US20080244966A1

US20080244966A1 - Fuel compositions

Info

Publication number: US20080244966A1
Application number: US11/828,929
Authority: US
Inventors: Claire Ansell; Richard Hugh CLARK; Richard John Heins; Johanne Smith; Trevor Stephenson; Robert Wilfred Matthews Wardle
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2006-07-27
Filing date: 2007-07-26
Publication date: 2008-10-09
Also published as: WO2008012320A1; CN101517044A; AR062133A1; EP2046923B1; AU2007278172A1; CN101517044B; EP2046923A1; JP2009544787A; JP5426375B2; BRPI0715106A2

Abstract

Use of a Fischer-Tropsch derived fuel component, in a fuel composition, is provided reducing the tendency of the composition to dissolve metals; increasing its thermal stability; reducing the concentration of a metal deactivator, antioxidant or detergent additive in the composition; or increasing the storage stability of the composition. The composition is preferably a diesel fuel composition.

Description

FIELD OF THE INVENTION

The present invention relates to certain types of fuel compositions.

BACKGROUND OF THE INVENTION

The thermal stability of middle distillate fuels has traditionally been a cause for concern in the aviation industry. Aviation fuels (kerosene fractions) are subjected to high levels of thermal stress during use.
For automotive diesel fuels, thermal stability has historically been less of a concern. However, trends in modern engine design, to comply with ever tightening emissions legislation, may change this. New common rail or unit injectors subject fuels to much more severe conditions than more traditional diesel engines, for example pressures of up to 2000 bar and temperatures above 100° C. Under these conditions, instability reactions are much more likely to occur.
Fuel thermal instability reactions are recognised to result from a combination of hydrocarbon oxidation reactions and interactions between polar species present in the fuel. These processes can be affected by two competing chemical trends. On the one hand, increasingly low fuel sulphur levels are resulting in lower levels of polar species (typically, the processes used to remove sulphur from a fuel will also result in a reduction in the level of other polar species such as nitrogen containing compounds and oxygenates), and hence a lower level of natural antioxidancy; this in turn can increase the extent to which oxidation reactions can occur, in particular when a fuel is subjected to thermal stress. On the other hand, polar species are often the bridging moieties which form fuel lacquers in thermal instability reactions; thus, lower levels of polar species can to some extent help to reduce the number of thermal instability reactions occurring.
Poor thermal stability in a fuel will result in an increase in the products of thermal instability reactions such as gums, lacquers and other insoluble components. These in turn can block engine filters, foul fuel injectors and valves and hence be detrimental to engine efficiency and emissions control. Fuel instability is also thought to lead to increased soot production in engine exhausts, which could lead to overloading of particulate traps. Thus, it is desirable for a fuel to have as high as possible a thermal stability, in particular in systems (such as common rail or unit injector diesel engines, or indeed aircraft engines) in which the fuel is subjected to a significant level of thermal stress.
It has also been seen, in the aviation industry, that thermal instability of a fuel can be exacerbated by the presence of trace catalytic metals—for example copper—which can occur if the fuel is able to dissolve such metals from the engine hardware, or from storage tanks or transportation equipment.

