Patent Publication Number: US-2012046506-A1

Title: Diesel fuel composition

Description:
The present invention relates to a diesel fuel composition. More specifically, it relates to a diesel fuel composition with superior oxidation stability while being constituted essentially only of paraffins. 
     Various kinds of research have been undertaken in recent years to make use of Fischer-Tropsch fuels (referred to below as FT fuels) to fuel cars in Japan. What is meant by FT fuels are fuels obtained by synthesis from raw materials such as natural gas, coal or biomass, using the Fischer-Tropsch process via synthesis gas, a mixture of carbon monoxide and hydrogen. They are often used under names corresponding to the raw material. For example, for those where natural gas is the raw material the name GTL is often used, if coal is the raw material the name CTL may be used, and if biomass is the raw material, the name BTL may be used. The term GTL is also sometimes used as a generic name for fuels obtained by the Fischer-Tropsch process, but in the present invention the term FT fuels is used for fuels obtained by the Fischer-Tropsch process and FT fuels are deemed to include GTL, CTL and BTL. 
     As mentioned above, these FT fuels are expected to be used as alternatives to petroleum because they are synthesised from raw materials such as natural gas, coal and biomass and also, because they do not contain sulphur or aromatic hydrocarbons, they are expected to be used as diesel fuels which are better for the environment, in that they limit the emission of sulphur oxides and particulate matter (PM) from engines. They have already been made available commercially in some areas, as reported for example in “The marketability of liquid fuels from natural gas (GTL)”, [Energy Economics], November 2001 issue. 
     It is acknowledged that diesel fuels, when oxidised, change colour, form sludge and have increased viscosity, for example, and it is also known that peroxides formed by the oxidation cause deterioration of elements (rubbers and metals) in a vehicle&#39;s fuel system. For this reason, oxidation stability is an important indicator for evaluation of the qualitative stability of a diesel fuel, and it is desirable for a diesel fuel to have superior oxidation stability. In recent years, diesel engines have been equipped with common rail fuel injection systems as a means of reducing particulates (or particulate matter, referred to below as PM) in exhaust gases. The structure of these common rail fuel injection systems is such that surplus fuel that has been conveyed under pressure to the injectors but that has not been injected into the combustion chamber is returned to the fuel tank via a return pipe. Since this fuel returning to the fuel tank (the return fuel) is at an elevated temperature, oxidation of the diesel fuel within the fuel tank is promoted, so that there is even more than hitherto a requirement to increase the oxidation stability of diesel fuels, including FT fuels. 
     As regards the oxidation stability of diesel fuels, it is widely and generally known that oxidation stability can be improved by the addition of amine-based and phenol-based anti-oxidants of various kinds, and there have been attempts to add oxidation stabilisers also to the fractions corresponding to the diesel oils of FT fuels (referred to below as FT diesel oils). As an example of this, mention may be made of JP-A-2008-214369. There, it is disclosed that it is possible to increase the oxidation stability at elevated temperatures by blending an anti-oxidant into a fraction corresponding to diesel oil for GTL fuels (GTL diesel fuel). However, the amount of anti-oxidant required in order to obtain the desired effect becomes large for fuels that have poor oxidation stability and production costs are raised. In addition, as the amount of anti-oxidant increases, there are problems in that the anti-oxidant is prone to separate out at lower temperatures, or conversely, if the amount added is small, problems in that, after the effect of the anti-oxidant has been used up during oxidation, the oxidation stability will deteriorate significantly and there may be a deleterious impact such as corrosion of metallic elements of the vehicle&#39;s fuel system. 
     JP-A-2008-266617 has, therefore, proposed maintaining the oxidation stability of diesel fuels without adding anti-oxidants. There, at least one kind of polycyclic aromatic compound selected from the group comprised of anthracenes and dialkylnaphthalenes is blended with an FT diesel oil to ensure oxidation stability. 
     However, it is known that polycyclic aromatic hydrocarbons in fuels are connected with the polycyclic aromatic hydrocarbons in diesel exhaust gases which are acknowledged as carcinogenic in many substances, and it is more preferable if the content of polycyclic aromatic hydrocarbons in fuels is as low as possible. Also, one of the features of FT diesel oils is that they do not give rise to the environmental problems associated with aromatics and sulphur, and that feature is attributable to their being formed only of paraffins. There have been problems in the prior art in dealing with the oxidation stability of FT diesel oils by blending in fuel compositions other than FT diesel oils, in that this basic feature of FT diesel oils is not sufficiently brought out. 
     The aim of the present invention, therefore, is to offer a diesel fuel composition with superior oxidation stability while being constituted essentially only of paraffins. 
     The diesel fuel composition according to the present invention essentially comprises only paraffins, and is characterised in that 
     1) the normal paraffins with 18 or fewer carbons constitute not less than 12% by mass, and
 
