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Measurement of nanoparticles of organic carbon in non-sooting flame conditions
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Proceedings of the
Proceedings of the Combustion Institute 32 (2009) 689–696
Combustion Institute www.elsevier.com/locate/proci
Measurement of nanoparticles of organic carbon in non-sooting flame conditions L.A. Sgro a,*, A.C. Barone a, M. Commodo a, A. D’Alessio a, A. De Filippo a, G. Lanzuolo a, P. Minutolo b a
Dipartimento di Ingegneria Chimica, University of Naples Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy b Istituto di Ricerche sulla Combustione, CNR, Piazzale Tecchio 80, 80125 Napoli, Italy
Abstract In this work we compare the results of several nanoparticle measurement techniques with the aim of investigating the formation of nanoparticles in non-sooting to slightly sooting flames. In slightly sooting conditions there is quite good agreement between Differential Mobility Analyser (DMA), Atomic Force Microscopy (AFM), and optical measurements on particle size and concentration. However, in rich flames below the onset of soot, DMA measures a strong drop-off in the total particle volume fraction at low fuel to air mixtures, which is not observed in optical or AFM measurements that detect a more gradual decrease in particle concentration with decreasing C/O and almost constant spectroscopic properties. The disagreement is significantly larger than experimental error and is only observed when the particle size distribution includes solely particles smaller than about 3 nm. Particle losses in the DMA sampling system does not seem to be the only possible reason for justifying the discrepancy with the other techniques. Further investigations are necessary in order to characterize chemically and physically this class of nanoparticles which constitute the earliest stage in the formation of particulate carbon. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Particle inception; DMA; AFM; UV optical
1. Introduction While particle growth and the dynamics of soot particles are relatively well understood, particle inception and the early stages of particle growth are still open areas of research in the field of combustion science [1–3]. Recent advances in nanoparticle measurement methods are providing new information on the *
Corresponding author. Fax: +39 081 593 6936. E-mail address: [email protected] (L.A. Sgro).
size, concentration and chemical nature of incipient nanoparticles, which helps develop and test theoretical models that describe particle inception and growth processes at high temperature. ‘‘In situ” optical measurements indicated that a first particles mode, named NOC, is formed in fuel rich non-sooting flames, starting from a critical equivalence ratio, /, value of about 1.5, and in sootforming flames prior to the formation of soot [4,5]. NOC mass concentrations are comparable to soot in flames near the onset of soot formation, suggesting that soot inception is due to the fast coagulation of NOC and a contemporaneous aro-
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.06.216
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matization/carbonization process that transforms coagulated NOC to graphitic soot, while in these flame conditions surface growth is a secondary process [6–8]. A series of in situ and off-line optical measurements in the UV, visible, and IR show that the chemical structure of NOC seems to consist of 2– 3 ring aromatics in a polymer-like structure linked with aliphatic bonding and possibly also oxygen [6–9]. The size distribution of incipient particles was also directly measured off-line by Atomic Force Microscopy (AFM) [7,10], and Transmission Electron Microscopy (TEM) [11,12], on-line by Differential Mobility Analysis (DMA) [13–15] and molecular beam mass spectroscopy [13,16,17], and ex situ and in situ by Time Resolved Fluorescence Polarization Anisotropy [18,19]. There is relatively good agreement on the size of the smallest particle measured, with a model diameter of about 2 nm. However, there is considerable disagreement on the quantity of such small particles in flame conditions prior to the onset of soot [13]. Specifically, DMA measures a very small or negligible amount of nanoparticles in the 1– 4 nm size range in flames with / lower than the soot threshold limit (/ < 2). Also, DMA measurements show a strong increase in particle concentration with flame residence time, suggesting that particle inception continues in the post-flame zone and surface growth can play a role in soot formation also in slightly sooting flames [15]. A better understanding of the reason for the discrepancy between these two measurements is important for obtaining reliable information for the theoretical modeling of particle inception and growth processes at high temperature. In this work, we compare measurements of particle size and concentration elaborated from different diagnostic methods with demonstrated sensitivity to combustion generated particles as small as 1 nm in a wide range of flame conditions aiming to shed light on the particle formation before the soot threshold. 2. Experimental methods Atmospheric pressure ethylene/air flames were stabilized on a McKenna burner, d = 6 cm, with a cold gas velocity, V = 10 cm/s. Temperature has been measured using uncoated type R thermocouples, with a spherical junction bead, d = 200 lm, following the procedures and corrections reported by Shaddix [20]. UV–vis optical measurements, off-line Atomic Force Microscopy (AFM) analysis of particles deposited on cool substrates by thermophoresis, and on-line DMA measurements were taken at a constant height above the burner, z = 10 mm, in fuel/air mixtures ranging from stoichiometric (C/ O = 0.33) to moderately sooting (C/O = 0.77) conditions.
2.1. In situ UV–visible optical measurements A De lamp was the light source for spectral light absorption measurements. The light beam was focalised in the flame, collected by two lenses and detected by an ICCD camera coupled with a spectrograph. In the high temperature flame environment, CO2 and water are the only major species that can absorb light at wavelengths larger than 200 nm. Their light absorption spectra at high temperature were calculated from their molar concentration, evaluated by the chemkin modeling code, and the spectral absorptivity reported in the literature [21] and was subtracted from the measured flame absorption spectra. To estimate the amount of light absorption due to gas phase PAHs it is necessary to know the concentration of each compound and their absorbance at flame temperature. Since these quantities are unknown, we roughly estimated PAH absorption from their concentrations measured in analogous flames reported in literature [22,23] and using the molar absorptivity at k = 266 nm of 2600 cm1/(mole/l). The estimated contribution of PAHs to light absorption increases with C/O ratio, and it is on the order of a few percent in leaner conditions and about 10% in the richest one (C/O = 0.8). Since these quantities are low and because of the large uncertainty, the PAH contribution was not subtracted. The residual light absorption spectra, K 0abs ðkÞ, can be attributed to high molecular mass species and elaborated within the Mie theory under the Rayleigh approximation. To elaborate the spectra, we made a further approximation considering the presence of only two classes of particles, soot and NOC, with optical properties that do not depend on particle size or flame conditions. Spectral extinction measurements can distinguish between NOC, which absorbs light in the UV but not in the visible, and soot, which has a continuous absorption curve throughout the UV–visible wavelength spectrum with a broad maximum at about 230 nm, and is the only species absorbing in the visible [8]. Therefore, the whole absorption spectrum due to soot, K 0abs-soot ðkÞ, was easily determined from the measurement of K 0abs ðkvis Þ, and the light absorption due to NOC was derived by subtracting K 0abs-soot ðkÞ from K 0abs ðkÞ. Both the volume fractions, fvNOC and fvsoot , were then determined using the volume attenuation coefficient of soot, Kabs-soot/fv, of 8.8 104 cm2/cm3 in the visible, according to literature data [24]. K 0abs-NOC /fv (k = 266 nm) = 3.5 104 cm2/cm3 for NOC, which corresponds to the refractive index equal to mNOC(k = 266 nm) = 1.35 0.09i [25]. The laser light source used for Laser Induced Fluorescence (LIF) was the fourth harmonic of
