Comparative Shock Tube and Kinetic Study on High-Temperature

Sep 6, 2016 - The ignition delay times of 2,3-dihydrofuran (23DHF) and 2,5-dihydrofuran (25DHF) were investigated over the temperature range of 1100 t...
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Comparative Shock Tube and Kinetic Study on High-Temperature Ignition of 2,3-Dihydrofuran and 2,5-Dihydrofuran Xiangshan Fan, Xibin Wang,* Jingshan Wang, and Kangkang Yang School of Energy and Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China S Supporting Information *

ABSTRACT: The ignition delay times of 2,3-dihydrofuran (23DHF) and 2,5-dihydrofuran (25DHF) were investigated over the temperature range of 1100 to 1635 K with pressure of 1.2, 4, and 10 atm for lean (φ= 0.5), stoichiometric, and rich (φ= 2) fuel/ O2/Ar mixtures. 23DHF shows shorter ignition delay times than 25DHF under the above conditions. A modified model (M_Tran model) was presented to improve the prediction of DHF ignition. Kinetic analysis indicated that most 23DHF transforms to cyclopropane carboxaldehyde (CPCA) and further to croton aldehyde (CA) by isomerization while most 25DHF dehydrogenates to furan. Some reactions involving CA and propene show strong sensitivity for 23DHF ignition. Some reactions of furan present strong effect on 25DHF ignition. Ignition delay data between furan, 23DHF, and 25DHF were compared to reveal the effect of number and location of carbon double bonds on the ignition characteristics. The bond dissociation energies of DHF are not as strong as that of furan and are significantly influenced by the locations of carbon double bonds, causing the differences in structure stability. As a result, the ignition trends of furan and DHF in this research are furan < 25DHF < 23DHF. In 1986 and 1989, Lifshitz et al.21,22 developed general reaction schemes of dihydrofuran pyrolysis by exploring the pyrolysis of 23DHF and 25DHF in shock tube and discovering the main pyrolysis products. Dubnikova and Lifshitz23 studied the isomerizations between 23DHF, cyclopropane carboxaldehyde (CPCA), and cis-croton aldehyde (c-CA) based on density functional theory and evaluated the reaction rates. The products formed during dissociation of 23DHF were investigated by Karunatilaka et al.24 using chirped-pulse Fourier transform microwave spectroscopy. The formation enthalpies of dihydrofuran and dihydrofuran radicals were calculated by Simmie25 using high-level methods. The bond dissociation energies for the C−H bonds of DHF and the derived cetane numbers were obtained in the research of Sudholt et al.11 Matsubara et al.26 investigated infrared multiphoton dissociation and isomerization reactions of 23DHF, CPCA, and CA with free electron laser and stated that CA generated from 23DHF is hopefully a cis-conformer. They also performed quantum chemical calculations at the MP2/cc-pVDZ level to explain their experiment results. Although the above studies have significantly improved our understanding into the combustion chemistry of DHF, the general reaction mechanism of DHF combustion has not been validated by enough experiments. The ignition delay times which are essential parameters in developing and validating chemical kinetics of fuel oxidation, have never been researched. In present study, the ignition delay times of 23DHF and 25DHF were measured behind reflected shock waves over the temperature range of 1640 to 1080 K, pressure of 1.2 atm to 10 atm, and equivalence ratio of 0.5 to 2. As far as we know, this is the first research on DHF ignition. An optimized model for

