Electron Microscopy Investigation of CarbonaceousParticulate Matter Generated by Combustion of Fossil

Fuels

Yuanzhi Chen, Naresh Shah,* Artur Braun, Frank E. Huggins, andGerald P. Huffman

Consortium for Fossil Fuel Science, Department of Chemical and Materials Engineering,University of Kentucky, 533 South Limestone Street, Lexington, Kentucky 40508-4005

Received October 19, 2004. Revised Manuscript Received April 19, 2005

The morphology, microstructure, and composition of individual carbonaceous particles generatedduring combustion of coal, residual oil, and diesel fuel were investigated by various electronmicroscopy (EM) techniques, including scanning electron microscopy (SEM), high-resolutiontransmission electron microscopy (HRTEM), selected area electron diffraction (SAED)/dark-fieldimaging, electron energy loss spectroscopy (EELS), scanning transmission electron microscopy(STEM), and energy-dispersive X-ray spectroscopy (EDS). Carbonaceous components of all ofthese fossil-fuel-derived particulate matter (PM) samples contain ultrafine soot aggregates withfractal chain morphologies and spherical primary particles (sizes ∼30-50 nm) that exhibit aconcentric arrangement of stacked graphitic layers around the particle center. Some submi-crometer soot aggregates with larger primary particles (∼100-400 nm) were also found in coal-derived carbonaceous PM. Larger spherical or irregular-shaped porous char particles dominatethe micrometer-sized fraction of coal and residual oil PM samples. The relative fractions of sp2

hybridized carbon atoms in carbonaceous particles were estimated from EELS measurements ofthe carbon K-edge taken at the magic angle. Although the carbonaceous particles generated bycombustion of various fossil fuels may have quite different morphologies and microtextures, theypossess similar internal structural parameters, such as the dimensions of basic structural units(BSUs) and the average hybridization state of the carbon atoms. Compositional differencesbetween soot aggregates and char particles were observed, and such information could be usedas fingerprints to identify the source of different types of carbonaceous particles.

Introduction

The correlation between the concentration of airborneparticulate matter (PM) <2.5 µm in mean diameter(PM2.5) and human morbidity and mortality due torespiratory and cardiovascular disease has been well-established.1,2 Available ambient measurements of PM2.5suggest that anthropogenic combustion sources, fires,and other emitters of condensable and secondary originscontribute a large percentage of the overall ambientPM2.5 mass in most areas.3 Not only is the carbonaceousmass a major contributor to total PM2.5 mass in mostareas of the United States, but a majority of the excessamount of PM2.5 in urban areas with high populationdensities compared to nearby rural areas is due tocarbonaceous matter.4 Generally, carbonaceous PMin ambient aerosols is subdivided into two fractions:

organic carbon (OC) and elemental carbon (EC). Glo-bally, 45% of all EC emitted and 55% of all primary OCemitted are estimated to originate from fossil fuelburning.5 Carbonaceous PM is also believed to havesignificant impact on global climate. For example, ECmay be the second most important direct forcing com-ponent of global warming, after CO2.6 Carbonaceousparticles have been found to comprise about half of thesubmicrometer mass in many natural aerosols and mayact as carriers of toxic chemicals.7 In addition, unburntresidual carbon can also affect many aspects of fossil-fuel-fired power plant performance and economy, in-cluding boiler efficiency, electrostatic precipitator op-eration, and the value of the fly ash as a commercialproduct.8 The composition and microstructure of com-bustion-generated carbonaceous PM could have signifi-cant effects on their roles and fates in the environment.The results obtained from conventional bulk analyses* Corresponding author. Phone: 859-257-4027. Fax: 859-257-7215.

E-mail: naresh@uky.edu.(1) Dockery, D. W.; Pope, C. A., III.; Xu, X.; Spengler, J. D.; Ware,

J. H.; Fay, M. E.; Ferris, B. G., Jr.; Speizer, F. E. N. Engl. J. Med.1993, 329, 1753-1759.

(2) Avakian, M. D.; Dellinger, B.; Heidelore, F.; Gullet, B.; Koshland,C.; Marklund, S.; Oberdorster, G.; Safe, S.; Sarofim, A. F.; Smith, K.R.; Schwartz, DE.; Suk, W. A. Environ. Health Persp. 2002, 110, 1155-1162.

