Shock Tube Study on Propanal Ignition and the Comparison to

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Shock Tube Study on Propanal Ignition and the Comparison to Propane, n‑Propanol, and i‑Propanol Ke Yang, Cheng Zhan, Xingjia Man, Li Guan, Zuohua Huang,* and Chenglong Tang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China ABSTRACT: High temperature ignition characteristics of propanal/oxygen mixtures diluted with argon were studied in a shock tube for temperatures ranging from 1050 to 1800 K, pressures ranging from 1.2 to 16.0 atm, fuel concentrations of 0.5, 1.25, 2.0%, and equivalence ratios of 0.5, 1.0, and 2.0. A detailed kinetic model consisting of 250 species and 1479 reactions was developed and validated against experimental results. To clarify the influence of functional groups and their positions on the oxidation, previously measured ignition delay times of propane, n-propanol, and i-propanol were employed for comparison. It was found that ignition delays are in the order of propane > i-propanol > n-propanol > propanal. Reaction pathway analysis indicated that the intermediate species of propane and i-propanol are rather stable, while the products of n-propanol and propanal are more reactive, which leads to the decreased ignition delay times. Sensitivity analysis demonstrated that some fuelspecific reactions exhibit relatively large sensitivity during the ignition of the four C3 fuels.

1. INTRODUCTION The depletion of fossil fuel and reduction of air pollution are major interests to the combustion community in the past decades. To resolve these issues, biofuels have been investigated as clean alternative fuels. Among all kinds of biofuels, alcohols have been widely used as additives in engines. However, engine studies show that the usage of alcohols leads to an increase in emissions of toxic compounds such as aldehydes, which is an unregulated emission.1 Fundamental studies on the oxidation of alcohols also identified the aldehydes to be the key stable intermediates.2−5 However, fundamental combustion and relevant oxidation kinetics for the aldehydes were not fully studied. Few experimental and theoretical studies were reported on the oxidation of small molecular aldehydes such as formaldehyde6,7 and acetaldehyde.8,9 The C3 aldehyde, propanal, has been identified as a key stable intermediate species during the oxidation of n-propanol10,11 or n-butanol.12 However, the propanal submodel in most models was only roughly handled and as a result gave poor prediction.13 C3 oxygenated fuels mainly include alcohol isomers, aldehydes, and ketones. Combustion and oxidation characteristics vary in different molecular structures. In the early 1990s, Norton and Dryer studied the influence of functional groups on combustion and concluded that higher concentrations of propanal enhanced the overall reactivity of n-propanol, while acetone decreased the overall reactivity as an intermediate of ipropanol oxidation.2 Li et al. investigated both lean and rich premixed flames for the three C3-oxygenated hydrocarbons (acetone, n-propanol, and i-propanol) at low pressure using tunable synchrotron photoionization and molecular-beam mass spectrometry.14 Veloo et al. studied flame propagation and extinction of n-propanol, i-propanol, and propane in the counterflow configuration under atmospheric pressure for an unreacted fuel-carrying stream temperature of 343 K to clarify the effects of the hydroxyl group.15 Burluka et al. measured the laminar burning velocities of three C3H6O isomers (propylene oxide, propionaldehyde, and acetone),16 but the modeling © 2015 American Chemical Society

results only show qualitative agreement with the measurements. Ranzi et al. hierarchically and comparatively reviewed experimental data on the laminar flame speeds of hydrocarbon and oxygenated fuels, including C3 alcohol isomers, propanal, and acetone.17 Recently, Benjamin et al.18 studied relatively high temperature ignition behavior of the selected C3 (propanal, acetone, and i-propanol) oxygenated hydrocarbons behind reflected shock waves. However, their experimental data were limited and the influence of functional groups on the oxidation of the hydrocarbons was not systematically studied. In this study, specific experiments were conducted on the oxidation of propanal behind reflected shock waves. An optimized propanal submodel was proposed and then coupled to the propanol model developed by Man et al.19 The influences of the presence and location of the C−O bond and CO double bond on ignition characteristics were analyzed on the basis of the improved kinetic model.

