Experimental and Modeling Study of n-Butanol Oxidation at High

Article pubs.acs.org/EF

Experimental and Modeling Study of n-Butanol Oxidation at High Temperature Jiaxiang Zhang, Liangjie Wei, Xingjia Man, Xue Jiang, Yingjia Zhang, Erjiang Hu, and Zuohua Huang* State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Peopleʼs Republic of China S Supporting Information *

ABSTRACT: Ignition delay times of n-butanol/oxygen diluted with argon were measured behind reflected shock waves. Experiments were carried out in the temperature range 1200−1650 K, at 2 and 10 atm, and at equivalence ratios of 0.5, 1.0, and 2.0. Correlations of ignition delay times were constructed on the basis of measured data through multiple linear regression. A modified kinetic model for the oxidation of n-butanol at high temperature was developed, based on previous models by adding and modifying some key reactions. The modified model shows good prediction of the measured data under all measured conditions. This model was also validated against jet-stirred reactor (JSR) data obtained from the literature, and fairly good agreement was observed. A fair improvement on the simulation of aldehydes (acetaldehyde and butyraldehyde) was found compared to the original model. Finally, reaction pathway and sensitivity analysis indicate that the H-abstraction reactions play a dominant role in the consumption of n-butanol, while unimolecular decomposition reactions become more important with increasing temperature. High-level accurate investigation of the rate constants of H-abstraction reactions and unimolecular decomposition reactions is required to further improve n-butanol oxidation kinetics. consumption pathway for n-butanol oxidation. Black et al.26 also developed a detailed model for n-butanol that was validated against ignition delay times and JSR data obtained by Dagaut et al.14 As computed by these authors, the C−H bond at the α-carbon has the weakest bond energy due to the effect of alcohol group. Heufer et al.28 measured the highpressure ignition delay times of stoichiometric n-butanol/air mixtures and found a non-Arrhenius relationship at temperatures less than 1000 K. Sarathy et al.29 recently developed a detailed model including the low-temperature chemistry for four butanol isomers. Their model showed reasonable prediction of the shock tube data and RCM data at intermediate and low temperature. As shown by the measurements of Noorani et al.,27 ignition delay times of ethanol, npropanol, and n-butanol collapse, while methanol shows comparable ignition delay times but slightly lower activation energy than those of the other three alcohols. Recently, Stranic et al.30 measured the ignition delay times of four butanol isomers at high temperature. However, the measurments present some disagreement with those of Black et al.26 and Moss et al.25 Although ignition delay times of n-butanol at high temperature have been measured behind reflected shock waves,25−27,30 there are still some discrepancies among these experiments due to the specific techniques and data analysis approaches. In addition, kinetic models established on the basis of these experimental data also present some noticeable discrepancies in predicting ignition delay times. Therefore, more comprehensive and reliable experimental data of n-butanol are needed for the validation of chemical kinetic models. A more accurate model is

1. INTRODUCTION n-Butanol, as one of the most promising biofuels, has received increasing attention due to its renewability and advantages over other biofuels. It has been intensively investigated that nbutanol can be produced via acetone−butanol−ethanol (ABE) fermentation1 by using various feed stocks, such as corn, wheat, sugar beet, and sugar cane. Compared to ethanol, the most widely used biofuel currently, n-butanol holds several advantages including higher energy density, lower vapor pressure, lower affinity to water, and less corrosive.2,3 However, ABE butanol still cannot compete on a commercial scale with butanol produced synthetically for cost issues, the relatively low-yield and sluggish fermentations, as well as problems caused by end-product inhibition and phage infections.4 Recently, Atsumi et al.5 developed Escherichia coli strains with genes coding for two enzymes that convert keto acids into aldehydes and subsequently into n-butanol. The company, Gevo, acquired an exclusive license to commercialize this new technology in 2008.4 Several engine studies have been conducted to investigate the effect of n-butanol blending on spark-ignition or compressionignition engines.6−12 Until recently, the pyrolysis and oxidation of n-butanol have been widely studied through various techniques including jet-stirred reactor (JSR),13−16 lowpressure sampling via electron inonization and photoionization molecular-beam mass spectrometry (EI- and PI-MBMS),17 diffusion flames,16−18 laminar flame speeds,19−21 rapid compression machines22−24 and shock tubes.25−28,30 Additionally, a number of kinetic models14,16,18,19,25,26,29,31 have been developed based on the above experimental investigations. Specifically, Moss et al.25 measured the ignition delay times for four isomers of n-butanol and developed a detailed model that agrees fairly well with the experimental data. Kinetic modeling indicates that H-abstraction is the dominant © 2012 American Chemical Society

Received: March 23, 2012 Revised: May 9, 2012 Published: May 18, 2012 3368

dx.doi.org/10.1021/ef3005042 | Energy Fuels 2012, 26, 3368−3380

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(Yokogava, scopecorder-DL750). The temperatures behind the reflected shock are calculated with the reflected shock module in the software Gaseq.33 It is estimated that the uncertainty of the temperature is about ±25 K. Prior to the measurements of n-butanol, ignition delay times of nheptane and ethanol were tested under the same conditions as

also needed to predict the new measured data. In this study, ignition delay times of n-butanol were systematically measured over a wide range of equivalence ratios, pressures, and temperatures and carefully compared with the previous data. An Arrhenius correlation was given by multiple linear regression of the measured data. Furthermore, a modified model was proposed, based on previous high-temperature chemical kinetic models. Finally, reaction pathway and sensitivity analysis were conducted to gain insight into the high-emperature oxidation kinetics of n-butanol.

