reduction of soot emissions by iron pentacarbonyl in isooctane diffusion flames1

7/29/2019 Reduction of Soot Emissions by Iron Pentacarbonyl in Isooctane Diffusion Flames1

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Combustion and Flame 154 (2008) 164180

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Reduction of soot emissions by iron pentacarbonyl inisooctane diffusion flames

K.B. Kim, K.A. Masiello, D.W. Hahn

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA

Received 8 August 2007; received in revised form 20 December 2007; accepted 21 January 2008Available online 3 April 2008

Abstract

Light-scattering measurements, in situ laser-induced fluorescence, and thermophoretic sampling with transmis-sion electron microscopy (TEM) analysis, were performed in laboratory isooctane diffusion flames seeded with4000 ppm iron pentacarbonyl. These measurements allowed the determination of the evolution of the size, numberdensity, and volume fraction of soot particles through the flame. Comparison to unseeded flame data provideda detailed assessment of the effects of iron addition on soot particle inception, growth, and oxidation processes.

Iron was found to produce a minor soot-enhancing effect at early residence times, while subsequent soot particlegrowth was largely unaffected. It is concluded that primarily elemental iron is incorporated within the soot parti-cles during particle inception and growth. However, iron addition was found to enhance the rate of soot oxidationduring the soot burnout regime, yielding a two-thirds reduction in overall soot emissions. In situ spectroscopicmeasurements probed the transient nature of elemental iron throughout the flame, revealing significant loss ofelemental iron, presumably to iron oxides, with increasing flame residence, suggesting catalysis of soot oxidationvia iron oxide species. 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Soot oxidation; Fuel additives; Iron pentacarbonyl; Soot emissions control

1. Introduction

Particulate matter (PM) is the term describingsmall particles found in the ambient air, such as dust,marine-derived particles, liquid droplets, smog com-ponents, and soot. PM ranges in size from a fewnanometers to tens of micrometers. Because of thesmall size of these particles (notably PM2.5, whichconsists of particles less than 2.5 m), they are able

to permeate and impact the deepest parts of the lungs,and are associated with incidences of asthma, chronic

* Corresponding author. Fax: +1 352 392 1071.E-mail address: [email protected] (D.W. Hahn).

bronchitis, and heart disease. Therefore, various ef-forts have been made to search for possible solutionsto decreasing the production rates of such fine parti-cles.

Soot particles, which compose a significant por-tion of PM2.5, are rich in amorphous carbon andpolycyclic aromatic hydrocarbons (PAHs), are wellknown to be carcinogenic [1], and can play key rolesin the global climate due to their influence on the

planets radiative transfer. Because soot from com-bustion processes is a major source of PM, there hasbeen significant interest in studying soot formationmechanisms and methods of soot reduction. Whilethe reduction of harmful soot emissions from com-

0010-2180/$ see front matter 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.combustflame.2008.01.011
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K.B. Kim et al. / Combustion and Flame 154 (2008) 164180 165

bustion processes is desirable, overall combustor per-formance (notably for propulsion systems) cannot becompromised, and large-scale combustor modifica-tions may not be practical in many situations. With

such goals and constraints in mind, the reduction ofsoot emissions via fuel additives remains an attractivepath for PM emissions control. This paper will focuson metallic-based additives, namely iron-based com-pounds.

The use of metallic fuel additives in combustionapplications has a long and at times controversial his-tory. For example, from the first half of the previ-ous century through the 1970s, tetraethyl lead was apopular additive in gasoline to enhance octane levels(i.e., reduce engine knock). However, due to envi-

ronmental and health concerns, the EPA ultimatelybanned leaded gasoline for on-road vehicles. Morerecently, MMT (methylcyclopentadienyl manganesetricarbonyl) has continued to be used as a fuel additivefor gasoline in many countries, although it remainsof interest to regulatory agencies, given the listing ofmanganese as a Clean Air Act metal.

As an alternative to additives containing Pb andMn (which target combustion performance), metal-lic additives have also been explored with the goalof altering sooting characteristics. Such additives in-

clude the alkali metals Li, Na, K, and Cs and thealkaline earth metals Ca, Sr, and Ba. In addition, tran-sition metals are of interest, particularly Mn and Fe.The history of iron-based additives dates back to the1960s, with Shayeson demonstrating a reduction in jetengine exhaust smoke with several additives, includ-ing iron compounds [2]. A comprehensive summaryof the early additives work, including discussion ofpractical combustors and laboratory flames, is pre-sented by Howard and Kausch, in which Fe and Mnare noted among the most effective additives under

heavily sooting conditions [3]. In general, additiveperformance results vary significantly, depending onfuel type, additive concentration, stoichiometry, andcombustor configuration. Interest in the current studyis limited to iron, due primarily to previous research,summarized below, that suggests an important rolein soot control, as well as the relatively benign na-ture of iron with respect to environmental and healthconcerns. Pertinent literature regarding iron-based ad-ditives is briefly summarized below, with attentiongiven to sootiron interactions and iron speciation.

Several laboratory studies using laminar, premixedflames have explored the roles of iron, as either fer-rocene Fe(C5H5)2 or iron pentacarbonyl Fe(CO)5,in hydrocarbon flames. Ritrievi et al. [4] studiedlaminar premixed ethylene flames seeded with fer-rocene, Fe(C5H5)2, for dopant concentrations of0.0050.14% Fe by weight of fuel, and for flame C/Oratios of 0.710.83. In studies of a similar premixed

ethylene flame seeded with ferrocene conducted byFeitelberg et al. [5], iron was added to the fuel in200 ppm concentrations on a molar basis (0.13% Feby weight of fuel). Hahn and Charalampopoulos [6]

studied a premixed propane flame seeded with ironpentacarbonyl, Fe(CO)5, with fuel equivalence ratiosof 2.4 and 2.5. Iron pentacarbonyl was added in con-centrations of 0.160.32% by weight of iron to thefuel. While exact results vary somewhat with fuel typeand stoichiometry in the above studies, the consistentfindings were that the addition of iron tended to in-crease the amount of soot formed (i.e., soot volumefraction), including increases in both soot particlesize and number density. These results are all con-sistent with previous work performed by Bonczyk [7]

and Haynes et al. [8]. The consensus of such studiesis that iron-based compounds nucleate prior to sootinception, providing increased surface area for sootformation and growth, ultimately leading to enhancedsoot emissions, although questions were raised as tothe exact nature of the iron-based compounds and fateof iron species. In efforts to answer these questions,various means of modeling, extractive sampling, andchemical analysis were explored in the above stud-ies to determine the chemical states of iron within thesoot.

The research groups discussed above reportedcombinations of Fe and FeO [4] and Fe2O3 [6],based on extractive sampling and subsequent chemi-cal analysis, although no in situ measurements havebeen made to date. The overall conclusion of theabove studies is that any soot suppression effects mustbe limited to the soot burnout (i.e., oxidation) regionsof the flame, which are absent in the premixed flameconfigurations. In actual combustors, the final stagein the soot emission cycle is generally soot oxidationor burnout, driven by the diffusion of additional oxy-

gen following primary combustion. In this regime, thesoot particles are partially or completely destroyed,generally via OH and O2 oxidation, yielding CO orCO2 as products. Therefore, while flat-flame burn-ers are an excellent laboratory tool, further attentionmust be given to alternative flame configurations thatare closer to the practical combustor configurations inwhich diffusion flames often play a key role.

