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Shock-Tube Measurements and Kinetic Modeling Study of Methyl Propanoate Ignition Zihang Zhang, Erjiang Hu,* Lun Pan, Yizhen Chen, Jing Gong, 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 S Supporting Information *

ABSTRACT: Ignition delay times of methyl propanoate (MP) were measured in a shock tube over the temperature range of 1040−1720 K, pressures of 1.2−10 atm, fuel concentrations of 0.5−2.0%, and equivalence ratios of 0.5−2.0. Through multiple linear regression, a correlation for the tested ignition delay times was obtained, and the measured data were also compared to the previous data. Two available MP models (Princeton model and Westbrook model), were used to simulate the experimental data. Results suggest that further modifications on the available MP models are necessary. The modified MP model, consisting of 318 species and 1668 reactions, was proposed on the basis of previous studies, and it gives better prediction on the MP ignition delay times under all tested conditions than those of the other two available models. The modified MP model was further validated against the MP pyrolysis data and laminar flame speeds, and reasonable agreements were achieved. Sensitivity analysis reveals that the small radical reactions play key important roles in MP high-temperature ignition, while some fuel-specific reactions also exhibit relatively large sensitivity coefficients. Reaction pathway analysis indicates that MP is dominantly consumed through the H-abstraction reactions.

1. INTRODUCTION The significant increase in the number of vehicles has brought large energy consumption and air pollution. Therefore, the combustion and engine community has been spending much effort to find engine alternative fuels. Recently, biodiesel has attracted more attention because of its clean combustion characteristics, low sulfur content, low greenhouse effect, and renewability.1,2 Nearly all diesel engines can use a biodiesel blend with diesel, with the mole fraction of biodiesel up to 20%, and the engines with some modifications can use even higher percentage biodiesel blends. Up to now, there have many investigations on the long-chain fatty acid methyl esters,3−15 which are the main components of biodiesel. However, because of their long carbon chain, large esters exhibit similar chemical kinetic properties compared to the alkanes with the same chain length.16 Thus, it is difficult to have a thorough comprehension on the specific reaction kinetics associated with the ester group only with the study of large esters. Therefore, in-depth investigations on the small esters, which exhibit unique chemical kinetic properties compared to relative alkanes because of their ester groups, are of great significance. Moreover, research on small esters is essential to the development of a reliable kinetic model for the large esters and biodiesels, and thus, related studies are needed. Although fundamental combustion data of small esters, such as methyl formate (MF), methyl acetate (MA), and methyl butanoate (MB), have been extensively reported in the previous literature,17−29 few studies focused on methyl propanoate (MP). Zhao et al.30 studied the MP pyrolysis using a laminar flow reactor, and a kinetic model for MP pyrolysis at a low pressure was developed. Farooq et al.31 investigated the decomposition pathways of three small methyl esters, i.e., MA, MP, and MB, in shock tubes. Because the recorded CO2 © 2014 American Chemical Society

time-history profiles of MB were significantly higher than that predicted by the three previous mechanisms,21,22,25 they concluded that these biodiesel fuel models may have to be revised. Akih-Kumgeh et al.32 measured the ignition delay times of methyl esters from MF to MB in a shock tube. They reported that ignition delay times of MA are the longest, while those of MP are the shortest. Lam et al.33 investigated the overall reaction rates for the reactions of these four small esters with hydroxide radical (OH) behind reflected shock waves, and rate constants of these reactions in Arrhenius form were obtained. Dievart et al.16 measured the diffusive extinction limits of some methyl ester flames in the counterflow configuration. They concluded that the global reactivity of methyl esters is similar to that of n-alkanes, with the exception of methyl ester, which is smaller than MB. They also proposed a reduced model for methyl esters smaller than methyl pentanoate. Wang et al.34 investigated the laminar flame speeds of seven small esters under a variety of conditions. However, many available models generally give overprediction, indicating that further modification on the available models is needed. As an important intermediate during the oxidation of biodiesel and large esters, the comprehension on the combustion process of MP is still limited; thus, a further study is worthwhile. Ignition delay times are essential to the development and validation of the chemical kinetics. Up to now, limited studies have focused on the ignition delay times of MP; thus, the understanding on MP ignition is still lacking. In this study, the ignition delay times of MP/O2/Ar mixtures were measured at different pressures, temperatures, equivalence ratios, and Received: July 6, 2014 Revised: October 21, 2014 Published: October 22, 2014 7194

