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Alternative jet fuels from vegetable oils

ARTICLE JANUARY 2001

DOI: 10.13031/2013.6988

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Robert Dunn

United StatesDepartment of Agriculture

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Transactions of the ASAE

Vol. 44(6): 17511757 2001 American Society of Agricultural Engineers ISSN 00012351 1751

ALTERNATIVEJETFUELSFROMVEGETABLEOILS

R. O. Dunn

ABSTRACT.

Air quality standards set forth by the Clean Air Act and its amendments have established guidelines for reductionof harmful ground level emissions from the aviation sector. Biodiesel, defined as the monoalkyl esters of fatty acids derivedfrom vegetable oil or animal fat, in application as an extender for combustion in compression ignition (diesel) engines, hasdemonstrated a number of promising characteristics including reduction of exhaust emissions. This work examines the fuelproperties of BioJet fuel blends consisting of 0.100.30 vol. frac. methyl soyate (SME) in JP8 and in JP8+100 jet fuels.Testing of cold flow properties indicated that blends with as little as 0.10 vol. frac. SME may limit operation of aircraft tolower altitudes where ambient temperature remains warmer than 29C. Treatment of SME with cold flow improver additivesmay decrease this limit to 37C. Blends with winterized SME gave the best results, reducing the limit to as low as 47C,a value that meets the standard fuel specification for JP8. Water reactivity studies indicated that SME/JP8 blends absorbedvery little water from buffered solution following contact with the oil phase. Although interface ratings for blends with upto 0.50 vol. frac. SME were 1b (clear bubbles covering not more than 50% of the interface) or better, separation ratingsno better than (3) (formation of cloudy suspensions in the oil layer) were observed. Even though fatty derivatives such asbiodiesel undergo oxidative degradation more readily than jet fuels, careful production, transport, and storage of BioJet fuelsshould not present a significant problem.

Keywords. Biodiesel, Cloud point, Cold filter plugging point, Flash point, Jet fuels, Kinematic viscosity, Methyl soyate, Oilstability index, Water reactivity.

he effects of ground level emissions from

commercial, military and general aviation on localair quality have earned considerable national andinternational attention in recent years. Although

improvements in technology and stricter regulatoryrequirements are predicted to stabilize or decrease harmfulemissions from most transportation sources by 2010, groundlevel emissions from commercial and military aircraft areexpected to continue rising. For example, the aircraft

component of mobile source nitrogen oxides emissions areexpected to increase from 0.63.6% in 1990 to 1.910.5% by2010 based on forecasted growth in ten major urban areas ofthe U.S. (U.S. EPA, 1999).

As early as 1995, the Federal Aviation Administration(FAA) recognized the need to develop strategies for reducingground level emissions from commercial aircraft. One optionis to increase scheduling of lowemissions aircraft to operatein areas with air quality problems. Aircraft would be rankedaccording to their minimum emissions per unit payload or perunit thrust, each measured with respect to one landing/takeoffcycle. Another approach is to minimize the number ofengines in operation during taxiin and taxiout(singleengine taxis). Other strategies such as derated

Article was submitted for review in June 2001; approved forpublication by the Food & Process Engineering Institute of ASAE inOctober 2001.

Mention of company or trade names is for description only and does not

imply endorsement by the U.S. Department of Agriculture.The author isRobert O. Dunn, Chemical Engineer, USDA Agricultural

Research Service, National Center for Agricultural Utilization Research,

1815 N. University St., Peoria, IL 61604; phone: 3096816101; fax:3096816340; email: [email protected].

power takeoffs and reducing the use of reversethrust uponlanding will generally require longer landing strips (FAA,1995).

