Blends of Soybean Biodiesel with Petroleum Diesel: Advantages

Article pubs.acs.org/IECR

Blends of Soybean Biodiesel with Petroleum Diesel: Advantages George W. Mushrush,†,‡ Janet M. Hughes,‡ and Heather D. Willauer†,* †

Material Science and Technology Division, Code 6300.2, Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, United States ‡ Department of Chemistry and Biochemistry, George Mason University, 4400 University Drive, Fairfax, Virginia 22030, United States ABSTRACT: The use of bioderived materials both as fuels and/or as blending stocks becomes more attractive as the price of hydrocarbon transportation fuels continues to rise. Historically, many biomass and agricultural derived materials have been suggested. Middle distillate transportation fuels for military applications have more severe restrictions than for the regular commercial consumer. One of the most difficult requirements to meet is that of fuel storage stability. In the present research, Soygold, a soybean derived fuel, was added in concentrations of 10−20% to both stable and unstable petroleum middle distillate fuels. The addition of the soy liquid was shown by ASTM methods to enhance fuel combustion properties, greatly improve the stability of unstable fuels, and dramatically increase fuel lubricity.

1.0. INTRODUCTION Many schemes have been proposed to produce middle distillate fuels from renewable and/or nonrenewable sources as diverse as plastic residues, trash, used automobile tires, and plants. The quantities of fuels required by United States consumers make these schemes daunting in both cost and complexity. If a nonpetroleum derived diesel fuel can not be produced with present technology in the quantities required, can a blending stock be produced in significant quantities? Soybean derived oils have several advantages over other vegetable oils as fuel liquids. Some of these advantages are that soybean oil is cheaper than other vegetable oils (17−20 cents/pound), the infrastructure to meet demand is in place for the large fuels market, and soybeans contain considerably more oil, 20%, in comparison to other crops such as corn, at 5% useable oil.1 The single largest consumer of middle distillate fuels in the United States is the Department of Defense.2 Fuels used for military applications have more severe restrictions than those used by the commercial consumer. As a further complication, military fuels are often stored for a year or more before use. This is in contrast to most typical civilian consumer uses, where middle distillate fuels are used practically in the same time frame in which they are produced. Thus, the materials added to a middle distillate fuel for military applications must be stable in storage. The blending of any material to a petroleum middle distillate fuel must be done with considerable care for many reasons; among these are the solubility in the fuel at both ambient as well as low temperatures (winter operations in cold climates), flash point considerations, and the type of engine in which the fuel will be burned. Most important, however, is that the added blending stock must not induce chemical instability in the fuel itself. The most significant and undesirable instability change in fuel liquids, with time, is the formation of sludge and other solids.3−7 These solids can plug nozzles, filters, coat heat exchanger surfaces, and otherwise degrade engine performance. In the present research, we report the results from the addition of a soybean fuel liquid, Soygold, in concentrations up to 20%, to both stable and unstable middle distillate fuels. The © 2012 American Chemical Society

soy-derived processed fuel liquid makes an excellent blending stock because it has neither organo-nitrogen nor organo-sulfur compounds. It is the presence of these compounds that initiate the chemical reactions of the degradation process.5,6 We also look at oxidative stability of the Soygold fuel liquid both at ambient conditions and under accelerated storage conditions. The fact that alternative fuels are becoming attractive because of price and the inherent characteristics of biofuels all point to the need for a timely investigation of soybean derived oils as alternative transportation fuels and/or heating oil replacement.

