Sustainable Aviation Fuels: A Comprehensive Review of Production Pathways, Environmental Impacts, Lifecycle Assessment, and Certification Frameworks
Institute of Powertrains and Aviation, Faculty of Civil and Transport Engineering, Poznan University of Technology, 60-965 Poznan, Poland
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Author to whom correspondence should be addressed.
Energies 2025, 18(14), 3705; https://doi.org/10.3390/en18143705
Submission received: 10 June 2025
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Revised: 4 July 2025
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Accepted: 11 July 2025
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Published: 14 July 2025
(This article belongs to the Section A: Sustainable Energy)
Abstract
Sustainable aviation fuels (SAFs) are currently considered a key element in the decarbonization of the aviation sector, offering a feasible solution to reduce life cycle greenhouse gas emissions without requiring fundamental changes in aircraft or infrastructure. This article provides a comprehensive overview of the current state of SAFs, including their classification, production technologies, economic aspects, and environmental performance. The analysis covers both currently certified SAF pathways, such as HEFA and FT-SPK, and emerging technologies like alcohol-to-jet and power-to-liquid, assessing their technological maturity, feedstock availability, and scalability. Economic challenges related to high production costs, investment risks, and policy dependencies are discussed, alongside potential mechanisms to support market deployment. Furthermore, the article reviews SAFs’ emission performance, including CO2 and non-CO2 effects, based on existing life cycle assessment (LCA) studies, with an emphasis on variability caused by feedstock type and production method. The findings highlight that, while SAFs can significantly reduce aviation-related emissions compared to fossil jet fuels, the magnitude of benefits depends strongly on supply chain design and sustainability criteria. There are various certified pathways for SAF production, as well as new technologies that can further contribute to the development of the industry. Properly selected biomass sources and production technologies can reduce greenhouse gas emissions by more than 70% compared to conventional fuels. The implementation of SAFs faces obstacles related to cost, infrastructure, and regulations, which hinder its widespread adoption. The study concludes that although SAFs represent a promising pathway for aviation climate mitigation, substantial scaling efforts, regulatory support, and continued technological innovation are essential to achieve their full potential.
1. Introduction
In recent years, sustainable aviation fuels (SAFs) have become a key element in the decarbonization strategy of the aviation sector. Their potential to reduce carbon dioxide emissions by 60–90% compared to conventional jet fuel positions them as a significant step toward achieving climate goals. In addition to reducing carbon emissions, SAFs also offer the opportunity to decrease fossil fuel consumption, as their production is partially based on renewable feedstocks. Nevertheless, the production and implementation of SAF face numerous challenges related to cost, supply, policy, and infrastructure. The targets set for the aviation sector are highly ambitious and will require substantial financial investment in the development of SAF production technologies [1].
The European Union has introduced the ReFuelEU Aviation regulation, which mandates fuel suppliers to gradually increase the share of SAF in aviation fuel. Starting in 2025, this share is to reach 2%, and by 2050, it is expected to rise to 70%. Additionally, from 2030, a minimum share of 1.2% of synthetic aviation fuels will be required, increasing to 35% by 2050. These ambitious targets are intended to stimulate the SAF market; however, their realization depends on production capacities and political support [2]. The aspect of synthetic fuels appears particularly problematic due to the limited availability of production technologies. Moreover, the production of synthetic fuels requires green hydrogen, which is currently very expensive to produce.
Global SAF production in 2024 was expected to reach 1.95 million tons, nearly doubling compared to the previous year. Europe accounts for over half of this production, although the pace of growth in the region is slower than the global average. In 2024, SAF production in Europe was expected to increase by 29% to 1.04 million tons, and in 2025, it is projected to reach 1.6 million tons. Despite these increases, SAF still constitutes only a small fraction of total aviation fuel consumption, which highlights the need for further expansion of production capacity. The limited supply of SAF presents a major challenge in light of the assumptions regarding its utilization [3].
SAF production encounters numerous difficulties, including the limited availability of raw materials such as waste oils and fats, and competition from other sectors for the same resources. Furthermore, the development of new production technologies, such as power-to-liquid (PtL), requires significant investment and time to reach commercial scale. Additionally, airport infrastructure must be adapted to handle and distribute SAFs, which entails further costs and logistical challenges.
