Reduction of PAH and Soot Precursors in Benzene Flames by Addition of Ethanol

Reduction of PAH and Soot Precursors in Benzene Flamesby Addition of EthanolDjemaa Golea, Yacine Rezgui,* Miloud Guemini, and Soumia Hamdane

Laboratoire de Chimie Appliquee et Technologie des Materiaux, Universite d'Oum El Bouaghi, B.P. 358, Route de Constantine,Oum El Bouaghi 04000, Algerie

ABSTRACT: A one-dimensional premixed flame model(PREMIX) and schemes resulting from the merging ofvalidated kinetic schemes for the oxidation of the componentsof the present mixtures (benzene and ethanol) were used toinvestigate the effect of oxygenated additives on aromaticspecies, which are known to be soot precursors, in fuel-richbenzene combustion. The specific flames were low-pressure(45 mbar), laminar, premixed flames at an equivalence ratio of2.0. The blended fuels were formed by incrementally adding4% wt of oxygen (ethanol) to the neat benzene flame and bykeeping the inert mole fraction (argon) and the equivalenceratio constants. Special emphasis was directed toward thecauses 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, especiallyregarding trends to form PAH. The modeling results indicated that the concentration of acetylene and propargyl radicals, themain PAH precursors, as well as the PAH amounts were lower in the flame of the ethanol−benzene fuel mixture than in the purebenzene flame and that all of the formed PAHs were issued from the phenyl radical. Finally, the modeling results providedevidence that the PAH reduction was a result of simply replacing “sooting” benzene with “nonsooting” ethanol withoutinfluencing the combustion chemistry of the benzene.

1. INTRODUCTIONPolycyclic aromatic hydrocarbons (PAHs) and soot, as anairborne contaminant in the environment, are generated bycombustion processes such as transportation, power generation,and waste incineration. Their escape from these processes intothe atmosphere can cause both acute and long-term respiratoryeffects, and their deposition on soil and plants and in thesurface water constitutes an important contamination of thehuman food chain.1,2 Thus, these compounds (PAHs) in sootare classified as a “known human carcinogen” by theInternational Agency for Research on Cancer (IARC). Inaddition to their direct health hazardous effects, strongevidence for the key role of PAHs in the formation of soothas been accumulated in recent years.3−5 Considering a yearlyemission 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 furtherimprovements will require a much more fundamental under-standing of soot production and evolution to meet theseregulations, which means that soot emissions from combustionprocesses continue to be a serious environmental concern.Consequently, in the past few years, a large amount of attentionhas been paid to the emissions of PAHs and soot from fossilfuel combustion,7,8 and the research results have pointed outthat modification of fuel composition through the use ofadditives can significantly reduce particulate matter (PM)

emissions from combustion processes.9 The addition of avariety of oxygenated compounds to diesel fuel has beenreported by numerous researchers. The compounds studiedinclude ethanol and tert-butyl alcohol, dimethyl and diethylcarbonates, dimethyl ether and other ethers, diglymes, ketones,and esters such as acetates and maleates.10−36 All of thesestudies agree on the fact that from the chemical point of view,there is an urgent need to (i) define the key reactionmechanisms responsible for observed reductions in PAHs,PM, unburned hydrocarbons, and carbon monoxide whenoxygenated fuels are used as replacements for conventionalfuels and (ii) understand the processes leading to potentialincreases in the emissions of other regulated hazardous airpollutants including aldehydes (formaldehyde, acetaldehyde,and propanal) and 1,3-butadiene that may originate from theuse of oxygenated fuels.19

2. SELECTED COMPOUNDSBecause of its presence in fuels in significant amounts, bothexperimental and theoretical investigations of the combustionof the simplest aromatic hydrocarbon benzene have beenstudied by different research groups around the world over the

Received: November 25, 2011Revised: March 17, 2012Published: March 19, 2012

Article

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past few decades.37−53 However, recent legislative action in theUnited States has limited its usage to a maximum of 1% due toits carcinogenic effects.37 Despite its limited presence in fuels,it is one of the primary intermediates that form during thecombustion of higher aromatics. Furthermore, benzene isclosely linked to the sooting process in the practical com-bustion systems especially under locally oxygen-starvedcombustion conditions. As mentioned by Therrien et al.,13

such conditions, both premixed and nonpremixed with air, canbe found in diesel engines, in the novel direct injection gasolineengines, and in aero-turbines. It was reported that the higherPM (particle number counts and mass) was produced with thedirect injection diesel engines and that conventional gasolineengines generate negligible particulate amounts; however, thePM emissions from the novel gasoline direct injection (GDI)engines can be significant.13

Benzene flames have been preferred for all of the above-mentioned facts and taking into account that the uncertaintiesassociated with the formation of the first ring can be avoided.Thus, an attractive way to investigate PAH formation pathwaysis the analysis of one-dimensional laminar premixed low-pressure benzene flame structures, where the first aromatic ringdoes 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 renewableresources (bioethanol) and because it can be used as a fuelextender for petroleum-derived fuels, as an oxygenate, as anoctane enhancer, and as a pure fuel.29,54 Besides, the results ofnumerous recent studies on practical engines have shown thatethanol with diesel or gasoline provided a significant reductionin particulate emissions, with no substantial increase in othergaseous emissions.55−59

Given the advantages that the presence of ethanol shows onthe emission of contaminants and taking into account thetechnical difficulties on diesel−ethanol blending, it is interestingto carry out research work that allows us to understand theinfluence of ethanol addition on the behavior of sootprecursors, such as small hydrocarbons and aromaticcompounds.

