Article pubs.acs.org/EF
Development of a Surrogate Mixture for Algal-Based Hydrotreated Renewable Diesel Dianne J. Luning Prak,*,† Jim S. Cowart,‡ Leonard J. Hamilton,‡ David T. Hoang,† Eva K. Brown,† and Paul C. Trulove† †
Chemistry Department, and ‡Mechanical Engineering Department, United States Naval Academy, Annapolis, Maryland 21402, United States S Supporting Information *
ABSTRACT: In this study, the chemical composition and physical properties of an algal-based hydrotreated renewable diesel (HRD) fuel were used to develop a surrogate mixture containing commercially available hydrocarbons. Analysis of the chemical composition of the algal HRD showed a small quantity of low-molecular-weight components and a high quantity of four highmolecular-weight components: n-pentadecane, n-hexadecane, n-heptadecane, and n-octadecane. Using these four components, a fifth branched component was added to match the physical properties of the algal HRD. Candidates for the fifth component were 2-methyloctane, 2-methylnonane, isooctane, and isododecane. The isooctane- and isododecane-based surrogates were tested in a Yanmar engine along with algal HRD and petroleum F76 diesel to assess the start of ignition, start of combustion, ignition delay, maximum rate of heat release, and overall combustion duration. The surrogate that best matches the physical properties of the flash point, density, viscosity, and surface tension as well as most closely reflecting the combustion metrics is one containing isododecane, n-pentadecane, n-hexadecane, n-heptadecane, and n-octadecane.
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INTRODUCTION As the price of petroleum oil rises and nations strive for oil independence, researchers have been developing production methods to make alternative fuels. One alternative fuel, algalbased hydrotreated renewable diesel (algal HRD), has been the focus of the U.S. Navy, which recently demonstrated that a mixture of 50% algal HRD and petroleum F76 diesel could be used in warships.1,2 Derived from extracted algal oil, HRD has been hydrotreated to remove oxygen, resulting in a fuel mixture containing predominantly linear and branched chained hydrocarbons and lacking in the aromatic compounds found in conventional petroleum-based fuels. Because algal HRD can be used in a variety of diesel engines, laboratory engine testing of algal HRD has begun to examine the emission benefits3 and the engine performance of these fuels in terms of ignition delay (IGD), peak cylinder pressure, brake torque, and brake fuel consumption.4 In conjunction with the engine tests, numerical modeling efforts strive to understand the combustion process of these fuels. As with many fuels, modeling and combustion efforts can be simplified if a surrogate mixture can be found that matches the properties of the algal fuel. Development of a surrogate reduces the myriad of chemical compounds found in fuels to a handful of simple components with known concentrations and properties aiding in modeling efforts because numerical models of real fuels would require a model too large for current computational resources.5 Such models require information on not only each component of the fuel but also all of the chemical kinetic rate constants, reaction pathways, and thermodynamic parameters that are not currently available. In addition to validating numerical models, surrogate mixtures can be used in the laboratory to provide a standard system that can be compared to real fuels, whose composition varies depending upon fuel sources or refinery, This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society
and for determining what fuel properties are most important for enhancing engine combustion.5 Researchers have sought to develop surrogates for rocket propellants,6 aviation fuels,7−9 coal-derived liquid fuel,10 petroleum diesel fuel,5,11−13 and biodiesel.14 Edwards and Maurice15 divided surrogate development into two approaches: one in which the physical properties of the surrogate are closely matched to the original fuel, called a “physical surrogate”, and a second in which the chemical and combustion characteristics are matched, called a “chemical surrogate”. In development and characterization of physical surrogates, several studies have used a distillation curve or advanced distillation curve in conjunction with other properties, including density, viscosity, speed of sound, cetane number, lower heating value, and thermal conductivity.8−10,13,16,17 These studies have resulted in surrogates ranging from 3 to 9 components. Wood et al.7 developed a 14-component surrogate for JP-4 from composition, cost, and the distillation curve and then measured physical and chemical properties (specific gravity, viscosity, surface tension, and hydrogen weight percentage) and combustion behavior in terms of smoke point, heat of combustion, droplet size, and velocity in a non-reacting spray chamber and axial field velocity and thermal field in a model combustor. Other researchers used chemical and physical properties together. Mueller et al.11 developed an 8-component surrogate for diesel fuel by minimizing the differences between the ignition quality (derived cetane number), carbon type, volatility (advanced distillation curve), and density of the surrogate and those of the diesel fuel. Received: November 19, 2012 Revised: January 11, 2013 Published: January 17, 2013 954
