Shock Tube Study on Propanal Ignition and the Comparison toPropane, n‑Propanol, and i‑PropanolKe 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 shocktube 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 wasdeveloped and validated against experimental results. To clarify the influence of functional groups and their positions on theoxidation, previously measured ignition delay times of propane, n-propanol, and i-propanol were employed for comparison. Itwas found that ignition delays are in the order of propane > i-propanol > n-propanol > propanal. Reaction pathway analysisindicated that the intermediate species of propane and i-propanol are rather stable, while the products of n-propanol andpropanal are more reactive, which leads to the decreased ignition delay times. Sensitivity analysis demonstrated that some fuel-specific reactions exhibit relatively large sensitivity during the ignition of the four C3 fuels.

1. INTRODUCTIONThe depletion of fossil fuel and reduction of air pollution aremajor interests to the combustion community in the pastdecades. To resolve these issues, biofuels have beeninvestigated as clean alternative fuels. Among all kinds ofbiofuels, alcohols have been widely used as additives in engines.However, engine studies show that the usage of alcohols leadsto an increase in emissions of toxic compounds such asaldehydes, which is an unregulated emission.1 Fundamentalstudies on the oxidation of alcohols also identified thealdehydes to be the key stable intermediates.2−5 However,fundamental combustion and relevant oxidation kinetics for thealdehydes were not fully studied.Few experimental and theoretical studies were reported on

the oxidation of small molecular aldehydes such as form-aldehyde6,7 and acetaldehyde.8,9 The C3 aldehyde, propanal, hasbeen identified as a key stable intermediate species during theoxidation of n-propanol10,11 or n-butanol.12 However, thepropanal submodel in most models was only roughly handledand as a result gave poor prediction.13

C3 oxygenated fuels mainly include alcohol isomers,aldehydes, and ketones. Combustion and oxidation character-istics vary in different molecular structures. In the early 1990s,Norton and Dryer studied the influence of functional groups oncombustion and concluded that higher concentrations ofpropanal enhanced the overall reactivity of n-propanol, whileacetone decreased the overall reactivity as an intermediate of i-propanol oxidation.2 Li et al. investigated both lean and richpremixed flames for the three C3-oxygenated hydrocarbons(acetone, n-propanol, and i-propanol) at low pressure usingtunable synchrotron photoionization and molecular-beam massspectrometry.14 Veloo et al. studied flame propagation andextinction of n-propanol, i-propanol, and propane in thecounterflow configuration under atmospheric pressure for anunreacted fuel-carrying stream temperature of 343 K to clarifythe effects of the hydroxyl group.15 Burluka et al. measured thelaminar burning velocities of three C3H6O isomers (propyleneoxide, propionaldehyde, and acetone),16 but the modeling

results only show qualitative agreement with the measurements.Ranzi et al. hierarchically and comparatively reviewedexperimental data on the laminar flame speeds of hydrocarbonand oxygenated fuels, including C3 alcohol isomers, propanal,and acetone.17 Recently, Benjamin et al.18 studied relativelyhigh temperature ignition behavior of the selected C3(propanal, acetone, and i-propanol) oxygenated hydrocarbonsbehind reflected shock waves. However, their experimental datawere limited and the influence of functional groups on theoxidation of the hydrocarbons was not systematically studied.In this study, specific experiments were conducted on the

oxidation of propanal behind reflected shock waves. Anoptimized propanal submodel was proposed and then coupledto the propanol model developed by Man et al.19 Theinfluences of the presence and location of the C−O bond andCO double bond on ignition characteristics were analyzed onthe basis of the improved kinetic model.

2. EXPERIMENTAL AND NUMERICAL APPROACHES2.1. Experimental Approach. All experiments were carried out in

a shock tube which has been described in detail previously.20 Theshock tube with a 11.5 cm inner diameter is separated into a 4.0 mlong driver section and a 4.8 m long driven section by a 0.06 m flangesection with double PET (polyester terephthalate) diaphragms.Diaphragms with different thicknesses were chosen to achieve differentpressures. Before each experiment, the driven section was evacuated tothe pressure below 10−5 kPa by a Nanguang vacuum system.

