As technological progresses are happening at a fast pace, energy is playing a more significant role in the daily lives of all people and social- economic development of every country 3R, 4R.

Industrialization, an increasing world population, globalization, more urban developments and other factors are the key reasons for request on more natural resources and energy.

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The worldwide growing urge to reduce the dependency on fossil fuels, and to realize better pollution control requires the improvement of alternate energy resources such as biofuels 2R.

Environmental concerns about global warming and the estimated declining of fossil reserves are the main drivers in the search of these alternate renewable energy resources to keep up the current and increasing level of energy use. The energy challenge refers to the obstacles that need to be overcome in the conversion from a fossil-dependent to a reliable, sustainable and competitive energy system that depends on renewable energy sources. Although altered forms of renewable energy will increasingly collaborate in the energy supply, it is expected that liquid transportation fuels will remain essential in the transportation sector in the coming decades. Such transportation fuels are presently mainly derived from crude oil feedstock 5R.

According to reports from experts, the aviation industry is a substantial contributor of the greenhouse gases (GHG) with an estimation of approximately 3% of all the emissions by various sectors in general. Where there has been a considerable rising in the concentration of pollutants in the atmosphere, mainly greenhouse gases which are, to a great extent, responsible for climate change. In latest years, the results of this phenomenon have been observed, and they involve the acceleration of melting of the polar ice caps, deviations in weather patterns and even the extinction of animal species. Amongst the greenhouse gases, CO2 is the main responsible of climate change.

According to the Intergovernmental Panel on Climate Change, the CO2 emissions enlarged by 80% annually between 1970 and 2004; this increment is interpreted in terms of the growth rate of CO2 equivalent, which from 1995 to 2004 was more than double than that of the former period from 1970 to 1994 6R.

Different stakeholders are focusing on various ways to reduce the gases and turning on the green energy sources as an alternative method. Other than the pollution, there is fear that at one point in time the petroleum that is the primary source of energy currently will be depleted since they are just extracted and not renewable 8R.
In 2014 and 2015, global energy consuming increased about by 1.0%, much below its 10-year average of 1.9%. Fossil fuels are the main sources of the energy to date. 544.284 GJ energy consumed in 2015 consisted of 32.94% petroleum, 29.2% coal, 23.85% natural gas, 4.44% nuclear energy and 9.57% other renewable energy resources (hydro, solar, wind and e.g.) 7R.

Over the past three years, an enormous number of experts from 24 countries and 82 institutions have collaborated to analyze a range of concerns associated with the sustainability of bioenergy production and use. The resulting assessment Bioenergy & Sustainability: Bridging the Gaps was started at a debate at the World Bank on September 28, 2015, in Washington DC, USA. During the debate, authors underlined key findings and discussed chances and challenges for sustainable energy in developing areas as well as the role of bioenergy in the 2030 and 2050 time horizons 1R.

In 2014, about 2627.02 million tons of oil have been required the transport sector, representing 27.9% of the total energy produced in the world. Furthermore, it is estimated that the consumption of energy in this section will increase between 80% and 130% in the next decades (2010–2050), hence CO2 emissions due to transportation will raise from 16% to 79% 9R. It is worth to mention that the rise in CO2 emissions is much smaller than the growth in fuel consumption due to improving efficiency of vehicles engines. The big expected growth for the transport sector is due to the doubling of international air traffic, along with the increasing by 50% of transportation of goods by road. Jet fuel and diesel will be the major fuels necessary for the growing of the transport sector.

The International Energy Agency considers that by 2050 biofuels will show about 27% of all fuels in the transport sector, particularly as a replacement of diesel and jet fuel 6R.

Biojet fuel has recently started to attract interest, and it has been identified as the most promising strategy to reduce CO2 emissions in the aviation sector 10R, 35R, where the use of bio-based jet fuel could reduce greenhouse gas emissions by up to 89% 11R.
As a measure to boundary the “Global Warming” some countries have submitted to impose “carbon-tax” on the airlines consuming aviation fossil fuel such as Jet A-1. The Association members of the International Air Transport (IATA) have undertaken the next goals:
• Effectiveness improving of fuel by 1.5% per year over the decade to 2020.

• To make all aviation industry progression carbon-neutral by 2020.

• To drop net releases of carbon dioxide by 50% by 2050, compared to 2005 levels 12R, 25R.
According to study from the experts in the field, technological improvements are expected to extensively play a major role, and the aviation industry is expected to make significant strides by the year 2050. Most of the results and tests presented are especially from the year 2005 to 2014 with companies competing in the field to ensure they are among the first to transform fully 13R.

Biofuels are gaining interest and public acceptance. This is due to their benefits especially since the prices of fossil fuels are unpredictable and governments are even offering subsidies to companies interested in the biofuel production.

Biofuel Definition
Biofuel is an alternative source of energy that is constantly created from biological carbon fixation, and it incorporate fuels that are made from biomass conversion and liquid fuels like cooking oil and different type of gases. Even though fossil fuels comprise carbon in them, they are not considered biofuels by the general definitions since it contains some carbons that are considered to be out of the carbon cycle for a very long time. Some of the sources that are known for the production of the biofuel including sorghum, corn, sugar, vegetable oil and sugar among others 8R, 14R.

