- Email: [email protected]
Autoignition study of methyl decanoate using a rapid compression machine
Fuel 266 (2020) 117060
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Autoignition study of methyl decanoate using a rapid compression machine ⁎
T
Wenyu Wang, Liang Yu , Yuan Feng, Yong Qian, Dehao Ju, Xingcai Lu Key Laboratory for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong University, Shanghai 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Methyl decanoate Ignition delay time Rapid compression machine Mechanism Kinetic analysis
Methyl decanoate (MD), a widely used surrogate of biodiesels, was investigated for its autoignition characteristics using a heated rapid compression machine (RCM). In this study, the ignition delay times (IDTs) of MD were measured at the compressed pressures of 5–20 bar, equivalence ratios varying from 0.53 to 1.60 and compressed temperatures of 633–855 K. An obvious two-stage ignition behavior was observed at low temperatures and typical negative temperature coefficient (NTC) phenomenon of total IDT was experimentally captured. The influences of compressed pressure, fuel and oxygen content, and nitrogen concentration on ignition delay times were systematically studied. The simulation results of Herbinet’s mechanism and Grana’s mechanism under variable volume simulation were compared with the experimental data. It is found that the two mechanisms qualitatively predict the autoignition features of MD but still need further optimization. In addition, reaction pathway analysis and sensitivity analysis were conducted to offer further insight into the low-to-intermediate temperature autoignition chemistry of MD.
1. Introduction With the increasing intensity of fossil energy usage, research on renewable energy has received increasing attention. As an emerging renewable energy source, biodiesel not only reduces oil extraction and imports but also relieves greenhouse gas and pollutant emissions, such as carbon monoxide, unburned hydrocarbons, and particulate matter [1–4]. Methyl decanoate is considered to be a favorable biodiesel surrogate with sufficient carbon chain length to mimic important longchain components in biodiesel such as C16-C22 fatty acid methyl esters (FAMEs) [5,6]. From a chemical point of view, MD possesses a carbomethoxy group which is the typical characteristic of biodiesel. Besides, MD has a cetane number (CN) of about 47 [7], at the lower bound of the CN range (45–60) of biodiesels. In addition, MD also has many similar reaction characteristics to actual biodiesel [8,9]. Herbinet et al. [9] first proposed a detailed mechanism for MD oxidation. The mechanism was validated against some engine data [10] and jet stirred reactor (JSR) results of rapeseed oil methyl ester (RME) [11]. Herbinet et al. stated that the mechanism captures the early characteristics of biodiesel oxidation, i.e. the characteristics of CO2 formation in the early stage. With the deepening of MD research, Glaude et al. [12,13] used EXGAS software to develop a new mechanism of FAMEs, and expanded the research scope of the mechanism to cover FAMEs from C7 to C19. Since the thermal decomposition of MD is very important for understanding the detailed mechanism of MD,
⁎
Herbinet et al. [14] conducted experimental research on MD decomposition based on JSR test platform, and used EXGAS software to generate the mechanism of MD decomposition. Seshadri et al. [6] used direct relation graph (DRG) method to simplify the Herbinet’s mechanism [9] and established a skeleton mechanism to simulate the ignition of MD in non-premixed flows. Sarathy et al. [15] improved the detailed mechanism of Herbinet’s [9] and reduced it to a skeleton mechanism using DRG method. Luo et al. [16,17] proposed a skeleton mechanism of MD covering low-to-high temperature conditions. Diévart et al. [18] developed a detailed mechanism of MD based on the mechanism of small FAMEs and validated the mechanism using a large number of experimental data. Grana et al. [19] extends the mechanism of MB [20] to develop a lumped mechanism of MD, which includes both high-temperature pyrolysis mechanism and low-temperature oxidation mechanism. In terms of fundamental combustion experiments, Szybist et al. [10] found that MD produced more CO2 than n-heptane and diesel during low-temperature (low-T) heat releasing stage in a motor engine. Glaude et al. [12] carried out an experimental study of JSR oxidation of FAMEs. Results reveal that C7-C11 FAMEs have similar reactivity, and their combustion characteristics are similar to those of linear alkanes. Seshadri et al. [6] studied the flameout and ignition of MD in non-premixed, non-uniform flows. Sarathy et al. [15] obtained experimental data from opposed-flow diffusion flame and used the data to validate their skeletal mechanism of MD. Wang et al. [21] investigated the
Corresponding author. E-mail address: [email protected] (L. Yu).
https://doi.org/10.1016/j.fuel.2020.117060 Received 15 September 2019; Received in revised form 9 December 2019; Accepted 10 January 2020 Available online 23 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
Fuel 266 (2020) 117060
W. Wang, et al.
compressed temperatures (TC). The compressed pressure (PC) was measured by a Kistler 6125B transducer combined with a Kistler 5015 charge-amplifier. According to the adiabatic core assumption, heat dissipation only occurs between the thermal boundary layer and the wall, and the core region of combustion chamber can be deemed as isentropic [38,39]. Therefore, the compressed temperature can be calculated by the following formula:
oxidation of three surrogate biodiesel fuels including MD in a counterflow configuration with laminar premixed and non-premixed flames. Pyl et al. [22] studied the thermal decomposition of MD in a pyrolysis set-up equipped with a dedicated on-line analysis section, which enables quantitative and qualitative on-line analyses of the entire reactor effluent. The reactor temperature was varied from 873 K to 1123 K at a fixed pressure of 1.7 bar for both high (N2/MD = 10) and low (N2/ MD = 0.6) nitrogen dilution. Haylett et al. [23] gave shock tube test results of MD at high temperatures (1185–1308 K), low pressure (about 8 atm), argon dilution (21%), and lower equivalence ratios (Φ = 0.09–0.17). Wang and Oehlschlaeger [24] studied the autoignition characteristics of MD/air mixtures in a heated shock tube at pressures of 15–16 atm, over a wide temperature range of 653–1336 K and multiple equivalence ratios of 0.5, 1 and 1.5. Campbell et al. [25,26] measured IDTs of a variety of biodiesel surrogates including MD using an aerosol shock tube. Reflected shock conditions covered initial temperatures from 1026 to 1388 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.3 to 1.4. Zhai et al. [27] investigated the pyrolysis of MD in a flow reactor at the pressures of 30 and 760 Torr and the temperatures ranging from 773 to 1198 K. Talukder et al. [28] measured laminar flame speeds and Markstein lengths of MD-air mixtures at elevated pressures and temperatures in outwardly propagating spherical flames. According to the above introduction, the previous experiments of MD focus on oxidation in the high-temperature range, and laboratory data at low-to-intermediate temperatures is urgently needed to facilitate the validation of low-temperature mechanism. This paper gives the experimental data of the ignition delay times of MD under low-tointermediate temperatures measured by a rapid compression machine. Besides, Herbinet’s mechanism [9] and Grana’s mechanism [19] were validated in this study and were used to mimic and analyze MD autoignition. Table 1 summarizes the existing experimental and modeling study on MD.
TC γ (T ) dT P ln ⎛ c ⎞ = ∫ ⎝ P0 ⎠ T0 γ (T ) − 1 T ⎜
⎟
In the formula, γ represents the specific heat ratio of the gaseous mixture, which can be derived from the thermodynamic data of methyl decanoate, nitrogen, and oxygen. TC is calculated by the measured compressed pressure (PC), initial pressure (P0), and initial temperature (T0) in the case of a certain mixture composition. In order to reproduce accurately the compression process and heat dissipation of RCM experiments, N2 was used to replace O2 of the reactant mixtures to obtain non-reactive pressure curves which subsequently are transformed into variable volume profiles that will be used in simulation. 2.2. Test conditions The methyl decanoate (CAS 110-42-9) used in this experiment was provided by Aladdin with a purity greater than 99%. The purity of both nitrogen and oxygen is greater than 99.999%. Preheating must be performed to obtain a fuel concentration that satisfies the requirements of the gas-phase experiment. The preheating temperature is 120-140℃ depending on the equivalence ratio. In this experiment, the maximum partial pressure of methyl decanoate in the mixing tank is less than half of the saturated vapor pressure [40], ensuring that the fuel can be completely vaporized. Before the experiment, liquid fuel was injected into the evaporated mixing tank with a syringe, and the mass was measured by an analytical balance with a minimum scale of 0.001 g. Nitrogen and oxygen are then added to the tank controlled precisely by a pressure gauge. Finally, the mixing tank was placed for more than 4 h to ensure uniformity. Table 2 lists the composition of the reactant mixture and the conditions of this experiment. The variation of IDT with temperature, pressure and component content was systematically studied. Each temperature point is repeated at least 3 times to ensure that IDT error is
2. Experiment and simulation specification 2.1. Experiment specification The IDTs of MD were studied in a heated rapid compression machine that has been used to measure ignition delay times of various fuels [29–37]. In this study, the compression ratio (CR) was changed by adjusting the length of the connecting rod to obtain different Table 1 Summary of the fundamental combustion experimental and modeling study on MD. Researchers
Mechanisms
Experiments
Herbinet et al. [9] Glaude et al. [12,13] Herbinet et al. [14] Seshadri et al. [6]
Detailed mechanism of MD Detailed mechanism using EXGAS (C7 to C19 FAMEs) Detailed mechanism of MD using EXGAS software Reduced mechanism of Herbinet’s mechanism [9] using DRG method Improved version of Herbinet’s mechanism [9]; skeleton mechanism using DRG method. Skeleton mechanism reduced from Herbinet’s mechanism [9] using DRG derived method Detailed mechanism of MD Lumped mechanism of MD
JSR oxidation (1.06 bar, 500–1100 K, residence times 1.5 s)
Sarathy et al. [15] Luo et al. [16,17] Diévart et al. [18] Grana et al. [19] Wang et al. [21]
Laminar flame speeds and local extinction strain rates in counterflow configuration (MD/air mixture, 1 atm, 403 K) Thermal decomposition in pyrolysis set-up (1.7 bar, 873 K–1123 K, N2/MD = 10, 0.6) ST autoignition (1185–1308 K, about 8 atm, Φ = 0.09–0.17, 21% argon dilution) ST autoignition (MD/air mixtures, 653–1336 K, 15–16 atm, Φ = 0.5, 1 and 1.5) ST autoignition (1026–1388 K, 3.5 and 7.0 atm, Φ = 0.3–1.4) Pyrolysis in flow reactor (30 and 760 Torr, 773–1198 K, 1% MD, 99% Ar) Flame speed in constant volume combustion chamber (423 K and 473 K, 2–6 atm) RCM autoignition (5–20 bar, 633–855 K, Φ = 0.53–1.6)
Pyl et al. [22] Haylett et al. [23] Wang and Oehlschlaeger [24] Campbell et al. [25,26] Zhai et al. [27] Talukder et al. [28]
JSR thermal decomposition (106.6 kPa, 773–1123 K, residence times 1 and 4 s) extinction and ignition in laminar non-premixed flows of counterflow configuration Opposed-flow diffusion flame (1 atm, 1.8% MD, 42% O2)
Detailed mechanism for MD pyrolysis
This study
2
Fuel 266 (2020) 117060
W. Wang, et al.
Table 2 Reactant mixtures and experimental conditions used in this study. No.
