Combustion and Flame 157 (2010) 1290–1297

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Combustion and Flame

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Multimodal ultrafine particles from pulverized coal combustion in a laboratoryscale reactor

Francesco Carbone a,*, Federico Beretta a, Andrea D’Anna b

a Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, piazzale Vincenzo Tecchio 80, 80125 Napoli, Italyb Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy

a r t i c l e i n f o

Article history:Received 10 August 2009Received in revised form 31 January 2010Accepted 31 March 2010Available online 22 April 2010

Keywords:CoalFly-ashesUltrafine particlesParticle size distribution functionsOxy-combustion

0010-2180/$ - see front matter � 2010 The Combustdoi:10.1016/j.combustflame.2010.03.015

* Corresponding author. Fax: +39 815936936.E-mail address: francesco.carbone@irc.cnr.it (F. Ca

a b s t r a c t

Particle size distribution functions have been measured in a ethanol fueled flame reactor fed with a lowamount of pulverized coal particles. The reactor is operated in low (5.0 vol.%) and high (76.5 vol.%) oxy-gen concentrations using two high volatile bituminous Colombian and Indonesian coals. A carbon blackpowder is also oxidized in the same conditions. Generated particles are sampled using rapid-dilutionprobes and the size distribution functions are measured on-line by a high resolution Differential MobilityAnalyzer. Results clearly show that ultrafine particles, those with sizes lower than 100 nm, have a mul-timodal size distribution function. These particles have huge number concentrations in both investigatedconditions whereas their formation is enhanced in the oxygen enriched condition. Ultrafine particles arealmost totally dominated in number by the fraction having sizes below 30 nm. Nanoparticles also accountfor a significant fraction of total particle mass and slowly coagulate in the reactor. The shape of the sizedistribution functions is not affected by the coal type, at least for the two investigated coals. Results sug-gest that ultrafine particles form through the vaporization–nucleation–growth pathway involving inor-ganic ashes. Moreover the contribution of carbonaceous particles seems particularly important for sizesmaller than 5 nm.

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

1. Introduction combustion conditions are still under investigation [6]. This is par-

The increasing demand of energy in the last decades, because ofemerging countries such as China and India has been largely satis-fied by coal combustion. Coal is also considered to be the majorfuel for power generation in the next decades [1]. However, themain drawback of the extensive use of coal for power generationis greenhouse gas, primarily carbon dioxide, and pollutants suchas nitrogen and sulfur oxides and particulate matter.

Up to recent times, air blown, atmospheric-pressure pulverizedcoal furnace has been the principal reactor design for coal combus-tion. Next generation coal power plants require carbon dioxidecapture and sequestration (CCS) to be environmental sustainable[2]. Recently, oxy-combustion technologies are becoming widelyused also because of the opportunity to retrofit existing air blownfurnaces. Retrofitting of existing plants requires oxygen enrichedconcentrations (>28 vol.%) in recirculated flue gases to maintainthe same heat flux [3].

Oxygen enriched environments in which coal is burned affect themain oxidation process as well as the formation of pollutants. Partic-ulate emission from conventional pulverized coal combustion sys-tems has been extensively studied [4,5] whereas the effects of oxy-

ion Institute. Published by Elsevier

rbone).

ticular true for ultrafine ashes whose formation, deriving from avaporization–nucleation–growth pathway, is strongly affected bycombustion temperature and oxygen concentration [7]. Indeed met-als undergo vaporization and subsequently undergo homogenousnucleation or heterogeneous condensation, usually in the form ofrefractory oxides. The passage to the gas phase can occur directly,for volatile or organically bonded metals (M), or via reduction to vol-atile sub-oxides (MOx�1), for refractory minerals (MOx) [8]:

MOx þ CO!MOx�1 þ CO2

Vaporization occurs in the high temperature, fuel rich boundarylayer surrounding the burning coal particles. It involves a complexchemistry and can be promoted by some elements, among all chlo-rine, that enhance the formation of volatile metal compounds [9].Nucleation occurs when burning products move towards lowertemperature and oxidizing conditions. This occurs, first, whenescaping from the boundary layer surrounding coal particles and,subsequently, at the furnace exhaust where lower boiling pointcompounds condense. At higher cooling rates, homogenous nucle-ation is favored over heterogeneous condensation [9].

The process involving ash transformation is, to some extent,coupled with carbonaceous particle inception due to pyrolysis oftar and volatile organic compounds released during coal devolatil-ization [10].

Inc. All rights reserved.

Table 1Properties of the used Colombian and Indonesian coals.

