Formulation of Surrogate Fuel Mixtures Based on Physical and

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Formulation of Surrogate Fuel Mixtures Based on Physical and Chemical Analysis of Hydrodepolymerized Cellulosic Diesel (HDCD) Fuel Dianne Jeanne Luning Prak, Peter J. Luning Prak, Paul C. Trulove, and Jim S. Cowart Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01114 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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Graphical Abstract Ignition delay(deg)

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Formulation of Surrogate Fuel Mixtures Based on Physical and Chemical Analysis of Hydrodepolymerized Cellulosic Diesel (HDCD) Fuel

Dianne J. Luning Prak1,* Peter J. Luning Prak2 Paul Trulove1 Jim S. Cowart2

1) Department of Chemistry U.S. Naval Academy 572 M Holloway Road Annapolis, MD, 21402

2) Department of Mechanical Engineering U.S. Naval Academy 590 Holloway Rd Annapolis, MD, 21402 *corresponding author [email protected] Tel: 410-293-6349 Fax: 410-293-2218

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ABSTRACT Surrogate fuel mixtures for a hydrodepolymerized cellulosic diesel (HDCD) fuel were formulated based on HDCD’s physical properties and chemical composition. The HDCD was found to contain alicylic, cyclic, and aromatic compounds. Surrogate mixtures composed of transdecahydronaphthalene (trans-decalin) and 1,2,3,4-tetrahydronaphthalene (tetralin) matched HDCD’s speed of sound, density, and bulk modulus. Diesel engine experiments were conducted on mixtures containing petroleum diesel fuel (60% and 80% volume fraction) mixed with HDCD, tetralin, transdecalin, or a mixture with 0.42 mass fraction of tetralin in trans-decalin. At both volume fractions, the startup performance of the 2-component surrogate/petroleum fuel mixtures matched that of the HDCD/petroleum mixture. The trans-decalin/petroleum fuel mixtures started faster while the tetralin/petroleum fuel mixtures started more slowly than those containing HDCD. These results show that speed of sound, density, and bulk modulus can be used as metrics to design surrogate fuel mixtures that match fuel startup performance in diesel engines.

INTRODUCTION Military readiness depends on having a secure source of fuel with properties that allow it to be run in engines of various sizes. The worldwide nature of military operations requires that fuel be purchased from numerous sources and pumped through commercial pipelines; this, in turn, has the potential to result in the use of fuels that contain components derived from non-petroleum sources (i.e., alternative fuels). It is important, therefore, for the military to test fuels from alternative sources in their engines to insure proper operation. Some alternative fuels can combust alone (pure or neat form) in military engines, while the composition of other alternative fuels requires them to be mixed with petroleum-based fuels to insure stable combustion in diesel engines.1-10 One such fuel is Hydrodepolymerized Cellulosic Diesel (HDCD) fuel that recent combustion experiments have shown must be mixed with at least 60% petroleum-based fuel for the engine start up time to be within military requirements.9 This fuel represents a new class of alternative fuels that are derived from cellulose and lignin.11-18 As with petroleum-based fuels, fuels from cellulose and lignin contain a large number of compounds.9,13 Researchers who strive to model the combustion of such complex fuels currently do not have the detailed combustion kinetic data (thermodynamic parameters, chemical kinetic rate constants, and reaction pathways) for the hundreds of compounds in these fuels. Furthermore, the computation cost for modeling a large number of reactions is extremely high.19 Understanding and modeling 2 ACS Paragon Plus Environment

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combustion kinetics can be simplified by formulating and testing mixtures with a few select components, called surrogate mixtures, whose properties match those of the fuel of interest.20-68 The goal of this study was to characterize HDCD, formulate a surrogate mixture for HDCD, and compare the combustion performance of the surrogates with that of HDCD in a military diesel engine. Surrogate mixture development and testing have been approached from combustion experiments, physical property measurements, and a combination of the two. The combustion performance of surrogate mixtures has been tested in diesel and gas engines, rapid compression machines, burners, shock tubes, variable flow reactors, and flame test rigs.23-41 Iyer et al.41 formulated binary and ternary surrogate mixtures for JP-8 by optimizing total soot index, derived cetane number, molecular weight, and the ratio of hydrogen to carbon and then showed that these surrogates could match the soot production in flame and model gas turbine combustor experiments. Surrogate mixtures containing from one to fourteen components have also been developed based on composition, flash point, speed of sound, lower heating value, derived cetane number, research octane number, density, viscosity, thermal conductivity, and volatility characteristics from the distillation curve (D-86) or advanced distillation curve.42-57 Mueller et al.54 formulated surrogate mixtures of several diesel fuels by matching the surrogate and diesel fuel’s density, mole fraction of each carbon type (11 carbon types were identified), derived cetane number, and volatility from the advanced distillation curve within a range of target values for each matching parameter. These surrogates contained from four to ten components. Luning Prak et al.24 used composition based information, speed of sound, viscosity, density and bulk modulus to develop a 2-component surrogate for an alcohol-to-jet fuel. The startup performance of the surrogate mixed with conventional petroleum-based jet fuel, JP-5, observed in a diesel engine was the same as that of the alcohol-to-jet fuel mixed with JP-5. Computer simulations have been applied to determine both the physical properties of surrogate mixtures and to their combustion behavior.58-68 Mooney et al.68 used molecular dynamics simulation to determine the density, bulk moduli, and heat of vaporization of single components and surrogate fuel mixtures. Several researchers have created combustion-based numerical models and have assessed the success of their simulations by comparing their results for surrogate mixtures with those reported by experimentalists for various fuels.58-67 Some simulations have included all the components of the surrogate mixture, while others have used only selected components or compounds from hydrocarbon families, such as linear alkanes. The goal of this study was to formulate surrogate mixtures for HDCD whose properties would match the fuel’s properties that previous work has indicated impact start-up in diesel engines (density, viscosity, surface tension, speed of sound, and bulk modulus) and to quantify properties that impact 3 ACS Paragon Plus Environment