Equivalence ratio
N2/O2
MD mole content
O2 mole content
PC (bar)
1 2 3 4 5
0.53 0.75 (dilute) 1.07 1.60 0.75
3.76 5.66 8.52 8.52 3.76
0.7192 0.7192 0.7192 1.075 1.004
20.9 14.9 10.4 10.4 20.8
15,10,7 15,10,7 20,15,10,7 15,10,7 11,10,7.2,7,5
less than 5%, and an experimental pressure trace closest to the average was selected as a representative one. All the reported IDTs are obtained from the representative pressure traces. 2.3. Simulation specification
Fig. 2. The total IDT and first-stage IDT of MD at the condition of Φ = 1.07 and PC = 10 bar.
Two mechanisms are adopted in this paper. Herbinet’s mechanism [9] consists of 2878 species and 8555 reactions. It was developed on the basis of n-decane mechanism, which has already been validated against IDT data and OH evolution profiles in shock tube experiments [41,42]. According to the author, the overall reactivity of MD has many similarities to that of n-decane. In general, MD oxidation is mainly dominated by the long alkyl chain structure while the presence of ester group limits the low-temperature reactivity. Another mechanism adopted is proposed by Grana et al. [19], with a size of 350 species and 1000 reactions. This lumped mechanism is extended from the methyl butyrate model via layered and modular methods. Grana et al. demonstrated that despite significant simplification, the model was able to correctly reproduce the pyrolysis and oxidation of MD, in both low-temperature and flame conditions. CHEMKIN PRO software [43] is used for simulation. The volume profiles calculated from non-reactive pressure curves are imported to calculated the IDT, which can be obtained from the attachment.
stage ignition. The uncertainty of the measured IDT is estimated to be less than 15%. 3.1. Dependence on temperature Fig. 2 shows the Arrhenius plots of total IDT and first-stage IDT at a pressure of 10 bar and an equivalence ratio of 1.07. It is shown that the NTC behavior of total IDT begins at the temperature of about 730 K, similar to that of n-decane [44]. With the increase of temperature, the total IDT decreases in the low-T region, rises in the NTC region, and decreases again in the intermediate-T region. The IDTs at low-T region are approximately distributed in a straight line in the Arrhenius-type plot, but the first-stage IDT exhibits a larger slop compared to the total IDT. Another important finding is that the first-stage IDT still rapidly decreases with rising temperature at the early stage of the NTC region. Fig. 3 compares the pressure curves at different temperature ranges under the same condition of Fig. 2. In the low-T range (T < 729.5 K), it is obvious that the first-stage ignition moment gradually approaches the end of compression (EOC) point as temperature rises. At the same time, the pressure rise in the first-stage ignition is almost unaffected (about 1 bar). In the NTC region (729.5–801.4 K), as the temperature increases, the first-stage pressure rise gradually decreases until it no longer occurs, which indicates a transition of the reaction pathway. At the intermediate-T zone, similar to the low-T zone, the total IDT continues to decrease with rising temperature, while no two-stage ignition phenomenon occurs. Fig. 4 compares Wang and Oehlschlaeger's shock tube experimental
3. Results and discussion The typical pressure curves obtained from the RCM experiment are shown in Fig. 1. The time before 0 indicates the compression process, and the moment of the maximum first derivative of the pressure is the ignition time. It can be seen that MD exhibits similar ignition phenomena as macromolecular alkanes [44], that is, two stages of ignition at low temperatures. τ1 is the first-stage ignition delay time and τ donates the total ignition delay time. The first-stage ignition timepoint is defined as the local dp/dt maximum prior to the final ignition. In the case of NTC and intermediate temperatures, MD only shows a single-
Fig. 1. The definition of MD ignition delay time in RCM experiment at the condition of Φ = 1.07 and PC = 15 bar. (a) Two-stage ignition at TC = 654.9 K; (b) Singlestage ignition at TC = 785.8 K. 3
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 3. Experimental pressure curves of MD autoignition at the condition of Φ = 1.07 and PC = 10 bar.
curves almost coincide before first-stage ignition, which indicates that the compression process can be well reproduced by the simulation. The simulated first-stage IDTs of Herbinet’s mechanism are closer to the experimental result compared to those of the Grana’s mechanism. However, the ignition intensity of the first-stage ignition of the simulation is greater than that of the experiment, and the simulated firststage pressure rise is also greater than that of the experimental measurement. This phenomenon may be due to the increase of heat dissipation caused by the sudden temperature rise during the first-stage ignition process, which cannot be adequately addressed by the simulation. The total ignition delay time of the experiment is slightly larger than the simulated value of the Herbinet’s mechanism but the difference is marginal. 3.2. Dependence on compressed pressure Fig.4. Comparison of the measured IDTs of shock tube by Wang and Oehlschlaeger [24] and that of the RCM in this study.
Fig. 6 compares the experimental and simulated total IDTs and firststage IDTs with varying temperature under different pressures. Obviously, it’s shown that the total IDT and the first-stage IDT decrease with the increase of pressure. The increase of pressure increases the concentration of the reactants, therefore accelerates the autoignition. It is shown that the total IDT is more sensitive to pressure change at NTC region where the dependence of IDT on pressure increases with the rise of temperature. Although the variation of first-stage IDT with varying temperature is larger than that of the total IDT, the dependence of firststage IDT on pressure is much weaker than that of the total IDT. That is to say that the first-stage IDT is less sensitive to pressure than the total IDT. In the low-temperature region, the simulation results of Herbinet’s
data [24] and the new RCM IDTs of MD in this study. It can be seen that this study is an important complement to the existing data. Besides, it is noteworthy that compared with shock tube data, NTC phenomenon in this study finished at a lower temperature (about 800 K), while that of shock tube data ends at about 900 K. This difference can be attributed to facility effect. Fig. 5 is a comparison of the experimental pressure curves and the variable volume simulation results at a compressed pressure of 10 bar, an equivalence ratio of 1.07, and compressed temperatures of 669.2 K and 775.7 K. It can be seen that the measured and simulated pressure
Fig. 5. Experimental and simulated pressure curves of MD autoignition at the condition of Φ = 1.07 and PC = 10 bar, (a) TC = 669.2 K and (b) 775.7 K. 4
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 6. RCM-measured and simulated total IDTs and first-stage IDTs at different compressed pressures for equivalence ratios of 0.53 (a, b), 0.75 (c, b), 1.07 (e, f) and 1.60 (g, h). Symbol: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
mechanism are basically consistent with the experimental results in both total and first-stage IDTs under different pressures. In contrast, the simulated IDTs using the Grana’s mechanism is obviously larger than
the experimental results. In the NTC region, however, Grana’s mechanism predicts the experimental results better than Herbinet’s mechanism which obviously underpredicts the total IDTs of MD. This 5
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 7. RCM-measured and simulated total and first-stage IDTs at different oxygen mole fractions of 10.4%, 14.9%, 20.9% for the compressed pressures of 15 (a, b), 10 (c, d) and 7.0 (e, f) bar. The mole fraction of MD is kept at 0.7192%. Symbols: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
14.9%, and 20.9%, respectively. Obviously, it is shown in Fig. 7 that both the total and first-stage IDTs decrease with the increase of O2 content at the investigated conditions. It is worth noting that at low temperatures, O2 content has little effect on the IDT. For example, at the pressure of 10 bar and temperature of about 645.5 K, the total IDT of 14.9% O2 mixture is only 1% lower than that of 10.4% O2 mixture, and the difference in first-stage IDT is only about 7%. As the temperature increases, the effect of O2 content on IDT begins to increase. For example, in the NTC region, the oxygen content of the mixture with an equivalence ratio of 0.53 (756.7 K) is 1.43 times that of the mixture with an equivalence ratio of 0.75 (757.9 K), while the IDT difference is
result is consistent with the results of Wang's shock tube experiment [24].
3.3. Dependence on O2 and fuel content The reactions of molecule oxygen addition to fuel radicals and hydrogen free radical are key pathways for low-T and intermediate-T autoignition chemistry. Fig. 7 compares the total and first-stage IDTs of MD with equivalence ratios of 0.53, 0.75 and 1.07 at compressed pressures of 15, 10 and 7 bar. The three reactant mixtures keep the same fuel content of 0.7192%, while their oxygen contents are 10.4%, 6
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 8. RCM-measured and simulated total IDTs and first-stage IDTs at different fuel mole fractions of 0.7192% and 1.075% for the compressed pressure of 15 (a, b), 10 (c, d) and 7.0 (e, f) bar. The mole fraction of oxygen is kept at 10.4%. Symbol: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
longer than the experimental results, which is responsible for the overpredicted total IDT. Fuel molecule directly affects the rate of the H-atom abstraction reaction of methyl decanoate to form methyl decanoate radical, which plays an important role in MD oxidation. Fig. 8 compares the Arrhenius plots of the MD ignition delay time at equivalence ratios of 1.07 and 1.60 with the compressed pressures of 15, 10 and 7.0 bar. The two reactant mixtures maintain the same oxygen content of 10.4%, while the fuel mole fraction is 0.7192% and 1.075%, respectively. It can be seen that the increase of fuel content results in a decrease in ignition delay time. Besides, with the increase of temperature, the dependence
nearly twice. This is because at the NTC region, the changes of O2 content not only influence the reaction rate of O2 relevant reactions but also influence reaction pathways (low-T chain branching pathways or chain prorogation pathways). We can see from the simulation results that the two mechanisms can well capture this rule, that is, the increase of oxygen content has little effect on IDT at low temperatures, but a larger influence at the NTC temperatures. Herbinet’s mechanism predicts total IDTs better at the low-T region and Grana’s mechanism performed better at NTC and intermediate-T region. In addition, Herbinet’s mechanism well predicts the first-stage IDT of MD. As a contrast, the simulated first-stage IDTs of Grana’s mechanism are significantly 7
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 9. Experimental pressure histories of MD autoignition at the condition of PC = 10 bar, (a) XMD = 0.7192%, TC = 640.8 K, and XMD = 1.075%, TC = 642.7 K; (b) XMD = 0.7192%, TC = 775.4 K and XMD = 1.075%, TC = 778.9 K. The mole fraction of oxygen is kept at 10.4%.
