Experimental and Modeling Studies on Ignition Delay Times of Methyl

Article pubs.acs.org/EF

Experimental and Modeling Studies on Ignition Delay Times of Methyl Hexanoate/n‑Butanol Blend Fuels at Elevated Pressures Yue Wang, Zheng Yang, Xin Yang, Dong Han, Zhen Huang, and Xingcai Lu* Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ABSTRACT: Ignition delay times for n-butanol/methyl hexanoate blend fuels under an O2/N2 atmosphere were measured in a rapid compression machine at compressed pressures of 11, 15, and 20 bar with a compressed temperature between 660 and 830 K. A kinetic model for n-butanol/methyl hexanoate fuel blends was built to predict the ignition delay and simulate the combustion process. The main reaction pathways involved in the oxidation process of fuel blends under low−mediumtemperature ranges were recognized. Over the conditions researched in this study, ignition delay of fuel blends showed threestage oxidation, namely, cool flame, negative temperature coefficient range, and high-temperature oxidation. It is found that the ignition delay times of fuel blends decrease with the increase of the pressure at top dead center and the proportion of n-butanol in blend fuel. Both methyl hexanoate and n-butanol show two-stage heat release in a low compressed temperature range and single-stage heat release in a high-temperature range. n-Butanol has a suppressing effect on the reaction pathways of methyl hexanoate in low-temperature oxidation and a slight effect in high-temperature oxidation.

1. INTRODUCTION The internal combustion engine consumes 60% of world fossil energy and is a major contributor to CO, CO2, hydrocarbons (HCs), NOx, particulate matters, and other harmful substances. To solve the problem of energy shortage and environmental problem, countries around the world committed to the development of new alternative fuels. Recently, biodiesel received wide attention as an alternative fuel for diesel engines. Biodiesel refers to all types of saturated and unsaturated fatty acid methyl esters and ethyl ester produced from the transesterification reactions between plant oils, animal fats, or waste edible oil with methanol or ethanol. Although the high moisture content and cold flow problem may bring troubles, biodiesel can significantly reduce the dependence upon fossil fuels without harming the environment and has become a potential alternative to fossil fuels. Biodiesel has similar ignition characteristics and physical properties to fossil diesel fuel and has the priority of high oxygen content, widespread sources, and other characteristics. Besides, the engine does not have to be significant adjusted to meet the use of biodiesel. It is wellknown that biodiesel can significantly reduce engine CO, HC, and soot emissions but modestly increase NOx.1−3 In comparison to the diesel/biodiesel blend, alcohol addition helps reduce NOx but modestly increase CO and HC.4,5 Biodiesel combined with n-butanol port fuel injection in premixed charge compression ignition (PCCI) and lowtemperature combustion (LTC) are very effective in simultaneously reducing soot and NOx at idling speeds.6 Butanol addition also showed lower total particle-phase polycyclic aromatic hydrocarbon (PAH) emission.7 To better understand the combustion process and emission formation of the biodiesel engine and make a further reduction in nitrogen oxide emissions, deep studies of basic combustion characteristics and chemical kinetic mechanisms are needed. Generally, the main components of biodiesel are C12−C22 methyl esters and contain 0−3 unsaturated double bonds, © 2014 American Chemical Society

while the length of the carbon chain and number of unsaturated bonds are determined by the feedstock oils. As early as 2001, Fisher et al.8 developed a chemical kinetic model of oxidation of methyl butanoate and verified the test in an airtight container at low-temperature ranges. Metcalfe et al.9 measured the ignition delay of methyl butyrate and methyl propionate in a shock tube and developed a detail mechanism of methyl butanoate on the basis of the model by Fisher et al. Dooley et al.10 studied the ignition delay times of methyl butyrate in the shock tube and set a kinetic model that can simulate the ignition delay times of methyl butyrate more accurately under different conditions. In these studies, it is found that methyl butyrate presents no negative temperature coefficient (NTC) phenomenon from the low- to medium-temperature range, which goes against with the oxidation of biodiesel characteristics. In addition, many investigations found that the oxygen atom of the biodiesel ester group does not fully participate in the oxidation process of C/H, and CO2 was found in the low-temperature reaction. Furthermore, the affection of unsaturated double bonds on the reaction paths of the oxidation mechanism and the combustion products has not yet been recognized. For these reasons, in recent years, researchers continue to study the oxidation and pyrolysis mechanisms of methyl ester and its isomers in various flame conditions to develop their accurate reaction mechanism, which reveals the function of methyl ester groups and unsaturated double bonds. Apart from this, researchers continue to seek an appropriate surrogate that can be used to study the oxidation and pyrolysis mechanisms of biodiesel fuel. Gail̈ et al.11 studied different concentrations of the binary mixture of methyl hexanoate and other chemical substances in a jet-stirred reactor and established the reaction mechanism. Received: May 9, 2014 Revised: July 15, 2014 Published: July 16, 2014 5515