2) the proportion of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm is less than 6.5% in proton nuclear magnetic resonance ( 1 H-NMR) spectra,
 
and satisfies the condition that the oxidation index OI represented by the following equation is less than 1.10:
 
       OI=0.247 X− 0.001 Y− 0.053 
     wherein X is the proportion (%) of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm in proton nuclear magnetic resonance ( 1 H-NMR) spectra; and Y is the content (mass %) of normal paraffins with 18 or fewer carbons. 
     It is undesirable here if the normal paraffins with 18 or fewer carbons are less than 12% by mass, if the proportion of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm is 6.5% or more in proton nuclear magnetic resonance ( 1 H-NMR) spectra, or if the oxidation index OI is 1.10 or more, because then the oxidation stability is impaired. To increase the oxidation stability even further, it is preferable if the oxidation index OI is less than 0.9, and more preferably less than 0.7. 
     The diesel fuel composition relating to the present invention may also be such that the increase in the total acid number between before and after an oxidation stability test is not more than 1.3 mg KOH/g. The preferred increase in the total acid number between before and after an oxidation stability test is 1.0 mg KOH/g, but more preferably 0.9 mg KOH/g. What is meant by an oxidation stability test in the present invention is an oxidation test performed as in ASTM D2274 under conditions of oxygen bubbling for 16 hours but with the test temperature changed to 115° C. 
     In addition, what is meant in the present invention by being essentially composed only of paraffins is that the main constituent does not contain styrene compounds or diene compounds, or condensed polycyclic aromatics. Containing compositions of other than paraffins as impurities is tolerated. For example, an FT diesel oil in which the total mass or volume of isoparaffins and normal paraffins is not less than 99% of the whole, excluding tiny impurities, is a diesel fuel composition constituted essentially only of paraffins suitable for the present invention. Also, it is possible to add suitable additives within a range that does not go beyond the scope of the present invention, for example a range that does not cause the problems of cost or separation in the prior art. 
     As additives, mention may be made of lubricity improvers to prevent wear of, for example, fuel feed-pump parts. It is also possible to use, as the lubricity improvers, any known lubricity improvers provided they are miscible in paraffins. Typical lubricity improvers are commercial acid-based lubricity improvers which have fatty acids as their main constituent, and ester-based lubricity improvers which have as their main constituent glycerin mono fatty acid esters. These compounds may be used singly or in combinations of two or more kinds. The fatty acids used in these lubricity improvers are preferably those that have as their main constituent a mixture of unsaturated fatty acids of approximately 12 to 22 carbons, but preferably about 18 carbons, that is oleic acid, linolic acid and linolenic acid. The lubricity improver may be added so that the wear scar WS 1.4 value in an HFRR (High Frequency Reciprocating Rig) of the fuel composition after addition of the lubricity improver is not more than 500 μm, but preferably not more than 460 μm, and the amount thereof is usually 50 to 1000 ppm. The WS 1.4 value in an HFRR here refers to the value obtained in accordance with the Japanese Petroleum Institute standard JPI-5S-50-98 “Gas oil—Method for testing lubricity”. 
    
