A Review of Alternative Aviation Fuels

Abstract

One of the most promising mid-term solutions for reducing GHG emissions from the aviation sector is alternative aviation fuels, especially sustainable aviation fuels (SAFs). Regulations imposed by the Fit for 55 package to use 38% of SAFs until 2050 require a comprehensive analysis of SAFs and production pathway development with increased blending limits of alternative fuel. Within this review, a summary of key aspects of alternative aviation fuels is presented. The review contains a description of the certification process and certified production pathways with an analysis of feedstocks used for SAF production. SAF emissions also have been analyzed based on available research. SAFs reduce particulate matter emissions significantly, even by 70%, compared to fossil fuels. The emission of gaseous exhaust compounds, such as carbon monoxide, unburned hydrocarbons and nitrogen oxides, also is discussed. Alternative aviation fuels have a lower LCA compared to conventional aviation fuel and the LCAs of specific feedstocks are presented.

1. Introduction

Decarbonization and emission reduction are some of the biggest challenges for the aviation sector. Initiatives such as the European Commission’s Green Deal have been implemented to achieve carbon neutrality across all industries by 2050. Despite the aviation sector accounting for only 2–3% of global carbon emissions in 2019 [1], the pressure to innovate more environmentally friendly aircraft is expected to rise due to predicted growth in air traffic until 2050. Major contributors to aviation emissions are short- and medium-range aircraft, responsible for approximately two-thirds of the total CO₂ emissions in aviation [2]. The aviation sector also generates non-CO₂ emissions harmful to the environment, such as hydrocarbons, carbon monoxide, sulfur oxides and particulate matter. Depending on the altitude of the flight and the atmospheric conditions in the vicinity, various exhaust gas byproducts like nitrogen oxides (NO_x) and a mixture of soot particles and water vapour can lead to the generation of ozone, which is detrimental to the climate, and contrails [2,3]. Alternative aviation fuels can significantly reduce greenhouse gases (GHGs) and non-carbon emissions from the aviation sector and are intensively developed to replace conventional aviation fuels in the next years. Alternative aviation fuels contain sustainable aviation fuels, made mostly from biomass, synthetic fuels and hydrogen. According to ReFuelEU Aviation, as a part of the Fit for 55 package, alternative aviation fuels should be widely used until 2050: a minimum of 5% of SAFs until 2030, with a minimum 0.7% of e-jet; a minimum of 20% of SAFs until 2035 with a minimum of 5% of e-jet; a minimum of 32% of SAFs until 2040 with a minimum of 8% of e-jet; a minimum of 38% of SAFs until 2045 with a minimum of 11% of e-jet; and a minimum of 63% of SAFs until 2050 with a minimum of 28% of e-jet [4].

Sustainable aviation fuels (SAFs) represent a highly promising solution in the medium term for the reduction of greenhouse gas emissions from the aviation sector. According to ICAO Annex 16, SAF is defined as “a renewable or waste-derived aviation fuels that meets sustainability criteria” [5]. Sustainable aviation fuel is a broader term than biofuels, as it uses as a feedstock not only biological raw materials, but also non-biological materials. Sustainable aviation fuels, to be considered ‘sustainable’, must adhere to several criteria: reduce carbon dioxide emissions in the entire life cycle of the fuel, feedstock used for production of SAFs should not compete with food crops for water and land, and must have a limited need for fresh water and no need for deforestation. Not every raw material used in the production of alternative aviation fuel meets all of these requirements. As SAF is a term broadly used by international aviation organizations, there is also another term for sustainable aviation fuels: CORSIA-eligible fuel (CEF). This kind of fuel should meet more criteria than SAFs, and the requirements are not only related to the environment. These requirements will be described in the next sections.

One of the key element in the global energy transition and achievement of the 2050 climate targets are e-fuels. E-fuels, also known as Power-to-Liquid (PtL) or synthetic fuels, are different solutions for GHG emissions reduction, which also can be used as SAFs [6]. E-fuels are produced from green hydrogen generated via water electrolysis utilizing renewable power sources and carbon dioxide obtained from industrial sources or direct air capture. E-fuels can be gaseous or liquid and, by converting power energy into chemical energy, synthetic fuels are easy to transport, can use existing infrastructure and allow the storage of renewable energy over extended periods [6,7]. Synthetic fuels are produced in 4 main stages: electrolysis of water, capturing CO₂, synthesizing the product and upgrading the product. E-fuels synthesized by hydrogen and carbon dioxide can be processed by the Fischer–Tropsch pathway, which is described in the next chapters, or methanol synthesis [6,8].

Another solution to reduce GHG emissions and reach climate targets is the usage of hydrogen in aviation. In the transportation sector, zero-carbon ammonia and hydrogen are identified as highly favourable alternatives for conventional fuels, which possess the capability to serve as fuel devoid of producing carbon-based residues, such as carbon dioxide, in the process of combustion [9,10]. Biohydrogen can be generated from various biomass resources through thermal and biochemical processes. Hydrogen can be used in fuel cells, which offers the possibility of powering onboard electrical systems or an electric propulsion systems, or hydrogen can be a liquid fuel for turbo engines. When considering its application in engines, significant modifications to aircraft are necessary due to low volumetric energy density. This includes the installation of cryogenic tanks for storing liquid hydrogen, which will result in additional weight and in higher energy consumption [4].

Within this review, a summary of SAF-certified production pathways with certification process is presented, including physicochemical parameters included in ASTM D7566 [11]. Raw materials used for SAF production also have been described, with division into the generation of biofuels. As currently there is also CORSIA-eligible fuel, requirements of this fuel are also presented, with Life Cycle Assessment analysis for selected production pathways. Based on available research, the review of gaseous exhaust emissions and particulate matter emissions has been outlined.

2. Certification Process

A new fuel that is going to be used in aviation must meet the specific requirements for conventional fuel and be approved by the ASTM D4054 process—Standard Practice for Evaluation of New Aviation Turbine Fuels and Fuel Additives [12,13]. The standard certification process usually takes from 3 to 5 years and costs more than 5 million dollars, but there are processes, which take a longer time and cost over 10 million dollars [12]. The certification process steps are shown in Figure 1. The first phase is Tier 1—Fuel specification properties and Tier 2—Fit-for-purpose properties. Tiers 1 and 2 are laboratory tests, taking about 6 months and costing approximately 50,000 dollars. The quantity of neat fuel needed for this Tier 1 is 40 L and for Tier 2 is 40–400 L [12]. After Tier 1 and 2, the ASTM Research Report is made and the end of the first phase is the Original Equipment Manufacturer (OEM) Review—this phase lasts from 6 months to 1 year and costs about 350,000 dollars, but the costs are covered by OEMs [12]. The second phase is divided in Tier 3—Component and rig testing and Tier 4—Aircraft and engine testing. Tiers 3 and 4 last from 2 to 3 years and cost about 4 million dollars. The quantity of neat fuel needed in Tier 3 ranges from 950 to 40,000 L, and for Tier 4, it is up to 850,000 L [12]. After Tiers 3 and 4, there is one more ASTM Research Report and at the end of phase 2 there is an OEM Review and Approval—this phase lasts from 6 months to 1 year and costs about 1 million of dollars, costs are covered by OEM [12]. The third phase starts from FAA (Federal Aviation Administration) Review, followed by an ASTM Balloting Process, which covers ASTM Review and Ballot; at this step, the pathway can be accepted or rejected. After the Balloting Process, the production pathway is added to the ASTM Specification and the certification process is finished [12,14].

