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PAH emissions from high-temperature oxidationof vaporized anthracene

Jennifer DeCoster a, Ali Ergut a, Yiannis A. Levendis a,*,Henning Richter b, Jack B. Howard b, Joel B. Carlson c

a Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115, USAb Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

c

US Army SBCCOMNatick Soldier Center, Natick, MA 01760, USA

Abstract

The presence of polycyclic aromatic hydrocarbons/compounds (PAH/PAC) in the exhaust of combus-tion equipment as well as their growth to soot particles depends on the balance of formation and depletionreaction pathways. Detailed understanding of PAH oxidation is necessary for a correct quantitativedescription of combustion processes. This study focuses on the high-temperature oxidation of anthracene,a 3-ring PAH, by molecular oxygen. By reacting this already formed PAH, under non-sooting conditions,its oxidative depletion processes were isolated from its formation processes, as well as from soot formationpathways. Anthracene powder, fluidized in nitrogen, was introduced into a two-stage electrically heatedlaminar-flow drop-tube furnace. In the first stage, kept at 900 C, anthracene was vaporized. Thereafter,upon mixing with preheated oxygen-containing gases in a venturi, gaseous anthracene was oxidized inthe second furnace at oxygen mole fractions of 15% and gas temperatures of 9501050 C. Gas temper-ature and velocity profiles in the furnaces, as well as anthracene particle vaporization trajectories wereobtained with a computational fluid dynamics code. Sampling, followed by chemical analysis, was conduct-ed at three locations within the oxidizing furnace. Trends with residence time show increasing CO and CO 2yields in the furnace and gradually decreasing O2. CO increased with higher oxidizing temperatures andoxygen concentrations. Acetylene was the most abundant light hydrocarbon, followed by methane,ethylene, benzene, propylene and ethyl acetylene. Prevalent PAC included fluorene-9-one, 9,10-anthracen-odione, biphenylene, dibenzofuran, fluorene, indene, naphthalene, anthrone, xanthone and 2-naphthalen-

ecarboxaldehyde. Most species yields increased with rising oxidation temperatures. A global anthracenereaction rate with oxygen was deduced, and an activation energy of 71 kJ/mol was calculated. An anthra-cene oxidation reaction scheme was proposed. 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Anthracene; Oxidation; PAH; Rate constant; Reaction pathways

1. Introduction

Concentrations of polycyclic aromatic hydro-carbons (PAH) in the exhaust gases of combustorsare the result of competing growth and depletion

1540-7489/$ - see front matter 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.proci.2006.07.211

*

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Proceedings of the Combustion Institute 31 (2007) 491499

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processes. Depending on parameters such asequivalence ratio and temperature, depletion ofPAH formed in the reaction zone can occur bysubsequent oxidation [13] or conversion to soot[3,4]. Reactions with O, OH and O2 are knownto be important in oxidation of PAH [13] but

the details are unclear, especially in reactions withhigh molecular weight PAH. Partially oxidizedPAH are likely intermediates prior to decomposi-tion to smaller hydrocarbon units andundercertain conditionscomplete conversion to CO,CO2 and H2O. Due to the role of PAH as sootprecursors, the lack of understanding of their oxi-dation chemistry limits the quantitative descrip-tion of particulate formation. Few solid kineticdata on the gas phase oxidation of PAH are avail-able, and in most cases the reaction pathways andeven the major intermediates are unknown.

Rates of some of the elementary steps in PAHoxidation have been measured directly, but usual-ly these are only available for the very first step inthe complex reaction cascade. For example, kinet-ics of reactions of benzene and naphthalene withOH radicals were measured in a laser photolysis/resonance fluorescence system and the stabilityof the C10H8OH adduct was found to be higherthan that of C6H6OH [5], suggesting that resonantstabilization of the p-radicals formed by additionreactions may be more important for PAH thanfor benzene. However, no information is availableon the subsequent reactions of the C10H8OH

adduct. Also, relative contributions of hydrogen-abstraction and direct addition oxidating speciesare expected to depend strongly on temperature,pressure and the size of the molecule. Reactionsof biphenyl, methylnaphthalenes and phenan-threne with O(3P) were measured in a temperaturerange from 298 to 873 K in a low-pressure micro-wave discharge flow system with mass spectromet-ric detection [6]. Formation of phenol-typeadducts was suggested to be the major reactionchannel and rate constants were found to increasein the following order: benzene, biphenyl, phenan-

threne, methylnaphthalenes. Reactivity of phen-anthrene was similar to that of naphthalene.