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Sarathy et al.12 studied methyl crotonate in a jet-stirred reactor and convection flame generator. HadjAli et al.13 studied fatty acid methyl esters with the carbon chain of C4−C8 and nalkanes with the carbon chain of C4−C7 on the rapid compression machine (RCM) and did a comparative analysis of impact of the ester groups on the burning of n-alkane. Dayma et al.14 conducted a methyl heptanoate combustion test in a shock tube and completed the combustion mechanism of heptanoate. Togbe et al.15 studied the ignition delay times of the binary mixture of methyl octanoate/ethanol in the shock tube to establish the dynamic model, and the simulation results showed good agreement with the experimental results. Dayma et al. studied a mixture of methyl octanoate and n-butanol in a jet-stirred reactor16 and shock tube15 to establish the reaction mechanism. Glaude et al. 17 established a combustion mechanism of methyl esters, simulated the combustion of methyl decanoate, and found that the low-temperature reaction is similar to alkanes. Herbinet et al.18 proposed a detailed chemical reaction mechanism of soybean rapeseed oil on the basis of a large number of published experimental data, which is, thus far, the most accurate reaction mechanism to simulate biodiesel. In addition, Boehman from Penn State University studied oxidation of saturated and unsaturated methyl esters of C6−C10 using cooperative fuels research (CFR) and measured their critical compression ratio to find out the effects of the unsaturated double bond and its position on the oxidation kinetics and substance generation.19−22 A large number of studies have proven that the ignition and combustion characteristics of methyl esters whose length of carbon chain is shorter than C5 are too different to real biodiesel to directly explain the combustion of biodiesel. Meanwhile, those fatty acid methyl esters whose length of carbon chain is longer than C10, which can be good enough to present the combustion characteristics of biodiesel, are difficult to be studied for the reason for vaporization difficulty and complicated mechanism. Thus, C6−C8 methyl esters are very suitable as surrogate fuel of biodiesel. Because oxidation of the biodiesel mechanism at a low−medium temperature has a significant impact on the ignition delay times and emission generation, while a detailed study is lacking worldwide, methyl hexanoate was selected as the model fuel to study the ignition delay times and reaction paths of the binary mixture of nbutanol/methyl hexanoate in a RCM. As a fatty acid methyl ester with seven carbon atoms, methyl hexanoate can be used as a better model fuel to simulate the ignition characteristics of biodiesel than methyl propionate and methyl butyrate in a RCM. In this paper, the authors measure the ignition delay times of the butanol/methyl hexanoate mixture at different concentrations and compressed pressures in a RCM. Reaction pathways and sensitive analysis about both fuels at low− medium-temperature ranges are studied using CHEMKINPRO to elaborate the impact of n-butanol on the ignition and combustion mechanisms of blend fuels.

connecting rod between the RCM piston and the IPC. Further details of the RCM are available in refs 25−27 A creviced piston is used in this study. The design of this piston is based on studies conducted by other researchers.28−31 The boundary vortex caused by the movement of the piston can be squeezed into the clearance volume around the piston skirt, reducing the influence of the boundary vortex on the internal flow field. Thus, the “adiabatic core” hypothesis32−34 can be used more accurately to calculate the temperature at the top dead center (TDC). The experimental system diagram is shown in Figure 1.

