Measurement of Laminar Flame Speed and Flammability Limits of a

Sep 8, 2016 - Validation of the experimental setup and methodology was achieved by measuring the laminar flame speed of n- heptane/air mixtures at ...
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Measurement of Laminar Flame Speed and Flammability Limits of a Biodiesel Surrogate Carlos A. Gomez Casanova,† Edwin Othen,‡ John L. Sorensen,‡ David B. Levin,§ and Madjid Birouk*,† †

Department of Mechanical Engineering, ‡Department of Chemistry, §Department of Biosystems Engineering, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ABSTRACT: Laminar flame speed of a newly developed biodiesel surrogate, 1,3-dimethoxyoctane, has been experimentally determined at standard atmospheric pressure and 433 K using a spherical combustion chamber. The 29 L combustion chamber has two pairs of fused silica windows for optical access and is also equipped with 4 pairs of axial fans for generating isotropic turbulence, which is used here to prepare homogeneous combustible mixtures of fuel and air. Schlieren technique was used to visualize and document the temporal evolution of the outwardly propagated spherical 1,3-dimethoxyoctane/air flame. An inhouse developed Matlab code was employed for postprocessing of the flame front images and determining the time histories of its radius. Validation of the experimental setup and methodology was achieved by measuring the laminar flame speed of nheptane/air mixtures at atmospheric pressure and 353 K, and compared with published reports. The results revealed that the proposed biodiesel surrogate, 1,3-dimethoxyoctane, has an overall flame speed comparable to that of published biodiesel surrogates. Furthermore, the flammability limits of the newly developed biodiesel surrogate were similar to those of gaseous hydrocarbons (e.g., methane, ethane) and higher than those of liquid fuels (e.g., gasoline, diesel).

1. INTRODUCTION Growing demand for liquid transportation and industrial fuels has raised concerns about fuel supply sustainability. While new oil reserves continue to be discovered, many of them are in remote locations that are very difficult, and thus costly, to access. In addition, increasing concentrations of carbon dioxide and other greenhouse gases in the atmosphere are a consequence of fossil fuel combustion and contribute to the alarming rate of climate change. Thus, there is growing urgency to develop novel renewable fuels to displace or supplement hydrocarbon-based fuels.1,2 In this regard, biodiesel, a liquid biofuel composed of methyl esters of vegetable-derived oils, has been suggested as a feasible and environmentally compatible alternative to petroleum-derived diesel fuel. Biodiesel is produced by transesterification of triglycerides, most commonly derived from oil seed crops, with methanol to produce the fatty acid methyl ester (FAME).3 Biodiesel fuels have low to zero sulfur content and have also been shown to reduce carbon monoxide and particulate emissions. Biodiesel fuels are also renewable, sustainable, and biodegradable. However, biodiesel fuels do have some inherent drawbacks, such as lower heating value compared to liquid hydrocarbon fuels (e.g., diesel, gasoline), high viscosity, and high boiling temperature.3,4 Several studies have been directed to develop viable biodiesel candidates to replace petroleumbased diesel fuels as a stand-alone combustible, or to blend biodiesel with petroleum-based diesel to power CI engines.5,6 Low-temperature combustion, which enables the operation of combustion engines at fuel lean conditions with outstanding performance and relatively low emissions, is a promising, but still emergent technology.7,8 In this regard, homogeneous charge compression ignition engines (HCCI) have received increased attention in recent years. HCCI engines combine the low fuel consumption of premixed fuels, which were originally developed for spark ignition engines (SI), and the outstanding © XXXX American Chemical Society