After ASTM D4054 tests, the approval process for ASTM D7566 begins, Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons [12]. At this stage, the properties and characteristics of new alternative fuels are compared with conventional aviation fuels, and if the fuel meets all the requirements, a new production pathway is added to ASTM D7566 [12]. Fuels meeting the ASTM D7566 specifications are appropriate for use in aircraft propulsion systems requiring compliance with the D1655 standard for aviation fuel. Alternative aviation, which is certified is a ‘drop in’ fuel, meaning it does not require any changes in engine construction or fuel infrastructure and distribution systems. A drop in fuel can be used in the engine in the same way as conventional aviation fuel. Currently there are eight production pathways that are certified for ASTM D7566 and are described in the following chapters. ASTM D7566 defines the maximum blending limit for each production pathway, and the maximum blending limit that is currently possible to achieve is 50% in volume of alternative fuel and 50% of conventional aviation fuel: Jet A-1 or Jet A. The general procedure for obtaining D7566 certification consists of the following stages presented in Figure 2:

Since January 2020, the “Fast Track to D4054” has been approved, which allows the certified production process to proceed much faster, but the maximum blending limit with conventional aviation fuel is only 10%. In the Fast Track pathway, there are also requirements for chemical composition of alternative fuels, for example the concentration of cycloparaffin must be less than 30 wt%, and the aromatics composition must be less than 20 wt% [12]. Fast Track includes the following steps: Compositional and Processing Screening, OEM Review, Lite Test Program, Fast Track Research Report, OEM review, FAA Review, ASTM Balloting Process and ASTM Specification [12].

3. Certified Production Pathways

3.1. Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK)

FT-SPK was approved in 2009 with a blending volume limit up to 50%. The range of raw materials for the Fischer–Tropsch Synthetic Paraffinic Kerosene process is wide, but mostly this is biomass and residues, like wood waste, agricultural residues, municipal solid waste and grass crops. A diverse range of biogenic residues serves as suitable raw material for the gasification process, which in turn supplies syngas to the FT route [4]. The fuel is a mixture of iso-alkanes and n-alkanes derived from syngas, which is a product of reforming natural gas or gasifying biomass [12]. The syngas is a mixture of carbon monoxide and hydrogen. According to Baxter G. et al. [15], the FT-SPK fuel is non-toxic with zero emissions of nitrogen oxides, carbon dioxide, hydrocarbons and reduces particulate matter emissions [15].

3.2. Hydroprocessed Ester and Fatty Acids (HEFA-SPK)

HEFA-SPK was approved in 2011 with a blending volume limit up to 50%. For this process, oily biomass, like algae, jatropha and camelina are used. The range of the raw material used in this production pathway is still being developed by producers, and currently a used cooking oil is playing a significant role in HEFA production. HEFA refines vegetable oils, waste oils or fats into SAF in a process of hydrogenation. In the first step of the process, oxygen is removed by hydrodeoxygenation. Then, simple paraffin molecules are broken down and isomerized to the length of the aviation fuel chain. The process is similar to that used to produce hydrotreated renewable diesel, but with sharper cracking of the longer chain carbon molecules [4]. Also the HEFA-SPK is a mixture of iso-alkanes and n-alkanes [12].

3.3. Hydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP)

HFS-SIP was approved in 2014 with a blending volume limit of 10%. It uses bacterial conversion of sugars into hydrocarbons. HFS-SIP is a biological platform thanks to which microorganisms convert C6 sugars into farnesene, which, after treatment with hydrogen, can be used as an SAF. Fernesene is a 15-carbon hydrotreated sesquiterpene, and the HFS-SIP is a single molecule. Currently this kind of fermentation is used in higher-value applications, like personal care [12].

3.4. Fischer-Tropsch Synthetic Paraffinic Kerosene with Aromatics (FT-SPK/A)

FT-SPK/A was approved in 2015 with a maximum blending volume limit up 50%. The raw material used in this process is renewable biomass, like municipal solid waste, wood waste and agricultural waste. FT-SPK/A contain less than 20 wt% of aromatics [16]. Fuel produced by this pathway has similar standard specifications as FT-SPK and HEFA-SPK, except for aromatics content and a minor difference requirement in density [16].

3.5. Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK)

ATJ-SPK was approved in 2016 with a blending volume limit up to 50%. The raw materials are mostly agricultural waste, e.g., corn shoots, grass and straw, and cellulosic biomass. The feedstock in these pathway is mostly raw material that can be broken down into sugar molecules [17]. Biomass is converted in alcohol, which has from 2 to 5 carbon atoms in the chains, and the alcohol is then subjected to a dehydration process, oligomerisation process in which the molecules are converted into longer-chain hydrocarbons, hydrogenation process and fractionation process. the final product is paraffins SPK.

3.6. Catalytic-Hydrothermolysis Synthesized Kerosene (CH-SK or CHJ)

CHJ was approved in 2020 with blending volume limit up to 50%. It uses vegetable or animal fats, greases and oils as a raw material. CHJ converts fatty acid esters and free fatty acids into SAF by catalytic hydrothermolysis followed by any combination of hydrotreating, hydrocracking or hydroisomerization and fractionation. The fuel contains n-alkanes, iso-alkanes, cyclo-alkanes and aromatics [12].

3.7. Hydroprocessed Hydrocarbons, Esters and Fatty Acids Synthetic Paraffinic Kerosene (HHC-SPK or HC-HEFA-SPK)

HHC-SPK was also approved in 2020 with a blending volume limit up to 10%. Feedstock used in this process are hydrocarbons of biological origin, free fatty acids, Botryococcus braunia algae and fatty acid esters [12]. Bio-based hydrocarbons, free fatty acids and fatty acid esters are refined in an HC-HEFA process similar to the HEFA process: they are treated with hydrogen to saturate the hydrocarbon molecules and remove substantially all the oxygen. The fuel contains a high concentration of iso-alkanes [12].

3.8. Alcohol to Jet Synthetic Kerosene with Aromatics (ATJ-SKA)

ATJ-SKA was approved in 2023 with a blending limit up to 50%. ATJ-SKA is produced by Swedish Biofuels AB. The composition of the fuel primarily consists of iso and n-alkanes, exhibiting minimal levels of cycloparaffinic hydrocarbons and polyaromatics [18].

