Article pubs.acs.org/EF
Fuel Properties of Highly Polyunsaturated Fatty Acid Methyl Esters. Prediction of Fuel Properties of Algal Biodiesel Gerhard Knothe*,† †
National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, Peoria, Illinois 61604, United States ABSTRACT: Biodiesel, defined as the monoalkyl esters of vegetable oils and animal fats, can be derived from other triacylglycerol-containing feedstocks. Algae are being considered for this purpose due to their claimed high production potential. However, there are no comprehensive reports regarding the fuel properties of biodiesel obtained from algal oils. Algal oils, with examples of some exceptions also mentioned here, often contain significant amounts of saturated and highly polyunsaturated (≥4 double bonds) fatty acid chains which influence fuel properties of the resulting biodiesel. In this connection, the relevant fuel properties of biodiesel from algal oils and the important fuel properties of highly polyunsaturated fatty acid methyl esters as they would occur in many biodiesel fuels obtained from algal oils, have not yet been reported. To fill this gap, in the present work for the first time two neat highly polyunsaturated fatty acid methyl esters with more than three double bonds, methyl 5(Z),8(Z),11(Z),14(Z)-eicosatetraenoate (C20:4) and methyl 4(Z),7(Z),10(Z),13(Z),16(Z),19(Z)-docosahexaenoate (C22:6), were investigated. The cetane numbers were determined as 29.6 for C20:4 and 24.4 for C22:6. Kinematic viscosity values were observed as 3.11 mm2/s for C20:4 and 2.97 mm2/s for C22:6, while oxidative stability values were below 0.1 h for both by the Rancimat test while densities were above 0.9 g/cm3. Two polyunsaturated C20 methyl esters, methyl 11(Z),14(Z)eicosadienoate and 11(Z),14(Z),17(Z)-eicosatrienoate, were also studied for kinematic viscosity, density, and oxidative stability to expand the database on these properties of C20 compounds and provide additional data to predict the properties of other highly polyunsaturated fatty acid methyl esters. Properties of biodiesel from algal oils are predicted with cetane numbers expected in the low to upper 40s and kinematic viscosity between 3 and 4 mm2/s for most algal biodiesel.
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INTRODUCTION Biodiesel1,2 is a biogenic alternative to petroleum-derived diesel fuel (petrodiesel) that can be obtained from vegetable oils, animal fats, used cooking oils, or other triacylglycerolcontaining materials by a transesterification reaction with a monohydric alcohol, usually methanol. Thus biodiesel is generally defined as the monoalkyl esters of vegetable oils or animal fats.3 Standards for biodiesel have been established in many countries worldwide with the American standard ASTM (American Society for Testing and Materials) D67513 and the European standard EN 142144 often serving as guidelines for development of standards elsewhere. Biodiesel is technically competitive with conventional, petroleum-derived diesel fuel (petrodiesel) and requires virtually no changes in the fuel distribution infrastructure. While biodiesel faces some technical challenges such as reduction of NOx exhaust emissions, improving cold flow properties, and enhancing oxidative stability, advantages of biodiesel compared to petrodiesel include reduction of most exhaust emissions, biodegradability, higher flash point, domestic origin, and inherent lubricity.1,2 Biodiesel production and use have generally increased over the years as concerns over the finite supply of petroleum mount. In recent years, the output of research papers on biodiesel from algae has increased exponentially as has the number of review and commentary papers written under a variety of perspectives, a selection if which are.5−31 A major driving force behind this work have that even when combining all possible vegetable oil and animal fat feedstocks, biodiesel can replace only a few percent of petroleum-derived diesel fuel (petrodiesel) with algae having been identified as a feedstock This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society
with high production potential. Critics state, however, that this production potential is often considerably exaggerated. Another issue that plays a role in promoting algae as potential biodiesel feedstock is the perceived avoidance of the so-called food vs fuel issue. An original extensive research program on fuel from algae was performed by the U.S. Department of Energy well before the current intensified interest in algae,32 and a roadmap in light of the renewed interest was recently established.33 Despite these efforts and reports in popular media on the use of algae-derived fuels for demonstration purposes or even acquisition by government agencies for larger-scale use, the availability of algal biodiesel has remained limited as has technical information on its fuel properties. A recent publication, however, discusses the effect of varying amounts of methyl eicosapentaenoate and methyl docosahexaenoate on algal fuel properties.34 The same authors state in a paper on exhaust emissions when operating a diesel engine with algaebased fatty acid methyl esters, actually a mixture of vegetable oil and fish oil was used as the authors stated that “algal oil is currently a scarce commodity”.35 These authors continued that “it was cost prohibitive to obtain algal fuel samples large enough to accommodate the necessary emissions text matrix”.35 These statements coincide with the general observation that no comprehensive reports seem to exist on the fuel properties of biodiesel from algal oils. Thus, besides simulation the Received: April 25, 2012 Revised: June 28, 2012 Published: June 28, 2012 5265
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Table 1. Specifications in Biodiesel Standards That Are Largely Affected by the Fatty Ester Profilea standard ASTM D6751 (United States)
EN 14214 (Europe)
specification
test method
unit
limit
cetane number kinematic viscosity oxidative stability cloud point cold-filter plugging point density fatty acid profile linolenic acid content content of FAME with 4 or more double bonds iodine value
D613/D6890 D445 EN 15751 D2500
mm2/s h °C
47 min 1.9−6.0 3 min report
test method
EN ISO 5165 EN ISO 3104 EN 14112 EN 15751 EN ISO 3675 EN ISO 12185 EN 14103 EN 14111
unit
limit
mm2/s h °C kg/m3
51 min 3.5−5.0 6 min variableb 860−900
% (m/m) % (m/m) g iodine/100 g
12 1 120
a
ASTM = American Society for Testing and Materials; ISO = International Standards Organization. bDepends on geographic location and time of year.
