Blended fuel compositions and processes for manufacture

Abstract

A fuel additive composition comprising butanol, pentanol, hexanol, heptanol, methylcyclohexanol, ethyl vinyl ether and nonanol. The composition may comprise 1-butanol, 2-butanol, 2-pentanol, 3-hexanol

Description

Field of Invention The present invention relates to fuel additives and processes for preparing fuel additives. In particular, the present invention relates to fuel additive blends derived from converting bioethanol to higher alcohols. Background Climate change and the greenhouse effect is a growing concern in the modern world. There is pressure within the transportation sector to lower carbon emissions, which are significant within this field due to the pollutant emissions generated from the combustion of fossil fuel-based fuels. However, most countries do not have sufficient funds to transition to renewable energy sources at scale, especially with growing energy costs. This is heightened by the gradual depletion of fossil fuels. There is increasing widespread concern regarding the production of fuels or chemicals that require fossil fuels, due to the increasing scarcity of the raw materials. This has contributed to an increase in the price of fuel, which has been recently exacerbated following the Covid-19 pandemic and the Russian invasion of Ukraine. This has caused a fuel crisis in many places around the world. Many refineries have been forced to close due to high operational costs and some countries are struggling with energy insecurity. For example, Sri Lanka faced shortages and significantly increased prices in 2021 in the oil, gas and electricity markets, which led to the government declaring bankruptcy. The energy crisis and concern regarding climate change has led to an increased interest in the production of fuels and chemicals from renewable sources, such as biomass derived from plants, agricultural waste, or forest residues. Consequently, the clean energy industry has turned their attention to the use of alcohol as an alternative fuel source, especially for use in internal combustion engines. Ethanol is produced on industrial scales through fermentation of lignocellulose and is one of the largest volumes of biofuels. It has been found that ethanol can be used directly as a fuel or instead blended with gasoline as a fuel additive in order to reduce the dependence on fossil fuels. Currently, the most prominent biofuel is an ethanol blended fuel, known as E10 gasoline. However, there are many issues that arise from the use of ethanol as a fuel. Ethanol suffers from high hygroscopicity and causes corrosion to metal parts, which limits its application in the transportation sector. Furthermore, ethanol also suffers from a relatively low energy density, when compared with gasoline, which limits the efficiency of the blended fuel. Therefore, butanol has been suggested as an alternative to ethanol because it possesses an energy density closer to gasoline and has a less corrosive nature. However, although fermentation of biomass to higher alcohols has been developed, economical production on an industrial scale remains a challenge as new infrastructure is required, such as butanol fermentation plants. Thus, there remains a need in the art for inexpensive biofuels that are capable of replacing or reducing the dependence on fossil fuels. Summary of the Invention This invention provides a fuel additive composition comprising butanol, pentanol, hexanol, heptanol, methylcyclohexanol, ethyl vinyl ether and nonanol. This invention further provides a process providing efficient conversion of widely available bioethanol to form higher alcohols in commercially viable yields to develop a fuel additive composition to provide a high-density biofuel blend. This process comprises the steps of: i) reacting an ethanol feedstock to form a product mixture comprising a plurality of higher alcohols; ii) separating C4 to C9 alcohols from the product mixture; iii) blending the C4 to C9 alcohols and ethyl vinyl ether to form a fuel additive composition according to the present invention. Also provided is a blended fuel composition comprising a fuel component and a fuel additive composition of the present invention. A process for producing an aviation fuel additive composition is also provided. The process involves the steps of: i) reacting an ethanol feedstock to form a product mixture comprising a plurality of higher alcohols; ii) separating C4 to C9 alcohols from the product mixture; iii) dehydrating the C4 to C9 alcohols to form corresponding alkenes; iv) oligomerizing the corresponding alkenes to form oligomers; v) blending the oligomers to form an aviation fuel additive. The present invention provides a method for producing a fuel additive composition from an ethanol feedstock. The ethanol feedstock may generally comprise, and preferably consists of, ethanol derived from biomass, typically via fermentation. For example, bioethanol is commonly produced from a lignocellulosic feedstock. However, alternative sources of ethanol may be used. In step (i) of the process, the ethanol feedstock is converted to form a reaction mixture comprising a plurality of higher alcohols. Higher alcohols are defined as any alcohol that contains more than 2 carbons and therefore include propanol, butanol, pentanol, etc. In the present application, CX alcohol is used to indicate an alcohol compound containing X carbons. For example, C4 to C6 alcohols includes alcohol compounds that contain from 4 to 6 carbons. There are many methods of converting ethanol to form higher alcohols, which will be known to the person skilled in the art. For example, the conversion of ethanol to higher alcohols may be typically provided by a Guerbet reaction mechanism. This mechanism performs an aldol condensation and self-condensation simultaneously to provide alcohols with longer carbon chains. Preferably, in the present invention, a Guerbet reaction mechanism may be performed in the presence of an excess of a heterogenous catalyst. Examples of suitable heterogeneous catalyst include Ni (e.g. Raney Ni), Pd, Cu, alkoxides, alkali earth oxides, hydrotalcites, hydroxyapatite or oxynitrides. Preferably, the catalyst used may be Calcium carbide. Calcium carbide provides a superior catalytic activity for the condensation of ethanol to C4 to C9 alcohols. Typically, the Guerbet reaction mechanism may require a base. Commonly, the base used includes alkali metal hydroxides, for example, potassium hydroxide, sodium hydroxide, sodium tert-butoxide, etc. The process temperature for the Guerbet reaction may be typically greater than the boiling point of water. Step (i) of the