Reduction of PAH and Soot Precursors in Benzene Flames by

Article pubs.acs.org/JPCA

Reduction of PAH and Soot Precursors in Benzene Flames by Addition of Ethanol Djemaa Golea, Yacine Rezgui,* Miloud Guemini, and Soumia Hamdane Laboratoire de Chimie Appliquée et Technologie des Matériaux, Université d'Oum El Bouaghi, B.P. 358, Route de Constantine, Oum El Bouaghi 04000, Algérie ABSTRACT: A one-dimensional premixed flame model (PREMIX) and schemes resulting from the merging of validated kinetic schemes for the oxidation of the components of the present mixtures (benzene and ethanol) were used to investigate the effect of oxygenated additives on aromatic species, which are known to be soot precursors, in fuel-rich benzene combustion. The specific flames were low-pressure (45 mbar), laminar, premixed flames at an equivalence ratio of 2.0. The blended fuels were formed by incrementally adding 4% wt of oxygen (ethanol) to the neat benzene flame and by keeping the inert mole fraction (argon) and the equivalence ratio constants. Special emphasis was directed toward the causes for the concentration-dependent influence of the blends on the amount of polycyclic aromatic hydrocarbons (PAHs) formed. The effects of oxygenate addition to the benzene base flame were seen to result in interesting differences, especially regarding trends to form PAH. The modeling results indicated that the concentration of acetylene and propargyl radicals, the main PAH precursors, as well as the PAH amounts were lower in the flame of the ethanol−benzene fuel mixture than in the pure benzene flame and that all of the formed PAHs were issued from the phenyl radical. Finally, the modeling results provided evidence that the PAH reduction was a result of simply replacing “sooting” benzene with “nonsooting” ethanol without influencing the combustion chemistry of the benzene. emissions from combustion processes.9 The addition of a variety of oxygenated compounds to diesel fuel has been reported by numerous researchers. The compounds studied include ethanol and tert-butyl alcohol, dimethyl and diethyl carbonates, dimethyl ether and other ethers, diglymes, ketones, and esters such as acetates and maleates.10−36 All of these studies agree on the fact that from the chemical point of view, there is an urgent need to (i) define the key reaction mechanisms responsible for observed reductions in PAHs, PM, unburned hydrocarbons, and carbon monoxide when oxygenated fuels are used as replacements for conventional fuels and (ii) understand the processes leading to potential increases in the emissions of other regulated hazardous air pollutants including aldehydes (formaldehyde, acetaldehyde, and propanal) and 1,3-butadiene that may originate from the use of oxygenated fuels.19

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) and soot, as an airborne contaminant in the environment, are generated by combustion processes such as transportation, power generation, and waste incineration. Their escape from these processes into the atmosphere can cause both acute and long-term respiratory effects, and their deposition on soil and plants and in the surface water constitutes an important contamination of the human food chain.1,2 Thus, these compounds (PAHs) in soot are classified as a “known human carcinogen” by the International Agency for Research on Cancer (IARC). In addition to their direct health hazardous effects, strong evidence for the key role of PAHs in the formation of soot has been accumulated in recent years.3−5 Considering a yearly emission rate of 1.6 million tons in combustion processes, PAHs and soot also contribute significantly to global warming. While significant reductions have been achieved in recent years, legislated limits are being steadily tightened,6 and further improvements will require a much more fundamental understanding of soot production and evolution to meet these regulations, which means that soot emissions from combustion processes continue to be a serious environmental concern. Consequently, in the past few years, a large amount of attention has been paid to the emissions of PAHs and soot from fossil fuel combustion,7,8 and the research results have pointed out that modification of fuel composition through the use of additives can significantly reduce particulate matter (PM) © 2012 American Chemical Society

2. SELECTED COMPOUNDS Because of its presence in fuels in significant amounts, both experimental and theoretical investigations of the combustion of the simplest aromatic hydrocarbon benzene have been studied by different research groups around the world over the Received: November 25, 2011 Revised: March 17, 2012 Published: March 19, 2012 3625

dx.doi.org/10.1021/jp211350f | J. Phys. Chem. A 2012, 116, 3625−3642

The Journal of Physical Chemistry A

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past few decades.37−53 However, recent legislative action in the United States has limited its usage to a maximum of 1% due to its carcinogenic effects.37 Despite its limited presence in fuels, it is one of the primary intermediates that form during the combustion of higher aromatics. Furthermore, benzene is closely linked to the sooting process in the practical combustion systems especially under locally oxygen-starved combustion conditions. As mentioned by Therrien et al.,13 such conditions, both premixed and nonpremixed with air, can be found in diesel engines, in the novel direct injection gasoline engines, and in aero-turbines. It was reported that the higher PM (particle number counts and mass) was produced with the direct injection diesel engines and that conventional gasoline engines generate negligible particulate amounts; however, the PM emissions from the novel gasoline direct injection (GDI) engines can be significant.13 Benzene flames have been preferred for all of the abovementioned facts and taking into account that the uncertainties associated with the formation of the first ring can be avoided. Thus, an attractive way to investigate PAH formation pathways is the analysis of one-dimensional laminar premixed lowpressure benzene flame structures, where the first aromatic ring does not have to be formed. Benzene was chosen, in this work, as a representative for aromatic fuels. Among the different alcohols studied, ethanol has attracted widespread interest because it is easily obtained from renewable resources (bioethanol) and because it can be used as a fuel extender for petroleum-derived fuels, as an oxygenate, as an octane enhancer, and as a pure fuel.29,54 Besides, the results of numerous recent studies on practical engines have shown that ethanol with diesel or gasoline provided a significant reduction in particulate emissions, with no substantial increase in other gaseous emissions.55−59 Given the advantages that the presence of ethanol shows on the emission of contaminants and taking into account the technical difficulties on diesel−ethanol blending, it is interesting to carry out research work that allows us to understand the influence of ethanol addition on the behavior of soot precursors, such as small hydrocarbons and aromatic compounds.

