Effects of Biodiesel Contamination on Oxidation and Storage Stability

Article pubs.acs.org/EF

Effects of Biodiesel Contamination on Oxidation and Storage Stability of Neat and Blended Hydroprocessed Renewable Diesel Jinxia Fu* and Scott Q. Turn Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii 96822, United States

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 12, 2015 | http://pubs.acs.org Publication Date (Web): August 5, 2015 | doi: 10.1021/acs.energyfuels.5b01260

S Supporting Information *

ABSTRACT: The present work investigates the effects of biodiesel contamination on conventional and renewable marine diesels, i.e., NATO F-76 and HRD-76. The physical properties and chemical composition of F-76, HRD-76, and biodiesel were measured, including viscosity, density, peroxide value, heat of combustion, and acid number. Long-term (ASTM D4625) and accelerated (ASTM D5304) test methods were used to investigate the influence of biodiesel contamination on storage stability of HRD-76, F-76, and their blends. The impact of biodiesel contamination on oxidation stability of neat and blended fuels was also studied by conducting test ASTM D2274. In addition, the influence of biodiesel contamination on physicochemical properties after stressing the fuel was investigated according to the ASTM methods. The presence of biodiesel at contaminant levels did not have significant effects on the fuel properties or storage and oxidation stability of neat and blended HRD-76 or F-76. Properties of contaminated samples were determined to meet MIL-DTL-16884N specifications. The storage stability of the HRD-76/B100 blends was also investigated on the basis of the ASTM D5304 method.

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INTRODUCTION Renewable biofuels are of considerable interest to the United States Navy (USN) to decrease the reliance on foreign sources of oil and increase environmental sustainability.1 In 2012, the United States Department of Navy (DoN) demonstrated a Green Strike Group (a carrier strike group powered by nuclear energy and biofuels).2 By 2016, DoN plans to sail the Great Green Fleet, and by 2020, 50% of total energy consumption of DoN will come from alternative sources.1,2 Currently, both first- and second-generation biofuels have been deployed by DoN for aviation, marine, and ground transportation. Firstgeneration biofuels, such as ethanol and biodiesel [fatty acid methyl esters (FAME)], are produced from food crops and generally used for onshore transportation,3−5 whereas the second-generation biofuels are produced from nonfood feedstocks, such as agricultural byproducts and microalgae,6,7 and generally applied for offshore operations.1,2 Biodiesel is produced by transesterification of edible and non-edible vegetable oils, recycled waste vegetable oils, and animal fats and can be used in pure form or blended with petroleum diesel, such as B5 and B20 with 5 and 20% biodiesel, respectively. In 2012, approximately 991 million gallons of biodiesel was produced in the United States, and this value increased to over 1300 million gallons in 2013.8 Currently, there are 94 biodiesel plants in the U.S. with capacity of 2.0 billion gallons per year.8 Two of the second-generation biofuels of particular interest to the USN are hydroprocessed renewable diesel (HRD-76) derived from algae or animal fats and hydroprocessed renewable jet fuel derived from Camelina (HRJ-5) for blending with petroleum marine diesel (NATO F-76) and jet fuel (JP-5), respectively.2 To clarify terminology, military specification MIL-DTL-16884N defines hydroprocessed or hydrotreated renewable diesel (HRD) as “synthesized paraffinic diesel (SPD) produced from mono-, di-, and triglycerides, free fatty acids, © 2015 American Chemical Society

