An ignition delay time and kinetic study of 2-methyltetrahydrofuran at high temperatures

Fuel 186 (2016) 758–769

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Full Length Article

An ignition delay time and kinetic study of 2-methyltetrahydrofuran at high temperatures Jingshan Wang, Xibin Wang ⇑, Xiangshan Fan, Kangkang Yang, Yingjia Zhang School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

h i g h l i g h t s
Ignition delays of 2-methyltetrahydrofuran (2-MTHF) were measured using a shock tube.
Two existed models (Mech I and Mech II) were validated.
A new model (Mech III) was developed by combining Mech I and AramcoMech_1.3 from NUI.
Ignition delays and soot precursors of 2-MTHF were compared with those of 2-methylfuran.

a r t i c l e

i n f o

Article history: Received 1 July 2016 Received in revised form 27 August 2016 Accepted 30 August 2016 Available online 13 September 2016 Keywords: Ignition delay time 2-Methyltetrahydrofuran Kinetic model

a b s t r a c t Kinetic analyses were performed based on the experimental results of ignition delay times of 2methyltetrahydrofuran (2-MTHF) using shock tube technique at temperatures of 1050–1800 K, equivalence ratios of 0.5–2.0, fuel mole concentrations of 0.25–1.0%, and pressures of 1.2–10 atm. A new kinetic model of 2-MTHF oxidation named Mech III was established according to the analysis of simulation using two published models (Mech I from Ravi Fernandes group and Mech II from Battin-Leclerc group) and the experimental data in this work. Comparison between simulation and experimental data indicated that Mech II shows remarkable under-prediction while Mech I gives a good agreement with ignition delay times under most conditions except for underprediction on fuel-rich mixtures at relative low temperature around 1250 K. Sensitivity analysis indicated that both models underestimated ignition delay times for the reactions of C0-C4 molecules, so Mech III was formed by introducing such reactions and can demonstrate improved simulation performance under all conditions. Reaction pathway analysis of Mech III showed that 2-MTHF is mainly consumed through fuel decomposition at high temperatures (around 1550 K), and H-atom abstraction reactions at lower temperatures (around 1250 K), respectively. The comparative experimental and kinetic study between 2-MTHF and 2-methylfuran (MF) indicated that 2-MTHF has higher ignition delay times under the same conditions in this work, while the disparity decreases as the temperature increases, and 2-MTHF produces less soot precursors under high temperature conditions. Ó 2016 Published by Elsevier Ltd.

1. Introduction Bio-fuels have been widely used as fossil fuel surrogates due to their advantages on regeneration and environmental protection. Cyclic ethers, produced from non-edible biomass [1–3], are second generation bio-fuels and becoming current research focuses. Compared with traditional bio-fuels such as methanol and ethanol, they have higher heat values and produce lower pollutants. Unsaturated cyclic ethers [4–11] (mainly furan-family fuels) have been widely studied. However, only limited studies have been per⇑ Corresponding author. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.fuel.2016.08.104 0016-2361/Ó 2016 Published by Elsevier Ltd.

formed on saturated cyclic ethers, which are ethers or their derivatives with a saturated cyclic structure containing one oxygen atom and 2–5 carbon atoms. Among these saturated cyclic ethers, tetrahydrofuran (THF)-family fuels are the most promising because THF is an important trap of greenhouse gases such as CO2 and CH4 [12], and many THF-family fuels are excellent solvents of most fuels [13]. They can be used either as co-solvent of alcohol blends such as methanol-diesel and methanol-gasoline or as fossil fuel surrogates. And they are also important intermediates during combustion of gasoline, diesel and biodiesel. In the case of alkanes, especially for those with straight chains, the formation of cyclic ethers including THF-family molecules from QOOH is the key reaction to control low temperature chemistry, leading to negative

J. Wang et al. / Fuel 186 (2016) 758–769

temperature coefficient (NTC) behavior of alkanes [14–16]. Ribaucour et al. [17] investigated auto-ignition characteristics of n-pentane, and found 2-MTHF is the most abundant saturated cyclic ether produced during low temperature oxidation. The formation of 2-MTHF dominates the combustion process. Zhang et al. [18] found that 2-MTHF is an immediate product during the low temperature combustion of methyl esters of fatty acids. Therefore, it is of significance to study the combustion and emission characteristics of THF-family fuels especially for 2-MTHF. Lucas et al. [19] tested emission characteristics of gasoline and the 2-MTHF-gasoline blend with 60% 2-MTHF in volume on a sixcylinder gasoline engine. It was found that the addition of 2-MTHF can significantly reduce HC, CO, ozone and benzene emissions but increase NOx emission slightly. Rudolph et al. [20] investigated combustion and emission characteristics of unleaded gasoline and its blends with different biomass derived fuels such as 2MTHF, methyl tert-butyl ether (MTBE), ethanol and methanol on a gasoline engine. They found that the addition of 2-MTHF yields similar combustion and emission characteristics to gasoline. Thewes et al. [21] carried out a comparative study on combustion and emission characteristics of different fuels including gasoline and 2-MTHF on a downsized direct injection gasoline engine. Compared with gasoline, HC and PM emissions of 2-MTHF decrease dramatically, while the NOx emission does not change significantly. Investigation of Janssen et al. [22] on IC engine using 2MTHF-DnBE blends found NOx and soot decrease remarkably under all conditions and even almost soot-free under the greatest EGR rates. Besides engine studies, autoignition characteristics of 2-MTHF have also attracted much attention. Leppard et al. [23] investigated low temperature autoignition characteristics of saturated cyclic ethers such as THF, 2-MTHF and tetrahydropyran (THP) using the motored-engine technique, and found that low temperature autoignition of saturated cyclic ethers is mainly determined by oxygen addition reactions producing plenty of HO2 radicals. Sudholt et al. [24] conducted a comparative study on auto-ignition characteristics of furans, THFs, dihydrofurans (DHFs), furfuryl alcohols, and their blends with n-heptane and diesel in an Ignition Quality Tester. They found that side chains have little effect on auto-ignition characteristics of furans due to C@C bonds, but have great effect on the auto-ignition of THFs due to the absence of C@C bonds. The derived cetane number (DCN) of THFs increases with the increase of side chain carbon number. Compared with gasoline and diesel, THFs with a short chain are recommended to be utilized on gasoline engines, while the fuels with long chains are suitable for diesel surrogates. Brassat et al. [25] measured ignition delay times of 1-butanol, 2-butanol, 2-MTHF and MF using a shock tube and a rapid compression machine (RCM) under engine conditions and found 2-MTHF has the highest enleanment potential, leading to better thermal efficiency and lower NOx emission. The octane number and ignition delay times of 2-MTHF are comparable to those of 1-butanol, and no obvious NTC phenomenon was observed for 2-MTHF at 20 bar. To further investigate the combustion phenomenon, detailed chemical kinetic studies are also required. Tran et al. [26] measured products of THF and 2-MTHF in a premixed laminar flame, and proposed possible reaction pathways for these fuels. Moshammer et al. [27] determined quantitative species profiles in laminar, premixed, low-pressure and fuel-rich 2-MTHF flames by a combination of EI- and PI-MBMS, and established a detailed kinetic model (Mech I) which has good prediction for intermediate products at high temperatures. More recently, Tran et al. [28] developed a detailed high temperature chemical kinetic model of THF (Mech II) validated against species profiles in a premixed low-pressure flame, laminar flame speeds, as well as ignition delay times measured using a shock tube.