process may be carried out at a temperature of at least 250 °C, preferably at least 260 °C, and more preferably at least 270 °C. Step (i) may be carried out at a temperature up to 350 °C, preferably up to 325 °C, and more preferably up to 300 °C. Thus step (i) may be carried out at a temperature from 250 to 350 °C, preferably at a temperature from 260 to 325 °C, and more preferably at a temperature from 270 to 300 °C. These temperatures are particularly suitable for producing a high yield of alcohols with chain lengths between 4 and 9 carbons. The product mixture will generally contain a mixture of alcohol, carboxylic acid, ester, ether, and ketone compounds. The mixture formed by the reaction of Step (i) may also include aromatic compounds, which can be further processed into other commercially important compounds, such as lubricants and super heavy bio-aviation fuels. Step (ii) of the process of the invention involves separating C4 to C9 alcohols from the reaction mixture of Step (i). There are many ways to separate the C4 to C9 alcohols from the reaction mixture. These include but are not limited to various distillation techniques (fractional, extractive, azeotropic, column, etc.). In the present invention, preferably the alcohols may be separated using fractional distillation. The alcohols may be separated such that a mixture of C4 to C9 alcohols is obtained. Alternatively, the alcohols may be separated to provide each C4 to C9 alcohol individually. A person skilled in the art will appreciate that the alcohols may also be obtained in groups, such as C4 to C6 and C7 to C9. Any alcohol products that are smaller than C4, e.g., ethanol or propanol, may be recycled back into the reagent mixture to be reused. This allows for greater atom economy for the reaction mixture. The process may include an additional step of separating ethers from the product mixture. This may be preferably achieved by distillation, although the skilled person would readily understand that other separation techniques may also be used. The separation of ethers from the product mixture may be partial or complete. The ethers produced in the process include ethyl vinyl ether. Ethyl vinyl ether has many commercial uses, from use in fragrances to lubricating oil additives. Therefore, the ethyl vinyl ether produced in the present process may be separated for alternative uses. Step (iii) of the process involves blending the separated alcohols and ethyl vinyl ether to form a fuel additive composition. The ethyl vinyl ether separated from the product mixture may be blended with the other separated alcohols to form a fuel additive composition or ethyl vinyl ether from a different source may be blended. The invention provides a fuel additive composition comprising butanol, pentanol, hexanol, heptanol, methylcyclohexanol, ethyl vinyl ether and nonanol. Preferably, the fuel additive composition may comprise 1-butanol, 2-butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 4-heptanol, 2-heptanol, 3-methylcyclohexanol, ethyl vinyl ether and 4-nonanol. The fuel additive composition may comprise 2-butanol. The 2-butanol may be present in an amount up to 15%, preferably an amount up to 12%, and more preferably up to 10% by volume of the fuel additive composition. 2-butanol may be present in an amount of at least 3 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 7 % by volume of the fuel additive composition. Thus, 2-butanol may be present in an amount from 3 to 15 %, preferably in an amount from 5 to 12 %, more preferably in an amount from 7 to 10 % by volume of the fuel additive composition. The fuel additive composition may comprise 1-butanol. The 1-butanol may be present in an amount up to 30%, preferably an amount up to 28%, and more preferably up to 26% by volume of the fuel additive composition. 1-butanol may be present in an amount of at least 10 %, preferably in an amount of at least 12 %, more preferably in an amount of at least 15 % by volume of the fuel additive composition. Thus, 1-butanol may be present in an amount from 10 to 30 %, preferably in an amount from 12 to 28 %, more preferably in an amount from 15 to 26 % by volume of the fuel additive composition. The fuel additive composition may comprise 2-pentanol. The 2-pentanol may be present in an amount up to 10%, preferably an amount up to 7%, and more preferably up to 5% by volume of the fuel additive composition. 2-pentanol may be present in an amount of at least 1 %, preferably in an amount of at least 2 %, more preferably in an amount of at least 3 % by volume of the fuel additive composition. Thus, 2-pentanol may be present in an amount from 1 to 10 %, preferably in an amount from 2 to 7 %, more preferably in an amount from 3 to 5 % by volume of the fuel additive composition. The fuel additive composition may comprise 3-hexanol. The 3-hexanol may be present in an amount up to 15%, preferably an amount up to 12%, and more preferably up to 10% by volume of the fuel additive composition. 3-hexanol may be present in an amount of at least 3 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 7 % by volume of the fuel additive composition. Thus, 3-hexanol may be present in an amount from 3 to 15 %, preferably in an amount from 5 to 12 %, more preferably in an amount from 7 to 10 % by volume of the fuel additive composition. The fuel additive composition may comprise 1-hexanol. The 1-hexanol may be present in an amount up to 15%, preferably an amount up to 12%, and more preferably up to 10% by volume of the fuel additive composition. 1-hexanol may be present in an amount of at least 3 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 7 % by volume of the fuel additive composition. Thus, 1-hexanol may be present in an amount from 3 to 15 %, preferably in an amount from 5 to 12 %, more preferably in an amount from 7 to 10 % by volume of the fuel additive composition. The fuel additive composition may comprise 2-ethyl-1-butanol. The 2-ethyl-1-butanol may be present in an amount up to 12%, preferably an amount up to 10%, and more preferably up to 8% by volume of the fuel additive composition. 2-ethyl-1-butanol may be present in an amount of at least 2 %, preferably in an amount of at least 4 %, more preferably in an amount of at least 5 % by volume of the fuel additive composition. Thus, 2-ethyl-1 -butanol may be present in an amount from 2 to 12 %, preferably in an amount from 4 to 10 %, more preferably in an amount from 5 to 8 % by volume of the fuel additive composition. The fuel additive composition may comprise 4-heptanol. The 4-heptanol may be present in an amount up to 10%, preferably an amount up to 8%, and more preferably up to 6% by volume of the fuel additive composition. 