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safety, pumping, and fuel transport (flash point, viscosity) and test these surrogates in a military diesel engine. In Heywoods’ seminal engine text81, the fuel’s viscosity, density and surface tension are shown to directly affect diesel spray formation. Bulk modulus has been shown to influence the start of injection event, and thereby ignition delay.23 Matching all properties of complex fuel mixtures (e.g. petroleumbased fuels) often requires a surrogate formulation with numerous components in the mixture. To minimize the components in the surrogate mixture, bulk modulus along with density and speed of sound were the properties chosen in order to optimize the surrogate formulation. Additionally, the surrogates were tested in blends with Navy petroleum diesel F-76 because diesel engine experiments showed poor start-up performance for blends containing more than 40% HDCD by volume.9 In the current study, the surrogates were tested in blends with two volume percentages: one set of mixtures contained 80% volume F-76, and the other set of mixtures contained 60% by volume F-76. The approach used to select the compounds in the surrogate fuel consisted of characterizing the HDCD in terms of its physical and chemical properties and its chemical composition. These properties were also compared with those of Navy petroleum-based diesel. Various components found in the HDCD were then used to create formulations until one was found that matched the properties of interest, namely speed of sound, density, and bulk modulus of HDCD. Blends of F-76 and various surrogates (single compounds or mixtures) were then tested in a diesel engine.

EXPERIMENTAL SECTION

Materials. In this study, pure component organic compounds were used to determine some of the components of HDCD including hexane (Aldrich,>99%), butyl cyclohexane (Aldrich, 99%) , indane (Aldrich, >99%), trans-decahydronaphthalene (trans-decalin) (Aldrich, 99%), 1,2,3,4tetrahydronaphthalene (tetralin) (Aldrich, 99%), bicyclohexyl (Aldrich, 99%), 2,7dimethyltetrahydronaphthalene (ChemSampCo, > 95%), 1,2,3,4,5,6,7,8-octahydroanthracene (Aldrich, rare chemical library, no purity given), indene (Aldrich, > 99%), 3-phenyl-1-butene (Aldrich, 98%), cisdecahydronaphthalene (TCI, 98%), and 1,2,4-trimethylbenzene (Aldrich, 98%). Surrogates were made with trans-decahydronaphthalene (TCI, >98%) and 1,2,3,4-tetrahydronaphthalene (TCI, >98%). The Navy F-76 diesel fuel (lot 8576) and the hydrodepolymerized cellulosic diesel (HDCD, lot 6338) were supplied by the Naval Fuels Team located at Patuxent River, Maryland. HDCD fuel was produced using the following approach. First lignocellulose feedstocks were thermocatalytically converted to a biocrude product through a proprietary technology. The biocrude was then hydrotreated to remove oxygen and other atoms and fractionated to produce a diesel-like product; tests of early lots of HDCD showed it 4 ACS Paragon Plus Environment

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contained mostly aromatic and alicyclic compounds.9,13,18 In the current work, production lot 6338 of HDCD was analyzed. Physical and Chemical Analysis. The organic compounds present in the HDCD were determined using gas chromatography/mass spectrometry. The GC/MS system included an Agilent 5975 inert mass selective detector for a quadrapole mass separator and an Agilent 6890N Gas Chromatograph. This GC contained a ZebronTM ZB-5MS column (0.25µm, 5% diphenyl-arylene-95% dimethylpolysiloxane, 30 m long, 0.25 mm in diameter), and the helium was pumped through this column at a flowrate of 1.5 mL/min. The compounds were separated using a program that ramps temperature from 60.0 °C to 250 °C at a rate of 10 °C/min. Electron impact ionization was used, and the mass to charge ratio (m/z) was scanned from 30 to 600. The method included sample dilution in hexane and a solvent delay in collection of the mass spectra to avoid causing saturation and damage of the detector. The system was also equipped with software that matches the mass spectral found in a sample to patterns in the NIST/EPA/NIH Mass Spectral Library (Version 2.0 g) to determine the “best fit”. This GC/MS method was used to determine the retention times and mass fragmentation pattern of commercially-purchased hydrocarbons and the HDCD components. The amount of each component was not determined in this study. Measurements of the fuels’ physical properties followed procedures employed in previous studies.6,9,23,24,36,49-50,55,69-74 Briefly, speed of sound, density, viscosity, surface tension, and flash point were measured using Density and Sound Analyzer (Anton Parr DSA 5000), Stabinger Viscometer (Anton Parr SVM 3000), Axisymmetric drop shape analyzer (Kruss DS100), and Setaflash Series 8 flash point tester (Stanhope-Seta Model 82000-0, closed-cup). The calibration of each instrument and their accuracy determination was accomplished using NIST-traceable and certified standards as previously described.69,70 The DS100 calculates surface tension by fitting the Young-LaPlace equation to an image of a droplet formed within the magnification window using air and drop density. The scale of the measurements for magnification is set by inputting the diameter of the needle through which the droplet is dispensed. The flash point tester was operated using temperature ramping conditions. Note that the manufacturer states that this unit conforms to various ASTM methods including E502, D3278, and D7236. The precision was determined by taking the average of 2 to 80 measurements. The measured speed of sound (c) and density (ρ) were used to calculate isentropic bulk modulus by Ev /Pa = (c2 /m2·s-2)(ρ/kg·m-3)

