WO2013012983A1

WO2013012983A1 - Aviation gas turbine fuel with improved low temperature operability

Info

Publication number: WO2013012983A1
Application number: PCT/US2012/047294
Authority: WO
Inventors: Paul P. Wells; Krystal B. Wrigley; Dennis Harold Hoskin; Roger Grant Gaughan
Original assignee: Exxonmobil Research And Engineering Company
Priority date: 2011-07-20
Filing date: 2012-07-19
Publication date: 2013-01-24
Also published as: US20140007498A1; CA2842193A1; EP2734607A1; SG10201605846XA

Abstract

The addition of biodiesel to petroleum-based kerosene jet fuels in very low concentrations can lower the temperature at which crystals appear in the fuel The fuels can comprise a blend of a hydrocarbon base fuel component and, for example, up to 1000 ppm, v/v of the total fuel, of a biodiesel component comprising a lower alkyl ester of a fatty acid of natural origin having from 8 to 24 carbon atoms; these blends can be characterized by improved low temperature flow properties, especially of Cloud Point (ASTM D 2500), which can be lower than that, of the petroleum fuel component without the alkyl ester, even in the presence of dissolved water up to the saturation level.

Description

AVIATION GAS TURBINE FUEL WITH IMPROVED LOW

TEMPERATURE OPERABILITY

FIELD OF THE INVENTION

[0001] This invention relates to fuels useful in aviation gas turbine engines and more particularly, to aviation gas turbine fuels with improved low temperature operability characteristics.

BACKGROUND OF THE INVENTION

[0002] Kerosene type fuels (kerojet) are well-established for use in aviation gas turbines, typically going under the designations such as Jet A, Jet A- l, JP-5, JP-8, NATO F-34, or NATO F-44. The various specifications impose a number of different requirements on the respective fuels, including flash point, distillation, maximum aromatics (coupled with Smoke Point), viscosity (-20°C), sulfur content, net heat of combustion, and density. Aircraft which fly at high altitude or encounter extremely cold environments have the potential for fuel freezing and, consequently, catastrophic failure of the fuel system; the low temperature properties of the fuel are therefore important factor in acceptability, and a number of tests have been developed to quantify the behavior of kerosene fuels at the low temperatures typically encountered in aviation use. The Cloud Point (ASTM D 2500) test measures the temperature at which paraffin crystals start to precipitate and therefore provides an indication of the temperature at which incipient system plugging problems arise. Low Cloud Point is therefore highly desirable in a kerosene jet fuel.

[0003] In recent years, energy crises and their consequent increase in prices have led to increased interest in alternative fuels, for example, Fischer- Tropsch liquids, coal liquefaction products, and bio-derived fuels. One type of bio-fuel which has received significant interest in recent years as a possible alternative to conventional petroleum diesel is biodiesel, and this has also been considered as an alternative aviation turbine fuel, in view of its character similar to petroleum-derived middle distillates, such as petroleum diesel (petrodiesei). Biodiesel is generally taken to be an ester produced by the transesterifieation of triglycerides found in naturally occurring oils and fats.

Various crops and animal sources are used in different parts of the world, depending on local availability: soybean oil is widely used in the U.S. and in Europe; and rapeseed oil and palm oil or coconut oil in Asia. Tallow (animal fat ) is also a source of triglycerides which have been converted to lower alkyl esters for biodiesel. Mixtures of oil may also be used. The lower alcohol normally used to effect the transesterifieation is methanol although other alcohols, such as ethanol, propanol, butanol, etc., may also be used. During the transesterifieation, the triglycerides of the long chain fatty acids in the natural oil are converted to fatty acid esters of the lower alcohol used in the esterification process with glycerol as a by-product. The fatty acid methyl ester products are generally referred to as FAMEs (Fatty Acid Methyl Esters) and include such classes as tallow methyl esters (TME), soybean methyl esters (SME), rapeseed methyl esters (RME), and palm oil methyl esters (PME).