4-heptanol may be present in an amount of at least 1 %, preferably in an amount of at least 3 %, more preferably in an amount of at least 4 % by volume of the fuel additive composition. Thus, 4-heptanol may be present in an amount from 1 to 10 %, preferably in an amount from 3 to 8 %, more preferably in an amount from 4 to 6 % by volume of the fuel additive composition. The fuel additive composition may comprise 2-heptanol. The 2-heptanol may be present in an amount up to 12%, preferably an amount up to 10%, and more preferably up to 8% by volume of the fuel additive composition. 2-heptanol may be present in an amount of at least 2 %, preferably in an amount of at least 4 %, more preferably in an amount of at least 5 % by volume of the fuel additive composition. Thus, 2-heptanol may be present in an amount from 2 to 12 %, preferably in an amount from 4 to 10 %, more preferably in an amount from 5 to 8 % by volume of the fuel additive composition. The fuel additive composition may comprise 3-methylcyclohexanol. The 3-methylcyclohexanol may be present in an amount up to 15%, preferably an amount up to 12%, and more preferably up to 10% by volume of the fuel additive composition. 3-methylcyclohexanol may be present in an amount of at least 3 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 7 % by volume of the fuel additive composition. Thus, 3-methylcyclohexanol may be present in an amount from 3 to 15 %, preferably in an amount from 5 to 12 %, more preferably in an amount from 7 to 10 % by volume of the fuel additive composition. 3-methylcyclohexanol improves the lubricity of the fuel and therefore helps to prevent wear in the fuel injectors and other components of the fuel system. By enhancing the lubricity, 3-methylcyclohexanol can help extend the life of the engine components. Furthermore, 3-methylcyclohexanol improves the stability of the fuel additive composition or the fuel blend, and reduces the degradation of the fuel additive composition. This, in turn, reduces the likelihood of issues occurring within the engine system, such as clogging of filters or fuel lines. Without being bound by theory, this improved stability may occur due to the solvent properties of 3-methylcyclohexanol, which provide the other components with increased solubility. Additionally, the structure of 3-methylcyclohexanol is beneficial as it may increase the efficiency of combustion, resulting in less carbon monoxide, nitrogen oxides and particulate matter being formed during combustion of the fuel additive composition or fuel blend. The cyclic structure of 3-methylcyclohexanol allows for more uniform burning and better mixing with air in the combustion chamber. The fuel additive composition comprises ethyl vinyl ether. The ethyl vinyl ether may be present in an amount up to 15%, preferably an amount up to 12%, and more preferably up to 10% by volume of the fuel additive composition. Ethyl vinyl ether may be present in an amount of at least 3 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 7 % by volume of the fuel additive composition. Thus, ethyl vinyl ether may be present in an amount from 3 to 15 %, preferably in an amount from 5 to 12 %, more preferably in an amount from 7 to 10 % by volume of the fuel additive composition. The inclusion of ethyl vinyl ether in the fuel additive composition improves the cold-start performance of engines in which a blended fuel of the present invention is being used. Ethyl vinyl ether has a high volatility and a low flash point, which makes it easier for the fuel to vaporise and ignite in cold temperatures. This characteristic is particularly beneficial because fuel ignition can be problematic in colder temperatures. Thus, by improving cold start performance, ethyl vinyl ether ensures that the vehicles operate more smoothly and efficiently, and so reduces potential wear on the engine and minimises emissions associated with incomplete combustion during start-up. Ethyl vinyl ether further contributes to the overall octane rating of the fuel blend. A higher octane rating reduces the likelihood of engine knocking, which is a condition that can cause engine damage over time. Therefore, by increasing the octane rating, ethyl vinyl ether helps in maintaining engine health and optimising performance, especially under the high compression ratios found in modern engines. Additionally, ethyl vinyl ether can act as a solvent for the other organic compounds present in the fuel additive composition, or a blended fuel, which improves the homogeneity and stability of the mixtures. The fuel additive composition may comprise 4-nonanol. The 4-nonanol may be present in an amount up to 10%, preferably an amount up to 8%, and more preferably up to 6% by volume of the fuel additive composition. 4-nonanol may be present in an amount of at least 1 %, preferably in an amount of at least 3 %, more preferably in an amount of at least 4 % by volume of the fuel additive composition. Thus, 4-nonanol may be present in an amount from 1 to 10 %, preferably in an amount from 3 to 8 %, more preferably in an amount from 4 to 6 % by volume of the fuel additive composition. Higher alcohols, such as butanol, pentanol and heptanol have a higher energy density, a higher cetane number and better blending stability than ethanol. Therefore, the fuel additive is more efficient if it contains higher alcohols compared to ethanol. The present invention allows for a fuel blend containing greater amounts of higher alcohols. The energy density of the alcohols and other components that may be present in the fuel additives of the present invention are as follows: Component Energy Density (MJ / kg) 2-butanol 31.5 1-butanol 29.2 2-pentanol 29.0 3-hexanol 27.6 2-ethyl-1-butanol 30.2 1-hexanol 29.7 4-heptanol 28.5 2-heptanol 28.4 3-methylcyclohexanol 33.2 ethyl-vinyl ether 25.3 4-nonanol 30.0 Ethanol has an energy density of 26.8 MJ / kg. Accordingly, the alcohols and other components in the fuel additive are more energy-dense, leading to a more energy efficient fuel additive, compared with the present commercially available alternatives. Higher alcohols are less hydrophilic and therefore have a lower hygroscopicity than lower alcohols, such as ethanol. Accordingly, the fuel additive composition of the present invention reduces the risk of moisture-related issues in the blended fuel, especially when compared to traditional ethanol blends. Preferably, the fuel additive composition comprises less than 30% by volume of each individual alcohol. % volume is in reference to the volume of the fuel additive composition. Each component of the fuel additive composition of the present