3.9. Co-Processing

In addition to the production pathways certified in ASTM D7566, there is also a co-processing pathway, which is described in ASTM D1655-20b [19]. This standard allows for co-processing of up to 5% of free fatty acids, fatty acid esters, mono-glycerides, di-glycerides and tri-glycerides or up to 5% of FT hydrocarbons [12]. Raw materials are subjected to hydrotreating or hydrocracking and fractionation [12].

4. Physicochemical Requirements of Alternative Aviation Fuels

Aviation fuels are required to adhere to various standards concerning their physicochemical characteristics for utilization in aircraft engines. Conventional aviation fuels are derived from distillation of crude oils and are characterized by a complex composition. An average jet fuel is composed of numerous hydrocarbon compounds, with aromatics making up approximately up to 20 wt% of the total. The distillation profile of typical jet fuels provides insights into the existence of high molecular weight components [20]. Standard jet fuel contains from 8 to 16 carbon atoms in molecules and Jet A contains from 55 to 60% of n-alkanes and iso-alkanes [9]. The composition of alternative fuels varies depending on the production process; for example FT-SPK, HEFA-SPK contains iso and n-alkanes, CHJ has also cyclo-alkanes, and ATJ-SPK based on isobuthanol contains only iso-alkanes of 8, 12 or 16 carbons [12].

The physicochemical parameters of aviation fuels play a crucial role in ensuring safe and efficient flight operation. Physicochemical properties determine the combustion parameters and also have a significant impact on gaseous exhaust emissions and particulate matter emissions. The aromatic content of fuel plays a crucial role in PM emissions, as alternative aviation fuels do not contain aromatics or the value is very low, the PM emission is also reduced compared to conventional aviation fuel. According to Jeyashekar et al. [21], a combination of aromatic fuel content, fuel lubricity and thermal effects can have an impact on seal swell. Fuel parameters have an impact on incomplete combustion; for example, density and surface tension have an influence on inefficient atomization and vaporization [22]. A crucial role in the formation of particulate matters is also played by fuel density and viscosity, surface tension, smoke point, cetane number and H to C ratio [18].

Some of the SAFs production pathways use catalysts in the process. Catalysts increase the reaction rate without undergoing permanent alterations, due to acceleration of the rate of chemical reaction, lowering the required activation energy. This objective is accomplished through a variety of mechanisms, including the weakening of current bonds, generating intermediate species, offering alternative pathways for reactions, and stabilizing transition states [23]. SAFs produced from vegetable oils use alumina-supported metal sulfides, fuels from lignocellulose and wastes in FT process use Fe- and CO-based supported catalysts. For ATJ fuels, the catalysts are heterogenous and homogenous acids [24].

The properties of selected SAFs are presented in Table 1. Depending on the research, the values of selected SAF are slightly different, and blends of specific SAF with conventional aviation fuel have different parameters depending on the content of SAF in the blend. According to the requirements included in ASTM D7566 and tested in Tier 1 in the certification process, not every parameter of SAF presented in Table 1 meets the standard’s requirements. Blends of specific fuel with Jet A or Jet A-1 will change the parameters and blends with maximum limits of SAF in the mixture will meet the ASTM requirements. For SIP fuel, the viscosity at −20 °C is higher than the value in ASTM D7566, but blending the limit of SIP is only 10%. The density of HEFA, ATJ-SPK and FT-SPK has a lower range of density than required in D7566. Lubricity HEFA is also higher than 0.85. The rest of the parameters are suitable with values in the ASTM standards.

Combustion characteristics of SAFs and conventional aviation fuel may vary depending on the chemical composition of the fuel. According to [25], fuels with higher values of n-paraffin in molecules will increase reactivity and thus will have higher derived cetane numbers. Alternative aviation fuels contain significantly lower aromatics than fossil fuels, so the derived cetane number of alternative fuels will be higher [25]. The composition of the fuel also affects the ignition behaviour, and research shows that conventional aviation fuels have longer ignition delays than alternative fuels [25].

Table 1. Physicochemical properties of selected SAFs according to ASTM D7566 standard [26,27,28,29].

Property	Unit	Limits ASTM D1655	HEFA [28,29]	CHJ [29]	ATJ-SPK [26,28,29]	ATJ-SKA [29]	FT-SPK [28]	FT-SKA [28]	SIP [28,29]
Heat of combustion	MJ/kg	min 42.8	44.154	43.202	43.89	43.396	-	-	43.5
Smoke point	mm	min 18	-	22.5	27.0	23.0	-	-	-
Viscosity in −20 °C	mm2/s	max 8.0	4.801	3.977	4.779	3.421	-	-	14.13
Viscosity in −40 °C	mm2/s	max 12	-	-	9.037	-	-	-	-
Freezing point	°C	max −40	−54.4	−41.3	−40	<−80	−40	−40	−60
Density at 15 °C	kg/m3	775–840	730–770	805.2	730–770	785.9	730–770	755–800	765–780
Flash point	°C	Min 38	42.0	42.5	47	48.5	38	38	107.5
Distillation:
10% recovered	°C	Max 205	162.9	171.4	-	-	205	205	-
50% recovered	°C	report	510.3	200.1	-	-	-	-	-
90% recovered	°C	report	270.8	244.8	-	-	-	-	-
Final boiling point	°C	max 300	148.9	152.1	-	-	300	300	247
Residue	vol%	max 1.5	1.2	1.5	-	1.1	-	-	-
Loss	vol%	max 1.5	1.1	0.9	-	1.1	-	-	-
T50-T10	°C	min 15	47.4	28.7	-	-	-	-	-
T90-T10	°C	min 40	107.9	73.4	-	-	22	22	-
Total sulfur	m%	0.3	<0.001	<0.001	<0.001	<0.001	-	-	<0.001
Lubricity	mm	max 0.85	0.906	0.570	-	0.606	-	-	0.562
Naphthalenes	vol%	max 3.0	-	0.35	-	0.08	-	-	-
Aromatics	vol%	8–25	-	0.014	0.015	15.8	0.015	0.015	0.5

Table 1. Physicochemical properties of selected SAFs according to ASTM D7566 standard [26,27,28,29].