thermal efficiency of compression ignition engines (CI), to generate a combustion process mainly governed by autoignition and combustion rates that are highly dependent on fuel chemical kinetics rather than fuel atomization and evaporation.9,10 An emergent variation of the HCCI engine denominated reactivity controlled compression engine (RCCI), which combines the outstanding environmental features of premixed combustion with the injection of small portions of fuel during the compression cycle to control autoignition, can also be suggested as a possible application for biodiesel surrogates.11 Similarly, dual-fuel engine, which operates on a premixed gaseous fuel/air mixture ignited with the injection of a pilot high cetane number liquid fuel (e.g., diesel), is another promising technology for which biodiesel surrogates could be implemented.12 Even though biodiesel appears to be a feasible and sustainable alternative fuel source, published experimental data on some of the combustion properties (e.g., laminar flame speed) are still very scarce. The conditions required to measure such combustion properties are quite challenging experimentally owing to high boiling temperature, condensation, formation of different thermal zones, and thus make them almost impractical to determine in a laboratory setting.13 To overcome these issues, biodiesel surrogates, which are shorterchained carbon oxygenated compounds with chemical kinetic behaviors that are similar to their heavier-chained emulated fuels (e.g., palm, rapeseed, soybean methyl esters) may be used.5,14,15 Certain criteria for selecting appropriate surrogates (e.g., H/C ratio, flame speed, and ignition characteristics) have been established to select biofuel candidates that resemble (i.e., Received: June 20, 2016 Revised: September 8, 2016

A

DOI: 10.1021/acs.energyfuels.6b01513 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Two step synthesis of 1,3-dimethoxyoctane (DMO) from methyl 3-hydroxyoctanoate.

alone fuel or as a blend. Additionally, the flammability limits of this newly developed biodiesel surrogate was also determined. The combustible mixture test conditions are atmospheric pressure and 433 K. This elevated temperature is required to enable fuel evaporation and also to avoid condensation.

have similar properties to biodiesel) biodiesel fuel.15 Coniglio et al.5 compiled recent numerical and experimental research on biodiesel surrogates, such as methyl butanoate, crotonate, octanoate, and decanoate. The properties of a fuel are influenced by its chemical composition. Consequently, a complete understanding of the chemical kinetics involved in the generated combustion reactions is essential for evaluating its feasibility for combustion applications.15 For instance, laminar flame speed is an important combustion property that provides fundamental information about the reactivity of a fuel. Empirical data (e.g., measurement of laminar flame speed) is essential for numerical simulations that enable the validation of chemical kinetic mechanisms, as well as for predicting engine performance and pollutant emissions.16,17 Several published studies reported experimental data of different biodiesel surrogates. Wang et al.4 measured the laminar flame speed of three monoalkyl methyl estersbutanoate (C5H10O2), crotonate (C5H8O2), and decanoate (C11H22O2)in a counterflow-jet configuration at 403 K and atmospheric pressure. They observed similar flame speeds for methyl decanoate and methyl crotonate, which were both slightly higher than that of methyl butanoate, which peaked at about 60 cm/s at an equivalence ratio near 1.1. Laminar flame speed of methyl butanoate was reported by Liu et al.,18 Dooley et al.,19 and Golovitchev and Yang.20 They all performed measurements at atmospheric pressure and initial temperatures ranging between 298 and 353 K for different equivalence ratios, and reported about 20% lower flame speed than that measured by Wang et.al.4 This discrepancy is mainly caused by the difference in the initial temperature of their mixtures. In numerical studies, Alviso et al.21 and Dievart et al.22 reported laminar flame speed of methyl decanoate, at 1 bar and 403 K, with a peak value of about 60 cm/s at slightly fuel rich conditions similar to that found by Wang et al.4 Rotavera et al.23 measured laminar flame speed of methyl octanoate (C9H18O2) and methyl cyclohexane at atmospheric pressure and 443 K using the outwardly propagating spherical flame approach, and reported comparable flame speed to the three surrogates reported by Wang et al.4 The only experimentally measured laminar flame speed of a pure biodiesel was reported by Chong and Hochgreb.24 They adopted the jet-wall stagnation configuration to measure laminar flame speed of diesel, biodiesel and their blends at atmospheric pressure and 470 K, and showed that their experimental results are 30% higher than those of biodiesel surrogates, which could be due to the difference in the initial temperature of the mixture. The literature briefly reviewed above showed that biodiesel is indeed a viable candidate to replace diesel in many engineering power generation applications. However, development of the right formulation of biodiesel to improve its overall properties merits further investigation. Studies of biodiesel surrogates, which are more convenient for research, can be used as a model for biodiesel fuels and therefore can contribute to the knowledge-base of biodiesel combustion. Therefore, we evaluated the laminar flame speed of a novel biodiesel surrogate, 1,3-dimethoxyoctane (DMO), as a potential stand-