and O2 contents of Φ = 0.75 (N2/O2 = 3.76) mixture is 1.4 times greater than that of Φ = 0.75 (N2/O2 = 5.66) mixture. Therefore, the former mixture at a compressed pressure of P (5 bar, 7.2 bar or 11 bar) and the latter mixture at a compressed pressure of 1.4 P (7 bar, 10 bar, or 15 bar) have the same fuel and O2 concentration but different N2 concentration. Although the two cases of having different pressure, the higher pressure of the latter mixture is caused by the excessive N2. The comparison of IDTs of the above two cases can therefore be used to reveal the influence of N2 on MD autoignition. Only two effects of N2 need to be considered: one is chemical effect and the other is thermal effect. For the aspect of chemical effect, nitrogen only participates in elementary reactions as a third body. As for thermal effect, the latter mixture contains more N2 than the former mixture, therefore has a larger heat capacity. For the same heat release, the temperature rise of the latter will be smaller than that of the former, which will lead to IDT difference of the two cases. The experimental and simulated IDTs of the two cases are shown in Fig. 11. It is found that the IDTs of the higher N2 concentration case are longer than those of the lower N2 concentration case, where the trend is more apparent in the NTC region than in the low-T region. In addition, note that the firststage IDT is very insensitive to N2. The IDT dependence on N2 can be well captured by the two mechanisms, and Grana’s mechanism shows obviously better performance than Herbinet’s mechanism. There is no doubt that thermal effect is primarily responsible for this result, and the reaction promoting effect of chemical effect is completely obscured. At the condition that the reactant concentration is constant, the higher N2 concentration case ignites later compared to the lower N2 concentration
of IDT on fuel content rises, which is similar to the IDT dependence on oxygen content. At temperatures lower than NTC, the reactivity is governed by the low-T branching reaction pathways in which fuel molecule participates in H-abstraction and O2-addition reactions. In the NTC region, fuel content plays a more important role through reaction sequence of MD + HO2 = RMDX + H2O2 and H2O2 (+M) = OH + OH (+M) therefore have a greater influence on ignition delay time. Simulation results of both mechanisms can correctly reflect the effect of fuel content on MD IDT, including the enhanced IDT dependence in the NTC region. Fig. 9 compares the pressure curves of MD autoignition at different fuel mole fraction. It is found that the mixture with an equivalence ratio of 1.6 has a more intense first-stage ignition and hot ignition at the temperature of about 640 K. At the temperature of about 777 K, there is no large difference in pressure rise between different fuel concentrations. Fig. 10 compares Arrhenius plots of MD IDTs at equivalence ratios of 0.53 and 0.75 with the compressed pressure of 10 bar and 7.0 bar. Obviously, despite the equivalence ratios of the two mixtures are less than 1, the dependence on fuel content displays a similar regulation with that at equivalence ratios of 1.07 and 1.60. 3.4. Influences of N2 on MD autoignition The influence of N2 on fuel autoignition is an interesting research point, but it is often not easy to be separated from experimental results. In this study, we design an experimental scheme to investigate the influence of N2 on MD autoignition. As is interpreted in Table 2, the fuel
Fig.10. RCM-measured and simulated total IDTs and first-stage IDTs at equivalence ratios of 0.75 and 0.53 with the compressed pressure of 10 bar (a) and 7.0 bar (b). Symbol: experimental data; solid line: results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism. 8
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 11. Comparison of RCM-measured and simulated IDTs at the same fuel and O2 concentrations but different N2 concentration. Symbol: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
4. Kinetic analysis
due to its higher heat capacity. The conclusion is also supported by the comparison of pressure curve, as shown in Fig. 12. As stated above, those higher pressure cases can be seen as the addition of N2 to those lower pressure cases. It is shown that after the addition of nitrogen, both the total and first-stage IDTs increase. The lower N2 concentration case exhibits a much stronger first-stage pressure rise than the higher N2 concentration case, which is a reflection of thermal effect, since the two cases should have similar low-T heat release. Besides, it is interesting that although the compressed pressures are different, their maximum pressure is very close.
4.1. Reaction pathway analysis To clarify the main reaction pathways and important intermediates during autoignition, a reaction path analysis based on Grana’s mechanism is carried out. The reaction pathways at the time point of 10% fuel consumption for three temperatures (650 K, 780 K, and 850 K) were compared at an equivalence ratio of 1.07 and a pressure of 10 bar. The results are shown in Fig. 13. The arrows represent the direction of the reaction, and the numbers in different colors on the arrows
9
Fuel 266 (2020) 117060
W. Wang, et al.
650 K, the proportion of this reaction is less than 0.1% and only 0.8% at 850 K. At 650 K, almost all QMDOOH continues to combine with an oxygen molecule to form peroxy-alkylhydroperoxy radical (ZMDOOH) which then decomposes into OH and Ketohydropreoxide (MDKETO). Finally, MDKETO decomposes into several species, including at least two free radicals, which is the only branching step of the low-T oxidation pathways. According to the site of the CeC bond breaking, half of the MDKETO decomposes into C7H17COCHO, CH3OCO and one OH radical. Another half decomposes into OH, RMBX, Aldehyde and other small radicals. At NTC and intermediate temperatures, the importance of the low-temperature pathways decreases significantly. For example, about 82.8% of MD at 650 K is finally converted to MYOKETO, while at 780 K, the proportion is reduced to 58.0%, and at 850 K, only 31.6% remains. At NTC and intermediate temperatures, apart from the low-T O2addition pathway, QMDOOH radical has another three types of propagation pathways, as depicted in Fig. 13. At low temperatures, the proportions of those pathways are negligible. For example, at 650 K, the sum of the proportion of the three types of reactions is less than 1%. At 780 K, the proportion increases to 21.3%, and at 850 K, the proportion reaches nearly half (48.3%). The occurrence of those reaction pathways disturbed the low-T branching pathways and suppresses the formation of reactive free radicals, which is the main cause of NTC behavior of ignition delay time. Due to the existence of the ester group, RMDX radical has a stronger tendency to undergo reaction RMDX + O2 = UME10 + HO2 compared to n-decane. This is one of the reasons why the low-T reactivity of MD is weaker than n-decane. Nevertheless, a proportion of 16.4% seems too large at the low temperature of 650 K. As a contrast, the simulation result of Glaude mechanism on JSR oxidation at the same temperature is only 5.6% [12]. Even the two simulations were performed based on different apparatus, such a big difference is quite unusual. Considering the fact that Grana’s mechanism generally overpredicts the low-T IDTs of MD, as discussed in Section 3, there is a great possibility that the rate of this reaction is
Fig. 12. Experimental pressure histories of MD autoignition for an equivalence ratio of 0.75 at the condition of PC = 10.7 bar, TC = 662.6 K, N2/O2 = 3.76 and PC = 15 bar, TC = 661.2 K, N2/O2 = 5.66.
represent the proportion of the reactant consumption through corresponding reactions. On the whole, the reaction pathway of MD oxidation is very similar to that of long-chain n-alkanes. Firstly, a hydrogen atom of MD molecule is abstracted by free radicals such as OH, CH3, and H, forming RMDX free radicals, among which H-atom abstraction reactions via OH radical are dominant. With the increase of temperature, the proportion of OH radical H-atom abstraction decreases. There are three pathways to consume RMDX. The dominant pathway is oxygen molecule addition to RMDX to form alkyl-ester peroxy radicals (RMDOOX), and RMDOOX then isomerizes to hydroperoxy alkyl-ester radicals (QMDOOH) via inner H transformation. Another important reaction pathway is the H-atom abstraction of RMDX by oxygen, producing methyl decenoate (UME10) and HO2. In addition, RMDX also undergoes β-decomposition to form a series of olefins and small molecular esters directly, which mainly occurs at high temperatures. At
Fig. 13. Reaction pathway analysis of MD autoignition at 10% conversion of fuel for the equivalence ratio of 1.07 and pressure of 10 bar at temperatures of 650 K, 780 K and 850 K by constant volume simulation of Grana’s mechanism. 10
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 14. Sensitivity analysis of total IDT performed at temperatures of 650 K and 780 K for an equivalence ratio of 1.07 and a pressure of 10 bar.
Fig. 15. Sensitivity analysis of first-stage IDT performed at temperatures of 650 K for an equivalence ratio of 1.07 and a pressure of 10 bar.
pre-exponential factor of ith reaction. Si is the sensitivity coefficient of IDT on the ith reaction. According to the definition, a positive sensitivity indicates that the reaction inhibits autoignition, while a negative sensitivity indicates that the reaction promotes ignition. The larger the absolute value of Si, the greater the promotion or inhibition effect of the reaction on autoignition. Fig. 14 shows the reactions with large sensitivity on total IDT at temperatures of 650 K and 780 K, respectively. It can be seen that at both temperatures, O2 + RMDX ⇒ HO2 + UME10 has the largest positive sensitivity. The enhancement of this reaction will reduce the proportion of O2 addition reactions and subsequent low-temperature pathways, which greatly reduces the reactivity and therefore suppresses
overestimated, and further optimization is needed. 4.2. Sensitivity analysis In order to find out specific reaction controlling MD autoignition, sensitivity analysis of IDT was carried out based on Grana’s mechanism at 650 K and 780 K. CHEMKIN PRO software and constant volume model are used for the calculation. Sensitivity is defined as follows:
Si =
τ (2ki ) − τ (ki ) τ (ki )
In the formula, τ represents ignition delay time, and ki represents the 11
Fuel 266 (2020) 117060
W. Wang, et al.
MD autoignition. At 650 K, ignition promoting reactions includes the isomerization reaction RMDOOX ⇒ QMDOOOH, decomposition reaction ZMDOOOOH ⇒ OH + MDKETO and a series of branching reactions such as MDKETO ⇒ OH + C2H5CHO + CH2CO + 1/6 RMDX + 5/6 RMBX. The enhancement of those reactions will significantly accelerate the autoignition. At 780 K, the sensitivity of IDT on reaction O2 + RMDX ⇒ HO2 + UME10 increases significantly, while the two important low-T branching reactions MDKETO ⇒ OH + CH3CHO + C2H4 + CH2CO + RMBX and MDKETO ⇒ OH + C2H5CHO + CH2CO + 1/6RMDX + 5/6RMBX exhibit negligible sensitivity. It is found that the sensitivities of all ignition-inhibiting reactions at 780 K are larger than those at 650 K, which agrees with the observation that the influence of compressed pressure, oxygen and fuel concentration in NTC region are larger than that in low-temperature region. In addition, it is shown in Fig. 15 that at 650 K almost all the important reactions of first-stage IDT are consistent with those of the total IDT.
Feng: Validation, Formal analysis. Yong Qian: Investigation, Data curation. Dehao Ju: Investigation. Xingcai Lu: Supervision, Project administration, Funding acquisition.