Property	Unit	Limits ASTM D1655	HEFA [28,29]	CHJ [29]	ATJ-SPK [26,28,29]	ATJ-SKA [29]	FT-SPK [28]	FT-SKA [28]	SIP [28,29]
Heat of combustion	MJ/kg	min 42.8	44.154	43.202	43.89	43.396	-	-	43.5
Smoke point	mm	min 18	-	22.5	27.0	23.0	-	-	-
Viscosity in −20 °C	mm2/s	max 8.0	4.801	3.977	4.779	3.421	-	-	14.13
Viscosity in −40 °C	mm2/s	max 12	-	-	9.037	-	-	-	-
Freezing point	°C	max −40	−54.4	−41.3	−40	<−80	−40	−40	−60
Density at 15 °C	kg/m3	775–840	730–770	805.2	730–770	785.9	730–770	755–800	765–780
Flash point	°C	Min 38	42.0	42.5	47	48.5	38	38	107.5
Distillation:
10% recovered	°C	Max 205	162.9	171.4	-	-	205	205	-
50% recovered	°C	report	510.3	200.1	-	-	-	-	-
90% recovered	°C	report	270.8	244.8	-	-	-	-	-
Final boiling point	°C	max 300	148.9	152.1	-	-	300	300	247
Residue	vol%	max 1.5	1.2	1.5	-	1.1	-	-	-
Loss	vol%	max 1.5	1.1	0.9	-	1.1	-	-	-
T50-T10	°C	min 15	47.4	28.7	-	-	-	-	-
T90-T10	°C	min 40	107.9	73.4	-	-	22	22	-
Total sulfur	m%	0.3	<0.001	<0.001	<0.001	<0.001	-	-	<0.001
Lubricity	mm	max 0.85	0.906	0.570	-	0.606	-	-	0.562
Naphthalenes	vol%	max 3.0	-	0.35	-	0.08	-	-	-
Aromatics	vol%	8–25	-	0.014	0.015	15.8	0.015	0.015	0.5

5. Feedstock Used in Production of Alternative Aviation Fuels

Biofuels can be categorized into three generations based on their feedstock: the first generation is primarily associated with edible food crops, the second generation is characterized by non-edible biomass, and the third generation encompasses algae. Some research describes a fourth generation, which contains non-biological resources and genetically modified organisms [30].

5.1. First Generation of Biofuels

First-generation feedstock includes raw materials that are useful for animals and people, for example oil palm, sugarcane, corn, sugar beets and wheat. Oil or fats are derived from these crops and can be readily transformed into aviation fuel, as well as ethanol, which is another product obtained from the first generation of biofuels through the fermentation of sugars. This generation exhibits considerable water and fertilizer requirements and competes for arable land with food crops that are grown to meet the dietary requirements of humans and animals [30,31,32]. Thus, it cannot be perceived as ‘sustainable’. Palm oil as a raw material for SAF does not meet the requirement of ‘no need for deforestation’, so this is also an example of unsustainable first generation of biofuels. Additionally, approximately 30–40% of newly established palm oil plantations in Malaysia and Indonesia are on peatlands, which plays a significant role in methane (CH₄) emissions. The converting process of palm oil also results in the formation of liquid organic waste material, which generates large amounts of methane during the anaerobic digestion [33].

5.2. Second Generation of Biofuels

The feedstock of the second biofuel generation is rich in oil or sugar and consists of two main groups: waste biomass and energy crops [30]. The second generation of biofuel has no food–energy conflict with food crops, but some raw materials in this group require arable land or forest. Also, the feedstock of second-generation biofuel needs advanced processing technologies because of the high content of hemicelluloses and lignin. Potable water is also needed during the cultivation stage [34]. Distribution of second-generation raw materials is shown in Figure 3, taking into account specific raw materials belonging to a given subgroup:

Oil-seed plants are the primary and most renowned group of energy crops. Jatropha (Jatropha curcas) is the most frequently discussed oil plant utilised in aviation biofuel production, in which every seed contains a significant oil proportion of between 27 and 40% of its weight in oil and is toxic to both humans and animals [36,37]. Jatropha does not pose a threat to arable land typically designated for food crops, as it can be cultivated in challenging environments such as non-arable and arid regions [35,37]. When adequately supplied with moisture, the jatropha plant can sustain crop production for four decades once established, and needs from 4 to 5 years to be highly productive [36,38]. Subsequent to an oil extraction procedure, jatropha undergoes conversion into bio-oil, which is further processed with hydrogen to generate HRJ (Hydroprocessed Renewable Jet) [39]. Despite the fact that the oil extraction residues contain substances harmful to humans, residue from the oil extraction process can be utilised as organic fertilizers because they are nutrient-rich in nitrogen, potassium and phosphorus [35]. Jatropha cultivation occurs in the southern hemisphere, specifically in Africa, America and Southeast Asia [40].

Camelina (Camelina sativa) is another frequently mentioned oil plant, which also has significant oil proportion in every seed of between 38 and 43% [37]. Like jatropha, it can be cultivated in challenging regions, like infertile soil or marginal lands, but also cold regions, as it is frost tolerant, and in tropical climates [41]. Camelina cultivation is conducted in a crop rotation system with wheat and other cereals, necessitating minimal inputs. The by-products from the oil extraction process can also serve as a nutritional supplement for animals in modest quantities. Refs. [37,42] is a short-season crop, from 85 to 100 days with low demand for nutrition [35]. Camelina is a weed that is widespread, particularly in Europe, North America and Central Asia [41,43].

The last plant in the group of oil-seed energy crops is castor bean (Ricinus communis L.), which is one of the most promising sources of renewable feedstock for biofuels. Castor beans are also nonedible and can grow in difficult marginal areas, which are not suitable for food plants [44]. From each seed, from 50 to 50% of the seed weight can be obtained [30]. To produce biofuel from castor beans, it must undergo processes such as transesterification, catalytic cracking (pyrolysis) or hydroprocessing [30]. The plant can grow in a variety of ecosystems, from rugged tropical deserts to wet forests, because is resistant to salinity and drought [44]. Castor beans are cultivated at latitudes from 40° South to 40° North, but can also be found in higher latitudes in the northern hemisphere [44]. The optimal altitude for growth is from 300 to 1800 m a.s.l., but they can also grow from sea level to 2000 m a.s.l. [44]. The castor bean is another plant that can be cultivated in marginal lands, which are too difficult to cultivate food crops, so the plant does not compete with food crops for land.

Another group of energy crops are grasses: Miscanthus, elephant grass (napier grass) and switch grass. Miscanthus is a plant from Asia and Africa, which also has been brought to Europe. Nowadays, it is also commonly naturalized in North and South America [45]. Some kinds of miscanthus have high productivity and, according to studies [46], have greater bioenergy potential than switch grass. Miscanthus grows in tropical to temperate climate and it is cold-resistant. It has a moderate demand for nutrients and a moderate to high demand for water [30,46]. Napier grass is cultivated in tropical climates and has high yields and a high demand for nutrients and water, but is also drought-resistant. Napier grass can be a good feedstock in the production of solid and liquid biofuels [30]. Switch grass is native to North America, grows in temperate climate, has a low demand for nutrients and a moderate demand for water [30].

The last group of energy crops are trees: poplar, willow and eucalyptus. Poplars and willows grow in temperate climates and have low demand (poplar) and low to moderate demand (willow) for nutrients. Poplar has a low to moderate demand for water, and willow has a moderate to high demand for water. Eucalyptus is a fast-growing tree from Australia that grows in zones from dry tropical to subtropical and has a low demand for nutrients but moderate to high demand for water [30].

