Sustainable alternative fuels in aviation

Energy xxx (2017) 1e9

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Sustainable alternative fuels in aviation Nadir Yilmaz a, *, Alpaslan Atmanli b a b

Department of Mechanical Engineering, Howard University, Washington DC, USA National Defense University, Istanbul, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 December 2016 Received in revised form 2 July 2017 Accepted 6 July 2017 Available online xxx

In recent years, renewable energy resources have become more important due to the limited number of regions for production of petroleum-based fuels, which are continuously depleting. The aviation sector in terms of commercial and cargo transportation has an increasing need for conventional, as well as, alternative fuels. Derivatives of petroleum fuels used in aviation have negative impacts on air quality. Factors causing greenhouse gas emissions (GHG) in the aviation sector must be reduced. However, such fuels used in the aviation sector are not sustainable. Biofuels which have the potential to replace petroleum fuels and help with emissions are heavily investigated in developed countries for independency, creating a better environment and sustainability. Biofuels which are already used for ground vehicles could also be implemented in the aviation sector to reduce fuel cost and emissions. Overall, aviation fuels made of sustainable resources would also support social and economic development. Numerous industrial initiatives have emerged to find alternative ways to attain bio-aviation fuels. Therefore, there is an increasing level of research with regards to alternative aviation fuels made of biomass in recent years. It is important to obtain basic feedstocks and to develop biofuel production processes in a cost-effective way. This study examines the necessity and the types of biofuels in the aviation sector. By designing unique fuel systems for air vehicles, it is possible to formulate biofuels which can be used for both air and ground vehicle applications. This type of consensus would help with sustainability and a better environment. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Aviation Biofuels Environment Sustainability

1. Introduction As technological developments are happening at a fast pace, energy is playing a more important role in the daily lives of all people and social-economical development of every country [1,2]. Industrialization, an increasing world population, globalization, more urban developments and other factors are the main reasons for demand on more natural resources and energy. Cost-effective, secure and clean energy has become one of the most important challenges nowadays and developing countries are in need of more energy due to their social-economic growth [2e4]. In 2014 and 2015, global energy consumption increased about by 1.0%, much below its 10-year average of 1.9%. Fossil fuels are the primary 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.) [5e7].

* Corresponding author. E-mail address: [email protected] (N. Yilmaz).

Petroleum based liquid fuels are used in the transportation sector. In 2011, the world's petroleum resources were consumed by air, ground, sea and railroad transportations (54%), industry (18%), commercial and agriculture (11%), petro chemistry (10%) and electric production (7%) [5,8e10]. It is projected that 74% of petroleum based energy will be consumed in the transportation sector by 2020. In addition, the need for oil in the transportation sector is growing rapidly and expected to increase by 1.3% per year until 2030. Based on the current statistics, 1.2 billion motor vehicles are used world-wide and this number is anticipated to reach to 2 billion until 2035 [6,11]. Thus, it is important to monitor and know the number of vehicles per person by the oil producers. As of 2009, the statistics shows that 70% of 870 million cars in the world belonged to The Organization for Economic Co-operation and Development (OECD) countries [11e13]. Pollutants are released to the atmosphere due to production and consumption of energy worldwide. Most of these pollutants occur because of fossil fuels [11]. With the increasing number of motor vehicles in the developing countries, it is necessary to take preventive measures to reduce exhaust emissions [14]. As the number

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of motor vehicles increases, the amount of pollutants from those vehicles increases with a higher percentage of contribution. Even a small change in atmospheric gas balance could lead to climate change. Carbon dioxide (CO2) has a great impact on climate change and global warming based on the Kyoto protocol [11,12]. CO2 emissions increased 4.6% in 2010 as compared to 2009. In 2010, 36% of total CO2 emissions was petroleum based and 22% of that was due to the transportation sector [13]. By 2030, it is expected to have 80% increase in carbon emissions from the transportation sector as well as the energy requirement [13]. The aviation section is an important part of the transportation sector. Developed and developing countries have made the most investment on the aviation sector. Like automobiles, aircrafts depend on fuels derived from fossil fuels and thus, the aviation sector needs environmental and economic regulations. With the increasing standards of the modern world, the aviation sector has to take an important role to protect the environment [14e16]. Thus, air and ground vehicles need to be supported by sustainable energy resources. The most important resource for sustainable energy that will meet the need for liquid fuels for these sectors should be domestic and easily producible sources [17e19]. Biomass has been effectively used over the years for the production of alternative fuels for ground transportation [1,3,17,20]. Similarly, biomass will be the best potential for use in the aviation sector which is one of most the common forms of transportation. Any agriculture and animal feed based biological materials with carbohydrate are considered biomass energy resources. And, fuels made of biomass are called biofuels [18e21]. Biomass, which are sustainable energy resources, reduce greenhouse effects, improve air quality, reduce oil dependency and produce new job opportunities [20]. Countries with biomass potential usually have large resources of vegetable oils, inedible materials and bioalcohols. There are two ways to use biomass: 1) direct combustion of biomass for electric or heat generation 2) conversion to biochemicals and thermochemical process such as biodiesel or other fuels [22]. Fig. 1 shows biomass feedstock and consumption [23,24]. Petroleum fuels have limited reserves in the world. Sudden jump of oil prices, limited resources, greenhouse effects, environmental issues and other important aspects, force the use of renewable energy resources [25]. With that, biomass is an important source to produce biofuels which are advantages as compared to petroleum based fuels and can be used for air and ground vehicles [26]. The aviation sector which is primarily used for commercial transportation do contribute to atmospheric pollution. Air vehicles produced 2% of CO2 emissions in 2012 worldwide with a projected amount of 3% by 2050 [27]. There are numerous measures which have been taken to reduce CO2 emissions [28]. Better engines and fuel technologies are contributing factors to reduce emissions and use fuels more efficiently in the aviation sector as compared to forty

