Variations of the Soot Precursors Chemical ... - ACS Publications

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Variations of the Soot Precursors Chemical Composition Induced by Ethanol Addition to Fuel Maurin Salamanca, Mauricio Velásquez, Fanor Mondragón, and Alexander Santamaría* Institute of Chemistry, University of Antioquia, A.A.1226, Medellin, Colombia ABSTRACT: In this study, conventional analytical methods such as infrared spectroscopy, 1H NMR, elemental analysis, and vapor pressure osmometry were combined to calculate the average structural parameters of the soot precursor material present in the soluble organic fraction (SOF) of young soot of aliphatic and aromatic inverse diffusion flames doped with ethanol. The structural parameters were classified in two groups: the first one is the parameters that describe the aromatic character, and the second describes the aliphatic character of the samples. The results of this study indicated that the addition of ethanol to aliphatic or partially aliphatic fuels produces less SOF due to an increase in the oxidation process, whereas the addition of ethanol into aromatic fuels caused an increase in this fraction. Additionally, it was observed that the ethanol addition to aromatic flames caused an increase in the fraction of aromatic carbon substituted by hydrogen (faHa) of SOF samples, which leads to a reduction of the average size of the aromatic cluster (Ra). This result, along with an increase of low molecular weight PAHs, could explain the increase in chloroform solubility. A similar behavior occurs in the aliphatic flames, where the aromatic component of SOF decreases due to ethanol addition; however, the reduction of the SOF only can be explained by an increase in the soot precursors oxidation caused by the OH radical coming from ethanol. Although the oxidation and pyrolysis are parallel processes in combustion, the extension of each of one will depend on the nature of the initial fuel, so that the ethanol addition will increase the oxidation process in aliphatic flames and promotes the formation of low molecular weight PAHs in the aromatic ones. H 2O + H → OH + H 2

1. INTRODUCTION In the lasts decades, the use of biofuels has received wide public acceptance because it contributes to the reduction of the dependence from petroleum and at the same time reduces the greenhouse gases emissions from fossil fuels. The development of low-priced processes for obtaining biofuels and the increment on petroleum prices has made biofuels an interesting choice.1,2 Lately, a variety of oxygenated compounds such as alcohols, ethers, and acetals have been studied because of their capacity to change the combustion dynamics of the fuels; especially, because these compounds can introduce different kinds of radicals that could increase particulate matter precursors or change the species involved in oxidation processes.3−5 The most studied oxygenated additive so far is ethanol, because it is produced from a renewable source at a reasonable cost.6,7 The use of ethanol as an additive has been reported in different flame configurations such as diffusion and premixed flames. For instance, Ni et al.8 studied the effect of different alcohols in ethylene diffusion flames and found a more effective reduction of particulate matter when methanol was added to the flame compared to the result obtained by the addition of ethanol. They proposed that the high reduction of soot obtained with methanol was due to an increase in the oxidation processes. Methanol pyrolysis leads mainly to an increase in the OH radical concentration, whereas in the case of ethanol the main decomposition products are ethylene and water. A possible explanation of this behavior was derived from the pyrolysis of benzene/ alcohol mixtures carried out in shock tubes; it was proposed that the ethanol decomposition in shock tubes can produce OH radicals through the following reaction sequence: C2H5OH → C2H4 + H 2O © 2012 American Chemical Society

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In this case, the production of C2H4 can enhance the formation of the molecular precursors previously mentioned. Whereas the synergistic effect between the oxidation by OH radicals and the removal of H atoms to form H2O can cause a slowdown in the soot formation process.9 These results agree with previous droplet flame studies done by Randoph and Law.10 They studied the gasification mechanisms and sooting characteristics of free-falling droplets of alchohol/oil blends and water/oil emulsions by microphotography to determine the instantaneous droplet composition and the gas-phase soot level. According to them, the dilution effect is a dominant factor responsible for the reduction of soot through alcohol blending, and their results suggested that the hydroxyl group in hexanol did not have a significant effect in soot formation. Whereas, dilution and thermal effects are important for water emulsified fuels. Recently, Maricq11 evaluated the effect of ethanol on the particulate matter generated in gasoline diffusion flames, using scanning mobility particle sizer (SMPS). In this research, it was found that the particle size distribution changes along the flame. For instance, the particle size distribution of gasoline/ethanol doped flames E0, E20, and E50 (where number that follows the letter “E” indicates the amount of ethanol used) are bimodal at a height above 5 mm and the particle diameter is twice as large as the one found in the E85 flame. Also, as the ethanol level increases, soot accumulation becomes slower. However, in samples taken at 20 mm the size particle distributions are nearly identical. As the hight of the sampling point increases, the soot Received: May 28, 2012 Revised: September 26, 2012 Published: September 26, 2012

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dx.doi.org/10.1021/ef300926y | Energy Fuels 2012, 26, 6602−6611

