Experimental Study on Ethane Ignition Delay Times and Evaluation of

Jun 15, 2015 - The ignition delay times of ethane were measured using a high-pressure shock tube at different pressures (p = 1.2, 5.0, and 20.0 atm) a...
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Experimental Study on Ethane Ignition Delay Times and Evaluation of Chemical Kinetic Models Erjiang Hu,* Yizhen Chen, Zihang Zhang, Xiaotian Li, Yu Cheng, and Zuohua Huang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ABSTRACT: The ignition delay times of ethane were measured using a high-pressure shock tube at different pressures (p = 1.2, 5.0, and 20.0 atm) and equivalence ratios (ϕ = 0.5, 1.0, and 2.0) with different argon diluent ratios. Correlations of the measured ignition delay times were provided. The measurements were compared to calculations from several representative chemical kinetic models to evaluate their performances. Results showed that Aramco Mech 1.3 could well reproduce the measured ignition delay times over a wide range, while GRI Mech 3.0 significantly overpredicted the measurements at stoichiometric and high equivalence ratio. To find out the reasons for the differences and similarities of the mechanisms on calculating the ignition delay time of ethane, sensitivity analysis and reaction pathway analysis were conducted. It is observed that the mechanisms have similar pathway for ethane consumption, while they have significant differences for ethyl radical decomposition reactions. Results also indicated that the incompleteness of the C2H5 + O2 reaction channels and the underestimation of the rate constant of reaction C2H4 + H (+M) = C2H5 (+M) might be responsible for the overestimation of GRI Mech 3.0 on ethane ignition. The mechanisms studied give similar prediction at a high equivalence ratio because reactions with a significant difference on rate constants do not show a high sensitivity coefficient at the condition.

1. INTRODUCTION Ethane is an important component in natural gas, an alternative fuel that is widely used in internal combustion engines and many other combustion devices. Moreover, ethane is a critical intermediate generally generated in the oxidation and pyrolysis processes of hydrocarbon fuels; therefore, its kinetic mechanism is fundamental for the hierarchical construction of higher hydrocarbon fuel models. Therefore, an accurate and reliable kinetic mechanism that can describe its combustion process under wide ranges is beneficial to our understanding on the combustion of ethane and other hydrocarbons. However, the working conditions of practical combustion devices are much broader than the applicable range of most mechanisms that are currently used. Given that the experimental studies on ethane combustion are not as extensive as for methane, it is worthwhile to extend the ethane ignition database, which can serve for the validation of chemical kinetic modeling. A mechanism can be used with credence if it is validated by a wide range of experimental data. Shock-tube studies on the ignition delay time of ethane can be backed to the 1970s.1−3 In 1981, Hidaka et al.4 measured the ignition delay times of ethane along with other C2 hydrocarbons in a shock tube at pressures below 0.5 atm. Lamoureux et al.5 obtained the ignition delay data of low hydrocarbons (methane, ethane, and propone) behind reflected shock waves. They compared their data to the calculation results of GRI Mech 3.0,6 reporting its overestimation on ethane ignition delay times for rich-fuel mixtures. Tranter et al.7 obtained the concentration data of intermediate species during ethane pyrolysis and oxidation processes at very high pressures (34.0 and 61.3 MPa) in a high-pressure single-pulse shock tube. They also studied the oxidation on rich and stoichiometric ethane mixtures at 40 atm using the same apparatus8 and reported that neither of the mechanisms that they studied could well © XXXX American Chemical Society

reproduce their experimental data at a rich ethane condition. In 2007, de Vries et al.9 summarized previous ethane ignition data in the literature and compared them to their measurements. They reported the convergence of different experimental data and advised further studies on the ethane oxidation modeling study at fuel-rich and high-pressure conditions. Recently, Aul et al.10 measured the ignition delay times of methane, ethane, and methane/ethane mixtures. The ignition delay data were used to validate Aramco Mech 1.0,11 and the rate constants of some important reactions were discussed in the paper. Many studies have been published concerning the chemical kinetic mechanisms of ethane. The mechanism published by Kilpinen et al.12 contained reactions describing ethane oxidation. Dagaut et al.13 built a detailed kinetic mechanism for ethane oxidation with a strong hierarchical structure after measuring the concentration profiles of species at high temperatures (800−1200 K) and high pressures (1−10 atm) in a jet-stirred reactor. The mechanism was based on previous studies and validated with a jet-stirred reactor and shock-tube data. Hunter et al.14 noticed the importance of other reaction channels of C2H5 + O2 in addition to the C2H4 + HO2 production. They expanded GRI Mech 1.1 with a C2 submechanism and adjusted the reaction rate constants of two key reactions according to their measured species profiles in a flow reactor. Hidaka et al.15 introduced a model with 157 reactions and 48 species, which could both simulate ethane oxidation and pyrolysis. In this model, certain reaction rate constants were adjusted on the basis of their measured species concentration Received: March 3, 2015 Revised: June 11, 2015