The core of the reaction mechanism, used to describe pyrolysis and oxidation of the benzene and PAH formation and oxidation, was gathered from the work of Richter and Howard.46 This model was developed initially for benzene oxidation and included the formation of PAHs, and then, it was extended and tested for the combustion of acetylene and ethylene. The model was developed and tested using species concentration profiles reported in the literature from molecular beam mass spectrometry measurements in four unidimensional laminar premixed low-pressure ethylene, acetylene, and benzene flames at equivalence ratios (Φ) of 0.75 and 1.9 (C2H4),61,62 2.4 (C2H2),63 and 1.8 (C6H6).38,64 As mentioned by the authors, predictive capabilities of the model were found to be at least fair and often good to excellent for the consumption of the reactants, the formation of the main combustion products, and the formation and depletion of major intermediates including radicals. Richter's model describes reactions of species up to C16H10 and consists of 157 chemical species and 872 reactions. This reaction mechanism is provided for low-pressure and atmospheric pressure conditions and takes into account the pressure dependence of chemically activated reactions. To model the ethanol oxidation, additional reactions were added to the core mechanism from Marinov's model.65 The selected reactions from the alcohol kinetic scheme were the initial reactions of the molecules themselves such as hydrogen abstraction and unimolecular decomposition, as well as reactions of the resulting products that eventually produced species present in the benzene mechanism. The ethanol mechanism developed by Marinov65 has been validated against a variety of experimental data sets: laminar flame speed data (obtained from a constant volume bomb and counter flow twin-flame),66 ignition delay data behind a reflected shock wave,67,68 and ethanol oxidation product profiles from a jet-stirred69 and turbulent flow reactor.70 Good agreement was found in modeling of the data sets obtained from the five different experimental systems. Furthermore, it should be mentioned that the required input data were obtained via the combination of the thermodynamic and transport data of the two studied species and that within the combined mechanism, all reactions and values of the rate coefficients were kept unchanged as compared to those in the base mechanisms. The combined mechanism is composed of 172 species and 1045 reactions. To verify that the base mechanisms had not changed in any substantial way by the added reactions, a comparison of benzene oxidation results with the augmented mechanism and the base mechanism was conducted. Only minor differences were observed between the two kinetic schemes. Finally, it is noteworthy that the neat benzene flame has been previously studied experimentally by Vandooren and coworkers48,52 (see Table 1). The blended fuels were formed by incrementally adding 4% wt of oxygen (ethanol) to the neat benzene flame and by keeping the inert mole fraction (argon) and the equivalence ratio constants. It is well known from the literature that the overall flame temperatures (temperature profile) decreased upon increasing the proportion of ethanol in the fuel mixture.13 Thus, to isolate the temperature effects from those of mixture proportions, the temperature profiles of all of the flames were kept nearly constant by adjusting the total cold gas velocity. This task was accomplished by means of the PREMIX code with solving the energy equation.

3. OBJECTIVES As mentioned by Song et al.,9 in engines and even flames, verification of proposed mechanisms for soot reduction is difficult because it requires measurements of radical species, which may be at very low concentrations. Also, it is often difficult to control all experimental parameters to eliminate unwanted effects. Therefore, in the present study, numerical modeling was used as a mean to assess the effect of oxygenates. Specifically, the effects of oxygenated additives on aromatic species, which are known to be precursors to soot, were investigated. The objective of the given study is to investigate the mechanism of ethanol influence on soot formation in premixed rich benzene flames and to determine the mechanism by which aromatic species are reduced. 4. MODELING APPROACH Kinetic modeling was conducted using the PREMIX code from the CHEMKIN II package.60 Mass flow rate through the burner, gas composition, pressure, temperature, and estimated initial solution profile were used as inputs. 3626



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Table 1. Parameters of the Used Ethanol−Benzene Flames composition (mole fractions) flame

equivalence ratio (Φ)

G (10−3 g cm−2 s−1)

C6H6

C2H5OH

O2

Ar

52

2 2 2 2 2 2

3.102 3.203 3.264 3.326 3.651 3.992

0.12 0.1056 0.0931 0.0802 0.0670 0.0534

0 0.0233 0.0471 0.0716 0.0968 0.1225

0.44 0.4311 0.4198 0.4082 0.3963 0.3841

0.44 0.4400 0.4400 0.4400 0.4400 0.4400

neat flame flame with flame with flame with flame with flame with

4% oxygen 8% oxygen 12% oxygen 16% oxygen 20% oxygen

Figure 1. Effect of ethanol on ethylene, acetylene, and propargyl mole fractions.

5. RESULTS AND DISCUSSION

study on the effect of ethanol on propene flames. This disagreement may be due to the difference in the initial conditions; in our work, the equivalence ratio, the inert mole fraction, and the temperature profile were kept constant, whereas in the work of Wang and co-workers, these parameters were allowed to change. In the case of acetylene, the modeling results showed that the shape of the mole fraction profiles was similar in all flames; this parameter increased with distance above the burner, reached a maximum at about 1.25 cm from the burner surface, and decreased thereafter. It is noteworthy that the presence of the peak mole fractions of C2H2 at this distance suggests that these species penetrated into the postflame zone, thus indicating their role in PAH formation chemistry. In comparison with the pure benzene fuel, C2H2 displayed a decrease in mole fraction with an increase in the ethanol amount, especially in the postflame zone. Similar trends were reported by Inal and Senkan71 during their study on the reduction of PAH and soot in premixed n-heptane−air flames by the addition of ethanol and by Wang and co-workers in the case of ethanol−propene flames.19 However, it should be noted that this finding is in disagreement with expectations from the ethylene trends, since the concentration of this latter increased with increasing ethanol proportion and ethylene can decompose to form acetylene.

