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Investigating the Role of Methane on the Growth of Aromatic Hydrocarbons and Soot in Fundamental Combustion Processes

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Investigating the Role of Methane on the Growth of Aromatic Hydrocarbons and Soot in Fundamental Combustion Processes Combustion and Flame 134 (2003) 249–260 Investigating the role of methane on the growth of aromatic hydrocarbons and soot in fundamental combustion processes J. F. Roeslera,*, S. Martinota, C.S. McEnallyb, L.D. Pfefferleb, J.-L. Delfauc, C. Vovellec aIFP, 1 et 4 av. de Bois-Pre´au, 92852 Rueil-Malmaison Cedex, France bDepartment of Chemical Engineering and Center for Combustion Studies, Yale University, New Haven, CT 06520-8286 USA cLaboratoire de Combustion et Syste`mes Re´actifs, CNRS, 45701 Orle´ans Cedex 2, France Received 30 November 2002; received in revised form 24 March 2003; accepted 1 April 2003 Abstract Experimental results are presented on the effect of methane content in a non-aromatic fuel mixture on the formation of aromatic hydrocarbons and soot in various fundamental combustion configurations. The systems considered consist of a laminar flow reactor, a laminar co-flow diffusion flame burner, and a laminar, premixed flame burner, all of which operate at atmospheric pressure. In the flow reactor, the experiments are performed at 1430 K, constant C-atom flow rates, 98% nitrogen dilution, C/O ratio ϭ 2, and with fuel mixtures consisting of ethylene and methane. The diffusion flames are performed with fuel mixtures of methane and ethylene diluted in nitrogen to maintain a constant adiabatic flame temperature. The premixed flame experiments are performed with n-heptane and methane mixtures at a C/O ratio ϭ 0.67 with nitrogen-impoverished air. The results show the existence of synergistic chemical effects between methane and other alkanes in the production of aromatics, despite reduced acetylene concentrations. This effect is attributable to the ability of methane to enhance the production of methyl radicals that will then promote production channels of aromatics that rely on odd-carbon- numbered species. Benzene, naphthalene, and pyrene show the strongest sensitivity to the presence of added methane. This synergy on aromatics trickles down to soot via enhanced inception and surface growth rates by polycyclic aromatic hydrocarbon condensation, but the overall effects on soot volume-fraction are smaller due to a compensating reduction in surface growth from acetylene. These results are observed under the very fuel-rich environments existing in the flow reactor and diffusion flames. In the premixed flames, however, instabilities did not permit investigation of conditions with sufficiently high equivalence ratios to perturb the aromatic and soot-growth regions. © 2003 The Combustion Institute. All rights reserved. Keywords: Soot; Aromatic hydrocarbons; PAH; Methane; Combustion; Flow reactor; Diffusion flame; Premixed flame 1. Introduction tion are complex. Early mechanistic models [1,2] suggested that the first aromatic ring was formed by The molecular growth-processes leading to poly- the addition of acetylene to C radicals and that the cyclic aromatic hydrocarbons (PAH) and soot forma- 4 key growth process toward increasingly larger PAH occurred by continuous addition of C2H2. Bi-aryl * Corresponding author. Tel: ϩ(33)1-47-52-5811; fax: formation by aromatic-ring addition was considered ϩ(33)1-47-52-7069. feasible only in the case of aromatic fuels. This type E-mail address: [email protected] (J.F. Roesler). of pioneering model was able to reproduce most of 0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved. doi:10.1016/S0010-2180(03)00093-2 250 J.F. Roesler et al. / Combustion and Flame 134 (2003) 249–260 the major PAH and soot trends observed experimen- The product samples are quenched and collected tally in combustion processes [1,3,4]. However, it along the center-line axis by means of an oil-cooled neglected other reaction pathways that are increas- probe at 180°C. Only 20% of the total flow is sam- ingly shown to be of importance. pled; therefore, the measurements correspond to the In 1990, Miller and Mellius [5] published work products along the center-line and not to the bulk that suggested that benzene could be formed, in large mean. part, from the re-combination of the odd-carbon- At the probe exit, the soot is filtered at 180°C, and numbered, resonantly-stabilized propargyl radicals. condensable species are then trapped. In a first pass, While this specific reaction is now well-recognized the trap is at ambient temperature, and the low-mo- and has been implemented in many recent kinetic lecular-weight gas-phase species, up to benzene, are models [6-9], there is growing evidence that many analyzed on-line with a Fourier-transform infrared other such radical species are