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Oxidation and Polymerization of Soybean Biodiesel/Petroleum Diesel Blends James Ball, James E Anderson, Bruno P. Pivesso, and Timothy J. Wallington Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02729 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Oxidation and Polymerization of Soybean Biodiesel/Petroleum Diesel Blends

James C. Ball*, James E. Anderson†*, Bruno P. Pivesso+, Timothy J. Wallington* *

Research & Advanced Engineering, Ford Motor Company, Dearborn, MI 48124

+

São Carlos Institute of Chemistry (IQSC), São Carlos, Brazil

†

Corresponding author: [email protected], 313-248-6857

Abstract

Fuels in modern diesel engine fuel systems are exposed to highly oxidizing conditions and it is important to understand their degradation mechanisms. A range of chemical and physical properties were monitored during the oxidation and polymerization of soybean methyl ester biodiesel (B100), a diesel fuel (B0) and their blends (B10, B30) at 90°C with aeration. The initial rapid oxidation of polyunsaturated fatty acid methyl esters (FAMEs) provided a transient pool of peroxides that led to the formation of aldehydes, ketones, and acids as secondary products. Monounsaturated and saturated FAMEs were oxidized concurrently with polyunsaturated FAMEs in B10, B30, and B100, but only B100 showed significant oxidation reactions continuing after the polyunsaturated FAMEs were depleted. New esters were a major oxidation product, eventually comprising 40-60% of the incorporated oxygen. Carboxylic acids and alcohols react to form esters and water, with vaporization of water driving the equilibrium

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towards ester formation. Polymers with ester linkages are likely contributors to the higher molecular weight materials formed and resulting increase in viscosity under these conditions.

Introduction The use of biodiesel has seen steady growth over the last few decades, having the potential to provide a variety of environmental, economic, and social benefits as a renewable fuel. Biodiesel is typically used in blends with petroleum diesel fuel up to 20% by volume. Biodiesel can improve the cetane number, lubricity, and tendency to form particulate emissions as compared to petroleum diesel.1-3 However, biodiesel is more susceptible to oxidation and degradation in quality during storage. Modern light-duty vehicle diesel engines use fuel that is pumped at high pressure (e.g. 180 MPa)4 to facilitate short injection durations, good spray characteristics, and the desired airfuel mixing. Fuel is continuously pumped through the common fuel rail that houses the injectors with excess fuel returning to the fuel tank. The recirculating fuel provides some cooling for the fuel injection system, but also results in some degree of fuel aeration and mixing in the fuel tank. The pumping work done on the fuel combined with elevated engine compartment temperatures and high ambient temperatures can result in fuel temperatures reaching 60-150°C.5-10 Biodiesel consists of saturated fatty acid methyl esters (FAMEs) and unsaturated FAMEs. Biodiesel from common feedstocks, e.g. soybean oil, is composed of FAMEs with alkyl chains having 16 or 18 carbon atoms (e.g. C16 or C18) and 0-3 double bonds (e.g. C16:0, C18:3) with the double bond typically in the cis configuration.11 Oxidation occurs initially at the methylene (-CH2-) positions allylic (adjacent) to the double-bonded carbon atoms with abstraction of a hydrogen atom by a reactive oxygen species.1, 11-18 Addition of O2 to alkyl radicals is rapid and Page 2 of 39 ACS Paragon Plus Environment