Global oxidation rates of selected PAH havebeen recently determined in this laboratory, inthe afterburner of two-stage batch-combustionof polystyrene [7]. As expected, oxidation ratesincreased for all species with gas temperature,but it was difficult to rationalize the observed dif-ferences in the oxidation rates of the variousPAH. Nevertheless, the observed fast oxidationof fluorene suggests a correlation between molec-ular structure and oxidation rates, e.g., 9-fluore-

none could be a plausible initial product.Indeed, 9-fluorenone and other PAH ketones havebeen detected mixed with PAH in diesel emissionsand in the environment, and some of the PAH areknown to be readily oxidized to ketones even atlow temperature [8]. The most detailed study of

the gas-phase oxidation of a PAH was performedby Shaddix et al. [9] who measured the stableintermediates formed in the oxidation of 1-meth-ylnaphthalene at 11601200 K, in an atmosphericpressure flow reactor. They constructed a kineticmodel consistent with the data [10]. Reaction

pathway analysis indicated that the primary reac-tion was H-abstraction from the methyl side-chain, but additions of O and H atoms to thearomatic rings were also important. Reactionpathways will likely be considerably different forun-substituted PAH, which do not have any read-ily abstractable H atoms. Also, the chemicallyactivated reactions that follow addition of a radi-cal to a PAH are expected to show strong temper-ature dependence [11].

Anthracene, a three-ring PAH, was selected forthis study because aromatics containing theanthracene configuration are known for high ther-mal reactivity [12,13] and propensity for soot for-mation [14,15]. Moreover, because of itsmolecular symmetry (it contains only three uniqueperipheral carbon positions: 9, 1, and 2) the num-ber of possible products of thermal treatment ofanthracene is limited, and this simplifies productanalysis [16].

Although there has been an investigation on theproducts of high-temperature pyrolysis of anthra-cene [16], high-temperature oxidation of anthra-cene in particular, and PAH in general, has verylittle experimental history. Exploratory work on

anthracene oxidation was recently conducted as amaster thesis carried out at MIT by the visitingstudent I. Griful Millet, under the supervision ofH. Richter and J.B. Howard [17]. In that work,transient (batch) oxidation of anthracene wasinvestigated in a muffle furnace at temperaturesbetween 500 and 800 C, and gas compositionsfrom 2.5 to 40% O2 in helium. Major productsidentified by GC/MS were anthrone and 9,10-anthracenodione, i.e., single- and double-oxidizedanthracene. Also xanthone and other oxygenatedPAC formed after loss of one to three carbon

units, including 2-naphthalenecarboxaldehyde, 1-H-phenalen-1-one, fluorene-9-one, 4-hydroxy-9-fluorenone and 1-hydroxy-9,10-anthracenodionewere detected.

Herein, the exploratory work of Griful Millet[17] on batch combustion of anthracene wasextended to obtain time-resolution of the interme-diate species formations and products. Anthra-cene was pulverized, fluidized in nitrogen andintroduced into a vaporizer furnace. The effluentof this furnace was then mixed with oxygen andoxidation of this premixed gaseous charge

occurred in a secondary furnace. Reactionsoccurred therein at steady-state/steady-flow,high-temperatures and known equivalence ratios,/. Oxygen mole fractions were low (15%), asthe determination of reaction rates requires theconsumption of a quantifiable amount of reactant

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but without its complete depletion, otherwise thereaction time would be unknown. Low oxygenmole fractions also prevented ignition of the char-ge and, because of the low oxidizing temperatures,formation of soot was avoided which would havebeen an anthracene depletion pathway competing

with the oxidation reactions. Sampling along thecenterline of the furnace allowed temporal-resolu-tion of the evolved products.

2. Experimental techniques and procedure

2.1. Experimental apparatus and conditions

Anthracene powder from Aldrich Chemicalwas crushed and sieved to 110150 lm. Tests wereconducted in a two-stage electrically heated, drop-tube furnace (ATS Systems) at 1 atm. The appara-tus consisted of an upper furnace where theanthracene particles vaporized and a lower fur-nace where oxidation occurred (Fig. 1). Particleswere introduced into the vaporizing furnace viaa long hypodermic tube (1.1 mm i.d.; Micro-Group) with a vibrated glass vial coupled to a syr-inge pump (Harvard Apparatus). The tubing wasvibrated to its natural frequency by two vibrators(Vibro-Graver by Alltech) to warrant a steadyflow of anthracene particles to the vaporizing fur-nace, in a 2 L min1 stream of nitrogen. Despiteanthracenes propensity to form soot, a previous

study on anthracene pyrolysis in argon reportedthat there was little evidence of soot (

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products were quantified using calibration gasmixtures.