[0004] Studies have been made on the use of biodiesel as an alternative jet fuel, and it has been concluded that, although significant technical and logistical hurdles need to be overcome, the task is not insurmountable and no single issue makes biofuel unfit for aviation use ("Alternative Fuels and Their Potential Impact on Aviation", NASA Report TM-2006-214365, Daggett et a!.). The cost of bio-derived fuels is a major logistical consideration: the 2003 report from the Imperial College Centre for Energy Policy and Technology ("The Potential for Renewable Energy Sources in Aviation", Saynor et ah, with rapeseed methyl esters projected to cost from US$33.5/GJ to USS52.6/GJ as compared to approximately US$4.6/GJ for petroleum kerojet. The greatest technical problem, as noted in the NASA report, with biodiesel is its need for warm temperatures: at normal flight temperatures, bio-derived jet fuel tends to freeze, and, for this reason, the amount of bio-derived fuel that can be blended with petroleum- based fuels is normally limited, typically to 10-20% of the blend. Blended with kerosene, biodiesel is known to raise the fuel's cloud point significantly. According to the Imperial College report, the addition of just 10 wt% biodiesel to kerojet raises the cloud point from -51°C to -29°C, a level which is unacceptable for the military JP-8, which requires fuels to operate at -47°C.

[0005] The presence of water in the fuel affects the Cloud Point, since not only do wax crystals appear at the Cloud Point but also ice crystals are prone to precipitate at even higher temperatures. A great deaf of care is therefore taken to ensure petroleum products are transported throughout the distribution system as dry as possible and that they are essentially dry when loaded onto the plane. For example, warm, potentially wet product from the refinery is allowed to cool and settle before transport, thereby reducing the amount of dissolved and/or finely dispersed free water. Storage tanks are regularly sumped to remove any water bottoms. Floating suction is an industry best practice and helps ensure dry product enters the distribution system. Even with these procedures in place, dissolved water can and does come out of solution as ambient temperature is reduced. The resulting free water can then cause corrosion, encourage microbiological growth, or freeze and block downstream fuel filters. Water is normally removed by passing the jet fuel through filter/coal escer and separator systems, normally a filter/coal escer cartridge and a separator cartridge specified by API/IP 1581 3^ld edition or 5^th edition (Category C, Category M, or Category M100) at several points in the fuel distribution system, usually at least when the fuel is transferred into and out of airport storage facilities. Mo-plane jet fuel water content standards are either 15 ppm v/v (ATA- 103) or 30 ppm v/v (LATA), as cited in the airline operator's handling standards, where ATA- 103 is commonly cited in the U.S. and IATA elsewhere (outside the former Soviet Union and China).

[0006] Anti-icing agents are currently used to improve the low temperature performance by reducing the incidence of solids formation. For example, di- ethylene glycol nionomethyl ether (DiEGME) is added to the military jet fuel JP-8. DiEGME, however, is expensive, added at 0.15 vol% (or 1 ,500 ppm v/v), and is incompatibile with filter monitors commonly used in the distribution system to remove free water from the fuel as it is loaded onto the plane.

[0007] Various proposals for improving the low temperature performance of bio- derived distillate range fuels have been made. U.S. Patent Application Publication No. 2006/0229222 relates to methods for improving the low temperature storage and performance properties of fatty acids and their derivatives, as well as of composition containing them, by the use of stabilizers selected from branched chain fatty acids, cyclic fatty acids, and polyamides. Jet fuels and diesel are mentioned as blend components for fatty acid compositions.

[0008] U.S. Patent Application Publication No. 2008/0163542 discloses blends of petroleum based fuels with renewable fuels to enhance the low temperature operability of the blends. Various performance indices, such as the Cold Filter Plugging Point, the Low Temperature Flow Test, Pour Point, and Cloud Point, are taken as measures of the low temperature performance characteristics of fuels such as kerosene-type aviation fuels, e.g., JP-5, JP-8, Jet A, and Jet A-l . The bio-derived component in the blend is stated to be no more than 50% v/v in typical cases and more typically up to 35%v/v; very low proportions down to 0.5% are mentioned but with no advantage shown for such blends.

[0009] U.S. Patent Application Publication No. 2010/0005706 discloses fuel oil compositions based on blends of renewable and petroleum fuels with additives to enhance the resistance to forming particulates during low temperature storage.

[0010] U.S. Patent Application Publications Nos. 2010/00058651 and

2011/0023352 disclose mixtures of fatty acid methyl esters useful as biofuels such as biodiesel and which are stated to have improved properties both at low and high temperatures.

[0011] Trial flights with blended jet fuels have been reported by commercial airlines including Air New Zealand, Japan Airlines, and military units such as the U.S. Air Force and U.S. Navy.

