Effect of 2,5-dimethylfuran addition on ignition delay times of n-heptane at high temperatures

Abstract

The shock tube autoignition of 2,5-dimethylfuran (DMF)/n-heptane blends (DMF0-100%, by mole fraction) with equivalence ratios of 0.5, 1.0, and 2.0 over the temperature range of 1200–1800 K and pressures of 2.0 atm and 10.0 atm were investigated. A detailed blend chemical kinetic model resulting from the merging of validated kinetic models for the components of the fuel blends was developed. The experimental observations indicate that the ignition delay times nonlinearly increase with an increase in the DMF addition level. Chemical kinetic analysis including radical pool analysis and flux analysis were conducted to explain the DMF addition effects. The kinetic analysis shows that at lower DMF blending levels, the two fuels have negligible impacts on the consumption pathways of each other. As the DMF addition increases to relatively higher levels, the consumption path of n-heptane is significantly changed due to the competition of small radicals, which primarily leads to the nonlinear increase in the ignition delay times of DMF/n-heptane blends.

Keywords

ignition delay time / shock tube / kinetic model / 2,5-dimethylfuran (DMF) / n-heptane

Introduction

The strengthening of emission regulations and rapid depletion of fossil fuels are the primary issues in the energy field, and increasing the percentage of alternative fuels in global transport is considered as a prospective way to address the issues. Biofuels, being renewable as they are, have attracted great interest as transportation fuels, of which, 2,5-dimethylfuran (DMF) offers several attractive combustion features. For example, the relatively higher research octane number (119) of 2,5-DMF leads to a higher engine knock resistance than gasoline [¹]. A higher energy density (30 MJ/L) makes it more competitive as an automotive energy carrier compared to other conventional biofuels, such as ethanol (20 MJ/L). One of the key problems related to the use of alternative fuels is the potential environmental damage. Thanks to its lower water solubility, DMF is perceived as more environment friendly in the view of reduction in water contamination [²,³]. In the past several years, however, DMF received little attention due to its difficulty in fuel preparation. Thanks to the significant breakthroughs made in the production methods [⁴,⁵], it became possible for DMF to be widely applied as a main automotive alternative fuel.

Many studies have been conducted to investigate the potential of using DMF as a gasoline substitute or additive. Xu et al. [¹,⁶] have carried out investigations on the use of DMF as a biofuel in an engine for the first time. The comparison of the performance and emissions between DMF and commercial gasoline in a single-cylinder, spark-ignition (SI), gasoline direct-injection (GDI) engine have showed that, DMF exhibits very similar combustion and emission characteristics to gasoline. These findings offer the potential of using DMF as a gasoline surrogate without significantly modifying the engines. Daniel et al. [⁷] have comparatively investigated the emissions characteristics in the combustion process of bio-alcohols, DMF and gasoline. DMF is found to give the lowest emissions of acetaldehyde, formaldehyde, and carbonyl compounds. Measurements of the knocking propensity of DMF blended with gasoline fuel have been performed by Rothamer and Jennings [⁸]. The results indicate that the knocking resistance is enhanced significantly by a lower level of DMF addition, with major pollutants such as particulate matter, HC, and NO_x being inhibited. Similar findings can be also found in the study of Gouli et al. [⁹].

The effect of DMF addition on the emission characteristics using a modified single cylinder compression-ignition (CI) engine has been investigated by Zhang et al. [¹⁰]. The results show that when the DMF addition level increases up to 40%, the NO_x/soot trade-off disappears and soot emissions reduce to approximately zero. Meanwhile, little effect on emissions of CO and THC is observed. Compared to the extensive studies in spark-ignition (SI) engines, studies concerning the use of DMF as a diesel substitute or additive in CI engines are extremely scarce.

