Combustion and Flame 157 (2010) 33–42

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

Chemical and morphological characterization of soot and soot precursorsgenerated in an inverse diffusion flame with aromatic and aliphatic fuels

Alexander Santamaria a,*, Nancy Yang b, Eric Eddings c, Fanor Mondragon a

a Institute of Chemistry, University of Antioquia, AA 1226, Medellín, Colombiab Sandia National Laboratories, Livermore, CA 94551-0969, USAc Department of Chemical Engineering, University of Utah, Salt Sake City, UT 84112, USA

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

Article history:Received 23 January 2009Received in revised form 6 June 2009Accepted 24 September 2009Available online 31 October 2009

Keywords:SootSoot precursorsYoung sootIDF sootAromaticity

0010-2180/$ - see front matter � 2009 The Combustdoi:10.1016/j.combustflame.2009.09.016

* Corresponding author. Address: Institute of ChemA.A. 1226, Medellín, Colombia. Fax: +57 4 219 6565.

E-mail address: alsp04@mateamaticas.udea.edu.co

Knowledge of the chemical and physical structure of young soot and its precursors is very useful inunderstanding the paths leading to soot particle inception. This paper presents chemical and morpholog-ical characterization of the products generated in ethylene and benzene inverse diffusion flames (IDF)using different analytical techniques. The trend in the data indicates that the soot precursor materialand soot particles generated in the benzene IDF have a higher degree of complexity than the samplesobtained in the ethylene IDF, which is reflected by an increase in the aromaticity of the chloroformextracts observed by 1H NMR and FT-IR, and shape and size of soot particles obtained by TEM and HR-TEM. It is important to highlight that the soot precursor material obtained at the lower positions inthe ethylene IDF has a significant contribution of aliphatic groups, which play an important role in theparticle inception and mass growth processes during the early stages of soot formation. However, thesegroups progressively disappear in the samples taken at higher positions in the flame, due to thermaldecomposition processes.

� 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Soot formation is a very complex phenomenon involving homo-geneous and heterogeneous processes as well as competitionamong oxidation, formation, and bond scission reactions [1–7].Although the process of soot evolution has been investigated bothexperimentally and theoretically in many studies, there are stillsome questions that need to be addressed regarding the chemicalstructure of the various compounds formed in the flame, particu-larly during the early stages of soot formation.

Dobbins et al. [8,9] characterized soot precursor particles (youngsoot) and carbonaceous soot taken at the centerline of an ethylenenormal diffusion flame (NDF) using laser microprobe mass spec-trometry (LMMS). Based on this analytical technique, they reportedC/H ratios of 1.8 for precursor particles obtained lower in the flame,and 5.6 for carbonaceous soot obtained higher up in the flame.These results were also confirmed by TEM images where the pre-cursor particles presented an irregular shape covered by tar mate-rial which makes them transparent to the electron beam, whereasthe carbonaceous particles are more opaque. It has been suggestedthat the precursor particles observed in diffusion flames can under-

ion Institute. Published by Elsevier

istry, University of Antioquia,

(A. Santamaria).

go conversion to carbonaceous soot in the high temperature regionsof the flame by the process of carbonization.

In a study published by Ciajolo et al. [10,11], the extractablematerial of young soot obtained in the inception region of an eth-ylene premixed flame was chemically characterized using UV–VISand FT-IR analyses. The results indicated that the extractable frac-tion of the soot demonstrated not only aromatic but also aliphaticcharacteristics, which were observed both before and after the sootinception point. For example, at low flame positions just below thesoot inception point, a significant contribution of aromatic hydro-gen was observed and reached its maximum value at the inceptionpoint. However, after this point, the aromatic hydrogen contribu-tion decreases, while the aliphatic hydrogen content due to bothCH2 and CH3 groups were much more significant. Similar resultswere obtained by Oktem et al. [12], using mass spectrometry.The results indicated that the aliphatic component present in thesemivolatile material has an important contribution in the sootgrowth process just after soot inception, or once the first particleshave been formed. Although these results made a significant con-tribution to understanding the chemistry and morphology of sootprecursor particles, there are still many questions concerning thechemical structure of these compounds for which a detailed anal-ysis requires larger amount of sample.

Up to now, most of the studies that involve the early period ofsoot formation have been carried out in premixed and normal

Inc. All rights reserved.

34 A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42

diffusion flames due to their experimental simplicity. Unfortu-nately, some of the disadvantages that these type of configurationspresent are that the soot particles that are collected are much morecarbonized and the amount of young soot under these conditions isoften minimal, which limits the chemical characterization [8,9,13].However, the flame configuration known as an inverse diffusionflame (IDF) appears to be a good alternative for studying the chem-istry of the soot inception process, since the combustion productsformed on the fuel side of the flame will never enter the oxidationzone and can thus escape unoxidized from the flame. As a result,the precursor material and young soot particles can be collectedfrom the surroundings without the need to invade the flame withthe sampling probe [14–16].