In a study using an isooctane diffusion flame op-erating above its smoke point, Bonczyk [9] notedboth soot-enhancing and -suppressing characteristics

of ferrocene added in concentrations of 0.09% Fe byweight of fuel. At early flame residence times, thesoot particle size and number density all increased,resulting in enhanced soot production similar to thatin premixed flames. However, these same parameterswere observed to decrease through the soot burnoutregimes of the flame, with the net effect being a re-duction in soot when compared to unseeded flames.


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166 K.B. Kim et al. / Combustion and Flame 154 (2008) 164180

Postflame sampling and subsequent chemical analy-sis revealed that Fe2O3 was the primary condensatephase. The study concluded that iron might be ini-tially reduced (via reaction with solid carbon) from

an oxide to elemental iron within the soot particles,eventually enhancing carbon deposition due to thecatalytic effects of Fe. At later residence times, ele-mental iron might be oxidized, for example to formFe2O3. The net result of this cycle is the oxidation ofcarbon to CO.

Such a method of iron reduction and subse-quent oxidation is also supported by Zhang andMegaridis [10], who studied an ethylene diffusionflame seeded with ferrocene, as well as by Kasperet al. [11], whose investigation included ferrocene-

seeded methane/argon and acetylene/argon flames. Inparticular, Zhang and Megaridis [10,12] performeddetailed chemical microanalysis of sampled soot par-ticles, verifying the presence of elemental iron withinthe soot agglomerate matrix for particles samplednear the fuel-rich burner axis, while the presence ofboth iron and oxygen suggestive of iron oxides wasfound in aggregates sampled from other parts of theflame. They concluded that iron nucleation prior tosoot formation and subsequent incorporation of iron-rich nuclei into the soot matrix was a critical compo-

nent for soot suppression. A related study of the roleof iron addition in flame inhibition in both premixedand laminar flames offered additional insight into ironnucleation [13,14]. The authors reported the forma-tion of iron-rich particles early in the flame, wheretheir calculations predicted the presence of Fe, FeO2,FeO, Fe(OH)2, and FeOH. Flame residence time, lo-cal stoichiometry, and temperature were all found tobe influential parameters, although the former was themost important factor.

There is not a significant amount of literaturebridging the gap between practical combustors andlaboratory flames. Toward this end, the effects of fuelspecification and fuel additives on soot formation wasreported in complex flow swirl combustors, includingthe effects of ferrocene addition to JP-8, isooctane,and blends of isooctane [15]. The study reported dis-parate soot suppressing effects with fuel type, andfurther concluded that simple smoke point measure-ments are not good predictors of additive performancein a complex flow field. A more contemporary studyreported similar findings, namely that the use of thesmoke lamp for assessing the soot-reducing poten-tial of metal-containing additives is not effective [16].Iron-based additives (ferrocene and iron naphthenate)were found to reduce soot emission only when oxy-gen was available, thereby supporting the hypothesisof soot-incorporated iron species as oxidative cata-lysts.

It is important to note that the findings charac-terized above are readily summarized by an increasein the burnout efficiency of soot particles in seededflames, not by an inhibition of soot formation with the

introduction of iron. In fact, seeded flames are likelyto realize peak soot production levels higher thanthose in unseeded flames due to increased surface areafor soot formation in the particle inception regimes, asquantified in the premixed flame studies noted above.More recent studies have added to these findings. Kimet al. [17] performed a unique set of experiments toexplicitly quantify the oxidation properties of flame-generated soot in the presence of metallic additives.They found that the addition of iron (as iron pentacar-bonyl) reduced the activation energy of soot oxidation

from 162 to 116 kJ/mol. Mass spectrometry of col-lected soot particles revealed strong signals indicativeof elemental iron, although relatively weak signals ofFeO+, Fe2O+, and Fe2O+3 were also reported. Tosummarize, the consensus of the literature is that thenet reduction of soot is realized by apparent enhance-ments in soot oxidation throughout the burnout (i.e.,oxidation) regime, including the catalysis of solid car-bon oxidation by various combinations and cycles ofFe and iron oxides [4,6,7,9,18].

An important point is made regarding the addi-

tive studies reviewed above, namely, that all analysesof soot species were ex situ (i.e., based on extrac-tive sampling); hence all methodologies are subjectto possible changes in iron states when soot sam-ples are removed from the true flame environmentprior to analysis. Therefore, a thorough understand-ing of the role of iron additives on soot emissionsis still somewhat limited by the unknown nature, in-cluding transient behavior, of the iron species withinthe flame environment. The current study involves de-tailed measurements of the soot inception, growth,

and soot oxidation regimes using diffusion flame con-figurations, combined with in situ diagnostics, withthe goals of further understanding the roles and mech-anisms of iron and iron speciation in soot suppression.

2. Experimental methods

2.1. Burner systems

All experiments were performed using prevap-

orized isooctane/oxygen diffusion flames, either un-seeded or seeded with iron pentacarbonyl, Fe(CO)5.Isooctane (HPLC-grade, Fisher Scientific) and ironpentacarbonyl (99.5%, Alfa Aesar) were generallymixed just prior to use to prevent any dissociationof the iron pentacarbonyl. Overall, isooctane providesa liquid fuel in which iron pentacarbonyl is readilysoluble, thereby ensuring reliable introduction of the


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Fig. 1. Concentric diffusion burner schematic (side view and top view). See Table 1 for dimensions of Burner 1 and Burner 2.

additive. Furthermore, the current use of isooctane iscomplementary to the detailed work of Bonczyk [9],although the wide variety of fuel types explored inthe literature (see above) provides no clear consensus

choice for fuel, but rather, the results tend to followthe flame configuration (i.e., diffusion vs premixed).

Two different concentric tube burners were em-ployed to investigate the soot inception, growth, andoxidation regimes with sufficient spatial resolution.The first system, designated Burner 1, was specif-ically designed for studying the soot inception andgrowth regimes, while the second system, designatedBurner 2, was designed for exploring the soot burnoutregime. A representative schematic of the burners isshown from side and top views in Fig. 1, and the exactdimensions of the two burners, along with flame pa-rameters, are summarized in Table 1. The liquid fuelwas prevaporized using a vaporization system con-sisting of a stainless steel tube, packed with stainlesssteel balls, maintained at 100 C, which was above theboiling point for the liquid isooctane (99 C). A nitro-gen coflow stream was introduced into the vaporiza-tion system at the head of the preheat zone at a rateof 0.8 l/min to assist in maintaining a steady output.For Burner 1, the liquid fuel was delivered to the va-porization system via a peristaltic pump at a rate of1 ml/min, which corresponds to an isooctane vaporflow rate of 0.18 l/min at atmospheric pressure. Themixture of fuel vapor and nitrogen was then passedthrough the center tube of the burner. A flow rate of9 l/min of pure oxygen was passed through the outerconcentric tube, resulting in a laminar, sooting diffu-sion flame. The resulting flame diameter was about

Table 1Concentric diffusion burner dimensions and flame parame-ters

Burner 1 Burner 2

Inner tube ID(fuel)

7.04 mm 1.5 mm

Outer tube ID 16.56 mm 6.96 mmArray tubes

(oxygen)2.54 mm ID 0.3 mm ID12 holes 9 holes

Flame stabilization Mesh stabilized UnstabilizedFlame height 5.5 cm 31 cmC8H18 (liquid) 1 ml/min 1.5 ml/minN2 coflow 0.8 l/min 0.8 l/minO2 9.0 l/min 2.6 l/minFroude number 0.1 11.8Fe(CO)5 fuel

seeding4000 ppm (mass) 4000 ppm (mass)

7.5 mm at the burner lip, increasing to a diameter ofnearly 12 mm at a height of 41 mm above the burnerexit. A stainless steel mesh flame holder was placed55 mm above the lip of the Burner 1 to promote flamestability of the buoyancy-driven flame (Froude num-ber of 0.1), thereby enabling spatially resolved light-scattering measurements.