2. EXPERIMENTAL APPROACH The experimental apparatus and its validation have been described previously.32 All measurements were carried out in a shock tube with 11.5 cm inner diameter. The shock tube is divided into a 4.0 m long driver section and a 4.8 m long driven section by double diaphragms. The pressures behind reflected shock waves are changed by employing different thickness PET (polyester terephthalate) films. The driver section is filled with helium (99.999%) as driver gas. High-pressure air is used to blow away the PET fragments left in the last experiment to avoid disturbance and contamination. The driven section can be evacuated to pressure below 10−6 mbar by a Nanguang vacuum system. Fuel mixtures are prepared manometrically in a 128 L stainless steel tank and allowed to mix for at least 12 h by molecular diffusion. Longer mixing times were tested and no appreciable difference in ignition delay times was observed. The partial pressure of n-butanol in all mixtures is below 50% of its saturated vapor pressure to ensure complete evaporation of all liquid fuel. The ignition delay time is defined as the time interval between the arrival of the incident shock at the endwall and the intercept of maximum slope of endwall CH* radical emission curve with the initial

Figure 2. Comparison of shock tube data to previous data and simulation results for n-heptane (0.4% n-heptane, ϕ = 1.0, and p = 2 atm) and ethanol (1.5% ethanol, ϕ = 1.0, and p = 2 atm). previous studies. Figure 2 shows the comparison between the present measured ignition delay times with those of n-heptane measured by Horning et al.34 and ethanol by Noorani et al.,27 as well as simulation results using various detailed models. These models (Curran model35 for n-heptane oxidation, Marinov model36 and Leplat model37 for ethanol oxidation) are well-established and have been validated against various experimental targets. The good agreement between the measurements of this shock tube facility and others as well as the simulation results provides confidence in performing measurements for n-butanol. In this study, endwall CH* emission, OH* emission, and pressure are used to define the ignition delay times and investigate the influence of various diagnostics. The definition of ignition delay time using OH* emission is the same as that with CH* emission mentioned above, while the endwall pressure method is defined as the time interval between the arrival of shock wave at the endwall and the time of maximum slope of the endwall pressure. Comparison of these three methods is given in Figure 3. It is noted that although Hall et al.38 mentioned that different diagnostics for ignition delay time could lead to some discrepancies, different diagnostic methods employed tend to agree well in this study. It is well-known that nonideal effects behind the reflected shock wave are unavoidable, particularly for long ignition delay times. However, these effects can be reduced to a negligible level.39 These effects mainly include boundary-layer effect and nonideal breakage of diaphragms, which will lead to the rise of temperature and pressure behind reflected shock waves. For shock tubes with large diameter, shock attenuation caused by the boundary layer can be remarkably reduced. In this study, the observable shock attenuation is less than 2% due to the relatively large diameter of the shock tube. If it is assumed that the pressure rise is isentropic, the pressure rise, on average 2%/ ms, can cause a temperature rise of 0.75%/ms. For ignition delay times of 50−800 μs in this study, the estimated temperature rise is about 1− 8 K, which is negligibly small. Simulation of ignition delay times was conducted by use of the Chemkin II40 software in a constant-volume, adiabatic, and zerodimensional reactor. The simulated ignition delay time is defined as the time interval between the beginning of reaction and the time of peak CH concentration. Compared to the slower rise in the measured

Figure 1. Endwall pressure and CH* emission profile for 0.5% nbutanol/3.0% O2/96.5% Ar at p = 9.53 atm and T = 1289 K. signal level, as shown in Figure 1. The arrival of incident shock at the endwall is extrapolated by the measured incident shock velocity and sidewall pressure rise at the location 10 cm from the endwall. The incident shock velocity is determined by four pressure transducers (PCB, 113B26) located along the shock tube with the same length interval of 30 cm. The three time intervals are recorded by three time counters (Fluke, PM6690). The incident shock velocity at the endwall is obtained by linear extrapolation of three incident shock velocities calculated from the three time intervals. The CH* emission selected by a narrow filter centered at 430 ± 10 nm is measured with a photomultiplier (Hamamatsu, CR131) located at the endwall. The high voltage of the photomultiplier is set to 100 V in all experiments. All data acquisition is carried out by useof a digital recorder 3369


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where τ is the ignition delay time in microseconds, A is a constant, p is the pressure in atmospheres, Xfuel is the fuel mole fraction, ϕ is the equivalence ratio, Ea is the activation energy in kilocalories per mole, R = 1.986 × 10−3 kcal/mol·K is the universal gas constant, and T is the temperature in kelvins. This correlation is similar to that employed by Horning et al.34 and can reflect the equivalence ratio and pressure dependence of ignition delay times explicitly. Table 2 lists the correlations of previous studies and the current study under various experimental conditions. It is noted that Moss et al.25 and Noorani et al.27 correlated the ignition delay times in different formats in their studies, while Black et al.26 and Stranic et al.30 did not propose the correlations. So in this study, all correlations here are made in format 1 for better comparison. In addition, only low-pressure data of Stranic et al. are used for correlation because of the wide pressure range and proximate equivalence ratios of high pressure data. Coefficient R2 is used as the criterion to reflect the regression quality. High value of R2 indicates a good regression result. As shown in Table 2, correlations by Moss et al., Stranic et al. and the current study have good regression (R2 > 0.98), while correlations by Black et al. and Noorani et al. show slightly poor regression (R2 < 0.95). It also can be seen that the correlations of Noorani et al., Stranic et al., and the current study have comparable pressure dependence. The correlations of Stranic et al. and the current study have the same activation energy and are larger than those of other authors by 4−6 kcal/mol.

Figure 3. Comparison of various diagnostic methods for 0.6% nbutanol at p = 2.6 atm and ϕ = 1.0. CH* concentration, the simulated CH concentration rise is relatively sharp. Therefore, the ignition delay times determined from peak CH* concentration are consistent to the definition in the experiment. The pressure rise caused by nonideal effects was not considered in the simulation. Five mixtures of n-butanol/O2/Ar were tested in the study. To minimize the nonideal gas effects in the shock tube, high-level dilution