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a Nd:YAG pulsed laser (k0 = 266 nm). The light beam was attenuated to avoid particle photo-fragmentation/vaporization. The LIF signal was measured with a spectrograph coupled with an ICCD camera 90 deg to the incident excitation light and calibrated by comparison with the scattering signal from room temperature gas with a known scattering cross section, like methane. In flame conditions that form only NOC, the UV light absorption due to particles and fluorescence signal are linearly proportional, and the UV fluorescence in sooting flames is mostly due to NOC [6,26]. Therefore, the profile of the volume fraction of NOC, fv,NOC versus C/O ratio was also determined by UV fluorescence measurements at k = 330 nm (after calibrating the fluorescence signal with fv,NOC determined by K 0abs at a single point measured in a non-sooting flame condition). 2.2. AFM Thermophoretic sampling was originally developed by Dobbins and Megaridis [27] to measure the size and morphology of flame generated soot. Thereafter, the technique was improved in order to determine the local soot volume fraction in flames [28], and to estimate the adhesion efficiency of nanoparticles smaller than 10 nm by comparison to the volume fraction estimated by optical methods [7]. Thermophoretic sampling has been here used to extract particles from flames with C/O ranging from 0.33 to 0.65 at z = 10 mm and determine their volume fraction in flame. Particles were collected on mica substrates. The mica disks (dmica = 3 mm, thickness = 0.2 mm) were adhered to a knife edge stainless steel holder (thickness = 0.2 mm, width = 4 mm) that was inserted vertically, parallel to the flow field, to minimize flame disturbance. The sampling time was ts = 30 ms. The central portion of the mica discs was analyzed by AFM operated in tapping-mode in order to minimize convolution, plastic deformation and displacement of the sample by the tip. Resolution was about 1–2 nm for the x and y axes and below ˚ for the z axis in low-noise conditions. A 1A deconvolution algorithm and an advanced image processing software were employed to calculate the particle volumes and the diameter of a volume equivalent sphere, ED. Thermophoresis, the phenomenon which drives particles toward colder regions in a gas environment with a temperature gradient, is the dominant mechanism causing particle deposition on a cold surface immersed in a flame [29]. When a cold sampling surface is inserted into a relatively hot particle-containing gas environment, particle mass transfer will occur across the thermal boundary layer. Following Ko¨ylu¨ et al. [28], the thermophoretic mass flux of particles to the sampling probe can be estimated as
J 00w
" 2 # Nux Tw 1 ffi DT q p fv 2x Tg
691
ð1Þ
where DT is the thermophoretic diffusivity, Nux is the local Nusselt number for heat transfer at x, which is the vertical position of the sampling probe measured from the lower edge, Tw and Tg are the probe surface and gas temperatures, respectively, qp is the particle density and fv is the local particle volume fraction. If all particles arriving at the mica plate adhere to its surface, the thermophoretic mass flux is equal to the total mass of particles deposited at the center of the mica plate per unit image area, A, and sampling time, ts. Since the kinetic energy of carbonaceous nanoparticles with mean sizes lower than 10 nm at the flame temperature may exceed the depth of the interaction potential of the interaction with the mica surface and the time spent in the region of the interaction potential is also small, these particles can rebound after collision. This thermal rebound effect causes the size distribution function of the particles adhered to the mica plate measured by AFM, dN/dlnD, to be proportional to the size distribution of the particles approaching the mica plate, and the proportionality factor is the adhesion efficiency c(D). The adhesion efficiency has been evaluated by the following expression [7]: U0 ðDÞ U0 ðDÞ cðDÞ ¼ 1 1 þ exp ð2Þ KT KT where U0 is the minimum of the Lennard–Jones interaction potential between a particle with diameter D and the mica plate, which was evaluated considering a Hamaker constant of 3 1020 J. From the total volume of particles measured by AFM and the expression of the thermophoretic mass flux the local particle volume fraction in the flame environment is determined by P 3 " 2 #1 c1 i dN i Di 2 x Tw i fv ffi 1 : ð3Þ Tg Ats DT Nux The particles in the flame can be considered isolated spherical particles in the free molecular regime, so that DT = 3/4(1 + pamom/8)1mg, with amom = 1 being the momentum accommodation coefficient and mg ¼ 1:29 109 T g1:65 m2 =s being the kinematic viscosity of the host gas [28]. The center of the mica disk, where images were analyzed, was positioned at x = 2 mm. Finally, Nux ¼ 0:332Pr1=3 Re1=2 x , Pr ffi 0.7 and Rex = ugx/mf where ug is the axial gas velocity in the flame, and mf is the gas kinematic viscosity at Tf = (Tg + Tw)/2. 2.3. DMA To prevent particle coagulation, flame products were sampled through an orifice
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(ID = 0.3 mm, thickness = 0.5 mm) into a turbulent flow dilution tube probe operating with N2 as the diluent with flow = 29.61 /min, as described in detail in earlier work [15] and used in many combustion studies [13,14]. For all conditions reported in this paper, the sample flow was approximately isokinetic. The size distributions of flame products were measured with a TapCon (or Vienna) DMA equipped with a Faraday Cup Electrometer (FCE) with rated sensitivity to measure particles/macromolecules in the size range dmobility = 0.6–28 nm [30]. Number concentrations were corrected for Dilution Ratio, DR, (calibrated by measuring CO2 concentrations) and reduced (by a factor of 300 K/1000 K) to account for the change in gas volume sampled at the probe orifice and measured in the DMA. Flame generated particles/macromolecules smaller than 1.5 nm cannot be accurately quantified due to a large interfering peak in the size range of 0.6– 1.4 nm (mobility diameter) that is due to molecular clusters formed by ion-induced nucleation in the bipolar charger [31]. We used the semi-empirical equation given by Ferna´ndez de la Mora, et al. to calculate the actual diameter, dactual, of particles from the measured electrical mobility, Z, in a sheath flow of a carrier gas with molecular mass, m, temperature, T and pressure, p, 1=2 q kTm Z ¼ 0:441 ð4Þ pðd actual þ d 0 Þ2 where k is the Boltzmann constant and d0 can be thought of as the effective diameter of the gas in which the aerosol is immersed (d0, air(273 K) = 0.5 nm) [32]. The dactual calculated by Eq. (4) is equal to mobility diameter, determined by the Milikan–Fuchs’ equation, when d0 = 0. All data in the paper are reported in terms of dactual. Finally, we estimated particle losses due to collisions with the walls in the sampling orifice for particles in a laminar flow, and in the dilution probe for particles in a turbulent flow. Particle losses in the turbulent flow of the sample line after dilution were evaluated using the expressions reported by Hinds [33]. Assuming that the high dilution occurs rapidly in sample line, losses in the turbulent diluted sample flow are very low compared to particle losses in the orifice prior to dilution [15]. The presence of particle rebound during penetration of nano-sized aerosol particles in laminar flow through tubes is still under debate [34,35]. Therefore, we determined particle losses to the orifice walls in two ways. First, we assumed the absence of thermal rebound: every collision of particles with a wall results in a loss for all sizes of particles measured. In this case, we used the formula for penetration in laminar flow derived by Gormley and Kennedy [36]:
P ¼ 0:8191 expð3:657bÞ þ 0:0975 expð22:3bÞ þ 0:0325 expð57bÞ for b P 0:0312; P ¼ 1 2:56b2=3 þ 1:2b þ 0:177b4=3 b < 0:0312
ð5Þ for
where b = pDL/Q, D is the particle diffusion coefficient, L is the tube length, and Q the aerosol flow rate. The temperature of flame products entering the probe orifice was estimated as Torifice = 1000 ± 250 K by extrapolation of the measured axial flame temperature profile. The large uncertainty in Torifice is because the slope in the temperature profile within 2 mm of the probe is changing and steep. We also calculated particle penetration allowing for possible particle rebound after collision with the wall. Particle sticking efficiency, c(D), was evaluated by the interaction potential of particles and orifice walls, similarly to the interaction of particles with the mica disk as discussed in AFM section, considering an Hamaker constant of 1 1019 J estimated for organic particles and a metal wall. The effect of particle rebound has been taken into account considering that the number of collisions with the walls producing particle losses is equal to the number of collisions multiplied by the particle sticking efficiency. 3. Results and discussion The axial temperature profile of the flames studied without the sampling probe, Tno-probe has a maximum at the flame front (located at z = 2–3 mm) and decreases by about 100 K over the post-flame zone (z = 3–12 mm). In the C/O range 0.50–0.71, the maximum flame temperature ranges from 1760 to 1700 K. Temperature measurements show that the probe significantly cools the post-flame zone near the orifice inlet underneath the probe, in agreement with Zhao et al. [15]. Tprobe is about 500 K cooler than Tno-probe 1 mm beneath the probe surface and Tprobe = Tno-probe 4 mm below the probe surface. The local temperature difference (Tprobe–Tno-probe) is more than an order of magnitude larger than the slight change in the maximum flame temperature in non-sooting flames that produce NOC. The AFM images of the samples on mica plates collected 10 mm above the burner surface in two flames with C/O = 0.56 and 0.65 are reported in Fig. 1. For both conditions, the residual light absorption spectrum, after gas contribution subtraction, K 0abs ðkÞ, and LIF spectrum measured in situ are also reported. Both images show the presence of very small particles. Even though the size of the particles is sensibly different
L.A. Sgro et al. / Proceedings of the Combustion Institute 32 (2009) 689–696
(a C/O=0.56) (a)
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Fig. 1. AFM images of samples collected at z = 10 mm and the corresponding in situ light absorption and LIF spectra for a C/O = 0.56 flame (a) and C/O = 0.65 flame (b).