For Burner 2, isooctane was prevaporized at a rateof 1.5 ml/min with a nitrogen coflow of 0.8 l/min.A flow rate of 2.6 l/min of pure oxygen was passedthrough the outer concentric tube array. The result-ing flame was operated above the smoke point, with atotal flame length of 31 cm and an average flame di-ameter slightly below 1 cm. However, no flame holderwas employed for these experiments, as it was desired


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Fig. 2. Schematic of the light-scattering configuration (top view).

to record soot profiles through the entire flame length.Therefore, the dimensions of the burner lip were re-duced to enable the burner to provide a more stable,momentum-driven flame (Froude number of 11.8)

conducive to measurements throughout the flame oxi-dation regime. Given the dilution of the fuel flow withnitrogen, the use of pure oxygen as opposed to air wasfound to promote flame stability. The significant res-idence time of both flames, combined with the lackof any shroud flow, suggests considerable diffusion ofambient air, minimizing any effects due to the use ofpure oxygen. The flame temperatures were recordedusing a 1/16-diameter type K thermocouple probecorrected for radiation loss. The maximum flame tem-perature was 1850 K, recorded at the height of 1.8 cm,

after which the temperature decreased steadily, drop-ping to 1200 K at 15 cm (the start of significant sootoxidation), and to 1000 K at 22 cm, above which thereis a precipitous drop approaching the flame tip.

For all iron-seeded flame experiments, the ironpentacarbonyl was added to the liquid isooctane at aconcentration of 4000 ppm by mass (0.11% Fe permass of fuel), which was found to be the most effi-

cient value for investigating soot suppression effectsin this study, as described below.

2.2. Optical soot diagnostics

Spatially resolved light-scattering and transmis-sion measurements were carried out using a fre-quency-doubled (532-nm) Q-switched (8 ns fwhm)Nd:YAG laser. The laser beam was vertically polar-ized with respect to the horizontal scattering plane.The laser was operated at a pulse repetition rate of10 Hz, with a pulse energy of 0.3 mJ/pulse, andwith a beam cross-sectional area of 0.0033 cm2 atthe center of the flame. The corresponding laser flu-ence was equal to 0.09 J/cm2, which was determined

in a previous study of pulsed laser/soot interactionsto produce no soot vaporization [19]. Fig. 2 showsthe optical setup for the light-scattering system. Thelaser beam first passed through an aperture to removethe edges of the Gaussian laser intensity profile be-fore it was directed through the primary focusinglens (f= 25 cm), such that the focused beam passedthrough the center of the flame. After exiting the


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flame, the beam was terminated in a low-reflectancelaser beam dump. The burner apparatus was enclosedwith an opaque Plexiglas box that provided largeopenings (20 by 10 cm) along the laser axis and a

5-cm-diameter hole in line with the PMT. The scat-tered light from the soot particles was collected at90 with respect to the incident beam and imagedonto the surface of a photomultiplier tube with unitymagnification using a 100-mm focal length bicon-vex lens. Apertures (0.5 mm in diameter) were usedboth in front of the biconvex lens and in front of thePMT inlet to further define the scattering volume. Inaddition, a 532-nm narrow-line laser filter and a po-larizer oriented to pass vertically polarized light withrespect to the horizontal scattering plane were used

to reject stray light, as well as to ensure the mea-surement of verticalvertical scattering, as discussedbelow. Finally, neutral density (ND) filters were usedas needed to ensure PMT signal linearity, which waschecked prior to all measurements. As implemented,the light-scattering collection system ensured that thescattered light was collected only from the precisescattering volume defined within the flame, with asolid collection angle of about 0.05 sr. The PMT sig-nal was recorded with a digital oscilloscope (500 Hz,4 GS/s). A precision high-voltage supply was used

to drive the photomultiplier tube, typically at 650 V.Individual measurements were recorded as the ensem-ble average of 300 laser pulses, which correspondsto a 30-second average at the 10-Hz laser repetitionrate.

Absolute differential scattering coefficients (K VV= N VV) were measured by performing a calibra-tion using ultra-high-purity methane gas at ambientpressure and temperature as a calibration standard.Corrections for stray light were made by recording thescattering ratio of the calibration methane signal to a

high-purity nitrogen signal, noting that the ideal ratiois 2.17 in the absence of any stray light for the currentparameters. For all experiments, stray light calibrationmeasurements were taken prior to every flame studyto determine an experiment-specific stray light value.Over the range of experimental measurements, the av-erage contribution of stray light was about 1/3 of themethane calibration signal. In addition, flame trans-mission measurements were recorded using a laserpower meter fitted with a narrowband 532-nm laserline filter. This setup is identical to the scattering sys-

tem shown in Fig. 2, except that the beam dump isreplaced by the laser power meter and aperture. In ad-dition to the use of a narrowband filter, correctionswere made for flame emission at all heights, althoughthe contribution was not significant.

Laser-induced fluorescence (LIF) measurementswere performed to assess the chemical state of theiron species. An optical parametric oscillator (OPO)

was pumped using an injection-seeded, 355-nmNd:YAG laser and the resulting laser pulse was fre-quency-doubled to realize the desired excitation fre-quency. LIF signals were recorded by collecting emis-

sion along the direction of beam propagation (i.e.,backscatter) using a pierced mirror and a 50-mm-diameter collection lens. Collected light was fiber-coupled to a 0.275-m spectrometer and recorded us-ing an intensified CCD array detector that was syn-chronized to the laser pulse. A 200-ns intensifier gatecentered on the laser pulse (8 ns fwhm) was usedfor all LIF experiments, which effectively eliminatedflame emission during the narrow detector gate.

2.3. Soot sampling

To complement the in situ light-scattering mea-surements, thermophoretic sampling was conductedover a range of flame heights for Burner 2. Thesoot aggregates were extracted from the flame usingthermophoretic sampling, and subsequent transmis-sion electron microscopy (TEM) analysis was imple-mented to determine the size and morphology of sootaggregates. Soot samples were collected directly onFormvar-carbon coated 150-mesh copper TEM grids(Electron Microscopy Sciences, Hatfield, PA) at 12different heights along the vertical axis of both theunseeded and seeded flames using Burner 2. For sam-pling, TEM grids were affixed to a holder and thenpassed through the flame on a horizontally swing-ing arm. It is necessary that the exposure time ofthe grids be long enough to capture an appropriateamount of soot, but short enough to avoid damagingthe grid or oversampling the aggregates. The sam-pling times were controlled manually in the presentstudy. As a result, each sample had a slightly differentresidence time in the flame, but care was taken thatall exposure times of the films were on a timescale ofms, as based on the total translation time and flamewidth.

TEM grids were analyzed using a JEOL 2010Fanalytical electron microscope system (point reso-lution of 0.19 nm). TEM analysis yielded primaryparticle diameters, overall agglomerate dimensions,and numbers of primary particles per agglomerate,which were subsequently used to calculate fractal di-mensions. Magnifications used for the present mea-

surements ranged from 50,000 to 330,000. Foranalysis, individual soot aggregates were randomlyselected at low magnification and then analyzed at theoptimum magnification. Aggregate dimensions andnumber of primary particles were manually extractedfrom individual TEM micrographs using a light box, amicrometer, and the calibrated scale bar on each TEMimage.