5. Conclusions
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117060.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51961135105) and China Postdoctoral Science Foundation (No. 2019M660087). Appendix A. Supplementary data
In this paper, the autoignition characteristics of methyl decanoate under low-to-intermediate temperatures were studied at wide compressed pressures and equivalence ratios. The experiment used a heated RCM to study the total IDT and the first-stage IDT with compressed pressures of 5–20 bar, equivalence ratios of 0.53–1.60, and temperatures of 625–855 K. Sufficient low-T IDT data and complete NTC region of methyl decanoate autoignition were obtained. It is found that methyl decanoate exhibits two-stage ignition characteristics under low temperatures and only one-stage ignition under intermediate temperatures. Variable volume simulation was carried out based on Herbinet’s and Grana’s mechanisms. In the low-T region, the simulation results of Herbinet’s mechanism is closer to the experiment results than the Grana’s in both the total IDT and the first-stage IDT, but the trend is reversed in the intermediate temperature region. Furthermore, the dependence of IDT on compressed pressure, oxygen and fuel content, equivalence ratios and N2 concentration are studied. The negative relation between IDT and compressed pressure, oxygen, and fuel content is concluded. It is found that the dependence of first-stage IDT on pressure is much weaker than that of the total IDT. In the low-temperature region, oxygen and fuel content have little effect on the total IDT and the first-stage IDT. As the temperature rises, the effects of oxygen and fuel concentration IDT increase. In the case where the fuel and O2 concentration is constant, the higher N2 concentration results in longer total IDTs, where the trend is more apparent in the NTC region than in the low-T region, which can be attributed to thermal effect. In addition, the first-stage IDT is found to be insensitive to N2 concentration. Reaction pathway analysis and sensitivity analysis based on Grana’s mechanism are performed at different temperatures. Results show that the reaction pathways of MD are very similar to those of n-alkanes. It believed that the rate rule of reaction RMDX + O2 = UME10 + HO2 may be overestimated, which may be responsible for the overprediction of Grana’s mechanism on the low-T IDT of MD. It is found that most of the ignition promoting reactions at 650 K are the reactions involved in the low-temperature branching pathways. Reaction O2 + RMDX ⇒ HO2 + UME10 has the largest positive sensitivity, because it disturbs the low-T branching pathway. Besides, the increased sensitivity of all ignition inhibiting reactions at 780 K agrees with the increased influence of compressed pressure, oxygen and fuel content on the IDT in the NTC region compared to that in the low-T region.
References [1] Demirbas A. Importance of biodiesel as transportation fuel. Energy Policy 2007;35:4661–70. [2] Lai JYW, Lin KC, Violi A. Biodiesel combustion: advances in chemical kinetic modeling. Prog Energy Combust Sci 2011;37:1–14. [3] Tsolakis A, Megaritis A, Wyszynski M, Theinnoi K. Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation). Energy 2007;32:2072–80. [4] Agarwal AK. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog Energy Combust Sci 2007;33:233–71. [5] Dooley S, Curran HJ, Simmie JM. Autoignition measurements and a validated kinetic model for the biodiesel surrogate, methyl butanoate. Combust Flame 2008;153:2–32. [6] Seshadri K, Lu T, Herbinet O, Humer S, Niemann U, Pitz WJ, et al. Experimental and kinetic modeling study of extinction and ignition of methyl decanoate in laminar non-premixed flows. Proc Combust Inst 2009;32:1067–74. [7] Murphy MJ, Taylor JD, Mccormick RL. Compendium of Experimental Cetane Number Data. Office of Scientific & Technical Information Technical Reports; 2004. [8] Pitz WJ, Westbrook CK, Herbinet O. Chemical kinetic modeling of advanced transportation fuels. Fuel Injection; 2009. [9] Herbinet O, Pitz WJ, Westbrook CK. Detailed chemical kinetic oxidation mechanism for a biodiesel surrogate. Combust Flame 2008;154:507–28. [10] Szybist JP, Boehman AL, Haworth DC, Koga H. Premixed ignition behavior of alternative diesel fuel-relevant compounds in a motored engine experiment. Combust Flame 2007;149:112–28. [11] Dagaut P, Gaı¨L S, Sahasrabudhe M. Rapeseed oil methyl ester oxidation over extended ranges of pressure, temperature, and equivalence ratio: experimental and modeling kinetic study. Proc Combust Inst 2007;31:2955–61. [12] Glaude PA, Herbinet O, Bax S, Biet J, Warth V, Battin-Leclerc F. Modeling of the oxidation of methyl esters-Validation for methyl hexanoate, methyl heptanoate, and methyl decanoate in a jet-stirred reactor. Combust Flame 2010;157:2035–50. [13] Herbinet O, Biet J, Hakka MH, Warth V, Glaude PA, Nicolle A, et al. Modeling study of the low-temperature oxidation of large methyl esters from C11 to C19. Proc Combust Inst 2011;33:391–8. [14] Herbinet O, Glaude PA, Warth V, Battin-Leclerc F. Experimental and modeling study of the thermal decomposition of methyl decanoate. Combust Flame 2011;158:1288–300. [15] Sarathy SM, Thomson MJ, Pitz WJ, Lu T. An experimental and kinetic modeling study of methyl decanoate combustion. Proc Combust Inst 2011;33:399–405. [16] Luo Z, Lu T, Maciaszek MJ, Som S, Longman DE. A reduced mechanism for hightemperature oxidation of biodiesel surrogates. Energy Fuels 2010;24:656–60. [17] Luo Z, Plomer M, Lu T, Som S, Longman DE. A reduced mechanism for biodiesel surrogates with low temperature chemistry for compression ignition engine applications. Combust Theor Model 2011;16:369–85. [18] Diévart P, Won SH, Dooley S, Dryer FL, Ju Y. A kinetic model for methyl decanoate combustion. Combust Flame 2012;159:1793–805. [19] Grana R, Frassoldati A, Saggese C, Faravelli T, Ranzi E. A wide range kinetic modeling study of pyrolysis and oxidation of methyl butanoate and methyl decanoate – Note II: lumped kinetic model of decomposition and combustion of methyl esters up to methyl decanoate. Combust Flame 2012;159:2280–94. [20] Grana R, Frassoldati A, Cuoci A, Faravelli T. Ranzi E, A wide range kinetic modeling study of pyrolysis and oxidation of methyl butanoate and methyl decanoate. Note I: lumped kinetic model of methyl butanoate and small methyl esters. Energy 2012;43:124–39. [21] Wang YL, Feng Q, Egolfopoulos FN, Tsotsis TT. Studies of C4 and C10 methyl ester flames. Combust Flame 2011;158:1507–19. [22] Pyl SP, Van Geem KM, Puimège P, Sabbe MK, Reyniers M-F, Marin GB. A
CRediT authorship contribution statement Wenyu Wang: Software, Validation, Investigation, Data curation, Writing - original draft. Liang Yu: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition. Yuan 12
Fuel 266 (2020) 117060
W. Wang, et al.
[34] Yu L, Mao Y, Li A, Wang S, Qiu Y, Qian Y, et al. Experimental and modeling validation of a large diesel surrogate: autoignition in heated rapid compression machine and oxidation in flow reactor. Combust Flame 2019;202:195–207. [35] Yu L, Mao Y, Qiu Y, Wang S, Li H, Tao W, et al. Experimental and modeling study of the autoignition characteristics of commercial diesel under engine-relevant conditions. Proc Combust Inst 2019;37:4805–12. [36] Yu L, Wang S, Wang W, Qiu Y, Qian Y, Mao Y, et al. Exploration of chemical composition effects on the autoignition of two commercial diesels: Rapid compression machine experiments and model simulation. Combust Flame 2019;204:204–19. [37] Yu L, Wang W, Wang S, Feng Y, Qian Y, Lu X. An experimental and modeling study of n-hexadecane autoignition under low-to-intermediate temperatures. Sci China Technol Sci 2019. [38] Goldsborough SS, Santner J, Kang D, Fridlyand A, Rockstroh T, Jespersen MC. Heat release analysis for rapid compression machines: challenges and opportunities. Proc Combust Inst 2019;37:603–11. [39] Goldsborough SS, Hochgreb S, Vanhove G, Wooldridge MS, Curran HJ, Sung C-J. Advances in rapid compression machine studies of low- and intermediate-temperature autoignition phenomena. Prog Energy Combust Sci 2017;63:1–78. [40] https://app.knovel.com/web/poc/ms/profile.html?cid=kc9V1ZITRT&prop= prSVC. [41] Davidson DF, Herbon JT, Horning DC, Hanson RK. OH concentration time histories in n-alkane oxidation. Int J Chem Kinet 2001;33:775–83. [42] Pfahl U, Fieweger K, Adomeit G. Self-ignition of diesel-relevant hydrocarbon-air mixtures under engine conditions. Combust Inst 1996;26:781–9. [43] CHEMKIN-PRO 15131 R D, San Diego; 2013. [44] Kumar K, Mittal G, Sung CJ. Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions. Combust Flame 2009;156:1278–88.
comprehensive study of methyl decanoate pyrolysis. Energy 2012;43:146–60. [23] Haylett DR, Davidson DF, Hanson RK. Ignition delay times of low-vapor-pressure fuels measured using an aerosol shock tube. Combust Flame 2012;159:552–61. [24] Wang W, Oehlschlaeger MA. A shock tube study of methyl decanoate autoignition at elevated pressures. Combust Flame 2012;159:476–81. [25] Campbell MF, Davidson DF, Hanson RK. Ignition delay times of very-low-vaporpressure biodiesel surrogates behind reflected shock waves. Fuel 2014;126:271–81. [26] Campbell MF, Davidson DF, Hanson RK. Scaling relation for high-temperature biodiesel surrogate ignition delay times. Fuel 2016;164:151–9. [27] Zhai Y, Ao C, Feng B, Meng Q, Zhang Y, Mei B, et al. Experimental and kinetic modeling investigation on methyl decanoate pyrolysis at low and atmospheric pressures. Fuel 2018;232:333–40. [28] Talukder N, Lee KY. Laminar flame speeds and Markstein lengths of methyl decanoate-air premixed flames at elevated pressures and temperatures. Fuel 2018;234:1346–53. [29] Sun S, Yu L, Wang S, Mao Y, Lu X. Experimental and kinetic modeling study on selfignition of α-methylnaphthalene in a heated rapid compression machine. Energy Fuels 2017;31:11304–14. [30] Yu L, Wu Z, Qiu Y, Qian Y, Mao Y, Lu X. Ignition delay times of decalin over low-tointermediate temperature ranges: rapid compression machine measurement and modeling study. Combust Flame 2018;196:160–73. [31] Yu L, Sun S, Tao W, Xingcai L. n-Decane autoignition characteristics under low-tointermediate temperature. Combust Sci Technol 2017;23:325–30. [32] Wang S, Yu L, Wu Z, Mao Y, Li H, Qian Y, et al. Gas-phase autoignition of diesel/ gasoline blends over wide temperature and pressure in heated shock tube and rapid compression machine. Combust Flame 2019;201:264–75. [33] Yu L, Qiu Y, Mao Y, Wang S, Ruan C, Tao W, et al. A study on the low-to-intermediate temperature ignition delays of long chain branched paraffin: Iso-cetane. Proc Combust Inst 2019;37:631–8.