for the two flames, the shape of the absorption and fluorescence spectra remain almost the same while the intensity of light absorption and fluorescence is about a factor of 3 higher in the flame with C/O = 0.65 than in the flame with C/ O = 0.56. It’s interesting to note that at 10 mm above the burner, even in the richer flame reported in the figure, no evidence of soot particle absorption or laser induced incandescence can be measured. Direct evidence that particles effectively emit fluorescence was obtained by analyzing the time resolved depolarization of fluorescence in the ns time scale [18]. Fluorescing particles were detected in material sampled in water from a non-sooting flame (C/O = 0.56), and the particle mean sizes were determined to be in the range of 1.2– 2.5 nm [18]. These measurements, performed in liquid phase and at ambient temperature, evidenced a different spectroscopic behavior between particles smaller and larger than 2 nm which can be indicative of a different chemical composition. It is worth noting that oxygen functionalities were detected in sampled particles by Surface Enhanced Raman Spectroscopy [9]. Figure 2 compares size distributions determined by on-line DMA and AFM. AFM particle size distributions are reported without the correction for particle adhesion efficiency, empty symbols, or with this correction, full symbols. In the C/O = 0.56 flame, the DMA does not detect any particles while the AFM measurement finds 1011 particles/cm3, or an even larger concentration if the effect of adhesion efficiency for particle deposition on the mica plate is considered
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Fig. 2. Particle size distribution function measured by DMA (triangle), and AFM (empty square) in various flames: C/O = 0.56 (a), C/O = 0.61 (b), and C/O = 0.65 (c). Full square report the AFM size distribution corrected for sticking efficiency c.
(Fig. 2a). In this flame, almost all particles are smaller than 3 nm and the modal diameter is at about 2 nm. Figure 2b shows the PDF measured in the C/ O = 0.61 flame. AFM measures slightly larger particles than the DMA; the modal diameter is larger than 2 nm, and the size distribution extends up to d = 5 nm. In this flame, the DMA also starts to detect particles, and the size distribution measured by DMA is reported in Fig. 3b with triangles. The first mode smaller than 1.5 nm may be due both to carbon compounds produced in flames and molecular clusters formed in the ionization process in particle free gasses [14]. This smallest peak in on-line DMA measurements (denoted with a gray background) was not considered for the analysis in this work. The size distribution measured by DMA covers the same size range as the AFM one, but the modal diameter measured by DMA is slightly smaller, about 1.5 nm, and significantly less particles are
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L.A. Sgro et al. / Proceedings of the Combustion Institute 32 (2009) 689–696 1.E-06
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Fig. 3. Total particle volume fraction, fv, versus flame C/O ratio measured by (a) light absorption at z = 10 mm (full diamonds) and z = 6 mm [4] (empty diamonds), LIF at z = 6 [4] (empty triangles) and at z = 10 (full triangles). The gray region indicate a range interval accounting for experimental uncertainty, repeatability of the measurements and variation with z to compare with ex situ results. (b) AFM (empty squares) and AFM with correction for sticking efficiency c compared to the optically determined fv (full squares). (c) MA (empty triangles) compared to optically determined fv.
detected. The particle concentration measured by DMA is lower then 1011 particles/cm3, while AFM estimates 1011–1012 particles/cm3 depending on whether the measurement is corrected for the size-dependent adhesion efficiency, c. Figure 2c reports size distributions measured in a richer flame, with C/O = 0.65, close to the soot inception threshold. In this case, DMA detects more particles than AFM, 5 1012 particles/cm3, and the DMA size distribution agrees very well with that obtained by AFM considering the effect of the particle adhesion efficiency. Both AFM and DMA find a modal diameter of about 1.5–2 nm, and no particles larger than 5 nm are detected.
It’s worthwhile to underline that although the size distribution measured in a near-sooting flame (Fig. 2c) is sensibly different than that measured in a non-sooting flame (Fig. 2a), where DMA does not measure any particles and the size distribution is limited to only d < 3 nm particles, no substantial differences in spectroscopic properties were detected as shown in Fig. 1. Figure 3 plots the total particle volume fraction, fv, determined by optical, AFM, and on-line DMA measurements as a function of C/O ratio. The volume fraction determined by LIF accounts only for the fluorescing species so that in sooting conditions it does not account for the soot components and is therefore lower than fv estimated by the other techniques, which measure the total particulate volume fraction. When fv is measured by off-line experiments using a sampling probe, results are typically spatially shifted to lower height above the burner, z, to account for the probe cooling the flame [37]. While we did not shift our measurements in z, Fig. 3a, reports fv measured by in situ optical methods at z = 10 mm and 6 mm [4], providing a range interval to compare with ex situ results. The range interval has been evidenced in the figure by two continuous lines, which also takes into account measurement uncertainties. The error bars in Fig. 3 account for the smallest volume fraction that can be measured by each technique greater than instrumental noise and differences in measurements made on different physical burners or set up. If we define the soot formation threshold, C/Osoot-thr, as the C/O value where light absorption in the visible and Laser Induced Incandescence are detected, for the examined flame conditions the C/Osoot-thr ffi 0.7. The reason why the fvLIF curves do not follow the continuous line drawn in Fig. 3a for C/O > 0.7 is because it neglects the contribution of soot particles. The onset of NOC by optical measurements can be considered to be the condition in which the measured scattering signal, Qvv, or light absorption and fluorescence start to noticeably increase above the gas phase background. In a previous paper, the threshold for NOC formation was estimated to be C/ONOC-thr ffi 0.5 mainly on the bases of scattering measurements [4]. Recent results of light absorption and LIF, using more recent data on gas phase properties [21], indicate that below C/O = 0.55 data uncertainty is too large to detect the presence of particles. We did not perform enough AFM measurements to precisely measure the onset C/O for NOC. However, AFM measures no detectable particles in stoichiometric flames (C/O = 0.33), while the size distributions measured by AFM for C/O = 0.5–0.61 are similar within experimental error. Figure 3b compares the optical measurements with the AFM ones. The volume fractions fvAFM
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determined by AFM measurements are lower than the optical ones, but the trend with C/O is similar. Furthermore, the differences in the absolute value of fv are strongly smoothed over when AFM results are corrected for the particle adhesion efficiency, c. The main difference between the three measurements in Fig. 3 is that the DMA measurements find a drastic drop-off in particle volume fraction for C/O < 0.65 (Fig. 3c), and no particles are detected by the DMA for C/O < 0.6 flames. This drop-off is not seen in the AFM or optical measurements, that measure a significant amount of particles in the range C/O = 0.5–0.65 (Fig. 3a and b). It’s interesting to note that the drop-off in the concentration of NOC is measured by DMA at low C/O where the size distributions show only the smallest mode (d < 3 nm) NOC particles. Since very small particles can be easily lost during probe sampling due to their large diffusivity and consequent number of collisions to the walls, we estimated the effect of losses of particles in the sampling line as described in the experimental section. Figure 4 shows that diffusional particle losses in the sample line (assuming rapid mixing of the sample and diluent flows) are negligible, and their correction minimally affects the f DMA curve. v Instead, losses in the orifice prior to dilution are significant, and the fvDMA curve corrected for losses in the orifice and sampling line is noticeably different than the uncorrected curve (Fig. 4). Since diffusion losses are stronger for smaller particles, loss correction sensibly effects the particle concentration measured by DMA for low C/O ratio but
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Fig. 4. Total particle volume fraction measured by DMA (full diamonds), DMA corrected for line losses (full triangles), DMA corrected for losses in line and orifice (full circles), DMA corrected for losses in line and orifice with correction for sticking efficiency c (letter x) compared to fv measured by optical methods.