Different studies have demonstrated that the soot particlesemitted from IDFs are chemically and morphologically similar tothe precursor particles found in normal and premixed flames. Forinstance, Blevins et al. [16] found that soot from IDFs was viscousand transparent and contained higher hydrogen content than sootfrom NDFs. Oh et al. [17] also studied an ethylene IDF, observingthat particles collected at low axial positions were tar-like andnon-agglomerated material, but at increasing axial position sootparticles aggregated and carbonized slightly, increasing in C/Hratio.

The purpose of this paper is to obtain some insight into thechemical structure and morphology of the soot precursor particlesgenerated in aromatic and aliphatic IDFs as a function of heightabove the burner surface.

2. Experimental

2.1. Burner

The IDF burner used in this work is similar to the one used byBlevins et al. [16], and consists of three concentric brass tubes,which are distributed as follows: a central tube for the air jet, anintermediate annular tube used for supplying the fuel, and an outerannular tube for a N2 stream that is used as a shield to prevent theformation of flames with the room air. The whole system wasmounted on a multi-axis screw mechanism and the flow rates ofthe gaseous systems were set to 27, 117 and 289 cm3/s for air, fuel(ethylene or benzene/N2) and N2 respectively, which were mea-sured at standard conditions (293 K and 85 kPa), such that the vis-ible flame height in a dark room was approximately 60 mm, adescription of the experimental setup can be found elsewhere[14,15].

To generate the benzene IDF, the liquid fuel was delivered withan HPLC vacuum pump at the flow rate of 1.0 mL/min into a glasscontainer, which was heated up to 120 �C. Then, a N2 stream at117 cm3/s was used to transport the benzene vapors through theheated lines toward the burner face where it was ignited.

2.2. Aromatic and aliphatic fuels

Ethylene (C2H4) and benzene (C6H6) were used as fuels to gen-erate the inverse diffusion flames studied here, and these fuelswere selected for three reasons. First, both fuels can produce suffi-cient soot in different flame configurations; second, ethylene andbenzene have been widely used in studies of soot formation, so alarge database exists for the main chemical reaction mechanisms[18–21]. A final consideration in the selection of ethylene and ben-zene is to compare and to understand the young soot formation inaromatic and aliphatic IDFs, since it is has been proposed that aro-matic fuels can produce more carbonized soot with particle sizediameters greater than the soot particles obtained from aliphaticflames [19,20].

2.3. Sampling procedure

Soot samples were taken at different heights at the lateral axisclose to the outer edge of the visible flame, using a 10 cm longstainless steel probe with a capillary tip of 1 mm. The samplingtime for each experiment depended on the amount of sample thatcould be collected on the filter, which was different at each posi-tion in the flame and varied from 20 min to 1 h.

2.4. Flame temperature

The temperature profiles along the lateral axis of the flame(sampling position) were obtained using a R type thermocouple(Pt:Pt–13% Rh, 75 lm with a bead diameter of about 150 lm),which was placed in the flame by a rapid insertion method in orderto reduce the exposure time of the thermocouple in the sooting re-gion. Radiation corrections of the thermocouple readings were ta-ken into account to calculate the gas temperature at each location.The uncertainty in the flame temperature measurements wasdetermined to be no greater than ±30 K.

2.5. Characterization methods

For a qualitative FT-IR analysis, a small amount of soot samplecollected on the Teflon filter was taken to prepare a 1% KBr pellet.Each spectrum was the result of a 300 scan accumulation, a valuethat provided the best signal/noise ratio. A Nicolet Magna 560spectrometer was used with a MCT/A detector operated in a wave-number range of 600–4000 cm�1. Three replicates of each samplewere taken from the flame to estimate reproducibility of the meth-od. In general, the uncertainty in the IR measurements was lessthan 5%.

It is important to highlight that the soot particles and the sootprecursor material extractable in chloroform give similar peak pat-terns in the infrared, since the graphite-like structure present insoot particles does not absorb in this range. For this reason, anychemical change detected in the soot particles themselves ismainly due to the presence of precursor material absorbed on theirsurface. Therefore, the IR signals coming from soot samples areindependent of the sample matrix and refer to either the soot par-ticles or the soot precursor material [14].

1H NMR spectra of the soluble fraction of soot samples collectedon the filter at different flame heights were taken in order to obtaina qualitative description of the chemical progression of soot parti-cle precursor material as a function of height. The extraction pro-cess was carried out with chloroform in an ultrasonic bath for15 min. Then, the soluble fraction was separated and the solventwas evaporated until dryness in inert atmosphere using a vacuumstove at 40 �C. Finally, spectra of the dried material re-dissolved indeuterated chloroform were taken using a Bruker AMX 300 spec-trometer. However, before starting the analysis, each spectra wascorrected for phase and baseline and then integrated manually atleast four times and the results were averaged to reduce the uncer-tainty (less than 5%) generated by the manual adjustment.

Elemental analysis of the soot extracts was carried out by thecombustion method using a Perkin Elmer CHN analyzer. Hydrogenand carbon were determined directly, while the oxygen contentpresent in the samples was calculated by difference assuming abase of 100%. These experiments were repeated three times persample with very good reproducibility. The estimated error wasless than 2% in all cases.