F. Carbone et al. / Combustion and Flame 157 (2010) 1290–1297 1291

Coagulation is mainly responsible for particle growth whereascondensation and sintering greatly influence their morphologyand surface composition [11].

Ultrafine ashes, those with sizes lower than 100 nm, representjust a small percentage of total fly ash mass at the exhaust of airblown pulverized coal furnaces [12] but it is of concern [13] be-cause recent studies have shown that particle toxicity increaseswith decreasing particle sizes and strongly depends on particlenumber more than mass concentration [14]. Moreover, ultrafineparticles are not easily removed by aftertreatment devices becausefiltering efficiency decreases with decreasing particle sizes in theultrafine range [15]. Recent results obtained by using advanceddiagnostics for particle size distribution measurements showedthat at high temperature, particle coagulation efficiency dramati-cally drops with decreasing particle size below 10 nm. This behav-ior was observed both for carbonaceous [16] and metal oxide [17–19] particles so that nano-ash emission from pulverized coal fur-naces might be a problem.

The aim of this paper is to measure the particle size distributionfunctions of ultrafine and nanosized particles formed in a laboratoryscale, pulverized coal combustion reactor. The reactor was firstlyoperated in conventional conditions, typical of air blown pulverizedcoal combustion, and, subsequently, in extremely high oxygen con-centration. The latter was selected to amplify the effects of oxygenenriched conditions on particle formation. Preliminary informationon the nature and behavior of the ultrafine and nanometric fractionswere obtained by comparing the results obtained in both conditionsand using two types of coals and a carbon black powder.

Colombian coal Indonesian coal

As received Dry As received Dry

Proximate analysisMoisture (%) 4.90 3.11Volatile (%) 38.31 40.28 40.20 41.49Ashes (750 �C) (%) 6.11 6.42 7.40 7.64Fixed carbon (%) 50.68 53.3 49.29 50.87HHV (kcal/kg) 6862 7216 7245 7478LHV (kcal/kg) 6560 6957 6938 7198

Ultimate analysis, wt.%C 70.95 74.60 71.68 73.98H 5.32 5.03 5.62 5.44N 1.32 1.39 1.38 1.42S 0.40 0.53

Ash analysis, wt.% on ash basisSiO2 47.05 44.47Al2O3 23.94 22.28Fe2O3 5.99 9.67MgO 1.43 1.41CaO 3.29 1.98Na2O 2.03 1.14K2O 1.40 2.31MnO 0.04 0.04

Minor elements in coal as received, ppmTi 176 114V 21 16Cr 7 8Co 56.1 24.5Ni 10 8.7Cu 5.7 9.7Zn 7.8 13.7As 2.5 1.9Se 4 0.4Mo 2.1 0.7Sr 71.8 96Zr 9.6 4.4Cd 0.1 0Sn 0.2 0.3Ba 241 62.7Hg 2.7 6.8Pb 2.3 3.5Cl 101 164F 16 45

2. Experimental apparatus

2.1. Flame reactor and operating conditions

A fuel lean flat laminar premixed flame operated at atmosphericpressure is used as reactor. The flame is sustained by droplets ofethanol, monodisperse in size and it is homogeneously doped withpulverized coal particles, not significantly modifying the flamestoichiometry. The reactor attempts to mimic local more than glo-bal conditions in furnaces and it allows investigating the kinetic ofash formation from single coal particle combustion in a well con-trolled environment.

The coals and the carbon black powder are first milled in a Plan-etary Mono Mill (Pulverisette 6, Fritsch) using tungsten carbidebowl and balls, and subsequently suspended in ethanol. The sus-pension, containing about 1.5 wt.% of coal/carbon black, is inten-sively sonicated to fragment agglomerated coal particles. Morethan 95% of the suspended particles have size smaller than30 lm, as verified by dispersion granulometric analyses (Hydro2000S, Malvern Instrument).

A Berglund–Liu-type Vibrating Orifice Aerosol Generator (VOAGmodel 3450, TSI), generating 80 lm monodisperse suspensiondroplets, is used to feed the flame. It supplies a constant flow of0.30 cm3/min through a 40 lm orifice oscillating at a frequencyof 20 KHz. The VOAG syringe pump is immersed into an ultrasonicthermostatic bath at 40 �C, to prevent particle sedimentation oragglomeration and the consequent orifice clogging. The resultingcoal/carbon black flow rate, added into the ethanol flame, is ofabout 4.5 mg/min.

The suspension droplets, once generated, are suddenly dis-persed and diluted in an oxidizing gas mixture with a flow rateof 2.3 l/min (STP), preventing their coalescence. The resultinggas/droplet mixture is fed to the burner.