759

So far, the mechanism of 2-MTHF (Mech I) only has been validated against experiment data of low-pressure premixed flame at an equivalence ratio of 1.7 and a pressure of 40 mbar, and limited data related to laminar flame speed and ignition delay time of 2MTHF are available in the literature. Therefore, the mechanism should be modified at a wider range of conditions for engine application. Experimental study has been carried out on ignition delay time of 2-MTHF in our previous work [29], and here more detailed research was performed experimentally at temperatures of 1050– 1800 K, equivalence ratios of 0.5–2.0, fuel mole concentrations of 0.25–1.0%, and pressures of 1.2–10 atm. Subsequently, a new kinetic mechanism of 2-MTHF oxidation was proposed and the reaction pathways were analyzed accordingly. Then, the autoignition and soot precursor production was compared to MF to reveal the mechanism of their combustion and emission characteristics. Finally, possible low temperature reaction pathways of 2MTHF were proposed. 2. Experimental setup and methods Experiments were conducted using a shock tube with a diameter of 11.5 cm, as shown in Fig. 1. Detailed parameters of the facility were described in previous publications [29,30]. The shock tube was divided into driver section and driven section with length of 2.0 m and 7.0 m, respectively. Diaphragms with different thicknesses can produce different reflected pressures. The entire tube was evacuated to a pressure below 104 Torr. Four fast-response pressure transducers (PCB 113B26) with a fixed distance of 300 mm were installed on the sidewall. Three time interval counters (FLUKE, PM6690) were used to record the time intervals and further to calculate the incident shock velocity. The reflected shock temperature was calculated using Gaseq [31] software. The thermodynamic data of 2-MTHF was employed from Burcat’s database [32]. A photomultiplier (Hamamatsu CR 131) with a filter of 307 ± 10 nm and a pressure transducer (PCB 113B03) were mounted on the endwall to measure OH⁄ signal and endwall pressure, respectively. The typical uncertainty of reflected shock temperature is ±20 K and the ignition delay time uncertainty is less than 15%. Detailed calculation process has been described in the Ref. [33]. Ignition delay time is defined as the time interval between the arrival of incident shock at endwall and the onset of ignition determined by extrapolating the maximum slope of OH⁄ signal to the baseline, as shown in Fig. 2. And the definition is in accordance with the sharp increase of the pressure profile. For further understanding of the mechanism of 2-MTHF ignition and reaction kinetics, more detailed experimental conditions than our previous work [29] were applied as listed in Table 1. Mixtures of 2-MTHF (with the purity of 99% from Aladdin), oxygen and argon (with the purity of 99.999% and 99.999% separately) were prepared in an evacuated 128 L stainless tank based on partial pressures. In order to ensure complete vaporization, partial pressure of the fuel should be less than one third of its vapor pressure (13.6 kPa for 2-MTHF at 20 °C [13]). Mixtures were let to rest for over 12 h to allow for mixing by diffusion process. 3. Results and discussion 3.1. Experimental results The work covers a wider range of conditions than low pressure premixed flame of 2-MTHF investigated by Moshammer et al. [27]. Tables of the ignition delay times measured during this study are provided in the supplementary material (S1). And a correlation in an Arrhenius form is proposed based on the experimental data to

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J. Wang et al. / Fuel 186 (2016) 758–769

Fig. 1. Schematic of the shock tube.

2-MTHF%= 0.5% p= 4.15atm T= 1319K = 1.0

0.20

12

Endwall OH* signal Endwall pressure signal

10

8

0.15

6 0.10

Ignition delay time

4

1037 μs

0.05

and Ru is universal gas constant with the value of 1.986  103 kcal/ (mol K). The regression coefficient R2 is 0.968, which indicates a good agreement between correlation and experimental data. In Eq. (1), negative exponential factors of pressure and fuel concentration indicate that ignition delay times of 2-MTHF decrease with the increase of these parameters. The positive exponential index of equivalence ratio indicates the inhibiting effect of increasing equivalence ratio on auto-ignition of 2-MTHF. The overall activation energy of 2-MTHF derived from data is 37.2 kcal/mol, and is higher than that of MF with 32.4 kcal/mol in similar conditions [7]. Detailed description about the effects of temperature, pressure, equivalence ratio and concentration on ignition delay time will be proposed in the following section.

OH* signal

Relative pressure signal

0.25

2

0

0.00 1200

1400

1600

1800

2000

2200

2400

time/μs

3.2. Comparison of chemical kinetic mechanisms of 2-MTHF

Fig. 2. Definition of ignition delay times.

describe effects of temperature, pressure, equivalence ratio and fuel concentration on the ignition delay time of 2-MTHF under temperatures from 1050 to 1800 K, equivalence ratios from 0.5 to 2.0, fuel mole concentrations from 0.25% to 1.0% and pressures of 1.2–10 atm.

s ¼ 4:79  105 pð0:590:04Þ X ð0:650:06Þ /ð1:240:06Þ  exp

  37:2  0:6 kcal=mol Ru T

ð1Þ

where s is ignition delay time in ls, / is equivalence ratio, p is pressure in atm, X is mole fraction of 2-MTHF, T is temperature in Kelvin

There are two chemical mechanisms involving 2-MTHF oxidation at high temperatures in the literature. They are Mech I from Ravi Fernandes group [27] and Mech II from Battin-Leclerc group [28]. Mech I containing 185 species and 1412 reactions was released in 2013. It includes a newly constructed H2AO2 sub mechanism from Burke [34], a CO sub-mechanism from Haas et al. [35], a detailed THF sub-mechanism from the discussion with Naik [36] and a 2-MTHF sub-mechanism involving 25 species and 168 reactions. The THF sub-mechanism consists of the C1-C3 submechanism from Lowry [37], the C4 sub-mechanism from Healy [38], and cyclic structure sub-mechanism from Narayanaswamy [39]. Rate constants of reactions in 2-MTHF sub-mechanism were determined by THF and other similar species as Simmie suggested [40]. Mech II containing 255 species and 1723 reactions was developed using the EXGAS software in 2015. Firstly, a C0-C6 base mech-

Table 1 Conditions of the experiment. Mixture

/

2-MTHF (%)

O2 (%)

Ar (%)

Pressure (atm)