3. OBJECTIVES

As mentioned by Song et al.,9 in engines and even flames,verification of proposed mechanisms for soot reduction isdifficult because it requires measurements of radical species,which may be at very low concentrations. Also, it is oftendifficult to control all experimental parameters to eliminateunwanted effects. Therefore, in the present study, numericalmodeling was used as a mean to assess the effect of oxygenates.Specifically, the effects of oxygenated additives on aromaticspecies, which are known to be precursors to soot, wereinvestigated. The objective of the given study is to investigatethe mechanism of ethanol influence on soot formation inpremixed rich benzene flames and to determine the mechanismby which aromatic species are reduced.

4. MODELING APPROACH

Kinetic modeling was conducted using the PREMIX code fromthe CHEMKIN II package.60 Mass flow rate through theburner, gas composition, pressure, temperature, and estimatedinitial solution profile were used as inputs.

The core of the reaction mechanism, used to describepyrolysis and oxidation of the benzene and PAH formation andoxidation, was gathered from the work of Richter andHoward.46 This model was developed initially for benzeneoxidation and included the formation of PAHs, and then, it wasextended and tested for the combustion of acetylene andethylene. The model was developed and tested using speciesconcentration profiles reported in the literature from molecularbeam mass spectrometry measurements in four unidimensionallaminar premixed low-pressure ethylene, acetylene, andbenzene 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 mentionedby the authors, predictive capabilities of the model were foundto be at least fair and often good to excellent for theconsumption of the reactants, the formation of the maincombustion products, and the formation and depletion ofmajor intermediates including radicals. Richter's modeldescribes reactions of species up to C16H10 and consists of157 chemical species and 872 reactions. This reactionmechanism is provided for low-pressure and atmosphericpressure conditions and takes into account the pressuredependence of chemically activated reactions. To model theethanol oxidation, additional reactions were added to the coremechanism from Marinov's model.65 The selected reactionsfrom the alcohol kinetic scheme were the initial reactions of themolecules themselves such as hydrogen abstraction andunimolecular decomposition, as well as reactions of theresulting products that eventually produced species present inthe benzene mechanism.The ethanol mechanism developed by Marinov65 has been

validated against a variety of experimental data sets: laminarflame speed data (obtained from a constant volume bomb andcounter flow twin-flame),66 ignition delay data behind areflected shock wave,67,68 and ethanol oxidation productprofiles from a jet-stirred69 and turbulent flow reactor.70

Good agreement was found in modeling of the data setsobtained from the five different experimental systems.Furthermore, it should be mentioned that the required inputdata were obtained via the combination of the thermodynamicand transport data of the two studied species and that withinthe combined mechanism, all reactions and values of the ratecoefficients were kept unchanged as compared to those in thebase mechanisms. The combined mechanism is composed of172 species and 1045 reactions.To verify that the base mechanisms had not changed in any

substantial way by the added reactions, a comparison ofbenzene oxidation results with the augmented mechanism andthe base mechanism was conducted. Only minor differenceswere observed between the two kinetic schemes.Finally, it is noteworthy that the neat benzene flame has been

previously studied experimentally by Vandooren and co-workers48,52 (see Table 1). The blended fuels were formedby incrementally adding 4% wt of oxygen (ethanol) to the neatbenzene 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 increasingthe proportion of ethanol in the fuel mixture.13 Thus, to iso-late the temperature effects from those of mixture proportions,the temperature profiles of all of the flames were kept nearlyconstant by adjusting the total cold gas velocity. This task wasaccomplished by means of the PREMIX code with solving theenergy equation.