Another approach that has gained recent attention isdevelopment of cleaner, greener alternative fuels byblending jet fuel (JP5 or JP8) with biodiesel (hereafterreferred to as BioJet fuels). Biodiesel, defined as fatty acidmonoalkyl esters derived from vegetable oil, used fryingoil, or animal fat, has a number of potential advantages in

applications such as jet fuel extenders. Biodiesel is produceddomestically and is renewable, nonflammable, and relativelysafe to store and handle. Biodiesel has kinematic viscosity ()and gross heat of combustion characteristics comparable tothose of No. 2 diesel fuel (D2). Biodiesel enhances lubricityand cetane number of conventional diesel fuels (Goeringetal.,1982; Schwab et al., 1987). Biodiesel reduces harmfulexhaust emissions such as particulate matter, volatile organiccompounds, polycyclic aromatic hydrocarbons, carbonmonoxide, and smoke (Clark et al., 1984; Masjuki et al.,1993; Scholl and Sorenson, 1993; Krahl et al., 1996).Biodiesel has a negative carbon dioxide balance and apositive energy balance in excess of 2:1 (Krahl et al., 1996).

Application of biodiesel as a jet fuel extender also raises

several concerns. First, biodiesel increases cloud point (CP)in blends with No. 1 diesel fuel (D1) and D2 (Dunn andBagby, 1995); therefore, BioJet fuel blends will be moresusceptible to operational problems such as clogged fuellines than neat jet fuels. Another concern is its reaction tocontact with water. Pumping of fuels through pipelines overlong distances may lead to contact with moisture in the pipes.In addition, in military applications water pumped into emptystorage tanks to maintain ballast on aircraft carriers mayleave behind aqueous residues when it is removed. A thirdconcern is reduction in nitrogen oxides emissions because

T
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1752 TRANSACTIONSOFTHEASAE

biodiesel is generally ineffective on these types of emissionsfrom compression ignition engines (Clark et al., 1984;Chang et al., 1996). Finally, biodiesel such as methyl soyate(SME) typically contains in excess of 80 wt% unsaturatedfatty acid esters, creating concerns with respect to oxidationduring longterm storage.

The work reported herein is a preliminary studyinvestigating fuel characteristics and their impact on some ofthe aforementioned concerns for BioJet fuels. Blendsconsisting of 0.100.30 vol. frac. SME in JP8 or JP8+100

(+100 referring to addition of thermal stability additives)were tested for effects on cold flow properties, waterreactivity, flash point, and viscosity with respect tononblended jet fuels. Some blends with enhanced cold flowpropertyesters were also tested. Oxidative stability ofblends is also discussed.

MATERIALSANDMETHODSSME samples were from the following two sources:

Interchem (Overland Park, Kansas) through the NationalBiodiesel Board (Jefferson City, Mo.), and AgEnvironmental Products (AEP, Lenexa, Kansas). Gaschromatography (Autosystem GC, PerkinElmer, Norwalk,Conn.; 25 m0.32 mm ID BPX70 column from SGE, Austin,Texas) analyses of the samples showed 10.7 wt% palmitic(C16 carbons:0 double bonds), 3.6% stearic (C18:0), 22.8%oleic (C18:1), 55.5% linoleic (C18:2), and 7.5% linolenic(C18:3) for InterchemSME. Analysis of AEPSME gavesimilar results (11.2% C16:0, 4.1% C18:0, 24.1% C18:1,52.6% C18:2, and 7.0% C18:3). JP8 and JP8+100 werefrom the Air National Guard stationed at Greater PeoriaRegional Airport and used as received. Cold flow improverswere DFI200 from DuPont (Wilmington, Del.) andWinterflow from Starreon Corp. (Englewood, Colo.). Theseadditives were mixtures of ethylene vinyl acetatecopolymers, naphthenic distillates, and proprietarycompounds designed to improve cold flow properties ofdiesel fuels.

Closedcup flash point (FP), pour point (PP), , and CPdata were measured in accordance with correspondingAmerican Society for Testing and Materials (ASTM)standard methods D93, D97, D445, and D2500 (ASTM,1995a through 1995d). Viscosities were measured at 20Cin accordance with the standard specification for jet aviationfuels (U.S. DOD, 1992). Apparatus and procedures formeasuring cold filter plugging point (CFPP) were describedpreviously (Dunnand Bagby, 1995).