1 2 3 4 5

1.0 1.0 1.0 0.5 2.0

0.25 1.0 0.5 0.5 0.5

1.75 7 3.5 7 1.75

98 92 96 92.5 97.75

1.2, 4, 10 4, 10 4, 10 4, 10 4, 10

761

anism involving C0-C2 sub-mechanism and C3-C6 sub-mechanism of unsaturated species is proposed. The THF mechanism contains reactions of light aromatic compounds up to ethylbenzene, ethenol sub-mechanism, and reactions consuming cyclopropanecarboxaldehyde, butanal and 2-butenal, as well as fulvene reactions leading to benzene formation. The second part is the construction of THF sub-mechanism involving unimolecular and bimolecular initiations, H-atom abstraction reactions, b scissions, isomerizations and displacement reactions. Rate constants of H-atom abstraction and b scission reactions were calculated using a CBS-QB3 level of theory. It has good performance on the prediction of low pressure premixed flame species concentrations and ignition delay times while over-predicts laminar flame speeds of THF. Mech II includes a 2-MTHF sub-mechanism mainly referred from Mech I, and it is different from Mech I in the following aspects:

Ignition delay time (μs)

J. Wang et al. / Fuel 186 (2016) 758–769

Ignition delay time ( s)

1000

1.2atm-exp 4atm-exp 10atm-exp 1.2atm-Mech I 4atm-Mech I 10atm-Mech I 1.2atm-Mech II 4atm-Mech II 10atm-Mech II

100

2-MTHF%= 0.25% = 1.0

10

0.55

0.60

0.65

0.70

0.75

0.80

0.85

-1

1000/T (K ) Fig. 3. Effects of pressures on ignition delay times of 2-MTHF.

(a) 100

2-MTHF%= 0.5% p= 4atm

10 0.50


There are no reactions involving 2-MTHF and HO2;
Mech II does not contain 2-MTHF decomposition reactions forming C4H8-1, C3H6 and C2H4;
There are no cross reactions of THF and 2-MTHF;
Rate constants of methyl group decomposition and H-atom abstraction for 2-MTHF are different from those of Mech I;

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

-1

1000/T (K )

=2.0-exp =1.0-exp =0.5-exp =2.0-Mech I =1.0-Mech I =0.5-Mech I =2.0-Mech II =1.0-Mech II =0.5-Mech II

1000

Ignition delay time ( s)

All chemical kinetic calculations were performed using SENKIN code [41] in CHEMKIN II package [42], with a zero-dimensional model and a constant volume assumption. Non-ideal effect (dp/dt of 4%/ms) was taken into consideration to simulate ignition delay times at relative low temperatures below 1250 K. Figs. 3–5 show the simulated results of Mech I and Mech II, as well as measured ignition delay times at different pressures, equivalence ratios and fuel mole concentrations. Fig. 3 shows both experimental and simulated ignition delay times for mixture 1 at elevated pressures from 1.2 to 10 atm. It can be seen that ignition delay times decrease with the increase of pressure in the whole temperature range. Mech I agrees well with measured data at all pressures, while Mech II underpredicts ignition delay times apparently, and the simulated ignition delay times are only about 1/3 to 1/4 of measured data at most conditions. Fig. 4 gives the effect of equivalence ratio (0.5–2.0) on ignition delay times of 2-MTHF with fuel concentration of 0.5% at 4.0 and 10 atm, respectively. It can be seen that increasing equivalence ratios significantly inhibit 2-MTHF ignition, especially for the case of 10 atm. This phenomenon is similar to furan-family fuels and can be explained that, for a fixed fuel concentration, oxygen concentrations increase with the decrease of equivalence ratios, and

1000

φ =2.0-exp φ =1.0-exp φ =0.5-exp φ =2.0-Mech I φ =1.0-Mech I φ =0.5-Mech I φ =2.0-Mech II φ =1.0-Mech II φ =0.5-Mech II

(b)

100

2-MTHF%= 0.5% p= 10atm

10 0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

-1

1000/T (K ) Fig. 4. Effects of equivalence ratios on ignition delay times of 2-MTHF.

enhance the reactivity of reaction system by promoting chain branching reaction H + O2 = O + OH, which dominates the high temperature oxidation. Meanwhile, the dilution effect of argon plays an increasingly important role as the equivalence ratio increases, and the competition between 2-MTHF + H reactions and H + O2 reactions also contributes to this result. Mech I well predicts ignition delay times of 2-MTHF under most conditions and shows over 80% agreements with the measured data, while the prediction of Mech II is much lower than experimental data at high temperatures and shows less than 10% agreements with the measured data. Fig. 5 shows the effect of fuel concentration on ignition delay times of stoichiometric mixtures at 4.0 and 10 atm. It can be seen that ignition delay times of 2-MTHF decrease with the increase of fuel concentrations. Under a certain equivalence ratio, when the fuel concentration increases, oxygen concentration increases and it promotes the reaction of H + O2 = O + OH. In addition, the dilution effect of argon weakens, and the competition between 2MTHF + H reactions and H + O2 reactions changes with the increase of fuel concentrations. Above all, the interaction of these factors leads to shorter ignition delay times. Mech I shows a good agreement (over 90%) with experimental data at high temperatures for different fuel concentrations but under-predicts ignition delay times at lower temperatures below 1250 K, especially under fuelrich conditions (60–80% agreements with the measured data). Mech II under-predicts the ignition delay times under most conditions and shows less than 10% agreements with the measured data.

J. Wang et al. / Fuel 186 (2016) 758–769

Ignition delay time ( s)

762

3.3. Chemical kinetic analysis

2-MTHF%=0.25%-exp 2-MTHF%=0.5%-exp 2-MTHF%=1.0%-exp 2-MTHF%=0.25%-Mech I 2-MTHF%=0.5%-Mech I 2-MTHF%=1.0%-Mech I 2-MTHF%=0.25%-Mech II 2-MTHF%=0.5%-Mech II 2-MTHF%=1.0%-Mech II

1000

To explain different performance of these mechanisms, sensitivity analysis and fuel flux analysis are presented in the following section.

100

3.3.1. Sensitivity analysis In order to recognize the key reactions during the oxidation of 2-MTHF, sensitivity analyses on all reactions were conducted. The sensitivity coefficient S is defined as:

(a) = 1.0 p= 4atm

10 0.55

0.60

0.65

0.70

0.75

0.80

S¼

0.85

-1

1000/T (K )

= 1.0 p= 10atm

Ignition delay time ( s)

1000

(b) 100

2-MTHF%=0.25%-exp 2-MTHF%=0.5%-exp 2-MTHF%=1.0%-exp 2-MTHF%=0.25%-Mech I 2-MTHF%=0.5%-Mech I 2-MTHF%=1.0%-Mech I 2-MTHF%=0.25%-Mech II 2-MTHF%=0.5%-Mech II 2-MTHF%=1.0%-Mech II

10

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

-1

1000/T (K ) Fig. 5. Effects of fuel mole concentrations on ignition delay times of 2-MTHF.