The burner consists of two coaxial stainless steel tubes and it issimilar to that used by Arabi-Katbi et al. [20]. The inner tube(18 mm ID) is used to carry the reacting mixture and a 40 mm long

Mullite Zirconia honeycomb (400CPSI, CTI s.a.) is placed on its top.Inner tube wall temperature was kept constant at 90 �C to preventethanol condensation. The ring (24 mm ID and 34 mm OD), sup-plied by the outer tube, has a flow of 9.0 l/min (STP) of sheath ar-gon to minimize surrounding air perturbation and reduce flameflicker. A flat plate is also placed at 90 mm from the burner mouth.

Ethanol droplets completely evaporate through the honeycomband the vapor burns stabilizing a flat premixed flame on the hon-eycomb mouth (cold gas velocity is 16.5 cm/s). Coal/carbon blackaggregates, less than 20 lm in sizes, are homogenously carried intothe ethanol flame where they devolatilize, ignite and oxidize. Theparticle combustion environment can be adjusted by controllingthe ethanol flame stoichiometry and dilution allowing coal/carbonblack particles to react in different conditions in term of gas com-position and temperature. Further details on the flame reactor arereported elsewhere [19,21,22].

Measurements have been performed using two high volatilebituminous Colombian and Indonesian coals whose propertiesare reported in Table 1. Ash composition and minor elements con-centrations, measured by Agilent 7500 ICP-MS are also listed in Ta-ble 1. A Carbon Black powder (N762, Phillips Petroleum Co.) withprimary particle size between 76 nm and 100 nm, has been alsooxidized in the same conditions to evaluate the unburnt carbonand to analyze the role of carbonaceous particles in the build-upto the particle size distribution.

1292 F. Carbone et al. / Combustion and Flame 157 (2010) 1290–1297

Both air and pure oxygen are used as oxidizing gases. Theresulting flame equivalence ratios are approximately 0.75 and0.15, respectively. The compositions of post flame gases in whichparticle combustion occurs are reported in Table 2. They have beencalculated by a mass balance whereas excess oxygen was mea-sured at the system exhaust. Gas composition typical of air blownpulverized coal furnaces is obtained in the air flame whereas theflame fed with pure oxygen provides very high oxygen concentra-tion, larger than that commonly used in pulverized coal oxy-com-bustion reactors.

Temperature along the reactor axis has been measured using a250 lm Pt/Pt-13%Rh thermocouple (Type R, Omega Engineering)and it has been properly corrected for radiative losses. Pure etha-nol, without coal or carbon black, is used during temperature mea-surements to avoid particle deposition on the thermocouplejunction. The temperature profiles of the two flames are similarbut the oxygen flame temperature is lower relative to the air flamebecause the diluent oxygen has higher specific heat than nitrogen.Particle residence time (RT) in the reactor has been estimatedassuming that particle velocity equals that of the hot gases, takinginto account thermal expansion and neglecting buoyancy effects.

2.2. Particle size distribution function measurement technique

Particle size distribution (PSD) functions of the formed particlesare measured on-line using a dilution probe and a TapCon 3/150Differential Mobility Analyzer (DMA).

The DMA system is operated at the highest allowed aerosol(5.0 l/min) and sheath (50 l/min) flow rates and it is equipped witha diffusion charger neutralizer (Am-241 bipolar) and a Faraday CupElectro-meter detector [21–23]. Measurements are performedoperating the DMA in two modalities by changing the maximumvoltage applied to the electrostatic classifier. The nominal mobilitydiameter (MD) ranges from 0.6 nm to 28 nm and from 2.1 nm to100 nm in the low and high voltage modalities, respectively.

A horizontal, rapid-dilution probe (8 mm ID, 0.5 mm wall thick-ness) is used to suddenly cool and dilute sampled aerosol and de-liver it to the DMA [24]. The aerosol is drawn through a pinhole,drilled on the probe walls, while 29.5 l/min of particle free dilutionnitrogen steadily flows into the tube. The dilution ratio (DR) is fi-nely regulated performing a slight underpressure (DP) in theprobe, manually controlled with a needle valve connected to a ro-tary vane vacuum pump [25]. Dilution is necessary to avoid parti-cle coagulation in the probe and to keep detected particleconcentration in the operating range of the Faraday cupelectrometer.

Two probes with 0.3 mm and 1.5 mm sampling pinhole diame-ters are used to obtain DRs which vary by more than one order ofmagnitude.