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3. Results

3.1. Burner 1: soot formation and growth regime

To spatially resolve the soot formation and growthprocesses, light-scattering and transmission measure-ments were made between the heights of 8.7 and41.0 mm using Burner 1. Radial measurements wererecorded from the central axis outward to a radiusof 3.2 mm, using 0.64-mm steps. At each flameposition, the absolute differential scattering coeffi-cients K VV (cm

1 sr1) were determined from thedirect calibration procedure. The transmission mea-surements were reduced using a three-point Abelinversion routine [20], yielding an extinction coeffi-

cient (cm1

) at each radial position. By pairing themeasured extinction and differential scattering co-efficients, classical light-scattering theory was usedto solve for the equivalent spherical particle diam-eter and the particle number density. As describedbelow, thermophoretic sampling was combined withRayleighDebyeGans (RDG) theory to invert thelight-scattering data for Burner 2. However, the ra-dially resolved light-scattering measurements per-formed for Burner 1 were not compatible with ra-dially resolved soot particle sampling. In addition,

significant agglomeration is considered less importantin the nucleation and initial growth regimes probedwith Burner 1. With these factors and limitations inmind, the Burner 1 data were inverted using Mietheory, utilizing a zeroth-order logarithmic size dis-tribution and a refractive index m = 2.0 0.35i asreported for flame-generated soot [21]. In order tocharacterize the scattering and extinction propertiesof a polydisperse particle system, a suitable distribu-tion function is necessary. Espenscheid et al. [22] pro-posed a zeroth-order lognormal distribution (ZOLD)to characterize a polydisperse system of particles. TheZOLD function is defined as

(1)p(r)= exp(2o /2)

2 ormexp

(ln r ln rm)222o

,

where r is the particle radius, rm is the modal valueofr , and o is a dimensionless measure of the widthand skewness of the distribution. The mean radius ris related to rm by

(2)r = rm exp

3

22o

,

and the true standard deviation of the ZOLD is re-lated to o and rm by

(3)= rmexp

42o

exp32o 1/2.While the ZOLD was originally presented as an alter-native to the well-known lognormal distribution, care-ful examination reveals the ZOLD to in fact represent

the traditional lognormal distribution, only character-ized by the mode rather than the mean, and by thestandard deviation of the distribution of the log. Fordata analysis, o was fixed at a value of 0.2, which is

close to the value expected for a self-preserving sizedistribution (o = 0.285), and was found to provide asufficiently broad region of solution space character-ized by a single value for data inversion. Specifically,the ratio of the differential scattering coefficient tothe extinction coefficient is reduced to a function ofmodal diameter alone for a given value of o. How-ever, in general, the functional relationship becomesmultivalued beyond a certain modal diameter, leadingto ambiguity during data inversion. For the presentanalysis, the selected complex refractive index and

skewness parameter o yielded an acceptable rangeof K VV/Kext with regard to the monotonic solutionrange for data inversion.

The optical properties of soot, notably flame-generated soot, have been the focus of much researchin the combustion community, and, in fact, remain acontemporary issue. In general, the exact nature ofoptical properties can be expected to vary with flameconditions (e.g., stoichiometry) and fuel type; hence,researchers are faced with no clear choice when se-lecting a suitable index value for data inversion. A re-

cent review of light-absorbing carbon provides a de-tailed treatment of the role of optical properties in ab-sorption and light scattering [23]. A unified inversionscheme for determining the optical properties of ag-gregates has also been reported [24]. The use of par-ticular index values was explored in a paper by Smythand Shaddix, which demonstrates the complexitiesand at times ambiguities associated with the selectionof a particular index value [25]. Although the valueselected for the present study, namely 2.0 0.35i,is consistent with range of recommended values [23]

for the real component, namely 1.9 to 1.95, the imag-inary component lies in the lower range of recom-mended values. To quantitatively assess the influenceof soot optical properties on data inversion, the cur-rent data were reanalyzed using an alternative valueofm= 1.57 0.56i (see Ref. [25]). The overall sootparameters such as the volume fraction, as discussedbelow, were found to increase by about 28% with thealternative index value; however, the final percentagereduction in soot volume fraction found with iron ad-dition was completely unchanged. In earlier studies,

Charalampopoulos et al. examined the effect of ironinclusion on the effective optical properties of sootagglomerates [26,27]. It was found through detailedanalysis using the MaxwellGarnett effective indexmodel that modest iron inclusion volume fractionsconsistent with the current study (see TEM analy-sis below) did not appreciably alter the scatteringresponse as compared to homogeneous soot-particle


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Fig. 3. Measured soot particle modal diameters as a func-tion of radial position recorded for Burner 1 at a height of8.7 mm. Data are reported for unseeded and iron seeded(4000 ppm) flames.

models. More recently, Shu and Charalampopoulosexamined methodologies for measuring the effectiverefractive indices of combustion-synthesized iron ox-ide agglomerates [28], although such an approach is

considered beyond the current scope of work. In con-cert, the above comments support the conclusion thatno findings reached in the current study will be al-tered by the use of alternative soot optical propertiesor refractive index models.

The experimentally measured scattering-to-ex-tinction ratio is related to Mie scattering theory usingthe relationship

(4)K vvKext

= N

r=0 vvp(r)dr

N

r

=0 extp(r)dr

,

where the differential scattering cross section (vv)and the extinction cross section (ext) are integratedusing the size distribution function p(r), per Eq. (1),as the weighting function. Note that the overall parti-cle number density N (cm3) cancels from Eq. (4),making the relationship a function of the modal par-ticle size and the skewness of the distribution func-tion. The full Mie solution was used to evaluate thescattering and extinction cross sections, and the inte-gration was performed numerically using the trape-

zoid rule. Rather than iteratively solve Eq. (4) foreach extinction-to-scattering ratio, a third-order poly-nomial curve was fit to the function of K VV/Kextvs modal diameter. This relation was then invertedto yield a straightforward solution of modal diam-eter for each experimental K VV/Kext ratio. Fig. 3shows the measured modal particle diameters for boththe unseeded and iron-seeded flames as a function

of radial positions for a height of 8.7 mm above theburner tip. In general, the current modal diametervalues in the range of 20 to 80 nm are consistentwith soot particle/agglomerate sizes from hydrocar-

bon flames inverted with light-scattering theory. Thelight-scattering results reveal an increase in soot parti-cle size at the lower flame heights with iron addition,including a factor of 2 increase in particle size nearthe center axis, as shown in Fig. 3. The increase insoot particle size at lower flame heights is concludedto arise from the presence of additional surface areafor soot nucleation and growth, which results from thenucleation of iron species (Fe or Fex Oy ) in advanceof soot nucleation. Hence, soot inception and initialgrowth processes are advanced by the availability of

what are essentially seed nuclei that provide addi-tional surface area. However, such gains are eventu-ally offset as the soot particles are altered via surfacegrowth at higher flame heights, thereby envelopingthe iron-rich nuclei within soot particles. Toward thehigher regions of the flame, the growth rates of the un-seeded and seeded flames were found to converge, asthe initial increase in surface area with iron additionis lost as compared to the unseeded flame followingsignificant soot growth.

