Cycloparaffins are present in significant quantities in conventional fuels. They could be present with fractions as high as 35% in diesel, 25% in jet fuel, and 10% ...
7th US National Technical Meeting of the Combustion Institute Hosted by the Georgia Institute of Technology, Atlanta, GA March 20-23, 2011
Detailed Chemical Kinetic Modeling of Decalin Combustion Chitralkumar V. Naik Reaction Design, San Diego, California 92121, USA
A detailed chemical kinetic mechanism for combustion of decalin under engine conditions has been developed. The mechanism is developed based on rate rules for alkanes and cyclic alkanes, and built on an existing self-consistent surrogate mechanism for the combustion of gasoline and diesel, including many other fuel components covering n- and iso-alkanes, olefins, cycloalkanes, and aromatics. The decalin mechanism contains newly added 172 species and 484 reaction steps in addition to the starting more than 3500 existing species and 15000 reaction steps in the base mechanism [1]. The resulting mechanism has been validated using the available published fundamental data on oxidation and pyrolysis. It has also successfully been tested to predict the ignition behavior of decalin under HCCI engine conditions. Rate and sensitivity analysis revealed that the low-temperature kinetics schemes similar to those for non-cyclic alkanes are also important for decalin. Hydrogen abstraction reactions from decalin by OH and HO2 radicals are also very important.
1.
Introduction and Background
Multicomponent surrogates for fossil fuels proposed in recent studies [2, 3] usually contain at least one representative component from each major fuel class from the overall composition of conventional fuels. Cycloparaffins are present in significant quantities in conventional fuels. They could be present with fractions as high as 35% in diesel, 25% in jet fuel, and 10% in gasoline worldwide [4, 5]. In addition, fuels derived from oil sands are expected to have a higher cycloalkane content, which will affect the ignition quality of the fuel and soot emissions from its use in engines. Cycloparaffins can play an important role in soot production since they can potentially form polycyclic aromatic hydrocarbon (PAH) directly via dehydrogenation [6] that can lead to increased soot production. Among the cycloalkanes present in conventional fuels, dicycloparaffin content can be 20 to 50% of the total cycloparaffins, particularly in jet fuels [7]. Therefore, it is important to include a dicycloparaffin component in fuel surrogates. Decalin is the simplest dicycloparaffin that is a good choice of component to be used in complex surrogates for diesel and other conventional fuels. The goal of this study is to develop a detailed chemical kinetic mechanism for decalin combustion applicable to engine operating conditions. Decalin is attractive as an endothermic propulsion fuel because of its high thermal stability and because, as a hydrogen donor, it suppresses carbon deposits when pyrolyzed, preventing fouling of fuel delivery systems. Therefore, several literature studies have focused on decalin pyrolysis. However, available experimental data on decalin oxidation are limited. Zhu et al. [8] measured the conversion of fuel in the gas phase oxidative pyrolysis of decalin in a heated flow reactor and also measured conversion for the overall hydrocarbon class of products. Zeppieri et al. [9] used an atmospheric flow reactor to study pyrolysis and oxidation of decalin at 1000-1200 K. Roesler et al. [10] also performed similar experiments to study decalin pyrolysis at 1050 K. Oehlschlaeger et al. [11] measured autoignition time measurements in a shock tube at temperatures above 1100 K and pressures of 12 and 40 atm. Yang and Boehman [12] measured the peak temperatures and pressures at various compression Paper RK76
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ratios in a variable compression ratio HCCI engine. These data can be used to validate the ignition characteristics of decalin under engine conditions. There are two structural isomers of decalin, as shown in Fig. 1: cis and trans. The cis isomer has lower thermal stability, and isomerizes to the trans form at temperatures lower than those typically encountered with fuel combustion in IC engines [13]. For this reason, most combustion studies considered trans-decalin [11, 14, 15]. However, in reality decalin is typically made of 40/60 cis/trans isomer fractions. Cetane numbers for cis-decalin and trans-decalin are 41.6 and 32, respectively, [16] resulting in an approximate cetane number of 35.8 for nominal decalin with isomeric blends.
Fig. 1. Cis (left) and trans structural isomers of decalin. 2.