A

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Energy & Fuels data in a shock tube. Naik and Dean16,17 developed a detailed mechanism of ethane oxidation and pyrolysis with 863 reactions and 112 species. The mechanism incorporated the results of ab initio studies of the important low-temperature pathways of the C2H5 + O2 reaction and applied many pressure-dependent rate coefficients. In 2013, Zhang et al.18 observed the underestimation of USC Mech 2.019 on the ignition delay times of ethane while studying C1−C4 alkanes. They replaced the rate constant of C2H4 + H (+M) = C2H5 (+M) by the rate constant in Aramco Mech 1.320 and found that the modified mechanism could well reproduce the experimental data. The similar phenomenon was also noticed by Pan et al.21 when calculating the ignition delay times of ethane using LLNL C4Mech.22 Although much research has been devoted to ethane ignition in a shock tube, its ignition delay data are still far from comprehensive. Therefore, one of our main purposes for this research is to extend the ethane ignition database by providing ignition delay data covering a broad parameter range by varying pressure, equivalence ratio, and diluent ratio. Two experiments were duplicated first to ensure the reliability of our experimental measurement. The validated data were thus used to evaluate the performances of currently accepted mechanisms. Key reactions were identified by sensitivity analysis and reaction pathway analysis and then discussed in detail.

Figure 1. Typical end-wall pressure and OH* chemiluminescence measurements with the corresponding ignition delay time for a stoichiometric ethane/O2/Ar mixture at 1.1 atm and 1255 K.

Table 1. Compositions of the Testing Mixtures

2. EXPERIMENTAL AND NUMERICAL METHOD

mixture

ϕ

Xethane (%)

XO2 (%)

XAr (%)

p (MPa)

1 2 3 4 5 6

0.5 1.0 2.0 1.0 1.0 1.0

1.0 1.0 1.0 0.75 1.5 2.0

7.0 3.5 1.75 2.625 5.25 7.0

92.0 95.5 97.25 96.625 93.25 91.0

0.12, 0.5, 2.0 0.12, 0.5, 2.0 0.12, 0.5, 2.0 0.5 0.5 0.2, 0.5

Diego Mech.30 They are summarized in Table 2. Two software tools, CHEMClean and CHEMDiffs,31 were used to compare the species and reactions in the mechanisms studied.

All measurements in this study were conducted in a shock tube, which has been described in detail in previous literature.23,24 Therefore, only a brief introduction of our experimental apparatus and method is given here. In this study, the testing mixtures were prepared manometrically in a 128 L stainless-steel tank and then mixed spontaneously by molecular diffusion for 12 h. The purities of ethane, oxygen, and argon were 99.9, 99.995, and 99.999%, respectively. The shock tube is 8.8 m long with an inner diameter of 11.5 cm. It is separated by polyester terephthalate (PET) films into two sections: a 4.0 m long driver section and a 4.8 m long driven section. The testing mixture (C2H6/ O2/Ar mixtures) and driver gas (N2 and Ar) are entered into the driven section and the driver section, respectively. Sudden venting of the gas between the two films triggers the diaphragm rupture and the generation of the shock wave. The shock wave reaches the reaction zone and ignites the mixture. PET films with different thicknesses should be used for conditions with different driver pressures. Four pressure transducers (PCB, 113B26) are placed on the side wall with the same length interval of 30 cm to calculate the velocity of the incident shock wave. A photomultiplier (Hamamatsu, CR131) installed at the end wall can filter the OH* emission centered at 430 ± 10 nm. A digital recorder (Yokogava, scopecorder DL750) is used to record all measurements. The temperature behind reflected shock waves is calculated using the reflected shock module in the software Gaseq25 with an uncertainty of ±25 K. The definition of ignition delay time is the interval between the time when the incident shock wave arrives at the end wall and the point at the zero baseline by extrapolating the steepest slope of the OH* chemiluminescence signal measured at the end wall, as shown in Figure 1. The C2H6/O2/Ar mixtures were tested at 1.2−20 atm, and their compositions are shown in Table 1. We used the constantvolume, adiabatic, and zero-dimensional reactor in CHEMKIN II package26 with the SENKIN/VTIM approach27,28 to carry out our simulations. The boundary effect of the facility (dp/dt) was considered in the calculation, and the dependent pressure rise is set to 4%/ms according to the earlier research.29 The ignition delay time in the simulation is defined as the time interval between the beginning of the simulation and the maximum rate of temperature rise (max dT/dt). Chemical kinetic mechanisms employed in this study are GRI Mech 3.0,6 USC Mech 2.0,19 Aramco Mech 1.3,20 LLNL C4Mech,22 and San

