Autoignition of Methyl Decanoate, a Biodiesel Surrogate, under High

Article pubs.acs.org/EF

Autoignition of Methyl Decanoate, a Biodiesel Surrogate, under High-Pressure Exhaust Gas Recirculation Conditions Zhenhua Li,†,‡ Weijing Wang,‡ Zhen Huang,† and Matthew A. Oehlschlaeger*,‡ †

Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ‡ Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *

ABSTRACT: The autoignition of methyl decanoate, a standalone biodiesel surrogate or surrogate component, is studied under high-pressure exhaust gas recirculation (EGR) conditions relevant to internal combustion engines. Ignition delay times were determined in reflected-shock experiments using measured pressure and electronically excited OH chemiluminescence for stoichiometric methyl decanoate/air/EGR mixtures containing 0−60% EGR at 900−1300 K and 20 and 50 atm. Ignition delay time dependence upon pressure and EGR fraction was found to obey τ ∝ P−0.8 and τ ∝ (1 − EGR %/100)−1 for the conditions studied. Experimental results are compared to three comprehensive kinetic models for methyl decanoate oxidation from the recent literature: Herbinet et al. (Herbinet, O.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2008, 154, 507−528 and Herbinet, O.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2010, 157, 893−908), Glaude et al. (Glaude, P. A.; Herbinet, O.; Bax, S.; Biet, J.; Warth, V.; Battin-Leclerc, F. Combust. Flame 2010, 157, 2035−2050), and Diévart et al. (Diévart, P.; Won, S. H.; Dooley, S.; Dryer, F. L.; Ju, Y. Combust. Flame 2012, 159, 1793−1805). Model−experiment comparisons are generally favorable, with some differences in model performance because of variations in C0−C2 chemistry, of predominate sensitivity at the conditions studied, and methyl decanoate hydrogen-atom abstraction rates. Ignition delay times for stoichiometric mixtures containing EGR, defined as the complete products of combustion, and synthetic EGR, defined here as pure N2, are indiscernible, indicating that the primary influence of EGR is to simply displace fuel and O2, thereby decreasing radical branching, a conclusion that is supported by the kinetic models.

1. INTRODUCTION Biodiesel, a potentially renewable liquid transportation fuel comprised of long-chain alkyl ester compounds, has several advantageous properties for use in diesel engines, including a relatively high cetane number, lubricity, and flash point. Biodiesels have been shown to improve combustion efficiency and reduce the emissions of particulate matter, carbon monoxide, and unburned hydrocarbons.1−4 They can be blended with petroleum diesel in addition to being used as standalone fuels.5 Biodiesel when produced from vegetable oil, animal fat, or waste oil via a transesterification reaction with methanol is composed of a variety of fatty acid methyl esters (FAMEs), where the structure of the methyl esters is controlled by structures of the fatty acids present in the feedstock.6,7 Because of the fact that biodiesels are comprised of multiple alkyl ester compounds, it is common to choose a single- or reducedcomponent surrogate to emulate biodiesel chemical and/or physical properties in combustion simulations and experiments. Methyl decanoate [MD, CH3(CH2)8COOCH3] has been considered as a biodiesel surrogate in the recent literature.8,9 MD is a reasonable surrogate for FAME biodiesel because it contains the requisite methyl ester functionality and a sufficiently long alkyl chain (10 carbons) to be representative of the long carbon chains typical of FAME biodiesel components (15−21 carbons) while still providing a sufficiently low boiling point (496 K at 1 atm and 381 K at 10 torr) to © 2012 American Chemical Society