(3) EPA Clearing House for Inventories and Emission Factors(CHIEF) website: http://www.epa.gov/ttn/chief/eiip/pm25inventory/index.html.

(4) National Air Quality and Emissions Trends Reports2003 specialstudies edition, US EPA report No. 454/R-03-005. Also available athttp://www.epa.gov/airtrends/.

(5) Seinfeld, J. H.; Pankow, J. F. Annu. Rev. Phys. Chem. 2003, 54,121-40.

(6) Jacobson, M. Z. Nature 2001, 409, 695-697.(7) Katrinak, K. A.; Rez, P.; Buseck, P. R. Environ. Sci. Technol.

1992, 26, 1967-1976.(8) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Mitchell,

G. D. Fuel 1995, 74, 1297-1306.

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10.1021/ef049736y CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 05/26/2005

are the average contributions of different particle typesthat have different physical and chemical properties.Studies based on analyses of individual particles provideadditional information that complements conventionalbulk analyses.

Scanning electron microscopy (SEM) has been rou-tinely used to characterize morphologies of carbon blackparticles.9,10 High-resolution transmission electron mi-croscopy (HRTEM), which is capable of providing struc-tural information at an atomic level resolution, has beenused to investigate the microstructures of diesel soot11,12

and coal char.13,14 Dark-field imaging has been used tocharacterize the structures and microtextures of varioustypes of carbonaceous materials, especially those ofpoorly organized materials.15-17 Electron energy lossspectroscopy (EELS) has been demonstrated to be auseful tool for obtaining information about bondingtypes and hybridization states of carbon atoms.7,18 Onelimitation of electron microscopy (EM) techniques is thatinformation about highly volatile components cannot beobtained, since those volatile species cannot withstandthe vacuum and electron beam. If significant, suchfactors may complicate the analysis of the OC compo-nents but should not affect the analysis of EC compo-nents. Nevertheless, we still can use EM techniques toobtain abundant information that cannot be achievedfrom conventional bulk analyses. In this study, we usea series of EM techniques including SEM, HRTEM,selected area electron diffraction (SAED)/dark-fieldimaging, EELS, scanning transmission electron micros-copy (STEM), and energy-dispersive X-ray spectroscopy(EDS) to characterize the morphology, microstructure,and composition of individual carbonaceous particlesgenerated from combustion of coal, residual oil, anddiesel fuel. Such information along with that obtainedfrom other techniques19 should be useful for evaluatingthe environmental and health impacts of carbonaceousparticles produced by combustion of fossil fuels.

Experimental Section

Samples. The coal and residual oil PM samples wereprepared in combustion experiments conducted at the USEPA’s National Risk Management Research Laboratory (NRM-RL) using a down-fired, refractory-lined laboratory pulverizedcoal combustor and a North American three-pass fire-tubepackage boiler, respectively. Detailed descriptions of thecombustor and boiler are given elsewhere.20,21 The PM sampleswere separated aerodynamically by a cyclone into a fine PM2.5

and a coarse PM2.5+ fraction. Only the fine fraction samples,PM2.5, were used in this study. The coal and residual oil usedin the combustion experiments were a high-volatile bituminouscoal from Ohio and a high-sulfur No.6 residual oil.

The diesel PM sample was produced at the diesel test enginefacility at the University of Utah using a two-stroke diesel testengine (Kubota Z482B, 482 cm3 displacement, 2200 rpm, 6 ft-lb load). The fuel/air ratio was 0.013. The diesel fuel testedwas a 50:50 mixture of the Chevron/Phillips reference fuelsT-22 and U-15, with an average cetane number of 46.7 andsulfur content of 79 ppm. Particles were collected on quartzfilters and then separated from the filter for further analyses.