Figure 1. RCM system. 2.2. Experimental Procedure and Definition of Ignition Delay. In this paper, the ignition delay times of 20% n-butanol/80% methyl hexanoate (mole ratio, M-but20), 40% n-butanol/60% methyl hexanoate (mole ratio, M-but40), 60% n-butanol/40% methyl hexanoate (mole ratio, M-but60), and neat methyl hexanoate (MHX) at different compressed pressures and equivalence ratios over wide temperature ranges were measured in the RCM. In this work, anhydrous n-butanol (99.9%) and high-purity methyl hexanoate (99.5%) were selected. The initial temperature is 353 ± 0.5 K. To precisely control the TDC temperature, high-purity N2, O2, and Ar were used for mixture preparation as follows: nitrogen, 99.99%; oxygen, 99.99%; and argon, 99.999%. Pressure−time data were measured using a Kistler 6125B transducer with a 5015 charge amplifier and recorded by a NI PCI-6132 data-acquisition board. Each test point was repeated 5−8 times to ensure the reliability of test data. Because of the high speed of the compression process, it is difficult to measure the instantaneous temperature. Thus, the “adiabatic core” hypothesis can be used more accurately to calculate the temperature at the TDC. T0 and P0 are the initial values of the temperature and pressure. P0 is the pressure at TDC. γ is the heat capacity of the mixture.

∫T

Tc

0

⎛P ⎞ γ dT = ln⎜ c ⎟ γ−1 T ⎝ P0 ⎠

The definitions of ignition delay used in this study are illustrated in Figure 2. dP/dt, named the rate of pressure rise, is the derivative of the pressure trace. The ignition delay, τ, is the time difference between the end of compression and the point of maximum dP/dt of the combustion process.

2. EXPERIMENTAL SYSTEM AND PARAMETER DEFINITIONS

3. KINETIC MODELING

2.1. Experimental System. The RCM, which had been introduced in our previous papers,23,24 is composed mainly by a driving system, braking system, combustion system, mixture preparing system, data acquisition, and control system. The combustion chamber is cylindrical with a 50 mm bore. The impact cylinder (IPC) has a bore of 120 mm and 0.3−0.8 MPa working pressure. The compression ratio can be changed in the range of 6.5−25 by adjusting the length of the

Simulations of the RCM experiment are further completed using CHEMKIN-PRO.35 A detailed blend mechanism proposed by Togbe et al.,15 consisting of 1098 species and 4545 reactions, is used in this work. Under the type of constrain volume and solve energy equation, this paper uses the compress and heat-transfer model developed by Tanaka et al.36 to deal with heat loss. In the simulations, the ignition 5516

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mixtures show a similar tendency with a temperature increase. Ignition delay times decrease as the temperature increases at low temperatures and then show NTC behavior at the middletemperature range. Because of limitations of the test conditions and physical parameters, the compressed temperature can only reach 830 K, and thus, the range of high temperatures is limited. With increasing the n-butanol mole fraction in the mixtures, ignition delay times increase at the same compressed temperature and pressure. It can be seen from the results of this experiment that n-butanol has a suppressing effect on the ignition delay times of methyl hexanoate. Figure 4 shows the experimental pressure traces for stoichiometric n-butanol/methyl hexanoate mixtures of different n-butanol mole fractions at a compressed pressure of 15 bar with compressed temperatures of 700 and 803 K. It can be seen that the addition of n-butanol can significantly affect ignition delay times of methyl hexanoate at both low and high temperatures. The ignition delay times of the binary mixture increase with the increase of the n-butanol content, while the maximum pressure decreases. n-Butanol addition contributes to assuaging knock tendency in combustion. 4.2. Effect of the Equivalence Ratio on the Ignition Delay of Blend Fuel. Figure 5 shows the comparison of

Figure 2. Definition of ignition delay (τ). delay time is determined on the basis of the maximum rate of pressure rising rate.

4. RESULTS AND DISCUSSION 4.1. Effect of the n-Butanol Content on the Autoignition on Blend Fuel. Figure 3 shows the ignition delay

Figure 5. Effect of the equivalence ratio on the ignition times of nbutanol/methyl hexanoate blend fuel.