allow for kinetic measurements in the gas phase. Additionally, MD has a cetane number of 47,10 at the lower end but within the range reported for FAME biodiesels from soy and rapeseed oils.11 In the last several years, MD has been the subject of a number of experimental and modeling studies focused on understanding its combustion properties and, in particular, its oxidation reaction kinetics. Szybist et al.8 studied the premixed combustion of MD, as well as a variety of diesel, biodiesel, and surrogate fuels in a motored engine study, reporting heat release profiles and chemical characterization of exhaust gaseous products and condensate. In 2008, Herbinet et al.9 published a detailed kinetic model to describe the oxidation of MD and compared their modeling predictions to the motoredengine speciation measurements by Szybist et al.,8 jet-stirred reactor (JSR) speciation measurements made during rapeseed oil methyl ester oxidation by Dagaut et al.,12 n-decane shocktube ignition delay measurements by Pfhal et al.,13 and OH radical time-history shock-tube measurements made during ndecane oxidation reported by Davidson et al.14 Herbinet et al. constructed their model based on the similarity between MD and n-alkane reaction kinetics and suggested that the overall reactivity of MD is controlled by the alkyl chain and not Received: May 24, 2012 Revised: July 3, 2012 Published: July 5, 2012 4887

dx.doi.org/10.1021/ef3009019 | Energy Fuels 2012, 26, 4887−4895

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Table 1. Experimental Conditions for MD Ignition Delay Time Measurements

a

MD (%)

O2 (%)

N2 (%)

CO2 (%)

H2O (%)

EGR (%)

ϕ

P (atm)

T (K)

1.34 1.34 1.07 1.07 0.802 0.802 0.802 0.535

20.73 20.73 16.58 16.58 12.44 12.44 12.44 8.29

77.94 77.94 76.87 76.87 75.80 75.80 86.76 91.17

0 0 2.74 2.74 5.48 5.48 0 0

0 0 2.74 2.74 5.48 5.48 0 0

0 0 20 20 40 40 40a 60a

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

20 50 20 50 20 50 20 20

± ± ± ± ± ± ± ±

919−1247 974−1204 907−1299 982−1255 890−1299 1005−1306 914−1240 963−1317

3 5 2 5 3 4 2 2

Synthetic EGR: CO2 and H2O replaced with N2.

In the present study, we report, for the first time, the influence of exhaust gas recirculation (EGR) and high pressures (up to 50 atm), found under internal combustion engine operation, on MD autoignition in a shock tube. Few fundamental experimental studies performed under wellcharacterized reaction conditions (i.e., outside of engines) exist in the literature to elucidate the influence of EGR on autoignition of transportation fuels or their surrogates or, for that matter, the influence of EGR on laminar burning velocities26,27 and other fundamental combustion properties. Given that modern spark-ignition and diesel engines as well as engines operating under other modes of combustion, such as stratified-charge compression-ignition (SCCI), homogeneouscharge compression-ignition (HCCI), and other low-temperature combustion (LTC) modes,28 are reliant on EGR for optimal performance, experimental information regarding the influence of EGR on autoignition is needed. Notable studies in which autoignition has been investigated under controlled (non-engine) EGR conditions include the shock-tube study by Gauthier et al.,29 who characterized the autoignition of nheptane, gasoline, and a ternary gasoline surrogate under HCCI conditions (15−60 atm and 800−1250 K) with EGR (CO2, H2O, O2, and N2) at loadings of 0, 20, and 30%. Vandersickel et al.30 also studied the autoignition of n-heptane as well as two kerosene-like fuels under EGR conditions (0, 30, and 50% EGR loading) in a shock tube at 20−65 bar and 700−1100 K. Healy et al.31 investigated the autoignition of n-heptane/α-methylnaphthalene mixtures, a diesel surrogate, under EGR conditions in a rapid compression machine at 860−990 K and 10 bar.