Fig. 1. Biomass feedstock and consumption [23].

years ago. Jet fuels constitute a large sector for the consumption of fossil fuels. World jet fuel demand is expected to increase by about 38% until 2015 [29]. Commercial airliners are facing aviation fuel cost as a major expenditure out of their total operational cost. However, more aerodynamic and lighter aircrafts, more efficient turbine engines and major improvements in the efficiency of the air traffic control system are some of the reasons for 70% more fuel efficient airline industry as compared to 40 years ago [30,31]. Fuel consumption per 100 km/traveler used to be 3.5 L which was reduced to 3 L in recent years. The fuels used in the aviation sector need to have high energy content, good flow characteristics and thermal stability [32]. Energy resources which meet these requirements should not challenge the food production and ecosystem while not harming the environment nor causing deforestation. It is the expectation of companies that aviations fuels are economical as well [33,34]. Besides lower fuel consumption, it is also important to release neutral-carbon emission. In addition, alternative aviation fuels must offer low carbon emission over their lifecycles. In a limited number of studies, non-edible oil crops such as camelina, jatropha, algae, halophytes, municipal and sewage wastes, forest residues etc were used aviation fuel production process [35e37]. Biofuel production uses thermo chemical and biochemical techniques. After 2008, numerous test flights were performed with biofuels and ASTM standards were achieved in 2011 in order to allow aircraft and engine manufacturers to use biofuels in air vehicles [34]. It is anticipated that consumption of second generation biofuels will be 6% in the aviation sector, which is supported by Boeing [38,39]. It is emphasized that aviation biofuel production will show progress with respect to the developed standards by national and international organizations. International Air Transport Association (IATA) predicts 30% biofuel in jet fuel by 2030 [39,40]. If this goal is achieved, biofuel use will result in a great deal of positive impact on economy and environment in developing countries. Biofuels, produced from various raw materials that are easy and widely available, improve the fuel properties. This will increase the variety of alternative fuels to be used throughout the world. Thus, biofuels which can be maintained economically, ecologically and socially have the potential to spread [41]. In coming years, alternative fuels used in both air and ground transportation will support the energy policy of countries [42]. Due to economical and environmental disadvantages of fossil fuels, alternative energy resources have also been investigated to reduce dependency on petroleum based fuels and protect environment. Overall, manufacturers of air and ground vehicles focus on design of engines and vehicles compatible with alternative fuels [43e45]. More research is needed for the compatibility of alternative fuels in ground vehicles which use mostly liquid fuels. In this regard, it is also important to note that internal combustion engines and gas turbines could potentially use fuels of similar properties with new fuel injection technologies and designs. Gasoline engines use alcohol based alternatives fuels in ground transportation. Diesel engines, which are used more widely than gasoline engines for transportation, use alcohols, biodiesel and synthetic fuels, which have been compared over the years against one another in terms of energy value and engine characteristics. With respect to the economic development of countries, air transportation and aviation has become an important transportation part of the system. This sector needs a special fuel whose characteristics are placed in between those of gasoline and diesel fuels. Thus, the need for alternative fuels in the aviation sector has led to economical developments but also new resources for raw fuels. Like in the transportation sector, fuels used in the aviation sectors must be tested over the years and reliable fuels in terms of environmental and performance impacts. As a result, bio-aviation