2. EXPERIMENTAL AND NUMERICAL APPROACHES 2.1. Experimental Approach. All experiments were carried out in a shock tube which has been described in detail previously.20 The shock tube with a 11.5 cm inner diameter is separated into a 4.0 m long driver section and a 4.8 m long driven section by a 0.06 m flange section with double PET (polyester terephthalate) diaphragms. Diaphragms with different thicknesses were chosen to achieve different pressures. Before each experiment, the driven section was evacuated to the pressure below 10−5 kPa by a Nanguang vacuum system. All reactant mixtures were prepared manually in a 128 L stainless steel mixing tank and allowed to mix for at least 12 h by molecular diffusion to ensure sufficient mixing. To avoid fuel condensation, the partial pressure of the fuel was controlled at less than half of its saturated vapor pressure at the tank temperature. n-Propanol (99.5%), i-propanol (99.5%), and propanal (99%) were injected to the evacuated tank to their respective partial pressure, then researchgrade oxygen and argon were manometrically charged to ensure the Received: November 19, 2015 Revised: December 21, 2015 Published: December 26, 2015 717

DOI: 10.1021/acs.energyfuels.5b02739 Energy Fuels 2016, 30, 717−724

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Energy & Fuels desired equivalence ratio and fuel fractions. The main properties of the fuels tested are shown in Table 1.

measured behind reflected shock waves under the pressures of 1.2, 5.0, and 16.0 atm, equivalence ratios of 0.5, 1.0 and 2.0, and temperatures of 1050−1800 K. The effect of fuel concentration (0.50, 1.25, and 2.00% propanal in the mixture) on ignition delay times was also investigated at the pressure of 5.0 atm. The compositions of mixtures used in this study are listed in Table 2, in which Φ and p refer to the equivalence ratio and tested pressure, respectively.

Table 1. Properties of the Fuels Tested

Table 2. Compositions of the Mixture in This Study

The ignition delay time is defined as the time interval between the arrival of the shock wave at the endwall and the extrapolation of the steepest rise in the endwall OH* chemiluminescence signal to the zero baseline, as show in Figure 1. The arrival of the shock wave is

mixture

Φ

propanal (%)

O2 (%)

Ar (%)

P (atm)

1 2 3 4 5

0.5 1.0 2.0 1.0 1.0

1.25 1.25 1.25 0.50 2.00

10.00 5.00 2.50 2.00 8.00

88.75 93.75 96.25 97.50 90.00

1.2, 5.0, 16.0 1.2, 5.0, 16.0 1.2, 5.0, 16.0 5.0 5.0

For all tested mixtures, the measured ignition delay times show a strong Arrhenius temperature dependence, and it can be correlated to the following Arrhenius formula by using the multiregression method −0.86 ± 0.07 τign = 2.76 ± 0.10 × 10−3p−0.60 ± 0.03 ϕ0.83 ± 0.04χ fuel

exp(31.29 ± 0.55 kcal/mol/RT ) (1)

where τign is ignition delay time in μs, p is pressure in atm, Φ is equivalence ratio, χfuel is fuel mole fraction in the mixture, T is temperature in K, and R = 1.986 × 10−3 kcal/mol/K is the universal gas constant. The fitting shows a high correlation coefficient, R2 = 0.966, indicating a strong linear relationship between the logarithmic IDT and inverse temperature. The correlation parameters, exponents in eq 1, indicate that the IDTs decrease with the increase of pressure, the decrease of equivalence ratio, and the increase of fuel concentration. However, this empirical formula is fitted on the basis of the present experimental data. Attention should be paid when it is applied to other experimental conditions. Two experimental cases conducted by Akih-Kumgeh et al.18 were repeated for comparison, as shown in Figure 2. It is demonstrated that the present data agree fairly well with those