SUMMARY OF THE INVENTION

[0012] As noted above, biofuels, especially biodiesel are generally considered to create low temperature performance problems. We have now found, however, that the addition of a relatively small amount of biodiesel, a widely available and inexpensive material, to petroleum-based kerosene jet fuels can facilitate a lowering in the temperature at which crystals can appear in the fuel. This property is quite unexpected in view of the known propensities of these materials.

[0013] According to the present invention, petroleum-based aviation gas turbine fuels comprise a blend of a hydrocarbon base petroleum fuel component and a non-zero amount up to about 1 percent by volume of the total fuel, of a biodiesel comprising a lower alkyl ester of a fatty acid of natural origin having from 8 to 24 carbon atoms; these blends can advantageously be characterized by a Cloud Point (ASTM D 2500) lower than that of the petroleum fuel component without the alkyl ester.

[0014] The amount of the lower alkyl ester is usually less than the maximum 1 vol% noted herein, e.g. , from 10 to 1000 ppm v/v, from 10 to 500 ppm v/v, from 10 to 50 ppm v/v, from 50 to 500 ppm v/v, or in certain cases from 100 to 300 ppm v/v of the total fuel. The bulk of the fuel can thus typically comprise or be a conventional petroleum-based kerojet, e.g. , having a middle distillate boiling range (initial to final boiling point or optionally either a T2 to T98 range or a T5 to T95 range), such as from 180°C to 350°C.

[0015] The fuel can generally be manufactured to comply with established aviation turbine fuel standards according to the user and thus should generally meet the specifications for Jet A and/or Jet A-l (Def. Stan. 91-91 and/or ASTM D 1 6551 JP-5 (NATO F-44)(MIL-T-5624 U), and/or JP-8 (NATO F-35)(MLL-T-83133E). Free water {i.e., water present as discrete droplets within the fuel, with droplet size typically varying from ~1 micron to ~30 microns, depending upon the presence of optional surfactants) may be present in fuel when loaded on-plane in the amounts allowed by the relevant specification, e.g., either 15 ppm v/v under ATA -103 and/or 30 ppm v/v (IATA), according to the operator's handling standards, as determined by APL'EI 1581 (formerly API/IP 1581). GOST specifications can generally apply in Russia, China, and FSU countries. Surprisingly, it has been found that the addition of the exceedingly small amounts of biodiesel (e.g., as low as about 10 ppm v/v of a lower alky! ester) can result in a lowered temperature for crystal formation in the petroleum fuel component, even when the total amount of dissolved water approaches the saturation le vel (typically about 75 ppm v/v at ambient temperature) in the petroleum component.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] The major component of the present fuel blends can advantageously include or be a hydrocarbon middle distillate. Though typically the hydrocarbon middle distillate can be derived solely or mostly from mineral (non-renewable) sources (e.g., crude oil, shale oil, and the like), additionally or alternately at least a portion of the hydrocarbon middle distillate can be derived from a renewable (non-mineral) source (e.g., plants and/or animals, such as vegetables/crops, domesticated land/air animal, aquatic organisms including fish and/or algae, and/or other uni- or multi- cellular organisms capable of producing the appropriate hydrocarbon molecules and/or precursors). The hydrocarbon middle distillate can have certain desired boiling point characteristics, e.g., an initial to final boiling point range from 180°C to 350°C, respectively. In certain embodiments, though, the hydrocarbon middle distillate may have a relatively small proportion of molecules whose boiling points are outside that desired range, such that it may be more universally applicable to describe the Tl to T99 boiling point range, the T2 to T98 boiling point range, or the T5 to T95 boiling point range, which can collectively, or each individually, range from 180°C to 350°C. As used herein, the expression "T[number]", with reference to a composition, shall be understood to refer to a situation where approximately [number] percent by weight of the composition has a boiling point (under atmospheric pressure). For instance, a composition has a T10 boiling point of about 200°C if approximately 10% by weight of the composition has boiled at a temperature of about 200°C.