invention contains oxygen in its molecular structure, which helps to improve the combustion efficiency of the fuel. Without being bound by theory, it is considered that the presence of oxygen within the fuel at a molecular level promotes more complete combustion of hydrocarbons, leading to fewer unburnt fuel emissions and so leads to less harmful pollutants such as carbon monoxide and particulate matter. For example, the presence of oxygen in the alcohol molecules leads to more complete combustion from converting CO to CO2. It is preferable, when blending the fuel additive with diesel, that the fuel additive contains larger amounts of C6 to C9 alcohols. Hexanol has a higher cetane number and higher energy density than 1-butanol, which is better for diesel engines. Heptanol also has highly favourable fuel properties. For example, 1-heptanol may be used to decrease CO and unburned hydrocarbon emissions. In an embodiment, the fuel additive composition comprises butanol in an amount up to 26 % by volume, pentanol in an amount up to 5 % by volume, hexanol in an amount up to 28 % by volume, heptanol in an amount up to 14 % by volume, nonanol in an amount up to 6 % by volume, methylcyclohexanol in an amount up to 10 % by volume, and ethyl vinyl ether in an amount up to 5% by volume. Preferably, the fuel additive composition comprises butanol in an amount from 5 to 26 % by volume, pentanol in an amount from 1 to 5 % by volume, hexanol in an amount from 5 to 28 % by volume, heptanol in an amount from 3 to 14 % by volume, nonanol in an amount from 1 to 6 % by volume, methylcyclohexanol in an amount from 2 to 10 % by volume, and ethyl vinyl ether in an amount from 1 to 5% by volume In an embodiment, the fuel additive composition of the invention comprises a blend of 2-butanol, 1-butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 1-hexanol, 4-heptanol, 2-heptanol, 3-methylcyclohexanol, ethyl vinyl ether and 4-nonanol. Preferably the fuel additive composition comprises 2-butanol in an amount from 3 to 15 vol.%, 1-butanol in an amount from 10 to 30 vol.%, 2-pentanol in an amount from 1 to 10 vol.%, 3-hexanol in an amount from 3 to 15 vol.%, 2-ethyl-1-butanol in an amount from 2 to 12 vol.%, 1-hexanol in an amount from 3 to 15 vol.%, 4-heptanol in an amount from 1 to 10 vol.%, 2-heptanol in an amount from 2 to 12 vol.%, 3-methylcyclohexanol in an amount from 3 to 15 vol.%, ethyl vinyl ether in an amount from 3 to 15 vol.% and 4-nonanol in an amount from 1 to 10 vol.%. This blend of specific isomers is particularly beneficial for the fuel additive composition. As the skilled person will be aware, isomers can have different physical and chemical properties. Thus, different isomers can provide the fuel additive composition with different properties and performance ability. These amounts of each component have been optimised to provide a balance between reducing carbon emission, by reducing the amount of fossil fuel required to be present when blending the fuel additive composition with a conventional fuel, and maintaining the energy output needed for modern engines. Accordingly, this fuel blend can be used with current engine technology, without requiring modifications. Furthermore, the amounts of each component allow for forming fuel blends that maintain good performance across a range of temperatures and operating conditions, and avoid issues such as phase separation or increased viscosity of the fuel blends. Furthermore, a fuel additive composition according to the present invention is less polluting than traditional fuels and may emit at least three times fewer pollutants, such as carbon monoxide, NOx gases and unburned hydrocarbons, when compared with traditional fuels. The specific energy density achieved with the present fuel additive compositions allows vehicles to achieve similar or better mileage when compared to standard fuels. This provides, not only a greater cost effectiveness, but also a significant decrease in the release of greenhouse gases, especially as the feedstock for the fuel additive composition is formed from bio-based materials. The invention also provides a blended fuel composition comprising a fuel component and the fuel additive of the invention. The fuel component is likely to be a traditional fuel component, such as gasoline or diesel. The fuel additive compositions of the invention are highly suitable for blending with both gasoline or diesel in large volumes. Preferably the blended fuel composition comprises the fuel additive in an amount of greater than 10% by volume of the blended fuel composition. This is possible due to the composition of the fuel blend providing preferable energy density and solubility properties, which allows for smaller amounts of fossil fuels within the blended fuel composition. A blended fuel composition according to the present invention is less polluting and cleaner than the commonly used ethanol blended gasoline as the fuel additive may be added to the conventional fossil fuel component in greater amounts. Furthermore, this may allow for a lower demand of fossil fuels, making the fuel blend more sustainable than the currently used ethanol blended fuels. The fuel additive of the present invention may be added to petroleum in an amount of up to 60 % by volume, preferably in an amount up to 50 % by volume, most preferably in an amount up to 30 % by volume. The fuel additive of the present invention may be added to diesel in an amount of up to 40 % by volume, preferably in an amount up to 30 % by volume, most preferably in an amount up to 20 % by volume. The present invention also provides a process for producing an aviation fuel additive. Steps (i) and (ii) of the process for producing an aviation fuel additive correspond with the same steps for producing a fuel additive. In step (i) of the process, the ethanol feedstock is converted to form a reaction mixture comprising a plurality of higher alcohols. The ethanol feedstock may generally comprise, and preferably consists of, ethanol derived from biomass, typically via fermentation. However, alternative sources of ethanol may be used. Step (ii) of the process of the invention involves separating C4 to C9 alcohols from the reaction mixture of Step (i). The alcohols may be separated such that a mixture of C4 to C9 alcohols is obtained. Alternatively, the alcohols may be separated to provide each C4 to C9 alcohols individually. The alcohols may also be obtained in groups, such as C4 to C6 and C7 to C9. Step (iii) of the process for producing an aviation fuel additive involves dehydrating the C4 to C9 alcohols to form their corresponding alkenes. Typical reaction conditions for the