Waste biomass is a second group next to energy crops and includes agricultural and forestry residues, food and municipal wastes. This group has no land requirement, no economic value, and a very low and low water footprint compared to energy crops [30,47,48]. Emissions counted in the life cycle assessment for wastes and residues have no land use change factor and the core life cycle assessment for aviation fuel starts when wastes and residues start to be aviation fuel feedstock.

In the waste biomass group, there are agricultural and forestry residues, which are typically lignocellulosic by-products, such as corn stover, wheat and rice straw, sugarcane bagasse, rice hull, palm kernel, wood pulp, wood chips, sawdust, cutter shavings and unprocessed parts of felled trees [30,49].

The other group of waste biomass is food and municipal wastes, which are mentioned in Figure 4. One of the most used feedstocks from this group is used cooking oil (UCO), which is a raw material mostly in HEFA [30]. UCO is collected from households and restaurants and many flights have been made on fuel based on used cooking oil [50].

Another raw material in this group is animal fats, such as tallow, yellow grease. According to research, biofuels produced from this raw material potentially have better combustion quality than biofuels produced from oil-seed crops [52].

One of the promising feedstocks is also Municipal Solid Waste (MSW). Fuel based on municipal solid waste can harness the high energy content of MSW and address issues related to landfills. MSW can be a feedstock in the Fischer–Tropsch pathway or can be converted to alcohol [30].

5.3. Third Generation of Biofuels

The third-generation of feedstock is algae. Algae have no food value and no food–energy conflicts with food crops, are totally renewable and are high in energy. Criteria are comparatively minimal since they demand sunlight, basic nutrients, carbon dioxide and have the ability to thrive in contaminated water [30]. Because of their rapid growth, algae has good use of land for cultivation and can be cultivated in open spaces and photobioreactors, in seawater, unproductive drylands, marginal farmlands or wastewater [34]. Additionally, the conversion of algae is easy and algae do not require fertilizers [53]. Algae clean the air and water in which they grow by absorbing significant amounts of carbon dioxide and other contaminants [54]. Approximately 1.83 kg of carbon dioxide can be reduced by one kilogram of biomass from algae during cultivation [55]. Disadvantages of the third generation are that there is insufficient biomass production for commercialization, large carbon and nitrogen sources are required, solar energy is available only in the daytime and the harvesting process is expensive [34,56].

5.4. Other Feedstock Used in SAF Production

Other raw materials used in the production of alternative aviation fuels are sometimes considered fourth-generation biofuels (FGB), depending on the source [30,34]. In the fourth generation can be included non-biological raw materials, like carbon dioxide, renewable electricity and water, but also genetically modified organisms [30]. Other feedstock included are e-fuels or synthetic fuels described in the Introduction. One of the raw materials in FGB is genetically modified microalgae, which are modified to improve photosynthetic efficiency, reduce photoinhibition and increase light penetration [57]. Microalgae have a high ability to adapt to extreme environmental conditions, such as salinity, ultraviolet radiation, photo-oxidation, drought and others [34]. The fourth generation of biofuels has no food–energy conflicts with food crops; genetically modified microalgae are easy to convert, have a low demand for water, can be cultivated in harsh environmental conditions and can absorb large amounts of CO₂. In contrast, the cultivation of genetically modified microalgae requires huge amount of carbon and nitrogen sources, harvesting is expensive and complicated and there is insufficient biomass production for commercialization of fuels from microalgae [34,56].

A different raw material is plastic waste, which has also been widely tested in recent years as a feedstock for alternative aviation fuel. Plastic production has increased rapidly in recent years and it is one of the most problematic wastes for the environment, as the recycling rate of plastic is low, and approximately 55% of global plastic residues is disposed of in landfills or discarded [58]. Plastics, being a non-renewable resource, have the potential to be transformed into waste-derived aviation fuel via methods like pyrolysis and depolymerization. The utilization of plastic waste in the production of waste-derived jet fuel brings about ecological advantages by eradicating the necessity for landfill disposal [23].

6. CORSIA Eligible Fuels

Alternative aviation fuels also need to meet several requirements to be considered ‘sustainable’ fuels. To certify aviation fuel as Sustainable Aviation Fuel regarding the requirements of the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), the ISCC CORSIA (International Sustainability & Carbon Certification, ISCC) has been developed [59]. As mentioned in previous chapters, aviation non-fossil fuels have many terms, including sustainable fuels, alternative fuels, aviation biofuels and others. To the precise terms used in aviation alternative fuels, in Annex 16 ICAO Volume IV, three new definitions have been provided [5]:

• “CORSIA eligible fuel (CEF)—a CORSIA sustainable aviation fuel or a CORSIA lower carbon aviation fuel, which an operator may use to reduce their offsetting requirements,
• CORSIA lower carbon aviation fuel—a fossil-based aviation fuel that meets the CORSIA Sustainability Criteria under this Volume (Annex 16 ICAO, Volume IV),
• CORSIA sustainable aviation fuel—a renewable or waste-derived aviation fuel that meets the CORSIA Sustainability Criteria under this Volume” [8].

The ISCC CORSIA Document 202 “Sustainability Requirements” describes the main sustainability criteria, which can be divided into Major Musts and Minor Musts [59]. Fuel has to meet all Major Musts and 60% of Minor Musts, but if fuel does not meet the first principle, no changes can be made. If fuel does not meet one of the principles 2–6 and 60% of Minor Musts, changes have to be made in 40 days [59]. Six ISCC CORSIA requirements are described below:

• Protection of Land with High Biodiversity Value or High Carbon Stock.
  Feedstock used in the production of CORSIA-eligible fuel shall not be acquired from land that was previously primary forest, peat lands or wetlands, and after 1 January 2008 was converted into area for biomass cultivation [59]. The first principle protects carbon reach areas (as mentioned above) as well as lands that are ecologically or culturally important or that allow the protection of threatened or vulnerable species [59].
• Environmentally, Responsible Production to Protect Soil, Water and Air.
  The use of the soil should be coherent with national and local laws relating to soil, but conservation of natural resources and biodiversity is also required, as well as avoidance of deterioration or damage of habitats. A few other requirements related to the principle second are: maintenance of natural vegetation areas and natural water rivers, streams and others; all highly invasive species or genetically modified plants have to be allowed to grow in specific regions or countries; and maintenance and improvement of soil fertility, prevent erosion and salinization [59].
• Safe Working Conditions.
  A company involved in the cultivation of raw materials and the production of alternative aviation fuel should be familiar with the local regulations regarding working conditions. Employees responsible for individual sectors related to the production of fuel should be properly trained and competent, be familiar with plant protection and hazardous substances used in cultivation, waste management, and also be qualified for hazardous or complex work, if necessary [59].
• Compliance with Human, Labour and Land Rights.
  This principle refers to avoiding negative environmental, economic and social impact, but also respect for a living wage and social environment. The production of biomass must not harm local food cultivation and if the prices of local food products increase due to the cultivation of biomass in the area, the company should establish mitigation measures [59]. This principle describes in detail the guidelines on basic human and worker rights in the biomass farm and the terms of employment, prohibiting discrimination and forced labour, providing legal employment.
• Compliance with Law and International Treaties.
  This principle requires, among others, legal ownership of used land, respect for existing land rights, and that the producer should be aware of his responsibilities according to the law [59].
• Good Management Practices and Continuous Improvement.
  The last principle points to economic stability, a documentation system and full compliance by subcontractors [59].

