Formation of Polycyclic Aromatic Hydrocarbons and Their Radicals in a Nearly Sooting Premixed Benzene Flame

Proceedings of the Combustion Institute, Volume 28, 2000/pp. 2609–2618 FORMATION OF POLYCYCLIC AROMATIC HYDROCARBONS AND THEIR RADICALS IN A NEARLY SOOTING PREMIXED BENZENE FLAME HENNING RICHTER, TIMOTHY G. BENISH, OLEG A. MAZYAR, WILLIAM H. GREEN and JACK B. HOWARD Department of Chemical Engineering Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA Polycyclic aromatic hydrocarbons (PAH) are associated with health hazardous effects, and combustion processes are major sources of their presence in atmospheric aerosols. In the present work, chemical reaction pathways of PAH formation have been investigated by means of the modeling of a nearly sooting, ס low-pressure, premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio ␾ kPa). This flame has 2.67 ס pressure ,1מcm s 50 ס argon, gas velocity at burner at 298 K v 30% ,1.8 been investigated by Bittner and Howard using molecular-beam sampling coupled to mass spectrometry. More recently, Benish extended the set of available data for radicals up to 201 amu and for stable species up to 276 amu using nozzle-beam sampling followed by radical scavenging with dimethyl disulfide and subsequent analysis by gas chromatography–mass spectrometry. An existing kinetic model has been refined. Density functional theory computations were used to update the thermodynamic database, while transition state theory followed by a bimolecular quantum Rice-Ramsperger-Kassel analysis allowed for the determination of kinetic data relevant for the present study. The reaction of phenylacetylene radicals with acetylene is shown to be limiting for the concentration of 1-naphthyl radicals, while naphthalene is formed mainly by self-combination of cyclopentadienyl. The insufficient consumption of PAH as well as acetylene beyond the reaction zone gives some evidence of the need of additional PAH growth pathways involving acetylene but thermodynamically more favorable than subsequent hydrogen-abstraction/acetyleneaddition reactions. A new pathway for acenaphthylene formation is suggested and consists of benzyne recombination followed by hydrogen attack and isomerization. Introduction molecular-beam sampling coupled to mass spectrometry for different fuels such as acetylene [6] and In epidemiological studies, air pollution has been benzene [6,7]. Molecular-beam sampling allows the positively associated with death from lung cancer measurement of radical intermediates, which is ex- and cardiopulmonary disease [1]. Combustion pro- tremely valuable for the testing of kinetic models, cesses are major sources of airborne species of but, unfortunately, sooting conditions are prohibitive health concern; and a possible explanation of the for this technique. Probe sampling and subsequent health hazardous effect of atmospheric aerosols is analysis by gas chromatography coupled to mass their association with polycyclic aromatic hydrocar- spectrometry (GC-MS) allowed the measurement of bons (PAH) [2]. Many PAH identified in aerosols concentration profiles for stable species up to coro- have been found to be mutagenic [3], and their role nene (C24H12) in premixed propane, acetylene, and as precursors of soot has been discussed [4,5]. benzene flames at reduced pressure [8], and up to Therefore, a better understanding of chemical re- pyrene (C16H10) in premixed methane, ethane, and action pathways leading to PAH formation is an es- propane flames at atmospheric pressure [9]. Con- sential issue in order to reduce their environmental centration profiles of PAH up to ovalene (C32H14) impact and to improve combustion processes. Due and for the fullerenes C60,C70,C76,C78, and C84 to the complexity of such processes, the assessment have been measured using gas and liquid chroma- of chemical reaction networks requires experiments tography (GC-MS and HPLC) [10]. Detailed kinetic in well-defined flow systems such as well-stirred re- models describing PAH growth have been devel- actors, plug-flow reactors, or premixed flames. Such oped and tested for premixed methane [11], ethane systems allow the use of numerical modeling, taking [11], acetylene [12], and ethylene [12,13] flames. into account sets of chemical reactions sufficiently The present work focuses on the detailed descrip- large to describe individual reaction steps. Flame tion of the first steps of the growth of PAH in a nearly structures of laminar, premixed, fuel-rich, low- sooting, low-pressure, premixed, laminar, one-dimen- pressure flames have been measured by means of sional benzene/oxygen/argon flame (equivalence 2609 2610 SOOT FORMATION AND DESTRUCTION Fig. 1. Comparison between experimental mole fraction profiles and model predictions in a nearly ס sooting benzene/oxygen flame (␾ 2.67 ,1מcm s 50 ס argon, v 30% ,1.8 