[0017] The volatility and other characteristics of the present fuel blends can be selected according to the fuel specification, e.g., by the specifications for Jet A, Jet A- l,JP-5, and/or JP-8, according to end user. Thus, for example, with Jet A and Jet A-l , a maximum T10 boiling point is typically 185°C, and a maximum final boiling (end) point is typically 340°C (Sim. Dis., ASTM D 2887). Also for Jet A and Jet A-l , the minimum Flash Point (ASTM D 56 or 3828) is typically 38°C, with similar specifications for JP-8. For JP-5, the maximum final boiling (end) point is typically 330°C (ASTM D 2887), and the minimum Flash Point is typically 60°C. The hydrocarbon middle distillate (fuel) component may be manufactured by any suitable (such as conventional) methods known in the art.

[0018] ^'The biodiesel component can be a renewable fuel of natural origin, typically having a middle distillate boiling range. As used herein, the phrase "of natural origin", with reference to a fatty acid herein, should be understood to mean not chemically synthesized by the hand of man. For clarification, regarding fatty acids, this means that the acyl portion of the fatty acid (i.e., the carbonaceous portion of the fatty acid, which constitutes all the carbons in the chain, including the carbon and oxygen atoms trom the carbonyl bond, optionally, but not necessarily, including the acid oxygen and hydrogen atoms), whether existing in a free acid, acid salt, and/or (tri-) glyceride form, originates from an organism. The organism may be either naturally occurring or genetically modified, naturally and/or by man's intervention, and still be considered "of natural origin", so long as the acyl portion of the fatty acid is produced by and/or through the organism. [0019] Biodiesel is described officially by the National Biodiesel Board (USA) according to ASTM D 6751 as a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats; European Standard EN 14214 describes the requirements and test methods for FAME biodiesel, Biodiesel is typically produced by a reaction of a vegetable oil or animal fat with a lower alcohol such as methanol or ethanol, optionally in the presence of a catalyst, to yield the desired lower mono-alkyl esters and glycerin, which can be advantageously removed as a by-product. As used herein, the term "lower", only as it refers to alcohols and alkyl esters, should be understood to mean 1 to 5 carbon atoms, for example 1 to 4 carbon atoms or 1 to 2 carbon atoms.

[0020] Biodiesel molecules can be blended with petroleum based diesel fuels for use in existing diesel engines, usually with little or no modification to the engine or fuel systems, and can thus be seen as distinct from the vegetable and waste oils that have been suitably modified to be used in diesel engines, Biodiesel molecules are also capable of being blended with conventional petroleum-derived kerojet to make a fuel useful in aviation gas turbines when the appropriate specifications are met.

[0021] The vegetable/crop oils conventionally used in the manufacture of biodiesel can vary, often according to local availability: rapeseed and soybean oils can be commonly used, with soybean oil alone accounting for about ninety percent of ail production in the U.S. Vegetable/Crop oils can additionally or alternately be obtained from corn, castor, olive, linseed, mustard, peanut, safflower, sunflower, and/or the like, as well as from other sources such as tall oil and/or field pennycress; in tropical zones, oils such as palm oil, coconut oil, hemp oil, honga oil, jatropha, and/or the like may be favored. Waste vegetable oil (WVG), or the oil remaining after use of the oil in food preparation, can additionally or alternately constitute a potential source. Animal fats, such as tallow (beef fat), lard (hog fat), chicken fat, duck fat, fish oil, and the like, are further additionally or alternately potential sources. The vegetable/crop oil sources can be preferred over land animal/fowl fat sources, for example, particularly in situations where low temperature properties are important. However, in situations where relatively small amounts of biodiesel component are used in the present blends, the relative impact of such molecules on the low temperature properties of the present blends can be small or insignificant, in which situations there may be no particular biodiesel source preference. Of course, combinations of various (natural) oils/fats are contemplated.

[0022] The selected (natural) oils/fats can be converted to their corresponding mono -esters, e.g., by a transesterification process using a lower alkanol as the esterifying agent. Methanol is normally preferred to make the meihyl esters of the fatty acid components (Fatty Acid Meihyl Ester - FAME), as it is the cheapest lower alcohol available, although ethanol can be used to produce an ethyl ester (Fatty Acid Ethyl Ester - FAEE) that can still be useful as biodiesel; higher alcohols, e.g., n-propanol, isopropanol, and/or butanols, even up to Ce or Cg alkanols, have also been used. Using increasingly higher carbon number alcohols can often improve the cold flow properties of the resulting alkyl ester, but generally at the cost of an increasingly less efficient and more costly transesterification reaction. Heat, as well as an acid or a base, can be used to catalyze the reaction. The predominant method for commercial-scale biodiesel is the base-catalyzed process, as it is seen as the most economical process for treating virgin vegetable oils, requiring relatively low temperatures and pressures and producing as high as 98+% conversion yield, if the starting oil is relatively low in moisture and free fatty acid content. Biodiesel produced from animal fats and other sources or by other methods may work better with acid catalysis, which can be slower.