7. Emissions from Sustainable Aviation Fuels

7.1. Emission of Gaseous Exhaust Compounds

As CO₂ and other GHG emissions are widely considered in Life Cycle Assessment (LCA), other gaseous emissions are mostly analyzed in terms of fuel combustion in jet engines. Emissions of carbon monoxide (CO), nitrogen oxides (NO_x), hydrocarbons (HC) and CO₂ vary by different flight levels, atmospheric conditions and engines parameters [16,60]. To compare emissions from different engines, it is useful to analyze emission indexes calculated on used fuel or parameters of the engine. A summary of research in emissions of gaseous exhaust compounds and particulate matter from alternative aviation fuels has been presented in

Table 2. A summary of research in emission of gaseous exhaust compounds and PM for alternative aviation fuels.

Research	Tested Engine	Tested Fuel	Emission Change in Relation to Conventional Aviation Fuel
[61]	T63-A-700	100% FT-SPK	CO2 and NOx similar
			UHC reduction by 20–30%
			CO reduction by 10–25%
			PM reduction
[62]	GTM 140	50%, 100% ATJ	CO rise
			CO2 slight reduction
			HC reduction for thrust over 50% and rise for thrust below 50%
		5%, 20%, 30% HEFA	CO and NOx rise
			HC reduction
	DGEN 380	5%, 20%, 30% ATJ	CO rise
			CO2 slight reduction
		5%, 20%, 30% HEFA	CO and CO2 rise
[63]	GTM 140	50% HEFA	CO and CO2 reduction
			NOx rise
[64]	CFM 56-3 (CFD simulation)	100% Jatropha biofuel	CO2 reduction by 20%
			UHC rise
		100% Algae biofuel	NOx reduction
			CO2 and NOx reduction
		100% Sunflower biofuel	UHC reduction for thrust below 80%
			CO2, UHC, NOx reduction
[65]	CFM56-5C (calculations)	Algae biofuel	CO2 reduction by 6%
			NOx reduction by 44%
			HC reduction by 32%
			CO reduction by 49%
		Jatropha biofuel	CO2 reduction by 9%
			NOx reduction by 6%
			HC similar
			CO increase by 132%
[66]	CFM56-7B	100% FT-SPK	PM number reduction by 52%
			PM mass reduction by 62%
		50% FT-SPK	PM number reduction by 34%
			PM mass reduction by 39%
[67]	CFM56-2-C1	50% HEFA	PM number and mass reduction by 50–70%
[68]	GTM 120	30% ATJ	PM number reduced by 51%
			PM mass reduced by 53%
[26]	CFM56-5C4	CHJ	PM increase
		100% ATJ	PM reduced by 70%
[69]	CFM56-7	100% FT-SPK	CO reduction by 20%
			NOx reduction by 10%
			PM number reduction by 70%
[70]	CFM56-7B	32% HEFA	PM GMD and GSD reduction
			nvPM number reduction by 60% (idle)
			nvPM mass reduction by 70%
			nvPM number reduction by 10% (take-off)
[71]	GE CF-700-2D-2	100% CH-SKA	PM reduction by 7–25%
		50% HEFA	PM reduction by 40–60%
		100% FT-SPK	PM reduction by 70–95%

.

Research by Corporan et al. [61] showed that emissions of CO₂ and NO_x from FT-SPK were similar to conventional aviation fuel, but emissions of CO and unburned hydrocarbons (UHC) were from 10 to 25% lower than for conventional aviation fuel [61].

According to research of Przysowa et al. [62] made on microturbine engine GTM-140 of ATJ and Jet A-1 blends, the findings demonstrated, that the addition of ATJ led to a rise in CO emissions in the entire engine operation range, and slight decrease in CO₂ emissions compared with conventional aviation fuel. At high engine loads (thrust over 50%), the ATJ addition decreased the HC emission in relation to neat Jet A-1, but for engine loads lower than 50% of thrust, the HC emission was higher for ATJ blends than for neat Jet A-1. Przysowa et al. also tested HEFA fuel on the same microturbine engine, and the results showed, that addition of HEFA in the blend with Jet A-1 increased the CO and NO_x emissions in entire engine operation points. Emissions of hydrocarbons were much lower for 20 and 30%vol of HEFA in blend than for neat Jet A-1 in every engine operating points [62].

For tests made on DGEN 380 turbofan carried out by Przysowa et al. [62], for ATJ blends with Jet A-1, emissions of CO were higher for blends with higher content of ATJ, and addition of ATJ to fuel blend slightly decreased CO₂ emissions compared to conventional aviation fuel. At the same engine there were also tests of HEFA, and for this fuel, the addition of HEFA resulted in an increase in CO and CO₂ emissions [62].

Different studies made by Gawron et al. [63] on microturbine engine GTM 140 fueled with HEFA based on camelina and HEFA based on used cooking oil, showed, that as well as fuel consumption and CO emissions were lower for HEFA blends than for Jet A-1. Emissions of CO were the lowest for HEFA based on camelina for lower rotational speeds, and for higher rotational speeds CO emissions were the lowest for HEFA based on used cooking oil. An inverse relationship was observed for CO₂ emissions, where the lowest values were for HEFA based on used cooking oil in lower rotational speeds, and for higher rotational speeds the lowest CO₂ emissions were for HEFA based on camelina. For NO_x emissions, an increase for both biofuels has been observed, compared to Jet A-1, with the highest values of HEFA based on camelina [63].

According to Oliveira and Brojo [64], who made computational fluid dynamics simulation on CFM56-3 engine of combustion of Jet A-1 and 100% of biofuels, made from jatropha, algae and sunflower. Jatropha biofuels decreased CO₂ emissions by 20% compared to neat Jet A-1, but biofuels from algae and sunflower also significantly reduced CO₂ emissions compared to Jet A-1. The emission index of unburned hydrocarbons (UHC) was the lowest for biofuel made from sunflower in entire engine loads, and also very similar to sunflower was UHC emission index of algae but only from 7 to 80% of thrust, and for 100% thrust the emission index for algae were higher than for Jet A-1. For jatropha biofuel the UHC emission index was higher than for Jet A-1 from 30% to 100% of thrust. NO_x emission index was also significantly lower for biofuels than for Jet A-1, and was the lowest for algae biofuel [64].

