269

14Jet Fuel

14.1Introduction

Aviation turbine fuel, or jet fuel, was introduced during the Second World War for militaryaircraft and its use has since become widespread for both military and civilian aircraft. Itdisplaced aviation-gasoline (Section 13.4) as the main fuel type for aircraft, as the power sourceof most aircraft changed from spark-ignition engines to turbine engines.

Jet fuel is a kerosene range fuel. Since jet fuel is used in a turbine engine, it requiresvery different fuel properties than aviation-gasoline. Combustion is not initiated in a closedcombustion chamber, but must be sustained in an open chamber. The energy content andcombustion quality of the fuel are key performance properties. The jet fuel is directly andcontinuously combusted with compressed air in a combustion chamber and the hot gasesare used to drive a turbine. Poor combustion will not only lead to energy loss and highhydrocarbon emissions, but also to the generation of particulate matter that can damage theturbine.

Poor engine performance in an aircraft has more disastrous consequences than poor engineperformance during road transportation. Furthermore, aircraft are not restricted to a singlecountry. The specifications for jet fuel are therefore mostly of an international nature, with-out much of the local variations in legislation seen for motor-gasoline and diesel fuel. Forthe same reasons, it is difficult to make changes to jet fuel specifications, and these speci-fications are not subject to political pressures in the same way as motor-gasoline and dieselfuel are.

From a refining point of view, jet fuel is an easy fuel to produce. It has less risk of significantspecification changes, but it has a smaller market base. Depending on the refinery design, thekerosene range material used for jet fuel can be blended with road transportation fuels andfluctuations in jet fuel demand can be easily absorbed.

The structure and aim of this chapter is similar to that of the chapters dealing withmotor-gasoline (Chapter 13) and diesel fuel (Chapter 15). The jet fuel specifications will berelated to the molecular requirements of the fuel and the demands placed on Fischer–Tropschrefineries. More detailed descriptions of jet fuels can be found in the literature [1, 2].

Fischer–Tropsch Refining, First Edition. Arno de Klerk. 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

270 14 Jet Fuel

14.2Jet Fuel Specifications

The jet fuel originally used for turbine engines was illuminating kerosene, which was producedfor wick lamps. It was initially thought that turbine engines are relatively insensitive to fuelproperties and kerosene was chosen because of its availability. Later, a wider cut that includedsome lighter material was used, but it was abandoned for general use because of greaterevaporative losses at high altitude and the safety risks involved in dealing with such a volatilefuel.

The British DEF STAN 91-91 (previously DERD 2494) [3] is widely recognized as the interna-tional standard to specify the civil aviation turbine fuel Jet A-1 (Table 14.1). Country-specific jet fuelspecifications are closely aligned with the Jet A-1 specification published in the DEF STAN 91-91.

The United States still uses Jet A for national air travel, which is defined by the ASTM D 1655standard [4]. The main difference between Jet A and Jet A-1 is in the freezing point requirement.

Table 14.1 Selected specifications for civilian aviation turbine fuels.

Property Jet A-1 Jet A Jet B

Net heat of combustion (MJ kg−1), min. 42.8 42.8 42.8Density at 15 ◦C (kg·m−3) 775–840 775–840 751–802Freezing point (◦C), max. −47 −40 −50Vapor pressure (kPa), max. – a – a 21Flash point (◦C), min.b 38 38 – a

Viscosity at −20 ◦C (cSt), max. 8 8 – a

Smoke point (mm), min. 25c 25c 25Existing gums (mg/100 ml), max. 7 7 7Lubricity, BOCLE (mm), max.b 0.85d – a – a

Composition, max.Aromatic content (vol%)b 25 25 25Naphthalene content (vol%)e 3 3 3Sulfur content (mass%) 0.3 0.3 0.3Thiol content (mass%) 0.003 0.003 0.003Acid content (mg KOH/g) 0.1 0.1 – a

Distillation (◦C), max.b

IBP Report – a – a

T10 205 205 – a

T20 – a – a 145T50 Report Report 190T90 Report Report 245FBP 300 300 – a

aNot limited by the specification.bDifferent specification requirements for Fischer–Tropsch-derived synthetic jet fuel.cRequirement can be lowered to 19 mm (Jet A-1) or 18 mm (Jet A) if used in conjunction with naphthalene content.dOnly a requirement if the jet fuel contains >95% hydroprocessed material.eNot a separate requirement if the smoke point is higher than 25 mm.