Table 2. Summary of the Mechanisms Employed in This Study mechanism

author

update date

number of reactions

number of species

GRI 3.06 USC 2.019 LLNL C422 San Diego30 Aramco 1.320

Smith et al. Wang et al. Marinov et al. William et al. Curran et al.

1999 2007 2004 2014 2013

325 784 689 244 1542

53 111 155 50 253

3. RESULTS AND DISCUSSION In this section, we first repeated an experimental condition of the previous study9 for the purpose of validation of our measurement. Then, we presented our data of ethane ignition delay times at various pressures, equivalence ratios, and diluent ratios. Finally, calculations were conducted employing different mechanisms and compared to experimental data for evaluating their performances. Important reactions and reaction pathways were analyzed as well. 3.1. Comparison to Previous Data. Figure 2 gives the comparison between the present measurement and previous data under identical experimental conditions: p = 2.0 atm, ϕ = 1.0, and Xethane = 2%. It can be seen that our measurement is highly consistent with the previous study.9 Therefore, the present ignition data can be used with confidence in evaluating the performances of kinetic mechanisms. 3.2. Ignition Delay Time Measurements. Figure 3 summaries the experimental conditions (pressure, dilution, and equivalence ratio) of the present and previous studies on B

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where τign is the ignition delay time in microseconds, concentrations are in moles per centimeter cubed, T is the temperature in kelvin. From Figure 4, we also observed the influences of initial parameters to ethane ignition. Figure 4a gives the ignition delay times of ethane at three pressures (p = 1.2, 5.0, and 20 atm) for the fuel-lean (ϕ = 0.5) mixture. It is observed that the same mixture has a shorter ignition delay time at a higher pressure. A similar phenomenon is observed in panels b and c of Figure 4 for stoichiometric (ϕ = 1.0) and fuel-rich (ϕ = 2.0) mixtures. Figure 4d shows the ignition delay times of the stoichiometric mixtures with different diluent ratios at the same pressure (p = 5.0 atm). The ignition delay time increases with the increase of the diluent ratio. Figure 4e rearranges and presents the ignition data for different equivalence ratios at the same pressure, showing that the increasing equivalence ratio increases the ignition delay time. To summarize, the rising of the pressure and the decrease of the equivalence ratio and diluent ratio have a promoting effect on ethane ignition. The same conclusion can also be found from the equation of the correlation, where the indexes of the composition concentrations and their sum represent their influences on the ignition delay. In general, the measured ignition delay times show good Arrhenius exponential dependence upon the reciprocal temperature. 3.3. Comparison to Chemical Kinetic Models. One of the objectives of this study is to use our measured ethane ignition data to evaluate the performances of existing models at different ranges. Current chemical kinetic mechanisms are commonly constructed with a hierarchical structure; therefore, the chemical kinetic models of hydrocarbon are inevitably dependent upon the accuracy of ethane chemistry. In this study, we tested several generally accepted hydrocarbon models, namely, GRI Mech 3.0,6 USC Mech 2.0,19 Aramco Mech 1.3,20 LLNL C4Mech,22 and San Diego Mech,30 to evaluate their performances on ethane ignition prediction. Figure 5 gives the calculation results of different models on the ignition delay times of ethane against the measurement at various conditions. It is seen that Aramco Mech 1.3 and San Diego Mech yield relatively satisfactory agreement, while USC Mech 2.0 and LLNL C4Mech generally underpredict the ethane ignition delay. The underestimations of USC Mech 2.0 and LLNL C4Mech have been reported before.18,21 As shown in Figure 5, GRI Mech 3.0 gives a significant overprediction on the ignition delay times of ethane, especially for fuel-lean and stoichiometric mixtures. GRI Mech 3.0 is a widely accepted mechanism for its valid and accurate simulation of methane and natural gas. Given its poor prediction on ethane ignition, it should be prudent not to use it for simulations on natural gas with a non-negligible fraction of ethane. Although the poor agreement between the GRI Mech 3.0 calculation and the experimental data has been noticed before,5 no chemical analysis or reasons for the phenomenon were given in the literature. A preliminary comparison of the mechanisms using ChemClean and ChemDiffs is given in Tables 3 and 4. The values in parentheses are the number of species or reactions in the mechanism, and the values in the table present the number of identical species or reactions between two mechanisms with exactly the same reactants and products. In Table 4, the former value is the number of reactions with exactly the same Arrhenius parameters, while the latter value is the number of identical reactions but with different reaction rate constants. It is observed that the relative similarity of the mechanisms