SUMMARY OF THE INVENTION

Accordingly there is provided in one embodiment a method for formulating a fuel composition, which method comprising blending together a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components, in which the JFTOT breakpoint of such fuel composition is greater than 300° C.
There is provided in another embodiment the method for formulating a fuel composition, which method comprising blending together a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components, the peroxide level of such fuel composition is 10 mg/kg or less after a period of storage of 8 weeks under storage temperature of at least 40° C.
Further a method of operating a fuel composition made by such methods are provided.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a fuel composition, and/or components for use in a fuel composition, which can overcome or at least mitigate the above described problems.
It has been found that a Fischer-Tropsch derived fuel component can have a much lower tendency to dissolve metals, in particular catalytic metals such as copper, than do conventional petroleum derived fuels. This in turn has been shown to result in a higher thermal stability. Moreover, Fischer-Tropsch derived fuel components of the invention appear to have high intrinsic thermal stabilities compared to petroleum derived fuels, thereby increasing the thermal stability of the fuel composition.
That this is possible is not necessarily obvious, since Fischer-Tropsch derived fuel components are well known to contain low levels of polar species, which might be expected to lead to an increased susceptibility to oxidation and hence a poorer thermal stability.
A certain level of thermal stability may be desirable in order for a fuel composition to meet current fuel specifications, and/or to comply with local regulations, and/or to satisfy consumer demand, and/or to ensure efficient or at least adequate operation of a fuel consuming system to be run on the composition. According to the present invention, such standards may still be achievable, due at least in part to the use of the Fischer-Tropsch derived fuel component.
Since it may be desirable to include a Fischer-Tropsch derived component in a fuel composition for other reasons, for example to reduce emissions from a fuel-consuming system (typically an engine) running on the fuel composition, or to reduce the level of sulphur and/or aromatics and/or other polar components in the composition, the ability to use a Fischer-Tropsch component for the additional purpose of reducing the uptake by the composition of catalytic metals, and improving the thermal stability of the composition, can provide significant formulation advantages.
The present invention may additionally or alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with either thermal stability or tendency to dissolve metals, for example storage stability (as described below); tendency to produce degradation products such as gums, lacquers and other deposits; tendency to discolour (which may in turn be due to the formation of degradation products); and/or detrimental effect on an engine or other fuel-consuming system, for instance on its efficiency and/or emissions and/or on components of the system such as its catalytic system.
In the context of the present invention, “use” of a Fischer-Tropsch derived component in a fuel composition means incorporating the component into the composition, optionally as a blend (i.e. a physical mixture) with one or more other fuel components. In one embodiment of the present invention, the Fischer-Tropsch derived fuel component may be the only fuel component present in the composition, optionally with one or more fuel additives. The Fischer-Tropsch derived component will conveniently be incorporated before the fuel composition is introduced into an engine or other system which is to be run on the composition. Instead or in addition the use of the Fischer-Tropsch derived fuel component may involve running a fuel-consuming system, typically a diesel engine, on a fuel composition containing or consisting of the Fischer-Tropsch component, typically by introducing the composition into a combustion chamber of an engine.
“Use” of a Fischer-Tropsch derived fuel component in the ways described above may also embrace supplying such a component together with instructions for its use in a fuel composition to achieve any of the purposes described above, for instance to reduce the tendency of the composition to dissolve metals and increase its thermal stability. The Fischer-Tropsch derived fuel component may itself be supplied as part of a formulation suitable for and/or intended for use as a fuel additive, in which case the Fischer-Tropsch component may be included in such a formulation for the purpose of influencing its effects on the metal solubilisation capability of a fuel composition, and its the thermal stability.
Thus, the Fischer-Tropsch derived component may be incorporated into an additive formulation or package along with one or more fuel additives selected for instance from detergents, lubricity enhancing additives, ignition improvers and static dissipaters.
The fuel composition used in the present invention may be for example a naphtha, kerosene or diesel fuel composition, in particular a kerosene or diesel fuel composition. It may be a middle distillate fuel composition, such as a heating oil, an industrial gas oil, an automotive diesel fuel, a distillate marine fuel or a kerosene fuel such as an aviation fuel or heating kerosene. It may be for use in an engine such as an automotive engine or an aircraft engine. In one embodiment it is for use in an internal combustion engine; for instance it may be an automotive fuel composition, such as a diesel fuel composition which is suitable for use in an automotive diesel (compression ignition) engine.
As described above, the Fischer-Tropsch derived fuel may be the only fuel component in a composition prepared according to the present invention. Alternatively, such a fuel composition may contain, in addition to the Fischer-Tropsch derived fuel component, one or more non-Fischer-Tropsch derived base fuels such as petroleum derived base fuels. In this case the fuel composition prior to incorporation of the Fischer-Tropsch derived component may contain a major proportion of, or consist essentially or entirely of, a base fuel such as a distillate hydrocarbon base fuel. A “major proportion” means typically 80% v/v or greater, or 90 or 95% v/v or greater, or even 98 or 99 or 99.5% v/v or greater. Such a base fuel may for example be a naphtha, kerosene or diesel fuel, preferably a kerosene or diesel fuel, such as a diesel fuel.
A naphtha base fuel will typically boil in the range from 25 to 175° C. A kerosene base fuel will typically boil in the range from 140 to 260° C. A diesel base fuel will typically boil in the range from 150 to 400° C.
The base fuel may in particular be a middle distillate base fuel, in particular a diesel base fuel, and in this case it may itself comprise a mixture of middle distillate fuel components (components typically produced by distillation or vacuum distillation of crude oil), or of fuel components which together form a middle distillate blend. Middle distillate fuel components or blends will typically have boiling points within the usual middle distillate range of 125 to 550° C. or 140 to 400° C.
A diesel base fuel may be an automotive gas oil (AGO). 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 140 or 150 to 400 or 550° C., depending on grade and use. They will typically have densities from 0.75 to 1.0 g/cm³, preferably from 0.8 to 0.9 or 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 40° C. (ASTM D445) might suitably be from 1.5 to 4.5 mm²/s.
Such fuels are generally suitable for use in a compression ignition (diesel) internal combustion engine, of either the indirect or direct injection type.
A diesel fuel composition which results from carrying out the present invention may also fall within these general specifications. It may for instance comply with applicable current standard specification(s) such as for example EN 590 (for Europe) or ASTM D975 (for the USA). By way of example, the fuel composition may have a density from 0.82 to 0.845 g/cm³at 15° C.; a T₉₅boiling point (ASTM D86) of 360° C. or less; a cetane number (ASTM D613) of 51 or greater; a kinematic viscosity (ASTM D445) from 2 to 4.5 mm²/s at 40° C.; a sulphur content (ASTM D2622) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons (PAH) content (IP 391 (mod)) of less than 11%. Relevant specifications may however differ from country to country and from year to year and may depend on the intended use of the fuel composition.
A petroleum derived gas oil may be obtained by 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. This also tends to reduce the content of other polar species such as nitrogen-containing species.
In the present invention, a base fuel may be or contain a so-called “biofuel” component such as a vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester) or another oxygenate such as an acid, ketone or ester. Such components need not necessarily be bio-derived.