4. KINETIC MODELING The chemical kinetic model used here is based on the n-butanol model developed by Black et al.26 The Black model was developed by adding an n-butanol submodel to the C4 chemistry41 and showed reasonable prediction of ignition data and JSR data. By adding the absent reactions in the Black model and updating some reaction rate constants, the original n-butanol submodel has been modified to give better prediction of the new experimental results. The modified n-butanol mechanism consists of 243 species and 1475 reactions. The detailed mechanism including thermochemical data are available as Supporting Information. It is noted that the modification is mainly on the n-butanol submodel, and the original C4 chemistry41 is not changed at all. Details of the modified n-butanol submodel are listed in Table 3. Below is a description of the modified n-butanol submodel. 4.1. Unimolecular Decomposition. In the Black model, simple (R1329−R1333) and complex (R1334) fission were considered as unimolecular decomposition reactions. For simple fission reactions, although the fissions of various C−C bonds play dominant roles in the consumption of n-butanol due to their lower BDEs (bond dissociation energies) compared to those of C−H bonds, the fission reactions of C−H should be included for detailed mechanism. However, only one fission reaction of O−H (R1333) was considered in the Black model. In fact, the O−H bond has the highest BDE among these C−H and O−H bonds. Therefore, the fission reactions (R1335−R1338) of other C−H bonds should also be included. In this study, a common rate constant of 1.0 × 1014 cm3·mol−1·s−1, the same as R1333, was used for these reactions. For complex fission, another possible and important reaction channel (R1339) was added to the Black model. The reaction rate by Grana et al.18 was adopted. 4.2. Hydrogen Abstraction. It has been already found that H-abstraction is the major pathway for the consumption of nbutanol.14,16,25,26 Among these reactions, H-abstractions by small radicals (OH, H, and HO2) from the α-, β-, γ-, and δcarbon positions and the alcohol group of n-butanol play a key role in the n-butanol oxidation.

Table 1. Test Mixtures and Experimental Conditions mixture

n-butanol (mol %)

ϕ

p5 (atm)

ref

1 2 3 4 5

0.6 1.0 0.8 0.67 0.5

1.0 1.0 1.0 1.0 0.5, 1.0, 2.0

2.6 1.2 2.0 3.0 2, 5, 10

26 25 27 30 this study

of fuel mixtures with argon was used in all experiments. Table 1 lists all the experimental conditions. Mixtures 1−4 and their corresponding experimental conditions replicate the conditions employed in the previous studies. Ignition delay times of mixture 5 with various equivalence ratios were measured at high temperatures and elevated pressures in this study.

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Comparison with Previous Studies. As mentioned above, mixtures 1−4 completely replicated the conditions in previous studies for more accurate comparison. Figure 4 shows the comparisons of ignition delay times between the measured data and previous studies. As shown in Figure 4a,b, the present data show shorter ignition delay times than those of Black et al.26 and Moss et al.,25 especially at relatively higher temperatures. In addition, the activation energy of the present data is noticeably higher than that of the two previous studies. Compared to the data of Noorani et al.,27 shown in Figure 4c, the present data shows reasonable agreement but slightly higher activation energy. Although endwall OH* emission signal was employed to define the ignition delay time in the study of Stranic et al.,30 very good agreement in ignition delay times and can be observed over the whole temperature range in Figure 4d. 3.2. Ignition Delay Time Correlations. Ignition delay time depends on mixture composition, temperature, and pressure. Ignition delay time correlation can be obtained by regression analysis on the measured data. In this study, the ignition delay time is correlated as

τ = Apa X fuel bφc exp(Ea /RT )

(1) 3370


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Figure 4. Comparison between measured data for mixtures 1−4 and previous studies25−27,30.

Table 2. Summary of Correlations in Format 1 a

b

c

Ea (kcal/mol)

p (atm)

Xfuel (%)

ϕ

T (K)

R2

−6

9.26 × 10 1.07 × 10−4 4.63 × 10−5

−0.92 ± 0.08 −0.82 ± 0.07 −0.53 ± 0.08

−1.14 ± 0.63 −0.51 ± 0.05 −0.86 ± 0.17

1.30 ± 0.06 0.46 ± 0.04 0.46 ± 0.21

34.0 ± 1.0 35.0 ± 0.7 31.6 ± 1.3

1−8 1.2−4 2−12

0.6−0.75 0.25−1.0 0.5−2.0

0.5−2.0 0.25−1 0.5−2.0

1150−1900 1200−1700 1100−1600

0.949 0.986 0.924

1.85 × 10−6

−0.58 ± 0.04

−1.00 ± 0.11

1.33 ± 0.15

38.5 ± 0.8

1−3

0.5−0.75

0.5−1.0

1200−1500

0.991

6.52 × 10−6

−0.49 ± 0.02

−0.75 ± 0.05

1.06 ± 0.04

38.5 ± 0.5

1.2−10

0.5−1.0

0.5−2.0

1200−1650

0.982

A

ref 26

Black et al. Moss et al.25 Noorani et al.27 Stranic et al.30 this study

Moc and Simmie42 recently computed the barrier heights of H-abstractions by OH radical from n-butanol and gave theorder Cα < Cγ < Cβ < Cδ < OH. In the Black model, the branching ratio of H-abstractions by OH radical from n-butanol across the measured temperature range follows the order Rγ > Rδ > Rβ > Rα > ROH, which is inconsistent with the computational results of Moc and Simmie. In fact, due to the effect of OH group, the Cα−H bond has the weakest BDE. As a result, H-abstraction should have the largest branching ratio at the α-carbon position. In the modified model, rate constants for Habstractions by OH radical from n-butanol calculated by Zhou et al.43 based on G3 potential energy surface were employed for their good agreement with the experimental data by Vasu et al.44 Calculations by Zhou et al.43 for the temperature range 1200−1600 K indicated that the branching ratios of these five reactions are in the order Rα > Rδ > Rγ > Rβ

> ROH, which is still slightly different from the computation of Moc and Simmie.42 Until recently, no high-level computation was carried out on H-abstractions by H radical from n-butanol. Again, the Black model presents a branching ratio order inconsistent with the computational results of barrier heights. In this study, the rate constants of H-abstraction reactions used in the Grana model18 were employed. In order to obtain similar branching ratio with OH radical and better prediction of ignition delay times, the Afactor of reaction R1342 was reduced by a factor of 3. H-abstraction by HO2 radical is also very important for nbutanol oxidation, particularly at low temperatures. In this study, the rate constant calculated by Zhou et al.45 was adopted. As the calculation of Zhou et al.45 shows, H-abstraction by HO2 radical from the α-position is dominant in the whole temperature range. In this study, H-abstractions by C2H3, O2, 3371


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Table 3. n-Butanol Submodela NUM R1329 R1330 R1331 R1332 R1333 R1334 R1335 R1336 R1337 R1338 R1339 R1340 R1341 R1342 R1343 R1344 R1345 R1346 R1347 R1348 R1349 R1350 R1351 R1352 R1353 R1354