14.2 Jet Fuel Specifications 271

Jet A has a maximum freezing point specification of −40 ◦C, whereas for Jet A-1 it is −47 ◦C,which makes Jet A-1 more suitable for long international flights. The wider cut, more volatile JetB is used only in Arctic regions, mainly due to its better cold-flow properties.

The specifications listed in Table 14.1 do not include all the stability and additive requirements.Additives are strictly regulated. In Jet A-1, antioxidants are required in any fuel composition thathas been hydroprocessed. The approved antioxidants for aviation fuel are hindered phenols witha maximum allowable concentration of 24 mg·l−1. The addition of a metal deactivator is allowed,and the only approved metal deactivator is N,N′-disalicylidene-1,2-propane diamine.

Jet fuel additives are a very important part of producing final on-specification jet fuel. Thesemolecules are added during blending and do not affect jet fuel refining. The limited discussionis therefore not a reflection on their importance; additives are crucial, but it is a reflection ontheir impact on refinery design.

14.2.1Synthetic Jet Fuel

The marketing effort to differentiate Fischer–Tropsch-derived fuels from crude-oil-derived fuelshad some unintended consequences for jet fuel derived from syncrude. The perception wascreated that the molecules in syncrude are somehow different from their counterparts in crudeoil.

Although the jet fuel refined from HTFT syncrude falls well within the composition andspecification limits defined for crude-oil-derived jet fuel, it required a lengthy qualificationprocess before it was allowed for use in jet fuel. In addition to the specifications for Jet A-1refined from conventional crude oil, Jet A-1 refined from syncrude had to meet a subset of morestringent requirements (Table 14.2) [3].

The ‘‘marketing’’ differentiation of synthetic jet fuel had a knock-on effect on the ASTM D 1655specification also [4]: ‘‘Jet fuels containing synthetic hydrocarbons have been previously allowedunder Specification D 1655. However, the fraction of these hydrocarbons was not limited, and

Table 14.2 Supplementary civil aviation turbine fuel specifi-cations specific to synthetic jet fuels containing material thatwere obtained from Fischer–Tropsch synthesis.

Property Jet A-1 specifications

Semisynthetic Fully synthetic

Fischer–Tropsch material (vol%), max. 50 100Flash point (◦C) – a 38–50Lubricity, BOCLE (mm), max. 0.85 0.85Aromatic content (vol%) 8–25 8–25Distillation (◦C), min.

(T50 – T10) – a 20(T90 – T10) – a 40

aNo additional specification restrictions beyond that required for conventional Jet A-1.

272 14 Jet Fuel

there were no requirements or restrictions placed on either these hydrocarbons or the final blend.It has been recognized that synthetic blends represent a potential departure from experienceand from key assumptions on which the fuel property requirements . . . have been based.’’ TheASTM D 1655 specification was aligned with the DEF STAN 91-91, and the potential use ofnon-Sasol Fischer–Tropsch-derived jet fuel as civil aviation turbine fuel was effectively scuttled.