strongly influenced by reactions of the methyl ester functionality, noting agreement between n-decane experimental ignition delay results13 and their modeling predictions for MD ignition delay. The Herbinet et al.9 model was also shown to satisfactorily reproduce early CO2 formation from the ester functionality observed in both the motored-engine study by Szybist et al.8 and the JSR measurements by Dagaut et al.12 In 2010, Herbinet et al.15 updated their detailed MD model and added models for unsaturated methyl decenoates; the original 2008 and updated 2010 Herbinet et al. MD models predict nearly identical high- to moderate-temperature MD ignition delay times, the focus of the present study. Further detailed kinetic modeling studies of MD oxidation include the work by Glaude et al.,16 who reported a model based on the EXGAS automated method for generating kinetic and thermochemical parameters, and Diévart et al.,17 who developed a detailed MD oxidation model based on extension of the kinetics of smaller methyl esters (methyl butanoate), as opposed to the extension of n-alkane kinetics as was used by Herbinet et al.9,15 Diévart et al. showed that different prescriptions of kinetic parameters for reactions involving the methyl ester functionality result in appreciably different MD oxidation pathways, intermediate species reactivity, and global model predictions. Experimental studies carried out on MD flames include those by Seshadri et al.,18 who investigated non-premixed flame extinction and ignition, Wang et al.,19 who studied laminar premixed flames reporting flame speeds and extinction strain rates, Sarathy et al.,20 who reported speciation in an opposedflow diffusion flame, and Dooley et al.,21 who investigated diffusion flame extinction limits. For the purposes of modeling MD oxidation kinetics in flames, reduced or skeletal versions of the Herbinet et al.9 MD kinetic model have been reported by Seshadri et al.,18 Sarathy et al.,20 and Luo et al.22 Previous shock-tube studies of the MD ignition delay have been reported by Haylett et al.23 and Wang and Oehlschlaeger.24 Haylett et al. performed MD ignition delay measurements using an aerosol-fuel-loading technique at lean (MD/O2/argon at ϕ = 0.09−0.17) and high-temperature (1185−1308 K) conditions around 8 atm. Wang and Oehlschlaeger used a heated (gas-phase) shock-tube technique, also used in the present study, to measure ignition delay times for ϕ = 0.5, 1.0, and 1.5 MD/air mixtures at temperatures from 653 to 1336 K and at pressures around 15−16 atm. In recent work relating to MD pyrolysis, Pyl et al.25 reported speciation measurements for more than 150 products of MD pyrolysis in a tubular reactor study at 873−1123 K and 1.7 bar. Comparisons of the Pyl et al. pyrolysis speciation measurements with kinetic model predictions from Herbinet et al.9 and Glaude et al.16 showed reasonable agreement.

2. EXPERIMENTAL SECTION Ignition delay time measurements were carried out at Rensselaer Polytechnic Institute in a heated shock tube (5.7 cm inner diameter, 4.11 m long driven section, and 2.59 m driver section) using techniques previously described;24,32,33 here, only details pertinent to the current study are given. Ignition delay times were measured using the reflected-shock technique at temperatures ranging from approximately 900 to 1300 K and at nominal pressures of 20 and 50 atm; the specific conditions for which experiments were carried out are given in Table 1. Mixtures studied include stoichiometric MD/air, stoichiometric MD/air/EGR mixtures with 20 and 40% EGR, and two stoichiometric MD/air/N2 mixtures, where the EGR has been replaced by pure N2, termed here synthetic EGR, with N2 dilution corresponding to 40 and 60% EGR. For the present MD autoignition studies, the shock tube and mixture preparation vessel and manifold were maintained at a uniform temperature of 140 °C. Reactant mixtures comprised of stoichiometric MD/air, MD/air/EGR, and MD/air/N2 mixtures were prepared via partial pressures in a heated mixing vessel. For the formulation of MD/ air and MD/air/N2 mixtures, liquid MD was directly injected into the heated mixing vessel and allowed to evaporate. After MD evaporation, N2 and then O2 were added from compressed gas cylinders; air was 4888


