High Temperature Compact Mechanism 1 Princeton 6th International Development for Large Alkanes: University Conference on Chemical n-Hexadecane Mechanical & Aerospace Kinetics Engineering Department 1 1 1 July 25-29, 2005 Marcos Chaos , Andrei Kazakov , Frederick L. Dryer , Zhenwei Zhao1, and Stephen P. Zeppieri2 2 United National Institute of Technologies Standards and Technology [email protected] Research Gaithersburg, MD http://www.princeton.edu/~combust Center

Introduction Mechanism Development Improving internal combustion engine efficiency while minimizing pollutant emissions is a topic of continued interest. To this end, robust chemical The current mechanism was built in a hierarchal manner. A high temperature hexadecane submechanism was kinetics models and more accurate numerical tools are needed to improve our understanding of the complex physical and chemical processes taking place combined with our recent update of the H2/O2 kinetic model [3] along with C1-C4 kinetics including the in combustion systems. This study focuses on the development of a high temperature mechanism for the oxidation and pyrolisis of n-hexadecane. modifications listed in [4]. In the hexadecane submechanism, five classes of reactions are treated: Hexadecane is a primary reference fuel for cetane rating of diesel fuels and is a major component in simple mixtures used to emulate the autoignition and Table 1. H-atom abstraction rates. ™ C-C, C-H bond homolysis: Rate constants for these combustion behavior of distillate-type fuels utilized in advanced engine designs (e.g. direct injection and homogeneous charge compression ignition (cm3-mol-s-cal) unimolecular fuel decomposition reactions were diesel engines). Models of this large hydrocarbon molecule [1, 2] are scarce and involve a large number of reactions and species which make them not Radical Log (A)/H b Ea calculated from the recombination reactions of two radical readily applicable to the simulation of multi-dimensional fluid flow problems. Recognizing the problem of mechanistic complexity versus predictive Primary sites species to form the stable parent molecule using robustness, this study aims to develop a partially reduced skeletal mechanism for large alkane molecules. H 6.97 2.00 7,700 thermochemistry. For reactions leading to hexadecyl + H, O 5.62 2.40 5,500 a recombination rate constant of 1x1014 cm3/(mol s) was OH 6.37 1.80 974 assumed [5]. For reactions involving CH the -2 3 10 13 3 HO2 3.90 2.55 16,500 recombination rate was assumed to be 2x10 cm /(mol s) CO CO CO CO CH3 11.34 0.00 11,600 whereas for decompositions where the smallest product is 2 CO2 CO2 CH O -3 CH O -3 CH O 12 2 10 2 10 2 O2 12.82 0.00 50,900 ethyl or larger, the recombination rate was set to 7x10 -3 H H H 10 2 2 2 3 C2H5 10.72 0.00 12,300 cm /(mol s) [5].

C2H3 11.22 0.00 18,000 ™ H-atom abstraction: Abstraction by H, O, OH, HO2, aC3H5 11.12 0.00 20,500 -4 -4 CH , O , C H , C H , and aC H is considered. Rates are 10-4 10 10 3 2 2 3 2 5 3 5 Secondary sites as listed in Table 1. H 6.64 2.00 5,000 O 4.76 2.60 1,769 ™ Isomerization: 1,4/1,5/1,6 internal hydrogen transfers -5 10 -5 -5 were considered. Rates follow the rules of [6]. 10 10 OH (C-C-C-) 5.70 2.00 -596 CH 4 10-3 (-C-C-C-) 5.47 2.00 -1,391 ™ β-scission: The alkyl radicals formed are assumed to C2H6 HO 3.68 2.60 13,900 1,3-C4H6 2 decompose through β-scission into a 1-olefin and a C H / 2 -4 2 4 10 CH 11.3 0.00 9,500 smaller alkyl radical which, in turn, also decomposes -4 -4 3 10 C H 10 3 6 O 13.3 0.00 47,590 aC H 2 following this β-scission pattern. The decomposition of action 3 4 action action r r r C2H5 10.10 0.00 10,400 all radicals larger than ethyl is treated in this manner. -5 10 -5 C H 11.30 0.00 16,800 Rate constant were calculated from the reverse reaction,