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forms peroxy radicals that can abstract hydrogen either from another unsaturated FAME or via intramolecular isomerization forming a hydroperoxide and a new alkyl radical that propagates the oxidation. The hydroperoxides formed decompose into alkoxy and hydroxyl radicals.13 Alkoxy radicals can decompose into aldehydes and alkyl radicals and can be further oxidized to form acids or react with alkenes to form polymers.1, 11, 13-16, 18 Hydroxyl radicals initiate oxidation of FAMEs via addition to double bonds and hydrogen atom abstraction leading to hydroxyl alkyl and alkyl radicals which add O2 to give peroxy radicals. The process is terminated by self- and cross-reactions of peroxy radicals to form non-radical molecular products (alcohols, aldehydes, and ketones) and by recombination of alkyl and alkoxy radicals.14, 19, 20 Several studies have contributed to the characterization of biodiesel oxidation at relevant diesel engine fuel system temperatures. Chuck et al. (2012) aged biodiesel at elevated temperatures (90 and 150°C) and showed increases in oxidized products, high molecular weight material, and viscosity.7 Pereira et al. (2015) studied the ratio of hydroperoxides to secondary products in soybean biodiesel and concluded that the rate of hydroperoxide formation increases faster with increasing temperature compared to the rate of hydroperoxide degradation.21 Monyem et al. (2000) tracked peroxides, acids, viscosity, and filter performance for biodiesel and biodiesel-diesel blends thermally aged at 60°C.5 Vega-Lizama et al. (2015) used thermogravimetry (TGA), viscosity, and other techniques to show that soybean biodiesel yielded high molecular weight residues with increasing reaction temperatures.22 Stavinoha and Howell (1999) evaluated a variety of methods to characterize biodiesel and biodiesel-diesel blend stability under accelerated (heated) conditions.9 Christensen and McCormick (2014) aged biodiesel and biodiesel/diesel fuels for up to 39 weeks at 43 °C to simulate long-term product storage and characterized how FAME composition, prior oxidation reactions, and antioxidants

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affected the onset of the peroxide formation.23 Østerstrøm et al. (2015) studied the oxidation of a rapeseed methyl ester (RME) biodiesel-diesel blend (30% v/v, B30) heated to 70 and 90°C with different aeration rates over a six week period and tracked changes in peroxide values (PV), total acid number (TAN), oxygen incorporation, fuel mass and FAME composition.24 Anderson et al. (2015) measured the effect of partially oxidized soybean methyl ester (SME) biodiesel fuel blend (B30) on the degradation of fresh SME biodiesel blends, simulating the impact of residual aged fuel in a vehicle fuel tank after refueling. It was shown that fuel aged to a high peroxide value had the greatest effect on the stability of fresh fuel with the primary effect being a greatly reduced oxidation reserve.25 In the present study, the oxidation of a diesel fuel, SME biodiesel, and their blends (B10, B30) was characterized. In the U.S., blends up to B10 are available for many retail consumers. Blends of at least B20 can be used in medium- and heavy-duty vehicles to assist in compliance with alternative fuel vehicle requirements.26 Accordingly, certain diesel engine vehicles are designed to accommodate up to B20.27 The B30 fuel used in the present study was selected to be conservatively protective for B20 fuels. In the present study, the oxidation of the fuels and the formation of higher molecular weight polymer products were assessed at 90°C with 100 mL/min aeration with dry air.24 The characterization included FAME composition, PV, anisidine values, TAN, ester values, Fourier transform infrared spectroscopy (FTIR), oxygen incorporation, gross heat of combustion, density, viscosity, thermogravimetric analysis (TGA) and gel permeation chromatography (GPC). The time-course experiments lasted approximately six weeks and were considerably longer than what would be typical in a diesel vehicle fuel system. The extended duration allowed a better assessment of the formation of high molecular weight products.


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Materials and Methods Diesel fuel, SME biodiesel, and their blends (10% and 30% v/v biodiesel) were obtained in a form without added antioxidants from Gage Products (Ferndale, MI). The diesel fuel was a U.S. Tier 2 diesel certification fuel (3000 cm-1), consistent with reactions involving this bond in unsaturated FAMEs.

Oxygen Incorporation


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Oxygen (Figure 6) was incorporated into all of the biodiesel-containing fuels over the first 10 days consistent with the formation of peroxides (Figure 2) and subsequent oxidation products (Figures 3 and 4). After this initial rapid oxidation period for the B10 and B30 fuels, oxygen content stabilized or grew slowly as the polyunsaturated FAMEs had been depleted and oxidation of the other FAMEs had greatly slowed or stopped altogether (Figure 1). For B100, oxygen consumption continued after the polyunsaturated FAMEs were depleted (at day 20), consistent with continued oxidation of methyl oleate and the saturated FAMEs, and additional oxidation of the products. Oxygen is incorporated into FAMEs via the formation of hydroperoxides followed by their decomposition and further oxidation to form aldehydes, acids, alcohols, ethers, esters and other oxygenated species.7, 14, 18, 44 Previous work on RME B30 showed an increase of 5-6% oxygen with similar thermal and aeration conditions and duration.24 Likewise, in a study of soybean oil oxidation,45 polymers formed from aeration-enhanced oxidation at 60°C and 30°C contained 21% oxygen compared to 12% oxygen in the original oil. In comparison, the B100 fuel in the present study showed >25% oxygen without isolation of the polymeric fraction (Figure 6). The contributions of the different oxidation products to the measured total oxygen content are compared in Figure 7. Acids, measured as TAN, comprised an increasing fraction of the additional oxygen content, accounting for 20-25% at day 35. Peroxides initially comprised 20-30% of the additional oxygen, but eventually became a negligible fraction. These contributions are similar to that reported for RME oxidation.24 The measured oxygen increase is likely an underestimate of the total amount of oxygen actually incorporated during these oxidation reactions. Oxygenated volatile compounds such as short-chain acids, aldehydes, esters