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fuels and bio-road fuels have to be examined for the readily and widely available raw materials (renewable hydrocarbons such as bio-oils, bioalcohols) which can be used in aviation and ground transportation sectors. The purpose of this study is to compare the characteristics of different alternative sources of energy and to discuss the suitability for both aviation and ground transportation. 2. Discussions and remarks While the depletion of fossil fuels increases, sustainable alternative fuels and energy resources have become necessary in the aviation sector, which has a growth rate of 5% along with 3% extra fuel consumption every year [43,44]. The most important factors when dealing with alternative fuels is to find; (1) sustainability (2) renewability (3) carbon dioxide recycling (4) eco-friendly technology and (5) less dependency on petroleum supplying countries [46]. Such alternative fuels could be made of biomass, which consist of alcohols, biodiesel made of animal waste, agriculture products and waste oils, and Fischer-Tropsch (FT) synthetic oils. Use of these alternative resources in the aviation sector would be beneficial in terms of environmental and economic aspects. 2.1. Potential sources of biofuel Biofuels made of biomass are clean, environment-friendly and efficient renewable energy resources. Biofuels are consumed as gas, liquid and solid. Liquid biofuels which have the potential to replace liquid fossil fuels are vegetable oils, biodiesel, biomass and bioalcohols. Biomass has an important role in replacement of fossil fuels, which in return would have an impact on the environment and economy and potential reduction of greenhouse gases [43]. Many of developed and developing countries have various processes of biofuels in order to decrease dependency on fossil fuels, reduce greenhouse gases and increase development of rural areas. There are two production processes creating primary and secondary fuels. Primary biofuels are natural and directly used for heating, cooking and electric production without any chemical process. Examples include waste oils, animal fats and wood [3,17]. Secondary types of biofuels are used to improve properties of primary biofuels. Vegetable oils, biodiesel, ethanol, methanol and biogas are some examples of secondary biofuels. Production of secondary biofuels from renewable sources with chemical processes is divided in to three areas as first, second and third generation [20]. Fig. 2 shows biofuels based on production methods. Secondary biofuels can be blended with fossil fuels or directly be used in internal combustion engines. 10% of bioethanol can be blended with petroleum based fuels and directly used in an internal

Fig. 2. Biofuels based on production methods.

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combustion engine without any engine modification. Bioethanol blend ratios can be increased with engine modifications. E85 (85% ethanol) and flexible are often used in Brazil and USA, respectively. Nowadays, second and third generation alternative fuel resources are continuously used to reduce emissions from fossil fuels and increase the percentage of such fuels [18,19]. Such research studies, depending on the country of origin where research is performed, show the extensive use of canola, soybean, cotton, palm and coconut oils [25]. Some of the mentioned oils are edible oils used in food industry. However, animal fats and waste oils are important resources to produce biodiesel, 20% of which can be blended with diesel fuel and safely used in internal combustion engines [7,10]. In addition, third generation fuels such as synthetic and algae based fuels are important alternative fuels for the near future. One of the most important reasons for USA and Europe to use biofuels is to reduce carbon based emissions. The European Union (EU) targets 10% of biofuel use by 2020. Thus, second and third generation fuels are supported for biofuel production, where such fuels are made of inedible resources, and thus, do not affect the food chain [47e49]. The biofuels, produced from biomass with inedible quality, support economic development. In recent years, investigations using inedible resources such as jatropha, camelina, algae and different wastes have shown a great potential for the transportation sector. When examined the basic properties of these materials; Jatropha is a non-edible energy crop which does not compete with food crops. It is drought and pest resistant, and grows quickly even under rough soil and climate conditions [29]. Jatropha has a permanent pattern of high yield oil production [1,2]. Jatropha is poisonous and contains 30e40% oil in each grain. Jatropha plant can continue yielding for 40 years once the growth starts with a small amount of moisture [3]. South Africa, South and Central America and South East Asia are the main reasons for cultivation of jatropha. Camelina is a non-food energy crop with high oil content, with the average about 38e43% [1]. It can grow on infertile soil or marginal land. Its cultivation happens as a rotational crop for wheat and cereals and it needs minimum input [29]. Its left over after the oil extraction can be used to feed animals. Algae has high lipid content, high rate of CO2 absorption, low land use and faster rate of growth [1,2]. As compared to other oils, algae has 60% oil by weight which make it advantageous [50]. Another advantage of algae is that it reduces food-fuel competition since it does not affect crop cultivation because of no need for land or water to survive [51]. Algae can produce large quantities of lipids and carbohydrate by using sunlight, waste water and CO2 and thus can play a crucial role in wastewater treatment [52]. As compared to other energy crops, it produces 30 times more yields per acre [53]. Algae can be processed into a variety of renewable fuels [54]. Current research shows bio-jet fuel from microalgae can reduce life cycle greenhouse gas emissions by 76% [29]. Wastes of various origins such plant or animal are dependable feedstocks. Examples include paper, wood products, municipal wastes and industrial wastes which can be converted to biofuel through different routes [1,2]. While these wastes potentially convert to biofuels, such conversion and energy production also helps with waste management and reduce harmful products. The use of waste materials for the production of biofuels can overcome many difficulties such as need of fertilizer, irrigation, land and labour [29,55]. These sources can be converted into products as biodiesel, bioalcohols and synthetic fuels due to improved biofuel production methods (thermochemical and biochemical) [56]. In the aviation sector, because the consumption of fuels in the aviation sector is higher than that in the ground transportation sector, it is aimed to balance environmental and economical aspects [57]. These fuels