All reactant mixtures were prepared manually in a 128 L stainlesssteel mixing tank and allowed to mix for at least 12 h by moleculardiffusion to ensure sufficient mixing. To avoid fuel condensation, thepartial pressure of the fuel was controlled at less than half of itssaturated vapor pressure at the tank temperature. n-Propanol (99.5%),i-propanol (99.5%), and propanal (99%) were injected to theevacuated tank to their respective partial pressure, then research-grade oxygen and argon were manometrically charged to ensure the

Received: November 19, 2015Revised: December 21, 2015Published: December 26, 2015

Article

pubs.acs.org/EF

© 2015 American Chemical Society 717 DOI: 10.1021/acs.energyfuels.5b02739Energy Fuels 2016, 30, 717−724

desired equivalence ratio and fuel fractions. The main properties of thefuels tested are shown in Table 1.

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

measured by a pressure transducer (PCB 113B26) located at theendwall. The incident shock velocity at the endwall is determined bylinear extrapolation of three time intervals recorded by three timecounters (Fluke PM6690). The OH* chemiluminescence selected by anarrow filter centered at 306 ± 10 nm is measured using aphotomultiplier (Hamamatsu, CR131) fixed at the endwall. A digitalrecorder (Yokogawa, scopecorder-DL750) is used to record all data.The temperature behind the reflected shock is calculated by the use ofthe reflected shock module in the chemical equilibrium programGaseq.21 The uncertainty of the temperature is about ±25 K. Therepeatability of ignition delay time measurement has been discussed indetail in ref 22.2.2. Simulation Approach. The simulation of the ignition delay

time was performed using the Chemkin II package and the Senkincode. The definition of simulated ignition delay time is defined as thetime interval between the beginning of simulation and the point of themaximum temperature rise rate (max dT/dt). The pressure riseinduced by the nonideal effect in a shock tube has been discussed indetail previously in ref 20; when the ignition delay time is less than 2ms, the pressure rise rate observed in measurement is mainly caused byheat release instead of shock tube facility effect.

3. RESULTS AND DISCUSSION3.1. Ignition Delay Time Measurements and Correla-

tions on Propanal. Ignition delay times of propanal were

measured behind reflected shock waves under the pressures of1.2, 5.0, and 16.0 atm, equivalence ratios of 0.5, 1.0 and 2.0, andtemperatures of 1050−1800 K. The effect of fuel concentration(0.50, 1.25, and 2.00% propanal in the mixture) on ignitiondelay times was also investigated at the pressure of 5.0 atm. Thecompositions of mixtures used in this study are listed in Table2, in which Φ and p refer to the equivalence ratio and testedpressure, respectively.

For all tested mixtures, the measured ignition delay timesshow a strong Arrhenius temperature dependence, and it can becorrelated to the following Arrhenius formula by using themultiregression method

τ ϕ χ= ± ×

±

− − ± ± − ±p

RT

2.76 0.10 10

exp(31.29 0.55 kcal/mol/ )

ign fuel3 0.60 0.03 0.83 0.04 0.86 0.07

(1)

where τign is ignition delay time in μs, p is pressure in atm, Φ isequivalence ratio, χfuel is fuel mole fraction in the mixture, T istemperature in K, and R = 1.986 × 10−3 kcal/mol/K is theuniversal gas constant. The fitting shows a high correlationcoefficient, R2 = 0.966, indicating a strong linear relationshipbetween the logarithmic IDT and inverse temperature. Thecorrelation parameters, exponents in eq 1, indicate that theIDTs decrease with the increase of pressure, the decrease ofequivalence ratio, and the increase of fuel concentration.However, this empirical formula is fitted on the basis of thepresent experimental data. Attention should be paid when it isapplied to other experimental conditions.Two experimental cases conducted by Akih-Kumgeh et al.18

were repeated for comparison, as shown in Figure 2. It isdemonstrated that the present data agree fairly well with those

Table 1. Properties of the Fuels Tested

Figure 1. Definition of ignition delay time.

Table 2. Compositions of the Mixture in This Study

mixture Φ propanal (%) O2 (%) Ar (%) P (atm)

1 0.5 1.25 10.00 88.75 1.2, 5.0, 16.02 1.0 1.25 5.00 93.75 1.2, 5.0, 16.03 2.0 1.25 2.50 96.25 1.2, 5.0, 16.04 1.0 0.50 2.00 97.50 5.05 1.0 2.00 8.00 90.00 5.0

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 fuelconcentration.