The Journal of Physical Chemistry A Article

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5. RESULTS AND DISCUSSION

5.1. Species Mole Fractions. The dependence of ethylene,acetylene, and propargyl radical mole fractions on ethanolproportion in the mixture fuel is depicted in Figure 1. It can beseen that regardless of the ethanol percentage, the concen-tration of ethylene increased at the beginning of the flame up toits peak value and then decreased in the postflame zone.Besides, C2H4 levels showed a dramatic change in peak heightwith doping, increasing by a factor of 4 in the case of the low-content oxygenated fuels and by a factor of 17 in the case of thehigh-content ones. This finding is consistent with the resultsreported by Korobeinichev et al.11 in the case of ethylene-doped ethanol flames and by Therrien et al.13 in the case ofethylbenzene−ethanol blends. Similar trends were alsoreported by Ergut et al.23 during their study on the PAHformation in one-dimensional premixed fuel-rich atmosphericpressure ethylbenzene and ethyl alcohol flames. It wasdemonstrated, 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, moreethanol will be available for conversion to ethylene. However,Wang et al.19 reported a different C2H4 behavior during their

study on the effect of ethanol on propene flames. Thisdisagreement may be due to the difference in the initialconditions; in our work, the equivalence ratio, the inert molefraction, and the temperature profile were kept constant,whereas in the work of Wang and co-workers, these parameterswere 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; thisparameter increased with distance above the burner, reached amaximum at about 1.25 cm from the burner surface, anddecreased thereafter. It is noteworthy that the presence of thepeak mole fractions of C2H2 at this distance suggests that thesespecies penetrated into the postflame zone, thus indicating theirrole in PAH formation chemistry. In comparison with the purebenzene fuel, C2H2 displayed a decrease in mole fraction withan increase in the ethanol amount, especially in the postflamezone. Similar trends were reported by Inal and Senkan71 duringtheir study on the reduction of PAH and soot in premixedn-heptane−air flames by the addition of ethanol and by Wangand co-workers in the case of ethanol−propene flames.19

However, it should be noted that this finding is in disagreementwith expectations from the ethylene trends, since theconcentration of this latter increased with increasing ethanolproportion and ethylene can decompose to form acetylene.

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

neat flame52 2 3.102 0.12 0 0.44 0.44flame with 4% oxygen 2 3.203 0.1056 0.0233 0.4311 0.4400flame with 8% oxygen 2 3.264 0.0931 0.0471 0.4198 0.4400flame with 12% oxygen 2 3.326 0.0802 0.0716 0.4082 0.4400flame with 16% oxygen 2 3.651 0.0670 0.0968 0.3963 0.4400flame with 20% oxygen 2 3.992 0.0534 0.1225 0.3841 0.4400

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



Thus, one can expect that with increasing ethylene, a higherlevel of acetylene should be observed, which is in contrary toour findings.Concerning the propargyl radical (C3H3), the modeling

results showed that the same trends were observed regardless ofthe ethanol proportion in the fuel mixture, in that the C3H3mole fraction increased upon increasing the height above theburner, passed through a maximum, and then decreased.Besides, C3H3 concentrations tend to be lower as ethanol isadded.Because numerous researchers have noted that changes in

concentrations of radical species such as H, O, and OH canhave a significant impact on the competition between oxidationpathways and pathways leading to aromatic species andsoot,24,72,73 the effects of the ethanol addition on theconcentrations of these radicals as well as on the carbonmonoxide mole fraction were investigated, and the results aredepicted 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 toreach a plateau at distances beyond 2.2 cm (Figure 2). Besides,the CO mole fraction decreased monotonically and linearly

with increasing the percentage of the ethanol in the mixture.Carbon monoxide amounts were the highest in the 0% oxygenflame (0.323) and the lowest in the 20% oxygen flame (0.258),which means a decrease of 20.12%. These trends are inqualitative agreement with the data reported by Inal andSenkan,71 who observed a decrease of 4% in the CO molefraction 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 isincreased in the fuel mix, a bigger fraction of the fuel mixgenerates carbon monoxide, and consequently, the CO molefraction increased with an increase in the ethanol proportion inthe fuel mix. While this prediction is valid for their flames, thesituation is different in the ethanol−benzene flames studiedhere, where the equivalence ratio (Φ = 2), the temperatureprofile, 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 thepostflame 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% oxygenflame (3.24 × 10−3), which means an increase of 53.6%. Thesefindings are in disagreement with the data reported by Wu et al.24

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



and by Song et al.,9 who observed a decrease in the “H” molefraction upon increasing the ethanol percentage. It is note-worthy that a plausible explanation of this discrepancy is thedifference in the initial conditions between our study and thetwo investigations. While the equivalence ratio, the inert gasmole fraction, and the temperature profile were kept constantin our study, some of these parameters were allowed to changein the Wu and co-workers work (temperature profile wasvariable) as well as in Song et al. investigation (inert gas and Φwere variables).As in the case of the radical “H”, the radical OH mole

fraction was boosted with a rise in the ethanol proportion in thefuel mix especially at the postflame zone (height above theburner >1.2 cm) (Figure 2). The neat benzene flame exhibitedthe lowest OH radical concentrations (4.982 × 10−4), whereasthe flame containing 20% oxygen displayed the highest molefractions (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 somewhatstrange behavior; it increased to pass through a maximum at4 wt % O2, then decreased to reach a minimum at 12 wt % O2,and then increased again with increasing the ethanol proportionin the fuel mix (Figure 2).Besides the above-mentioned compounds, the PAHs are

