Combustion and Flame 157 (2010) 1959–1965

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Combustion and Flame

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The effect of temperature on soot properties in premixed methane flames

M. Alfè a,*, B. Apicella a, J.-N. Rouzaud b, A. Tregrossi a, A. Ciajolo a

a Istituto di Ricerche sulla Combustione, C.N.R., Napoli, Italyb Laboratoire de Géologie, Ecole Normale Supérieure, UMR CNRS 8538, 24 Rue Lhomond, 75231 Paris Cedex 05, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 December 2009Received in revised form 25 January 2010Accepted 9 February 2010Available online 4 March 2010

Keywords:SootPremixed flamesTEM

0010-2180/$ - see front matter � 2010 The Combustdoi:10.1016/j.combustflame.2010.02.007

* Corresponding author. Address: Istituto di RicercP.le Tecchio, 80, 80125 Napoli, Italy. Fax: +39 081 59

E-mail address: alfe@irc.cnr.it (M. Alfè).

The effect of flame temperature on soot properties was studied in premixed methane/oxygen flamesburning at a constant mixture composition (C/O = 0.60, U = 2.4) and different cold-gas flow velocities(4 and 5 cm s�1). Temperature and concentration profiles of stable gases and condensed phases combus-tion products were measured along the flame axis. It was found that the high flame temperature condi-tions cause a larger decomposition of methane into hydrogen and C2–C4 hydrocarbons, thereby reducingthe formation of benzene and condensed phases including condensed species and soot. Soot propertieswere studied by UV–Visible absorption spectroscopy, thermogravimetry and H/C elemental analysis. Adescription of soot nanostructural organization was also performed by means of high-resolution trans-mission electron microscopy. Different properties and nanostructures were found to develop in the soot,depending on the temperature and on soot aging associated. Soot dehydrogenation occurred to a largerextent in the high flame temperature conditions. As soot dehydrogenates the mass absorption coeffi-cients of soot exhibited an increasing trend along the flame axis. However, mature soot retained a rela-tively high H/C ratio and low absorption coefficients with respect to other less hydrogenated fuels even inhigh temperature conditions. This indicates that the aromatization/dehydrogenation of soot in premixedflames is more dependent on the fuel characteristics rather than on the flame temperature.

Generally, it was assessed that mature soot produced from diverse hydrocarbon fuels with similarflame temperatures and flame types possess a different chemical composition and structure. To thisregard the H/C atomic ratio and mass absorption coefficients appeared to be signatures of soot propertiesand structural evolution.

� 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

The soot size distribution and the relationship between mor-phology, H/C atomic ratio, oxidation reactivity and aromaticity ofyoung and mature soot sampled in the inception and in the post-oxidation zone of fuel-rich premixed laminar flames of diversehydrocarbon fuels has been recently studied [1,2]. Soot propertieshave shown to strongly depend on the soot aging process and somesignificant differences related to the fuel used have been observed.In particular, it has been found that soot having low hydrogen con-tent, higher aromaticity and structural order is rapidly produced byburning cyclohexane and benzene fuels [2] and attributed to thehigher tendency of these fuels to form aromatic precursors alreadyin the flame oxidation region. It was also noteworthy that thedehydrogenation and carbonization processes associated to thepassage from young to mature soot has shown to be much less evi-dent in the methane flame.

ion Institute. Published by Elsevier

he sulla Combustione, C.N.R.,36936.

These findings suggested that fuel characteristics strongly affectthe soot structure and properties. However, it has to be underlinedthat the results have been obtained in different C/O ratio and flametemperature conditions that are known to affect both soot loadingas well as the distribution of the combustion products (includinggaseous and condensed phases) [3,4]. It has been argued that theeffect of flame temperature has to be considered to verify the influ-ence of the fuel characteristics on soot properties (see B. Haynescomment and authors reply to [2] in Ref. [5]).

To the aim of clarifying this aspect, the effect of the temperatureon the chemical structures of the flame and on the characteristicsof soot produced along the axis of methane flames burning at thesame C/O ratio (0.60) and different cold-gas velocity (4 and5 cm s�1) has been studied in this work.

The chemical structures of the methane flames have been stud-ied by measuring along the flame axis the temperature and con-centrations profiles of gaseous species and condensed phases.Soot sampled along the flames and separated from the condensedphases by means of dichloromethane (DCM) extraction has beenanalyzed by high-resolution transmission electron microscopy(HRTEM), UV–Visible absorption spectroscopy, thermogravimetricand elemental analysis.