(1)


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Diesel Engine Combustion Analysis. A L100V Yanmar single cylinder diesel engine connected to an electrical generator (6kW) was used to test the fuel mixtures in this study. Yanmar engine tests were performed using the mixtures containing 60% or 80% (volume) fractions of petroleum F-76 with the remaining percentage being HDCD, the optimal 2-component surrogate, or the individual components in the surrogate. The engine with generator was installed in the laboratory and was instrumented with standard temperature and flow sensors as well as fast response fuel line pressure and fast response combustion chamber Kistler sensors, as described in Bermudez et al.9 High speed experimental engine operational data were collected. Following the experiments, the data were analyzed in order to quantify and compare the standard in-cylinder combustion metrics of each fuel blend. The combustion analysis approach is based on the MIT energy conservation approach during the compression, combustion and expansion stokes of the engine.75,76 The metrics quantified include maximum rate of fuel energy release, duration of combustion, combustion phasing (CA50 = Crank Angle where one-half, 50%, of the fuel is burned), the start of injection (SOI), the start of combustion (SOC), and the ignition delay (IGD, which is conventionally defined as the difference between SOI and SOC). Of these diesel engine combustion metrics, combustion phasing and IGD are the most commonly used by the engine research community in order to characterize diesel combustion with changing fuels. Another goal of this study is to compare diesel engine combustion characteristics of the surrogate fuel blends with those of the base fuel mixtures of HDCD and F-76. The Derived Cetane Number (DCN) was also measured in order to further aid in understanding the combustion behavior of the fuel blends. DCN was measured using the Advanced Engine Technology Ltd.'s (AET) Ignition Quality Tester (IQT) unit following the ASTM D6890 Standard Test Method.77 DCN values of tetralin reported in the literature vary widely (8.0, 13, and 21.3), 82,85 suggesting that the IQT is less accurate at low cetane numbers and making it difficult to use these lower values to predict mixture behavior. While DCN has been used in surrogate development for some fuels, it would not be appropriate for surrogate development for HDCD.

RESULTS & DISCUSSION The goals of this study were to characterize HDCD by determining its major chemical components and by measuring its physical properties, to prepare surrogate hydrocarbon mixtures for HDCD that are based upon its major components and closely matches its key physical properties, and to combust the surrogate fuel mixtures in an instrumented diesel engine. The properties chosen in this work to match the HDCD fuel were bulk modulus, density, and speed of sound because preliminary work on the 6 ACS Paragon Plus Environment

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engines used in this study showed that these properties are important in the engine start up process. The flash point, viscosity, and surface tension were also measured. Measurements for petroleum F-76 and HDCD fuel properties. Figure 1 and 2 show the viscosity and density measurements over a narrow range of temperatures. The HDCD fuel is more viscous and more dense than petroleum F-76. The F-76 fuel meets the diesel fuel military specifications,79 but the HDCD is too dense, as shown in Table 1. The surface tension, bulk modulus, and speed of sound of the petroleum HDCD are higher than those of petroleum F-76 (Table 1), but the flash point is similar. The speed of sound, density, and bulk modulus of HDCD were used along with the chemical composition (as described below) to select which component were used in the surrogates. Composition of HDCD. The HDCD fuel possesses a large number of components as shown in Figure 3. Most of these components encompass a narrower boiling point range as compared to the components of petroleum F-76, as suggested by the presence of a larger number of peaks in the HDCD chromatogram with retention times less than 12 min. This lot of HDCD has a similar chromatogram to the chromatogram for a different lot of HDCD,9 which was found to consist of predominantly alicyclic, cyclic, and aromatic compounds. To determine the components in the HDCD, the mass fragmentation pattern for each peak on the gas chromatogram in Figure 3 was analyzed and matched to compounds in the NIST mass spectral database. Table 2 contains the peak retention times and the compounds with the greatest percentage match. These compounds are alicyclic, cyclic, and aromatic in nature. French et al.13 also reported that hydrotreated pyrolytic lignin contained alkyl-cyclohexanes, hydro-1H-indenes and hydro-naphthalenes. Kloekhorst et al.18 also reported the presence of these types of compounds. To confirm the identity of some of the components, the mass spectra and retention times of several commercially available compounds (indane, trans-decahydronaphthalene, 1,2,3,4-tetrahydronaphthalene, bicyclohexyl, butylcyclohexane, 2,7-dimethyltetrahydronaphthalene, 1,2,3,4,5,6,7,8-octahydroanthracene, indene, 3phenyl-1-butene, cis-decahydronaphthalene, and trimethylbenzene) were compared with the mass spectra and retention time of compounds found in HDCD. Six of the pure compounds analyzed had mass spectra and retention times that matched those of compounds found in the HDCD (Figure 4). These compounds are butyl cyclohexane, indane, trans-decahydronaphthalene, 1, 2, 3, 4tetrahydronaphthalene, bicyclohexyl, and 2,7-dimethyltetrahydronaphthalene. The similarities between the mass spectra of each compound and the compounds found in HDCD are shown in Figure 5. Formulation of surrogate mixture. Since the HDCD contained of a variety of alicyclic, cyclic, and aromatic compounds, a surrogate should be developed that contains these types of compounds that can be purchased at a reasonable cost in quantities large enough to burn in a diesel engine, and that 7 ACS Paragon Plus Environment