Since neither Mech I nor Mech II can well predict ignition delay times of 2-MTHF at all experimental conditions, a new mechanism named Mech III is proposed by replacing C0-C4 base mechanism of Mech I with AramcoMech_1.3 from NUI. And the AramcoMech_1.3 has been validated under a wide range of conditions. Mech III contains 282 species and 1712 reactions, and the 2-MTHF submechanism extracted from Mech I consists of 29 species and 172 reactions. The mechanism is submitted as a supplemental material (S3). Comparison between Mech III and Mech I under current conditions is shown in Fig. 6. It is clear that Mech III has better performance, especially for mixture 2 at temperatures below 1250 K. The modified mechanism can well predict ignition delay times of 2-MTHF under all experimental conditions. To test the performance of Mech III in predicting species mole fractions of premixed, laminar low-pressure flames, both simulated results of Mech III and Mech I are compared with original flame data of Moshammer et al. [27], and they are shown in the supplemental material (S2). In S2, both mechanisms well predict mole fractions of main species such as CO, H2O, H2, CO2, O2 and Ar, as well as 2-MTHF. Compared with Mech I, Mech III improves the performance in predicting species including C2H2, C3H3, propyne, allene, C3H6, C4H6, C4H7, C2H2O, C2H4O and C5H8O, and has similar performance in predicting species such as C2H4 and C4H6O. As to species such as C3H4 (tested with ElectronIonization molecular beam mass spectrometry), C3H5, C4H8, CH2O and C3H6O, the prediction of Mech III is less accurate than Mech I. On the whole, Mech III has better performance than Mech I.

sð2ki Þ  sð0:5ki Þ 1:5sðki Þ

ð2Þ

where s is ignition delay time and ki is the rate constant of the i-th element reaction. Negative sensitivity coefficient indicates a promoting effect on the corresponding reaction and vice versa. Fig. 7(a) shows the results of mixture 2 under temperatures of 1250 K and 1550 K at 10 atm using Mech I. It can be seen that H + O2 = O + OH is the most important promoting reaction at both temperatures, similar to the combustion chemistry of most fuels at temperatures above 1200 K. At 1550 K, reactions involving small species such as C2H3 + O2 = CH2CHO + OH, CH3 + HO2 = CH3O + OH and C2H4 + CH3 = C2H3 + CH4 have significant promoting effects on the consumption of 2-MTHF. For fuel related reactions, 2-MTHF = CYCCCCJO + CH3 significantly promotes the reactivity due to the production of CH3 radical. Other reactions such as 2-MTHF (+M) = C4H8-1 + CH2O(+M) and 2-MTHF + H = C5H9O-1 + H2 also promote the combustion process. 2-MTHF(+M) = C3H6 + CH3CHO (+M) and CH3 + HO2 = CH4 + O2 are the most important inhibiting reactions. C3H5-A + H(+M) = C3H6(+M), C3H6 + H = C3H5-A + H2, and fuel related reaction 2-MTHF + H = C5H9O-5 + H2 also inhibit the oxidation of 2-MTHF. It is interesting that CH3 + HO2 = CH3O + OH and CH3 + HO2 = CH4 + O2 have same reactants but show quite different effects. It is because the former one produces more reactive radicals like CH3O and OH, and the second reaction is a termination reaction, transforming radicals into stable molecules. In addition, fuel decomposition reactions have more important effects than H-atom abstraction reactions. C3H5-A + H(+M) = C3H6(+M) and C3H6 + H = C3H5-A + H2 are chain reactions to consume H radical, and they inhibit oxidation significantly. As temperature decreases to 1250 K, the promoting effect of 2-MTHF = CYCCCCJO + CH3 weakens, but H-atom abstraction reactions such as 2-MTHF + HO2 = C5H9O-1 + H2O2, 2-MTHF + OH = C5H9O-1 + H2O and 2-MTHF + H = C5H9O-5 + H2 have more significant influence. It’s notable that the most important inhibition reaction is C2H3O1-2 = CH2CHO, and 2HO2 = H2O2 + O2 becomes more important for the autoignition of 2-MTHF. Meanwhile, CH2CHO + O2 ) CH2O + CO + OH has a sharp increase on sensitivity at 1250 K, leading to shorter ignition delay times at temperatures below 1250 K predicted by Mech I. Fig. 7(b) shows the sensitivity analysis with Mech III at the same condition of Mech I. As the figure presents, similar to Mech I, H + O2 = O + OH is the most important promoting reaction at both 1250 K and 1550 K. At 1550 K, reactions such as 2-MTHF = CYCCCCJO + CH3, C2H3 + O2 = CH2CHO + OH, CH3 + HO2 = CH3O + OH and C2H4 + CH3 = C2H3 + CH4 are still of importance to promote oxidation of the fuel. Meanwhile, 2-MTHF + (M) = C3H6 + CH3CHO(+M), CH3 + HO2 = CH4 + O2, C3H5-A + H(+M) = C3H6(+M) and C3H6 + H = C3H5-A + H2 are also the most important reactions to inhibit oxidation of 2-MTHF. As for the Mech I case, fuel decomposition reactions are more important to affect ignition delay times than H-atom abstraction reactions. As the temperature decreases to 1250 K, H-atom abstraction reactions are more important, and 2-MTHF + H = C5H9O-5 + H2 is the most important reaction to inhibit oxidation of the fuel. Unlike Mech I, C2H3O1-2 = CH2CHO and

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J. Wang et al. / Fuel 186 (2016) 758–769

2-MTHF%= 0.5% p= 10atm

1000

Ignition delay time (μs)

Ignition delay time (μs)

2-MTHF%= 0.25% φ = 1.0

(a) 1.2atm-exp 4atm-exp 10atm-exp 1.2atm-Mech I 4atm-Mech I 10atm-Mech I 1.2atm-Mech III 4atm-Mech III 10atm-Mech III

100

10

1000

(c) φ =2.0-exp φ =1.0-exp φ =0.5-exp φ =2.0-Mech I φ =1.0-Mech I φ =0.5-Mech I φ =2.0-Mech III φ =1.0-Mech III φ =0.5-Mech III

100

10

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.55

0.60

0.65

Ignition delay time (μs)

Ignition delay time (μs)

(b) φ =2.0-exp φ =1.0-exp φ =0.5-exp φ =2.0-Mech I φ =1.0-Mech I φ =0.5-Mech I φ =2.0-Mech III φ =1.0-Mech III φ =0.5-Mech III

100

10 0.50

0.60

0.65

0.70

0.85

0.90

(d) 2-MTHF%=0.25%-exp 2-MTHF%=0.5%-exp 2-MTHF%=1.0%-exp 2-MTHF%=0.25%-Mech I 2-MTHF%=0.5%-Mech I 2-MTHF%=1.0%-Mech I 2-MTHF%=0.25%-Mech III 2-MTHF%=0.5%-Mech III 2-MTHF%=1.0%-Mech III

100

0.75

0.80

0.85

0.90

0.55

0.60

0.65

-1

0.70

0.75

0.85

-1

1000/T (K )

1000/T (K )

φ = 1.0 p= 10atm

1000

Ignition delay time (μs)

0.80

φ = 1.0 p= 4atm

1000

10

0.55

0.75

1000/T (K )

2-MTHF%= 0.5% p= 4atm 1000

0.70

-1

1000/T (K-1)

(e) 2-MTHF%=0.25%-exp 2-MTHF%=0.5%-exp 2-MTHF%=1.0%-exp 2-MTHF%=0.25%-Mech I 2-MTHF%=0.5%-Mech I 2-MTHF%=1.0%-Mech I 2-MTHF%=0.25%-Mech III 2-MTHF%=0.5%-Mech III 2-MTHF%=1.0%-Mech III

100

10

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

-1

1000/T (K ) Fig. 6. Comparison of Mech III and Mech I in predicting ignition delay times of 2-MTHF.