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very early in the oxidation region, which were no longer detected at the end of the flame, whereas for the aliphatic fuels, the presence of both aromatic molecules and freshly nucleated soot particles were observed downstream of the flame. A few studies have been focused on the influences of additives in the chemical composition of soluble organic fraction of soot, also referred as SOF. Santamarı ́a et al.,21 using combined information obtained by nuclear magnetic resonance (NMR), infrared spectroscopy (IR), vapor pressure osmometry (VPO), and elemental analysis, described the SOF coming from benzene and ethylene flames in terms of average structural parameter such as chain length, number of fused aromatic rings, etc. They found that benzene produces SOF with a larger number of fused aromatic rings compared to ethylene, whereas ethylene produces SOF with a larger aliphatic chain length. The purpose of this study was to evaluate the chemical effect of ethanol concentration (0, 10, and 20%) in the SOF chemistry of soot generated in aliphatic (hexane), partially aliphatic (diesel surrogate), and aromatic flames produced in aromatic (benzene and toluene), in terms of average structural parameters.

accumulation mode continues to grow with a larger particle diameter. However, there are a few differences among E0, E20, and E50. At 40 mm above the burner, there was a concurrent decline of the nucleation mode, which persists in the reference flame (E0). Moreover, it was observed that the ethanol addition above 20% can cause a significant reduction in the particulate matter emission and at the same time can induce some changes in the chemical particle composition, as was observed for the flame with 85% ethanol, where a strongly reduction in the volatile organic fraction of particulate matter was obtained. McEnally and Pfefferle,12 using laser induced incandescence and photoionization mass spectroscopy, found an increase of benzene and total soot volume fraction when ethanol and dimethyl ether were added to the fuel in a normal diffusion ethylene flame. The authors suggested that these additives increase soot concentrations because they can decompose to methyl radical, promoting the formation of propargyl radical and consequently the formation of benzene. Dimethyl ether has a stronger effect than ethanol because it decomposes into two methyl radicals. Wu et al.13 studied the effect of ethanol on a flat premixed ethylene flame for two different equivalent ratios using laser induced fluorescence and incandescence. Under the conditions investigated, the authors found a reduction in soot and soot precursor compounds of two or three aromatic rings. Moreover, the effect of ethanol addition declined when the equivalence ratio was higher for the same percentage of ethanol added to the flame. The authors concluded from the carbon flux analysis that only about one-half of the carbon in ethanol is converted into species that contribute to the production of aromatic species. Zhang et al.14 studied the effect of ethanol and methanol in the emissions of a direct-injection diesel engine. Although they found that NOX and particulate matter were reduced, the emissions of HC, CO, and NO2 were increased, especially when methanol was used. On the other hand, Lee et al.15 measured the particle size distribution and pollutants emissions in an engine operated with gasoline/ethanol blends (0, 10, and 85%). They obtained a total particulate matter reduction of 37% and observed that the particulate matter emission with diameter between 30−200 nm was strongly reduced when the engine was operated with 85% of ethanol. Other studies have found that the differences in particle nanostructure, as well as soot oxidation rate, are due to differences on chemical nature of initial fuel. Vander Wal and Tomasek16 suggest that fuel identity governs the soot nanostructure under specific temperature combinations and residence times as dictated by the flow rates and found that, at a temperature of 1650 °C and under a high flow rate, acetylene, benzene, and indene fuels produced a highly curved nanostructured soot, whereas at low flow rate they produced a more graphitic soot. Similarly, when ethanol was used as fuel, it was obtained that the soot particles nanostructure was curved regardless of the flow rate. Ciajolo and co-workers17−20 have been performed several studies with the aim to obtain structural information of carbonaceous materials, especially soot. For example, they studied the molecular weight distribution and structural properties of carbon particles formed in methane, ethylene, and benzene fuel rich premixed flames using UV−vis spectroscopy, chemical analysis, and size exclusion chromatography. The evolution of the C/H ratio, molecular weight distribution and the UV−vis properties showed that benzene flame produces aromatic precursors and nascent soot particles

2. MATERIALS AND METHODS 2.1. Burner and Sampling Procedure. The ethanol effect was evaluated using pure hexane, benzene, and toluene flames and a diesel surrogate with a chemical composition consisting of 5% benzene, 17% toluene, 28% hexane, 30% cyclohexane, and 30% iso-octane. Each fuel was blended with 10 and 20% of ethanol. An inverse diffusion flame (IDF) was used in this research, which can produce young soot, which can be chemically characterized. The burner is composed of three concentric stainless steel tubes; the central tube was used to supply the air, the annular tube to supply the fuel, and the outer tube to supply nitrogen to produce a shield to avoid the interaction of the flame with the surrounding air. A detailed description of the burner was given by Santamarı ́a et al.22 The liquid fuels were delivered by a HPLC (high pressure liquid chromatography) pump to a vaporizer at 150 °C, and then, the vaporized fuel was mixed with nitrogen and carried toward the burner mouth. The experimental conditions to obtain a 60 mm flame height in all cases are shown in Table 1.