Currently, the bioenergy is supposed to account for approximately 10% of the world energy, and it can be in solid form and liquid as well as gases 15R. However, the literature mostly recognizes only a small definition of biofuels to mean the liquid part of it since the world is concerned on replacing the fuel in the transport industry which is mostly liquid. Though the use of bioenergy has caught the attention of many in the recent two decades, biofuels have been in use initially especially in the developing countries where biomass sources such as firewood, animal dung, and charcoal among others have been considered to be cheaper than other fuels 8R. Initially, biofuel was used in homes only through the use of wood pellets and fuelwood just to mention a few. The other category is the secondary biofuels which are the processed biomass, and they include ethanol as well as biodiesel which can be for industrial purposes and in the vehicle and aviation industry 15R.

Biofuel Production
Even though the biofuels are known to have an advantage over the fossil fuels, its production requires a lot of consideration before it starts to be produced in large quantities or before it fully takes over the energy industry. The main reasons are because very little is known about the viability as well as high costs of production due to scarce raw materials. The primary focus is on the viability to ensure that it fulfills the goal of providing the energy gain and there will be no energy vacuum or shortage due to a shortage in the supply. Secondly, biofuels are at times produced from crops which can be used in human foods. It is, therefore, paramount to ensure that there is enough food supply and not everything is directed towards the energy industry leaving the world population to starve 16R. In dealing with the problem, latest advances are focused on the next generation of biofuels. Generally, innovative biofuels referred to second or third generation biofuels which are created from a broad spectrum of mainly non-edible biomass feedstock. Some of these are attainable biofuels can be applied to present distribution infrastructure and engine platforms. By-products of advanced production of biofuel encompass bioelectricity, bio heat, bio-chemicals and protein based feed. On the other hand, some bioenergy experts are skeptical to use the term of “first generation” as many of the plants are very advanced and have very good carbon footprint 17R.

Unlike the first generation biofuel which is produced from feedstocks that can be used for human consuming, the next generation biofuel feedstocks cannot be consumed by human beings. The third target in the manufacturing of the fuels is to ensure that it will bring greater economic earnings compared to the fossil fuels. One of the causes for shifting focus to the alternative fuels is due to the unstable prices of fossil fuels in the market. Hence, the green energy should provide stability and the economic gain expected 8R.

Feedstock Potential in Biofuel Production
Biochemical and thermochemical conversion of feedstocks are the main techniques which can be used to output Biofuels. Where, enzymes and other microorganisms are used to transform feedstocks to sugars in biochemical process prior to their fermentation to yield ethanol; while thermochemical – where technologies of pyrolysis and gasification yield a synthesis gas (CO+H2) which is used to produce a wide-ranging biofuel of long carbon chains such as synthetic diesel or aviation fuel 24R.
Numerous factors are considered as soon as determining to convert a particular biomass feedstock to biofuel, such as the obtainability and cost of the feedstock, the recycle waste possibility or residuals as biomass feedstock, and how the transformation process may add value to the biomass feedstock by generating innovative or chosen yields. The selected feedstock may require pre-processing, as well as drying, densification and biochemical pretreatment, to rise the energy efficiency and diminish the cost of transporting bulky biomass to a bio refinery 18R. Fig. (1), shows three pathways classification for organic wastes conversion into biofuels which are: (i) biochemical, (ii) physiochemical, and (iii) thermo-chemical methods 55R.

Fig. (1). Organic wastes into biofuels conversion processes.

Biofuels have previous been categorized broadly into three generations based on the feedstock kind which is being used for its production Fig. (2) 19R, 20R. The first generation of biofuels and biochemical is produced from edible feedstock while from non-edible feedstock the third generation biofuel are produced.

Fig. (2). Classification of biomass feedstock in three generations.

On the other hand, biofuels can be classified as a primary and secondary biofuel as is shown in Fig. (3) 21R.
Fig. (3). Biofuel production sources.

Biofuels produced from the first generation have distinguished economic, environmental and political interest as the mass production of biofuel needs more arable agricultural lands resulting in reduced lands for human and animal food production. Moreover, first generation production process biofuels is also in charge of environmental degradation. Consequently, enthusiasms first-generation biofuels have been demised. As biofuels from first generation class are not viable, researchers concentrated on the second generation biofuels class. Because of costly and sophisticated technologies of the second generation biofuels production process, the biofuel production from the second generation is not gainful for commercial production. Hence, the researchers focused on third-generation biofuels. Microalgae, as shown, is the major component of third-generation biofuels. Currently, it is considered to be a reasonable alternative renewable energy resource for biofuel production overcoming the disadvantages of the two classes of first and second generation biofuels 21R, 22R, 23R. A widespread range of organic wastes can be used for advanced biofuels production which shows intrinsic opportunities for diversified production processes and biofuel potentials. Organic wastes examples include kitchen, garden, forest, lignocellulosic, solid biogenic, farm, animal, paper, sewage sludge also municipal wastes comprising essentially organic fractions 36R, 37R, 38R.

Sustainable Aviation Biofuel
Renewable fuels plays a crucial role into aviation sector to diminish greenhouse gases emissions and to introduce a sustainable fuel that can substitute the classical jet fuel. In order to relieve the CO2 emissions to face the global challenge of climate change 26R, 27R, 28R, 30R.

The submitting of biofuels into aviation sector must not alter the aircraft engine performance and the fuel must keep its properties during the wide range of atmospheric conditions during flights 29R.