Apparatus and procedures for winterization of biodiesel(neat) have also been described elsewhere (Dunn et al .,1997). Winterization was conducted by equilibrating thesample for approximately 16 hours (h) at 0C and filtering outthe solid crystals. This process was repeated at 2Cincrements until the remaining solution could withstand abath temperature of 10C for a period of 3 h. Winterizationof neat AEPSME (without additives) gave a final CP =12C with a relatively poor product yield = 0.32 gwinterized product collected per 1 g starting material.Winterization of InterchemSME + 2000 ppm DFI200 gaveCP = 11C with yield = 0.87 g/g; winterization ofInterchemSME + 2000 ppm Winterflow gave CP = 11Cwith yield = 0.80 g/g.

Water reactivity data were measured in accordance withASTM method D1094 (ASTM, 1999). Measured quantitiesof fuel sample (80 mL) and aqueous buffer solution (20 mL)were placed in a graduated mixing cylinder and stoppered.The contents were gently shaken at room temperature for 2min (5 s) then immediately placed on a vibration freesurface and allowed to settle undisturbed for 5 min. Waterreactivity was determined with respect to the change involume (V) of the aqueous layer and appearance of theoilwater interface following settling of the mixture.

Water penetration into BioJet fuel blends followinglongterm contact between oil and aqueous phases was alsomeasured. Measured quantities of fuel sample and distilleddeionized water (10 mL each) were placed in a mixingcylinder and equilibrated in a constant temperature bath.Samples were initially agitated and allowed to settle backinto two layers during equilibration. Following exposure for28 days (d), the oil layer was separated and tested for totalmoisture content by Karl Fischer titration. Samplesequilibrated at 10C, 20C, and 30C were tested for waterpenetration.

RESULTSANDDISCUSSIONCOLDFLOWPROPERTIES

In general, aircraft fuel tanks are not insulated. This meansthere is only the thin tank wall separating the fuel from airtemperatures that decrease as altitude increases. Jet fuelspecifications require the fuel to resist formation of solidcrystals at temperatures as low as 47C for JP8 (U.S. DOD,1992). This limit corresponds to an altitude of 9500 maccording to standard atmospheric tables used to makeengineering calculations for aircraft (Anon., 2000).

Results from CP and PP measurements for blends withSME are shown in figures 1 and 2. Both CP and PPpredictably increase with increasing vol. frac. SME. Forblends with at least 0.30 vol. frac., increases in CP and PPwere nearly linear. The PP results show very little increase forblends up to 0.20 vol. frac., followed by a significant increasefrom 0.20 to 0.30 vol. frac. This may be an indication of aphase transition from a solute (SME)solvent (jet fuel) typeto a cosolvent type solution. Nevertheless, CP results showthat blends with as little as 0.10 vol. frac. SME increased CPfrom 50C to 30C. This temperature corresponds to amaximum standard atmospheric altitude of 7000 m (Anon.,2000). Unless cold flow properties of biodiesel aresignificantly improved, BioJet fuel blends may limit aircraftoperation to lower altitudes where ambient temperaturesexceed the CP.

Effects of treating SME with 1000 ppm cold flowimprovers (before blending) on CP of 0.10 vol. frac. BioJetfuel blends are shown in figure 3. For both JP8 andJP8+100 based blends, additives decreased CP to the range35C to 37C, corresponding to a maximum standardatmospheric altitude of 8000 m (Anon., 2000). Testing ofneat (nonblended) JP8 and JP8+100 treated with1000 ppm of each additive showed no significant effect onCP. Therefore, the additives appeared to be beneficial withrespect to inhibiting nucleation and crystalline growth of themethyl ester molecules when blended with jet fuels.
https://www.researchgate.net/publication/226685806_Methyl_and_ethyl_soybean_esters_as_renewable_fuels_for_diesel_engines?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/226685806_Methyl_and_ethyl_soybean_esters_as_renewable_fuels_for_diesel_engines?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/225675548_Low-Temperature_Properties_of_Triglyceride-Based_Diesel_Fuels_Transesterified_Methyl_Esters_and_Petroleum_Middle_DistillateEster_Blends?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/225675548_Low-Temperature_Properties_of_Triglyceride-Based_Diesel_Fuels_Transesterified_Methyl_Esters_and_Petroleum_Middle_DistillateEster_Blends?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/226685806_Methyl_and_ethyl_soybean_esters_as_renewable_fuels_for_diesel_engines?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/226685806_Methyl_and_ethyl_soybean_esters_as_renewable_fuels_for_diesel_engines?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==