The first synthetic jet fuel blend to be qualified as fuel for civilian aircraft was a semisyn-thetic jet fuel. This blend was allowed to contain at most 50% material derived fromFischer–Tropsch synthesis. In fact, the DEF STAN 91-91 specification is explicit in limitingthe Fischer–Tropsch-derived material in semisynthetic jet fuel to hydrogenated oligomers fromthe Sasol Synfuels facility (Figure 9.11). These hydrogenated oligomers, which are also callediso-paraffinic kerosene (IPK), are obtained from solid phosphoric acid-catalyzed oligomerization ofC3 –C4 HTFT condensate followed by hydrogenation over a sulfided base metal catalyst. Since thisqualification process was completed before 2004, many of the subsequent change to the refinery(Section 9.5) altered the composition of the IPK. The DEF STAN 91-91 specification was alsoexplicit in prohibiting synthetic aromatics in this material. It is not clear why these restrictionswere imposed, since there are no obvious technical grounds for this. In fact, considering thestricture of the specification, it is not clear whether future semisynthetic jet fuel production bySasol Synfuels is technically in violation of the DEF STAN 91-91 specification due to changes inrefinery configuration and operation.

Fully synthetic jet fuel has been qualified for use since April 2008 [3]. As in the case of thesemisynthetic jet fuel, the composition of fully synthetic jet fuel is limited specifically to a blendof light distillate, heavy naphtha, and IPK (Figure 9.12). The light distillate and heavy naphthastreams are hydrotreated, straight-run HTFT products. Again, the technical grounds for thestricture are not clear.

The qualification process nevertheless highlighted some important requirements that shouldhold true for synthetic and crude-oil-derived jet fuel. These are as follows:

1) The jet fuel must have a reasonably wide and even boiling point distribution. The temptationto selectively remove certain carbon numbers as chemicals and the selective addition of largevolumes of narrow boiling range materials must be avoided.

2) Fuel systems that were exposed to aromatics cannot be reliably used with aromatics-freefuels. There must be a minimum amount of aromatics.

3) Synthetic jet fuel is naturally more refined and attention must be paid to lubricity, which istypically destroyed during hydroprocessing.

14.2.2Fuel for Military Use

Military aircraft rely on the supply logistics of their country or alliance of origin. In this respect,such aircraft are not subject to the same need for international specifications as civilian aircraft.Different subspecifications may therefore exist to accommodate specific military applications(Table 14.3) [2].

Fischer–Tropsch-derived fuels, with their associated supply security independent of crude oil,are attractive for military applications. The concept of a single Battlefield Use Fuel of the Future(BUFF) is also a way to simplify supply logistics. Since kerosene is mainly a light distillate

14.3 Jet Fuel Properties 273

Table 14.3 United States military jet propulsion (JP) avia-tion turbine fuels and their corresponding maximum freezingpoint and minimum flash point specifications.

Fuel Freezing point (◦C) Flash point (◦C) RVP (kPa) Fraction Comments

JP-1 −60 43 – Kerosene Obsolete, introduced in 1944JP-2 −60 – <14 Wide cut Obsolete, introduced in 1945JP-3 −60 – 34–48 Wide cut Obsolete, introduced in 1947JP-4 −72 – 14–21 Wide cut Air Force fuel (Jet B analog)JP-5 −46 60 – Kerosene Navy fuel (aircraft carrier use)JP-6 −54 – – Kerosene Obsolete, XB-70 program in 1956JP-7 −43 60 – Kerosene Lower volatility, higher thermal stabilityJP-8 −47 38 – Kerosene Air Force fuel (Jet A-1 analog)

RVP, Reid vapor pressure.

fraction, a paraffinic kerosene can in principle be refined to meet specifications for both turbineengines and compression-ignition engines [5].

No standard has been agreed upon for such a BUFF, but it is clear that Fischer–Tropsch-derivedkerosene fractions may be well suited for such an application. A number of studies have reportedefforts to refine Fischer–Tropsch syncrude and coal liquids to JP-5 type jet fuels [6–10].

14.3Jet Fuel Properties

The composition of a large number of kerosene blends, mostly in the 190–230 ◦C boiling range,were compared to jet fuel properties routinely used to specify jet fuel quality [11]. The kerosenefractions were obtained from different crude oils and coal liquefaction. This work is remarkable inthat it was able to clearly show which compound classes are required to produce on-specificationjet fuel.

The compound classes were lumped into three main groups: the n-alkanes [n], the branchedalkanes and cycloalkanes [BC], and the aromatics [Ar]. Each of the jet fuel properties waslinearly correlated to the jet fuel composition expressed in terms of the mass fraction of theaforementioned three compound classes (Equation 14.1).