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is less relevant for sooting flames with size distributions that include also larger particles. With loss corrections, DMA results agree much better with the optical ones. Nevertheless, this result is in contrast with the observation that the orifice clogs in sooting flame conditions and not in non-sooting flames. Furthermore, the particle size distribution corrected for losses no longer presents a maximum at 1.5–2 nm; instead, it continuously decreases from d = 1 nm, where the number concentration becomes extremely high (1013–1014 particle/cm3). Since these results are not realistic, and in disagreement with other works that also report a particle mode at 1.5 nm [38], we also investigated the effect of possible thermal rebound in calculating particle penetration in the orifice. Allowing for thermal rebound, the fvDMA curve again increases for non-sooting flames but the drop-off of the curve remains relevant (Fig. 4). It’s worth noting that there are severe uncertainties in the calculations of particle penetration. First, corrections for losses depends on the temperature in the probe orifice, which was not measured directly, but rather extrapolated from axial temperature measurements. Second, the flow through the orifice is actually creeping and not quite laminar (Re 0.8) so losses may be higher than those calculated from laminar flow theory. Finally, since it is quite unlikely that the small sample flow (1 ml/min) mixes quickly and thoroughly with the large diluent flow (29.6 l/min), losses near the orifice are probably underestimated. 4. Conclusions The main objective of this work was to investigate particulate formation in premixed ethylene/ air flames from non-sooting to slightly sooting conditions. To this aim, we compared measurements of particle size and concentration elaborated from different diagnostic methods with demonstrated sensitivity to combustion generated particles as small as 1 nm. All of the techniques are in quite good agreement in flames above the soot formation threshold. In flames with C/ O < 0.65, DMA measures a strong drop-off in particle concentration, and no particles are detected for C/O < 0.6. In contrast, a much lower decrease in particle concentration has been detected by optical measurements and AFM analysis of particles collected by thermophoretic sampling. In blue flames optical measurements detect particles with similar spectroscopic properties to those detected in richer flames, and the only difference is that their size distribution measured by AFM shows only particles smaller than 3 nm. The small size of the particles alone does not justify the disagreement with DMA at low C/O since both DMA and AFM detect the same
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concentration of d < 3 nm particles in the C/ O = 0.65 flame. Also corrections for particles losses in the probe used for DMA measurements do not appear to justify the discrepancies. Further investigation in particle composition across the soot threshold is necessary to understand the motivation of a different detection efficiency between DMA, based on particles mobility measurements and thermophoretic-AFM and optical methods. The effect of a different chemical composition, percentage of aliphatic–aromatic functionalities or oxygen inclusions, together with the very small size of the particles should be investigated in more detail to determine possible effects on particles diffusivity, ionization efficiency, optical properties or interaction potential which could help to merge the gap between the current measurements. In summary, the differences of results by techniques based on different effects stimulate a more detailed analysis of the properties of nanoparticles produced at different C/O ratios in premixed flames. This analysis is important for modern combustion systems, which emit low to undetectable levels of soot since local and instantaneous C/O ratios above stoichiometry would produce high molecular mass structures (nanoparticles) that can be emitted to the atmosphere.
References [1] A. D’Anna, A. Violi, A. D’Alessio, A.F. Sarofim, Combust. Flame 127 (2001) 1995–2003. [2] M. Frenklach, H. Wang, in: H. Bockhorn (Ed.), Soot Formation in Combustion, Springer-Verlag, Heidelberg, 1994, p. 265. [3] J. Singh, R.I.A. Patterson, M. Kraft, H. Wang, Combust. Flame 145 (2006) 117–127. [4] P. Minutolo, G. Gambi, A. D’Alessio, Proc. Combust. Inst. 27 (1998) 1461–1469. [5] A. D’Alessio, P. Minutolo, G. Gambi, A. D’Anna, Ber. Bunsenges Phys. Chem. 97 (1993) 1574–1582. [6] P. Minutolo, G. Gambi, A. D’Alessio, S. Carlucci, Atmos. Environ. 33 (1999) 2725–2732. [7] A. D’Alessio, A.C. Barone, R. Cau, A. D’Anna, P. Minutolo, Proc. Combust. Inst. 30 (2005) 2595–2603. [8] G. Basile, A. Rolando, A. D’Alessio, A. D’Anna, P. Minutolo, Proc. Combust. Inst. 29 (2002) 2391– 2397. [9] G. Rusciano, A.C. De Luca, A. D’Alessio, P. Minutolo, G. Pesce, A. Sasso, Carbon 46 (2) (2008) 335–341. [10] A.C. Barone, A. D’Alessio, A. D’Anna, Combust. Flame 132 (2003) 181–187. [11] C.M. Megaridis, R.A. Dobbins, Combust. Sci. Technol. 66 (1989) 1–16. [12] R.A. Dobbins, H. Subramaniasivam, in: H. Bockhorn (Ed.), Soot Formation in Combustion, Springer-Verlag, Berlin, 1994, p. 290.
[13] M. Thierley, H.-H. Grotheer, M. Aigner, et al., Proc. Combust. Inst. 31 (2007) 639–647. [14] L.A. Sgro, A. De Filippo, G. Lanzuolo, A. D’Alessio, Proc. Combust. Inst. 31 (2007) 631–638. [15] B. Zhao, Z. Yang, J. Wang, M.V. Johnston, H. Wang, Aerosol Sci. Technol. 37 (2003) 611. [16] M. Bachmann, W. Wiese, K.-H. Homann, TwentySixth Symposium (International) on Combustion, 1996, pp. 2259–2267. [17] P. Weilmunster, A. Keller, K.-H. Homann, Combust. Flame 116 (1999) 62–83. [18] A. Bruno, C. de Lisio, P. Minatolo, A. D’Alessio, Combust. Flame 151 (2007) 472–481. [19] A. Bruno, F. Ossler, P. Minutolo, C. de Lisio, J. Opt. A: Pure Appl. Opt., in press. doi:10.1088/14644258/10/6/064016. [20] C.R. Shaddix, Proceedings of the Thirty-Third Natural Heat Transfer Conference, Albuquerque, NM, 1999. [21] T. Joutsenoja, A. D’Anna, A. D’Alessio, M.I. Nazzaro, Appl. Spectrosc. 55 (2) (2001) 130–135. [22] R. Barbella, A. Ciajolo, A. D’Anna, A. Tregrossi, Proceedings of the Joint Meeting of the Italian and Portuguese Sections of the Combustion Institute, Salsomaggiore Terme, Italy, September 20–23, 1994, p. III-13. [23] T.R. Melton, A.M. Vincitore, S.M. Senkan, Twenty-Seventh Symposium (International) on Combustion, 1998, pp. 1631–1637. [24] K.C. Smyth, C.R. Shaddix, Combust. Flame 107 (1996) 314–320. [25] D. Cecere, L.A. Sgro, G. Basile, A. D’Alessio, A. D’Anna, P. Minutolo, Combust. Sci. Technol. 174 (2002) 377–398. [26] M. Commodo, S. Violi, A. D’Anna, et al., Combust. Sci. Technol. 179 (2007) 387–400. [27] R.A. Dobbins, C.M. Megaridis, Langmuir 2 (1987) 254–259. ¨ . Ko¨ylu¨, C.S. McEnally, D.E. Rosner, L.D. ¨ .O [28] U Pfefferle, Combust. Flame 110 (1997) 494–507. [29] A.D. Eisner, D.E. Rosner, Combust. Flame 61 (1985) 153–166. [30] G.P. Reischl, J.M. Makela, J. Neid, Aerosol Sci. Technol. 27 (1997) 651–672. [31] W. Winklmayr, G.P. Reischl, A.O. Lindner, A. Berner, J. Aerosol Sci. 22 (1991) 289–296. [32] J. Ferna´ndez de la Mora, L. de Juan, K. Liedke, A. Schmidt-Ott, J. Aerosol Sci. 34 (2003) 79–98. [33] W.C. Hinds, Aerosol Technology, second ed., John Wiley & Sons, New York, NY, 1999. [34] H.C. Wang, G. Kasper, J. Aerosol Sci. 22 (1991) 31–41. [35] M. Alonso, Y. Kousaka, T. Hashimoto, N. Hashimoto, Aerosol Sci. Technol. 27 (1997) 471– 480. [36] P.G. Gormley, M. Kennedy, Proc. R. Irish Acad. 52A (1949) 163–169. [37] B. Zhao, Z. Yang, M.V. Johnston, H. Wang, A.S. Wexler, M. Balthasar, M. Kraft, Combust. Flame 133 (2003) 173–188. [38] T.G. Baquet, H.-H. Grotheer, M. Aigner, Rapid Commun. Mass Spectrom. 21 (2007) 4060–4064.