proposed as important key species during the formation pro-cess of soot, especially concerning their leading role for sootprecursor formation. Thus, as for hydrogen, acetylene,propargyl radical, and so on, the dependence of the PAHconcentration on the amount of the blending compound isinspected in the following.The modeling data showed clearly that the same PAH

species were produced in all of the studied flames, although theamounts of these species decreased as oxygenate levelsincreased. For both neat benzene and blended ethanol−benzene flames, all of the PAH concentrations exhibited amaximum inside the main oxidation region, suggesting that theoxidation and pyrolytic zones overlap; this is a characteristic

feature of benzene flames.74,75 Differences can be noted in therelative locations of the concentration peaks of some aromaticspecies. The dominant aromatics were indene, naphthalene,biphenyl, acenaphthalene, anthracene, and phenanthrene (seeFigures 3 and 4).Figure 3 portrays the dependence of the phenylacetylene

(C8H6), indene (C9H8), and naphthalene (C10H8) molefractions on the ethanol percentage. It can clearly be seenthat regardless the ethanol proportion, the C8H6 mole fractionincreased to reach a maximum and then declined. Withincreasing ethanol percentage, the phenylacetylene molefraction was lowered. The addition of oxygenate significantlyreduced the mole fractions of C8H6 up to about 70.6%lowering, with respect to the levels in neat benzene flame, wasobserved. On the other hand, it is clear that for all flames,indene (C9H8) peaked at or above 0.83 cm from the burnersurface (Figure 3) and that the maximum mole fractions of thisspecies were more than three times higher in the case of theneat benzene flame as compared to the flame containing 20%oxygen. In addition, it can be seen from the data depicted inFigure 3 that the peak concentrations of naphthalene occurredat nearly the same distance for the six studied fuels and that upto 86% reduction was obtained in this species mole fractionwith an increase in oxygenate concentration in fuel blends(Figure 3). Similar qualitative trends were reported by Inal andSenkan71 during their study on the effect of oxygenateconcentration on species mole fractions in premixed n-heptaneflames.Concerning acenaphthalene (C12H8) (Figure 4), the

obtained results showed that whatever the ethanol concen-tration in the fuel mix, the mole fraction of this compoundincreased to reach a maximum, and then, it decreased slightlyfor distances above the burner surface higher than 1.4 cm.Besides, it is shown that acenaphthalene concentrationdisplayed 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

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



flame (1.335 × 10−4), which means a decrease of 83.3%. On theother hand, it was found that the addition of the oxygenatedcompound decreased the biphenyl (C12H10) peak concen-tration by 7% in the case of the low content oxygenated fuel(fuel with 4% oxygen) and by 33% in the case of the highcontent ones (fuel with 20% oxygen).Anthracene and phenanthrene (C14H10) showed similar

trends. For all flames, anthracene (C14H10) peaked at orabove 1.08 cm from the burner surface, whereas the peaks ofphenanthrene concentrations were localized at or above 1.02cm (Figure 4). Pronounced decreases upon ethanol additionwere observed for the two species, and up to 91% reductionswere obtained for both pollutants.5.2. Pathway Analysis. Because of the complexity of

chemical and physical conditions, a reaction flux analysis wasperformed to track the main reaction pathways of the fuelblends, as well as the competing (i.e., formation anddecomposition) reactions for the selected light gases andaromatic compounds, which are suspected soot precursors. Inaddition, identification of any differences observed between theblended fuels and the neat benzene flame was analyzed. Theanalysis was performed using the appropriate subroutines in theChemkin package (CKQYP, CKCONT), which systematicallycompute the rate of production/consumption of each species.60

According to Lamoureux et al.,76 the involved reactions, forspecies k, are sorted out with respect to their maximumabsolute rate, and their sign indicated the species consumptionor formation. In this respect and to describe the benzene andthe ethanol−benzene oxidation pathways, rates of consumptionand production were computed for every species. It isnoteworthy that in this section, only the main reactions thathave an important role in chemicals belonging to the studiedsystem will be presented. The numbers in parenthesescorrespond to the numbers of reaction in the combinedethanol−benzene mechanism.In the case of the neat benzene flame and 4% oxygen-

blended fuel mix, analysis of the benzene consumption fluxshows 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 phenoxyradicals and unimolecular decomposition to propargyl radical[C3H3 + C3H3 = C6H6 (reverse of Rev 638)] to be thedominant depletion pathways, whereas the nonabstractivechannel leading to phenol formation via the hydroxyl attackon the ring [C6H6 + OH = C6H5OH + H (652)] appeared toplay a minor role (approximately 2.4% of initial benzene wasconsumed by this reaction). These results are in agreementwith the data reported by Richter et al.46 The authors found