Figure 1. Definition of ignition delay time. measured by a pressure transducer (PCB 113B26) located at the endwall. The incident shock velocity at the endwall is determined by linear extrapolation of three time intervals recorded by three time counters (Fluke PM6690). The OH* chemiluminescence selected by a narrow filter centered at 306 ± 10 nm is measured using a photomultiplier (Hamamatsu, CR131) fixed at the endwall. A digital recorder (Yokogawa, scopecorder-DL750) is used to record all data. The temperature behind the reflected shock is calculated by the use of the reflected shock module in the chemical equilibrium program Gaseq.21 The uncertainty of the temperature is about ±25 K. The repeatability of ignition delay time measurement has been discussed in detail in ref 22. 2.2. Simulation Approach. The simulation of the ignition delay time was performed using the Chemkin II package and the Senkin code. The definition of simulated ignition delay time is defined as the time interval between the beginning of simulation and the point of the maximum temperature rise rate (max dT/dt). The pressure rise induced by the nonideal effect in a shock tube has been discussed in detail previously in ref 20; when the ignition delay time is less than 2 ms, the pressure rise rate observed in measurement is mainly caused by heat release instead of shock tube facility effect.

3. RESULTS AND DISCUSSION 3.1. Ignition Delay Time Measurements and Correlations on Propanal. Ignition delay times of propanal were

Figure 2. Comparison with previous data of Akih-Kumgeh et al.18 at Φ = 1.0, pressures of 1.2 and 12.0 atm for 1.25% propanal fuel concentration. 718

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Energy & Fuels in publication18 under the pressure of 12 atm, but give significantly lower values at 1.2 atm. Many repeated experiments at 1.2 atm were performed, and the difference at low pressure still existed. In this study, the driven section is evacuated to pressure below 10−5 kPa and the typical leakage rate is less than 5 × 10−5 kPa/min. Therefore, the present measurements are of high validity. 3.2. Validation and Optimization of Propanal Model. Up to now, there are two detailed propanal mechanisms available, the Veloo model23 and the NUIG model.24 The Veloo model was developed by Veloo et al. for the interpretation of JSR data, which includes 113 species and 995 reactions. The NUIG model involving 330 species and 2012 reactions was developed by Pelucchi et al. for modeling the pyrolysis and oxidation of n-C3−C5 aldehydes in a shock tube. However, these two models have large deviation in the prediction of the ignition delay time of propanal. The Veloo model shows overprediction under the fuel-rich (Φ = 2.0) condition, while the NUIG model shows poor prediction under fuel-lean (Φ = 0.5) and fuel stoichiometric (Φ = 1.0) conditions, especially at relatively low temperatures. Therefore, the propanal submechanism needs to be modified and then coupled into the propanol mechanism.19 The measured IDTs in this study mainly locate in the high temperature regions, and mechanism modification also focuses on the high temperature part. Recently, da Silva and Bozzelli25 calculated the bond dissociation energies (BDEs) corresponding to homolytic C−H and C−C bond cleavage in propanal using CBS-APNO 298 K enthalpies, while Akih-Kumgeh and Bergthorson determined the BDEs of propanal by atomization using the CBS-QB3 method in Gaussian 09, as shown in Figure 3a,b. It is observed that the BDE of R-CH2CHO (83.7 kcal/