Samples for SEM characterization were deposited on poly-carbonate filters and coated with a thin layer of an Au/Pd alloy.Samples for transmission electron microscopy (TEM) analysiswere prepared by ultrasonicating small amounts of the PMpowder in acetone and transferring several drops of thesuspensions onto holey carbon TEM grids. The holey carbonTEM grids were precoated with a layer of Au/Pd alloy todifferentiate carbonaceous contaminations present on the laceycarbon film from the real sample particles.

Analytical Procedure. The morphologies of coarse par-ticles were characterized by using an Hitachi S-3200 scanningelectron microscope. A 200 kV field emission analyticaltransmission electron microscope (JEOL JEM-2010F) equippedwith an Oxford energy-dispersive X-ray spectrometer, a scan-ning (STEM) unit with high-angle annular dark field (HAADF)detector, and a Gatan imaging filter (GIF)/PEELS system wasused for the in-depth analysis of individual particles. Bright-field imaging was used to visualize particle morphologies andsizes, while SAED/dark-field imaging was used to characterizethe orientation and distribution of microcrystallites. HRTEMimages were recorded under optimal focus condition at atypical magnification of 400K-500K. The profiles of the basicstructural units (BSUs) were obtained by using 002 latticefringe images. EELS spectra were recorded in diffraction mode(image-coupled mode) with an energy resolution of 1 eV (fullwidth at half-maximum of zero-loss peak) and a dispersion rateof 0.2 eV/channel. The background at the carbon K-edge wassubtracted from the EELS spectra by using a power lawapproximation, and a Fourier deconvolution was also con-ducted to remove the contribution of plural inelastic scattering.To avoid possible interference from the lacey carbon film, onlyunsupported particles hanging over the gaps in the laceycarbon support were analyzed. EDS analyses were conductedin STEM mode using an analytic probe with a size of 1.0 nm.Digital Micrograph software was used for image and EELSdata recording and processing, while ES Vision Emispecsoftware was used for EDS analysis and dark-field imaging.

Results and Discussion

Morphologies. Micrographs of different types ofcarbonaceous particles are shown in Figure 1. Sootaggregates dominate the ultrafine (<0.1 µm) and sub-micrometer (0.1-1 µm) fractions. Typically they havespherical primary particles with sizes of ∼30-50 nmand fractal-like chain structures that can extend to amicrometer or more (Figure 1a). This type of soot(referred to as ultrafine soot in the text) was observedin combustion products of diesel, residual oil, and coal.The second type of soot (Figure 1b) also has sphericalprimary particles, but with significantly larger sizes(∼0.1-0.4 µm). This type of soot (referred to as submi-crometer soot in the text) was observed in samples

(9) Stoffyn-Egli, P.; Potter, T. M.; Leonard, J. D.; Pocklington, R.Sci. Total Environ. 1997, 198, 211-223.

(10) Fernandes, M. B.; Skjemstad, J. O.; Johnson, B. B.; Wells, J.D.; Brooks, P. Chemosphere 2003, 51, 785-795.

(11) Saito, K.; Gordon, A. S.; Williams, F. A.; Stickle, W. F. Combust.Sci. Technol. 1991, 80, 103-119.

(12) Ishiguro, T.; Takatori, Y.; Akihama, K. Combust. Flame 1997,108, 231-234.

(13) Sharma, A.; Kadooka, H.; Kyotani, T.; Tomita, A. Energy Fuels2002, 16, 54-61.

(14) Shim, H.-S.; Hurt, R. H.; Yang, N. Y. C. Prepr. Symp.sAm.Chem. Soc., Div. Fuel Chem. 1998, 43, 965-969.

(15) Vanderwal, R. L. Combust. Sci. Technol. 1998, 132, 315-323.(16) Chen, H. X.; Dobbins, R. A. Combust. Sci. Technol. 2000, 159,

109-129.(17) Rouzaud, J. N. Fuel Process. Technol. 1990, 24, 55-69.(18) Jager, C.; Henning, Th.; Schlogl, R.; Spillecke, O. J. Non-Cryst.