2.4. PAH extraction and analysis by gas chroma-tographymass spectrometry

Following the oxidation experiments, the filtersand XAD-4 resins were spiked with a 50 lL inter-nal standard containing 50 lg each of naphtha-lene-d8, acenaphthene-d10, anthracene-d10,chrysene-d12 and perylene-d12. A Dionex ASE200 Accelerated Solvent Extractor was used toextract the organic compounds from the filtersand the resins together. XAD-4 resin and filterblanks as well as combustion blanks were extract-ed and analyzed. Details of the extraction tech-nique are given in Ref. [18]. Identified aldehydecomponents were spiked onto XAD-4 resin and

the recovery from XAD-4 was calculated. Theresponses of the aldehydes/ketones were used todevelop a relative response factor (RRF), whichwas used to calculate the aldehyde/ketone concen-tration, based on the response from the appropri-ate internal standard.

Chemical analysis of extracts was conductedby means of gas chromatography coupled to massspectrometry (GCMS) using a Hewlett-Packard(HP) Model 5890 gas chromatograph equippedwith a HP Model 5971 mass selective detector.Sample preparation, the operating conditions of

the GCMS system and the data reduction weredescribed previously [11,18]. Species identificationwas based on the NIST and Wiley mass spectrallibraries, and known retention times for isomerswere used. In order to assess the reproducibilityof the PAH analysis, triplicate oxidation experi-ments were performed. The experimental proce-dure was kept consistent in all evaluations toensure the validity of comparative values and rel-ative trends.

The extraction efficiency for the PAH species isthe amount of standard recovered from the sam-

ple divided by the amount of the deuterated com-pounds in the internal standard solution. TheNational Functional Guidelines recommend arecovery of greater than 50% for the aforemen-tioned five internal standards which was exceededherein (59, 74, 85, 98, and 100%).

3. Results and discussion

3.1. Gas temperature measurements

Temperature profiles of the gas in the vaporiz-er and oxidizer furnaces were obtained along theirvertical centerline using a 48 inch type-K Omegathermocouple, see Fig. 2. Product sampling wasperformed in the nearly-constant temperatureregion of the oxidizer furnaces heated zone,

where variations of not more than 10 K wereobserved.

3.2. Anthracene particle vaporization calculations/simulations

Calculations were performed in order to deter-mine whether vaporization of the injected anthra-cene particles (110150 lm) was to be expectedwithin the heated zone of the first-stage furnace.The distance and the time that the largest diame-ter anthracene particles need to travel to vaporize

completely were evaluated by accounting for theheat-up to the boiling temperature and for theensuing vaporization/sublimation at constanttemperature, as outlined in [19,20]. Results fromthese calculations were compared to computationsexecuted with the finite-difference numerical codeFluent 6.0, see Fig. 3.

Both methods yielded similar particle time-of-flight outcomes, i.e., $175 ms. There was somedisagreement, however, on the prediction of parti-cle penetration in the vaporizer furnace (8.5 vs.11.5 cm), which may be because the calculationswere conducted assuming constant gas tempera-ture while, for computations, Fluent takes into

Temperature Profile of the Vaporizer and Oxidizer

vaporizer oxidizer

vaporizer oxidizer

reziropavehtfo

ecnartne

reziropav

ehtfotixe

irutnev

f

oretnec

rezidixo

ehtfo

ecn

artne

Sampling Pointsrezidixoehtfotixe

0

200

400

600

800

1000

1200

Distance from the Injection Point (cm)

Temperatu

re(C)

.

.

0 20 40 60 80

Fig. 2. Temperature profiles in vaporizer and oxidizerfurnaces.

0

50

100

150

Elapsed time (ms)

(retemaiD

)m

CalculationNumerical computation

a

Distance traveled (cm)

b

0 50 100 150 0 4 8 12

Fig. 3. Diameter of a 150 lm anthracene particle vs. (a)time to vaporize and (b) traveled distance to vaporize;results of both the numerical computations (Fluent) andcalculations are superimposed.


