Particulate Formation in Premixed and Counter-flow Diffusion

Article pubs.acs.org/EF

Particulate Formation in Premixed and Counter-flow Diffusion Ethylene/Ethanol Flames Maurin Salamanca,† Mariano Sirignano,‡ and Andrea D’Anna‡,* †

QUIREMA, Institute of Chemistry, University of Antioquia, A.A. 1226, Medellín, Colombia Università Federico II di Napoli, Piazzale Tecchio 80, 80125 Napoli, Italy

‡

ABSTRACT: The role of ethanol, as substituent to ethylene, on the formation of particulate matter has been investigated in different flame configurations by using in situ optical techniques. Laser induced fluorescence and incandescence signals, correlated to small precursor nanoparticles and large soot particles, respectively, have been measured in premixed and counterflow diffusion flames of ethylene doped with different amounts of ethanol, covering a wide range of ethanol addition, much higher than that reported in the literature. In premixed flames, the addition of ethanol reduces nanoparticle and soot particle formation. The effect is stronger for the soot particles, that is, particles with sizes larger than 10 nm, and in flames operated close to the soot threshold limit. The addition of 30% of ethanol reduces soot particle concentration below the detection limit, whereas nanoparticles are still formed in large amounts. The counter-flow diffusion flame configuration shows a different behavior. In the fuel side of the flame, a noticeable increase of both nanoparticle and soot particle concentrations is detected for amounts of ethanol added up to 20 vol %, whereas a decrease of the particulate formation, both nanoparticles and soot, is observed for larger amounts of ethanol added. Conversely, in the oxidizer side of the flame, particulate matter is always reduced when ethanol is added to the fuel. The results show that the role of ethanol in the formation of nanoparticles and soot is not always in the sense of reduction, but it also strongly depends on the combustion conditions.

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Salamanca et al.13 found that the reduction of soot was associated with a consistent increase of the particle number density and a decrease of the average diameter of the particles. The behavior of ethanol in diffusion flames depends on the experimental setup and on the amount of the additive to the initial fuel. Ethanol decomposes at high temperatures producing ethylene and methyl radicals and thus might promote, in some conditions, the formation of aromatics and soot.14,15 However, many studies report that also in nonpremixed combustion configurations particle formation is reduced when ethanol is added to the fuel.3,16−18 It is clear that the oxidation and pyrolysis of ethanol and the environment in which it reacts have an impact on the amount, morphology, and chemical characteristics of the formed particles. Most of the previous studies on the addition of ethanol were focused on the total amount of particulate matter produced in combustion whereas less attention was paid to the size distribution of the particles and on their composition. Recently, the effect of ethanol addition to the fuel on the amount and the size distribution functions of the formed particles were investigated by Maricq18 in nonpremixed flames and by Salamanca et al.13 in premixed flame conditions, by using a particle differential mobility analyzer. Maricq18 found that, in diffusion flames, the addition of small quantities of ethanol to gasoline had little effect on flame properties, whereas the flame with larger quantities of ethanol was qualitatively different. Semivolatile organic compound formation was strongly reduced, as well as the mass of the particle relative to the pure gasoline flame.

INTRODUCTION Biofuels are gaining increased attention as alternative to fossil fuels. The main motivation is the mitigation of climate change and energy security objectives. Biofuels may produce less net carbon dioxide emissions than oil-based conventional fuels. Moreover, there is also a general idea about their ability to reduce exhaust emissions, particularly polycyclic aromatic hydrocarbons (PAHs) and particulate matter.1 For these reasons, European Community has promoted the use of biofuels as alternative motor vehicle fuels.2 Member states are instructed to ensure that a minimum proportion of biofuels and other renewable fuels should be placed on their markets. In spite of this general belief, experimental results performed in laboratory-scale experiments and in engines have shown that the effect of biofuels on the formation of aromatic compounds and particulate matter is controversial. Biofuels produce these pollutants similar to conventional hydrocarbon fuels. Their formation can be promoted or reduced with respect to hydrocarbon molecules having the same number of C-atoms according to operative conditions.3,4 Moreover, whereas an overall reduction of the emitted particles is obtained, the effect on health of the formed particles might be even worse with respect to common hydrocarbon generated particulate matter.5 Ethanol, a typical representative of biofuel, is one of the most used oxygenate additives because it can be obtained from biomass at reasonable cost.6 Different studies have been performed aimed to understand the role of ethanol on combustion features and particulate emission. Studies performed in shock-tube pyrolysis experiments7,8 and in premixed flames9−12 showed a beneficial effect of ethanol addition on particulate emission: the addition of the oxygenated additive reduces the amount of aromatics, including benzene and PAHs, and soot particles. Recently, © 2012 American Chemical Society