5.1. Species Mole Fractions. The dependence of ethylene, acetylene, and propargyl radical mole fractions on ethanol proportion in the mixture fuel is depicted in Figure 1. It can be seen that regardless of the ethanol percentage, the concentration of ethylene increased at the beginning of the flame up to its peak value and then decreased in the postflame zone. Besides, C2H4 levels showed a dramatic change in peak height with doping, increasing by a factor of 4 in the case of the lowcontent oxygenated fuels and by a factor of 17 in the case of the high-content ones. This finding is consistent with the results reported by Korobeinichev et al.11 in the case of ethylenedoped ethanol flames and by Therrien et al.13 in the case of ethylbenzene−ethanol blends. Similar trends were also reported by Ergut et al.23 during their study on the PAH formation in one-dimensional premixed fuel-rich atmospheric pressure ethylbenzene and ethyl alcohol flames. It was demonstrated, by means of chemical kinetic computations, that as much as 18% of the ethanol (on a molar basis) decomposed to ethylene at the conditions investigated therein. Thus, as the ethanol percentage is raised in the fuel, more ethanol will be available for conversion to ethylene. However, Wang et al.19 reported a different C2H4 behavior during their 3627



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Figure 2. Effect of ethanol on H, OH, O, and CO mole fractions.

with increasing the percentage of the ethanol in the mixture. Carbon monoxide amounts were the highest in the 0% oxygen flame (0.323) and the lowest in the 20% oxygen flame (0.258), which means a decrease of 20.12%. These trends are in qualitative agreement with the data reported by Inal and Senkan,71 who observed a decrease of 4% in the CO mole fraction upon an increase in the ethanol percentage. However, Therrien et al.13 and Wu et al.24 reported an opposite behavior. The authors postulated that as the proportion of ethanol is increased in the fuel mix, a bigger fraction of the fuel mix generates carbon monoxide, and consequently, the CO mole fraction increased with an increase in the ethanol proportion in the fuel mix. While this prediction is valid for their flames, the situation is different in the ethanol−benzene flames studied here, where the equivalence ratio (Φ = 2), the temperature profile, and the inert gas mole fraction were kept constants. A rise in the radical “H” mole fraction was observed when rising the ethanol proportion in the fuel mix especially at the postflame zone (height above the burner >1.2 cm) (Figure 2). Hydrogen radical mole fractions were the lowest in the 0% oxygen flame (2.11 × 10−3) and the highest in the 20% oxygen flame (3.24 × 10−3), which means an increase of 53.6%. These findings are in disagreement with the data reported by Wu et al.24

Thus, one can expect that with increasing ethylene, a higher level of acetylene should be observed, which is in contrary to our findings. Concerning the propargyl radical (C3H3), the modeling results showed that the same trends were observed regardless of the ethanol proportion in the fuel mixture, in that the C3H3 mole fraction increased upon increasing the height above the burner, passed through a maximum, and then decreased. Besides, C3H3 concentrations tend to be lower as ethanol is added. Because numerous researchers have noted that changes in concentrations of radical species such as H, O, and OH can have a significant impact on the competition between oxidation pathways and pathways leading to aromatic species and soot,24,72,73 the effects of the ethanol addition on the concentrations of these radicals as well as on the carbon monoxide mole fraction were investigated, and the results are depicted in Figure 2. As expected, regardless of the amount of ethanol in the fuel mixture, carbon monoxide was the major combustion product, and its mole fraction increased at the beginning of the flame to reach a plateau at distances beyond 2.2 cm (Figure 2). Besides, the CO mole fraction decreased monotonically and linearly 3628



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Figure 3. Effect of ethanol on phenylacetylene, indene, and naphthalene mole fractions.

and by Song et al.,9 who observed a decrease in the “H” mole fraction upon increasing the ethanol percentage. It is noteworthy that a plausible explanation of this discrepancy is the difference in the initial conditions between our study and the two investigations. While the equivalence ratio, the inert gas mole fraction, and the temperature profile were kept constant in our study, some of these parameters were allowed to change in the Wu and co-workers work (temperature profile was variable) as well as in Song et al. investigation (inert gas and Φ were variables). As in the case of the radical “H”, the radical OH mole fraction was boosted with a rise in the ethanol proportion in the fuel mix especially at the postflame zone (height above the burner >1.2 cm) (Figure 2). The neat benzene flame exhibited the lowest OH radical concentrations (4.982 × 10−4), whereas the flame containing 20% oxygen displayed the highest mole fractions (6.322 × 10−4), which means an increase of 26.9%. In contrast to H and OH radicals, the “O” radical mole fraction at the end of the postflame zone displayed a somewhat strange behavior; it increased to pass through a maximum at 4 wt % O2, then decreased to reach a minimum at 12 wt % O2, and then increased again with increasing the ethanol proportion in the fuel mix (Figure 2). Besides the above-mentioned compounds, the PAHs are proposed as important key species during the formation process of soot, especially concerning their leading role for soot precursor formation. Thus, as for hydrogen, acetylene, propargyl radical, and so on, the dependence of the PAH concentration on the amount of the blending compound is inspected in the following. The modeling data showed clearly that the same PAH species were produced in all of the studied flames, although the amounts of these species decreased as oxygenate levels increased. For both neat benzene and blended ethanol− benzene flames, all of the PAH concentrations exhibited a maximum inside the main oxidation region, suggesting that the oxidation and pyrolytic zones overlap; this is a characteristic