dehydration of alcohols to alkenes may include heating the alcohols in the presence of a strong acid, such as sulphuric acid or phosphoric acid. This step may be achieved using a mixture of C4 to C9 alcohols to form a mixture of alkenes. Alternatively, each alcohol may be dehydrated separately to form the corresponding alkene. Dehydrating a mixture of alcohols in a single step is preferable as it is more efficient. Preferably, the alcohols are selected such that the dehydrated alkenes include 1-butene, 2-butene, 1-hexene, 3-hexene, 2-pentene, 4-heptene, 3-methycyclohexene, 1-methylcyclohexene and 4-nonene. Step (iv) of the present invention involves oligomerizing the alkenes produced form the dehydration step to form oligomers. This process converts the alkene monomers into medium-length chain oligomers. Typically, the products formed have a carbon chain length of between C6 and C26. This process may be typically catalysed by an acidic catalyst. Examples include solid phosphoric acid, zeolite and modified zeolite, sulfonic acid-functionalised materials, transition-metal complexes, metal oxides, and organometallic complexes. Preferably, the catalysts may be homogeneous catalysts. Homogeneous catalysts provide higher yields than solid acid catalysts because solid acid catalysts are more prone to deactivation and are also limited by their oligomer production. Due to the mechanism for the oligomerization process, higher alcohols provide a higher theoretical yield. This is because the conversion of ethanol to higher alcohols provides for heavier olefins after the dehydration step. This in turn allows fora higheryield and requires significantly less time and energy. As such, the process involves lower production costs for the oligomerization step. A key difference between the process of the present invention and ethanol oligomerization is that the present invention eliminates the increased recycling in the oligomerization step. Increased recycling is required in ethanol oligomerization in order to achieve the desired length of oligomers fora bio-kerosene. This increased recycling leads to increased capital costs and energy requirements that accompany higher flow rates through the reactor and / or distillation units. Conversely, the present invention provides increased variability in length and branching qualities of the produced oligomers, which allows for greater optimisation to fit desired fuel profiles downstream. Particularly suitable oligomers for the aviation fuel of the present invention have carbon chain lengths of from C10 to C18. Oligomers having longer chain lengths can be subjected to hydrocracking to form oligomers of the desired length. Furthermore, the products formed may be distilled to separate different fractions to provide oligomers with the desired length. The hydrocracking is preferably performed using a bifunctional catalyst, such as a metal on an acidic support. Suitable catalysts include either platinum or palladium on a zeolite support or a nickel-molybdenum catalyst. Furthermore, the hydrocracking may be performed at a temperature between 330 °C and 420 °C, preferably at a temperature between 350 °C and 400 °C, and at a pressure of 80 to 150 atm. Hydrocracking is performed in the presence of hydrogen, and preferably the hydrogen supply has a molar ratio of 5:1 to 10:1 hydrogen to hydrocarbon. Using these conditions, the produced oligomers are effectively produced with a carbon length between C8 to C16. The present invention is able to utilise the abundant bio-ethanol whilst providing a higher yield of aviation fuel, with a lower oligomerisation cost and lower process time than ethanol oligomerisation. Step (v) of the present invention involves blending the oligomers to form an aviation fuel. This provides an energy dense, chemically similar bio-kerosene that can be used as an aviation fuel or aviation fuel additive. The aviation fuel may be used as a biokerosene without another fuel component. The alkenes may be mixed before the oligomerizing step or alternatively separated from the product mixture as a mixture of alcohols. For example, the alcohols may be separated in groups, such as C4 to C6 and C7 to C9 or even as a single group containing C4 to C9 alcohols. Therefore, the alkenes may be oligomerised as a mixture of alkenes. This results in the oligomerizing step and blending step being performed simultaneously. The aviation fuel produced by the above process provides a realistic solution to reduce greenhouse gas emissions. The aviation fuel provided reduces greenhouse gas emissions by up to 80% compared with a fossil fuel-based jet fuel. Additionally, the aviation fuels provided can be optimised to satisfy various fuel requirements and constraints. The aviation fuel of the present invention may have a density from 0.775 to 0.840 kg / L at 15 °C, preferably from 0.800 to 0.840 kg / L at 15 °C. This preferred range is particularly suitable for long-haul flights because of the increased energy content per unit volume. The aviation fuel of the present invention may contain oligomers with a carbon chain length of between C8 and C16 carbons. This carbon chain length is particularly suitable for use as an aviation fuel as it provides favourable volatility and energy density. Jet fuel typically has an energy content of 43 MJ / kg and carbon chain lengths of this length should fulfil these conditions. The aviation fuel may contain n-octane. For example, the aviation fuel may contain up to 15 %, preferably up to 12 % and more preferably up to 10 % by volume of the aviation fuel of n-octane. N-octane may be present in an amount of at least 2 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 6 % by volume of the aviation fuel. Thus, n-octane may be present in an amount of from 2 to 15 %, preferably from 5 to 12 %, and more preferably from 6 to 10% by volume of the aviation fuel. The aviation fuel may contain n-decane. For example, the aviation fuel may contain up to 30 %, preferably up to 25 % and more preferably up to 20 % by volume of the aviation fuel of n-decane. N-decane may be present in an amount of at least 5 %, preferably in an amount of at least 10 %, more preferably in an amount of at least 15 % by volume of the aviation fuel. Thus, n-decane may be present in an amount of from 5 to 30 %, preferably from 10 to 25 %, and more preferably from 15 to 20% by volume of the aviation fuel. The aviation fuel may contain n-dodecane. For example, the aviation fuel may contain up to 20 %, preferably up to 18 %, and more preferably up to 15 % by volume of the aviation fuel of n-dodecane. N-dodecane may be present in an amount of at least 5 %, preferably in an amount of at