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matches the speed of sound, density, and bulk modulus of HDCD. The 2,7dimethyltetrahydronaphthalene was prohibitively expensive, so 1, 2, 3, 4-tetrahydronaphthalene was selected as a less costly alternative that is also partially aromatic and similar in overall structure. Mixtures of 1, 2, 3, 4-tetrahydronaphthalene and bicyclohexyl have speeds of sound (> 1397.9 m/s) and bulk moduli (> 1895 MPa) that exceed the values for HDCD, so bicyclohexyl would not be a good cyclic compound.74 The physical properties reported for mixtures 1, 2, 3, 4-tetrahydronaphthalene and trans-decahydronaphthalene suggest that a surrogate for HDCD could be made from mixtures of these compounds.78 Based on those data, a mixture containing 0.41 mass fraction of 1, 2, 3, 4tetrahydronaphthalene in trans-decahydronaphthalene would have a density at 293.15 K that would match the value given in Table 3 for HDCD. To match the speed of sound in Table 3 for HDCD, the mass fraction of 1, 2, 3, 4-tetrahydronaphthalene in trans-decahydronaphthalene would have to be 0.44. A surrogate mixture containing 0.42 mass fraction of 1, 2, 3, 4-tetrahydronaphthalene in transdecahydronaphthalene was prepared, and its bulk modulus was found to match that of HDCD as shown in Table 3 with the density slightly higher and the speed of sound slightly lower than the these property values for HDCD. This “optimal” surrogate mixture was used in the combustion experiments. Table 3 also shows the property values of the pure components for comparison. Surrogate mixtures/F-76 blends for diesel engine testing. Prior diesel engine combustion testing with a different lot of HDCD showed that diesel engine combustion could not be initiated with the neat fuel.9 However, blends containing up to 40% by volume HDCD with petroleum F-76 fuel could combust. Therefore, we prepared mixtures using 80% and 60% by volume F-76 and the remaining percentage being HDCD, the optimal surrogate mixture (containing 0.42 mass fraction of tetralin in trans-decalin), trans-decalin, or tetralin. A comparison of the chemical and physical properties of these blends with those of 80% F-76 diesel and 20% HDCD, reveals that the F-76 fuel blend containing the 0.42 mass fraction of tetralin in trans-decalin had a bulk moduli, density, and speed of sound that were close to the these property measurements for the HDCD blend (bold font in Table 3). The Derived Cetane Number for the 80/20 blend containing HDCD was measured to be 50, which agrees with the 51 DCN measured for the surrogate mixture. The 80/40 blend containing trans-decalin had a higher DCN of 55, and the one containing tetralin had a lower DCN of 47, than the base 80/20 blend. The similarity in properties was also found for mixtures with 60% by volume mixtures of F-76 with HDCD (Table 3, bold font). DCN for the 60/40 blend containing HDCD was measured to be 42, which is the same as the surrogate mixture/blend. The 60/40 blend containing trans-decalin had a higher DCN of 46, and the one containing tetralin had a lower DCN of 32, than the base 60/40 blend. These results confirm that the 8 ACS Paragon Plus Environment

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tetralin is much less reactive than the trans-decalin in the constant volume combustion chamber environment of the IQT cetane measurement, as suggested by their individual DCN values. Diesel Engine Combustion Results. Steady-state and startup diesel engine testing was performed using a the Yanmar diesel engine-generator laboratory unit described above. Each fuel blend used in this study was cold-started at least twice such that consistency was confirmed. The engine speed (RPM) startup results are shown in Figure 6 (A) and (B), where (A) shows the data from the 60% F76 with 40% HDCD fuels, and (B) with the 80% F76 with 20% HDCD fuels. It can be seen that the F76/HDCD blends (black dashed line) give a start time when engine speed (RPM) attains a rated speed of 3600 rpm in nominally ten seconds with 60% F76, and a faster four seconds with 80% F76 blend. These values are slower than that of neat F76, which has been shown to have a startup times of three seconds in this engine.9 Next, the red data lines (dash with two dots) in both figures shows the startup results with tetralin as the single-component HDCD surrogate. It can be seen that the engine takes longer to attain its rated speed with this surrogate. With the 40% concentration after thirty seconds the tetralin does not even reach rated speed. With the 20% volume fraction the start time is nominally twelve seconds as compared to the base fuel blend (HDCD/F76) four seconds start time. The other single-component surrogate is the trans-decalin (green dotted data lines). In both 40% and 20% concentrations it shows moderately faster start times than the base fuel blends (black lines). Finally the ideal surrogate is shown with the blue solid line in the engine rpm startup figures. These results give start times that are very similar to the base fuel blend (black data) start times, within approximately one-second similarity. It is the authors’ experience from this and previous work that the repeatability of the cold start engine experiments using lower cetane fuels results in a standard deviation of nominally one second in start-up times. Therefore, the contribution of trans-decalin alone shows a higher cetane effect, while the contribution of tetralin alone show a low cetane - delayed starting effect. The higher cetane effect for trans-decalin and the lower cetane effect for the tetralin are supported by DCN numbers of trans-decalin and tetralin as shown in Table 3 and by DCN number for the mixtures, where the DCNs are 46, 42, 42, and 32 for 60/40 blend’s containing trans-decalin, the optimal surrogate, HDCD, and tetralin, respectively. Next are shown the engine-based Ignition Delay (IGD) results in Figure 7(A) and (B) for the four fuels containing the 40% HDCD and its surrogates as well as the 20% HDCD and its surrogates. IGD is determined analytically using the fuel injection line pressure data (an indicator of start of injection) along with the in-cylinder combustion chamber pressure sensor (an indicator of start of combustion). IGD is defined as the difference from the Start Of Injection (SOI) to the Start Of Combustion (SOC). These combustion metrics are analytically determined using an engine based energy-heat release first 9 ACS Paragon Plus Environment