feature of benzene flames.74,75 Differences can be noted in the relative locations of the concentration peaks of some aromatic species. The dominant aromatics were indene, naphthalene, biphenyl, acenaphthalene, anthracene, and phenanthrene (see Figures 3 and 4). Figure 3 portrays the dependence of the phenylacetylene (C8H6), indene (C9H8), and naphthalene (C10H8) mole fractions on the ethanol percentage. It can clearly be seen that regardless the ethanol proportion, the C8H6 mole fraction increased to reach a maximum and then declined. With increasing ethanol percentage, the phenylacetylene mole fraction was lowered. The addition of oxygenate significantly reduced the mole fractions of C8H6 up to about 70.6% lowering, with respect to the levels in neat benzene flame, was observed. On the other hand, it is clear that for all flames, indene (C9H8) peaked at or above 0.83 cm from the burner surface (Figure 3) and that the maximum mole fractions of this species were more than three times higher in the case of the neat benzene flame as compared to the flame containing 20% oxygen. In addition, it can be seen from the data depicted in Figure 3 that the peak concentrations of naphthalene occurred at nearly the same distance for the six studied fuels and that up to 86% reduction was obtained in this species mole fraction with an increase in oxygenate concentration in fuel blends (Figure 3). Similar qualitative trends were reported by Inal and Senkan71 during their study on the effect of oxygenate concentration on species mole fractions in premixed n-heptane flames. Concerning acenaphthalene (C12H8) (Figure 4), the obtained results showed that whatever the ethanol concentration in the fuel mix, the mole fraction of this compound increased to reach a maximum, and then, it decreased slightly for distances above the burner surface higher than 1.4 cm. Besides, it is shown that acenaphthalene concentration displayed a decreasing trend upon increasing ethanol fraction: mole fractions were the highest in the benzene neat flame (7.996 × 10−4) and the lowest in the 20% oxygen-blended 3629



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Figure 4. Effect of ethanol on acenaphthalene, biphenyl, anthracene, and phenanthrene mole fractions.

flame (1.335 × 10−4), which means a decrease of 83.3%. On the other hand, it was found that the addition of the oxygenated compound decreased the biphenyl (C12H10) peak concentration by 7% in the case of the low content oxygenated fuel (fuel with 4% oxygen) and by 33% in the case of the high content ones (fuel with 20% oxygen). Anthracene and phenanthrene (C14H10) showed similar trends. For all flames, anthracene (C14H10) peaked at or above 1.08 cm from the burner surface, whereas the peaks of phenanthrene concentrations were localized at or above 1.02 cm (Figure 4). Pronounced decreases upon ethanol addition were observed for the two species, and up to 91% reductions were obtained for both pollutants. 5.2. Pathway Analysis. Because of the complexity of chemical and physical conditions, a reaction flux analysis was performed to track the main reaction pathways of the fuel blends, as well as the competing (i.e., formation and decomposition) reactions for the selected light gases and aromatic compounds, which are suspected soot precursors. In addition, identification of any differences observed between the blended fuels and the neat benzene flame was analyzed. The analysis was performed using the appropriate subroutines in the Chemkin package (CKQYP, CKCONT), which systematically compute the rate of production/consumption of each species.60

According to Lamoureux et al.,76 the involved reactions, for species k, are sorted out with respect to their maximum absolute rate, and their sign indicated the species consumption or formation. In this respect and to describe the benzene and the ethanol−benzene oxidation pathways, rates of consumption and production were computed for every species. It is noteworthy that in this section, only the main reactions that have an important role in chemicals belonging to the studied system will be presented. The numbers in parentheses correspond to the numbers of reaction in the combined ethanol−benzene mechanism. In the case of the neat benzene flame and 4% oxygenblended fuel mix, analysis of the benzene consumption flux shows hydrogen abstraction with OH [C6H6 + OH = C6H5 + H2O (647)], H [C6H6 + H = C6H5 + H2 (644)], and O [C6H6 + O = C6H5O + H (646)] leading to phenyl and phenoxy radicals and unimolecular decomposition to propargyl radical [C3H3 + C3H3 = C6H6 (reverse of Rev 638)] to be the dominant depletion pathways, whereas the nonabstractive channel leading to phenol formation via the hydroxyl attack on the ring [C6H6 + OH = C6H5OH + H (652)] appeared to play a minor role (approximately 2.4% of initial benzene was consumed by this reaction). These results are in agreement with the data reported by Richter et al.46 The authors found 3630