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Shimadzu GC 17A. The GC was equipped with a Restek Rxi-5 ms column (30 m, 0.25 mm, 0.25 μm, 5% diphenyl/95% dimethyl polysiloxane) that was operated at a helium flow rate of 1.5 mL/min. Separation was accomplished by a temperature ramping program starting at 60.0 °C and climbing at a rate of 10 °C/min to 250 °C. An electron impact (EI) ionization method was used for the mass spectrometer with a m/z scan from 30 to 600. Samples of the pure components and fuel were prepared by pipetting 0.1 μL of the fuel into 10.0 mL of hexane. The mass spectral analysis was delayed until the solvent peak had passed. The retention times of the various hydrocarbons were determined for comparison. No standards were prepared for quantitative measurements. The flash point was measured using a Setaflash Series 8 closed cup flash point tester model 82000-0 (Stanhope-Seta) under temperature ramping and no flash/flash settings. Per the literature of the manufacturer, this flash point model conforms to ASTM D3828 (gas ignition option), ASTM D1655 (gas ignition option), ASTM D3278, ASTM D7236, and ASTM E502. For comparison purposes, the measured flash point n-dodecane, 80.0 ± 1.0 °C, is in agreement with the literature value of 79 °C, while the measured flash point for nnonane, 32.0 ± 1.0 °C, agrees with the literature value of 34 °C.22 An SVM 3000 Stabinger viscometer (Anton Paar) was used to measure the density and viscosity of the algal HRD, petroleum F76 diesel, and the organic liquid mixtures, while a DSA 5000 density and sound analyzer (Anton Paar) was used to measure the speed of sound and also density. The accuracy of the SVM 3000 Stabinger viscometer was tested with a certified viscosity reference standard (Standard S3, Cannon Instrument Company) and recleaned and retested if the density deviated by more than 0.1% from the reference value and if the viscosity deviated by more than 1% from the reference value. Duplicate samples of each individual liquid or liquid mixture were measured at five temperatures between 20 and 100 °C, and these duplicates were used to determine the precision of the measurement. The DSA 5000 analyzer was checked before use with degassed distilled water and recalibrated if it failed the check, as specified by the manufacturer. Duplicate samples of each individual liquid or liquid mixture were measured at eight temperatures between 20 and 50 °C, and these duplicates were used to determine the precision of the measurement. Using the density (ρ) and speed of sound (c) measurements from the DSA 5000, the isentropic bulk modulus, Ev, was calculated at each temperature and ambient pressure by
Some researchers have tested and, in some cases, modified surrogates based on combustion characteristics, including IGD in a high-pressure shock tube,14 combustion products in a jetstirred reaction,18 soot induction delay and soot yield,12 and autoignition, soot fraction, and extinction in a laminar nonpremixed counterflow system.19 Lemaire et al.20 compared soot volume fraction (laser-induced fluorenscence) of 2-component diesel surrogate with diesel using a Holthius burner and then reformulated the 2-component surrogate to better match the volume fraction of soot. The soot products for the surrogate differed from those of the diesel fuel. Using n-decane, isooctane, and toluene as a starting point, Dooley et al.21 used threshold sooting index, hydrogen/carbon ratio, and “derived” cetane number to develop a surrogate. They then measured and modeled combustion heat release and reaction gases, such as CO, CO2, O2, and H2O, and IGD in a shock tube and in a rapid compression machine. Their surrogate behaved similarly to the fuel. For the current study, the goal was to develop surrogate mixtures that would match several fuel properties, such as flash point, density, and viscosity, which are important to the U.S. Navy for fuel handling and pumping, and to select the best mixture based on combustion of these surrogates in a military diesel engine. Military diesel must conform to specific standards for flash point, density, and viscosity. The method used to develop the surrogate mixture involved determining the major components of the algal HRD fuel by chemical analysis and measuring physical properties of the algal HRD fuel, including density, viscosity, flash point, and surface tension. The physical properties of petroleum F76 diesel were also measured for comparison to the algal fuel. Using the major components of the algal fuel as a starting point, an additional lighter branched component was added to the surrogate mixture to match the flash point. Then, the viscosity, density, and surface tension of the surrogate mixtures were measured. Two of the surrogate mixtures were tested in a military diesel engine.