The dilutions performed at several underpressures (DP) werecalibrated for both probes, measuring the oxygen and carbon diox-ide concentration into the probe during sampling. The calibrateddilution (±5% uncertainty) ratios are of the order of 1 � 103 and50 with the 0.3 mm and 1.5 mm pinhole probes, respectively.The corresponding sampling flow rates (Qsam) agree very well withthose calculated using the Bernoulli equation, accounting for gasthermal compression. Sampling velocity (Vs) only depends on the

Table 2Composition of the gas mixture in which coal particle combustion occurs.

Vol.% ± 0.5% Air flame Oxy flame

O2 5.0 76.5CO2 9.5 9.5H2O 14.0 14.0N2 71.5 0.0

underpressure once the gas temperature (Tor) and thus density(q), into the orifice is evaluated.

Q sam ¼p4

D2pin � VS ¼

p4

D2pin �

2 � DPqðTorÞ

� �0:5

The residence time of the sampled aerosol before entering theDMA is relatively short being of the order of milliseconds in thepinhole and of tenths of a second in the probe after dilution andcooling. The aerosol cooling rate, because of its mixing with nitro-gen, is of the order of 105–106 Ks�1.

Earlier studies on sooting laminar premixed flames examinedthe dilution probe effects and found that particle coagulation intothe probe may significantly change the shape of the PSD functions[26,27]. Particle coagulation in the sampling system can be sup-pressed by varying the DR in order to attain a critical value, abovewhich the shape of the size distribution does not change. However,following this procedure it was found that the dilution required tofollow faithfully the size distribution of nanoparticles smaller than10 nm reduced the concentration of larger particles to concentra-tions at the limit of the dynamic range of the electrometer.

To obtain the entire PSD function of ultrafine particles, the high-est DRs, preventing smallest nanoparticle coagulation, are usedwith the DMA operating in the 0.6–28 nm range while measure-ments with the DMA operating in the nominal MD range between2.1 nm and 100 nm are performed at lower dilution ratios. This al-lows the detection of larger particles but it does not prevent thecoagulation of smaller particles.

Number PSD function measurements are performed positioningthe sampling pinhole on the axis at 50 mm from the burner outlet.Results are averaged on about 10 scans and corrected for the ap-plied dilution ratio. The inherent uncertainty on number concen-tration obtained by each scan is very narrow (±15%) consideringthe discontinuous nature of the investigated phenomenon.

The number PSD functions were fitted by a sum of lognormaldistributions:

dNd ln D

ðMDÞ ¼X

i

Ni

lnðriÞffiffiffiffiffiffiffi2pp exp �1

2

ln MDhDii

� �lnðriÞ

24

35

28><>:

9>=>;

where Ni, hDii and ri are the total number concentration, the med-ian diameter and the width of the ith lognormal distribution func-tion, respectively.

Volume PSD functions are obtained by multiplying each point ofthe number PSD distribution functions by the corresponding parti-cle volume. The latter is calculated with the spherical approxima-tion considering that MD is larger than the particle sphericalequivalent diameter because of the DMA sheet gas effective diam-eter (D0 = 0.5 nm) [28].

dfvd ln D

ðMDÞ ¼ p6� ½MD� D0�3 �

dNd ln D

ðMDÞ

The particle number concentration and volume fraction (fv) arecalculated by integrating the obtained size distribution functionsin the desired MD intervals.

3. Results

An overall view of the flame reactor burning coal particles with5.0 vol.% and 76.5 vol.% of oxygen is reported in Fig. 1. Single coalparticle combustion trajectories can be quite well seen in the pic-tures because of light emission from the hot burning char surface.

The flat premixed flame zone is evidenced by the blue lightemission at the bottom of the reactor due to ethanol vapor com-bustion. Coal ignition seems to occur immediately downstreamof the flame front (RT � 1 ms) in both cases whereas the endpoint

1000 1200 1400 1600 18000

10

20

30

40

50

60

T [K]

HAB[m

m]

Fig. 1. Photographs and axial temperature profiles of the flame reactors (� air flame; J oxygen flame). The estimated value of particles residence time are also indicated asfunction of the height above the burner (HAB).

F. Carbone et al. / Combustion and Flame 157 (2010) 1290–1297 1293

of the luminous strikes, occurs at about 45 mm (RT � 50 ms) in theair flame and in a few millimeters (RT � 3 ms) in the oxygen flamebecause of the higher burning rate associated with enhanced oxy-gen concentration [29]. Light emission disappearing correspondswith inhibition of metal vaporization because the residual particletemperature drops to that of the surrounding gas. Char particlecombustion temperature seems to be significantly higher for thehigh oxygen condition as evidenced by the stronger yellow emis-sion which can be distinguished in the picture compared to thered emission observed in the low oxygen condition.