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account the input experimental temperature pro-file along the axis of the vaporizer. Nevertheless,both methods indicated that the injected anthra-cene particles, (110150 lm) were expected tovaporize inside the vaporizer (upper) furnace,whose hot zone extends 19 cm from the injection

point. Even so, due to the nature of this powder,clumping of particles while feeding into the injec-tor was difficult to completely avoid.

Although ensuring vaporization of the anthra-cene particles was important, re-condensation atthe venturi was also a concern. The partial pres-sure of anthracene vapors in the vaporizer furnacewas calculated to be $0.023 atm, correspondingto the vapor pressure of anthracene at $195 C[21]. Figure 2 shows the gas temperature dippingto this point only at the center of venturi whereadditional gas is mixed, decreasing the anthracenepartial pressure. Hence, recondensation of anthra-cene at the venturi is not expected.

3.3. Emission yields

Yields of major gas products (CO, CO2 andO2) and light hydrocarbons, sampled inside theoxidizer furnace, are shown in Fig. 4. Yields, inmg g1 of anthracene injected, of CO and CO2increased from 20 (1217 ppm) to 72 (4040 ppm)and from 74 (2900 ppm) to 184 (6500 ppm),respectively, at 1.2% O2, 950 C. CorrespondingO2 yields (mg g

1 of anthracene injected)

decreased from 258 (1.40%) to 214 (1.05%), seeFig. 4a. At 1.2% O2, 1050 C, CO and CO2 yieldsvaried only mildly in the sampling zone, around80 and 160 mg/g, respectively (Fig. 4b). At 5% ini-tial O2, the absolute values of both CO and CO2increased by factors of 1.5 and 2, respectively,compared to the latter case (Fig. 4c). Oxygen

was consumed faster at the higher furnace temper-ature, even before the first sampling point.

The most prevalent light volatile hydrocarbonwas acetylene, followed by ethylene, methane,benzene and propylene, see Fig. 4. In most casesthe concentration of light hydrocarbons increased

along the furnace axis, especially at the lower ini-tial oxygen concentration (1.2%), see Figs. 4a andb. Furthermore, the mole fractions of light hydro-carbons increased with furnace temperature, andat 1050 C, propyne was also detected. As inputoxygen concentration increased from 1.2 to 5%,the yields of prevalent hydrocarbons remainednearly unchanged, whereas benzene, propyneand propylene yields decreased by factors of upto 10, 5 and 6, respectively. Ethane was detectedonly at 5% initial O2 case. Ethyl acetylene wasdetected in all tests but was not quantified.

Polycyclic aromatic compounds (PAC) werecaptured both in the gaseous and condensed phas-es. The combined PAC yields in the two phasesare presented. Yields of unreacted anthracenetogether with other detected PAC are shown inFig. 5. Under all experimental conditions, fluo-rene-9-one, 9,10-anthracenodione, biphenylene,dibenzofuran, fluorene, 1H-phenalene, indene,anthrone, 2-naphthalenecarboxaldehyde, ace-naphthylene, phenanthrene, phenylacetylene and4-methylphenanthrene were the most prevalentPAC. This composition of the product mixtureis very similar to that detected in Millets thesis

[17]. Differences amounted to 1-hydroxy-9,10-anthracenodione which was detected therein butnot herein; and to biphenylene, dibenzofuran, flu-orene, indene, acenaphthylene and naphthalene,which were present in all experiments herein, butwere not reported by Millet [17].

At the oxidizer temperature of 950 C and1.2% O2, most PAC yields were in the range of501000 lg/g of anthracene, see Fig. 5a. Majorproduct PAC yields over 1000 lg/g of anthracenewere 9,10-anthracenodione, naphthalene and fluo-rene-9-one.