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must meet various conditions so that they can be efficiently used in the aviation sector. The properties of alternative aviation fuels are (1) inedible (2) renewable resources (3) reduced greenhouse gas emission (4) sustainability and clean burning (5) compatibility with conventional fuel [29,58e64]. According to this, all of the above renewable bioresources could be used to produce alternative aviation fuels.

Table 2 Potential biofuels for various types of vehicles. Vehicle type

Fossil fuel type

Potential biofuel type

Light vehicles Heavy vehicles Equipment/Machinery Marine Aviation Remote generation

Gasoline/LPG/diesel Diesel Diesel Diesel/fuel oil Jet fuel (JP) Diesel/naturel gas

Alcohol/Biodiesel Biodiesel Biodiesel Biodiesel Renewable and Synthetic fuels Biodiesel/Biogas

2.2. Potential alternative fuels for the aviation sector Alternative fuels that can be used in aviation must be compatible with existing fuel systems, storage conditions and fuel transfer process [43e45,64]. For this reason, it is necessary to very-well know the basic properties of aviation fuels [40,41]. The aviation sector uses petroleum based kerosene which is also called jet fuel. Such fuels are called jet fuel for gas turbines and aviation gasoline for piston-based engines such as United States (US) Jet A (European Jet A-1), Jet Propellant JP-4 (European F-40), JP-5 (European F-44), JP-7 (US only), JP-8 (F-34), JP-TS (US only), and JP-8 I 100 (US only) [29]. Kerosene based fuels are Jet A, Jet A-1, JP 5 and JP 8 with 8e18 carbons. Naphtha type fuels are Jet B and JP-4 with 5e15 carbons. These fuels are also used as blends of kerosene-naphtha and kerosene-gasoline (JP-4) [44]. Table 1 shows basic fuel properties of biodiesel, ethanol, nbutanol and jet fuel. Similar fuel properties indicate that some of the fuels can replace one another. In internal combustion engines, studies have shown biodiesel and alcohols are the primary alternative fuels which can be produced from biomass. Nowadays, alcohols (e.g. ethanol and butanol) can be produced via fermentation from the raw material which is actually used for biodiesel production. It is important to note that alternative fuels, which are produced from the same raw materials, have similar fuel properties and characteristics. For instance, in diesel engines, ethanol or butanol can be added to diesel fuel/biodiesel blends. In addition, diesel/butanol blends have shown similar engine characteristics and emissions as diesel/biodiesel blends in diesel engines. Therefore, Table 1 shows the fuels which are used frequently for studies nowadays and can be used in the same engine in the form of various combinations of blends. Being able to mix such liquids is another indicator that the fuels have similar miscibility and physical characteristics. Table 2 shows potential bio fuels for different types of vehicles used nowadays. Table 3 shows fuel criteria based on commercial aviation operations for safety purposes. In aviation fuels, high heat content, good atomization, rapid evaporation, good burning characteristics, low explosion risk, being free from contaminants, minimum carbon formation, low viscosity, good thermal stability, and good storage should be obtained. Fuels have to pass laboratory, storage and flight tests for operational purposes and are then finally certified in the end [43,44]. Therefore, aviation fuels have to meet the standards of ASTM (American Society for Testing and Materials) D1655 in USA and DSA