1. INTRODUCTION New biofuels synthesized by second generation methods of production from agricultural wastes could relieve the emission of greenhouse gases and the shortage of energy. Furans and tetrohydrofurans (THFs) as second generation biofuels can be produced by agricultural wastes,1−8 such as bagasse and corncob, and have been widely studied. P-series fuels,9 in which renewable energy is largely contained, are suitable for cold-weather conditions and contain 17.5−32.5% methyl tetrohydrofuran (MTHF).10 “Tailor-Made Fuels from Biomass” project was proposed by RWTH Aachen University to develop possible fuel candidates evaluated by their physical properties, chemical kinetics, and engine performance. Furans and THFs were selected as potential candidates for transportation fuels in this project.11 Many experimental and kinetic studies for furans and THFs have been done in recent years. Several combustion mechanisms were developed, such as models for furan, 2methylfuran and 2,5-demethylfuran from Somers et al.12−14 and Liu et al.,15−17 model for furan oxidation from Tian et al.,18 model for high temperature THF combustion from Tran et al.,19 and model for MTHF from Moshammer et al.20 Fundamental understanding relating combustion properties to molecular structures could be acquired from these studies. Dihydrofuran (DHF) is also a kind of cyclic ether with the similar molecular structure to furan and THF. In every model mentioned above, 2,3-dihydrofuran (23DHF) and 2,5-dihydrofuran (25DHF) were served as intermediates and integrated in these models with detailed chemistry. However, none of these models explicitly attempted to model the combustion of dihydrofurans, causing the uncertainty of DHF sub model. Study on DHF combustion could not only improve the kinetic schemes of these bio-derived fuels, but also could help understanding the effect of double bonds on the combustion properties of cyclic ethers. © XXXX American Chemical Society

Received: June 7, 2016 Revised: September 1, 2016

A

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Energy & Fuels DHF ignition at this research range was acquired based on the work of Tran et al.19 The ignition data were compared with that of furan for further understanding of the ignition characteristics of DHF and kinetic analysis was carried out to discover the influence of molecular structures.

Table 1. Mixture Compositions

2. EXPERIMENTAL APPARATUS AND METHODS The purpose of this experiment is to measure the ignition delay times of DHF/O2/Ar mixtures heated by reflected shock waves. The present shock tube facility has been specified in detail in previous works27,28 and will be described here briefly. The shock tube consists of a 2 m-long driver section and a 7.3 mlong driven section with a diameter of 11.5 cm. Two sections are separated by a connecting flange with PET (polyester terephthalate) diaphragms on both sides. Before every experiment, the driver section and the driven section were filled with mixture of nitrogen, helium, and fuel/O2/Ar mixture. When an experiment began, the PET diaphragms were broken by the high pressure and a shock wave generated and fully developed in the driven section. A digital recorder, three time interval counters, and four fast response piezoelectric pressure transducers in the end of the driven section could determine the accurate speed of the shock wave. From the wave speed and initial condition, the temperature and pressure of the fuel mixture behind the reflected shock wave could be calculated. Besides, a narrow band-pass filter, a photomultiplier, and a pressure sensor are mounted on the end wall of the tube, thus the ignition delay times can be measured by detecting OH* emission as well as pressure surge. The ignition delay time is defined as the time interval between the arrival of incident shock wave at the end wall and the extrapolation of the maximum slope of the OH* radical curve to the zero baseline, as shown in Figure 1.

φ (%)

23DHF, 25DHF (%)

O2 (%)

Ar (%)

P (atm)

0.5 1 2

0.5 0.5 0.5

5 2.5 1.25

94.5 97 98.25

4 1.2, 4, 10 4

temperature. All the simulation and kinetic analysis were conducted using the CHEMKIN II package.29 The interaction between reflected shock wave and boundary layer can cause the attenuation of the shock wave and should be taken into account, so the modified parameter with pressure rise of 4% per millisecond was used in all the simulations to compensate for the inevitable error.

3. RESULTS AND DISCUSSION 3.1. Ignition Delay Time Measurement and Correlation. The experiments were conducted at the fixed fuel concentration of 0.5%, temperature of 1640−1080K, pressure of 1.2−10 atm, and equivalence ratio of 0.5−2. Figure 2

Figure 1. Definition of the ignition delay time.

By measuring partial pressures, the DHF/O2/Ar mixtures were prepared precisely in a stainless steel tank at constant temperature of 20 °C. The mixture compositions were tabulated in (Table 1). Additional time was allowed for the gases to mix to ensure homogeneity. The 23DHF and 25DHF obtained from Aladdin were of 97%, 99% purity, respectively. The Ar and O2 were ultrahigh purity grade (99.999%). To simulate the ignition delay and fuel pyrolysis, an adiabatic, zero-dimensional homogeneous reactor was assumed. The calculated ignition delay time was defined as the time interval from the beginning of the reaction to the steepest rise of the