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and alcohols, as well as water formed during the reaction to form esters, would be lost by purging with air at elevated temperatures. The amount of oxygen from lost water can be estimated from the amount of new esters formed, e.g., approximately 3 wt. % oxygen for B100 through day 43. Ester content was the single greatest contributor to the increase in oxygen content, ultimately comprising 40-60% of the added oxygen. The contribution of aldehydes and ketones as measured by the anisidine value could not be directly compared, as discussed earlier. Together, acids and esters accounted for 80-90% of the added oxygen for B10 and B30, and nearly 60% for B100. (The lower percentage for B100 may be because the initial oxidation reactions and TAN formation were still ongoing.) Thus, the majority of the product types that have incorporated oxygen in this study have been identified. Gross Heat of Combustion Gross heat of combustion (higher heating value) provides another aggregate measure of fuel oxidation (Figure 8).35 The heating values for all of the fresh and aged fuels were closely correlated (r2 = 0.98) with their oxygen content (Supporting Information Figure S3). The heating value of the diesel fuel (B0) increased slightly over time (0.5% after 43 days), likely due to the volatilization of some lighter hydrocarbons and incorporation of small amounts of oxygen (only detected as peroxides in the analyses conducted). The greatest reduction in heating value was observed for the B100 fuel with a 19.5% decrease at the termination of the experiment (43 days). The heating value reductions for B30 (9.1%) and B10 (6.1%) were intermediate to the B0 (~0%) and B100 (19.5%) cases. Note that these changes are probably conservative measures of oxidation in this system. The loss of highly oxidized, volatile products (e.g., formic or acetic


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acid) and the formation of esters with elimination of water would tend to increase the measured heating value and decrease the oxygen content of the aged fuel. The gross heat of combustion of the biodiesel-containing fuels declined most rapidly during the period when peroxide values were elevated and polyunsaturated FAMEs were reacting with oxygen. The latter, slower phase is consistent with the slower oxidation of methyl oleate and hydrocarbon chains of the saturated FAMEs, as well as other secondary oxidation reactions to produce acids, aldehydes, and ketones. Even though a greater percentage of the FAMEs were degraded in B100 than in B30 and B10 (Figure 1), the heating value and oxygen content data indicate a greater overall extent of oxidation in the B10 and B30 fuels than would be inferred by the differences in biodiesel content alone. If only biodiesel had oxidized in the B30 and B10 fuels, and each had oxidized to the extent seen in B100, then the decline in heat of combustion for B30 would be approximately 30% of the decline in B100 (30% x 19.5% = 5.9%) and 10% of the decline in B100 (10% x 19.5% = 2.0%). However, these values are much less than the observed decline in heating values for these two fuels. A similar pattern is seen for oxygen incorporation. These observations can are explained best by oxidation of diesel hydrocarbons in the B30 and B10. Diesel fuel alone showed only small changes in heating value and oxygen content, suggesting that little oxidation of diesel fuel occurred in the absence of biodiesel. Taken together, these data suggest that biodiesel oxidation likely promotes the oxidative degradation of the diesel hydrocarbons in these blends under the conditions of this study. Density Fuel density increased throughout the aging experiment (Figure 9). For diesel fuel, the increase was small but consistent over time. The density of the biodiesel blends followed a Page 14 of 39 ACS Paragon Plus Environment