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1753Vol. 44(6): 17511757

Figure 1. Cloud point (CP, in C) of BioJet fuel blends. Legend: squares =JP8 blends; triangles = JP8+100 blends. Source for methyl soyate(SME): Ag Environmental Products.

Figure 2. Pour point (PP, in C) of BioJet fuel blends. Legend: squares =JP8 blends; triangles = JP8+100 blends. Source for SME: Ag Environ-

mental Products. See Fig. 1 for other abbreviations.

Figure 3. CP (C) of BioJet fuel blends prepared with 0.10 vol. frac. addi-

tivetreated SME. Additive loading = 1000 ppm (before blending). Sourcefor SME: Interchem. See figure 1 for abbreviations.

Previous CP studies with SME/D2 blends (Dunnet al.,1996)showed little or no reduction in CP following treatmentwith either DFI200 or Winterflow. On the other hand, thesame study reported that blends with D1, a fuel that closelyresembles jet fuel with respect to composition and physicalproperties, showed decreases in CP of up to 10C for blendratios of 0.10 vol. frac. SME.

Effects of using winterized SME on CP of 0.10 vol. frac.BioJet fuel blends are shown in figure 4. AEPSME was usedfor winterization of neat SME and InterchemSME forwinterization of SMEadditive (2000 ppm beforewinterization) mixtures. For blends with winterized neatSME, CP decreased to 47C in JP8 and 51C inJP8+100, values that appear to meet freezing pointspecifications for jet fuels. For blends with winterizedSMEDFI200, CP decreased to 40C and 42C for JP8and JP8+100, corresponding to maximum standard

atmospheric altitudes of 8900 m (Anon., 2000). For blendswith winterized SMEWinterflow, CP decreased to 36Cand 40C for JP8 and JP8+100. Comparison withcorresponding results in figure 3 suggests that, whilewinterization of SMEDFI200 offered some benefit forreducing CP, winterization of SMEWinterflow was onlybeneficial for SME/JP8+100 blends.

Blends with winterized neat SME had CP values that werelower than those with winterized SMEadditive mixtures.Prior to blending with jet fuel, the winterized SMEadditivemixtures each had CP = 11C, while winterized neat SMEhad CP = 12C. Given the aforementioned observation thatadditives had essentially no effect on CP of nonblended jetfuels, the results shown in figure 4 were puzzling.

Earlier winterization studies (Dunn et al., 1997) suggestedit is unlikely that fatty acid group profiles for winterizedSMEadditives mixtures differed significantly from that ofwinterized neat SME. It equally unlikely that the solubilitylimits for the additives in jet fuel solvent were exceededfollowing blending with winterized SMEadditivecomponent. Assuming no loss of additive during stepwisewinterization, the maximum additive loading for winterizedSMEjet fuel blends was approximately 250 ppm. Theaforementioned observation that additives had essentially noeffect on CP of nonblended jet fuels suggests that thesolubility limit exceeds 1000 ppm unless it was decreased bypresence of SME. If this were the case, results in figure 3would be similar to those in figure 4. The effects of cold

improver additives on CP of BioJet fuels blended withwinterized as well as nonwinterized SME may merit furtherinvestigation.