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Fig. 1. Temperature and concentration profiles of reactants and main combustionproducts measured in methane flames at v = 4 cm s�1 (upper panel) and atv = 5 cm s�1 (lower panel).

1960 M. Alfè et al. / Combustion and Flame 157 (2010) 1959–1965

2. Experimental

Combustion products were isokinetically sampled and quanti-fied along the axis of two premixed fuel-rich laminar flat flamesstabilized on a commercial McKenna burner (Holthuis & Associ-ated, Sebastopol, CA) at atmospheric pressure burning methane/O2 at constant mixture composition (54.55/45.45, C/O = 0.60,U = 2.4) and different cold-gas flow velocities (4 and 5 cm s�1). De-tails of the experimental combustion system have been describedelsewhere [6].

Flame temperature was measured with a fast-response thermo-couple (silica-coated 25 lm Pt/Pt-13%Rh) by using a fast-insertionprocedure and corrected for radiative losses. The uncertainty of themeasured temperatures was estimated to be as high as 100 K.

Combustion products were collected along the flame axis bymeans of a stainless-steel water-cooled probe vertically insertedin the flame. Stable C1–C6 hydrocarbons were analyzed on-line tothe sampling probe by an HP5890A gas chromatograph equippedwith 7515 Chrompack Al2O3/KCl capillary column and flame ioni-zation detector (FID); CO2, CO, H2, N2 were analyzed on aHP5700A gas chromatograph with an 8700 Altech coaxial columnand thermoconductibility detector (TCD).

Condensed phases collected on the probe wall, on a Teflon filterand in an ice-cooled trap placed along the sampling line, were ex-tracted with DCM to separate the condensed species from theinsoluble carbonaceous material (soot).

Condensed species were analyzed by means of gas chromatog-raphy/mass spectrometry (GC–MS) for the quantification of poly-cyclic aromatic hydrocarbons (PAH) up to 300 u. GC–MS wasperformed on an HP5890 gas chromatograph equipped with anHP-5MS crosslinked 5% PhMe siloxane 30 m � 0.25 mm � 0.25 lmfilm thickness column coupled with an HP5989A mass spectrome-ter with an electron impact/chemical ionization ion source.

H/C atomic ratio of soot was measured by a Perkin-Elmer 2400CHNSO elemental analyzer.

Thermogravimetric analyses (TG) of soot were performed on aPerkin-Elmer Pyris 1 TGA thermogravimetric analyzer by heatingthe soot in oxidative environment (air, 30 ml min�1) from 30 �Cup to 850 �C at a rate of 10 �C min�1.

UV–Visible spectra of soot, dissolved/suspended in N-meth-ylpyrrolidinone (NMP) by means of ultrasonic agitation, were ac-quired on an HP 8452A Diode Array spectrophotometer in the200–800 nm wavelength range by using 1 cm path-length quartzcuvette.

TEM images were acquired on a JEOL 2011 electron microscopeoperating at 200 kV using LaB6 filament (resolution in the latticefringe mode 0.144 nm). Soot, dispersed by ultrasonic agitation inethanol, was placed on the electron microscope TEM grid (20-nmthick carbon lacey coated Cu grids, Spi Supplies). The soot nano-structural organization was analyzed at magnifications of500,000�. In these images, each fringe represents the profile of apolyaromatic layer, i.e. of a graphene plane; the pixel size is0.01384 nm. Each image was analyzed by reducing the backgroundnoise and subsequently by transforming the image in a binary one.Then, this binary image was skeletonized: each fringe being onepixel-large. Each fringe can be analyzed individually by a lab-madesoftware [7,8]. Fringes shorter than an aromatic ring (i.e. 0.246 nm)are first eliminated, since without physical sense. Then, the lengthof all the fringes, L, i.e. the extent of the aromatic layers, was indi-vidually measured, by considering that a layer could be curved un-til to a distortion ratio arbitrarily fixed at 40%. Coherent domains,defined as stacks of N parallel planes, also referred as BSU (basicstructural units), as defined by Oberlin and Thrower [9] are ana-lyzed. The planes are considered stacked when the fringe disorien-tation is lower than 15% and when the interfringe distance lower

than 0.6 nm (above this value the Van der Waals interactions areassumed to be negligible). Our software provides the percentageof non-stacked layers (nsl) and the domain characteristics: numberN of stacked layers, their diameter La and height Lc, and the meaninterlayer spacing d. The error on the structural parameters de-pends on the pixel size (<0.15 nm in our case).