R1405 R1406 R1407 R1408 R1409 R1410 R1411 R1412 R1413 R1414 R1415 R1416 R1417 R1418 R1419 R1420 R1421 R1422 R1423 R1424 R1425 R1426 R1427 R1428 R1429

A

reaction

Unimolecular Decomposition n-C4H9OH (+M) ⇔ CH3 + C3H6OH (+M) 3.79 × 1024 n-C4H9OH (+M) ⇔ C2H5 + p-C2H4OH (+M) 5.53 × 1024 n-C4H9OH (+M) ⇔ n-C3H7 + CH2OH (+M) 3.02 × 1023 n-C4H9OH (+M) ⇔ OH + p-C4H9 (+M) 6.33 × 1020 n-C4H9OH (+M) ⇔ H + p-C4H9O (+M) 6.04 × 1014 n-C4H9OH (+M) ⇔ C4H8-1 + H2O (+M) 3.52 × 1013 H + C4H8OH-4 ⇔ n-C4H9OH 1.00 × 1014 H + C4H8OH-3 ⇔ n-C4H9OH 1.00 × 1014 H + C4H8OH-2 ⇔ n-C4H9OH 1.00 × 1014 H + C4H8OH-1 ⇔ n-C4H9OH 1.00 × 1014 n-C4H9OH ⇔ n-C3H7CHO + H2 5.00 × 1013 Hydrogen Abstraction n-C4H9OH + H ⇔ C4H8OH-4 + H2 7.22 × 106 n-C4H9OH + H ⇔ C4H8OH-3 + H2 4.81 × 106 n-C4H9OH + H ⇔ C4H8OH-2 + H2 1.60 × 106 n-C4H9OH + H ⇔ C4H8OH-1 + H2 7.22 × 106 n-C4H9OH + H ⇔ p-C4H9O + H2 2.41 × 106 n-C4H9OH + OH ⇔ C4H8OH-4 + H2O 2.15 × 104 n-C4H9OH + OH ⇔ C4H8OH-3 + H2O 5.09 × 101 n-C4H9OH + OH ⇔ C4H8OH-2 + H2O 8.29 × 10−1 n-C4H9OH + OH ⇔ C4H8OH-1 + H2O 4.56 × 103 n-C4H9OH + OH ⇔ p-C4H9O + H2O 5.88 × 102 n-C4H9OH + HO2 ⇔ C4H8OH-4 + H2O2 8.80 × 10−2 n-C4H9OH + HO2 ⇔ C4H8OH-3 + H2O2 2.76 × 10−4 n-C4H9OH + HO2 ⇔ C4H8OH-2 + H2O2 7.51 × 10−3 n-C4H9OH + HO2 ⇔ C4H8OH-1 + H2O2 3.50 × 10−5 n-C4H9OH + HO2 ⇔ p-C4H9O + H2O2 6.47 × 10−7 n-C4H9OH + R ⇔ C4H8OH-4 + RH n-C4H9OH + R ⇔ C4H8OH-3 + RH n-C4H9OH + R ⇔ C4H8OH-2 + RH n-C4H9OH + R ⇔ C4H8OH-1 + RH n-C4H9OH + R ⇔ p-C4H9O + RH Isomerization of First-Formed Radicals p-C4H9O ⇔ C4H8OH-4 1.32 × 10−1 p-C4H9O ⇔ C4H8OH-3 5.32 × 10−10 C4H8OH-4 ⇔ C4H8OH-1 3.30 × 10−19 Decomposition of First-Formed Radicals C4H8OH-4 ⇔ C2H4 + p-C2H4OH 2.37 × 1012 C4H8OH-4 ⇔ C4H7OH[1−4] + H 2.65 × 1012 C4H8OH-3 ⇔ C3H6 + CH2OH 4.23 × 1010 C4H8OH-3 ⇔ C4H7OH[1−4] + H 6.04 × 1011 C4H8OH-3 ⇔ C4H7OH[2−1] + H 1.43 × 1012 C4H8OH-2 ⇔ C3H5OH + CH3 7.56 × 1010 C4H8OH-2 ⇔ C4H8-1 + OH 6.67 × 1015 C4H8OH-2 ⇔ C4H7OH[2−1] + H 1.43 × 1012 C4H8OH-2 ⇔ C4H7OH[1−1] + H 4.07 × 1011 C4H8OH-1 ⇔ C2H3OH + C2H5 1.52 × 1012 C4H8OH-1 ⇔ n-C3H7CHO + H 3.07 × 1014 C4H8OH-1 ⇔ C4H7OH[1−1] + H 4.07 × 1011 p-C4H9O ⇔ n-C3H7CHO + H 8.89 × 1010 C4H8OH-1 + O2 ⇔ n-C3H7CHO + HO2 5.28 × 1017 C4H8OH-1 + O2 ⇔ C4H7OH[1−1] + HO2 7.62 × 102 C4H8OH-2 + O2 ⇔ C4H7OH[1−1] + HO2 7.62 × 102 C4H8OH-2 + O2 ⇔ C4H7OH[2−1] + HO2 7.62 × 102 C4H8OH-3 + O2 ⇔ C4H7OH[1−4] + HO2 7.62 × 102 C4H8OH-3 + O2 ⇔ C4H7OH[2−1] + HO2 7.62 × 102 C4H8OH-4 + O2 ⇔ C4H7OH[1−4] + HO2 7.62 × 102 Keto−Enol Isomerization C2H3OH ⇔ CH3CHO 3.80 × 107 C4H7OH[1−1] ⇔ n-C3H7CHO 3.80 × 107 3372

n

Ea

−2.23 −2.23 −1.88 −1.37 0.10 0.00 0.00 0.00 0.00 0.00 0.00

88 070 89 010 85 710 94 930 103 800 67 230 0 0 0 0 69 500

2.00 2.00 2.00 2.00 2.00 2.77 3.35 3.74 2.81 2.82 4.31 4.76 4.52 5.26 5.30

6526 3951 3951 3951 6526 185 −4351 −4067 −3680 −583 17 300 11 900 14 700 8270 10 500