Table 1. Flame Conditionsa flame

fuel flow (mL/min)

air flow (L/min)

N2 flow on fuel (mL/min)

hexane benzene toluene synthetic fuel

1.50 0.75 0.75 0.75

2.40 1.30 1.30 1.76

250 250 250 230

a The different flows were adjusted until a stable flame of 60 mm was obtained.

The sampling process was carried out at different heights along the visible border flame using PTFE filters of 0.2 μm pore diameter in line with a vacuum pump system connected to the probe (see Figure 1). To ensure reproducibility, the sampling process was carried out under the same conditions keeping constant the suction flow rate and time. Soot sampling was done at different heights above the burner mouth at visible border of the flame; the heights used to do the sampling are shown in Figure 1 (right side). The soot inception was determined visually as the point where the luminosity of the flame changes from blue to yellow color. It is important to mention that the location of the inception point in the flames evaluated in this study was not affected by the ethanol addition and depended only on the fuel used, being 13 mm for hexane, 3 mm for aromatics, and 5 mm for surrogate. The sampling time was kept constant for all experiments (5 min each). However, sometimes the sampling time in the hexane flame was three 6603


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Figure 1. Inverse diffusion flame setup used in this study (left side) and the height above the burner used to do the sampling (right side).

Table 2. Structural Parameters Definitions (Taken from Santamaria et al.23)a Abbreviation fa

fal

fα

faHa

%Ci

Structural Parameter Definition aromatic atomic carbon fraction [C* − (0.33Hγ* + 0.4H β*1 + 0.5(H β*2 + Hα* + Hf*) + HO* + Cq)] fa = C* aliphatic atomic carbon fraction [0.33Hγ* + 0.4H β*1 + 0.5(H β*2 + Hα* + Hf*)] fal = C* atomic fraction of α type carbon attached to aromatic ring [0.5Hα*] fα = C* atomic fraction of aromatic carbon substituted by aromatic hydrogen H* faHa = a C* weight fraction of Ci carbon with i = a, al, α %Ci = fi (%C) i = a , al , α

#Ci

avg. no. of Ci carbons per average structural unit i = a, al, α,... #Ci = (%Ci*)MW/1200

#Oq,e

avg. no. of oxygen present per avg. structural unit #Oq,e = (0.5*%O)MW/1600

#Cas

avg. no. of aromatic carbons substituted by alkyl and oxygenated groups

#CaS = #Cα + #CO + #Cf + #CaHa + #Cq + #Oe (C/H)al

C/H molar ratio in aliphatic groups [(0.33Hγ* + 0.4H β*1 + 0.5(H β*2 + Hα* + Hf*))] ⎛ H ⎞* ⎛ C ⎞* ⎜ ⎟ ⎜ ⎟ = ⎝ H ⎠al ⎝C⎠ (Hγ* + H β*1 + H β*2 + Hα* + Hf*)

L

chain length no. of carbon atoms per avg. structural unit L = (#Cal − #Cf )/#Cα

Ra

no. of fused aromatic rings per avg. structural unit R a = 1 + (#Ca − #Ca s)/2

C*: molar fraction of carbons obtained by elemental analysis. H*: molar fraction of hydrogen obtained by elemental analysis. O*: molar fraction of oxygen obtained by elemental analysis. MW: average molecular weight of the samples. Hγ: aliphatic hydrogen in methyl group on γ position or further to an aromatic ring. Hβ1: alicyclic hydrogen in β position to two aromatic rings. Hβ2: aliphatic hydrogen in methyl or methylene groups in β position to an aromatic ring. Hα: hydrogen of CH, CH2, CH3 on α position to aromatic rings. Hf: hydrogen fluorene type. HO: oleofinic hydrogen. Ha: hydrogen on aromatic rings. a

times longer than the others in order to ensure enough amount of sample for chemical analysis. In all experiments, the sampling was carried out at least five times to ensure reproducibility. 2.2. Chemical Characterization of the Soluble Organic Fraction (SOF). The chemical analysis was done on the soot soluble

organic fraction in chloroform. The fourier transform infrared spectroscopy (FT-IR) analysis was carried out using the KBr pellet method in a Nicolet Magna IR 560 spectrometer with a MCT/A detector operated at 77 K with wavenumber range from 600 to 4000 cm−1. Each spectrum was taken at least three times to estimate reproducibility. For 1H NMR 6604