In fact, DMF could be a promising candidate in reactivity controlled compression ignition (RCCI) combustion. In RCCI combustion, fuels with a higher reactivity such as diesel are used to regulate the ignition and combustion of fuels with low reactivity such as gasoline, alcohols, etc. As an advanced low temperature combustion (LTC) technique, RCCI combustion offers prominent benefits in terms of simultaneous reduction of NO_x and particulate matter (PM) [¹¹], achieved by directly injecting the high-reactivity fuel into the cylinder and supplying the low-reactivity fuel to the intake manifold [¹²,¹³]. Gasoline and diesel are usually used as the low-reactive and high-reactive fuels, respectively, for RCCI combustion research [¹⁴–¹⁶]. The work of Curran et al. [¹⁷] has showed that NO_x and PM emission decrease by more than 90% using the diesel/gasoline combination to achieve RCCI combustion, demonstrating the great potential of this strategy to control in-cylinder fuel reactivity for improved efficiency and lower emissions. Due to the similarity of combustion characteristics between DMF and gasoline, as mentioned previously, DMF can be a promising candidate as a low-reactivity fuel blended with diesel for RCCI combustion. Hence, it is worthwhile to conduct studies on the ignition and combustion kinetics involved in the diesel/DMF fuel blends combustion, which have seldom been the subject of research. Since commercial diesel fuel consists of many types of components, n-heptane has been widely used as a diesel surrogate. Thus, n-heptane is adapted as the substitute for diesel fuel in this study.

Many works have also been published regarding fundamental combustion researches of furans, most of which focused on the measurement of global combustion parameters such as flame, ignition, and pyrolysis and theoretical calculation of the key reactions [³,¹⁸–²³]. A detailed DMF chemical kinetic model has been developed by Sirjean et al. [³]. Ignition delay time measurements of DMF/O₂/Ar have been performed using a shock tube over a temperature range of 1300–1831 K, at pressures of 1.0 atm and 4.0 atm, and equivalence ratio ranges of 0.5–1.5. The validation results demonstrate reasonable agreements between experimental values and simulations. Somers et al. [²] have updated this model by combining the furan sub-model extracted from the work of Sirjean et al. [³] with an existing foundational model. Validation experiments have been conducted at higher pressures of 20 atm and 80 atm using a shock tube. The experimental results show that the model of Somers et al. achieves better agreement with the measured ignition data than that of Sirjean et al. at higher pressures. Since the two models share the same DMF sub-model, the improvement should be attributed to the refined C1–C5 base model. Recently, Qian et al. [²⁴] have presented a comprehensive review of the recent progress in the development of DMF, including scission reaction mechanism, laminar flame speed, shock tube ignition delay, and low pressure premixed flame, etc. They conclude that further exploration of the detailed and simplified model of DMF is needed. Particularly, the model development of DMF blends with gasoline and diesel is highlighted.

As a primary reference fuel (PRF), n-heptane has been extensively investigated in a range of experimental facilities, including shock tube [²⁵–³⁰]. Much effort has been devoted into the development of its kinetic model at both high and low temperatures to make its reaction process better understood since 1979 [²⁶]. Westbrook et al. [³¹] have developed a kinetic model describing the oxidation of iso-octane and n-heptane. This model consists of both high- and low-temperature chemistry, and is validated against a series of experimental data including shock tube data. A systematic study of the n-heptane oxidation has been conducted by Curran et al. [³²], and a new model has been released with overall performance improved. Besides, a successful frame for the model development process has been set by categorizing the reaction classes. Mehl et al. [³³] have further refined and adopted this model in their work of kinetic development for gasoline substitutes at engine relevant conditions. This model achieves a good agreement between the simulation and experiment across wide temperature and pressure ranges.

More recently, researchers still keep their interest in providing a better kinetic model describing the n-heptane combustion [³⁴–³⁶]. However, all of the models focus on the improvement of the model performance at lower temperatures (JSR and RCM conditions). The high temperature chemistry of n-heptane has been well understood by current kinetic models.

In this paper, high temperature shock tube measurements have been performed for n-heptane, DMF, and DMF/n-heptane blends (20/80 mol%, 50/50 mol%, 80/20 mol%) at conditions of various equivalence ratios, pressures, over a range of temperatures. Then, a DMF/n-heptane blend kinetic blend model has been constructed to better understand the chemical phenomenon behind the ignition process. This paper may contribute to further simulation works with regards to the combined ignition behaviors of DMF/n-heptane.