Figure 2. Experimental ignition delay times (symbols) with 20% error bar and fitting equations (dashed line) of stoichiometric 23DHF (a) and 25DHF (b).

presents the ignition delay times of stoichiometric 23DHF and 25DHF in various pressures and the results are of good logarithmic relation at wide temperature range. As shown in Figure 2, shorter ignition delay times were measured at higher pressures. This is a general trend for fuel reactivity and could be explained as the reaction rate increases with concentration. B

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Energy & Fuels For most furans and THFs, among the equivalence ratio range of 0.5−2, the ignitions in richer mixture were slower at relative high temperatures and dilution ratios,14,19,30−32 and the same phenomena appear for DHF, as shown in Figure 3. But

Figure 4. Ignition delay times and fitting equations of stoichiometric DHF and furan30,31 at 4 atm with fuel concentration set at 0.5%.

Figure 5. Derived cetane numbers (DCN) of (hydro) furanic species investigated in an ignition quality tester by Sudholt et al.11 The DCN corresponding to a RON 90 fuel is indicated as reference value for a gasoline fuel. Figure 3. Experimental ignition delay times (symbols) with 20% error bar and fitting equations (dashed line) of 23DHF (a) and 25DHF (b) in 4 atm.

molecule structure significantly affects the ignition characteristics of cyclic ethers. The mechanism of the number and location of double bonds affecting the ignition has been discovered by kinetic analysis in this article. 3.2. Model Modification. As far as we know, no model has been explicitly attempted modeling the combustion of dihydrofuran, but many models for (hydro) furans were integrated by DHF sub model which is basically from the work of Tian et al.18 and contains general reactions for DHF. The pyrolysis and gradual oxidation of some intermediates, such as CPCA and CA, are also deliberately considered in the work of Tran et al.19 and Moshammer et al.20 But up to now, no experiment has been done for DHF oxidation, so the chemistry scheme of DHF combustion has rarely been validated and the rate of many reactions is of high uncertainty. In present work, models from Tian et al.,18 Somers et al.,13 Tran et al.,19 and Moshammer et al.20 were tried to simulate the ignition delay times of DHF. The calculation results are generally similar among these models but none presented satisfactory prediction for ignition delay times and activation energy in this temperature range, as shown in the Supporting Information. However, the deviation of prediction from current experimental data reveals the deficiency in DHF reaction scheme. Therefore, several modifications were done based on the Tran model19 to settle these issues.

when referring to furans under low dilution ratio and intermediate temperature conditions, fuel-rich mixtures ignite faster than fuel-lean ones.33−35 That is because at relative high temperatures and dilution ratios, the chain branching reaction H + O2 = O + OH dominates the ignition process and exhibits greater influential at higher temperatures. The O2 concentration reduces with the increase of equivalence ratio, leading to the decrease of the rate of this key reaction and delaying the overall ignition. As furan, 23DHF, and 25DHF are all cyclic ethers with similar five-number oxygen heterocyclic structures and different numbers and locations of carbon double bonds, comparison of ignition characteristics among the three fuels were performed to evaluate the effect of the double bonds. As shown in Figure 4, ignition delay times of 23DHF and 25DHF are compared with that of furan at 4 atm. The comparison indicated that the ignition of 25DHF is slower than that of 23DHF and furan is the slowest in ignition among the three fuels. The results are consistent with the work of Sudholt et al.11 On an ignition quality tester they discovered that the derived cetane numbers of the three kinds of fuels are furans < DHFs < THFs, as displayed in Figure 5. Such phenomenon indicated that the C

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Energy & Fuels Table 2. Modifications Based on the Model of Tran et al. num

reactions

1 2 3 4 5 6 7 8 9 10 11 12

C4H5O-3 + H = 23DHF C4H5O-3 + H = 25DHF 23DHF = C2H4OE#3 + C2H2T 23DHF = C2H4Z + CH2COZ 23DHF = C3H6Y + CO furan + H = C4H5O-3 CA = >C3H6Y + B2CO O2 + CH3 = HCHO + OH HO2 + CH3 = CH3O + OH C3H6Y + O2 = C3H5Y + HO2 CHO + M = H + CO + M 2CH3(+M) = >C2H6(+M)