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



that hydrogen abstraction via reaction 647 was the mostimportant in the benzene consumption pathway and thatformation of phenol and hydrogen radicals, via reaction 652,did not contribute significantly to the overall reaction rate ofC6H6 + OH. This latter observation was also reported byMadronich and Felder.77 In addition, it is well-known from theopen literature that O atom addition to C6H6 leads to a C6H6Oadduct, which either can follow a rearrangement to producephenol or suffer a ring hydrogen abstraction resulting inphenoxy. Alzueta et al.44 and Bajaj and Fontijn78 concludedfrom a flow reactor study that phenoxy is the main primaryproduct of this step, which confirms our results.The same reactions governed the benzene depletion in 8−20%

oxygen-blended fuels; however, the contributions of themwere changed. In the case of 8% oxygen, the reactions 647 and644 contributed with 38.20 and 25.97%, respectively, whereastheir contributions, in the case of blended fuel mix containing20% oxygen, were 41.51 and 29.56%, respectively. On the otherhand, the obtained data indicated that for blended fuels, theunimolecular C6H6 decomposition leading to C3H3 radicals(Rev 638) was more important than the benzene reaction withO atoms (646) and that the contribution of these two reactionsto the benzene consumption decreased with increasing theethanol proportion in fuel mix. From these results, it could beconcluded that regardless of the ethanol proportion in the fuelmix, phenyl, phenoxy, and to some extent propargyl radicalswere 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 ethanolconversion. In the case of 4% oxygen flame, about 64% ofethanol was consumed via this reaction, whereas this pathwaywas responsible for the decomposition of about 70% of theethanol present in the 20% oxygen flame. The amount ofethanol consumed by unimolecular decomposition in the 8, 12,and 16% oxygen flames fell in between these values, leading tothe fact that the higher the ethanol concentration in the fuelblend the higher the contribution of this decompositionpathway. These findings are in contrast to those reported byTherrien et al.,13 who found a decreasing trend of this reactionwith an increase of the ethanol proportion in the fuel mix. Thisdiscrepancy is potentially due to the difference in the fuelsburned as well as in the combustion conditions between ourwork 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 unimoleculardecomposition of the cyclopentadienyl radical [C5H5 = C3H3 +C2H2 (574)] as the most important reaction. In addition, itwas found that the following set of reactions contributed toacetylene production:

‐ + = +n C H C H C H H4 3 2 2 6 4 (Rev 623)

= +HCCCHO C H CO2 2 (329)

The rates of these reactions exhibited a decreasing trend withincreasing ethanol proportion in the fuel mix (this decrease was33.7, 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 thetwo reactions, which displayed a decreasing trend with a rise inthe ethanol amount:

= +C H C H H2 3 2 2 (213)

‐ + = +n C H C H C H H4 3 2 2 6 4 (623)

The independence of the C2H2 formation on the C2H4concentration could explain the fact that although the ethyleneconcentration increased with increasing the ethanol proportionin the mixture, the acetylene mole fraction showed an oppositebehavior.On the other hand, the flux analysis results showed that

whatever the ethanol concentration in the fuel mix, the majorityof acetylene consumption occurred through oxygen atomattack, leading to HCCO, HCH, CO, and H:

+ = +C H O HCCO H2 2 (211)

+ = +C H O HCH CO2 2 (210)

Increasing the amount of ethanol led to a lower consumptionrates. From these results, it is obvious that raising the ethanolinitial concentration induced a decrease in the rates of bothformation 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 uponraising the ethanol amount.Our computation showed that the propargyl radical (C3H3)

was mainly formed by the unimolecular decomposition ofbenzene and cyclopentadienyl radical as well as thecombination of propargylen triplet (C3H2) and hydrogenradicals. Rates of these reactions were lowered with increasingthe concentration of the blending compound:

+ =C H C H C H3 3 3 3 6 6 (Rev 638)

= +C H C H C H5 5 3 3 2 2 (574)

= +C H C H H3 3 3 2 (Rev 312)

These results are in accordance with the findings of Detilleuxand Vandooren,58 who reported that reaction Rev 638 was thepredominant channel in producing C3H3, and with those ofVourliotakis and co-workers,53 who reported that the propargylradical was mainly a product of C5 chemistry, throughisomerization of cyclopentadienyl to the 1-pentyn-3-en-5-ylradical 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 theformation reactions, these reactions were lowered upon raisingthe ethanol proportion:

+ = +C H H C H H3 3 3 2 2 (313)

+ =C H OH CH CHCHO3 3 2 (333)

+ = +C H O C H CH O3 3 2 2 (323)

From these results, it is obvious that an increase in the ethanolconcentration led to a decrease in both formation andconsumption reactions of C3H3. However, the net effect[summation of the effect on the formation reactions minussummation of the effect on the consumption reactions (seeFigure 5)] exhibited a decreasing trend upon increasingbenzene replacement percentage by oxygenate additive, whichmeans that the decrease in the formation reactions was morenoticeable than the one of the consumption ones, andconsequently, the C3H3 concentration decreased uponincreasing the ethanol amount.Concerning phenylacetylene (C8H6), it was found that in

mixtures containing from 0 to 8% oxygen, this species waspredominantly formed by H abstraction from H2 byethynylphenylene (reverse of reaction Rev 729), by the direct