Received: June 27, 2012 Revised: September 3, 2012 Published: September 4, 2012 6144

dx.doi.org/10.1021/ef301081q | Energy Fuels 2012, 26, 6144−6152

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Table 1. Premixed Flame Conditions Ethanol Added (%) 0 2 4 6 8 10 20 30 0 2 4 6 8 10 20 30 0 2 4 6 8 10 20 30 0 2 4 6

Ethylene (L/min) Equivalence 1.85 1.81 1.78 1.74 1.70 1.67 1.48 1.30 Equivalence 1.90 1.87 1.83 1.79 1.75 1.71 1.52 1.33 Equivalence 1.96 1.92 1.88 1.84 1.80 1.76 1.56 1.37 Equivalence 2.08 2.04 2.00 1.96

Air (L/min) ratio = 1.89 14.00 14.00 14.00 14.00 14.00 14.00 14.00 14.00 ratio = 1.95 13.95 13.95 13.95 13.95 13.95 13.95 13.95 13.95 ratio = 2.01 13.90 13.90 13.90 13.90 13.90 13.90 13.90 13.90 ratio = 2.16 13.77 13.77 13.77 13.77

Liquid Ethanol (mL/min)

Ethanol Added (%)

0 0.089 0.178 0.267 0.355 0.444 0.888 1.333

8 10 20 30 0 2 4 6 8 10 20 30

0.000 0.091 0.183 0.274 0.365 0.457 0.913 1.370

0 2 4 6 8 10 20 30

0.000 0.094 0.188 0.281 0.375 0.469 0.938 1.407

0 2 4 6 8 10 20 30

0.000 0.100 0.200 0.300

Ethylene (L/min) Equivalence 1.92 1.87 1.87 1.46 Equivalence 2.21 2.16 2.12 2.07 2.03 1.99 1.77 1.54 Equivalence 2.33 2.28 2.24 2.19 2.14 2.10 1.86 1.63 Equivalence 2.45 2.40 2.35 2.30 2.25 2.20 1.96 1.71

Air (L/min) ratio = 2.16 13.77 13.77 13.77 13.77 ratio = 2.31 13.65 13.65 13.65 13.65 13.65 13.65 13.65 13.65 ratio = 2.46 13.52 13.52 13.52 13.52 13.52 13.52 13.52 13.52 ratio = 2.61 13.40 13.40 13.40 13.40 13.40 13.40 13.40 13.40

Liquid Ethanol (mL/min) 0.399 0.499 0.999 1.498 0.000 0.106 0.212 0.318 0.423 0.529 1.058 1.588 0.000 0.112 0.223 0.335 0.447 0.559 1.117 1.676 0.000 0.117 0.235 0.352 0.470 0.587 1.175 1.762

ethylene. Ethylene is chosen as representative of hydrocarbon fuels because it is well-known that at the high temperatures typical of the combustion process hydrocarbons decompose forming ethylene and methane and their radicals. 19,20 Consequently, ethylene is a good surrogate of hydrocarbon fuels when investigating the combustion characteristics. The amount of ethanol added is varied from 2 to 30% of total carbon fed in premixed flames and from 5% to 60% in nonpremixed conditions, a range much wider than that reported in the literature. Moreover, premixed flames are operated in fuel-rich conditions with equivalence ratios across the soot threshold limit, 1.89 to 2.61, to enhance the formation of precursor nanoparticles with respect to soot particles. Lower values of the equivalence ratios do not form particulates, whereas larger ones are typical of pyrolysis conditions, and these condition are investigated in the counter-flow flame configuration. This wide range of equivalence ratios allows to study the transition between nonsooting and sooting regimes and better understanding the impact of ethanol addition on the formation of small nanoparticle as well as of large soot particles.