In addition to soot particle size, it is useful to ex-

amine the soot particle number density and the corre-sponding soot loading (i.e., volume fraction) betweenflame conditions. Solution of Eq. (4) for the soot par-ticle modal diameter readily allows calculation of theoverall soot particle number density N from the rela-tion N= ext/Kext. With the particle modal diameterand number density known, the particle volume frac-tion is determined from

(5)fv =4

3N r3m exp

15o

2

,

where rm and o are ZOLD parameters described pre-viously. Equation (5) is realized by the product of theparticle number density and the weighted integral ofthe single-particle volume using the size distributionp(r) as the weighting function. Fig. 4 shows the mea-sured soot particle volume fractions (i.e., total soot)at the greatest height measured in Burner 1, namely41 mm above the surface. The Fig. 4 data reveal aslight increase in soot volume near the burner axis,although such differences are well within the exper-imental uncertainty. Specifically, Abel inversion can

compound data inversion errors, notably those asso-ciated with the innermost data point; hence consider-able uncertainty can be expected for the central node,as reflected by the relatively large error bar corre-sponding to this location in Fig. 4. Examination of therate of change of the integrated soot volume fractionswith respect to flame height (i.e., soot growth rate)revealed continuous soot growth throughout the mea-


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Fig. 4. Measured soot particle volume fractions as a functionof radial position recorded for Burner 1 at a height of 4.1 cm.Data are reported for unseeded and iron-seeded (4000 ppm)flames.

surement region of Burner 1. Recall that the Burner 1configuration was designed to probe the soot incep-tion and growth regimes with high resolution; hence itwas expected that no significant soot oxidation wouldbe observed.

Overall, it is concluded that the roles of the ironadditive are primarily limited to iron nucleation andsubsequent incorporation into the soot particulateswithin the inception and growth regimes. TEM analy-sis, reported below, will further define the role of ironincorporation. The exact nature of the iron chemicalstate remains to be determined, and additional discus-sion will be done in combination with the soot oxida-tion region measurements following the next section.

3.2. Burner 2: soot oxidation regime

The investigation of the soot oxidation (i.e., burn-out) regime was performed using the Burner 2 con-figuration, which provided sufficient residence timeto observe soot burnout. Because the strong radialdependence of soot properties gives way to a nearlyuniform radial profile at relatively great flame heights(as verified experimentally), combined with the en-hanced spatial variability of the unstabilized flame,only axial (i.e., = 0) light-scattering measurementswere recorded.

The unstabilized Burner 2 configuration providedaccess to flame residence all the way to the flametip, which was necessary to examine the oxidationregime to the full extent. This flame was also use-ful for selecting an optimal concentration of ironpentacarbonyl by examining the smoke point depen-dence. Specifically, the smoke point was determinedby adjusting the oxygen flow rate, for a fixed fuel

Fig. 5. Measured smoke point (expressed as fuel equivalenceratio) as a function of Fe(CO)5 concentration recorded forBurner 2. Error bars represent one standard deviation.

flow rate and iron pentacarbonyl concentration, un-til the point at which visible smoke at the flame tipfirst occurred. These measurements were repeated foriron pentacarbonyl concentrations ranging from 0 to

20,000 ppm, with the results presented in Fig. 5.An increased fuel equivalence ratio (i.e., increasinglyfuel-rich) at the smoke point corresponds to a reducedpropensity to smoke; hence the Fig. 5 data demon-strate that the addition of iron pentacarbonyl providesbenefits regarding the breakthrough of visible sootfrom the flame tip (i.e., smoke). Furthermore, the datareveal a breakpoint near a seeding concentration of4000 ppm iron pentacarbonyl, beyond which addi-tional seeding concentrations provide only marginalchange in the measured smoke point. The Fig. 5 data

also provide insight into the relative roles of diffu-sion and surface reaction, suggesting the importanceof the former. Given the complexity of the current ex-periments, it was desirable to explore a single optimalvalue of iron pentacarbonyl seeding rather than per-form a broad, parametric study; hence, 4000 ppm wasselected as the final value for the iron pentacarbonylseeding concentration.

In a manner similar to that followed before, ab-solute differential scattering coefficients were re-corded between a height of 9.4 cm above the burner

tip and a height of just over 25 cm, which was es-sentially the flame tip. Fig. 6 presents the averagevalues of the differential scattering coefficients alongwith the standard deviation (N = 10 observationsover multiple days) for the unseeded and iron-seededflames. The differential scattering coefficients tend topeak in both cases near a height of about 14 cm abovethe burner surface, and then steadily decrease with ad-


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Fig. 6. Measured differential scattering coefficients as afunction of height above burner recorded for Burner 2.Data are reported for unseeded and iron-seeded (4000 ppm)flames. Error bars represent one standard deviation.

ditional height (i.e., increasing residence time). Thisbehavior indicates that the soot profile has transi-tioned from the soot growth regime to the oxidation

regime above this height, which is characteristic ofdiffusion flames. Regarding the comparison of theunseeded and the iron-seeded flames within the sootgrowth regime (i.e., lower flame heights), the scatter-ing coefficients are essentially identical (within ex-perimental variability) for both seeded and unseededflames. Such behavior is in excellent agreement withthe spatially resolved Burner 1 data, which like-wise probed the soot formation and growth regime,supporting the conclusion that iron particulates arequickly incorporated into soot agglomerates, giving

way to typical (i.e., unseeded) soot growth and sootyields. Meanwhile, the prominent deviation betweenthe two flames in scattering coefficient is observedin the soot oxidation zone, increasingly so at greaterflame heights approaching the flame tip. The Fig. 6data support the conclusion that iron addition actswithin the soot oxidation regime of the flame. It isnoted that soot processes are generally considered tooccur in concentric regions between the flame centerand edge zone [29], which is consistent with the radi-ally resolved measurements of Burner 1 (see Fig. 3).

However, for Burner 2, radial scattering measure-ments did not reveal any significant radial structurethroughout the oxidation zone (>15 cm); hence thecurrent in situ measurements and extractive samplingdata are considered representative of the relevant sootprocesses (e.g., oxidation).

To support quantitative analysis of the light-scattering measurements, soot particles were sampled

at axial locations using thermophoretic sampling, asdescribed above, and subsequently analyzed usingTEM. Typical soot particle micrographs are presentedin Fig. 7 for lower and upper flame positions, for both

the unseeded and iron-seeded flames. The TEM im-ages were analyzed to extract the relevant parameters,including agglomerate size and fractal dimensions,and the primary soot particle size and primary particlenumber density (i.e., particles per agglomerate). Foreach flame height investigated, 375 primary soot par-ticles were randomly selected from a number of sootagglomerates, to provide a statistical average of theprimary particle diameters. Consistent with the resultsfrom Burner 1, the soot primary particle parameterswere similar in both size and morphology at the lower

flame heights. Specifically, the average primary sootparticle diameters at the lower heights (averaged be-tween 9.4 and 11.7 cm above burner) were equal to 31and 29 nm in the unseeded and iron-seeded flames, re-spectively. The relative standard deviations averagedto 6.5% for these data; hence, the difference is on theorder of the experimental uncertainty. In contrast, atthe greatest residence time investigated (H= 25 cm),the primary particle diameters were reduced to anaverage of 24 and 20 nm in the unseeded and iron-seeded flames, respectively. The overall decrease in

particle size with increasing flame height (i.e., flameresidence) is consistent with significant soot oxidationin both flames. However, the 37% greater reductionin final soot particle size in the iron-seeded flame ascompared to the unseeded flame is characteristic ofenhanced soot oxidation rates and overall soot sup-pression. Overall, the primary particle diameters, asmeasured from the TEM micrographs, depict the tran-sition from soot growth to soot oxidation along thelength of the flame. As described above, no radialflame resolution was realized for the current soot sam-

pling. It is difficult to assess the overall uncertainlyassociated with the radially averaged soot sampling;however, the overall uniformity of the observed sootagglomerate TEM data (e.g., 6.5% RSD for primarysoot particles) provides some quantitative assessmentof variability. To further quantify the overall differ-ences between the two flame conditions, it is useful toexamine the total soot volume fractions.