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According to the scientific literature,24,25 it has been established that the pyrolysis of pure ethanol can produce ethylene and water. However, when ethanol is used as a fuel additive, its thermal decomposition products could play a different role in the soot formation process depending on the initial fuel. For instance, when ethanol is added to aliphatic or partially aliphatic flames (hexane or surrogate), it can induce the formation of ethylene and water, which at the same time leads to a decrease in the soot growing velocity. The water coming from the combustion can consume the hydrogen radical necessary to activate the HACA (H abstraction C2H2 addition) mechanism to form molecular hydrogen and OH according (see eq 2) to the Frenklach model.9 Also, it is expected that the increase in the OH radical concentration can contribute to an increase in the soot precursor’s oxidation process. In the aromatic flames (benzene and toluene) the opposite occurs, since the C/H ratio for aromatic flames is higher than its aliphatic counterpart, a fact that counteracts the production of water and increases the production of low molecular weight precursors due to presence of C2 radical coming from ethanol decomposition. It is important to recognize that this mechanism is alternative to the polymerization process that already exists in aromatic flames. 3.2. Infrared Spectroscopy. Figure 3 shows the infrared spectra of the SOF coming from the reference fuels used in this

analysis, the extracts were dried to remove the chloroform solvent, and then, they were redissolved in CDCl3 containing trace amounts of tetramethylsilane (TMS), which was used as an internal chemical shift reference. All spectra were taken in a Bruker AMX 300 spectrometer. For each spectra, the baseline was corrected and integrated manually at least four times and the results were averaged to reduce the uncertainty (which was less than 5%) generated by the manual adjustment. The elemental analysis of the SOF samples was carried out by the combustion method by using a CHNSO Leco Instruments analyzer. The average molecular weight data of the extractable material of soot generated in the flame was determined by vapor pressure osmometry (VPO) in a Knauer K7000 osmometer using chloroform as solvent and benzyl as calibration standard. All measurements were carried out using sample and standard solutions of 1 g/kg and 0.0160 mol/kg, respectively. 2.3. Structural Parameters. The 1H NMR spectrum of each sample was separated into characteristic regions according to Santamaria’s work.22 Then, this information was correlated with molecular weight and elemental analysis data to obtain structural parameters. Table 2 shows the equations used to calculate the structural parameters.

3. RESULTS AND DISCUSSION 3.1. Soot Soluble Organic Fraction. Figure 2 shows the mass percentage of soluble organic fraction (SOF) as function

Figure 3. FT-IR spectra of the SOF coming from aromatic and aliphatic flames gathered at 10 mm height. Benzene (−), toluene (− (gray)), hexane (--), and surrogate (-- (gray)).

study. It was observed that the most characteristic signals of the soot coincide with those reported by Santamariá et al.22 and Cain et al.26 The most significant differences among the spectra were found to be related to the CHal stretching mode of aliphatic groups as well as the CHar and CC stretching modes of aromatic molecules. In general, it was observed that saturated aliphatic fuels or fuels with high aliphatic content can produce SOF with long aliphatic chains, probably bonded to small PAHs (polycyclic aromatic hydrocarbons), whereas the aromatic flames can produce SOF with highly condensed aromatic species or large PAHs that can be adsorbed on the surface of soot particles. Figure 4 shows that the CHal stretching signal of SOF coming from the surrogate soot decreases as a function of flame height, as a result of both thermal annealing/oxidation and chemical growing process of soot. Moreover, contrary to the tendency observed for the CHal stretching signal, the CHar stretching mode of aromatic groups showed a constant increment up to 40 mm and then a reduction at 60 mm. The

Figure 2. SOF in chloroform of the samples (A) benzene and toluene (B) hexane and surrogate with the corresponding ethanol blends.

of flame height and ethanol addition. The amount of SOF decreases as a function of flame height for both reference and ethanol-doped flames. This reduction is mainly caused by competition between thermal and oxidative processes of soot particles upstream of the flame. However, upon comparing the amount of SOF coming from the ethanol doped aromatic flames with the reference ones at a particular position; it was observed an increment of SOF as function of ethanol amount. This behavior was opposite to that observed in hexane and surrogate flames. 6605


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not explain the solubility data, where the soot from hexane and surrogate flames showed to be less soluble than the soot coming from aromatic flames when ethanol was added. Therefore, the effect of ethanol on the chemical characteristics of SOF will depend on the chemical nature of the initial fuel. In some cases, the addition of ethanol can produce an increase in the oxidation of precursor species causing the reduction in solubility, as was observed in hexane and surrogate fuels. Whereas in aromatic flames, the oxidation process of SOF due to the ethanol addition was not so evident compared to the alkylation process and the small PAHs formation through a C2 path parallel to the rapid polymerization process that already exist in aromatic flames, a fact that justifies the increase in solubility. These results also corroborate the data obtained by 1H NMR where a similar trend was observed. Figure 6 shows the region between 1900 to 1500 cm−1. The most remarkable effect of the ethanol addition is the increase of the carbonyl signal (1730 cm−1). In the case of 20% ef ethanol added to aromatic flames, the change of intensity is more pronounced. This is an evidence of the oxygen incorporation into the soot structure produced from fuel-ethanol IDFs. This result agrees with the elemental analysis results. 3.3. 1H-Nuclear Magnetic Resonance. In the Figure 7A, the 1H NMR spectra of SOF taken at the inception point and 60 mm height of hexane flame with and without ethanol are shown. The assigned groups (see Figure 7A.) correspond to hydrogen on aromatic rings (Ha 6.5−9.0 ppm), olefinic hydrogen (HO 4.5−6.0 ppm), hydrogen on carbon connecting two aromatic units or fluorene type (Hf 3.7−4.5 ppm), hydrogen of CH, CH2, and CH3 groups on α position to aromatic rings, and the acetylenic hydrogen of phenylacetylene-like structure (Hα 2−3.7 ppm), hydrogen of CH2− naphthenic type groups on β position to aromatic rings (Hβ1 1.4−2.0 ppm), hydrogen of CH, CH2, and CH3 groups on β position to aromatic rings (Hβ2 1.0−1.4 ppm), and hydrogen of methyl groups on γ or greater positions to aromatic rings (Hγ 0.5−1.0 ppm). The hydrogen classification is based on the work develop by Guillen et al.27 It is observed that ethanol addition did not significantly affect the aromatic and aliphatic hydrogen fraction of SOF with height, corroborating what has been observed by FT-IR for SOF of aliphatic flames. Therefore, the decline in solubility observed for SOF samples coming from ethanol-doped hexane flame can be explained by an increase in soot precursorś oxidation mainly by OH radicals, instead of an increment of the aromatic condensation during pyrolysis. On the other hand,