Analysis made by Pawlak and Kuźniar [65] for cruise phase based on CFM56-5C engine, showed, that use of algae biofuel can reduce CO₂ emissions by almost 6% and jatropha biofuel—by almost 9%, compared to Jet A-1. Biofuel based on algae can reduce NO_x emissions by 44% and based on jatropha—by 16%, compared to J A-1. HC emissions were reduced by 32% with the use of algae fuel, and there were negligible changes for jatropha biofuel. For CO emissions, algae biofuel can decrease CO by 49% and jatropha biofuel can increase CO emissions by 132% in relation to conventional aviation fuel [65].

Timko et al. [69] carried out test on CFM56-7 engine fueled 50% and 100% of FT fuel. The results showed that neat FT fuel reduced CO emissions by 20% and NO_x emissions by 10%, and 50% of FT fuel reduced NO_x emissions by 5% compared to Jet A [69].

7.2. Emissions of Particulate Matter

Particles which are emitted from engines fueled with SAFs are generally 35% smaller than from conventional aviation fuels. This is a result of decreased concentration of soot nuclei that were available for the process of surface growth and agglomeration [16]. The reduction in PM emissions is mostly related with fuel hydrogen content and aromatics [72]. According to Corporan et al. [61], alternative aviation fuels that are aromatic-free have a notably lower soot production because the rate growth of molecular soot and subsequent particle nucleation in at lower combustion temperatures is relatively low. The relative influence of aromatics on soot creation decreases when the temperature of combustion increases, because of the increase in the chemical rates of soot production from paraffinic compounds [61]. The concentration of particles can also be affected by a decrease in unburned hydrocarbons, as UHC might condense on soot nuclei or generate nuclei-sized particles within sampling lines [61]. PM emissions can be also affected by the ratio of iso- to normal paraffin, as the Isoparaffins produce higher particles concentration emissions than normal paraffins [61]. Properties which have an impact on particles formation are density, viscosity, surface tension, ratio of hydrogen to carbon, cetane number, smoke point, boiling point, aromatic content and aromatic type, ignition delays and molecular weight of fuel components [22]. According to Braun-Unkoff [73], influence on formation of particles have also pressure, adiabatic flame temperature, equivalence ratio and ignition time delay. A crucial role in PM emissions play also engine operation parameters, as PM is a results of incomplete combustion and incomplete chemical reactions. To incomplete chemical reaction are caused by inefficient atomization and vaporization, which is related with density and surface tension of the fuel [22,74].

In a study conducted by Lobo et al. [66], a notable 52% decrease in the number of particles and a 62% decrease in particle mass emissions were observed during a CFM56-7B engine test replicating the landing and take-off cycle, when utilizing neat Fischer-Tropsch Synthetic Paraffinic Kerosene, fuel in comparison to Jet A-1 fuel. The decline in PM emissions was linked to the absence of aromatic compounds in FT fuel, contrasting with the 18.5 vol% of aromatics found in Jet A-1. While a decrease in the aromatic content within a synthetic fuel mixture may lead to reduced soot formation, it was noted that challenges like inadequate swelling of engine seals emerged [75]. For 50% of FT-SPK utilised in engine, the reduction of the number of PMs was 34% and PM mass was 39%, where the 50% of FT-SPK blend with conventional fuel has 9.25 vol.% of aromatics [66].

According to Moore et al. [67], blend of HEFA based on camelina and Jet A in 50% of volume, has reduced the volatile and non-volatile particles by 50–70%. The tests were carried out in-flight, using NASA DC-8 aircraft and sampling jet [67].

Research by Jasiński et al. [68] carried out on the miniature jet engine GTM 120 and fueled by ATJ-SPK and Jet A-1 blend showed that the reduction of particles and mass increases with an increase in ATJ in the blend. The median value across the entire thrust range for the PM number emission index for 30% vol. of ATJ was 51% of the emission index of Jet A-1, and for PM mass emission index was 53% compared to neat Jet A-1. The addition of ATJ to Jet A-1 reduced the formation of the soot mode, which is related to the reduction of sulfur and aromatic content in the ATJ [68].

In a study conducted by Schripp et al. [26], where CHJ and ATJ were tested in a CFM56-5C4 engine, the emissions of PM from neat ATJ fuel was reduced by 70%, but from CHJ fuels, there was an increase in PM emissions. Neat ATJ fuel is characterized by a minimal aromatics content of less than 1%, whereas the CHJ fuel contains 20.9 vol% of aromatics and contains a lower furl hydrogen content [26].

Research carried out by Corporan et al. [61] on 50% of synthetic paraffinic kerosene (SPK) and 50% of hydroprocessed renewable jet (HRJ) in blend with conventional fuel showed that FT-SPK fuel and HRJ fuel based on tallow or camelina have significantly lower particles in the number emission index than conventional aviation fuel. The utilization of SPK fuels results in a reduction of the number of particles by 90–98% in the idle phase and by 60–80% in the cruise phase compared to conventional aviation fuel.

Durdina et al. [70] carried out tests on CFM56-7B26 engine fueled with 32% HEFA-SPK with Jet A-1, which contained 11.3 vol% of aromatics. The results showed that the geometric mean diameter (GMD) and geometric standard deviation (GSD) of particles were reduced while utilizing the HEFA blend at each point of the engine operation. Emissions of non-volatile particulate matter (nvPM) were reduced the most when idle, and the reduction was by 80% of the nvPM mass and 60% of the number of nvPms. For LTO cycle the nvPM mass was reduced by 20% and the number of nvPMs by 25% compared to conventional aviation fuel [70].

Chan et al. [71] tested alternative aviation fuels in engine GE CF-700-2D-2. The tested fuels were neat FT-SPK fuel, 50% HEFA and neat CH-SKA fuel, produced by catalytic thermolysis [71]. The distribution trends for tested SAF fuel were similar. For entire operating points, the results are as follows: 50% HEFA-SPK reduced the number of PM emissions by 40–60%, 100% FT-SPK reduced the number of PM emissions by 70–95% and 100% CH-SKA reduced the number of PM emissions by 7–25% [71].