The average molecular weight data (MW) of the soot extractswere determined by Vapor Pressure Osmometry (VPO) in a Knauerosmometer using chloroform as solvent and benzyl as calibrationstandard. Then, the molecular weight was obtained from thekcal/Ks ratio, where kcal is the calibration constant for benzyl

A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42 35

expressed in kg/mol and Ks is the value measured for the sample,which is expressed in kg/g.

Finally, soot particle morphology was evaluated at three loca-tions along the lateral axis of the flames by means of TransmissionElectron Microscopy (TEM). The first location was approximately at6 mm above the base of the flame immediately after the visiblesoot inception point, the second one was located at 35 mm, wherethe soot maximum peak was observed and the third one was takenat the exhaust.

3. Results

3.1. Temperature and soot solvent extraction

Fig. 1 shows the temperature profiles taken at the sampling po-sition (6 mm from the centerline) for both ethylene and benzeneIDF flames. The flame temperature was measured at locationsabove the burner base ranging from 2 mm to 60 mm (flame tip)using thermocouples [22,23].

At the 15 mm sampling position, the temperature of the ethyl-ene flame approaches 1400 K, whereas the temperature peak of thebenzene flame is shifted toward the burner face with a maximumtemperature no higher than 1300 K. After passing this point, thetemperature of both flames decreases rapidly down to 800 K atthe flame tip due to convective heat transfer to the cooler sur-rounding gas.

In general, the differences in temperature observed for thesetwo flames are mainly due to differences in their heats of combus-tion. In addition, it is well known that aromatic fuels have the ten-dency to produce much more soot than aliphatic fuels. Therefore,the higher the soot concentration, the more energy is radiated bysoot, which results in lower flame temperature [24]. Also, it is wor-thy to mention that the benzene flame temperature can be affectedby the fuel dilution with N2, which in turn increases the dilutionratio, causing a temperature reduction of the system.

The sampling temperature of the flames evaluated in this studyis relatively high, but not as high as the temperatures found at thecenterline of premixed and normal diffusion flames. Thus, it isanticipated that the global oxidation–carbonization process expe-rienced by the soot particles at the lateral axis in an inverse diffu-sion flame will be less drastic compared to its counterpart since thetime–temperature history for these processes is different and

600

800

1000

1200

1400

1600

0 10 20 30 40 50 60

E-IDFB-IDF

Tem

pera

ture

(K)

Height (mm)

Fig. 1. Temperature profiles at the lateral axis or sampling position of ethylene andbenzene IDF flames as a function of height above the burner base.

because the combustion products of IDFs are transported directlytoward the fuel side, where no oxygen is present [16,25].

An important aspect directly related to the flame temperature isthe degree of condensation of the combustion products, which canbe estimated through a simple extraction test with organic sol-vents. Fig. 2 shows the weight percentage variation of soot extract-able material in chloroform as a function of height above theburner surface for the ethylene and benzene flames respectively.In general, the amount of extractable material of soot obtainedfrom these flames is high close to the burner mouth and then de-creases very rapidly during the first 25 mm mainly due to thermaldecomposition processes, since the temperature at low flame posi-tions is higher, promoting the loss of some labile material and anincrease in the aromaticity of the samples. These observationsare in agreement with the results obtained in the pioneering workof Dobbins that suggests that liquid-like precursor particles ob-served at the fuel side of NDFs undergoes metamorphosis to formcluster aggregates in the high temperature region by the process ofcarbonization caused by a reduction in the hydrogen fraction [9].

On the other hand, after a height of 25 mm in the flame, eachprofile in Fig. 2 reaches a constant value around 51% for ethyleneand around 36% for benzene, values that are consistent with thosereported in literature and indicate that the chemical compositionand the degree of maturing of soot obtained in this range of heights(20 mm – exhaust) may be very similar [16]. This behaviour wasalso observed by Mikofski et al. [26], who worked with an ethyleneIDF configuration. They reported that after the high temperaturereaction zone at the axial position of the flame, soot and precursorscan continue to grow at temperatures below 1200–1300 K throughsurface reactions and scavenging of PAHs. This observation wasassociated with a slight increment in the slope of the radially inte-grated soot PLII (planar laser-induced incandescence) and PAH PLIF(Planar Laser-Induced Fluorescence) signals. However, furtherdownstream (above 45 mm from the burner face), the integratedsignals approached constant values indicating that soot formation,growing mechanisms and other associated chemical reactionsapparently had become frozen.

It is also possible to observe that the amount of soot precursormaterial (soluble in chloroform) obtained in the ethylene IDF is lar-ger than the amount gathered from the benzene flame, indicatingthat the former flame produces younger soot with a significantamount of low molecular weight material, while the benzene flame

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 200

E-IDF B-IDF

% E

xtra

ctio

n

Height (mm)

Fig. 2. Weight percentage of extractable material of soot gathered from ethyleneand benzene inverse diffusion flame.