of these reactions do not include the pressure dependency item. Pelucchi et al.24 calculated the temperature and pressure dependency of unimolecular decomposition reactions of propanal using a three-frequency version of Quantum Rice− Ramsperger−Kassel theory (QRRK/MSC) in Tore form. The decomposition of propanal is mainly initiated by C−C decomposition, while C−H bond cleavage contributes a little to the initiation. Therefore, the rate constant of the decomposition of C−C, recommended by Pelucchi et al.,24 was used in the modified model. Reactions that break down the H atom from each C atom site are also added with a reverse rate constant of 1.0 × 1014 cm3 mol−1 s−1. An analogical method can be used for the assignment of rate constants of the H-abstraction reactions by small radicals like H, OH, and O. For H-abstraction from the α position, the rate constant recommended by Veloo et al.23 was used, while the rate constants for H-abstraction from β and γ positions were obtained on analogy with the counterparts of the ethyl propanoate model by Metcalfe et al.26 or the butanol model by Sarathy et al.13 Fuel radicals (such as C 2 H 5 CO, CH 3 CHCHO, and CH2CH2CHO) decompose primarily through β-scission. For this type of reactions, the kinetic parameters in the modified model were directly taken from the butanol model by Sarathy et al.13 Additionally, as mentioned by Mereau et al.,27 carbonyl radicals (R-CO) can not only decompose through β-scission but also decompose into R and CO (RCO + M = R + CO + M). Thus, the missed reactions (C2H5CO + M = C2H5 + CO + M) are added with rate constants recommended by Dayma5 used in the reaction CH3CO + M = CH3 + CO + M. The main reactions mentioned above are listed in Table 3, in which R represents small radicals such as CH3 and HCO. Thermodynamic data of all species in the modified propanal submodel are obtained from the butanol model by Sarathy et al.13 The modified propanal submodel first validates against measured ignition delay times. Figure 4 gives the measured ignition delay times with those of calculations using the modified model, Veloo model,23 and NUIG model.24 Pressure dependence is shown in Figure 4a−c at three equivalence ratios (0.5, 1.0, and 2.0). It can be seen that the modified model captures well the trend of pressure dependence and quantitatively agrees with the experimental data at 5.0 and 16.0 atm. However, predictions of the modified model are slightly higher than the measurement at 1.2 atm and relatively low temperatures. This deviation is considered resulting from the uncertainties of both prediction and measurement; therefore, an error bar was added to reflect this uncertainty, as shown in Figure 4a. Results show that the Veloo model and NUIG model can qualitatively predict the pressure dependence. However, their computed results are significantly higher than the measured values, especially in relatively low temperature region. Besides pressure dependence, the effect of fuel concentration was also investigated, as shown in Figure 4d. Similar results are obtained in the predictions of the three models for a fuel concentration of 0.50%. With the increase of fuel concentration, both the Veloo model and NUIG model demonstrate poor prediction under relatively low temperature, while the modified model not only captures the dependence of fuel concentration but also predicts well the measured ignition delay times. In general, the modified propanal model gives a better prediction than those of the Veloo model and NUIG model under most conditions.

Figure 3. Bond dissociation energies (kcal/mol) of propanal.

mol) is almost identical to that of R-CHO (83.8 kcal/mol). Among all C−H bonds, the C−H bond at the carbonyl group has the weakest bond energy, which is 89.3 kcal/mol, while the C−H bond on the terminal primary carbon has the strongest bond energy (102.4 kcal/mol), which approximately equals the C−H bond energy in propane (ca.102 kcal/mol). It indicates that the carbonyl group exerts little influence on the terminal primary carbon (γ-carbon). Therefore, an analogous method can be used in the construction of the propanal submodel before accurate rate constants of the unimolecular decomposition and H-abstraction reactions are available. The propanal submodel is a part of the alcohol model, where many reactions have already been included in the alcohol model, such as isomerization reactions. The main modification in this study focuses on the initiation reactions of propanal, including unimolecular decomposition, H-abstraction, and radical decomposition reactions. For unimolecular decomposition reactions, these reactions usually have strong pressure dependency; however, in most of the models, the rate constants 719

DOI: 10.1021/acs.energyfuels.5b02739 Energy Fuels 2016, 30, 717−724

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Energy & Fuels Table 3. Propanal Submodel (Units: cm, mol, s, cal, K) reaction C2H5CHO ⇄ C2H5 + HCO Low/ Troe/0.2491 × 10−02 C2H5CHO ⇄ CH3 + CH2CHO Low/ Troe/0.2491 × 10−02 CH3CHCHO + H ⇄ C2H5CHO CH2CH2CHO + H ⇄ C2H5CHO C2H5CO + H ⇄ C2H5CHO C2H5CHO + H ⇄ C2H5CO + H2 C2H5CHO + H ⇄ CH3CHCHO + H2 C2H5CHO+H ⇄ CH2CH2CHO + H2 C2H5CHO + O ⇄ C2H5CO + OH C2H5CHO + O ⇄ CH3CHCHO + OH C2H5CHO + O ⇄ CH2CH2CHO + OH C2H5CHO + OH ⇄ C2H5CO + H2O C2H5CHO + OH ⇄ CH3CHCHO + H2O C2H5CHO + OH ⇄ CH2CH2CHO + H2O C2H5CHO + R ⇄ C2H5CO + RH C2H5CHO + R ⇄ CH3CHCHO + RH C2H5CHO + R ⇄ CH2CH2CHO + RH