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equivalence ratio and tested pressure, respectively. The thermodynamic data of MP, O2, and Ar were obtained through the Burcat database.38 The temperature behind the reflected shock wave was calculated by Gaseq,39 a software for combustion equilibrium calculations. The uncertainty of the temperature behind the reflected shock wave was estimated to be less than 22.1 K in the current experiment, and the uncertainty of the ignition delay times measured in this study was generally within 19%. The detailed uncertainty analysis was provided as the Supporting Information.

dilution ratios. A modified MP model was proposed and validated against different literature data. Sensitivity analysis and reaction pathway analysis were conducted to obtain insight on the ignition chemistry of MP.

2. EXPERIMENTAL SECTION The experiments were performed in a shock tube that has been described in detail in the previous publications.35−37 Briefly, the shock tube has a diameter of 11.5 cm. Two diaphragms were used to separate the tube into a 2 m long driver section and a 7.3 m long driven section. Different thicknesses of diaphragms were selected depending upon the magnitude of the nominal reflected pressure. Prior to each experiment, the driven section was evacuated by a Nanguang vacuum system to a pressure below 10−3 Torr. Four fast-response sidewall pressure transducers (PCB 113B26), which were located along the end part of the driven section with the equal interval of 300 mm, were used to trigger three time counters (FLUKE, PM6690), and the time intervals were then recorded. By linear extrapolation of these time intervals, the endwall incident shock velocity was then obtained. The OH* chemiluminescence and endwall pressure were detected by an endwall photomultiplier (Hamamtsu, CR 131) and a pressure transducer (PCB 113B03), respectively. All data were recorded by a Yokogawa DL750 digital recorder. Figure 1 shows the definition of the measured ignition delay time, i.e., the time interval between the endwall pressure profile rising

3. IGNITION DELAY TIME MEASUREMENTS AND CORRELATIONS For all tested mixtures, a correlation of ignition delay times as a function of the fuel mole fraction, oxygen mole fraction, pressure, and temperature was obtained through the regression analysis as follows: τ = 4.33 × 10−4XMP 0.218 ± 0.077XO2−0.786 ± 0.042p−0.442 ± 0.026 exp(32.93 ± 0.49/R uT )

(R2 = 0.974)

(1)

where τ is the ignition delay time in microseconds, XMP and XO2 are the mole fractions of MP and O2, respectively, p is the pressure in atmospheres, Ru = 1.986 × 10−3 kcal mol−1 K−1 is the universal gas constant, and T is the temperature in kelvin. Because of the experimental conditions of this study, eq 1 is suitable to the temperature range of 1040−1720 K, pressure range of 1.2−10 atm, equivalence ratio range of 0.5−2.0, and fuel mole fraction range of 0.5−2.0%. From Figure 2, it can be seen that the fitted values agree well with the measured data. As expected, ignition delay times increase with an increase in the pressure, as shown in Figure 2a. This is consistent with the negative exponent of pressure in eq 1. Panels b and c of Figure 2 indicate that the ignition delay times decrease with a decrease in the equivalence ratio or an increase in the fuel concentration. This is reasonable as the O2 concentration is increased and the most important ignition promoting reaction R1 (H + O2 = O + OH) is then accelerated; thus, the overall reactivity is increased. Akih-Kumgeh et al.32 also measured the ignition delay time of MP. Through the linear regression analysis, the following correlation was obtained in their study: τ = 2.2 × 10−5ϕ−0.22 ± 0.08D1.10 ± 0.06p−0.93 ± 0.04

Figure 1. Definition of ignition delay times.