Figure 4. FT-IR spectra of the SOF coming from surrogate fuel as function of the flame height: 5 mm (−), 10 mm (− (gray)), 20 mm (--), 40 mm (-- (gray)), and 60 mm (...).

behavior observed before 40 mm is due to the generation of low molecular weight PAHs that introduces aromatic hydrogen in the SOF samples, while above 40 mm either of these two cases may occur: (1) the condensation process of small aromatic units are favored at this stage, which leads to a reduction in the amount of aromatic hydrogen, or (2) the oxidation of small PAHs becomes important. The same behavior was observed for all fuels and ethanol blends. This result agrees with those obtained by 1H NMR where the aromatic hydrogen at low flame position is higher than that obtained at high positions of the flame. On the other hand, Figure 5 shows the infrared spectra of SOF from hexane, benzene, and surrogate blended with ethanol taken at the flame height of 10 mm. For all fuels, C−Har signal remains almost identical for 10% ethanol, accompanied by a slight reduction of this signal when 20% of ethanol was used. In these flames, the ethanol addition does not affect the aromatic fraction of the soot soluble part. Another interesting feature observed in Figure 3 is that the ethanol effect is reflected in an increase in the aliphatic fraction of SOF as a function of ethanol amount. Toluene samples showed a similar behavior to the one observed in benzene samples. Also, all SOF samples showed a reduction in the C−Har/ C−Hal ratio when ethanol was added to the corresponding flame, this reduction is mainly due to an increase in the aliphatic character of SOF samples promoted by the C2 radical coming from ethanol, since the signal corresponding to aromatic species, C−Har, was unaffected. However, this result alone does

Figure 5. FT-IR spectra of SOF for (A) hexane, (B) benzene, and (C) surrogate of samples taken at 10 mm. Pure fuel (−), 10% ethanol (--), and 20% ethanol (− (gray)). 6606


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Figure 6. FT-IR spectra of SOF for (A) hexane, (B) benzene, and (C) surrogate of samples taken at 10 mm. Pure fuel (−), 10% ethanol (--), and 20% ethanol (− (gray)).

Figure 7. 1H NMR spectra of SOF taken at the soot inception and 60 mm height of (A) hexane, (B) benzene, and (C) surrogate flames with 20% ethanol (−) and without ethanol (− (gray)).

contrary to what is observed in the hexane flame, the 1H NMR spectra of SOF from aromatic flames (Figure 7B) show an increment in the aromatic hydrogen fraction due to ethanol addition, especially at a flame height of 60 mm. This result indicates that the addition of ethanol to aromatic flames induces the formation of small aromatic species (one or two rings) through C2 radicals in a parallel mechanism oriented in the aromatic polymerization process that exists under these flame conditions. Also, it is important to highlight that the formation of molecules of one or two aromatic ring can increase the solubility of SOF, since the fraction of aromatic carbon substituted by aromatic hydrogen increases. Figure 7C shows the 1H NMR spectra of SOF coming from surrogate flames with and without ethanol. At the inception point, it is observed an increase in the aromatic hydrogen fraction due to ethanol addition, followed by a reduction of the hydrogen signal intensity between 2 and 3 ppm, which represents the oleofinic and naphthenic functional groups attached to aromatic rings. At 60 mm, it was found that the aliphatic fraction remains almost

constant due to ethanol addition, while the aromatic hydrogen fraction showed a slight reduction. We believe that the conjunction of both aromatic and aliphatic components in the initial fuel could explain the chemical behavior of surrogate SOF, since from one side, the aromatic part of the fuel (20% by weight) favors the polymerization process of SOF, whereas the aliphatic component of the surrogate fuel favors the alkylation process, so we think that the ethanol effect in this flame could be dual. To get some insights of the ethanol effect, each spectrum was integrated in the regions described in Figure 7A and then normalized against its total hydrogen area. Figure 8 shows the results of the hydrogen area normalization of SOF obtained at 60 mm for hexane, benzene, and surrogate flames. For the surrogate flame (Figure 8C), it was observed that the ethanol addition caused a slight decrease of the aromatic character (aromatic hydrogen, Ha) as function of the ethanol amount, followed by an increment in the aliphatic hydrogen content (aliphatic hydrogen: Hα, Hβ2, Hγ) with the exception of the alicyclic hydrogens (Hβ1) content, which remained constant in 6607