    
     Furthermore, as examples of other additives, mention may be made of detergents such as amine salts of alkenyl succinate derivatives, metal deactivators such as salicylidene derivatives, de-icing agents such as polyglycol ethers, rust inhibitors such as aliphatic amines and alkenyl succinate esters, anti-static agents such as anionic and cationic amphoteric surfactants, and silicone-based defoaming agents. These additives may be used singly or in combinations of two or more kinds. The amount added may be selected according to use, but will be, for example, not more than 0.2% by mass relative to the fuel oil composition. 
     According to the present invention, it is possible to obtain a diesel fuel composition with excellent oxidation stability although comprised essentially only of paraffins. It having been inferred that the proportion of branches relative to the length of linear molecules is connected with the oxidation stability, and various trials having been carried out, it has been discovered that there was a correlation there with the chemical shifts in proton nuclear magnetic resonance ( 1 H-NMR) spectra. In addition, whilst it is generally believed that oxidation stability is subject to an influence that depends on the molecular weight and that if the molecular weight is large, this has a favourable impact on oxidation stability, it has been discovered that if the normal paraffins with 18 or fewer carbons are not less than a specified proportion, this imparts an improvement in oxidation stability. The present invention is based on these novel findings. 
     Low-temperature flow characteristics are required of diesel fuel compositions, to take account of use during winter or in cold regions. From the standpoint of improving these low-temperature flow characteristics, it is generally preferable if there are more iso-paraffins. On the other hand, from the standpoint of oxidation stability it is preferable to have more normal paraffins. In other words, low-temperature flow characteristics and oxidation stability show opposing behaviours in the composition. However, because it is possible to improve low-temperature flow characteristics by means of additives, it is possible to achieve improvement in oxidation stability while taking low-temperature flow characteristics into account by suitable use of additives. 
     For low-temperature fluidity improvers it is possible to use any known low-temperature fluidity improvers provided they are miscible with paraffins. Typical low-temperature fluidity improvers are commercial low-temperature fluidity improvers such as ethylene-vinyl acetate copolymers, ethylene-alkylacrylate copolymers, alkenyl succinamides, chlorinated polyethylenes, or polyalkyl acrylates. These compounds may be used singly or in combinations of two or more kinds. Of these, ethylene-vinyl acetate copolymers and alkenyl succinamides are especially preferred. As to the amount of the low temperature fluidity improver, for example a suitable amount may be blended in so as to satisfy the pour points and cold filter plugging points specified in JIS K 2204, which is the JIS standard for diesel fuel, but normally the amount will be 50 to 1000 ppm. The pour point here refers to the pour point obtained in accordance with JIS K 2269 “Testing methods for pour point and cloud point of crude oil and petroleum products”, and the cold filter plugging point refers to the cold filter plugging point obtained by JIS K 2288 “Petroleum products—Diesel fuel—Determination of cold filter plugging point”. 
     Examples of the diesel fuel composition according to the present invention are explained here. However, the present invention is not limited by the examples of given below. 
     The following base materials were used to produce diesel fuel compositions comprising hydrocarbon fuel oils and FT fuels comprised only of polykerosene (paraffin) base materials, being oil mixtures with adjusted distillation characteristics and compositions. Table 1 shows the characteristics of the base materials, while Tables 2 and 3 show the characteristics and compositions of the diesel fuel compositions obtained. Table 3 also shows an FT fuel of the prior art as a reference example. 
     Base material A, base material B (synthetic paraffins): Taking as the raw materials by-product gases (butane and butylene fractions) which have as their main constituent light hydrocarbons with a carbon number of 4 obtained, for example, from fluid catalytic cracking apparatus and thermal cracking apparatus during petroleum refining, an oligomerisation treatment was carried out by means of the IFP/Axens Polynaphtha process, and after conversion selectively to hydrocarbon fractions of 10 to 24 carbons, paraffin base materials with differing distillation characteristics and compositions were obtained via desulphurisation, hydrogenation treatment of olefins and the distillation process. 
     Base material C, base material D, base material E, base material F, base material G (FT fuels): natural gas was partially oxidised by means of the SMDS (Shell Middle Distillate Synthesis) process, and after synthesising the syngas derived from carbon monoxide and hydrogen (CO+H 2 ) to a waxy straight-chain alkyl hydrocarbon by means of a Fischer-Tropsch reaction, hydrocracking and isomerisation were carried out over a catalyst, and base materials being mixtures of normal paraffins and isoparaffins with differing distillation characteristics and compositions were obtained. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Units 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Density 
                 g/cm 3   
                 0.769 
                 0.783 
                 0.750 
                 0.774 
                 0.777 
                 0.785 
                 0.757 
               