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defined as pure N2 and O2 at a molar ratio of 3.76−1.0. For mixtures with added EGR, EGR was defined as the complete products of stoichiometric MD/air combustion (CO2, H2O, and N2 at 13.7, 13.7, and 72.6% by moles, respectively) and the EGR % was defined as the amount of the MD/air mixture displaced by the complete stoichiometric combustion products. For the formulation of MD/ air/EGR mixtures, liquid MD was first injected into the heated mixing vessel and allowed to evaporate, followed by the injection and evaporation of distilled water. After the addition of these two liquids, CO2, N2, and O2 were added from compressed gas cylinders in that order. In all cases, measurements of partial pressures were used to define mixture fractions and mixtures were mechanically mixed inside the mixing vessel with a rotating vane assembly for at least 20 min prior to experiments. Methyl decanoate was purchased from Aldrich at 99+% purity; O2, N2, and CO2 were purchased from Noble Gas Solutions at 99.995% purity; and H2O distilled at Rensselaer was used for mixture preparation. Shock waves were produced by bursting polycarbonate diaphragms with either helium or helium−nitrogen driver gases; tailored helium− nitrogen driver gases were used for extended reflected-shock test times longer than approximately 1.5 ms. The post-incident- and postreflected-shock conditions were determined using the normal shock relations with measured incident-shock velocity, known initial conditions (temperature, pressure, and reactant mixture), and thermodynamic properties for the reactant mixture species taken from Goos et al.34 The incident-shock velocity was measured using a series of five piezoelectric pressure transducers spaced over the last meter of the shock-tube driven section. The temperature and pressure immediately following the reflected shock are reported as the experimental temperature and pressure and have uncertainties of approximately ±1 and ±1.5%, respectively. Because of non-ideal viscous gas dynamics, the pressure rises slowly in the post-reflectedshock region at a rate measured to be (dP/dt)(1/P0) = 1−3% ms−1. Assuming an isentropic relationship between pressure and temperature in the reflected-shock region, experimentally validated in previous works,35,36 the corresponding temperature rise rate is (dT/dt)(1/T0) = 0.3−0.8% ms−1. Ignition delay times were measured behind reflected shocks using pressure and electronically excited hydroxyl radical chemiluminescence (OH*) observed at the shock-tube end wall. An example experiment is illustrated in Figure 1. Ignition delay times were defined as the time interval between shock reflection at the end wall and the onset of ignition at the end wall, defined by extrapolating the maximum slope in the OH* signal to the baseline. Reported ignition delay times have an estimated uncertainty of ±20% based on the contributions to ignition delay uncertainty from uncertainties in initial reactant mixture

composition (∼2% uncertainty in all reactant concentrations), reflected-shock temperature and pressure (∼1 and ∼1.5% uncertainty, respectively), and determining ignition delay from recorded pressure and chemiluminescence signals (5−20 μs, depending upon the length of ignition delay time).