2.0. EXPERIMENTAL METHODS 2.1. Chemicals and Materials. Chemicals were reagent grade and were obtained from Aldrich Chemical Co., Milwaukee, WI, and were used without additional purification. 2.2. Gas Chromatography/Mass Spectrometry. The soy blending stock and the soy−petroleum fuel mixtures were analyzed by combined capillary column gas chromatography/ mass spectrometry. The gc/ms system consisted of an Agilent (Hewlett-Packard) 6890A gas chromatograph configured for splitless injection (1 μL), and a Finnegan INCOS 50B mass spectrometer. The gc was equipped with an all glass inlet system and was used in conjunction with a 0.32 mm × 30 m fused silica capillary column, HP cross-linked 5% phenyl methyl silicone. The injector temperature was 250 °C, and the detector was 320 °C. The column flow was 1 mL/min. The temperature program started with an initial temperature of 60 °C for 5 min, a ramp of 5 °C/min, to a final temperature of 260 °C. The mass spectrometer was operated in the electron impact ionization mode (70 eV) with continuous scan acquisition from 50 to 350 amu at a cycling rate of approximately 1 scan/s. The parameters were set up with the electron multiplier at 1050 V, source temperature of 200 °C, and transfer line temperatures at 290 Received: Revised: Accepted: Published: 1764

October 19, 2012 December 12, 2012 December 17, 2012 December 17, 2012 dx.doi.org/10.1021/ie302865x | Ind. Eng. Chem. Res. 2013, 52, 1764−1768

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°C. The INCOS 50 data system and ITDS software were used to process mass spectral information. 2.3. Nuclear Magnetic Resonance. 1H and 13C NMR spectra were acquired in CDCl3 at 300 and 75.0 MHz, respectively, on a Bruker 300 MHz spectrometer. 1H chemical shifts are reported in units of ppm downfield from TMS (set at 0.0 ppm). 13C chemical shifts are listed in (ppm) relative to chloroform (set at 77.0). 2.4. Simulated Burner. A simulated burner was constructed to test nozzle plugging and soot formation from the soy−fuel mixtures. The brass burner consisted of an 0.125 in. id nozzle orifice surrounded by a 0.5 in. air mixing chamber. The outer chamber was drilled and tapped for a 0.125 in. air line. The fuel was pumped through a 0.25 in. tube to the nozzle orifice by two syringe pumps, each with a 50 mL capacity syringe. The air was operated at 5 psig or less. The burn of the various soy−fuel mixtures was for a period of 15−30 min at which time the burn was suspended and the nozzle examined for deposit formation. A quantitative procedure for measuring soot was used. 2.5. Storage Stability Tests. The soy−fuel mixtures were tested for fuel instability and chemical incompatibility reactions. The mixtures were tested for fuel sedimentation by a gravimetric technique described in ASTM D5304-99.8 The samples were then filtered and sediment determined by a gravimetric procedure as described in D5304-99. 2.6. Soygold Oxidative Stability Test. The Soygold liquid was subjected to a steady stream of ambient air for 1 week. The reaction was carried out in a 1 L vacuum flask connected to an aspirator protected by a safety bottle. The air at ambient conditions was filtered through a drying tube filled with anhydrous CaSO4 with fiberglass plugs before passing through the soy liquid. 2.7. Lubricity Tests. Fuel blends of 10 and 20 vol % Soygold petroleum diesel were prepared. The neat fuel (stable American No. 2 diesel) and blends were measured for lubricity by two ASTM methods. Method one was ASTM Method D6078 Scuffing Load Ball-on-Cylinder Lubricity Evaluator, SLBOCLE.9 In brief, this method may be described as a 50 mL fuel sample placed in a reservoir and the temperature adjusted to 25 °C. A 500 g mass was placed on a load arm holding a nonrotating steel ball. A steel ring rotating at 525 rpm was partially immersed in the test fuel. The ball was kept in contact with the ring for 60 s after which it was moved 0.75 mm away from the ring so that a new load could be applied. The tangential frictional force was recorded and the friction coefficient calculated. The minimum load required to give a frictional coefficient greater than 0.175 was considered to be a measure of the lubricating properties of the fuel. Fuels with SLBOCLE values below 2000 g may cause accelerated wear in the fuel lubricated rotary-type injection pump. Method two was ASTM Method D6079 High Frequency Reciprocating Rig, HFRR.11 This method can be described as follows: 2 mL samples were placed in the test reservoir at 60 °C. A fixed steel ball was held in a vertically mounted chuck and forced against a horizontally mounted stationary steel plate with a 200 g applied load. The test ball was oscillated at 50 Hz with a stroke length of 1 mm for 75 min while in contact with the steel plate. The wear scar on the test ball was considered to be a measure of the lubricating property of the fuel. Wear scars of 450 μm or lower at 60 °C measured by this method should be able to protect fuel injection equipment.11