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pattern similar to that for TAN, with a rapid initial increase as oxygen was incorporated via peroxides (Figure 2), acids (Figure 4), and other oxidation products. Subsequently, there was a stable to slow increase in density for B10 and B30, and a significant but slowing increase for B100. These density changes likely relate to a combination of factors including oxygen incorporation (Figure 6) and increased hydrogen bonding of carboxylic acids and other oxygenated compounds. Viscosity

The viscosity of the four fuels increased over time (Figure 10) with the magnitude of the change increasing with the FAME content of the fuel. The B100 fuel showed the greatest change, increasing exponentially over time. The B10 and B30 fuels showed an initial rise in viscosity followed by a slower, relatively steady increase. The timing of the initial viscosity increase was similar to that for TAN with the onset occurring shortly after the onset of rapid peroxide production. This is consistent with polymerization reactions known to occur after initial FAME oxidation and likely involve radical-radical recombination reactions including ether formation or ester formation from the condensation reaction of alcohols and acids leading to crosslinks.14, 46, 47 Waynick (2005)18 reported that TAN and viscosity continue to increase with oxidation and are closely correlated, and suggested that the formation of polymeric compounds and increased viscosity was dependent on the formation of carboxylic acids in aged biodiesel samples. The data presented here (Figure 11) similarly show correlation between TAN and viscosity through day 11, but the relationship changes after this point. Specifically, for B10 and B30, TAN stabilizes but viscosity continues to increase, suggesting that acids are no longer


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required for the viscosity increase, or that acids are consumed in these reactions as fast as they are being produced in other reactions. The viscosity increases observed in this system likely arise from covalent bond formation between smaller molecules to generate larger molecules as well as hydrogen bonding that increase the strength of interactions between molecules. Additional analyses were conducted to better characterize the viscosity increases and its causes. Gel Permeation Chromatography (GPC) GPC allows a more quantitative assessment of molecular size distribution than TGA. GPC has been used to distinguish unreacted or partially reacted products during FAME synthesis by transesterification48 and to characterize FAME polymer formation due to heating.7, 48 Figure 12 shows the GPC chromatograms of the B100 fuel over the 43 day aging period. GPC chromatograms for B30, B10, and B0 are shown in the Supporting Information (Figures S5-S7). Biodiesel FAMEs in the B100 and B30 show a relatively narrow distribution around 36 mL elution volume (~300 Da). The FAME peak is less evident in B10 due to the low concentration of the biodiesel against the diesel fuel background. Diesel-range hydrocarbons, being lower in molecular weight, elute later. For B100, the FAME peak initially declines rapidly and then more slowly through the end of the study, generally matching the combined data for the individual FAMEs in Figure 1. Higher molecular weight compounds are formed with aging, some of which later disappear, while still higher molecular weight compounds are formed, consistent with a pattern of gradual polymerization by FAMEs and/or their oxidation products. For example, an intermediate product (33.2 mL, ~830 Da) is formed and later depleted. Assuming an average FAME molecular weight of 295 Da, this represents a polymer of ~3 FAME monomers. The highest molecular weight material detected by GPC elutes at 30 mL, Page 16 of 39 ACS Paragon Plus Environment

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consistent with an approximate molecular weight of 3900 Da, representing a polymer of 13-14 FAME monomers.14, 44 For the B30 and B10 fuels (Figures S5 and S6), the same pattern is observed albeit on a reduced scale and obscured by the larger diesel fraction. The FAMEs exhibit an initial rapid decline in concentration followed by the formation of higher molecular weight materials. The peak observed at 32 mL (~1300 Da) suggests a polymer with 4-5 FAME monomers. The diesel portion of B30, B10, and B0 (Figures S5-S7) chromatograms show relatively minor changes in peak shape and height consistent with the loss of the more volatile diesel hydrocarbons. The observed formation of higher molecular weight products following biodiesel oxidation is consistent with prior studies,7, 48 but extends these observations to blends of biodiesel in diesel fuel and to a greater degree of oxidation. Similar trends were seen using TGA, though with less resolution (Figure S4).