3. RESULTS All raw ignition delay measurements are shown in Figure 2 and have been appended in tabular form in the Supporting Information. While negative-temperature-coefficient (NTC) behavior was not observed, for decreasing temperatures from 1300 to 900 K, there is clear reduction in the apparent activation energy (local slope on the Arrhenius axes), a result of the transition from high-temperature oxidation chemistry to intermediate-temperature chemistry at the entrance to the NTC region. In previous lower pressure (∼15 atm) MD/air measurements at variable equivalence ratios, we have shown that a NTC region exists from approximately 900 to 750 K,24 in agreement with the predictions of detailed kinetic models containing low- and high-temperature chemistry.9,15−17 Over the range of temperatures studied (900−1300 K), a secondorder polynomial fits the ignition delay data well and, in Figure 2, helps to illustrate differences in ignition delay for different mixtures. As exhibited in Figure 2, increased EGR fractions result in increased ignition delay for stoichiometric MD/air/EGR mixtures. The ignition delay dependence upon mixture composition can be represented with a simple power-law relationship determined through regression analysis to be τ ∝ (1 − EGR %/100)−1, when expressed in terms of EGR fraction, which for the stoichiometric mixtures studied is equivalent to τ ∝ (MD (%)/100)−1 or τ ∝ (O2 (%)/100)−1, when expressed in terms of MD or O2 fraction. As generally observed for hydrocarbon fuels, increased pressure results in decreased ignition delay; in this study, the pressure dependence was found, via regression analysis, to obey τ ∝ P−0.8, very similar to the pressure scaling measured in our laboratory for several pure hydrocarbons37,38 and multi-component non-oxygenated fuels33 at similar experimental conditions. In Figure 3, ignition delay times have been scaled to common pressures and EGR fractions. The relatively small amount of scatter about the second-order polynomial fits [±10% rootmean-square (rms) scatter] illustrates the effectiveness of the two power-law scaling factor. In Figure 3a, the pressure-scaled 20 and 50 atm data are also compared to previous measurements for MD/air made at 16 atm.24 When the two studies are scaled to account for pressure differences, they are in very good agreement. While the reported scaling factors are effective within the condition space of the current study (900− 1300 K, 20−50 atm, ϕ = 1, and EGR = 0−60%) and, as shown in Figure 3a, at slightly lower pressures (16 atm), they should not be used far outside of the present condition space because of the complex temperature−pressure−mixture dependencies characteristic of hydrocarbon oxidation. 4. KINETIC MODELING AND DISCUSSION Several detailed9,15−17 and reduced18,20,22 kinetic models for MD oxidation have been reported in the literature. Here, three detailed models are compared to the measured ignition delay times in Figures 4 and 5, those developed by Herbinet et al. [Lawrence Livermore National Laboratory (LLNL)],9,15 Glaude et al. (Nancy Université),16 and Diévart et al. (Princeton University)17 and detailed in Table 2. Ignition

Figure 1. Example reflected-shock ignition delay time measurement. Profiles are side-wall pressure (2 cm from the shock-tube end wall) and OH* emission viewed through the end wall. 4889


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Figure 2. Measured ignition delay times for stoichiometric MD/air/EGR mixtures at (a) 20 atm and (b) 50 atm. Lines are second-order polynomial best fits to data sets.

Figure 3. Ignition delay times scaled to common conditions: (a) 20 and 50 atm MD/air results (EGR = 0%) scaled to 20 atm with τ ∝ P−0.8 and Wang and Oehlschlaeger24 16 atm MD/air results scaled to 20 atm using the same pressure exponent and (b) 20 and 50 atm MD/air/EGR results scaled to 0% EGR with τ ∝ (1 − EGR %/100)−1.

overpredicts the 20 atm ignition delay times by on average 50% and at most a bit more than a factor of 2 (Figure 4c). Additionally, the Glaude et al. model predicts a stronger EGR influence on ignition delay than measured or predicted by the other two models. In Figure 5, experiment−model comparisons are shown at 50 atm. At the higher pressure, the Herbinet et al.9,15 model underpredicts ignition delay by 50−100% and the Glaude et al.16 and Diévart et al.17 predictions are in excellent agreement with deviations near the ±20% uncertainty limits. At 50 atm, all three models appear to predict the EGR dependence well, within our ability to discern the EGR dependence experimentally. Inspection of the model-predicted pressure dependencies (τ ∝ Pn) in the high-temperature region of constant activation energy reveals that both the Herbinet et al. and Diévart et al. models predict a pressure exponent of approximately n = −0.9 and the Glaude et al. model predicts approximately n = −1, while the experiment yields n = −0.8. It is also apparent in Figures 4 and 5 that the three models predict different temperature-dependent behavior (“curvature”) around 900−1000 K. The Glaude et al.16 model predicts an