3. Results and discussion

3.1. Gas and condensed-phases concentration profiles

The concentrations of reactants and main combustion products(gas and condensed phases) measured in fuel-rich methane/O2

flames (C/O = 0.60) burning at two different cold-gases flow veloc-ities (4 and 5 cm s�1), i.e. at two different flame temperatures, arereported in Figs. 1–6. The lines reported in these figures are justused to follow the experimental data trends. To account for probeeffects, the concentration profiles were shifted placing the maxi-mum of CO formation rate in correspondence with the maximumtemperature. The shift of the concentration profiles was 3.3 mmand 2.5 mm upstream for v = 4 cm s�1and v = 5 cm s�1 flames,respectively.

The maximum temperature, measured at 2.2 mm of height abovethe burner, was 1650 K for the low-temperature flame (v = 4 cm s�1)and 1770 K for the high-temperature flame (v = 5 cm s�1) as shownin Fig. 1 where temperature and concentration profiles of the reac-tants and main combustion products are reported. The completeoxygen consumption and the maximum formation of the main com-bustion products (CO, CO2 H2) occur at about 3.5 mm height aboveburner (HAB) for both the flames.

The main difference in the chemical structure of the flames con-sists in the larger CH4 consumption in the high-temperature flame(v = 5 cm s�1) occurring at the end of the main oxidation region.

Fig. 2. Concentration profiles of ethylene and acetylene measured in methaneflames at v = 4 cm s�1 (upper panel) and at v = 5 cm s�1 (lower panel).

Fig. 4. Concentration profiles of the most abundant C3 (upper panel) and C4

hydrocarbons (lower panel) measured in the methane flame at v = 5 cm s�1.

M. Alfè et al. / Combustion and Flame 157 (2010) 1959–1965 1961

The consumption increase of methane for effect of the higher tem-perature corresponds to the higher formation of typical thermaldecomposition products as H2 (Fig. 1) and C2H2 (Fig. 2) for bothflames.

In the main oxidation zone of both flames the C2H4 profiles, alsoreported in Fig. 2, present the typical rise-decay trend of reaction

Fig. 3. Concentration profiles of the most abundant C3 (upper panel) and C4

hydrocarbons (lower panel) measured in the methane flame at v = 4 cm s�1.

intermediates being readily consumed and dehydrogenated toacetylene that reaches the maximum concentration later on.

The concentration profiles of minor C3 and C4 hydrocarbons arereported in Figs. 3 and 4 for both flames. C3 and C4 hydrocarbonsare mainly composed of unsaturated hydrocarbons as propyne,

Fig. 5. Concentration profiles of cyclopentadiene and benzene measured in themethane flames at v = 4 cm s�1 (upper panel) and at v = 5 cm s�1 (lower panel).

Fig. 6. Concentration profiles of GC-PAH, CS and soot measured in the methaneflames at v = 4 cm s�1 (upper panel) and at v = 5 cm s�1 (lower panel).

Fig. 7. Temperature of the maximum combustion rate and H/C atomic ratio ofmethane soot sampled in the low-temperature flame, v = 4 cm s�1 (d), and in thehigh-temperature flame, v = 5 cm s�1 (s).

1962 M. Alfè et al. / Combustion and Flame 157 (2010) 1959–1965

propene, 1,2-propadiene, butadiene, diacetylene and 1-butene-3-yne. Propene, 1,2-propadiene, 1-butyne and butadiene are formedin the oxidation zone and readily destroyed at the end of the oxi-dation region. Propyne, 1-butene-3-yne and diacetylene areformed later on persisting in the pyrolysis region of the flame. Gen-erally, the concentration levels of C3–C4 hydrocarbons do not showsignificant differences. However, it is noteworthy that in the low-temperature flame (v = 4 cm s�1) the most abundant C3 hydrocar-bon, propyne, remains at higher concentration levels downstreamof the low-temperature flame. Just the larger concentration of pro-pyne as well as the larger concentration of unburned methane(Fig. 1) are responsible for the continuous and larger formationof benzene downstream of the low-temperature flame. This canbe seen in Fig. 5 where the concentration profiles of benzene andcyclopentadiene are reported together for both flames. In thehigh-temperature flame where methane is more consumed(Fig. 1), the benzene concentration reaches a quite constant valuejust downstream of the main oxidation zone. In both flames, cyclo-pentadiene presents a rise-decay profile in the main oxidation re-gion peaked before the maximum formation rate of benzene. Thisis consistent with the cyclopentadiene role in benzene oxidationinvolving phenoxy formation and further decomposition in C5H5

and CO [10]. The role of cyclopentadiene is also considered innaphthalene formation process through the self-combination ofresonantly stabilized cyclopentadienyl radicals [11–13].