Furthermore, it is estimated that the biojet fuel in the aviation sector will permit, at least, partial fuel independence so industries are currently working on developing optimum processes that utilize sustainable feedstocks and can be economically feasible produced 26R.

Nevertheless biofuel in the aviation has been researched for a while now, the change is a bit slow due to the conditions and required standards of the fuels. The aviation industries is one of the most precise and dangerous sectors, and safety is of ultimate importance 31R.

For that reason, any new change being introduced has to be quite tested with various necessities being observed to ensure that there are no threats to the aircrafts as well as the passengers. In addition, aircraft are quite costly, and the airlines would like to use them for an expanded period. Hence, they have to confirm that the fuel is properly tested and it will not be a threat to their investment by damage their aircraft. In other words, the aviation industry is looking for an alternate fuel that will directly substitute the current jet fuels without compromising on the safety as well as providing high-performance fuel are stringent requirements of the jet fuels 8R,32R. The biological origin feedstocks are used for the production of alternative aviation fuels and hence are renewable. Jatropha, camelina, algae, halophytes, municipal and sewage wastes, forest residues etc., are non-edible oil crops that considered as the foremost available resources for the energy production process. Several technologies have appeared for the aviation bio fuel production from the biomass resources 33R.

A framework has advanced to select the greatest proper feedstock and conversion procedures deserving of additional policy support. The following are the criteria key used in the assessment undertaken 34R:
• Waste and residue feedstocks priority.

• Consideration of the classification of land used for feedstock production (if applicable)
• Competing uses and potential substitute resources
• Lifecycle profile of GHG emission profile
• Economic viability
• Feedstock availability
2. Biojet Technologies:

Numerous conversion technologies of bio-jet fuel, whether in the research phase, development, demonstration, or commercial platforms, are described in the literature. Studies on the conversion technologies that convert bio-based feedstocks to jet fuel have been worked out in the literature in areas including the availability of feedstock, upgrading technology, process economics, and commercial improvement 54R, 52R.

2.1. Background:
Biotechnology could potentially make a remarkable contribution to renewable jet fuel production as already observed in the production of bioethanol and biodiesel for piston engine vehicles.
Fuel represents one of the biggest operating costs in the aviation industry. Aviation fuel has stringent quality requirements specification than fuels used in road transport. According to a U.S. Department of Energy’s (DOE’s) Bioenergy Technologies Office 42R, 43R, 4gallons are of jet fuel produced from one barrel of crude oil. The worldwide aviation industry consumes approximately1.5–1.7billion barrels (47.25–53.55 billion gallons) of conventional jet fuel per year 39R, 40R, 41R. Some main challenges such as crude oil prices, national security, environmental impact, and sustainability make it difficult to have a long-range plan and budget for working expenses. As a result sustainable biofuels have been produced worldwide to submit a solution to these concerns where industries are currently working on developing optimum routes that utilize sustainable feedstocks and can be produced economically. Many process technologies of bio jet conversion are diverse and depend mightily on the type of feedstock 26R, 42R. Then, we will depict fossil jet fuel in order to present biojet fuel.

2.2. Jet Fuel and Biojet Properties and Specifications:
Jet fuel is a common name for aviation fuels which has a distillation fraction range of crude oil from 15 to 275oC used in aircraft gas turbine engines which is also a composite blend of up to >1000 different chemical compounds.

The essential components are linear and branched alkanes and cycloalkanes with a typical distribution of carbon chain-length (C6–C16) 44R. The composition of fossil jet fuel is: almost 20% paraffins, 20% naphthenes, 40% isoparaffins and 20% aromatics 6R. This composition affects on fuel physical properties, showed in Table (1), as freezing point (?47 °C) and energy content (43.28 MJ/kg) 6R,45R. The qualifications required for jet fuels are (1) appropriate minimum energy density, (2) maximum acceptable freeze point temperature,, (3) maximum allowable viscosity, (4) maximum permissible deposits in standard heating tests, (5) maximum allowable amount of wear in standardized test (6) allowable sulfur and aromatics content, (7) maximum acidity and mercaptan concentration, (8) minimum fuel electrical conductivity, and (9) minimum allowable flash point 26R, 48R.

Table (1): Some properties of fossil and renewable jet fuel.

Biojet fuel comprises of renewable hydrocarbons in the fossil jet fuel boiling range it means that it has a comparable composition to fossil jet fuel, and depending on the production process used it can contain aromatic combinations. The emitted particles from burning biojet fuel are lower than those caused by fossil jet fuel in case of aromatic compounds lack 6R. Though, the lack of aromatic compounds can cause wear problems in certain types of engines 46R, and they are required to swell O-rings and seals in engines. As a result, using 50% volume mixtures of fossil fuels and biojet has been established as a standard. On the other hand, amount of aromatic compounds can be added to biojet fuel, technically, in this case, it is reasonable to use it at 100% in aircraft engines 6R. Biojet fuel, also known as synthetic paraffinic kerosene (SPK), is comprised of renewable hydrocarbons which properties are practically identical, or in some cases supreme 47R, to those of fossil jet fuel. The combustion of SPKs produces lower carbon dioxide emissions than fossil jet fuel; besides, they have the characteristic of holding very minute sulfur content. Therefore, biojet fuel has been identified by the International Air Transport Association as the most workable alternative to replace the fossil fuels in aviation 6R, 49R.