delay time predictions were carried out using CHEMKIN PRO39 using the adiabatic homogeneous constant volume batch reactor model, commonly used for the simulation of reflected-shock experiments and valid here given the relatively modest amount of experimental non-ideal vicious pressure rise. Modeled ignition delay times were determined by extrapolating the maximum slope in the simulated OH profiles to the baseline (zero OH), in a method similar to that used for interpreting the experiments. In Figure 4, comparisons of all 20 atm experimental results and modeling predictions are shown. At 20 atm, the predictions of the Herbinet et al.9,15 model are in excellent agreement with the data (Figure 4b); specifically, the Herbinet et al. model is at most 25% shorter than the measured ignition delay times for MD/air and mostly within the experimental scatter for the MD/air/EGR mixtures. Similarly, the Diév art et al.17 predictions are in good agreement with the 20 atm measurements (Figure 4d), at most 30−40% longer than the measured ignition delay times. The Herbinet et al. and Diévart et al. models also accurately capture the measured ignition delay dependence upon EGR at 20 atm. The Glaude et al.16 model 4890


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Figure 4. Comparison of measured ignition delay times to the predictions of three literature kinetic models described in Table 2: (a) all three models for MD/air at 20 atm and (b) Herbinet et al.,9,15 (c) Glaude et al.,16 and (d) Diévart et al.17 models for MD/air/EGR at 20 atm.

0.1% CO, CH4, and H2 to MD/air/EGR mixtures only results in changes to predicted ignition delay times of −0.1, +0.1, and −0.3%, respectively. The influence of NOx and particulate matter could not be tested using the three kinetic models considered. Within the three kinetic models compared throughout the present study,9,15−17 the effect of EGR on autoignition is to primarily displace fuel and O2, thereby increasing ignition delay through the influence of MD and O2 concentrations on radical pool development. Secondarily, the addition of EGR and displacement of MD and O2 reduces heat release, also increasing ignition delay. The competition between the effect that EGR has on the radical pool development and heat release was examined by comparing simulations carried out with the three kinetic models9,15−17 under both the adiabatic constant volume constraint, providing ignition delay time sensitivity to the influence of heat release, and the constant temperature and pressure constraint, which eliminates the influence of heat release. The simulations illustrate that on average approximately 75% of the increase of ignition delay upon EGR addition is due to radical inhibition and 25% is due to heat release reduction. For the conditions studied, radical pool growth is predominately controlled by small-molecule radical branching mechanisms, some of which are affected by third-body collision efficiencies. The three kinetic models compared9,15−17 include

earlier (higher temperature) entrance to the NTC than the Herbinet et al. and Diévart et al. models. At both 20 and 50 atm, the experimental ignition delay times display temperature dependence at the limit of the experimental temperature range (900−1000 K), more in agreement with the Herbinet et al. and Diévart et al. models and not in support of the early entrance to NTC predicted by the Glaude et al. model. In the Figure 4 and 5 comparisons, it has been assumed that mixtures containing EGR (CO2, H2O, and N2) are equivalent to mixtures containing synthetic EGR (pure N 2), an assumption that is examined in Figure 6, where comparisons of 20 atm ignition delay times for MD/air/EGR mixtures containing 40% EGR and 40% synthetic EGR are made. Measurements show no discernible difference in the ignition delay time at the Figure 6 conditions for mixtures containing the two different diluting gases (EGR and synthetic EGR), in accordance with the three kinetic models. It should be noted that the observed similarity between EGR and N2 dilution (synthetic EGR) will not hold for EGR at lean conditions, where the EGR will contain O2. The present study also does not account for the influence minor species present in real EGR (CO, CH4 and unburned hydrocarbons, NOx, particular matter, etc.) may have on reactivity. Within the confines of the three kinetic models tested,9,15−17 the addition of CO, CH4, and H2 at reasonable real EGR concentrations was found to have a negligible effect on ignition delay. For example, the addition of 4891


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Figure 5. Comparison of measured ignition delay times to the predictions of three literature kinetic models described in Table 2: (a) all three models for MD/air at 50 atm and (b) Herbinet et al.,9,15 (c) Glaude et al.,16 and (d) Diévart et al.17 models for MD/air/EGR at 50 atm.