Jet fuel property = a1[n] + a2[BC] + a3[Ar] + a4 (14.1)

These correlations are helpful in understanding the relationship between jet fuel properties andthe composition of the blend. Using these relationships, it is possible to construct a diagramthat identifies the compositional space allowed by jet fuel specifications for synthetic Jet A-1(Figure 14.1).

The analysis has one inherent shortcoming when applied to Fischer–Tropsch-derived syn-thetic jet fuel, which relates to the lumping of branched alkanes and cycloalkanes as the[BC] group. The cycloalkanes have comparable freezing points to the branched alkanes, but

274 14 Jet Fuel

[BC] [Ar]

[n]

Minimum aromatics

Maximum aromatics

Maximum freezingpoint

Maximum density

Smoke pointOn-specificationsynthetic Jet A-1

Minimum density

Figure 14.1 Relationship between jet fuel composition andthe synthetic jet fuel specification domain. The compositionis defined by grouping the n-alkanes as [n], the branchedalkanes and cycloalkanes as [BC], and the aromatics as [Ar].

cycloalkanes on average have a 70–80 kg·m−3 higher density than the branched alkanes. Forcrude-oil- and coal-liquid-derived jet fuels, which contain a significant amount of cycloalkanes,the minimum density specification is not constraining (Figure 14.1), but for Fischer–Tropschliquids that may not be the case. Nevertheless, it does not detract from the analysis by Cooksonet al. [11].

14.3.1Net Heat of Combustion

The net heat of combustion is the amount of energy released when the fuel is completely oxidizedto produce carbon dioxide and water vapor. The standard method to determine the net heat ofcombustion is the ASTM D 4809 [12], which employs oxidation in a bomb calorimeter.

It should be noted that the aviation turbine fuel specification places a limit on the min-imum gravimetric energy content, not the volumetric energy content (Table 14.1). A jet fuelwith a low density and that is rich in alkanes will have a high gravimetric energy content.Fischer–Tropsch-derived jet fuels fit this description well and will easily meet the energy specifi-cation. Conversely, a fuel with a high density that is rich in aromatics will have a high volumetricenergy content but low gravimetric energy content.

In practice, there is a trade-off between using a fuel with a high gravimetric energy content asopposed to a fuel with a high volumetric energy content.

14.3 Jet Fuel Properties 275

1) Military applications. The fuel tank space in an aircraft is limited, which imposes a volumetricconstraint on flight range. Carrying fuel with a high volumetric energy content (high density)will improve the flight range for a given tank capacity. In military applications, a fuel withhigh volumetric energy content is consequently desirable. The extreme case is jet fuel formissiles, where a very dense fuel is beneficial. For example, JP-10 has an energy density of39.4 MJ·l−1 (For Jet A-1, it is typically around 35.1 MJ·l−1) [1].

2) Civilian applications. Selecting a fuel for commercial aircraft involves more trade-offs.Although the fuel tank capacity on civilian aircraft is also limited, aircraft usually take ononly enough fuel to reach their destination, with some additional fuel to give it an adequatesafety margin. Selecting a fuel with a high gravimetric energy content (low density) ismore efficient, because the energy weighs less and fuel tank capacity is not constraining.However, fuel is sold volumetrically. This implies that a jet fuel with high gravimetric energycontent costs more for the same energy content as a jet fuel with high volumetric energydensity. As a consequence, in most cases a fuel with high volumetric energy density (highdensity) is preferred, but airliners seldom have the luxury to pick the energy content of theirfuel.

14.3.2Density and Viscosity

The fuel viscosity and, to a lesser extent, the fuel density influence the spray pattern and dropletsize of the fuel when it is injected under high pressure into the combustion chamber of a turbineengine. The fuel system is designed to produce a fine spray that can easily evaporate when it ismixed with the hot air from the compression section.