The primary particles constituting soot agglomerates were less than half the size of their counterparts in the pure gasoline flame. Salamanca et al.13 found a general reduction of the total volume fraction of particulate when ethanol was added, with an increasing reduction as a function of the amount of ethanol added. The particulate matter formed from the ethanol-doped flames was essentially constituted by nanometer-size particles, those with sizes well below 10 nm. These latter particles do not contribute significantly to the mass of the particulate emitted but constitute almost the totality of the number concentration of the emitted particulate. Particulate matter generated during hydrocarbon combustion comprises a wide distribution of particle sizes from few nanometers to micrometers. Studies on the influence of oxygenated additives on particulate formation have been limited to the gas-phase aromatic compounds (e.g. PAHs) and to the larger soot particles. There are no studies focused on the nanosized fraction of the particulate matter and on the role of oxygenated additives on their formation mechanism. Nanosized particles have a stronger effect on health with respect to the larger soot particles and might become an issue to the diffusion of biofuels as an alternative to fossil fuels. In this paper, we use previously developed in situ spectroscopic diagnostics able to follow nanoparticle and soot formation, to investigate the changes induced in the combustion system when different amounts of ethanol are added to premixed and counter-flow diffusion flames of

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EXPERIMENTAL METHODS

Premixed Flames. Premixed ethylene/air flames with equivalence ratios (Φ) ranging from 1.89 to 2.61 were investigated. The flames were stabilized on a capillary burner (i.d. 5.8 cm) by means of a stainless steel plate located at 30 mm above the burner surface. The ethanol added was varied from 2 to 30% of the total carbon fed. 6145


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Figure 1. Experimental setup for laser diagnostic measurements and representative photographs of the flames studied. When ethanol was introduced into the fuel line, the ethylene flow rate was reduced in the appropriate proportion in order to keep the total carbon flow rate constant. The air flow rate was not changed in order to maintain the equivalence ratio of the flame constant, that is, fuel-toair ratio respect to the stoichiometric value. The velocity of the unburned gases at room conditions (1 bar and 298 K) was fixed at 10 cm/s. Table 1 reports the details of the premixed flame conditions. To avoid ethanol condensation in the burner, the temperature of the recirculating water at the burner was fixed at 75 °C. High purity air and ethylene were supplied by using mass flow controllers. Ethanol was supplied by a syringe pump that allowed having high repeatability of the measurements with stable and constant flow rate. Premixed gases were preheated at 150 °C, a temperature much higher than the boiling point of ethanol, in order to guarantee the total evaporation of the ethanol and to avoid condensation if cold points were present in the feeding line. Premixed gases fed to the burner react at the burner mouth forming a monodimensional flame structure in which the different heights above the burner correspond to different residence times, that is, the progress of the chemical reactions can be followed by performing measurements at different heights above the burner (HAB). A thin flame front, blue-colored, is stabilized at about 2−3 mm in all the examined conditions. The gases emerging from this zone enter the burned gas zone of the flame where they continue reacting, forming high molecular mass hydrocarbons and particles, depending on the operating conditions of the flame. This flame region is an almost transparent region followed by a zone with intense yellow-orange luminosity that becomes more evident as the equivalence ratio of the fuel/air mixture is increased.20 Figure 1 (upper part-left side) shows a picture of a fuel-rich ethylene premixed flame in which the different

flame zones are clearly evident. Measurements in the flame front were performed at 1.5 and 2.5 mm whereas measurements in the burned gas zone were performed with a spatial resolution of 0.5 mm stepwise from 3 to 15 mm heights above the burner. Counter-flow Flames. The counter-flow burner system consisted of two opposed jet nozzles (i.d. 2.54 cm) vertically positioned. The oxidizer stream was introduced from the upper nozzle while the fuel stream was introduced from the lower nozzle. Screens were used at the exit of each jet to establish uniform gas flow velocities in order to generate stable flat flames. Using a mild vacuum through the holes in the annular section of the bottom burner, combustion products and shield gas were vented out of the system. Flames were stabilized by feeding 25 vol % ethylene/ethanol mixtures (the total amount of ethanol and ethylene is maintained constant) and 75 vol % Ar. The oxidizer stream was composed by 22 vol % oxygen and 78 vol % Ar, and it was maintained constant for all the examined conditions. The oxidizer and fuel stream velocities were fixed at 16.12 and 13.16 cm/s at room conditions, respectively. These velocities have been chosen similar to that of previous studies21,22 from the same group on this flame and allow to have enough spatial resolution for the optical measurements. The distance between the two opposed jet nozzles was maintained constant at 1.5 cm for all measurements. Figure 1 (upper part-right side) shows a picture of the counter-flow flame of ethylene with indications of the fuel and oxidizer streams, the stagnation plane, and the flame front. In the selected flame configuration, the stagnation plane, that is, the location between the two opposed jets were the oxidizer and fuel streams velocities vanish, is located at about 5 mm from the fuel jet exit in all the examined conditions. The flame front is located in the oxidizer side at about 6146