2. MATERIAL AND FUEL PRODUCTION In order to overcome the limitations inherent in biodiesel, especially cold weather applications, the development of a surrogate molecule that could improve the physical and chemical properties of biodiesel was investigated. This investigation centered on the use of polyhydroxyalkonate polymers produced by fermentation cultures of Pseudomonas putida. We have been working with a strain of P. putida that is capable of producing a polymer that is primarily derived from 3hydroxyoctanoic acid monomers. The overarching goal was to develop chemical conversion technology that would convert solid poly-3hydroxyoctanoate into a liquid that could be used as a biodiesel additive or as a stand-alone fuel. We chose 1,3-dimethoxyoctane (DMO) as a target for this chemical conversion technology since we reasoned that this could be produced from poly-3-hydroxyoctanoate in two straightforward (Figure 1) steps. In order to determine the feasibility of 1,3-dimethoxyoctane as a biodiesel additive or a standalone fuel, we sought to investigate its combustion properties. Therefore, the present paper aims to evaluate the laminar flame speed of 1,3-dimethoxyoctane. Additionally, the flammability limits of 1,3dimethoxyoctane were also determined. The synthesis of 1,3-dimethyoxyoctane (DMO) is described in Figure 1. Methyl 3-hydroxyoctnaoate was chosen as a starting point of the synthesis of 1,3-dimethoxyoctane since this methyl ester can be readily synthesized using a published method.25 Methyl 3-hydroxyoctanoate offers a reasonable model compound that can be used for optimization of the chemical conversion technology for the poly-3hydroxyoctanoate produced by fermentation cultures of Pseudomonas putida. The synthesis of DMO involves an initial reduction of the methyl ester to 1,3-octanediol using sodium borohydride in THF. The deprotonation of this diol using sodium hydride was followed by an alkylation using methyl iodide in THF at room temperature. A final washing step removed the inorganic byproducts to produce 1,3dimethoxyoctane (DMO) in an average of 75% final yield from the methyl 3-hydroxyoctanoate over repeated runs. The complete experimental details and characterization data are provided below. 2.1. General Synthetic Methods. Unless otherwise noted, all of the commercial reagents were used without further purification and the solvents were not dried prior to use. 1H and 13C NMR spectra were recorded at 300 and 75 MHz, respectively, with CDCl3 (δ= 7.26 ppm, δ= 77.00 ppm) as reference. Multiplicities are indicated as follows: s (singlet); d (doublet); t (triplet); q (quadruplet); m (multiplet) and coupling constants were given in Hertz. The infrared spectra were recorded on a Bruker Alpha-P FT-IR spectrometer using an attenuated total reflectance sample technique (ATR) with a diamond ATR unit. The conversion of the starting materials was monitored by thin-layer chromatography (TLC) using silica gel plates (silica gel F-254, 0.20 mm), and the components were visualized by observation under UV light (254 nm) or by dipping in KMnO4 followed by heating. All reactions were quenched with a standard aqueous work up and the products were used without further purification. 2.2. Synthesis of 3-Hydroxy-1-octanol. In four separate 1 L round-bottom flask fitted with a stir bar was placed methyl 3hydroxyoctanoate (33.7 mL, 0.19 mol) which was dissolved in 500 mL of THF followed by the addition of sodium borohydride (14.3 g, 0.38 B

