A Comparison of the Effect of C 4 Submechanisms on the Rate Parameters Derived from a Cyclopentadiene Combustion Model

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1 th US National Combustion Meeting Organized by the Eastern States Section of the Combustion Institute and Hosted by the Georgia Institute of Technology, Atlanta, GA March 20-23, 2011 A Comparison of the Effect of C 4 Submechanisms on the Rate Parameters Derived from a Cyclopentadiene Combustion Model Yelena Kozachkova 1 and Robert G. Butler 1 1 Department of Natural Sciences, Baruch College, CUNY, New York, NY A detailed, quantitative, zero-dimensional kinetic C 5 submodel with thermodynamic parameters was integrated with literature submodels for 1-methyl naphthalene, toluene and two different models for C 4 oxidation. The models, one with 137 species and 720 reactions and the second with 138 species and 726 reactions, were subjected to comparison with the experimental atmospheric pressure flow reactor cyclopentadiene results at K, [c-c 5 ] = ppmv, = 0.6, 1.0, 1.6 and pyrolysis using flux and sensitivity analyses. The models include detailed oxidation and pyrolysis chemistry of methyl naphthalene, naphthalene, indene and alkyl aromatics that are important intermediates in the combustion scheme. A number of species and isomers have been included which were not dealt with in previous literature models. Reactants and products for five combination reactions of two C 5 species have been determined. Eleven reaction rate constants have been derived directly from the experimental data. The rate constant for one of the main reactions leading to the formation of cyclopentadienone, c-c 5 + OH c-c 5 H 4 OH + H was found to be 5x10 11 cm 3 mol -1 s -1 based on the first model and 2.5x10 11 cm 3 mol -1 s -1 based on the second, a change of -50%. Other changes in derived rates are discussed. Introduction Polycyclic Aromatics (PAHs), soot precursors and soot form during combustion, in both diffusion and pre-mixed systems. Soot refers to particulate matter (PM) aerosols formed from hydrocarbons; PM may be solid, liquid (tar) or a mixture. Hydrocarbon combustion is significant source of PM 2.5 and PM 0.1 emissions in the US and vehicular emissions are the major source of PM 0.1 [1]. Local particulate concentrations correlate directly with increased mortality due to cancer, cardiac and pulmonary diseases [2]. Understanding the combustion processes involving soot requires elucidation of the mechanism involving the resonantly stabilized cyclopentadienyl (CPDyl) radical, which is needed for accurate modeling of species profiles in combustion experiments [3], in flame speed measurements for aromatics [4] and autoignition in shock tubes [5]. The accepted mechanism [6] at flow reactor conditions for alkyl benzene oxidation is oxidative attack on the alkyl side chain leads to the formation of phenyl and then phenoxy. Phenoxy decomposes to CPDyl which subsequently reacts to form cyclopentadienoxy (c-c 5 O) and then undergoes ring opening. 1

2 The original model was developed to analyze the experimental cyclopentadiene (CPD) data from the Princeton adiabatic flow reactor at K, [CPD] = ppmv, = 0.6, 1.0, 1.6 and pyrolysis [7].The model included the C 1-4 submechanism of Marinov et al. [8] which proved adequate for the development of the C 5 submodel. Since developing the original model, considerable progress has been made in understanding the chemistry of CPDyl and we sought to update our model. Laskin et al. s [9] study of the oxidation of 1,3-butadiene affords us the opportunity to use their USC Mech Version II C 4 submodel validated in the same apparatus and under similar conditions as our experimental CPD data. Furthermore, since Marinov et al. s [8] work is at flame temperature we are concerned that certain reactions they assumed to be at equilibrium or reacting at the collision rate might not do so under our cooler and more dilute conditions. We replaced Marinov s et al. C 1-4 submodel and our H 2 -O 2, C 1-4 thermodynamics with the corresponding kinetics and thermodynamics of Laskin et al. [9]. 