exp(37.9 ± 0.9/R uT )

steeply for the first time and the onset of ignition, which is defined by extrapolating the maximum slope of the OH* chemiluminescence curve to the zero line. Driver gases used in this experiment were helium and nitrogen, both with a purity of 99.99%. Before each experiment, the reactant mixtures consisting of MP, O2, and Ar (99.9, 99.99, and 99.99% in purity, respectively) were prepared in a 128 L stainless-steel tank, standing for at least 12 h to ensure adequate mixing. In all experimental conditions, the partial pressure of MP was less than 2.66 kPa, which is half of its saturated vapor pressure at 300 K. Compositions of five mixtures tested in this study are listed in Table 1, in which ϕ and p are the

where ϕ and D are the equivalence ratio and dilution ratio, respectively. To make a insightful comparison between the two studies, the fitted values of eq 2 are also plotted in Figure 2. It is found that the activation energy obtained by Akih-Kumgeh et al. is obviously higher than that obtained in this study, and this can be partly explained by the more diluted reactants in the current study. As shown in Figure 2a, the correlated values of eq 2 agree well with current measurements at the pressure of 5 and 10 atm. However, at the low pressure of 1.2 atm, significant discrepancy is observed because the fitted value of eq 2 is higher than the experimental data, especially at the low temperature range. The discrepancy at a low pressure is further demonstrated in Figure 2b, where the fitted values of eq 2 are observed to be distinctly higher than the measurements at the equivalence ratios of 0.5, 1.0, and 2.0. Figure 2c shows that the two correlations exhibit similar values at 1 and 2% fuel concentrations, while a higher value of eq 2 is observed at 0.5% fuel concentration.

Table 1. Compositions of MP/O2/Ar Mixtures mixture

ϕ

MP (%)

O2 (%)

Ar (%)

p (atm)

1 2 3 4 5

0.5 1.0 2.0 1.0 1.0

1 1 1 0.5 2

10 5 2.5 2.5 10

89 94 96.5 97 88

1.2, 5, 10 1.2, 5, 10 1.2, 5, 10 5 5

(2)

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Figure 3. Comparison to previous data conducted by Akih-Kumgeh et al.32

dilution ratio, the activation energy obtained in the current shock tube is the higher activation energy. Although the tested pressure in the current experiment is 1.2 atm, which is a little higher than that by Akih-Kumgeh et al., the slight difference between the pressures is not responsible for the discrepancy. In conclusion, there are some deviations between the ignition delay time data obtained from the two studies.

4. CHEMICAL KINETIC MODEL Simulations were carried out using the Senkin40 codes in Chemkin II41 package with the constant volume homogeneous reactor. All models were assumed to be zero-dimensional and adiabatic. Two available chemical kinetic models, namely, the Princeton model16 and the Westbrook model,34 were used to simulate the ignition delay times. The Princeton model was constructed on the basis of a previous MB and methyl decanoate (MD) model developed by Dievart et al.42 The small molecular species reaction kinetic in the Princeton model was updated in accordance with the nC5_4943 model. However, most of the reactions in the MP submodel were prescribed in a manner consistent with the MB and MD subsets. The Westbrook model was obtained through combining several submodels into a single mechanism. While the submodels of MF, MA, EF, and EA26 and MB, EP, and methyl isobutanoate44 were developed in the previous studies, the reaction rates in the MP submodel were generally estimated. The onset of ignition was defined as the appearance of the maximum slope in the temperature profile (max dT/dt). A 4%/ ms in pressure rise was considered in all simulations for the existence of a boundary layer effect.35−37 Figure 4 gives the comparison between the measured and simulated ignition delay times. The calculated ignition delay times by the Westbrook model are significantly shorter than the measured data under all conditions, indicating the poor prediction on MP ignition by the Westbrook model. This may due to the estimation of the rate constants of the MP submodel in the Westbrook model and the absence of some important reactions. The Princeton model can well-predict the global activation energy under all tested conditions and the ignition delay times at the pressure of 10 atm; however, at lower pressures, the model overpredicts the ignition delays and a significant discrepancy appears at the pressure of 1.2 atm.

Figure 2. Measured and fitted ignition delay times of MP.