As the soot particles undergo a combination of sur-face growth and significant agglomeration with flameresidence time, their morphology evolves to a frac-

tal structured aggregate. Using either Rayleigh scat-tering theory or Mie scattering theory itself is nota reliable application for the large-sized and open-structured soot agglomerates. Such particles are moreaccurately modeled using the RayleighDebyeGans(RDG) scattering theory. The RDG theory has beenwidely used to interpret light-scattering data from ag-gregates, notably flame-generated soot [3033]. The


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Fig. 7. TEM images of sampled soot particles. H= 10 cm for unseeded (top left) and iron-seeded (top right) samples. H= 25 cmfor unseeded (bottom left) and iron-seeded (bottom right) samples.

major assumptions for RDG applicability include that

the soot agglomerates are fractal-like objects with amass fractal dimension of less than 2 and that the

primary particles are sufficiently small so that theysatisfy the Rayleigh scattering theory.

The morphological fractal features of soot ag-glomerates can be characterized by a power-law re-lationship between the number of primary particles inan aggregate and its projected area on the TEM im-age,

(6)Np

=kf(Rg/dp)

Df,

where Np is the number of primary particles per ag-gregate, kf is the fractal prefactor, Rg is the radius ofgyration of an aggregate, dp is the primary particlediameter, and Df is the mass fractal dimension im-plying the openness of the soot aggregate. Both theparameters Np and dp were directly determined us-ing transmission electron microscopy (TEM) analysis

of thermophoretically sampled soot aggregates. The

radius of gyration then can be calculated using a cor-relation [32], namely,

(7)(LW)1/2/(2Rg)= 1.17,where L is the geometric mean of maximum length,and W is the maximum projected width normal to L.With these parameters known, the fractal propertiesare obtained using a linear regression method with aleast-squares approach. In other words, when Np isplotted as a function of Rg/dp in logarithmic scale

for a set of aggregates, the fractal dimension de-scribes the slope, and the fractal prefactor determinesthe magnitude of the least-squares straight-line fit tothe data. The analysis of hundreds of individual ag-glomerates revealed no statistical difference in fractaldimension between the unseeded (1.84 0.08) andseeded flames (1.80 0.14), with the overall averagefractal dimension equal to 1.82 (6% RSD) for all data.


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This value is in excellent agreement with a number ofvalues reported for flame-generated soot, which rangefrom 1.54 to 1.85 [3033].

Using the primary soot particle parameters in com-

bination with RDG theory enables calculation of theaverage agglomerate differential scattering cross sec-tion. The differential scattering cross section (cm2/sr)of a single soot agglomerate is defined as

(8)agg =N2p pS(q),where the differential scattering cross section of theprimary soot particle within an agglomerate, p, isassumed to satisfy the criteria for Rayleigh scatter-ing theory, namely that the size parameter ( dp/)is much less than unity. The new parameter, S(q),

shown in Eq. (8) is denoted as the angular scatteringform factor, given by

(9)S(q)= C(qRg)Df,provided that the product qRg is less than unity. Inthese expressions, q is the modulus of the scatteringvector (cm1), defined as

(10)q = 4

sin(/2).

The measured differential scattering coefficient(cm1 sr1) is then a product of the calculated dif-ferential scattering cross section of the soot agglom-erates (Eq. (8)) and the overall soot agglomerate num-ber density, Nagg, namely,

(11)K vv,agg =Naggagg.Based on the above analysis, the TEM data were usedto calculate the average differential scattering crosssection of the soot agglomerates at each flame height.This value was then combined with the measured dif-

ferential scattering coefficient and Eq. (11) to yieldthe soot agglomerate number density as a function offlame height for both the unseeded and iron-seededflames. The overall soot volume fraction, fv, is thencalculated from the relation

(12)fv =

6d3p

NpNagg,

where Np and dp are the average number of primaryparticles per aggregate and the primary particle diam-eter, respectively, as directly measured per the TEM

analysis.The volume fraction data as a function of flameheight are presented in Fig. 8 for the unseeded andiron-seeded flames. Error bars represent the standarderror based on the formal propagation of experimen-tal error (as standard deviation) related to each of themeasured fundamental parameters, namely the differ-ential scattering coefficient, angular scattering form

Fig. 8. Measured soot volume fractions as a function ofheight above burner recorded for Burner 2. Data are re-ported for unseeded and iron-seeded (4000 ppm) flames.The dashed line represents the nominal transition from sootgrowth to soot oxidation. Error bars represent the standarderror.

factor, number of primary particles per aggregate, andprimary particle diameter. The Fig. 8 data reveal two

distinct regions with regard to the comparison of theunseeded and iron-seeded flames. In the lower regionsof the flame, below about 15 cm, the volume fractionsof the two flames are comparable within experimentaluncertainty (i.e., error bars). This is in agreement withthe Burner 1 results, and is the region where the ironis incorporated within the soot aggregates during theprimary growth regime, thereby exerting little influ-ence after inception. However, over the last 5 cm offlame residence, the iron-seeded flame is character-ized by a marked reduction in soot volume fraction.

Specifically, at a height of 25 cm, which is near theflame tip, the soot volume fraction is reduced from3.52 107 in the unseeded flame to a value of1.22 107 in the iron-seeded flame, correspond-ing to a 66% reduction in soot emissions. The Fig. 8data support the conclusion that the soot-suppressingrole of the iron additive functions primarily in the sootburnout regime, although additional discussion is re-quired as to the exact mechanisms of action.

3.3. Chemical speciation of iron species

To elucidate the processes through which the ironaddition apparently enhances the oxidation of soot,it is necessary to understand the chemical state ofthe iron species within the soot aggregates, as dis-cussed above. Several additional measurements wereperformed toward this goal. First, in the Burner 1configuration, rust-colored residue was noted on the


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Fig. 9. Raman spectra of iron oxide (Fe2O3) reference pel-let (upper spectrum) and of the red deposit observed on theflame holder of Burner 1 after iron-seeded operation (lowerspectrum).

surface of the flame holder with iron seeding of thefuel. Confocal micro-Raman spectroscopy (LabRamInfinity, Jobin Yvon) was used to analyze the state ofthe residual iron on the flame holder. The excitation

wavelength was 632.8 nm, generated from a He:Necontinuous wave (cw) laser. For comparison, refer-ence Fe2O3 Raman spectra were taken from bulk ironoxide powders pressed into a 3-cm-diameter disc ofapproximately 2 mm thickness.

The flame holder used during a seeded flame ex-periment was placed under the 100 objective of themicro-Raman spectrometer and a Raman spectrumwas recorded from the area corresponding to the rust-colored residue. Spectra were recorded using a 5-sintegration time averaged over eight individual acqui-

sitions. The resulting flame holder spectrum was con-sistent with the reference Fe2O3 spectra, with bothshown in Fig. 9. The Raman analysis was consistentand repeatable when recorded immediately after re-moving the flame holder from the flame, or whenrecorded after several days. In addition, no trace ofother iron oxides (FeO or Fe3O4) was observed, norwere any such iron oxide signals recorded from theflame holder following the unseeded flame experi-ments. It is noted that an additional peak centered at662 cm1 is also apparent from the Raman spectra

in Fig. 9. The 662 cm1

peak is not characteristic ofsoot or carbon, and is attributed to the flame holdersubstrate, as verified using control experiments.