Solids 1999, 258, 161-179.(19) Braun, A.; Shah, N.; Huggins, F. E.; Huffman, G. P.; Wirick,

S.; Jacobsen, C.; Kelly, K.; Sarofim, A. F. Fuel 2004, 83, 997-1000.

(20) Linak W. P.; Miller C. A.; Wendt J. O. L. J. Air Waste Manage.Assoc. 2000, 50, 1532-1544.

(21) Miller, C. A.; Linak, W. P.; King, C.; Wendt, J. O. L. Combust.Sci. Technol. 1998, 134, 477-502.

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derived from bituminous coal combustion, but not insamples derived from diesel or residual oil combustion.

Char particles that are dominant in the micrometer-size fraction have quite different morphologies comparedto those of the soot aggregates. Most char particles haveporous structures with spherical or irregular shapes(Figure 1c, 1e), while some char particles have a solidstructure and less porosity (Figure 1d). Such charparticles are present in both coal and residual oilsamples, but char particles from residual oil combustiontypically have more perfect spherical shapes, which nodoubt reflects the nature of liquid fuel droplets and thelack of any significant solid mineral matter in them.Cenospheres with porous surfaces or solid surfaces arevery typical for residual oil samples. PM from dieselcombustion rarely contains such char particles, althoughoccasionally large carbonaceous residual particles withundefined random shapes can be found.

The morphological differences between soot aggre-gates and char particles are mainly related to differ-ences in their formation. Primary soot particles areformed by generation of nuclei by condensation incooling postcombustion gaseous streams. Often theseprimary particles mature to larger sizes by condensationof volatile organic compounds. Further collisions of thesenuclei mode primary particles lead to accumulationmode aggregates.22 Relatively larger char particles, onthe other hand, are derived from pyrolysis and oxidationof fuel particles. The nature of the fuel, the fuel/air ratio,and the temperature and duration of combustion allplay a role in determining the morphologies of thesecarbonaceous particles.

Microstructural Observations. Two typical high-resolution images of primary particles of ultrafine sootaggregates are shown in Figure 2. In the first one,concentrically arranged stacked graphitic layers, roughly

parallel and equidistant, form a microtexture similarto the structure of an onion. This type of microstructurehas been previously reported in studies of carbonblacks23,24 and diesel soots.11,12 The second image dem-onstrates a similar outer arrangement to that of the firstone, but multiple spherical nuclei surrounded by severalgraphitic layers are present in the inner core, indicatingthat original small nuclei mode primary particles mayhave coalesced together, perhaps followed also by gas-phase surface growth, to form a larger particle. Thesemultiple nuclei or “growth centers” were also observedin an early study of carbon black23 and in a more recentstudy of diesel soots.12

A typical high-resolution image of a submicrometerprimary soot particle derived from the coal combustionis shown in Figure 3. The primary particles also exhibita concentric arrangement of graphitic layers. Some ofthe particles show a more ordered structure in whichthe graphitic layers are longer and consist of morestacked planes. Since the center of the particle is toothick for normal HRTEM observation conditions, itcannot be established whether this type of particlecontains multiple “growth centers”.

The larger char particles do not exhibit concentricmicrotextures such as those observed in primary sootparticles. They typically demonstrate an anisotropicarrangement of graphitic layers with varying length andthickness. Figure 4 shows the HRTEM image of the tipof a char particle. Short and discontinuous graphiticlayers are observed roughly extending along the tipdirection.

Quantitative characterization of the microstructuresof different types of carbonaceous particles was at-tempted by measuring their basic structural unit (BSU)

(22) Kittleson, D. B. J. Aerosol Sci. 1998, 29, 575-588.

(23) Ban, L. L.; Hess, W. M. Petroleum Derived Hydrocarbons; ACSSymposium Series 21; Deviney, M. L., O’Grady, T. M., Eds.; AmericanChemical Society: Washington, DC, 1976; pp 358-376.

(24) Donnet, J. B. Carbon 1982, 20, 267-282.