[0023] The fatty acids from which preferred esters can be made can be saturated (containing no carbon-carbon double bonds) and/or unsaturated (containing one or more carbon-carbon double bonds) and can have acyl chain lengths (pre-esterification acid -equivalent numbers of carbons) ranging from 8 to 24 carbons, for example, 8 to 22 carbons, 10 to 22 carbons, 12 to 22 carbons, typically predominantly (i.e., more than 50% by weight) 12 to 18 carbons or 14 to 18 carbons. Non-limiting examples of fatty acids can include, but are not limited to, caprylic acid (C8:0), capric acid (C10:0), lauric acid (C 12:0), myristic acid (C14:0), palmitic acid (CI 6:0), palmitoleic acid (C 16: l ), sapienic acid (CI 6:2), stearic acid (CI 8:0), oleic acid (CI 8: 1 ), linoleic acid (C18:2), linolenic acid (CI 8:3), arachidic acid (C20:0), eicosenoic acid (C20: l), eicosadienoic acid (C2():2), mead acid (C20:3), arachidonic acid (C20:4),

eicosapentanoic acid (C20:5), behenic acid (C22:0) emcic acid (C22: l ), lignoceric acid (C24:0), nervonic acid (C24: l ), and the like, and obviously combinations thereof. Unsaturated acids, typically obtainable from canola, linseed, camelina, mustard, soybean oils, and/or other vegetable/crop oils can be preferred in certain embodiments, e.g., for their improved flow properties.

[0024] In the present fuel compositions, the biodiesel component can be advantageously used in relatively small (additive) amounts, but in amounts still sufficient to contribute a significant c flees on the low temperature flow properties of the combination/blend of the biodiesel component and the hydrocarbon (middle distillate) fuel component, especially with reference to the blend Cloud Point (ASTM D 2500), The addition of the biodiesel can advantageously have an insignificant, or no observable, effect on other low temperature properties, such as Freeze Point (ASTM D5972) and Cold Filter Plugging Point (ASTM D6371 ), e.g., maintaining the values of these within 2°C, typically within 1°C, of the respective low temperature property value for the same fuel/blend when no biodiesel is present. When biodiesel is present in the fuel/blend in amounts up to 1000 ppm v/v, the Cloud Point has been found to be at least 1°C lower than that of the unadditized composition, typically at least 3°C lower, for example at least 5°C lower, or at least 8°C lower, than for the same fuel/blend without the biodiesel component. This effect can be observed in certain circumstances even with significant amounts of dissolved water present, e.g., up to the saturation level of water in the fuel/blend (kerojet fuel saturated at room temperature at sea. level can hold -75 ppm v/v dissolved water in solution, which can decrease as

temperature/pressure decrease),

[0025] Additionally or alternately, the present invention can include one or more of the following embodiments.

[0026] Embodiment 1. A petroleum-based aviation gas turbine fuel comprising a blend of a hydrocarbon base petroleum fuel component and a sufficient non-zero amount, up to 1% by volume of the total fuel, of a lower alkyl ester of a fatty acid of natural origin having from 8 to 24 carbon atoms, such that the blend exhibits a Cloud Point (ASTM D 2500) at least 1 °C lower than that of the petroleum fuel component without the alkyl ester.

[0027] Embodiment 2. An aviation gas turbine fuel according to embodiment 1, in which one or more of the following is satisfied: the amount of the lower alkyl ester is from 10 to 1000 ppm, v/v of the total fuel; the amount of the lower alkyl ester is from 10 to 50 ppm, v/v of the total fuel; the amount of the lower alkyl ester is from 100 to 300 ppm, v/v of the total fuel; and the blend exhibits a Cloud Point (ASTM D 2500) at least 3°C lower than that of the petroleum fuel component wi thout the alkyl ester.