Experimental specifications

Setup and procedures

Ignition delay times of n-heptane/DMF fuel blends are measured using a shock tube. The high-temperature shock tube with an inner diameter of 11.5 cm is composed of three parts: the driver section 2.0 m in length, the driven section 5.3 min length, and the flange section 0.06 m in length. The driver and driven sections are bolted together by the flange section while two polyethylene terephthalate (PET) diaphragms are used to divide the entire tube into three separate parts. Desired reflect shock pressures can be achieved by the instant burst of the double diaphragms with corresponding thickness. High-purity helium (99.999%) and nitrogen (99.999%) are used as the driver gas which bursts the diaphragms due to the large pressure differential between the driven and driver sections. Fuel mixtures in the driven section can be heated to a certain temperature in a very short time by the compression of the shock wave.

Four pressure transducers (PCB 113B26) are located along the driven section with an identical interval of 30 cm. These pressure transducers together with three time counters (Fluke PM6690) allow the measurement of the incident shock velocity, which can be used to determine the reflected shock temperature (T₅) through one-dimensional shock relations. At the end-wall, another piezoelectric pressure transducer (PCB 113B03) is used to monitor the pressure rise to determine the arrival of the incident shock wave. At the center of the end-wall, there is an optical window through which the light emission (OH*) can be detected by a photomultiplier (Hamamastu CR 131) together with a narrow band pass filter of 307 nm.

Reactant mixtures are prepared in a 128 L stainless steel mixing tank. The partial pressure of the fuel in the tank is assured to be less than 50% of its saturated vapor pressure at room temperature to exclude fuel condensation. The mixing time is at least 10 h to ensure homogeneity. Before each experiment, the driven section needs to be evacuated below 10⁻⁵ bar and the leak rate needs to be kept less than 10⁻⁶ bar/min.

Definition of ignition delay time and system validation

The onset of ignition in this study is determined by extrapolating the maximum rate of OH* emission increase to the baseline. Thus the ignition delay time is defined as the time interval between the arrival of the shock wave and the onset of ignition, as shown in Fig. 1. As a result of the boundary effect, a slight pressure rise (dp/dt≈ 4%/ms) can be found before ignition occurs. This effect has been considered when performing kinetic simulations by giving a pressure-time history corresponding to the pressure rise. The ignition delay measurements have an uncertainty of 18% and corresponding error bars are added to the measured ignition data.

To validate the shock tube and data processing method, the ignition delay time of n-heptane was measured under the same test condition as that of Horning et al. [³⁷]. A reasonable agreement between the two data sets as well as the simulations was achieved, as displayed in Fig. 2. This provides confidence in performing ignition delay measurements for the target fuels in this study.

Adopting the mixture preparation method of Herzler and Naumann [³⁸], the fuel/O₂/Ar mixtures (X_Ar/X_O2 = 79%/21%) are diluted with Ar (80% Ar/20% mixture), and the dilution ratio is kept constant for all mixtures. The detailed test conditions in this study are given in Table 1.

Results and discussion

Kinetic model construction

To perform deep kinetic analysis and to better understand the DMF/n-heptane blend ignition chemistry, a detailed kinetic blend model was developed. Simulations were conducted using the Chemkin package [³⁹] and Senkin [⁴⁰]. The first step for developing the blend model is to choose two well-developed individual models for the two fuel components. A good model for n-heptane (LLNL model, version 3.1) proposed by the research group of Lawrence Livermore National Laboratory (LLNL) [³³] was adopted because it has been validated by comparison to experiments in shock tubes. For DMF, the model released by Somers et al. [²], as mentioned earlier, was used. The coupling process started with comparing the two individual kinetic models, after which the common reactions between the two models were removed for the n-heptane model. Then, by incorporating the modified n-heptane model into the DMF model, the blend model consisting of 4442 reactions and 993 species was constructed. Thermo-chemical data was obtained by merging those of the two models and eliminating the replicated species.