involved in the produc- spectrometer (FTIR). The gas-phase species are then tion of at least the smaller PAH. This evidence is collected for subsequent analysis by gas chromatog- found in various theoretical analyses of reaction raphy (GC) with a flame ionization detector (FID) in channels and in experimental flame and modeling studies [10–14]. order to more accurately quantify the hydrocarbon If these reactions are indeed important, then add- gas-phase species with up to eight carbon atoms. The ing methane to the PAH and soot-forming regions in species quantified solely by FTIR are CO, CO2, and combustion systems should, under some conditions, H2O. The higher-molecular-weight species are col- promote their growth. The premixed flame results of lected on a second pass, with the trap at dry-ice Senkan and Castaldi [15] that show a methane, lam- temperature (Ϫ70°C) that is then washed with inar, premixed, flat flame to produce more benzene CH2Cl2. The detailed speciation of the PAH from the and PAH than a similar ethane flame confirm this line trap is obtained by GC-FID and GC-mass spectrom- of thought. More direct evidence of this effect of etry (MS) analysis. All other wetted surfaces (probe, methane was obtained in flow-reactor studies [16,17]. Teflon liner, and soot filter) are washed ultrasonically The addition of methane was found to enhance the with CH2Cl2. Analysis of the collected solutions formation of aromatics and soot in n-heptane oxida- showed Ͻ 5% loss of any one of the reported PAH to tive pyrolysis. Detailed reaction mechanisms from these surfaces. The total masses of condensable spe- the literature were found to reproduce these effects, cies (hereafter referred to as “tar”) of molecular provided PAH formation was appropriately described weight, corresponding roughly to masses greater than with odd-carbon-numbered radical species. acenaphthylene, and the mass of soot (non-dichlo- The present work further investigates experimen- romethane-soluble, condensable species) are deter- tally the influence of methane as a parameter to be mined gravimetrically. considered in the formation of soot and PAH. Our In the experiments presented here, the fuel was goal was to further quantify the effect, under flow- initially C H that was gradually substituted with reactor conditions, and to determine whether or not 2 4 CH at a constant total-carbon molar content of 3%. these were applicable to classic, laminar, premixed 4 and diffusion flame conditions. A mixture parameter characterizing the initial fuel composition is defined as: XCH 2. Experimental set-ups ␤ ϭ 4 (1) X ϩ 2X CH4 C2H4 2.1. The flow-reactor system and represents the fraction of fuel carbon injected as methane. This parameter was varied from 0 to 0.5. The flow-reactor device and the analytical tech- The oxygen content was also held constant to main- niques for quantifying the composition of the sam- ϳ pled products have been described in detail previ- tain a C/O ratio of 2.0, leading to an equivalence ously [17,18]. The reactor is made of quartz, with a ratio varying slightly between 6 and 7 due to added 30-mm inside diameter. It is inserted in a three-zone hydrogen in the presence of methane. The experi- furnace. The premixed reactants are rapidly heated in mental conditions investigated were for a tempera- the first zone through a coiled 1.5-m-long tube with ture of 1430 K, atmospheric pressure, and a total flow an inside diameter of 4 mm. The flow enters the rate of 0.54 slpm (standard liters per minute) with larger-diameter test section and is laminarized by nitrogen as the carrier gas. The product samples were passing through a porous quartz disk. The test section collected at a fixed position of 0.25 m downstream of is at atmospheric pressure and is isothermal within the porous quartz disk, corresponding to a bulk mean 5 K throughout a distance of 35 cm. residence time of 0.42 s. J.F. Roesler et al. / Combustion and Flame 134 (2003) 249–260 251 2.2. The laminar co-flow diffusion flame system Table 1 Volumetric flowrates for the laminar diffusion flames* The laminar co-flow diffusion flame burner and ␤ N CH C H Ar the analytical techniques have been described in de- 2 4 2 4 tail previously [19]. The flames are generated with an 0.00 500 0 220 7.27 atmospheric pressure, axisymmetric, co-flowing 0.03 484 11 213 7.15 burner. The fuel mixture flows from an un-cooled 0.05 468 23 206 7.04 12-mm-diameter vertical brass tube. The air flows 0.08 452 35 199 6.93 0.11 436 48 192 6.83 between this tube and a surrounding 108-mm inside- 0.14 417 61 183 6.68 diameter chimney. 0.18 397 75 175 6.53 Temperatures are measured with an un-coated 0.21 378 90 166 6.40 125-␮m wire-diameter/260-␮m junction-diameter 0.25 358 105 158 6.28 type R (Pt-Pt/13% Rh) thermo-couple. Further details 0.29 336 121 148 6.10 are provided elsewhere [20].
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