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synchronized with the laser pulse and by summing the ICCD counts over 150 scans. Data repeatability is 8%; thus, the reduction repeatability is 13%. Both fluorescence and incandescence signals were detected simultaneously. Depending on the flame structure and operating conditions, two maxima can be distinguished in the fluorescence spectra. The first one corresponds to the spectral region from 300 to 380 nm, that is, the near UV region. This signal is present, and it is quite strong in both premixed and counter-flow diffusion flames. The second one is in the spectral region from 380 to 450 nm. This fluorescence signal is more evident in the premixed flames with higher equivalence ratios and in the fuel side of the diffusion flames. D’Alessio and co-workers26−29 attributed the UV fluorescence measured in premixed flames to precursor nanoparticles made-up of aggregates of polycyclic aromatic hydrocarbons having “two-rings aromatic subunits connected by aliphatic or, eventually, oxygen bonding” more than to single, gas-phase PAHs. The build-up of fluorescence in the visible was interpreted by the same authors to “a progressive aromatization of the precursor structure in their transformation to soot nuclei”. Recently, by measuring time-resolved spectral emission, the visible fluorescence in the fuel-side of a counter-flow diffusion flame was attributed to highly packed, sandwich-like structures, that is, to the cluster of PAHs held together by van der Waals forces.22 The incandescence signal appears as a continuum in the visible region, and it is attributed to solid soot particles that are able to dissipate the acquired energy by thermal emission more than by fluorescence emission. In order to evaluate and subtract the contribution of this signal, we use the blackbody radiation curve at 4000 K,30 which matches the measured emission values in the region between 500 and 550 nm. The blackbody temperature only affects the fluorescence profile in the visible region. In the investigated conditions, the visible fluorescence signal does not change in shape and exhibits an uncertainty of about 5% whether 4000 or 3500 K were used.22

8 mm. Combustion products and pyrolysis species formed between the stagnation plane and the flame front are moved away from the oxidation region of the flame and transported toward the stagnation plane where they are exhausted without being oxidized by the flame front. Ethanol was added to the fuel stream in concentrations from 5 to 60 vol % of the total carbon fed. The fuel stream was preheated to allow ethanol to completely evaporate and was sent to the burner. The preheating temperature set point was fixed to maintain the outlet temperature of fuel stream at 150 °C while maintaining the temperature of recirculating water at 75 °C. Details of the operating conditions of the counter-flow flames are reported in Table 2. Measurements were performed at different

Table 2. Counter-flow Flame Conditions ethanol added (%)

ethylene (L/min)

argon/fuel (L/min)

oxygen (L/min)

argon/oxidant (L/min)

liquid ethanol (mL/min)