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law of thermodynamics approach. 81 To help see the trend in the IGD, a sixth-order fit of the data excluding the mis-fires was used and is shown in Figures 7(C) and (D). Figure 7 shows that the different fuel blends show some variability in the first 10 to 20 seconds following the start of cranking, especially with the 40% HDCD and its surrogates. IGD increases (e.g. longer overall in terms of crank degrees or time) with time since start of cranking during the first 10 seconds. The cause of this lengthening is a result of an increase in engine speed during the engine start up process, since the crank angle degrees occur faster in real time with increasing engine speed. When the IGD maximum is reached (in crank angle degrees), it can be seen that the IGD then begins to shorten for later times. This shortening effect (after the IGD reaches its maximum) is caused by the mechanical governor that injects less fuel (leaning out) into the engine once the rated speed is reached. Also, the increased thermal state of the engine is a factor in this effect. The data points on the abscissa (zero IGD) are engine cycles that did not fire (no combustion event occurred). For the lower cetane fuels, a general frequent pattern of misfires may occur with a single misfire event then being followed by a firing combustion event during the startup period. In general it can be seen that the 40% HDCD fuel blend and its surrogates show more variability in the IGD behavior than do the 20% blends. This leads to slower overall engine acceleration and the lengthening of engine start times. The green data points and green dotted lines in both IGD figures are with trans-decalin. This more reactive compound leads to shorter IGDs (with more ideally phased combustion) and thus aids in the quicker start times when this single component is used. The less reactive tetralin, in general, has longer IGDs (larger values), and thus leads to later combustion which has lower pressures due to its less than ideal combustion phasing. The base blend and 0.42 surrogate blend IGD trends are similar (blue and black data points and lines) with much scatter in the early phase of startup, and bounded by the two single component surrogates (green and red data points and lines). The engine location where one-half of the fuel has burned (CA50; Crank Angle of 50% fuel burned) is shown for the four fuel blends next in Figure 8. To help see the trend in the CA50, a sixthorder fit of the data was used and is shown in Figures 8 (C) and (D). As seen with the IGD results, a significant variability in CA50 can be observed during the first 20 seconds after the engine cranking begins. During this time the engine is accelerating in RPM towards the rated speed. The general trend is that CA50 delays or retards towards later crank degrees as the engine runs up towards rated speed. This effect results from an increase in engine speed and IGD as described earlier. As the engine gets hotter, the combustion IGD starts to shorten in length followed by the CA50 point advancing back towards then engine’s TDC location. In general, for both the 40% and 20% trans-decalin surrogates, the CA50 is earlier, more ideally phased (close to the engine’s Top-Dead-Center, TDC) and thus is the 10 ACS Paragon Plus Environment

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reason for the fast times with this single component in the fuel mixture. In the case of the single component tetralin (red data), the CA50’s are very late relative to TDC (larger values after TDC), and thus provide much less combustion effort and effect onto the engine’s piston, resulting in slower engine acceleration. The HDCD and 0.42 blends show similar behavior in combustion phasing (black and blue data points and lines), which further explains their close results with engine starting behavior.

SUMMARY AND CONCLUSON In this work, surrogate mixture development was approached by determining the chemical compounds in the fuel, formulating the mixture by matching fuel properties, and combusting the fuel blends in a military diesel engine. The work presented herein as well as previous studies show that this approach is successful.23,24 Other studies could also utilized this approach. Many of the strategies employed to develop surrogate mixtures use information on the fuel’s chemical composition. Some researchers select compounds for their mixtures from a list of all the components identified in the fuel while others select from hydrocarbon classes (paraffins, monocyloparaffins, alkylbenzenes, indenes, etc.) found in the fuel with more selective processes also matching carbon types (bonding and adjacent atoms). 21,22,23,24, 31,43, 45,47,54 For example, Wood et al.22 formulated their surrogate mixture for jet fuel by matching the volume percentages for each hydrocarbon class. For the paraffin class, their surrogate contained n-alkanes from 6 to 14 carbons long and totaled approximately 61% of the fuel. Mueller et al.42 utilized nuclear magnetic resonance imaging techniques to classify the types of carbon bonds found in diesel fuel samples and to enable better selection of components in their surrogate mixtures. Price is also a factor in limiting the potential compounds that can be used to make a surrogate mixture, especially when engine tests are to be run. Even though 2,2,4,4,6,8,8-heptamethylnonane is not found in petroleum-based fuels, it is often used in surrogate development because it is an inexpensive representative of branched alkanes.42 Surrogate mixture properties can be predicted from correlations used for multi-component systems or from other modeling approaches such as molecular dynamic simulations, and these approaches can be used to select the components based on weighting factors that have been found to be relevant to combustion systems.68,80 Care must be taken when developing and using correlations because the behavior of systems with many components can be different from systems with a few components. Fuels such as butanol-derived alcohol-to-jet fuel have been shown to have only a few components, while fuels such as HDCD have many components.24 The use of density and bulk modulus to govern the formulation of the surrogate mixture was successful in this study, but determining which specific properties to use to control surrogate development will always require careful consideration. In the current study, the higher bulk modulus 11 ACS Paragon Plus Environment