Conclusions Exposure of biodiesel-containing fuels to high temperatures and oxygen, conditions found in modern diesel engine fuel systems, resulted in oxidation of biodiesel FAMEs, first to hydroperoxides followed by secondary reactions leading to carboxylic acids, aldehydes, ketones, alcohols, esters and polymerized components. Monounsaturated and saturated FAMEs were oxidized concurrently with polyunsaturated FAMEs in B10, B30, and B100, however only B100 showed significant oxidation continuing after the polyunsaturated FAMEs were depleted.


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Formation of higher molecular weight materials, seen through viscosity increases, TGA, and GPC, was observed for all of the biodiesel-containing fuels. These were most rapid during the initial phase of the study, but continued at a slower rate throughout. B100 exhibited the most extreme changes, consistent with its continued oxidation activity. Ester content greatly increased for the biodiesel-containing fuels, comprising 40-60% of the incorporated oxygen, and was identified as a possibly significant contributor to the higher molecular weight materials and increased viscosity. While the reaction of carboxylic acids and alcohols to form esters and water are in equilibrium, any water formed would be vaporized and removed from this system, driving the equilibrium towards ester formation. The experimental conditions used in this study, while extreme in duration for engine operation, are representative of conditions that exist in modern diesel fuel systems.5-7, 9, 10 The results may also be instructive for oxidation of biodiesel that may accumulate in engine oil.49-52 Potential actions to mitigate these effects in vehicles operating with biodiesel-containing fuel include ensuring adequate oxidation stability of the fuel, moderating the system temperature rise, reducing the exposure duration, and limiting oxygen intrusion.

Acknowledgments The authors gratefully acknowledge the assistance of Travis Collings for conducting the GC analyses and Alane Moura Lira and Sherry Mueller for helpful discussions and laboratory support.

Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.xxxxxxx. Fuel property data (Table S1), GPC columns and standards (Table S2), additional data figures including TGA, FTIR, and additional GPC results (Figures S1–S7), and tabulated experimental data (Tables S3–S13). (PDF)

Notes Disclosure: Whereas this article is believed to contain correct information, Ford Motor Co. (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of their companies. This disclaimer may not be removed, altered, superseded or modified without prior Ford permission. The authors declare no competing financial interest.

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34. Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method), ASTM D4809; ASTM International: West Conshohocken, PA; 2013. 35. Thompson, C. J., Peterson, L. C., Reece, L. D., Beck, M. S. Trans. ASAE 1998, 41, 931. 36. Nimse, S. B., Pal, D. RSC Adv. 2015, 5, 27986-28006. 37. Lacoste, F., Lagardere, L. Eur. J. Lipid Sci. Technol. 2003, 105, 149-155. 38. Morrison, R. T., Boyd, R. N. Organic Chemistry; Allyn and Bacon: Boston, 1972. 39. Araújo, S. V., Rocha, B. S., Luna, F. M. T., Rola Jr, E. M., Azevedo, D. C. S., Cavalcante Jr, C. L. Fuel Process. Technol. 2011, 92, 1152-1155. 40. Leonardo, R., Valle, M. M., Dweck, J. J. Therm. Anal. Calorim. 2017, 1-7. 41. Wazilewski, W. T., Bariccatti, R. A., Martins, G. I., Secco, D., Souza, S. N. M. d., Rosa, H. A., Chaves, L. I. Ind. Crops Prod. 2013, 43, 207-212. 42. Yaakob, Z., Narayanan, B. N., Padikkaparambil, S., Unni K, S., Akbar P, M. Renewable Sustainable Energy Rev. 2014, 35, 136-153. 43. Silverstein, R. M., Bassler, G. C., Morrill, T. C. Spectometric Identification of Organic Compounds; 3rd ed. John Wiley & Sons, New York: 1974. 44. de Carvalho, A. L., Cardoso, E. A., da Rocha, G. O., Teixeira, L. S. G., Pepe, I. M., Grosjean, D. M. Fuel 2016, 173, 29-36. 45. Chang, S. S., Kummerow, F. A. J. Am. Oil Chem. Soc. 1954, 31, 324-327. Page 23 of 39 ACS Paragon Plus Environment

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46. Wexler, H. Chem. Rev. 1964, 64, 591-611. 47. Chang, S. S., Kummerow, F. A. J. Am. Oil Chem. Soc. 1953, 30, 403-407. 48. Knothe, G. J. Am. Oil Chem. Soc. 2006, 83, 823-833. 49. Richardson, D. E. J. Eng. Gas Turbines Power 2000, 122, 506-519. 50. Truhan, J. J., Qu, J., Blau, P. J. Tribol. Int. 2005, 38, 211-218. 51. Uy, D., Anderson, J., Gangopadhyay, A. SAE Int. J. Fuels Lubr. 2010, 3, 569-578. 52. Zdrodowski, R., Gangopadhyay, A., Anderson, J. E., Ruona, W. C., Uy, D., Simko, S. J. SAE Int. J. Fuels Lubr. 2010, 3, 579-597.