13
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Autoignition study of methyl decanoate using a rapid compression machine ⁎
T
Wenyu Wang, Liang Yu , Yuan Feng, Yong Qian, Dehao Ju, Xingcai Lu Key Laboratory for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong University, Shanghai 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Methyl decanoate Ignition delay time Rapid compression machine Mechanism Kinetic analysis
Methyl decanoate (MD), a widely used surrogate of biodiesels, was investigated for its autoignition characteristics using a heated rapid compression machine (RCM). In this study, the ignition delay times (IDTs) of MD were measured at the compressed pressures of 5–20 bar, equivalence ratios varying from 0.53 to 1.60 and compressed temperatures of 633–855 K. An obvious two-stage ignition behavior was observed at low temperatures and typical negative temperature coefficient (NTC) phenomenon of total IDT was experimentally captured. The influences of compressed pressure, fuel and oxygen content, and nitrogen concentration on ignition delay times were systematically studied. The simulation results of Herbinet’s mechanism and Grana’s mechanism under variable volume simulation were compared with the experimental data. It is found that the two mechanisms qualitatively predict the autoignition features of MD but still need further optimization. In addition, reaction pathway analysis and sensitivity analysis were conducted to offer further insight into the low-to-intermediate temperature autoignition chemistry of MD.
1. Introduction With the increasing intensity of fossil energy usage, research on renewable energy has received increasing attention. As an emerging renewable energy source, biodiesel not only reduces oil extraction and imports but also relieves greenhouse gas and pollutant emissions, such as carbon monoxide, unburned hydrocarbons, and particulate matter [1–4]. Methyl decanoate is considered to be a favorable biodiesel surrogate with sufficient carbon chain length to mimic important longchain components in biodiesel such as C16-C22 fatty acid methyl esters (FAMEs) [5,6]. From a chemical point of view, MD possesses a carbomethoxy group which is the typical characteristic of biodiesel. Besides, MD has a cetane number (CN) of about 47 [7], at the lower bound of the CN range (45–60) of biodiesels. In addition, MD also has many similar reaction characteristics to actual biodiesel [8,9]. Herbinet et al. [9] first proposed a detailed mechanism for MD oxidation. The mechanism was validated against some engine data [10] and jet stirred reactor (JSR) results of rapeseed oil methyl ester (RME) [11]. Herbinet et al. stated that the mechanism captures the early characteristics of biodiesel oxidation, i.e. the characteristics of CO2 formation in the early stage. With the deepening of MD research, Glaude et al. [12,13] used EXGAS software to develop a new mechanism of FAMEs, and expanded the research scope of the mechanism to cover FAMEs from C7 to C19. Since the thermal decomposition of MD is very important for understanding the detailed mechanism of MD,
⁎
Herbinet et al. [14] conducted experimental research on MD decomposition based on JSR test platform, and used EXGAS software to generate the mechanism of MD decomposition. Seshadri et al. [6] used direct relation graph (DRG) method to simplify the Herbinet’s mechanism [9] and established a skeleton mechanism to simulate the ignition of MD in non-premixed flows. Sarathy et al. [15] improved the detailed mechanism of Herbinet’s [9] and reduced it to a skeleton mechanism using DRG method. Luo et al. [16,17] proposed a skeleton mechanism of MD covering low-to-high temperature conditions. Diévart et al. [18] developed a detailed mechanism of MD based on the mechanism of small FAMEs and validated the mechanism using a large number of experimental data. Grana et al. [19] extends the mechanism of MB [20] to develop a lumped mechanism of MD, which includes both high-temperature pyrolysis mechanism and low-temperature oxidation mechanism. In terms of fundamental combustion experiments, Szybist et al. [10] found that MD produced more CO2 than n-heptane and diesel during low-temperature (low-T) heat releasing stage in a motor engine. Glaude et al. [12] carried out an experimental study of JSR oxidation of FAMEs. Results reveal that C7-C11 FAMEs have similar reactivity, and their combustion characteristics are similar to those of linear alkanes. Seshadri et al. [6] studied the flameout and ignition of MD in non-premixed, non-uniform flows. Sarathy et al. [15] obtained experimental data from opposed-flow diffusion flame and used the data to validate their skeletal mechanism of MD. Wang et al. [21] investigated the
Corresponding author. E-mail address: [email protected] (L. Yu).
https://doi.org/10.1016/j.fuel.2020.117060 Received 15 September 2019; Received in revised form 9 December 2019; Accepted 10 January 2020 Available online 23 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
Fuel 266 (2020) 117060
W. Wang, et al.
compressed temperatures (TC). The compressed pressure (PC) was measured by a Kistler 6125B transducer combined with a Kistler 5015 charge-amplifier. According to the adiabatic core assumption, heat dissipation only occurs between the thermal boundary layer and the wall, and the core region of combustion chamber can be deemed as isentropic [38,39]. Therefore, the compressed temperature can be calculated by the following formula:
oxidation of three surrogate biodiesel fuels including MD in a counterflow configuration with laminar premixed and non-premixed flames. Pyl et al. [22] studied the thermal decomposition of MD in a pyrolysis set-up equipped with a dedicated on-line analysis section, which enables quantitative and qualitative on-line analyses of the entire reactor effluent. The reactor temperature was varied from 873 K to 1123 K at a fixed pressure of 1.7 bar for both high (N2/MD = 10) and low (N2/ MD = 0.6) nitrogen dilution. Haylett et al. [23] gave shock tube test results of MD at high temperatures (1185–1308 K), low pressure (about 8 atm), argon dilution (21%), and lower equivalence ratios (Φ = 0.09–0.17). Wang and Oehlschlaeger [24] studied the autoignition characteristics of MD/air mixtures in a heated shock tube at pressures of 15–16 atm, over a wide temperature range of 653–1336 K and multiple equivalence ratios of 0.5, 1 and 1.5. Campbell et al. [25,26] measured IDTs of a variety of biodiesel surrogates including MD using an aerosol shock tube. Reflected shock conditions covered initial temperatures from 1026 to 1388 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.3 to 1.4. Zhai et al. [27] investigated the pyrolysis of MD in a flow reactor at the pressures of 30 and 760 Torr and the temperatures ranging from 773 to 1198 K. Talukder et al. [28] measured laminar flame speeds and Markstein lengths of MD-air mixtures at elevated pressures and temperatures in outwardly propagating spherical flames. According to the above introduction, the previous experiments of MD focus on oxidation in the high-temperature range, and laboratory data at low-to-intermediate temperatures is urgently needed to facilitate the validation of low-temperature mechanism. This paper gives the experimental data of the ignition delay times of MD under low-tointermediate temperatures measured by a rapid compression machine. Besides, Herbinet’s mechanism [9] and Grana’s mechanism [19] were validated in this study and were used to mimic and analyze MD autoignition. Table 1 summarizes the existing experimental and modeling study on MD.
TC γ (T ) dT P ln ⎛ c ⎞ = ∫ ⎝ P0 ⎠ T0 γ (T ) − 1 T ⎜
⎟
In the formula, γ represents the specific heat ratio of the gaseous mixture, which can be derived from the thermodynamic data of methyl decanoate, nitrogen, and oxygen. TC is calculated by the measured compressed pressure (PC), initial pressure (P0), and initial temperature (T0) in the case of a certain mixture composition. In order to reproduce accurately the compression process and heat dissipation of RCM experiments, N2 was used to replace O2 of the reactant mixtures to obtain non-reactive pressure curves which subsequently are transformed into variable volume profiles that will be used in simulation. 2.2. Test conditions The methyl decanoate (CAS 110-42-9) used in this experiment was provided by Aladdin with a purity greater than 99%. The purity of both nitrogen and oxygen is greater than 99.999%. Preheating must be performed to obtain a fuel concentration that satisfies the requirements of the gas-phase experiment. The preheating temperature is 120-140℃ depending on the equivalence ratio. In this experiment, the maximum partial pressure of methyl decanoate in the mixing tank is less than half of the saturated vapor pressure [40], ensuring that the fuel can be completely vaporized. Before the experiment, liquid fuel was injected into the evaporated mixing tank with a syringe, and the mass was measured by an analytical balance with a minimum scale of 0.001 g. Nitrogen and oxygen are then added to the tank controlled precisely by a pressure gauge. Finally, the mixing tank was placed for more than 4 h to ensure uniformity. Table 2 lists the composition of the reactant mixture and the conditions of this experiment. The variation of IDT with temperature, pressure and component content was systematically studied. Each temperature point is repeated at least 3 times to ensure that IDT error is
2. Experiment and simulation specification 2.1. Experiment specification The IDTs of MD were studied in a heated rapid compression machine that has been used to measure ignition delay times of various fuels [29–37]. In this study, the compression ratio (CR) was changed by adjusting the length of the connecting rod to obtain different Table 1 Summary of the fundamental combustion experimental and modeling study on MD. Researchers
Mechanisms
Experiments
Herbinet et al. [9] Glaude et al. [12,13] Herbinet et al. [14] Seshadri et al. [6]
Detailed mechanism of MD Detailed mechanism using EXGAS (C7 to C19 FAMEs) Detailed mechanism of MD using EXGAS software Reduced mechanism of Herbinet’s mechanism [9] using DRG method Improved version of Herbinet’s mechanism [9]; skeleton mechanism using DRG method. Skeleton mechanism reduced from Herbinet’s mechanism [9] using DRG derived method Detailed mechanism of MD Lumped mechanism of MD
JSR oxidation (1.06 bar, 500–1100 K, residence times 1.5 s)
Sarathy et al. [15] Luo et al. [16,17] Diévart et al. [18] Grana et al. [19] Wang et al. [21]
Laminar flame speeds and local extinction strain rates in counterflow configuration (MD/air mixture, 1 atm, 403 K) Thermal decomposition in pyrolysis set-up (1.7 bar, 873 K–1123 K, N2/MD = 10, 0.6) ST autoignition (1185–1308 K, about 8 atm, Φ = 0.09–0.17, 21% argon dilution) ST autoignition (MD/air mixtures, 653–1336 K, 15–16 atm, Φ = 0.5, 1 and 1.5) ST autoignition (1026–1388 K, 3.5 and 7.0 atm, Φ = 0.3–1.4) Pyrolysis in flow reactor (30 and 760 Torr, 773–1198 K, 1% MD, 99% Ar) Flame speed in constant volume combustion chamber (423 K and 473 K, 2–6 atm) RCM autoignition (5–20 bar, 633–855 K, Φ = 0.53–1.6)
Pyl et al. [22] Haylett et al. [23] Wang and Oehlschlaeger [24] Campbell et al. [25,26] Zhai et al. [27] Talukder et al. [28]
JSR thermal decomposition (106.6 kPa, 773–1123 K, residence times 1 and 4 s) extinction and ignition in laminar non-premixed flows of counterflow configuration Opposed-flow diffusion flame (1 atm, 1.8% MD, 42% O2)
Detailed mechanism for MD pyrolysis
This study
2
Fuel 266 (2020) 117060
W. Wang, et al.
Table 2 Reactant mixtures and experimental conditions used in this study. No.