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mixtures that contained tetralin had longer ignition delay. This higher bulk modulus can cause an earlier start of injection,23 which increases the ignition delay because the fuel enters the chamber at a lower temperature and needs to reside longer before combusting. The impact of fuel density on ignition delay is complex and depends on engine operating and design factors such as temperature and injector design. Kim et al.83 simulated the impact of the variation in several fuel properties, including density, on spray patterns and IGD. At a temperature of 900 K, IGD increased as the density increased.83 The results in the current study, where the temperature is likely to be close to 900 K, are consistent with this prediction in that the tetralin mixtures were the most dense and had the longest IGD. The complex impact of density is illustrated by Kim et al.83 who found that at a temperature of 750 K, which is too low for the engines used in the current study, the least dense material had the longest IGD. Correlation of spray patterns in engines with fuel properties also show the complex impact of density in that fuel density is given both explicitly in equations but also implicitly in fuel system pressure.84 Future work needs to be done to link specific physical properties of fuels directly with the resulting combustion behavior. It is important, therefore, to understand how chemical composition impacts fuel properties and their combustion in various types of engines. In this study, the physical properties and chemical composition of a Hydrodepolymerized Cellulosic Diesel fuel were used in order to formulate surrogate mixtures that contained hydrocarbons that could be readily and cheaply purchased and engine tests were run with blends containing the surrogate and F-76. A chemical composition analysis of the HDCD fuel revealed that it is composed mostly aromatic, cyclic, and alicyclic compounds. Using pure transdecahydronaphthalene and tetralin that were identified in the HDCD, a surrogate mixture that contained tetralin at a mass fraction of 0.42 in trans-decahydronaphthalene was prepared that had a bulk modulus, speed of sound, and density that were similar to those of HDCD. Diesel engine combustion experiments were conducted on eight mixtures which consisted of 60% or 80% (volume) petroleum F-76 with the remaining percentage being either HDCD, the optimal surrogate mixture, trans-decahydronaphthalene or tetralin. The engine results revealed combustion characteristics that were very similar for the HDCD and 0.42 mass fraction binary surrogate blends. The single component tetralin surrogate showed much later combustion, characteristic of low cetane, while the single component pure transdecahydronaphthalene surrogate showed much more reactive nature in the engine leading to faster overall start times. The similarity in combustion behavior between the binary surrogate and the HDCD shows the success of this surrogate development approach. These data will be useful in future studies that numerically model combustion behavior of fuel mixtures that consist of cyclic and aromatic hydrocarbons. 12 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS The authors wish to thank ONR’s (Office of Naval Research) NEPTUNE Project under the direction of Dr. Maria Medeiros (grant #N0001415WX01853) and NAVAIR for funding this project, and Bridget Lee for helping with mixture preparation.

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0.92 0.90

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T/K Figure 1. Comparison of the density of petroleum F-76 diesel fuel (■) and HDCD (□)

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T/ K Figure 2. Comparison of the kinematic viscosity of petroleum F-76 diesel fuel (■) and HDCD (□)


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12 15 18 21 Time (min)

Figure 3. Gas chromatography/Mass Spectrometry scan of petroleum diesel fuel (F-76) (top) and HDCD (bottom) both diluted 1/100 in n-hexane

Total ion count (×105)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6 5 4 3 2 1 0 5

7

9

11

13

Time (min)

Figure 4. Gas chromatogram of HDCD and various components that match the retention time and mass spectra of components in HDCD


A

1

B

-0.3

0.5

Relative Intensity

Relative Intensity

0.1

0.5

1.5

0.1 1

D

-0.3

0.5

0

-1.1

G

0.5

1.5

0.1 1

H

-0.3

0.5

I

-1.1 30 60 90 120 150 m/z

-0.7 0

2

0.9

0.1

0.5

-0.3 -0.7

0

-1.1 30 65 100 135 170 m/z

-1.1 30 60 90 120 150 m/z

1

J

-0.3

0.5

0.5

1.5

-0.7 0

F

-1.1

2

0.9

Relative Intensity

2

0.1 1

30 60 90 120 150 m/z

30 60 90 120 150 m/z

0.9 0.5

1.5

-0.7

-0.7 0

E

0.9

C

0.5

1.5

2

2

0.9

Relative Intensity

2

Relative Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Relative Intensity

Page 23 of 29

0.9

K

0.5

1.5

0.1 1

L

-0.3

0.5

-0.7

0

-1.1 30 65 100 135 170 m/z

Figure 5. Mass spectra of peak at retention time of 6.095 min for (A) butylcyclohexane (molar mass = 140.3) and (B) HDCD; peak at retention time of 6.132 min for (C) indane (molar mass = 118.2) and (D) HDCD; peak at retention time of 6.548 min for (E) trans-decahydronaphthalene (molar mass = 138.2) and (F) HDCD; peak at retention time of 8.162 min for (G) 1,2,3,4-tetrahydronaphthalene (molar mass = 132.2) and (H) HDCD; peak at retention time of 10.471 min for (I) bicyclohexyl (molar mass = 166.3) and (J) HDCD; and peak at retention time of 10.481 min for (K) 2,7-dimethyl 1,2,3,4tetrahydronaphthalene (molar mass = 160.3) and (L) HDCD.