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in publication18 under the pressure of 12 atm, but givesignificantly lower values at 1.2 atm. Many repeated experi-ments at 1.2 atm were performed, and the difference at lowpressure still existed. In this study, the driven section isevacuated to pressure below 10−5 kPa and the typical leakagerate is less than 5 × 10−5 kPa/min. Therefore, the presentmeasurements are of high validity.3.2. Validation and Optimization of Propanal Model.

Up to now, there are two detailed propanal mechanismsavailable, the Veloo model23 and the NUIG model.24 TheVeloo model was developed by Veloo et al. for theinterpretation of JSR data, which includes 113 species and995 reactions. The NUIG model involving 330 species and2012 reactions was developed by Pelucchi et al. for modelingthe pyrolysis and oxidation of n-C3−C5 aldehydes in a shocktube. However, these two models have large deviation in theprediction of the ignition delay time of propanal. The Veloomodel shows overprediction under the fuel-rich (Φ = 2.0)condition, while the NUIG model shows poor prediction underfuel-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 thencoupled into the propanol mechanism.19

The measured IDTs in this study mainly locate in the hightemperature regions, and mechanism modification also focuseson the high temperature part. Recently, da Silva and Bozzelli25

calculated the bond dissociation energies (BDEs) correspond-ing to homolytic C−H and C−C bond cleavage in propanalusing CBS-APNO 298 K enthalpies, while Akih-Kumgeh andBergthorson determined the BDEs of propanal by atomizationusing the CBS-QB3 method in Gaussian 09, as shown in Figure3a,b. It is observed that the BDE of R-CH2CHO (83.7 kcal/

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 grouphas the weakest bond energy, which is 89.3 kcal/mol, while theC−H bond on the terminal primary carbon has the strongestbond energy (102.4 kcal/mol), which approximately equals theC−H bond energy in propane (ca.102 kcal/mol). It indicatesthat the carbonyl group exerts little influence on the terminalprimary carbon (γ-carbon). Therefore, an analogous methodcan be used in the construction of the propanal submodelbefore accurate rate constants of the unimolecular decom-position 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 alcoholmodel, such as isomerization reactions. The main modificationin this study focuses on the initiation reactions of propanal,including unimolecular decomposition, H-abstraction, andradical decomposition reactions. For unimolecular decom-position reactions, these reactions usually have strong pressuredependency; however, in most of the models, the rate constants

of these reactions do not include the pressure dependency item.Pelucchi et al.24 calculated the temperature and pressuredependency of unimolecular decomposition reactions ofpropanal using a three-frequency version of Quantum Rice−Ramsperger−Kassel theory (QRRK/MSC) in Tore form. Thedecomposition of propanal is mainly initiated by C−Cdecomposition, while C−H bond cleavage contributes a littleto the initiation. Therefore, the rate constant of thedecomposition of C−C, recommended by Pelucchi et al.,24

was used in the modified model. Reactions that break down theH atom from each C atom site are also added with a reverserate 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 likeH, OH, and O. For H-abstraction from the α position, the rateconstant recommended by Veloo et al.23 was used, while therate constants for H-abstraction from β and γ positions wereobtained on analogy with the counterparts of the ethylpropanoate model by Metcalfe et al.26 or the butanol modelby Sarathy et al.13

Fuel radicals (such as C2H5CO, CH3CHCHO, andCH2CH2CHO) decompose primarily through β-scission. Forthis type of reactions, the kinetic parameters in the modifiedmodel were directly taken from the butanol model by Sarathyet al.13 Additionally, as mentioned by Mereau et al.,27 carbonylradicals (R-CO) can not only decompose through β-scissionbut 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 mainreactions mentioned above are listed in Table 3, in which Rrepresents small radicals such as CH3 and HCO. Thermody-namic data of all species in the modified propanal submodel areobtained from the butanol model by Sarathy et al.13