Mole F 10 2 3 Mole F Mole F Mole -5 10 aC3H5 10.70 0.00 16,100 that is, the addition of an alkyl radical across the double CH 4 C2H4 / 2 CH4 C2H4 / 2 C H C H C H C H 2 6 3 6 2 6 3 6 bond of an alkene (Log A = 11, Ea = 7,200 cal/mol [5]). 10-6 -6 1,3-C4H6 aC3H4 10 1,3-C4H6 aC3H4 ™ Olefin reactions: Olefins larger than C4 follow unimolecular decomposition pathways as well as H 1-C4H8 / 5 1-C4H8 / 5 1-C H / 5 abstraction (by H, O, OH, and CH ) from primary, secondary and vynilic sites with subsequent rapid 4 8 1-C5H10 3 1-C5H10 1-C5H10 1-C6H12 1-C6H12 decomposition. Rates listed in [7] and [8] were used. 1-C H -5 6 12 1-C7H14 10 -5 1-C7H14 10 1-C H -5 7 14 10 1-C8H16 1-C8H16 1-C8H16 1-C9H18 1-C9H18 1-C10H20 Mechanism Reduction 10-6 An approach similar to that previously applied in our laboratory for n-decane [9] is extended here for n- 10-6 ABC10-6 hexadecane. The originally developed detailed mechanism (above) included detailed treatment of the alkyl radicals formed, incorporating both internal hydrogen isomerization and β-scission reactions and consisted of 1000 1040 1080 1120 1160 1200 1000 1040 1080 1120 1160 1200 1000 1050 1100 1150 1200 1250 T (K) T (K) T (K) 151 species undergoing 1,155 reactions. In order to reduce the number of required species and simultaneously 2.5x10-4 maintain the desired mechanistic detail, it was assumed that each of the alkyl radicals in the system exists in 1.6x10-2 φ = 0.5 φ = 0.5 isomeric partial equilibrium above the C4 level. Using the corresponding isomerization rates, the relative -4 2.0x10 φ = 1.0 φ = 1.0 concentrations of each isomer within a given radical group were calculated as a function of temperature and an φ = 1.5 φ = 1.5 1.2x10-2 Arrhenius-type expression (i.e. k = ATb exp (E /RT)) was fitted to the results. The calculated Arrhenius 1.5x10-4 a action

r parameters were used to modify the rates associated with the decomposition of each isomer radical. As a result, 8.0x10-3 1.0x10-4 only a single radical species was needed to represent all the isomers associated with it and the respective Mole Fraction Mole Mole F Mole reaction channels associated with each isomer. Using this approach, the resulting reduced model consisted of -3 5.0x10-5 4.0x10 DE 98 species and 944 reactions. 0.0 0.0 1000 1040 1080 1120 1160 1200 1000 1040 1080 1120 1160 1200 T (K) T (K) 45 Figure 1. Comparison of experimental [4] and computed species profiles for the oxidation of hexadecane at atmospheric pressure in a Jet Stirred A Note on Laminar Flame Speed The laminar flame speed embodies the fundamental diffusive, 40 Reactor (JSR) with a mean residence time of 70 ms. (A) φ = 0.5, 0.03% C16H34 / 1.47% O2 / 98.5% N2; (B) φ = 1.0, 0.03% C16H34 / 0.735% O2 / 99.235% N ; (C) φ = 1.5, 0.03% C H / 0.49% O / 99.48% N ; (D) hexadecane consumption; (E) oxygen consumption. reactive, and exothermic properties of a given fuel/oxidizer 2 16 34 2 2 35 mixture and is commonly used to partially validate kinetic models. No hexadecane flame speed experimental data are 30

currently available in the literature. However, Davis and Law 25 0.30 H

2 (cm/s) Speed Flame n-pentane C2H6 [11] showed that normal alkanes (from ethane through heptane) Validation CH n-heptane 4 C H 20 0.25 3 6 C 2H4 / 2 have almost the same flame speeds (since they have similar n-hexadecane, model results ™ Oxidation: To date, the only detailed experimental study on the oxidation of hexadecane is 0.20 structure and flame temperature). It is safe to assume, thus, that 15 that of Ristori et al. [2]. They studied the gas–phase oxidation of C16H34 in a Jet Stirred 0.6 0.8 1 1.2 1.4 hexadecane flame speeds will be similar to those of other alkanes Equivalence Ratio Reactor (JSR) at atmospheric pressure over 1000-1250K temperature range and for 0.15 and, in fact, the current model shows this (Figure 3). The ability Figure 3. Calculated hexadecane laminar equivalence ratios spanning from 0.5 through 1.5. Concentration profiles were obtained for 0.10 of the mechanism to accurately predict these flame speeds flame speed compared with experimental major and minor species (up to decene). Figure 1 shows the results obtained from the depends on the small-molecule kinetics as discussed in [4]. data [11] for other large alkanes. current model compared against the experimental data [4]. As can be seen, the model 0.05

successfully captures reactivity trends as the equivalence ratio is varied. The model also 0