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Figure 2. Schematic diagram of the experimental setup. for visualization and imaging of the outwardly propagating flame generated in the center of the vessel. One pair has a diameter of 125 mm and the other 100 mm. These windows are large enough to enable capturing flame propagation over an extended radius. The test rig is also capable of generating homogeneous and isotropic turbulence via 4 pairs of axial fans arranged symmetrically inside the chamber, which are needed here only for preparing homogeneous fuel/air mixtures for laminar flame speed measurements. The spherical test rig is also equipped with a 2 kW bank of heating elements capable of heating the vessel up to a gas temperature of about 473 K. The combustion air is preheated in the air supply line using a 520 W heating tape covered with thermal insulation.27,31 Control of the gas temperature inside the vessel is achieved using three Omega K-type thermocouples having an accuracy of ±1.5%, which were placed at three different locations inside the combustion chamber. The gas temperature was varied (by trial and error) until the entire amount of liquid fuel injected into the chamber was vaporized. The corresponding (average of the 3 thermocouples’ readings) temperature was taken as the initial temperature of the mixture. The volumetric method was used for preparing combustible mixtures of fuel/air. Initially, preheated air was injected into the vacuumed (empty) hot vessel/chamber. Once the temperature inside the vessel exceeded the fuel boiling temperature, the chamber was evacuated. Afterward, liquid fuel was injected using a 10 mL syringe, which has a 100 mm long needle capable of depositing fuel drops inside the combustion vessel. The necessary mass fraction of combustion (preheated) air was then injected to form the desired equivalence ratio, and once the fuel/air mixture attained the set initial temperature, the mixture was ignited. The fuel/air mixture was controlled using partial pressure method where a piezoelectric pressure transducer operating between 0 and 10342.1 mbar was used for measuring the total pressure of the fuel-air mixture. The volumetric method was based on the ideal gas assumption32 and the theoretical chemical reaction of 1,3-dimethoxyoctane (DMO) (having a molecular formula of C10H22O2). The ignition of the fuel/air mixture was triggered in the center of the combustion chamber using two tungsten electrodes, each has a diameter of 2 mm and a length of 400 mm. The tip of each electrode was tapered to reduce its diameter (from 2 mm to 0.7 mm) in order to increase energy concentration at its tip. They were installed horizontally using two electrically insulated holders which allowed for a 2.5 mm gap separation between the two electrodes. This distance/gap was found ideal to generate suitable spark energy for igniting the mixture. The ignition spark was generated by a high voltage electrical discharge using an ignition transformer (model Franceformer 5 LAY 12) of 120 VAC power supply capable of

mol). The reaction was held at reflux in THF under an argon atmosphere for 8 h. After cooling to room temperature the solvent was removed under vacuum and excess sodium borohydride was quenched with concentrated brine solution (200 mL). After the evolution of hydrogen complete (approximately 30 min) the mixture was extracted with 200 mL of ethyl acetate. The organic layer was washed with brine (4 × 200 mL), dried over Na2SO4, the solvent was removed under vacuum, and the four fractions combined to produce the product in 91% yield (107.5 mL, 0.69 mol). The complete chemical characterization data for 3-hydroxy-1-octanol is provided below. 1,3-dihyrdroxyoctane (96.2 mL): Light amber oil. 1H NMR (300 MHz, CDCl3) δ: 0.89 (t, J = 6.9 Hz, 3H), 1.30 (m, 6H), 1.45 (m, 3H), 1.68 (m, 2H), 3.66 (s, 1H), 3.82 (m, 3 H); 13C (75 MHz, CDCl3) δ: 14.0, 22.6, 25.3, 31.8, 37.7, 38.3, 61.5, 72.0; FTIR cm−1: 606, 1051, 1457, 1743, 2858, 2928, 3323. 2.3. Synthesis of 1,3-Dimethoxyoctane (DMO). An oven-dried 1 L three-necked round-bottom flask was charged with NaH (30.3 g, 0.76 mol) in mineral oil dispersion. The mineral oil was removed by washing it with hexane (3 × 200 mL) under an argon atmosphere. At this point, a solution of 1,3-dihydroxyoctane in THF was added to the flask while being stirred at room temperature. A pressure-equalizing funnel containing neat CH3I (107.2 mL, 1.72 mol) was attached to the round-bottomed flask. The CH3I was added at a rate below which vigorous bubbling would be observed. After the CH3I was added the reaction was stirred at room temperature for 24 h under an argon atmosphere. The reaction was quenched with water concentrated under reduced pressure. The concentrated solution was then extracted with hexane (1 × 500 mL). The organic layer was then washed with water (1 × 300 mL), 1 M HCl (2 × 300 mL), water (2 × 300 mL), and concentrated brine (2 × 300 mL). The organic layer was dried (NaSO 4 ) and evaporated to leave a residue of pure 1,3dimethoxyocatane with a yield of >99% (150.0 mL, 0.68 mol). The complete chemical characterization data for 1,3-dimethoxyoctane (DMO) is provided below. 1,3-dimethoxyoctane (112.5 mL): Light amber liquid. 1H NMR (300 MHz, CDCl3) δ: 0.9 (t, J = 6.9 Hz, 3H), 1.30 (m, 6H), 1.47 (m, 2H), 1.74 (m, 2H), 3.28 (m 2H), 3.33 (s, 3H), 3.34 (s, 3H), 3.46 (m, 2H). 13C (75 MHz, CDCl3) δ: 14.1, 22.6, 24.8, 32.0, 33.7, 33.9, 56.7, 58.6, 69.5, 78.1; FTIR δ: 1091, 1190, 1379, 1460, 2859, 2928.