277 reactions were replaced; we have updated the models to include the most recent kinetic inputs of Marinov et al. [10]. Where reactions in Marinov et al. were not present in the USC mechanism, we have chosen to retain them. While this update is expected to improve the agreement with smaller species profiles, we also investigated how this change affected profiles of the fuel and higher species. We specifically looked at the sensitivity of the eleven reaction rates that were used to tune the original model. Experimental One of us, (RGB), used Princeton s turbulent (Re ~4000) adiabatic plug flow reactor to produce highly detailed, time-resolved, experimental data on the oxidation and pyrolysis of CPD. The mass flow rates, probe position and measured cold, non-reactive flow profile are used to determine the elapsed reaction time. A water-cooled probe axially samples and quenches the reactive mixture, protonating radical species to their parent species. Resonantly stabilized species, such as CPDyl, contribute significantly to the reported parent species concentration. Thus when comparing the modeling results to experimental data, we have added the concentration of the resonantly stabilized species to the parent species. A HP5890 Series II plus GC FID analyzes gas samples using a PoraPLOT Q column (10 m x 0.32 mm, Chromopack) and a DB-5 column (30 m x 0.32 mm, J&W Scientific). A GC FT- IR (Nicolet Series 800 Interface) was used to detect oxygenates that co-elute with other species. The detection limit of the FT-IR is approximately 20 ppmv [11], which is the maximum concentration of undetected species, such as cyclopentadienone (CPDone). Reproducibility measurements find O 2 and the C 1 4 aliphatics have a ±5% uncertainty. The fuel and one-ring aromatics have a ±10% uncertainty. Larger aromatics have a ±20% uncertainty. The limit of detection was ~2 ppmv carbon atoms. An absolute uncertainty in the reaction time is ±15%; relative uncertainties are much less (1 2%). Additional details of these experiments are presented elsewhere; [7][11][12]. 2

3 Mole Fraction x 10 6 (ppm) Mole Fraction x 10 6 (ppm) Paper # Provided from CI Office Model Emdee et al. s [12] EBG model successfully predicts the oxidation of benzene and toluene in the Princeton flow reactor. Since its publication, the EBG C 5 submechanism has been extensively used in the literature; [10], [13] and references therein. The experimental conditions were modeled as a zero-dimensional adiabatic plug-flow reactor using the CHEMKIN PRO software [14]. Regarding Figure 1, the consumption of CPD Comparison of Experimental (Symbols) and Modeling (Lines) Results for the EBG Model: Major Species phi = 1.0, init temp = 1150 K, init fuel = 2100 (Cyclopentadienone, C H O, was not observed experimentally.) by the EBG model is too rapid. The major Sum of C H and C hydrocarbon product was correctly predicted to be 2000 C H 2 2 acetylene, but at eight times the experimental 1500 concentrations. The second most abundant hydrocarbon was projected to be CPDone with a cyclopentadiene C H O C H ppmv concentration as compared to being C H experimentally undetectable (<20 ppmv). The 2 2 C H negligible production of the major oxidation species methane and naphthalene was unacceptable. Figure Time (ms) 1 Comparison of Experimental (Symbols) and Modeling (Lines) Results In order to expand the capabilities of the EBG Temperature Variation Oxidation Results: Mole Fractions of Sum of CH and C phi = 1.0, initial fuel = 2000 ppm model, the C 8-12 model of Pitsch [15] was added. The 2500 open: init temp = 1100 K majority of the C 1 -C 4 and H 2 -O 2 chemistry was filled: init temp = 1150 K checkered: init temp = 1200 K 2000 supplied by Marinov et al. [8]. The model was 1500 optimized by fitting eleven reactions to the