CH2CHO + O2 ) CH2O + CO + OH have little effect on ignition delay times of 2-MTHF, and it is the result of the different rates of reactions involving small radicals between two mechanisms. In fact, the replacement of reactions involving small radicals makes the prediction of Mech III more accurate than Mech I. Sensitivity analysis of ignition delay times of 2-MTHF with Mech II is presented in Fig. 7(c), and the condition is the same as that of Mech I. As the figure shows, the most important promoting reaction at 1550 K is H + O2 = O + OH. Meanwhile, O2 + CH3 = CH2O + OH and fuel related reactions such as 2-MTHF + H = C5H9O-5 + H2 and 2-MTHF + OH = C5H9O-1 + H2O also promote oxidation process of 2-MTHF. The most important inhibiting reactions are 2-MTHF + H = C5H9O-4 + H2, 2-MTHF + OH = C5H9O-4 + H2O and 2CH3 (+M) ) C2H6(+M). As the temperature decreases to 1250 K,

CYCCCCJO + CH3 = 2-MTHF, which has little effect on the ignition delay time of 2-MTHF at 1550 K, becomes the most important promoting reaction, different from the case of Mech I and Mech III. Other reactions such as H + O2 = O + OH, O2 + CH3 = CH2O + OH and 2-MTHF + OH = C5H9O-5 + H2O are also important reactions to promote oxidation of 2-MTHF. It is notable that HCO(+M) = H + CO(+M) promotes oxidation at 1250 K and inhibits oxidation at 1550 K. The most important inhibiting reaction is 2-MTHF + H = C5H9O-4 + H2 and the sensitivity index is higher than that of 1550 K. However, in the case of Mech I and Mech III, H-atom abstraction reactions producing C5H9O-4 have little effects on ignition delay times of 2-MTHF. The disparity is mainly due to the different kinetic rate of H-atom abstraction reactions on the side chain of 2-MTHF between Mech II and other mechanisms.

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J. Wang et al. / Fuel 186 (2016) 758–769

Fig. 7. Sensitivity analysis of 2-MTHF.

In general, sensitivity analysis of Mech I is similar to that of Mech III, and the difference between them is mainly the kinetic data of reactions involving small species. While the difference between Mech II and other mechanisms is obvious, and it is not only the result of different kinetic data of reactions involving small species, but also the result of different kinetic data of fuel related reactions. 3.3.2. Fuel flux analysis To compare the reaction pathways of Mech I, Mech III and Mech II, fuel flux analyses of mixture 2 were conducted at 10 atm and temperatures of 1550 K and 1250 K for 20% fuel consumption. As the 2-MTHF sub-mechanism of Mech III is taken from Mech I, reaction pathways of Mech I and Mech III are presented in one figure, as shown in Fig. 8. Reaction pathways at 1550 K and 1250 K are presented in italics and traditional forms respectively. It can be seen that, for Mech I, 2-MTHF is mainly consumed through decomposition reactions at 1550 K, taking up 91.0% of total fuel consumption. The most important fuel decomposition reaction is 2-MTHF(+M) = C3H6 + CH3CHO(+M), which produces a propylene and an acetaldehyde, taking up 39.1% of fuel consumption. And 2-MTHF(+M) = C4H8-1 + CH2O(+M) is also an important pathway to consume 2-MTHF, with fuel consumption of 31.9%. Furthermore, the CH3 and C2H4 produced from the fuel decomposition reactions also contribute much to the consumption of 2-MTHF at 1550 K. Similar to Mech I, fuel is mainly consumed through fuel decompo-

sition reactions (73.7%) for Mech III at 1550 K, but the percent is much lower than that of Mech I. For Mech III, most fuel is consumed through fuel decomposition reactions to produce C3H6 (31.7%) and C4H8-1 (25.9%). 21.1% of the fuel is consumed via Hatom abstraction reactions with H atom. At 1250 K, both Mech I and Mech III show that less fuel is consumed through decomposition. It only takes up 36.8% and 40.6% consumptions of 2-MTHF for Mech I and Mech III, respectively. And compared with Mech I (62.8%), less fuel is consumed through H-atom abstraction reactions for Mech III (57.0%) at 1250 K. As a result, more stable alkenes including C2H4, C3H6 and C4H8-1 are produced through decomposition for Mech III, leading to the inhibiting effect of the oxidation process and better prediction at this temperature. To clarify the 2-MTHF oxidation at 1250 K, a more detailed analysis is conducted with Mech III. The results indicate that the most important decomposition reaction at 1250 K is still 2-MTHF(+M) = C3H6 + CH3CHO(+M), consuming 19.2% of the fuel, while the fuel decomposition reaction producing 1-butene and formaldehyde is also important, taking up 15.8% of 2-MTHF consumption. Moreover, there are five C5H9O isomers produced via H-atom abstraction reactions of 2-MTHF with small radicals including H, OH, HO2 and CH3 at different carbon ring positions. Among the isomers, the most abundant species is C5H9O-5, taking up 21.1% of total fuel consumption, and H-atom abstraction reactions producing other C5H9O isomers all contribute to less than 10% of fuel consumption

765

J. Wang et al. / Fuel 186 (2016) 758–769 O

O

C2H4 + CH2CHO

17.4% 19.2% 39.1% 31.7%

O

CH3

+H(3.1% 4.5% 2.8% )

C2H4

3.1% 3.4% 7.2% 3.8%

CH2O

(+M)

(+M) 2.0% 2.2% 12.8% 10.3%

14.3% 15.8% 31.9% 25.9%

O

+H(3.3% 4.6% 2.8%)

+H(11.1% 15.8% 2.6% 10.4% )

+H(3.3% 4.6% 2.8%)

+OH(5.1% 3.1%)

+OH(5.1% 3.2%)

+OH(5.1% 3.1%)

+OH(5.1% 3.1%)

+HO2(1.8% 1.0%)

+CH3(1.6% 1.6%)

+OH(5.1% 3.1%)

+CH3(1.2% 1.2%)

+HO2(1.8% 1.0%)

+HO2(1.8% 1.0%)

+HO2(1.8% 1.0%)

O

(C5H9O-1)

1.2% 1.3% 1.4%

(C5H9O-2)

(total 10.0% 8.6%) (total 11.3% 9.7%) 80.7% 80.8% 75.1%

19.0% 18.8% 24.3%

O

O

97.3% 97.4% 93.2% O

22.3% 22.6% 19.4%

O

C2H4 +

C2H4 +

C2H4 +

O

O

2.1% 2.0% 1.9% (C5H9O-5)

(total 11.4% 9.9%)

17.0% 17.2% 15.6%

(total 19.9% 21.1%)

-H(50.9% 50.6% 54.3%)