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Figure 8. Hydrogen distribution of SOF coming from (A) hexane, (B) benzene, and (C) surrogate fuel and its blends with 10% and 20% of ethanol at 60 mm.

The ethanol effect is observed through an increase of both H/C and O/C ratios, which is consistent with the ethanol amount. This means that, as the ethanol amount increases, the total hydrogen and oxygen introduced in the SOF also increases, being more evident for the aromatic flames than for the aliphatic ones. Considering the results obtained by 1H NMR and FT-IR, it is seen to be that the aliphatic hydrogen fraction contributes significantly to the total hydrogen amount incorporated by the ethanol addition. 3.5. Structural Parameters. The structural parameters calculated for the SOF samples are shown in Table 4. It can be inferred from the 1H NMR spectra that the contribution of carbonyl, oleofinic, and fluorene type compounds around 2−3 ppm were not significant. Therefore, two global structural parameters (aliphatic chain length and number of fused aromatic rings) were taken into account to perform the analysis. Each of these parameters was calculated from atomic parameters, such as atomic fraction of aromatic carbon (fa), atomic fraction of aromatic carbon substituted by aromatic hydrogen (faHa), atomic fraction of aliphatic carbon (fal), atomic fraction of carbon attached to aromatic unit in α position (fα), C/H molar ratio in aliphatic groups (C/H)al as well as the weight fraction and number of each type of carbons (%Ci and #Ci with i = a, al, α) present in samples. Comparison of the values obtained at the soot inception point and at 60 mm high shows that the value of the different parameters related to the aromatic character of SOF samples (fa, faHa, %Ca, #Ca, and Ra) increases as the flame height increases, followed by a subsequent reduction on the aliphatic character, as it was observed on the values of fal, fα, %Cal, %Cα, #Cal, L, and (C/H)al. The increase of the aromatic fraction (fa) and number of fused aromatic rings (Ra) as function of flame height is the most significant evidence of the growth process of the aromatic units along the flame axes, a fact that can be explained by the HACA and polymerization mechanisms. It is important to take into account that the aromatic cluster is already large at the inception point of all flames, which means that its formation takes place very early in the flame. However, it is clear that the flames with aromatic compounds generate soot precursors with a higher degree of aromatic condensation (Ra), and higher aromatic carbon fraction ( fa), since the polymerization mechanism through aromatic moieties is faster. Similarly, the parameter associated with atomic fraction of aromatic carbon substituted by hydrogen (faHa) of almost all SOF samples, except for the one coming from surrogate flame,

all cases, results that support what have been observed by FT-IR. This suggests that ethanol promotes the formation of low molecular weight aromatic species (one or two aromatic rings) through the radical C2 and HACA mechanism.28 3.4. Elemental Analysis and Vapor Pressure Osmometry. Figure 9 shows the average molecular weight of the SOF coming from all fuels and the ethanol fuel blends at the inception point.

Figure 9. Average molecular weight of the SOF coming from all fuels and ethanol fuel blends at the inception point.

For benzene flames, the average molecular weight of SOF was ∼25% higher compared to SOF samples coming from the others flames (toluene, hexane, and surrogate fuels). This fact is attributable to the chemical nature of initial fuel and its aromatic polymerization rate, being faster for benzene, toluene, and surrogate, since hexane may require more time to generate the basic aromatic unit. On the other hand, ethanol addition caused a reduction of the average molecular weight for all the fuels tested, except for the surrogate, where the average molecular weight remained constant. Table 3 shows the H/C and O/C ratios obtained by elemental analysis of all SOF samples evaluated in this study, although there is not an apparent effect of ethanol on these parameters, the results indicate that the aromatic condensation degree consistently increases with flame height, since the H/C ratio is reduced. This agrees with the fact that H/C ratio for benzene is one, whereas for a graphene layer tends to zero. 6608


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Table 3. H/C and O/C Ratios Obtained by Elemental Analysis of SOF Hexane

Benzene

Height above the burner

0%

20%

0%

10%

Inception point 60 mm

0.75 0.71

0.75 0.71

0.47 0.43

0.51 0.46

Inception point 60 mm

0.035 0.074

0.045 0.069

0.007 0.002

0.008 0.006

Toluene 20% H/C 0.59 0.51 O/C 0.009 0.007

Surrogate

0%

10%

20%

0%

10%

20%

0.50 0.46

0.57 0.50

0.62 0.54

0.71 0.65

0.71 0.73

0.80 0.67

0.007 0.003

0.010 0.004

0.012 0.008

0.028 0.039

0.038 0.069

0.051 0.028

Table 4. Structural Parameter Calculated for SOF for the Different Fuels Used for the Inception Point and 60 mm Hexane structural param.