n

A 2.60 2.60 1.00 5.75 3.16 2.58

× × × × × ×

1013 1013 1016 1015 1013 1009

0 0 0 0 0 1.482

Ea (cal) 1560 1560 75800 69300 59700 2040

ref as C4H6Z2 + R1H = C4H7Y18 as C4H6Z2 + R1H = C4H7Y18 as C4H6O2344 21

as C4H8O46 13

deleted as acrolein42−44 3.80 × 1011 0 9000 7.70 × 1013 0 0 5.96 × 1019 −1.67 46192.1 4.80 × 1017 −1.2 17700 6.77 × 1016 −1.18 654 LOW/3.400 × 1041 −7.030 2762.00/ TROE/0.6190 73.20 1180.00 9999.00/ H2/2/H2O/6/CH4/2/B2CO/1.5/CO2/2/C2H6/3AR/0.7/

Tran model19 is a relatively new model developed for high temperature oxidation of THF and has been validated by the structure of laminar flames, adiabatic laminar burning velocities, and ignition delay times of THF.19 In the development of this model, automatic generation (EXGAS), Evans−Polanyi correlations, and CBS-QB3 theoretical calculations were combined, and some key kinetic parameters were updated to take into account the specificity of cyclic ethers combustion chemistry. The C0−C4 reaction base,36−38 sub mechanism for benzene formation,39 and reactions for vinyl alcohol40 were integrated in this mechanism. The rates of H abstraction reactions and important isomerization, decomposition reactions of DHF from Tian et al.18 were mostly obtained by analogy or experiment. DHF radicals are also the fuel radicals of furan and tetrohydrofuran whose reaction mechanism is very detailed and specific in Tran model. Detailed reactions for consumption of some intermediates, such as CPCA, butanal, and CA, are from the work of Moshammer et al.20 The fuel-specific reactions of DHF have been modified first. Decomposition reactions of 23DHF were added to capture the pyrolysis results of the shock tube study.21 Moreover, reaction rates of some initial H elimination reactions with certain degree of uncertainty were reconsidered. The modifications also consisted of the adjustment of the reactions of important intermediates, such as fuel radicals, CA, and propene. Several rate coefficients of small molecule reactions were also updated. As a consequence, an optimized reaction mechanism of DHF and better agreement with current experimental data were presented. All the modifications are listed in Table 2. In original model, CA is an important isomer of 23DHF. The rate for CA decomposition to propene and carbon monoxide is from the dissociation of trans-conformer through three- or four-membered cyclic transition states in low temperature (654−763.5 K) and pressure (55.5−150 Torr).41 However, cisCA is most likely the main isomer of 23DHF23,26 and the decomposition rate of trans-CA does not fit for that of cis-CA. According to the work of Chatelain et al.,42−44 there are no such decomposition reactions for acrolein, so the reaction was deleted in the modified model. As for HO2 + CH3 = CH3O + OH, when the rate of 7.7 × 1013 cm3 mol−1 s−1 was used, good agreement between experimental OH time-history and the computation by a detailed iso-octane mechanism was obtained in the work of He

47 45 44 48

GRI 3.049

et al.45 This rate has been adapted in present work to improve the prediction. Based on the modified model (M_Tran model), favorable improvement was obtained at predictions. The activation energy and the reactivity trends are captured at various equivalence ratios and pressures. The updated chemical kinetic model is able to predict the accurate ignition delay times for both 23DHF and 25DHF over this temperature range, as shown in Figure 6. Fair prediction of species distributions for

Figure 6. Experimental (symbols) ignition delay times of 23DHF (a) and 25DHF (b) with 20% uncertainty bars. Solid lines are calculations of M_Tran model; dashed line are calculations of Tran model. D

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Energy & Fuels DHF pyrolysis21,22 was also obtained. Deviation still remains in the prediction of distribution of main isomerization products in the pyrolysis of 23DHF, probably due to the absent of consumption pathways of CA. The prediction of DHF pyrolysis was shown in Figure 7 and in the Supporting Information. After the modification, the model still retains the ability to predict THF ignition. See the Supporting Information for details.