The fuel composition to which the present invention is applied may have a sulphur content of 1000 mg/kg or less. It may have a low or ultra low sulphur content, for instance at most 500 mg/kg, or at most 350 mg/kg, suitably no more than 100 or 50 or 10 or even 5 mg/kg, of sulphur.
By “Fischer-Tropsch derived” is meant that a fuel component is, or derives from, a synthesis product of a Fischer-Tropsch condensation process. A Fischer-Tropsch derived fuel may also be referred to as a GTL (Gas-to-Liquids) fuel. The term “non-Fischer-Tropsch derived” may be construed accordingly.
It is known to include such components in fuel compositions; in particular, Fischer-Tropsch derived gas oils have been included in automotive diesel fuels. What has not been appreciated before, to our knowledge, is their ability to influence the metal solubilisation capacity of a fuel composition and in turn its thermal stability.
The Fischer-Tropsch reaction converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons:
n(CO+2H₂)═(—CH₂—)_n +nH₂O+heat,
in the presence of an appropriate catalyst and typically at elevated temperatures (e.g. 125 to 300° C., preferably 175 to 250° C.) and/or pressures (e.g. 5 to 100 bar, preferably 12 to 50 bar). Hydrogen:carbon monoxide ratios other than 2:1 may be employed if desired.
The carbon monoxide and hydrogen may themselves be derived from organic or inorganic, natural or synthetic sources, typically either from natural gas or from organically derived methane. The gases which are converted into liquid fuel components using such processes can in general include natural gas (methane), LPG (e.g. propane or butane), “condensates” such as ethane, synthesis gas (CO/hydrogen) and gaseous products derived from coal, biomass and other hydrocarbons.
Gas oil, naphtha and kerosene products may be obtained directly from the Fischer-Tropsch reaction, or indirectly for instance by fractionation of Fischer-Tropsch synthesis products or from hydrotreated Fischer-Tropsch synthesis products. Hydrotreatment can involve hydrocracking to adjust the boiling range (see, e.g., GB-B-2077289 and EP-A-0147873) and/or hydroisomerisation which can improve cold flow properties by increasing the proportion of branched paraffins. EP-A-0583836 describes a two step hydrotreatment process in which a Fischer-Tropsch synthesis product is firstly subjected to hydroconversion under conditions such that it undergoes substantially no isomerisation or hydrocracking (this hydrogenates the olefinic and oxygen-containing components), and then at least part of the resultant product is hydroconverted under conditions such that hydrocracking and isomerisation occur to yield a substantially paraffinic hydrocarbon fuel. The desired gas oil fraction(s) may subsequently be isolated for instance by distillation.
Other post-synthesis treatments, such as polymerisation, alkylation, distillation, cracking-decarboxylation, isomerisation and hydroreforming, may be employed to modify the properties of Fischer-Tropsch condensation products, as described for instance in U.S. Pat. No. 4,125,566 and U.S. Pat. No. 4,478,955.
Typical catalysts for the Fischer-Tropsch synthesis of paraffinic hydrocarbons comprise, as the catalytically active component, a metal from Group VIII of the periodic table, in particular ruthenium, iron, cobalt or nickel. Suitable such catalysts are described for instance in EP-A-0583836 (pages 3 and 4).
An example of a Fischer-Tropsch based process is the SMDS (Shell Middle Distillate Synthesis) described by van der Burgt et al in “The Shell Middle Distillate Synthesis Process”, paper delivered at the 5th Synfuels Worldwide Symposium, Washington D.C., November 1985 (see also the November 1989 publication of the same title from Shell International Petroleum Company Ltd, London, UK). This process (also sometimes referred to as the Shell “Gas-To-Liquids” or “GTL” technology) produces middle distillate range products by conversion of a natural gas (primarily methane) derived synthesis gas into a heavy long chain hydrocarbon (paraffin) wax which can then be hydroconverted and fractionated to produce liquid transport fuels such as the gas oils useable in diesel fuel compositions. A version of the SMDS process, utilising a fixed bed reactor for the catalytic conversion step, is currently in use in Bintulu, Malaysia and its gas oil products have been blended with petroleum derived gas oils in commercially available automotive fuels.
Gas oils, naphthas and kerosenes prepared by the SMDS process are commercially available for instance from Shell companies. Further examples of Fischer-Tropsch derived gas oils are described in EP-A-0583836, EP-A-1101813, WO-A-97/14768, WO-A-97/14769, WO-A-00/20534, WO-A-00/20535, WO-A-00/11116, WO-A-00/11117, WO-A-01/83406, WO-A-01/83641, WO-A-01/83647, WO-A-01/83648 and U.S. Pat. No. 6,204,426.
By virtue of the Fischer-Tropsch process, a Fischer-Tropsch derived fuel has essentially no, or undetectable levels of, sulphur and nitrogen. Compounds containing these heteroatoms tend to act as poisons for Fischer-Tropsch catalysts and are therefore removed from the synthesis gas feed. This reduction in the level of polar species might be expected to reduce the thermal stability of a Fischer-Tropsch derived fuel, which makes the present invention all the more surprising.
Further, the Fischer-Tropsch process as usually operated produces no or virtually no aromatic components, which again might be expected to reduce the thermal stability of the resultant fuel. The aromatics content of a Fischer-Tropsch derived fuel, suitably determined by ASTM D4629, will typically be below 1% w/w, preferably below 0.5% w/w and more preferably below 0.2 or 0.1% w/w.
Generally speaking, Fischer-Tropsch derived fuels have relatively low levels of polar components, in particular polar surfactants, for instance compared to petroleum derived fuels. Such polar components may include for example oxygenates, and sulphur- and nitrogen-containing compounds. A low level of sulphur in a Fischer-Tropsch derived fuel is generally indicative of low levels of both oxygenates and nitrogen-containing compounds, since all are removed by the same treatment processes.
Where a Fischer-Tropsch derived fuel component is a naphtha fuel, it will be a liquid hydrocarbon distillate fuel with a final boiling point of typically up to 220° C. or preferably of 180° C. or less. Its initial boiling point may be higher than 25° C., in cases higher than 35° C. Its components (or the majority, for instance 95% w/w or greater, thereof) are typically hydrocarbons having 5 or more carbon atoms; they are usually paraffinic.
In the context of the present invention, a Fischer-Tropsch derived naphtha fuel may have a density of from 0.67 to 0.73 g/cm³at 15° C. and/or a sulphur content of 5 mg/kg or less, preferably 2 mg/kg or less. It may contain 95% w/w or greater of iso- and normal paraffins, preferably from 20 to 98% w/w or greater of normal paraffins. It may be the product of a SMDS process, suitable features of which may be as described below in connection with Fischer-Tropsch derived gas oils.
A Fischer-Tropsch derived kerosene fuel is a liquid hydrocarbon middle distillate fuel with a distillation range suitably from 140 to 260° C., preferably from 145 to 255° C., more preferably from 150 to 250° C. or from 150 to 210° C. It will have a final boiling point of typically from 190 to 260° C., for instance from 190 to 210° C. for a typical “narrow-cut” kerosene fraction or from 240 to 260° C. for a typical “full-cut” fraction. Its initial boiling point is preferably from 140 to 160° C., more preferably from 145 to 160° C.
A Fischer-Tropsch derived kerosene fuel may have a density of from 0.730 to 0.760 g/cm³at 15° C.—for instance from 0.730 to 0.745 g/cm³for a narrow-cut fraction and from 0.735 to 0.760 g/cm³for a full-cut fraction. It preferably has a sulphur content of 5 mg/kg or less. It may have a cetane number of from 63 to 75, for example from 65 to 69 for a narrow-cut fraction or from 68 to 73 for a full-cut fraction. It may be the product of a SMDS process, suitable features of which may be as described below in connection with Fischer-Tropsch derived gas oils.
A Fischer-Tropsch derived gas oil should be suitable for use as a diesel fuel, ideally as an automotive diesel fuel; its components (or the majority, for instance 95% w/w or greater, thereof) should therefore have boiling points within the typical diesel fuel (“gas oil”) range, i.e. from about 150 to 400° C. or from 170 to 370° C. It will suitably have a 90% w/w distillation temperature of from 300 to 370° C.
A Fischer-Tropsch derived gas oil will typically have a density from 0.76 to 0.79 g/cm³at 15° C.; a cetane number (ASTM D613) greater than 70, suitably from 74 to 85; a kinematic viscosity (ASTM D445) from 2 to 4.5, such as from 2.5 to 4.0 or from 2.5 to 3.7, mm²/s at 40° C.; and/or a sulphur content (ASTM D2622) of 5 mg/kg or less, in cases of 2 mg/kg or less.