Table 3 Basic criteria for aviation fuels. Operability

Environmental concerns

Supportability

Ignition delay Cold flow properties Thermal stability Altitude relight

Production process Low CO2 emission Low Particulates Non-Mutagenics

Wide availability Acceptable cost Easy use Good ground storage

(Defence Standards Agency) in England [45]. JP fuels for military use have to meet the NATO standards. Biofuels also have to pass tests and meet standards and thus, USA (US-RFS2) and Europe (EURED) have various fuel standards for sustainable/renewable alternative fuels [46]. Based on the above-mentioned definitions and standards, there have been initiatives to actively use biofuels in the aviation sector as replacements for conventional jet fuel [61e65]. The aviation sector has adapted the use of alternative fuels for fuel efficiency, less dependency on petroleum based fuels and environmental factors such as reduction of greenhouse gases. Thus, there is an aim to reduce the aviation sector's contribution of 2% of CO2 emissions to lower levels. The amount of fuels used in aviation is much less than that in ground transportation [47,66,67]. It is important that alternative aviation fuels have advantages in terms of choice of feedstock, production processes, fuel properties meeting standards, safe storage, easy transportation and extensive usage. As in ground transportation, biofuels have the potential to be used in the aviation sector with numerous advantages. Aviation fuel conversion and production methods are oil to jet fuel Hydroprocessed renewable jet fuels (hydroprocessed esters and fatty acids (HEFA)) and biodiesel, gas to jet fuel (Fischer-Tropsch), alcohol to jet fuel (ethanol to jet and butanol to jet), and sugar to jet fuel (fermentation of sugars to hydrocarbons) [47,68]. When the performance of these production methods and the properties of their products and the alternative fuel requirements of the aviation sector are examined with specifications seen in Table 3, the following can be noted. HEFA is generally paraffinic liquids. HEFA jet fuels are produced by the hydrodeoxygenation of vegetable oils, animal fats, waste grease, algal oil and bio-oil and the major side products are water and propane [29]. Jatropha oil, camelina oil, algal oil, bio-oil, animal

Table 1 Fuel properties of biodiesel, ethanol, n-butanol and jet fuel [8,10,29,44]. Items

Biodiesel (EN14214)

Ethanol

n-Butanol

Jet fuel

Molecular weight (kg/kmol) Density (g/ml@20  C) Kinematic viscosity (mm2/s@40  C) Low heating value (MJ/kg) Freezing point ( C) Flash point ( C) Auto ignition temperature ( C) Boiling point ( C) Vapour pressure (kPa@38  C) Oxygen content (%)

e 0,860e0,9 3,5-5 35e43 1e 15 >120 e e e e

46,07 0,789 1,08 26,8 114,3 8 434 78,4 13,8 34,8

74,12 0810 2,23 33,1 89,5 35 385 117,7 2,27 21,6

z185 0,775e0,84 8 42,80e43,02 47e 60 38 210 150e170 14e21 e

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fats and waste grease are some of the most promoting feedstock for the production of HEFAs [41,69]. The method of hydroprocessing includes treatment of fats and oils in the presence of hydrogen for the removal of oxygen from the feedstock. Hydrodeoxygenation is followed by isomerization and cracking to achieve desired fuel specifications such as low temperature properties [34,70]. The hydroprocessed renewable jet fuels are high energy biofuels which can be used even without blending. One of its advantages is to reduce carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) [29,71]. HEFAs are free of aromatics and sulphur and have high cetane number, high thermal stability and low tailpipe emissions [38,52]. These fuels are stable for storage and resistant to microbial growth [29,53,72]. HEFAs are suitable for conventional aircraft engines without further engine modification and do not raise any fuel quality issues. These fuels do not form deposit in engines [54,73]. HEFAs are advantageous for higher altitude flights because of cold flow properties [51]. Their lubricity is low because of the absence of oxygen and sulphur [55] which can be improved by blending with conventional jet fuel or other additives [52,74]. Cold flow properties like cold filter plugging point and cloud point are affected by higher paraffin content but it also depends on the type of feedstock [56]. Cetane number of hydroprocessed renewable jet fuels affects the fuel ignition in the engine as compared to conventional jet fuel but such problems can be easily fixed by blending HEFAs with conventional fuels [55]. Hydroprocessed renewable jet fuels are comparatively economical [23]. HEFA fuel that meets ASTM D7566 specification can be mixed with conventional jet fuel, up to a blend ratio of 50% [29,72e76]. Biodiesel, which is a major alternative fuel in ground transportation, can be made of various oils and used in diesel engines by limited blends with diesel fuel [1e4]. Biodiesel is alkyl esters of fatty acid and produced by the process of transesterification. Biodiesel production is inexpensive and does not create a food-fuel crisis because it can be made of non-edible oils. Biodiesel is