Figure 8. Reaction pathway analysis for the stoichiometric 23DHF(a) and 25DHF(b) at 10 atm, 0.5% fuel concentration, and 20% fuel consumption using the M_Tran model. Black font for 1250 K, red font for 1600 K.

21,22

Figure 7. Experimental species distributions (symbols) and model prediction for pyrolysis of 23DHF (a) and 25DHF (b). Solid line for M_Tran model; dashed line for model of Tran et al.

in Figure 9a, substantial amount of CPCA and CA are produced before the ignition. Propene and propene radicals are also generated in large quantities and this phenomenon is consistent with the studies of Matsubara et al.26 Compared with low temperature condition, the branching ratio of fuel decomposition reactions increases by nearly 10% at high temperature. However, the consumption of 23DHF is still mainly through isomerization to CPCA at high temperature. The reaction pathways of 25DHF are shown in Figure 8b. Most of the 25DHF is consumed by dehydrogenation yielding furan and a small portion is consumed by H-abstraction and Haddition reactions to generates 2,3-dehyrofuran-3-radical and tetrohydrofuran-3-radical. Portions of 2,3-dehyrofuran-3-radical dehydrogenates to furan subsequently. From Figure 9b, furan presents high mole fraction before the ignition, indicating it is an important intermediate in the ignition for its low reactivity. Compared with low temperature condition, the branching ratio of dehydrogenation to furan reduces by 6.5% at high temperature.

As it can well predict DHF ignition in various conditions, M_Tran model was adopted for further analysis. In the following sections, reaction pathway analysis and sensitivity analysis were done at the same pressure and temperatures to find out the key reactions for the two fuels to comprehensively understand the chemical kinetics of DHF ignition. 3.3. Reaction Path Analysis of 23DHF and 25DHF Oxidation. Based on the M_Tran model, reaction pathway analysis was conducted to figure out the main consumption ways of DHF and meaningful intermediates. Figure 8a shows the reaction pathways of stoichiometric 23DHF at 10 atm, 20% fuel consumption. Most of the 23DHF is consumed by isomerizing to cyclopropane caboxaldehyde (CPCA), or unimolecular decomposition to ethylene and vinyl aldehyde. Croton aldehyde (CA) is also yielded through the isomerization of 23DHF, but the primary generation pathway is isomerization from CPCA. The isomerization reactions between 23DHF, CPCA, CA have attracted a lot of attention18,19,23,26 in 23DHF reaction scheme. As illustrated E

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Figure 9. Calculated temperature and species mole fractions of 23DHF (a) and 25DHF (b) ignition at 10 atm, 1250 K, and 0.5% fuel fraction. Red dashed lines for temperature.

Figure 10. Reaction sensitivity analysis for the stoichiometric 23DHF (a) and 25DHF (b) using the M_Tran model at 10 atm, 0.5% fuel concentration, 1250 K and 1600 K, the same condition as at pathway analysis. Sensitivity coefficient of reaction H + O2 = OH + O is multiplied by 1/3 for clarity.

The difference in kinetic schemes could partly explain the observed trends in the ignition of DHFs. Most 23DHF is consumed by isomerization yielding CPCA and further to CA which could transform to small molecules in many ways. Nevertheless, 25DHF mainly dehydrogenates to stable furan which is liable to delay the ignition. As shown in Figure 9, furan remains longer time and higher content in ignition of 25DHF than that of CA in the ignition process of 23DHF under the same condition. In other words, 23DHF is faster in transforms to small molecules. That should be the reason for the slower ignition of 25DHF than that of 23DHF, as shown in Figure 4. 3.4. Sensitivity Analysis of 23DHF and 25DHF Oxidation. To find out the most influential reactions to the overall ignition of DHF, sensitivity analysis was done based on M_Tran model. The normalized sensitivity coefficient for the ith reaction in this work is defined as the following equation. Si =