0 5 10 15 20 30 40 50 60

1.27 1.21 1.15 1.08 1.02 0.89 0.76 0.64 0.51

3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9

0.0 0.153 0.305 0.458 0.611 0.916 1.222 1.527 1.833

locations between the opposed jet nozzles, from the fuel to the oxidizer, with a spatial resolution of 0.2 mm. Diagnostic System. Temperature was measured by using a fastinsertion procedure.23 It was applied to measure temperature reducing soot deposition on the thermocouple bead. A correction was applied to the raw temperature measurements in order to account for radiation losses from the thermocouple bead. The measured temperature is actually the temperature of the thermocouple bead, and thus, calculations are required to solve for the gas temperature. The gas temperature is calculated using an energy balance of the thermocouple bead. The bead is heated by convection and is cooled by radiating heat away to the cold air. Conduction through the thin, relatively long wires coming off from the thermocouple bead is ignored. The emissivity of the thermocouple bead is set equal to that of the clean metal whereas the transport properties of the gases surrounding the thermocouple bead are estimated as the molar average of the most abundant permanent gases, as determined from a kinetic calculation of the flames.24 Temperature data were highly repeatable; measurement uncertainty remained within 50 K. The fourth harmonic of a Nd:YAG laser at 266 nm has been used as the excitation source for spectrally and time-resolved laser induced emission (LIE) measurements (200−550 nm range). The laser beam diameter at focal point of 350 μmwas focused in the center of the flame. The emitted radiation at 90° with respect to the laser beam was focused onto the 280 μm entrance slit of a spectrometer and detected by an intensified CCD camera thermoelectrically cooled to −10 °C to reduce noise. The energy of the laser pulse was kept constant at 0.8 mJ with pulse duration of 8 ns. The chosen laser energy gave the better compromise, in the examined conditions, between laser induced emission signals and species fragmentation interference.22 Figure 1 shows a schematic representation of the optical lay-out used for LIE measurements. The measured spectra were corrected for the spectral response of the detection system and calibrated against the Rayleigh scattering of cold ethylene at 266 nm. Time-resolved measurements allowed us to distinguish a broadband and almost structureless short-lived fluorescence signal in the region between 300 and 500 nm superimposed to a long-lived incandescence emission. The incandescence signal appeared as a continuum in the visible region up to ICCD detection limit.25 For the study of the influence of ethanol addition to ethylene, the emission spectra were detected using a gate time of 100 ns

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RESULTS AND DISCUSSION

Premixed Flames. Figure 2 shows the spectral laserinduced fluorescence signals measured at 10 mm above the

Figure 2. Spectral laser-induced emission fluorescence signals measured at 10 mm above the burner in three ethylene/air flames with equivalence ratios (△) 1.89,(○) 2.16, and (*) 2.46. The spectra have been obtained by subtracting the contribution of the blackbody radiation curve at 4000 K matching the measured emission values in the region between 500 and 550 nm.

burner in three ethylene/air flames with equivalence ratios 1.89, 2.16, and 2.46. The spectra have been obtained by subtracting the contribution of the blackbody radiation curve at 4000 K, matching the measured emission values in the region between 500 and 550 nm. 6147


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In the nonsooting ethylene flame (Φ = 1.89), the fluorescence spectrum shows a broadband peak in the UV range centered around 340−350 nm. Another broadband peak in the visible range around 380−400 nm arises in the flames with equivalence ratios above the soot threshold limit (Φ = 2.16 and 2.46) and becomes the prevalent one in the fluorescence spectrum after subtracting the contribution of the incandescence signal. Figure 3 reports the three signals,

A slight increase of the temperature has been found when ethanol is added, as shown in Figure 4A. The increase remains

Figure 4. Temperature (A) and laser induced emission signals measured along the axis of the ethylene/ethanol premixed flames at Φ = 2.31: (B) LIF 350 nm, (C) LIF 400 nm, (D) LII. The percentages of ethanol added are 0% (■), 10% (△), 20% (●), and 30% (□).

within 50 K with respect to the pure ethylene flame. A more important effect has been found on the position of the maximum flame temperature, which moves of about 1 mm toward higher flame heights when the amount of ethanol added increases, except for the 10% flame. The addition of ethanol does not modify the qualitative trends of fluorescence and incandescence signals. The progressive addition of ethanol causes the reduction of the incandescence signal, which goes below the detection limit when 30% of ethanol is added, whereas the fluorescence signals is only slightly reduced (by about 30% for the larger amount of ethanol used). The effect reported for Φ = 2.31 flame is typical of all the flames investigated, that is, flames with Φ ranging from 1.89 to 2.61. Figure 5 summarizes the results of the flames with equivalence ratios from 1.89 to 2.61. It shows for each flame the percentage reduction of the maximum values of the fluorescence intensity with respect to the pure ethylene flames. The percentage reduction of the incandescence intensity with respect to the pure ethylene flame, calculated at 10 mm from the flame front, is also reported in Figure 5, but for the sooting flames. As already shown for the Φ = 2.31 flame, ethanol addition has a smaller effect on the fluorescence signals than on the incandescence ones. Furthermore, the data indicate that the effect is greater in the less rich flames, in agreement with literature data.10 Fluorescence in the UV range centered at 350 nm has been attributed to high-molecular-mass aromatic compounds with sizes of the order of 2−3 nm constituted by 2-, 3-fused-rings species connected by aliphatic bonds.27,29 This attribution was based on the simultaneous detection of scattering in excess of gas-phase compounds, that is, compounds of high-molecular