To make a rigorous comparison, especially at a relative low pressure condition, where large discrepancy exists, the condition of mixture E in their study was repeated using current shock tube facility, and the comparison result is shown in Figure 3. It can be seen that, for the same mixture, the AkihKumgeh et al.32 data are significantly higher than the current data, especially at relative high temperatures. Moreover, as mentioned above, the activation energy obtained by the current study is lower than that by Akih-Kumgeh et al. because the tested mixtures were more diluted. On the contrary, at the same 7196

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Figure 4. Comparison between measured and predicted ignition delay times using the Princeton and Westbrook models.

The Aramco Mech1.3 model,45 developed by Metcalfe et al. in 2013, is a newly developed chemical kinetic model for C1− C4 species. It was optimized and validated against various experimental data, and good agreement was generally obtained. In comparison to the nC5_4943 model, which was developed by their group in 2010, many reaction rates have been revised, according to the new values obtained in recent years. To improve the simulation performance of MP ignition delay times, the MP submodel in the Princeton model was attached to the Aramco Mech1.3 model. Moreover, to obtain better prediction, some reactions were modified, and the modifications are as follows: (1) The unimolecular decomposition reactions are very important reactions during the primary consumption of MP. The fuel mainly break through the β C−C bond (R1549, MP = ME2J + CH3), CH3−O bond (R1548, MP = PAOJ + CH3), and α C−C bond (R1550, MP = CH3OCO + C2H5). From the reaction pathway analysis at the primary time (not presented in the paper) presented by the Princeton model, the fuel is consumed dominantly through R1548, while R1549 just contributes to a very small portion of the total fuel consumption. This is not consistent with the bond dissociation energies of MP calculated by Akih-Kumgeh et al.32 As shown in Figure 5, the bond dissociation energies increase in the order of the β C−C bond, CH3−O bond, and α C−C bond. Thus, these illogical rates were replaced by the calculated rates proposed by Zhao et al.30 The reasonable effect of these substitutions can be

Figure 5. Bond dissociation energies of MP in kcal/mol, calculated by Akih-Kumgeh et al.32

further demonstrated in the reaction pathway analysis presented below. (2) The CH3OH elimination reaction, as pointed out in previous studies,30,32 is absent from the MP submodel. Thus, reaction R1551 (MP = CH 3 OH + CH3CHCO) was added, and the reaction rate was taken from Zhao et al.30 (3) The reaction rates of H-abstraction reactions in the previous MP submodel were generally obtained through the analogy to that of MB in the study by Dooley et al.,23 because of their similarity in molecular structure. However, Dooley et al. obtained these reaction rates mainly through the analogy to the relative H-abstraction reaction rates 7197

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Figure 6. Comparison between measured and predicted ignition delay times using the Princeton and modified MP models.

of methyl cyclohexane obtained by Orme et al.46 Thus, large uncertainties exist for these reaction rates because they were just roughly estimated values. Sensitivity analysis of the combined model (is not displayed in this study) showed that the two H-abstraction reactions, namely, R1567 (MP + H ↔ H2 + MP2J) and R1577 (MP + H ↔ H2 + MPMJ) exhibit a large inhibiting effect on the ignition at a relatively low pressure. Thus, modification on these analogical rates has great potential to improve the simulation at the pressure of 1.2 atm. When the pre-exponential factor was halved, rates of R1567 and R1577 were decreased by a factor of 2, bringing a promoting effect on MP ignition at low pressures. Thus, the modified model with 318 species and 1668 reactions named the modified MP model was proposed. The thermodynamic and transport data of the small molecular species and the MP submodel species in the modified MP model were taken from the Aramco Mech1.3 and Princeton models, respectively. Figure 6 shows the comparison between the Princeton and modified MP models. At three equivalence ratios, both models give good predictions on ignition delay times at the pressure of 10 atm. However, at lower pressures, significant improvement in prediction of the modified MP model is presented, while the Princeton model gives remarkable overprediction at a relatively low pressure. In general, the modified MP model gives good prediction under most conditions, and only slight discrepancy is

presented in the low-temperature range at the pressure of 1.2 atm. Thus, modifications are still needed to further improve the MP model.