τ(2k i) − τ(0.5k i) 1.5τ(k i)

demonstrate limited influence in ignition delay, probably because the instant of pathway analysis is at the very beginning of the ignition process while the sensitivity analysis is for the exploration of the overall ignition. The oxidation reactions of CH3 and acetylene show large sensitivity, especially at low temperature. According to reaction pathway analysis, CH3 and acetylene are the decomposition products of propene radical which is an important intermediate in 23DHF ignition. Figure 10 b illustrates the ignition sensitivity analysis of 25DHF. Compared with that of 23DHF, fuel reactions and primary product reactions of 25DHF presented relatively high sensitivity. 25DHF = furan + H2 and furan + H = C4H5O-3 are the two reactions that express the most prohibit effect on ignition at 1600 K. C4H5O-3 + H = 25DHF exhibits strong impact on promoting ignition. The sensitivity of 25DHF = furan + H2 shows considerable difference at high and low temperatures though wide reaction channel is kept for this reaction at both temperatures. The oxidation reactions of small species, such as acetylene, CH3, and CHO, show great acceleration effect on the ignition of both 23DHF and 25DHF. 3.5. Influence of Carbon Double Bond on Ignition Delay Time. The ignition delay times of 23DHF and 25DHF have been compared with that of furan in Figure 4. In this section, more ignition and structure data of the three cyclic

(1)

Where Si is the ignition delay time sensitivity, τ is the ignition delay time, and ki is reaction rate of the ith reaction in the mechanism. Figure 10a shows the ignition sensitivity analysis of 23DHF at 10 atm, the same condition as pathway analysis. The ignition of 23DHF is mainly sensitive to small molecule reactions involving H2−O2 and C1−C3 species, while the three decomposition reactions of 23DHF present notable influence. Although the isomerization reactions of 23DHF hold large percentage in initial reaction pathways, these reactions F

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Sudholt et al.11 summarized the boundary dissociation energies (BDEs) for furans and THFs and reported that the C−H bonds at unsaturated carbon atom are very strong, while the BDE of the H-abstraction from the saturated carbon atom closed to oxygen atom or double bond is significantly lower. As shown in Figure 12, the BDEs of ring-hydrogen bonds of DHF are more sensitive to location than that of furan and THF. Compared with oxygen atom, the adjacent double bond is more likely to cause the instability of α-H. Due to the adjacent double bond in the ring and the influence of the oxygen atom, the C2− H bond and C5−H bond of 25DHF are very weak, leading to the general tendency of H2 elimination and the generation of two more unsaturated carbon atoms. Consequently, furan is generated at large quantities and delayed the ignition of 25DHF. The molecular structure of the 23DHF is unsymmetrical. The double bond and the oxygen atom cause the low BDE of C2−O bond. Thus, 23DHF processes ring-opening isomerization to CPCA and CA through C−O bond scission with relatively lower activation energy.26 Moreover, H-shift occurs on C2 and C3 during the generation of CA. Subsequently, CPCA is largely isomerized to CA. After the isomerization, the subsequent decomposition and H-abstraction reactions of CA helped the transformation to small molecules and accelerated the ignition. The ignition delay time of 23DHF is shorter than that of 25DHF, suggesting the effect of the location of carbon double bond on ignition characteristic. From Figure 12, the BDEs of the ring-hydrogen bonds of furan are similar and quite high (higher than that in the corresponding aromatic species). Furthermore, the ring strain is very strong due to the double bonds on furan ring. As a consequence, the fairly stable cyclic structure of furan determines its relatively weak ignition performance. Without the double bonds, the BDEs of C−H bonds and C−C bonds on saturated THF ring are significantly lower, thus the decomposition and ignition of THF are more likely to occur. As Figure 12 shows, the structure stability of DHF is between that of furan and THF, as is the ignition property,11 revealing the effect of the number of the double bonds on ignition characteristic of the three species. Figure 13 shows the sensitivity analysis of the three fuels under the same condition. Some reactions of primary products present higher sensitivity and the reaction schemes of the three fuels are more distinctive at 4 atm than that at 10 atm. Both furan and 25DHF ignition are greatly prohibited by H elimination reaction of furan + H = C4H5O-3, which is the largest furan consumption reaction in laminar flames15 and ignition behind shock waves.30 This reaction displays notable influence on 25DHF ignition probably because furan is the most and the most stable intermediate in the ignition of 25DHF. For the same reason, many reactions of furan show strong sensitivity for 25DHF ignition, such as furan + OH = C2H3CHOZ + CHO and furan + H = C4H5O-3. 25DHF =

ethers were displayed to further perceive the influence of the number and location of carbon double bond on ignition delay. The ignition delay times and the correlations of furan, 23DHF, and 25DHF are shown in Figure 11. The activation