97.7% 97.8% 94.5%

-H(1.3% 1.3% 3.3%)

O

O

CH2O +

+HO2(1.6% ) O

1.5% 1.5% (C5H9O-4)

(total 10.2% 8.7%)

+CH3(2.1% 2.1%)

O

O

(C5H9O-3) 2.0% 1.9% 4.2%

Total fuel decompositions: 36.8% 40.6% 91.0% 73.7% Total H-abstractions: 62.8% 57% 2.6% 21.1%

(+M)

+H(2.8% 4.0% 2.3%)

O

CH2O +

C

CH3CHO

O

O

-H(5.8% 5.8% 8.4%)

C2H3 CH3CHO

CH2CHO

-H(2.6%)

-CH3(59.4% 56.4% 57.2%)

O

-H(2.7% 2.8% 4.0%)

O

4.6% 5.0% 4.6%

8.0% -H(24.0% 25.8% 26.3%) 8.7% 6.5% O

O

O

Mech I T= 1250K T=1550K Mech III: T= 1250K T=1550K CH2O

CH2O

Fig. 8. Flux analysis of 2-MTHF at high temperatures with Mech I and Mech III. (Black characters, Mech I; blue characters, Mech III. Roman characters, 1250 K; italic characters, 1550 K.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

separately. It should be noted that the least amount of C5H9O isomers produced is C5H9O-1, and it is determined by the bond energy between H atom and C atom at different sites [26]. The CAH bond energy on the side alkyl is the highest (102.9 kcal/mol), and it leads to less production of C5H9O-1. While as to C5H9O-5, the CAH bond energy on the 5th C atom is 93.6 kcal/mol, thus resulting in more production of C5H9O-5 species. Although all C5H9O isomers are mainly consumed via b scission, the bond rupture positions and products are different from each other. The main pathways of C5H9O-1 and C5H9O-2 are both via b scission on CAO bond to form opened ring structures, followed by the decomposition into small radicals such as formaldehyde and ethylene. C5H9O-5 is also mainly consumed via b scission on CAO bond, while the main product is propylene, and little C5H9O-5 is transformed to a cyclic structure with C@C bond via H-atom abstraction. Unlike isomers above, cyclic structures are

main productions of C5H9O-3. 56.4% of C5H9O-3 is consumed to produce 2,3-dihydrofuran by removing side methyl, and reactions producing 2,3-dihydro-5-methyl-furan and 2,5-dihydro-2-methylfuran contribute to 25.8% and 2.8% consumption of C5H9O-3 separately. Only 13.7% of C5H9O-3 is consumed via b scission to form formaldehyde. Additionally, small amount of C5H9O-3 is transformed into C5H9O-2 via an isomerization process. Similar to C5H9O-3, the oxidation pathway of C5H9O-4 includes H-atom abstraction reactions forming cyclic structures with C@C bond, as well as ring opening reactions via b scission. And part of C5H9O-4 is isomerized to form C5H9O-3 or C5H9O-5. However, unlike C5H9O-3, there are no reactions involving side methyl removal during the oxidation process of C5H9O-4. It is also noteworthy that, the ring opening process is mainly performed through the break of CAO bond (22.6%) instead of C-C bond (17.2%) because of the lower bond energy of CAO bond.

766

J. Wang et al. / Fuel 186 (2016) 758–769

Fig. 9 shows the reaction pathway of 2-MTHF with Mech II at the same condition as that of Mech I and Mech III. As shown in the figure, most fuel is consumed through H-atom abstraction reactions at 1550 K (77.7%), and it is different from the case of Mech I (2.6%) or Mech III (21.1%). This is because Mech II is lacking some fuel decomposition reactions forming stable species such as C2H4, C3H6 and C4H8-1, and it also does not contain reactions involving cyclic structures with C@C bond and isomerizations of C5H9O isomers. All these factors lead to the under-prediction of ignition delay times with Mech II.

Fuel%= 1.0% = 1.0

Ignition delay time (μs)

1000

MF-4atm-exp MF-10atm-exp 2-MTHF-4atm-exp 2-MTHF-10atm-exp MF-4atm-Sim MF-10atm-Sim 2-MTHF-4atm-Sim 2-MTHF-10atm-Sim

100

10

3.4. Comparison of ignition delay times and soot precursors with MF at high temperatures

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

-1

1000/T (K )

In the furan-family fuels, MF has a similar structure to 2-MTHF except for C@C bonds, and it can be applied as a gasoline surrogate as 2-MTHF. To figure out the difference between fuels with saturated and unsaturated structures on auto-ignition and emission characteristics, ignition delay times and soot precursors of 2-MTHF and MF are compared. As mentioned above, main oxidation products of 2-MTHF are aldehydes, alkenes, dihydrofurans and their derivatives. Tran et al. [26] found that 2-MTHF produces comparable formaldehyde with ethanol but less acetaldehyde. They also pointed out that main products of furan-family fuels are alkynes such as C3H4 and C4H4, which are important species in the formation of soot. Therefore, 2-MTHF has a potential to produce less soot compared to Furan-family fuels. Fig. 10 shows ignition delay times of stoichiometric mixtures of 2-MTHF and MF with the fuel concentration of 1.0% at pressures of 4.0 and 10 atm. Ignition delay times of MF and 2-MTHF are respec-

O

Fig. 10. Comparison of ignition delay times of MF and 2-MTHF at high temperatures.

tively fitted to an Arrhenius form relation against pressures, as presented in Eqs. (3) [7] and (4):

s ¼ 4:35  103 p0:662 expð16290=TÞ

ð3Þ

s ¼ 2:98  103 p1:090 expð18454=TÞ

ð4Þ

where s is ignition delay time in ls, p is pressure in atm, and T is temperature in Kelvin. Through the comparison of the two equations, the pressure exponent of 2-MTHF is higher than that of MF, which means that pressure plays a more important role for 2-MTHF under tested

O

C2H4 + CH2CHO

Mech II

Total fuel decompositions: 13.0% 29.1%

CH3

T= 1250K T=1550K

13.0% 29.1%

Total H-abstractions: 86.0% 77.7%

O

+H(6.5% 6.6% )

+H(5.9% 5.4%)

+OH(6.8% 3.9%)

+OH(6.8% 6.8%) +CH3(1.2%)

+H(6.8% 6.8%)

+H(23.2% 24.8% ) +OH(6.8% 3.0%)

+OH(6.8% 6.8%)

+CH3(1.6%) O

O

O

O

(C5H9O-1)

(C5H9O-2)

(total 13.9% 12.2%)

(total 13.3% 10.5%) 80.9% 75.7%

19.1% 24.3%

O

O

O

2.0% 4.3% O

O

C2H4 + O

63.3% 58.5%

36.7% 41.5%

(C5H9O-5)

(total 13.6% 13.6%)

(total 31.6% 27.8%)

43.3% 44.6%

56.7% 55.4%

O

O

99.0% 97.7%

O

O

C2H4 + CH2CO + CH3

CH2O +

(C5H9O-4) (total 13.6% 13.6%)

(C5H9O-3)

98.0% 95.7%

O

CH2O +

+H(6.8% 6.8%) +OH(6.8% 6.8%)

C2H3

CH3CHO

CH2O

Fig. 9. Flux analysis of 2-MTHF at high temperatures with Mech II. (Normal font, 1250 K; bold font, 1550 K.)