2.8. Soy Diesel Fuel. The soy-derived biodiesel fuel, Soygold, was supplied by Ag Environmental Products, 9804 Pflumm Road, Lenexa, KS 66215. This material was a light yellow in color, had a boiling point greater than 400 °F, negligible water solubility, a specific gravity of 0.88, a flash point of 425 °F, a cetane number >40, and no antioxidant present. 2.9. Petroleum Diesel Fuels. All storage stability testing was done by ASTM D5304.8 The petroleum middle distillate fuels used were from our inventory of tested fuels. The stable fuel (no. 2 diesel) was an American refined fuel that has been used as a stable fuel for comparison in our laboratory. This fuel yields 0.6 mg of solids/100 mL and, therefore, was classed as a very stable fuel. Three unstable fuels were used: fuel number 1 was a Spanish refined number 2 diesel; fuel number 2 was a refined number 2 diesel from Brazil, and fuel number 3 was a Kenya refined number 2 diesel. These fuels have historically been unstable forming 2.5 mg solids/100 mL for fuel number 1, 3.8 mg solids/100 mL for fuel number 2, and 5.1 mg solids/100 mL for fuel number 3 in the storage stability test procedure. This yield of total solids ranks these fuels as very unstable.

3.0. RESULTS AND DISCUSSION Diesel fuel is a liquid product distilling over the range 150−400 °C (300−750 °F). The carbon number ranges, on average, from about C13 to about C21. The chemical composition of a typical diesel fuel and how it applies to the individual specifications, °API gravity, distillation range, freezing point, and flash point are directly attributable to both carbon number and compound classes present in the finished fuel as given in Table 1, ASTM D396-96.12 Table 1. General ASTM Specifications for the Various Types of Diesel Fuels specification

militarya

no. 1-Db

no. 2-Dc

Soygold

°API gravity total sulfur % boiling point °C flash point °C pour point °C hydrogen wt % cetane number acid number

44.0 0.5 357 60 −6 12.5 43 0.3

34.4 0.5 288 38 −18

40.1 0.5 338 52 −6

44 0 204 218

40 0.3

40 0.3

12.3 >45 0

a

MIL-F-16884J. bHigh speed, high load engines. cLow speed, high load engines.

In addition to cost and availability, the soy−diesel fuel liquid offers several attractive advantages for use as a fuel blending stock as shown in Table 1. Further, Table 2 illustrates that the soy derived fuel has the same carbon number range as a typical diesel fuel. The military has many required specifications. Among these are those for color, water solubility, flash point, Table 2. Methyl Ester Concentration In Soygold Biodiesel methyl ester methyl methyl methyl methyl methyl a

1765

linoleate oleate linolenic stearate palmitate

carbon number

concentration (wt %)a

C18:2 C18:1 C18:3 C18:0 C16:0

53 24 3 10 10

Trace of other unidentified methyl esters. dx.doi.org/10.1021/ie302865x | Ind. Eng. Chem. Res. 2013, 52, 1764−1768