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that hydrogen abstraction via reaction 647 was the most important in the benzene consumption pathway and that formation of phenol and hydrogen radicals, via reaction 652, did not contribute significantly to the overall reaction rate of C6H6 + OH. This latter observation was also reported by Madronich and Felder.77 In addition, it is well-known from the open literature that O atom addition to C6H6 leads to a C6H6O adduct, which either can follow a rearrangement to produce phenol or suffer a ring hydrogen abstraction resulting in phenoxy. Alzueta et al.44 and Bajaj and Fontijn78 concluded from a flow reactor study that phenoxy is the main primary product of this step, which confirms our results. The same reactions governed the benzene depletion in 8−20% oxygen-blended fuels; however, the contributions of them were changed. In the case of 8% oxygen, the reactions 647 and 644 contributed with 38.20 and 25.97%, respectively, whereas their contributions, in the case of blended fuel mix containing 20% oxygen, were 41.51 and 29.56%, respectively. On the other hand, the obtained data indicated that for blended fuels, the unimolecular C6H6 decomposition leading to C3H3 radicals (Rev 638) was more important than the benzene reaction with O atoms (646) and that the contribution of these two reactions to the benzene consumption decreased with increasing the ethanol proportion in fuel mix. From these results, it could be concluded that regardless of the ethanol proportion in the fuel mix, phenyl, phenoxy, and to some extent propargyl radicals were the main intermediates formed during benzene depletion. The unimolecular decomposition to ethylene and water step was found to be dominant for the initiation reactions of ethanol conversion. In the case of 4% oxygen flame, about 64% of ethanol was consumed via this reaction, whereas this pathway was responsible for the decomposition of about 70% of the ethanol present in the 20% oxygen flame. The amount of ethanol consumed by unimolecular decomposition in the 8, 12, and 16% oxygen flames fell in between these values, leading to the fact that the higher the ethanol concentration in the fuel blend the higher the contribution of this decomposition pathway. These findings are in contrast to those reported by Therrien et al.,13 who found a decreasing trend of this reaction with an increase of the ethanol proportion in the fuel mix. This discrepancy is potentially due to the difference in the fuels burned as well as in the combustion conditions between our work and the study of Therrien and co-workers. In the case of 0−12% oxygen-blended fuels, reaction flux analysis of acetylene formation revealed a unimolecular decomposition of the cyclopentadienyl radical [C5H5 = C3H3 + C2H2 (574)] as the most important reaction. In addition, it was found that the following set of reactions contributed to acetylene production: n‐C4 H3 + C2H2 = C6H 4 + H HCCCHO = C2H2 + CO

The independence of the C2H2 formation on the C2H4 concentration could explain the fact that although the ethylene concentration increased with increasing the ethanol proportion in the mixture, the acetylene mole fraction showed an opposite behavior. On the other hand, the flux analysis results showed that whatever the ethanol concentration in the fuel mix, the majority of acetylene consumption occurred through oxygen atom attack, leading to HCCO, HCH, CO, and H:

(623)

(210)

C3H3 + C3H3 = C6H6

(Rev 638)

C5H5 = C3H3 + C2H2

(574) (Rev 312)

These results are in accordance with the findings of Detilleux and Vandooren,58 who reported that reaction Rev 638 was the predominant channel in producing C3H3, and with those of Vourliotakis and co-workers,53 who reported that the propargyl radical was mainly a product of C5 chemistry, through isomerization of cyclopentadienyl to the 1-pentyn-3-en-5-yl radical and unimolecular decomposition of the latter. Three reactions were seen to be about equally important in controlling the consumption of C3H3. As in the case of the formation reactions, these reactions were lowered upon raising the ethanol proportion: C3H3 + H = C3H2 + H2

(313)

C3H3 + OH = CH2CHCHO

(333)

C3H3 + O = C2H + CH2O

(323)

From these results, it is obvious that an increase in the ethanol concentration led to a decrease in both formation and consumption reactions of C3H3. However, the net effect [summation of the effect on the formation reactions minus summation of the effect on the consumption reactions (see Figure 5)] exhibited a decreasing trend upon increasing benzene replacement percentage by oxygenate additive, which means that the decrease in the formation reactions was more noticeable than the one of the consumption ones, and consequently, the C 3H 3 concentration decreased upon increasing the ethanol amount. Concerning phenylacetylene (C8H6), it was found that in mixtures containing from 0 to 8% oxygen, this species was predominantly formed by H abstraction from H2 by ethynylphenylene (reverse of reaction Rev 729), by the direct

The rates of these reactions exhibited a decreasing trend with increasing ethanol proportion in the fuel mix (this decrease was 33.7, 15.5, and 27.7% for 574, Rev 623, and 329, respectively). However, in the case of mixtures containing 16 and 20% oxygen, the predominant channels of C2H2 formation were the two reactions, which displayed a decreasing trend with a rise in the ethanol amount:

n‐C4 H3 + C2H2 = C6H 4 + H

C2H2 + O = HCH + CO

C3H3 = C3H2 + H

(329)

(213)

(211)

Increasing the amount of ethanol led to a lower consumption rates. From these results, it is obvious that raising the ethanol initial concentration induced a decrease in the rates of both formation and consumption reactions of acetylene. However, the first reactions were more influenced than the latter ones, which led to a lowering in the acetylene concentration upon raising the ethanol amount. Our computation showed that the propargyl radical (C3H3) was mainly formed by the unimolecular decomposition of benzene and cyclopentadienyl radical as well as the combination of propargylen triplet (C3H2) and hydrogen radicals. Rates of these reactions were lowered with increasing the concentration of the blending compound:

(Rev 623)

C2H3 = C2H2 + H

C2H2 + O = HCCO + H

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Figure 5. Effect of ethanol on C3H3 formation and consumption reactions.

C2H2 attack on the benzene ring (728), and finally by the H atom addition to ethynylphenylene (732) via the following series of reactions: C8H6 + H = C8H5 + H2

Scheme 1. Sequence Reactions for the Formation of Phenylacetylene (C8H6) in the Case of 0−8% OxygenBlended Fuels

(Rev 729)

C6H5 + C2H2 = C8H6 + H

(728)

C8H5 + H = C8H6

(732)

Hydrogen atoms abstraction from H2 (Rev 729) accounted for 52% of the phenylacetylene formation, whereas the bimolecular reaction of the phenyl radical with acetylene (728) contributed with 37% and the addition of ethynylphenylene (C8H5) and hydrogen radical (732) accounted for 11%. Besides, the pathway investigation demonstrated that ethynylphenylene formation occurred by the reaction sequence: C10H8 + H = C10H7 + H2, C10H7 = C8H5 + C2H2, which suggested that phenylacetylene was a product issued from naphthalene (see Scheme 1). 3632



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Figure 6. Effect of ethanol on C8H6 formation and consumption reactions.