data. Thermodynamic inputs came from Marinov [8], 1000 Burcat [16], and the THERM software [17].Sensitivity 500 and Flux analysis contributed to the building of the model and understanding its dynamics. A sample of the results can be seen in Figure 2. Time (ms) Figure 2 Discussion Eleven rate constants: k 123, k 128, k 134, k 154, k 169, k 176, k 197, k 218, k 220, k 221 and k 226 were derived from the data using the model. All reaction numbers refer to the original model [7] which is available from the authors at The reactants of the combination of two C 5 rings to ultimately yield naphthalene have previously been assumed by most workers to be two CPDyl radicals. The present work predicts that under our conditions, with relatively high CPD concentrations, that CPDyl reacts faster with its parent. Rate constants, reactants and products for the following five reactions of two C 5 rings were determined by tuning the model against the pyrolysis data from K with 2000 ppmv initial fuel to the profiles for naphthalene (k 128 ), indene (k 154 ), benzene and total C 4 (k 176 ), dihydronaphthalene (k 123 ) and total phenyl-c 4 (k 134 ). The best reactant pairs were found by matching the shape of the concentration profiles; see Figure 3. The product of reaction R128, c- C 10 H 9, rapidly decomposes to naphthalene and H. In order to match the shape of the naphthalene 3

4 Mole Fraction x 10 6 (ppm) Paper # Provided from CI Office profile, combination of CPD with CPDyl is indicated. In other systems, with lower CPD concentrations, the combination of two CPDyl moieties is likely to dominate. The substitution of the USC mechanism for the Marinov et al. mechanism results in increased CPDyl concentration compared to CPD. This has relatively little effect on k 128 ; see Table 1, which demonstrates the robustness of the C 5+ submechanism. Because of the decrease in the concentration of CPD, it is necessary to increase both k 123 and k 134 by 85%. The relatively smaller changes in k 154 and k 176 are in between those of k 128 and k 123, k 134, but they are opposite in sign. The increased fragility of n-c 4 in USC mechanism is partly responsible Comparison of Experimental (Symbols) and Modeling (Lines) Results: Naphthalene phi = 100, init temp = 1150 K, init fuel = 2100 ppm Experimental Data Has Been Shifted +53msec. Naphthalene Time (msec) Figure 3 Reactants: Solid Line: C 5 + C 5 Dashed Line: C 5 + C 5 Dotted Line: C 5 + C 5 c C 10 H 9 H H c C 9 H 8 CH 3 H n C 4 c C 10 H 10 H H C 4 H H (R128) (R154) (R176) (R123) (R134) Comparisons between the model and concentration of C 3 species in the pyrolysis experiments yielded an experimental branching ratio for k 220 : k 221 ; R221 was the major source for C 3 species. Roy et al. [18] [19] [20] previously measured the total rate and had derived a theoretical branching ratio that was somewhat lower than the value of the current work. H H 2 H a C 3 C 2 H 2 (R220) (R221) Substitution of the USC mechanism did not produce a significant change in the branching ratio. An upper limit was found for k 226 ; the rate of Emdee et al. [12] was reduced from 3x10 13 cm 3 mol -1 s -1 to the value of 5x10 11 cm 3 mol -1 s -1 using the Marinov et al. submechanism in order to limit the amount of CPDone produced by the decomposition of C 5 H 4 OH to < 20 ppmv. Substitution of the USC mechanism required another 50% reduction in k 226 due to the increased concentration of CPDyl during much of the reaction. OH c C 5 H 4 OH H (R226) 4