To further assess the state of iron within the sam-pled soot aggregates, high-resolution TEM in combi-nation with EDS analysis was used to probe individ-ual agglomerates. As can be observed in Fig. 7, aswell as additional high-resolution images, the TEM

images from the iron-seeded soot particles revealsmall inclusions (less than 10 nm) throughout the pri-mary soot particles. Energy-dispersive spectroscopy(EDS) was used to probe these individual inclusions

with nanometer spatial resolution. EDS revealed thepresence of both iron and oxygen from many of theinclusions, with Fe:O ratios (molar) that varied fromabout 1:0.5 to 1:4. About 40% of the measured inclu-sions revealed only a strong iron signal, with essen-tially no oxygen signal recorded. No clear trend wasobserved with regard to flame height. Analysis of thesoot structure in regions away from these inclusions(i.e., presumably pure soot) revealed no detectableiron or oxygen signals. Therefore, the TEM/EDSanalysis fully supports the nucleation of iron-rich par-

ticles and subsequent incorporation of these seed nu-clei (5 to 10 nm size) into the soot agglomerates,although no clear or consistent chemical species as-sociated with the iron-rich inclusions was found.

Finally, in an attempt to gain information as tothe chemical nature of iron and possible transfor-mations within the flame, laser-induced fluorescence(LIF) measurements were performed as an alterna-tive in situ probe. To optimize the LIF signal for ironspecies within a flame environment, preliminary stud-ies were performed in an iron-pentacarbonyl-seeded

CO diffusion flame, which is known to produce iron-rich agglomerates without the presence of interfer-ing soot [34]. Using such a flame, three primaryexcitation bands for LIF of elemental iron were ex-plored, namely 295.39 nm (704.034,547.2 cm1),296.69 nm (033,695.4 cm1), and 297.31 nm(704.034,328.7 cm1). Significant fluorescence wasobserved at 372.8, 376.4, and 378.8 nm for the firstwavelength, 373.5 nm for the second wavelength, and375.0 and 375.8 nm for the third wavelength, respec-tively. The strongest LIF scheme for iron was found to

be excitation of the resonant 296.69-nm transition andobservation of the 373.5-nm emission line (6928.333,695.4 cm1). Representative fluorescence spectraare shown in Fig. 10 for two different flame heights. Itwas also noted that no emission signal was recordedon or near the 373.5-nm spectral region when thelaser was tuned 0.5 nm off of the 296.69-nm reso-nant line, ensuring that no spurious signals or otheremission sources (e.g., broadband fluorescence) werepresent other than those signals arising from elemen-tal iron atoms. Using this scheme, LIF measurements

were recorded as a function of flame height (0 to25 mm) using Burner 2 for the iron-seeded flame con-dition. The LIF signal was quantified by integratingover the full width of the 373.5-nm emission line,and then normalizing the signal to the data recordedover the first five flame heights (0 to 4 mm), withthe results shown in Fig. 11. The data reveal a rathersteady but slow decay in elemental iron, as measured


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Fig. 10. Laser-induced fluorescence spectra recorded fromthe iron-seeded (4000 ppm) flame using Burner 2. The up-per spectrum was recorded 3.7 cm above the burner, and thelower spectrum was recorded 18.5 cm above the burner sur-face. The excitation line was 296.7 nm (033,695.4 cm1)for both measurements. The prominent peak at 373.5 nmcorresponds to the 6928.333,695.4 cm1 atomic iron tran-sition.

Fig. 11. Intensity of 373.5-nm laser-induced fluorescenceintensity as a function of height above burner for theiron-seeded (4000 ppm) flame using Burner 2. The excita-tion line was 296.7 nm. Error bars represent one standarddeviation.

by the LIF signal, through the soot nucleation andgrowth regime. This is followed by a rapid and signif-icant drop in iron signal throughout the soot oxidationregime, where the signal was observed to decrease bymore than two orders of magnitude between a flameheight of 15 cm and the flame tip.

Prior to detailed discussion of this trend, a fewcomments are noted concerning the LIF data. With

LIF, one must always consider quenching effects,especially in the presence of a variation in concomi-tant species and temperature, as in the current flameregimes. To independently corroborate the loss of the

LIF signal with increasing flame height, additionaltransmission measurements were performed usingan iron hollow-cathode lamp, noting that while LIFis subject to quenching, absorption spectroscopy isfree from this effect. Using this technique, absorptionmeasurements using the strong Fe resonant transitionat 271.9 nm were recorded as a function of flameheight. The transmission measurements revealed atrend identical to that in the LIF measurements,namely, low and consistent transmission through thesoot inception and growth regime (up to a height of

14 cm), followed by a rapid increase in transmis-sion to near unity (i.e., nonabsorbing) at the flametip. Therefore, the presence of a strong LIF signal atlow heights is the result of the presence of elemen-tal iron throughout the lower flame heights prior tothe onset of soot oxidation. In contrast, the markeddemise of the LIF signal through the soot oxidationregime (from 15 cm to the flame tip) is the resultof the loss of elemental iron, presumably to an ironoxide species, and not the result of preferential signalquenching.

Additional LIF measurements were performedto determine the presence of any FeO within theiron-seeded flames. Several resonant lines of FeO,as reported by Mavrodineanu and Boiteux [35],were investigated, including both 590.3-nm (87017,800 cm1) and 564.7-nm (87018,570 cm1) ex-citation schemes with corresponding fluorescence at621.9 nm for the former and 627.9 and 593.5 nm forthe latter. Using both of these schemes, no FeO LIFwas detected, leading to the conclusion that FeO wasnot present in a significant quantity (i.e., above the

detection limit) within the iron-pentacarbonyl-seededflame. It is noted, however, that the use of backscatterfor collection of the LIF signals in combination withthe unstabilized flame does not enable probing of theflame with any significant radial resolution.

4. Discussion of results

The current flame studies provide additional in-sight into the mechanisms of soot suppression with

iron-based fuel additives. Detailed light-scatteringmeasurements and TEM analysis of sampled soot par-ticles were performed throughout the soot inception,growth, and oxidation regions, revealing two distinctregimes for iron addition regarding the interactionswith sooting characteristics. The data strongly sup-port the role of iron-rich nucleus particles (5 nm insize) acting as early nucleation sites for subsequent


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soot inception and growth. This is consistent with ob-servations made in previous studies of iron-seededpremixed flames for several other fuels [58,18]. Asthe soot particles undergo surface growth through-

out the soot growth regime, the TEM images suggestthat these iron-rich nuclei become completely sur-rounded by soot, so that the resulting primary sootparticles are characterized by the appearance of typ-ical flame-generated soot, but with the addition ofdiscrete iron-rich clusters or inclusions. Soot volumefraction profiles at the end of the growth regime inthe iron-seeded flame were found to be typical of theunseeded flames, as the influence of the iron-rich in-clusions on the overall soot surface growth processeswas found to be negligible. Clearly the significant

influence of iron addition is not within the soot in-ception and growth regimes, other than incorporationof iron species, but rather is manifest within the sootoxidation or burnout regimes. However, the exact na-ture of the chemical state of the iron species is animportant question.