(C18:3) cause the expectation that the even more highly unsaturated fatty esters in algal oils may have similarly low, or even lower, CNs, affecting the overall CN of the biodiesel fuels. Therefore, determining this property is crucial. The kinematic viscosity, however, of highly unsaturated fatty acid alkyl esters would be expected to be unproblematic, but experimental data have been lacking to underscore this expectation. With regard to low-temperature properties, extremely low melting points can be expected for such unsaturated compounds based on literature data for which reason no cold flow studies specific to these HiPUFAMEs appear necessary currently. Besides determining the properties of such neat esters, it is of interest to extrapolate the results to other HiPUFAMEs found in algal oils and which were not available at reasonable cost or effort. Another issue is to predict the properties of algal biodiesel fuels using the experimental data obtained here instead of utilizing only data for fatty acid alkyl esters as they are observed in vegetable oils. To expand the property database, in this connection two additional polyunsaturated fatty acid methyl esters, methyl 11(Z),14(Z)-eicosadienoate (C20:2) and methyl 11(Z),14(Z),17(Z)-eicosatrienoate (C20:3), were studied to have a more complete set on C20 compounds available. It may be noted that besides these properties, EN 14214 limits specific fatty acids in biodiesel. The limiting specifications include fatty acids with three or more than three double bonds in the chain and the iodine value (IV). Kinematic viscosity and density specifications, however, in EN 14214 can also affect the fatty acid profiles permissible under this standard and therefore feedstocks. As a result of the transesterification reaction, biodiesel contains small amounts of glycerol, free fatty acids, partially reacted acylglycerols (mono- and diacylglycerols), as well as residual starting material (triacylglycerols). These contaminating trace materials are accordingly limited in biodiesel standards such as ASTM D6751 and the EN 14214 as well as other standards under development around the world. These issues will not be dealt with here for algal biodiesel as they transcend the scope of this work. To summarize the above aspects, this work deals with fuel properties of algal biodiesel and its components also in relation to the relevant specifications in biodiesel standards.
properties of what would be algae-derived biodiesel fuels need to be extrapolated from what is known about the properties of neat fatty esters and their mixtures as they exist in biodiesel fuels from other feedstocks such as vegetable oils.25 Besides saturated fatty acids, most notably palmitic acid (hexadecanoic acid; C16:0), many algal oils contain significant amounts of highly polyunsaturated fatty acids (HiPUFA; ≥ 4 double bonds), with the most common being methyl 5(Z),8(Z),11(Z),14(Z),17(Z)-eicosapentaenoate (C20:5; methyl arachidonate) and methyl 4(Z),7(Z),10(Z),13(Z),16(Z),19(Z)-docosahexaenoate (C22:6).7,25,32 For such esters in the neat form, however, to the best of our knowledge, no information is available regarding their fuel properties. To better predict these properties of algal oils, corresponding information on neat compounds is essential. For this purpose, two highly polyunsaturated fatty acid methyl esters (HiPUFAME), methyl 5(Z),8(Z),11(Z),14(Z)-eicosatetraenoate (C20:4; methyl arachidonate) and methyl 4(Z),7(Z),10(Z),13(Z),16(Z),19(Z)-docosahexaenoate (C22:6), were selected for fuel property testing. Although C20:5 together with 22:6 is probably the most common HiPUFAME in algal oils, it was not investigated due to the high costs associated with it in the neat form, so that the more affordable C20:4 methyl ester was studied instead as C20 HiPUFAME. Some algal species containing these HiPUFAs are Isochrysis and Nannochloropsis.7,25,34 It may be noted, however, that in a study extending the aforementioned “algal biodiesel” exhaust emissions work,35 fuel properties of simulated algal esters were investigated.34,36 Another issue is that the published fatty acid profiles of many algal oils do not add to 100% (or a value close to 100%) when evaluating fatty acid profiles