kPa). Temperature:—(experiment, ᭹ [7]); C2H2: (experiment [7], right scale), ---- (prediction, right scale); ᭿ C5H5: (experiment [7], left scale), ••• (prediction, left scale.) argon, gas velocity at burner at new set of experimental data measured recently by 30% ,1.8 ס ratio ␾ -kPa) by Benish [14] in a nearly sooting, low-pressure, pre 2.67 ס pressure ,1מcm s 50 ס K, v 298 means of kinetics modeling. This flame has been mixed benzene/oxygen/argon flame using nozzle chosen because of the availability of a large set of beam sampling followed by radical scavenging [15]. experimental data and different modeling studies for The computations were conducted with the PRE- comparison. Bittner and Howard [7] used molecu- MIX flame code [20] using an experimental tem- lar-beam sampling and measured concentration pro- perature profile [7], shown in Fig. 1. Thermody- files for stable species up to 202 amu and radicals namic and kinetic data of the model were critically up to 91 amu. A temperature profile was obtained reviewed, updated if necessary, and completed. For by means of thermocouple measurements, taking instance, the formation of benzoquinone (C6H4O2) into account heat losses by radiation. The successful [18,21] was added to the model. Subsequent ben- testing of the model for smaller stable and, in par- zoquinone pyrolysis and oxidation reactions [22] ticular, radical intermediates involved in the growth with acetylene and CO as major final products were process is essential for the confident application of implemented, as well as recent kinetic data on the a model to larger species and the assessment of po- consumption of cyclopentadiene [23] and cyclopen- tential errors and uncertainties. tadienyl [24], the latter leading via unimolecular de- Recently, the body of data available for Bittner and cay to C H and C H . This addition of supplemen- Howard’s nearly sooting benzene flame was ex- 2 2 3 3 tary pathways for the consumption of cyclic C6 and tended for radicals up to 201 amu and for stable C species and acetylene formation led in compari- species up to 276 amu [14], using nozzle beam sam- 5 son to the initial model [19] to a significant improve- pling followed by radical scavenging with dimethyl ment of the prediction capability for cyclopenta- disulfide (DMDS) and subsequent analysis by GC- dienyl, phenyl, and acetylene (Figs. 1 and 2). MS [15]. This technique allowed for the identifica- Possible reasons for the remaining overpredictions tion of specific PAH radicals, that is, identification of the carbon site where hydrogen abstraction oc- of cyclopentadienyl and phenyl are discussed later. curred, and their individual quantification using For species with unknown or poorly known ther- standard compounds for the analysis of the different modynamic properties, density functional theory cal- scavenging products. culations were carried out with the DGAUSS pack- Oxidation chemistry of this flame has been studied age [25] using the BLYP functional in conjunction by different authors using kinetic modeling [16–18]. with the DZVP basis set. The geometries were op- The starting point of the present work was a recently timized, followed by determination of entropies and published PAH model [19] which has been tested heat capacities after vibrational analysis. Heats of with encouraging results against Bittner and How- formation were taken from the NIST database [26] ard’s data [7], and also for a sooting, premixed ben- unless otherwise mentioned. For species for which no value was recommended, as was the case for most ס argon, v 10% ,2.4 ס zene/oxygen/argon flame (␾ kPa) studied experimentally by radical species, heats of formation were determined 5.33 ס p ,1מcm s 25 Grieco et al. [10]. by means of isodesmic reaction, that is, reactions which maintain the overall number and types of bonds. The thermodynamic data of species involved Approach in the studied chemical reaction pathways and de- An existing kinetics model describing the forma- duced in the present work are given in Table 1; cor- tion of PAH [19] was refined and tested against the responding structures are shown in Table 2. FORMATION OF PAH IN BENZENE FLAMES 2611 Fig. 2. Comparison between experimental mole fraction profiles and model predictions in a nearly ס sooting benzene/oxygen flame (␾ 2.67 ,1מcm s 50 ס argon, v 30% ,1.8 ᭹ kPa). C6H5: (experiment [14], left scale), —— (prediction, left scale); biphenyl: ᭿ (experiment [14], left scale), ---- (prediction, left scale); phenanthrene: ᭜ (experiment [14], right scale), •••• (prediction, right scale). TABLE 1 Thermodynamic data of species used in the present work 0 0 Species DHf Sf Cp, 300K Cp, 400K Cp, 500K Cp, 600K Cp, 800K Cp, 1000K Cp, 1500K a C2H2 54.19 48.66 11.32 12.47 13.39 14.15 15.32 16.21 17.94 Cylcopentadienyl

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