Figure 2. Comparison between present measured data and the measured data from ref 9.

Figure 3. Summary of the measurement conditions including previous and current studies of ethane ignition delay times.

the ignition delay times of ethane with a three-dimensional plot. It can be seen that most of the previous data were situated at the low- and middle-pressure ranges, while the current study broadens the database of ethane shock-tube studies by covering a wide range of experimental conditions. Figure 4 shows the measured ignition delay times of ethane with their correlations at different pressures, equivalence ratios, and diluent ratios. The correlations of the ignition delay times with the temperature and the concentrations of ethane, oxygen, and argon were obtained using the multivariable linear regression method. The activation energy acquired is 32.9 kcal/mol, which falls in the range of previous studies (29.9− 55.2 kcal/mol)9,21 ⎛ 16600 K ⎞ 0.57 ⎟[C H ] τign = 1.39−10 exp⎜ [O2 ]−1.02 [Ar]0.06 ⎝ T ⎠ 2 6 (1) C

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Figure 4. Measured ignition delay times of C2H6/O2/Ar mixtures at different pressures, equivalence ratios, and diluent ratios.

different mechanisms have a similar portion for C 2 H 6 consumption pathways at various conditions. This is because the mechanisms studied have an identical or a close reaction rate constant for important hydrogen abstraction reactions. For example, the reaction rate constants of C2H6 + H = C2H5 + H2 and C2H6 + OH = C2H5 + H2O are close, as shown in panels a and b of Figure 7. Figure 6 also presents the main consumption pathway of C2H5. For each mechanism studied, C2H5 (+M) = C2H4 + H (+M) and C2H5 + O2 = C2H4 + HO2 are critical consumption pathways of the ethyl radical. However, the portions of C2H5 consumed respectively through the two pathways vary at different conditions for different mechanisms. At a low equivalence ratio, the flux of C2H5 + O2 = C2H4 + HO2 of GRI Mech 3.0 is much higher than that of other mechanisms. For GRI Mech 3.0, the majority of the ethyl radical is consumed by hydrogen abstraction reactions with O2, while for

studied is not high. However, their performances differ at certain conditions, while they agree well at the others. In the following section, reaction pathway and sensitivity analysis were carried out to explain the similarities and differences of their prediction on the ignition delay times of ethane. 3.4. Sensitivity Analysis and Reaction Pathway Analysis. Reaction pathway analysis of ethane oxidation was performed using the five mechanisms at different pressures and equivalence ratios. From Figure 6, it can be seen that most C2H6 molecules are decomposed via hydrogen abstraction reactions with H, OH, O2, and CH3 radicals forming C2H5 molecules. At different equivalence ratios, the dominant consumption pathway differs. At a low equivalence ratio, the majority of C2H6 molecules is consumed via a hydrogen abstraction reaction with OH radicals, while at a high equivalence ratio, the main pathway of C2H6 consumption is a hydrogen abstraction reaction with H radicals. However, D

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Figure 5. continued

E

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Figure 5. Comparison of experimental and calculated ignition delay times of ethane at different pressures, equivalence ratios, and dilution ratios.