and fatty acid esters from plant, algal oils, or animal fats (for example, fatty acid methyl esters) that have been hydroprocessed to remove essentially all oxygen. HRD may also be referred to as hydroprocessed esters and fatty acids synthetic paraffinic diesel (HEFA-SPD) or paraffinic middle distillate (PMD)”. Unlike the first-generation bioethanol and biodiesel, these second-generation fuels are drop-in replacements that meet engine performance specifications,9−13 and their production processes can be realized using existing refineries.14 Hydroprocessed renewable diesels (HRDs) derived from microalgae oil and animal fats15 predominantly comprise linear and branched alkanes ranging from C15 to C18, with higher concentrations of n-C17 and n-C18.13,16,17 The hydroprocessing reaction involves two main steps: (i) saturation of the double bonds of the triglycerides and breaking the saturated triglycerides to fatty acids and (ii) conversion of fatty acids into alkanes through hydrogenation or decarboxylation processes.16,18 The hydroprocessing technology has been deployed in vegetable oil conversion, such as palm oil,19 rapeseed oil,18,20−23 soybean oil,24−26 sunflower oil,27−31 and waste cooking oil.32−35 The physicochemical properties, such as cetane number, cloud point, density, heat of combustion, speed of sound, surface tension, and viscosity, have been wellcharacterized for HRD-76,12,13,36,37 and the results indicate that HRD-76 has properties very similar to petroleum NATO F-76. Storage and oxidation stability tests illustrate that HRD-76 is more stable than F-76,13 and blending HRD-76 in F-76 will increase the fuel stability. The USN has identified a blend of 50% F-76 and 50% HRD-76 to meet alternative fuel goals,9−12 and approximately 350 000 gallons of HRD-76 were purchased in 2011 in preparation for the Great Green Fleet exercises.2 Received: June 5, 2015 Revised: July 15, 2015 Published: July 20, 2015 5176

DOI: 10.1021/acs.energyfuels.5b01260 Energy Fuels 2015, 29, 5176−5186

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Energy & Fuels

D2274 methods. The latter two included test durations that were extended beyond those required in the method, as described below. The fuel sample was dissolved in isooctane and reacted with potassium iodide to reduce the hydroperoxides in the fuel. Then, an equivalent amount of iodine was liberated by titrating with a sodium thiosulfate solution. The PV is expressed as milligrams of hydroperoxide per kilogram of sample. PV is an indication of the quantity of oxidizing constituents present in fuel samples and influences other measures of fuel quality, such as the cetane number (CN), density, viscosity, etc. A Parr 6200 Isoperibol calorimeter was used to measure the heat of combustion, i.e., heating value, based on the ASTM 4809 method.46 An approximately 0.3 g fuel sample was used for the test, and the O2 pressure of the bomb is ∼3 MPa. The measurements are taken with 0.0001 °C resolution over a 20−40 °C working range, and uncertainty of the measurement is 0.05−0.1%. The heat of combustion measured in this study is actually the high heating value (HHV). The ASTM D974 method47 was also employed to measure the AN of the fuel. Generally, this method is used to indicate the relative change of carboxylic acid groups in chemical compounds owing to the oxidation and quantify of the amount of acid present. The AN is expressed in milligrams of potassium hydroxide (KOH) that is required to neutralize the acid in 1 g of fuel sample. The fuel sample is dissolved in a titration solvent consisting of toluene, isopropyl alcohol, and a small amount of water to form a homogeneous single phase. The fuel solution is titrated at room temperature with a KOH solution, and the end point is indicated by the color change of the pnaphtholbenzein solution added. Storage and Oxidation Stability. The ASTM D4625,41 D5304,42 and D227443 methods were employed to test the storage and oxidation stability of F-76, HRD-76, and their blends contaminated with biodiesel. ASTM D462541 and D530442 were developed to evaluate the storage stability of middle-distillate petroleum fuel.48−51 ASTM D462541 requires that fuel be stored at 43 °C for 24 weeks with samples analyzed every 4 or 6 weeks to quantify the amount of filterable and adherent insolubles present. For this method, 1 week of storage at 43 °C is roughly equivalent to 1 month of storage at normal ambient temperature, 21 °C.41 This method has also been expanded to evaluate the storage stability of distilled and undistilled biodiesels.52−58 ASTM D5304 is an accelerated method compared to ASTM D4625. The storage stability of the fuel is evaluated after undergoing 16 h subjected to an oxygen atmosphere at 90 °C and 800 kPa. The 16 h test is expected to yield approximately the same amount of insolubles as 20 °C storage for 27 months under 101.325 kPa air pressure.42 ASTM D5304 has also been deployed to study the storage stability of biodiesels.59−61 ASTM D2274 is an accelerated method developed to quantify the oxidation stability of distillate fuel oil.48−50 This method also requires an elevated temperature, 95 °C, and continuous oxygen exposure, 3 L h−1 flow of pure oxygen. As with ASTM D4625 and D5304, this method was also employed for biodiesel samples.54−58,62 ASTM D4625,41 D5304,42 and D227443 methods quantify the amount of insolubles formed under controlled test conditions. The total insolubles include filterable and adherent insolubles; the filterable insolubles are solid particles formed during storage, which can be removed by filtration, and the adherent insolubles are gums formed during storage that remain tightly attached to the walls of the test vessel. All three test methods call for stressing the fuel followed by cooling for 60 ± 5 min in the dark at room temperature. In all cases, the aged samples were filtered using two Whatman nylon membrane filters (47 mm diameter and 0.8 μm pore size) to quantify filterable insolubles. Adherent insolubles were removed from the oxidation cell or storage bottle and associated glassware with trisolvent (a mixture of equal moles of acetone, methanol, and toluene). Dependent upon the method, the solvent was evaporated at 135 or 160 °C to obtain the adherent insolubles. Military specification MIL-DTL-16884N identifies maximum allowable total insolubles of 3.0 mg/100 mL for NATO F76 analyzed by ASTM D5304 and 1.5 mg/100 mL using ASTM D2274 analysis methods for 40 h, instead of the 16 h specified by ASTM.