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EXPERIMENTAL SECTION
Ev (Pa) = c 2 (m s−1)×ρ (kg m−3)
Materials. The pure organic compounds used in the composition analysis of the algal HRD fuel included octane (Sigma Aldrich, 99+% pure), 2-methyl octane (TCI Chemicals), 2-methyl nonane (TCI Chemicals), decane (Sigma Aldrich, 99+% pure), undecane (Sigma Aldrich), dodecane (Sigma Aldrich, 99+% pure), tridecane (Sigma Aldrich), 2-methyl tridecane, tetradecane (MP Chemicals), pentadecane (Sigma Aldrich), 2-methyl pentadecane (MP Chemicals), hexadecane (Sigma Aldrich, >99% pure), 2-methyl hexadecane (MP Chemicals), heptadecane (Sigma Aldrich), octadecane (Sigma Aldrich), eicosane (Sigma Aldrich), and hexane (Pharmco AAPR). Surrogates were made with the components mentioned above, 2,2,4trimethylpentane (isooctane, Sigma Aldrich, 99.5% pure), and isododecane (mixture of isomers, Alfa Aesar). The petroleum F76 diesel and an algal HRD (lot 10113-02250-000) were provided by the Naval Fuels and Lubricants Cross Function Team at Patuxent River, MD (PAX River). The algal HRD was produced by Solazyme and refined by Honeywell UOP. Solazyme uses proprietary algae grown in environmentally controlled reaction vessels. Honeywell UOP uses a two-stage process that first removes the heteroatoms and saturates the double bonds using a propriety catalyst and then isomerizes and selectively cracks the resulting products to increase the distribution of components. Analyses of Chemical and Physical Properties. To determine the composition of the algal HRD, gas chromatography/mass spectrometry (GC/MS) analysis was conducted and comparisons were made with pure organic compounds. In the analysis, a QP5050A single quadrapole mass spectrometer was used in conjunction with a
(1)
A Kruss DS100 axisymmetric drop-shape analyzer was used to measure the surface tension of the algal HRD, petroleum F76 diesel, and the organic liquid mixtures. In this analyzer, a droplet of the organic phase is formed in the air. The system takes an image of the droplet, enlarges it, and analyzes the droplet shape by fitting it with the Young−Laplace equation using the densities of the organic phase and air.23,24 Over 15 measurements were taken for three droplets of each liquid. Using this system, the surface tension of decane, 22.8 ± 0.1 dyn/cm, is slightly lower than the literature value of 23.8 dyn/cm at 20 °C.25 Manufacturers do not specify surface tension in their analysis, and trace impurities can cause variation in values. Analyses of Combustion in a Military Diesel Engine. Engine tests were run on the algal HRD, petroleum F76 diesel, and two surrogate mixtures in a laboratory-installed Yanmar L48 V singlecylinder diesel engine. This engine was coupled to a 20 hp MidwestDynamatic dynamometer (MW66), which allowed for testing various engine loads at different engine speeds. For this study, engine load sweeps were performed at 2000 and 3000 rpm. The engine− dynamometer test stand was heavily instrumented, including fast incylinder and fuel-line pressure sensors as well as conventional flow and temperature sensors. Details of this engine test stand can be found in the study by Arment et al.26 To compare one fuel to another, standard in-cylinder combustion metrics were analyzed from the high-speed experimental data.27 These include start of injection (SOI), start of combustion (SOC), IGD, maximum rate of heat release (maxROHR), and combustion duration. These combustion metrics are based on an 955
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Table 1. Selected Property Values of Fuelsa and Comparison with Military Specifications30
energy conservation analysis of the closed engine cylinder during compression, combustion, and expansion.28,29 IGD, maxROHR, and combustion duration are the most commonly used metrics to characterize diesel combustion. It is of interest to see how closely the combustion characteristics of surrogate fuel mixtures compare to that of algal HRD. Petroleum F76 diesel was also measured for reference.