Figure 3. Temperature (A) and laser induced emission signals measured along the axis of ethylene flames with different equivalence ratios Φ = 1.89 (●), Φ = 2.16 (△), Φ = 2.46 (■). (B) UV fluorescence, (C) visible fluorescence, (D) incandescence.

that is, the UV fluorescence at 350 nm, the visible fluorescence at 400 nm, and the incandescence signal, as a function of the flame height in the nonsooting (Φ = 1.89), slightly sooting (Φ = 2.16), and fully sooting (Φ = 2.46) ethylene flames. The onset of UV and visible fluorescence is located at 1.5 mm above the burner. After a sharp increase, the signals reach a local maximum. The maximum is located at around 2−3 mm above the burner for the three flames, and immediately after the maximum in the flame temperature profiles. Maximum flame temperature decreases for the richer flame by about 100 K with respect to the leaner one. In the Φ = 1.89 and 2.16 flames, the fluorescence signals slowly decrease at increasing heights above the burner, whereas in the Φ = 2.46 flame a reincrease of the signal is observed at 7 mm. The incandescence signal is negligible in the Φ = 1.89 flame, and it is just above the detection limit in the Φ = 2.16 flame. A clear incandescence signal is detected in the fully sooting flame (Φ = 2.46). It increases immediately after the first maximum of the fluorescence signal. The effect of ethanol addition on the three emission signals is reported in Figure 4 where data relative to the slightly sooting flame with Φ = 2.31 and different amounts of ethanol added to ethylene, specifically 10, 20, and 30 vol %, are shown. 6148


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is added and the almost complete depletion of soot with 30 vol % of ethanol addition. In the same operating conditions, the reduction of small nanoparticles does not exceed 30%. As a consequence, the total particulate reduction is achieved by adding ethanol, but the emitted particulate still contains a large fraction of nanoparticles. This finding is in agreement with our previous results obtained by using a particle differential mobility analyzer: the addition of ethanol depleted the soot particle mode in the particle size distribution functions but only slightly decreased the amount of the nanosized particle mode. Indeed, the particle size distributions measured in the same operating conditions of the present work clearly show the presence of large amount of small nanoparticles with sizes smaller than 10 nm, which are not strongly reduced by ethanol addition.13 The reasons for such selectivity of ethanol in reducing particles on the basis of their sizes is explained with a slowdown of the coagulation process leading to soot. The addition of ethanol to the ethylene enhances the fuel oxidation, thus reducing the formation of PAHs and of precursor nanoparticles. The following process of soot formation by coagulation and surface addition on the particle nuclei is less effective. The effect is more evident in slightly sooting conditions where the inception process controls the final amount of soot. In fully sooting conditions, where surface reactions control the final amount of soot,32 the role of ethanol addition on the reduction of soot is less effective. Counter-flow Flames. In the counter-flow configuration used in this work, the flame front is located in the oxidizer side of the stagnation plane in all the analyzed flames. Three regions can be distinguished in the flame: the fuel side, the stagnation zone, and the oxidation zone. The fuel side is characterized by a relatively low temperature, below ∼1000 K, by a high concentration of the fuel and by the complete absence of oxygen. Across the stagnation planelocated at about 5 mm in the pure ethylene flamethe flame temperature reaches values of the order of 1000−1500 K. Moving toward the oxidizer side, temperature increases toward the adiabatic flame temperature. Figure 6 shows the temperature profile and the fluorescence signals at 350 and 400 nm from the fuel side to the oxidizer size in the counter-flow flame of ethylene. Fluorescence emission is detected along the entire ethylene flame. Moving from the fuel side toward the stagnation plane, fluorescence intensity increases and reaches a maximum value in the fuel side. Thereafter, it decreases crossing the stagnation plane toward the oxidizer side of the flame. In this flame region, fluorescence intensity increases again reaching a second maximum value. Thereafter, it decreases moving toward the main oxidation zone. The laser induced incandescence signal is also reported in Figure 6B. The incandescence signal maximizes across the stagnation plane region close to the second maximum of the fluorescence signal, that is, that in the oxidation side of the flame. Replacement of ethylene with ethanol in the fuel stream modifies the intensities of the fluorescence and the incandescence signals. Figure 7 shows the fluorescence signals in the UV at 350 nm and in the visible at 400 nm, and LII for flames obtained adding increasing amounts of ethanol from 5% to 60%. The addition of ethanol changes the temperature in the fuel side and in the region across the stagnation plane by about 50− 100 K, but the location of the stagnation plane remains unchanged. Nevertheless, the addition of ethanol results in the enhancement of the fluorescence intensities in the UV and