3.63 6.20 8.64

2689 6710 5268

0.48 0.25 1.04 0.46 0.23 0.94 −0.87 0.23 0.40 0.60 −0.45 0.40 0.75 −1.64 2.45 2.45 2.45 2.45 2.45 2.45

28 890 35 710 28 170 36 560 36 340 31 380 27 660 36 340 35 430 29 120 34 700 35 430 21 060 839 −296 −296 −296 −296 −296 −296

26 added, 48 26 26 26 26 26 26 added, 48 26 26 26 added, 48 estimated, estimated, estimated, estimated, estimated, estimated, estimated,

1.53 1.53

51000 51 000

estimated, 50 estimated, 50

ref 26 26 26 26 26 26 added, added, added, added, added,

estimated estimated estimated estimated 18

18 18 estimated, 18 18 18 43 43 43 43 43 45 45 45 45 45

47 47 47

49 49 49 49 49 49 49


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Table 3. continued NUM R1430 R1431 R1432 R1433

A

reaction

Keto−Enol Isomerization C2H3OH + HO2 ⇔ CH3CHO + HO2 1.49 × 105 C4H7OH[1−1] + HO2 ⇔ n-C3H7CHO + HO2 1.49 × 105 C2H3OH + HOCHO ⇔ CH3CHO + HOCHO 2.81 × 10−2 C4H7OH[1−1] + HOCHO ⇔ n-C3H7CHO + HOCHO 2.81 × 10−2 butenol oxidation ethenol oxidation

n 1.67 1.67 3.29 3.29

Ea 6810 6810 −4509 −4509

ref 51 estimated, 51 52 estimated, 52 26 26

a R denotes a species (O, CH3, HCO, CH2OH, CH3O, CH3O2, C2H5, C2H3, O2, or C2H5O2). Among these species, C2H3 and O2 are added to the base model on the basis of refs 16 and 19, respectively, while n-butanol + C2H5O2 is estimated on the basis of n-butanol + CH3O2. Units are centimeters, moles, seconds, calories, and kelvins

importance for alcohol oxidation.52,54 Uncatalyzed in the gas phase, the isomerizations have large barrier heights (58 kcal/ mol for butenol/butanal isomerization54) and will only occur rapidly at high temperatures. The rate constants for these two isomerizations (R1428 and R1429) without catalyst were estimated to be identical to the propenol/propanal isomerization calculated by Akih-Kumgeh.55 In this study, keto−enol isomerization catalyzed by hydroperoxyl radical and formic acid were added on the basis of the computation for ethenol/ acetaldehyde isomerization conducted by da Silva et al.56,57

and C2H5O2 absent in the Black model were also added to improve the simulation. The rate constant of n-C4H9OH + C2H5O2 was estimated to be similar to that of n-C4H9OH + CH3O2 used in the Black model. 4.3. Isomerization of First-Formed Radicals. Alkyl radicals can transfer H atoms from one site to the radical site at rates that depend on the type of C−H bond (primary, secondary, and tertiary) broken and the ring strain energy barrier involved.46 In the Black model, the isomerizations of first-formed radicals proceed via 1−3 and 1−4 hydrogen shift reactions. In fact, the 1−3 hydrogen shift that occurs through four-membered ring transition structure has very large barrier height and is of less significance. In this study, only 1−4 and 1− 5 hydrogen shift reactions (R1405−R1407) were considered. Rate constants of these reactions with temperatures from 200 to 2500 K were derived from the calculation by Zheng and Truhlar47 by use of canonical variational theory with multidimensional tunneling contributions. 4.4. Decomposition of First-Formed Radicals. The firstformed radicals mainly decompose by β-scission. In the Black model, three possible reaction channels (R1409, R1416, and R1420) are absent for the decomposition of first-formed radicals. In the modified model, rate constants of R1409 and R1416 were estimated on the basis of the reaction of hydrogen atom addition to butene recommend by Curran.48 For reaction R1420, the rate constants was estimated on the basis of the reaction of hydrogen atom addition to propanal, also recommend by Curran.48 Although the reactions first-formed radicals + O2 → keto/ enol + HO2 were included in the Black model, their reaction rates were roughly estimated and did not reflect the significant influence of the OH group. In fact, according to the investigation of da Silva et al.49 on CH3CHOH + O2, acetaldehyde formation significantly predominates over ethenol formation across the whole temperature range. There is only a slight decrease of the reaction rate as the temperature increases. In view of the similarity in chemical structure between CH3CHOH and C4H8OH-1, the rate constant of R1421 was estimated to be identical to that of CH3CHOH + O2 ⇔ CH3CHO + HO2, while the rate constants of R1422−R1427 were estimated to be identical to that of CH3CHOH + O2 ⇔ CH2CHOH + HO2. 4.5. Keto−Enol Isomerization. Enols have been demonstrated as common intermediates in hydrocarbon oxidation.50−53 In the Black model, ethenol and butenol oxidation were included. However, as discussed by Weber et al.,23 the keto− enol isomerization reactions were completely absent in the Black model. In fact, keto−enol isomerization is of great

5. VALIDATION OF MODIFIED MODEL 5.1. Validation against Ignition Delay Times. In this section, only the Black model and new modified model were used for simulation of the experimental ignition data. Comparisons between the new modified model and other models16,19,25,29 are available as Supporting Information. Comparisons were conducted under various fuel fractions, pressures, and equivalence ratios for model validation. It is necessary to test the kinetic models under various fuel concentrations to improve the model performance in predicting other combustion processes. In fact, the effect of fuel concentration in this study is equivalent to that of dilution ratio employed by others.27,58 Figure 5 shows the ignition delay times of various n-butanol fractions and their simulation results. As expected, ignition delay times decrease with increasing fuel fraction. Both models well capture this trend and show good prediction of the activation energy. However, compared to the Black model, the modified model can quantitatively simulate

Figure 5. Comparison between experimental data and model prediction at various n-butanol fractions, ϕ = 1.0, and p = 2 atm. 3373


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the ignition delay times very well under all conditions. The Black model overpredicts the ignition delay times by about 30% across the whole temperature range. Besides the effect of fuel concentration, pressure dependence was also investigated in the study, as shown in Figure 6. It is

