WO2006067234A1

WO2006067234A1 - Altering properties of fuel compositions

Info

Publication number: WO2006067234A1
Application number: PCT/EP2005/057159
Authority: WO
Inventors: Elizabeth Helen Crawshaw; Lisa Caroline Lilley; Beverly Van Sluis
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2004-12-24
Filing date: 2005-12-23
Publication date: 2006-06-29
Also published as: ZA200704741B; NO20073861L; MY153137A; EP1841840A1; JP5275631B2; US20060156619A1; JP2008525556A; CA2591826A1; BRPI0519634A2

Abstract

A method for increasing the cetane number of a diesel fuel composition which contains a major proportion of a diesel base fuel, in order to reach a target cetane number X, which method comprises adding to the base fuel an amount x of a fatty acid alkyl ester (FAAE) having a cetane number B greater than the cetane number A of the base fuel, wherein x is less than the amount of the FAAE which would need to be added to the base fuel in order to achieve cetane number X if linear blending rules applied; and the use of a FAAE in a diesel fuel composition for the purpose of reducing the level of cetane improving additives in the composition. The concentration of the FAAE in the overall fuel composition is preferably from 0.05 to 25% v/v.

Description

ALTERING PROPERTIES OF FUEL COMPOSITIONS

The present invention relates to a method for increasing the cetane number of a diesel fuel composition .

The cetane number of a fuel or fuel composition is a measure of its ease of ignition . With a lower cetane number fuel a compression ignition (diesel ) engine tends to be more difficult to start and may run more noisily when cold . There is a general preference, therefore, for a diesel fuel composition to have a high cetane number, and as such automotive diesel specifications generally stipulate a minimum cetane number . Many diesel fuel compositions contain cetane boost additives , also known as ignition improvers , to ensure compliance with such specifications . It is known to include fatty acid alkyl esters ( FAAEs ) , in particular fatty acid methyl esters , in diesel fuel compositions . An example of a FAAE included in diesel fuels is rapeseed methyl ester (RME) . FAAEs are typically derivable from biological sources and may be added for a variety of reasons , including to reduce the environmental impact of the fuel production and consumption process or to improve lubricity .

It is also well known that FAAEs often have higher cetane numbers than typical diesel base fuels . For example, the cetane number of soy methyl ester ( SME) is generally ~ 55 , whereas that for a typical European diesel base fuel is ~ 51-55.

Following conventional fuel formulation principles , it would be expected that the cetane number of a base fuel/FAAE blend would vary linearly with FAAE concentration . In other words , the addition of a FAAE to a base fuel having a lower cetane number would be expected to increase the cetane number of the fuel to an extent directly proportional to the amount of FAAE added .

It has now been surprisingly discovered that FAAEs can produce a non-linear change in cetane number when blended with diesel base fuels . Based on this discovery, the present invention is able to provide a more optimised method for increasing the cetane number of a diesel fuel composition to reach a particular target value .

According to a first aspect of the present invention there is provided a method for increasing the cetane number of a diesel fuel composition which contains a major proportion of a diesel base fuel , in order to reach a target cetane number X, which method comprises adding to the base fuel an amount x of a fatty acid alkyl ester ( FAAE) having a cetane number B greater than the cetane number A of the base fuel , wherein x is less than the amount of the FAAE which would need to be added to the base fuel in order to achieve cetane number X if linear blending rules applied .

As described above, if linear blending rules applied then the cetane number of the base fuel/FAAE mixture would vary linearly with FAAE concentration . If this were the case, it would then be straightforward to calculate the amount of any given FAAE needed to increase the cetane number of the base fuel to reach the target X . However, it has now been found that , in particular at lower concentrations , a FAAE can actually "boost" the cetane number of a diesel base fuel above the level that would be expected if linear blending rules applied . This allows a lower amount of FAAE to be used to achieve any given target X, thus lowering any costs or other detrimental effects associated with inclusion of the FAAE .