It is noteworthy that the higher the ethanol proportion in the fuel mix, the lower the phenylacetylene formation and consumption reactions rates. The effect on production rates outweighed the one on the consumption reactions rates, which led to a decrease in the C8H6 amount (Figure 6). Calculations indicated that indene (C9H8) was exclusively formed via the reaction of indenyl (C9H7) with the radical H:

In the case of fuels blended with 12−20% oxygen, the reaction 728 surpassed Rev 729, and it contributed with 47% for 12% oxygen. These findings are in disagreement with those found by Castaldi et al.,79 who reported that the phenylacetylene production pathway occurs by ethylene addition to phenyl to form styrene and H-atom followed by styrene dehydrogenation by H-atoms. This discrepancy might be ascribed to the difference in the nature of the used fuel; in our study, an aromatic hydrocarbon (benzene) was used, whereas in the work of Castaldi and co-workers, an olefinic hydrocarbon (ethylene) was studied. When formed, phenylacetylene might subsequently undergo aryl H atom abstraction or C atom abstraction giving ethynylphenylene (730) and phenylcarbene (C6H5CH) radical (740), respectively: C8H6 + OH = C8H5 + H2O

(730)

C8H6 + O = C6H5CH + CO

(740)

indenyl + H = indene

(805)

A snapshot of indene formation paths, as given by means of the pathway analysis of the studied neat and blended fuels, is depicted in Figure 7. It can be seen that indene is a product of naphthalene (C10H8). Similar trends were reported by Castaldi et al.79 during their study on aromatic and PAH formation in a premixed ethylene flame. The authors mentioned that indene formation could occur by the reaction sequence: C10H8 + H = C10H7 + H2, C10H7 + O2 = C10H7O + O, C10H7O = indenyl + CO, and indenyl + H = indene. Once formed, indene was mainly consumed via its reactions with H, OH, and O radicals, giving benzyl radical (C6H5CH2), acetylene, indenyl, molecular hydrogen, water, and hydroxyl radical:

This phenylacetylene consumption route is in qualitative agreement with the results reported by Richter et al.,46 who mentioned that oxidation of phenylacetylene by O radicals leading to phenylcarbene had only a limited effect on phenylacetylene profiles in the postflame zone.

C6H5CH2 + C2H2 = indene + H 3633

(Rev 806)



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replacement percentage by ethanol. However, the effect was more pronounced in the case of the first reactions, which led to a net decrease in the indene amount (Figure 8). The modeling results indicated that the sole naphthalene formation route was the bimolecular self-combination of the cyclopentadienyl radicals (C5H5): C5H5 + C5H5 = C10H8 + 2H

The importance of resonance-stabilized radicals, such as cyclopentadienyl, was first identified by Marinov et al.80 when investigating PAH formation in methane and ethylene flames through both experimental and detailed kinetic modeling studies. In their study, the insufficiency of the acetylene addition processes in predicting the experimentally observed PAH levels was mentioned, and the inclusion of the naphthalene formation via cyclopentadienyl self-combination made their kinetic model precisely predictable. On the other hand, several authors have reported that, in addition to the HACA mechanism, the aromatic growth can proceed through different routes, and cyclopentadienyl (C5H5) moieties have been proposed as potential precursors due to their neutrality and reactivity at different sites.4,81−84 Particularly, Melius and co-workers,82 based on theoretical approach, proposed a nine-step

Figure 7. Indene formation pathway.

indene + H = indenyl + H2

(802)

indene + OH = indenyl + H2O

(803)

indene + O = indenyl + OH

(804)

(771)

As in the case of phenylacetylene, both the formation and the consumption reactions decreased with increasing the benzene

Figure 8. Effect of ethanol on indene formation and consumption reactions. 3634



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Scheme 2. Naphthalene (C10H8) Formation−Consumption Sequence Reactions

(see Scheme 2). In ethanol−benzene flames containing 20% oxygen, contributions of these reactions were 28.3, 26.2, 27.9, and 17.6%, respectively. It is noteworthy that in the case of 16 and 20% oxygen-blended fuels, the effect of the reaction 776 on naphthalene consumption was more pronounced than the one of 779. As for the other studied compounds, the modeling data showed an inhibiting effect of ethanol on consumption reactions rates. This effect was more pronounced in the case of the formation reactions leading to the fact that high concentrations of the blending compound favor the suppression of naphthalene (Figure 9). The obtained results showed that regardless of the ethanol amount, acenaphthalene (noted A2R5) formation was ascribed to the decomposition reaction of the BIPHENH radical:

reaction mechanism for the conversion of two cyclopentadienyl radicals to naphthalene via rearrangements involving threemembered ring closing and opening of resonance-stabilized radicals. This observation on the naphthalene formation route was also confirmed by Richter et al.,85 who reported that C5H5 radical self-recombination was the major pathway to naphthalene in benzene premixed flame. To gain more insight in naphthalene formation, the cyclopentadienyl radicals pathway was investigated. The modeling results showed that these species were issued from the reaction sequence: C6H5 + O2 = C6H5O + O

(629)

C6H6 + O = C6H5O + H

(646)

C6H5O = C5H5 + CO

(668)