A list of fuel specification standards is a guideline for producers and users by which they can depict and control the properties of fuel indispensable for appropriate and safe performance. ASTM standard specification D1655 defines specified types of jet fuels for civil use: Jet A and Jet A-1. It specifies that jet fuels shall be properly sampled and tested to examine their conformance to detailed desires as to composition, volatility, combustion, fluidity, thermal stability, contaminants, corrosions, and additives 50R. Some of the ASTM D1655 and DEF STAN91-91 specifications, for conventional Jet A-1 are given in Table (2).

Table (2): Jet A-1 aviation fuel specifications 12R, 51R:
In general, alternate jet biofuels are produced by three main routes: thermochemical, oleo chemical (a chemical compound derived industrially from animal or vegetable oils or fats) and biochemical, and some of existing developing upgrading pathways include: conversion of lignocellulosic materials through Fischer-Tropsch (FT), Hydrothermal liquefaction (HTL) and Pyrolysis; additional optional ways are Hydrotreatment of lipid/fatty acid feedstocks; Fermentation of sugars or Direct Sugars to Hydrocarbons (DSHC); as well as other hybrid technologies such as Alcohol-to-Jet (ATJ) and Aqueous Phase Reforming of sugars (APR) 41R,53R.

2.3. Bio-Jet Fuel Process Conversion Technologies Pathways:
A variation of pathways with a number of options for conversion technologies have been defined to produce alternative jet fuels from bio-based and waste materials. A pathway can be defined based on the used feedstock(s), the conversion method (es), and the output fuel(s). Chemical composition of the alternative jet fuels ultimately vary according to the pathway: some pathways can create synthetic paraffinic kerosene (SPK) without aromatics, whereas others can generate aromatic compounds as well. Accordingly, a great concern has been taken on blending grades with fossil-based conventional jet fuel 56R.

Though there is a global interest in producing alternatives to petroleum-sourced jet aviation fuel, the production technologies being used in 2014 that are agreeable by the recognized international authority are still limited. There are currently three ASTM-approved processes for producing biojet fuel Table (3),: the Fischer-Tropsch (FT) process based on use of a biomass feedstock (sometimes also called BTL -biomass to liquid), which was approved in 2009, the hydro-processed esters and fatty acids process (HEFA) which was approved in 2011, and the hydro-processed depolymerized cellulose to jet (HDCJ) process which is referred to as Synthesized Iso-Paraffinic (SIP) fuel, which was approved in June 2014. Some other pathways are well-advanced in the ASTM certification process including alcohol oligomerization to jet fuel (ATJ) 25R.

Fischer-Tropsch (F-T) Synthetic Paraffinic Kerosene, the Hydro treated Esters of Fatty Acids (HEFA) and hydro-processed depolymerized cellulose to jet (HDCJ) have been certified technologies for the production of biojet fuel (ASTM D7566 standard specification) to be used in blending of up to 50% with fossil jet fuel while (HDCJ) was approved by the ASTM by year (2014) and it could be blended at up to blends of 10% (by volume) with conventional jet fuel 56R, 57R.

Table (3): Biojet pathway summary 12R, 58R.

Fig. (4): A simplified schematic diagram of different technology pathways to bio-jet fuel
2.3.1. Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK):
The Fischer-Tropsch process was established in 1920 by Franz Fischer and Hans Tropsch. Fischer-Tropsch (F-T) process is the first approved production pathway for synthetic kerosene. In this process a group of chemical reactions have occurred that to convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. (F–T) process is often considered as the key technological component for transforming synthesis gas (or “syngas”) to transportation fuels and other liquid products 59R. Their process took a synthesis gas produced from coal and this was transformed to liquid fuels using a catalyst of alkalized iron chips and reactor conditions of temperature of 673K and pressure of 4100 bar. This technology is based on a catalytic process driving the reaction of carbon monoxide with hydrogen, with further steps producing liquid hydrocarbons 25R.

Fischer-Tropsch fuels are normally manufactured in a three-step procedure:
• Syngas generation: The feedstock is converted into synthesis gas (syngas) composed of CO and H2.

• Hydrocarbon synthesis: The syngas is converted into a mixture of liquid hydrocarbons and wax, generating a “synthetic crude” by using catalyst. This step is the true Fischer-Tropsch synthesis.

• Upgrading step: The mixture of F-T hydrocarbons is upgraded into the desired fuels as a results of some operations which are hydrocracking and isomerization and fractionation 60R.

The process is essentially established on the concept of Biomass to Liquid (BtL) conversion utilizing feedstocks like lignocellulosic biomass, , wood , cereal straw, and forest residues to biodiesel and biojet fuel as synthetic fuels Fig. (5).This process is consisted generally of five-step:
(a) Pretreatment of biomass. Where, biomass feedstocks are exposed to drying at first and then milled to decrease moisture content as well as the particle sizes 48R.

(b) Gasification or pyrolysis.

(c) Syngas purification.

Where, undesirable compounds from the synthesis gas stream are removed. After this step the concentrated CO2 stream is released to the atmosphere.
(d) Fischer-Tropsch (F-T) synthesis.

In this step, the synthesis gas is delivered over an iron- or cobalt-based catalyst under definite process conditions to form a broad hydrocarbons mixture of ranging from gases as ethane (a short chain hydrocarbon) to waxes (long chain hydrocarbons). By varying the reaction conditions of catalyst, temperature, pressure and time, the distribution of carbon length of the obtaining hydrocarbons can be shifted to maximize the in demand carbon chain length; so, for example, of middle kerosene distillates with a carbon chain length of C9-C15.