Since it may be desirable to add a FAAE to a diesel fuel composition for other reasons , for example to reduce its environmental impact ( including to reduce emissions ) and/or to improve its lubricity, the ability to use the FAAE for the additional purpose of increasing the cetane number can provide formulation advantages . Because the FAAE can cause an unexpectedly high cetane number, relatively small amounts may be used in some cases to replace, either partially or wholly, other cetane improvers which would otherwise be needed in the composition, thus reducing the overall additive levels in the composition and their associated costs .

In the present context , a "maj or proportion" of a base fuel means typically 80% v/v or greater, more suitably 90 or 95% v/v or greater, most preferably 98 or 99 or 99.5% v/v or greater . "Reaching" a target cetane number can also embrace exceeding that number .

The FAAE will typically be added to the fuel composition as a blend (i . e . a physical mixture) , conveniently before the composition is introduced into an internal combustion engine or other system which is to be run on the composition . Other fuel components and/or fuel additives may also be incorporated into the composition, either before or after addition of the FAAE and either before or during use of the composition in a combustion system. The amount of FAAE added will depend on the natures of the base fuel and FAAE in question and on the target cetane number . In general, the volume fraction v of FAAE in the resultant base fuel/FAAE mixture will be less than the volume fraction v ' which would be required if linear blending rules applied, wherein v ' would be defined by the equation :

X = A + v ' (B-A) . The volume fractions v and v ' must each have a value between 0 and 1. When carrying out the method of the present invention the actual volume fraction of FAAE, v, is preferably at least 0.02 lower than the "linear" volume fraction v ' , more preferably at least 0.05 or 0.08 or 0.1 lower, most preferably at least 0.2 , 0.3 or 0.5 lower and in cases up to 0.6 or 0.8 lower than v ^f . In absolute terms , the actual volume fraction v is preferably 0.25 or less , more preferably 0.2 or less , yet more preferably 0.15 or 0.1 or 0.07 or less . It may for example be from 0.01 to 0.25 , preferably from 0.05 to 0.25 , more preferably from 0.05 or 0.1 to 0.2.

Thus , as a result of carrying out the method of the present invention, the concentration of the FAAE in the overall fuel composition (or at least in the base fuel/FAAE mixture ) is preferably 25% v/v or less , more preferably 20% v/v or less , yet more preferably 15 or 10 or 7% v/v or less . As a minimum it may be 0.05% v/v or greater, preferably 1% v/v or greater, more preferably 2% or 5% v/v or greater, most preferably 7 or 10% v/v or greater .

Fatty acid alkyl esters , of which the most commonly used in the present context are the methyl esters , are already known as renewable diesel fuels ( so-called "biodiesel" fuels) . They contain long chain carboxylic acid molecules (generally from 10 to 22 carbon atoms long) , each having an alcohol molecule attached to one end . Organically derived oils such as vegetable oils (including recycled vegetable oils) and animal fats can be subj ected to a transesterification process with an alcohol (typically a C₁ to C₅ alcohol ) to form the corresponding fatty esters , typically mono-alkylated . This process , which is suitably either acid- or base- catalysed, such as with the base KOH, converts the triglycerides contained in the oils into fatty acid esters and free glycerol, by separating the fatty acid components of the oils from their glycerol backbone . In the present invention, the FAAE may be any alkylated fatty acid or mixture of fatty acids . Its fatty acid component ( s ) are preferably derived from a biological source, more preferably a vegetable source . They may be saturated or unsaturated; if the latter, they may have one or more double bonds . They may be branched or un-branched . Suitably they will have from 10 to 30 , more suitably from 10 to 22 or from 12 to 22 , carbon atoms in addition to the acid group ( s ) -CO₂H . A FAAE will typically comprise a mixture of different fatty acid esters of different chain lengths , depending on its source . For instance the commonly available rapeseed oil contains mixtures of palmitic acid (C₁₆) , stearic acid

(C₁₈ ) , oleic, linoleic and linolenic acids (C₁₈ , with one , two and three unsaturated carbon-carbon bonds respectively) and sometimes also erucic acid (C₂₂ ) ^~ of these the oleic and linoleic acids form the maj or proportion . Soybean oil contains a mixture of palmitic, stearic, oleic, linoleic and linolenic acids . Palm oil usually contains a mixture of palmitic, stearic and linoleic acid components .