*

0%

Benzene 20%

0%

fa fal fα faHa %Ca %Cal %Cα %CaHa %O* #Ca #Cal #Cα** #CaHa #Oq L Ra

0.772 0.147 0.026 0.374 69.24 13.2 2.32 33.5 1.65 18.8 3.58 1 9.12 0.86 3.36 5.85

0.782 0.145 0.014 0.391 69.68 13.0 1.25 34.8 2.07 17.2 3.19 1 8.58 0.98 3.06 5.29

0.950 0.021 0.008 0.429 90.68 2.0 0.80 40.9 0.34 35.6 0.78 1 16.05 0.26 0.94 10.77

fa fal fα faHa %Ca %Cal %Cα %CaHa %O* #Ca #Cal #Cα** #CaHa #Oq L Ra

0.756 0.140 0.014 0.360 65.3 12.1 1.17 31.1 3.32 21.4 3.96 1 10.2 2.09 3.73 6.61

0.812 0.098 0.009 0.470 70.5 8.52 0.79 40.8 3.14 21.3 2.58 1 12.3 1.82 2.42 5.50

0.979 0.009 0.003 0.404 94.2 0.88 0.30 38.9 0.09 41.5 0.39 1 17.1 0.08 0.77 13.17

Oxygen percentage obtained from elemental analysis.

Toluene 20%

Inception Point 0.937 0.032 0.014 0.496 88.31 3.0 1.30 46.8 0.40 30.5 1.05 1 16.18 0.29 1.49 8.18 60 mm 0.972 0.010 0.005 0.480 92.5 0.91 0.45 45.7 0.29 36.1 0.36 1 17.8 0.24 0.85 10.12

Surrogate

0%

20%

0%

20%

0.967 0.011 0.008 0.465 92.03 1.1 0.73 44.2 0.43 26.8 0.31 1 12.90 0.20 1.02 7.97

0.944 0.025 0.004 0.553 88.47 2.3 0.36 51.8 0.35 21.2 0.56 1 12.44 0.27 0.97 5.40

0.806 0.134 0.019 0.383 73.46 12.3 1.74 34.9 0.49 18.8 3.13 1 8.91 0.65 2.92 5.93

0.741 0.160 0.029 0.393 65.24 14.1 2.53 34.6 0.58 17.3 3.72 1 9.15 1.18 3.48 5.05

0.990 0.004 0.001 0.444 95.1 0.37 0.13 42.6 0.32 32.2 0.13 1 14.4 0.09 0.85 9.87

0.974 0.014 0.003 0.503 92.3 1.36 0.28 47.7 0.14 27.5 0.40 1 14.2 0.21 0.86 7.64

0.870 0.072 0.011 0.469 78.6 6.55 1.02 42.4 0.21 18.4 1.53 1 9.9 0.83 1.42 5.24

0.840 0.098 0.018 0.427 76.8 9.00 1.63 39.0 0.37 31.6 3.70 1 16.0 1.05 3.41 8.77

**

Normalized fraction of Cα in the average structure

decrease as flame height increases, due to maturing processes caused by the thermal gradient and oxidative radical present in the flame environment; however, the extent of the decomposition process of the aliphatic chains attached to the aromatic cluster depends on the composition and structure of initial fuel. For example, the aliphatic flames produce SOF with large substitution and large number of aliphatic carbons compared to the SOF produced in aromatic flames, result that agrees with the (C/H)al ratio and the chain length (L) observed for samples. When the ethanol was added to these flames, it was observed that the aliphatic fraction ( fal) of SOF increases at the soot

showed a decrease with flame height, suggesting that the aromatic cluster size is increasing according to the fuel type. For instance, aromatic flames produce more peri-condensed aromatic species than the aliphatic ones. On the other hand, to explain the behavior observed for the surrogate SOF, one should consider that this fact is related to fuel composition (20% aromatics and 80% aliphatics) and that thermal decomposition products can favor both the aromatic polymerization and pyrolysis condensation processes. Additionally, parameters associated with the atomic fraction of aliphatic carbons and the atomic fraction of carbon in α position to aromatic rings ( fal and fα) indicate that their values 6609