C2H2 attack on the benzene ring (728), and finally by theH atom addition to ethynylphenylene (732) via the followingseries of reactions:

+ = +C H H C H H8 6 8 5 2 (Rev 729)

+ = +C H C H C H H6 5 2 2 8 6 (728)

+ =C H H C H8 5 8 6 (732)

Hydrogen atoms abstraction from H2 (Rev 729) accounted for52% of the phenylacetylene formation, whereas the bimolecularreaction of the phenyl radical with acetylene (728) contributedwith 37% and the addition of ethynylphenylene (C8H5) andhydrogen radical (732) accounted for 11%. Besides, thepathway investigation demonstrated that ethynylphenyleneformation occurred by the reaction sequence: C10H8 + H =C10H7 + H2, C10H7 = C8H5 + C2H2, which suggested thatphenylacetylene was a product issued from naphthalene (seeScheme 1).

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

Scheme 1. Sequence Reactions for the Formation ofPhenylacetylene (C8H6) in the Case of 0−8% Oxygen-Blended Fuels



In the case of fuels blended with 12−20% oxygen, thereaction 728 surpassed Rev 729, and it contributed with 47%for 12% oxygen. These findings are in disagreement with thosefound by Castaldi et al.,79 who reported that the phenyl-acetylene production pathway occurs by ethylene addition tophenyl to form styrene and H-atom followed by styrenedehydrogenation by H-atoms. This discrepancy might beascribed to the difference in the nature of the used fuel; inour study, an aromatic hydrocarbon (benzene) was used,whereas in the work of Castaldi and co-workers, an olefinichydrocarbon (ethylene) was studied.When formed, phenylacetylene might subsequently undergo

aryl H atom abstraction or C atom abstraction givingethynylphenylene (730) and phenylcarbene (C6H5CH) radical(740), respectively:

+ = +C H OH C H H O8 6 8 5 2 (730)

+ = +C H O C H CH CO8 6 6 5 (740)

This phenylacetylene consumption route is in qualitativeagreement with the results reported by Richter et al.,46 whomentioned that oxidation of phenylacetylene by O radicalsleading to phenylcarbene had only a limited effect onphenylacetylene profiles in the postflame zone.

It is noteworthy that the higher the ethanol proportion in thefuel mix, the lower the phenylacetylene formation andconsumption reactions rates. The effect on production ratesoutweighed the one on the consumption reactions rates, whichled 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:

+ =indenyl H indene (805)

A snapshot of indene formation paths, as given by means of thepathway analysis of the studied neat and blended fuels, isdepicted in Figure 7. It can be seen that indene is a product ofnaphthalene (C10H8). Similar trends were reported by Castaldiet al.79 during their study on aromatic and PAH formation in apremixed ethylene flame. The authors mentioned that indeneformation 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 hydroxylradical:

+ = +C H CH C H indene H6 5 2 2 2 (Rev 806)

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



+ = +indene H indenyl H2 (802)

+ = +indene OH indenyl H O2 (803)

+ = +indene O indenyl OH (804)

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

replacement percentage by ethanol. However, the effect wasmore pronounced in the case of the first reactions, which led toa net decrease in the indene amount (Figure 8).The modeling results indicated that the sole naphthalene

formation route was the bimolecular self-combination of thecyclopentadienyl radicals (C5H5):

+ = +C H C H C H 2H5 5 5 5 10 8 (771)

The importance of resonance-stabilized radicals, such ascyclopentadienyl, was first identified by Marinov et al.80 wheninvestigating PAH formation in methane and ethylene flamesthrough both experimental and detailed kinetic modelingstudies. In their study, the insufficiency of the acetyleneaddition processes in predicting the experimentally observedPAH levels was mentioned, and the inclusion of thenaphthalene formation via cyclopentadienyl self-combinationmade their kinetic model precisely predictable. On the otherhand, several authors have reported that, in addition to theHACA mechanism, the aromatic growth can proceed throughdifferent routes, and cyclopentadienyl (C5H5) moieties havebeen proposed as potential precursors due to their neutralityand reactivity at different sites.4,81−84 Particularly, Melius andco-workers,82 based on theoretical approach, proposed a nine-step

Figure 7. Indene formation pathway.