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compounds. The results depicted in Table 3 show conclusively the oxidative stability of the soy-derived fuel. The conditions were quite rigorous with a stream of air continuously saturating the soy liquid for 168 h. The 1H NMR spectra shows that the concentrations of the methyl esters were unchanged after this extended time period. The storage stability results are depicted in Table 4. This ASTM procedure features severe reaction conditions.8 In brief,

and specific gravity, to mention a few. A comparison of the various fuels in Table 1 showed that this soy derived biofuel would not diminish the quality of a petroleum fuel to which it was added. Another important ASTM required standard is the color test,11 which is a color comparison test. Consumers, whether civilian or military, generally prefer a lighter colored product. As any fuel ages, it generally darkens in color. The soy derived fuel and the petroleum−soy mixtures were both observed to be color stable under both low temperature and thermal oxidative conditions for extended periods of time. It is important to note that a change in color may or may not be related to degradation. However, our laboratory has observed that a color change to dark brown can be directly correlated with an increasing hydroperoxide concentration and its subsequent reaction with both organo-sulfur and organonitrogen compounds.8 The soy-derived diesel did not darken with time even at temperatures of 70 °C and exposure to light over a 6 month time period. These observations could be a direct correlation to the chemical components present in the soy blending stock. Analysis by gc/ms showed that the soy blending stock was a mixture of methyl esters of long chain carboxylic acids as illustrated in Table 2. Approximately 75% of the total concentration of the methyl esters consisted of unsaturated compounds as determined by NMR integration. The single peak in the alkoxy region of the 1H NMR spectra, with no sign of splitting indicated the presence of only methoxy esters with no visible peaks from carboxylic acids. The identity of the unsaturated alkyl chain was further supported by the integration of the NMR spectra within the olefin region. No evidence of organo-sulfur or organo-nitrogen compounds was detected by gc/ms in the soy fuel. Alkyl esters of long chain fatty acids are also thermally stable. This was a further contributing factor to the observed stability of this blending stock and to enhance the storage stability results of the petroleum derived fuel to which it was added. The NMR analysis as illustrated in Table 3 shows that the soy blending stock consisted of approximately 75% unsaturated methyl ester

Table 4. Storage Stability of Soy-Derived Diesel Blends and Petroleum-Derived Middle Distillate Fuels15

stable diesel fuela unstable diesel fuels fuel 1b fuel 2c fuel 3d fuel blends 10% soy−90% fuel stable diesel fuel 1 fuel 2 fuel 3 20% soy−80% fuel stable diesel fuel 1 fuel 2 fuel 3 a

before oxidation CO2CH3 HCCH CH3 CH2CO2R CH2 after oxidation CO2CH3 HCCH CH3 CH2CO2R CH2

a

δ

intergration

signala

remarks

3.66 5.40−5.28 0.87 0.91 2.30 2.77 1.64−1.60 1.31−1.26

3 3 2.67 0.30 2 1.5 2.5 18

s m t t t t m m

only methyl ester no conjugation 89% alkyl 11% vinyl unconjugated γ−δ unsaturation alkyl alkyl

3.66 5.40−5.28 0.88 0.97 2.30 2.79 2.08−2.00 1.64−1.60 1.31−1.26

3 3 2.69 0.32 2 1.3 3.5 2 18

s m t t t t m m m

only methyl ester no conjugation 89% alkyl 11% vinyl unconjugated γ−δ unsaturation alkyl alkyl alkyl

0.6 3.8 3.7 5.1

0.3 0.9 1.8 1.5 0.2 0.5 1.3 2.4

American. bSpain. cBrazil. dKenya (all number 2 diesels).