(e) Hydroisomerization of F-T wax to Biojet, Biodiesel, and naphtha depending on needs.

After leaving the F-T section of the facility, the product of hydrocarbon is promoted to liquid aviation fuels using well-established methods frequently used in petroleum refineries. The outputs of the process can be constricted to middle distillates and naphtha, both of which have almost zero sulfur content. The middle distillates can be separated into a mix of automotive diesel and jet fuel.

The kerosene fraction obtained using the (BtL) process is of very good quality, free from sulfur and other impurities 12R.

Fig. (5): The Fisher-Tropsch Synthetic bio-kerosene scheme process.

The derived Fischer-Tropsch kerosene from a biomass feedstock would get fuel cycle CO2 benefits comparing with to mineral kerosene, and would also mostly preclude the emissions of sulfur oxides (SOx) since the fuel is virtually sulfur free. Fischer-Tropsch kerosene product is similar to mineral kerosene in chemical and physical properties. By this way, it is in suitable with current fuel storage and handling facilities with current jet engines 60R. On the other hand, there are some disadvantages of (F–T) fuels. Primarily, (F–T) kerosene has density with minimum requirement. Moreover, the deficiency of aromatic compounds may cause fuel leaks in the engine system. But, these disadvantages can be obviated by blending conventional jet fuel with synthetic fuel in various ratios to overcome such problems 25R, 59R.

2.3.2. Hydrogenated Esters and Fatty Acids (HEFA):
The renewable jet fuel process (HEFA) is a good model of innovative technology that may be proceeded in biofuels production for aviation purposes. HEFA stands for Hydro treated Esters and Fatty Acids which are commonly paraffinic liquids having chemical formula CnH2n+2 33R. This technology is based on the hydro-processing of natural oils and fats. Firstly in this process, oxygen is taken away from triglycerides by hydro-deoxygenation/decarboxylation, and double bonds are saturated by the addition of hydrogen to produce long straight-chain hydrocarbons 25R, 61R.

The process can transform a multiplicity of refined natural oils and fats wither these oils are edible and nonedible natural oils such as tallow, and algal oils 12R. (HEFA) fuels are created by hydroprocessing technique of esters and fatty acids, such as edible (Soybean, Canola) and non-edible (Camelina, Jatropha) plant oils, used cooking oil (yellow grease), oils from algae, and tallow (a hard fatty substance made from rendered animal fat). In (HEFA) process, the renewable oil is treated using hydrogen to produce a fuel in the distillation range of jet fuel diesel and naphtha via (hydroprocessing) 56R, 62R. Vegetable oils and fats are triglycerides, which mostly encompass fatty acids with carbon numbers in the range C14 to C20, but jet fuel contains hydrocarbons in the range of carbon atoms from C8 to C16. These straight hydrocarbons mostly fall into the diesel range and are improved into jet fuel by selective cracking and isomerization which consumes more hydrogen. The stages of this process are demonstrated in Fig. (6).

Fig. (6): Hydrogenated Esters and Fatty Acids process steps.

Chemically hydrotreated vegetable oil-based biojet fuels (i.e., the product of the HEFA processing) are a mixture of free of sulfur and aromatics paraffinic hydrocarbons. The low-temperature properties of HEFA can be adapted to meet the local necessities by adjusting the severity of the process or by additional catalytic processing 25R. Ultra-low sulfur jet fuel that confirmed Jet A-1 specifications can be achieved by adjusting the catalytic processes of the procedures of deoxygenation, isomerization, and selective cracking of the hydrocarbons present in the used natural oils and fats to attain a high quality product.
The produced biojet fuel from (HEFA) process is analogical to conventional petroleum in properties, moreover it has the advantages of higher cetane number, lower aromatic content, lower sulfur content, and potentially lower emissions of greenhouse gases. In addition, such advantage of hydroprocessing is very likable because have a real impact over production and investment costs, two very important drawback factors 42R, 66R, 67R.

Fig. (7): Possible pathway reactions during the conversion of a triglyceride molecule under catalytic hydrotreatment 63R.

The production pathway in Fig. (6)- is similar to traditional fuel produced from refinery process of fossil crude oils. Pretreatment is the first step where the bio-material is prepared for the veritable production. The prepared material from this step is reacted with hydrogen (hydrotreatment) in the next step. This is considered as a production step which gets rid of the oxygen and converts the material into hydrocarbons. Cracking and isomerization of these hydrocarbon yield a mixture of n-alkanes and iso-alkanes which give the desired good cold flow properties. Later, the output raw product is then distilled and fractionated into different products 61R.