Military fuel specifications require fuels used on Navy ships preserve quality under conditions presented by the shipboard environment and fuel−seawater ballasting practices.38 These necessarily withdraw the candidacy of biodiesel as a blending agent with NATO F-76 because it will result in higher water content.39 Potential for contamination arises from the extensive use of both biofuels and petroleum fuels in ports where the USN takes on fuel. Because F-76 and HRD-76 have more rigorous standards than biodiesel, the problem of major concern is the contamination of F-76, HRD-76, or their blends by biodiesel. When compared to diesel fuels, the stability of biodiesel is impacted by its content of unsaturated esters.40 As the unsaturation in the fatty acid chain portion increases, the biodiesel becomes more unstable.40 Therefore, the neat and blended marine diesel could become less stable if contaminated with biodiesel, potentially inducing other problems, such as property changes, fuel tank corrosion, or increased engine maintenance requirements. Although the properties of HRD-76, F-76, and their blends have been characterized,12,13,36,37 the influence of biodiesel contamination, especially on fuel physicochemical properties and storage and oxidation stabilities, has not been investigated. The present work reports results of measurements of physicochemical properties of HRD-76, F-76, and their blends contaminated by biodiesel. Two ASTM methods, ASTM D462541 and ASTM D5304,42 were used to investigate the storage stability of neat and contaminated fuel samples. ASTM D5304 was also employed to study the storage stability of HRD-76/B100 blends. ASTM D227443 was conducted to study the oxidation stability of these fuels. Additionally, fuel properties, such as density, viscosity, peroxide value (PV), heat of combustion, and acid number (AN), were also measured according to the ASTM methods after stressing the fuel samples. All ASTM methods and military specifications used in this study are listed in the Supporting Information.