Figure 1. Morphologies of carbonaceous particles derived from combustion of different fossil fuels: (a) ultrafine soot (diesel), (b)submicrometer soot (coal), (c) an irregular-shaped porous char (coal), (d) an irregular-shaped solid char (coal), (e) a spherical charwith a highly porous surface (residual oil), and (f) a char cenosphere with a smooth surface (residual oil).

1646 Energy & Fuels, Vol. 19, No. 4, 2005 Chen et al.

parameters, including the average distance of interlayerspacings (d002), the stacked layer length (La), and thelayer thickness (Lc) from the 002 lattice fringe images.About 10 particles in different fields, each with ∼5-8measuring points, were measured for each parameter.As shown in Figure 5, the average d002 values are inthe range of 3.65-3.75 Å, whereas La and Lc are in therange of 10-14 Å for all carbonaceous particles, al-though some individual values were far from the aver-age value. Typical stacked layer numbers are 3-5. Thedifferences among each category are not significant,although the primary particles of diesel soot aggregatesexhibit somewhat larger La and Lc and smaller d002values compared with soot aggregates derived fromother fuels.

Typical SAED patterns of soot aggregates and charparticles are shown in Figure 6. The rings or arcs areindexed to the crystalline structure of graphite.25 TheSAED pattern (Figure 6a) obtained from soot aggregatesshows broad and diffuse 002 and hk rings, whichindicates a microstructure consisting of randomly dis-tributed crystallites that have a fine size and do notpossess long range order. The SAED pattern in Figure6b was obtained from a char particle that has some ofthe crystalline domains in the Bragg condition for the002 reflection, but not randomly distributed over allazimuthal angles, and therefore, it exhibits the 002reflection as arcs. The SAED pattern in Figure 6c

(25) Beyssac, O.; Rouzaud, J.; Goffe, B.; Brunet, F.; Chopin, C.Contrib. Mineral. Petrol. 2002, 143, 19-31.

Figure 2. HRTEM images of an ultrafine soot primaryparticle showing a concentric arrangement of stacked graphiticlayers (a) and a particle with multiple nuclei (indicated byarrows in part b).

Figure 3. HRTEM image of a submicrometer primary sootparticle.

Figure 4. HRTEM image of a char particle.

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represents the case in which char particles have stackedlayer planes normal to the incident beam direction.However, since the 002 planes are not in the Braggcondition, only the 10 and 11 diffraction rings areobserved.

Dark-field imaging was used to visualize individualmicrocrystallites or BSUs and mutual orientationsamong them. Figure 7 shows typical bright-field and 002dark-field images of soot aggregate. The bright spots inthe dark-field image are produced by diffraction fromthe 002 planes of microcrystallites that fulfill the Braggcondition. An objective aperture was placed at the 9o’clock position of the 002 diffraction ring, so that onlymicrocrystallites corresponding to this position wouldbe imaged. By moving the objective aperture around the002 diffraction ring, microcrystallites at different azi-muthal angles can be imaged. All these dark-fieldimages show patterns in which bright spots are distrib-uted on the particles edge but are absent in the center.Such patterns are observed because there is a largerdensity of circumferential 002 lattice planes parallel tothe electron beam (with normal azimuthal orientation)on the edge than in the center. Since it is thermody-namically unfavorable to form circumferential arrange-ments of microcrystallites with small radii of curvaturein the core region, the core region possesses a lessordered arrangement. Higher solubility of the inner corein nitric acid than the rigid outer shell of diesel sootaggregate has been attributed to less ordering of thecore region.12

Figure 8 shows the bright-field and 002 dark-fieldimage of a char particle. Instead of showing the con-centric microtextures that were observed in soot ag-gregates, the char particle exhibits microtextures withdifferent orientations, which can be discerned from thenonuniform distribution of bright spots and dark regionsin the dark-field image. The sizes of the individualbright spots in the dark-field image, which approxi-mately represent the dimensions of BSUs, are in agree-ment with the HRTEM observations. These heteroge-neous microtextures are commonly seen in larger coaland residual oil char particles, and the formationmechanism depends on the chemical nature of parentfuel, pyrolysis temperature, and residence time. De-tailed microtextural models developed for coal carbon-ization are available.17,26