Table 3. Species in Commona GRI 3.0 (53) Aramco 1.3 (253) USC 2.0 (111) LLNL C4 (155) San Diego (50) a

GRI 3.0 (53)

Aramco 1.3 (253)

USC 2.0 (111)

LLNL C4 (155)

35 34 33 31

66 46 36

60 34

31

San Diego (50)

The values in parentheses are the number of species in the mechanism, and the values in the table present the number of identical species.

Table 4. Reactions in Commona GRI 3.0 (325) GRI 3.0 (325) Aramco 1.3 (1542) USC 2.0 (784) LLNL C4 (689) San Diego (247)

49; 93; 50; 21;

88 72 79 77

Aramco 1.3 (1542)

USC 2.0 (784)

LLNL C4 (689)

120; 124 57; 104 29; 80

85; 140 28; 89

27; 65

San Diego (247)

a

The values in parentheses are the number of reactions in the mechanism, and the former value in the table is the number of reactions with exactly the same Arrhenius parameters, while the latter value is the number of identical reactions but with different reaction rate constants.

ϕ = 2.0, where the models agree well. The sensitivity coefficient is calculated by the following formula using the definition given by Petersen et al.:32

other mechanisms, the ethyl radical is mainly consumed by decomposition to acetylene. At a high equivalence ratio, the C2H5 consumption pathway for all mechanisms studied is dominated by C2H5 (+M) = C2H4 + H (+M). At this condition, all mechanisms give a similar prediction of the ignition delay time and good agreement with experimental data. Sensitivity analysis was performed using GRI Mech 3.0 at the condition of T = 1250 K, p = 1.2 atm, and ϕ = 0.5, where a significant gap between the calculated and measured results was observed, and at the condition of T = 1250 K, p = 1.2 atm, and

Si =

τ(2.0ki) − τ(0.5ki) 1.5τ(ki)

(2)

where τ is ignition delay time of the combustible mixture, Si and ki are the sensitivity coefficient and rate constant of the ith reaction, respectively. A positive sensitivity coefficient demonF

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Figure 6. Reaction pathway analysis of ethane calculated by five mechanisms at three conditions.

strates that the corresponding reaction inhibits the overall ignition process and vice versa.

The elementary reactions with the highest sensitivity coefficients are listed in Figure 8. Given that GRI Mech 3.0 G

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Figure 7. Comparison of reaction rate constants: (a) C2H6 + H = C2H5 + H2 (GRI Mech 3.0, Aramco Mech 1.3, and USC Mech 2.0 have an identical rate constant, and LLNL C4Mech and San Diego Mech have an identical rate constant), (b) C2H6 + OH = C2H5 + H2O (USC Mech 2.0, LLNL C4Mech, and San Diego Mech have an identical rate constant), (c) C2H4 + H + M = C2H5 + M, and (d) C2H5 + O2 = C2H4 + HO2.

Reaction C2H6 + H = C2H5 + H2 is an important reaction controlling the ignition of ethane because it is the main pathway of ethane consumption. Figure 7a gives its reaction rate constant, and it can be seen that all of the mechanisms studied have an identical or a similar value for the reaction, which should not cause the difference of performance between mechanisms. Reaction C2H5 (+M) = C2H4 + H (+M) is a pressure-dependent reaction. Both the sensitivity and reaction pathway analyses prove their importance in the ethane oxidation process. The reaction rate constant employed in Aramco Mech 1.3 was originally derived from the research of Miller and Klippenstein,33 but with the high- and low-pressure limits multiplied by a factor of 0.7. Later studies18,21 indicated that the adjusted value gives better results when compared to experimental data. Further experimental and calculation studies may justify such adjustment or give a more accurate calculated value of the rate constant. As shown in Figure 7c, there are significant differences between the rate constants employed in the models and the value in Aramco Mech 1.3 is twice as large as the value in GRI Mech 3.0. The discrepancy in describing this reaction may be one of the reasons why the overall predictions differ for GRI Mech 3.0 and Aramco Mech 1.3. Many studies have been carried out on the detailed chemistry of the C2H5 + O2 reaction. Studies34−36 indicated that the reaction C2H5 + O2 may undergo a complicated transition state before producing C2H4 + HO2. The reaction first forms an unstable adduct C2H5O2, which may produce C2H4 + HO2 or react in the reverse direction. In addition to the channel C2H5 + O2 = C2H4 + HO2, other channels may also be important in the reaction. Aramco Mech 1.3 takes the value recommended by