Density is not really an independent variable. The jet fuel specifications allow a wide densityrange, and in practice density is not controlled, but a result of the limitations placed on thefuel composition. Fischer–Tropsch-derived materials produce jet fuels on the lower end of thedensity scale, mainly due to its high alkane and low cycloalkane content.

The spray pattern and the droplet size distribution are important to the performance of aturbine engine. When the viscosity is increased and the density is decreased, it will change thedroplet size distribution to produce larger droplets [13]. If the droplets are too large it affects theperformance of the engine. Larger droplets also pose a safety risk, because it makes it difficult torelight an engine in flight after flameout has occurred. (Flameout occurs when the air velocityexceeds the flame propagation velocity.)

Another effect of higher viscosity is to increase the pressure drop in the fuel lines. This isdetrimental to aircraft operation for a number of reasons. The pump duty required to maintaina constant fuel flow is increased if the viscosity is too high, and the fuel pump may not beable to supply the engine with the required amount of fuel. High viscosity will also reduce thecooling efficiency of the jet fuel as heat exchange fluid on aircraft (Section 14.3.7). The maximumviscosity in jet fuel is consequently regulated by specification.

On the lower end of the viscosity scale, too low a viscosity is not good either. The hydrodynamiclubricity of a fuel is influenced by its viscosity. A jet fuel with too low a viscosity may causeexcessive wear on pump parts and flow control units. Although there is no minimum limit forviscosity; the distillation profile effectively sets a lower viscosity limit.

276 14 Jet Fuel

Materials with high density and molecular mass usually have a higher liquid viscosity. Theheavy end of the distillation profile strongly influences the viscosity, and the inclusion of materialthat has a boiling point higher than 260 ◦C is limited by the viscosity requirements.

14.3.3Freezing Point Temperature

The freezing point is defined as the highest temperature at which all compounds in the fuelare still in the liquid phase. The primary reference method for freezing point determination ofaviation fuel is ASTM D 2386 [14], but other more modern methods may also be used.

Since the jet fuel is a mixture of many compounds, the fuel mixture does not become a solid atthe freezing point. As the temperature is lowered below the freezing point of the fuel, compoundswill start crystallizing out of solution to create a slush of fuel and solid hydrocarbons. This affectsthe ability of the fuel to be moved from the fuel tanks to the engine. Jet fuel remains pumpableonly to between 4 and 15 ◦C below its freezing point [1].

Fischer–Tropsch syncrude is rich in linear materials, which have high freezing points(Table 14.4). In order to meet the freezing point specification of jet fuel, the straight-run kerosenefraction of syncrude must be hydroisomerized (Chapter 18). Paraffinic kerosene suitable for jetfuel can also be produced from lighter material by oligomerization (Chapter 19) followed byhydrogenation, or from heavier material by hydrocracking (Chapter 21).

14.3.4Aromatic Content and Smoke Point

Fuel components that have a tendency to form carbonaceous particles during the early stagesof combustion should be avoided. In the event that such particles are formed, these particulatesmust be completely consumed before leaving the combustion chamber.

The formation of such carbonaceous particles in the combustion chamber is detrimentalin two ways. Firstly, the particles become incandescent at the high temperature and pressureconditions of the combustion chamber, which can cause hot spots on the chamber wall dueto the high additional heat transfer rate. This may cause cracks and lead to premature enginefailure. The particles can also block the holes in the combustion chamber wall that supply air tothe combustion section, thereby disrupting the flow pattern. Secondly, if these particles are notconsumed in the combustion chamber, they impinge on the turbine blades and stators, causingerosion of the turbine section [1]. Such particles are also responsible for visible smoke.

Aromatic compounds and especially naphthalenic compounds are more prone to the formationof such particles than aliphatic hydrocarbons. Both the total aromatic content and the totalnaphthalenic content of jet fuel are therefore regulated by the aviation turbine fuel specifications.The smoke point is a measure of the tendency of a fuel to form particles (black smoke) duringcombustion, and the method for its determination is described by the ASTM D 1322 standardtest method [15]. It is a simple test wherein the fuel is burned in a wick-fed lamp, and the smokepoint is the maximum flame height that can be achieved without smoke being formed.