Bonding Information. A transmission electron mi-croscope equipped with EELS becomes a powerfulspectroscopic technique that can be used to probe thechemical information and electronic structure in amaterial with high-spatial resolution. This techniquehas been employed to characterize various carbonmaterials, such as diamond films,27 carbon black,18 andurban carbonaceous PM.7 A detailed introduction toEELS may be found in ref 28. In carbonaceous materi-als, a carbon atom may adopt different hybrid electronicconfigurations, such as sp1, sp2, or sp3. In a sp2-bondedcarbon material, the carbon atoms have three strong σbonds formed in the basal plane and π (pz) orbitals outof the plane. Electronic transition from the 1s core levelto an unoccupied 2p(π*) state is manifested as a sharppeak at 285 eV in EELS and NEXAFS spectra.29 In sp3-bonded carbon materials, such transitions are absent.By quantitatively analyzing the carbon K-edge featuresand comparing them to standards such as graphite(100% sp2 in the basal plane) and polyethylene (0% sp2

bonding), quantitative information regarding the rela-tive abundance of the two hybridization states of thecarbonaceous component of PM can be obtained.

In a specimen that is crystallographically anisotropic(e.g. graphite), the exact direction of momentum transferand hence energy states available to the excited electrondepend on the specimen orientation with respect to theincident electron beam.30 Therefore, the intensity of thenear-edge features is related to the orientation of thebasal plane with respect to the incident electron beam.To alleviate this experimental anomaly, EELS spectramust be obtained under the so-called “magic angle”condition, where spectral details are independent oforientation.31-34 In this study, the magic angle condi-tions were empirically determined by collecting a seriesof carbon K-edge spectra of graphite particles underdifferent collection angles and then integrating the π*and π* + σ* peaks.

Typical EELS spectra of a char particle are shown inFigure 9a. Microtextures with preferred orientations arepresent in the char particle, which are revealed fromthe SAED patterns recorded from two different regions(marked by circles in Figure 9c). The EELS spectracollected under normal large collection angle conditionfrom these two regions show quite different 1s-π* peakintensities. Spectra collected at the magic angle (Figure9b) at the identical regions overlap each other becausethe magic angle condition reduces the orientationdependence of near-edge features and thereby providestruer bonding information.

Typical carbon K-edge EELS spectra of ultrafine sootaggregates from combustion of various fossil fuels are

(26) Rouzaud, J. N.; Vogt, D.; Oberlin, A. Fuel Process. Technol.1988, 20, 143-154.

(27) Bruley, J.; Williams, D. B.; Cuomo, J. J.; Pappas, D. P. J.Microsc. 1995, 180, 22-32.

(28) Egerton, R. F. Electron Energy Loss Spectroscopy in the ElectronMicroscope; Plenum Press: New York, 1996.

(29) Stohr, J. NEXAFS Spectroscopy; Springer Series in SurfaceScience 25; Springer-Verlag: New York, 1992.

(30) Leapman, R. D.; Fejes, P. L.; Silcox, J. Phys. Rev. B 1983, 28,2361-2370.

(31) Menon, N. K.; Yuan, J. Ultramicroscopy 1998, 74, 83-94.(32) Yuan, J.; Brown, L. M. Micron 2000, 31, 515-525.(33) Daniels, H. R.; Brydson, R.; Rand, B.; Brown, A. P. E. Inst. Phys.

Conf. Ser. 2001, 168, 291-294.(34) Daniels, H.; Brown, A.; Scott, A.; Nichells, T.; Rand, B.; Brydson,

R. Ultramicroscopy 2003, 96, 523-534.

Figure 5. BSU parameters of different types of carbonaceousparticles measured from 002 lattice images: (A) coal ultrafinesoot, (B) coal submicrometer soot, (C) coal char, (D) residualoil ultrafine soot, (E) residual oil char, (F) diesel ultrafine soot.(The error bars represent standard deviations of the meanvalues.)