Figure 8. Sensitivity analysis of ignition delays calculated by GRI Mech 3.0 at T = 1250 K, p = 1.2 atm, and ϕ = 0.5 and 2.0.

can give good agreement on the ignition delay of methane, the overestimation of ethane at a low equivalence ratio may be attributed to the ethane-specific reactions with high sensitivity coefficients, as listed below. R74:

C2H4 + H ( +M) = C2H5 ( +M)

R78:

C2H6 + H = C2H5 + H 2

R175:

C2H5 + O2 = C2H4 + HO2 H

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DeSain et al.,37 including multiple channels of the C2H5 + O2 reaction. However, the release date of GRI Mech 3.0 predated these studies and, thus, did not incorporate other channels into the model. A comparison of the values of the mechanisms is given in Figure 7d. The value given in GRI Mech 3.0 is about 1 time higher than the value of Aramco Mech 1.3. Therefore, the inaccuracy of GRI Mech 3.0 could be attributed to its incompetence in describing the reaction C2H5 + O2. At ϕ = 2.0, the mechanisms give reasonable agreement with each other and the experimental data. It can be seen from Figure 7 that those important reactions, such as C2H5 (+M) = C2H4 + H (+M) and C2H5 + O2 = C2H4 + HO2, do not have high sensitivity coefficients at this condition; thus, the differences of their reaction rate constants do not significantly affect the result of ethane ignition prediction. From Figure 6, the reactions with high sensitivity coefficients at ϕ = 2.0 are not the main pathway of reaction. Therefore, GRI Mech 3.0 gives a similar prediction to other mechanisms at a high equivalence ratio.