Figure 4. CP (C) of BioJet fuel blends prepared with 0.10 vol. frac. win-terized SME. Additive loading = 2000 ppm (before winterization).Sources for SME: Ag Environmental Products for winterized SME(neat); Interchem for winterized SMEadditive. See figure 1 for abbrevi-ations.
https://www.researchgate.net/publication/225514083_Improving_the_low-temperature_properties_Of_alternative_diesel_fuels_Vegetable_oil_derived_methyl_esters?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/225514083_Improving_the_low-temperature_properties_Of_alternative_diesel_fuels_Vegetable_oil_derived_methyl_esters?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/225514083_Improving_the_low-temperature_properties_Of_alternative_diesel_fuels_Vegetable_oil_derived_methyl_esters?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==https://www.researchgate.net/publication/225514083_Improving_the_low-temperature_properties_Of_alternative_diesel_fuels_Vegetable_oil_derived_methyl_esters?el=1_x_8&enrichId=rgreq-ab6c5c30-9b67-404b-9960-9ed2e620ca7a&enrichSource=Y292ZXJQYWdlOzI3MzIxNzUwNDtBUzoyNzE3NjEyOTYwNjQ1NTFAMTQ0MTgwNDM1MTk1Mw==


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Figure 5 is a plot comparing CFPP and CP results for0.10 vol. frac. BioJet fuels blended with untreated, additivetreated, or winterized SME. Leastsquares analysis yieldedthe following regression line:

CFPP = 0.7719 (CP) 10.2 (1)

where R2= 0.92 and standard error of the yestimate () =1.5C. With respect to the regression line drawn in figure 5,winterized SME/JP8 + 100 blends appear to show the small-est deviation from the regression line. Comparing measured

CFPP with corresponding values calculated from equation 1,deviations were in the range 0.23.3C. Thus, as reportedin earlier studies with biodiesel blends in D2 and D1 (Dunnand Bagby, 1995; Dunn et al., 1996), CFPP of BioJet fuelsmay be estimated from CP measurements.

Cold filter plugging point (CFPP) is a method originallydeveloped to test the rate at which a fluid passes undervacuum through a wire filter screen. The minimum operatingtemperature for an aircraft should be warmer than thetemperature where solid crystals plug or restrict flow throughfilters in the fuel system. The CFPPapparatus employed inthis work had a 45 m wiremesh screen. Results in figure 5and equation 1 indicate that filters this size will allow flowat temperatures equal to or less than CP as long as the CP of

the fuel is 45C or higher. The CFPP apparatus is designedto allow varying wiremesh sizes, making it adaptable to testlowtemperature filterability for a range of fuel filter types.

KINEMATICVISCOSITIESThe of JP8 should not exceed a value of 8.0 mm2/s

when measured at 20C (U.S. DOD, 1992). Results intable 1 indicate that 0.10 vol. frac. BioJet fuels blended withuntreated, additivetreated, or winterized SME do notexceed this limit. Results show that regardless of whether andhow SME was winterized prior to blending, values wereapproximately 1 mm2/s greater than those for nonblendedJP8 or JP8+100.

FLASHPOINTSAlso shown in table 1 are closedcup FP results for BioJet

fuels blended with 0.10 vol. frac. (nonwinterized) SME. Ingeneral, blending jet fuel with biodiesel increases FP by

Figure 5. Leastsquares fit of cold filter plugging point (CFPP, in C) ver-sus CP (C) data for 0.10 vol. frac. BioJet fuel blends. Legend: squares =additivetreated SME blends; triangles = winterized SME blends; closedsquares and closed triangles = JP8 blends; open squares and open

triangles = JP8+100 blends. See figure 1 for other abbreviations.