Mechanism Development The existence of structural isomers in decalin creates some complexity for kinetics modeling at low temperatures, which is important under IC engine conditions. To simplify the mechanism, we have not differentiated between these isomers but rather considered nominal decalin – treated as a 60/40 blend of cis/trans isomers. This approach is also conceptually similar to the approach used by Pitz et al. [17] for low-temperature oxidation mechanism for methylcyclohexane. They modified the rate-rules to account for the boat and chair structure of cyclohexylperoxy radical isomerization. For the decalin mechanism, similar rate-rules apply since no distinction was made between isomers of decalin in this study. The decalin oxidation mechanism includes all the reaction classes as described by Westbrook et al. [18] and also used by Pitz et al. [17] and Silke et al. [5] for the development of the detailed reaction mechanisms for cyclic-alkanes. Reaction classes 1 to 9 [18] are important for high-temperature oxidation while classes10 to 25 are relevant for low-temperature oxidation. Rate-rules used for decalin are similar to those used for the methylcyclohexane mechanism [17]. Thermodynamic properties were generated using the group additivity method [19]. As described elsewhere [1, 20], the decalin combustion mechanism has been merged with the base gasoline and diesel surrogate mechanisms to extend the choice of surrogate components and perform the simulations that are discussed later. The base mechanism for surrogates was developed based on the validated published mechanisms for compounds larger than C6 and including linear and branched alkanes such as n-heptane, n-decane, n-hexadecane, iso-octane, heptamethylnonane [18, 21-23], cyclic alkanes such as cyclohexane and methylcyclohexane [5, 17], and aromatics such as toluene [24]. It includes many other improvements beyond the scope of this article, several new component submechanisms such as 1-pentene, 1-hexene, 1methylnaphthalene, and already have been validated extensively [20, 25-27]. This provides a strong base to develop detailed reaction mechanisms for new fuel components and ensures self-consistency. The base master mechanism also included chemistry for emissions, contains over 3500 species and over 15000 reaction steps. The detailed chemical kinetic submechanism for decalin extends the base mechanism by 169 new species and 482 new reactions. 1 10
2 3
5 4
Fig. 2. Three different possible parent radical positions (1, 2, and 5) from decalin. Paper RK76
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To briefly discuss the underlying chemistry of decalin, at high -temperatures, fuel decomposition and hydrogen abstraction are the major initiation pathways. There are three possible parent fuel radicals of decalin at positions 1, 2, and 5, as shown in Fig. 2. Chae and Violi [14] performed density functional theory (DFT) calculations for decalin pyrolysis involving abstraction from all possible carbon sites and carbon-carbon bond cleavage leading to various pyrolysis products. They found that the abstraction reactions dominate the only important C(5)-C(10) carbon-carbon bond cleavage. In development of the high-temperature decalin oxidation mechanism, we have included H-atom abstractions from all three positions by different species and also considered the C(5)-C(10) bond cleavage reaction. In addition to C-C bond cleavage reactions, we also added a molecular elimination reaction (decalin cyclohexane + 2 ethylene) that directly forms cyclohexane and ethylene from decalin. This reaction is important for initiation under pyrolytic conditions. The rate coefficients used were similar to those recommended by Tsang [28] for eliminating two propene molecules from 1-hexene, but with the activation energy increased by 1 kcal/mol (well within uncertainty limits). A similar reaction was already included for cyclohexane [5]. We also included the dehydrogenation pathways to form tetralin and naphthalene from decalin. All isomers of decalene (the olefin based on the parent fuel) with carbon-carbon double bond at positions 1, 2, 4, and 5 (Fig. 2) were considered. It is worth noting that for simplicity, we lumped the isomers of di-olefin species that were generated from isomers of decalene in the mechanism. At low temperatures, molecular oxygen can add to three different parent radical sites, as shown in Fig. 2, for the equivalent of alkylperoxy (RO2•) radicals. These RO2• radicals can further isomerize and react with 2nd O2 to lead to chain-branching at low temperatures. Although the mechanism is built on rate-rules, one modification made to the rate-rules was for decalin+HO2 hydrogen-abstraction reactions. The rate constants used for this reaction were based on Carstensen and Dean [29] for smaller alkanes that had stronger temperature dependence than that based on the rate-rules. This shift from the rate-rules (still within uncertainty limits) improves the ignition time predictions at high temperatures under the shock-tube conditions of Oehlschlaeger et al. [11] as discussed later. 3.