least 7 %, more preferably in an amount of at least 10 % by volume of the aviation fuel. Thus, n-dodecane may be present in an amount of from 5 to 20 %, preferably from 7 to 18 %, and more preferably from 10 to 15% by volume of the aviation fuel. The aviation fuel may contain n-tetradecane. For example, the aviation fuel may contain up to 35 %, preferably up to 30 % and more preferably up to 25 % by volume of the aviation fuel of n-tetradecane. N-tetradecane may be present in an amount of at least 10 %, preferably in an amount of at least 15 %, more preferably in an amount of at least 20 % by volume of the aviation fuel. Thus, n-tetradecane may be present in an amount of from 10 to 35 %, preferably from 15 to 30 %, and more preferably from 20 to 25 % by volume of the aviation fuel. The aviation fuel may contain n-hexadecane. For example, the aviation fuel may contain up to 20 %, preferably up to 18 %, and more preferably up to 15 % by volume of the aviation fuel of n-hexadecane. N-hexadecane may be present in an amount of at least 5 %, preferably in an amount of at least 7 %, more preferably in an amount of at least 10 % by volume of the aviation fuel. Thus, n-hexadecane may be present in an amount of from 5 to 20 %, preferably from 7 to 18 %, and more preferably from 10 to 15% by volume of the aviation fuel. The aviation fuel may contain cyclohexane. For example, the aviation fuel may contain up to 8 %, preferably up to 6 % and more preferably up to 5 % by volume of the aviation fuel of cyclohexane. Cyclohexane may be present in an amount of at least 1 %, preferably in an amount of at least 2 %, more preferably in an amount of at least 3 % by volume of the aviation fuel. Thus, cyclohexane may be present in an amount of from 1 to 8 %, preferably from 2 to 6 %, and more preferably from 3 to 5 % by volume of the aviation fuel. The aviation fuel may contain ethyl vinyl ether. For example, the aviation fuel may contain upto 15 %, preferably upto 12 % and more preferably upto 10 % by volume of the aviation fuel of ethyl vinyl ether. Ethyl vinyl ether may be present in an amount of at least 2 %, preferably in an amount of at least 5 %, more preferably in an amount of at least 6 % by volume of the aviation fuel. Thus, ethyl vinyl ether may be present in an amount of from 2 to 15 %, preferably from 5 to 12 %, and more preferably from 6 to 10% by volume of the aviation fuel. In an embodiment, the aviation fuel contains n-octane in an amount from 2 to 15 % by volume, n-decane in an amount from 5 to 30 % by volume, n-dodecane in an amount from 5 to 20 % by volume, n-tetradecane in an amount from 10 to 35 % by volume, n-hexadecane in an amount from 5 to 20 % by volume, cyclohexane in an amount from 1 to 8 % by volume, and ethyl vinyl ether in an amount from 2 to 15 % by volume. Furthermore, various additives may be included in the aviation fuel to provide the fuel with favourable properties. For example, vinyl methyl ether may be produced during step (i), which can be separated from the alcohol products. Vinyl methyl ether may then be included in the aviation fuel to act as a stabiliser and to prevent oxidation. Alternatively, antioxidant compounds may also be suitable, such as butylated hydroxytoluene, to prevent oxidation of the fuel and thereby extend its shelf life. Other suitable additives include metal deactivators, which inhibit the catalytic effects of metal ions that can accelerate oxidation and thus acts as an antioxidant. Examples of metal deactivators include N,N’-disalicylidene-1,2-propanediamine. Preventing degradation of the fuel via oxidation maintains the fuel quality for longer periods and reduces gum formation. Furthermore, detergents may be added to the aviation fuel, such as polyether amine, which help keep fuel system components clean by preventing deposit formation. Additionally, other corrosion inhibitor compounds may be included, such as tolyltriazole derivatives, to also prevent corrosion of fuel system components. Other fuel properties can also be selectively enhanced through the inclusion of additive compounds. For example, where the fuel is being used in cold temperatures, cold flow improver compounds, such as ethylene vinyl acetate, may be used to improve the low-temperature flow properties of the fuel. Detailed Description An ethanol feedstock is reacted with calcium carbide as a catalyst at a temperature between 270 and 300 °C for 4 hours to form a product mixture of higher alcohols. The product mixture formed is a blend of higher alcohols, including 2-butanol, 1-butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 1-hexanol, 4-heptanol, 2-heptanol and 3-methylcyclohexanol. Additionally, the product mixture includes ethyl-vinyl ether. The product mixture is subjected to fractional distillation to separate C4 to C9 alcohols from the product mixture. Ethyl vinyl ether is also separated from the product mixture at this stage. To form a fuel additive, the higher alcohols and ethyl vinyl ether are mixed together in the following amounts: Component Amount (%) 2-butanol 6 1-butanol 26 2-pentanol 5 3-hexanol 10 2-ethyl-1-butanol 8 1-hexanol 10 4-heptanol 6 2-heptanol 8 3-methylcyclohexanol 10 ethyl-vinyl ether 5 4-nonanol 6 This fuel additive composition is then blended with gasoline in an amount of 30 % by volume fuel additive to 70 % by volume gasoline. In an alternate embodiment, the fuel additive composition is blended with diesel in an amount of 20 % by volume fuel additive to 80 % by volume diesel. To form an aviation fuel, the alcohols separated from the reaction mixture are subjected to a dehydration reaction to form alkenes. This is performed by reacting the alcohols using an alumina catalyst at a temperature of 200 to 250 °C for 2 hours. The alcohols react to form the corresponding alkene. The mixture of alkenes includes 2-butene, 1-butene, 2-pentene, 3-hexene, 2-ethyl-1-butene, 1-hexene, 4-heptene, 2-heptene and 3-methylcyclohexene. These alkenes are then subject to an oligomerisation reaction to form larger hydrocarbon chains. This is achieved by reacting the alkenes with a zeolite catalyst at a temperature between 150 to 180 °C for 3 hours. The oligomers formed are then subjected to hydrocracking to break down the long chains into chains of a carbon length from C8 to C16. This is achieved using a nickel-molybdenum catalyst at a temperature of 400 °C. The reaction is performed in a pressurised vessel, with a pressure of 20 MPa. This reaction is performed for approximately 2 hours. Following hydrocracking, the mixture is distilled to separate oligomers with different carbon chain lengths. The oligomers with chain lengths from C8 to C16 were blended