Equivalence ratio
N2/O2
MD mole content
O2 mole content
PC (bar)
1 2 3 4 5
0.53 0.75 (dilute) 1.07 1.60 0.75
3.76 5.66 8.52 8.52 3.76
0.7192 0.7192 0.7192 1.075 1.004
20.9 14.9 10.4 10.4 20.8
15,10,7 15,10,7 20,15,10,7 15,10,7 11,10,7.2,7,5
less than 5%, and an experimental pressure trace closest to the average was selected as a representative one. All the reported IDTs are obtained from the representative pressure traces. 2.3. Simulation specification
Fig. 2. The total IDT and first-stage IDT of MD at the condition of Φ = 1.07 and PC = 10 bar.
Two mechanisms are adopted in this paper. Herbinet’s mechanism [9] consists of 2878 species and 8555 reactions. It was developed on the basis of n-decane mechanism, which has already been validated against IDT data and OH evolution profiles in shock tube experiments [41,42]. According to the author, the overall reactivity of MD has many similarities to that of n-decane. In general, MD oxidation is mainly dominated by the long alkyl chain structure while the presence of ester group limits the low-temperature reactivity. Another mechanism adopted is proposed by Grana et al. [19], with a size of 350 species and 1000 reactions. This lumped mechanism is extended from the methyl butyrate model via layered and modular methods. Grana et al. demonstrated that despite significant simplification, the model was able to correctly reproduce the pyrolysis and oxidation of MD, in both low-temperature and flame conditions. CHEMKIN PRO software [43] is used for simulation. The volume profiles calculated from non-reactive pressure curves are imported to calculated the IDT, which can be obtained from the attachment.
stage ignition. The uncertainty of the measured IDT is estimated to be less than 15%. 3.1. Dependence on temperature Fig. 2 shows the Arrhenius plots of total IDT and first-stage IDT at a pressure of 10 bar and an equivalence ratio of 1.07. It is shown that the NTC behavior of total IDT begins at the temperature of about 730 K, similar to that of n-decane [44]. With the increase of temperature, the total IDT decreases in the low-T region, rises in the NTC region, and decreases again in the intermediate-T region. The IDTs at low-T region are approximately distributed in a straight line in the Arrhenius-type plot, but the first-stage IDT exhibits a larger slop compared to the total IDT. Another important finding is that the first-stage IDT still rapidly decreases with rising temperature at the early stage of the NTC region. Fig. 3 compares the pressure curves at different temperature ranges under the same condition of Fig. 2. In the low-T range (T < 729.5 K), it is obvious that the first-stage ignition moment gradually approaches the end of compression (EOC) point as temperature rises. At the same time, the pressure rise in the first-stage ignition is almost unaffected (about 1 bar). In the NTC region (729.5–801.4 K), as the temperature increases, the first-stage pressure rise gradually decreases until it no longer occurs, which indicates a transition of the reaction pathway. At the intermediate-T zone, similar to the low-T zone, the total IDT continues to decrease with rising temperature, while no two-stage ignition phenomenon occurs. Fig. 4 compares Wang and Oehlschlaeger's shock tube experimental
3. Results and discussion The typical pressure curves obtained from the RCM experiment are shown in Fig. 1. The time before 0 indicates the compression process, and the moment of the maximum first derivative of the pressure is the ignition time. It can be seen that MD exhibits similar ignition phenomena as macromolecular alkanes [44], that is, two stages of ignition at low temperatures. τ1 is the first-stage ignition delay time and τ donates the total ignition delay time. The first-stage ignition timepoint is defined as the local dp/dt maximum prior to the final ignition. In the case of NTC and intermediate temperatures, MD only shows a single-
Fig. 1. The definition of MD ignition delay time in RCM experiment at the condition of Φ = 1.07 and PC = 15 bar. (a) Two-stage ignition at TC = 654.9 K; (b) Singlestage ignition at TC = 785.8 K. 3
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 3. Experimental pressure curves of MD autoignition at the condition of Φ = 1.07 and PC = 10 bar.
curves almost coincide before first-stage ignition, which indicates that the compression process can be well reproduced by the simulation. The simulated first-stage IDTs of Herbinet’s mechanism are closer to the experimental result compared to those of the Grana’s mechanism. However, the ignition intensity of the first-stage ignition of the simulation is greater than that of the experiment, and the simulated firststage pressure rise is also greater than that of the experimental measurement. This phenomenon may be due to the increase of heat dissipation caused by the sudden temperature rise during the first-stage ignition process, which cannot be adequately addressed by the simulation. The total ignition delay time of the experiment is slightly larger than the simulated value of the Herbinet’s mechanism but the difference is marginal. 3.2. Dependence on compressed pressure Fig.4. Comparison of the measured IDTs of shock tube by Wang and Oehlschlaeger [24] and that of the RCM in this study.
Fig. 6 compares the experimental and simulated total IDTs and firststage IDTs with varying temperature under different pressures. Obviously, it’s shown that the total IDT and the first-stage IDT decrease with the increase of pressure. The increase of pressure increases the concentration of the reactants, therefore accelerates the autoignition. It is shown that the total IDT is more sensitive to pressure change at NTC region where the dependence of IDT on pressure increases with the rise of temperature. Although the variation of first-stage IDT with varying temperature is larger than that of the total IDT, the dependence of firststage IDT on pressure is much weaker than that of the total IDT. That is to say that the first-stage IDT is less sensitive to pressure than the total IDT. In the low-temperature region, the simulation results of Herbinet’s
data [24] and the new RCM IDTs of MD in this study. It can be seen that this study is an important complement to the existing data. Besides, it is noteworthy that compared with shock tube data, NTC phenomenon in this study finished at a lower temperature (about 800 K), while that of shock tube data ends at about 900 K. This difference can be attributed to facility effect. Fig. 5 is a comparison of the experimental pressure curves and the variable volume simulation results at a compressed pressure of 10 bar, an equivalence ratio of 1.07, and compressed temperatures of 669.2 K and 775.7 K. It can be seen that the measured and simulated pressure
Fig. 5. Experimental and simulated pressure curves of MD autoignition at the condition of Φ = 1.07 and PC = 10 bar, (a) TC = 669.2 K and (b) 775.7 K. 4
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 6. RCM-measured and simulated total IDTs and first-stage IDTs at different compressed pressures for equivalence ratios of 0.53 (a, b), 0.75 (c, b), 1.07 (e, f) and 1.60 (g, h). Symbol: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
mechanism are basically consistent with the experimental results in both total and first-stage IDTs under different pressures. In contrast, the simulated IDTs using the Grana’s mechanism is obviously larger than
the experimental results. In the NTC region, however, Grana’s mechanism predicts the experimental results better than Herbinet’s mechanism which obviously underpredicts the total IDTs of MD. This 5
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 7. RCM-measured and simulated total and first-stage IDTs at different oxygen mole fractions of 10.4%, 14.9%, 20.9% for the compressed pressures of 15 (a, b), 10 (c, d) and 7.0 (e, f) bar. The mole fraction of MD is kept at 0.7192%. Symbols: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
14.9%, and 20.9%, respectively. Obviously, it is shown in Fig. 7 that both the total and first-stage IDTs decrease with the increase of O2 content at the investigated conditions. It is worth noting that at low temperatures, O2 content has little effect on the IDT. For example, at the pressure of 10 bar and temperature of about 645.5 K, the total IDT of 14.9% O2 mixture is only 1% lower than that of 10.4% O2 mixture, and the difference in first-stage IDT is only about 7%. As the temperature increases, the effect of O2 content on IDT begins to increase. For example, in the NTC region, the oxygen content of the mixture with an equivalence ratio of 0.53 (756.7 K) is 1.43 times that of the mixture with an equivalence ratio of 0.75 (757.9 K), while the IDT difference is
result is consistent with the results of Wang's shock tube experiment [24].
3.3. Dependence on O2 and fuel content The reactions of molecule oxygen addition to fuel radicals and hydrogen free radical are key pathways for low-T and intermediate-T autoignition chemistry. Fig. 7 compares the total and first-stage IDTs of MD with equivalence ratios of 0.53, 0.75 and 1.07 at compressed pressures of 15, 10 and 7 bar. The three reactant mixtures keep the same fuel content of 0.7192%, while their oxygen contents are 10.4%, 6
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 8. RCM-measured and simulated total IDTs and first-stage IDTs at different fuel mole fractions of 0.7192% and 1.075% for the compressed pressure of 15 (a, b), 10 (c, d) and 7.0 (e, f) bar. The mole fraction of oxygen is kept at 10.4%. Symbol: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
longer than the experimental results, which is responsible for the overpredicted total IDT. Fuel molecule directly affects the rate of the H-atom abstraction reaction of methyl decanoate to form methyl decanoate radical, which plays an important role in MD oxidation. Fig. 8 compares the Arrhenius plots of the MD ignition delay time at equivalence ratios of 1.07 and 1.60 with the compressed pressures of 15, 10 and 7.0 bar. The two reactant mixtures maintain the same oxygen content of 10.4%, while the fuel mole fraction is 0.7192% and 1.075%, respectively. It can be seen that the increase of fuel content results in a decrease in ignition delay time. Besides, with the increase of temperature, the dependence
nearly twice. This is because at the NTC region, the changes of O2 content not only influence the reaction rate of O2 relevant reactions but also influence reaction pathways (low-T chain branching pathways or chain prorogation pathways). We can see from the simulation results that the two mechanisms can well capture this rule, that is, the increase of oxygen content has little effect on IDT at low temperatures, but a larger influence at the NTC temperatures. Herbinet’s mechanism predicts total IDTs better at the low-T region and Grana’s mechanism performed better at NTC and intermediate-T region. In addition, Herbinet’s mechanism well predicts the first-stage IDT of MD. As a contrast, the simulated first-stage IDTs of Grana’s mechanism are significantly 7
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 9. Experimental pressure histories of MD autoignition at the condition of PC = 10 bar, (a) XMD = 0.7192%, TC = 640.8 K, and XMD = 1.075%, TC = 642.7 K; (b) XMD = 0.7192%, TC = 775.4 K and XMD = 1.075%, TC = 778.9 K. The mole fraction of oxygen is kept at 10.4%.