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MATERIALS AND METHODS

Materials. Petroleum F-76 and HRD-76 were provided by the United States Navy Supply Center at Patuxent River, MD. The HRD76 fuel lot was refined by UOP.12 Biodiesel produced by Pacific Biodiesel was purchased from Carl’s Jr. 76 Station in Honolulu, HI. The biodiesel purchased is B99.99 with 0.01% impurity and was produced from waste cooking oil. The fuels were used as received, unless otherwise noted. Chemical and Physical Property Analyses. The composition of fuels was determined by gas chromatography/mass spectrometry (GC/MS). Fuel samples (1 μL) were dissolved in 1 mL of hexane and analyzed using a Bruker 436-GC gas chromatograph and SCION-MS select, single-quadrupole mass spectrometer. GC was equipped with an Agilent DB1701 column [low/mid-polarity, 60 m, (14% cyanopropylphenyl)-methylpolysiloxane], with a 15 m guard-column before the back-flush valve and operated at a helium flow rate of 1.5 mL/min. The guard column protects the analytical column by trapping nonvolatile residues and preventing them from entering the analytical column. The temperature program starts at 50 °C for 4 min, followed by an increase to 280 °C at a rate of 10 °C min−1, and a hold at the final temperature for 12 min. An Anton Paar SVM300 Stabinger viscometer was used to measure the viscosity and density of neat and blended fuels at given temperatures according to the ASTM D455 method.44 The accuracy of the viscometer was tested with a certified viscosity reference standard (Standard S3, Cannon Instrument Company), and the measurement repeatability was determined to be ±0.1% for viscosity, ±0.0002 g cm−3 for density, and ±0.005 °C for temperature. ASTM D370345 was employed to investigate the PV change after stressing the fuel by subjecting samples to ASTM D4625, D5304, and 5177


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Energy & Fuels To prepare the contaminated samples, each 400 mL of F-76, HRD76, and their blends was contaminated by adding 2 mL of biodiesel (B100).


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RESULTS AND DISCUSSION Properties and Characterization. Results of the GC/MS analysis of F-76, HRD-76, and B100 are presented in panels A, B, and C of Figure 1, respectively. The characteristics of F-76 and HRD-76 were published elsewhere.12,13,63,64 The biofuels, HRD-76 and B100, comprise fewer compounds, and the boiling points of their components span a narrower range than that of F-76. F-76 is composed of approximately 25% aromatics and 73% saturates (alkanes and cycloalkanes), with a small amount of olefins (alkenes, cycloalkenes, and some dienes),64 while HRD-76 is mainly composed of C15−C18 n-alkanes and branched monomethyl C16 and C17 alkanes. The molar composition of HRD-76 was estimated to be 11% npentadecane, 9% n-hexadecane, 22% n-heptadecane, 25% isoheptadecane, 15% n-octadecane, and 18% isooctadecane.13 B100 has five dominant peaks ranging in retention time from 26 to 29 min. A FAME reference standard purchased from AccuStandard (FAMQ-005) with 37 saturated and unsaturated esters from butyric acid methyl ester to cis-4,7,10,13,16,19docosahexaenoic acid methyl ester was used to identify B100 GC peaks. Results indicate that the purchased B100 mainly consists of five components, i.e., methyl palmitate, methyl oleate, methyl linoleate, methyl stearate, and methyl linolenate. The composition of each compound was calculated on the basis of the calibration curves, and the calculated mole fraction was 26.3% methyl palmitate, 27.8% methyl oleate, 30.4% methyl linoleate, 8.9% methyl stearate, and 6.6% methyl linolenate. The amount of unsaturated esters accounts for over 60% of the B100 sample, which highly influences the stability of the fuel sample. As the proportion (concentration and degree) of unsaturation in the fatty acid chains increases, the biodiesel becomes more unstable. Table 1 lists the selected physicochemical properties of F-76, HRD-76, and B100. The MIL-DTL-16884N specification requires a kinematic viscosity range from 1.7 to 4.3 mm2 s−1 at 40 °C and identifies a maximum allowable density and AN of 0.876 g cm−3 at 15 °C and 0.3 mg of KOH g−1 for NATO F-76, respectively. Although HRD-76 is less viscous compared to F76, it still meets the MIL-DTL-16884N specification. Because of the high density and viscosity, however, B100 cannot be directly used as a replacement biofuel for F-76. Additionally, the general quality of B100 is lower compared to HRD-76 and F76, owing to the high PV. The density and kinematic viscosity temperature dependence of F-76, HRD-76, and B100 were also measured over the temperature range of 20−100 °C, as shown in Figure 2. The variation in density with the temperature is linear, while the variation in kinematic viscosity with the temperature is exponential. The results for F-76 and HRD-76 are in good agreement with that reported by Prak et al.12 and Hsieh et al.63 The uncertainty values for density and kinematic viscosity are smaller than the plotting symbol. Storage Stability. The fuel storage stability is highly affected by its aromatic content, sulfur content, and fuel oxidative and thermal stability. ASTM D4625-04 is the one of most widely established methods for testing the storage stability of middle-distillate petroleum fuels and has been employed for estimating the storage stability of biofuels.13,52−58 F-76, HRD76, and their 50/50 blends contaminated by a trace amount of B100 were subjected to long-term storage tests, i.e., ASTM