BIPHENH = A2R5 + H

This means that the key step in the naphthalene production process was the phenyl oxidation by O2 as well as the benzene attack with O atoms (see Scheme 2). The importance of the reaction 629 in the PAH growth process was evoked by Castaldi and co-workers during their study on the formation of PAH in premixed ethylene flames.83 These observations suggest that the amount of naphthalene is directly related to the phenyl concentration. However, our modeling results indicated that the phenyl amount displayed a decreasing trend upon increasing the ethanol proportion in the fuel mix, and consequently, the naphthalene would behave similarly, and its concentration would be lowered with a rise in the benzene percentage replacement by oxygenate additive. Once formed, naphthalene was subsequently attacked by OH and H radicals leading to naphthyl radical (C10H7) isomers:

(827)

The rate of this reaction decreased with increasing ethanol proportion, and a rate decrease up to 73.6% was observed in the case of 20% oxygen-blended fuel as compared to the neat benzene flame (Figure 10). In addition, our modeling results indicated that acenaphthalene formation involved the reaction sequence: unimolecular decomposition of the phenyl radical 636 to give hydrogen atoms and benzyne (C6H4). Once formed, C6H4 underwent a self-combination 824 to yield biphenylene (noted BIPHEN), which at its turn underwent a reaction addition leading to the BIPHENH radical (reverse of the reaction Rev 825). Finally, the radical BIPHENH was decomposed to give acenaphthalene and hydrogen atoms via the reaction 827 (see Scheme 3): C6H5 = C6H 4 + H

(636)

(779)

C6H 4 + C6H 4 = BIPHEN

(824)

C10H8 + H = C10H7_1 + H2

(776)

BIPHENH = BIPHEN + H

C10H8 + H = C10H7_2 + H2

(777)

C10H8 + OH = C10H7_1 + H2O

(778)

C10H8 + OH = C10H7_2 + H2O

BIPHENH = A2R5 + H

In neat benzene flame, the contribution of the reaction 778 to the naphthalene decay was 30.3%, whereas the reactions 779, 776, and 777 accounted for 28.7, 23.7, and 17.2%, respectively

(Rev 825) (827)

This sequence suggests that acenaphthalene is mainly issued from the phenyl radical. 3635



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Figure 9. Effect of ethanol on naphthalene formation and consumption reactions.

On the other hand, it was found that whatever the ethanol amount, acenaphthalene was transformed to the naphthyl radical (C10H7_1) via its reaction with H atoms:

Scheme 3. Sequence Reactions for the Formation of A2R5 via C6H5

C10H7_1 + C2H2 = A2R5 + H

(Rev 828)

Scheme 4. Sequence Reactions for the Formation of Biphenyl (C12H10)

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Figure 10. Effect of ethanol on acenaphthalene formation and consumption reactions.

As in the case of the formation reaction, this reaction seems to be lowered by a rise in the ethanol proportion in the fuel mix (Figure 10). Concerning biphenyl, calculations showed that this species formation was governed by the following set of reactions (see Scheme 4): C6H5 + C6H6 = C12H10 + H

(765)

C6H5 + C6H5 = C12H10

(764)

C12H 9 + H = C12H10

(770)

In the neat benzene flame, the reaction between the phenyl radical and the benzene accounted for 70.3%, whereas the phenyl radicals self-recombination contributed with 16.4%, and finally, the addition reaction of hydrogen atoms with C12H9 radical accounted for 13.2%. These results are in accordance with those reported by Wang and Frenklach86 and by Appel and co-workers,87 who mentioned that the biphenyl (C12H10) formation from the addition of benzene (C6H6) to phenyl (C6H5) was the most important channel. The importance of this reaction was confirmed, especially under fuel-rich and highpressure88 conditions.

Figure 11. Effect of ethanol on biphenyl formation and consumption reactions. 3637



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Scheme 5. Sequence Reactions for the Formation of Phenanthrene via C6H6 and C6H5C2H (C8H6) Addition87

Figure 12. Effect of ethanol on phenanthrene formation and consumption reactions.

similarly, and a slight rise was observed until 4% oxygen followed by a monotonic decay. Finally, it is noteworthy that the amount of oxygen led to greater reductions in both net formation and consumption rates. However, the lowering in the net biphenyl formation was more noticeable than the one on the net consumption, inducing a net lowering in the biphenyl amount with a rise in ethanol concentration in the fuel mix (Figure 11). The modeling results indicated that in the case of the neat benzene flame and ethanol−benzene fuel containing 4% oxygen, the main reactions governing phenanthrene (noted A3: C14H10) formation were as follows:

After being formed, the prominent channels for the biphenyl consumption were its reactions with OH and H radicals (see Scheme 4): C12H10 + OH = C12H 9 + H2O

(767)

C12H10 + H = C12H 9 + H2

(766)

Contribution of the reaction with OH radical was found to be 59.6%, whereas that of the second reaction 766 was 40.6%. Flux analysis indicated that a monotonic decrease was observed on the reaction 765, whereas the other two formation reactions 764 and 770 increased to pass through a maximum at 4% oxygen, and then, they decreased monotonically. On the other hand, the two biphenyl consumption reactions behaved

indenyl + C5H5 = A3 + 2H 3638

(838)



Article

Figure 13. Effect of ethanol on anthracene formation and consumption reactions.