The FAAE used in the present invention is preferably derived from a natural fatty oil , for instance a vegetable oil such as rapeseed oil , soybean oil, coconut oil , sunflower oil , palm oil , peanut oil , linseed oil , camelina oil, safflower oil, babassu oil, tallow oil or rice bran oil . It may in particular be an alkyl ester ( suitably the methyl ester) of rapeseed, soy, coconut or palm oil .

The FAAE is preferably a C₁ to C₅ alkyl ester, more preferably a methyl , ethyl or propyl (suitably iso- propyl ) ester, yet more preferably a methyl or ethyl ester and in particular a methyl ester .

It may for example be selected from the group consisting of rapeseed methyl ester (RME, also known as rape oil methyl ester or rape methyl ester) , soy methyl ester ( SME, also known as soybean methyl ester) , palm oil methyl ester ( POME) , coconut methyl ester (CME) ( in particular unrefined CME; the refined product is based on the crude but with some of the higher and lower alkyl chains (typically the C₆, C₈ , C₁₀ , C₁₆ and C₁₈ ) components removed) and mixtures thereof . In general it may be either natural or synthetic, refined or unrefined ("crude" ) .

The FAAE suitably complies with specifications applying to the rest of the fuel composition, and/or to the base fuel to which it is added, bearing in mind the intended use to which the composition is to be put ( for example, in which geographical area and at what time of year) . In particular, the FAAE preferably has a flash point ( IP 34 ) of greater than 101°C; a kinematic viscosity at 40⁰C ( IP 71) of 1.9 to 6.0 centistokes , preferably 3.5 to 5.0 centistokes ; a density from 845 to 910 kg/m³, preferably from 860 to 900 kg/m³, at 15⁰ C ( IP 365 , EN ISO 12185 or EN ISO 3675 ) ; a water content ( IP 386 ) of less than 500 ppm; a T95 (the temperature at which 95% of the fuel has evaporated, measured according to IP 123 ) of less than 360°C; an acid number ( IP 139 ) of less than 0.8 mgKOH/g, preferably less than 0.5 mgKOH/g; and an iodine number ( IP 84 ) of less than 125 , preferably less than 120 or less than 115 , grams of iodine ( I₂ ) per lOOg of fuel . It also preferably contains (eg, by NMR) less than 0.2% w/w of free methanol , less than 0.02% w/w of free glycerol and greater than 96.5% w/w esters . In general it may be preferred for the FAAE to conform to the European specification EN 14214 for fatty acid methyl esters for use as diesel fuels .

The measured cetane number of the FAAE (ASTM D613 ) is suitably 55 or greater, preferably 58 or 60 or 65 or even 70 or greater .

Two or more FAAEs may be added to the base fuel in accordance with the present invention, either separately or as a pre-prepared blend, so long as their combined effect is to increase the cetane number of the resultant composition to reach the target number X . In this case the total amount x ' of the two or more FAAEs must be less than the amount of that same combination of FAAEs which would need to be added to the base fuel in order to achieve the target cetane number X if linear blending rules applied for both or all of the FAAEs .

The FAAE preferably comprises ( i . e . either is or includes ) RME or SME .

The FAAE may be added to the fuel composition for one or more other purposes in addition to the desire to increase cetane number, for instance to reduce life cycle greenhouse gas emissions , to improve lubricity and/or to reduce costs . The cetane number of a fuel composition may be determined in known manner, for instance using the Standard test procedure ASTM D613 ( ISO 5165 , IP 41 ) which provides a so-called "measured" cetane number obtained under engine running conditions .

More preferably the cetane number may be determined using the more recent and accurate "ignition quality test ( IQT ) " (ASTM D6890 , IP 498/03 ) , which provides a "derived" cetane number based on the time delay between inj ection and combustion of a fuel sample introduced into a constant volume combustion chamber . This relatively rapid technique can be used on laboratory scale ( ca . 100ml ) samples of a range of different diesel fuels .