Table 1. Kinematic viscosities () at 20C and closedcup flash points (FP) of 0.10 vol. frac. blends.[a]

Ester Jet fuel Additive[b]

(mm/s)FP

(C)

None JP8 None 5.0 64.5

None JP8+100 None 5.1 56.2

SME JP8 None 5.8 74.6

SME JP8 DFI200 5.8 59.8

SME JP8 Winterflow 5.8 56.1

SME JP8+100 None 5.8 63.8

SME JP8+100 DFI200 6.0 55.9

SME JP8+100 Winterflow 6.0 55.9

Winterized SME JP8 None 5.8[c]

Winterized SME JP8 Winterflow 5.8

Winterized SME JP8 DFI200 5.9

Winterized SME JP8+100 None 6.0[c]

Winterized SME JP8+100 Winterflow 6.0

Winterized SME JP8+100 DFI200 6.0

[a] Source for SME: Interchem, unless noted otherwise. Variances for data


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1755Vol. 44(6): 17511757

decreases in the aqueous layer of at least 3.80 mL, includinga decrease of 6.00 mL for neat JP8+100.

According to the fuel specification for JP8 (U.S. DOD,1992), qualitative interface rating results should be no worsethan 1b. With the exception of neat SME, all blends listedin table 2 met this criterion. On the other hand, each of thefour blends (0.100.50 vol. frac. SME in JP8) yielded aseparation rating of (3) due to formation of cloudysuspension in the oil layer. Similar tests (not shown) onJP8+100 blends resulted in formation of emulsions in the

fuel layer, leading to separation ratings no better than (3)for these blends.Results from water penetration following 28 d exposure of

equivalent volumes of SME/JP8 blends (0.00, 0.10, 0.20,0.30, and 1.00 vol. frac. SME) and aqueous phase are shownin table 3. Only one blend, 0.10 vol. frac. SME at 20C, gavea moisture content exceeding the maximum (0.050 wt%)allowed by the provisional fuel specification for biodiesel,PS121 (ASTM, 2000c). Neat SME samples also yieldedmoisture contents in excess of this maximum limit(0.1420.203 wt%) regardless of temperature. These resultsshow that at low blend levels, exposure of BioJet fuel blendsto a bulk aqueous phase for an extended period results in verylittle absorption of moisture into the bulk oil phase.

It is known that oxidation degradation during storage canincrease the acid value of biodiesel, leading to formation ofwaterinoil emulsions. It is likely that a substantial increasein free fatty acid content in biodiesel would be necessary tocause formation of emulsions in 0.100.30 vol. frac. blendswith jet fuel. Hence, it may be speculated that formation ofemulsions will be checked unless the total acid number of theblend exceeds the maximum value (0.015 mg KOH/g)allowable with respect to the fuel specification for JP8 (U.S.DOD, 1992). This speculation was confirmed whenlongterm exposure to nonbuffered aqueous phase resultedin no visible indications of emulsions for any of the oilsamples studied in this work.

STORAGESTABILITYStorage stability, particularly with respect to oxidative

degradation, is a concern for longterm storage of biodieselblended with petroleum middle distillate fuels includingthose used in aviation applications.

When biodiesel is in contact with ambient air for anextended period of time, it reacts with the oxygen present andundergoes degradation. Factors that influence oxidativedegradation include temperature, presence or absence oflight, storage container material, and presence of pro andantioxidizing contaminants (du Plessis et al., 1985;

Table 3. Karl Fischer titration of InterchemSME/JP8 blendsafter 28 d exposure to equivalent amount of distilled,

deionized water (10 mL oil + 10 mL H2O).[a]

Percent moisture (wt)[b]

en rat o

(vol. frac. SME) 10C 20C 30C

0.00 0.040 0.069 0.035

0.10 0.011 0.060 0.019

0.20 0.014 0.033 0.020

0.30 0.024 0.027 0.022

1.00 0.203 0.152 0.142

[a] See figure 1 for abbreviations.[b] Data are mean of two replicate measurements.

Bondioli et al., 1995; Thompson et al., 1998). Extended deg-radation can positively or negatively affect fuel quality of jetfuels with respect to total acid number, specific gravity, ce-tane number, viscosity and heat of combustion.