4000 3500 3000 2500 2000 1500 1000 500 0

Engine speed (rpm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Engine speed (rpm)

Energy & Fuels

A

0

5

10 15 20 time (sec)

25

30

Page 24 of 29

4000 3500 3000 2500 2000 1500 1000 500 0

B

0

5

10 15 20 time (sec)

25

30

Figure 6. Engine Speed (RPM – Revolutions Per Minute) during a cold engine start with the various tested fuels: (A) 60% F76 with 40% trans-decalin (····), 60% F76 with 40% HDCD (- - -), 60% F76 with 40% of the 0.42 surrogate (–––), and 60% F76 with 40% tetralin (– · · – · ·); (B) 80% F76 with 20% trans-decalin (····), 80% F76 with 20% HDCD (- - -), 80% F76 with 20% of the 0.42 surrogate (–– –), and 80% F76 with 20% tetralin (– · · – · ·).


60

50

50

Ignition delay(deg)

60 40 30 20 10 A

0

40 30 20 10

B

0 0

5

10 15 20 time (sec)

25

30

0

60

60

50

50

Ignition delay(deg)

Ignition delay(deg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Ignition delay(deg)

Page 25 of 29

40 30 20 10

C

0 0

5

10 15 20 time (sec)

25

30

5

10 15 20 time (sec)

25

30

40 30 20 10 D

0 0

5

10 15 20 time (sec)

25

30

Figure 7: Ignition Delay (IGD) in degrees with (A) 60% F76 with 40% trans-decalin (♦), 60% F76 with 40% HDCD (▲), 60% F76 with 40% of the 0.42 surrogate (●), and 60% F76 with 40% tetralin (■); (B) 80% F76 with 20% trans-decalin (♦), 80% F76 with 20% HDCD (▲), 80% F76 with 20% of the 0.42 surrogate (●), and 80% F76 with 20% tetralin (■). (C) 6th-order fit is used to show the general trend of 60% F76 with 40% trans-decalin (····), 60% F76 with 40% HDCD (- - -), 60% F76 with 40% of the 0.42 surrogate (–––), and 60% F76 with 40% tetralin (– · · – · ·); (D) 6th-order fit is used to show the general trend of 80% F76 with 20% trans-decalin (····), 80% F76 with 20% HDCD (- - -), 80% F76 with 20% of the 0.42 surrogate (–––), and 80% F76 with 20% tetralin (– · · – · ·).


Energy & Fuels

100 A

80

crank angle 50 (deg)


100

60 40 20 0 0

5

10 15 20 time (sec)

25

B

80 60 40 20 0

30

0

60

5

10 15 20 time (sec)

25

30

60 C

50



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

40 30 20 10 0 0

5

10 15 20 time (sec)

25

30

50

D

40 30 20 10 0 0

5

10 15 20 time (sec)

25

30

Figure 8: Location of 50% fuel burned (CA50) for (A) 60% F76 with 40% trans-decalin (♦), 60% F76 with 40% HDCD (▲), 60% F76 with 40% of the 0.42 surrogate (●), and 60% F76 with 40% tetralin (■); (B) 80% F76 with 20% trans-decalin (♦), 80% F76 with 20% HDCD (▲), 80% F76 with 20% of the 0.42 surrogate (●), and 80% F76 with 20% tetralin (■). (C) 6th-order fit is used to show the general trend of 60% F76 with 40% trans-decalin (····), 60% F76 with 40% HDCD (- - -), 60% F76 with 40% of the 0.42 surrogate (–––), and 60% F76 with 40% tetralin (– · · – · ·); (D) 6th-order fit is used to show the general trend of 80% F76 with 20% trans-decalin (····), 80% F76 with 20% HDCD (- - -), 80% F76 with 20% of the 0.42 surrogate (–––), and 80% F76 with 20% tetralin (– · · – · ·).


Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1. Selected property valuesa of Petroleum F-76 and HDCD and comparison with military specifications79 Density at 288.15 K (g mL-1)

Viscosity at 313.15 K (mm2·s-1)

Flash point (ºC)

Surface tension at 294 K (dyn/cm)

Speed of sound at 293.15 K (mˑs-1)

Bulk Modulus at 293.15 K (MPa)

1.7 < x > 4.3 > 60 ------------------------------Military spec < 0.876 b 0.8460 2.70 22.3 ± 0.2 1229.1 1580 61 ± 2 Petroleum F76 diesel 0.9106b 2.90 26.1 ± 0.3 1432.1 1860 61 ± 1 HDCD a The errors in the measurements are 0.02 kg·m−3, 0.06 mm2·s-1, 0.4 mˑs-1, 0.5 MPa, 0.1 mN·m-1, 1 K for density, viscosity, speed of sound, bulk modulus, surface tension, and flash point, respectively. b extrapolated


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

Table 2: Top matches for peaks at various retention times on GC and compounds in NIST databasea Octahydro-2-methyl pentalene