3. EXPERIMENTAL SETUP AND METHODOLOGY Experiments were performed using a spherical combustion chamber having a volume of 29 L (Figure 2). Since details of this rig (spherical vessel) were provided elsewhere,26−30 only a a brief description is given here. The spherical vessel is equipped with two pairs of optical fused-silica windows, which are capable of withstanding high pressure, C

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Energy & Fuels generating 10 000 VAC between the two electrodes. A solid state relay (model RS3−1D10−51) with 120−240 VAC load power controlled by a 3−32 VDC control voltage was used to trigger the ignition system. Labview was used to synchronize the ignition spark and the camera. Images of the outwardly propagated flame were captured using a Z-type configuration of Schlieren system.33 The present Schlieren setup consisted of a Nanosence MKIII high-speed CCD camera having a maximum of 1040 frames per second at full resolution of 1280 pixels × 1024 pixels, a pair of spherical mirrors each having a 152.4 mm diameter and 1.524 m focal length, and a LED source light. A Labview user interface was used to allow for synchronization of the spark generation and a 5-VDC TTL camera signal so that image capturing can be triggered to a 50 μs prior to spark activation. An inhouse developed Matlab code was used for postprocessing of the recorded flame images and determining the propagation speed. Several image processing techniques were used to track the flame edge and deduce flame radius at each instant of time during propagation. Laminar flame speed was extracted from the temporal evolution of the radius of the spherical outwardly propagating flame. The linear extrapolation of the stretched laminar flame speed for determining laminar flame speed at zero stretch conditions has been widely adopted in the literature.23,34−43 It is briefly summarized as follows: The stretched flame speed is expressed as dru dt

Sb =

these components of the experimental setup. The overall uncertainty of the flame speed measurements at a 95% confidence level is determined following the approach developed in refs 47 and 48.

4. RESULTS AND DISCUSSION Laminar flame speed and flammability limits of 1,3-dimethoxyoctane were measured at 1 bar and 433 K. Validation of the experimental methodology was performed by measuring the laminar flame speed of n-heptane/air mixtures at 1 bar and 353 K over an extended range of equivalence ratio ranging between 0.8 and 1.4 and compared with its counterparts’ published findings. The measurements of laminar flame speed and flammability limits were obtained by repeating the experiment three times at the same test conditions (i.e., equivalence ratio) and taking an average value. The repeatability of the experiments was found to be within ±5%). 4.1. Validation of the Experimental Methodology. In order to validate the experimental methodology developed in this study, the laminar burning velocity of a liquid fuel, nheptane/air mixture at 353 K and 1 bar, was measured at different equivalence ratios ranging from 0.8 to 1.4. The obtained results were compared with published experimental data at nearly similar test conditions.49−51 The density ratio (eq 5) used for determining laminar flame speed (eq 4) was obtained using the equilibrium combustion code of Olikara and Borman.52 The present experimental results of heptane/air mixture are presented in Figure 3. The present results show

(1)

The stretch rate is given as

α=

1 dA 2 dru 2 = = Sb A dt ru dt ru

(2)