The aromatic content of Fischer–Tropsch syncrude is low, and even in the heaviest aromaticfraction of HTFT syncrude the dinuclear aromatic content is low. Fischer–Tropsch-derived jetfuels are inherently more hydrogen-rich, have a high smoke point, and burn clean. In comparison

14.3 Jet Fuel Properties 277

Table 14.4 Selected physical properties of C9 –C15 hydrocarbons.

Compound Formula Boiling point (◦C) Density at 15 ◦C Freezing point (◦C)(kg·m−3)

n-Alkanesn-Nonane C9H20 150.8 721.9 −53.5n-Decane C10H22 174.2 734.2 −29.6n-Undecane C11H24 195.9 744.5 −25.6n-Dodecane C12H26 216.3 752.7 −9.6n-Tridecane C13H28 235.2 761.7 −5.4n-Tetradecane C14H30 253.8 763.3 5.9n-Pentadecane C15H32 270.7 772.2 9.9

Branched alkanes2-Methyloctane C9H20 143.3 717.7 −80.42-Methylnonane C10H22 167 730.6 −74.72,5-Dimethyloctane C10H22 158 737a −84.52-Methyldecane C11H24 189.9 736.9a −49.52,3-Dimethylnonane C11H24 186.8 747.1a −117.72-Methylundecane C12H26 210.2 745.6a −462,5-Dimethyldecane C12H26 198.1 747a −60.42-Methyldodecane C13H28 229.4 753.3a −262,4-Dimethylundecane C13H28 216.8 754.6a −75.52-Methyltridecane C14H30 247.4 760.4a −26.52,3-Dimethyldodecane C14H30 245.9 768.4a −57.92-Methyltetradecane C15H32 264 766a −8.92,4-Dimethyltridecane C15H32 250 767.1a −44

Cycloalkanesn-Propylcyclohexane C9H18 156.7 793.6a −94.9cis-1,2-Diethylcyclopentane C9H18 153.6 800.4 −118cis-Decalin C10H18 195.8 901.8 −43n-Butylcyclopentane C10H20 156.6 810.3 −108n-Butylcyclohexane C10H20 177 799a −79n-Pentylcyclohexane C11H22 203 804a −57.5n-Hexylcyclohexane C12H24 224 808a −43n-Heptylcyclohexane C13H26 244.9 811.2 −30.5n-Octylcyclohexane C14H28 263.6 817.2 −19.7

Aromaticsn-Propylbenzene C9H12 159.2 868.3 −99.5Cumene C9H12 152.4 868.5 −96n-Butylbenzene C10H14 183.3 866 −88sec-Butylbenzene C10H14 173.3 866.2 −75.5p-Cymene C10H14 177.1 860.7 −67.9p-Diethylbenzene C10H14 183.8 866.3 −42.8n-Pentylbenzene C11H16 205.4 862.9 −75n-Hexylbenzene C12H18 226.1 862.1 −61n-Heptylbenzene C13H20 246.1 860.8 −48n-Octylbenzene C14H22 264.4 860.2 −36

aDensity reported at 20 ◦C.

278 14 Jet Fuel

with crude-oil-derived JP-8 (Jet A-1 type) jet fuel, Fischer–Tropsch-derived synthetic jet fuel andblends thereof have considerably lower particulate matter emissions [16].

In general, the combustion quality of jet fuel is related to its hydrogen content, andin Fischer–Tropsch-derived jet fuels the hydrogen content is usually higher than mostcrude-oil-derived jet fuels. A high smoke point and low particulate emissions are thereforeto be expected on the basis of the high hydrogen content.

14.3.5Sulfur and Acid Content

The limitation on the sulfur content in jet fuel is mostly to reduce SOx emissions, but it hasthe added advantage of reducing microbial growth during fuel storage. A key aspect to limitingbiological activity is limiting free water, but reducing elemental nutrients like sulfur also helps.