C3H8 predicts well the formation of alkenes. Some disparities exist in the prediction of CO and 1-C H 1,3-C H 5 10 Summary 4 6 0.08 C H 1,3-butadiene. Such discrepancies have previously been reported [4, 9]; our laboratory is 1-C H 16 34 4 8 A partially reduced skeletal reaction mechanism for the high temperature oxidation and pyrolisis of n- currently further refining the mechanism below the C3 level in order to improve these results. 0.06 hexadecane has been developed consisting of 98 species and 944 reactions. By assuming that the alkyl radicals

The model, however, yields comparable results when compared to the model developed in action r formed during these processes are in isomeric partial equilibrium, a considerable reduction in the required [2] which consists of nearly 300 species and 1,800 reactions. 0.04 number of species was achieved while still maintaining the desired level of detail. The mechanism has been Mole F shown to reproduce the limited available experimental data and its performance is comparable to that of other ™ Pyrolisis: In industrial applications, alkenes are usually “manufactured” by pyrolisis of raw 0.02 much larger mechanisms available in the literature. The method described here can, thus, be successfully materials such as LPG and naphtas. In recent years, however, the use of higher-boiling applied to the generation of compact mechanisms for other large alkanes found in modern fuels. Future materials has increased, especially gas-oil. Hexadecane kinetics studies are, thus, beneficial 0

0.010 1-C6H12 measurements using Princeton’s Variable Pressure Flow Reactor (VPFR) will be performed to generate for these applications as the carbon content of gas-oil is similar to that of hexadecane (it is 1-C14H28 1-C8H16 Benzene additional validation data to further refine the mechanism. 1-C9H18 noted, however, that gas-oil is a complex mixture of a large number of paraffins, aromatics, 0.008 and naphtas). Figure 2 compares computed species profiles against available data [10] obtained during the thermal (steam) cracking of hexadecane at atmospheric pressure in a 0.006 References tubular quartz reactor. Data is plotted with respect to the percentage of fuel conversion, X = 1. C. Chevalier, W. J. Pitz, J. Warnatz, C. K. Westbook, and H. Melenk, Proc. Combust. Inst. 24, 93-101 (1992). 0.004 2. A. Ristori, P. Dagaut, and M. Cathonnet, Combust. Flame 125, 1128-1137 (2001). 100 [1 - ([C16]f / [C16]o)] where [C16]o is the initial hexadecane concentration and [C16]f is the 3. J. Li, Z. Zhao, A. Kazakov, and F. L. Dryer, Int. J. Chem. Kinet. 36, 566-575 (2004). amount of hexadecane recovered. Reasonable agreement is observed for major species. 0.002 4. Z. Zhao, J. Li, A. Kazakov, F. L. Dryer, and S. P. Zeppieri, Combust. Sci. Tech. 177, 89-106 (2005). Trends for large alkenes are also reproduced although it is noted that considerable 5. D. L. Allara and R. Shaw, J. Phys. Chem. Ref. Data 3, 523-559 (1980). 0 6. C. K. Westbrook and W. J. Pitz, pp.39-54 in Complex Chemical Reaction Systems: Mathematical Modeling and experimental uncertainty exists for these measurements since the concentration of species 65 70 75 80 85 90 Fuel Conversion(%) Simulation, Springer-Verlag, New York (1987). larger than 1-pentene was very low. The model does capture the relative higher 7. W. Tsang, J. Phys. Chem. Ref. Data, 17, 887-952 (1988). Figure 2. Experimental [10] and calculated 8. W. Tsang, J. Phys. Chem. Ref. Data, 20, 221-273 (1991). concentration of 1-hexene which is probably caused by the 1,5 isomerization of alkyl species profiles during the steam pyrolisis of 9. S. P. Zeppieri, S. D. Klotz, and F. L. Dryer, Proc. Combust. Inst. 28, 1587-1595 (2000). radicals. hexadecane at a temperature of 973 K and 10. D. Depeyre, C. Flicoteaux, and C. Chardaire, Ind. Eng. Chem. Proc. Dev. 24, 1251-1258 (1985). atmospheric pressure. 11. S. G. Davis and C. K. Law, Combust. Sci. Tech. 140, 427-449 (1998).