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List of Figures Captions Figure 1. Percent of initial FAME concentration of fuels aged at 90°C with aeration. Data presented are for single determinations. Figure 2. Peroxide values of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples. Figure 3. Anisidine values of fuels aged at 90°C with aeration. Data presented are for single determinations. Figure 4. Total acid number (TAN) of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples. Figure 5. Ester content in fuels aged at 90°C with aeration. Data presented are for single determinations. Figure 6. Oxygen content in fuels aged at 90°C with aeration. Data presented are for single determinations. Figure 7. Contributions to oxygen content in fuels aged at 90°C with aeration. Left panels show the measured total oxygen content as compared to the implied oxygen content contributions calculated from the measured ester content, TAN, PV, and their sum. Right panels show the contribution of additional esters, TAN, PV, and their sum as a percentage of the measured total oxygen content increase. Figure 8. Gross heat of combustion of fuels aged at 90°C with aeration. Data presented are for single determinations.


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

Figure 9. Density of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples. Figure 10. Viscosity of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples. Figure 11. TAN versus kinematic viscosity of fuels aged at 90°C with aeration for biodieselcontaining fuels. Figure 12. Gel permeation chromatograms of B100 aged at 90°C with aeration. The absorbance of 210 nm is plotted versus GPC elution volume (mL) and elapsed aging time (0–43 days). Smaller elution volume indicates greater molecular weight. Table 1. Fuel Properties


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Energy & Fuels

Table 1. Fuel Properties Property

B0

B10

B30

B100

Biodiesel content (vol. %)

0.0

10.3

29.9

100.0

Specific gravity, 60°F (15.6°C)

0.852 0.855 0.862 0.885 2

Kinematic viscosity, 40°C (mm /s)

2.33 a

2.45

2.75

4.10

Oxidation stability, Rancimat (h)

ND

9.8

8.5

6.5

Acid number (mg KOH/g)

0.00

0.02

0.10

0.20

Saturates (vol. %)

63.0

ND

ND

ND

Aromatics (vol. %)

33.9

ND

ND

ND

Olefins (vol. %)

3.1

ND

ND

ND

a. ND – not determined


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Figure 1. Percent of initial FAME concentration of fuels aged at 90°C with aeration. Data presented are for single determinations.


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Figure 2. Peroxide values of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples.


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Figure 3. Anisidine values of fuels aged at 90°C with aeration. Data presented are for single determinations.


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Figure 4. Total acid number (TAN) of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples.


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Figure 5. Ester content in fuels aged at 90°C with aeration. Data presented are for single determinations.


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Figure 6. Oxygen content in fuels aged at 90°C with aeration. Data presented are for single determinations.


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Figure 7. Contributions to oxygen content in fuels aged at 90°C with aeration. Left panels show the measured total oxygen content as compared to the implied oxygen content contributions calculated from the measured ester content, TAN, PV, and their sum. Right panels show the contribution of additional esters, TAN, PV, and their sum as a percentage of the measured total oxygen content increase.


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Figure 8. Gross heat of combustion of fuels aged at 90°C with aeration. Data presented are for single determinations.


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Figure 9. Density of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples.


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Figure 10. Viscosity of fuels aged at 90°C with aeration. Error bars are ± 1 standard deviation of the mean determined from duplicate samples.


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Figure 11. TAN versus kinematic viscosity of fuels aged at 90°C with aeration for biodieselcontaining fuels.


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Figure 12. Gel permeation chromatograms of B100 aged at 90°C with aeration. The absorbance of 210 nm is plotted versus GPC elution volume (mL) and elapsed aging time (0–43 days). Smaller elution volume indicates greater molecular weight.


Petroleum Diesel

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