36 A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42

produces a more complex material with a large aromatic cluster,which would explain the low solubility in chloroform as heightabove the burner increases.

500100015002000250030003500

Wavenumer (cm -1)

6 mm

15 mm

35 mm

60 mm

Exhaust

Fig. 4. FT-IR analysis of the soot collected in a benzene IDF flame.

Ethylene IDF-6 mm

500100015002000250030003500

Wavenumber (cm-1)

Benzene IDF-6 mm

Fig. 5. FT-IR spectra of the soot taken at 6 mm of the ethylene and benzene IDFflames.

3.2. FT-IR analysis soot collected in the filter

The infrared analysis was done on six soot samples taken at 6,15, 25, 35, 60 mm and exhaust for both ethylene and benzene IDFsin order to get information of the different functional groups pres-ent in samples. As stated in Section 2, the IR signals coming fromsoot samples are relatively independent of the matrix in whichthey were analyzed and can be referred to as either soot particlesor soot precursor material. For convenience and to make compari-sons with previous work [14], the IR analysis performed in thiswork refers to soot particles as a whole. Fig. 3 shows the FT-IRspectra of the samples taken at different positions of the lateralaxis of an ethylene flame. The main characteristic signals observedin the spectra correspond to C–H stretching of acetylenic(3300 cm�1), aromatic (3030 cm�1), carboxylic (1720 cm�1) andaliphatic groups (2975 cm�1, 2925 cm�1 and 2850 cm�1). The pres-ence of the C–H stretching in aliphatic groups comes mainly frommethyl, methylene, and methine groups bonded to aromatic ringson PAHs or methylene bridges (fluorene-type) maintaining theinterconnection of PAHs within the network [10,11,27].

As can be seen, all FT-IR spectra display the same absorptionpeak pattern; however, the chemical progression of soot as a func-tion of height is different, particularly at low flame positions,where a significant contribution due to aliphatic functional groupscan be found. These aliphatic groups start disappearing as heightabove the burner increases due to thermal decomposition pro-cesses caused by the high temperature (above 1200 K) recordedduring the first 25 mm along the sampling position. This behaviouris followed by an increment in the aromatic character, since thesignals corresponding to aromatic functional groups observed at3030 cm�1, 1595 cm�1, and 750–840 cm�1 increase.

Another feature that can be observed in the FT-IR spectra ofFig. 3 is that the signal at 1720 cm�1, corresponding to carbonylgroups disappears almost completely in the soot sample taken atthe exhaust. This change is accompanied by the broadening ofthe region from 1000 to 1300 cm�1. This region is a complex sec-tion of the infrared spectra where signals corresponding to aro-matic C–C and C–H plane deformation structures can overlapwith signals corresponding to ether C–O–C stretching groups[28]. Therefore, with the information currently available, a detailedanalysis cannot be done.

Fig. 4 shows the soot chemical progression as a function ofheight above the burner of the benzene flame, where a similar peak

Fig. 3. FT-IR analysis of the soot collected in an ethylene IDF flame.

pattern as described for the ethylene soot samples was observed,indicating that regardless of the fuel type used to generate theflame, the functional groups present in the soot particles and ob-served by FT-IR are similar.

The trend observed in the benzene soot samples indicates thatthe aromatic character increases faster than the ethylene soot sam-ples, especially during the first stages of soot formation, followedby a reduction in the aliphatic content, which is not very signifi-cant. Although the starting fuel is an aromatic compound, the pres-ence of aliphatic functional groups suggests that the thermaldecomposition process is different in this flame.

Some other characteristics exist in the FT-IR spectra for the sootsamples collected in the benzene flame that make them differentfrom the FT-IR spectra of soot samples taken in the ethylene flame.For example, the relative intensities of aromatic and aliphaticstretching are different in the samples produced in both flames.Particularly, at low positions of the benzene flame the aromatic/ali-phatic ratio of the soot samples is much larger than the aromatic/aliphatic ratio of the soot samples obtained in the ethylene flame(see Fig. 5), indicating that the ethylene soot at 6 mm can be clas-sified as ‘‘young soot”, whereas the benzene soot sample may havea higher degree of complexity even for the sample taken at 6 mm.

Also note in Fig. 5 that the contribution of acetylene groupsfound in the soot samples collected in the ethylene flame is greaterthan the contribution of acetylene groups found in the benzeneflame samples, which indicates that the HACA mechanism maynot be the main process that governs the formation and growthof soot particles in aromatic flames. An alternative explanationwould be that aromatic polymerization is the main chemical pro-cess taking place for these conditions.

024681 0ppm

( a )

( b )

Exhaust60453525156

Fig. 6. 1H NMR spectra of soot extractable material as a function of height above theburner of an ethylene IDF flame. (a) 6 mm and (b) exhaust.

0246810ppm

(a)

(b)

Exhaust60453525156

6789

Fig. 7. 1H NMR spectra of soot extractable material as a function of height above theburner of the benzene IDF flame. (a) 6 mm and (b) exhaust.