[0028] Embodiment 3. An aviation gas turbine fuel according to either of embodiments 1-2, in which the lower alkyl ester comprises one or more of the following: a methyl ester of a Cg to C₂₂ fatty acid of natural origin; a methyl ester of a C|2 to C22 fatty acid of natural origin; a methyl ester of a C₁₂ to C₁₈ fatty acid of natural origin; a methyl ester of a fatty acid derived from rapeseed oil; a methyl ester of a fatty acid derived from soybean oil; a methyl ester of a fatty acid derived from palm oil; a methyl ester of a fatty acid derived from tallow; and a blend of methyl esters of fatty acids derived from rapeseed oil, soybean oil, palm oil, and tallow.

[0029] Embodiment 4. An aviation gas turbine fuel according to any one of the pre vious embodiments, which conforms to one or more of the following specifications: Jet A (Def. Stan. 91-91); Jet A-l (ASTM D1655); JP-5 (MIL-T-5624 N); and JP-8 (MIL-T-83133C).

[0030] Embodiment 5. An aviation gas turbine fuel according to any one of the previous embodiments, which contains a total amount of water, wherein the petroleum component has a water saturation level , and wherein the total amount of water in the fuel is less than or equal to said water saturation level.

[0031] Embodiment 6. An aviation gas turbine fuel according to any one of the previous embodiments, which contains free water in an amount up to 30 ppm, v/v of the total fuel.

[0032] Embodiment 7. An aviation gas turbine fuel according to any one of the previous embodiments, wherein the fuel exhibits one or more of the following: a Freeze Point (ASTM 135972) within 1°C of a Freeze Point of the petroleum fuel component without the alkyl ester; a Cold Filter Plugging Point (ASTM D6371) within 1°C of a Cold Filter Plugging Point of the petroleum fuel component without the alkyl ester; and a Cloud Point (ASTM D2500) at least 5°C lower than a Cloud Point of the petroleum fuel component without the alkyl ester. EXAMPLES

Example 1

[0033] A dried kcrojet fuel (Jet A) was prepared by exposure to a 4A molecular sieve dessicant; this treatment has been shown to be very effective at removing solublized water. The Karl Fisher (D6304) water content of the dried Jet A sample was about 38 ppm w/w via Procedure A.

[0034] A sample of the Jet A fuel (not dried) was saturated by placing it in an epoxy-lined 5-US gallon (-19 L) can, containing a ~S L heel of distilled water, then sparging wet nitrogen over the headspace. Sparging was conducted overnight (for at feast '"10 hours) to fully saturate the sample.

[0035] The Cloud Points of the saturated fuel and the dried fuel were about -51°C and about -60°C, respectively. Even though the freezing point of water is significantly higher (~0°C), the water content apparently affected the results, causing Cloud Point results to be noticeably higher for the water-saturated fuel than for the dried fuel

[0036] To determine the effect of biodiesel concentration on low temperature operability, a sample of the saturated Jet A was additized with different concentrations (-50 ppm v/v, ~100 ppm v/v, -250 ppm v/v, and -400 ppm v/v) of a F AME mixture. The FAME mixture was prepared from equal volumes of soybean oil methyl ester (SME), rapeseed oil methyl ester (RME), palm oil methyl ester (PME), and tallow methyl ester (TME). The additized samples were then submitted for testing by the procedures of ASTM D2500 (Cloud Point), ASTM D5972 (Freeze Point), and ASTM D6371 (Cold Filter Plugging Point). The results are summarized in Table 1 together with the result of the dried fuel for comparison.

Table 1

* The ASTM method indicates that measurements should stop at -51°C; however, the samples in this Example were tested until CFPP was determined, for better comparison, even though these values required measuring CFPP at temperatures below -51°C.

[0037] Interestingly, it can be seen from Table 1 that the water content within the jet fuel clearly affected Cloud Point results with virtually no effect on the other two low- temperature properties measured, namely Freeze Point and CFPP. Indeed, addition of only -50 ppm v/v biodiesel appeared to be enough to engage the water and

significantly lower the cloud point of the fuel to a value comparable to that of the reference blend produced with dried fuel. Without being bound by theory, it appeared that the FAME component interacted somehow with whatever water was bound within the hydrocarbon jet fuel, with the end result being a measurably lower Cloud Point.

[0038] ASTM D2500-09 lists two different sets of repeatability (r) and

reproducibility (R) levels depending upon sample type (i.e., one set for petroleum products and another set for biodiesel blends). The levels are slightly greater for petroleum products (r = 2°C and R = 4°C). Applying these more stringent levels, the cloud point reproducibility bands for the Blend Nos. 1 and 2 might just intersect at about -55°C. The reproducibility bands for Blend Nos. 1 and 3, as well Blend Nos. 1 and 6, however, would not overlap.