Figure 8. Comparison between experimental data and model prediction at various equivalence ratios for 0.5% n-butanol in Ar at p = 10 atm.

observed that ignition delay times decrease with increasing pressure. This trend is verified by the negative pressure exponent in the ignition delay time correlation as shown in Table 2. Again the modified model well predicts the pressure dependence and shows good quantitative agreement with the experimental data. However, the modified model slightly overpredicts the ignition delay times at high pressure (p = 10 atm). This might be due to the absence of low-temperature chemistry of n-butanol in the current model. Similar overprediction at high pressure is observed for the Black model. In addition, the Black model overpredicts the experimental data by about 30% at all pressures. Experiments were conducted under lean (ϕ = 0.5), stoichiometric (ϕ = 1.0), and rich (ϕ = 2.0) conditions at pressures of 2 and 10 atm. Figures 7 and 8 explicitly indicate that the ignition delay times have a negative dependence on

equivalence ratio. It is well-known that the chain branching reaction, H + O2 ⇔ OH + O, plays a very important role at high temperature. Higher oxygen concentration leads to increased reaction reactivity and decreased ignition delay times. Compared to the Black model at low pressure (p = 2 atm), the modified model shows good prediction at various equivalence ratios. At high pressure (p = 10 atm), the modified model prediction also agrees very well with the measured data. However, it is observed that the deviation between the two models is more obvious for the lean mixture (ϕ = 0.5) than the others, which implies that the modified model presents a slightly higher equivalence ratio dependence than that of the Black model. According to the study of Noorani et al.,27 ethanol and nbutanol have comparable ignition delay times under the same conditions. This conclusion was consistent with the previous study of Veloo et al.,19 who reported that the flame speeds of methanol, ethanol, and n-butanol are comparable under lean to stoichiometric conditions. Figure 9 shows the comparison of experimental data and model prediction for ethanol and nbutanol under the same conditions. The ethanol data has a dilution ratio of 3.36 in this study. The definition of dilution

Figure 7. Comparison between experimental data and model prediction at various equivalence ratios for 0.5% n-butanol in Ar at p = 2 atm.

Figure 9. Comparison between experimental data and model prediction for 1.5% ethanol and 0.78% n-butanol with the same dilution ratio, D = 3.36, at ϕ = 1.0 and p = 2 atm.

Figure 6. Comparison between experimental data and model prediction at various pressures for 0.5% n-butanol, ϕ = 1.0.

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Figure 10. Experimental and computed species profiles for the oxidation of 0.1% n-butanol in a JSR at ϕ = 1.0, p = 1 atm, and τ = 7 s. Experimental data16 are shown by large symbols; simulation by the modified model is shown by lines with small symbols.

(800−1200 K). In this study, only JSR data for ϕ = 1.0 at two different pressures are selected for comparison. The validation against experimental JSR data for the Black model at p = 10 atm has been presented in ref 26. Figures 10 and 11 give the comparisons between the measured and computed species profiles at p = 1.0 and 10 atm, respectively. At p = 1.0 atm for the temperature range 950− 1150 K, the modified model presents good prediction in all species except NC 3H 7CHO, C 4H 8-1, C 2 H2 , and CO2 . NC3H7CHO and C4H8-1 are slightly underpredicted across the temperature range, while C2H2 is overpredicted significantly. CO2 is overpredicted at T > 1100 K and underpredicted at T < 1100 K. At temperatures below 950 K and over 1150 K, species concentrations are underpredicted by the modified model. These discrepancies are also observed for the Black model. Compared to the unsatisfactory prediction of nC3H7CHO and CO2 at p = 1.0 atm, these two species are well predicted at p = 10 atm. However, the other two species, nC4H9OH and C4H8-1, are slightly underpredicted across the whole temperature range at p = 10 atm. The modified model also overpredicts the C2H2 concentration. The overprediction is also present in the comparison of the Black model to the experimental data. ROP (rate of production) analysis shows that the rate of C2H2 is related to the small radicals in the C4

ratio is the same as that of Herzler and Naumann59 but a little different from that of Noorani et al.27 Therefore, the n-butanol data are correlated to a condition with the same dilution ratio, equivalence ratio, and pressure based on the correlation developed in this study for better comparison. Two wellestablished ethanol models, Marinov model36 and Leplat model,37 were employed for simulation. It can be observed that n-butanol shows comparable ignition delay times with those of ethanol. This trend is consistent with the observation of Noorani et al.27 Additionally, both the above ethanol models predict the ignition delay times well, particularly the Leplat model.37 The modified n-butanol model shows fairly good agreement with the experimental data and well captures the similarity between ethanol and n-butanol. Therefore, it is proved that the experimental data and modified model for nbutanol are reliable and accurate to a certain extent. However, the Black model overpredicts the experimental data. As a result, it does not well capture the similarity. 5.2. Validation against JSR Data. To further validate the performance of the modified model, comparison between predicted and measured JSR data by Dagaut et al.14 and Sarathy et al.16 was carried out. These two experiments were conducted at p = 1.0 atm and p = 10 atm, with equivalence ratios (lean, stoichiometric, and rich conditions) and temperature range 3375


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Figure 11. Experimental and computed species profiles obtained for the oxidation of 0.1% n-butanol in a JSR at ϕ = 1.0, p = 10 atm, and τ = 7 s. Experimental data14 are shown by large symbols; simulation by modified model is shown by lines with small symbols.

which is inconsistent with the observation of previous studies.52,53,60 In modified model, the reaction rates of firstformed radicals + O2 → keto/enol + HO2 were estimated on the basis of previous studies. In addition, the keto−enol isomerization reactions with and without catalyst were added. As Figure 12 shows, the modified model gives reasonable prediction on the measured data, while the Black model predicts obviously higher enol concentrations and lower aldehyde concentrations. According to the ROP analysis, nC3H7CHO is predominantly produced by reaction R1421 (see Table 3), which is mainly due to the effect of the OH group. This observation is consistent with the investigation of Zhang et al.53 With increasing temperature, the production of nC3H7CHO by dissociation of radicals becomes important. The slight underprediction of aldehyde at low temperature is probably caused by the absence of low-temperature chemistry.