A Fischer-Tropsch derived fuel component used in the present invention may for instance be a product prepared by a Fischer-Tropsch methane condensation reaction using a hydrogen/carbon monoxide ratio of less than 2.5, or of less than 1.75, or from 0.4 to 1.5, and suitably using a cobalt containing catalyst. It may have been obtained from a hydrocracked Fischer-Tropsch synthesis product (for instance as described in GB-B-2077289 and/or EP-A-0147873), or a product from a two-stage hydroconversion process such as that described in EP-A-0583836 (see above). In the latter case, suitable features of the hydroconversion process may be as disclosed at pages 4 to 6, and in the examples, of EP-A-0583836.
Suitably, a Fischer-Tropsch derived fuel component used in the present invention is a product prepared by a low temperature Fischer-Tropsch process, by which is meant a process operated at a temperature of 250° C. or lower, such as from 125 to 250° C. or from 175 to 250° C., as opposed to a high temperature Fischer-Tropsch process which might typically be operated at a temperature of from 300 to 350° C.
Suitably, in accordance with the present invention, a Fischer-Tropsch derived fuel component will consist of at least 70% w/w, or at least 80% w/w, or at least 90 or 95 or 98% w/w, or at least 99 or 99.5 or even 99.8% w/w, of paraffinic components, in particular iso- and normal paraffins. The weight ratio of iso-paraffins to normal paraffins will suitably be greater than 0.3 and may be up to 12; suitably it is from 2 to 6. The actual value for this ratio will be determined, in part, by the hydroconversion process used to prepare the gas oil from the Fischer-Tropsch synthesis product.
The olefin content of the Fischer-Tropsch derived fuel component is suitably 0.5% w/w or lower. Its aromatics content is suitably 0.5% w/w or lower.
In accordance with the present invention, the Fischer-Tropsch derived fuel component may be for example a naphtha, kerosene or diesel (gas oil) component, suitably a kerosene or diesel component, such as a diesel component.
A fuel composition prepared according to the present invention may contain a mixture of two or more Fischer-Tropsch derived fuel components.
The concentration of the Fischer-Tropsch derived fuel component, in a composition prepared according to the present invention, may be 1% v/v or greater, such as 2 or 5 or 10 or 15% v/v or greater, for example 20 or 25 or 30 or 40 or 50% v/v or greater. It may be up to 100% v/v (i.e. the fuel is entirely Fischer-Tropsch derived), or it may be up to 99 or 98 or 95 or 90 or 80% v/v, in cases up to 75 or 60 or 50% v/v. Suitably the proportion of Fischer-Tropsch derived fuel component(s) in the composition is up to 40 or in cases 30% v/v, or up to 25 or 20 or 15% v/v; for example it may be from 5 to 30% v/v.
The Fischer-Tropsch derived fuel component may be used in the fuel composition for one or more other purposes in addition to the desire to reduce metal dissolution capability and increase thermal stability, for instance to reduce emissions from a fuel-consuming system (typically an engine) running on the fuel composition, and/or to reduce the level of sulphur and/or aromatics and/or other polar components in the composition. Thus the present invention can be used to optimise the properties and performance of a fuel composition in a number of ways, and can therefore provide additional flexibility in fuel formulation.
The tendency of a fuel composition to dissolve metals refers to its tendency or ability to take up a metal from a metal surface, typically a part of an engine or other fuel consuming system, with which the composition is placed into contact, suitably during normal operation of the fuel consuming system. This tendency may suitably be assessed by measuring the amount of the relevant metal in the fuel composition after contact with the surface for a given period of time and under specified conditions, for instance as described in Example 1 below.
The test conditions may be designed to mimic those to which the fuel composition might be subjected when used in a fuel consuming system such as an internal combustion engine. They may for example involve increased temperature, for instance of 30° C. or higher or of 40° C. or higher, such as from 30 to 40° C. (to mimic conditions in a typical vehicle fuel tank during fuel recycling from an engine); from 40 to 80° C. (to mimic conditions in the high pressure pump and rail of a common rail injection system); from 80 to 100° C. (to mimic conditions in typical vehicle engine fuel injectors which are in thermal contact with the engine block); from 100 to 150° C. (to mimic conditions to which a fuel is subjected when close to an injector nozzle); and/or up to 250° C. (as in accelerated tests, such as at the metal tube surface in the JFTOT test described in the examples below).
The test conditions may involve a pressure from atmospheric (to mimic storage conditions in a typical fuel tank) to around 1000 or 1500 or even 2000 bar (to which a fuel composition might be exposed in a typical common rail diesel engine injection system). Suitably the test conditions involve increased pressure, i.e. a pressure above atmospheric, for example a pressure of up to 50 bar, such as around 33.3 bar as in the JFTOT test used in the examples below.
The present invention may be used to reduce the tendency of the fuel composition to dissolve any one or more metals. The metal may be a catalytically active metal, such as copper, iron, zinc, lead, silver, chromium, aluminium, magnesium, nickel or tin, in particular iron or copper which may be present in fuel storage systems. Its dissolution into the fuel composition may be from a metal or metal-containing (for instance a metal alloy) body, including a body containing a metal salt (for example, an oxide or sulphide or a corrosion product such as rust). Such a metal may be present in the fuel composition in an elemental or ionic (which includes complexed) form.
In the context of the first aspect of the present invention, the term “reducing” embraces any degree of reduction, including reduction to zero. The reduction may for instance result in the fuel composition containing at least 10% less of the relevant metal, after contact with a metal-containing surface, than would the same composition prior to incorporation of the Fischer-Tropsch derived fuel component, if contacted with the same surface for the same period of time and under the same conditions. This figure may in cases be at least 25 or 40 or 50%, in cases at least 60 or 70 or even 80%.
The reduction may be as compared to the metal dissolving tendency which the fuel composition would otherwise have exhibited prior to the realisation that a Fischer-Tropsch derived fuel component could be used in the way provided by the present invention, and/or that of an otherwise analogous fuel composition intended (e.g. marketed) for use in an analogous context, prior to adding a Fischer-Tropsch derived fuel component to it in accordance with the present invention.
The thermal stability of a fuel composition may in the present context be regarded as its thermal oxidation stability. It may be measured in any suitable manner, such as using the Jet Fuel Thermal Oxidation Tester (JFTOT) method, for instance as described in Examples 2 and 3 below. Thermal stability may be assessed with reference to a maximum temperature at which the fuel still fulfils specified criteria, as for example the JFTOT “breakpoint”.
Alternatively or additionally, the thermal oxidation stability of a fuel composition may be assessed by measuring the change in peroxide number of the composition (for example, using the standard test method ASTM D3703) following subjection to a specific (typically high temperature) event or condition.
The term “increasing”, in the context of thermal stability, embraces any degree of increase. The increase may for instance result in the fuel composition having a JFTOT breakpoint which is at least 5% higher than prior to incorporation of the Fischer-Tropsch derived fuel component. This figure may in cases be at least 8 or 10 or 25 or 50%. Again the increase may be as compared to the thermal stability of the fuel composition prior to the realisation that a Fischer-Tropsch derived fuel component could be used in the way provided by the present invention, and/or of an otherwise analogous fuel composition intended (e.g. marketed) for use in an analogous context, prior to adding a Fischer-Tropsch derived fuel component to it in accordance with the present invention.
In absolute terms, the JFTOT breakpoint of a fuel composition which results from carrying out the present invention may be greater than 300 or 350° C., or it may be 360° C. or greater, such as 370 or 380° C. or higher. Ideally the fuel composition has a JFTOT breakpoint within these ranges even when it contains up to 10 or even 15 ppbw (parts per billion by weight) of a dissolved metal such as copper.
Prior to incorporation of the Fischer-Tropsch derived component, the fuel composition may for instance have a JFTOT breakpoint of 350° C. or less, or 300° C. or less, or 250° C. or less.
