure compounds, but for complex mixtures of unknown molecular weight the specific absorption coefficients can be evaluated only on mass basis (m2/g). The mass absorption coefficients at 300 nm of hexane soot varies slightly from about 4 to 5 m2/ g, whereas the mass absorption coefficients of benzene soot extend in a larger range steeply increasing from 2 to 10 m2/g (upper part of Fig. 9). A similar trend is also exhibited in the visible as shown in the lower part of Fig. 9, where the
590
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
Fig. 9. Axial profiles of mass specific absorption coefficients of soot sampled in the hexane (open symbols) and benzene (black symbols) flames at 300 nm (upper part) and 500 nm (lower part).
absorption coefficients of soot at 500 nm are reported. It is worth noting that the absorption coefficients at 500 nm of soot range from 1 to 5 m2/g similar to those measured for carbon black [18], but generally lower than the values reported in literature for particulates produced from different fuels and combustion systems (7–10 m2/g) [19– 21]. The much slower and reduced increase of the absorption coefficients of hexane soot is likely due to a more complex soot formation mechanism. Indeed, soot inception and growth in the hexane flame occurs in a chemical environment rich in hydrogen, saturated and unsaturated pyrolytic hydrocarbons (Figs. 3 and 4) and large PAH (Fig. 6) which presumably together form soot particles with a relatively low aromatic content and a more disordered structure and with a larger variety of aromatic systems linked with aliphatic bridges. By contrast, the steep rise of the absorption coefficient of benzene soot up to very high values appears to reflect the more pure aromatization process occurring as soot nucleates and progressively ages in a very reactive (high oxygen and high temperature) environment poorer in hydrogen and hydrocarbon species (mainly acetylene) (Figs. 3 and 5) and richer in small aromatic reactants that produce particles with a narrower size distribution of aromatic systems. As mentioned, the narrowing of the spectral band width corresponds to a decrease of the size of aromatic clus-
ters as postulated by Robertson and Reilly [16] for graphitic planar systems. However, preliminary high resolution transmission microscopy (HRTEM) measurements of soot have shown the appearance of both bent subunits and graphene layers [22]. In the case of bent aromatic structures, the deviation from planar structures results in a lower electron density of the graphitic bond, thus the curvature increase has the same narrowing effect on the band widths observable due to the decrease of aromatic size for planar aromatic systems. Consequently, the higher absorption coefficient (Fig. 9) and the narrowing of the band width in the case of benzene soot (Fig. 8) can be attributed both to the decrease of the aromatic size unit of planar aromatic systems and to the increase of radius of curvature of bent substructures. HRTEM has also shown that benzene soot exhibits a much lower size [23] and a higher proportion of curved structures [22] compared with soot coming from aliphatic fuel flames. Hence, it can be concluded that the spectral features of the benzene soot have to be due to a different internal structure of primary particles in terms of both size and shape which in turn derives from a different soot inception mechanism. Further verification of the differences in soot structure is needed to yield valuable information about the soot inception process. 4. Conclusions The flame structures of atmospheric-pressure sooting premixed flames of aliphatic and aromatic hydrocarbons with the same carbon atom number (hexane and benzene) were studied at similar temperature and C/O ratio conditions by sampling and chemical and spectroscopic analysis. The larger formation of CO2 along with the lower formation of hydrocarbons (mainly acetylene) characterized the benzene oxidation in contrast with the large abundance of saturated and unsaturated hydrocarbons characterizing hexane oxidation. The prevalence of acetylene and simple aromatic reactants in the oxidation region of the benzene flame favours the early appearance of soot particles with respect to hexane where more time is necessary for the formation of the right mixture for soot inception (significant pool of aromatics, mainly PAH, and methane and acetylene) after oxygen consumption. The differences in soot inception mechanism is reflected in differences in the soot structure which, from UV–vis spectroscopic analysis, is much more ordered and aromatic with a narrower size of aromatic systems in the benzene flame. By contrast, there is a lower degree of ordering corresponding to more complex aliphatic/aromatic structures with a larger variety of aromatic systems in the hexane flame.