mechanism. Thus, the discrepancy in C2H2 prediction probably relies on the C4 mechanism, which is not modified in the modified model. Acetaldehyde and butanal are important intermediate species in the oxidation of n-butanol. In the study of Black et al.,26 the CH3CHO and NC3H7CHO species profiles are underpredicted significantly. The authors explained that the rapid keto−enol isomerization occurring somewhere between the reactor exit and the analytical system may contribute to the discrepancy. Li et al.52 reported acetaldehyde:ethenol:propanal:propenol(s) ratios of 40:5:125:1 in lean n-propanol flames and discussed that acetaldehyde can only be formed effectively from the isomerization of ethenol. More recently, Oßwald et al.60 found that negligible butenol species is identified in fuel n-butanol flame, while a relatively large amout of butanal species is detected. In addition, the detected acetaldehyde concentration is about 2.5 times higher than that of ethenol. Furthermore, Zhang and Boehman53 reported small concentrations of but-2en-1-ol and but-3-en-1-ol and 300 times higher concentrations of butanal in their engine ignition study. Then they inferred that butanal is primarily formed from α-hydroxybutyl rather than from but-1-en-1-ol isomerization. In the Black model, the keto−enol isomerization reaction channels were completely neglected. The reaction rates of first-formed radicals + O2 → keto/enol + HO2 are roughly estimated. As a result, the concentrations of enols are much higher than those of ketos,

6. REACTION PATHWAY AND SENSITIVITY ANALYSIS 6.1. Reaction Pathway Analysis. Reaction pathway analysis is complementary in determining main reaction pathways for species interested. As shown in Figures 13 and 14, reaction pathways of the Black model and the modified model are performed at three initial temperatures (1200, 1400, and 1600 K) with the same pressure (2 atm), equivalence ratio (ϕ = 1.0), and fuel concentration (0.5% n-butanol). The timing 3376


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Figure 12. Comparison between experimental data and model prediction for ketos and enols.

Figure 13. Reaction pathway diagram of Black model for 0.5% n-butanol in shock tube at T = 1200 K (boldface type), T = 1400 K (italic type), and T = 1600 K (roman type); ϕ = 1.0, p = 2 atm, 20% fuel consumption.

of 20% fuel consumption is selected for analysis like that by Black et al.26 and Weber at al.24 In both models at various temperatures, the consumption of n-butanol is dominated by H-abstraction reactions. Due to the

high activation energy, the unimolecular decomposition reactions become more important and the branching ratios increase to about 40% as temperature increases. Among the unimolecular decomposition reactions in both models, the 3377


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Figure 14. Reaction pathway diagram of modified model for 0.5% n-butanol in shock tube at T = 1200 K (boldface type), T = 1400 K (italic type), and T = 1600 K (roman type); ϕ = 1.0, p = 2 atm, 20% fuel consumption.

the Black model, the addition of reaction, p-C4H9O ⇔ nC3H7CHO + H, leads to another important reaction channel for the scission of p-C4H9O in the modified model. 6.2. Sensitivity Analysis. Sensitivity analysis of the modified model on temperature dependence is conducted in this study. The normalized sensitivity is defined as

highest branching ratio occurs at the fission of Cα−Cβ bond because of its weakest bond energy. An apparent difference between two models is that a complex fission channel, nC4H9OH ⇔ n-C3H7CHO + H2, is added in the modified model and plays a comparable role with the reaction of n-C4H9OH (+M) ⇔ C4H8-1 + H2O (+M). Although the fission of the C4H9−OH bond is included in both models, it is not illustrated in the reaction pathway diagrams for its negligible contribution to the initiation process due to the higher C4H9−OH bond energy compared to other C−C bond dissociation energies. Among the H-abstraction reactions of the Black model, the leading consumption pathway is H-abstraction from the γcarbon, followed by those from β-, δ-, and α-carbons and alcohol group. In the modified model, H-abstraction mainly occurs from the α-carbon. Generally, the branching ratios are in the order Rα > Rγ > Rδ > Rβ > ROH. According to the ROP analysis of n-butanol, H-abstraction by OH and H radicals are much more important than that by other radicals. Thus, it is important to experimentally and theoretically investigate the rate constants of H-abstraction from n-butanol by OH and H radicals. H-abstraction from the alcohol group is of the least importance for its highest bond dissociation energy. Compared to the Black model, H-abstraction from β-carbon plays a important role in the modified model. The reaction pathways of the radicals produced after initial decomposition of n-butanol also present some differences between the two models. In the Black model, the majority of C4H8OH-4 decomposes to ethylene (C2H4) and 2-hydroxyethyl (C2H4OH) though β-scission, with only 2% undergoing isomerization to form C4H8OH-1. In the modified model, the isomerization of C4H8OH-4 to form p-C4H9O plays a more important role with branching ratio up to about 30% at T = 1200 K. Ethenol (C2H3OH), which is a dominant product of the β-scission of C4H8OH-1, primarily undergoes isomerization to acetaldehyde (CH3CHO) in the modified model, while this pathway is completely absent in the Black model. Compared to

S=

τ(2ki) − τ(0.5ki) 1.5τ(ki)

(2)

where τ is ignition delay time and ki is the pre-exponential factor of the ith reaction. Negative S indicates a promoted effect on overall reaction rate. Figure 15 shows the sensitivity analysis of 0.5% n-butanol at ϕ = 1.0 and p = 2 atm at three different temperatures. The modified model presents an extremely high sensitivity to the main chain branching reaction R1. Small radicals have significant effect on ignition delay times, while only one fuelspecific reaction exists among the 12 most important reactions. Fuel-specific reaction R1346, as a H-abstraction reaction by OH radical, has negative value at low temperature and positive value at high temperature, which implies that increasing the reaction rate can decrease the activation energy of the ignition delay times. As shown in Figure 15, most reactions that have the positive values tend to form stable products or radicals, leading to reduced reactivity and longer ignition delay times. Although the sensitivity analysis does not well reflect the modifications in the modified model, which are mainly focused on fuel-specific reactions, some modified reactions out of the 12 most important ones indeed show relatively high sensitivity. Their accumulated effects contribute to the improvement of the modified model in predicting ignition delay times.