The thermal stability of a fuel composition may reduce during its storage and/or use, for example due to dissolution of one or more metals from a fuel consuming system in which it is stored or used. According to the present invention, a Fischer-Tropsch derived fuel component may be used in a fuel composition for the purpose of reducing the tendency of a fuel composition to suffer such a reduction in thermal stability during storage or use. It has been found that not only is a Fischer-Tropsch derived fuel component likely to dissolve less metal than other, for example petroleum derived, fuels, but that on uptake of dissolved metal it may suffer from less of a reduction in thermal stability than would a non-Fischer-Tropsch derived fuel.
A fuel composition to which the present invention is or has been applied may contain other standard fuel additives, many of which are known and readily available. The total additive content in the fuel composition may suitably be from 50 to 10000 mg/kg, such as below 5000 mg/kg.
Additives often included in fuel compositions are metal deactivators and corrosion inhibitors. As a result of carrying out the present invention, however, lower levels of such additives may be needed as the composition is likely to be less aggressive towards metals during use.
Thus, according to a second aspect, the present invention provides the use of a Fischer-Tropsch derived fuel component, in a fuel composition, for the purpose of reducing the concentration of a metal deactivator in the composition. The concentration of a corrosion inhibitor may also be reduced. The metal deactivator or corrosion inhibitor may be of any type. “Reducing” its concentration may embrace any degree of reduction, including to zero.
Another type of additive often included in fuel compositions is an anti-oxidant. Again as a result of carrying out the present invention, lower levels of such additives may be needed as the composition has a higher thermal oxidation stability.
Thus, according to a third aspect, the present invention provides the use of a Fischer-Tropsch derived fuel component, in a fuel composition, for the purpose of reducing the concentration of an antioxidant in the composition. The antioxidant may be of any type. “Reducing” its concentration may embrace any degree of reduction, including to zero.
Detergent additives are also often included in fuel compositions. The present invention may reduce the need for such additives, by reducing the level of deposits which are formed (and which therefore need to be dispersed) during storage and use of a fuel composition.
Thus, according to a fourth aspect, the present invention provides the use of a Fischer-Tropsch derived fuel component, in a fuel composition, for the purpose of reducing the concentration of a detergent additive in the composition. The detergent additive may be of any type. “Reducing” its concentration may embrace any degree of reduction, including to zero.
A fifth aspect of the present invention provides a method for formulating a fuel composition, which method involves blending together a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components (such as fuel additives), for the purpose of reducing the tendency of the blend to dissolve metals. The present invention also provides use in a fuel composition of a blend of a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components (such as fuel additives), for the purpose of reducing the tendency of the blend to dissolve metals. The thermal stability of the blend may also be increased.
The methods of the present invention may be used for the purpose of achieving a desired target (typically minimum) thermal stability for the fuel composition. This target may be a JFTOT breakpoint within the ranges quoted above.
According to a sixth aspect, the present invention provides a method of operating a fuel consuming system, which method involves introducing into the system a fuel composition prepared in accordance with any one of the first to the fifth aspects of the present invention. The fuel composition may be introduced for one or more of the purposes described above in connection with the first to the fifth aspects of the present invention, in particular to reduce the amount of metal which it takes up from parts of the system with which it comes into contact, and to improve the thermal stability of the fuel composition, and/or to reduce occurrence of effects associated (whether directly or indirectly) with fuel thermal instability, for example filter blocking or valve or injector fouling, or loss of system efficiency or emissions control.
In the context of the present invention, a “fuel consuming system” includes a system which transports (for example by pumping) or stores a fuel composition, as well as a system which runs on (and hence combusts) a fuel composition.
The system may in particular be an engine, such as an automotive or aircraft engine, in which case the method involves introducing the relevant fuel composition into a combustion area of the engine. It may be an internal combustion engine, and/or a vehicle which is driven by an internal combustion engine. The engine is preferably a compression ignition (diesel) engine. Such a diesel engine may be of the direct injection type, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or of the indirect injection type. It may be a heavy or a light duty diesel engine.
The present invention may be of particular use where the fuel consuming system is of the type which subjects a fuel composition to significant levels of thermal stress, for instance one which subjects a fuel composition to pressures in excess of 1000 or 1500 or 2000 bar and/or one which subjects a fuel composition to operating temperatures of 100° C. or greater or of 120 or 140° C. or greater. The fuel consuming system may for instance involve high pressure fuel injection.
According to a seventh aspect, the present invention provides a fuel composition preparable by, or which has been prepared by, a method according to any one of the first to the fifth aspects of the present invention.
In addition to relatively high intrinsic thermal stabilities, Fischer-Tropsch derived fuels are also now believed to have relatively high storage stabilities (typically, stability against oxidation), compared for instance to petroleum derived fuels. Moreover, since dissolved metals are also believed to impact on storage stability, the relatively low tendency of a Fischer-Tropsch derived fuel component to dissolve metals may also help to improve the storage stability of a fuel composition containing such a component.
Thus, according to an eighth aspect of the present invention, there is provided the use of a Fischer-Tropsch derived fuel component, in a fuel composition, for the purpose of increasing the storage stability of the composition.
All hydrocarbon fuels degrade to some extent during storage, the degradation rate depending on their composition and storage conditions. When it does occur, storage instability manifests itself as a darkening in the colour of the fuel and the formation of a fine organic sludge. If the fuel is subsequently stirred up, for instance during tank filling, this sludge is dispersed and can cause filter blockages if the fuel is used before the sludge has resettled.
Fuel instability may also lead to undesirable deposits in the pre-combustion and combustion areas of fuel injection systems, and/or to increased soot production in engine exhausts which in turn may lead to overloading of particulate traps.
Poor oxidation stability during storage or thermal stressing is known to lead to the accumulation of peroxides in a fuel. These in turn are associated with a number of undesirable side effects. For example, peroxides can attack and degrade elastomeric parts within an engine or other system in which the fuel is being used. Oxidation intermediates can also react with other species present in the fuel (for example, polar compounds) to produce gums and sludges, which in turn can block engine filters, foul fuel injectors and valves and hence be detrimental to engine efficiency and emissions control. Moreover, peroxides are themselves corrosive to metals, and their breakdown products acidic; thus higher peroxide levels can lead to increased corrosion within a fuel consuming system.
The storage stability of in particular automotive diesel fuels is likely to become increasingly problematic as fuel sulphur levels decrease. The presence of sulphur-containing species in a fuel can contribute a degree of natural antioxidancy, but as sulphur levels fall to meet with ever tightening emissions legislation (the adoption in 1996 of a low sulphur—0.05% w/w or less—specification for European automotive gas oils, followed by subsequent increasing pressure to reduce sulphur levels in cases to less than 10 mg/kg), there has been increasing concern about the impact this might have on the oxidation stability of the fuels. At sulphur levels of 50 mg/kg or less it is unlikely that fuels will possess sufficient natural antioxidancy to protect against oxidation reactions during typical storage periods.