Alternatively, cetane number may be measured by near infrared spectroscopy (NIR) , as for example described in US-A-5349188. This method may be preferred in a refinery environment as it can be less cumbersome than for instance ASTM D613. NIR measurements make use of a correlation between the measured spectrum and the actual cetane number of the sample . An underlying model is prepared by correlating the known cetane numbers of a variety of fuel samples ( in this case, for example, diesel base fuels , FAAEs and/or blends thereof) with their near infrared spectral data . The method of the present invention preferably results in a diesel fuel composition which has a derived cetane number ( IP 498 /03 ) of 50 or greater, more preferably of 51 or 52 or 53 or greater .

The method of the present invention may additionally or alternatively be used to adj ust any property of the diesel fuel composition which is equivalent to or directly associated with cetane number . The diesel base fuel used in the composition may be any known diesel base fuel , and it may itself comprise a mixture of diesel fuel components . It will preferably have a sulphur content of at most 2000 ppmw (parts per million by weight ) . More preferably it will have a low or ultra low sulphur content, for instance at most 500 ppmw, preferably no more than 350 ppmw, most preferably no more than 100 or 50 or even 10 ppmw, of sulphur . The resultant mixture of the base fuel and the FAME will also preferably have a sulphur content within these ranges .

In some cases it may be preferred that the base fuel is not a sulphur free ( " zero sulphur" ) fuel .

Typical diesel fuel components comprise liquid hydrocarbon middle distillate fuel oils , for instance petroleum derived gas oils . Such base fuel components may be organically or synthetically derived . They will typically have boiling points within the usual diesel range of 150 to 400 ° C, depending on grade and use . They will typically have densities from 0.75 to 0.9 g/cm³ , preferably from 0.8 to 0.86 g/cm³, at 15 ° C ( IP 365 ) and measured cetane numbers (ASTM D613 ) of from 35 to 80 , more preferably from 40 to 75 or 70. Their initial boiling points will suitably be in the range 150 to 230 " C and their final boiling points in the range 290 to 400 °C . Their kinematic viscosity at 40⁰ C (ASTM D445 ) might suitably be from 1.5 to 4.5 centistokes .

Such fuels are generally suitable for use in a compression ignition (diesel ) internal combustion engine, of either the indirect or direct inj ection type . Again, the fuel composition which results from carrying out the present invention will also preferably fall within these general specifications . In particular, its measured cetane number will preferably be from 45 to 70 or 80 , more preferably from 50 to 65 or at least greater than 50 or even 53 or 55 or 57.

A petroleum derived gas oil may be obtained from refining and optionally (hydro) processing a crude petroleum source . It may be a single gas oil stream obtained from such a refinery process or a blend of several gas oil fractions obtained in the refinery process via different processing routes . Examples of such gas oil fractions are straight run gas oil , vacuum gas oil , gas oil as obtained in a thermal cracking process , light and heavy cycle oils as obtained in a fluid catalytic cracking unit and gas oil as obtained from a hydrocracker unit . Optionally a petroleum derived gas oil may comprise some petroleum derived kerosene fraction .

Such gas oils may be processed in a hydrodesulphurisation (HDS ) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition . In the method of the present invention, the base fuel may be or contain another so-called "biodiesel" fuel component, such as an alcohol (in particular methanol or ethanol ) or other oxygenate or a vegetable oil or vegetable oil derivative . It may be or contain a Fischer-Tropsch derived fuel , in particular a Fischer-Tropsch derived gas oil . Such fuels are known and in use in diesel fuel compositions . They are, or are prepared from, the synthesis products of a Fischer-Tropsch condensation reaction, as for example the commercially used gas oil obtained from the Shell Middle Distillate Synthesis (Gas To Liquid) process operating in Bintulu (Malaysia) . The diesel fuel composition which results from the method of the present invention may if desired contain no, or only low levels of, other cetane improving ( ignition improving) additives such as 2-ethylhexyl nitrate (EHN) . In other words , the present invention embraces the use of a FAAE in a diesel fuel composition for the purpose of reducing the level of cetane improving additives in the composition . As described above, the amount of FAAE used to achieve a given reduction in additive level will be less than the amount that would be necessary if linear blending of the FAAE and base fuel applied .