               
                 @ 15° C. 
               
            
           
           
               
            
               
                 Distillation characteristics 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 IBP 
                 ° C. 
                 180.5 
                 186.0 
                 189.5 
                 258.5 
                 204.0 
                 208.5 
                 155.0 
               
               
                 10% 
                 ° C. 
                 188.5 
                 198.0 
                 199.0 
                 264.5 
                 226.0 
                 244.0 
                 172.0 
               
               
                 50% 
                 ° C. 
                 189.5 
                 208.5 
                 207.0 
                 270.5 
                 261.5 
                 295.0 
                 203.0 
               
               
                 90% 
                 ° C. 
                 192.5 
                 238.0 
                 221.5 
                 285.5 
                 305.0 
                 341.0 
                 295.0 
               
               
                 FBP 
                 ° C. 
                 198.5 
                 247.5 
                 233.5 
                 294.5 
                 315.5 
                 358.0 
                 312.0 
               
               
                 Cetane 
                   
                 57.9 
                 59.0 
                 79.4 
                 94.8 
                 83.7 
                 89.9 
                 66.6 
               
               
                 index 
               
               
                 Sulphur 
                 Mass 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
               
               
                 component 
                 ppm 
               
               
                 Paraffin 
                 Vol. % 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
               
               
                 component 
               
               
                 Aromatic 
                 Vol. % 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
               
               
                 component 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Units 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 A 
                 Vol. % 
                 50 
                   
                   
                   
                   
                   
               
               
                 B 
                 Vol. % 
               
               
                 C 
                 Vol. % 
                 50 
                 100 
               
               
                 D 
                 Vol. % 
                   
                   
                 100 
               
               
                 E 
                 Vol. % 
                   
                   
                   
                 100 
               
               
                 F 
                 Vol. % 
                   
                   
                   
                   
                 100 
               
               
                 G 
                 Vol. % 
                   
                   
                   
                   
                   
                 100 
               
               
                 Density @ 15° C. 
                 g/cm 3   
                 0.759 
                 0.750 
                 0.774 
                 0.777 
                 0.785 
                 0.757 
               
            
           
           
               
            
               
                 Distillation characteristics 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 IBP 
                 ° C. 
                 184.0 
                 189.5 
                 258.5 
                 204.0 
                 208.5 
                 155.0 
               
               
                 10% 
                 ° C. 
                 193.0 
                 199.0 
                 264.5 
                 226.0 
                 244.0 
                 172.0 
               
               
                 50% 
                 ° C. 
                 197.5 
                 207.0 
                 270.5 
                 261.5 
                 295.0 
                 203.0 
               
               
                 90% 
                 ° C. 
                 212.0 
                 221.5 
                 285.5 
                 305.0 
                 341.0 
                 295.0 
               
               
                 FBP 
                 ° C. 
                 228.5 
                 233.5 
                 294.5 
                 315.5 
                 358.0 
                 312.0 
               