In terms of fuel quality, the limitation placed on the thiol (mercaptan) content is more crucial.Thiols and organic acids can cause corrosion of some engine and fuel system components andare therefore limited by jet fuel specifications.

Fischer–Tropsch syncrude is sulfur-free but contains carboxylic acids. The acid content can bereduced by hydrotreating (Chapter 16).

14.3.6Volatility

The volatility of jet fuel is regulated by specification of the distillation profile. In the caseof synthetic jet fuel, this specification is more stringent (Table 14.2). The distillation profileindirectly controls other properties also, such as the minimum viscosity and combustion quality.

By specifying a minimum slope for the distillation profile of synthetic jet fuel, the gradualevaporation of the fuel over the whole boiling range is ensured. By doing so, it reduces the risk offlash evaporation associated with narrow boiling synthetic compounds, which would cause theformation of fuel-rich and air-poor pockets in the turbine engine.

14.3.7Stability

There are two stability aspects to consider for jet fuel. The first is storage stability, which refers tothe ability of the fuel to resist autoxidation during storage. The second is thermal stability, whichrefers to the ability of the fuel to resist the formation of particulates, gums, and deposits whenthe fuel comes into contact with hot surfaces.

The thermal stability of jet fuel is very important, since the fuel serves as a heat exchangemedium in the engine and airframe to cool engine oil, hydraulic fluid, and air conditioningequipment. It is a key fuel property, especially in high-performance aircraft, such as used by themilitary. Thermal stability is measured by the ASTM D 3241 standard test method [17].

Fischer–Tropsch hydrocarbons generally improve the thermal stability of jet fuel blendsunder mild oxidative stress conditions [18]. However, this beneficial effect is dependent on theamount of prestressing. Prestressing reduces the amount of antioxidants in the fuel, which leadsto faster subsequent oxidation [19]. Phenol (not hindered alkyl phenols) has been implicated

14.3 Jet Fuel Properties 279

in the formation of deposits during thermal stability testing, and a mechanism involvingphenol oxidation was proposed [20] that described the observations with various crude oil andFischer–Tropsch semisynthetic jet fuel mixtures reasonably well [18].

One of the advantages of Fischer–Tropsch-derived synthetic jet fuel for naval aviation is thevery low copper migration [8]. This is a direct consequence of the low sulfur level in Fischer–Tropsch-derived jet fuel. Copper ions are not the most efficient ions to catalyze hydroperoxidedecomposition [21–23], but by reducing the dissolution of copper, fuel stability is improved.

The contamination of syncrude with dissolved iron, either from the Fischer–Tropsch catalystor from carboxylic acid corrosion, must be prevented. Even at very low levels, iron can degradethe stability of jet fuel [24]. In this respect, cobalt is an even more potent autoxidation catalyst [23].Metal contaminants from Fischer–Tropsch syncrude must consequently be avoided in syntheticjet fuel production.

14.3.8Elastomer Compatibility and Lubricity

During normal operation with crude-oil-derived jet fuel, the elastomeric materials in contact withthe fuel absorb some of the jet fuel components, which results in seal swelling. This is a naturaland expected consequence of exposing the elastomers to the fuel. It is therefore not an issue thathas to be addressed by the jet fuel specifications.

Elastomer swelling also takes place while in contact with synthetic jet fuels. However, theamount of swelling in the presence of crude-oil-derived jet fuel and a paraffinic fuel can be quitedifferent [25, 26]. When an aircraft switches from conventional to synthetic jet fuel, or vice versa,it is important that the elastomeric materials retain their degree of swelling. When the degree ofelastomeric swelling changes, it can cause leaks and this must be avoided.

Semisynthetic jet fuel, irrespective of whether it is produced from IPK, avoids elastomercompatibility problems by blending with conventional jet fuel. In a more general sense, thisholds true for semisynthetic jet fuel blends with any paraffinic Fischer–Tropsch-derived material.Although the DEF STAN 91-91 specifications recognize only HTFT-derived IPK, in this regardthere is little difference between HTFT-derived IPK and an analogous IPK derived fromhydrocracked LTFT material.