Figure 1. GC/MS chromatograms of (A) F-76, (B) HRD-76, and (C) B100. 5178


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Table 1. Physicochemical Properties of F-76, HRD-76, and B100 fuel

ρ at 15 °C (g cm−3)

ν at 40 °C (mm2 s−1)

PV (mg kg−1)

HHV (MJ kg−1)

AN (mg of KOH g−1)

F-76 HRD-76 B100

0.8455 0.7787 0.8778

2.9182 2.7675 4.3725

1.87 ± 0.17 0.13 ± 0.02 16.59 ± 0.49

44.8292 ± 0.0733 46.4097 ± 0.0428 39.1707 ± 0.0422

0.11 ± 0.00 0.00 ± 0.00 0.23 ± 0.01

Figure 2. Kinematic viscosity and density of F-76, HRD-76, and B100.

D4625 for 24 weeks with analyses occurring at 0, 4, 8, 12, 16, 18, 20, and 24 weeks. Two samples of each material were analyzed according to the weekly schedule (e.g., two sample bottles containing 400 mL of HRD-76 plus 2 mL of B100 were removed from the 43 °C storage oven and analyzed at week 18). Panels A and B of Figure 3 show the amount of filterable and adherent insolubles found in the samples at each analysis interval. The amount of filterable and adherent insolubles formed after stressing the contaminated F-76 and 50/50 blend samples increases with the storage time, whereas no significant linear relationships were found between the amounts of insolubles formed and the storage time for the contaminated HRD-76 samples, as indicated by the very low R2 of linear regression. The amounts of filterable and adherent insolubles formed in the contaminated HRD-76 samples are all lower than those of contaminated F-76 samples, demonstrating that contaminated HRD-76 is more stable than contaminated F76 and has less unstable precursors. The concentration of insoluble compounds in the contaminated 50/50 blend is between that for contaminated F-76 and HRD-76 samples, illustrating the influence of F-76 on their formation. It is also worth noting that the total amount of insolubles formed in contaminated F-76 approaches the maximum allowance, 3 mg/ 100 mL, by the MIL-DTL-16884N specification after 24 weeks of storage. The results of biodiesel-contaminated fuel storage can also be compared to uncontaminated fuel samples. Panels A and B of Figure 4 present the differences between insoluble results from unadulterated and B100-contaminated samples. The data for uncontaminated samples were obtained from a previous study,13 in which values for 4, 8, 12, 16, and 24 week tests

Figure 3. (A) Filterable and (B) adherent insolubles formed in biodiesel-contaminated HRD-76, F-76, and a blend of 50% HRD-76 and 50% F-76, as determined by the ASTM D4625 test method over 24 weeks. Two samples were analyzed each time.

were reported. B100 contamination has negligible influence on HRD-76 storage stability, as demonstrated by the minimal differences in insoluble formation between contaminated and uncontaminated samples. However, the B100 contamination has significant impact on filterable insoluble formation in F-76 5179