0

0.1

0.2

0.3

HγHβ2 Hβ1

HγHβ2Hβ1

Alip

hati

c hy

drog

en f

ract

ion

Height (mm)

E-IDF B-IDF

0 20 40 60 200

Fig. 8. Aliphatic and aromatic tendenc

A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42 37

3.3. 1H NMR analysis

1H NMR analysis is a technique routinely used to characterizegasoline, heavy crude oils, and solvent soluble coal fractions. Thehydrogen groups classified by 1H NMR spectra can yield structuralinformation that allows the characterization of complex mixturescontaining hundreds of aromatic, naphthenic, paraffinic, olefinic,and isoparaffinic compounds [29,30].

Fig. 6 shows the soot progression in an ethylene IDF, which canbe followed through the extractable material characterization. Themain assigned groups correspond to: hydrogen on aromatic rings(Ha), hydrogen on b position to aromatic rings (Hb), and hydrogenof terminal methyl groups on c, d or greater positions to aromaticrings (Hb). More detailed information on the chemical shifts ob-served by 1H NMR is presented in Ref. [15].

As can be seen from Fig. 6a, the 1H NMR spectrum of theextractable material of the soot sample collected at 6 mm showsthat these products have a significant aliphatic component withpredominance of methyl and methylenic groups on b and c posi-tions to aromatic rings. It is important to observe that there is a sig-nificant fraction of hydrogen on the c position between 0.7 and0.8 ppm, which suggests the existence of large aliphatic chains orsaturated rings (naphthenic-type) joined to different types of aro-matic rings. The region between 3.5 and 5.0 ppm can correspond toolefinic hydrogen or CH2 groups that connect two aromatic units(for example fluorene-type).

Fig. 6b shows the 1H NMR spectrum of the chloroform-extract-able material of the soot sample obtained in the exhaust (49% sol-uble in chloroform). This spectrum shows a remarkable decrease inall the hydrogen signals of naphthenic, olefinic, and aliphatic char-acter (chemical shift region between 0.5 and 6.0 ppm), followed byan increase in the aromatic content (see the aromatic regionenlargement in the upper part of Fig. 6).

Fig. 7 shows the 1H NMR spectra of the soot extractable materialtaken at 6 mm (part a) and at the exhaust (part b) of the benzeneflame. These samples present a very similar behaviour to the onefound in the ethylene IDF samples, except that the intensities cor-responding to aliphatic functional groups are very much smaller,even for the soot sample taken at the lowest flame position(6 mm). However, the development in the aromatic character ofthis material occurs very early in this flame, indicating that thecombustion products of a benzene flame have a more complex aro-matic structure, which explains the low solubility.

Fig. 8 shows a summary of the trends of each hydrogen type ob-served by 1H NMR as function of the height above the burner ofethylene and benzene flames. Fig. 8a shows that the aliphatichydrogen fractions labelled Hb(Hb1 + Hb2), and Hc decrease with

0

0.2

0.4

0.6

0.8

1

0 20 40 60 200

E-IDFB-IDF

Aro

mat

ic h

ydro

gen

frac

tion

(H

a)

Height (mm)

ies obtained by 1H NMR analysis.

300

400

500

600

700

800

900

0 10 20 30 40 50 60 200

E-IDFB-IDF

Ave

rage

mol

ecul

ar w

eigh

t

Height (mm)

Fig. 9. Average molecular weight of the extractable material of soot collected as afunction of height of both ethylene and benzene IDF flames.

38 A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42

an increase in height above the burner surface. However, the mostdrastic reduction for the ethylene IDF samples took place on the bhydrogen (from 0.50 to 0.07). This observation is accompanied byan increase in the aromatic hydrogen fraction (from 0.26 to0.82), as shown in Fig. 8b. These results indicate that the observedaliphatic structures early in the flame undergo de-alkylation and/or cyclization reactions, leading to more compact structures.

In contrast, the aliphatic hydrogen fraction of the benzene IDFsamples contributes very little because the aromaticity repre-sented by the aromatic hydrogen content (Ha) is developed veryearly in the flame (Fig. 8b). For instance, the sample taken at thelowest position of this flame has an aromatic hydrogen fractionof about 0.80, while the b hydrogen contribution is only 0.092(Fig. 8a). However, as height about the burner increases, the bhydrogen content is reduced to 4%, while the Ha fraction reachesa constant value at about 0.90.

Recent studies have found that at lower heights along the cen-terline of premixed ethylene flames (below the soot inceptionpoint), the soot chemical composition was dominated by polycyclicaromatic hydrocarbons, whereas aliphatic compounds made anoticeable contribution to soot growth after the soot inceptionpoint where the particles become ‘‘more carbonized” [12]. Accord-ing to these observations, the results described above seem to becontradictory.

However, this is not the case since this study used a differentflame configuration where the time–temperature history for car-bonization and oxidation processes is different as compared to apremixed flame configuration. For a given nominal equivalence ra-tio, the soot collected from the periphery of an inverse diffusionflame (fuel-rich zone) would be less oxidized than the soot col-lected from a premixed flame. Thus, it is expected that soot andintermediate hydrocarbons produced in an inverse diffusion flamehave a different evolution history as height above the burner in-creases as compared to a premixed flame.