The qualification of fully synthetic jet fuel from HTFT syncrude (Section 14.2.1) demonstratedthat a synthetic jet fuel that naturally contains aromatic compounds does not result in elastomercompatibility problems. The minimum aromatic specification in synthetic jet fuel is to ensure elas-tomer compatibility. However, it was pointed out that it is not only the aromatic content but alsothe presence of some oxygenates in low concentration that is needed for the aromatics to achievethe required volume swell [25]. Crude-oil-derived jet fuel contains low concentrations of sulfurcompounds that perform the same function. Severely hydrotreated jet fuels may have differentelastomer swelling characteristics, even though they contain percentage levels of aromatics.

The heteroatom-containing compounds in jet fuel, irrespective of their origin, have an-other important function. Polar compounds provide boundary layer lubrication. In the case ofFischer–Tropsch-derived jet fuel, the low levels of oxygenates remaining in the jet fuel afterrefining are likely to be responsible for improved lubricity and elastomer swelling characteristics[27]. Severe hydrotreatment can remove these oxygenates and lead to poor lubricity as well asproblems with differential elastomer swelling, despite the presence of aromatics.

280 14 Jet Fuel

The inclusion of a lubricity specification for severely hydrotreated conventional jet fuels andsynthetic jet fuels is therefore understandable. This is a lesson that was learnt the hard way withdiesel fuels (Section 15.3.4). Lubricity is determined by the ball-on-cylinder lubricity evaluator(BOCLE), as outlined in the ASTM D 5001 standard test method [28].

14.4Future Jet Fuel Specification Changes

The nature of aircraft and jet engines, as well as the air transport industry in general, is suchthat changes in aviation turbine fuel specifications are difficult to make. There is a significantsafety aspect involved, and any change in jet fuel specification must be qualified on differentengine types before it can be accepted as safe for use. It is doubtful that significant changes inthe specification of Jet A-1 will be seen.

The most likely specification to come under scrutiny is the sulfur content. The specificationlimit of 0.3% is very high compared to the drive toward 0.001% in other transportation fuel types.For the crude oil refining industry, such a change will require a significant intervention, since alarge portion of the jet fuel is produced by sweetening, and not by hydrotreating.

Sweetening processes oxidatively convert thiols into disulfides and are not always followed bya disulfide extraction step. Even so, the sweetened kerosene will still contain other sulfur species,even when sweetening is followed by extraction. A different conversion step will be needed toremove the remaining sulfur.

Fischer–Tropsch-derived jet fuels are not affected by such a change and may even benefitindirectly. As the sulfur level in conventional jet fuels is decreased, the trace compound differencesbetween synthetic and conventional jet fuels will become smaller. This may pave the way for asingle jet fuel standard, irrespective of the refining origin.

Military applications of jet fuel may also help to unify jet fuel specifications. The developmentof BUFF is pertinent. Should the development of a BUFF be successful, it is possible that thequalifying tests done on military aircraft might give enough credence to also qualify such fuelsfor civilian aircraft. This in turn could provide the impetus to align the Jet A-1 specifications withBUFF specifications.

It is not yet clear whether bio-derived fuels will try to make inroads in the jet fuel market. Withsufficient political support this may well happen and, if it does, it may also catalyze a unificationof Jet A-1 specifications for conventional and synthetic fuel types.

References

1. Bacha, J., Barnes, F., Franklin, M., Gibbs, L.,Hemighaus, G., Hogue, N., Lesnini, D., Lind, J.,Maybury, J., and Morris, J. (2000) Aviation Fuels,Chevron, San Ramon, CA.

2. Bishop, G.J. (2008) in Handbook of Fuels (ed. B.Elvers), Wiley-VCH Verlag GmbH, Weinheim,pp. 321–341.

3. UK Ministry of Defence (2008) Turbine Fuel,Aviation Kerosine Type, Jet A-1. NATO Code:

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