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Figure 4. Differences between the measured quantities of (A) filterable and (B) adherent insoluble compounds in unadulterated HRD-76, F76, and their 50/50 blend and the virgin materials contaminated with B100 after testing according to ASTM D4625. Figure 5. Change of fuel (A) kinematic viscosity at 40 °C and (B) density at 15 °C at ASTM D4625 analysis intervals.

samples, and this impact is also reflected in the 50/50 blends. No relationship was observed for the adherent insoluble formation in any of the fuel samples. The influences of long-term storage and biodiesel contamination on fuel physicochemical properties were also investigated. Panels A and B of Figure 5 present the kinematic viscosity and density change after stressing the contaminated fuel samples. The density and viscosity of contaminated F-76 and 50/50 blends increase with storage time, but a notable relationship was not observed for contaminated HRD-76 samples. Although the fuels became denser and more viscous after aging, the kinematic viscosity and density of neat and blended fuels still meet the MIL-DTL-16884N specification. Additionally, Table 2 compares the fuel properties after 24 weeks of storage with the original (time = 0) samples. The results indicate that the long-term storage has more impact on

contaminated F-76 samples than HRD-76 samples, especially as shown in the PV. The PV of contaminated F-76 increases over an order of magnitude after 24 weeks of storage. The oxygen overpressure method, ASTM D5304, was also employed to study the oxidative stability of contaminated fuel samples. The advantages of this test method compared to ASTM D4625 is the greatly reduced testing time, 16 h versus 24 weeks, and reduced sample size, 100 versus 400 mL. The 16 h ASTM D5304 test yields approximately the same amount of insolubles as a 27 month storage test using air at atmospheric pressure and 20 °C.42 This method also allows for the oxygen consumption rate, reflected by the pressure reduction in the pressurized vessel, to be monitored during the test. Figure 6 illustrates the oxygen consumption rate of contaminated and 5180


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Table 2. Influence of 24 Weeks of Storage and B100 Contamination on Fuel Properties fuel

ρ at 15 °C (g cm−3)

ν at 40 °C (mm2 s−1)

400 mL of F-76 + 2 mL of B100 (0 week) 400 mL of 50/50 blend + 2 mL of B100 (0 week) 400 mL of HRD-76 + 2 mL of B100 (0 week) B100 (0 week) 400 mL of F-76 + 2 mL of B100 (24 weeks) 400 mL of 50/50 blend + 2 mL of B100 (24 weeks) 400 mL of HRD-76 + 2 mL of B100 (24 weeks) B100 (24 weeks)

0.8471 0.8135 0.7796 0.8778 0.8474 0.8136 0.7797 0.8781

2.9309 2.8708 2.8488 4.3725 2.9559 2.8645 2.7948 4.3787

PV (mg kg−1) 0.61 1.51 0.22 15.62 16.11 4.94 0.22 17.36

± ± ± ± ± ± ± ±

0.06 0.16 0.03 0.56 0.80 0.12 0.02 1.83

HHV (MJ kg−1) 44.8458 45.6167 46.6211 39.1707 45.0614 45.6190 46.4993 39.3535

± ± ± ± ± ± ± ±

0.1954 0.0050 0.2390 0.0422 0.2559 0.1559 0.1873 0.0428

AN (mg of KOH g−1) 0.10 0.07 0.00 0.23 0.11 0.07 0.01 0.32

± ± ± ± ± ± ± ±

0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01

HRD-76 might decrease the stability of fuel. Neat HRD-76 is more stable than F-76. Table 3 lists the physicochemical properties of contaminated and uncontaminated fuel samples after the 16 h test. The kinematic viscosity and PV increase after the test for all of the fuel samples, whereas the density and AN change is not significant. B100 contamination does not have a very significant influence on fuel properties after the 16 h test. Storage stability of HRD-76 and B100 blends was also tested using the accelerated ASTM D5304 method. Figure 7 shows

Figure 6. Oxygen consumption of contaminated and uncontaminated fuel samples using the ASTM D5304 test method. The oxygen consumption is calculated by the pressure change inside of the vessel.