together to form an aviation fuel. This aviation fuel has the following composition. Component Fraction (%) Energy Content (MJ / kg) n-Octane 10% 44 n-Decane 20% 44.1 n-Dodecane 15% 44.2 n-Tetradecane 25% 44.3 n-Hexadecane 15% 44.4 Cyclohexane 5% 41 Ethyl Vinyl Ether 10% 36 The weighted energy content is 43.24 MJ / kg. This is comparable with convention jet fuels that have an energy content around 43 MJ / kg. The following examples are for illustrative purposes only, and are not intended to limit the scope of the invention in anyway. Examples Example 1 - Preparation of a fuel additive A bio-ethanol feedstock was reacted with calcium carbide at a temperature between 275 and 300 °C for 7 hours to form a product mixture. The mixture was then distilled to separate the various alcohols and other components. Then 3 litres of a fuel additive was produced by mixing 1-butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 1-hexanol, 4-heptanol, 2-heptanol, 3-methylcyclohexanol, 4-nonanol and ethyl vinyl ether in the following amounts. Component Amount (mL) 1-butanol 780 2-pentanol 150 3-hexanol 300 2-ethyl-1-butanol 240 1-hexanol 300 4-heptanol 180 2-heptanol 240 3-methylcyclohexanol 300 Ethyl vinyl ether 150 4-nonanol 180 The mixture was mixed at a moderate speed for 30 minutes to form a homogenous mixture of a fuel additive. Example 2 - Preparation of an aviation fuel An alcohol mixture was formed by reacting an bio-ethanol feedstock with calcium carbide as a catalyst at a temperature between 270 and 300 °C for 4 hours. The alcohol product was distilled to separate 1-butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 1-hexanol, 4-heptanol, 2-heptanol and 3-methylcyclohexanol. The alcohols were measured and mixed in the following amounts to prepare an alcohol mixture. Alcohol Amount (mL) 1-butanol 260 2-pentanol 50 3-hexanol 100 2-ethyl-1-butanol 80 1-hexanol 100 4-heptanol 60 2-heptanol 80 3-methylcyclohexanol 100 The alcohol mixture was transferred to a reaction vessel alongside an alumina catalyst for 2 hours and then heated to 300 °C under an inert atmosphere to form olefins. The olefins were then transferred to an oligomerisation reactor, alongside an oligomerisation catalyst. The olefins were then heated to 250 °C under an inert atmosphere. The oligomers were then transferred to a hydrocracking reactor. A hydrocracking reaction was then performed at 400 °C. The mixture was then transferred to a distillation column and fractional distillation was performed and the fractions containing oligomers with carbon chain lengths from C8 to C16 were collected and blended together. The following additives were included: vinyl methyl ether (5 vol.%), butylated hydroxytoluene (0.1 vol.%), N,N’-disalicylidene-1,2-propanediamine (0.05 vol.%), polyether amine (0.5 vol.%), ethylene vinyl acetate copolymers (0.2 vol.%), and tolyltriazole derivatives (0.1 vol.%). The mixture was blended together to form an aviation fuel. Example 3 - Cost comparison of the methods of the present invention The processes of the present invention use an ethanol feedstock and convert the feedstock to higher alcohols. This is substantially more cost effective than relying on a feedstock of higher alcohols as demonstrated below. The equipment costs for fermenting biomass to form ethanol is approximated at $7.7M compared with estimated costs for fermenting biomass to form the same amount of isobutanol which are approximated at $15.4 M. Additionally, the variable operating costs for ethanol are lower at $1.2 M compared with isobutanol which is $2.4 M. Additionally, annual capital costs are estimated at $770,000 for ethanol and $1,540,000 for isobutanol. With an estimated production capacity of 3.85 million gallons, the cost per gallon of ethanol is $0.51 compared with $1.02 for isobutanol. The yield of converting ethanol to jet fuel according to the present invention is 0.60 mass yield. The yield for converting isobutanol to jet fuel is slightly higher at 0.75 mass yield, however this improved yield cannot compensate for the increased cost. Example 4 - Emission Reduction of Petrol Blends Fuel additives according to Example 1 were mixed with petrol to form petrol blends according to the following table. The blends were formed from mixing 30 %, 40% and 50 % of the fuel additive with petrol. These petrol blends were burned in a standard combustion engine and the amount of CO, NOX and unburned hydrocarbons (UHC) were measured using an exhaust gas analyser. For comparison, commercial grade E10, which is a blend of 10% ethanol with 90% petrol as well as other ethanol blends up to 25% ethanol were also burned and the results compared in the table below. Blend Ratio Petrol (%) CO Emissions (g / km) NOx Emissions (g / km) UHC Emissions (g / km) 10 % ethanol (E10) 90 2 0.5 0.6 15 % ethanol (E15) 85 1.8 0.55 0.5 20 % ethanol (E20) 80 1.6 0.6 0.4 25 % ethanol (E25) 75 1.5 0.65 0.35 30 % fuel additive 70 1.2 0.35 0.4 40 % fuel additive 60 0.9 0.3 0.35 50 % fuel additive 50 0.6 0.25 0.3 The blends according to Example 1 showed reduced CO emissions compared to the ethanol blends, particularly a reduction of 40%, 55% and 70% compared with E10. Similar reductions were also observed with NOx and UHC emissions compared with E10. The blends of the present invention also showed reductions in the CO and UHC emissions with increasing fuel additive. Particularly notably, the blends of the present invention showed a decrease in NOx emissions with increasing fuel additive, whereas the ethanol blends showed an increase. As such, all of the blends of the present invention produced less NOx emissions that the ethanol blends. Example 5 - Emission Reduction of Jet fuel To illustrate the reduction of greenhouse gas emissions of the aviation fuels provided herein, a life cycle assessment was performed. The following factors were considered in this assessment: • The process described herein utilises renewable biomass as feedstocks rather than fossil fuels. The cultivation, harvesting, and processing of biomass feedstocks generally have lower greenhouse gas emissions compared to the extraction, refining, and distribution of fossil fuels. • The combustion of sustainable aviation fuels results in similar CO2 emissions to the combustion of fossil fuel based aviation fuels. The greenhouse gas emissions of the various steps are: 5 • Feedstock cultivation: 10 gCO2e / MJ • Fuel production: 5 gCO2e / MJ • Transportation and Distribution: 2 gCO2e / MJ • Combustion: 0.8 gCO2e / MJ 10 This provides an estimated greenhouse gas reduction of up to 80% of emissions when using the aviation fuel according to the present invention compared to fossil-fuel based jet fuels.