Experiments at 1050 C and 1.2% O2 producedyields that were on average much higher thanthose at 950 C, some components increased bya factor of five, see Fig. 5b. Biphenylene and fluo-rene-9-one yields increased the most, whereas9,10-anthracenodione and 2-naphthalenecarbox-aldehyde increased only slightly with rising oxidiz-ing temperature. Biphenylene, was generated at500016,000 lg/g of anthracene, followed by fluo-rene-9-one, naphthalene, fluorene, acenaphthyl-ene, 9,10-anthracenodione, 1H-phenalene,phenanthrene, anthrone, dibenzofuran, phenyl-

acetylene, indene, etc.Experiments at 1050 C and 5% O2 producedyields that were overall comparable to those inthe lower oxygen case above, see Fig. 5c. Underthese conditions fluorene-9-one resulted in thehighest yield (900016,000 lg/g of anthracene)

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

)g/gm(dleiY

a

Sampling Point

b c

O2 CO CO2 Methane

Ethane Ethylene Propylene Acetylene

Propyne Benzene

1 2 3 1 2 3 1 2 3

Fig. 4. Plot of fixed gas and light hydrocarbon yields(mg/g of anthracene injected). Tvaporizer for all cases was900 C. (a) Toxidizer = 950 C, [O2] = 1.2%; (b) Toxidizer =1050 C, [O2] = 1.2%; (c) Toxidizer = 1050 C, [O2] = 5%.Dt between sampling points was $80 ms.

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followed by biphenylene and naphthalene. Someprominent PAC byproducts such as biphenylene,fluorene, phenanthrene and acenaphthylene,decreased in yields with increasing oxygen; others,especially oxygenated ones, such as fluorene-9-

one, 2-naphthalencarboxaldehyde and dibenzofu-ran increased significantly together withnaphthalene.

Generally, but not in all cases, PAC yieldsincreased along the centerline of the furnace, asanthracene was consumed. Moreover, some PACwere predominately detected in the gas-phase,others in the condensed-phase. For instance, inall three series of experiments, naphthalene,indene, phenylacetylene, benzaldehyde, benzofu-ran and styrene yields were found mostly asgaseous PAC. Biphenylene, acenaphthylene, 2-

methylnaphthalene, 1-methylnaphthalene, phen-anthrene and 1-methylphenanthrene were foundin both phases, whereas the remaining specieswere found nearly entirely in the condensed phase.

3.4. Oxidation of anthracene: reaction pathwaysand rate

Measurement of reaction products as functionof reaction time (Figs. 4 and 5) led to betterinsight into prevalent reactions involved inanthracene oxidation. The resulting reaction

scheme, given in Fig. 6, shows formation ofanthrone as a major initial oxidation product.Consistent with previous observations in combus-tion environments [3], isomerization to phenan-threne, which is oxidized subsequently,represents a competing pathway. Oxidation of

anthrone to 9,10-anthracenodione is a followingreaction step. It should be noted that 9,10-anthra-cenodione has structural features similar to p-benzoquinone which has been discussed as a

thermally stable intermediate of benzene oxida-tion [2,4]. Naphthalenecarboxaldehyde is anotherkey intermediate of continued anthracene deple-tion. While elemental reactions leading to its for-mation still need to be identified, unimolecularCO loss yields naphthalene, another major

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1 3

Sampling Point Sampling Point Sampling Point

PACY

ield(g/gAnthracene

Injected)

a

1

b

1 3

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

UnreactedAnthraceneYield(mg/gAnthraceneInjected)

c

1 3

Sampling Point

Unreacted Anthracene

Biphenylene

Naphthalene

Fluorene-9-one

Fluorene

Acenaphthylene

1H-Phenalene

9,10-Anthracenedione

Anthrone

Phenanthrene

Dibenzofuran

Phenylacetylene

Indene

2-Naphthalenecarboxaldehyde

4-Methylphenanthrene

2-Methylnaphthalene

Pyrene

Acenaphthene


Acephenanthrylene

Benzaldehyde

Xanthone

Benzofuran


9H-Xanthene

1-MethylnaphthaleneStyrene

2 2 3 2

Fig. 5. Plot of PAC yields (lg/g of anthracene injected). Tvaporizer for all cases was 900 C. (a) Toxidizer = 950 C,[O2] = 1.2%; (b) Toxidizer = 1050 C, [O2]=1.2%; (c) Toxidizer = 1050 C, [O2] = 5%. Dt between sampling points was$80 ms.

Fig. 6. Main products and proposed reaction pathwaysfor anthracene oxidation. (1) 2-Naphthalenecarboxalde-hyde, (2) 1H-phenalen-1-one, (3) anthracene, (4)anthrone, (5) fluorene-9-one, (6) 4-hydroxy-9-fluore-none, (7) phenanthrene, (8) 9,10-anthracenodione, (9)Xanthone, (10) 1-hydroxy-9,10-anthracenodione.