Example 2

[0039] A similar series of samples was prepared using Jet -A in its "as-received" or "as is" condition, containing about 54 ppm w/^'w water by the Karl Fisher (D6304) Procedure A (representing less than the saturation amount of water, typically -75 ppm v/v or -93 ppm w/'w). The FAME additive was then added to the "as is" jet fuel in the same relative amounts as in Example 1, and the Freeze Point (D5972), Cloud Point (D2500), and Cold Filter Plugging Point (ASTM D6371) were determined. The results are given in Table 2 below^r.

Table 2

[0040] Again, for the most part, Cloud Point values for the blends appeared to be significantly reduced by the addition of the biodiesel component with virtually no effect on the other measured low temperature properties.

Example 3

[0041 ] The effect on the Cloud Point of adding relatively small levels of biodiesel to Jet-A was investigated by adding different concentrations (-10 ppm v/v, -20 ppm v/v, -30 ppm v/v, and -40 ppm v/v) of the FAME mixture of Example 1 to the saturated Jet A sample of Example 1. The results are shown in Table 3 below.

Table 3

[0042] The effect of biodiesel in lowering the Cloud Point of the fuel/blend was notable, even at very low levels (-10 to -50 ppm v/v) and even in the presence of significant amounts (saturated levels) of dissolved water (-75 ppm v/v). [0043] While the present invention has been described and illustrated by reterenc to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the trite scope of the present invention

Claims

1. A petroleum-based aviation gas turbine fuel comprising a blend of a hydrocarbon base petroleum fuel component and a sufficient non-zero amount, up to 1% by v olume of the total fuel, of a lower alkyl ester of a fatty acid of natural origin having from 8 to 24 carbon atoms, such that the blend exhibits a Cloud Point (ASTM D 2500) at least i°C lower than that of the petroleum fuel component without the alkyl este

2. An aviation gas turbine fuel according to claim 1, in which one or more of the following is satisfied: the amount of the lower alkyl ester is from 10 to 1000 ppm, v/v of the total fuel; the amount of the lower alkyl ester is from 10 to 50 ppm, v/v of the total fuel; the amount of the lower alkyl ester is from 100 to 300 ppm, v/v of the total fuel; and the blend exhibits a Cloud Point (ASTM D 2500) at least 3°C lower than that of the petroleum fuel component without the alkyl

3. An aviation gas turbine fuel according to either of claims 1 -2, In which the lower alkyl ester comprises one or more of the following: a methyl ester of a Cs to C-n fatty acid of natural origin; a methyl ester of a Cj ? to C₂₂ fatty acid of natural origin; a methyl es ter of a C12 to Cis fatty acid of natural origin; a methyl ester of a fatty acid derived from rapeseed oil; a methyl ester of a fatty acid derived from soybean oil; a methyl ester of a fatty acid derived from palm oil; a methyl ester of a fatty acid derived from tallow; and a blend of methyl esters of fatty acids derived from rapeseed oil, soybean oil, palm oil, and tallow.

4. An aviation gas turbine fuel according to any one of the previous claims, which conforms to one or more of the following specifications: Jet A (Def. Stan. 91 - 91); Jet A-i (ASTM 131655); JP-5 (MIL-T-5624 N); and JP-8 (MIL-T-83133C).

5. An aviation gas turbine fuel according to any one of the previous claims, which contains a total amount of water, wherein the petroleum component has a water saturation level, and wherein the total amount of water in the fuel is less than or equal to said water saturation level.

6, An aviation gas turbine fuel according to any one of the previous claims, which contains tree water in an amount up to 30 ppm, v/v of the total fuel.

7. An aviation gas turbine fuel according to any one of the previous claims, wherein the fuel exhibits one or more of the following: a Freeze Point (ASTM D5972) within 1°C of a Freeze Point of the petroleum fuel component without the alkyl ester; a Cold Filter Plugging Point (ASTM D6371) within 1°C of a Cold Filter Plugging Point of the petroieum fuel component without the alkyl ester; and a C₁₀ud Point (ASTM D2500) at least 5°C lower than a Cloud Point of the petroleum fuel component without the alkyl ester.