of algal oils compiled in the literature.7,25 Fuel properties tested are prescribed in the biodiesel standards ASTM D67513 as well as EN 142144 and are directly affected by the properties of fatty acid alkyl esters. These properties are among the most critical for overall fuel performance. The limits on relevant properties in these standards are given in Table 1. Thus, cetane number (CN), kinematic viscosity, oxidative stability, and density were determined. The issue of oxidative stability has been one of the major technical issues affecting the use of biodiesel.37−39 Cetane number, on the other hand, has usually not been a problem for biodiesel derived from vegetable oils and animal fats, but the low CN of methyl esters such as methyl linolenate
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EXPERIMENTAL SECTION
Materials. All neat compounds (methyl 11(Z),14(Z)-eicosadienoate, 11(Z),14(Z),17(Z)-eicosatrienoate, methyl 5(Z),8(Z),115266
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Table 2. Property Data for Polyunsaturated Fatty Acid Methyl Esters As Found in Algal Oils and Typical Esters Found in Biodiesel from Vegetable Oilsa fatty acid methyl ester property
C20:2
C20:3
C20:4
C22:6
C16:0
C18:0
C18:1
C18:2
C18:3
cetane number kinematic viscosity (40 °C; mm2/s) oxidative stability (110 °C; h) density (g/cm3) 15 °C 40 °C melting point (°C)
NDb 4.59 0.84 (0.01) 0.884747 0.866907 ND
ND 3.87 0.1 (0.01) 0.895463 0.877433 ND
29.57 3.11 0.09 0.906397 0.888107 ND
24.35 2.97 0.07 0.923587 0.905160 ND
85.9 4.38 >24 0.8491 28.5
101 5.85 >24 37.7
59.3 4.51 2.79 0.87746 0.85937 20.2
38.2 3.65 0.94 0.89016 0.87195 −43.1
22.7 3.14 0.00 0.90166 0.883 −45.5
a
Data for C16:0, C18:0, C18:1, C18:2, and C18:3 from refs 40, 41, 50, and 62. All density values determined in the course of this work. Cetane numbers are derived cetane numbers (see the Experimental Section). bND = not determined.
(Z),14(Z)-eicosatetraenoate and methyl 4(Z),7(Z),10(Z),13(Z),16(Z),19(Z)-docosahexaenoate) were purchased from NuChek-Prep, Inc. (Elysian, MN) and were of purity >99% as confirmed by gas chromatography (GC). GC analysis was carried out with an Agilent Technologies (Santa Clara, CA) 6890 gas chromatograph equipped with a flame ionization detector and a DB-88 ((88%-cyanopropyl)methylarylpolysiloxane) column (30 m × 0.25 mm ID tmex 0.20 μm film thickness). The temperature program was an initial temperature of 150 °C held for 15 min, increased to 210 at 2 °C/min then 50 °C/ min to 220 °C, and held for 5 min with He as carrier gas at 9.6 mL/ min. The injector and detector temperatures were 240 and 280 °C, respectively. The identity of the neat compounds was additionally verified by 1H- and 13C NMR spectroscopy on a Bruker (Billerica, MA) Avance 500 spectrometer operating at 500 MHz (1H NMR) or 125 MHz (13C NMR) with CDCl3 as solvent. Polynomial regression was carried out with Origin software (OriginLab Corporation, Northampton, MA). Property Determination. The neat compounds (C20:2, C20:3, C20:4, and C22:6) were received in sealed ampules. These ampules were opened in a glovebox under nitrogen just before experimental property determinations in order to minimize exposure of the samples to air. Cetane numbers were determined as derived cetane numbers (DCN) using an Ignition Quality Tester (IQT) located at Southwest Research Institute (San Antonio, TX) as described previously.40 The IQT (ASTM D6890) is an approved alternative to the traditional CN standard (ASTM D613) in the biodiesel standard ASTM D6751. Kinematic viscosity was obtained with Cannon-Fenske viscometers employing the standard ASTM D445 as described previously.41 Oxidative stability measurements were carried out with a Rancimat instrument (Metrohm, Herisau, Switzerland) using the standard EN14112.42 The times determined by the Rancimat method are induction times correlating to the maximum rate change in the conductivity of water into which the effluent of the samples are swept with an air stream. It may be noted that oxidative stability determination of the C20:4 and C22:6 methyl esters by the Rancimat method causes an odor resembling fried fish. Density was determined with an Anton Paar (Anton Paar USA, Ashland, VA) DMA 4500 M densitometer.