A

n

unimolecular decomposition 0.1300 × 1027 0.2870 × 1085 376.8 0.1160 × 1026 0.1260 × 1088 372.5 1.00 × 1014 1.00 × 1014 1.00 × 1014 hydrogen abstraction 1.20 × 1014 1.30 × 1006 9.50 × 1004 5.85 × 1012 2.20 × 1013 9.81 × 1005 2.65 × 1012 1.15 × 1011 5.28 × 1009

β-scission C2H5CO + M ⇄ C2H5 + CO + M 2.75 × 1009 H2O/16.25/ CO/1.875/ CO2/3.75/ CH4/16.25/ C2H6/16.25/ H2/2.50/ AR/0.75/ C2H5CO ⇄ CH3CHCO + H 4.66 × 1010 CH3CHCHO ⇄ CH3CHCO + H 1.35 × 1013 CH3CHCHO ⇄ C2H3CHO + H 4.16 × 1012 CH3CHCHO + HO2 ⇄ CH3CHOCHO + OH 9.64 × 1012 CH3CHOCHO ⇄ CH3CHO + HCO 3.98 × 1013 CH3CHCO + OH ⇄ C2H5 + CO2 1.73 × 1012 CH3CHCO + OH ⇄ SC2H4OH + CO 2.00 × 1012 CH3CHCO + H ⇄ C2H5 + CO 4.40 × 1012 CH3CHCO + O ⇄ CH3CHO + CO 3.20 × 1012 CH2CH2CHO ⇄ C2H4 + HCO 1.26 × 1013 CH2CH2CHO ⇄ C2H3CHO + H 1.67 × 1013

3.3. Comparison of Ignition Delay Times of Propane, n-Propanol, i-Propanol, and Propanal. Figure 5 shows the ignition delay times of the four C3 fuels in the stoichiometric mixtures with a fuel concentration of 0.75% at the pressures of 1.2 and 16 atm. The experimentally measured ignition delay times of n-propanol and i-propanol from the previous study19 were used, and the ignition delay times of propane and propanal were calculated using the suggested correlation in ref 20 and the correlation of this study. It is observed that propane and i-propanol have the comparable, but longest, ignition delay times. Propanal has the slightly shorter ignition delay times than those of n-propanol. Meanwhile, numerical calculations for four fuels using the modified model are plotted in the figure, as shown in a solid line. The modified model captures well the relationship of the four fuels and quantitatively agrees with the experimental results. The global activation energies of the four fuels are obtained with the multiregression method. The activation energies of propane, n-propanol, i-propanol, and propanal are 40.44, 35.10, 40.06, and 31.29 kcal/mol, respectively, indicating that the temperature sensitivity of propanal ignition delay times is the weakest, while propane and i-propanol are comparable.

−3.000 −18.600 6.089 −2.800 −19.400 6.089 0.00 0.00 0.00

Ea

ref

86405.9 101060.0/ 4632./ 85718.2 101280.0/ 5252./ 0.00 × 1000 0.00 × 1000 0.00 × 1000

24

0.00 2.40 2.75 0.00 0.00 2.43 0.00 0.51 0.97

7.00 × 1003 4.47 × 1003 6.28 × 1003 1.81 × 1003 3.28 × 1003 4.75 × 1003 −7.30 × 1002 6.30 × 1001 1.59 × 1003

8 31 13 23 26 26 23 26 26

1.41

3.58 × 1004

5

4.26 × 1004 3.35 × 1004 3.42 × 1004 0.00 × 1000 9.70 × 1003 −1.01 × 1003 −1.01 × 1003 1.46 × 1003 −4.37 × 1002 3.03 × 1004 4.62 × 1004