One earlier study (Dunn, 1998) reported results from oilstability index (OSI, related to induction period)measurements of SME and SME/D1 blends. These studieswere performed in accordance with American Oil ChemistsSociety method Cd 12b92 (AOCS, 1993) modified for ablock temperature of 50C. For neat SME, OSI = 13.6 hours

(h), while winterization reduced OSI to 6.89.5 h. On theother hand, a 0.20 vol. frac. blend of SME with D1 increasedOSI to 77.0 h, while a 0.30 vol. frac. winterized SME blendwith D1 gave OSI = 51.7 h. Similar testing of nonblendedD1 resulted in OSI values exceeding the limit (500 h)imposed by the experimental apparatus. Given thesimilarities between D1 and jet fuels, BioJet fuel blends maybe expected to have a relatively robust oxidative stability.

COMPARISONOFBIODIESELANDJP8 FUELSPECIFICATIONS

Although corresponding ASTM fuel specifications forbiodiesel and jet fuels are similar in many respects, there areseveral notable differences. With respect to acid

neutralization, the specification for JP8 (U.S. DOD, 1992)gives a maximum total acid number of 0.015 mg KOH/g oilwhile the provisional specification for biodiesel (ASTM,2000c) gives a maximum acid number of 0.80 mg KOH/g oil.Thus, some care should be taken during processing andstorage of biodiesel prior to blending it with jet fuel.

As stated above, jet fuels have a maximum freezing pointmeasured by ASTM method D2386 (ASTM, 2000b)requirement of 47C (U.S. DOD, 1992). Although biodieselmay be tested for CP by the customer, PS121 (ASTM, 2000c)specifies no maximum value because this provisionalspecification was developed with the philosophy thatbiodiesel would be primarily applied as a fuel extender.Nevertheless, corresponding methods for measuring freezing

point and CP require very different test conditions. Forfreezing points, mixtures are stirred constantly and samplesare first tested by cooling in 1C increments until cloudinessis observed, then removed from the cooling bath andretested in 0.5C increments while warming up; themeasured freezing point is taken as the temperature wherecloudiness disappears. For CPs, mixtures remain quiescentand samples are tested during cooling in 1C increments; CPis taken as the temperature where cloudiness is first observedin the sample.

The third notable difference in ASTM guidelines betweenJP8 and biodiesel lies in how heats of combustion aremeasured and reported. For biodiesel as well as D1 and D2,grossheats of combustion, defined as the heat released by one

unit mass of fuel in a constant volume bomb withsubstantially all of the water condensed to the liquid state(ASTM, 2000a), are quantified. Clark et al.(1984) reportedgross heats of combustion of 39.8 MJ/kg for SME and45.2 MJ/kg for D2. Given that D1 as a lighter distillatefraction typically has a smaller heat content than D2,biodiesel should compare more favorably with jet fuelsbecause these fuels have composition and physical propertiessimilar to those of D1. To effectively compare heat contents,the netheat of combustion of biodiesel should be determined.Net heat of combustion is defined as the heat released by


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combustion of one unit mass of fuel at a constant pressure of1 atm. (0.1 MPa) with the water remaining in the vapor state(ASTM, 2000a). Jet fuel specification MILT83133D (U.S.DOD, 1992) reports a net heat of combustion of 42.8 MJ/kgfor JP8.

Another deviation between fuel guidelines is inmeasurement of Custrip corrosion properties (ASTMD130). For JP8, samples are tested for 2 h at 100C (U.S.DOD, 1992), while biodiesel samples are tested for 3 h at50C (ASTM, 2000c).

Finally, with respect to gum formation, the maximumvalue for existent gum is 7.0 mg/100 mL for JP8 (U.S. DOD,1992). There is no similar maximum value specified forbiodiesel. Clark et al. (1984) have reported a gum number of16,400 mg/100 mL for SME. Another factor in formation ofgums in biodiesel is the presence of glycerol, a coproductfrom transesterification of the parent oil. Glycerol in the fuelcan clog filters. For this reason, PS121 (ASTM, 2000c)specifies a maximum total glycerol content of 0.240 wt% forbiodiesel. Overall, care should be taken during processing,transportation, and storage of biodiesel to exclude excessiveamounts of gums and glycerine.