The chemical differences in soot and its precursors early in theflame, as observed by FT-IR and 1H NMR, indicate that the sootinception and growth processes are dependent on fuel type. For in-stance, it is evident that the formation of aliphatic species in theethylene IDF plays a significant role in the soot inception andgrowth processes that maintain the interconnection within the pri-mary particle. However, when these primary particles are still inthe reaction zone (high temperature region), the aliphatic compo-nent can undergo de-alkylation and cyclization reactions accompa-nied by hydrogen lost on b and c positions, leading to theformation of more stable structures. In contrast, the contributionof aliphatic structures formed during the combustion of benzeneis apparently not significant, which implies that the soot inceptionmechanism at low positions of this flame must be different fromthe one proposed for the ethylene flame. Some authors have pro-posed that the soot formation mechanism in a benzene flamemight occur through polymerization reactions of aril radicals [31].

It is worth mentioning that, although the FT-IR and 1H NMR arequalitative techniques, they provide very reliable informationabout the chemical changes that take place leading to soot forma-tion in aromatic and aliphatic flames and as will be seen later, theyalso agree with the quantitative results of this study.

3.4. Vapor pressure osmometry (VPO)

The average molecular weight obtained by VPO was carried outupon seven extracts of soot samples taken from both ethylene andbenzene IDF flames. Fig. 9 shows the average molecular weight dis-tribution of the extracts taken along lateral axis of the ethylene andbenzene IDFs. The sample taken at the lowest position of the eth-ylene flame shows the highest average molecular weight amongthe samples evaluated (MW = 817). This figure also shows that

the average molecular weight of the extractable material obtainedin the ethylene IDF flame diminishes with height mainly due tothermal decomposition processes (at low flame positions the tem-perature is much higher) until reaching a constant value after35 mm, which also indicates that the average chemical structuremay be very similar among these samples. If the data were normal-ized by the lowest molecular weight, it appears that all the samplesmay have a basic structural unit, that according to 1H NMR is anaromatic cluster of about 370 Daltons, a value that is in the massrange of PAHs found on particulate combustion products of diversefuel systems, including the mass range for precursor particles stud-ied by Dobbins in ethylene flames [9].

Therefore, the high molecular weight observed in the samplestaken at low flame positions of the ethylene IDF is not only dueto the presence of a well-defined aromatic cluster but also due tothe high contribution of oxygen and aliphatic structures presentin these samples. It is important to note that the basic unit (aro-matic clusters) is quickly formed in the flame as stated above. Ananalysis done by Mikofski and co-workers [32] showed that poly-cyclic aromatic hydrocarbon (PAH) formation in an ethylene in-verse diffusion flame takes place rapidly close to the burnersurface, which is consistent with our results.

On the other hand, Fig. 9 also shows the average molecularweight of the soot extracts taken in the benzene flame. Some ofthe main characteristics of these data are that the average molec-ular weight of all the samples is rather constant and close to 420Daltons, which implies that the structural unit and the chemicalcomposition may be very similar at the various heights sampled.The other point is that the average molecular weight of the extractstaken at the low positions of the benzene flame is almost half ofthat the molecular weight determined in the extracts of the ethyl-ene IDF; which is primarily due to the low aliphatic contribution inthe benzene samples.

3.5. Elemental analysis

The elemental analysis showed that the extracts of the sootsamples gathered in the ethylene and benzene IDFs consist basi-cally of C, H, and O (see Table 1). The carbon content of the extractstaken between 6 mm and the exhaust of an ethylene flame in-creases from 85% to 94%, accompanied by a corresponding reduc-tion in the hydrogen content, which changes from 8% to 5%. A

Table 2Elemental analysis of the soot precursor soot gathered from a benzene IDF.

Height (mm) wt.% C wt.% H wt.% O (C/H) total

6.0 95.4 ± 0.3 3.73 ± 0.3 0.87 ± 0.3 2.13 ± 0.215.0 95.9 ± 0.2 3.50 ± 0.2 0.60 ± 0.2 2.28 ± 0.125.0 96.2 ± 0.2 3.38 ± 0.1 0.42 ± 0.1 2.37 ± 0.135.0 96.6 ± 0.3 3.33 ± 0.3 0.09 ± 0.1 2.42 ± 0.245.0 96.3 ± 0.2 3.31 ± 0.2 0.39 ± 0.2 2.42 ± 0.160.0 96.3 ± 0.3 3.49 ± 0.2 0.24 ± 0.1 2.30 ± 0.1Exhaust 96.1 ± 0.2 3.59 ± 0.1 0.31 ± 0.1 2.23 ± 0.1

Table 1Elemental analysis of the soot precursor gathered from an ethylene IDF.