Figure 3. Effect of the n-butanol content on the ignition delay times of methyl hexanoate.

ignition delay times for the different equivalence ratios of Mbut40 at a pressure of 15 bar. It was found from the figure that the ignition delays of the rich binary mixture (Ø = 1.5) are slightly short when compared to the stoichiometric mixture over wide temperature ranges. While, for a lean fuel/air mixture

times for stoichiometric n-butanol/methyl hexanoate mixtures of different n-butanol mole fractions at a compressed pressure of 15 bar with a compressed temperature between 660 and 830 K. It is seen that ignition delay times of all of the binary

Figure 4. Effect of the n-butanol content on combustion pressures at low- and high-temperature conditions. 5517

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Figure 6. Effect of the equivalence ratios on combustion pressures at low and high temperatures.

to each other when the compressed temperature is below 750 K or above 950 K. In the temperature region of 750−950 K, the ignition delay times of the mixture fuel have little change when the temperature varies, showing NTC characteristics. The ignition delay times decrease with an increasing compressed pressure in both experiment and simulation at the same temperature. Besides, the simulation data are a reasonable match with experimental data at 11 and 15 bar, while discrepancies were observed at 20 bar. The mechanism may need to be changed when it comes to high pressures.

(Ø = 0.5), the ignition delay times are significantly longer than rich and stoichiometric mixtures during the experimental temperature ranges, but the variation tendencies are quite close to each other during the low−medium-temperature ranges. Figure 6 illustrates the pressure traces of 40% n-butanol/ methyl hexanoate (M-but40) at different equivalence ratios under the compressed pressure of 15 bar and temperature of 712 and 783 K. It is shown that the maximum in-cylinder pressures of the rich fuel/air mixture (Ø = 1.5) are much higher than those of the stoichiometric mixture not only at lower temperatures but also at higher temperatures. These results imply that blend fuels do significant improvement on the combustion speed under the rich-fuel/air mixture conditions. Moreover, it is easy to understand that the combustion speed and maximum in-cylinder pressure of the lean fuel/air are significant smaller than those of stoichiometric and rich mixtures, as shown in Figure 6. 4.3. Effect of the Compressed Pressure on Ignition Delay Times. Figure 7 shows that the experimental ignition

5. CHEMICAL ANALYSIS OF BLEND FUELS 5.1. Main Species of MHX and n-Butanol/MHX Blend Fuel. Figure 8 shows the temperature traces and some species concentrations for stoichiometric n-butanol/methyl hexanoate mixtures (M-but40) and MHX at a compressed pressure of 15 bar with a compressed temperature of 700 K. To more clearly show the changes of the concentration of each substance, the authors amplified some value of the concentration. It can be seen from Figure 8 that MHX shows a two-stage reaction not only in the MHX/air mixture but also in the M-but40/air mixture. While MHX was consumed more than two-thirds rapidly during the low-temperature stage for the MHX/air mixture, MHX reacted slowly for a long time and consumed only 50% during the low-temperature stage for the M-but40/air mixture. Another observation in Figure 8 is the significant difference of H2O2 traces for two fuel/air mixtures. For the MHX/air mixture, the H2O2 concentration increased rapidly at the end of the low-temperature reaction and further increased to a high level during the NTC region. Once the temperature achieves 1000−1100 K, H2O2 decomposed to OH in a short moment and triggered the hot ignition. For the M-but40/air mixture, H2O2 increases to a peak point at the end of the lowtemperature reaction and then decreases to a low level during the NTC region, but it increases slowly to a high level before the hot ignition. At last, the most interesting finding in Figure 8 is that the two-stage reactions and NTC region for n-butanol are observed for the M-but40/air mixture. Because both the test and simulation of pure n-butanol are not shown in two-stage reaction characteristics,37 the two-stage behavior of n-butanol in the blended fuels is mainly due to the reaction path changes of methyl hexanoate. The dehydrogenation abstracted by the OH radical is the initiate reaction of the low-temperature chainbranching reaction of n-butanol and methyl hexanoate. In comparison to n-butanol, methyl hexanoate easily releases OH radical, which makes it easier for n-butanol to obtain more OH radical for the low-temperature reaction, contributing to the two-stage exothermic phase of n-butanol.