The relationship between the stretched and unstretched laminar flame speed is given as S b0 = S b + L bα

(3)

Finally, laminar flame speed is obtained as ρ SL = S b0 b ρu

(4)

where the density ratio is calculated based on the mass conservation equation as34

ρb ρu

=

n uTu nbTb

(5)

The flammability limits of 1,3-dimethoxyoctane were determined as the minimum and maximum concentration of fuel in the fuel/air mixture at which a propagating flame can still occur.44,45 In the present study, these limits were determined following the visual criteria and methodology detailed in ASTM E681.46 Table 1 presents the uncertainty of the experimental measurement of laminar flame speed, which is attributed to bias and random uncertainty sources. The former is caused by the inherent errors of each component of the experimental setup (e.g., nonlinearity in pressure transducers, thermocouple reading, optical aberrations), and it cannot be determined by means of statistical methods. The latter is determined based on a series of repeated measurements at each of

Figure 3. Laminar flame speed of n-heptane/air mixtures at 1 bar and 353 K.

good agreement with their counterparts published data of Kelley et al.49 and Smallbone et al.,50 which were obtained at almost similar initial temperature. The results of Sileghem et al.51 are slightly higher at all equivalence ratios, which could be attributed to the difference in the initial temperature and also possibly to the experimental method adopted in each study. The employed test conditions in these studies along with the adopted experimental method are given in Table 2. 4.2. Laminar Flame Speed of 1,3-Dimethoxyoctane (DMO). Figure 4 presents the temporal evolution of the laminar flame radius of 1,3-dimethoxyoctane/air at different equivalence ratios. According to this figure, the temporal evolution of the laminar flame radius of 1,3-dimethoxyoctane/

Table 1. Uncertainties of the Experimental Measurements of Laminar Flame Speed ϕ

BSL [cm/s]

PSL [cm/s]

USL [cm/s]

SL [cm/s]

%

0.7 0.8 1.0 1.1 1.2 1.3

0.891 1.0601 1.2651 1.2995 1.3822 2.0978

0.3975 0.5032 0.6875 0.7448 0.6713 0.4877

1.2296 1.5081 1.9362 2.05189 1.9896 2.3413

22.29 28.31 38.91 42.23 37.81 25.17

5.5166 5.3273 4.9762 4.8587 5.2623 9.3020 D

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at 1 bar and 433 K. The unstretched laminar flame speed, as expressed in eq 3, corresponds to the linear extrapolation of S0b at zero stretch rate for each equivalence ratio. This figure shows that the stretched flame speed Sb varies quasi-linearly with the stretch rate. In fact, high stretch rate corresponds to early flame propagation stage where the influence of the ignition system is strong, and low stretch rate corresponds to the flame propagation near the vessel’s wall. In the present tests, however, the confinement effect near the chamber’s wall has not been observed within the range of flame’s radius imaged in the present study (up to 35 mm). This could be attributed to the fact that the volume of the present test rig (29 L) is substantially higher than most spherical vessels used for measuring laminar flame speed36,38−41,43 where the present largest captured flame radius remains very small compared to that of the chamber radius (that is, 20% of the chamber radius). The Markstein length of the laminar flame of DMO/air at different equivalence ratios is presented in Figure 6. Published

Table 2. Experimental Conditions of Published Data on Laminar Flame Speed of n-Heptane/Air Mixtures method

P [bar]

T [K]

ref.

spherical flame counterflow planar heat flux planar

1 1 1

353 350 358

49 50 51

Figure 4. Temporal evolution of laminar flame radius of 1,3dimethoxyoctane (DMO) at 1 bar and 433 K.

air exhibits a linear variation at radius higher than 10 mm. The nonlinearity at lower flame radius during the early stage is caused by the disturbances due to ignition when the electrical discharge was deposited at the central point of the combustion chamber. Therefore, a flame radius of 10 mm was chosen as the minimum for data analysis. The maximum flame’s radius imaged was 35 mm which is still far away from the chamber’s wall, which has a radius of 190 mm. That is why the wallconfinement effect has not been observed in Figure 4. Figure 5 shows the stretched laminar flame speed of a DMO/ air mixture as a function of the stretch rate (denoted as α) for seven different equivalence ratios ranging between 0.8 and 1.4