Proceedings of the
Proceedings of the Combustion Institute 32 (2009) 689–696
Combustion Institute www.elsevier.com/locate/proci
Measurement of nanoparticles of organic carbon in non-sooting flame conditions L.A. Sgro a,*, A.C. Barone a, M. Commodo a, A. D’Alessio a, A. De Filippo a, G. Lanzuolo a, P. Minutolo b a
Dipartimento di Ingegneria Chimica, University of Naples Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy b Istituto di Ricerche sulla Combustione, CNR, Piazzale Tecchio 80, 80125 Napoli, Italy
Abstract In this work we compare the results of several nanoparticle measurement techniques with the aim of investigating the formation of nanoparticles in non-sooting to slightly sooting flames. In slightly sooting conditions there is quite good agreement between Differential Mobility Analyser (DMA), Atomic Force Microscopy (AFM), and optical measurements on particle size and concentration. However, in rich flames below the onset of soot, DMA measures a strong drop-off in the total particle volume fraction at low fuel to air mixtures, which is not observed in optical or AFM measurements that detect a more gradual decrease in particle concentration with decreasing C/O and almost constant spectroscopic properties. The disagreement is significantly larger than experimental error and is only observed when the particle size distribution includes solely particles smaller than about 3 nm. Particle losses in the DMA sampling system does not seem to be the only possible reason for justifying the discrepancy with the other techniques. Further investigations are necessary in order to characterize chemically and physically this class of nanoparticles which constitute the earliest stage in the formation of particulate carbon. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Particle inception; DMA; AFM; UV optical
1. Introduction While particle growth and the dynamics of soot particles are relatively well understood, particle inception and the early stages of particle growth are still open areas of research in the field of combustion science [1–3]. Recent advances in nanoparticle measurement methods are providing new information on the *
Corresponding author. Fax: +39 081 593 6936. E-mail address: [email protected] (L.A. Sgro).
size, concentration and chemical nature of incipient nanoparticles, which helps develop and test theoretical models that describe particle inception and growth processes at high temperature. ‘‘In situ” optical measurements indicated that a first particles mode, named NOC, is formed in fuel rich non-sooting flames, starting from a critical equivalence ratio, /, value of about 1.5, and in sootforming flames prior to the formation of soot [4,5]. NOC mass concentrations are comparable to soot in flames near the onset of soot formation, suggesting that soot inception is due to the fast coagulation of NOC and a contemporaneous aro-
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.06.216
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matization/carbonization process that transforms coagulated NOC to graphitic soot, while in these flame conditions surface growth is a secondary process [6–8]. A series of in situ and off-line optical measurements in the UV, visible, and IR show that the chemical structure of NOC seems to consist of 2– 3 ring aromatics in a polymer-like structure linked with aliphatic bonding and possibly also oxygen [6–9]. The size distribution of incipient particles was also directly measured off-line by Atomic Force Microscopy (AFM) [7,10], and Transmission Electron Microscopy (TEM) [11,12], on-line by Differential Mobility Analysis (DMA) [13–15] and molecular beam mass spectroscopy [13,16,17], and ex situ and in situ by Time Resolved Fluorescence Polarization Anisotropy [18,19]. There is relatively good agreement on the size of the smallest particle measured, with a model diameter of about 2 nm. However, there is considerable disagreement on the quantity of such small particles in flame conditions prior to the onset of soot [13]. Specifically, DMA measures a very small or negligible amount of nanoparticles in the 1– 4 nm size range in flames with / lower than the soot threshold limit (/ < 2). Also, DMA measurements show a strong increase in particle concentration with flame residence time, suggesting that particle inception continues in the post-flame zone and surface growth can play a role in soot formation also in slightly sooting flames [15]. A better understanding of the reason for the discrepancy between these two measurements is important for obtaining reliable information for the theoretical modeling of particle inception and growth processes at high temperature. In this work, we compare measurements of particle size and concentration elaborated from different diagnostic methods with demonstrated sensitivity to combustion generated particles as small as 1 nm in a wide range of flame conditions aiming to shed light on the particle formation before the soot threshold. 2. Experimental methods Atmospheric pressure ethylene/air flames were stabilized on a McKenna burner, d = 6 cm, with a cold gas velocity, V = 10 cm/s. Temperature has been measured using uncoated type R thermocouples, with a spherical junction bead, d = 200 lm, following the procedures and corrections reported by Shaddix [20]. UV–vis optical measurements, off-line Atomic Force Microscopy (AFM) analysis of particles deposited on cool substrates by thermophoresis, and on-line DMA measurements were taken at a constant height above the burner, z = 10 mm, in fuel/air mixtures ranging from stoichiometric (C/ O = 0.33) to moderately sooting (C/O = 0.77) conditions.
2.1. In situ UV–visible optical measurements A De lamp was the light source for spectral light absorption measurements. The light beam was focalised in the flame, collected by two lenses and detected by an ICCD camera coupled with a spectrograph. In the high temperature flame environment, CO2 and water are the only major species that can absorb light at wavelengths larger than 200 nm. Their light absorption spectra at high temperature were calculated from their molar concentration, evaluated by the chemkin modeling code, and the spectral absorptivity reported in the literature [21] and was subtracted from the measured flame absorption spectra. To estimate the amount of light absorption due to gas phase PAHs it is necessary to know the concentration of each compound and their absorbance at flame temperature. Since these quantities are unknown, we roughly estimated PAH absorption from their concentrations measured in analogous flames reported in literature [22,23] and using the molar absorptivity at k = 266 nm of 2600 cm1/(mole/l). The estimated contribution of PAHs to light absorption increases with C/O ratio, and it is on the order of a few percent in leaner conditions and about 10% in the richest one (C/O = 0.8). Since these quantities are low and because of the large uncertainty, the PAH contribution was not subtracted. The residual light absorption spectra, K 0abs ðkÞ, can be attributed to high molecular mass species and elaborated within the Mie theory under the Rayleigh approximation. To elaborate the spectra, we made a further approximation considering the presence of only two classes of particles, soot and NOC, with optical properties that do not depend on particle size or flame conditions. Spectral extinction measurements can distinguish between NOC, which absorbs light in the UV but not in the visible, and soot, which has a continuous absorption curve throughout the UV–visible wavelength spectrum with a broad maximum at about 230 nm, and is the only species absorbing in the visible [8]. Therefore, the whole absorption spectrum due to soot, K 0abs-soot ðkÞ, was easily determined from the measurement of K 0abs ðkvis Þ, and the light absorption due to NOC was derived by subtracting K 0abs-soot ðkÞ from K 0abs ðkÞ. Both the volume fractions, fvNOC and fvsoot , were then determined using the volume attenuation coefficient of soot, Kabs-soot/fv, of 8.8 104 cm2/cm3 in the visible, according to literature data [24]. K 0abs-NOC /fv (k = 266 nm) = 3.5 104 cm2/cm3 for NOC, which corresponds to the refractive index equal to mNOC(k = 266 nm) = 1.35 0.09i [25]. The laser light source used for Laser Induced Fluorescence (LIF) was the fourth harmonic of