A1C2H_2 + C6H6 = A3 + H

(846)

A3_9 + H = A3

(858)

A3_2 + H = A3

(856)

A3_4 + H = A3

(857)

C12H 9 + C2H2 = A3 + H

(837)

A3_1 + H = A3

(855)

C8H6 + C6H5 = A3 + H

(845)

In the case of ethanol-blended fuel containing 8−16% oxygen, the same set of reactions governed phenanthrene formation; however, the reaction 837 surpassed the reaction 857 in the case of 8−12% oxygen. In the case of 20% oxygen, the primary route for the formation of phenanthrene is predicted to occur through the reaction 846, and alternative routes evolving A3_x, C12H9, and C8H6 species were found to be of less importance. These findings are in agreement with the results collected by Marinov and co-workers79,80,89 during their study on the PAH 3639



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the consumption ones, which induced a net lowering in the amount of anthracene (Figure 13).

formation in methane and ethylene flames. The authors have mentioned that the fusion of two five-membered rings in the reaction of indenyl with cyclopentadienyl (838) was the major route in the phenanthrene formation. On the other hand, Appel and co-workers87 reported that phenanthrene (A3) was primarily formed via ring−ring “condensation” reactions and mostly via the sequence (Scheme 5). The authors pointed out that this reaction pathway was initiated by a step analogous to the biphenyl (C12H10) formation from the addition of benzene (C6H6) to phenyl (C6H5) and that the H migration, presumed to be sufficiently rapid, makes it a more “direct” process. After its formation, the phenanthrene predominant consumption channels were its attack by OH and H radicals as well as its isomerization to anthracene: A3 + OH = A3_1 + H2O

(851)

A3 + OH = A3_4 + H2O

(853)

A3 + H = A3_1 + H2

(847)

A3 + OH = A3_9 + H2O

(854)

A3 + OH = A3_2 + H2O

(852)

A3 + H = A3_4 + H2

(849)

A3L = A3

6. CONCLUSIONS This study provides supporting evidence of the following: • Regardless of the ethanol proportion in the fuel mixture, the shape of the mole fraction profiles of ethylene, acetylene, and propargyl radical increased with increasing the height above the burner to reach a maximum and then began to decline. Upon raising the ethanol content in the fuel mix, the C2H4 mole fraction displayed an increasing trend, whereas concentrations of C2H2 and C3H3 exhibited an opposite behavior. • As expected, carbon monoxide was the major combustion product, and its mole fraction increased steadily with distance from the burner surface and leveled off at about 2.2 cm. Surprisingly, the CO mole fraction decreased monotonically and linearly with increasing the percentage of the ethanol in the mixture, and up to 20% lowering was observed in the case of 20% oxygen flame as compared to the neat benzene flame. In contrast, up to 53.6 and 26.9% rising were observed in the case of hydrogen atoms and OH radical, respectively. • For both neat and ethanol-blended benzene flames, all calculated PAH components peaked inside the main reaction zone. Ultimately, the mole fractions of some components subsided, whereas those of other components remained nearly constant in the postflame region. The dominant aromatics were indene, naphthalene, biphenyl, acenaphthalene, anthracene, and phenanthrene. • Analysis of the main pathways of the reactions leading to benzene and ethanol depletion in all flames indicated that regardless of the ethanol proportion in the fuel mix, phenyl, phenoxy, and to some extent propargyl radicals were the main intermediates formed during benzene depletion. On the other hand, the unimolecular decomposition to ethylene and water step was found to be dominant for the initiation reactions of ethanol conversion. • Pathway analysis of the reactions leading to PAH formation showed that both phenylacetylene and indene were products of naphthalene whose production process key step was the phenyl oxidation by O2. Also, acenaphthalene and biphenyl were mainly issued from the phenyl radical. On the other hand, it was found that the fusion of two five-membered rings in the reaction of indenyl with cyclopentadienyl was the major route in the phenanthrene formation and that the main formation routes of anthracene were the reverse of the phenanthrene isomerization reaction as well as the combination reactions of hydrogen radical with A3L isomers. • PAH concentrations displayed a decreasing trend upon increasing the ethanol percentage. As all of the formed PAHs were issued from the phenyl radical, it could be concluded that the PAH reduction was a result of a simply replacing “sooting” benzene with “nonsooting” ethanol without influencing the combustion chemistry of the benzene.

(Rev 872)

A3 + H = A3_9 + H2

(850)

A3 + H = A3_2 + H2

(848)

As can be seen, from the data depicted in Figure 12, phenanthrene formation and consumption reactions rates exhibited decreasing influence with increasing initial ethanol concentrations. This effect was more pronounced in the case of the formation reactions, leading to a net decrease in the phenanthrene amount. Modeling results indicated that anthracene (noted A3L) was mainly formed via the phenanthrene isomerization reaction Rev 872 as well as the combination reactions of hydrogen atoms with A3L isomers (871, 868, and 865): A3L = A3

(Rev 872)

A3L9 + H = A3L

(871)

A3L2 + H = A3L

(868)

A3L1 + H = A3L

(865)

On the other hand, anthracene was consumed by means of hydrogen abstraction reactions with OH and H radicals: A3L + OH = A3L_1 + H2O

(864)

A3L + OH = A3L_2 + H2O

(867)

A3L + H = A3L_1 + H2

(863)

A3L + OH = A3L_9 + H2O

(870)

A3L + H = A3L_2 + H2

(866)

A3L + H = A3L_9 + H2

(869)

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+213)7 73 21 39 69. Fax: (+213)32 42 10 36. E-mail: [email protected].

All of these reactions displayed decreasing trends upon raising the benzene replacement by the oxygenate additive. However, the formation reactions were more influenced by ethanol than

Notes

The authors declare no competing financial interest. 3640



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ACKNOWLEDGMENTS We acknowledge the “Centre de Développement des Energies Renouvelables” (CDER)-Algérie for its financial support.

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Reduction of PAH and Soot Precursors in Benzene Flames by

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