Height (mm) wt.% C wt.% H wt.% O (C/H) total

6.0 85.3 ± 0.5 7.71 ± 0.2 7.02 ± 0.5 0.92 ± 0.115.0 86.5 ± 0.7 6.42 ± 0.4 7.12 ± 0.6 1.12 ± 0.125.0 88.9 ± 0.9 5.44 ± 0.4 5.67 ± 0.9 1.36 ± 0.135.0 90.2 ± 0.8 4.90 ± 0.3 4.90 ± 0.8 1.53 ± 0.145.0 92.9 ± 0.4 4.89 ± 0.2 2.21 ± 0.6 1.58 ± 0.160.0 93.6 ± 0.4 5.24 ± 0.2 1.12 ± 0.3 1.49 ± 0.1Exhaust 93.7 ± 0.4 5.20 ± 0.2 1.12 ± 0.4 1.50 ± 0.1

A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42 39

detailed analysis shows that the total (C/H) atom ratio increasessignificantly with height, particularly during the first 35 mm,which implies an increase in the aromatic character of these sam-ples. Nevertheless, after this point the total (C/H) ratio reaches aconstant value of about 1.50, which indicates that the samples rep-resent a very similar chemical composition. Similar values of total(C/H) atom ratio have been found in the literature for soot particleprecursors obtained in ethylene premixed and diffusion flames[8,10,11,16].

Another important characteristic observed in the elementalanalysis reported in Table 1 is that all the extracts of soot samplestaken at low flame positions (below 35 mm) had significant oxygencontent, which decreases gradually with height from 7% to 5%.However, after 35 mm the oxygen content in the samples fallsdown to about 1%, value that remains constant even in the sampletaken at the exhaust. FT-IR analysis of these samples also identifiedthe presence of oxygenated functional groups, and that thesegroups correspond to carbonyl and ether groups. Other authorsusing near edge X-ray absorption fine structure (NEXAFS) Spectros-copy have also reported that the presence of oxygenated species insoot extracts coming from several sources, and reported that atleast 50% of the total oxygen content is present in benzoquinonetype carbonyl groups [28].

Table 2 summarizes the elemental analysis results of the sootextracts taken at different locations of the benzene flame. In thiscase, we can see that the hydrogen content of the extracts is muchlower than the hydrogen content of the ethylene flame samplesand is relatively constant at all heights measured, which is inagreement with the solubility results. Also, it is observed that theC/H ratio is higher (above 2) for these samples compared to theC/H ratio reported in Table 1 for the ethylene flame samples. Thisresult suggests that the precursor material obtained in the benzeneflames is structurally more complex than the precursor materialcollected in an aliphatic flame.

3.6. TEM ad HR-TEM analysis

Soot formation processes and morphology have been exten-sively investigated and a wealth of information has been accumu-lated in the last two decades. It is well known that soot particlesemitted from a diesel engine are usually observed as chain-likeaggregates (secondary particles) composed of several tens to hun-dreds of primary spherical particles [33].

Although many studies have been carried out in premixed andnormal diffusion flames, comparatively few have been performedin inverse diffusion flames, where soot samples obtained are muchyounger and can provide more detailed information of early stagesof the soot formation process. In this study, we attempted to obtainmorphological information for the soot collected from ethyleneand benzene IDFs and to verify that the soot produced in this con-figuration was in the early stages of formation.

Fig. 10 shows the transmission electron microscopy (TEM)images of soot samples taken at 6 mm, 35 mm and exhaust-levelheights of ethylene and benzene IDFs. All of the TEM images areat the same scale, with a 100 nm reference marker in order to facil-itate comparisons. TEM images taken at low positions in the ethyl-ene flame showed a film-like deposition and it was difficult todistinguish individual particles. This result suggests that the pri-mary particles can evolve from a chemical condensation of heavyPAHs given the characteristic of a liquid-like character at roomtemperature as it was observed in the TEM image.

In contrast, TEM images of soot particles taken at the same po-sition of a benzene IDF showed both liquid-like precursors and pri-mary soot particles. The individual particles are nearly sphericaland no chain-like structures were observed. However, as the heightabove the burner increases, some interesting characteristics can beobserved in both flames. First, the soot aggregates obtained at35 mm and at the exhaust of an ethylene IDF flame present anirregular shape and consist of very small particle-like knobs thatproject from an overlayer surface that consists of a heavy tarrymaterial (PAHs), very similar to the particles found by Blevinset al. [16]. In contrast, the soot particles of the benzene flame pres-ent very well-defined chain-like aggregates composed of nearlyspherical particles, partially covered with tarry material. Next,the size of aggregates produced in both flames increases as heightabove the burner increases; however, the soot aggregates obtainedin benzene IDF are much larger than the ones observed in the eth-ylene IDF. This observation can be explained by the greater rate ofaromatic polymerization, leading to the formation of aromaticclusters and then soot [31].

These results were compared with TEM images obtained fromsoot samples taken at the exhaust of normal diffusion (NDF) andpremixed ethylene flames. Fig. 11 shows that the soot sample ob-tained in an ethylene NDF exhibit typical long chain aggregates ofrounded particles and no apparent overlaid-coating is observed,which is different from what was observed in the ethylene IDF.