the conditions were 100 mL fuel at 90 °C, for 16 h, at 100 psig pure oxygen. A fuel that survives these drastic conditions should prove stable for more than 2 y in storage. The middle distillate fuels in this study were selected based on the intrinsic stability of the fuels. The solids results in Table 4 are the average of three runs that agree to within 5%, and the concentration range of the mixtures in Table 4 were selected based on the Navy’s recommendation that the maximum allowable concentration of bioderived fuel be 20%. The results in Table 4 demonstrate conclusively that both the 10% and 20% Soygold petroleum middle distillate blends were stable. Even the most unstable fuel (Kenya no. 2 diesel, 5.1 mg solids) in a 20% soy blend easily passed the ASTM stability procedure (1.5 mg solids). The high degree of positive synergism with this fuel was somewhat surprising. However, the added soy-derived fuel, Table 2, was composed of methyl esters of long chain fatty acids which are chemical and thermally table. More importantly, esters do not readily form hydroperoxide species, and without a significant concentration of these active oxygen species, a fuel is stable.6 The lack of peroxidation reaction under the forcing conditions employed illustrated that at ambient conditions the peroxidation reaction could be neglected. Furthermore no evidence of conversion of the methyl esters to the corresponding acid was detected by 1H NMR. Despite the slight shifts noted in Table 3 in chemical frequency, which was believed to be due to varying concentration of the sample, the composition of the oxidized fuel appeared consistent with the nonoxidized sample. There was a slight decrease in olefin character with the corresponding increase in alkyl functionality as would be expected when subjected to such reaction conditions. The majority of the olefin character remained,

Table 3. 1H NMR of Soygold Before Air Oxidation and After Air Oxidation for 168 h functionality

solids by gravimetric methods (mg solids/100 mL fuel)

fuels

s = singlet, t = triplet, m = multiplet. 1766

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longer and thus decrease consumer operating cost. Finally, this blending stock would also be an internal source of added oxygen to the petroleum-derived fuel with all of the benefits that this entails for combustion.

thus confirming the storage stability in the presence of moisture and air. For the stable American refined petroleum diesel fuel, a dramatic improvement in storage stability was noted, Table 4. The petroleum diesel fuel yielded 0.6 mg of solids while the blends gave 0.3 mg for the 10% blend and 0.2 mg for the 20% blend. There are at least two possible explanations for this. In the case of the 20% mixture, the fuel concentration is lowered significantly, but most important was that the soy-derived fuel itself was so stable. The simulated burner experiments were used to check for both deposits that form at the burner nozzle and for soot deposits that result from the flame. The highest concentration of blending stock (20%) was used for these tests. The nozzle was examined after a 30 min burn, and no deposits were observed. Prior experience with this burner indicates that if no deposits are observed at the burner nozzle within 30 min, the fuel can burn almost continuously deposit free.13 The filter paper that was used to monitor soot formation likewise showed no soot deposit by gravimetry. It is important to note that this procedure is for deposit formation due to high temperature oxidative conditions and is not necessarily related to storage instability reactions. The fuel blends were subjected to one other critical ASTM procedure, that of filterability.14 The 10% and 20% blends passed this filterability with ease. Filterability is generally considered to be a measure of fuel cleanliness. A biofuel that consists mainly of the ester functional group has another major benefit. Fatty acid esters are known to be lubricity enhancers. Table 5 shows that the addition of 10−20%

4.0. CONCLUSIONS The use of a biodiesel fuel such as Soygold as a blending stock for middle distillate fuels has many positive attributes. The blending stock meets or exceeds ASTM requirements and definitely showed a positive synergism when mixed with both stable and unstable middle distillate fuels. Blends in the 10− 20% concentration range yields fuels that are stable in storage for at least a year. Finally, esters are lubricity enhancers and consequently would have a positive synergism with fuel system components and also serve as an additional oxygen source, thus improving combustion.

■

Corresponding Author

*E-mail: [email protected]. Phone: 202-767-2673. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the Office of Naval Research both directly and through the Naval Research Laboratory.