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a Nd:YAG pulsed laser (k0 = 266 nm). The light beam was attenuated to avoid particle photo-fragmentation/vaporization. The LIF signal was measured with a spectrograph coupled with an ICCD camera 90 deg to the incident excitation light and calibrated by comparison with the scattering signal from room temperature gas with a known scattering cross section, like methane. In flame conditions that form only NOC, the UV light absorption due to particles and fluorescence signal are linearly proportional, and the UV fluorescence in sooting flames is mostly due to NOC [6,26]. Therefore, the profile of the volume fraction of NOC, fv,NOC versus C/O ratio was also determined by UV fluorescence measurements at k = 330 nm (after calibrating the fluorescence signal with fv,NOC determined by K 0abs at a single point measured in a non-sooting flame condition). 2.2. AFM Thermophoretic sampling was originally developed by Dobbins and Megaridis [27] to measure the size and morphology of flame generated soot. Thereafter, the technique was improved in order to determine the local soot volume fraction in flames [28], and to estimate the adhesion efficiency of nanoparticles smaller than 10 nm by comparison to the volume fraction estimated by optical methods [7]. Thermophoretic sampling has been here used to extract particles from flames with C/O ranging from 0.33 to 0.65 at z = 10 mm and determine their volume fraction in flame. Particles were collected on mica substrates. The mica disks (dmica = 3 mm, thickness = 0.2 mm) were adhered to a knife edge stainless steel holder (thickness = 0.2 mm, width = 4 mm) that was inserted vertically, parallel to the flow field, to minimize flame disturbance. The sampling time was ts = 30 ms. The central portion of the mica discs was analyzed by AFM operated in tapping-mode in order to minimize convolution, plastic deformation and displacement of the sample by the tip. Resolution was about 1–2 nm for the x and y axes and below ˚ for the z axis in low-noise conditions. A 1A deconvolution algorithm and an advanced image processing software were employed to calculate the particle volumes and the diameter of a volume equivalent sphere, ED. Thermophoresis, the phenomenon which drives particles toward colder regions in a gas environment with a temperature gradient, is the dominant mechanism causing particle deposition on a cold surface immersed in a flame [29]. When a cold sampling surface is inserted into a relatively hot particle-containing gas environment, particle mass transfer will occur across the thermal boundary layer. Following Ko¨ylu¨ et al. [28], the thermophoretic mass flux of particles to the sampling probe can be estimated as
J 00w
" 2 # Nux Tw 1 ffi DT q p fv 2x Tg
691
ð1Þ
where DT is the thermophoretic diffusivity, Nux is the local Nusselt number for heat transfer at x, which is the vertical position of the sampling probe measured from the lower edge, Tw and Tg are the probe surface and gas temperatures, respectively, qp is the particle density and fv is the local particle volume fraction. If all particles arriving at the mica plate adhere to its surface, the thermophoretic mass flux is equal to the total mass of particles deposited at the center of the mica plate per unit image area, A, and sampling time, ts. Since the kinetic energy of carbonaceous nanoparticles with mean sizes lower than 10 nm at the flame temperature may exceed the depth of the interaction potential of the interaction with the mica surface and the time spent in the region of the interaction potential is also small, these particles can rebound after collision. This thermal rebound effect causes the size distribution function of the particles adhered to the mica plate measured by AFM, dN/dlnD, to be proportional to the size distribution of the particles approaching the mica plate, and the proportionality factor is the adhesion efficiency c(D). The adhesion efficiency has been evaluated by the following expression [7]: U0 ðDÞ U0 ðDÞ cðDÞ ¼ 1 1 þ exp ð2Þ KT KT where U0 is the minimum of the Lennard–Jones interaction potential between a particle with diameter D and the mica plate, which was evaluated considering a Hamaker constant of 3 1020 J. From the total volume of particles measured by AFM and the expression of the thermophoretic mass flux the local particle volume fraction in the flame environment is determined by P 3 " 2 #1 c1 i dN i Di 2 x Tw i fv ffi 1 : ð3Þ Tg Ats DT Nux The particles in the flame can be considered isolated spherical particles in the free molecular regime, so that DT = 3/4(1 + pamom/8)1mg, with amom = 1 being the momentum accommodation coefficient and mg ¼ 1:29 109 T g1:65 m2 =s being the kinematic viscosity of the host gas [28]. The center of the mica disk, where images were analyzed, was positioned at x = 2 mm. Finally, Nux ¼ 0:332Pr1=3 Re1=2 x , Pr ffi 0.7 and Rex = ugx/mf where ug is the axial gas velocity in the flame, and mf is the gas kinematic viscosity at Tf = (Tg + Tw)/2. 2.3. DMA To prevent particle coagulation, flame products were sampled through an orifice
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(ID = 0.3 mm, thickness = 0.5 mm) into a turbulent flow dilution tube probe operating with N2 as the diluent with flow = 29.61 /min, as described in detail in earlier work [15] and used in many combustion studies [13,14]. For all conditions reported in this paper, the sample flow was approximately isokinetic. The size distributions of flame products were measured with a TapCon (or Vienna) DMA equipped with a Faraday Cup Electrometer (FCE) with rated sensitivity to measure particles/macromolecules in the size range dmobility = 0.6–28 nm [30]. Number concentrations were corrected for Dilution Ratio, DR, (calibrated by measuring CO2 concentrations) and reduced (by a factor of 300 K/1000 K) to account for the change in gas volume sampled at the probe orifice and measured in the DMA. Flame generated particles/macromolecules smaller than 1.5 nm cannot be accurately quantified due to a large interfering peak in the size range of 0.6– 1.4 nm (mobility diameter) that is due to molecular clusters formed by ion-induced nucleation in the bipolar charger [31]. We used the semi-empirical equation given by Ferna´ndez de la Mora, et al. to calculate the actual diameter, dactual, of particles from the measured electrical mobility, Z, in a sheath flow of a carrier gas with molecular mass, m, temperature, T and pressure, p, 1=2 q kTm Z ¼ 0:441 ð4Þ pðd actual þ d 0 Þ2 where k is the Boltzmann constant and d0 can be thought of as the effective diameter of the gas in which the aerosol is immersed (d0, air(273 K) = 0.5 nm) [32]. The dactual calculated by Eq. (4) is equal to mobility diameter, determined by the Milikan–Fuchs’ equation, when d0 = 0. All data in the paper are reported in terms of dactual. Finally, we estimated particle losses due to collisions with the walls in the sampling orifice for particles in a laminar flow, and in the dilution probe for particles in a turbulent flow. Particle losses in the turbulent flow of the sample line after dilution were evaluated using the expressions reported by Hinds [33]. Assuming that the high dilution occurs rapidly in sample line, losses in the turbulent diluted sample flow are very low compared to particle losses in the orifice prior to dilution [15]. The presence of particle rebound during penetration of nano-sized aerosol particles in laminar flow through tubes is still under debate [34,35]. Therefore, we determined particle losses to the orifice walls in two ways. First, we assumed the absence of thermal rebound: every collision of particles with a wall results in a loss for all sizes of particles measured. In this case, we used the formula for penetration in laminar flow derived by Gormley and Kennedy [36]:
P ¼ 0:8191 expð3:657bÞ þ 0:0975 expð22:3bÞ þ 0:0325 expð57bÞ for b P 0:0312; P ¼ 1 2:56b2=3 þ 1:2b þ 0:177b4=3 b < 0:0312
ð5Þ for
where b = pDL/Q, D is the particle diffusion coefficient, L is the tube length, and Q the aerosol flow rate. The temperature of flame products entering the probe orifice was estimated as Torifice = 1000 ± 250 K by extrapolation of the measured axial flame temperature profile. The large uncertainty in Torifice is because the slope in the temperature profile within 2 mm of the probe is changing and steep. We also calculated particle penetration allowing for possible particle rebound after collision with the wall. Particle sticking efficiency, c(D), was evaluated by the interaction potential of particles and orifice walls, similarly to the interaction of particles with the mica disk as discussed in AFM section, considering an Hamaker constant of 1 1019 J estimated for organic particles and a metal wall. The effect of particle rebound has been taken into account considering that the number of collisions with the walls producing particle losses is equal to the number of collisions multiplied by the particle sticking efficiency. 3. Results and discussion The axial temperature profile of the flames studied without the sampling probe, Tno-probe has a maximum at the flame front (located at z = 2–3 mm) and decreases by about 100 K over the post-flame zone (z = 3–12 mm). In the C/O range 0.50–0.71, the maximum flame temperature ranges from 1760 to 1700 K. Temperature measurements show that the probe significantly cools the post-flame zone near the orifice inlet underneath the probe, in agreement with Zhao et al. [15]. Tprobe is about 500 K cooler than Tno-probe 1 mm beneath the probe surface and Tprobe = Tno-probe 4 mm below the probe surface. The local temperature difference (Tprobe–Tno-probe) is more than an order of magnitude larger than the slight change in the maximum flame temperature in non-sooting flames that produce NOC. The AFM images of the samples on mica plates collected 10 mm above the burner surface in two flames with C/O = 0.56 and 0.65 are reported in Fig. 1. For both conditions, the residual light absorption spectrum, after gas contribution subtraction, K 0abs ðkÞ, and LIF spectrum measured in situ are also reported. Both images show the presence of very small particles. Even though the size of the particles is sensibly different
L.A. Sgro et al. / Proceedings of the Combustion Institute 32 (2009) 689–696
(a C/O=0.56) (a)
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Fig. 1. AFM images of samples collected at z = 10 mm and the corresponding in situ light absorption and LIF spectra for a C/O = 0.56 flame (a) and C/O = 0.65 flame (b).