Results Three types of recent experimental data have been used for validating the decalin mechanism: species profiles during oxidative pyrolysis of decalin with approximately 1170 ppm decalin with 150 ppm O2 in nitrogen, in an atmospheric plug flow reactor at various temperatures measured by Zeppieri et al. [9]; shock-tube ignition time of decalin/air mixtures measured by Oehlschlaeger et al. [11]; critical compression ratio suggesting the onset of lean decalin/air ignition in a CFR engine measured by Yang and Boehman [12]. All simulations were performed using various ideal reactor models in CHEMKIN-PRO [30]. Each simulation point takes only a few minutes to run using the large detailed mechanism. As seen in Fig. 3, decay of decalin under oxidative pyrolysis in the Zeppieri et al. flow reactor conditions is captured reasonably well for 1063 K. The major products observed and predicted are methane, ethylene, propene, 1,3-butadiene, cyclohexane, and benzene. Formation of aromatic products increases at higher temperatures. However, at higher temperatures there is discrepancy in the predicted and measured decalin profiles. The discrepancy is highest for intermediate temperatures near 1100 K and then again decreases at higher temperatures. There are several possible experimental issues mentioned by Zeppieri et al. that may cause such a discrepancy with predictions using an ideal plugflow reactor model. These issues involve the differences in fuel isomeric composition and effect of heterogeneous reactions of decalin with reactor surface under turbulent conditions. Additional experimental data are also needed to improve the mechanism and validate it. Comparisons of predicted autoignition delay time for decalin/air to that measured by Oehlschlaeger et al. at two different pressures, for equivalence ratio (φ) of 0.5 and 1.0 are shown in Fig. 4 and Fig. 5, Paper RK76
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respectively. Overall, the predictions are in good agreement with the data. Although the negative temperature coefficient (NTC) effect was predicted by the model, the data below 1000 K were not available. 1 Decalin pyrolysis, 1 atm, fuel~1170 ppm, O2~150 ppm in N2
Normalized decalin (ppm / ppm initial)
0.9 0.8
1193 K 1156 K 1108 K
0.7 0.6 0.5
1063 K 1015 K
0.4
1193 K 1156 K
0.3
1108 K 1063 K
0.2
1015 K
0.1 0 0
0.05
0.1
0.15
0.2
0.25
Time (s)
Fig. 3. Comparison of predicted decay of decalin (1170 ppm fuel, 150 ppm O2, balance N2) in an atmospheric flow reactor to that measured by Zeppieri et al. [9]. Lines represent model predictions and symbols represent the data. 1.0E-02
Autoignition time (s)
phi 0.5
1.0E-03
decalin, 12 atm decalin, 12 atm
1.0E-04
decalin, 40 atm decalin, 40 atm 1.0E-05 0.7
0.8
0.9
1
1.1
1.2
1000/T (1/K)
Fig. 4. Comparison of predicted autoignition delay time for decalin/air to that measured in a shock tube by Oehlschlaeger et al. [11] for equivalence ratio of 0.5 and two different pressures. Lines represent model predictions and symbols represent the data. Single zone IC Engine model was used for modeling the variable compression ratio HCCI engine experiments of Yang and Boehman. Same engine parameters were used for modeling – engine speed 600 RPM, equivalence ratio 0.25, inlet temperature 473 K. Woschni parameters for heat loss were used in the model. These parameter, though arbitrary, were consistent with other simulations of a CFR engine used elsewhere [26]. In addition, the single zone IC engine model is not capable of predicting the peak temperature and pressure, but good for capturing the onset of ignition. Therefore, comparison of the predictions to the measurements should be considered only semi-quantitative. As seen in Fig. 6, model captures the onset of ignition near compression ratio of 7.5 as indicated by sudden rise in peak pressures. Note that there seem to be some ambiguity in the measured values since the steep increase in peak temperature is around compression ratio of 8, which is different from that indicated by peak pressures.