The modified propanal submodel first validates againstmeasured ignition delay times. Figure 4 gives the measuredignition delay times with those of calculations using themodified model, Veloo model,23 and NUIG model.24 Pressuredependence is shown in Figure 4a−c at three equivalence ratios(0.5, 1.0, and 2.0). It can be seen that the modified modelcaptures well the trend of pressure dependence andquantitatively agrees with the experimental data at 5.0 and16.0 atm. However, predictions of the modified model areslightly higher than the measurement at 1.2 atm and relativelylow temperatures. This deviation is considered resulting fromthe 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 andNUIG model can qualitatively predict the pressure dependence.However, their computed results are significantly higher thanthe measured values, especially in relatively low temperatureregion.Besides pressure dependence, the effect of fuel concentration

was also investigated, as shown in Figure 4d. Similar results areobtained in the predictions of the three models for a fuelconcentration of 0.50%. With the increase of fuel concentration,both the Veloo model and NUIG model demonstrate poorprediction under relatively low temperature, while the modifiedmodel not only captures the dependence of fuel concentrationbut 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 modelunder most conditions.

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

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3.3. Comparison of Ignition Delay Times of Propane,n-Propanol, i-Propanol, and Propanal. Figure 5 shows theignition delay times of the four C3 fuels in the stoichiometricmixtures with a fuel concentration of 0.75% at the pressures of1.2 and 16 atm. The experimentally measured ignition delaytimes of n-propanol and i-propanol from the previous study19

were used, and the ignition delay times of propane andpropanal were calculated using the suggested correlation in ref20 and the correlation of this study. It is observed that propaneand i-propanol have the comparable, but longest, ignition delaytimes. Propanal has the slightly shorter ignition delay timesthan those of n-propanol. Meanwhile, numerical calculations forfour fuels using the modified model are plotted in the figure, asshown in a solid line. The modified model captures well therelationship of the four fuels and quantitatively agrees with theexperimental results. The global activation energies of the fourfuels are obtained with the multiregression method. Theactivation energies of propane, n-propanol, i-propanol, andpropanal are 40.44, 35.10, 40.06, and 31.29 kcal/mol,respectively, indicating that the temperature sensitivity ofpropanal ignition delay times is the weakest, while propane andi-propanol are comparable.

In terms of molecular structure, the oxygen atom connectedto the terminal carbon atom increases the chain length andmakes the breaking more readily when the molecule is attackedby the other molecules or radicals, generating more reactiveradicals and thus promoting the ignition. This explains why n-propanol and propanal are more likely to be ignited thanpropane. When the OH group is added to the middle carbonatom, the structure of the molecule becomes more compactrather than the increase of chain length, thus the molecule ismore 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. Itcan be seen that discrepancies among the four C3 fuels aresignificant, which have been captured by the modified model.Although the molecular structure of i-propanol is compact, theOH in i-propanol is easier to be split off than that of the Hatom in propane, leading to more reactive radicals andpromoting the overall reaction under high pressure for i-propanol. Figure 6 shows the bond dissociation energies of n-propanol and i-propanol at the CBS-QB3 level calculated by EI-Nahas et al.28 As shown in Figures 3a and 6a, the BDEs of thesemolecules are different, and propanal has comparatively weaker

Table 3. Propanal Submodel (Units: cm, mol, s, cal, K)

reaction A n Ea ref

unimolecular decompositionC2H5CHO ⇄ C2H5 + HCO 0.1300 × 1027 −3.000 86405.9 24

Low/ 0.2870 × 1085 −18.600 101060.0/Troe/0.2491 × 10−02 376.8 6.089 4632./

C2H5CHO ⇄ CH3 + CH2CHO 0.1160 × 1026 −2.800 85718.2 24Low/ 0.1260 × 1088 −19.400 101280.0/Troe/0.2491 × 10−02 372.5 6.089 5252./

CH3CHCHO + H ⇄ C2H5CHO 1.00 × 1014 0.00 0.00 × 1000 30CH2CH2CHO + H ⇄ C2H5CHO 1.00 × 1014 0.00 0.00 × 1000 30C2H5CO + H ⇄ C2H5CHO 1.00 × 1014 0.00 0.00 × 1000 30