M. Alfe` et al. / Proceedings of the Combustion Institute 31 (2007) 585–591
References [1] B.S. Haynes, H. Jander, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 84 (1980) 585–592. [2] S.J. Harris, A.M. Weiner, Combust. Sci. Technol. 38 (1984) 75–87. [3] K. Homann, H.Gg. Wagner, Proc. Combust. Inst. 11 (1967) 371–379. [4] J. Howard, J. Bittner, Structure of Sooting Flames, in: J.G. Lahaye (Ed.), Soot in combustion systems and its toxic properties, Prado Plenum Press, New York, London, 1983, pp. 57–93. [5] H. Bockhorn, F. Fetting, H.W. Wenz, Ber. Bunsenges. Phys. Chem. 87 (1983) 1067– 1073. [6] M. Brei, H. Jander, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 91 (1987) 30–36. [7] F. Takahashi, I. Glassman, Combust. Sci. Technol. 37 (1984) 1–19. [8] A. Tregrossi, A. Ciajolo, R. Barbella, Combust. Flame 117 (1999) 553–561. [9] A. Tregrossi, V. Raimo, R. Barbella, M. Alfe`, B. Apicella, in: Proceedings of the Joint Meeting of The Scandinavian-Nordic and Italian Sections of The Combustion Institute, Ischia September 18–21, Associazione Sezione Italiana del Combustion Institute, 2003, vol. I, 5.8.1. [10] J.H. Kent, H.Gg. Wagner, Combust. Sci. Technol. 41 (1984) 245–269. [11] A. Ciajolo, R. Barbella, A. Tregrossi, L. Bonfanti, Proc. Combust. Inst. 27 (1998) 1481– 1487.
591
[12] A. Ciajolo, A. D’Anna, R. Barbella, A. Tregrossi, A. Violi, Proc. Combust. Inst. 25 (1996) 2327–2333. [13] K. Brezinsky, Progress Energy Combust Sci 12 (1986) 1–24. [14] B. Apicella, M. Alfe`, R. Barbella, A. Tregrossi, A. Ciajolo, Carbon 42 (2004) 1583–1589. [15] E. Clar, Polycyclic Hydrocarbons, Academic Press, London, 1974. [16] J. Robertson, E.P. O’Reilly, Phys. Rev. B 35 (6) (1987) 2946–2957. [17] R.L. Vander Wal, A. Tomasek, Combust. Flame 136 (1–2) (2004) 129–140. [18] C. Jager, Th. Henning, R. Shlo¨gl, O. Spillecke, J. Non-Crystalline Solids 258 (1999) 161–179. [19] D.M. Roessler, F.R. Faxvog, R. Stevenson, G.W. Smith, in: D.C. Siegla, G.W. Smith (Eds.), Particulate Carbon: Formation During Combustion, Plenum Press, New York, 1981, pp. 57–85. [20] I. Colbeck, B. Atkinson, Y. Yohar, J. Aerosol Sci. 28 (5) (1987) 715–723. [21] M. Schnaiter, H. Horvath, O. Mo¨lher, K-H. Naumann, H. Saathoff, O.W. Scho¨ck, J. Aerosol Sci. 34 (2003) 1421–1444. [22] M. Alfe`, B. Apicella, R. Barbella, A. Ciajolo, P. Minutolo, L. Kennedy, W. Merchan–Merchan, A. Ciajolo, in: Proceedings of the GRICU Congress 2004, Ischia (Napoli) September 12–15, 2004. [23] M. Alfe`, B. Apicella, R. Barbella, A. Ciajolo, L. Kennedy, W. Merchan-Merchan, J.N. Rouzaud, A. Tregrossi, in: Proceedings of the 28th Meeting of The Italian Sections of The Combustion Institute, Napoli, July 4–6, 2005.