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1.0E-02
Autoignition time (s)
phi 1.0
1.0E-03
decalin, 12 atm decalin, 12 atm
1.0E-04
decalin, 40 atm decalin, 40 atm 1.0E-05 0.7
0.8
0.9
1
1.1
1.2
1000/T (1/K)
Fig. 5. Comparison of predicted autoignition delay time for decalin/air to that and measured in a shock tube by Oehlschlaeger et al. [11] for equivalence ratio of 1.0 and two different pressures. Lines represent model predictions and symbols represent the data. Decalin/air phi 0.25, Tinlet 200 C at 1 atm, 600 rpm CFR engine 45
2000 Pmax (bar) Pmax (bar) Tmax (K)
Peak pressure (bar)
35
1800 1600
Tmax (K)
30
1400 25 1200 20 1000
15
Peak temperature (K)
40
800
10 5
600 4
5
6
7
8
9
10
Compression ratio
Fig. 6. Comparison of predicted peak temperature and pressure at various compression ration in a CFR engine using a single-zone IC engine model to that measured by [12] with decalin fuel. Lines represent model predictions and symbols represent the data. 4.
Discussion To understand the mechanism of decalin oxidation and pyrolysis, we performed rate and sensitivity analyses under several reactor conditions. Molecular structures of the species names used in the mechanism and in some of the figures here are shown in Nomenclature. Fig. 7 shows major pathways from decalin under the oxidative-pyrolysis conditions of Zeppieri et al. at 10% conversion for both 1015 K and 1193 K. At both temperatures, exactly the same reaction pathways are identified as important. Ethylene (c2h4) is produced directly from decalin via a molecular elimination channel, whereas all other products, such as 1,3-butadiene (c4h6), are produced via other pathways that involve parent radicals and cyclohexene. Fig. 8 shows sensitivity analysis for decalin at 1015 K and 1193 K. Decalin mainly decomposes via a molecular elimination channel to produce cyclohexane (cychexene). This reaction is the single most important pathway for decalin decay, especially at 1193 K. This reaction is also the dominating reaction based on sensitivity analyses. Hydrogen abstraction from decalin by H and CH3 radicals also consume the fuel to a smaller extent. Based on sensitivity analyses, these reactions are slightly more important at lower temperatures than at higher temperatures. Surprisingly, at 1015 K, the reaction of vinyl (c2h3) with O2 is the second-most sensitive reaction under the oxidative-pyrolysis conditions of Zeppieri et al. [9] Paper RK76
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Reaction path diagrams for decalin oxidation at 12 atm, equivalence ratio of 1, and 10% conversion, for 800 K and 1200 K are shown in Fig. 9 and Fig. 10. Hydrogen-atom abstraction from decalin by H, OH, and HO2 radicals initiates the oxidation process. The parent radicals 1-decalyl and 2-decalyl are produced in higher quantities than 5-decalyl. At 800 K, addition of O2 to parent radicals is a dominant pathway that leads to chain branching and two major products, ethylene and cyclohexane (cychexene). CO and formaldehyde are also major intermediates, mainly produced from subsequent reactions of the 2-oxy-decalin (dcl2oj) radical. At 1200 K, ring opening followed by dissociation reactions dominates overall kinetics. Radical 1-decalyl leads to several major products such as ethylene and 3methylidenecyclohex-1-ene (ch2*chxe), mostly through several dissociation steps. Radical 2-decalyl leads to 1,3-butadiene and cyclohexene, whereas radical 5-decalyl leads to ethylene as well as 1,3cyclohexadiene (x13cychex).
C2H4
Fig. 7. Major reaction pathways during oxidative-pyrolysis of decalin under the flow-reactor conditions of Zeppieri et al. [9]
Fig. 8. Sensitivity analysis of decalin during oxidative pyrolysis under the flow-reactor conditions of Zeppieri et al. at 10% conversion. Top chart shows the analysis for 1015 K and the bottom chart shows the analysis at 1193 K. Paper RK76
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Temperature sensitivity analyses at 10% fuel conversion for initial temperatures of 800 K and 1200 K under shock-tube ignition of a stoichiometric decalin/air mixture are shown in Fig. 11 and Fig. 12. As expected, reactions of decalin-based RO2 species dissociation and isomerization to QOOH (e.g., dcl2o2 = dcl2qr10), as well as O2QOOH (e.g., dcl2q10o2 = dcl2o10q+oh) leading to chain branching at low temperatures are the most important reactions at 800 K. At 1200 K, due to a shift in kinetics, reactions of hydrogen and core (C2 and C4 hydrocarbons) component kinetics begin to impact the ignition process. Hydrogen-atom abstraction from decalin by OH and HO2 radicals also plays a role at various conditions of the oxidation process.