2OH + M, and other small-molecule termolecular and decomposition reactions are dependent upon the collision efficiencies of the bath gas components, the models predict that the inclusion of CO2 and H2O at the fractions contained in EGR (e.g., 5.48% H2O and 5.48% CO2 by moles in a MD/air/ 40% EGR mixture) does not appreciably influence MD ignition delay, as observed for hydrogen-containing mixtures diluted with large concentrations of water vapor.40,41 This modeling prediction is in agreement with the present experimental observation of no discernible difference between ignition delay times for stoichiometric mixtures containing EGR and synthetic EGR. In shock-tube and rapid compression machine studies, bath gas composition can also influence the measured ignition delay through the dependence of the temperature history, following heat release on bath gas heat capacity. Differences in ignition delay of tens of percents have been observed in shock-tube and rapid compression machine measurements using nitrogen- and argon-diluted fuel/O2/diluent mixtures because of the differences in the specific heats of nitrogen and argon.42,43 In the present study, the differences in the heat capacity for EGR and synthetic EGR are slight (e.g., the heat capacities for mixtures with 40% EGR and 40% synthetic EGR differ by less than 5%)

specific collision efficiencies for N2, CO2, and H2O for termolecular and decomposition reactions found within the C0 (H2/O2) and C1−C3 oxidation submodels but do not include collider-specific collision efficiencies for termolecular and decomposition reactions involving larger hydrocarbons or MD. At the higher temperatures encountered in this study, H + O2 → OH + O is an important chain-branching reaction and reduction in the concentrations of O2 and MD, which produces H atoms through β-scission chemistry proceeded at the present conditions primarily by hydrogen abstraction, directly reduces reactivity. At the lower temperatures studied, H + O2 + M → HO2 + M competes with H + O2 → OH + O. Much of the HO2 that is formed goes on to abstract an H atom from MD, forming hydrogen peroxide, MD + HO2 → R + H2O2. Hydrogen peroxide decomposition follows, producing two hydroxyl radicals, H2O2 + M → 2OH + M. This radicalproducing sequence again is slowed by the displacement of fuel and O2 by EGR, through the dependence of the formation of HO2 upon O2 and H atoms, which come from MD. Of course, other reactivity-promoting mechanisms are dependent upon the concentration of fuel fragments and O2 (e.g., CH3 + HO2 → CH3O + OH, followed by CH3O + M → CH2O + H) and are also slowed by the reduction in fuel and O2 concentrations. While the rates of H + O2 + M → HO2 + M, H2O2 + M → 4892


The 2008 Herbinet et al. model9 was updated in 2010.15 At the present high- to moderate-temperature conditions, the 2008 and 2010 versions of the model produce nearly identical ignition delay predictions.

Figure 6. MD/air/EGR stoichiometric ignition delay time measurements at 20 atm for 40% EGR and 40% synthetic EGR (i.e., pure N2 dilution) with kinetic modeling comparisons.9,15−17

and do not influence ignition delay, within experimental resolution. Examination of the three models considered in the present study, those listed in Table 2, indicates that the models contain hundreds of differences of varying influence on ignition delay. However, as pointed out by Diévart et al.,17 MD ignition delay times at the high- to moderate-temperature conditions studied here are primarily sensitive to small-molecule oxidation chemistry (C0−C2 submodels), and differences in the C0−C2 chemistry are mostly responsible for the differences in predicted ignition delay and predicted pressure and EGR dependencies exhibited by the three models. An example of sensitivity analysis is given for the Herbinet et al. and Diévart et al.17 models in Figure 7. The sensitivity analysis, carried out for

Figure 7. Ignition delay sensitivity for the Herbinet et al.9,15 and Diévart et al.17 models for stoichiometric MD/air at 1200 K and 20 atm.