A careful observation allows us to distinguish some larger andmore persistent incandescent coal particles on the edge of the reac-tor. They are larger agglomerates formed on burner walls, carriedinto the flame by cold gases. Measurements with DMA are not af-fected by these random perturbations because particle samplingsare performed on axis.

Fig. 2 shows a magnification of a typical PSD function not cor-rected for the dilution ratio, measured with the 0.3 mm pinholeprobe when the DMA is operated in the nominal size range be-tween 0.6 nm and 28 nm. It shows a high number concentrationof particles with sizes smaller than 3 nm. The first task is to under-stand if the acquired signal is associated with particles in the nano-meter size range or to artifacts of the measuring procedures. Tothis end, background measurements were performed in ambientair and the coal free flame. The background signals are always be-low the electrometer sensitivity operating the DMA in the high

0,0E+00

1,0E+08

2,0E+08

3,0E+08

4,0E+08

5,0E+08

6,0E+08

7,0E+08

8,0E+08

0.5 1.0 1.5 2.0 2.5

dN/d

lnD

in th

e pr

obe

[cm

-3]

MD [nm]

Coal Free Flame

Coal Doped Flame

Fig. 2. Background raw signals measured in the coal free flame (D) compared tothat measured in a coal doped flame (J).

voltage mode while that measured for the coal free flame in thelow voltage mode is reported in Fig. 2 for comparison. Below1 nm the signals are comparable indicating that the signals below1 nm have to be attributed to other compounds than particles, i.e.to molecular clusters formed in the neutralizer from particles freegas because of the high free ion concentration [23]. The measuredsize distribution functions are reported for sizes not lower than1 nm for this reason.

Fig. 3 reports the number PSD functions corrected for the DRmeasured on the axis at 50 mm above the burner in the lower oxy-gen concentration condition using the Colombian coal. The PSDfunction is firstly measured at the highest DRs with the 0.3 mmpinhole probe to prevent particles coagulation in the probe operat-ing the DMA in the nominal size range between 0.6 nm and 28 nm.The measured size distribution function is plotted in Fig. 3 (circles).The PSD shows a very large number of particles with sizes between1 nm and 3 nm while particle number concentration falls by aboutfour orders of magnitude with increasing particle sizes, reachingthe limit of the detection system for mobility diameters of the or-der of 10 nm. The detected particle number concentration is3 � 1011 cm�3 while their volume fraction is 3 � 10�10.

Fig. 3 also reports the size distribution function measured at thelowest dilution, with the 1.5 mm pinhole probe in the size range upto 100 nm (triangles). It clearly shows a mode centered at about3 nm and a decrease by about four orders of magnitude moving to-wards increasing sizes. The number concentration of detected par-

1,0E+05

1,0E+06

1,0E+07

1,0E+08

1,0E+09

1,0E+10

1,0E+11

1,0E+12

100101

dN/d

lnD

[cm

-3]

MD [nm]

Fig. 3. Number PSD functions measured in the air flame feeding the Colombiancoal, corrected for the dilution ratio: s DR � 1 � 103; D DR � 50.

1294 F. Carbone et al. / Combustion and Flame 157 (2010) 1290–1297

ticles is 1 � 1010 cm�3 while their volume fraction is 3.5 � 10�10,larger than that measured at higher dilution ratio because of thepossibility to detect larger particles.

The two PSD functions agree well for sizes larger than 7 nm.Coagulation in the probe, due to lower dilution, affects only smallerparticles causing the decrease in their number concentration. Theentire ultrafine PSD function can be obtained by merging that mea-sured at the highest dilution for sizes smaller than 10 nm and thatmeasured at the lowest dilution for larger sizes.

Fig. 4 reports the reconstructed PSD functions in number and vol-ume. Particle number concentration quickly falls by six orders ofmagnitude with increasing sizes from 1.5 nm to 60 nm while itslightly increases for larger size. The number concentration is3 � 1011 cm�3, almost all due to particles smaller than 10 nm. In-deed, the number concentration of larger particles is four orders ofmagnitude lower (7.5 � 106 cm�3) while those larger than 30 nmare another order of magnitude less abundant (1.5 � 106 cm�3).The ultrafine particle volume fraction (5 � 10�10) is mainly due totwo classes of particles, those smaller than 10 nm representingabout 60% and those larger than 30 nm (2 � 10�10).