hydrogen abstractionC2H5CHO + H ⇄ C2H5CO + H2 1.20 × 1014 0.00 7.00 × 1003 8C2H5CHO + H ⇄ CH3CHCHO + H2 1.30 × 1006 2.40 4.47 × 1003 31C2H5CHO+H ⇄ CH2CH2CHO + H2 9.50 × 1004 2.75 6.28 × 1003 13C2H5CHO + O ⇄ C2H5CO + OH 5.85 × 1012 0.00 1.81 × 1003 23C2H5CHO + O ⇄ CH3CHCHO + OH 2.20 × 1013 0.00 3.28 × 1003 26C2H5CHO + O ⇄ CH2CH2CHO + OH 9.81 × 1005 2.43 4.75 × 1003 26C2H5CHO + OH ⇄ C2H5CO + H2O 2.65 × 1012 0.00 −7.30 × 1002 23C2H5CHO + OH ⇄ CH3CHCHO + H2O 1.15 × 1011 0.51 6.30 × 1001 26C2H5CHO + OH ⇄ CH2CH2CHO + H2O 5.28 × 1009 0.97 1.59 × 1003 26C2H5CHO + R ⇄ C2H5CO + RHC2H5CHO + R ⇄ CH3CHCHO + RHC2H5CHO + R ⇄ CH2CH2CHO + RH

β-scissionC2H5CO + M ⇄ C2H5 + CO + M 2.75 × 1009 1.41 3.58 × 1004 5H2O/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 0.79 4.26 × 1004 13CH3CHCHO ⇄ CH3CHCO + H 1.35 × 1013 −0.17 3.35 × 1004 13CH3CHCHO ⇄ C2H3CHO + H 4.16 × 1012 −0.02 3.42 × 1004 13CH3CHCHO + HO2 ⇄ CH3CHOCHO + OH 9.64 × 1012 0.00 0.00 × 1000 13CH3CHOCHO ⇄ CH3CHO + HCO 3.98 × 1013 0.00 9.70 × 1003 13CH3CHCO + OH ⇄ C2H5 + CO2 1.73 × 1012 0.00 −1.01 × 1003 13CH3CHCO + OH ⇄ SC2H4OH + CO 2.00 × 1012 0.00 −1.01 × 1003 13CH3CHCO + H ⇄ C2H5 + CO 4.40 × 1012 0.00 1.46 × 1003 13CH3CHCO + O ⇄ CH3CHO + CO 3.20 × 1012 0.00 −4.37 × 1002 13CH2CH2CHO ⇄ C2H4 + HCO 1.26 × 1013 0.00 3.03 × 1004 13CH2CH2CHO ⇄ C2H3CHO + H 1.67 × 1013 0.00 4.62 × 1004 13

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corresponding bonds. The Cα−Cβ bond in propanal is weakerthan that in n-propanol by about 2 kcal/mol; thus propanalwould undergo the C−C bond cleavage reactions more likely.

3.4. Reaction Pathway Analysis. Figure 7 gives thereaction pathway on the basis of the modified model for thefour C3 fuels at the pressures of 1.2 and 16.0 atm and atemperature of 1300 K for the stoichiometric mixtures with afuel concentration of 0.75%. The instant of 20% fuelconsumption is chosen to analyze the reaction fluxes asemployed in ref 29.As shown in Figure 7, the major consumption for the four C3

fuels is from the H-abstraction reactions, while reactions ofunimolecular decomposition show a small contribution. Forpropanal and i-propanol, as shown in Figure 7b,d, thecontribution of unimolecular decomposition reactions to fuelconsumption is less than 3%. The increase of pressure does notlead to a significant change in the contribution of each pathway,but the results show an increase in the contribution of H-

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

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

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

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abstraction reactions and a decrease in the contribution ofunimolecular decomposition can still be observed.After β-scission reactions, the main intermediate species of

propane are ethene and propene. For n-propanol, the mainintermediates include ethenol, ethene, and propene. The mainintermediates of i-propanol are propene and acetone. Forpropanal, the main intermediates include C2H5 radicals, 1-propenal, 2-propenal, and ethene. It is shown that theintermediates of propane and i-propanol are the rather stablespecies which are relatively difficult to decompose for furtheroxidation. However, ethanol, a product during the oxidation ofn-propanol, is an unstable product which will undergo theinternal isomerization to yield acetaldehyde. Similarly, 1-propenal and 2-propenal of propanal are also unstableintermediates and will rapidly decompose to form C2H3 andC2H5 radicals. It is well-known that C2H5 is an unstable radicalwhich could readily undergo decomposition to form C2H4 anda H atom. The fast production of H atoms accelerates the rateof the main chain branching of H + O2 = OH + O and leads tomore 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 tothe terminal carbon, it will produce unstable molecules whichcould readily decompose to promote the ignition duringoxidation. When it attaches to the middle carbon, it willgenerate rather stable ketones and contribute a little to the

ignition. In addition, the existence of the CO double bondwill make the whole structure more unstable and produce moreunstable intermediates or radicals during the oxidation, leadingto the enhancement of ignition. Reaction pathway analysisreveals the autoignition characteristics of the four C3 fuels. Interms of the length of ignition delay times, they follow in theorder of propane > i-propanol > n-propanol > propanal.