The concentration profiles of condensed phases in terms of sootand condensed species are reported in Fig. 6. In agreement with pre-vious work [4], a larger formation of both condensed species andsoot can be observed in the low-temperature flame (v = 4 cm s�1)associated with the larger formation of benzene (Fig. 5) and to thelower formation of acetylene and diacetylene (Figs. 2–4).

Overall, it can be assessed that the higher flame temperature fa-vors the formation of hydrogen, acetylene and diacetylene therebyreducing the formation of benzene and condensed phases. It is thusevident that the formation of condensed phases in methane flamesis strictly correlated to the benzene trend (Fig. 5) [14]. The analysis

of condensed species showed the presence of PAH up to 300 u ana-lyzed by GC–MS (GC-PAH) (Fig. 6) and of aromatic species up to800–1000 u identified by size exclusion chromatography and laserdesorption mass spectrometry [15–17].

3.2. Soot characterization

The H/C atomic ratios of soot sampled along the flames are re-ported in Fig. 7. The H/C atomic ratios of soot are lower in the high-temperature flame and decrease along the axis of both flames asconsequence of the dehydrogenation process associated to the sootaging process. Soot carbonization occurs to a larger degree in thehigher temperature conditions consistently with the higher forma-tion of hydrogen, acetylene and diacetylene observed in the high-temperature flame (Figs. 1 and 2).

Thermogravimetric analysis of soot in air flow gives out thetemperature of the maximum combustion rate (i.e. the tempera-ture to which the maximum weight loss occurs). This temperature,considered as indicator of the soot oxidation reactivity, is also re-ported in Fig. 7. In a previous work, it has been shown that the tem-perature of the maximum combustion rate of methane soot islower with respect to that measured for soot produced from otherhydrocarbon fuels [2]. This has been found consistent with thehigher H/C ratio of methane soot. In both temperature conditionsan increase along the flame of the maximum combustion temper-ature from 620 to 660 �C, is representative of the decrease of sootoxidation reactivity during the soot formation/maturation processin correspondence of the H/C ratio decrease (Fig. 7). However, itis noteworthy that soot formed in the high temperature conditions,in spite of the lower H/C ratio, does not exhibit a significant loweroxidation reactivity with respect to soot formed in low tempera-ture conditions. This indicates that there is not a unique correlationbetween oxidation reactivity and H/C ratio, but other effects possi-bly related to the soot morphology and/or structure should be con-sidered as suggested in the following structural analysis.

The UV–Visible absorption is a useful indicator of the internalcarbon structure of carbonaceous materials. In previous studyUV–Visible spectra of soot caught on quartz plates inserted inthe same methane flames have been measured [18]. By means ofTauc analysis [18,19], three classes of absorbers having a differentoptical gap, Eg, indicative of different aromatic island sizes havebeen detected. The decrease of the optical gap of the class havingthe lower Eg, (from 1 to 0.5 eV), i.e. the larger aromatic island size,has shown to be correlated with the decrease of the soot H/C ratio.This demonstrates that aromatization occurs in correspondence ofsoot dehydrogenation. Moreover, minor changes of the maximum

Fig. 8. Specific mass absorption coefficient (m2 g�1) of methane soot at v = 4 cm s�1

measured at 300 nm (N) and 500 nm (d) and at v = 5 cm s�1 measured at 300 nm(4) and 500 nm (s).

M. Alfè et al. / Combustion and Flame 157 (2010) 1959–1965 1963

absorbance position at 210–230 nm have been detected along theflame indicating the occurrence of small changes in the carboninternal structure in terms of sp3/sp2 carbon hybridization [20].

The mass absorption coefficient of soot is another importantspectral parameter related to both morphological and internal car-bon structure [21], but it can not be measured on the particulatedeposited on quartz plates. Thus, the mass absorption coefficients(m2 g�1) of soot have been measured on NMP suspensions of sootof known concentration in the spectral region above 290 nm,where the NMP solvent does not interfere. The profiles of the massabsorption coefficients in the UV (300 nm) and in the visible(500 nm) are reported in Fig. 8 for both the methane flames. It isworth to underline that the contribution of DCM-soluble con-densed species (mainly constituted of PAH) to the mass absorption

Fig. 9. TEM images (25,000�) of methane soot sampled in the low (v = 4 cm s

coefficient of soot is avoided since the mass absorption coefficienthas been evaluated on soot pre-extracted with DCM.