Model validation against experimental measurements

A blend model should give as accurate predictions as the original ones for pure components. Or at least, there should not be any significant deterioration. Figure 3 is a comparison between the experimental data and the calculating results for neat DMF using both the blend model and the original Somers model. It can be seen that the blend model gives virtually the same predictions as that of the Somers model, which is unsurprising since the two models are combined based on the Somers model. To be specific, quite a few reactions exist in both the DMF model and the n-heptane model, with different rate coefficients. Regarding these reactions, the blend model adopted those presented in the Somers model. A comparison of predictions for n-heptane ignition is presented in Fig. 4. As shown in Fig. 4, both the blend model and LLNL model give reasonably good predictions of the ignition delay times of n-heptane. Overall, the blend model can well reproduce the measured ignition data of both pure DMF and n-heptane. The calculated ignition delay times using the blend model along with the experimental measured data at different levels of DMF addition (0%–100%, by mole fraction) are depicted in Fig. 5. Since the ignition delay times at an equivalence ratio of 2.0 among these fuel mixtures are very close, only pure DMF, DMF/n-heptane blends (1:1, by mole fraction), and pure n-heptane are plotted, as shown in Fig. 5(d). As the results show, the blend model gives reasonable predictions on the ignition delay times of binary fuel blends (20/80 mol%, 50/50 mol%, 80/20 mol%) at different equivalence ratios and two pressures.

Overall, the modeling results agree well with the experimental values for both pure components and binary fuel blends. This suggests that the blend model can well describe the ignition chemistry of the two fuels, which allows a deeper insight into the chemical process behind the ignition behaviors.

Kinetic explanation for DMF blending effect

As demonstrated in Fig. 5, an increase of DMF mole fraction in the fuel blends results in an increasing ignition delay times at all test conditions. Generally, the reactivity of fuel blends lies between their two neat components. In this work, as seen from Fig. 5, n-heptane is the most reactive (shortest ignition delay) while DMF is the least reactive (longest ignition delay). The reason for this is that, at high temperatures, the n-heptane is attacked by small radicals such as O, H, OH, and HO₂ in the first step. The alkyl radicals (C₇H_15–n, n = 1, 2, 3, 4) formed in this way decompose to smaller alkyl radicals through fast thermal elimination of alkenes. DMF primarily (approximately 50%) decomposes via H-abstraction at the methyl side to give DMFA252J at higher temperatures according to Xu et al. [⁴¹]. DMFA252J is consumed dominantly by yielding products such as C₅H₆ and C₆H₅OH, with reactive radicals consumed and aromatics generated, leads to a decrease in the system reactivity. In addition, the markedly high C-H and R-CH₃ bond dissociation energies of alkylfurans (DMF included), has been pointed out by Simmie and Curran [²³]. This reduces the chances of the bond breakage at these sites, which, therefore, makes the autoignition of DMF more difficult to happen and eventually leads to a longer ignition delay time.

To elucidate the effect of DMF blending on the autoignition, kinetic analyses were performed. It has been widely accepted that small radicals such as O, OH, H, etc., are essential for high temperature ignition [⁴²,⁴³]. In other words, ignition would not happen until these small radicals are fully developed. Hence, radical pool (sum of OH, O, H, C₂H₅, CH₃, and HO₂) analysis may help to understand the blending effect. Evolution profiles of radical pool concentration for fuel mixtures at different DMF addition levels were calculated using the blend model, as exhibited in Fig. 6. The drastic rise of the curves stands for the onset of ignition and the period before ignition is called induction period. The simulation results demonstrate a decrease in the radical pool concentration with an increasing DMF addition level, which results in longer ignition delay times.

Dominant reactions involved in the ignition of the DMF/n-heptane blends can be reflected by sensitivity analysis. Here, the sensitivity coefficient was calculated by respectively increasing and decreasing the pre-exponent constant by a factor of 2:

(1)S={\displaystyle \frac{{\tau }_{\mathrm{2}k}-{\tau }_{\mathrm{0.5}k}}{{\tau }_k}},

where S is the sensitivity coefficient, τ is the ignition delay time, and k is the rate constant of a certain reaction. Figure 7 presents the sensitivity coefficients of the most sensitive reactions at T = 1300 K, p = 2.0 atm, and f = 1.0. A positive S value implies that the ignition delay time increases with the increasing rate constant of a particular reaction and vice versa. In other words, the reactions with the sensitivity coefficient>0 inhibits ignition and those with a negative one have a promoting effect on ignition. It can be seen that the sensitivity of most ignition promoting reactions decreases remarkably with an increase in the DMF addition level, which indicates a reduction of the importance of these reactions.