CO
C2H4
CH2O
Fig. 9. Major reaction pathways during the oxidation of decalin/air mixture at 800 K, equivalence ratio of 1, 12 atm, at 10% conversion. Simulation conditions are similar to those used in Fig. 5.
C2H4
Fig. 10. Major reaction pathways during the oxidation of decalin/air mixture at 1200 K, equivalence ratio of 1, 12 atm, at 10% conversion. Simulation conditions are similar to those used in Fig. 5. Paper RK76
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dcl2q10o2dcl2o10q+oh dcl2o2dcl2qr10 dcl1o2dcl1qr3 dcl1q3o2dcl1o3q+oh dcl2q4o2dcl2o4q+oh dcl2o2dcl2qr4 decalin+ohdclr2+h2o decalin+ohdclr5+h2o
At 10% fuel conversion
dcl1o2dcl1qr10
800 K, phi 1, 12 atm
dcl2qr10=>c*chxccc*o+oh -0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
Normalized temperature se nsitivity coefficient
Fig. 11. Temperature sensitivity analysis for the oxidation of decalin/air mixture at 800 K, equivalence ratio of 1, 12 atm, at 10% conversion. Simulation conditions are similar to those used in Fig. 5. c4h71-4+o2c4h6+ho2 c2h3+o2ch2cho+o x13cychex+hcycc6h91 c4h71-4c4h6+h h+o2o+oh decalin+ho2dclr1+h2o2 decalin+ho2dclr2+h2o2 h2o2(+M)2oh(+M)
At 10% fuel conversion 1200 K, phi 1, 12 atm
-0.005 -0.004 -0.003 -0.002 -0.001 0 0.001 0.002 0.003 Normalized temperature sensitivity coefficient
0.004
Fig. 12. Temperature sensitivity analysis for the oxidation of decalin/air mixture at 1200 K, equivalence ratio of 1, 12 atm, at 10% conversion. Simulation conditions are similar to those used in Fig. 5. 5.
Concluding remarks A detailed chemical kinetic mechanism for the combustion of decalin under engine conditions has been developed that contains newly added, fuel-specific, 172 species and 484 reaction steps. The mechanism is based on rate-rules developed in published studies for cyclic alkanes, and extends a previously developed mechanism for gasoline and diesel surrogates to ensure self-consistency among different components. The mechanism has been validated using the fundamental data in published studies on oxidation and pyrolysis. Different sets of reactions contribute under different reactor conditions as found in the rate of production and sensitivity analyses. Onset of ignition under an HCCI engine conditions has also been successfully validated. Acknowledgements The author acknowledges the partial financial support from the Model Fuels Consortium II (www.modelfuelsconsortium.com).
6. [1] [2] [3]
References C. V. Naik; K. V. Puduppakkam; E. Meeks, SAE Int. J. Engines 3 (1) (2010) 241-259. A. Agosta. Thesis on Development of a Chemical Surrogate for JP-8 Aviation Fuel Using a Pressurized Flow Reactor. Drexel University, 2002. J. T. Farrell; N. P. Cernansky; F. L. Dryer; C. A. Hergart; C. K. Law; R. M. McDavid; C. J. Mueller; A. K. Patel; H. Pitsch, SAE Technical Papers 2007-01-0201 (2007).
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NOMENCLATURE Name
Structure
Name
Structure
Name
decalin
dcl1q3o2
c*cchxccj
dcl-je
dcl2q4o2
c*cchxj
dclr1
dcl2q10o2
c*cchxej
dclr2
dcl1o3q
c*cccchxj
dclr5
dcl2o4q
cychexene
dcl1o2
dcl2o10q
x13cychex
dcl2o2
chx1e3ccccj
c*chxccc*o
dcl1qr3
chx1ej3cccc
cycc6h91
dcl1qr10
ch2*chxe
c4h71-4
dcl2qr4
chx1e3ccj
c4h6
dcl2qr10
c*cchxe
nc3h7
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Structure
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