Soot absorption coefficients slightly increase along both theflames as aggregation and carbonization of soot particles occur. Itcan be, however, noticed that in both temperature conditions thefinal value of methane soot absorption at 500 nm is much lower(about 2 m g�1) with respect to other fuels as ethylene and ben-zene (about 5 m g�1) [2] and lies well below the lower absorptioncoefficient of most carbon materials measured at or near combus-tion sources (5–13 m g�1) [21]. The low absorption coefficientscorrespond to the low absorption peak position (210–230 nm)and relatively high optical gap values of soot deposited on quartzplates inserted in the same flames [18]. This demonstrates thatmethane soot does not undergo a large aromatization/carboniza-tion during soot aging, even at high temperature. Overall, it canbe assessed that a very moderate aromatization process occurs inmethane flames independently on the temperature level. Hence,soot characteristics appears to be not strictly related to the tem-perature nor to the soot loading, but the fuel characteristics, affect-ing the surrounding hydrocarbon and hydrogen environment,affect the formation process of soot and its properties.

The TEM images of young and mature soot sampled in bothmethane flames, reported in Fig. 9, exhibit the coexistence of largeglobular coalesced amorphous material and of small spherical par-ticles with a narrow size distribution, connected in loose, openchain-like aggregates. The presence of amorphous material ap-pears particularly evident in the young soot of both flames and per-sists in the mature soot of the low-temperature flame. Thisamorphous material is ascribed to high molecular weight aromaticspecies, strongly absorbed on soot surface and not extracted withDCM, whose contribution is particularly high in the soot inceptionregion and remains quite significant downstream of the low-tem-perature flame [2,22].

�1, upper panel) and high-temperature flames (v = 5 cm s�1, lower panel).

Fig. 10. HR-TEM images of methane soot at low (v = 4 cm s�1, (a–d, adapted from [1]) and high flame temperature (v = 5 cm s�1, e–h).

1964 M. Alfè et al. / Combustion and Flame 157 (2010) 1959–1965

The HR-TEM images of methane soot particles, shown in Fig. 10,exhibit a different nanoscale organization in dependence on thetemperature and soot aging. The methane soot particles producedat lower temperature (v = 4 cm s�1) appear as very disordered car-bons (Fig. 10a and b), and no significant change in the soot nano-structure, moving from young to mature soot (Fig. 10c and d),can be observed. Also in the high-temperature flame (v = 5 cm s�1),young soot appears as a disordered carbon (Fig. 10e and f), whereasa better structural order can be observed in the mature soot(Fig. 10g and h); the development of the classical onion-like struc-tures in the best structured areas can be also noted.

Table 1 summarizes the structural parameters obtained by anal-ysis of the skeletonized images that allows obtaining a semi-quan-titative measurement of the qualitative observations inferred bythe visual inspection of the HR-TEM images. L corresponds to themean fringe length and has been evaluated by considering all thefringes larger than the size of one aromatic ring (L > 0.246 nm)and all the fringes larger than the size of two fused aromatic rings(L > 0.49 nm), La and Lc correspond to the diameter and to theheight of the coherent domains, respectively, i.e. of the BSU,formed by N stacked layers with an interlayer spacing of d.

The BSU structural parameters reported in Table 1 represent themean values evaluated on 15–20 images representative of the beststructured areas (areas of 16 � 16 nm sampled on the externalshell of the nanoparticles). The number of analyzed fringes is more

Table 1Structural parameters by HR-TEM image analysis, nm.

Methane shoot v = 4 cm s�1 [1]

Younga Matureb

L (>2.5, h = 40%) 0.45 0.50L (>4.9, h = 40%) 0.70 0.74La (>2.5, h = 15%) 0.28 0.34d (>2.5, h = 15%) 0.39 0.38N (>2.5, h = 15%) 0.23 0.26Lc (>2.5, h = 15%) 0.49 0.62% nsl 66.8 51.1

a Height above number: 3.7 mm.b Height above number: 10.7 mm.c Height above number: 4.6 mm.d Height above number: 11.6 mm.

than 3000 for each soot sample. The percentage of nsl (singlefringes) is also reported in Table 1.

The interlayer spacing, d, typical of poorly ordered carbons(0.39 nm in comparison with the 0.3354 nm graphite value), issimilar for both the temperature conditions. The histograms ofthe fringe lengths L for both the methane flames, evaluated admit-ting a 40% of fringe tortuosity and considering all the fringes largerthan the size of one aromatic ring (L > 0.25 nm), are reported inFig. 11.