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intermediate (Fig. 5, not shown in Fig. 6). Oxida-tion of naphthalene to naphthol or napthoxy, fol-lowed by another unimolecular CO loss step [3],explains the formation of indene which is abun-dant in the product mixtures (Fig. 5). Benzene islikely to be product of the further indene decom-

position/oxidation. Its lower yields at the higheroxygen concentration of 5% is consistent with itsfast subsequent oxidation. A detailed quantitativereaction mechanism describing benzene oxidationhas been discussed previously [2,4] and explainssemi-quantitatively the presence of acetylene, pro-pylene, CO and CO2 in the present work.

Well-defined experimental conditions, includ-ing good temperature control, allowed for thedetermination of global rate constants describinganthracene oxidation at different temperatures.Due to the excess of oxygen, leading to its deple-tion by not more than 25% (Fig. 4) between sam-pling points, anthracene oxidation was treated asa pseudo-first order chemical reaction:

PRODUCTSOk

2

Rate constants were obtained for the differenttemperatures and oxygen partial pressures fromthe experiments conducted herein in the drop-tubefurnace and from those of Millet [17] in the mufflefurnace. Reaction rates were determined betweenthe highest and the middle sampling points, andthen again between the middle and the lowestsampling points and averages were taken. Gastemperatures along the sampling locations wereassumed constant, as the profile in Fig. 2 showsvariations of not more than 10 K. Rate con-stants for the reactions were obtained as follows:

dAnthracene

dt kAnthraceneO2

[O2] was assumed to be constant throughout thereaction, and was set at the average value between

two sampling points, i.e., O2 O2 fO2 0

2. The rate

constant (k) was obtained by integrating theabove equation,

k ln

AnthracenefAnthracene0

O2Dt

The residence time, Dt, between the samplingpoints is calculated based on the integrated veloc-

ity profile at the centerline as obtained by the Flu-ent code, using the experimentally obtained gastemperature profile as input. Moreover, Roesler[22], using classical reactor theory, suggested thatthe chemistry in laminar flow reactors with fullydeveloped velocity profiles follows plug flow

behavior along the centerline stream on the time-scale defined by the local velocity. For systems,such as the one herein, where the Damkoehlernumber exceeds 40, radial mass transport be-comes locally negligible.

In this study, two oxidizing furnace tempera-

tures were explored, therefore, two rate constantswere calculated. The same approach was appliedto 1.2 and 5% O2 concentration cases at1050 C. Results are consistent with the previouswork of Millet [17]. Based on both data setsdepicted in Fig. 7 (from steady-state combustionin the drop-tube and from unsteady combustionin the muffle furnace), and assuming Arrheniusbehavior, a rate constant of 1 1010exp(71 kJ mol1/RT) was deduced,where R = 0.008314 kJ mol1 K.

Finally, it should be noted that, under mostconditions, anthracene oxidation by O2, describedquantitatively by the global reactions deducedhere, occurs in addition to direct oxidation bythe following sequence:

O2 anthracene ! HO2 anthracyl

anthracyl O2 ! products

Observation of significant quantities of ace-phenanthrylene and fluoranthene (Fig. 5) as wellas of acetylene (Fig. 4) is consistent with the for-mation of the aceanthrylene by addition of acety-lene to 1- and 9-anthracyl radicals followed byring closure and thermal interconversion [23].Similarly, acenaphthylene, also observed in thepresent work, is formed via the reaction of 1-naphthyl radicals with acetylene.

Quantitative assessment of the relative contri-butions of both pathways requires sufficiently pre-cise kinetic data describing hydrogen abstractionand anthracyl concentrations. However, compila-tion of a kinetic model, including this reactionsequence followed by the comparison of predictedtime-dependent concentrations of anthracene andmajor intermediates with determination of

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

7 8 9 10 11 12 13

1/T x 104

(K-1

)

mc(k

3

elom

1-s

1-)

Muffle Furnace (Millet)

Drop-tube Furnace (5%O2)

Drop-tube Furnace (1%O2)

k= Ae-Ea/RT

= 1x1010

e-8551(1/T)

Activation Energy (E a)= 71 kJ/mol

Pre-exponential (A)= 1x1010

Fig. 7. Plot of the global rate constant, k vs. 1/T foroxidation of anthracene.