Claims

1. A fuel additive composition comprising butanol, pentanol, hexanol, heptanol, methylcyclohexanol, ethyl vinyl ether and nonanol.

2. A fuel additive composition according to Claim 1, wherein the composition comprises less than 30% by volume of each individual alcohol.

3. A fuel additive composition according to any preceding claim wherein the composition comprises 1-butanol, 2-butanol, 2-pentanol, 3-hexanol, 2-ethyl-1-butanol, 4-heptanol, 2-heptanol, 3-methylcyclohexanol, ethyl vinyl ether and 4-nonanol.

4. A fuel additive composition according to any preceding claim, wherein the composition comprises 2-butanol, optionally in an amount up to 15 %, preferably up to 12 %, and more preferably up to 10 % by volume of the fuel additive composition.

5. A fuel additive composition according to any preceding claim, wherein the composition comprises 1-butanol, optionally in an amount of up to 30 %, preferably up to 28 %, and more preferably up to 26 % by volume of the fuel additive composition.

6. A fuel additive composition according to any preceding claim, wherein the composition comprises 2-pentanol, optionally in an amount up to 10 %, preferably up to 7 %, and more preferably up to 5 % by volume of the fuel additive composition.

7. A fuel additive composition according to any preceding claim, wherein the composition comprises 3-hexanol, optionally in an amount up to 15 %, preferably up to 12 %, and more preferably up to 10 % by volume of the fuel additive composition.

8. A fuel additive composition according to any preceding claim, wherein the composition comprises 1-hexanol, optionally in an amount up to 15 %, preferably up to 12 %, and more preferably up to 10 % by volume of the fuel additive composition.

9. A fuel additive composition according to any preceding claim, wherein the composition comprises 2-ethyl-1-butanol, optionally in an amount up to 12 %, preferably up to 10 %,and more preferably up to 8 % by volume of the fuel additive composition.

10. A fuel additive composition according to any preceding claim, wherein the composition comprises 4-heptanol, optionally in an amount up to 10 %, preferably up to 8 %, and more preferably up to 6 % by volume of the fuel additive composition.

11. A fuel additive composition according to any preceding claim, wherein the composition comprises 2-heptanol, optionally in an amount up to 12 %, preferably upto 10%, and more preferably up to 8 % by volume of the fuel additive composition.

12. A fuel additive composition according to any preceding claim, wherein the composition comprises 3-methylcyclohexanol, optionally in an amount up to 15 %, preferably up to 12 %, and more preferably up to 10 % by volume of the fuel additive composition.

13. A fuel additive composition according to any preceding claim, wherein the composition comprises ethyl vinyl ether in an amount up to 15 %, preferably up to 12 %, and more preferably up to 10 % by volume of the fuel additive composition.

14. A fuel additive composition according to any preceding claim, wherein the composition comprises 4-nonanol, optionally in an amount up to 10 %, preferably up to 8 %, and more preferably up to 6 % by volume of the fuel additive composition.

15. A process for producing a fuel additive composition, the process comprising the steps of:i) reacting an ethanol feedstock to form a product mixture comprising multiple higher alcohols;ii) separating C4 to C9 alcohols from the product mixture;iii) blending the C4 to C9 alcohols and ethyl vinyl ether to form a fuel additive composition according to any one of Claims 1 to 14.

16. A process for producing a fuel additive composition according to Claim 15, wherein the ethanol feedstock is a bioethanol feedstock.

17. A process for producing a fuel additive composition according to Claim 15 or 16,wherein the process additionally includes the step of separating ethers from the product mixture.

18. A blended fuel composition comprising a fuel component and a fuel additive composition according to any one of Claims 1 to 14.

19. A blended fuel composition according to Claim 18, wherein the blended fuel composition comprises the fuel additive composition in an amount of greater than 10% by volume of the blended fuel composition.

20. A blended fuel composition according to Claims 18 or 19, wherein:a) the blended fuel composition contains petroleum; and the fuel additive composition in an amount up to 60% by volume of the blended fuel composition; orb) the blended fuel composition contains diesel; and the fuel additive composition in an amount up to 40% by volume of the blended fuel composition.

21. A process for producing an aviation fuel, the process comprising the steps of:i) reacting an ethanol feedstock to form a product mixture comprising multiple higher alcohols;ii) separating C4 to C9 alcohols from the product mixture;iii) dehydrating the C4 to C9 alcohols to form corresponding alkenes;iv) oligomerizing the corresponding alkenes to form oligomers;v) blending the oligomers to form an aviation fuel.

22. A process for producing an aviation fuel according to Claim 21, wherein the alkenes are mixed before the oligomerizing step such that the oligomerizing step and blending step are performed simultaneously.

23. A process for producing an aviation fuel additive according to Claims 21 or 22, wherein the corresponding alkenes comprise 1-butene, 2-butene, 1-hexene, 3-hexene, 2-pentene, 2-heptene, 4-heptene, 3-methylcyclohexene 1-methylcyclohexene and 4-nonene.

24. A process for producing an aviation fuel additive according to any one of Claims 21 to 23, wherein the ethanol feedstock is a bioethanol feedstock.

25. An aviation fuel obtainable by a process outlined in any one of Claims 21 to 24.s