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biodegradable and has excellent lubricity [7e11]. Some of its advantages include miscibility with petroleum fuels, high flash point, no sulphur in its composition, reduced greenhouse gas emissions and non-toxic [64e67]. The energy density of biodiesel is very low compared to conventional jet fuels and this it is not sufficient to use as an aviation fuel [68]. Biodiesel as aviation fuel does not need further engine rebuilding or modification [69]. Some of its disadvantages include biodegradability causing biological growth during storage which affects the stability [1], high freezing point which is a concern for high altitude flight, presence of polyunsaturated and unsaturated fatty acids [1,2], polarity which causes the formation of emulsion [7e10] and insufficient feedstock supply [29]. First generation biofuel, biodiesel, is suitable for use in commercial airlines. This fuel does not, however, meet the standards in terms of fuel performance and safety. Raw material needed for biofuel production for use in aviation should be inedible and ecofriendly [47,48,77]. Thus, vegetable oils such as camelina, jatropha and algae oils are options for production of aviation biofuels. It is important to note that some of those fuels have poor cold flow properties and oxidation problems within six months, and may not be used as aviation fuels. Some preliminary research studies used bio-synthetic parafinic kerosene made of jatropha, algae, and camelina oils (Fig. 3) at high temperatures [49,78]. While the above-mentioned fuels cannot replace the jet fuel, they can be blended with the jet fuel between 5 and 20% ratios and used as part of a jet fuel. Although they have advantages in terms of production and environment, some of their poor fuel properties prevent them from being used as jet fuels. Thus, they have to go through additional processes for the improvement of their fuel properties, but such processes would increase the cost [27,79,80]. In 2014, ASTM approved the use of 5 mg/kg biodiesel in jet fuels [38,81]. FT fuels, produced by advanced technological production processes (thermochemical), are important alternative fuels for internal combustion engines. They are hydrocarbon-based fuels, which

Fig. 3. Raw materials for bio-jet fuel production.

Fig. 4. Process of biofuel and synthetic fuel production.

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are produced by catalytic conversion of syn gas (CO and H2) [29,82]. Fig. 4 shows the process of biofuel and synthetic fuel production. The range of hydrocarbons depends on the catalyst, pressure and temperature conditions of the process [83e85]. A wide range of biomass feedstock can be used for the syn gas generation [86]. The FT fuels are usually clean burning, high value fuels [29] and characterized by non-toxicity, no emission of nitrogen oxides, low sulphur and aromatic content, reduced particulate emission and high cetane number. Also, the fuel combustion is free of CO2 and hydrocarbons [87]. Jet fuel produced by the FT process from different feedstock shows similar properties and is characterized by no sulphur and aromatics [80,88]. Differences in fuel properties of FT fuels are mainly because of operational conditions rather than the nature of the feedstock [29,57]. The FT process is expensive and the efficiency of the process ranges between 25% and 50% [59]. FT fuels have low power and low fuel economy because of low energy density [61] in additional to having low lubricity due to no sulphur [37,68]. However, fuel lubricity is improved when FT fuels are blended with conventional fuels [80,81]. According to the ASTM D7566 standard, FT-fuel is approved for a 50% blend with Jet-A1 fuel [72,89]. In addition, FT bio-synthetic parafinic kerosene (FT-SPK) has been studied and used as an alternative. FT-SPK is produced from solid biomass or coal with the order of synthesis gas generation, FT synthesis, hydrocracking and synthetic product [45]. This synthetic fuel has similar properties as jet fuel and can potentially be used as an aviation fuel, but its production process is not as environmentally effective [27]. GTL (gas to liquids) jet fuel from natural gas is another way to produce an alternative fuel [80,82]. Alcohols became important alternatives for internal combustion engines in recent years. Alcohols made of biomass can be used in both gasoline and diesel engines and counted as next generation fuels. Bio alcohols are produced by thermal and biochemical fermentation of carbohydrates which are formed by the hydrolysis of biomass [29,48,52]. Direct sugar sources are subjected to fermentation to produce alcohol [90]. Wood, agricultural wastes, forest residues and wastes are some of biomass feedstocks for the production of bioalcohol [91]. Ethanol and n-butanol have potential alternative fuels for ground vehicles. However, because of their high volatility, low flash point, low energy density and low temperature properties, both alcohols are not viable for aviation [2,7]. In addition, alcohols need specific delivery infrastructure and storage system for use in the aviation industry [7e10]. Alcohols as jet fuels require engine modifications [9,29]. Safety concerns arise at high altitude flights because of the high volatility of ethanol [19] and because of poor fuel properties of ethanol, blending with conventional jet fuels is not a feasible option [31,43]. Although alcohols have low calorific value and high heat of vaporization, which are important hurdles toward being aviation fuels, it may be possible to change chemical structures of alcohols with technological developments to make them suitable alternatives in the aviation sector [28,34,47]. In order to promote the use of biofuel in the aviation sector, it is necessary to improve production methods and to improve fuel standards developed by international organizations in collaboration with industry. Thus, IATA and international civil aviation organization (ICAO) have led collaborative research in this field [35e40]. Studies in this area started around 2007e2008 and have accelerated through 2015. In 2013, IATA, in agreement with the ICAO, approved a resolution setting out ambitious targets for addressing carbon emission reduction entitled ‘Aviation CarbonNeutral Growth Strategy (CNG2020)’. The strategy involves the use of sustainable biofuels (drop-in), aiming to achieve benefits from environmental, social, and economic perspectives [91]. One of the most important problems while conducting this type of