7. CONCLUSIONS Ignition delay times of n-butanol were measured over a range of equivalence ratios of 0.5−2.0, pressures of 1.2−10 atm, and 3378


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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 (Grant 51136005) and State Key Laboratory of Engines (SKLE201101).

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(1) Ramey, D. Butanol advances in biofuels; http://www.lightparty. com/Energy/Butanol.html, 2004. (2) Nigam, P. S.; Singh, A. Prog. Energy Combust. Sci. 2011, 37 (1), 52−68. (3) Harvey, B. G.; Meylemans, H. A. J. Chem. Technol. Biotechnol. 2011, 86 (1), 2−9. (4) European Biofuels Technology Platform (EBTP), Biobutanol; http://www.biofuelstp.eu/butanol.html, 2009. (5) Atsumi, S.; Hanai, T.; Liao, J. C. Nature 2008, 451 (7174), 86− 89. (6) Yacoub, Y.; Bata, R.; Gautam, M. Proc. Inst. Mech. Eng. A: J. Power Energy 1998, 212 (5), 363−379. (7) Szwaja, S.; Naber, J. D. Fuel 2010, 89 (7), 1573−1582. (8) Yao, M.; Wang, H.; Zheng, Z.; Yue, Y. Fuel 2010, 89 (9), 2191− 2201. (9) Wallner, T.; Miers, S. A.; McConnell, S. ASME Conf. Proc. 2008, 2008 (48132), 129−139. (10) Rakopoulos, D. C.; Rakopoulos, C. D.; Giakoumis, E. G.; Dimaratos, A. M.; Kyritsis, D. C. Energy Convers. Manage. 2010, 51 (10), 1989−1997. (11) Rakopoulos, D. C.; Rakopoulos, C. D.; Hountalas, D. T.; Kakaras, E. C.; Giakoumis, E. G.; Papagiannakis, R. G. Fuel 2010, 89 (10), 2781−2790. (12) Rakopoulos, D. C.; Rakopoulos, C. D.; Papagiannakis, R. G.; Kyritsis, D. C. Fuel 2011, 90 (5), 1855−1867. (13) Dagaut, P.; Togbé, C. Fuel 2008, 87 (15−16), 3313−3321. (14) Dagaut, P.; Sarathy, S. M.; Thomson, M. J. Proc. Combust. Inst. 2009, 32, 229−237. (15) Dagaut, P.; Togbé, C. Energy Fuels 2009, 23 (7), 3527−3535. (16) Sarathy, S. M.; Thomson, M. J.; Togbe, C.; Dagaut, P.; Halter, F.; Mounaim-Rousselle, C. Combust. Flame 2009, 156 (4), 852−864. (17) McEnally, C. S.; Pfefferle, L. D. Proc. Combust. Inst. 2005, 30 (1), 1363−1370. (18) Grana, R.; Frassoldati, A.; Faravelli, T.; Niemann, U.; Ranzi, E.; Seiser, R.; Cattolica, R.; Seshadri, K. Combust. Flame 2010, 157 (11), 2137−2154. (19) Veloo, P. S.; Wang, Y. L.; Egolfopoulos, F. N.; Westbrook, C. K. Combust. Flame 2010, 157 (10), 1989−2004. (20) Gu, X.; Huang, Z.; Li, Q.; Tang, C. Energy Fuels 2009, 23 (10), 4900−4907. (21) Gu, X.; Huang, Z.; Wu, S.; Li, Q. Combust. Flame 2010, 157 (12), 2318−2325. (22) Zhang, Y.; Kumar, K.; Sung, C.-J. Autoignition of binary fuel blends of n-butanol and n-heptane in a rapid compression machine . 49th AIAA Aerospace Sciences Meeting, Orlando, FL, January 4−7, 2011; paper AIAA-2011-92. (23) Weber, B. W.; Kumar, K.; Zhang, Y.; Sung, C.-J. Combust. Flame 2011, 158 (5), 809−819. (24) Weber, B.; Kumar, K.; Sung, C.-J. Autoignition of butanol isomers at low to intermediate temperature and elevated pressure. 49th AIAA Aerospace Sciences Meeting, Orlando, FL, January 4−7, 2011; paper AIAA-2011-316. (25) Moss, J. T.; Berkowitz, A. M.; Oehlschlaeger, M. A.; Biet, J.; Warth, V.; Glaude, P. A.; Battin-Leclerc, F. J. Phys. Chem. A 2008, 112 (43), 10843−10855. (26) Black, G.; Curran, H. J.; Pichon, S.; Simmie, J. M.; Zhukov, V. Combust. Flame 2010, 157 (2), 363−373. (27) Noorani, K.; Akih-Kumgeh, B.; Bergthorson, J. Energy Fuels 2010, 1989−2004.

Figure 15. Sensitivity analysis for 0.5% n-butanol at three different temperatures, ϕ = 1.0, and p = 2 atm.

temperatures of 1200−1650 K. An ignition delay time correlation is established by multiple linear regression on the measured data. A modified n-butanol submodel is developed, based on the Black model, by adding absent reactions and updating some reaction rates. Compared to the Black model, the modified model shows better agreement with the experimental ignition data under all measured conditions. Comparison was also conducted between ignition delay times of ethanol and nbutanol. It is observed that these two alcohols present comparable ignition data and the modified model well captures this similarity. The modified model was also validated against experimental JSR data at pressures of 1 and 10 atm at ϕ = 1.0. Most of the species were well predicted. Particularly, reasonable improvements were archived for the simulation of acetaldehyde and butanal by optimizing the reaction rates of hydroxybutyl + O2 and adding keto−enol isomerization reactions. According to the reaction pathway analysis for the modified model, the consumption of n-butanol is dominated by Habstraction reactions. The modified model presents a different branching ratio order of H-abstraction reactions from that of the Black model and has the largest branching ratio at the αcarbon position. The new branching order is more consistent with the order of the bond dissociation energies and the computational results by previous studies. It is also observed that unimolecular decomposition reactions become more important as temperature increases. The sensitivity analysis for ignition delay times shows that the main branching reaction R1 has extremely high sensitivity under all conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Shock tube data, reaction mechanism, associated thermochemistry, and comparison to other models. This material is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone 86-29-82665075; fax 86-29-82668789; e-mail [email protected]. 3379


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