The eighth aspect of the present invention provides the use, in a fuel composition, of a Fischer-Tropsch derived fuel component, for the purpose of improving the storage stability of the composition.
It has been found that a Fischer-Tropsch derived fuel component can accumulate significantly lower levels of peroxides, on storage, than a conventional petroleum derived fuel. This implies a higher storage stability for the Fischer-Tropsch derived fuel.
Moreover, it is believed that not only the presence of natural antioxidancy (for instance, due to sulphur containing species), but also the hydrocarbon structure, can be relevant to the oxidation stability of a fuel. The ability to form stable hydrocarbon radicals can promote the radical driven autoxidation process and hence decrease storage stability. Radical stability is believed to be greater for aromatic species than for cyclic and iso-paraffins, and lower still for normal paraffins. Thus, it is now believed that autoxidation processes could proceed more readily in a fuel with higher levels of for instance aromatic components, with a consequent detrimental effect on its oxidation stability.
The balance between the two competing influences on oxidation stability—on the one hand the presence of polar species contributing to the natural antioxidancy of a fuel and on the other the ability of species such as aromatics to help promote radical driven autoxidation processes—is not yet fully understood. It is not therefore straightforward to predict the oxidation stability of any given fuel component.
Fischer-Tropsch derived fuels tend to contain relatively low levels of aromatic species and of sulphur containing species. This might be expected to lead to a lower natural antioxidancy and hence to a lower storage stability. In the past, it has often been thought necessary to blend Fischer-Tropsch derived fuels with other fuel components, and/or to process them in particular ways, in order to improve their storage stability (see for example U.S. Pat. No. 6,162,956 in which a Fischer-Tropsch fuel is blended with a raw gas field condensate distillate fraction or a mildly hydrotreated condensate fraction in order to improve its oxidation stability, and WO-A-97/14768 and WO-A-97/14769 in which a high stability diesel fuel is prepared by separating a Fischer-Tropsch derived fuel into two fractions, one of which is hydrotreated prior to recombining with the non-hydrotreated fraction).
At the same time, however, Fischer-Tropsch derived fuels also tend to contain low levels of aromatic species and of cyclic paraffins, and relatively low ratios of iso- to normal paraffins. It has now been found that, in the case of these particular fuel components, this appears to counter the low inherent antioxidancy and results, overall, in increased storage stability. This in turn may be used to increase the storage stability of a fuel composition to which a Fischer-Tropsch derived fuel is added.
Preferred features of the eighth aspect of the present invention, for instance the nature(s) of the fuel component(s) and optionally of any additives present in the fuel composition, and the nature and concentration of the Fischer-Tropsch derived fuel component, may be as described above in connection with the first to the fifth aspects of the present invention.
In particular, the Fischer-Tropsch derived fuel component preferably has an olefin content of 0.5% w/w or lower, more preferably 0.1% w/w or lower. It suitably has an iso- to normal-paraffins ratio (i:n) of from 3:1 to 4:1. It may have a kinematic viscosity at 40° C. of from 2.5 to 4.0 mm²/s.
The concentration of the Fischer-Tropsch derived fuel component, in a composition prepared according to the eighth aspect of the present invention, may also be as described above in connection with the first to the fifth aspects of the invention. Suitably it may be from 5 to 30% v/v. In some cases the fuel composition may consist solely or essentially (for instance, optionally with one or more fuel additives) of the Fischer-Tropsch derived fuel component. Again, a mixture of two or more Fischer-Tropsch derived fuel components may be used together in accordance with the eighth aspect of the present invention.
This aspect of the present invention may additionally or alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with storage stability, for example to reduce its tendency to accumulate peroxides and/or acidic species and/or gums and sludges, and/or to reduce its corrosivity.
The storage stability of a fuel composition may in the present context be regarded as its oxidation stability, typically during normal conditions of storage and use. It may be assessed in any suitable manner, such as by reference to the peroxide content of the composition following a fixed period of storage and/or use under specified conditions (peroxide content may be measured using standard test method ASTM D3703). Instead or in addition, storage stability may be assessed using standard test method ASTM D2274 (oxidation stability by accelerated method).
The terms “increasing” and “improving”, in the context of storage stability, embrace any degree of increase or improvement. The increase may for instance result in the fuel composition having a peroxide level which is at least 10% lower than that of the same composition without the Fischer-Tropsch derived fuel component, after a specified period of storage under specified conditions. This figure may in cases be at least 25 or 50 or 75 or 80 or in some case 90 or 95 or even 98 or 99%. The specified storage period may for example be 4 weeks or 8 weeks or 12 weeks or 18 weeks, if the fuel is stored for example at 40° C. or higher (e.g. at 43° C. as in many standard fuel storage tests) or 60° C. or higher. The storage period may be 2 years or more, for example from 2 to 4 years, in particular if the fuel is stored under normal ambient conditions, for example at from 20 to 25° C.
The increase in storage stability may be as compared to the storage stability of the fuel composition prior to the realisation that a Fischer-Tropsch derived fuel component could be used in the way provided by the present invention, and/or of an otherwise analogous fuel composition intended (e.g. marketed) for use in an analogous context, prior to adding a Fischer-Tropsch derived fuel component to it in accordance with the present invention.
In absolute terms, the peroxide level of a fuel composition prepared according to the present invention is preferably 10 mg/kg or less, more preferably 5 or 2 or even 1 mg/kg or less, after a period of storage of one year under normal ambient conditions, and/or after a period of storage of 8 or 12 weeks under storage at 40° C. or higher.
A ninth aspect of the present invention provides a method for formulating a fuel composition, which method involves blending together a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components (such as fuel additives), for the purpose of increasing the storage stability of the blend. The method of either the eighth or the ninth aspect of the present invention may be used for the purpose of achieving a desired target (typically minimum) level of storage stability for the fuel composition.
According to a tenth aspect, the present invention provides a method of operating a fuel consuming system, which method involves introducing into the system a fuel composition prepared in accordance with the eighth or the ninth aspect of the present invention. The fuel composition may be introduced for one or more of the purposes described above in connection with the eighth and ninth aspects of the present invention, in particular to improve the storage stability of the fuel composition and/or to reduce occurrence of effects associated (whether directly or indirectly) with fuel storage instability, for example filter blocking or valve or injector fouling, or increased soot production or increased corrosivity (to metals and/or elastomers).
Again a “fuel consuming system” includes a system which transports (for example by pumping) or stores a fuel composition, in particular one which causes a physical disturbance to the composition (such as by pumping) which might serve to disperse sludges.
According to an eleventh aspect, the present invention provides a fuel composition preparable by, or which has been prepared by, a method according to the eighth or ninth aspect of the present invention.
Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to”, and do not exclude other moieties, additives, components, integers or steps.
Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
Preferred features of each aspect of the present invention may be as described in connection with any of the other aspects.
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 any accompanying claims and drawings). Thus features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the present invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.
Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.
The following examples illustrate the properties of fuel compositions prepared in accordance with the present invention, in particular their ability to dissolve catalytic metals and their thermal and storage stabilities.