Specifically, the reaction R1357 is usually considered as the most important ignition promoting reaction at high temperatures and it experiences a dramatic decrease in the sensitivity coefficient as DMF addition increases. Reactions R1686, R1658, and R1673 are associated with the species C₂H₄, which is primarily the products of β-scission of n-heptane fuel radical generated from H-abstraction at the terminal carbon atom.

The calculated ignition delay times using the well validated blend model versus DMF addition levels in the fuel/O₂/Ar mixtures is plotted at T = 1300 K, as shown in Fig. 8. The ignition delay times increase with increasing DMF fraction at all test conditions. What is noteworthy is that the ignition delay increase is nonlinear as DMF fraction increases. For instance, only around 5% increment of ignition delay time (f = 1.0, p = 2.0 atm) was observed when the DMF addition level increases from 0% to 20%. Nevertheless, another 20% addition of DMF (addition level increases from 80% to 100%) at the condition results in an increase of nearly 30% in the ignition delay times.

From another perspective, the nonlinear blending effect can be interpreted as the fact that the ignition can be obviously promoted by a small amount of n-heptane added into pure DMF while adding a small amount of DMF into pure n-heptane has little effect on the ignition. During the induction period, as seen from Fig. 6, DMF has the lowest radical pool concentration. When it is blended with 20% n-heptane, the concentration of radical pool is significantly promoted. Yet, there is no obvious change between the fuel with 20% DMF addition and pure n-heptane. These simulated observations are consistent with the experimental values.

To deeply investigate the interaction effects between the two fuels during the ignition process, normalized fuel consumption rates are simulated. It can be seen from Fig. 9 that n-heptane consumes faster than DMF. The consumption rate of n-heptane hardly changes when 20% DMF is blended, while that of DMF is promoted remarkably. This shows that n-heptane plays an overwhelmingly dominant role in the blend ignition process at a DMF addition level of 20%. The promoted consumption of DMF should be the result of an early rise in temperature compared to pure DMF, as shown in Fig. 10. When the addition level of DMF is increased up to 80%, as shown from Fig. 11, the consumption rate of both DMF and n-heptane is obviously changed.

Reaction pathway analysis was performed under the same condition for fuel mixtures with 0%, 20%, and 80% of DMF addition (pure n-heptane). The main consumption path of n-heptane is plotted in Fig. 12. As shown in Fig. 12, for pure n-heptane, the majority (70.9%) would undergo H-abstraction to yield fuel radicals through the attack by radicals such as CH₃, OH_, H, and O. With 20% DMF addition, the H-abstraction is still the most prominent fuel consumption path with a total contribution of 69.8%, which is almost the same as that of pure n-heptane. This indicates that an addition of 20% DMF hardly influences the chemical process when n-heptane plays a dominant role in the ignition. However, this value falls drastically to 39.8% when the DMF fraction increases to 80%. As noted previously, this case can be treated as 20% addition of n-heptane to DMF, where DMF dominates the ignition. The results suggest that under this condition, n-heptane and DMF compete for active radicals such as CH₃, OH and H. Because DMF dominates in content, a large amount of these radicals (H especially) are consumed, inhibiting the first step breakage of n-heptane as discussed previously. This promotes the consumption of DMF and eventually the ignition. This helps to better understand the chemical phenomenon behind the nonlinear effect of DMF addition on the ignition of DMF/n-heptane blends.

Conclusions

This paper presented novel shock tube experiments on the high temperature autoignition of n-heptane, DMF and their binary blends at several blending levels. A blend model of DMF/n-heptane was constructed, with a good agreement between the model and experiment. Kinetic analyses were performed to better understand the mutual effect between the two components.

Generally, DMF addition inhibits the ignition of DMF/n-heptane fuel blends. Noteworthy is that the ignition delay demonstrates a nonlinear increase trend versus the DMF addition level. At relatively lower addition levels of DMF, n-heptane dominates the blend ignition. An addition of 20% DMF hardly influences the chemical consumption channel when n-heptane plays a dominant role in the ignition. As the addition increases to relatively higher levels where DMF dominates the ignition chemistry, a large amount of reactive small radicals such as CH₃, OH, and H are consumed by DMF, inhibiting the first step breakage of n-heptane. This promotes the consumption of DMF and eventually leads to a shorter ignition delay. This helps to explain the nonlinear effect of DMF addition on the ignition of DMF/n-heptane blends.

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