Hydrogenation process means the saturation of double bonds in a molecule through addition of hydrogen in a reactor at certain conditions of temperatures and pressures and by using a suitable catalyst. Triglyceride (for example) with three chain of fatty acids, being linoleic (C18:2), oleic (C18:1) and stearic acid (C18:0), this molecule is said to be completely hydrogenated when all of the unsaturated fatty acids are changed into saturated one, resulting in three chains of stearic acid. Furthermore, another term often used in the literature when upgrading a triglyceride molecule is Hydrotreatment, where hydrogen, alongside a hydrotreatment catalyst, the carbonyl group is added after hydrogenation, and three more reactions can take place instantaneously, according to a selective the process 63R, as it can be seen in Fig.( 7). So, after saturation, addition of excess hydrogen causes the breaking of the glycerol compound forming propane and a chain of free fatty acids; the carboxylic acid group that stays attached to the free fatty acid must be removed to form straight chain alkanes and can be achieved through three ways: the hydrodeoxygenation (HDO) route, in which it reacts with hydrogen to produce a hydrocarbon with the same number of carbon atoms as the fatty acid chain and two moles of water; the decarboxylation (DCOX) pathway, which produces one carbon atom hydrocarbon (less than the fatty acid chain and a mole of CO2 ); and the decarbonylation (DCO) route which also produces a hydrocarbon with one carbon atom less, as well as a mole of CO and water. Depending of the composition of the ultimate n-alkanes produced, they need to be exposed to either isomerization, cracking (cyclization), to advance its combustion properties, obtaining isoalkanes, lighter hydrocarbons and aromatics, respectively 64R,65R. Final products attained when hydroprocessing most of the vegetable oils, include organic liquid product, water, and gases (H2, H2S, CO, CO2, C3H8, CH4, and other light hydrocarbons), however their distribution could vary according to the process 63R. A wide-ranging information of all this chemical reactions means a real chance for taking advantage of such information to apply it to jet biofuel synthesis. One important factor in this process is that hydrogen requirement for the reactions differs from one vegetable oil to another 42R.

2.3.3. Alcohol Oligomerization to Jet Fuel (ATJ):
ATJ fuel also termed alcohol oligomerization, where fuel is produced from alcohol with a short chain such as methanol, ethanol, butanol, and also from long chain fatty acid alcohols. Ethanol from 10%-15% is considered as the maximum used percent for the majority of gasoline-powered vehicles on the road today, which generates a blend wall that makes it difficult to obtain extra market penetration of ethanol as a blend stock for gasoline. Therefore, ethanol upgrading to jet fuel blend stock submits a prospective pathway for improving drop-in or fungible fuels in the market for jet fuel 48R.
To make alcohols as alternative jet fuel, the differences in the physical and chemical properties between alcohols and traditional jet fuel have to be minimized. In the United States, anhydrous ethanol, at 99.5–99.9% purity, is requested to blend with gasoline to obviate separation 68R. The alcohol to jet process (ATJ) process commonly starts with an alcohol with the general formula ROH, where R is a saturated alkyl group with the chain length from 2 to 5 carbon atoms and (–OH) is a hydroxyl group. 71R. Nevertheless, for upgrading stage to jet fuel products, the high-purity ethanol is still uncertain requirements. A typical three-steps of (ATJ) process that converts alcohols to jet fuel has been established. The process includes removing of water in alcohol dehydration step, oligomerization, and then hydrogenation. The overall diagram for alcohol to jet fuel process is shown in Fig. (8). in the next oligomerization step, the gaseous material is oligomerized into a higher molecular weight of unsaturated compounds which consistent approximately with jet fuel then they are separated and further processed. In the third major step, hydrogenation over a solid-phase catalyst with hydrogen gas.

The advantage of this pathway is that all of these process steps have been confirmed on a commercially pertinent scale and the scaling-up risk is predictable to be reduced. However, it is a necessity to improve and integrate the process of the biomass-derived intermediates. 69R.

Fig. (8). Alcohol oligomerization to jet fuel process steps (ATJ) 25R.

The production of ATJ fuel involves two main separate steps: the alcohol production, and conversion of the alcohol to the desired fuel. These steps are in principle separate from each other and can take place at different locations. Whereas the alcohol source possess a decisive importance from a point of view of sustainability, the conversion process of the alcohol to jet fuel (ATJ) is of relevance for technical certification 61R.
In the final step, the hydrogenated product from the previous step is distilled to yield the final products of which kerosene is one of them 70R. Currently, fermentation of lignocellulosic residues is a popular approach, but in principle, the feedstock can be all kinds of biomass, or even be inorganic substances. In this route, jet fuel range hydrocarbons are prepared from alcohols such as ethanol or butanol by oligomerization. The alcohols used as a starting material can be produced from sugars or lignocellulosic biomass, and thus have greater potential to produce jet fuel in very large volumes, compared to use of vegetable oils/fats via the (HEFA) process 25R, 75R. A lot of the probable feedstock can be used for the process, including starch, sugar, cellulose and waste 72R. The method is very economical due to the inexpensive feedstocks and the small amount of energy is needed in this process 73R. Direct fermentation of sugar and starch transform them into alcohol, but in the biomass case, pretreatment step must be involved to obtain sugar, which is then immediately fermented to alcohol or exposed to gasification followed by gas fermentation 74R.

2.3.4.: Direct Sugar to Hydrocarbons (DSHC):
The process pathway of direct Sugar to Hydrocarbons (DSHC) transforms sugar into a pure paraffin molecule which is then can be blended with fossil jet fuel produced from conventional route. An advanced fermentation process is used to achieve the conversion. Unlike ‘traditional’ fermentation of sugars to ethanol, this biological conversion process is carried out under aerobic conditions of any cellulosic material which can be used as a feedstock. The feedstock material is pre-treated using enzymatic hydrolysis. A simple sugars resulting juice is obtained after hydrolysis then it is filtered in order to get rid of lignin-rich solids and to purify it 25R.
Consequent to solids removal the stream of sugar could be sent directly to the stage of biological conversion or further processed to concentrate the sugars by the mean of evaporation or other means. Next to the bio conversion step, the end-product is obtained by separation it from the water phase. Here, the produced paraffin has an advantage over ethanol fermentation that because of the low solubility of long hydrocarbon chains in water and easy separation of the two phases easily. The process steps are illustrated in Fig. (9).