military specification petroleum F76 diesel algal HRD
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RESULTS AND DISCUSSION Surrogate development requires determining the major components of the algal HRD and preparing a hydrocarbon mixture that matches physical properties as closely as possible. The properties selected in this work for matching were flash point, density, and viscosity. Property Measurements for Petroleum Fuel and Algal HRD. Density and viscosity were measured over the range of temperatures from 20 to 100 °C, as shown in Figures 1 and 2.
density at 15 °C (g mL−1)
viscosity at 40 °C (mm2 s−1)
flash point (°C)
60
surface tension (dyn cm−1)
0.851
2.47
62 ± 1
26.7 ± 0.2
0.772
2.30
64 ± 1
24.6 ± 0.3
The error in the density is 0.0001 g mL−1, and the error in viscosity is 0.01 mm2 s−1. a
The flash points of 62 and 64 °C for petroleum F76 diesel and algal HRD, respectively, are slightly higher than the 60 °C specification. The surface tension values of the algal HRD and petroleum F76 diesel are similar and vary by 2 dyn cm−1, as shown Table 1. These values were measured at a temperature slightly below 21 °C (20.5−20.8 °C). The algal chemical composition and the flash point were used to select the composition of the surrogates. Composition of Algal HRD. The algal HRD has components that span a smaller boiling point range than the petroleum F76 diesel, as shown in the GC/MS scan in Figure 3.
Figure 1. Comparison of the density of petroleum F76 diesel (■) and algal HRD (□).
Figure 3. GC/MS scan of (A) petroleum F76 diesel and (B) algal HRD.
The petroleum F76 diesel had more than 15 dominant peaks ranging in retention time from 3.5 to 21.5 min, while the algal HRD only had 4 dominant peaks at 12.39, 13.66, 15.10, and 16.35 min, with smaller peaks at lower boiling points. To determine the potential identity of the components, the retention times of pure component alkanes were measured and their retention times are given in Table 2. The components that best match the retention times of the algal HRD are npentadecane, n-hexadecane, n-heptadecane, and n-octadecane with retention times of 12.4, 13.7, 15.1, and 16.37 min, respectively. The widths of the pure component peaks were larger than those of the algal HRD; therefore, small differences in retention times are not significant. To help confirm the identity, the mass spectra of these compounds were compared, as shown in Figures 4−7. The mass spectra of the pure components match the spectra in the algal HRD fairly well. The lack of exact match may be caused by the presence of other compounds in the algal HRD at the same retention time. On
Figure 2. Comparison of the kinematic viscosity of petroluem F76 diesel (■) and algal HRD (□).
The petroleum F76 diesel is denser and more viscous than the algal HRD, but both fuels fall within the military specifications for diesel fuel,30 as shown in Table 1. The variation in density with the temperature is linear. A least-squares fitting program (Excel, 2010), was used to generate equations density (petroleum F76 diesel, g mL−1) = −7.150 × 10−4T (°C) + 0.8621
(2)
density (algal HRD, g mL−1) = −7.048 × 10−4T (°C) + 0.7859
(3)
The flash point of petroluem F76 diesel and algal HRD fall within the specifications of the U.S. Navy, as shown in Table 1. 956
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Table 2. Physical Property Information and Retention Time Data for Pure Hydrocarbons molar mass
retention time (min)
organic liquid
formula
boiling point (K)31−33
n-octane 2-methyl octane 2-methyl nonane n-decane n-undecane 2-methyl undecane n-dodecane n-tridecane 2-methyl tridecane n-tetradecane n-pentadecane 2-methyl pentadecane n-hexadecane 2-methyl hexadecane n-heptadecane n-octadecane n-eicosane
C8H18 C9H20 C10H22 C10H22 C11H24 C12H26
398.7 416.1 440 447 468 483
114 128 142 142 156 170
2.3 3.13 4.63 5.2 6.79 7.77
C12H26 C13H28 C14H30
489 507 518 ± 3
170 184 198
8.3 9.77 10.7
C14H30 C15H32 C16H34
523 540 550 ± 3
198 212 226
11.1 12.4 13.3
C16H34 C17H36
554 564 ± 3
226 240
13.7 14.6
240 254.5 282.5
15.1 16.37 18.5
C17H36 C18H38 C20H42
576 589 616
Figure 6. MS scan of (A) 15.1 min peak on algal HRD gas chromatogram and (B) heptadecane.