Figure 5. Reduction calculated based on the laser induced emission signals in ethylene/ethanol premixed flames with different percentages of ethanol added to the flames: (A) UV fluorescence, (B) visible fluorescence, (C) incandescence. Φ = 1.89 (▲), Φ = 1.95 (▽), Φ = 2.01 (●), Φ = 2.16 (○), Φ = 2.31 (■),Φ = 2.46 (□), Φ = 2.61(⧫).

mass, and UV fluorescence, that is, the fluorescence of 2-, 3fused-rings aromatics. Broadband fluorescence in the visible range around 400 nm has been attributed to nanoparticles similar to those fluorescing in the UV, but with a number of fused-rings higher than 2 or 3 and/or to highly packed, sandwich-like structuresa cluster of PAHs held together by van der Waals forces.20,22 Atomic force microscopy and particle differential mobility analysis measurements have confirmed the optical measurements showing the presence of 2−3 nm particles in the region of the flame characterized by fluorescence signals and absence of incandescence.20,31 The incandescence signal has been attributed to particles with a strong solid state feature.22,25 On the basis of the literature attributions, by following the three laser induced emission signals in the different flame conditions, it is possible to follow the evolution of different classes of aromatic compounds: • 2−3 nm particles constituted by high-molecular mass oligomers of very small PAHs, associated to UV fluorescence; • nanoparticles constituted by oligomers of larger PAHs or by clusters of PAHs, associated to visible fluorescence; • soot particles with sizes larger than 10 nm, associated to incandescence. In agreement with literature data,10 our measurements confirm the strong reduction of soot particles when ethanol 6149


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Figure 8. Relative intensity with respect to the pure ethylene flame of the (■) UV fluorescence, (●) visible fluorescence, and (△) incandescence as function of the amount of ethanol added. (A) fuel side, (B) oxidant side. Figure 6. Temperature (A) and laser induced emission signals (B) measured in a counter-flow diffusion ethylene flame, UV fluorescence (□), visible fluorescence (●) and incandescence (▲). The stagnation plane is located at 5 mm.

side of the flames. It shows the ratio of the maximum intensity of the fluorescence and incandescence signals in the ethanol doped flames with respect to the same signals in the pure ethylene flame as function of the amount of ethanol added. Fluorescence signals detected in the fuel side are attributed to nanoparticles of organic carbon deriving from PAH coagulation in the fuel-side of the flame.22 PAHs are formed with a slow chemical process due to the pure pyrolytic conditions of this flame region. The process of aromatic growth, and the subsequent coagulation leading to nanoparticles, is enhanced by the radical rich environment created by the addition of ethanol. OH and H radicals deriving from ethanol decomposition might activate radical reactions involving C2H4 and CH3 which then reacts to yield aromatic compounds.1 The radical rich flame zone created by the addition of ethanol favors also the molecular growth of small PAHs through radical-molecule reactions, leading to the formation of high-molecular mass aromatics contributing to the increase of the fluorescence signals. The larger amount of aromatic compounds formed in the fuel side and the radical rich environment also promote the transformation of nanoparticle into compounds having a solid state nature, thus contributing to the incandescence signal in the fuel side of the flame. As a consequence, nanoparticles constituted by clusters of PAHs and incipient soot particles are formed in the fuel side for effect of the addition of ethanol up to 20%. The successive reduction for larger percentage of ethanol can be attributed to the prevalence of the main oxidation pathways promoted by the presence of ethanol. As a result, larger amounts of ethanol added to the ethylene cause a larger fraction of the fuel to be converted to partial oxidation compounds, according to literature analysis of kinetic pathways. A partial oxidation of the fuel is hence favored already in the fuel side of the flame, resulting in a decreased formation of particulate. Fluorescence signals in the oxidizer side is somewhat similar to those found in premixed flames. It is derived from similar conditions in terms of temperatures (above 1500 K) and radical concentrations. These flame conditions favor the formation of high-molecular mass aromatics constituted by aromatic hydrocarbons connected by aliphatic bonds. These compounds can