8. Life Cycle Assessment of Alternative Aviation Fuels

The Life Cycle Assessment of sustainable aviation fuel involves estimating the greenhouse gas emissions when compared to conventional aviation fuel. The LCA of conventional aviation fuel as Jet A-1 is calculated as 89 gCO₂e/MJ [76,77] and sustainable aviation fuel or CORSIA-eligible fuel should reduce CO₂ emissions throughout the entire life cycle compared to conventional aviation fuel [76,77]. According to the ICAO Document—CORSIA Methodology For Calculating Actual Life Cycle Emissions Values [78], the total emissions in the Life Cycle Assessment are a combination of the Core LCA and ILUC (Induced Land Use Change) values. The Core LCA value covers all emissions connected to each stage of sustainable aviation fuel production, transportation, and utilization in aircraft. This includes emissions from feedstock cultivation, processing, collection, recovery, extraction, transportation to processing facilities, conversion to fuel, fuel transportation, distribution, and combustion in aircraft engines. The equation for Core LCA is shown below (1):

Core LCA value [gCO₂e/MJ] = e_{fe_c} + e_{fe_hc} + e_{fe_p} + e_{fe_t} + e_{fefu_p} + e_{fu_t} + e_{fu_c},

(1)

where:

e_{fe_c}—emissions from feedstock cultivation;

e_{fe_hc}—emissions from feedstock harvesting and collection;

e_{fe_p}—emissions from feedstock processing;

e_{fe_t}—emissions from feedstock transportation;

e_{fefu_p}—emissions from conversion process;

e_{fu_t}—emissions from fuel transportation and distribution;

e_{fu_c}—emissions from fuel combustion.

ILUC value refers to emissions resulting from potential land use change for the production of sustainable aviation fuel feedstock. In some cases, the ILUC value is equal to “0”. These feedstocks include waste, residues, and by-products; feedstocks that do not lead to the expansion of global agricultural land use and feedstocks with significantly higher yields per unit area compared to land-based crops, such as certain types of algae [78].

According to studies, the least emission stage of the SAF life cycle is during the transportation and distribution of the fuel, and the primary sources of the greenhouse gas emissions are agricultural activities, feedstock cultivation and fuel production. One of the reasons for high emission indexes during the feedstock cultivation is the use of the fertilizers and pesticides [79].

Life cycle emission factors presented in CORSIA Default Life Cycle Emissions Values for CORSIA Eligible Fuels [51] were adopted and presented in Figure 4. The ICAO document mentioned describes the LCA values for raw materials from different regions and every value has been presented on the graph. The final value of GHG emissions is a sum of Induced Land Use Change and Core LCA, also described in the ICAO Document. CORSIA differentiates alcohol-to-jet based on isobutanol (ATJ) and is based on ethanol (ETJ). As can be seen, the lowest life cycle emission factor from every analyzed pathway has miscanthus in FT-SPK pathway, but for miscanthus, the results are also very low for ATJ or ETJ. The highest life cycle emission value is for ETJ based on corn grain. Very close to Jet A-1 baseline is corn grain from ATJ based on isobutanol. For the HEFA pathway, the highest emission values are for palm oil, rapeseed oil and soybean oil. The lowest values in HEFA are for jatropha oil and used cooking oil. It is worth mentioning that agricultural and forestry residues, MSW and also used cooking oil have ILUC values equal to zero. Comparing the median values of LCA for presented pathways, the lowest median value has an FT-SPK pathway and is equal to 6.8 and the highest median value is for HFS-SIP, which has only two raw materials from different regions, and its median value is equal to 44 [80].

The range of the main results for LCA for HEFA fuel is shown in Figure 5. The results are only for core LCA; no land use change was included in the considered studies. The summarized results from different studies based on LCA for HEFA fuel allowed comparing different feedstocks used in HEFA production. In terms of core LCA GHG emissions, almost all pathways have lower core LCA values than conventional aviation fuel (89 gCO₂e/MJ), with the exception of microalgae and tallow. The differences result from various cultivation processes, used fertilizers and transportation methods of feedstock. Camelina and microalgae are some of the best-tested raw materials in terms of LCA, which are described in research. Results show that camelina from various regions and different research has a lower LCA value than Jet A-1, but in a few cases, the values are close to 89 gCO₂e/MJ. The lowest core LCA values from analysed research has jatropha, which is equal to −134 gCO₂e/MJ [81]. Total LCA emissions are largely affected by land use changes; for example, the core LCA of soybean is from 30 to 70 gCO₂/MJ, but with land use change value the total LCA can be equal to 564 gCO₂e/MJ for conversion from rainforest [82].

9. Conclusions

The primary advantage of alternative aviation fuels is their capacity to notably diminish greenhouse gas emissions within the aviation industry and effectively contribute to climate change mitigation. This factor stands as a critical aspect in the advancement of SAF technology [94]. The review presents the available alternative aviation fuel production pathways with a review of emission results and life cycle assessments of selected fuels, and a description of certification processes of SAFs. As alternative aviation fuels are one of the main solutions to reduce GHG and PM emissions from the aviation sector, they should be investigated throughout the entire life cycle. As the electrification of aviation and the use of hydrogen require comprehensive changes in aircraft and infrastructure, drop-in fuels such as SAF based on biomass or e-fuels can be used in engines without any changes. The requirements imposed by ReFuelEU Aviation as a part of the Fit for 55 package, assuming the use of a much more SAFs in the coming years, require the development and increase in the scale of SAF production processes and a significant reduction of SAF prices. Certifying the use of 100% SAF in aircraft engines or increasing blending limits is also a challenge for the aviation sector. The maturity of SAF production processes also affects the ability to meet the volume of aviation fuel production in the future [94]. Some production processes of SAFs are still in the certification process and may be certified in the coming years, so the possibilities of SAF production will expand. Such pathways are, for example, Shell IH² (Integrated hydropyrolysis and hydroconversion), ReOIL (Pyrolysis of non-recyclable plastics), and Virent SAK (Synthesized Aromatic Kerosene). Some of them are tested in Phase 1 of the certification process, and some in Phase 2. The ASTM D7566 is constantly updated with new processes [12].

In terms of sustainability, not every raw material used for alternative fuel production is sustainable and meets CORSIA requirements. The first group of biofuels which was described in the article does not meet SAF and CORSIA requirements but was widely used as a feedstock at the beginning of the development of aviation biofuels, so much research refers to non-sustainable raw materials, such as corn, sunflower oil or palm oil. The range of raw materials used as a feedstock for alternative aviation fuel production is constantly expanding and more residues and wastes are being used. Not every SAF is a CEF, as some stages in fuel production do not meet CORSIA requirements. As CORSIA is a program that requires compensation for generated CO₂ emissions during international flights (with some exceptions), through the use of CEFs as a main fuel, an aviation operator has the opportunity to realize advantages in the form of decreased CORSIA CO₂ offsetting obligations [95]. The growing demand for sustainable fuels will result in greater interest in SAF fuels and may contribute to the acceleration of the 100% SAF or CEF blend certification and may accelerate the maturity of some pathways.

Sustainable aviation fuels reduce CO₂ emissions in the entire life cycle of fuel, compared to conventional aviation fuel, and significantly reduce the number of particles and mass emissions, whiih was widely described in the article. Life cycle assessment of SAF is strongly related to used feedstocks for the production of SAF, but even one kind of raw material can have significantly different emissions depending on the regions where it is cultivated. Almost every analyzed feedstock has a lower LCA value than conventional aviation fuel. This shows the potential for GHG emission reduction due to the use of alternative aviation fuels.