Figure 7. MS scan of (A) 16.4 min peak on algal HRD gas chromatogram and (B) octadecane.
estimated to be 13% n-pentadecane, 20% n-hexadecane, 39% nheptadecane, and 28% n-octadecane. A more accurate determination was not necessary because these were not the only components of the surrogate. Surrogate Mixture Development. A surrogate containing only 13% pentadecane, 20% hexadecane, 39% heptadecane, and 28% octadecane does not represent the algal HRD very well because the lower boiling point components that elute earlier in the chromatogram are not represented and the measured flash point of this 4-component mixture was 141 °C, which is much higher than 64 °C measured for the algal HRD. The density at 20 °C (0.7768 mg/L) and viscosity at 40 °C (3.23 mm2/s) of the 4-component mixture were also higher than the 0.7715 mg/ L and 2.30 mm2/s values of the algal HRD. To lower the flash point, several branched alkanes (2-methyl octane, 2-methyl nonane, isooctane, or isododecane) were added individually to the 4-component mixture. Branched components were selected because hydrotreating of biofuels tends to form branched as well as linear alkanes.34 The branched component was added until the flash point of the surrogate mixture matched that of the algal HRD. Because of the expense of the 2-methyl alkanes, initial tests added decane at varying concentrations to match the flash point, as shown in Figure 8 (□). The mass percentage of 2-methyl nonane required to match the algal HRD flash point was similar to that of decane, but the mass of 2-methyl octane was much less (Figure 8). The mixture flash point was very sensitive to the amount of isooctane but not as sensitive to the amount of isododecane (Figure 9). The surrogate mixture compositions that had flash points within 1 °C of the 64 ± 1 °C of the algal HRD are given in Table 3. An additional value is given for isooctane to show that a 0.3% change in the isooctane mass percentage can lower the flash point by 3 °C. Of the five
Figure 4. MS scan of (A) 12.4 min peak on algal HRD gas chromatogram and (B) pentadecane.
Figure 5. MS scan of (A) 13.7 min peak on algal HRD gas chromatogram and (B) hexadecane.
the basis of the chemical analysis, n-pentadecane, n-hexadecane, n-heptadecane, and n-octadecane were selected as the major starting components of the surrogate. To determine the relative amounts of these four components, their peak heights and peak areas were used. It was assumed that the detector response is the same for all compounds. The mass percentages were 957
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Table 4. Comparison of Physical Property Information of Surrogates and Algal HRDa density at 20 °C (g mL−1) algal HRD initial nalkane mixture surrogate 1 surrogate 1A surrogate 2 surrogate 3 surrogate 4
Figure 8. Determination of the percentage of the fifth component added to the pentadecane, hexadecane, heptadecane, and octadecane mixture to match the flash point of that found for algal HRD: decane (□), 2-methyl octane (■), and 2-methyl nonane (▲).
viscosity at 40 °C (mm2 s−1)
surface tension at room temperature (dyn cm−1) 24.6 ± 0.3 not measured
0.7715 0.7768
2.30 3.23
0.7757 0.7752 0.7714 0.7628 0.7684
3.13 3.12 2.76 2.15 2.37 ± 0.04
25.0 24.5 21.4 24.0 24.1 ± 0.4
The error is 0.0001 g mL−1 or less for density, 0.01 mm2 s−1 or less for viscosity, and 0.1 dyn cm−1 or less for surface tension, unless otherwise indicated. a
Figure 9. Determination of the percentage of isooctane (□) and isododecane (■) added to the pentadecane, hexadecane, heptadecane, and octadecane mixture to match the flash point of that found for algal HRD.