In this context , "use" of a FAAE in a fuel composition means incorporating the FAAE into the composition, typically as a blend ( i . e . a physical mixture) and optionally with one or more other fuel components ( such as diesel base fuels ) and optionally with one or more fuel additives . The FAAE is conveniently incorporated before the composition is introduced into an engine or other combustion system which is to be run on the fuel composition . Instead or in addition the use may involve running a diesel engine on the fuel composition containing the FAAE, typically by introducing the composition into a combustion chamber of the engine .

The term "reducing" embraces reduction to zero ; in other words , the FAAE may be used to replace one or more cetane improving additives either partially or completely. The reduction may be as compared to the level of the relevant additive ( s ) which would otherwise have been incorporated into the fuel composition in order to achieve a desired target cetane number, for instance in order to meet government fuel specifications or consumer expectations . Thus the FAAE can help in reducing the overall additive levels in the composition and their associated costs .

Preferably the FAAE is used to reduce the w/w concentration of the relevant additive ( s ) in the fuel composition by at least 10% , more preferably by at least 20 or 30% , yet more preferably by at least 50 or 70 or 80 or even 90% or, as described above, by 100% .

It may for instance be used to replace cetane improving additive ( s ) to an extent that the concentration of cetane improving additives remaining in the fuel composition is 300 ppmw or less , preferably 200 ppmw or less , more preferably 100 or 50 ppmw or less . Most preferably it may be used to replace cetane improving additive ( s ) substantially entirely, the fuel composition being nearly or essentially free of such additives and containing for example 10 or 5 ppmw or less, preferably 1 ppmw or less , of cetane improving additives .

(All additive concentrations quoted in this specification refer, unless otherwise stated, to active matter concentrations by mass . The term "cetane improving additive" refers to additives , other than the FAAE, which can increase the cetane number of a fuel or otherwise improve its ignition quality . ) Subj ect to the above, a diesel fuel composition prepared according to the present invention may contain other components in addition to the diesel base fuel and the FAAE . Typically such components will be incorporated in fuel additives . Examples include detergents such as polyolefin substituted succinimides or succinamides of polyamines , for instance polyisobutylene succinimides or polyisobutylene amine succinamides , aliphatic amines , Mannich bases or amines and polyolefin (e . g . polyisobutylene ) maleic anhydrides . Succinimide dispersant additives are described for example in GB-A-960493 , EP-A-0147240 , EP-A-0482253 , EP-A-0613938 , EP-A-0557516 and WO-A-98 /42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides .

The additives may contain other components in addition to the detergent . Examples are lubricity enhancers such as amide-based additives ; dehazers , e . g . alkoxylated phenol formaldehyde polymers ; anti-foaming agents (e . g . polyether-modified polysiloxanes ) ; anti-rust agents (e . g . a propane-1 , 2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative , the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms , e . g . the pentaerythritol diester of polyisobutylene- substituted succinic acid) ; corrosion inhibitors ; reodorants ; anti-wear additives ; anti-oxidants (e . g . phenolics such as 2 , β-di-tert-butylphenol , or phenylenediamines such as N, N ' -di-sec-butyl-p- phenylenediamine) ; metal deactivators ; and combustion improvers . Where the fuel composition contains one or more ignition improvers ( cetane improvers ) , these may be selected from for example 2-ethylhexyl nitrate (EHN) , cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in US-4208190 at column 2 , line 27 to column 3 , line 21.

The fuel composition will suitably contain only a minor amount of such additives . Thus unless otherwise stated, the (active matter) concentration of each such additional component in the overall fuel composition is preferably up to 1% w/w, more preferably in the range from 5 to 1000 ppmw, advantageously from 75 to 300 ppmw, such as from 95 to 150 ppmw . It is particularly preferred that a lubricity enhancer be included in the fuel composition, especially when it has a low (e . g . 500 ppmw or less ) sulphur content . The lubricity enhancer is conveniently present at a concentration from 50 to 1000 ppmw, preferably from 100 to 1000 ppmw, based on the overall fuel composition . Additives may be added at various stages during the production of a fuel composition; those added at the refinery for example might be selected from anti-static agents , pipeline drag reducers, flow improvers (e . g . ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers ) and wax anti-settling agents . When carrying out the method of the present invention, the diesel base fuel may already contain such refinery additives . Other additives may be added downstream of the refinery .