               
                 Cetane index 
                   
                 68.3 
                 79.4 
                 94.8 
                 83.7 
                 89.9 
                 66.6 
               
               
                 Sulphur 
                 Mass ppm 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
               
               
                 component 
               
               
                 Paraffin 
                 Vol. % 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
                 ≧99 
               
               
                 component 
               
               
                 Aromatic 
                 Vol. % 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
                 ≦1 
               
               
                 component 
               
               
                 n-paraffins of 
                 Mass % 
                 47.0 
                 94.0 
                 93.8 
                 17.9 
                 12.1 
                 30.3 
               
               
                 18 or fewer 
               
               
                 carbons 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Comparative 
                 Reference 
               
               
                   
                 Units 
                 Example 
                 Example 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 A 
                 Vol. % 
                   
                 — 
               
               
                   
                 B 
                 Vol. % 
                 85 
                 — 
               
               
                   
                 C 
                 Vol. % 
                   
                 — 
               
               
                   
                 D 
                 Vol. % 
                 15 
                 — 
               
               
                   
                 E 
                 Vol. % 
                   
                 — 
               
               
                   
                 F 
                 Vol. % 
                   
                 — 
               
               
                   
                 G 
                 Vol. % 
                   
                 — 
               
               
                   
                 Density @ 15° C. 
                 g/cm 3   
                 0.781 
                 0.786 
               
            
           
           
               
            
               
                 Distillation characteristics 
               
            
           
           
               
               
               
               
               
            
               
                   
                 IBP 
                 ° C. 
                 189.5 
                 209.0 
               
               
                   
                 10% 
                 ° C. 
                 199.5 
                 240.0 
               
               
                   
                 50% 
                 ° C. 
                 212.0 
                 290.0 
               
               
                   
                 90% 
                 ° C. 
                 252.0 
                 341.0 
               
               
                   
                 FBP 
                 ° C. 
                 274.5 
                 355.0 
               
               
                   
                 Cetane index 
                   
                 61.5 
                 85.9 
               
               
                   
                 Sulphur 
                 Mass ppm 
                 ≦1 
                 ≦1 
               
               
                   
                 component 
               
               
                   
                 Paraffin 
                 Vol. % 
                 ≧99 
                 ≧99 
               
               
                   
                 component 
               
               
                   
                 Aromatic 
                 Vol. % 
                 ≦1 
                 ≦1 
               
               
                   
                 component 
               
               
                   
                 n-paraffins of 
                 Mass % 
                 14.1 
                 10.9 
               
               
                   
                 18 or fewer 
               
               
                   
                 carbons 
               
               
                   
                   
               
            
           
         
       