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reaction fluxes, can provide additional insight.Development of such detailed model will be partof future work.

4. Conclusions

Vaporization of anthracene particles and sub-sequent oxidation of gas-phase anthracene tookplace in a two-stage laminar-flow, drop-tube fur-nace at fuel-rich, atmospheric conditions. Theupper stage was a vaporizer furnace where anthra-cene particles were injected, heated and vaporizedin nitrogen. This stage was followed by a venturimixer where oxygen was introduced. The bottomstage was an oxidizer furnace where anthracenevapors oxidized at fuel-rich conditions. Samplingwas conducted at three constant-temperaturelocations along the centerline of the oxidizer fur-nace. Analysis of the sampled gases was per-formed to assess the rate of consumption ofanthracene and to identify the by-products of oxi-dation. Yields of CO and CO2 showed increasingtrends with time or distance traveled in the oxidiz-ing furnace at the lower temperature, whereasthey did not significantly change at the highertemperature. The average values of CO steadilyincreased with higher oxidizing temperatures andoxygen concentrations. On the other hand, CO2exhibited trends like those of CO, but were moresensitive to oxygen concentration than tempera-

ture. As the oxidizing temperature increased,more oxygen was consumed. GCMS analysisrevealed many prevalent PAC, including unre-acted anthracene, fluorene-9-one, 9,10-anthracen-odione, 1-H-phenalene, biphenylene, naphthalene,dibenzofuran, fluorene, acenaphthylene andanthrone. Other products resulted in lower yields.

PAC product yields increased by up to anorder of magnitude when the oxidizer was operat-ed 100 C higher. Biphenylene and fluorene-9-oneyields increased the most, whereas 9,10-anthracen-odione only slightly increased with the rise in oxi-

dizing temperature. Most major oxygenatedspecies, such as 2-naphthalenecarboxaldehyde,dibenzofuran and fluorene-9-one, together withnaphthalene, increased with oxygen concentra-tion. The yields of other PAC with no oxygenatom in their molecules (e.g. biphenylene, fluo-rene, acenaphthylene, and 1H-phenalene)decreased. The most abundant light volatilehydrocarbons were acetylene, ethylene, methane,benzene and propylene. When operating with1.2% oxygen concentration, benzene, propyneand propylene yields were higher than at the 5%

conditions. Yields of light hydrocarbons increasedwith furnace temperature; acetylene yieldsincreased, more than any other species, as temper-ature and oxygen concentration increased.

Based on the aforementioned detected oxida-tion byproducts an anthracene oxidation pathway

scheme was proposed. Moreover, a global rateexpression for oxidation of anthracene wasdeduced to be; ln{[Anthracene]f/[Anthra-cene]0} = 1 10

10exp(8551/T(K)) [O2]0 Dt,at temperatures in the broad neighborhood of1000 C, which are pertinent to practical incinera-

tor systems (afterburners).

Acknowledgments

The authors acknowledge technical assistance/discussions from Dr. K. Taghizadeh, Dr. W.H.Green, E.F. Plummer, C. Korber-Goncalves, J.Jordan and J. Pantalone.

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Comment

Judy Wornat, Louisiana State University, USA. Youreport biphenylene as one of your higher yield polycy-clic aromatic products. What evidence do you havethat the C12H8 product you have assigned to bebiphenylene is really biphenylene and not either theC12H8 2-ethynylnaphthalene or the C12H8 1-ethyny1-naphthalene? In systems that have phenylacetylene,such as I believe you have, an ethphylnaphthaleneseems a more likely C12H8 product. Since the massspectra of biphenylene, tethynylnaphthalene, and 2-

ethynylnaphthalene are very similar, one needs toshow that the two ethynylnaphthalenes can be ruledout before assuming that the C12H8 product observedis piphenylene. Note that a typical NIST mass spectralibrary, used with most GC-MS instruments, does not

contain the mass spectra of the ethynylnaphthalenes,so the fact that an automated search might present amatch with biphenylene is not sufficient substantia-tion of biphenylenes presence.

Reply. We currently use both the Wiley and NISTsearches. Retention times were used to compare the stan-dards of biphenylene with those of the unknowns. Theretention time windows are adjusted to ensure that thereis no retention time overlap for these isomers. Moreover,

since over the years the analytical study involved manysamples of different origins and the retention timesmatched (samples from other fuels tested in the samelab in the past), we are confident of the results providedin the paper.


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