As shown in the HR-TEM images reported in Fig. 10, the meth-ane soot formed in low temperature conditions exhibits small BSU(Table 1) with only few fringes beyond 0.75 nm in length (Fig. 11a).This reveals its poor graphene layer extent and organization (thelayers are frequently non-stacked and only a very little numberof layers are stacked). The fringe length distribution for the meth-ane soot formed at higher temperature extends to much longergraphene layers dimensions (>1 nm). The larger variation of BSUparameters, in terms of Lc and La increase and the decrease of %nsl, for methane soot formed in high temperature conditions (Ta-ble 1 and Fig. 11) indicates the increase of the structural order thataccompanies its maturation process.

The development of different nanostructures during the sootgrowth and maturation (Figs. 9 and 10) and the decrease of H/C ra-tio (Fig. 7) observed as consequence of the different flame temper-atures, justify the decrease of sot reactivity towards oxygen in both

Methane shoot v = 5 cm s�1 [1]

Youngc Matured

0.46 0.570.72 0.820.30 0.390.40 0.390.24 0.280.54 0.69

59.6 38.2

Fig. 11. Histograms of the fringe lengths L for methane soot at low (v = 4 cm s�1,adapted from [1]) and high flame temperature (v = 5 cm s�1) (obtained admitting40% of fringe tortuosity).

M. Alfè et al. / Combustion and Flame 157 (2010) 1959–1965 1965

flames (Fig. 7). A correlation between soot nanostructure and sootreactivity toward oxidation has been proposed [2,23] and rational-ized in terms of a large accessibility of carbon in edge sites (muchmore reactive than the basal plane carbon atoms) for effect of cur-vature possibly due to five-member rings as in fullerene-likestructures.

The accessibility of carbon in edge sites is, indeed, influenced bythe curvature radii of the soot graphene layers: the more curvatureof the soot, the faster the reactivity toward oxidation [23]. Thehigher curvature of the graphene layers of the mature soot of thehigh-temperature flame relative to the low-temperature flame(Fig. 10), should lead to a higher reactivity.

On the other hand, the lower H/C atomic ratio in the case of thehigh-temperature flame (Fig. 7), associated to a lower number ofactive C–H sites available for oxidation attack, counteracts theenhancing effect of higher curvature on soot oxidation reactivity.The interplay of the two effects could give as a result that no signif-icant changes of reactivity toward oxidation are observable in thehigh-temperature flame with respect to the low-temperatureflame (Fig. 7).

4. Final remarks

The effect of temperature on the flame structure and on sootproperties has been studied in methane premixed rich flames (C/O = 0.60) burning at two different cold-gas flow velocities corre-sponding to different maximum flame temperatures.

The higher flame temperature conditions favor the formation ofhydrogen, acetylene and diacetylene, thereby reducing the forma-tion of benzene and condensed phases including condensed spe-

cies and soot. Thus, it appears that benzene is the rate limitingfactor in soot formation.

Soot carbonization occurs to a larger degree in the high flametemperature conditions consistently with the higher formation ofhydrogen, acetylene and diacetylene. In correspondence of sootdehydrogenation, the mass absorption coefficients of soot exhibitan increasing trend. However, mature soot retains a relatively highH/C ratio and low absorption coefficients with respect to the otherfuels [2] even in high flame temperature conditions. This indicatesthat the aromatization/carbonization of soot is more dependent onthe fuel characteristics rather than on the flame temperature.

Soot oxidation reactivity and morphology have also shown sig-nificant differences in relation to soot aging process. The soot oxi-dation reactivity has been found to decrease along the flame as H/Cdecreases and as fringe lengths increase.

Generally, it can be assessed that mature soot produced fromdiverse hydrocarbon fuels with similar flame temperatures andflame types possess a different chemical composition and structureand in this sense, the H/C atomic ratio and mass absorption coeffi-cients appear to be good signatures of soot properties and struc-tural evolution. Further work to disentangle the complexity ofthe formation process of soot through a deeper analysis of thehydrogen quality, aromaticity character and morphological fea-tures is necessary.

Acknowledgments

This work is supported by the ‘‘Accordo di Programma CNR-MSE, Gruppo Tematico Carbone Pulito – Fondo per il Finanziamen-to Attivita‘ di Ricerca e Sviluppo di Interesse Generale per il Siste-ma Elettrico Nazionale”.

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