Figure 6. Markstein length of 1,3-dimethoxyoctane (DMO) and hydrocarbon fuels at 1 bar.

corresponding data of isooctane/air and that of n-heptane/air collected in the present study are also plotted in this figure for comparison. The Markstein length, which is determined as the slope of the laminar flame speed versus the stretch rate plotted in Figure 5, characterizes the effect of flame stretch and curvature on flame speed.53 Markstein length is an indication of the flame stability where high Markstein length implies more stable flames.34,35 Figure 6 shows that the DMO/air flame has a Markstein length that is slightly higher but still comparable to that of n-heptane/air mixture at any equivalence ratio indicating a slightly more stable DMO/air flame than that of n-heptane/ air. On the other hand, compared with isooctane/air mixture,54 it is clearly seen that DMO/air flame has higher flame stability at fuel lean conditions but only slightly higher at fuel rich conditions, confirming the strength of DMO at fuel lean and near stoichiometric conditions. Figure 7 presents the laminar flame speed of DMO/air mixture flame at atmospheric pressure and an initial temperature of 433 K for equivalence ratio ranging between 0.8 and 1.4. Because of the lack of the enthalpy of formation of DMO, which is one of the input parameters needed for calculating the adiabatic flame temperature based on the equilibrium approach

Figure 5. Stretched laminar flame speed of 1,3-dimethoxyoctane (DMO) at 1 bar and 433 K. E

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that, at any equivalence ratio, DMO exhibits a slightly lower laminar flame speed than that of most biodiesel surrogates but still comparable to that of methyl butanoate.4 This slight difference could be partly attributed to the small difference in the initial temperature between the DMO/air mixture and that of biodiesel surrogates reported in this figure. This difference could also be attributed to the single C−O bonds present in DMO as opposed to the double C−O bonds present in these biodiesel surrogates. In general, double oxygen bonds are capable of releasing more energy than single C−O bonds. That is why DMO, which has single C−O bonds, has slightly lower reactivity (and hence slightly lower laminar flame speed) than that of these biodiesel surrogates which have double C−O bonds.57 4.3. Flammability Limits of 1,3-Dimethoxyoctane. Table 4 shows the lower (LFL) and upper flammability limits Table 4. Flammability Limits of 1,3-Dimethoxyoctane LFL

Figure 7. Laminar burning velocity of 1,3-dimethoxyoctane (DMO), n-decane, n-dodecane, and biodiesel surrogates as a function of the equivalence ratio.

developed by Olikara and Borman,52 methyl octanoate (C9H18O2) was selected as the fuel for calculating the density ratio (eq 5) because of the availability of the enthalpy of formation and also due to its similar chemical structure to DMO. The laminar flame speed of 1,3-dimethoxyoctane (DMO)/air is also compared in this figure with their counterparts published data of different biodiesel surrogates,4,23 n-decane and n-dodecane.55 Test rigs along with initial conditions used in these studies are listed in Table 3. This

P [bar]

T [K]

1

433

spherical flame

1

403

methyl crotonate

1

403

methyl decanoate

1

403

methyl octanoate methyl cyclohexane decane

1 1 1

443 443 400

dodecane

1

400

counterflow planar counterflow planar counterflow planar spherical flame spherical flame counterflow planar counterflow planar

fuel

method

[% vol]

ϕ

[% vol]

ϕ

0.86

0.62

13.84

1.75

(UFL) of DMO measured at 1 bar and 433 K. These measurements were obtained following the visual criterion of ASTM E-681 which expresses each limit in percent volume of the fuel in the air/fuel mixture and its corresponding equivalence ratio.46 The adopted method consisted of the following. For determining the upper flammability limit, the fuel concentration at rich conditions was increased gradually until the mixture still ignites but the flame does no propagate. The average of the last two consecutive fuel concentrations corresponding to a propagated and nonpropagated flame was determined as the upper flammability limit. Similar approach was used at fuel lean conditions to determine the lower flammability limit but with the fuel concentration reduced gradually.46 Comparison of the flammability limits of DMO with common gaseous and liquid hydrocarbon fuels45 is presented in Figure 8. The DMO fuel exhibits comparable flammability