The multimodal behavior of the PSD function has been repro-duced with four lognormal distribution functions to obtain anacceptable fit of the experimental data. Fig. 4 reports as solid linethe fitted curve and as dashed lines the contributing functions.Parameters of the fitting procedure are reported in Table 3.

The same experimental and fitting procedures described for thelow oxygen condition have been used for the analysis of the PSDfunctions of the particles formed in high oxygen concentrationconditions using the Colombian coal. Fig. 5 reports the measured

1,0E+05

1,0E+06

1,0E+07

1,0E+08

1,0E+09

1,0E+10

1,0E+11

1,0E+12

100101

dN/d

lnD

[cm

-3]

MD [nm]

1,0E-12

1,0E-11

1,0E-10

1,0E-09

100101

dFv/

dlnD

MD [nm]

Fig. 4. Number (top) and volume (bottom) size distribution functions of ultrafineash in the air flame feeding the Colombian coal. Crosses represent that obtained bymerging the measurements at highest and lowest dilution. Solid line shows thefitting performed summing four lognormal modes that are also plotted in dashedlines.

number PSD functions. As in the previous case, the number sizedistribution functions are firstly measured operating the DMA inthe nominal size range between 0.6 nm and 28 nm and using thehighest DRs to prevent particle coagulation into the probe. Thisfunction is plotted with circles in Fig. 5. Also in the high oxygenconcentration condition, the particle number concentration is veryhigh in the size range between 1 and 2 nm and it suddenly falls atincreasing sizes. The number concentration of detected particle is3 � 1011 cm�3 while their volume fraction is 5.5 � 10�10.

Fig. 5 also reports with triangles the PSD measured at the lowestdilution. It clearly shows a mode centered between 3 nm and 4 nmand a decrease by four orders of magnitude with increasing sizesup to 100 nm. The number concentration of detected particles is1 � 1010 cm�3 while their volume fraction is 3 � 10�9.

As previously described, appreciable differences between thePSD functions measured at the lowest and the highest dilution ra-tios are observed for sizes smaller than 15 nm while they wellagree for larger sizes. The entire ultrafine particle size distributionfunctions is obtained merging that measured at the highest dilu-tion for size smaller than 20 nm with that measured at the lowestdilution for larger sizes and it is plotted in Fig. 6 in number and vol-ume. The PSD function falls five orders of magnitude with increas-ing sizes up to 100 nm. The huge number concentration of ultrafineparticles again depends on particles smaller than 10 nm whereasall sizes contribute significantly to the ultrafine particle volumefraction. About 35% (9.5 � 10�10) is due to particles smaller than30 nm while those smaller than 10 nm (2 � 10�10) are also a signif-icant fraction, about 7.5%, of ultrafine volume.

Once again, the PSD function is multimodal and four lognormaldistribution functions are required to perform the fitting reportedin Fig. 6 by a solid line. Each of the four lognormal functions is alsoplotted in Fig. 6 by dashed line whereas the parameters of the log-normals are reported in Table 3.

The same procedure described in detail for ultrafine ashes of theColombian coal is followed to obtain the size distribution functionsof particles generated from the Indonesian coal and from the car-bon black powder. Table 3 also reports the parameter to reproducethe Indonesian coal experimental data.

4. Comparative discussion

The fitted PSD functions obtained for the Colombian and Indo-nesian coals in low and high oxygen conditions are compared inFig. 7 in terms of particle number yields on coal basis and volumeyields on ash basis. PSD shapes are not largely affected by the typesof coal but instead depend on the gas composition in which coalcombustion occurs. Fig. 7 also shows the size distribution functionsresulting from carbon black oxidation in the same conditions ofoxygen concentrations. Oxidation of carbon black in the flamereactor leaves less than 0.01% of particles not oxidized at 50 mmabove the burner. These particles have sizes in the 1–5 nm rangeand could derive from fragmentation of black carbon, as observedfor soot oxidation in a flow reactor [30] or surface oxidativedetachment. The size distributions function of carbon black gener-ated particles is almost the same as that from coal combustion inthe size range below 5 nm.

Ultrafine particles are generated in a huge number of the orderof 1012 per microgram of burnt coal. Volume yields on ash basiscan be approximately identified with the mass percentage on totalash of particles assuming unit density for ultrafine particles. In thishypothesis, ultrafine particles larger than 30 nm (those commonlydetected) in conventional conditions are approximately 1% of totalash mass, a value not too far from the typical percentage of ultra-fine ashes detected in laboratory air blown pulverized coal fur-naces [5,31]. This is particularly true considering that the

Table 3Lognormal distribution parameters used to fit the measured size distribution functions.