eicosatrienoate were also investigated. Table 2 contains the experimentally determined data for these compounds in comparison to previously determined data for C16 and C18 FAME as they occur in biodiesel from vegetable oils and other feedstocks. Cetane Number. The CNs (as DCNs) of C20:4 and C22:6 were determined as 29.6 and 24.4, respectively. Cetane numbers of C20:2 and C20:3 were not determined for cost reasons. The CN values of C20:4 and C20:6 thus are in the range of or even slightly higher than that of methyl linolenate, which may be somewhat surprising in light of the well-known cetane-lowering effect of increasing number of double bonds in a fatty acid chain.40,43 However, when comparing FAMEs such as methyl oleate, methyl linoleate, and methyl linolenate it may be noted that the decrease in CN decreases with every additional double bond. For example, the decrease in CN from methyl stearate to monounsaturated methyl oleate is about 40, that from methyl oleate to diunsaturated methyl linoleate about 18 to 20, and that from methyl linoleate to triunsaturated methyl linolenate about 15 to 18. Thus, in the case of C20:4, the cetane-lowering effect of the additional double bond vs methyl linolenate is likely offset by the increasing chain length, actually even overcompensated as the CN of C20:4 is slightly higher than that of C18:3. This is additionally demonstrated by the CN of monounsaturated FAMES. The CN of methyl oleate is approximately 58, while the CNs of methyl 11(Z)eicosenoate and the CNs of methyl 11(Z)-eicosenoate (methyl erucate) are >70.44 Overall, a general observation derived from these results is that two additional carbon atoms in the chain compensate for an additional double bond in terms of CNs. In very recently published work on formulations simulating algal biodiesel it is was reported,34 on the other hand, that the overall CN depends on HiPUFAME content and that the double bonds in the HiPUFAMEs have a stronger effect on CN than chain length. Therefore, the CNs of other C20 and C22 HiPUFAMEs are also likely higher than might have been expected. To approximate the CNs of these other HiPUFAMEs, an empirical approach using polynomial regression was applied to the experimental data and the values for the unavailable methyl esters extrapolated. The results showed that third- or fourthorder polynomial regression gives the best fit for determining CNs of unavailable compounds. An example with relevant data is given in Figure 1 for C20 compounds. This result is similar to that of third-order polynomial regression giving the best prediction of kinematic viscosity (40 °C) of neat compounds which are either solids at that temperature or unavailable in sufficient amounts in the neat form for this property determination.45
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RESULTS AND DISCUSSION The properties relevant to fuel use of two HiPUFAMEs, methyl eicosatetraenoate (arachidonate) and methyl docosahexaenoate, were determined. As mentioned above, numerous other HiPUFAMEs exist in algal oils; however, the C20:4 and C22:6 esters were selected for reasons of cost and availability with C22:6 being one of the most common HiPUFAMEs in algal oils. This is also, to our knowledge, the first investigation of fuel properties of neat fatty esters with more than three double bonds. The properties of these neat compounds are used in the prediction of fuel properties of biodiesel fuels obtained from algal oils. To extend the database on properties of FAME, methyl eicosadienoate and methyl 5267
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“migrate(s)” toward one end of the chain.43 This effect is not accommodated for in the present calculation of CNs for compounds for which experimental data do not exist. This effect, however, is relatively minor and even less for CNs of mixtures such as biodiesel. Cetane numbers of mixtures can be generally calculated from the equation CNmix =
The regression analysis suggests that the CN of C22:4 could be approximately 42, while the CN of C22:3 could be around 55 (Table 3). These may also appear to be somewhat Table 3. Prediction of Cetane Number for C16, C18, C20, and C22 PUFAMEs and HiPUFAMEs by Third-Order Polynomial Regressiona number of double bonds
C16 C18 C20 C22
0
1
2
3
4
5
6
85.9 101
52 59.3 73.2 74.2
32 38.2 49 68
22.7 36 55
16 29.6 42
22 32
18 24.4
(1)
in which CNmix is the CN of the mixture, AC = the relative amount (vol.-%) of an individual neat ester in the mixture, and CNC is the CN of the individual neat ester. It may be noted that this is a generalization of the equation given in the CN standard ASTM D613 for the calculation of CNs of mixtures of hexadecane (= cetane) and 2,2,4,4,6,8,8-heptamethylnonane which is CN = % cetane +0.15 (% HMN).46 Using eq 1, the CNs of biodiesel from algal oils with fatty acid profiles given in the literature appear to range largely from the low to upper 40s. Thus biodiesel from some algal oils would likely not meet the minimum CN specification (47) in ASTM D6751, while even less algal biodiesel fuels would not meet the higher minimum CN specification (51) in EN 14214. This conclusion is confirmed by the work on simulated algal biodiesel in which the CN (as DCN as also used here) varied between 37.0 and 48.4.36 It is stated there36 that the eicosapentaenoic and docosahexaenoic species would need to be removed to meet the minimum CN value prescribed in ASTM D6751. Another approach, however, would be the use of cetane improver additives although no information is available on the interaction of these additives with algal biodiesel. Besides CN, exhaust emissions are related to the issue of combustion. They are not addressed in standards, rather in regulations affecting engines. In a recent study35 it was found that simulated algal biodiesel fuels reduced NOx emissions despite the presence of HiPUFAMEs. This result is unexpected since higher levels of unsaturated compounds have previously been associated with increased NOx.47,48 Recent work on the cause of the NOx increase, however, has shown this effect to be more complex, being related to several coupled mechanisms which could reinforce or cancel one another depending on fuel and combustion characteristics with stoichiometric mixtures at ignition and in the standing premixed autoignition zone close to the flame lift-off length likely being major factors.49 It may be surmised that the presence of saturated fatty esters offsets the expected increase caused by HiPUFAMEs. The particulate matter was shown to decrease to levels similar to vegetable oilbased biodiesel fuels.35 Kinematic Viscosity. The kinematic viscosity values of methyl arachidonate and methyl docosahexanoate at 40 °C were determined to be 3.11 and 2.97 mm2/s, respectively. These values can be compared to those of several other methyl esters available from the literature. Important to note in this connection is that the viscosity of a fatty acid chain decreases with an increasing number of cis double bonds and increases with chain length, usually an increasing number of CH2 groups. The kinematic viscosity values of both C20:4 and C22:6 are close to the value for C18:3 (Table 2). This observation demonstrates both the effect of chain length and unsaturation on viscosity as the viscosity-increasing effect of greater chain length is offset by the viscosity-reducing effect of cis double bonds and is similar to that for CNs. To enhance the database
Figure 1. Polynomial regression for cetane data of C20 compounds with y = 100.573 − 48.977x + 9.943x2 − 0.740x3 (R2 = 0.99). Note that this leads to a CN > 100 for the saturated compound (CN of the C18 saturated methyl ester, methyl stearate, is approximately 100).