5. MODIFIED MP MODEL VALIDATION The modified MP model yields good agreement with the measured ignition delay times under the tested conditions. For further validation of the applicability of the modified model, simulations on other experimental data were conducted. 5.1. Validation against MP Pyrolysis Data. Zhao et al.30 studied the MP pyrolysis in a laminar flow reactor over temperatures from 1000 to 1500 K at a low pressure of 30 Torr. Mole fraction profiles of MP, hydrocarbon products, and oxygenate products were detected. In this study, numerical profiles of some main species predicted by the modified and Princeton models were plotted for the comparison to the experimental data. Figure 7 gives the comparison between the predicted and measured mole fraction profiles of MP and some main species. Although discrepancy still exists, both models yield fairly good prediction on most species. For CH3OH, CH3CHCO, CH2O, and C2H2, a significant better prediction was observed for the modified MP model compared to the Princeton model. Generally speaking, the modified MP model can give a good prediction on MP pyrolysis. 5.2. Validation against Flame Speed Data. Wang et al.34 coupled some small ester models into a single mechanism, 7198

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Figure 7. Comparison between experimental and calculated mole fraction profiles of MP and some main species from MP pyrolysis (symbols, experimental data; black lines, simulation using the Princeton model; and red lines, simulation using the modified MP model).

which was then attached to two different C0−C4 submodels, namely, the submodel developed by Bourque et al.47 and another submodel taken from USC Mech II.48 The two coupled models, are then named as model I and model II. Figure 8 shows the laminar flame speeds measured by Wang et al.,34 along with the predicted data with model I, model II, Princeton model, and modified MP model. Even though an overprediction from all models is still presented, the modified

MP model gives closer prediction than the other three models, especially at relatively large equivalence ratios.

6. SENSITIVITY ANALYSIS AND REACTION PATHWAY ANALYSIS 6.1. Sensitivity Analysis. To ascertain the reactions that mostly affect the ignition chemistry, the sensitivity analysis was performed on the basis of the modified MP model. The sensitivity coefficient is defined as S=

τ(2.0ki) − τ(0.5ki) 1.5τ(ki)

(4)

where τ is the ignition delay time of MP and ki is the rate constant of the ith reaction. A positive sensitivity coefficient indicates that the reaction exhibits an inhibiting effect on ignition and vice versa. Figure 9 lists the most influential reactions for the ignition prediction at 1% fuel concentration, equivalence ratio of 1.0, pressure of 1.2 atm, and two temperatures of 1200 and 1500 K. As expected, the chain-branching reaction R1 exhibits the largest negative sensitivity coefficient; thus, it has the highest promotion effect on MP ignition. Generally, other reactions that exhibit negative values generate more active radicals, leading to the increased reactivity and decreased ignition delay time. In contrast to this, the reactions having positive values tend to produce stable products or radicals. Although small radicals are dominant in the MP ignition, the four fuel-specific reactions among the 15 most important reactions also play an important role. The sensitivity coefficient values of Habstraction reactions R1557, R1567, R1568, and R1577 at

Figure 8. Validation against laminar flame speeds for MP at Tu = 333 K and p = 1.0 atm. 7199

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models at different temperatures (1200 and 1500 K), p = 1.2 atm, equivalence ratio of 1.0, and 20% fuel consumption. In general, the two models give similar prediction on the reaction pathways. At both temperatures, MP is dominantly consumed through the H-abstraction reactions via H, OH, HO2, and CH3 attacking. Among these reactions, the abstraction of the H atom from α-C, which produces CH3CHCOOCH3 (MP2J), exhibits the highest percentage in MP consumption because it only breaks the weakest C−H bond. MP2J mainly decomposes through the two routes: one produces CH2CHCOOCH3 (MP2D) and H atom, and another produces CH3CHCO and CH2O. Similarly, the H atom in the CH3O group can also be easily abstracted, and the product CH3CH2COOCH2 (MPMJ) is then exclusively decomposed to CH3CHCO and CH2O. Additionally, the less important Habstraction reaction product CH2CH2COOCH3 (MP3J) mainly decomposes via the β-scission reaction and forms C2H4 and CH3OCO. In general, the branch ratios of Habstraction reactions are in the order of Rα > RCH3O > Rβ, which are consistent with the values of C−H bond energy calculated by Akih-Kumgeh et al.,32 as shown in Figure 5. Reaction pathway analysis of the modified MP model shows a negligible MP (about 1%) consumption through the unimolecular decomposition reactions at 1200 K, while the value increases to 10.9% when the temperature increases to 1500 K, which implies the importance of the unimolecular decomposition reaction on the MP ignition at a higher temperature because of its relatively high activation energy. A total of 6.6 and 2.3% MP decompose to CH2COOCH3 (ME2J) + CH3 and CH3CH2COO + CH3, respectively, and a total of 2.0% MP goes through the CH3OH elimination reaction to produce CH3OH and CH3CHCO. In general, the branch ratios of unimolecular decomposition reactions are consistent with the values of relative bond energy, as shown in Figure 5. However, reaction pathway analysis with the Princeton model implies that MP ↔ PAOJ + CH3 is the most important decomposition pathway at 1500 K; thus, it fails to capture this