Mode Colombian coal Indonesian coal

Number concentration Ni (cm�3) Median diameter hDii (nm) Width ri Number concentration Ni (cm�3) Median diameter hDii (nm) Width ri

Air flameI 2.8 � 011 1.4 1.4 2.8 � 1011 1.45 1.4II 4.2 � 108 3.8 1.5 7.0 � 108 3.8 1.5III 5.6 � 106 15 1.6 1.7 � 107 12.5 1.6IV 1.1 � 106 100 1.7 2.0 � 106 100 1.7

Oxygen flameI 2.0 � 1011 1.3 1.3 2.0 � 1011 1.3 1.3II 1.8 � 109 3.8 1.5 5.0 � 109 3.8 1.5III 2.8 � 108 15 1.6 5.0 � 108 10 1.6IV 1.7 � 107 40 1.7 2.2 � 107 40 1.7

1,0E+05

1,0E+06

1,0E+07

1,0E+08

1,0E+09

1,0E+10

1,0E+11

1,0E+12

100101

dN/d

lnD

[cm

-3]

MD [nm]

Fig. 5. Number PSD functions measured in the oxygen flame feeding the Colombiancoal, corrected for the dilution ratio: s DR � 1 � 103; D DR � 50.

1,0E+05

1,0E+06

1,0E+07

1,0E+08

1,0E+09

1,0E+10

1,0E+11

1,0E+12

100101

dN/d

lnD

[cm

-3]

MD [nm]

1,0E-12

1,0E-11

1,0E-10

1,0E-09

1 10 100

dFv/

dlnD

MD [nm]

Fig. 6. Number (top) and volume (bottom) size distribution functions of ultrafineash in the oxygen flame feeding the Colombian coal. Crosses represent thatobtained by merging the measurements at highest and lowest dilution. Solid lineshows the fitting performed summing four lognormal modes that are also plotted indashed lines.

F. Carbone et al. / Combustion and Flame 157 (2010) 1290–1297 1295

investigated flame reactor inhibits coarse particles formation be-cause coal particles are fed with size smaller than 20 lm. Anywaynanoparticles smaller than 30 nm represent a significant fraction ofthe ultrafine ashes in both conventional and oxygen enriched con-ditions, being much higher in the latter condition. The ultrafineparticle yields in both flames are almost independent of the typeof coal used. A correlation between the yields of nano sized parti-cles (D < 30 nm) and the coal chlorine content in oxygen enrichedconditions have been observed elsewhere [21]. Results in conven-tional conditions and in the entire ultrafine range reported in thispaper seem to follow this trend.

The ultrafine PSD functions are multimodal both in conven-tional and oxygen enriched conditions and they can be fitted byfour lognormal distributions in all investigated cases. The firstthree modes are roughly centered at almost the same diameters,namely 1.5 nm, 4 nm and 15 nm, below the resolution of com-monly used diagnostics. The fourth mode is centered at about100 nm for the air flames, in reasonable agreement with modalsizes of the ultrafine ash detected at the exhaust of air blown pul-verized coal furnaces [5]. The center of the fourth mode is at signif-icantly smaller sizes (�50 nm) for the oxygen flames.

The other main difference between the PSD functions in con-ventional and oxygen enriched conditions concerns the contribu-tion of each mode to the entire PSD function. The amounts ofparticles in the first and second modes are comparable, both innumber and in volume. Particles belonging to the third mode aremore abundant (more than one order of magnitude) in the oxygenflame relative to the air flame. The fourth mode contains a consid-erably larger number of particles in the oxygen flame but it ac-counts for a smaller contribution to volume yields relative to theair flame because of the smaller particle sizes.

The comparison of these results clearly shows that oxygen en-riched conditions enhance the formation of small particles, in thesize range between 5 nm and 80 nm, regardless of the type of coalexamined in this study and in agreement with behavior reported inthe literature [6]. This behavior could be related to the higher charburning temperature which enhances metal vaporization from theprecursor coal particles. Moreover the higher surface temperaturecauses a stronger cooling rate of such vapors while they escapefrom the boundary layer surrounding coal particles. The highercooling rate favors homogeneous nucleation, which leads to theformation of smaller particles relative to condensation growth [9].