CH2CHO

J. Wang et al. / Fuel 186 (2016) 758–769

conditions. In addition, 2-MTHF has a higher overall activation energy (36.7 kcal/mol) than that of MF (32.4 kcal/mol) at similar conditions in this work. As the figure shows, at the same pressure, 2-MTHF exhibits longer ignition delay times than MF at temperatures below 1300 K. When the temperature increases, the gap decreases until a cross of the profiles appears. This can be explained that, as Tran et al. presented in their work [26], the CAH bond energy on the methyl group of MF (86.2 kcal mol1) is lower than CAH bond energy on either cyclic carbons (higher than 92 kcal mol1) or methyl group (102.9 kcal mol1) of 2-MTHF, and the week CAH bond energy on the methyl group of MF leads to the lower ignition delay times of MF than 2-MTHF at experimental conditions in this work. To clarify the phenomena, kinetic simulation is conducted. 2-MTHF is simulated with Mech III, and MF is simulated with modified Somer’s mechanism [43] as Wei et al. proposed [7]. Fig. 10 shows that both mechanisms agree well with the experimental data. Profiles of calculated temperatures and small radicals are presented in Figs. 11–13. Fig. 11 gives profiles of temperature history for 2-MTHF and MF under stoichiometric condition with fuel concentration of 1.0%, the pressure of 4.0 atm and temperatures of 1250 and 1450 K. When the initial temperature is 1250 K, the temperature profile of 2MTHF rises sharply at about 1200 ls, later than that of MF at about 800 ls, and it indicates a longer ignition delay time of 2-MTHF. When the initial temperature is 1450 K, both profiles of 2-MTHF and MF rise almost synchronously, and it indicates that two fuels have similar ignition delay times at this initial temperature, as shown in Fig. 10. Fig. 12 respectively illustrates profiles of small free radicals including O, OH, H and HO2 as a function of time during the oxidation of 2-MTHF and MF. It can be seen that, at 1250 K, all free radicals of 2-MTHF present the peak value later than those of MF. The equilibrium concentrations of most free radicals produced during the oxidation of 2-MTHF are larger than those of MF, meaning that fewer free radicals are consumed through H-atom abstraction reactions and more 2-MTHF is consumed through decomposition than MF. Therefore, intermediate products are difficult to be oxidized and ignition delay times of 2-MTHF are longer than MF. When the temperature increases, more 2-MTHF and MF are consumed through decomposition. The oxidizations of their products become easier. Considering the structure without C@C bond, the decomposition of 2-MTHF is easier than that of MF, resulting in a smaller gap of ignition delay times between 2-MTHF and MF. When the temperature is 1450 K, free radicals of both fuels get

3000

to peak values almost synchronously, indicating similar ignition delay times of two fuels. 2-MTHF and MF are both novel oxygenate fuels. Engine tests indicated that the addition of both fuels into traditional fuels can reduce soot production and emission [22,25]. To further understand the mechanism of soot production and destruction, comparative study of soot precursors for both fuels is of significance. Fig. 13 illustrates ethylene and acetylene profiles of two fuels at 1450 K. It can be seen that the peak value of ethylene of 2-MTHF is higher than that of MF, while acetylene shows the opposite trend. This is in consistent with Tran’s test result [26] that THFfamily fuels can produce more ethylene and less acetylene than Furan-family fuels. As a result, 2-MTHF produces less soot than MF because ethylene is less likely to produce benzene than acetylene. 3.5. Validation of Mech III under engine operating conditions Ignition delay time comparison between simulated results with Mech III and experimental data by Brassat et al. [25] was done to check the validity of Mech III under engine conditions (stoichiometric 2-MTHF-air mixtures at 20 atm), as shown in Fig. 14. It can be seen that ignition delay times of 2-MTHF decrease with the increase of temperature at the whole range of temperatures, while the profile rises rapidly at temperatures above 1000 K or below 770 K, and the slope of the profile becomes smaller under temperatures between 770 and 1000 K. This might be the result of different dominant reactions of 2-MTHF oxidation at different temperature regions. Though Mech III well predicts ignition delay times of 2-MTHF at temperatures above 1000 K, it under-predicts evidently the ignition delay time at temperatures below 1000 K, and the simulated result considering the rise of pressure (dp/dt = 4%/ms) does not make much improvement. And Mech III can’t simulate ignition delay times of 2-MTHF at temperatures below 900 K due to the lack of low temperature oxidation reactions. Kerschgens et al. [44] simulated ignition delay times of 2-MTHF at temperatures below 1000 K with surrogates involving proper proportions of n-heptane, DME, ethanol, ethane and phenol, and the constructed mechanism agreed well with the measurements. However, this model can’t predict intermediate products. Leppard et al. [23] investigated the possible oxidation pathways of 2-MTHF and THF, and measured the intermediate products. They suggested that gamma valerolactone (GVL) was an important intermediate product for low temperature oxidation of 2-MTHF. Therefore, detailed low temperature mechanism of 2-MTHF should contain reactions involving cyclic species like GVL, as well as b scission reactions, which are important during the low temperature combustion of most fuels. And this would be investigated in our future work. 4. Conclusions

2500

Temperature/K

767

= 1.0 p = 4atm Fuel% = 1.0%

2000

2-MTHF-1250K MF-1250K 2-MTHF-1450K MF-1450K

1500

0

200

400

600

800

1000 1200 1400 1600 1800 2000

time/μs Fig. 11. Temperature profiles of MF and 2-MTHF with time.

Ignition delay times of 2-MTHF were measured at a wide range of pressures, temperatures, equivalence ratios and fuel concentrations. Based on experimental data and two previous mechanisms (Mech I and Mech II), a new mechanism (Mech III) which well predicts ignition delay times at all experimental conditions was developed. Main conclusions are summarized as follows: (1) Ignition delay times of 2-MTHF were measured behind reflected shock waves at a wide range of conditions. Results show that ignition delay times of 2-MTHF decrease with the increase of temperature, pressure and fuel mole concentration, and increase with the increase of equivalence ratio. And a correlation was obtained as:

768

J. Wang et al. / Fuel 186 (2016) 758–769 -3

-3

= 1.0 p = 4atm Fuel% = 1.0%

-3

6.0x10

8.0x10 2-MTHF-1250K MF-1250K 2-MTHF-1450K MF-1450K

-3

5.0x10

(a)

-3

4.0x10

O radical

-3

3.0x10

-3

2.0x10

-3

1.0x10

OH radical mole concentration

O radical mole concentration

7.0x10

OH radical

-3

6.0x10

(b)