Fig. (9). Direct Sugar to Hydrocarbons (DSHC).

In this pathway, sugarcane (or any other plant sugars including from sugar beet, sweet sorghum or cellulosic sugars) is exposed to a yeast fermentation process producing the unsaturated fermentation product (farnesane) which is a set of six closely related chemical compound ?-Farnesene and ?-farnesene isomers, differing by the location of one double bond. Then it is submitted to another transformation process. However, in commercial aviation field this new biojet product is used only in blends of up to 10 per cent with conventional jet kerosene 25R. The production of Synthesized Iso-Paraffinic (SIP) fuel which consists of two foremost steps. In the first step, microorganisms are used to ferment sucrose and produce farnesene. In the next step, farnesene is changed into the respective alkane, a molecule with no double bonds, via hydrogen reaction in a catalytic bed. The resulting product of saturated alkane is then purified by distillation to yield an aviation grade fuel. The final product fuel ideally comprises mostly of farnesane, even though some traces of remaining farnesene and olefins (incompletely hydrogenated farnesene) may be existing in the ultimate product as well as some other trace by-products 61R, 76R, 77R.

Farnesene is a biochemically synthesized olefin (1, 6, 10-Dodecatrienes, C15H24). It is insoluble in water and soluble in alcohols. Its Pour point ?76 °C and b.p 250 °C, density 0.83 (15 °C). The physico-chemical properties of farnesene or its isomers have an advantage over ethanol and butanol as biofuels due to its non-hygroscopic property, very low pour point and density, flash point which meets the specifications of aviation fuel 12R.

2.3.4. Hydrotreated Depolymerized Cellulosic Jet (HDCJ):
Hydro-treated depolymerized cellulosic jet (HDCJ) is a modern technology developed to transform the cellulosic biomass into renewable gasoline, diesel, and jet fuels 80R. This process (also known initially as HPO) Hydrogenated Pyrolysis Oil is based on the produced oil from lignocellulosic biomass fast pyrolysis 25R. The term of (HDCJ) includes the pyrolysis as a production pathway but also includes comparable pathways where depolymerization step are implemented by alternative processes 61R.

Pyrolysis means thermally decomposition of dry biomass (in the absence of air) into solids, liquids and gases, which consequently leads to the bio oil and methane formation with other side products 33R, 78R. Pyrolysis oils can be upgraded by breaking down the larger molecules and then reduction at high temperatures with a catalyst. This can either be done by feeding them as a small percentage of feedstock into the fluid catalytic cracking unit of a petroleum refinery, or in stand-alone hydrotreating plant. Latest years have seen increasing attainment in demonstrating these technologies. Renewed attention has been motivated by the prospect of converting the pyrolysis oil intermediates into biojet fuels 79R.

ASTM has not yet approved the HDCJ jet fuel. Bio- oils from the pyrolysis process undergo a sequences of hydro-treating processes to yield jet-fuel-range products. If no catalytic additions upgrading is applied, pyrolysis oils undergo hydro-treating and fractionation to form jet blend stocks, as shown in Fig. (10). Three major steps are involved in the HDCJ production pathway. Where, it starts with lignocellulosic biomass (a combination of lignin, cellulose, and hemicellulose) as a feedstock which is essentially considered as complex polymers comprised of carbon, hydrogen, and oxygen. In the foremost step of depolymerization, the feedstock is the complex polymers are cracked down into smaller fragments. Depolymerization is conducted by heating the material in an oxygen-free atmosphere (as mentioned before) in the case of pyrolysis. Hydrothermal or catalytic approaches or combination with pyrolysis are the other possible means of depolymerization. In the second step, the depolymerized crude product is then passed to be hydroprocessed and get rid of oxygen besides converting the oxygenates to hydrocarbons and to some extent saturating aromatic compounds. In the final step is distillation where the produced hydrocarbon is distilled to yield the ultimate products of which kerosene is one 61R, 90R.

Fig. (10): Hydrotreated Depolymerized Cellulosic Jet (HDCJ).

The feedstock for the pyrolysis process could be any dried and granulated carbon-rich feedstock including biomass. The outputs are CO2, flammable gases (mainly CO, CH4, H2, C2H6, and C2H4), bio-oil of oxygenated organic species mixture having carbons ranging from C1 to C21+ 48R, char (carbon) and ash. The pyrolysis process conditions that favor liquid pyrolysis oil production are determined by the temperature, residence time, and rate of heating also other reaction parameters. In producing biojet fuel by this pathway the products from the pyrolysis process are gasified in the existence of an oxidizing agent (typically steam) to produce syngas of H2 gas and CO, volatiles of CH4 and CO2. After the accomplished of the gasification process, the syngas can be further processed into hydrocarbon chains of changing length, which are then refined to isolate the desired jet fuel using the techniques usually existed and carried out in petroleum refineries. The biojet produced from pyrolysis oil contains a definite amount of aromatic compounds which are presently required in jet fuel system to avoid the engine sealing problems. 25R.
Fast pyrolysis occurs under conditions of high temperatures (up to 500 oC) and atmospheric pressure and the reaction takes place in seconds. The pyrolysis conditions affect the ratio of the aqueous phase to the bio oil where, a higher temperature produces a higher yield of bio oil. The output char byproduct can be sold or can be fed back into the system to help in conserving high temperatures during the pyrolysis step. Drying the used feedstock in this process is required.