It is important to remember that measurements have beenperformed at the same height of 50 mm above the burner in both

1,0E+06

1,0E+07

1,0E+08

1,0E+09

1,0E+10

1,0E+11

1,0E+12

100101

1/M

coal

·dN p

artic

les/d

lnD

[µg-

1 ]

MD [nm]

Air Flame - Carbon BlackOxygen Flame - Carbon BlackAir Flame - ColombiaOxygen Flame - ColombiaAir Flame - IndonesiaOxygen Flame - Indonesia

1,0E-04

1,0E-03

1,0E-02

1,0E-01

100101

1/M

ash·

dVpa

rticl

es/d

lnD

[cm

3 /g]

MD [nm]

Fig. 7. Number yields on coal basis (top) and volume yields on ash basis (bottom)size distribution functions of ultrafine particles. Dashed and solid lines show thatobtained for the Colombian coal in the air and oxygen flames, respectively. Dottedand solid-dotted lines are the fit of results obtained for the Indonesian coal in the airand oxygen flames, respectively. Triangles an diamonds are particles generatedfrom oxidation of carbon black in the air and oxygen flame, respectively.

1296 F. Carbone et al. / Combustion and Flame 157 (2010) 1290–1297

conditions regardless of the different residence times evidenced bythe visual analysis of Fig. 1. Indeed, PSD functions are measured inproximity of the char burnout/quenching zone in the low oxygenconditions and far downstream this zone in the high oxygen condi-tions. Once generated, particles have a longer residence time in thehigh oxygen conditions relative to the low oxygen conditions be-fore they are sampled. Nanoparticles smaller than 10 nm seem tosurvive at high temperatures despite the long residence time inthe reactor probably because of their low coagulation efficiency.Coagulation may be partially responsible for the particle accumu-lation in the second mode when switching from the air to the oxy-gen flame.

The yields of particles smaller than 5 nm are not largely affectedby oxygen concentration and thus by char surface temperature.Moreover particles formed during carbon black oxidation havethe same sizes of the first mode of the size distribution resultingfrom coal combustion suggesting that first mode mainly containscarbonaceous particles. Particles with sizes between 5 nm and80 nm could be generated through selective nucleation of com-pounds having different vapor pressures. Their concentrations arestrongly affected by oxygen concentration and so by char surfacetemperature. The largest fraction of detected ultrafine particleshas sizes typical of ultrafine ashes at the exhaust of coal furnaces[9] so that they could be formed through smaller particle growthbeing an incipient multicomponent accumulation mode. These dif-ferent mechanisms could be responsible for the multimodal sizedistribution functions shown in this paper.

A size dependent chemical characterization of the particles isrequired to further speculate on the origin of each particle sizemode. PSD function measurements at several heights above the

burner, to follow particles dynamic in the flame, could also beuseful.

5. Conclusion

The determination of the size distribution function of ultrafineparticles, with sizes below 100 nm, formed during pulverized coalcombustion has been carried out in a laboratory scale flame reac-tor, operated at low (50 vol%) and high (76.5 vol%) oxygen concen-trations. The measurements are performed on-line by a highresolution Differential Mobility Analyzer using a rapid-dilutionprobe sampling technique.

The detected ultrafine particles have huge number concentra-tions in both conditions. Nanoparticles smaller than 30 nm com-prise almost all the number density while they also constitute asignificant fraction of ultrafine ash in volume. The nanometric frac-tion seems to survive coagulation.

The measured particle size distribution functions have been fit-ted with a sum of lognormal distributions to clearly show theirmultimodal nature. Four modes are observed for ultrafine ashesin both conventional and oxygen enriched conditions. The firstthree modes are centered at 1.5 nm, 4 nm and 15 nm regardlessof the operating conditions. The fourth mode is centered at100 nm in conventional conditions, corresponding to the size ofcommonly detected ultrafine particles, whereas it shows a quitesmaller size (50 nm) in oxygen enriched conditions.

Results obtained by carbon black oxidation suggested that car-bonaceous particles having sizes smaller than 5 nm largely accountfor the smallest mode of coal ultrafine ashes in both low and highoxygen concentrations. The larger modes are probably generatedbecause of selective nucleation of compounds having different va-por pressures and growth mechanisms.

The effect of enhanced oxygen concentration is very strongcausing the formation of a larger amount (more than four times)of ultrafine particles with smaller size relative to that generatedin conventional conditions. This behavior can be explained by thehigher coal burning temperature which promotes the metal vapor-ization–nucleation pathway.

Acknowledgments

The work was partially supported by Ministry of EconomicalDevelopment (MSE, Italy) under the Accordo di Programma CNR–MSE ‘‘Carbone Pulito”. The authors are grateful to R. Pagliara, F.Montagnaro, V. Stanzione, M. Urciuolo and R. La Gala for the exper-imental support.

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