1648 Energy & Fuels, Vol. 19, No. 4, 2005 Chen et al.

shown in Figure 10. These spectra show somewhatbroadened 1s-π* peaks at 285.4 eV and 1s-σ* peaksat 292.4 eV compared to that of graphite. Although somespectra show minor deviations, significant differencesbetween these spectra were not observed. Carbon K-edge spectra of submicrometer soot aggregates from coalsamples show very similar features to those of ultrafinesoot aggregates, although some primary particles show-ing more amorphous characteristics can also be found.

Soluble organic species that may be associated withcarbonaceous particles are assumed to be dissolved inthe acetone during the dispersion process and thereforethey do not interfere with the analysis. Compared tothe obvious morphological and microtextural differencesamong the different carbonaceous types, the variationin the sp2 fraction is not large, with values around 80%.This implies that there is an intrinsic similarity in thebasic constitution of all carbonaceous particles derivedfrom fossil-fuel combustion, which may serve to dif-

ferentiate them from those formed by other processes,such as diamond-like carbonaceous materials (very lowcontents of sp2 fraction) produced by laser ablation orion beam sputtering.

Elemental Compositions. Elemental compositionsof these carbonaceous particles, at high spatial resolu-tion, were obtained using STEM/EDS. The ultrathin-window detector provides compositional information forall elements heavier than beryllium.

Figure 11a shows the typical EDS spectrum andSTEM/HAADF image of a coal ultrafine primary sootparticle. Besides the dominant carbon, much weaker butdiscernible O and S can also be detected (estimated tobe less than 1%). The EDS spectra of coal submicrome-ter soot and residual-oil ultrafine soot aggregates arebasically similar to those of coal ultrafine soot ag-gregates, although some coal submicrometer soot par-ticles may have somewhat higher O contents. Dieselultrafine soot aggregates typically exhibit much lower

Figure 6. Typical SAED patterns of soot aggregates (a), chars with preferential orientation of basal planes of BSUs approximatelyparallel to incident beam (b), and chars with basal planes of BSUs approximately normal to incident beam (c).

Figure 7. Bright-field image (left) and corresponding 002 dark-field image (right) of an ultrafine soot aggregate.

Figure 8. Bright-field image (left) and corresponding 002 dark-field image (right) of a char particle.

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S contents compared to coal and residual oil sootaggregates, which is related to the lower S content ofdiesel fuels. One of the compositional differences be-tween soot and char particles is that some char particlescontain more discernible sulfur and metallic elements(e.g., Al and Ti in coal chars and V in residual oil chars)than soot particles. Typical EDS spectrum and STEM/HAADF image of a coal char particle are shown inFigure 11b. The S content of the coal char particle ishigher than that of the coal ultrafine soot aggregateshown in Figure 11a. Furthermore, the EDS spectrumof the coal char particle also exhibits a weak Al peak,which is absent in all EDS spectra of the coal sootaggregates examined in this study. Similarly, residual-oil char particles typically exhibit higher sulfur contentsthan residual oil soot aggregates. V and P, which arebasically indiscernible in the residual oil soot ag-gregates, are frequently observed in the residual-oil charparticles.35 The metallic elements may originate fromorganically bound species in the parent fuel. S K-edgeX-ray absorption fine structure (XAFS) spectroscopyindicates that thiophene is the major organic sulfur formin coal fly ash and residual oil fly ash (ROFA).36

The compositional differences between soot aggre-gates and char particles can be explained by theirformation mechanisms. Since the char particles do notundergo a vaporization-condensation process, inorganicelements that originate from the parent fuels could bemore easily preserved in the char particles, whereas forthe soot particles, the heterogeneous elements (e.g., S,Al, and V) are not conducive to forming graphitic layersduring the formation of the primary particles.