Figure 7. Experimental and simulation ignition delay times at different compressed temperatures.

delay times vary with the temperature for stoichiometric Mbut40 at compressed pressures of 11, 15, and 20 bar. For a fixed fuel/air mixture, the ignition delay times at different compressed pressures can only be obtained at limited temperature ranges. Thus, simulated ignition delays were displayed over wide operating ranges. A detailed blend mechanism proposed by Togbe et al.15 is used in this paper. It can be seen that ignition delay times at different compressed pressures are close 5518

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Figure 8. Comparison on the temperature history and species concentration of MHX and MHX/n-butanol blend fuel at low-temperature conditions (Ø, 1; Pc, 15 bar; and Tc, 700 K).

Figure 9. Comparison on the temperature history and species concentration of MHX and MHX/n-butanol blend fuel at high-temperature conditions (Ø, 1; Pc, 15 bar; and Tc, 950 K).

Figure 10. Sensitivity analysis of methyl hexanoate and 40% n-butanol/methyl hexanoate (Pc, 15 bar; Tc, 700 K).

HO2 and H2O2 gradually accumulated before the fuel was ignited and consumed rapidly. However, the maximum H2O2 concentration for the MHX/air mixture is significantly higher than that of the M-but40/air mixture. 5.2. Sensitivity Analysis for n-Butanol/Methyl Hexanoate Blend Fuel. To further study the specific reactions that affect the ignition of n-butanol/methyl hexanoate, chemical kinetic mechanisms should be used to do analysis sensitivity about the mixture. Figure 10 compares the sensitivities of ignition delay times for stoichiometric MHX and n-butanol/ methyl hexanoate blend fuel (M-but40) at a compressed pressure of 15 bar with a compressed temperature of 700 K.

Figure 9 shows the temperature history and main species concentrations for stoichiometric n-butanol/methyl hexanoate mixtures (M-but40) and MHX at a compressed pressure of 15 bar with a compressed temperature of 950 K. It can be found that reactants for neat fuel and binary mixture present a singlestage exothermic behavior at the temperature of 950 K. The main reaction pathways for two mixtures at high temperature are decomposition reactions, which will be analyzed in detail later. The decomposition reaction stimulated by high temperature will generate some small molecule and radical group, which sensitively accelerate reaction and reduce ignition delay times. Moreover, as seen in Figure 9, both concentrations of 5519

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Figure 11. Sensitivity analysis of methyl hexanoate and 40% n-butanol/methyl hexanoate (Pc, 15 bar; Tc, 950 K).

Figure 12. Reaction pathways of methyl hexanoate for 40% n-butanol/methyl hexanoate blend fuel at lower temperatures (Pc, 15 bar; T, 750 K).

For comparison, the sensitivity coefficients of ignition delay times were normalized; thus, a positive coefficient means that it is suppressive for ignition and vice versa. It can be found from the sensitivity analyses that, for MHX, MHX + O2 ⇄ MHX2j + HO2 [MHX2j = CH3(CH2)3CH·COOCH3] has a very prominent negative sensitivity coefficient, which means that it will improve ignition tremendously, while MHX + OH ⇄ MHX3j + H2O shows the largest positive sensitivity coefficient. Because this reaction consumes one OH radical but produces MHX3j and H2O, it has a suppression effect on the ignition. For n-butanol/methyl hexanoate blend fuel, not only does R574: MHX + O2 ⇄ MHX2j + HO2 [MHX2j = CH3(CH2)3CH· COOCH3] have the largest negative sensitivity coefficient, but R3544: C4H9OH + OH ⇄ H2O + pC4H9O, R738: MHX3OO ⇄ MHX3OOH2j, and other reactions also play important an effect on the improvement of the ignition. In addition, there are many reactions that show suppression on the ignition process; the main reactions are shown as follows:

MHX 2j + O2 ⇄ MHX 2OO

(R693)

MHX + OH ⇄ MHX 2j + H 2O

(R592)

MHX 2OO ⇄ MHX 2OOH5j

(R740)

MHX3OO ⇄ MHX3OOH5j

(R736)

MHX 5OOH 2 · O ⇄ MB4j2 · O + CH3CHO + OH (R1006)