Figure 7. Laser induced emission signals measured in counter-flow diffusion ethylene/ethanol flames with different percentages of ethanol added: 0% (■), 5% (□), 20% (●), 40% (○), 60% (▲). (A) UV fluorescence, (B) visible fluorescence, (C) incandescence.

visible in the fuel side, for lower amounts of ethanol added (below 20%), and a suppression of the fluorescence signals, for larger amounts of ethanol added (more than 20%). The same effect is evident for the incandescence signal in the fuel side, that is, in the flame region between 2−5 mm. Incandescence is hardly detected in the fuel side of the pure ethylene flame, whereas it is strongly enhanced for lower addition of ethanol. Figure 8A summarizes the effect of the addition ethanol on the formation of the particulate in the fuel 6150


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ethanol to the fuel leaves nanoparticle formation almost unaffected. The net result is the emission of particulate matter enriched in harmful nanoparticles. In some particular combustion conditions, the addition of ethanol to the fuel might even result in an enhanced reactivity of ethylene and in an increased formation of combustion generated particle. This is particularly true in partial premixing combustion conditions typical of modern engines in which the use of oxygenated additives might be detrimental for the environment.

rapidly evolve in soot particles when transported by convection from the main oxidation zone to the stagnation plane, giving rise to the incandescence signal in that zone of the flame. The transformation process of high-molecular mass aromatics into soot particles is quite fast in the examined flame conditions. In fact, fluorescence and incandescence signals maximize almost at the same flame location testifying this fast transition. The addition of ethanol has a reducing effect on both incandescence and fluorescence signals in the oxidizer side of the flame (Figure 8B). Differently from the premixed flame configuration, fluorescence and incandescence signals are strongly reduced for effect of ethanol addition, that is, both nanoparticles and soot particle formation are reduced. Ethanol addition of 60% completely inhibits soot formation and reduces to very low values the formation of nanoparticles in counter-flow flames.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS This work focuses on the effect of ethanol, as substituent to ethylene, on the formation of nanoparticles and soot in different flame configurations. In premixed flames, the addition of ethanol reduces nanoparticle and soot formation. The effect is stronger for the larger soot particles and in flames across the soot threshold limit. The addition of 30% of ethanol reduces soot below the detection limit, but nanoparticles are still formed in large amounts. The reason for such selectivity of ethanol in reducing particles on the basis of their sizes is explained with a slowdown of the coagulation process leading to soot. The addition of ethanol to the ethylene enhances the fuel oxidation, thus reducing the formation of PAHs and precursor nanoparticles. The following process of soot formation by coagulation and surface addition on the particle nuclei is less effective. The effect is more evident in slightly sooting conditions where the inception process controls the final amount of soot. In fully sooting conditions, where surface reactions control the final amount of soot, the role of ethanol addition on the reduction of soot is less effective. The counter-flow diffusion flame shows a different behavior. An increase of both nanoparticle and soot in the fuel side of the flame is detected for lower amounts of ethanol added (up to 20 vol %). A decrease of the particulate matter formation is observed for larger amounts of ethanol added. The addition of ethanol in the fuel stream increases the fuel reactivity already in the pyrolysis regions of the flame. As result, the pyrolytic zone becomes rich in radicals and the PAH formation and growth are enhanced. The formation of small nanoparticles by chemical and physical mechanisms is enhanced as well. After the inception, particles rapidly coagulate and grow forming soot particles before reaching the stagnation plane. At increasing amounts of ethanol added in the fuel mixture, the oxidation of the fuel prevails on the molecular growth also in the fuel side, causing a reduction of the amount of particle formed in the fuel side. In the oxidizer side, the decrease of particle formation for effect of ethanol addition is most likely due to processes similar to those occurring in premixed flames: an increase in the oxidation rate of the hydrocarbons and a reduced formation of aromatics. Results of this study show that particle formation is controlled not only by the nature of the fuel but it also strongly depends on combustion conditions. The presence of bonded oxygen in ethanol reduces the molecular growth process and thus the formation of particulate matter, but in both premixed and nonpremixed conditions, the addition of

ACKNOWLEDGMENTS The authors acknowledge the financial support for this work provided by MIUR under the PRIN08 “Polveri ultrafini ed effetti sulla salute” and MSE, Accordo di Programma MSECNR: “Cattura della CO2 e utilizzo pulito dei combustibili fossili”. M. Salamanca thanks COLCIENCIAS and the University of Antioquia for the PhD scholarship.

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