and O2 contents of Φ = 0.75 (N2/O2 = 3.76) mixture is 1.4 times greater than that of Φ = 0.75 (N2/O2 = 5.66) mixture. Therefore, the former mixture at a compressed pressure of P (5 bar, 7.2 bar or 11 bar) and the latter mixture at a compressed pressure of 1.4 P (7 bar, 10 bar, or 15 bar) have the same fuel and O2 concentration but different N2 concentration. Although the two cases of having different pressure, the higher pressure of the latter mixture is caused by the excessive N2. The comparison of IDTs of the above two cases can therefore be used to reveal the influence of N2 on MD autoignition. Only two effects of N2 need to be considered: one is chemical effect and the other is thermal effect. For the aspect of chemical effect, nitrogen only participates in elementary reactions as a third body. As for thermal effect, the latter mixture contains more N2 than the former mixture, therefore has a larger heat capacity. For the same heat release, the temperature rise of the latter will be smaller than that of the former, which will lead to IDT difference of the two cases. The experimental and simulated IDTs of the two cases are shown in Fig. 11. It is found that the IDTs of the higher N2 concentration case are longer than those of the lower N2 concentration case, where the trend is more apparent in the NTC region than in the low-T region. In addition, note that the firststage IDT is very insensitive to N2. The IDT dependence on N2 can be well captured by the two mechanisms, and Grana’s mechanism shows obviously better performance than Herbinet’s mechanism. There is no doubt that thermal effect is primarily responsible for this result, and the reaction promoting effect of chemical effect is completely obscured. At the condition that the reactant concentration is constant, the higher N2 concentration case ignites later compared to the lower N2 concentration
of IDT on fuel content rises, which is similar to the IDT dependence on oxygen content. At temperatures lower than NTC, the reactivity is governed by the low-T branching reaction pathways in which fuel molecule participates in H-abstraction and O2-addition reactions. In the NTC region, fuel content plays a more important role through reaction sequence of MD + HO2 = RMDX + H2O2 and H2O2 (+M) = OH + OH (+M) therefore have a greater influence on ignition delay time. Simulation results of both mechanisms can correctly reflect the effect of fuel content on MD IDT, including the enhanced IDT dependence in the NTC region. Fig. 9 compares the pressure curves of MD autoignition at different fuel mole fraction. It is found that the mixture with an equivalence ratio of 1.6 has a more intense first-stage ignition and hot ignition at the temperature of about 640 K. At the temperature of about 777 K, there is no large difference in pressure rise between different fuel concentrations. Fig. 10 compares Arrhenius plots of MD IDTs at equivalence ratios of 0.53 and 0.75 with the compressed pressure of 10 bar and 7.0 bar. Obviously, despite the equivalence ratios of the two mixtures are less than 1, the dependence on fuel content displays a similar regulation with that at equivalence ratios of 1.07 and 1.60. 3.4. Influences of N2 on MD autoignition The influence of N2 on fuel autoignition is an interesting research point, but it is often not easy to be separated from experimental results. In this study, we design an experimental scheme to investigate the influence of N2 on MD autoignition. As is interpreted in Table 2, the fuel
Fig.10. RCM-measured and simulated total IDTs and first-stage IDTs at equivalence ratios of 0.75 and 0.53 with the compressed pressure of 10 bar (a) and 7.0 bar (b). Symbol: experimental data; solid line: results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism. 8
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 11. Comparison of RCM-measured and simulated IDTs at the same fuel and O2 concentrations but different N2 concentration. Symbol: experimental data; solid line: simulation results of Herbinet’s mechanism; dash line: simulation results of Grana’s mechanism.
4. Kinetic analysis
due to its higher heat capacity. The conclusion is also supported by the comparison of pressure curve, as shown in Fig. 12. As stated above, those higher pressure cases can be seen as the addition of N2 to those lower pressure cases. It is shown that after the addition of nitrogen, both the total and first-stage IDTs increase. The lower N2 concentration case exhibits a much stronger first-stage pressure rise than the higher N2 concentration case, which is a reflection of thermal effect, since the two cases should have similar low-T heat release. Besides, it is interesting that although the compressed pressures are different, their maximum pressure is very close.
4.1. Reaction pathway analysis To clarify the main reaction pathways and important intermediates during autoignition, a reaction path analysis based on Grana’s mechanism is carried out. The reaction pathways at the time point of 10% fuel consumption for three temperatures (650 K, 780 K, and 850 K) were compared at an equivalence ratio of 1.07 and a pressure of 10 bar. The results are shown in Fig. 13. The arrows represent the direction of the reaction, and the numbers in different colors on the arrows
9
Fuel 266 (2020) 117060
W. Wang, et al.
650 K, the proportion of this reaction is less than 0.1% and only 0.8% at 850 K. At 650 K, almost all QMDOOH continues to combine with an oxygen molecule to form peroxy-alkylhydroperoxy radical (ZMDOOH) which then decomposes into OH and Ketohydropreoxide (MDKETO). Finally, MDKETO decomposes into several species, including at least two free radicals, which is the only branching step of the low-T oxidation pathways. According to the site of the CeC bond breaking, half of the MDKETO decomposes into C7H17COCHO, CH3OCO and one OH radical. Another half decomposes into OH, RMBX, Aldehyde and other small radicals. At NTC and intermediate temperatures, the importance of the low-temperature pathways decreases significantly. For example, about 82.8% of MD at 650 K is finally converted to MYOKETO, while at 780 K, the proportion is reduced to 58.0%, and at 850 K, only 31.6% remains. At NTC and intermediate temperatures, apart from the low-T O2addition pathway, QMDOOH radical has another three types of propagation pathways, as depicted in Fig. 13. At low temperatures, the proportions of those pathways are negligible. For example, at 650 K, the sum of the proportion of the three types of reactions is less than 1%. At 780 K, the proportion increases to 21.3%, and at 850 K, the proportion reaches nearly half (48.3%). The occurrence of those reaction pathways disturbed the low-T branching pathways and suppresses the formation of reactive free radicals, which is the main cause of NTC behavior of ignition delay time. Due to the existence of the ester group, RMDX radical has a stronger tendency to undergo reaction RMDX + O2 = UME10 + HO2 compared to n-decane. This is one of the reasons why the low-T reactivity of MD is weaker than n-decane. Nevertheless, a proportion of 16.4% seems too large at the low temperature of 650 K. As a contrast, the simulation result of Glaude mechanism on JSR oxidation at the same temperature is only 5.6% [12]. Even the two simulations were performed based on different apparatus, such a big difference is quite unusual. Considering the fact that Grana’s mechanism generally overpredicts the low-T IDTs of MD, as discussed in Section 3, there is a great possibility that the rate of this reaction is
Fig. 12. Experimental pressure histories of MD autoignition for an equivalence ratio of 0.75 at the condition of PC = 10.7 bar, TC = 662.6 K, N2/O2 = 3.76 and PC = 15 bar, TC = 661.2 K, N2/O2 = 5.66.
represent the proportion of the reactant consumption through corresponding reactions. On the whole, the reaction pathway of MD oxidation is very similar to that of long-chain n-alkanes. Firstly, a hydrogen atom of MD molecule is abstracted by free radicals such as OH, CH3, and H, forming RMDX free radicals, among which H-atom abstraction reactions via OH radical are dominant. With the increase of temperature, the proportion of OH radical H-atom abstraction decreases. There are three pathways to consume RMDX. The dominant pathway is oxygen molecule addition to RMDX to form alkyl-ester peroxy radicals (RMDOOX), and RMDOOX then isomerizes to hydroperoxy alkyl-ester radicals (QMDOOH) via inner H transformation. Another important reaction pathway is the H-atom abstraction of RMDX by oxygen, producing methyl decenoate (UME10) and HO2. In addition, RMDX also undergoes β-decomposition to form a series of olefins and small molecular esters directly, which mainly occurs at high temperatures. At
Fig. 13. Reaction pathway analysis of MD autoignition at 10% conversion of fuel for the equivalence ratio of 1.07 and pressure of 10 bar at temperatures of 650 K, 780 K and 850 K by constant volume simulation of Grana’s mechanism. 10
Fuel 266 (2020) 117060
W. Wang, et al.
Fig. 14. Sensitivity analysis of total IDT performed at temperatures of 650 K and 780 K for an equivalence ratio of 1.07 and a pressure of 10 bar.
Fig. 15. Sensitivity analysis of first-stage IDT performed at temperatures of 650 K for an equivalence ratio of 1.07 and a pressure of 10 bar.
pre-exponential factor of ith reaction. Si is the sensitivity coefficient of IDT on the ith reaction. According to the definition, a positive sensitivity indicates that the reaction inhibits autoignition, while a negative sensitivity indicates that the reaction promotes ignition. The larger the absolute value of Si, the greater the promotion or inhibition effect of the reaction on autoignition. Fig. 14 shows the reactions with large sensitivity on total IDT at temperatures of 650 K and 780 K, respectively. It can be seen that at both temperatures, O2 + RMDX ⇒ HO2 + UME10 has the largest positive sensitivity. The enhancement of this reaction will reduce the proportion of O2 addition reactions and subsequent low-temperature pathways, which greatly reduces the reactivity and therefore suppresses
overestimated, and further optimization is needed. 4.2. Sensitivity analysis In order to find out specific reaction controlling MD autoignition, sensitivity analysis of IDT was carried out based on Grana’s mechanism at 650 K and 780 K. CHEMKIN PRO software and constant volume model are used for the calculation. Sensitivity is defined as follows:
Si =
τ (2ki ) − τ (ki ) τ (ki )
In the formula, τ represents ignition delay time, and ki represents the 11
Fuel 266 (2020) 117060
W. Wang, et al.
MD autoignition. At 650 K, ignition promoting reactions includes the isomerization reaction RMDOOX ⇒ QMDOOOH, decomposition reaction ZMDOOOOH ⇒ OH + MDKETO and a series of branching reactions such as MDKETO ⇒ OH + C2H5CHO + CH2CO + 1/6 RMDX + 5/6 RMBX. The enhancement of those reactions will significantly accelerate the autoignition. At 780 K, the sensitivity of IDT on reaction O2 + RMDX ⇒ HO2 + UME10 increases significantly, while the two important low-T branching reactions MDKETO ⇒ OH + CH3CHO + C2H4 + CH2CO + RMBX and MDKETO ⇒ OH + C2H5CHO + CH2CO + 1/6RMDX + 5/6RMBX exhibit negligible sensitivity. It is found that the sensitivities of all ignition-inhibiting reactions at 780 K are larger than those at 650 K, which agrees with the observation that the influence of compressed pressure, oxygen and fuel concentration in NTC region are larger than that in low-temperature region. In addition, it is shown in Fig. 15 that at 650 K almost all the important reactions of first-stage IDT are consistent with those of the total IDT.
Feng: Validation, Formal analysis. Yong Qian: Investigation, Data curation. Dehao Ju: Investigation. Xingcai Lu: Supervision, Project administration, Funding acquisition.