13 13 13 13 13 13 13 13 13 13 13

0.79 −0.17 −0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

24

30 30 30

In terms of molecular structure, the oxygen atom connected to the terminal carbon atom increases the chain length and makes the breaking more readily when the molecule is attacked by the other molecules or radicals, generating more reactive radicals and thus promoting the ignition. This explains why npropanol and propanal are more likely to be ignited than propane. When the OH group is added to the middle carbon atom, the structure of the molecule becomes more compact rather than the increase of chain length, thus the molecule is more difficult to decompose and subsequently ignite. Ignition delay times were measured for the stoichiometric mixtures at a high pressure of 16 atm, as shown in Figure 5b. It can be seen that discrepancies among the four C3 fuels are significant, which have been captured by the modified model. Although the molecular structure of i-propanol is compact, the OH in i-propanol is easier to be split off than that of the H atom in propane, leading to more reactive radicals and promoting the overall reaction under high pressure for ipropanol. Figure 6 shows the bond dissociation energies of npropanol and i-propanol at the CBS-QB3 level calculated by EINahas et al.28 As shown in Figures 3a and 6a, the BDEs of these molecules are different, and propanal has comparatively weaker 720

DOI: 10.1021/acs.energyfuels.5b02739 Energy Fuels 2016, 30, 717−724

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Figure 4. Comparison between measured and simulated ignition delay times under various conditions: (a) pressure effect at Φ = 0.5; (b) pressure effect at Φ = 1.0; (c) pressure effect at Φ = 2.0; (d) fuel concentration effect at 5.0 atm.

Figure 6. Bond dissociation energies in kcal/mol of (a) n-propanol and (b) i-propanol.

corresponding bonds. The Cα−Cβ bond in propanal is weaker than that in n-propanol by about 2 kcal/mol; thus propanal would undergo the C−C bond cleavage reactions more likely. 3.4. Reaction Pathway Analysis. Figure 7 gives the reaction pathway on the basis of the modified model for the four C3 fuels at the pressures of 1.2 and 16.0 atm and a temperature of 1300 K for the stoichiometric mixtures with a fuel concentration of 0.75%. The instant of 20% fuel consumption is chosen to analyze the reaction fluxes as employed in ref 29. As shown in Figure 7, the major consumption for the four C3 fuels is from the H-abstraction reactions, while reactions of unimolecular decomposition show a small contribution. For propanal and i-propanol, as shown in Figure 7b,d, the contribution of unimolecular decomposition reactions to fuel consumption is less than 3%. The increase of pressure does not lead to a significant change in the contribution of each pathway, but the results show an increase in the contribution of H-

Figure 5. Ignition delay times of propane, n-propanol, i-propanol, and propanal: (a) 1.2 atm; (b) 16.0 atm. 721

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Figure 7. Reaction pathway diagrams of the present model for 0.75% fuel concentration in a shock tube at pressures of 1.2 atm (normal font) and 16.0 atm (italic bold font); Φ = 1.0, T = 1300 K, 20% fuel consumption.

abstraction reactions and a decrease in the contribution of unimolecular decomposition can still be observed. After β-scission reactions, the main intermediate species of propane are ethene and propene. For n-propanol, the main intermediates include ethenol, ethene, and propene. The main intermediates of i-propanol are propene and acetone. For propanal, the main intermediates include C2H5 radicals, 1propenal, 2-propenal, and ethene. It is shown that the intermediates of propane and i-propanol are the rather stable species which are relatively difficult to decompose for further oxidation. However, ethanol, a product during the oxidation of n-propanol, is an unstable product which will undergo the internal isomerization to yield acetaldehyde. Similarly, 1propenal and 2-propenal of propanal are also unstable intermediates and will rapidly decompose to form C2H3 and C2H5 radicals. It is well-known that C2H5 is an unstable radical which could readily undergo decomposition to form C2H4 and a H atom. The fast production of H atoms accelerates the rate of the main chain branching of H + O2 = OH + O and leads to more reactive radicals and promotes the ignition. It can be deduced that the presence of the OH functional group changes the intermediate products. When it attaches to the terminal carbon, it will produce unstable molecules which could readily decompose to promote the ignition during oxidation. When it attaches to the middle carbon, it will generate rather stable ketones and contribute a little to the

ignition. In addition, the existence of the CO double bond will make the whole structure more unstable and produce more unstable intermediates or radicals during the oxidation, leading to the enhancement of ignition. Reaction pathway analysis reveals the autoignition characteristics of the four C3 fuels. In terms of the length of ignition delay times, they follow in the order of propane > i-propanol > n-propanol > propanal. 3.5. Sensitivity Analysis. To further interpret the oxidation chemistry of the four C3 fuels, sensitivity analysis was performed on the basis of the modified model under the same conditions with those of pathway analysis, as shown in Figure 8. The sensitivity coefficient is defined as S=