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



reaction mechanism for the conversion of two cyclopentadienylradicals to naphthalene via rearrangements involving three-membered ring closing and opening of resonance-stabilizedradicals. This observation on the naphthalene formationroute was also confirmed by Richter et al.,85 who reportedthat C5H5 radical self-recombination was the major pathway tonaphthalene in benzene premixed flame.To gain more insight in naphthalene formation, the

cyclopentadienyl radicals pathway was investigated. Themodeling results showed that these species were issued fromthe reaction sequence:

+ = +C H O C H O O6 5 2 6 5 (629)

+ = +C H O C H O H6 6 6 5 (646)

= +C H O C H CO6 5 5 5 (668)

This means that the key step in the naphthalene productionprocess was the phenyl oxidation by O2 as well as the benzeneattack with O atoms (see Scheme 2). The importance of thereaction 629 in the PAH growth process was evoked byCastaldi and co-workers during their study on the formation ofPAH in premixed ethylene flames.83 These observationssuggest that the amount of naphthalene is directly related tothe phenyl concentration. However, our modeling resultsindicated that the phenyl amount displayed a decreasing trendupon increasing the ethanol proportion in the fuel mix, andconsequently, the naphthalene would behave similarly, and itsconcentration would be lowered with a rise in the benzenepercentage replacement by oxygenate additive.Once formed, naphthalene was subsequently attacked by OH

and H radicals leading to naphthyl radical (C10H7) isomers:

+ = _ +C H OH C H 1 H O10 8 10 7 2 (778)

+ = _ +C H OH C H 2 H O10 8 10 7 2 (779)

+ = _ +C H H C H 1 H10 8 10 7 2 (776)

+ = _ +C H H C H 2 H10 8 10 7 2 (777)

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

(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 16and 20% oxygen-blended fuels, the effect of the reaction 776 onnaphthalene consumption was more pronounced than the oneof 779.As for the other studied compounds, the modeling data

showed an inhibiting effect of ethanol on consumptionreactions rates. This effect was more pronounced in the caseof the formation reactions leading to the fact that highconcentrations of the blending compound favor the suppres-sion of naphthalene (Figure 9).The obtained results showed that regardless of the ethanol

amount, acenaphthalene (noted A2R5) formation was ascribedto the decomposition reaction of the BIPHENH radical:

= +BIPHENH A R5 H2 (827)

The rate of this reaction decreased with increasing ethanolproportion, and a rate decrease up to 73.6% was observed inthe case of 20% oxygen-blended fuel as compared to the neatbenzene flame (Figure 10). In addition, our modeling resultsindicated that acenaphthalene formation involved the reactionsequence: unimolecular decomposition of the phenyl radical636 to give hydrogen atoms and benzyne (C6H4). Onceformed, C6H4 underwent a self-combination 824 to yieldbiphenylene (noted BIPHEN), which at its turn underwent areaction addition leading to the BIPHENH radical (reverse ofthe reaction Rev 825). Finally, the radical BIPHENH wasdecomposed to give acenaphthalene and hydrogen atoms viathe reaction 827 (see Scheme 3):

= +C H C H H6 5 6 4 (636)

+ =C H C H BIPHEN6 4 6 4 (824)

= +BIPHENH BIPHEN H (Rev 825)

= +BIPHENH A R5 H2 (827)

This sequence suggests that acenaphthalene is mainly issuedfrom the phenyl radical.

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



On the other hand, it was found that whatever the ethanolamount, acenaphthalene was transformed to the naphthylradical (C10H7_1) via its reaction with H atoms:

_ + = +C H 1 C H A R5 H10 7 2 2 2 (Rev 828)

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

Scheme 3. Sequence Reactions for the Formation of A2R5via C6H5

Scheme 4. Sequence Reactions for the Formation ofBiphenyl (C12H10)



As in the case of the formation reaction, this reaction seems tobe 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 (seeScheme 4):

+ = +C H C H C H H6 5 6 6 12 10 (765)

+ =C H C H C H6 5 6 5 12 10 (764)

+ =C H H C H12 9 12 10 (770)

In the neat benzene flame, the reaction between the phenylradical and the benzene accounted for 70.3%, whereas thephenyl radicals self-recombination contributed with 16.4%, andfinally, the addition reaction of hydrogen atoms with C12H9radical accounted for 13.2%. These results are in accordancewith those reported by Wang and Frenklach86 and by Appeland 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 ofthis reaction was confirmed, especially under fuel-rich and high-pressure88 conditions.

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

Figure 11. Effect of ethanol on biphenyl formation and consumptionreactions.



After being formed, the prominent channels for the biphenylconsumption were its reactions with OH and H radicals (seeScheme 4):

+ = +C H OH C H H O12 10 12 9 2 (767)

+ = +C H H C H H12 10 12 9 2 (766)

Contribution of the reaction with OH radical was found to be59.6%, 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 formationreactions 764 and 770 increased to pass through a maximum at4% oxygen, and then, they decreased monotonically. On theother hand, the two biphenyl consumption reactions behaved

similarly, and a slight rise was observed until 4% oxygenfollowed by a monotonic decay. Finally, it is noteworthy thatthe amount of oxygen led to greater reductions in both netformation and consumption rates. However, the lowering in thenet biphenyl formation was more noticeable than the one onthe net consumption, inducing a net lowering in the biphenylamount 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 (notedA3: C14H10) formation were as follows:

+ = +indenyl C H A 2H5 5 3 (838)

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.