proposed surrogates, the surrogates containing n-methyloctane (surrogate 2) and isododecane (surrogate 4) match the density of the algal HRD better than the others, while the surrogate mixtures containing 2-methylnonane (surrogate 3) and isododecane (surrogate 4) match the viscosity of the algal HRD better than the others (Table 4). Additional density and viscosity data over a temperature range from 20 to 100 °C can be found in the Supporting Information. Military Diesel Engine Tests. The 2-methyl branched components are only commercially produced in small quantities; therefore, engine testing on the surrogate mixtures containing 2-methyloctane and 2-methylnonane was costprohibitive. The performance of petroleum F76 diesel, algal HRD, the 5-component surrogate containing isooctane, and the 5-component surrogate containing isododecane was compared using the metrics of the SOI, SOC, and IGD, maxROHR, and combustion duration in the Yanmar engine. The measured SOI experimental results are shown in Figure 10. Gross mean effective pressure (GMEP) is shown as the independent variable. GMEP is the work per engine cycle as determined by the integrated in-cylinder pressure times cylinder volume change divided by the engine cylinder volume. Dividing the work per cycle by the displaced volume of the
Figure 10. SOI for petroleum F76 diesel (■), algal HRD (□), surrogate with isododecane (▲), and surrogate with isooctane (×). The error bars are the standard deviation.
engine leads to units of pressure and, thus, the characteristic name of “effective pressure”. GMEP is directly proportional to engine torque. The ordinate shows the SOI as measured by the fast Kistler fuel-line pressure sensor located just upstream of the diesel fuel injector. The “negative” degrees reflect the engine position before top center (TC). Thus, it can be seen that U.S. Navy petroleum F76 diesel fuel arrives the earliest (greatest negative value). This is due to petroleum F76 diesel having the highest bulk modulus of elasticity when compared to HRD and the two engine tested surrogates, as shown in Figure 11. When the engine is fueled with petroleum F76 diesel, this conventional diesel fuel arrives in the engine cylinder more than one crank angle degree earlier than the other fuels. The SOI results for algal HRD are very similar to the modestly lagging surrogates, which start injection within a degree of algal HRD. At the 95% confidence interval (CI) using the standard
Table 3. Algal HRD Surrogate Mixtures (Percentage by Mass) with Flash Points That Match the Algal HRD Value of 64 ± 1 °C component (% by mass) surrogate
n-C15H32
n-C16H34
n-C17H36
n-C18H38
initial n-alkane mixture 1 1A 2 3 4
13 13.17 13.13 12.2 9.8 9.3
20 19.59 19.54 18.2 14.9 13.8
39 37.81 37.70 34.7 27.3 26.5
28 27.49 27.40 26.3 20.7 19.3
isooctane
2-methyl octane
2-methyl nonane
isododecane
1.94 2.23 8.6 27.2 31.1 958
flash point (°C) 141 63 60 63 63 65
± ± ± ± ± ±
2 1 1 1 1 1
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reactive by detailed chemical kinetic modeling.35 The algal HRD starts burning a few tenths of a degree after the isooctanebased surrogate, followed by the isododecane-based surrogate by approximately 1/2 of a crank angle degree. These differences are modest; however, it is expected that approximately 1/3 branched alkane content of the isododecane-based surrogate would delay the SOC for this fuel because of the less reactive nature of the highly branched alkanes. Finally, petroleum F76 diesel fuel is the last to start burning. Conventional diesel fuel is comprised of nominally equal portions of aromatic and cycloparaffinic hydrocarbon compounds (along with normal and branched alkanes), which are more resistant to autoignition compared to the straight-chain paraffins.35 The combined effect of the time (or crank angle degrees) from SOI to SOC is captured by the IGD combustion metric. IGD is a critically important metric for diesel engine operation. If IGD is too short combined with the SOC near-piston TC, then very high pressure combustion can result, potentially leading to engine overload damage. If IGD is too long, partial combustion or no combustion may occur, leading to the engine stalling. Thus, when a new fuel is evaluated, often the changes in IGD are evaluated as a reflection of fuel suitability. IGD as a function of engine load (GMEP) is shown in Figure 13 for the fuel and fuel surrogates tested in this study. In
Figure 11. Bulk modulus for petroleum F76 diesel (■), algal HRD (□), surrogate with isododecane (▲), and surrogate with isooctane (×). The standard deviations in the calculated values are smaller than the symbols.