Table 4 Target for sustainable jet fuel use in various countries [39]. Country/region

Organization

Blend target

Timeframe

World USA EU Australia Germany Netherlands Israel Indonesia Nordic countries

Boeing FAA EC AISAF Aireg Bioport Holland FCI Government NISA

1% 5% 3%e4% 50% 10% 1% 20% 2% 3%e4%

2016 2018 2020 2050 2025 2015 2025 2016 2020

research was economic factors. Lower oil prices and oscillations negatively affect the projection studies for the development of alternative jet fuels. For example, while oil price was $100 dollars per barrel during 2011e2013, it went down as much as $40 dollars in recent years. Thus, cost of alternative fuels must be comparable to petroleum based fuels in addition to environmental advantages. Overall, such studies are being conducted in USA as Europe as well as Brazil, South Africa, China, Indonesia and Japan with the support and collaboration of companies as Airbus, Boeing and Gulfstream. Japan targets the use of alternative fuels in all flights for the Tokyo Olympics by 2020. Indonesia and the USA have done collaborative research for the development of alternative aviation fuels. Similarly, South Africa is in the process of developing fuels which meet RSB standards, and Europe has solar-jet projects. Yet despite numerous successful test flights, aviation bio-fuels have yet to become widely commercialized. As seen in Table 4, those countries have put forward a target for the implementation and use of sustainable jet fuel [39]. In order to reach the target, numerous tests have been conducted. World-wide, the aviation sector uses 1.5e1.7 billion barrels of Jet A1 per year. It shows the importance of even 1% use of biofuels in the aviation sector in terms of the positive economic and environmental outcomes [82e84,87].

2.3. Environmental effects of alternative fuels Sustainable environment is one of the most important factors in developing alternative fuels. High production and wide-range use are indispensable factors for alternative fuels to compete with fossil fuels. Thus, alternative fuels gain more importance in terms of greener environment and higher competition against conventional fuels [58e63]. For alternative fuels, there are a number of challenges to overcome such as; (1) environmental challenges, (2) production issues, (3) distribution problems, (4) feedstock availability and sustainability, (5) compatibility with conventional fuel. Of course, the most important obstacle is to protect the environment [29,50e54,71,72]. In international projections, reduction of CO2 is critical for the future [70,78,88]. In both air and ground transportations, this need has been recognized and legal implications have been placed. 2.5% of man-made CO2 was from the aviation sector in 2005 as it is expected to become 4e4.7% by 2050 [50,54]. Emissions from aircraft engines affect the radiative balance of the atmosphere, and therefore the climate system. Biofuels are the first feasible option for greenhouse gas emission reduction in aviation. Thus, alternative fuels in the aviation sector must meet all aviation fuel standards. Otherwise, the use of such fuels will be limited [28,47]. The uprising demand for bio-jet fuels affects the fertility of soil and biodiversity and cause increase in atmospheric CO2 [56]. In the long term, use of agricultural land for crop cultivation for biofuel production will cause food scarcity and reduce soil quality and water availability in the soil. However, biofuel made of inedible biomass would not harm the food chain. The use of

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Fig. 5. Airline companies using alternative fuels in 2008e2013 [39].

fertilizers and insecticides will result in soil destruction and water pollution [57]. The fuel development should not arise any human health issues [62]. In spite of all these potential disadvantages, production of alternative fuels for the aviation sector is still a new initiative and research is ongoing. While alternative jet fuels have similar origins, their fuel properties must meet standards based on flight and climate conditions [85]. Fuels for the purpose of aviation and ground transportation can be made of the same source. Since 2011, 22 airline companies have used biofuels made of various resources in more than 2000 commercial flights [38,39]. Fig. 5 shows airline companies using alternative fuels in 2008e2013 according to IATA [39]. In 2008, for the first time ever, Virgin Atlantic used biofuel made of coconut oil in a flight from London to Amsterdam. By 2011, with the approval of use of 50% of biofuels in commercial flights, interest in biofuels has increased in the aviation sector. However, in order to promote the use of these fuels in all airline companies, these alternative fuels must be competitive in terms of price, production, and social, economic, environmental and technological perspectives. With regards to the environment, use of alternative fuels is projected to allow for a 85% decrease of greenhouse gases and other pollutants due to no sulphur. ICAO was directed by the Kyoto Protocol to explore pathways for mitigating aviation emissions [42]. IATA indicated the target of 50% reduction of CO2 caused by air vehicles by 2050 [41,83,87]. In order to achieve this target, there are 15 aviation standards for biofuel production and processes by EU-RED, US-RFS and ASTM. Increasing the number of these standards and studies will be important toward more use of alternative aviation fuels, which contribute to a more ecofriendly environment. 3. Conclusions Air transport is a developing business sector, with rapidly increasing rates in transport loads and fuel demand. Aircraft emissions are impacting GHG emissions and hence inducing climate change. Since reduction of carbon emissions is recognized as important, bio-jet fuels can contribute to this goal substantially in the short and medium-term. However, Alternative fuel production requires establishment of new industries with time and efforts. There is a need to explore new opportunities for the production of renewable jet fuel. Materials and production technologies used for making biofuels for ground transportation are suitable for the development of sustainable aviation fuels. Aviation bio-fuel properties must meet the standards based on engine operation conditions, storage, environmental and safety aspects. The fuel standards for bio-jet fuels have successfully been established during the last several years. To date, the research shows that alternative fuels have been safely used in the aviation sector. Among the various processes and alternative fuels, HEFA and FT fuels, have the