-3

4.0x10

2-MTHF-1250K MF-1250K 2-MTHF-1450K MF-1450K

-3

2.0x10

0.0

0.0 0

200

400

600

0

800 1000 1200 1400 1600 1800 2000

200

400

600

800 1000 1200 1400 1600 1800 2000

time /μs

time/μs

8.0x10 7.0x10 6.0x10 5.0x10 4.0x10 3.0x10 2.0x10 1.0x10

-3

-4

-3 -3

2-MTHF-1250K MF-1250K 2-MTHF-1450K MF-1450K

(c)

-3

H radical

-3

2-MTHF-1250K MF-1250K 2-MTHF-1450K MF-1450K

1.2x10

= 1.0 p = 4atm Fuel% = 1.0%

-3

-3 -3 -3

HO2 radical mole concentration

H radical mole concentration

9.0x10

= 1.0 p = 4atm Fuel% = 1.0%

-4

1.0x10

-5

(d)

= 1.0 p = 4atm Fuel% = 1.0%

8.0x10

-5

6.0x10

-5

4.0x10

HO2 radical

-5

2.0x10

0.0

0.0 0

200

400

600

800 1000 1200 1400 1600 1800 2000

0

200

400

600

time/μs

800

1000

1200

1400

time/μs

Fig. 12. Small free radicals of MF and 2-MTHF with time.

100000

2-MTHF-C2H4 T= 1450K

MF-C2H4

4000

2-MTHF-C2H2 MF-C2H2

= 1.0 p = 4atm Fuel% = 1.0%

3000

2000

1000

Ignition delay time ( s)

mole concentration of C2H2 and C2H4/ppm

5000

= 1.0 p= 20atm

10000

1000

2-MTHF-exp 2-MTHF-sim-with dp/dt 2-MTHF-sim-without dp/dt

100

10 0 1 0

20

40

60

80

100

120

140

160

180

200

time/μs Fig. 13. Comparison of soot precursors produced by MF and 2-MTHF.

s ¼ 4:79  105 pð0:590:04Þ X ð0:650:06Þ /ð1:240:06Þ  exp

  37:2  0:6 kcal=mol Ru T

(2) Mech II under-predicts ignition delay times of 2-MTHF, while the disparity decreases at lower temperatures. Mech I well predicts ignition delay times of 2-MTHF under most conditions except for rich mixtures at low temperatures around 1250 K. With the combination of Mech I and AramcoMech_1.3 from NUI, a newly constructed mechanism (Mech III) was proposed. The new mechanism can well predict ignition delay times of 2-MTHF under all experimental conditions.

0.6

0.8

1.0

1.2

1.4

1.6

1000/T Fig. 14. Ignition delay times of 2-MTHF at 20 atm in air. (Symbols, measurements; solid lines, predictions with dp/dt; dashed lines, predictions without dp/dt.)

(3) Chemical kinetic analyses were made on three mechanisms. The sensitivity analysis of Mech I shows that fuel decomposition reactions are more important than H-atom abstraction reactions during the auto-ignition of 2-MTHF at high temperatures. Comparison of sensitivity at different conditions indicates that the reaction of CH2CHO + O2 ) CH2O + CO + OH is responsible for the poor prediction. Mech III is similar to Mech I, but sensitivity of some reactions involving small radicals are different from that of Mech I. Pathway analysis of Mech III indicates that 2-MTHF is mainly consumed by decomposition at high temperatures around

J. Wang et al. / Fuel 186 (2016) 758–769

1550 K, but by H-atom abstraction reactions at low temperatures around 1250 K. (4) Compared to MF, 2-MTHF has higher ignition delay times under the same conditions in this work. The disparity decreases as the temperature increases. Chemical kinetic analysis shows 2-MTHF produces less soot precursor–acetylene than MF, and it explains why engines using 2-MTHF produce less soot emission than using MF. Conflict of interest The authors declare no competing financial interest. Acknowledgements This study is supported by the National Natural Science Foundation of China (61235003). Appendix A. Supplementary material Ignition delay times, species mole fraction profiles, and the combined mechanism (Mech III) are included in the supplementary material. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.fuel.2016.08.104. References [1] Hu L, Lin L, Liu S. Chemoselective hydrogenation of biomass-derived 5hydroxymethylfurfural into the liquid biofuel 2,5-dimethylfuran. Ind Eng Chem Res 2014;53:9969–78. [2] Alonso DM, Wettstein SG, Dumesic JA. Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem Soc Rev 2012;41:8075–98. [3] Tran L-S, Sirjean B, Glaude PA, Fournet R, Battin-Leclerc F. Progress in detailed kinetic modeling of the combustion of oxygenated components of biofuels. Energy 2012;43:4–18. [4] Wei H, Feng D, Shu G, Pan M, Guo Y, Gao D, et al. Experimental investigation on the combustion and emissions characteristics of 2-methylfuran gasoline blend fuel in spark-ignition engine. Appl Energy 2014;132:317–24. [5] Ma X, Xu H, Jiang C, Shuai S. Ultra-high speed imaging and OH-LIF study of DMF and MF combustion in a DISI optical engine. Appl Energy 2014;122:247–60. [6] Pan M, Shu G, Pan J, Wei H, Feng D, Guo Y, et al. Performance comparison of 2methylfuran and gasoline on a spark-ignition engine with cooled exhaust gas recirculation. Fuel 2014;132:36–43. [7] Wei L, Tang C, Man X, Huang Z. Shock-tube experiments and kinetic modeling of 2-methylfuran ignition at elevated pressure. Energy Fuels 2013;27:7809–16. [8] Xu N, Tang C, Meng X, Fan X, Tian Z, Huang Z. Experimental and kinetic study on the ignition delay times of 2,5-dimethylfuran and the comparison to 2methylfuran and furan. Energy Fuels 2015;29:5372–81. [9] Cheng Z, Xing L, Zeng M, Dong W, Zhang F, Qi F, et al. Experimental and kinetic modeling study of 2,5-dimethylfuran pyrolysis at various pressures. Combust Flame 2014;161:2496–511. [10] Xu N, Wu Y, Tang C, Zhang P, He X, Wang Z, et al. Experimental study of 2,5dimethylfuran and 2-methylfuran in a rapid compression machine: comparison of the ignition delay times and reactivity at low to intermediate temperature. Combust Flame 2016;168:216–27. [11] Xu N, Gong J, Huang Z. Review on the production methods and fundamental combustion characteristics of furan derivatives. Renew Sustain Energy Rev 2016;54:1189–211. [12] Sowjanya Y, Prasad PSR. Formation kinetics & phase stability of double hydrates of C4H8O and CO2/CH4: a comparison with pure systems. J Nat Gas Sci Eng 2014;18:58–63. [13] Aycock DF. Solvent applications of 2-methyltetrahydrofuran in organometallic and biphasic reactions. Org Process Res Dev 2007;11:156–9. [14] Zádor J, Taatjes CA, Fernandes RX. Kinetics of elementary reactions in lowtemperature autoignition chemistry. Prog Energy Combust Sci 2011;37:371–421. [15] Herbinet O, Bax S, Glaude PA, Carre V, Battin-Leclerc F. Mass spectra of cyclic ethers formed in the low-temperature oxidation of a series of n-alkanes. Fuel 2011;90:528–35.

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