CONCLUSIONS BioJet fuel blends with SME will require significant

reduction in freezing point to allow aircraft operation athigher altitudes. BioJet with 0.10 vol. frac. SME will belimited to altitudes of 7000 m based on standardatmospheric conditions.

BioJet fuels blends with SMEadditive mixtures yieldedCP reductions of 69C for 0.10 vol. frac. blends.Additives (at 1000 ppm) did not significantly affect CP ofneat (nonblended) JP8 and JP8+100 fuels.

BioJet fuel blends with winterized SME yieldedsubstantial CP reductions. In some cases, BioJet fuelsgave CPs below 47C, the specified maximum freezingpoint for JP8.

BioJet fuels blends with winterized SMEadditivesshowed CP reductions to as low as 42C. This reductionmight allow aircraft operating altitudes to increase to8900 m based on standard atmospheric conditions.

BioJet fuels with up to 0.30 vol. frac. SME may beformulated without compromising viscosity (measured at20C), with respect to jet fuel specifications.

BioJet fuels tend to have elevated FPs with respect tononblended jet fuels. On the other hand, addition of coldflow improvers such as DFI200 or Winterflow decreasedFP.

BioJet fuels with up to 0.50 vol. frac. SME in JP8indicated very little propensity to absorb water frombuffered solutions. Although water reactivity interfaceratings were 1b or better for blends, separation ratingswere rated at (3) due to formation of a cloudysuspension in the oil layer. Blends in JP8+100 exhibitedhigher degrees of waterabsorption and tended to formemulsions.

Longterm exposure to an equivalent volume ofnonbuffered aqueous phase yielded very littlemeasurable water penetration in blends with up to0.30 vol. frac. SME in JP8.

RECOMMENDATIONSThe studies reported herein are preliminary and should be

followed up with a more extensive evaluation to confirm thefeasibility of applying biodiesel in alternative jet fuel blends.Examples of future studies within this context include thefollowing: Testing for freezing point (ASTM D2386) of neat

biodiesel and BioJet fuel blends including comparisonwith CP and CFPP results such as those reported in thiswork.

Develop lowtemperature filterability tests to evaluatefuels with respect to filter screens common in aircraft fuelsystems.

Oxidative and thermal stability testing of BioJet fuels topredict effects of longterm storage degradation. Thismay require development of test methods including thoserequiring accelerated experimental conditions.

Testing for Custrip corrosion (D 130) of BioJet fuelsunder conditions stipulated for aviation fuels (2 h, 100C).

Completion of these studies should provide a basis for devel-oping a database on fuel properties of BioJet fuel blends.Once the database has been established, the next step shouldinitiation of performance and emissions testing of BioJet fuelblends in jet turbine engines.

ACKNOWLEDGEMENTSH. Khoury, B. Mernick, A. Callison, and D. Ehmke

provided technical assistance for experimental studies andanalyses. J. Cummings, Naval Air Systems Command(Patuxent River Naval Air Station, MD), provided jet fuelspecification data. L. A. Dockman, Cardinal Aircraft Corp.(Townsend, MD), provided advice and guidance fordesigning the studies behind this work.

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Methods and Practices of the AOCS, Vol. 1. Champaign, Ill.:American Oil Chemists Society.

ASTM. 1995a. D93. Standard test method for flash point byPenskyMartins closed cup tester. InAnnual Book of ASTMStandards, Vol. 05. West Conshohocken, Pa.: American Societyfor Testing and Materials.

_____. 1995b. D97. Standard test method for pour point ofpetroleum products. InAnnual Book of ASTM Standards, Vol.05. West Conshohocken, Pa.: American Society for Testing andMaterials.

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_____. 2000a. D240. Standard test method for heat of combustionof liquid hydrocarbon fuels by bomb calorimeter. InAnnual

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Clark, S. J., L. Wagner, M. D. Schrock, and P. G. Piennar. 1984.Methyl and ethyl soybean esters as renewable fuels for dieselengines.J. Am. Oil Chem. Soc.61(10): 16321638.

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