     
     The various characteristics shown in Tables 1 and 2 were measured by the methods below. 
     Density @ 15° C. 
     Density at 15° C. measured in accordance with JIS K 2249 “Crude oil and petroleum products—Determination of density and density/mass/volume conversion tables”. 
     Distillation Characteristics 
     Distillation characteristics obtained in accordance with JIS K 2254 “Petroleum products—Distillation test methods”. 
     Cetane Index 
     Refers to the cetane index measured in accordance with JIS K 2280 “Petroleum products—Fuel oils—Determination of octane number and cetane number, and method for calculation of cetane index, 8. Method of calculating cetane index using the four-variable equation”. However, reference values were recorded in the case of FT fuels because they lie outside the recommended appropriate scope of the cetane index calculation. 
     Sulphur Component 
     Sulphur content obtained in accordance with JIS K 2541-2 “Crude petroleum and petroleum products—Determination of sulphur content, Part 2: The microcoulometric titration-type oxidation method”. 
     Paraffin Component 
     Paraffin component measured in accordance with JPI-5S-49-97 “Petroleum products—Determination of hydrocarbon types—High performance liquid chromatography”. 
     Aromatic Component 
     Sum of monocyclic aromatic and dicyclic aromatic and tri- or higher cyclic aromatic hydrocarbon components measured in accordance with JPI-5S-49-97 “Petroleum products—Determination of hydrocarbon types—High performance liquid chromatography”. 
     Proportion of n-Paraffins 
     For the content of normal paraffins with 18 or fewer carbons, gas chromatography in accordance with ASTM D 2887 “Standard test method for boiling point range distribution of petroleum fractions by gas chromatography” was used, and the normal paraffin content was obtained from the peak area values of different carbon numbers from the chromatograms thus obtained. 
     The type of column in the gas chromatography was HP5 (length: 30 m, inside diameter: 0.32 mm, liquid layer thickness: 0.25 μm), and the analysis conditions were as follows. 
     Column tank temperature rise condition: 35° C. (5 minutes)→10° C./minute (temperature rise) 320° C. (11.5 minutes) 
     Specimen volatilisation chamber conditions: 320° C. fixed, split ratio 150:1 
     Detector part: 320° C. 
     In the case of Examples 1 to 6, the Comparative Example and the Reference Example, proton nuclear magnetic resonance ( 1 H-NMR) spectra analysis was carried out, and the proportion of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm was obtained. In addition, the oxidation index OI was obtained from the proportion of the total peak area obtained and the proportion of normal paraffins of carbon number 18 or less. 
     The results are shown in Tables 4 and 5. 
     As regards the oxidation stability test of Examples 1 to 6, the Comparative Example and the Reference Example, the increment in the total acid number between before and after the acceleration test was measured (referred to below as the acid number). The oxidation stability test was performed in accordance with ASTM D2274 at 115° C. under conditions of oxygen bubbling for 16 hours. These results, too, are shown in Tables 4 and 5. The total acid number was measured in accordance with JIS K 2501 “Petroleum products and lubricants—Determination of neutralisation value”. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Units 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Proportion of 
                 % 
                 1.4 
                 0.6 
                 0.7 
                 3.0 
                 2.9 
                 2.8 
               
               
                   1 H-NMR 
               
               
                 peak area 
               
               
                 Oxidation 
                   
                 0.25 
                 0.00 
                 0.03 
                 0.67 
                 0.65 
                 0.61 
               
               
                 index OI 
               
               
                 Δ Acid 
                 mg KOH/g 
                 0.17 
                 0.07 
                 0.06 
                 0.50 
                 0.85 
                 0.37 
               
               
                 number 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Comparative 
                 Reference 
               
               
                   
                 Units 
                 Example 
                 Example 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Proportion of 
                 % 
                 6.5 
                 4.7 
               
               
                   
                   1 H-NMR 
               
               
                   
                 peak area 
               
               
                   
                 Oxidation 
                   
                 1.54 
                 1.10 
               
               
                   
                 index OI 
               
               
                   
                 Δ Acid 
                 mg KOH/g 
                 1.51 
                 1.36 
               
               
                   
                 number 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 4, although all Examples 1 to 6 were constituted only of paraffins, their Δ acid number was smaller than the Reference Example, which was an FT fuel of the prior art, and it was confirmed that the oxidation stability was superior. The A acid number of the Comparative Example was above the Reference Example and it may be concluded that the oxidation stability was not improved. 
     It was confirmed that all the Examples of the present invention were characterised in that 
     1) the normal paraffins with 18 or fewer carbons constituted not less than 12% by mass, and
 
2) the proportion of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm was less than 6.5% in proton nuclear magnetic resonance ( 1 H-NMR) spectra, and the oxidation index OI represented by the following equation was less than 1.10.
 
       OI=0.247 X− 0.001 Y− 0.053 
     wherein X is the proportion (%) of the total peak area of the peak group at chemical shifts of 1.45 to 2.25 ppm relative to the total peak area of the peak group at chemical shifts of 1.00 to 1.45 ppm in proton nuclear magnetic resonance ( 1 H-NMR) spectra; and Y is the content (mass %) of normal paraffins with 18 or fewer carbons).