for the two flames, the shape of the absorption and fluorescence spectra remain almost the same while the intensity of light absorption and fluorescence is about a factor of 3 higher in the flame with C/O = 0.65 than in the flame with C/ O = 0.56. It’s interesting to note that at 10 mm above the burner, even in the richer flame reported in the figure, no evidence of soot particle absorption or laser induced incandescence can be measured. Direct evidence that particles effectively emit fluorescence was obtained by analyzing the time resolved depolarization of fluorescence in the ns time scale [18]. Fluorescing particles were detected in material sampled in water from a non-sooting flame (C/O = 0.56), and the particle mean sizes were determined to be in the range of 1.2– 2.5 nm [18]. These measurements, performed in liquid phase and at ambient temperature, evidenced a different spectroscopic behavior between particles smaller and larger than 2 nm which can be indicative of a different chemical composition. It is worth noting that oxygen functionalities were detected in sampled particles by Surface Enhanced Raman Spectroscopy [9]. Figure 2 compares size distributions determined by on-line DMA and AFM. AFM particle size distributions are reported without the correction for particle adhesion efficiency, empty symbols, or with this correction, full symbols. In the C/O = 0.56 flame, the DMA does not detect any particles while the AFM measurement finds 1011 particles/cm3, or an even larger concentration if the effect of adhesion efficiency for particle deposition on the mica plate is considered
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Fig. 2. Particle size distribution function measured by DMA (triangle), and AFM (empty square) in various flames: C/O = 0.56 (a), C/O = 0.61 (b), and C/O = 0.65 (c). Full square report the AFM size distribution corrected for sticking efficiency c.
(Fig. 2a). In this flame, almost all particles are smaller than 3 nm and the modal diameter is at about 2 nm. Figure 2b shows the PDF measured in the C/ O = 0.61 flame. AFM measures slightly larger particles than the DMA; the modal diameter is larger than 2 nm, and the size distribution extends up to d = 5 nm. In this flame, the DMA also starts to detect particles, and the size distribution measured by DMA is reported in Fig. 3b with triangles. The first mode smaller than 1.5 nm may be due both to carbon compounds produced in flames and molecular clusters formed in the ionization process in particle free gasses [14]. This smallest peak in on-line DMA measurements (denoted with a gray background) was not considered for the analysis in this work. The size distribution measured by DMA covers the same size range as the AFM one, but the modal diameter measured by DMA is slightly smaller, about 1.5 nm, and significantly less particles are
volume fraction, cm3/cm3
volume fraction, cm3/cm3
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L.A. Sgro et al. / Proceedings of the Combustion Institute 32 (2009) 689–696 1.E-06
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fv (Kabs z=6mm) fv (DMA) 0.55
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Fig. 3. Total particle volume fraction, fv, versus flame C/O ratio measured by (a) light absorption at z = 10 mm (full diamonds) and z = 6 mm [4] (empty diamonds), LIF at z = 6 [4] (empty triangles) and at z = 10 (full triangles). The gray region indicate a range interval accounting for experimental uncertainty, repeatability of the measurements and variation with z to compare with ex situ results. (b) AFM (empty squares) and AFM with correction for sticking efficiency c compared to the optically determined fv (full squares). (c) MA (empty triangles) compared to optically determined fv.
detected. The particle concentration measured by DMA is lower then 1011 particles/cm3, while AFM estimates 1011–1012 particles/cm3 depending on whether the measurement is corrected for the size-dependent adhesion efficiency, c. Figure 2c reports size distributions measured in a richer flame, with C/O = 0.65, close to the soot inception threshold. In this case, DMA detects more particles than AFM, 5 1012 particles/cm3, and the DMA size distribution agrees very well with that obtained by AFM considering the effect of the particle adhesion efficiency. Both AFM and DMA find a modal diameter of about 1.5–2 nm, and no particles larger than 5 nm are detected.
It’s worthwhile to underline that although the size distribution measured in a near-sooting flame (Fig. 2c) is sensibly different than that measured in a non-sooting flame (Fig. 2a), where DMA does not measure any particles and the size distribution is limited to only d < 3 nm particles, no substantial differences in spectroscopic properties were detected as shown in Fig. 1. Figure 3 plots the total particle volume fraction, fv, determined by optical, AFM, and on-line DMA measurements as a function of C/O ratio. The volume fraction determined by LIF accounts only for the fluorescing species so that in sooting conditions it does not account for the soot components and is therefore lower than fv estimated by the other techniques, which measure the total particulate volume fraction. When fv is measured by off-line experiments using a sampling probe, results are typically spatially shifted to lower height above the burner, z, to account for the probe cooling the flame [37]. While we did not shift our measurements in z, Fig. 3a, reports fv measured by in situ optical methods at z = 10 mm and 6 mm [4], providing a range interval to compare with ex situ results. The range interval has been evidenced in the figure by two continuous lines, which also takes into account measurement uncertainties. The error bars in Fig. 3 account for the smallest volume fraction that can be measured by each technique greater than instrumental noise and differences in measurements made on different physical burners or set up. If we define the soot formation threshold, C/Osoot-thr, as the C/O value where light absorption in the visible and Laser Induced Incandescence are detected, for the examined flame conditions the C/Osoot-thr ffi 0.7. The reason why the fvLIF curves do not follow the continuous line drawn in Fig. 3a for C/O > 0.7 is because it neglects the contribution of soot particles. The onset of NOC by optical measurements can be considered to be the condition in which the measured scattering signal, Qvv, or light absorption and fluorescence start to noticeably increase above the gas phase background. In a previous paper, the threshold for NOC formation was estimated to be C/ONOC-thr ffi 0.5 mainly on the bases of scattering measurements [4]. Recent results of light absorption and LIF, using more recent data on gas phase properties [21], indicate that below C/O = 0.55 data uncertainty is too large to detect the presence of particles. We did not perform enough AFM measurements to precisely measure the onset C/O for NOC. However, AFM measures no detectable particles in stoichiometric flames (C/O = 0.33), while the size distributions measured by AFM for C/O = 0.5–0.61 are similar within experimental error. Figure 3b compares the optical measurements with the AFM ones. The volume fractions fvAFM
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determined by AFM measurements are lower than the optical ones, but the trend with C/O is similar. Furthermore, the differences in the absolute value of fv are strongly smoothed over when AFM results are corrected for the particle adhesion efficiency, c. The main difference between the three measurements in Fig. 3 is that the DMA measurements find a drastic drop-off in particle volume fraction for C/O < 0.65 (Fig. 3c), and no particles are detected by the DMA for C/O < 0.6 flames. This drop-off is not seen in the AFM or optical measurements, that measure a significant amount of particles in the range C/O = 0.5–0.65 (Fig. 3a and b). It’s interesting to note that the drop-off in the concentration of NOC is measured by DMA at low C/O where the size distributions show only the smallest mode (d < 3 nm) NOC particles. Since very small particles can be easily lost during probe sampling due to their large diffusivity and consequent number of collisions to the walls, we estimated the effect of losses of particles in the sampling line as described in the experimental section. Figure 4 shows that diffusional particle losses in the sample line (assuming rapid mixing of the sample and diluent flows) are negligible, and their correction minimally affects the f DMA curve. v Instead, losses in the orifice prior to dilution are significant, and the fvDMA curve corrected for losses in the orifice and sampling line is noticeably different than the uncorrected curve (Fig. 4). Since diffusion losses are stronger for smaller particles, loss correction sensibly effects the particle concentration measured by DMA for low C/O ratio but
volume fraction, cm3/cm3
1.E-06
1.E-07
1.E-08
1.E-09
1.E-10
0.5
0.55
0.6
0.65
0.7
0.75
0.8
C/O
Fig. 4. Total particle volume fraction measured by DMA (full diamonds), DMA corrected for line losses (full triangles), DMA corrected for losses in line and orifice (full circles), DMA corrected for losses in line and orifice with correction for sticking efficiency c (letter x) compared to fv measured by optical methods.