τ(2.0ki) − τ(0.5ki) 1.5τ(1.0ki)

(2)

where ki is the pre-exponential factor in the rate constant of the ith reaction and τ is ignition delay time. A positive sensitivity coefficient indicates an increase in the ignition delay time and an inhibiting effect on the overall reactivity with the increase of reaction rate, and vice versa. It is shown that, for both low and high pressures, similar results for the four C3 fuels are presented. The reaction with the maximum negative sensitivity coefficient is the chain branching reaction H + O2 = O + OH. The H-abstraction reactions by H and OH radicals from the fuel have high positive coefficients, which indicates an inhibition on the reactivity because these 722

DOI: 10.1021/acs.energyfuels.5b02739 Energy Fuels 2016, 30, 717−724

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Figure 8. Sensitivity analysis for 0.75% fuel concentration in shock tube at pressures of 1.2 and 16.0 atm, Φ = 1.0, T = 1300 K.

and i-propanol, a comparative study of high temperature ignition behavior of the four C3 fuels was conducted. The main conclusions are summarized as follows: 1. Ignition delay times of propanal show a typical Arrhenius dependence on temperature. Like most of the hydrocarbon fuels, ignition delay times increase with the decrease of pressure, the increase of equivalence ratio, and the decrease of fuel concentration. 2. A modified propanal submodel was proposed by updating the latest rate constants or adding some absent reactions on the basis of literature review. Compared to the Veloo model and NUIG model, the modified model shows better prediction on the ignition delay times, except for the low pressure condition. 3. Ignition delay times of the four C3 fuels are in the order of propane > i-propanol > n-propanol > propanal. The discrepancy among the four C3 fuels is more obvious under high pressure conditions. The modified model can capture well the ignition behaviors of the four C3 fuels. 4. Reaction pathway analysis on the basis of the modified model shows that consumption of the four C3 fuels is dominated by H-abstraction reactions. Propane mainly produces ethene and propene. n-Propanol primarily

reactions consume high reactive H radicals and produce less reactive radicals and stable hydrogen. Furthermore, the reactions with relatively high positive coefficients are mainly the recombination reactions, and those with relatively negative coefficients are mainly the chain propagating reactions of small radicals. However, there are some differences in high sensitivity reactions among the four C3 fuels. The recombination reaction CH3 + CH3 (+M) = C2H6 (+M) has a relatively high positive sensitivity coefficient in the oxidation of propane and ipropanol, but it is not included in the controlling reactions of npropanol and propanal. The ignition promoting reactions are mostly the small radical reactions in the oxidation of propanal, while some fuel-specific reactions are found to enhance the reactivity of n-propanol. The reactions of small radicals have stronger enhancement on the reactivity. This further explains the shorter ignition delay times of propanal compared to npropanol.

4. CONCLUSIONS A study on ignition delay times of propanal was conducted behind reflected shock waves under different conditions. Coupling with the previous work on propane, n-propanol, 723

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Energy & Fuels

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produces ethenol besides ethene and propene. i-Propanol mainly produces propene and acetone. Propanal mainly produces C2H5 radicals, 1-propenal, 2-propenal, and ethene. 5. Sensitivity analysis shows that the main chain branching reaction R1 dominates the reactivity of the four C3 fuels and H-abstraction reactions from the fuel molecule have relatively large coefficients. However, the recombination reactions, CH3 + CH3 (+M) = C2H6 (+M) and C3H5-A + H = C3H6, inhibit the reactivity of propane and ipropanol; however, inconspicuous influence of these reactions is observed on the ignition of n-propanol and propanal.

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (91441203 and 51406159) and the National Basic Research Program of China (2013CB228406). The State Key Laboratory of Engines, Tianjin University (K2015-01), is also acknowledged.

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DOI: 10.1021/acs.energyfuels.5b02739 Energy Fuels 2016, 30, 717−724