deviation from approximately 50 engine cycles collected at each GMEP, the differences in SOI between the algal HRD and the two surrogates are not statistically different. The two surrogates also have similar bulk modulii to the algal HRD; however, they both have higher viscosities. Because this Yanmar engine uses a Bosch unit pump and injector fuel system with a 25 cm connecting high-pressure line, higher viscosity fuel will tend to delay SOI because of slower fuel transport from the pump to the injector. Both surrogates show slightly delayed SOI compared to algal HRD. SOC results for the four fuels tested in this study are shown in Figure 12. SOC is determined by analyzing the in-cylinder
Figure 13. Injection delay as a function of GMEP (normalized engine torque) at 2000 rpm for petroleum F76 diesel (■), algal HRD (□), surrogate with isododecane (▲), and surrogate with isooctane (×). The error bars are the standard deviation.
general, IGD is seen to moderately increase in duration (crank angle degrees) as load increases. This is due to the increased relative importance of fuel evaporative cooling as more fuel is injected (e.g., load increased). All four fuels tested show this trend. IGD in this study is defined as the time (crank angle degrees) from the SOI to the SOC. Again, as was the case for SOC, the highly paraffinic isooctane-based surrogate has the shortest IGD, which would be characteristic of a very high cetane fuel. This surrogate is nominally 3−6% faster than algal HRD (significant at the 90% CI). Next, algal HRD and the isododecane-based surrogate show very similar (within 1% of each other, not statistically different) IGD behavior, being slightly slower compared to isooctane-based surrogate by nominally one crank angle degree. Lastly, petroleum F76 diesel fuel has the longest IGD because of its much lower ignition quality (e.g., lower cetane number) compared to the other fuels. The petroleum F76 diesel is delayed by approximately 2− 3 crank angle degrees. This corresponds to nominally a significant 20% difference compared to algal HRD and its surrogates.
Figure 12. SOC as a function of GMEP (normalized engine torque) at 2000 rpm for petroleum F76 diesel (■), algal HRD (□), surrogate with isododecane (▲), and surrogate with isooctane (×). The 95% CIs (not shown) range from 0.2 to 0.5 degrees, which means that the SOC values for algal HRD and the two surrogates are not statistically different from each other at all GMEP. These SOC values do differ from those of the petroleum F76 diesel.
pressure curve for deviations beyond the expected compression and expansion pressure trace in the absence of fuel injection (motoring pressure trace). For this study, SOC was precisely defined as 5% of the maximum combustion heat release, which was analyzed after the data were collected. In Figure 12, it can be seen that the surrogate containing isooctane starts combustion first (greatest negative value, before TC), despite the fact that it arrived in the combustion chamber after most of the other fuels (Figure 10). This surrogate starts burning first because of its strongly paraffinic nature, consisting of 98% linear alkanes. Straight-chain alkanes have been shown to be the most 959
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of the fuel has burned. It can be seen that all of the fuels show a somewhat similar behavior. In general for diesel engines, long IGDs lead to significant fuel−air premixing and, thus, shorter overall burn durations. This trend is also true in this study. Petroleum F76 diesel, which has the longest IGD, also has the shortest overall burn duration. The high cetane algal HRD and its surrogates all have longer burn durations because of their shorter IGDs, and thus, more time is required to burn the fuel completely. When comparing the burn duration of the isooctane- and isododecane-based surrogates, the differences are very modest; however, with small burn duration standard deviations, the results are statistically significantly different at some but not all GMEP. On the basis of these engine measurements, the surrogate containing the isododecane more closely reflects the combustion behavior of algal HRD fuel by being a better match to the IGD and maxROHR characteristics of HRD. The isooctane-based surrogate also shows similar combustion characteristics to HRD but, in general, shows faster ignition characteristics and lower rates of heat release.