uncontaminated fuel samples in identical vessels. The oxygen consumption rate of the fuel samples is high in the first 2 h of the 16 h test period, owing to the oxidation of unstable precursors. As these become fully reacted, the rate slows and becomes constant. The slope of the trend line of the sample 16 h data demonstrates that HRD-76 is generally more stable than F-76 samples even with B100 contamination, which is inconsistent with results obtained in the ASTM D4625 investigation. B100 contamination increases the O2 consumption rate of HRD-76, F-76, and their 50/50 blends as a result of the existence of unsaturated fatty acid esters. It is also worth noting that the O2 consumption rate of the 50/50 blend is faster than the F-76 samples, indicating that blending F-76 with

Figure 7. Percentage of original oxygen head space pressure as a function of time during tests of B100 and HRD-76 blends according to the ASTM D5304 method.

the oxygen consumption rate of different B100/HRD-76 blends in the 16 h test. The oxygen consumption rate is slow in the

Table 3. Influence of Long-Term Storage and B100 Contamination on Fuel Properties Determined after the 16 h ASTM D5304 Test fuel

ρ at 15 °C (g cm−3)

ν at 40 °C (mm2 s−1)

F-76 50/50 blend HRD-76 400 mL of F-76 + 2 mL of B100 400 mL of 50/50 blend + 2 mL of B100 400 mL of HRD-76 + 2 mL of B100

0.8457 0.8123 0.7788 0.8473 0.8134 0.7794

2.9780 2.8098 2.7746 2.9385 2.8351 2.7802 5181

PV (mg kg−1) 4.38 2.44 0.26 3.06 2.80 0.41

± ± ± ± ± ±

0.16 0.16 0.02 0.07 0.09 0.04

AN (mg of KOH g−1) 0.11 0.07 0.01 0.10 0.05 0.01

± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01


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Energy & Fuels


first 8 h for all of the fuel blends, owing to the presence of antioxidants, and the oxygen consumption rate of 100% B100 accelerates thereafter. The initiation of the oxygen consumption rate transition depends upon the B100 concentration in the fuel blend, and the transition time increases with the reduction of the B100 content. For the fuel blends with 25 and 10% (v/v) B100, the oxygen consumption rate is constant during the entire 16 h test period. Two tests using lower B100 concentration samples (25 and 10%, v/v) were conducted for extended periods (40 and 90 h, respectively) to determine whether a transition point would ultimately occur. Figure 8 displays the transition time and

Figure 9. Kinematic viscosity and density change of B100 and HRD76 blends after 16 and 40 h ASTM D5304 tests at 40 and 15 °C, respectively. Note that error bars for density data points are ±0.0002 g cm−3.

Table 4. PV of B100 and HRD-76 Blends after 16 and 40 h ASTM D5304 Tests B100 (%, v/v) 0 10 25 50 75 90 100

Figure 8. Transition time of the oxygen consumption rate and corresponding amount of oxygen consumed by the B100 and HRD-76 fuel blends at the transition point as a percentage of the initial oxygen pressure in the test vessel.