3-phenyl-1-butene

1,2,3,4-tetrahydro naphthalene (tetralin)

1,2,3,4-tetrahydro-2,7dimethyl-naphthalene CH3

H3C

CH3

CH3

5.465 min

6.901 min

8.162 min

3-Methyl-1-(1-methylethyl)cyclohexane

1-hexadecyl-2,3-dihydro 1H-indene

CH3

H3C

2-methyl-,cis-1,1’bicyclohexyl

CH3

CH3

H3C

3-methyl 2-butenyl benzene

10.481 min

H3C CH3

5.591 min

6.964 min

Butyl cyclohexane CH3

8.489 min

1,4-dimethyl 2octadecyl-cyclohexane

3-methyl 2-butenyl benzene

11.022 min 2-heptynyl benzene CH3

H3C

CH3 CH3

H3C

CH3

6.095 min 2-cyclohexyl eicosane CH3 CH3

7.027, 7.179, 7.191 min

8.540 min

1-(cyclohexylmethyl)-4ethyl-transcyclohexane

2,3-dihydro,1,3-dimethyl 1Hindene

12.081 min 1,1’-(1,2-ethanediyl)bis cyclohexane

H3C H3C

6.107 min (1-methylpropyl)cyclohexane CH3

7.179 min

8.678 min

1-methyl 2-(2-propenyl)benzene

1,2,3,4-tetrahydro-2-methylnaphthalene

CH3

H3C

6.107 min Indane

CH3

12.837 min 1,2,3,4,5,6,7,8octahydrophenanthrene

CH3

CH2

7.809 min

9.006, 9.094 min

4-ethenyl-1,2-dimethyl benzene

1-ethyl-2,3-dihydro-1Hindene

15.433 min 1,2,3,4,5,6,7,8octahydroanthracene

CH3

6.132 min Octahydro-5-methyl1H-indene

7.821 min

9.208 min

2,3-dihydro,4-methyl 1H-indene

1,2,3,4-tetrahydro-5-methylnaphthalene

15.433 min 9-methyl-soctahydroanthracene CH3

CH3

H3C

6.271 min Transdecahydronaphthalene (trans-decalin)

CH3

7.821, 7.96 min

9.724 min

1-hexadecyl-2,3-dihydro 1H-indene

1,1’bicyclohexyl

16.051 min 10,10-birnorabieta-8,11,13-triene CH3 H3C

CH3 CH3

CH3

H3C CH3

6.536 min

7.935 min

10.317 min

17.702, 18.710 min


Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

a

The compounds in red font are those whose presence was confirmed by matching the mass spectra and retention time of the compound in the fuel with pure compounds.

Table 3. Comparison of physical property information of fuels, pure components, the optimal surrogate containing 0.42 mass fraction of tetralin in trans-decalin, and mixtures containing 80% F-76 and 20% of the surrogates or the HDCD and mixtures containing 60% F-76 and 20% of the surrogates or the HDCDa Substance or Mixture

Density / kg·m−3

Trans-decalin Tetralin F-76 HDCD 0.42 mass fraction binary surrogate

Speed of sound/ mˑs-1 1397.9b 1489.5b 1373.6 1432.1 1430.7

Bulk Modulus/ MPa 1699b 2149b 1590 1860 1860

Surface tension/ mN·m-1 30.5b 36.1b 27.8 30.3 31.8

Flash point/ °C 53.6b 74.6b 61.0 61.0 60.8

Derived Cetane Number

869.75b 968.8b 842.5 907.8 908.8

Kinematic Viscosity/ mm2·s-1 2.46b 2.32b 4.20 4.65 2.22

80% F-76 /20%HDCD 80% F-76 /20% 0.42 mass fraction binary surrogate 80% F-76 / 20% trans-decalin

855.2 855.8

4.27 3.55

1384.8 1384.4

1640 1640

28.1 28.1

62.7 60.8

50 ± 1 51 ± 1

848.2

3.60

1378.1

1611

28

59

55 ± 1

80% F-76 / 20% tetralin

867.3

3.47

1394.3

1686

28.6

63.7

47 ± 1

60% F-76 / 40% HDCD 60% F-76 / 40% 0.42 mass fraction binary surrogate 60% F-76 / 40% trans-decalin

868.4 869.1

4.35 3.06

1396.6 1395.7

1694 1693

28.5 29.2

63.0 61.5

42 ± 2 42 ± 2

853.6

3.17

1382.6

1632

28.5

57.3

46 ± 1

60% F-76 / 40% tetralin

892.4

3.00

1416.1

1790

30.5

68.2

32 ± 2

31.8c, 32c, 46d 8.9 c, 13d, 21.3c 55 ± 1 29 ± 2 25 ± 2

a

The measurements are reported at 293.15 K except for surface tension, which is reported at 294 K. The errors in the measurements are 0.02 kg·m−3, 0.06 mm2·s-1, 0.4 mˑs-1, 0.5 MPa, 0.1 mN·m-1, and 1 K for density, viscosity, speed of sound, bulk modulus, surface tension, and flash point, respectively. DCN variability is estimated based on the authors’ experience.breference 78. The bolded numbers are intended to emphasize the similarities in properties between mixtures containing HDCD and mixtures containing the 0.42 mass fraction binary solvent. creference 82. dreference 85.


Formulation of Surrogate Fuel Mixtures Based on Physical and

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