EXAMPLE 1

This example assessed the ability of four different automotive diesel fuel compositions to solubilise catalytic metals when in contact with metal surfaces. The compositions were stored over a copper billet at 43° C. and atmospheric pressure, samples being taken monthly to determine their copper content by Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
The fuels used were:

- F1 a commercially available ultra low sulphur automotive diesel fuel (petroleum derived), sourced in the UK;
- F2 & F3 commercially available zero sulphur automotive diesel fuels (petroleum derived), sourced in Sweden and Germany respectively; and
- F4 a Fischer-Tropsch derived gas oil (ex. Shell).

The four fuels had the properties listed in Table 1 below.

TABLE 1

Fuel
property	Test method	F1	F2	F3	F4

Cetane	ASTM D613	60.2	58.6	52.0	>74.8
number
Density @	IP 365/	0.8312	0.8112	0.832	0.7852
15° C.	ASTM
(g/cm³)	D4052
Kinematic	IP 71/		2.041	2.86	3.606
viscosity @	ASTM D445
40° C.
(mm²/s)
Cloud point	IP 219	−6	−34	−9.0	+2
(° C.)
CFPP (° C.)	IP 309		−36		(−1)
					(+1)
Distillation	IP 123/
(° C.):	ASTM D86
IBP		171.8	188.8	172.2	211.5
10%		211.2	207.0	209.2	249.0
recovered
20%		230.7	211.5	227.1	262.0
30%		250.5	219.8	243.8	274.0
40%		264.8	228.0	258.8	286.0
50%		276.9	235.8	272.8	298.0
60%		287.9	243.2	287.0	307.5
70%		298.7	250.6	301.8	317.0
80%		311.2	259.0	318.1	326.5
90%		328.1	270.3	338.8	339.0
95%		345.2	279.3	354.2	349.0
FBP		358.7	290.3	363.7	354.5
Sulphur	ASTM	39	<5	8.0	<5
content	D2622
WDXRF)
(mg/kg)
Aromatics	IP 391
(% m)	(mod)
Mono			3	22.1	0.1
Di			<0.1	2.6	<0.1
Tri			<0.1	0.3	<0.1
Total			3	25.0	0.1

The results of the copper solubilisation tests are shown in Table 2 below.

	TABLE 2

	Copper content (ppbw)

	Sulphur	Day	Day	Day	Day	Day	Day
Fuel	(mg/kg)	0*	28	54	84	112	140

F1	39	17	80	160	500	880	810
F2	<5	4	50	80	148	190	220
F3	<10	<3	20	30	93	170	190
F4	<5	<3	<20	15	34	60	95

(*= prior to storage)
(ppbw = parts per billion by weight)

It is clear from Table 2 that the Fischer-Tropsch derived fuel F4 has a significantly lower propensity to dissolve the copper than any of the more conventional, petroleum derived, diesel fuels.

EXAMPLE 2

In this example, the intrinsic thermal stabilities of the four fuels F1 to F4 were assessed using the Jet Fuel Thermal Oxidation Tester (JFTOT), according to the standard test method ASTM D3241 (IP 323). This technique, developed for the evaluation of jet fuels, involves pumping fuel over a heated tube at a specified flow rate for a specified period of time. The JFTOT “breakpoint” is the highest temperature (measured to the nearest 5° C.) at which the fuel passes the JFTOT test criteria, which relate to tube appearance and test filter pressure differential. The JFTOT test was chosen as it subjects a fuel to higher temperatures than those typically observed in a diesel engine, and thus provides a relatively stringent assessment of a fuel's stability. It can also, being an accelerated test method, yield stability data in a relatively short time period.
The results for the four fuels are shown in Table 3.

TABLE 3

		JFTOT
	Sulphur	breakpoint
Fuel	(mg/kg)	(° C.)

F1	39	240
F2	<5	350
F3	<10	285
F4	<5	>380

Table 3 shows that the Fischer-Tropsch derived fuel F4 is significantly more thermally stable than any of the petroleum derived diesel fuels, even the zero sulphur diesels F2 and F3 which have comparable levels of sulphur. Even when tested at 380° C. (the highest temperature achievable using the JFTOT), the Fischer-Tropsch fuel still passed the test criteria.