PV (mg kg−1) for 0h 0.13 1.12 4.37 8.21 13.02 12.01 16.59

± ± ± ± ± ± ±

0.02 0.10 0.12 0.50 0.12 0.89 0.49

PV (mg kg−1) for 16 h 0.26 4.44 19.89 1389.59 1141.66 1468.93 1147.90

± ± ± ± ± ± ±

0.02 0.18 0.93 55.32 11.53 81.36 35.58

PV (mg kg−1) for 40 h 0.28 6.44 833.35 1088.62 1030.31 1198.37 1008.15

± ± ± ± ± ± ±

0.03 0.07 86.23 3.21 114.25 71.07 52.88

blend after the 16 h D5304 test, at which point the transition time has not yet been reached. Generally, the oxygen consumption rate transition and significant PV, viscosity, and density change of the fuel blend all result from the biodiesel oxidation, which involves a radical chain reaction.65 The biodiesel used in this study contains a higher mole fraction of polyunstaturated methyl esters (approximately 37%) than monounsaturated esters (approximately 28%), and the oxidation rate of biodiesel is mainly affected by the number of bis-allylic methylene groups adjacent to the double bond compared to the allylic methylene groups; i.e. polyunstaturated esters are more susceptible to be oxidized than the monounsaturated esters, owing to the presence of additional bis-allylic methylene configuration.40,65 As a result of the oxidation, acids, aldehydes, alcohol, ketones, and peroxides are produced. Additionally, because of the high testing temperature, 90 °C, of ASTM D5304, highly stable conjugated structures are formed by the Diels−Alder reaction between the conjugated diene groups in the chain and polyunstaturated olefinic group from the nearby chain,66 which demonstrates the increase of density and kinematic viscosity after stressing the samples. Oxidation Stability. The accelerated ASTM D2274 method is the standard method for the testing of the oxidation stability of middle-distillate petroleum fuel and biofuel. This method was also employed to investigate the effect of biodiesel

amount of oxygen consumed at the transition point of different fuel blends. The uncertainty of the transition time is approximately ±1.5 h. The data exhibit an exponential increase in transition time with decreasing B100 content and a concomitant decrease in the amount of oxygen consumed at the transition point. The influence of B100 oxidation on fuel blend physicochemical properties was also investigated, particularly kinematic viscosity, density, and PV. Figure 9 compares the kinematic viscosity and density of the fuel blends before and after the ASTM D5304 tests. Generally, kinematic viscosity and density of the B100/HRD-76 fuel blend meets the MIL-DTL-16884N specification, except for the blend with >90% (v/v) B100. However, the kinematic viscosity of the fuel samples increases significantly after 16 and 40 h ASTM D5304 tests, and the viscosity increase is related to the B100 concentration in the fuel blend. Higher composition of B100 in the fuel induces a higher viscosity increase. A similar trend was also observed for the fuel density. Table 4 lists the PV of the fuel blends before and after 16 and 40 h D5304 tests. As with kinematic viscosity and density, the PV of the fuel blend is highly affected by B100 composition. The PV increases sharply if the testing time is longer than the transition time, which is demonstrated by the PV of 25:75 B100/HRD-76 samples. The PV of the blend after the 40 h D5304 test is about 24 times higher than that of the 5182


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the contaminated samples compared to the uncontaminated samples. The formation of filterable insolubles is not significant. As seen in the results obtained from the ASTM D4625 test, longer oxidation times and an increasing F-76 fraction induce larger amounts of insoluble formation. It is also important to note that the insolubles formed in F-76 samples (D2274) approach the limit of the MIL-DTL-16884N specification and B100 contamination decreases the adherent insolubles formed in the fuel. The influence of B100 contamination on the fuel properties was also investigated. Panels A and B of Figure 11 show the kinematic viscosity and density change of the contaminated and uncontaminated fuel samples after 16 and 40 h oxidation tests. As expected, the kinematic viscosity increased with the oxidation time and F-76 fraction, while the density increase was negligible. The B100 contamination has less influence on


contamination on F-76, HRD-76, and their blends. Insoluble formation in contaminated F-76, HRD-76, and their 50/50 blend was tested for 16 and 40 h (panels A and B of Figure 10),

Figure 10. Insolubles measured in contaminated and uncontaminated fuel (F76, HRD-76, and 50/50 blend) with and without B100 contamination after undergoing the ASTM D2274 test for (A) 16 h and (B) 40 h. FI is filterable insolubles; AI is adherent insolubles; and TI is total insolubles.

and the values obtained were compared to that of uncontaminated samples published previously.13 MIL-DTL16884N requires a 40 h oxidation period and specifies that total insolubles formed must be

Effects of Biodiesel Contamination on Oxidation and Storage Stability

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