MD/air at 1200 K and 20 atm, shows strong sensitivity to a number of C0−C2 reactions largely controlling reactivity at this condition, including H2O2 + M → 2OH + M, H2O2 + O2 → 2HO2, H + O2 → OH + O, CH3 + HO2 → CH3O + OH, and CH3 + HO2 → CH4 + O2. At the lower temperatures encountered in the present study (∼900−1000 K), sensitivity analysis shows sensitivity to C0−C2 chemistry as well as sensitivity to hydrogen abstractions from MD by HO2 and OH, which have different rate coefficients in the three models

a

Glaude et al.16 (Nancy Université) Diévart et al.17 (Princeton University)

1251/7171 2276/7123

validation

Dagaut et al.12 rapeseed oil methyl ester JSR speciation (800−1400 K and 1−10 atm) Pfahl et al.13 n-decane shock-tube ignition delay (650−1300 K and 13 and 50 atm) Davidson et al.14 n-decane shock-tube OH time histories (1360−1710 K and 2 atm) Szybist et al.8 n-heptane and MD motored Code of Federal Regulations (CFR) engine exhaust speciation Glaude et al.16 MD JSR speciation (500−1100 K and 1 atm) Glaude et al.16 MD JSR speciation (500−1100 K and 1 atm) Haylett et al.23 MD shock tube ignition delay (1180−1300 K and 8 atm) Wang and Oehlschlaeger24 MD shock tube ignition delay (650−1340 K and 16 atm) Wang et al.19 MD laminar burning velocity Dooley et al.21 MD diffusion flame extinction limits Sarathy et al.20 MD diffusion flame speciation 2878/8555

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number of species/reactions study

Herbinet et al.9,15 (LLNL)a

Table 2. Literature MD Kinetic Models

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considered9,15−17 because of different MD bond strength prescriptions used in those models; see Diévart et al.17 for a thorough analysis of the implications of the bond strength prescriptions for MD on modeling predictions for a number of global kinetic targets. The strong influence of C0−C2 chemistry on ignition delay predictions at high temperatures (1200 K and greater) and the increasing influence of hydrogen abstractions from the fuel at moderate temperatures (∼1000 K) is characteristic of ignition delay sensitivity, regardless of the parent fuel.

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5. SUMMARY Ignition delay time measurements have been made for MD, a standalone biodiesel surrogate or surrogate component, at engine-relevant high-pressure EGR conditions. Measurements were conducted behind reflected-shock waves at temperatures of 900−1300 K and pressures of 20 and 50 atm. Mixtures of stoichiometric MD/air/EGR were studied for EGR fractions of 0−60%. Over the range of conditions studied, the ignition delay time dependencies upon pressure and EGR fraction were found to obey simple power-law relationships, determined by regression analysis to be τ ∝ P−0.8 and τ ∝ (1 − EGR %/100)−1. Comparisons of measured ignition delay times to the predictions of three comprehensive kinetic models for MD oxidation from the recent literature9,15−17 are globally in very good agreement, in many cases with deviations within or near the experimental uncertainty limits and at worst within approximately a factor of 2. At the conditions of the present experimental study and within the kinetic modeling schemes examined, ignition delay exhibits the strongest sensitivity to C0−C2 chemistry, with hydrogen abstraction from MD by HO2 and OH playing a role at the lower temperatures encountered (∼900−1000 K). Ignition delay times for mixtures containing EGR, defined as the complete products of combustion, and synthetic EGR, defined here as pure N2, were found to be indiscernible, in agreement with kinetic modeling, indicating that the primary influence of EGR is to simply displace fuel and O2, thereby decreasing radical branching.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data in tabular form (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 518-276-8115. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation under Grant CBET-1032453. Zhenhua Li is grateful to the Graduate School of Shanghai Jiao Tong University and the National Natural Science Foundation Key Project (50936004) for support.

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Energy & Fuels

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