695
is less relevant for sooting flames with size distributions that include also larger particles. With loss corrections, DMA results agree much better with the optical ones. Nevertheless, this result is in contrast with the observation that the orifice clogs in sooting flame conditions and not in non-sooting flames. Furthermore, the particle size distribution corrected for losses no longer presents a maximum at 1.5–2 nm; instead, it continuously decreases from d = 1 nm, where the number concentration becomes extremely high (1013–1014 particle/cm3). Since these results are not realistic, and in disagreement with other works that also report a particle mode at 1.5 nm [38], we also investigated the effect of possible thermal rebound in calculating particle penetration in the orifice. Allowing for thermal rebound, the fvDMA curve again increases for non-sooting flames but the drop-off of the curve remains relevant (Fig. 4). It’s worth noting that there are severe uncertainties in the calculations of particle penetration. First, corrections for losses depends on the temperature in the probe orifice, which was not measured directly, but rather extrapolated from axial temperature measurements. Second, the flow through the orifice is actually creeping and not quite laminar (Re 0.8) so losses may be higher than those calculated from laminar flow theory. Finally, since it is quite unlikely that the small sample flow (1 ml/min) mixes quickly and thoroughly with the large diluent flow (29.6 l/min), losses near the orifice are probably underestimated. 4. Conclusions The main objective of this work was to investigate particulate formation in premixed ethylene/ air flames from non-sooting to slightly sooting conditions. To this aim, we compared measurements of particle size and concentration elaborated from different diagnostic methods with demonstrated sensitivity to combustion generated particles as small as 1 nm. All of the techniques are in quite good agreement in flames above the soot formation threshold. In flames with C/ O < 0.65, DMA measures a strong drop-off in particle concentration, and no particles are detected for C/O < 0.6. In contrast, a much lower decrease in particle concentration has been detected by optical measurements and AFM analysis of particles collected by thermophoretic sampling. In blue flames optical measurements detect particles with similar spectroscopic properties to those detected in richer flames, and the only difference is that their size distribution measured by AFM shows only particles smaller than 3 nm. The small size of the particles alone does not justify the disagreement with DMA at low C/O since both DMA and AFM detect the same
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concentration of d < 3 nm particles in the C/ O = 0.65 flame. Also corrections for particles losses in the probe used for DMA measurements do not appear to justify the discrepancies. Further investigation in particle composition across the soot threshold is necessary to understand the motivation of a different detection efficiency between DMA, based on particles mobility measurements and thermophoretic-AFM and optical methods. The effect of a different chemical composition, percentage of aliphatic–aromatic functionalities or oxygen inclusions, together with the very small size of the particles should be investigated in more detail to determine possible effects on particles diffusivity, ionization efficiency, optical properties or interaction potential which could help to merge the gap between the current measurements. In summary, the differences of results by techniques based on different effects stimulate a more detailed analysis of the properties of nanoparticles produced at different C/O ratios in premixed flames. This analysis is important for modern combustion systems, which emit low to undetectable levels of soot since local and instantaneous C/O ratios above stoichiometry would produce high molecular mass structures (nanoparticles) that can be emitted to the atmosphere.
References [1] A. D’Anna, A. Violi, A. D’Alessio, A.F. Sarofim, Combust. Flame 127 (2001) 1995–2003. [2] M. Frenklach, H. Wang, in: H. Bockhorn (Ed.), Soot Formation in Combustion, Springer-Verlag, Heidelberg, 1994, p. 265. [3] J. Singh, R.I.A. Patterson, M. Kraft, H. Wang, Combust. Flame 145 (2006) 117–127. [4] P. Minutolo, G. Gambi, A. D’Alessio, Proc. Combust. Inst. 27 (1998) 1461–1469. [5] A. D’Alessio, P. Minutolo, G. Gambi, A. D’Anna, Ber. Bunsenges Phys. Chem. 97 (1993) 1574–1582. [6] P. Minutolo, G. Gambi, A. D’Alessio, S. Carlucci, Atmos. Environ. 33 (1999) 2725–2732. [7] A. D’Alessio, A.C. Barone, R. Cau, A. D’Anna, P. Minutolo, Proc. Combust. Inst. 30 (2005) 2595–2603. [8] G. Basile, A. Rolando, A. D’Alessio, A. D’Anna, P. Minutolo, Proc. Combust. Inst. 29 (2002) 2391– 2397. [9] G. Rusciano, A.C. De Luca, A. D’Alessio, P. Minutolo, G. Pesce, A. Sasso, Carbon 46 (2) (2008) 335–341. [10] A.C. Barone, A. D’Alessio, A. D’Anna, Combust. Flame 132 (2003) 181–187. [11] C.M. Megaridis, R.A. Dobbins, Combust. Sci. Technol. 66 (1989) 1–16. [12] R.A. Dobbins, H. Subramaniasivam, in: H. Bockhorn (Ed.), Soot Formation in Combustion, Springer-Verlag, Berlin, 1994, p. 290.
[13] M. Thierley, H.-H. Grotheer, M. Aigner, et al., Proc. Combust. Inst. 31 (2007) 639–647. [14] L.A. Sgro, A. De Filippo, G. Lanzuolo, A. D’Alessio, Proc. Combust. Inst. 31 (2007) 631–638. [15] B. Zhao, Z. Yang, J. Wang, M.V. Johnston, H. Wang, Aerosol Sci. Technol. 37 (2003) 611. [16] M. Bachmann, W. Wiese, K.-H. Homann, TwentySixth Symposium (International) on Combustion, 1996, pp. 2259–2267. [17] P. Weilmunster, A. Keller, K.-H. Homann, Combust. Flame 116 (1999) 62–83. [18] A. Bruno, C. de Lisio, P. Minatolo, A. D’Alessio, Combust. Flame 151 (2007) 472–481. [19] A. Bruno, F. Ossler, P. Minutolo, C. de Lisio, J. Opt. A: Pure Appl. Opt., in press. doi:10.1088/14644258/10/6/064016. [20] C.R. Shaddix, Proceedings of the Thirty-Third Natural Heat Transfer Conference, Albuquerque, NM, 1999. [21] T. Joutsenoja, A. D’Anna, A. D’Alessio, M.I. Nazzaro, Appl. Spectrosc. 55 (2) (2001) 130–135. [22] R. Barbella, A. Ciajolo, A. D’Anna, A. Tregrossi, Proceedings of the Joint Meeting of the Italian and Portuguese Sections of the Combustion Institute, Salsomaggiore Terme, Italy, September 20–23, 1994, p. III-13. [23] T.R. Melton, A.M. Vincitore, S.M. Senkan, Twenty-Seventh Symposium (International) on Combustion, 1998, pp. 1631–1637. [24] K.C. Smyth, C.R. Shaddix, Combust. Flame 107 (1996) 314–320. [25] D. Cecere, L.A. Sgro, G. Basile, A. D’Alessio, A. D’Anna, P. Minutolo, Combust. Sci. Technol. 174 (2002) 377–398. [26] M. Commodo, S. Violi, A. D’Anna, et al., Combust. Sci. Technol. 179 (2007) 387–400. [27] R.A. Dobbins, C.M. Megaridis, Langmuir 2 (1987) 254–259. ¨ . Ko¨ylu¨, C.S. McEnally, D.E. Rosner, L.D. ¨ .O [28] U Pfefferle, Combust. Flame 110 (1997) 494–507. [29] A.D. Eisner, D.E. Rosner, Combust. Flame 61 (1985) 153–166. [30] G.P. Reischl, J.M. Makela, J. Neid, Aerosol Sci. Technol. 27 (1997) 651–672. [31] W. Winklmayr, G.P. Reischl, A.O. Lindner, A. Berner, J. Aerosol Sci. 22 (1991) 289–296. [32] J. Ferna´ndez de la Mora, L. de Juan, K. Liedke, A. Schmidt-Ott, J. Aerosol Sci. 34 (2003) 79–98. [33] W.C. Hinds, Aerosol Technology, second ed., John Wiley & Sons, New York, NY, 1999. [34] H.C. Wang, G. Kasper, J. Aerosol Sci. 22 (1991) 31–41. [35] M. Alonso, Y. Kousaka, T. Hashimoto, N. Hashimoto, Aerosol Sci. Technol. 27 (1997) 471– 480. [36] P.G. Gormley, M. Kennedy, Proc. R. Irish Acad. 52A (1949) 163–169. [37] B. Zhao, Z. Yang, M.V. Johnston, H. Wang, A.S. Wexler, M. Balthasar, M. Kraft, Combust. Flame 133 (2003) 173–188. [38] T.G. Baquet, H.-H. Grotheer, M. Aigner, Rapid Commun. Mass Spectrom. 21 (2007) 4060–4064.