number of C in fatty acid chain
∑ A C × CNC
a
Predicted values in italics, experimental values when available (see Table 2) are also given.
surprisingly high values, but this can be explained as discussed above. On the other hand, it may be surmised that the CN of methyl stearidonate (C18:4; methyl 6(Z),9(Z),12(Z),15(Z)octadecatetraenoate) would be among the lowest of all HiPUFAMEs, approximately 16, as there is no increase in chain length to offset the cetane-lowering effect of the fourth double bond when comparing this to C18:3. Determining the CN and other fuel properties of methyl stearidonate was not possible, however, due to the significant costs of obtaining this material in the necessary amount in the neat form. Furthermore, the results imply by polynomial regression that the CNs of methyl eicosapentaenoate (20:5; 5(Z),8(Z),11(Z),14(Z),17(Z)-eicosapentaenoate) would be close to that of C18:3. It may be noted that some algal oils also contain C16:3 and C16:4 fatty acid chains,25 whose CN would also likely be very low (Table 3), probably even lower than C18:4, as there is no chain extension to offset the effect of the additional double bonds. For CN predictions it is very important to note that they are not absolute as experimental CN determinations show some variability. Furthermore, CNs depend on double bond positions, with CNs increasing when the double bond(s) 5268
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that the kinematic viscosity is also affected by the double bond position as shown for C18:1 fatty acid methyl esters (FAME).41 The fatty acid profiles of many algal oils show fatty acid chains with positional isomerism of the double bonds, a full discussion of which is beyond the scope of this work. The overall effect, however, on kinematic viscosity is relatively minor and therefore the ω-9 isomers, being by far the most common among the monounsaturated fatty acid chains, were used in this work as being representative. It appears that the kinematic viscosity of most biodiesel from algal oils with significant amounts of HiPUFAMEs would likely be in the range of 3.0−4.0 mm2/s. Biodiesel from algal oils would thus be in the range specified in the standard ASTM D6751, but some, especially with high amounts of HiPUFAMEs, may not meet the minimum specification in EN 14214. However, it appears that there is no technical justification for the high minimum viscosity specification in EN 14214 as the viscosity of most petrodiesel fuels is lower than 3.5 mm2/s as there is no reason why the viscosity of biodiesel could not be the same as that of petrodiesel. The present predictions on the kinematic viscosity of the methyl esters of algal oils correlate with the results reported on simulated esters, although some higher viscosities, whose origin is not clear, were also reported there.36 It is not clear, however, why the kinematic viscosity increased with increasing unsaturation in algal biodiesel simulations.36 Similar to the discussion of CNs above, the kinematic viscosity of most algal biodiesel fuels is likely lower than that of most vegetable oil-derived biodiesel fuels is underscored by the following general observations. Saturated fatty compounds possess higher viscosity than those with cis unsaturation. The most common saturated fatty acid in algal oils is palmitic acid whose methyl ester has a kinematic viscosity of 4.38 mm2/s. Saturated fatty acids with >16 carbons occur only in minor amounts in algal oils. Thus C16:0 is the fatty acid with the greatest viscosity contribution among the saturated fatty acids, the short chain esters such as 14:0 possessing lower viscosity. Similarly observations for the mono- and polyunsaturated fatty acids show that oleic acid (or other C18:1 isomers) whose methyl ester has a kinematic viscosity of 4.51 mm2/s is the most viscous component. As the kinematic viscosities of C16:0 and C18:1 methyl ester are similar and the kinematic viscosities of the other major components of algal biodiesel fuels lower, the kinematic viscosity of algal biodiesel fuels is likely lower than that of most other biodiesel fuels, especially those obtained from vegetable oils with more conventional fatty acid profiles. Oxidative Stability. The oxidative stabilities as measured by Rancimat induction times of C20:4 and C22:6 methyl esters were less than 0.1 h at 110 °C. These are low values that can be considered to be in the realm of expectations as oxidative stability decreases with the number of (methylene-interrupted) double bonds. Previously, the oxidative stability of methyl linolenate had been determined to be 0.0 h.50 No neat unsaturated fatty ester, however, has been found to meet the specifications for oxidative stability in biodiesel standards which are 3 h in ASTM D6751 and 6 h in EN 14214 with the oxidative stability of neat methyl oleate per the Rancimat test being 2.79 h.50 When methyl oleate is assigned a relative oxidation rate of 1, relative rates of oxidation for unsaturated fatty esters are 41 for C18:2, 98 for C18:3, and 195 for C20:4 as given in ref 51. Another issue is that minor amounts of highly