Figure 9. Sensitivity analysis for 1% MP in a shock tube at high and low temperatures, p = 1.2 atm, and ϕ = 1.0.

1500 K are distinctly smaller than those at 1200 K. This is reasonable because MP is consumed exclusively through the Habstraction reactions at 1200 K, while the unimolecular decomposition reactions play another important role at 1500 K (as shown in the reaction pathway analysis in the next section). 6.2. Reaction Pathway Analysis. Reaction pathway analysis was conducted to detect main reaction pathways in MP ignition in a shock tube. Figure 10 gives the main reaction pathways for 1.0% MP with the Princeton and modified MP

Figure 10. Reaction pathway analysis for 1.0% MP in a shock tube using the Princeton model (red bold font) and the modified MP model (black normal font) at T = 1200 K (values outside parentheses) and T = 1500 K (values inside parentheses), p = 1.2 atm, ϕ = 1.0, and 20% fuel consumption. 7200

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trend. In the Princeton model, the CH3OH elimination reaction is absent.

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CONCLUSION A study on ignition delay times of MP was conducted using a shock tube in the temperature range of 1040−1720 K, pressures of 1.2, 5.0, and 10.0 atm, MP concentrations of 0.5, 1.0, and 2.0%, and equivalence ratios of 0.5, 1.0, and 2.0. Main conclusions are summarized as follows: (1) Correlation of ignition delay times as a function of the fuel mole fraction, oxygen mole fraction, pressure, and temperature for the tested mixtures was provided using the multiple regression analysis. A comparison of ignition data between the current study and the previous study was conducted, and the result shows that discrepancy exists between the two studies. (2) Simulation performed with the Princeton and Westbrook models shows that the Westbrook model fails to predict the MP ignition under all tested conditions, while the Princeton model can give good prediction at a high pressure of 10 atm but significant discrepancy still exists at relatively lower pressures. (3) A modified MP model was proposed and a remarkable improvement was obtained for the low-pressure prediction, especially at a relatively high temperature. The modified model was further validated against MP pyrolysis data and flame speeds, and a reasonable agreement was obtained. (4) Sensitivity analysis based on the modified MP model shows that small radical reactions are of great importance in MP ignition, while four fuel-specific reactions, namely, R1557, R1567, R1568, and R1577, also show large sensitivity coefficients. Reaction pathway analysis shows that MP is dominantly consumed through the H-abstraction reactions at both low and high temperatures, while unimolecular decomposition reactions become important at an elevated temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Measured ignition delay times of MP, uncertainty analysis of the experimental ignition delay times, and mechanism files of the modified MP model. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected]. *Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the National Natural Science Foundation of China (51306144), the National Basic Research Program (2013CB228406), and the State Key Laboratory of Engines at Tianjin University (SKLE201302). The support from the Fundamental Research Funds for the Central Universities is also appreciated. The authors also thank Prof. Westbrook for providing the latest ester model.

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REFERENCES

(1) Hill, J.; Nelson, E.; Tilman, D.; Polasky, S.; Tiffany, D. Environmental, economic, and energetic costs and benefits of biodiesel 7201

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dx.doi.org/10.1021/ef501527z | Energy Fuels 2014, 28, 7194−7202