ASTM D-6079

fuel sample

SLBOCLE, g

HFRR, μm

American fuel−neat 10% Soygold 20% Soygold neat Soygold

1700 5050 5300 6000

600 115 75 155

REFERENCES

(1) http://agrimoney.com/2012 (accessed Jan 2013). (2) Defense Energy Support Center. In Petroleum Quality Information Report, Fort Belvoir, Virginia, 2009; DESC-BP. (3) Hazlett, R. N.; Hall, J. M. In The Chemistry of Engine Combustion Products, Ebert, L. B., Ed.; Plenum Press: New York, 1985; p 245. (4) Hiatt, R. R. In Organic Peroxides, Wiley Interscience: New York, 1971, Vol. II, Chapter 1. (5) Hucknall, D. J. In Selective Oxidation of Hydrocarbons; Academic Press: New York, 1974. (6) Mayo, F. R. In The Chemistry of Fuel Deposits and Their Precursors; Final Report, Naval Air Systems Command, 1972; N00019-72-C10161. (7) Mushrush, G. W.; Speight, J. G. 1998. A Review of the Chemistry of Incompatibility in Middle Distillate Fuels. Rev. Process Chem. Eng. 1998, 1 (1), 5−29. (8) ASTM, American Society for Testing Materials. Standard Test Method for Assessing Distillate Fuel Storage Stability by Oxygen Over pressure. In Annual Book of ASTM Standards; ASTM: Philadelphia, 1999; Part 05.03, ASTM D5304-99, pp 569−572. (9) ASTM, American Society for Testing Materials. Standard Test Method for Evaluating Lubricity of Diesel Fuels by the Scuffing Load Ball on Cylinder Lubricity Evaluator, SLBOCLE. In Annual Book of ASTM Standards; ASTM: Philadelphia, 2000; Part 05.04, ASTM D6078-99. (10) ASTM, American Society for Testing Materials. Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High Frequency Reciprocating Rig, HFRR. In Annual Book of ASTM Standards; ASTM:Philadelphia, 2000, Part 05.01, ASTM D6079-99. (11) ASTM, American Society for Testing Materials. Standard Specification for Fuel Oils. In Annual Book of ASTM Standards; ASTM: Philadelphia, 1997; Part 05.01, ASTM D396-96, pp 165−168. (12) ASTM, American Society for Testing Materials. Standard Specification for Fuel Oils. In Annual Book of ASTM Standards; ASTM: Philadelphia, 1997; Part 05.01, ASTM D975-96, pp 323−334. (13) Mushrush, G. W.; Beal, E. J.; Hardy, D. R.; Hughes, J. M. Napalm as An Energy Resource. J. Hazardous Materials 1999, A69, 13−21.

Table 5. Diesel and Biodiesel Blends and Lubricity ASTM D-6078

AUTHOR INFORMATION

of the soy liquids as a blending stock dramatically enhanced the lubricity of the petroleum derived fuel by both the SLBOCLE and HFRR ASTM methods. The results for the SLBOCLE showed a dramatic improvement (20% blend 5300 g) from the neat fuel, 1700 g. Fuels with values below 2000 g for this method have been found to cause accelerated wear. The values for the HFRR method should decrease as the lubricity increases. This method measures the wear scar and the lower the values, the better the fuel lubricity. It is obvious from Table 5 that the fuel blends are significantly better than the neat fuel. HFRR wear scar values higher than 450 μm cause accelerated wear. The improvement with the added Soygold is dramatic. The selection of the concentration ranges for the Soygold was based on the Navy’s proposal to allow up to 20% bioderived materials to be added to some diesel fuels.15 A lower concentration, 10%, was also used. This lower value was predicated on the basis that since bioderived materials are to be used to extend the quantity of middle distillate, it did not make sense to consider lower concentrations. The Soygold fuel liquid contained an antioxidant; however even at added levels of this diluent, the antioxidant concentration would be exceeding low in the fuel blends. The enhanced lubricity of these blends ensures that fuel pumps and other fuel components would last 1767

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(14) ASTM, American Society for Testing Materials. Standard Test Method for Determining Filterability of Distillate Fuel Oils. In Annual Book of ASTM Standards; ASTM: Philadelphia, 1999; Part 05.03, ASTM 426-99. (15) MIL-STD, Military Standards Detail Specification Fuel, Naval Distillate F-76. In MIL-DTL 16884M; Naval Sea System Command: Washington, DC, August 14, 2012.

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