Figure 11. Experimental data and the fitting equations of ignition delay times for furan,30,31 23DHF, and 25DHF at stoichiometric mixture, 0.5% fuel concentration, and approximate pressure of 1.2 atm (a), 10 atm (b).

energy of furan is the highest among the three fuels owing to its steepest slope. While the effects of temperature on ignition delay times are similar between 23DHF and 25DHF, meaning that the active energies of 23DHF and 25DHF are very close. The ignition delay times of 25DHF is more sensitive to pressure among the three fuels. Moreover, the ignition delay time of furan is the longest, while that of 23DHF is the shortest under this research conditions. This is consistent with the data in Figure 4 and experiment results of Sudholt et al.11 The ignition characteristic is affected by molecular structure through chemical kinetics, so the kinetics and structure of the three species were compared in the following to study the influence of molecular structure on the ignition of these cyclic ethers.

Figure 12. Bond dissociation energies (kcal/mol) for the ring hydrogen bonds of furan,50 DHF,11 and THF.25 G

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both 23DHF and 25DHF are mainly oxidation reactions for small molecules, such as CH3, CHO, and acetylene.

4. CONCLUSIONS The present study investigated ignition delay times for 23DHF and 25DHF at equivalence ratios of 0.5, 1.0, 2.0; pressure of 1.2, 4, 10 atm; and temperature range of 1640 to 1080 K with the fixed fuel concentration of 0.5%. The results show a pleasing consistency and considerably expand the available data for cyclic ethers. A modified model (M_Tran model) based on Tran model was presented to improve the prediction of ignition delay data of 23DHF and 25DHF. The ignition delay times of 23DHF are shorter than that of 25DHF in all the experiment conditions. Reaction pathway analysis and sensitivity analysis using M_Tran model provide insight into the ignition process of the two fuels. Most of 23DHF is consumed by isomerization reactions. However, 25DHF is mostly consumed by H2 elimination yielding stable furan and delays the ignition. The ignition delay times of 23DHF and 25DHF are mainly sensitive to small molecule reactions and few fuel-specific reactions. Some H abstraction reactions of CA and propene also show strong sensitivity for 23DHF ignition at 4 atm. Many reactions of furan present strong effect on 25DHF ignition. The sensitivity of 25DHF = furan + H2 shows considerable difference in high and low temperatures at 10 atm though wide reaction channel is kept for this reaction at both temperatures. The ignition delay times and kinetic schemes of furan and DHF were compared to discover the mechanism of the number and location of double bonds affecting the ignition of cyclic ethers. Furan has the longest ignition delay in this high temperature range owing to its stable molecular structure with two double bonds. Due to the adjacent double bond and oxygen atom, the C−H bond at C2 and C5 of 25DHF are the weakest, leading to the general occurrence of H2 elimination yielding stable furan which delays the ignition of 25DHF. Influenced by double bond and oxygen atom on the ring, the boundary dissociation energy of C2−O bond of 23DHF is relatively lower, resulting in the ring-opening isomerization to CPCA and further to CA. Subsequent decomposition and dehydrogenation of CA accelerate the overall transformation to little molecules and boost the ignition of 23DHF.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01332. (PDF)

Figure 13. Reaction sensitivity analysis for the stoichiometric furan30(a), 23DHF (b), and 25DHF (c) at 4 atm, 0.5% fuel concentration, and 1250 K. Reaction H + O2 = OH + O is omitted for clarity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

furan + H2 expresses the most prohibited effect on ignition at 1600 K, 10 atm, but the sensitivity of this reaction was not distinct at 1250 K, 4 atm. The H abstraction reactions of CA and propene show impressive influence on 23DHF ignition at 4 atm, illustrating the importance of these species during the ignition process. Reactions performing remarkable influence on

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the funding supports from National Natural Science Foundation of China (61235003). H

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DOI: 10.1021/acs.energyfuels.6b01332 Energy Fuels XXXX, XXX, XXX−XXX