on experimental values available for prediction, the kinematic viscosity values of C20:2 and C20:3 were also determined (Table 2). Thus, the kinematic viscosity values of C20:4 and C22:6 can be correlated with the kinematic values of other C20 and C22 esters in relation to C18 and other esters. By such an approach, the data can also be used to qualitatively predict by third-order polynomial regression the kinematic viscosity values of not only C16 and C18 esters45 but also the other HiPUFAMEs of interest here. An example for C22 compounds is shown in Figure 2. The corresponding predicted kinematic viscosity values of unavailable HiPUFAMEs are given in Table 4.
Figure 2. Polynomial regression for kinematic viscosity data of C22 compounds with y = 9.293 − 2.149x + 0.242x2 − 0.01x3 (R2 = 0.99).
Table 4. Prediction of Kinematic Viscosity (40 °C; mm2/s) for C16, C18, C20, and C22 HiPUFAMEs by Third-Order Polynomial Regressiona number of double bonds number of C in fatty acid chain
0
C16 C18 C20 C22
4.38 5.85 7.40 9.31
1 3.67 4.51 5.77 7.33
(Δ7) (Δ9) (Δ11) (Δ13)
2
3
4
5
6
3.10 3.65 4.59 5.93a
2.70 3.14 3.87 4.80
2.45 2.80 3.11 3.90
2.50 2.80 3.35
2.97
a
Predicted values in italics, experimental values when available (see Table 2) are also given. Experimental values and predictions for saturated fatty from refs 41 and 45.
The kinematic viscosity of mixtures νmix can be calculated45 from the equation νmix =
∑ A C × νc
(2)
in which νmix is the kinematic viscosity of the biodiesel sample (mixture of fatty acid alkyl esters), and νc is the kinematic viscosity of the individual compounds in the mixture. The experimental values of monounsaturated fatty acid chains used for predicting the kinematic viscosity of the HiPUFAMEs were those of the cis ω-9 species. It may be noted 5269
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Prediction of Properties of Algae-Derived Biodiesel Fuels. For this purpose, some algal fatty acid profiles compiled in previous publications7,25 are evaluated. The properties predicted here are CN and kinematic viscosity. The effect of saturated fatty esters on cold flow properties is discussed above, and no formal prediction is made here as the HiPUFAMEs evaluated here probably have no significant influence on this property. Similarly, oxidative stability is not evaluated as there is no direct quantitative relationship between the amount of a specific unsaturated fatty ester in a biodiesel fuel and its oxidative stability by the Rancimat method. However, small amounts of highly unsaturated compounds reduce oxidative stability more than the small amounts indicate. The species Isochrysis has found considerable interest as potential algal feedstock for biodiesel. Evaluation of some fatty acid profiles shows that biodiesel from an Isochrysis galbana species (ref 116 in the present ref 25) with 10.9% C14:0, 13.7% C16:0, 16:1% C16:1, 8.8% C18:4, 26% C20:5, and 11.8% C22:6 would likely have a CN in the low 40s. By the fatty acid profile, the CN would be approximately 39, but the fatty acid profile given adds to only about 93% so that some accommodation is needed for the “missing” 7% of the profile, assuming that the “missing” part of the profile would contribute to the properties proportionally to the fatty acid profile accounted for. The kinematic viscosity would be around 3 mm2/s accounting for the “missing” 7%, while the fatty acid profile as given would provide for 2.85 mm2/s. An algal oil from an Isochrysis galbana species (ref 111 in the present ref 25) with 16.8% C14:0, 11.1% C16:0, 5.1% C16:1, 9.7% C18:1, 3.5 C18:2, 16.3% C18:3, 19.8% C18:4, and 12% C22:6 would likely exhibit a CH around 44 considering that around 96% of the fatty acid profile is accounted for in the reference. The kinematic viscosity of biodiesel from this oil would be around 3.3 mm2/s when accommodating the “missing” part of the profile while the profile as given would provide for a kinematic viscosity of about 3.26 mm2/s. On the other hand, another Isochrysis galbana sample reported (ref 110 in the present ref 24) with 19.2% C14:0, 13.7% C16:0, 4.7% C16:1, 17.5% C18:1, 6.7% C18:2, 6.7% C18:3, 13.5% C183, 2.2% C22:5, 9.5% C22:6, and >99% of the fatty acid profile accounted for would likely exhibit a CN around 49.5. The kinematic viscosity of biodiesel from this oil would be approximately 3.55 mm2/s. Another species that has found considerable interest is Nannochloropsis. For example, a Nannochloropsis species with 6.9% C14:0, 30.5% C16:0, 9.6% C16;1, 4.4% C18:1, 4.6% C20:4, and 30.1% C20:5 with approximately 86% of the fatty acid profile accounted for (ref 111 in the present ref 25) would lead to CN of approximately 52 when accommodating the “missing” part of the profile (the reported part of the profile accounting for a CN of approximately 47) and a kinematic viscosity of approximately 3.4 mm2/s when accommodating for the “missing” part of the profile (the reported part of the profile accounting for a kinematic viscosity of 3.10 mm 2/s). Interestingly, in this case the CN requirement in biodiesel standards may be met, due to the high content of saturated fatty esters with high CN, which on the other hand would lead to poor cold flow properties. Very recently, Nannochloropsis gaditana has been suggested as a “model for oleaginous algal biofuel production”.59 A fatty acid profile provided in other literature60 for this species with 5.3% C14:0, 33.2% C16:0, 28.0 C16;1, 2.1% C18:0, 6.0% C18:1, and 16.9% C20:5 with a total of approximately 94% of the fatty acid profile defined would give a CN for the methyl esters of approximately 60 and