5. Conclusions
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117060.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51961135105) and China Postdoctoral Science Foundation (No. 2019M660087). Appendix A. Supplementary data
In this paper, the autoignition characteristics of methyl decanoate under low-to-intermediate temperatures were studied at wide compressed pressures and equivalence ratios. The experiment used a heated RCM to study the total IDT and the first-stage IDT with compressed pressures of 5–20 bar, equivalence ratios of 0.53–1.60, and temperatures of 625–855 K. Sufficient low-T IDT data and complete NTC region of methyl decanoate autoignition were obtained. It is found that methyl decanoate exhibits two-stage ignition characteristics under low temperatures and only one-stage ignition under intermediate temperatures. Variable volume simulation was carried out based on Herbinet’s and Grana’s mechanisms. In the low-T region, the simulation results of Herbinet’s mechanism is closer to the experiment results than the Grana’s in both the total IDT and the first-stage IDT, but the trend is reversed in the intermediate temperature region. Furthermore, the dependence of IDT on compressed pressure, oxygen and fuel content, equivalence ratios and N2 concentration are studied. The negative relation between IDT and compressed pressure, oxygen, and fuel content is concluded. It is found that the dependence of first-stage IDT on pressure is much weaker than that of the total IDT. In the low-temperature region, oxygen and fuel content have little effect on the total IDT and the first-stage IDT. As the temperature rises, the effects of oxygen and fuel concentration IDT increase. In the case where the fuel and O2 concentration is constant, the higher N2 concentration results in longer total IDTs, where the trend is more apparent in the NTC region than in the low-T region, which can be attributed to thermal effect. In addition, the first-stage IDT is found to be insensitive to N2 concentration. Reaction pathway analysis and sensitivity analysis based on Grana’s mechanism are performed at different temperatures. Results show that the reaction pathways of MD are very similar to those of n-alkanes. It believed that the rate rule of reaction RMDX + O2 = UME10 + HO2 may be overestimated, which may be responsible for the overprediction of Grana’s mechanism on the low-T IDT of MD. It is found that most of the ignition promoting reactions at 650 K are the reactions involved in the low-temperature branching pathways. Reaction O2 + RMDX ⇒ HO2 + UME10 has the largest positive sensitivity, because it disturbs the low-T branching pathway. Besides, the increased sensitivity of all ignition inhibiting reactions at 780 K agrees with the increased influence of compressed pressure, oxygen and fuel content on the IDT in the NTC region compared to that in the low-T region.
References [1] Demirbas A. Importance of biodiesel as transportation fuel. Energy Policy 2007;35:4661–70. [2] Lai JYW, Lin KC, Violi A. Biodiesel combustion: advances in chemical kinetic modeling. Prog Energy Combust Sci 2011;37:1–14. [3] Tsolakis A, Megaritis A, Wyszynski M, Theinnoi K. Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation). Energy 2007;32:2072–80. [4] Agarwal AK. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog Energy Combust Sci 2007;33:233–71. [5] Dooley S, Curran HJ, Simmie JM. Autoignition measurements and a validated kinetic model for the biodiesel surrogate, methyl butanoate. Combust Flame 2008;153:2–32. [6] Seshadri K, Lu T, Herbinet O, Humer S, Niemann U, Pitz WJ, et al. Experimental and kinetic modeling study of extinction and ignition of methyl decanoate in laminar non-premixed flows. Proc Combust Inst 2009;32:1067–74. [7] Murphy MJ, Taylor JD, Mccormick RL. Compendium of Experimental Cetane Number Data. Office of Scientific & Technical Information Technical Reports; 2004. [8] Pitz WJ, Westbrook CK, Herbinet O. Chemical kinetic modeling of advanced transportation fuels. Fuel Injection; 2009. [9] Herbinet O, Pitz WJ, Westbrook CK. Detailed chemical kinetic oxidation mechanism for a biodiesel surrogate. Combust Flame 2008;154:507–28. [10] Szybist JP, Boehman AL, Haworth DC, Koga H. Premixed ignition behavior of alternative diesel fuel-relevant compounds in a motored engine experiment. Combust Flame 2007;149:112–28. [11] Dagaut P, Gaı¨L S, Sahasrabudhe M. Rapeseed oil methyl ester oxidation over extended ranges of pressure, temperature, and equivalence ratio: experimental and modeling kinetic study. Proc Combust Inst 2007;31:2955–61. [12] Glaude PA, Herbinet O, Bax S, Biet J, Warth V, Battin-Leclerc F. Modeling of the oxidation of methyl esters-Validation for methyl hexanoate, methyl heptanoate, and methyl decanoate in a jet-stirred reactor. Combust Flame 2010;157:2035–50. [13] Herbinet O, Biet J, Hakka MH, Warth V, Glaude PA, Nicolle A, et al. Modeling study of the low-temperature oxidation of large methyl esters from C11 to C19. Proc Combust Inst 2011;33:391–8. [14] Herbinet O, Glaude PA, Warth V, Battin-Leclerc F. Experimental and modeling study of the thermal decomposition of methyl decanoate. Combust Flame 2011;158:1288–300. [15] Sarathy SM, Thomson MJ, Pitz WJ, Lu T. An experimental and kinetic modeling study of methyl decanoate combustion. Proc Combust Inst 2011;33:399–405. [16] Luo Z, Lu T, Maciaszek MJ, Som S, Longman DE. A reduced mechanism for hightemperature oxidation of biodiesel surrogates. Energy Fuels 2010;24:656–60. [17] Luo Z, Plomer M, Lu T, Som S, Longman DE. A reduced mechanism for biodiesel surrogates with low temperature chemistry for compression ignition engine applications. Combust Theor Model 2011;16:369–85. [18] Diévart P, Won SH, Dooley S, Dryer FL, Ju Y. A kinetic model for methyl decanoate combustion. Combust Flame 2012;159:1793–805. [19] Grana R, Frassoldati A, Saggese C, Faravelli T, Ranzi E. A wide range kinetic modeling study of pyrolysis and oxidation of methyl butanoate and methyl decanoate – Note II: lumped kinetic model of decomposition and combustion of methyl esters up to methyl decanoate. Combust Flame 2012;159:2280–94. [20] Grana R, Frassoldati A, Cuoci A, Faravelli T. Ranzi E, A wide range kinetic modeling study of pyrolysis and oxidation of methyl butanoate and methyl decanoate. Note I: lumped kinetic model of methyl butanoate and small methyl esters. Energy 2012;43:124–39. [21] Wang YL, Feng Q, Egolfopoulos FN, Tsotsis TT. Studies of C4 and C10 methyl ester flames. Combust Flame 2011;158:1507–19. [22] Pyl SP, Van Geem KM, Puimège P, Sabbe MK, Reyniers M-F, Marin GB. A
CRediT authorship contribution statement Wenyu Wang: Software, Validation, Investigation, Data curation, Writing - original draft. Liang Yu: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition. Yuan 12
Fuel 266 (2020) 117060
W. Wang, et al.
[34] Yu L, Mao Y, Li A, Wang S, Qiu Y, Qian Y, et al. Experimental and modeling validation of a large diesel surrogate: autoignition in heated rapid compression machine and oxidation in flow reactor. Combust Flame 2019;202:195–207. [35] Yu L, Mao Y, Qiu Y, Wang S, Li H, Tao W, et al. Experimental and modeling study of the autoignition characteristics of commercial diesel under engine-relevant conditions. Proc Combust Inst 2019;37:4805–12. [36] Yu L, Wang S, Wang W, Qiu Y, Qian Y, Mao Y, et al. Exploration of chemical composition effects on the autoignition of two commercial diesels: Rapid compression machine experiments and model simulation. Combust Flame 2019;204:204–19. [37] Yu L, Wang W, Wang S, Feng Y, Qian Y, Lu X. An experimental and modeling study of n-hexadecane autoignition under low-to-intermediate temperatures. Sci China Technol Sci 2019. [38] Goldsborough SS, Santner J, Kang D, Fridlyand A, Rockstroh T, Jespersen MC. Heat release analysis for rapid compression machines: challenges and opportunities. Proc Combust Inst 2019;37:603–11. [39] Goldsborough SS, Hochgreb S, Vanhove G, Wooldridge MS, Curran HJ, Sung C-J. Advances in rapid compression machine studies of low- and intermediate-temperature autoignition phenomena. Prog Energy Combust Sci 2017;63:1–78. [40] https://app.knovel.com/web/poc/ms/profile.html?cid=kc9V1ZITRT&prop= prSVC. [41] Davidson DF, Herbon JT, Horning DC, Hanson RK. OH concentration time histories in n-alkane oxidation. Int J Chem Kinet 2001;33:775–83. [42] Pfahl U, Fieweger K, Adomeit G. Self-ignition of diesel-relevant hydrocarbon-air mixtures under engine conditions. Combust Inst 1996;26:781–9. [43] CHEMKIN-PRO 15131 R D, San Diego; 2013. [44] Kumar K, Mittal G, Sung CJ. Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions. Combust Flame 2009;156:1278–88.
comprehensive study of methyl decanoate pyrolysis. Energy 2012;43:146–60. [23] Haylett DR, Davidson DF, Hanson RK. Ignition delay times of low-vapor-pressure fuels measured using an aerosol shock tube. Combust Flame 2012;159:552–61. [24] Wang W, Oehlschlaeger MA. A shock tube study of methyl decanoate autoignition at elevated pressures. Combust Flame 2012;159:476–81. [25] Campbell MF, Davidson DF, Hanson RK. Ignition delay times of very-low-vaporpressure biodiesel surrogates behind reflected shock waves. Fuel 2014;126:271–81. [26] Campbell MF, Davidson DF, Hanson RK. Scaling relation for high-temperature biodiesel surrogate ignition delay times. Fuel 2016;164:151–9. [27] Zhai Y, Ao C, Feng B, Meng Q, Zhang Y, Mei B, et al. Experimental and kinetic modeling investigation on methyl decanoate pyrolysis at low and atmospheric pressures. Fuel 2018;232:333–40. [28] Talukder N, Lee KY. Laminar flame speeds and Markstein lengths of methyl decanoate-air premixed flames at elevated pressures and temperatures. Fuel 2018;234:1346–53. [29] Sun S, Yu L, Wang S, Mao Y, Lu X. Experimental and kinetic modeling study on selfignition of α-methylnaphthalene in a heated rapid compression machine. Energy Fuels 2017;31:11304–14. [30] Yu L, Wu Z, Qiu Y, Qian Y, Mao Y, Lu X. Ignition delay times of decalin over low-tointermediate temperature ranges: rapid compression machine measurement and modeling study. Combust Flame 2018;196:160–73. [31] Yu L, Sun S, Tao W, Xingcai L. n-Decane autoignition characteristics under low-tointermediate temperature. Combust Sci Technol 2017;23:325–30. [32] Wang S, Yu L, Wu Z, Mao Y, Li H, Qian Y, et al. Gas-phase autoignition of diesel/ gasoline blends over wide temperature and pressure in heated shock tube and rapid compression machine. Combust Flame 2019;201:264–75. [33] Yu L, Qiu Y, Mao Y, Wang S, Ruan C, Tao W, et al. A study on the low-to-intermediate temperature ignition delays of long chain branched paraffin: Iso-cetane. Proc Combust Inst 2019;37:631–8.
13