In contrast, an ethylene premixed flame produces primary par-ticles and aggregates 3–4 times smaller than those observed in theNDF, which are smaller than the soot particles gathered from theethylene IDF. The reason for this is related to the residence timeof these particles in the fuel stream of a premixed flame which isshorter than their counterparts, which also limits their growth.

Fig. 12 shows HR-TEM images of the soot samples obtainedfrom the three different types of ethylene flame configurations.In the upper part of Fig. 12, it is possible to observe the progressionof soot samples as a function of height in an ethylene IDF. At lowflame positions (6 mm), a relatively thin overlayer coating withoccasional knobs projecting was observed. HR-TEM of the knobsshowed very little or no crystallinity. These knobs may be just ran-dom protrusions of the overlayer coating. The soot sample takenabove 35 mm of height showed more aggregation but the knobshave a medium degree of crystalline fringe formation.

For comparative purposes, the lower part of Fig. 12 shows twoHR-TEM images of soot samples taken at the exhaust of both nor-mal and premixed ethylene flames. The sample taken from an eth-ylene NDF showed that the particles have a fairly well developedonion ring morphology in their crystalline structure, whereas HR-TEM image of soot particles taken in an ethylene premixed flame,the crystallinity is tending toward onion ring, but not as well

Fig. 10. TEM images of soot particles collected at different positions of both ethylene and benzene IDFs.

Fig. 11. Soot morphology obtained from different flame configurations.

40 A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42

developed as in the NDF. The above information suggests that thesoot produced in an ethylene inverse diffusion flame appears to be

less structured than the soot obtained in ethylene NDF and pre-mixed flames as well as in the benzene IDF flame.

Fig. 12. HR-TEM images of soot particles collected at different position of the ethylene IDF and at the exhaust of premixed and normal diffusion flames.

A. Santamaria et al. / Combustion and Flame 157 (2010) 33–42 41

4. Conclusions

Tests of solubility in chloroform indicate that the percentage ofextractable soot in an ethylene IDF flame diminishes from 94% inthe sample taken near the burner face to 51% in the exhaust sam-ple; whereas, the percentage of extractable soot in the benzene IDFflame was smaller and below 50% for all heights sampled. These re-sults suggest that the degree of aromaticity of the condensed mate-rial in soot samples depends on the initial fuel (aliphatic oraromatic). This observation is also reflected in the average molec-ular weight, since the soot precursor material obtained at 6 mm inthe ethylene flame showed the highest molecular weight (817 Dal-tons) among all the samples. However, as height above the burnerincreases, the molecular weight decreases and reaches a constantvalue around 370 Daltons. On the other hand, the precursor mate-rial obtained in the benzene flame shows a uniform average molec-ular weight for the whole range of heights measured, whichsuggests the existence of a common structural unit.

1H NMR results of extractable soot precursor material taken atlow positions in the ethylene flame indicate a significant contribu-tion of hydrogen from aliphatic species (about 50%), which dimin-ishes as height above the burner increases, a fact that is stronglyreflected in the reduction of Hb1 and Hb2 content. In the benzeneIDF flame, the contribution of hydrogen to aliphatic species issmall. However, the development in the aromatic character ofthe extractable material from the benzene flame is quite signifi-cant, even for the sample taken very low in the flame. Similar re-sults were obtained by FT-IR.

The chemical differences for soot and its precursors early in theflame, as observed by FT-IR and corroborated by 1H NMR, indicatethat the soot inception and growth processes are fuel type depend-able. For instance, it is evident that the formation of aliphatic spe-cies at the low portions of the ethylene IDF plays a significant rolein the soot inception and growth processes that maintain the inter-connection within the primary particle. However, when these pri-mary particles are still in the reaction zone (high temperatureregion), the aliphatic component undergoes de-alkylation andcyclization reactions accompanied by hydrogen loss at the b and

c positions, leading to the formation of more stable structures. Incontrast, the contribution of aliphatic structures formed duringthe combustion of benzene is apparently not significant, which im-plies that the soot inception mechanism at low positions in thisflame must be different from the one proposed for the ethyleneflame.

Finally, the TEM and HR-TEM micrographs of soot samples ta-ken in benzene and ethylene flames show that the morphologyof the particles from these two flames is completely different andagrees well with the characterization of the soot precursors men-tioned above. For example, the soot particles obtained from theethylene flame present irregular aggregates of small particleswhich are apparently covered by a liquid-like material or precursormaterial, whereas the soot obtained in the benzene IDF flameshows well-defined aggregates of nearly spherical particles, whichare larger than the soot particles observed in the ethylene IDFflame and have a high degree of crystallinity, similar to that ob-served in NDF and premixed flames.

Acknowledgments

The authors would like to thank the Sostenibilidad program ofthe University of Antioquia 2009–2010 and to COLCIENCIAS throughthe Project 1115-405-20283 for financial support. A.S. thanks COL-CIENCIAS and the University of Antioquia for the PhD scholarshipand the Center for the Simulation of Accidental Fires and Explosionsat the University of Utah (funded by the US Department of Energyunder Contract No. LLL B341493) for the visiting research grant.

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