Figure 11 shows sensitivities of ignition delay times for MHX and stoichiometric n-butanol/methyl hexanoate blend fuel (Mbut40) at a compressed pressure of 15 bar with a compressed temperature of 950 K. For methyl hexanoate, the most important reactions that improve the ignition are R991: H2O2 +M ⇄ 2OH + M, R42: MHX + HO2 ⇄ MHX2j + H2O2, and others, while the largest positive coefficient reaction, which shows an important effect on the suppression of the ignition delay, is R990: H2O2 + O2 ⇄ HO2 + HO2. For n5520

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Figure 13. Reaction pathways of methyl hexanoate for 40% n-butanol/methyl hexanoate at high temperatures (Pc, 15 bar; T, 1000 K).

butanol could be ignored. This shows an agreement with the summary by Dayma et al.11 about the high-temperature reaction pathways of pure methyl hexanoate. Besides, this is also proven by the close curves of ignition delay times at different n-butanol contents.

butanol/methyl hexanoate blend fuel, the largest positive and negative coefficient reactions are similar to those of MHX. 5.3. Reaction Pathway Analysis of n-Butanol/Methyl Hexanoate Blend Fuel. Figure 12 shows the reaction pathways of methyl hexanoate for stoichiometric n-butanol/ methyl hexanoate mixtures (M-but40) at a compressed pressure of 15 bar with a compressed temperature of 724 K and an in-cylinder temperature of 750 K. The main lowtemperature reactions are followed by H-atom abstraction, addition of O2 to fuel radicals, and ROO radical isomerization. Part of the isomerization product will be added O2 and then decomposed into alkenes, aldehydes, and other radical groups, while the others will generate intermediates of cyclic ether, etc., and then decomposed into alkenes, aldehydes, and other radical groups. The study about the low temperature of hydrocarbon fuel indicates that ignition delay times are determined by those following reaction rates: Ri + O2 = RiOO, RiOO = Qj(OOH)I, Qj(OOH)i + O2 = OOQj(OOH)I, OOQj(OOH)i = HOOQ′O + OH, of which the cold flame is activated by the decomposition of OOQj(OOH)i after added O2 the second time.38 OOQj(OOH)i = HOOQ′O + OH will generate a sum of OH, while RH + OH = Ri + H2O will consume a large number. The position of the NTC region largely depends upon the competition between the productions of OH with OHconsuming reactions. Figure 13 shows the reaction pathways of methyl hexanoate for stoichiometric n-butanol/methyl hexanoate mixtures (Mbut40) at a compressed pressure of 15 bar with a compressed temperature of 724 K and an in-cylinder temperature of 1000 K. The ignition reaction at high temperatures share the same reaction pathway as the low-temperature reaction, while the main reaction is C−C scission that happened at the β position of methyl hexanoate to produce unsaturated methyl ester/ methyl ester radical or unsaturated alkyl/alkane. Because the carbon chain length has reached seven, methyl hexanoate decomposes easier than n-butanol and the effect coursed by n-

6. CONCLUSION Ignition delay times of n-butanol/methyl hexanoate mixtures were measured on a RCM at various compressed pressures and equivalence ratios. Sensitivity analysis and reaction pathway analysis were performed on the basis of a detailed chemical kinetics model of binary fuel. Some conclusions can be obtained as follows: (1) Ignition delay times of n-butanol/ methyl hexanoate increase with the compressed pressure decreases, show clearly three-stage combustion characteristics with the temperature, and behave as NTC phenomena. (2) The increasing of the n-butanol fraction will contribute to the increase of ignition delay times and alleviate the knock tendency. (3) Both methyl hexanoate and n-butanol of blend fuels show two-stage heat release at lower compressed temperatures but single-stage heat release at higher compressed temperatures. (4) n-Butanol addition has a more significant impact on the low-temperature reaction pathways of methyl hexanoate and little effect on the high-temperature reaction pathways.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-34206039. Fax: +86-21-34205949. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5521

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ACKNOWLEDGMENTS This work was sponsored by the Shanghai Pujiang Program (14PJD021) and the National Basic Research Program (Grant 2013CB228405)

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dx.doi.org/10.1021/ef5010489 | Energy Fuels 2014, 28, 5515−5522