5 The consumption of two trace species was treated by assigning minimum values to k 169 for benzofuran and k 197 to 2-cyclopentenone. O + O O O c-c 5 O C 2 H 4 + C 2 H 2 + CO + CHO + CO (R169) (R197) The values k 167 or k 197 were the same with both submechanisms. The oxidation model was calibrated with the ad hoc chain branching reaction R218. The reduction from the literature value [21] of k 218 from exp (-86651/RT) to 4x10 6 regardless of submechanism in the present work is a mark of improvement. Our most recent modeling effort eliminates this reaction entirely [22]. O 2 c 2,4 C 5 O OH (R218) Table 1: Comparison of rates and rate constants for the two C 4 submodels. Reaction # k original k new Original Rate at 1150 K New Rate at 1150 K % Change x10 11 T 2.79 e /RT 9.63x10 12 xt 2.38 exp /RT 7.86x x % x10 11 T 2.70 e /RT 6.37x10 12 xt 2.09 exp /RT 1.90x x % x10 10 T 2.80 e /RT 5.10x10 11 xt 1.68 exp /RT 6.24x x % x10 13 T 1.63 e /RT 4.58x10 13 xt 2.58 exp /RT 4.50x x % x10 9 T 2.29 e /RT 1.28x10 14 xt 2.87 exp /RT 1.69x x % Units: moles, cm 3, joules, K Conclusion The lack of significant change in the main consumption reaction of CPD under pyrolysis conditions demonstrates the robustness of the C 5 submechanism. Nevertheless, this crude sensitivity experiment again demonstrates the autocatalytic nature of combustion systems, such that even in hierarchical models, the lower tiers can have a significant effect on fuel consumption and intermediate formation. We intend to continue upgrading our CPD mechanism and apply it to additional data sets. Acknowledgments The authors thank Professors Irvin Glassman and Kenneth Brezinsky for their advice and support during the experimental phase of this project and during the development of the original model. Thomas Toshkoff, Eugene Tsypin, Liz Mechel, Ryan Closson and Vik Gil contributed to 5

6 the model database and data processing. The results presented are part of a research program supported by PSC-CUNY Award and sponsored by the Department of Energy, Office of Basic Energy Science, through grant DE-FG02-86ER3554 and are gratefully acknowledged. References 1 D. W. Dockery, C. A. Pope III, X. Xu, J. D. Spengler, J. H. Ware, M. E. Fay B. G. Ferris Jr. F. E. Speitzer, NEJM, 329 (24) (1993) J. A. Araujo, B. Barajas, M. Kleinman, X. Wang, B. J. Bennett, K. W. Gong, M. Navab, J. Harkema, C. Sioutas, A. J. Lusis, A. E. Nel, Circ. Res. (2008) 102, , DOI: /CIRCRESAHA H. Wang, M. Frenklach, Eastern States Section Meeting, Combustion Institute, Princeton, NJ, 1993, pp S. G. Davis, H. Wang, K. Brezinsky, C. K. Law, Proc. Combust. Inst. 26 (1996) M. Yahyaoui, M. H. Hakka, P.A. Glaude, F. Battin-LeClerc, Int. J. Chem. Kinet 40 (2008) I. Glassman, R. A. Yetter (2008). Combustion (4th ed.), Academic Press, Inc., MA. 7 R. G. Butler (2001). Combustion Chemistry of 1,3-Cyclopentadiene. Ph.D. Thesis, Department of Chemistry, Princeton University, Princeton, NJ. 8 N. M. Marinov, P. J. Pitz, C. K. Westbrook, M. J. Castaldi, S. M. Senkan, Comb. Sci. Tech (1996) pp A. Laskin, H. Wang, C. K. Law, Int. J. Chem. Kinet. (2000) 32(10) pp H. Wang, X. You, A. V. Joshi, S. G. Davis, A. Laskin, F. Egolfopoulos, C. K. Law, USC Mech Version II. High-Temperature Combustion Reaction Model of H2/CO/C1-C4 Compounds. May N. M. Marinov, P. J. Pitz, C. K. Westbrook, A. M. Vincitore, M. J. Castaldi, S. M. Senkan, C. F. Melius, Comb. Flame, (1998) Vol. 114 (1/2), pp R. G. Butler, I. Glassman, Proc. Combust. Inst., 32(1), (2009) pp , doi: /j.proci J. L. Emdee, K. Brezinsky, I. Glassman, J. Phys. Chem. 96 (1992) A. Burcat, M. Dvinyaninov, Int. J. Chem. Kinet., 29 (1997) Software by Reaction Design, 6440 Lusk Boulevard, Suite D-205, San Diego, CA 92121, 15 H. Pitsch, Twenty-Sixth Symposium (International), The Combustion Institute, (1996) pp A. Burcat, B. McBride Ideal Gas Thermodynamic Data for Combustion, Air Pollutant Use. TAE 732, January 1995, Technion-Israel Institute of Technology, Faculty of Aerospace Engineering. 17 E. R. Ritter, J. W. Bozzelli. Int. J. Chem. Kinet. 23 (1991) K. Roy, P. Frank, Th. Just, Israeli J. Chem., 36 (1996) K. Roy, Ch. Horn, P. Frank, V. Slutsky, Th. Just, Proc. Combust. Inst. 27 (1998) K. Roy, M. Braun-Unkhoff, P. Frank, Th. Just, Int. J. Chem. Kinet., 33(12) (2001) D.A. Bittker, Comb. Sci. Tech. 79 (1991) pp Abstract submitted to the 7 th International Conference on Chemical kinetics, July 10-14, 2011, MIT, Cambridge, MA. 6 6