The method of the present invention may form part of a process for, or be implemented using a system for, controlling the blending of a fuel composition, for example in a refinery . Such a system will typically include means for introducing a FAAE and a diesel base fuel into a blending chamber, flow control means for independently controlling the volumetric flow rates of the FAAE and the base fuel into the chamber, means for calculating the volume fraction of the FAAE needed to achieve a desired target cetane number input by a user into the system and means for directing the result of that calculation to the flow control means which is then operable to achieve the desired volume fraction in the product composition by altering the flow rates of its constituents into the blending chamber .

In order to calculate the required volume fraction, a process or system of this type will suitably make use of known cetane numbers for the base fuel and FAAE concerned, and conveniently also a model predicting the cetane number of varying concentration blends of the two according to linear blending rules . The process or system may then, according to the present invention, select and produce a FAAE volume fraction lower than that predicted by the linear blending model to be necessary . It may use a so-called quality estimator which will provide , using a model , a real-time prediction of the cetane number of each resulting blend from available raw process measurements , such as for example the NIR measured cetane numbers and the volumetric flow rates of the constituents . More preferably such a quality estimator is calibrated on-line by making use of, for example, the method described in WO-A-02/06905. The method of the present invention may thus conveniently be used to automate, at least partially, the formulation of a diesel fuel composition, preferably providing real-time control over the relative proportions of the FAAE and base fuel incorporated into the composition, for instance by controlling the relative flow rates of the constituents .

In accordance with the present invention, "use" of a FAAE in the ways described above may also embrace supplying a FAAE together with instructions for its use in a diesel fuel composition to increase the cetane number to a particular target and/or to improve the ignition quality of the composition and/or to reduce the level of cetane improving additives in the composition . The FAAE may be supplied as a component of a formulation suitable and/or intended for use as a diesel fuel additive, in which case the FAAE may be included in the formulation for the purpose of influencing its effects on the ignition quality of a diesel fuel composition .

A second aspect of the present invention provides a diesel fuel composition prepared, or preparable, using a method according to the first aspect . This composition contains a maj or proportion of a diesel base fuel which preferably has a low sulphur content ( for example, less than 400 or preferably 300 ppmw) and/or a measured cetane number (ASTM D613 ) of from 48 to 52.

The present invention also provides a method of operating a diesel engine , and/or a vehicle which is driven by a diesel engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition according to the second aspect . The fuel composition may be used in this way for the purpose of improving ease of fuel ignition during use of the engine .

Preferred features of the second and other aspects of the present invention may be as described above in connection with the first aspect .

Other features of the present invention will become apparent from the following examples . Generally speaking the present invention extends to any novel one, or any novel combination, of the features disclosed in this specification ( including the accompanying claims ) . Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose . Examples

These Examples demonstrate the effects of fatty acid alkyl esters, in particular fatty acid methyl esters , on the cetane numbers of various typical diesel base fuels . The fatty acid methyl esters ( FAMEs ) tested were rapeseed methyl ester (RME) and soy methyl ester ( SME) .

The base fuels tested were a typical German specification sulphur-free ( "zero sulphur" ) diesel fuel Fl , a US specification diesel fuel F2 , a summer grade European specification ultra low sulphur diesel fuel F3 (without additives ) and a diesel fuel F4 which was the same as F3 but contained a standard refinery treatment additive ( single dose treat rate; contained no cetane improvers) . The specifications for the base fuels Fl to F3 are shown in Table A.

Table A

Various blends of the fatty acid methyl esters ( FAMEs ) and the base fuels were prepared, to assess the effect of FAME concentration on the ignition quality of the resultant fuels .

Derived cetane numbers were determined for most samples , using the ignition quality test ( IQT) method IP 498 /03. For some samples , measured (engine ) cetane numbers were also obtained according to the CFR Cetane

Engine method, ASTM D613.

Example 1 - effect of RME on cetane number

The effect of RME on both the measured and the derived cetane numbers of various diesel base fuels was assessed as described above . The results are shown in Tables 1 to 4 for the base fuels Fl to F4 respectively.