unsaturated species may have a greater effect on oxidative stability than the minor amounts may indicate. The oxidative stability values per the Rancimat test of the C20:2 and C20:3 esters (Table 2) agree with the other results, providing additional confirmation on their validity. However, no further induction times are calculated here as it is obvious that they would be very similar and low, probably around 0.1 h. Therefore, antioxidants would need to be added to any algal biodiesel fuel. As insufficient quantities of biodiesel from this source are available, no information on antioxidants in actual algal biodiesel is available. One study,34 in which formulations simulating algal biodiesel were studied for this purpose, indicates that tert-butylhydroquinone (TBHQ) may be effective. In any case, further research would be needed on actual algal biodiesel fuels. Density. Density is not specified in ASTM D6751 but is included as a specification in EN 14214. It may be surmised that the primary reason for the inclusion of density in EN 14214 is to exclude vegetable oils as triacylglycerols usually display density values >0.90 g/cm3. Furthermore, similar to CN and kinematic viscosity, the density of a mixture ρmix can be given by an equation ρmix =
∑ A C × ρc
(3)
in which ρmix is the density of the biodiesel sample (mixture of fatty acid alkyl esters), and ρc is the density of the individual compounds in the mixture. Due to their high degree of unsaturation and therefore greater C:H ratio, both C20:4 and C22:6 display higher density than the typical C16 and C18 esters found in most vegetable oil-based biodiesel fuels. Although not predicted here, it is clear from these values that both C20:5 and C22:5 will exhibit density values >0.90 g/cm3 (900 kg/m3). For comparison purposes, a compilation of density values of some common FAME can be found in the literature.52 Although the density of C20 and C22 HiPUFAMEs (Table 2) exceeds the maximum specification in the European standard EN 14214, the density of biodiesel from algal oils should be in the specified range. Usually sufficient other fatty acids with lower density are present to offset the higher density of the HiPUFAMEs. Cold Flow. The influence of HiPUFAMEs on cold flow was not investigated as the cold flow properties depend largely on the nature and amount of the higher-melting saturated FAMES.53 For example, the cloud point (CP, the temperature at which the first solids form when cooling a fuel) of palm biodiesel with approximately 46% methyl palmitate and 4% methyl stearate was reported as 16 °C54 while that of soy biodiesel with approximately 10% methyl palmitate and 5% methyl stearate was reported as 0 °C.55 As most algal oils have saturated fatty acid contents within the range of soy to palm oils, it can be assumed that the CP of algal biodiesel fuels can be observed between the values for soy and palm biodiesel. It may be noted that trace components with high melting points such as monoacylglycerols and sterol glucosides also play a role in the low-temperature properties of biodiesel.56,57 Besides these observations, biodiesel from some algal oils may contain materials not yet observed elsewhere affecting cold flow properties. For example, Isochrysis galbana contains highmelting alkenones which can affect the cold flow properties of the resulting biodiesel by raising the CP.58 5270
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kinematic viscosity of approximately 3.8 mm2/s when accommodating for the unknown part of the fatty acid profile (the reported profile providing for a CN of about 58 and a kinematic viscosity of about 3.63 mm2/s). However, the high content of saturated fatty esters that imparts the high CN for this algal biodiesel would also provide for poor cold flow properties with a high CP. Another species, Chlorella vulgaris, has little C20 and C22 HiPUFAMES, but due to greater amounts of C18:2 and C18:3, the CN of the resulting biodiesel would be around 46 for a reported species25 and the kinematic viscosity would be approximately 3.7 mm2/s. In any case, the above examples show that the CN and kinematic viscosity values of most algal biodiesel fuels would fall into the ranges discussed above. More calculations on reported fatty acid profiles were carried out to confirm this (data not reported for the sake of brevity). Feedstock Restrictions. EN 14214 contains several specifications that directly restrict some types of fatty acids in the fatty acid