_ + = +A C H 2 C H A H1 2 6 6 3 (846)

_ + =A 9 H A3 3 (858)

_ + =A 2 H A3 3 (856)

_ + =A 4 H A3 3 (857)

+ = +C H C H A H12 9 2 2 3 (837)

_ + =A 1 H A3 3 (855)

+ = +C H C H A H8 6 6 5 3 (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 thecase of 8−12% oxygen. In the case of 20% oxygen, the primaryroute for the formation of phenanthrene is predicted to occurthrough 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 byMarinov and co-workers79,80,89 during their study on the PAH

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



formation in methane and ethylene flames. The authors havementioned that the fusion of two five-membered rings in thereaction of indenyl with cyclopentadienyl (838) was the majorroute in the phenanthrene formation. On the other hand, Appeland co-workers87 reported that phenanthrene (A3) wasprimarily formed via ring−ring “condensation” reactions andmostly 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 sufficientlyrapid, makes it a more “direct” process.After its formation, the phenanthrene predominant con-

sumption channels were its attack by OH and H radicals as wellas its isomerization to anthracene:

+ = _ +A OH A 1 H O3 3 2 (851)

+ = _ +A OH A 4 H O3 3 2 (853)

+ = _ +A H A 1 H3 3 2 (847)

+ = _ +A OH A 9 H O3 3 2 (854)

+ = _ +A OH A 2 H O3 3 2 (852)

+ = _ +A H A 4 H3 3 2 (849)

=A L A3 3 (Rev 872)

+ = _ +A H A 9 H3 3 2 (850)

+ = _ +A H A 2 H3 3 2 (848)

As can be seen, from the data depicted in Figure 12,phenanthrene formation and consumption reactions ratesexhibited decreasing influence with increasing initial ethanolconcentrations. This effect was more pronounced in the case ofthe formation reactions, leading to a net decrease in thephenanthrene amount.Modeling results indicated that anthracene (noted A3L) was

mainly formed via the phenanthrene isomerization reaction Rev872 as well as the combination reactions of hydrogen atomswith A3L isomers (871, 868, and 865):

=A L A3 3 (Rev 872)

+ =A L9 H A L3 3 (871)

+ =A L2 H A L3 3 (868)

+ =A L1 H A L3 3 (865)

On the other hand, anthracene was consumed by means ofhydrogen abstraction reactions with OH and H radicals:

+ = _ +A L OH A L 1 H O3 3 2 (864)

+ = _ +A L OH A L 2 H O3 3 2 (867)

+ = _ +A L H A L 1 H3 3 2 (863)

+ = _ +A L OH A L 9 H O3 3 2 (870)

+ = _ +A L H A L 2 H3 3 2 (866)

+ = _ +A L H A L 9 H3 3 2 (869)

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

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

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 abovethe 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 concen-trations of C2H2 and C3H3 exhibited an opposite behavior.• As expected, carbon monoxide was the major combustion

product, and its mole fraction increased steadily with distancefrom the burner surface and leveled off at about 2.2 cm.Surprisingly, the CO mole fraction decreased monotonicallyand linearly with increasing the percentage of the ethanol in themixture, and up to 20% lowering was observed in the case of20% oxygen flame as compared to the neat benzene flame. Incontrast, up to 53.6 and 26.9% rising were observed in the caseof hydrogen atoms and OH radical, respectively.• For both neat and ethanol-blended benzene flames, all

calculated PAH components peaked inside the main reactionzone. Ultimately, the mole fractions of some componentssubsided, whereas those of other components remained nearlyconstant in the postflame region. The dominant aromatics wereindene, naphthalene, biphenyl, acenaphthalene, anthracene, andphenanthrene.• Analysis of the main pathways of the reactions leading to

benzene and ethanol depletion in all flames indicated thatregardless of the ethanol proportion in the fuel mix, phenyl,phenoxy, and to some extent propargyl radicals were the mainintermediates formed during benzene depletion. On the otherhand, the unimolecular decomposition to ethylene and waterstep was found to be dominant for the initiation reactions ofethanol conversion.• Pathway analysis of the reactions leading to PAH formation

showed that both phenylacetylene and indene were products ofnaphthalene whose production process key step was the phenyloxidation by O2. Also, acenaphthalene and biphenyl weremainly issued from the phenyl radical. On the other hand, itwas found that the fusion of two five-membered rings in thereaction of indenyl with cyclopentadienyl was the major routein the phenanthrene formation and that the main formationroutes of anthracene were the reverse of the phenanthreneisomerization reaction as well as the combination reactions ofhydrogen radical with A3L isomers.• PAH concentrations displayed a decreasing trend upon

increasing the ethanol percentage. As all of the formed PAHswere issued from the phenyl radical, it could be concluded thatthe PAH reduction was a result of a simply replacing “sooting”benzene with “nonsooting” ethanol without influencing thecombustion chemistry of the benzene.

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.



mailto:[email protected]

■ ACKNOWLEDGMENTS

We acknowledge the “Centre de Developpement des EnergiesRenouvelables” (CDER)-Algerie for its financial support.

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