REFERENCES

(1) Bowman, C. T. An experimental and analytical investigation of the high-temperature oxidation mechanisms of hydrocarbon fuels. Combust. Sci. Technol. 1970, 2 (2−3), 161−172. (2) Burcat, A.; Crossley, R. W.; Scheller, K.; Skinner, G. B. Shock tube investigation of ignition in ethane−oxygen−argon mixtures. Combust. Flame 1972, 18 (1), 115−123. (3) Cooke, D.; Williams, A. Shock tube studies of methane and ethane oxidation. Combust. Flame 1975, 24, 245−256. (4) Hidaka, Y.; Tanaka, Y.; Kawano, H.; Suga, M. Mass spectrometric study of C2 hydrocarbons oxidation in shock waves. J. Mass Spectrom. Soc. Jpn. 1981, 29 (2), 191−198. (5) Lamoureux, N.; Paillard, C.-E.; Vaslier, V. Low hydrocarbon mixtures ignition delay times investigation behind reflected shock waves. Shock Waves 2002, 11 (4), 309−322. (6) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C., Jr. GRI-Mech 3.0, 1999; http://www.me.berkeley. edu/gri_mech/. (7) Tranter, R. S.; Sivaramakrishnan, R.; Brezinsky, K.; Allendorf, M. D. High pressure, high temperature shock tube studies of ethane pyrolysis and oxidation. Phys. Chem. Chem. Phys. 2002, 4 (11), 2001− 2010. (8) Tranter, R.; Amoorthy, H. R.; Raman, A.; Brezinsky, K.; Allendorf, M. High-pressure single-pulse shock tube investigation of rich and stoichiometric ethane oxidation. Proc. Combust. Inst. 2002, 29 (1), 1267−1275. (9) de Vries, J.; Hall, J. M.; Simmons, S. L.; Rickard, M. J.; Kalitan, D. M.; Petersen, E. L. Ethane ignition and oxidation behind reflected shock waves. Combust. Flame 2007, 150 (1), 137−150. (10) Aul, C. J.; Metcalfe, W. K.; Burke, S. M.; Curran, H. J.; Petersen, E. L. Ignition and kinetic modeling of methane and ethane fuel blends with oxygen: A design of experiments approach. Combust. Flame 2013, 160 (7), 1153−1167. (11) Healy, D.; Donato, N.; Aul, C.; Petersen, E.; Zinner, C.; Bourque, G.; Curran, H. n-Butane: Ignition delay measurements at high pressure and detailed chemical kinetic simulations. Combust. Flame 2010, 157 (8), 1526−1539. (12) Kilpinen, P.; Glarborg, P.; Hupa, M. Reburning chemistry: A kinetic modeling study. Ind. Eng. Chem. Res. 1992, 31 (6), 1477−1490. (13) Dagaut, P.; Cathonnet, M.; Boettner, J. C. Kinetics of ethane oxidation. Int. J. Chem. Kinet. 1991, 23 (5), 437−455. (14) Hunter, T.; Litzinger, T.; Wang, H.; Frenklach, M. Ethane oxidation at elevated pressures in the intermediate temperature regime: Experiments and modeling. Combust. Flame 1996, 104 (4), 505−523. (15) Hidaka, Y.; Sato, K.; Hoshikawa, H.; Nishimori, T.; Takahashi, R.; Tanaka, H.; Inami, K.; Ito, N. Shock-tube and modeling study of ethane pyrolysis and oxidation. Combust. Flame 2000, 120 (3), 245− 264. (16) Naik, C. V.; Dean, A. M. Detailed kinetic modeling of ethane oxidation. Combust. Flame 2006, 145 (1), 16−37. (17) Naik, C. V.; Dean, A. M. Modeling high pressure ethane oxidation and pyrolysis. Proc. Combust. Inst. 2009, 32 (1), 437−443. (18) Zhang, J.; Hu, E.; Zhang, Z.; Pan, L.; Huang, Z. Comparative study on ignition delay times of C1−C4 alkanes. Energy Fuels 2013, 27 (6), 3480−3487. (19) Wang, H.; You, X.; Joshi, A. V.; Davis, S. G.; Laskin, A.; Egolfopoulos, F.; Law, C. K. USC Mech Version II. High-Temperature Combustion Reaction Model of H2/CO/C1−C4 Compounds, 2007; http://ignis.usc.edu/USC_Mech_II.htm. (20) Metcalfe, W. K.; Burke, S. M.; Ahmed, S. S.; Curran, H. J. A hierarchical and comparative kinetic modeling study of C1−C2 hydrocarbon and oxygenated fuels. Int. J. Chem. Kinet. 2013, 45 (10), 638−675. (21) Pan, L.; Zhang, Y.; Zhang, J.; Tian, Z.; Huang, Z. Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures. Int. J. Hydrogen Energy 2014, 39 (11), 6024−6033.

4. CONCLUSION Ethane chemistry is of great importance in chemical kinetic modeling of hydrocarbons. Ethane ignition delay times were measured using a shock tube at different pressures (p = 1.2, 5.0, and 20.0 atm) and equivalence ratios (ϕ = 0.5, 1.0, and 2.0) with different argon diluent ratios. A correlation of ignition delay times as a function of the pressure, temperature, and mole fractions of fuel and oxygen was calculated using the multiple regression method. Experimental results were compared to the calculated results of several chemical kinetic mechanisms. The results suggested that Aramco Mech 1.3 and San Diego Mech can give good agreement with experimental data. A significant overestimation of GRI Mech 3.0 was observed. Reaction pathway analysis indicated that the first step of ethane ignition is hydrogen abstraction reactions with H, OH, O, and CH3 radicals, and most ethane molecules are consumed by reaction C2H6 + H = C2H5 + H2 for mechanisms studied. However, the consumption pathway of the ethyl radical differs at different conditions and for different mechanisms. Sensitivity analysis suggested that C2H5 (+M) = C2H4 + H (+M) and C2H5 + O2 = C2H4 + HO2 are controlling reactions in ethane oxidation at ϕ = 0.5. The significant differences of their reaction rate constants cause the difference of performance on ethane ignition. The sensitivity coefficients of the two reactions are not significant at a high equivalence ratio, and thus, the predictions of the mechanisms studied agree well with themselves.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51306144 and 91441118) and the National Basic Research Program (2013CB228406). The authors also appreciate the funding support from the Fundamental Research Funds for the Central Universities. I

DOI: 10.1021/acs.energyfuels.5b00462 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.5b00462 Energy Fuels XXXX, XXX, XXX−XXX