potential to replace the conventional jet and road fuels. These fuels can be used in tanks, pipelines, pumps and automobiles without any changes nowadays. The direct synthesis of aviation fuels from renewable biomass is of great interest in research and industrial applications. The main problems with biodiesel and bioalcohols as aviation fuels are their poor fuel properties. Aviation biofuel options will increase as the fuel properties of such alternative fuels are improved. In addition, streamlining the production process of the existing biofuels will spread bio-jet fuel use in the aviation sector. Parallel to these developments, organizations such as IATA and ICAO must work with airline companies and aircraft manufacturers to reduce CO2 emissions in commercial flights and to advocate for more use of alternative fuels in aviation. The use of waste materials from different feedstocks can contribute to the issues related to feedstock costs. Sustainable aviation fuel will be economically more competent as better biofuel production technologies are developed and cost of production decreases, along with more incentives and research. References [1] Mahmudul HM, Hagos FY, Mamat R, Adam AA, Ishak WFW, Alenezi R. Production, characterization and performance of biodiesel as an alternative fuel in diesel engines-A review. Renew Sustain Energy Rev 2017;72:497e509. [2] Hajjari M, Tabatabaei M, Aghbashlo M, Ghanavati H. A review on the prospects of sustainable biodiesel production: a global scenario with an emphasis on waste-oil biodiesel utilization. Renew Sustain Energy Rev 2017;72:445e64. [3] Demirbas¸ A. Green energy and technology biofuels. London: Springer Verlag; 2009. p. 1e4. [4] US Energy Information Administration (EIA). Annual energy outlook 2012 with projections to 2035. 2012. p. 17e63. [5] Dudley B. BP statistical rewiev world of energy. 65 th edition June 2016. p. 6e19. [6] Salvi BL, Subramanian KA, Panwar NL. Alternative fuels for transportation vehicles: a technical review. Renew Sustain Energy Rev 2013;25:404e19. [7] Yilmaz N, Morton B. Comparative characteristics of compression ignited engines operating on biodiesel produced from waste vegetable oil. Biomass Bioenergy 2011;35:2194e9. [8] Atmanli A. Comparative analyses of dieselewaste oil biodiesel and propanol, n-butanol or 1-pentanol blends in a diesel engine. Fuel 2016;176:209e15. [9] Yilmaz N, Sanchez TM. Analysis of operating a diesel engine on biodieselethanol and biodiesel-methanol blends. Energy 2012;46:126e9. [10] Yilmaz N. Comparative analysis of biodiesel-ethanol-diesel and biodieselmethanol-diesel blends in a diesel engine. Energy 2012;40:201e3. [11] World Energy Council. Energy full report. 2016. p. 4e20. [12] US Energy Information Administration (EIA). Key world energy statistics. 2012. p. 6e11. [13] Organization of the Petroleum Exporting Countries (OPEC). World oil outlook. 2012. p. 25e62. [14] Yilmaz N, Davis SM. Polycyclic aromatic hydrocarbon (PAH) formation in a diesel engine fueled with diesel, biodiesel and biodiesel/n-butanol blends. Fuel 2016;181:729e40. [15] Yilmaz N, Vigil FM, Donaldson AB, Darabseh T. Investigation of CI engine emissions in biodieseleethanolediesel blends as a function of ethanol concentration. Fuel 2014;115:790e3. [16] US Energy Information Administration (EIA). CO2 emission from fuel combustion. 2012. p. 37e9. [17] Sidibe SS, Blin J, Vaitilingom G, Azoumah Y. Use of crude filtered vegetable oil

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