EXAMPLE 3

This example assessed the impact of copper pick-up on the thermal stability of diesel fuels. Fuels F2 to F4 (those having comparably low sulphur levels) were assessed using the JFTOT method as outlined in Example 1, after doping with an appropriate quantity of copper naphthenate. The doping levels were chosen in each case to approximate to those found in the fuels after 8 weeks' storage in contact with a copper billet, as observed in Example 2. Thus, 50 ppbw of copper was aimed for in fuels F2 and F3, this level being midway between the 80 ppbw and 30 ppbw that were respectively detected in these fuels at day 54. For the Fischer-Tropsch derived fuel F4, a dosing level of 20 ppbw was aimed for.
The JFTOT results are shown in Table 4.

TABLE 4

		Cu	Cu
		content	content	Neat fuel	Cu-doped
		(ppbw) -	(ppbw) -	JFTOT	JFTOT
	Sulphur	target	measured	breakpoint	breakpoint
Fuel	(mg/kg)	level	level	(° C.)	(° C.)

F2	<5	50	55	350	345
F3	<10	50	60	285	220
F4	<5	20	15	>380	>380

As seen in Table 4, the Fischer-Tropsch derived fuel F4 still had excellent thermal stability despite the copper which it might for instance have dissolved after 8 weeks' contact with a copper-containing surface. After storage under similar conditions, the two petroleum derived diesel fuels have taken up significantly more copper and this appeared to have affected their thermal stability, fuel F3 in particular showing a significant decrease in its JFTOT breakpoint compared to the neat fuel.
Thus, even when exposed to catalytic metals during storage, and/or during use in a fuel consuming system such as a diesel engine, a Fischer-Tropsch derived fuel component appears less likely to suffer from a reduction in thermal stability than is a petroleum derived diesel fuel.
A Fischer-Tropsch derived component may therefore be incorporated into a fuel composition, according to the present invention, in order to lessen its metal pick-up susceptibility and hence improve its thermal stability.

EXAMPLE 4

This example assessed the storage stability of five different automotive diesel fuel compositions, with reference to their tendency to accumulate peroxides during storage.
The compositions were stored at 43° C. and atmospheric pressure, in air, for 24 weeks. Samples were taken at monthly intervals to determine peroxide content, using a modified version of ASTM D3703 so as to avoid the use of halogenated solvents. The relatively high storage temperature was intended to mimic longer storage periods under normal ambient conditions.
The fuels used were:

- F1 & F2 commercially available ultra low sulphur (<50 mg/kg) petroleum derived automotive diesel fuels, both sourced in the UK;
- F3 a commercially available “zero sulphur” (<5 mg/kg) petroleum derived automotive diesel fuel, sourced in Sweden; and
- F4 & F5 two Fischer-Tropsch derived gas oils (ex. Shell), both with sulphur contents of <5 mg/kg.

Their aromatics contents were between 20 and 30% m for F1 and F2, less than 5% m for F3 and <0.5% m for the Fischer-Tropsch derived gas oils F4 and F5.
The peroxide contents of the extracted fuel samples are shown in Table 5 below.

	TABLE 5

	Peroxide content (mg/kg)

	Sulphur	0	4	8	12		20
Fuel	(mg/kg)	weeks	weeks	weeks	weeks	18 weeks	weeks	24 weeks

F1	45	0.2	3	0.6	23.7	21.3	NT	27.2
F2	<50	0.3	0.6	0.6	49.7	0.3	NT	0.8
F3	<5	0.1	0.05	0.9	50	0.3	NT	0.1
F4	<5	0.3	0.9	0.9	0.4	NT	1.3	NT
F5	<5	0.2	0.3	1.6	0.6	NT	0.9	NT

(NT = not tested)

The Table 5 data show fluctuations in peroxide levels throughout the storage period, as a result of both the test methodology and the fact that peroxides can themselves decay to other oxidation products. Nevertheless, overall the data show that for the conventional petroleum derived diesel fuels F1 to F3, peroxide levels increase significantly after only eight to twelve weeks' storage. Those for the Fischer-Tropsch derived gas oils F4 and F5, however, remain low (at effectively the detection limit of the test method) throughout a 20 week storage period. This indicates a far higher oxidation stability for the Fischer-Tropsch derived fuels.

EXAMPLE 5

As discussed above, a number of factors are now believed to influence the storage stability of a fuel. These include not only the degree of natural antioxidancy inherent in the fuel, but also its hydrocarbon structure. The ability to form stable hydrocarbon radicals will promote radical driven autoxidation processes and hence decrease the storage stability of a fuel. Radical stability is believed to decrease in the order aromatics>cyclic and iso-paraffins>normal paraffins.
Table 6 below compares the composition of the Fischer-Tropsch derived gas oil F5 used in Example 4 with that of a commercially available petroleum derived ultra low sulphur diesel fuel F6, sourced in the UK.

	TABLE 6

	Composition (% w/w)

	Component	F5	F6

Normal and iso-	99.77	43.84
paraffins
Cyclic paraffins	0.22	24.55
Dicyclic paraffins	0.00	8.46
Mono-aromatics	0.01	18.29
Di- & poly-	0.00	4.86
aromatics
Total	100.00	100.00

Overall, it can be seen that the petroleum derived fuel F6 has a far higher concentration of the fuel components (for example, aromatic species and cyclic paraffins) which are likely to be able to form stable radicals and hence promote autoxidation. The Fischer-Tropsch derived fuel, in contrast, contains only a trace of cyclic paraffins and virtually no aromatic components, its composition being mainly normal and iso-paraffins. This means that the fuel will form much lower levels of stable radical species, which in turn is believed to contribute to its significantly higher storage stability.
A Fischer-Tropsch derived fuel may therefore be used, in accordance with the present invention, to improve the overall storage stability of a fuel composition into which it is incorporated.

Claims

1. A method for formulating a fuel composition, which method comprising blending together a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components, in which the JFTOT breakpoint of such fuel composition is greater than 300° C.

2. The method of claim 1 which the JFTOT breakpoint is greater than 350° C.

3. The method for formulating a fuel composition, which method comprising blending together a non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuel component, optionally with other fuel components, the peroxide level of such fuel composition is 10 mg/kg or less after a period of storage of 8 weeks under storage temperature of at least 40° C.

4. A method of operating a fuel consuming system, which method involves introducing into the system a fuel composition prepared according to claim 1.

5. A method of operating a fuel consuming system, which method involves introducing into the system a fuel composition prepared according to claim 2.

6. A method of operating a fuel consuming system, which method involves introducing into the system a fuel composition prepared according to claim 3.

7. The method of claim 2 wherein the concentration of the Fischer-Tropsch derived fuel component in the fuel composition is from 5 to 30% v/v.