In addition to the intrinsic elements, coal-derivedcarbonaceous particles (either soot or char particles)may be coated/mixed with external inorganic speciesthat contain alkali and alkaline earth elements, suchas Na, Mg, Ca, and Ba, whereas residual-oil-derivedcarbonaceous particles are coated/mixed with speciescontaining transition metals, such as V, Ni, Fe, and Zn.Diesel-derived carbonaceous particles have many lessinorganic inclusions than those derived from coal andresidual oil. These external inorganic species, along withthe intrinsic elements in the carbonaceous particles,may constitute an important fingerprint that could beused to identify their possible combustion and fuelsources.

Conclusions

Ultrafine soot aggregates, which are believed to formvia a vaporization-condensation mechanism, are oneof two major forms of carbonaceous particles derivedfrom combustion of fossil fuels. In this paper, they havebeen identified in the combustion products of coal,residual oil, and diesel, but they are more abundant indiesel-derived samples. These ultrafine soot aggregatestypically have a fractal-like chain structure with spheri-cal primary particles having sizes between 30 and 50nm and microtextures consisting of concentrically stackedgraphitic layers. In addition to ultrafine soot aggregates,submicrometer soot aggregates have been observed inthe bituminous-coal-derived sample. These also have a

(35) Chen, Y.; Shah, N.; Huggins, F. E.; Huffman, G. P. Environ.Sci. Technol. 2004, 38 (24), 6553-6560.

(36) Huggins, F. E.; Huffman, G. P. Int. J. Soc. Mater. Eng. Resour.2002, 10, 1-13.

Figure 9. (a) Carbon K-edge EELS spectra obtained from region 1 (dashed line) and region 2 (solid line) of the char particleshown in part c at normal large collection angle. (b) Under magic angle conditions, carbon K-edge EELS spectra from the sameregions overlap, overcoming the angular dependency of the graphitic crystalline anisotropy. (c) TEM bright-field image of thechar particle. Insets are SAED patterns recorded from regions 1 and 2.

Figure 10. Typical EELS carbon K-edge spectra (collectedunder magic angle conditions) of ultrafine soot derived from(a) coal, (b) residual oil, and (c) diesel, respectively.

1650 Energy & Fuels, Vol. 19, No. 4, 2005 Chen et al.

similar concentric layered microtexture but a signifi-cantly larger primary particle size, between 100 and 400nm. The other major carbonaceous type is porous orsolid char particles with shapes varying from nearlyspherical to highly irregular. These particles dominatethe micron-size fraction in the coal and residual oilderived samples. Compared to the relatively homoge-neous arrangement of BSUs in soot particles, the charparticles typically exhibit microtextures with preferredorientations. The HRTEM observation shows that theBSUs in these carbonaceous particles consist of severalroughly parallel stacked graphitic layers with interlayerspacing larger than that of graphite and dimensionstypically ranging between 10 and 14 Å.

The carbon atom hybridization state was examinedby EELS using the magic angle approach. EELS experi-ments at such angles greatly reduce the orientationdependence of carbon 1s near-edge features and makethe analysis of carbon particles with heterogeneousmicrotextures possible. All types of carbonaceous par-

ticles investigated in this study exhibit approximately80% sp2 fraction with no significant variation amongdifferent carbonaceous types. EDS spectra recorded fromindividual particles indicate that diesel soot aggregatescontain much less sulfur than coal and residual oil sootaggregates. Coal char and residual oil char particlestypically exhibit much more sulfur and metallic ele-ments than coal soot and residual oil soot aggregates.

Acknowledgment. The authors are grateful to Dr.William P. Linak and Dr. C. Andrew Miller of theUnited States Environmental Protection Agency (EPA)for generating the coal and residual oil samples, Ms.Kerry Kelly of University of Utah for generating dieselsoot samples, and Dr. Alan Dozier for his assistance inthe use of the TEM. Financial support by the NationalScience Foundation under CRAEMS grant CHE-0089133is also acknowledged.

EF049736Y

Figure 11. EDS spectra and STEM/HAADF images of (a) coal ultrafine soot aggregates and (b) a coal char particle. The spectraare normalized to the height of the carbon peak for comparison. The beam positions are indicated by the circles, and Cu peaks arefrom TEM grids.

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