The maxROHR is an important combustion metric. If the maxROHR is too fast, excessive engine noise may result or, in the limit, engine damage may occur. In general, low cetane number fuels can lead to long IGDs and, thus, large maxROHR. This is true for petroleum F76 diesel, which has the greatest maxROHR seen in this study, as shown in Figure 14. Because
Figure 14. MaxROHR as a function of GMEP (normalized engine torque) at 2000 rpm for petroleum F76 diesel (■), algal HRD (□), surrogate with isododecane (▲), and surrogate with isooctane (×). The error bars are the standard deviation.
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CONCLUSION
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ASSOCIATED CONTENT
In this study, the chemical composition and physical properties of an algal HRD fuel were used to develop a surrogate mixture containing commercially available hydrocarbons and the combustion of a subset of surrogates was tested in a Yanmar engine. Analysis of the chemical composition of the algal HRD showed a small quantity of low-molecular-weight components and a high quantity of four high-molecular-weight components: n-pentadecane, n-hexadecane, n-heptadecane, and n-octadecane. Using these four components, a fifth branched component was added to match the physical properties of the algal HRD. Candidates for the fifth component included 2-methyloctane, 2methylnonane, isooctane, and isododecane. The isooctane- and isododecane-based surrogates were tested in a Yanmar engine along with algal HRD and petroleum F76 diesel to assess SOI, SOC, IGD, maxROHR, and overall combustion duration. The surrogate that best matched the flash point, density, viscosity, and surface tension was one containing isododecane, npentadecane, n-hexadecane, n-heptadecane, and n-octadecane. Combustion of algal HRD, petroleum F76 diesel, and the five component surrogates containing either isooctane or isododecane showed that the algal HRD behaved differently from the petroleum F76 diesel, with the algal fuel having shorter SOI, longer SOC, shorter IGD, lower maxROHR, and longer overall burn duration. When comparing the combustion metrics for the two surrogate mixtures, the SOI, SOC, and overall combustion event duration were similar to each other and to that of the algal HRD but the IGD and maxROHR for the isododecane-based surrogate mixture more closely matched the algal HRD than the isooctane-based surrogate mixture. On the basis of the physical properties and combustion metrics, the surrogate mixture containing isododecane, n-pentadecane, nhexadecane, n-heptadecane, and n-octadecane best matched the algal HRD.
the maxROHR results of algal HRD are significantly less than petroleum F76 diesel, the potential for engine damage because of excessive pressure rise is unlikely. The algal HRD and the two surrogate mixtures are seen in the lower section of the figure. Algal HRD and the isododecane-based surrogate have very similar maxROHR, within 1%. The isooctane-based surrogate has the lowest maxROHR, nominally 20% lower than the algal HRD and the isododecane-based surrogate at the higher engine loads. The dominant normal alkane (paraffin) nature of the isooctane-based surrogate leads to short IGDs and, thus, low overall heat release rates during combustion, as will be described next. Finally, overall burn duration is shown in Figure 15. This metric is defined as the crank angle degree duration to burn most of the injected fuel. Overall, burn duration coupled with its phasing affect engine output torque. For this study, the burn duration is defined to start when 10% of the fuel has burned as determined from the heat release analysis. The end of the burn duration is defined as the point in the engine cycle where 90%
S Supporting Information *
Figure 15. Overall combustion event burn duration as a function of GMEP (normalized engine torque) at 2000 rpm for petroleum F76 diesel (■), algal HRD (□), surrogate with isododecane (▲), and surrogate with isooctane (×). The standard deviations of burn duration are equal to or smaller than the size of the symbol.
Additional density and viscosity data (Tables S1 and S2). This material is available free of charge via the Internet at http:// pubs.acs.org. 960
dx.doi.org/10.1021/ef301879g | Energy Fuels 2013, 27, 954−961
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 410-293-6349. Fax: 410-293-2218. E-mail: prak@ usna.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Office of Naval Research for funding this project. REFERENCES
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