The present study suggests several answers re-garding the evolution of iron species within theiron-seeded flames. The ex situ Raman spectroscopyanalysis of the flame holder for the Burner 1 experi-ments is consistent with the transition from elemental

iron to iron oxide species (e.g., Fe2O3) within thelatter regions of iron-seeded flames, although severalimportant comments are noted with regard to the ex-act species. The measurements only correspond to thestabilized flame-holder configuration and to the sootactually deposited and aged on the flame holder. Thisprovides considerable residence time to age and there-fore oxidize iron species, and therefore provides nodirect evidence as to the state of iron species withinthe inception, growth, and oxidation regimes. In addi-tion, thermodynamic equilibrium calculations suggest

that FeO may be a dominant species under the broaderrange of flame conditions, notably at higher tempera-tures, although it is difficult to model the exact oxygenmole fractions given the diffusion flame configura-tion. Also of importance is the difficulty of ensuringthat equilibrium conditions are in fact reached giventhe dynamic nature of the flame species profiles andthe rather short residence time of iron species, namelyabout 50 ms. With these comments in mind, and theoverall fuel-rich nature of the flame based on inletflow rates of fuel and oxygen, it is quite probable that

elemental iron is a significant component of the sootparticles resulting from the iron-seeded flames priorto entering the soot oxidation regime.

The presence of Fe at early times is in full agree-ment with the in situ LIF measurements of the presentstudy, and also is consistent with recent work by Kimet al. [17], which reported evidence of considerableelemental iron in soot sampled from an ethylene dif-

fusion flame. The rather marked demise of the ele-mental iron LIF signal through the oxidation regimein the current study may be explained by the forma-tion of iron oxide species (FeO, Fe2O3, or Fe3O4).

At earlier residence times, the iron is largely protectedfrom oxidation by encapsulation within the soot parti-cles and by the competition of hydrocarbon oxidationfor available oxygen. However, at longer residencetimes, (1) the oxygen concentrations are significantlyincreased as additional oxygen diffuses into the flameand within the soot particles, (2) the significant oxi-dation of carbon (i.e., soot) further exposes the ironto additional oxygen, and (3) the longer time scalesmay allow the approach to equilibrium conditions. Inthe aggregate, the above comments suggest a transi-

tion from elemental iron to iron oxide species and thesubsequent interaction of these new iron species withsoot throughout the soot burnout regime.

The presence of significant elemental iron enteringthe soot oxidation zone combined with the increasedrate of soot oxidation (as compared to the unseededflame) throughout the burnout region provides directevidence of the soot reduction role of iron addition.The direct oxidation of soot by iron oxide species,such as the following,

Csolid+

FeO

CO+

Fe, (13)

can be considered unimportant for several reasons.First, the data suggest a transition from elemental ironto some other iron species (e.g., iron oxides) withinthe oxidation region, not the transition to elementaliron. Secondly, the amount of soot reduction achievedin the present study is approximately two orders ofmagnitude greater than could be obtained by directoxidation, as based on an overall mass balance as-suming that all iron atoms are incorporated in sootand can participate. Discounting of direct oxidation

leads to the consideration of catalytic soot oxidationby iron species through reactions such as the follow-ing processes,

Csolid + O2 + Fe CO2 + Fe, (14)Csolid + OH + Fe CO + H + Fe. (15)These reactions would occur on or near the surfaceof the iron-rich nuclei dispersed throughout the sootparticles, noting that the transition metals such as ironhave been widely studied as catalyst materials. In ad-dition to oxygen, the role of OH as a soot oxidizer

may be important in the presence of iron catalysts,where, for example, the following surface reactionsmay be responsible for the creation of OH,

2Fe(s) + O2 O(s) + O(s), (16)Fe(s) + H H(s), (17)O(s) + H(s) OH, (18)


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where Fe(s) denotes a free iron-surface site, and O(s)and H(s) indicate surface species. Finally, OH radi-cals are desorbed from the surface of the iron andare available to oxidize the solid carbon via Reac-

tion (15). In addition to catalysis by elemental iron,it is perhaps more reasonable to assume oxidation ofelemental iron and the subsequent catalysis by ironoxide species:

(19)aFe+ bO2 Fex Oy ,(20)Csolid + Fex Oy +O2 CO2 + Fex Oy .

Such a transition is consistent with the increase inavailable oxygen in the latter flame regions and is inagreement with the decreased elemental iron LIF sig-

nals, as noted above. Hence the results most likelyindicate iron oxide formation concomitant with en-hanced soot oxidation.

The relative importance of Reaction (19), as wellthe overall partitioning of iron species, was furtherexplored using STANJAN (Version 3.89) to assessequilibrium mole fractions as a function of flame tem-perature. The code was run based on the input fuel,oxidizer, metal additive, and diluent flow rates, withno attempts to correct for species diffusion of ambi-ent air with flame residence time. For temperatures

near the maximum flame temperature (1800 K), thedominant iron-containing species are predicted to beFe(OH)2 and FeO (vapor and liquid), which are con-sistent with the findings of Rumminger and Linteris[13,14], as well as recent work by Guo and Kennedyusing iron-pentacarbonyl-seeded hydrogen diffusionflames [36]. At the present recorded flame temper-atures of 1000 to 1200 K, corresponding to theflame oxidation region (>15 cm), predicted domi-nant species include FeO and Fe3O4, with the latterrapidly increasing with decreasing temperature. At

even lower temperatures, the oxide Fe2O3 becomesdominant, which may explain the presence of this par-ticular oxide as the species recorded directly on theflame holder. The failure to find evidence of FeO viaLIF measurements, notably at early flame heights, issignificant in terms of the equilibrium calculations,and is perhaps evidence of lack of equilibrium at earlyheights as discussed above.

The combination of experimental measurements,namely the LIF data, and the equilibrium calculations,suggests the importance of iron oxide formation (e.g.,

Fe3O4 and Fe2O3) with increasing flame residencetime in the acceleration of soot oxidation via Reaction(20) within the iron-seeded flames. While the role ofelemental iron is considered important during soot in-ception and early growth, the observed rapid demiseof Fe during the region of enhanced soot oxidationsuggests that the primary role of iron is as a precursorto formation of catalyzing iron oxide species.

5. Conclusions

In summary, the soot-suppressing role of iron pen-tacarbonyl suggests the importance of iron species,

notably elemental iron, at early flame residence times,where iron-rich nuclei provide surface area for sootinception and growth, and ultimately lead to iron-richinclusions within the flame-generated soot particles.The actual soot-suppressing mechanisms are thenmanifest in the soot oxidation regime, whereby thecurrent LIF measurements support a transformationof elemental iron to iron oxide species with increas-ing flame residence. These iron oxide species thenpresumably catalyze the oxidation of soot, therebyresulting in reduced soot emissions (66% in the cur-

rent studies) from the overall flame. Overall, the ex-act flame configuration, combustor residence times,and combustor fuel type and stoichiometry are allexpected to play key roles in the soot-suppressing ef-fects realized with iron-based additives such as ironpentacarbonyl. The specific roles of iron species nu-cleation and subsequent incorporation into the sootparticle matrix, as well as chemical transformations,are fundamental steps for practical realization of ben-eficial soot reduction with metallic fuel additives.

Acknowledgment

This work was supported in part by the U.S. NavyOffice of Naval Research, Physical Sciences S&T Di-vision, through Contract N00014-0210838.

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reduction of soot emissions by iron pentacarbonyl in isooctane diffusion flames1

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