3.5. Sensitivity Analysis. To further interpret theoxidation chemistry of the four C3 fuels, sensitivity analysiswas performed on the basis of the modified model under thesame conditions with those of pathway analysis, as shown inFigure 8. The sensitivity coefficient is defined as

τ ττ

=−

Sk k

k(2.0 ) (0.5 )

1.5 (1.0 )i i

i (2)

where ki is the pre-exponential factor in the rate constant of theith reaction and τ is ignition delay time. A positive sensitivitycoefficient indicates an increase in the ignition delay time andan inhibiting effect on the overall reactivity with the increase ofreaction 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 themaximum negative sensitivity coefficient is the chain branchingreaction H + O2 = O + OH. The H-abstraction reactions by Hand OH radicals from the fuel have high positive coefficients,which indicates an inhibition on the reactivity because these

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) and16.0 atm (italic bold font); Φ = 1.0, T = 1300 K, 20% fuel consumption.

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reactions consume high reactive H radicals and produce lessreactive radicals and stable hydrogen. Furthermore, thereactions with relatively high positive coefficients are mainlythe recombination reactions, and those with relatively negativecoefficients are mainly the chain propagating reactions of smallradicals.However, there are some differences in high sensitivity

reactions among the four C3 fuels. The recombination reactionCH3 + CH3 (+M) = C2H6 (+M) has a relatively high positivesensitivity coefficient in the oxidation of propane and i-propanol, but it is not included in the controlling reactions of n-propanol and propanal. The ignition promoting reactions aremostly the small radical reactions in the oxidation of propanal,while some fuel-specific reactions are found to enhance thereactivity of n-propanol. The reactions of small radicals havestronger enhancement on the reactivity. This further explainsthe shorter ignition delay times of propanal compared to n-propanol.

4. CONCLUSIONS

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

and i-propanol, a comparative study of high temperatureignition behavior of the four C3 fuels was conducted. The mainconclusions are summarized as follows:

1. Ignition delay times of propanal show a typical Arrheniusdependence on temperature. Like most of the hydro-carbon fuels, ignition delay times increase with thedecrease of pressure, the increase of equivalence ratio,and the decrease of fuel concentration.

2. A modified propanal submodel was proposed byupdating the latest rate constants or adding some absentreactions on the basis of literature review. Compared tothe Veloo model and NUIG model, the modified modelshows 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 orderof propane > i-propanol > n-propanol > propanal. Thediscrepancy among the four C3 fuels is more obviousunder high pressure conditions. The modified model cancapture well the ignition behaviors of the four C3 fuels.

4. Reaction pathway analysis on the basis of the modifiedmodel shows that consumption of the four C3 fuels isdominated by H-abstraction reactions. Propane mainlyproduces ethene and propene. n-Propanol primarily

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.

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produces ethenol besides ethene and propene. i-Propanolmainly produces propene and acetone. Propanal mainlyproduces C2H5 radicals, 1-propenal, 2-propenal, andethene.

5. Sensitivity analysis shows that the main chain branchingreaction R1 dominates the reactivity of the four C3 fuelsand H-abstraction reactions from the fuel molecule haverelatively large coefficients. However, the recombinationreactions, CH3 + CH3 (+M) = C2H6 (+M) and C3H5-A+ H = C3H6, inhibit the reactivity of propane and i-propanol; however, inconspicuous influence of thesereactions is observed on the ignition of n-propanol andpropanal.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zhhuang@mail.xjtu.edu.cn. Phone: +86 29 82665075.Fax: +86 29 82668789.NotesThe authors declare no competing financial interest.

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

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Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b02739Energy Fuels 2016, 30, 717−724

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