The derived cetane number for the neat RME was 58.1.

Table 1 - RME in base fuel Fl

(X = not measured)

Table 2 - RME in base fuel F2 κ O>

(X = not measured)

Table 3 - RME in base fuel F3

(X = not measured)

Table 4 - RME in base fuel F4

(X = not measured)

These data show a non-linear change in cetane number with RME concentration, for all the base fuels tested. In particular, they show a marked "boost" in cetane number at lower RME concentrations , such as 20% v/v or below . Thus in this regime, for any given RME concentration the cetane number of the base fuel/RME blend is higher than linear blending rules would have predicted . Correspondingly, in order to achieve any given target cetane number, a lower amount of the RME is needed than if linear blending rules applied .

The trend is highlighted by the higher accuracy IQT data .

The presence of the refinery additive in base fuel F4 appears to have no significant impact on the ability of the RME to enhance cetane number .

For the zero sulphur fuel Fl , it appears that slightly higher FAME concentrations (e . g . 10% v/v or greater) are needed to achieve such a significant cetane number boost . Example 2 - effect of SME on cetane number

The effect of SME ( soy methyl ester) on both the measured and the derived cetane numbers of the four base fuels was assessed as described above . The results are shown in Tables 5 to 8. The derived cetane number for the neat SME was 71.4.

Table 5 - SME in base fuel Fl

(X = not measured)

Table 6 - SME in base fuel F2

(X = not measured)

Table 7 - SME in base fuel F3

(X = not measured)

Table 8 - SME in base fuel F4

(X = not measured)

Again these data show a non-linear boost in cetane number at lower SME concentrations , for all base fuels . As for RME, there is no statistically significant difference in this effect between the additivated ( F4 ) and unadditivated ( F3 ) fuels .

It can therefore be seen that a target increase in cetane number may be achieved, for the base fuels , by incorporating a FAAE in an amount smaller than the amount that would be needed if linear blending applied . For example in the case of the SME/F2 blends ( see Table 6 ) , a target cetane number of 51.7 can be achieved using only 2% v/v SME, whereas if linear blending applied, one would expect 15.8% v/v of SME to be needed to achieve the same cetane number . Similarly a target cetane number of 56.5 can be achieved using only 7% v/v SME, whereas linear blending rules would predict that 36.3% v/v of SME would be needed to achieve this effect . (These figures are derived cetane numbers, measured by the IP 498 /03 method . )

Claims

C L A I M S

1. A method for increasing the cetane number of a diesel fuel composition which contains a maj or proportion of a diesel base fuel, in order to reach a target cetane number X, which method comprises adding to the base fuel an amount x of a fatty acid alkyl ester ( FAAE) having a cetane number B which is greater than the cetane number A of the base fuel , wherein x is less than the amount of the FAAE which would need to be added to the base fuel in order to achieve cetane number X if linear blending rules applied .

2. A method according to claim 1 , wherein the volume fraction of FAAE added to the base fuel , v, is at least 0.05 lower than the volume fraction v ' which would be needed if linear blending rules applied .

3. A method according to claim 1 or claim 2 , wherein the concentration of the FAAE in the overall fuel composition is from 0.05 to 25% v/v .

4. A method according to claim 3 , wherein the concentration of the FAAE in the overall fuel composition is from 1 to 15% v/v.

5. A method according to any one of the preceding claims , wherein the FAAE is a fatty acid methyl, ethyl or iso-propyl ester .

6. A method according to any one of the preceding claims , wherein the FAAE is selected from the group consisting of rapeseed methyl ester, soy methyl ester, palm oil methyl ester, coconut methyl ester and mixtures thereof .

7. A method according to any one of the preceding claims , wherein the diesel fuel composition contains less than 50 ppmw of other cetane improving ( ignition improving) additives .

8. A method according to any one of the preceding claims , wherein the derived cetane number ( IP 498 /03 ) of the diesel fuel composition, as a result of use of the FAAE in the composition, is 50 or greater .

9. Use of a FAAE in a diesel fuel composition, for the purpose of reducing the level of cetane improving additives in the composition .