Table 3. Test Conditions of Published Laminar Flame Speed of Biodiesel and Biodiesel Surrogates 1,3-dimethoxyoctane (DMO) methyl butanoate

UFL

ref. present work 4 4 4 23 23 55 55

figure shows several interesting observations depending on the range of equivalence ratio. The present results indicate that laminar flame speed of DMO/air mixtures is slightly lower (up to a maximum of 15%) but still comparable to those of ndecane and n-dodecane reported by Kumar and Sung.55 However, the variation of the profile of the DMO/air flame speed as a function of the equivalence ratio is similar to those of different biodiesel surrogates.4,23 The saturated chemical structures present in DMO promote higher flame temperatures.24,56 This consequently results in a flame speed of DMO/air mixture comparable to their counterparts’ biodiesel surrogate4,23 Nevertheless, it is possible to observe in Figure. 7

Figure 8. Flammability limits of 1,3-dimethoxyoctane (DMO) along with other fuels.

limits to short-chained gaseous hydrocarbons (e.g., methane, ethane) but noticeably wider than common liquid fuels and their surrogates (e.g., diesel, gasoline, kerosene) and substantially lower than alcohol fuels (e.g., methanol, ethanol). This is very advantageous considering that DMO has a comparable heating value to that of canola biodiesel and methyl decanoate but higher than that of most biodiesel surrogates with nearly F

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Energy & Fuels similar density and molecular weight (see Table 5).58−61 In addition, some of our preliminary laboratory tests have shown that DMO has higher volatility than biodiesel.58,62 Table 5. Fuel Properties of Selected Biodiesel Surrogates fuel

molecular weight [g/mol]

1,3-dimethoxyoctane (DMO) methyl butanoate methyl crotonate methyl octanoate methyl decanoate

density [g/mL]

LHV kJ/mol

174.28

0.826

6579.0758

102.1059 100.1159 158.2359 186.2959

0.89859 0.94459 0.87759 0.87359

2945.5060

■

5523.7661 6832.2461

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5. CONCLUSION The measured laminar flame speed of DMO is found only slightly lower than that of biodiesel surrogates over the explored equivalence ratios. This finding suggests that DMO could be suitable for use as an additive to conventional hydrocarbon fuels, or as a stand-alone surrogate fuel owing to its remarkable reactivity. Moreover, the present results suggest that DMO/air mixture could be ideal for applications in dualfuel engine where DMO can be used as the pilot fuel to ignite a gaseous/fuel mixture (e.g., methane, methanol, ethanol, biogas). This is supported by the fact that DMO lacks carbon double bonds which improves the cetane number63 and consequently ignition properties better than those of diesel fuel.64 The wide flammable limit/range of DMO, which was found comparable to that of paraffin hydrocarbons (e.g., methane, ethane), is a confirmation that dual-fuel IC engines would be a suitable application due to the more extended equivalence ratio over which DMO is flammable. This is also supported by the fact that DMO is more volatile than biodiesel and diesel which can be an advantage for improving fuel/air mixtures. However, additional combustion characteristics must be determined in order to ascertain the suitability of DMO for IC engine applications.

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Sb=Stretched flame speed SL=Laminar flame speed t=Time T=Temperature Tb=Adiabatic flame temperature Tu=Initial temperature USL=Overall uncertainty α=Stretch rate ρb=Density of burned gases ρu=Density of unburned gases ϕ=Equivalence ratio

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Telephone: 204-4748482. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by BiofuelNet Canada, and the Centre for Emerging Renewable Energy Inc. (CERE).

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NOMENCLATURE A=Flame area BSL=Total bias uncertainty LHV=Lower heating value Lb=Markstein length nb=Moles of burned gases nu=Moles of unburned gases P=Pressure PSL=Total random uncertainty ru=Flame radius S0b=Unstretched flame speed G

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