Upgrading of the crude bio-oil is an essential process before using it as a transportation fuel. Strategies for upgrading process technology differ between the producing companies but, there are two main common stages of the upgrade process 81R, 82R.

In general, the production conditions of char comprising slow heating rates by ultimate temperatures lower than 450 °C. Nevertheless, gases are produced in case of using high temperatures greater than 800 °C with rapid heating rates. Fast pyrolysis of biomass is achieved at the temperature range between 400 and 600 °C within few seconds as residence time. Thereafter, quick quenching or cooling of the produced gas is undertaken where the gas is sent to a condenser to yield bio-oil 84R, 85R, 86R. About 80% yields of upgraded bio oil can be achieved with fast pyrolysis. The oil quality has been improved by using zeolite catalyst during the pyrolysis process. The aromatic content can be increased by the catalyst as well the oxygen can be decreased. The catalytic pyrolysis produces high quality oil compared to the non-catalytic one moreover it needs less upgrading steps. Consequently, the cost of catalytic pyrolysis is efficient compared to the non-catalytic 87R. Such process is considered to be active and satisfactory option for conversion of algae biomass into Jet fuel but the drying of biomass is an added cost which needs to be analyzed and optimized. On the other hand, slow commercialization of the fast pyrolysis suffers from some correlated problems due to the deficiency of an effective market concerning crude bio-oil 83R. Therefore, there is a necessity for optimization and improvement of modern and low costs upgrading processes. Through the development of the present hydrodeoxygenation or deoxygenation processes, this target may be possible. A lot of researches are still being undertaken for improving this technique which comprise the appropriateness of several catalysts as well as their life cycle, reduction of hydrogen intake and the estimation of the required desired deoxygenation to co-process bio-oil in classical refineries. 88R. these modern styles are presently just at the laboratory and pilot scale levels 83R, 89R. Generally, it is vital to confirm that the pyrolysis process of any biomass to produce crude bio-oil without upgrading is a very reasonable technology 83R.

3. Issues Limiting the Deployment of Biojet Fuels on a Global Scale:
The opportunities to deploy cost-effective biojet fuels on a worldwide scale are limited by a number of issues such as technical restrictions, high production costs, price and contend uses for feedstock, production capacity, lack of policy incentives, and the real potential of waste and residues 12R.

3.1. High Cost of Production:
Biojet fuel is presently costlier than petro Jet-A1 fuel owing to uncertain and poor supply chain of feedstock such Jatropha oil, camellia oil etc. in the international market. Essentially cost of biojet fuel is dependent on (a) input cost and composition of feedstock, (b) process technologies, (c) conversion efficiency and product output, (d) value-added coproducts, (e) process energy efficiency.

Biojet fuel produced by HEFA process was projected about US$ 4.5–5.4/gal which is almost double the current price of petroleum Jet A fuel (NREL Technical Report 2016). It was found that Feedstock and hydrogen represent almost 50–70% of the Biojet fuel price. Therefore, sustainable supply of prospect feedstock is the key factor for cost reduction 12R.

3.2. Technology and Plant Capacity:
Many processes have been developed for biojet fuel are in the pilot or demo scale. Some of them still require technical maturation. Application of waste/ residues as feedstock more benefit on net greenhouse gas reduction comparing other potential feedstock; however, their availability and supply chain strategy has not yet established internationally.

High capacity stand-alone production plants requisite high capital and operating cost. Feasibility of use of biojet fuel in aviation sector is potential as drop-in fuels in near future. thus, such plants should build in the vicinity of petroleum refinery or other biofuel plants 12R.

3.2. Shortage of Policy Incentive:
Government of numerous countries has stated incentives on blending of biodiesel and bioethanol with petrol or diesel respectively to make parity with the conventional fuels. Though, no such action has been initiated for use of biojet fuel in the aviation sector. This condition may potentially create global variations in price and availability of feedstock and biojet fuel.

4.4 Certification:
The other area of challenge in the alternate jet fuel is the certification process which is lengthy and costly. The aviation industry is one of the riskiest sectors, and therefore, it involves a lot of necessities before a new product is approved for safety purpose 91R . According to those who have already went through the process of certification with ASTM, it takes between 3 to 5 years at a very high cost of between 10 to 15 million USD 92R. From the numbers presented regarding the time taken and the costs, there is a important need for both to be reviewed as it is scaring away the concerned stakeholders 8R.

4. Conclusion
Road map for development and application of biojet fuel as drop-in fuel in the aviation purpose has been laid down. Biojet is considered an important part of the aviation industry’s GHG emission reduction strategy. Using it to reach a GHG emissions goal of 2050 emissions at one-half of the 2005 level would involve significant development of standards and regulatory approvals, feedstocks, conversion facilities, transportation infrastructure, and logistics. Initial use of biojet has proceeded in commercial and military applications 12R.