profile of the feedstocks that can be used for biodiesel. These restrictions include limiting linolenic acid to a maximum of 12% and fatty acids with four or more double bonds to a maximum of 1%. These restrictions obviously would affect most algal oils, formally eliminating the affected species as biodiesel feedstocks when using EN 14214 as guiding standard. A third restriction is the IV, which is a measure of the total number of double bonds in a sample. It formally arises through the addition if iodine to the double bonds of unsaturated fatty acids and can be calculated directly from the molecular weight and number of double bonds of a compound. Generally, the more double bonds in a fatty acid chain, the higher the IV, although the IV is molecular weight-dependent and decreases with increasing MW but the same number of double bonds. Overall, the IV has significant problems when applied to biodiesel61 and can largely be considered outdated as it does not provide any information that a full fatty acid profile does not provide. Despite these disadvantages, it has found its way into EN 14214 and therefore must be taken into some consideration.Table 5 gives the IVs of neat HiPUFAMES and
points increasing with saturation level. The formation of some trans isomers is also possible, leading to a similar effect on cold flow properties as trans isomers exhibit higher melting points than their cis counterparts. Exceptions. It is important to note that some algal oils by their reported fatty acid profiles resemble vegetable oils. Examples are Chlorella protothecoides and, especially, Trichosporon capitatum [ref 25, see also corresponding references therein], the biodiesel fuels of which would likely meet CN and kinematic viscosity limits in biodiesel standards. This shows that not only are the growing conditions of algae essential for an acceptable resulting biodiesel fuel but also the species selected.
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CONCLUSIONS Most current biodiesel fuels from algal oils would have difficulty meeting several specifications in biodiesel standards. The CNs of many algal biodiesel fuels would require the use of cetane improving additives to attain the minimum values prescribed in biodiesel standards. Similarly, the use of antioxidants would be necessary to meet the minimum requirements in standards, but this is also the case for biodiesel from vegetable oils and other feedstocks. Nature and amounts of the most acceptable antioxidants would still need to be determined. Kinematic viscosity should not be a technical problem with algal biodiesel fuels although some fuels may not meet the minimum requirement in EN 14214, a requirement for which, however, no technical justification exists. Cold flow is likely problematic for most algal biodiesel fuels. Beyond the property specifications, feedstock restrictions in EN 14214 affect algal biodiesel fuels. Some exceptions from these observations exist. Other issues not discussed here are the presence of heteroelements limited in biodiesel standards. These elements are calcium, magnesium, phosphorus, potassium, sodium, and sulfur. The origin of these specifications may vary, including catalyst (sodium or potassium hydroxide or methoxide), phospholipids, glucosinolates (sulfur; mainly in rapeseed or other brassicas), or contact with extraneous materials (especially for animal fats). However, these elements are of concern as they can degrade engine combustion or poison catalytic exhaust emissions aftertreatment systems. Little to no information appears to be available yet how algal biodiesel fuels would fare in this respect.
Table 5. Iodine Values of HiPUFAMEs and Amounts Fulfilling the Requirement of Maximum IV of 120 in EN 14214a HiPUFAME
iodine value
amount needed to attain the maximum IV of 120 in EN 14214
18:3 16:4 18:4 20:4 20:5 22:4 22:5 22:6
260.4 386.9 349.5 318.8 401 293 368.3 444.6
46.1% 31% 34.3% 37.6% 29.9% 41% 32.6% 27%
a
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AUTHOR INFORMATION
Corresponding Author
*Phone: (309) 681-6112. Fax: (309) 681-6524. E-mail:
[email protected]. Corresponding author address: USDA/ARS/NCAUR, 1815 N. University St., Peoria, IL 61604, USA. Notes
Disclosure: Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.
For comparison purposes, the values for C18:3 are also given.
also the amount of each HiPUFAME that in a fatty acid profile would suffice to attain the limit of IV = 120 in the EN 14214 standard. It may be noted that the IV in mixtures such as biodiesel is a result of the IV of all components added taking their relative amounts into consideration. The feedstock restrictions could be reduced or avoided through a process such as hydrogenation, but this would add cost besides imparting even poorer cold flow properties due to melting
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ACKNOWLEDGMENTS The author thanks Kevin Steidley for excellent technical assistance, Dr. Karl Vermillion for obtaining the NMR spectra, 5271
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