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Also, it was found that aromatic flames produce SOF with more condensed aromatic molecules, a fact that was evidenced in the increase of the Ra number, whereas the aliphatic or partially aliphatic flames produce SOF with a reduced aromatic nucleus accompanied by an aliphatic chain length up to 4 carbon atoms. It was found that the effect of ethanol added to these flames induced not only a changes in the extension of SOF solubility in chloroform but also changes in the chemical character of the samples. We suggest that these changes are related to the chemical characteristic of the initial fuel; for example, it was found that the addition of ethanol to aliphatic or partially aliphatic fuels produce less SOF due to an increase in the oxidation process, whereas the addition of ethanol into aromatic fuels caused an increase in this fraction. In the last case, it is thought that the addition of ethanol favors the HACA mechanism over the oxidation, since the C2 radical from ethanol decomposition can lead to the formation of low molecular weight PAHs. On the other hand, it was observed that the ethanol addition to aromatic flames caused an increase in the fraction of aromatic carbon substituted by hydrogen (faHa) of SOF samples, which leads to a reduction of the average size of the aromatic cluster (Ra). This result along with an increase of low molecular weight PAHs could explain the increase in chloroform solubility. A similar behavior occurs in the aliphatic flames, where the aromatic component of SOF decreases due to ethanol addition; however, the reduction of the SOF only can be explained by an increase in the soot precursors oxidation caused by the OH radical coming from ethanol. Although the oxidation and pyrolysis are parallel processes in combustion, the extension of each of one will depend on the nature of the initial fuel, so that the ethanol addition will increase the oxidation process in aliphatic flames and promotes the formation of low molecular weight PAHs in the aromatic ones.

inception point of all aromatic flames, including for the surrogate fuel flame that contains 20% of aromatics in its composition. Nevertheless, although the increment was evident for this parameter, the aliphatic chain length did not show to be very sensitive with the ethanol addition. By contrast, in the hexane flame, the aliphatic fraction decreases with the ethanol addition, indicating that the oxidation process and fragmentation of the aliphatic moieties is taken place, a fact that would partially explain the reduction in solubility at the soot inception point when the ethanol is added to the flame. At 60 mm high of the flame, the tendency of these parameters was kept, but in a lesser extent, as product of thermal annealing generated at low flame position, causing the reduction of the aliphatic character. On the other hand, although the aromatic carbon substituted by hydrogen (faHa) shows an increase due to ethanol addition, the number of fused aromatic rings (Ra) showed the opposite, especially for the aromatic fuel flames, where the ethanol addition promotes the formation of small aromatic molecules (one or two rings) able to incorporate aromatic hydrogen into the SOF sample, a fact that would explain the reduction of the average value of Ra. According to the calculation of the average number of oxygen (#O) based on elemental analysis, It is presumed that the oxygen present in soot or soot precursorś (SOF) is linked to carboxylic, carbonyl, and ether groups. From our experimental data, it was observed the amount of oxygen incorporated in SOF samples increases as a function of ethanol amount added to aromatic and surrogate flames, whereas for pure hexane flame this tendency was not observed, which indicates that the degree of oxidation also depends on the chemical nature of fuel. In general, the average structural parameters evaluated in the SOF samples coming from an aliphatic, two aromatic, and a surrogate inverse diffusion flame suggest that the ethanol addition causes a change of the chemical nature of soot precursors. However, the extent of the change will depend on the chemical nature and composition of the initial fuel. For example, a reduction of aliphatic component represented by fal, fα, L, and (C/H)al along with a decrease in the average number of fused aromatic rings was observed in the SOF generated from hexane/ethanol IDF, whereas the same parameters shown an opposite behavior in the samples obtained in the aromatics and surrogate flames with ethanol. Therefore, any change in the chemical characteristics of soot precursors caused by the ethanol addition will change the chemical characteristics and environmental impacts of soot particles themselves.

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AUTHOR INFORMATION

Corresponding Author

*Address: Carrera 53 No 61-30, Torre 2, Laboratorio 334. Telephone: (57) 4 2196654. Fax: (57) 4 2196565. E-mail: alex. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to COLCIENCIAS and the University of Antioquia for the financial support given through the Project No. 1115-405-20283. The authors also would like to thank the Sostenibilidad Program 2013−2014 of the University of Antioquia for financial support. M. Salamanca and M. Velasquez thank COLCIENCIAS and the University of Antioquia for the PhD scholarship and financial support, respectively.

4. CONCLUSIONS The average structural analysis gives us information of hypothetical structures that are useful to describe the physicochemical properties of complex samples, such as the soluble organic fraction of soot (SOF) evaluated in this study. Similarly, this type of methodology can be used to evaluate, in a qualitative and semiquantitative way, the changes induced by the effect of additives on soot precursor’s evolution along the flame height. In general, the chemical evolution of the soot precursors along the flames evaluated in this study was described in terms of average structural parameters, which were classified in two main groups, (1) parameters that describes the aromatic character (fa, faHa, %Ca, #Ca, and Ra) and (2) parameters describing the aliphatic character of samples (fal, fα, %Cal, %Cα, #Cal, L, and (C/H)al). It was found that the aromatic character of samples increases as a function of flame height, followed by a subsequent reduction on the aliphatic character due to thermal annealing.

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REFERENCES

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Variations of the Soot Precursors Chemical ... - ACS Publications

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