Despite these challenges, forecasts indicate continued growth in SAF production in the coming years. However, achieving the targets set by the EU will require coordinated efforts from governments, the aviation industry, and the energy sector. Investments in research and development, political support, and the creation of financial incentives will be necessary to scale up production and lower SAF costs. Only through joint efforts will it be possible to achieve sustainable development in the aviation sector and meet ambitious climate goals. It is also important that aviation decarbonization plans do not significantly affect air ticket prices. The obligation to use SAF rests with the airline operators, who may pass on the increased operational costs to passengers. Any potential exclusion of parts of society due to financial constraints is certainly undesirable.
The aim of this study is to investigate sustainable aviation fuels (SAFs) as a key component in the decarbonization of the aviation sector. In response to growing climate concerns and the urgent need to reduce greenhouse gas emissions, SAFs provide a viable alternative to conventional jet fuels, enabling continued aviation operations without requiring major changes to existing infrastructure.
This article offers a comprehensive review of current SAF production technologies, classification systems, and environmental impacts, facilitating a better understanding of their potential within the framework of sustainable aviation. In addition, the study identifies key challenges related to the implementation of SAF—economic, technological, and regulatory—and proposes policy recommendations to support their broader adoption. Ultimately, the paper aims to contribute to the existing body of knowledge and to highlight practical directions for advancing SAF development and integration in the aviation industry.
2. Sustainable Aviation Fuel Policy
2.1. Current Situation and Global Approach to SAF
Air transport has a significant impact on the environment and climate. Aviation is one of the major sources of greenhouse gases, accounting for approximately 2.3–2.5% of global carbon dioxide emissions. It also produces large amounts of nitrogen oxides and particulate matter, which contribute to air pollution. Due to the rapidly growing aviation market and the increasing number of flights, the issue of harmful emissions and pollution is becoming more severe. It is essential to implement initiatives and new solutions to mitigate the damage caused by this phenomenon [4,5,6,7,8].
One of the fastest-growing initiatives for climate improvement is the introduction of sustainable aviation fuels (SAFs). The use of alternative fuels, due to their raw material sources and production processes, enables significant decarbonization of aviation. SAF implementation helps reduce carbon dioxide emissions by an average of 70–80%, particulate matter emissions by 50–90%, and sulfur oxides to nearly zero. Additionally, due to lower combustion temperatures and a more efficient burning process, it allows for a reduction in nitrogen oxide emissions. A key advantage of SAF is its production from renewable resources, aligning with the circular economy model, where waste is transformed into valuable products [4,5,6].
There are numerous materials suitable for SAF production, including plant biomass (various vegetable oils, grains, and sugar crops), organic waste and residues (used cooking oils, animal fats, wood, forestry residues, straw), industrial and municipal waste (plastics, paper, coal waste), recycled raw materials (waste gases), energy crops (camelina, miscanthus), and microorganisms (microalgae). As a result, fuel production is not dependent on petroleum, whose reserves are limited [4,5,6,9,10,11,12]
Importantly, implementing SAF does not require airport infrastructure modifications or fleet replacements. This allows airlines to adopt these fuels relatively quickly and without significant financial investment. Furthermore, such actions align with the sustainability requirements set by aviation organizations, fulfill environmental and climate protection goals, and meet societal and corporate expectations. Public awareness of climate protection is growing, and there is increasing pressure from society, organizations, and investors for airlines to reduce their carbon footprint. The use of SAF technology helps fulfill international obligations, secure funding for further development, enhance brand reputation, and build customer trust. By adopting SAF, it is possible to combine rapid economic growth with sustainable development and environmental protection [4].
However, the actual implementation of sustainable fuels comes with challenges and limitations that need to be addressed. One of the main obstacles is the high production cost of SAF, which exceeds that of conventional aviation fuels. Due to advanced technology, production processes and infrastructure are highly expensive in purchase and maintenance. Additionally, to achieve the desired emission reduction effect, production must be scaled up to an industrial level, ensuring that changes extend beyond a small region or a few companies and are adopted globally. Another issue is the availability of raw materials, which compete with other sectors, including the food, energy, agricultural, and road biofuel industries. This competition increases market value and acquisition costs [4,5,6,10,13].
Airlines are often reluctant to adopt SAF due to its financial unprofitability. To address this, international aviation organizations and governments have introduced various initiatives to promote its development. These efforts mainly focus on financial aspects, such as grants, investment programs, and tax incentives. Additionally, they support research and development initiatives to scale up production, improve efficiency, and diversify available raw materials while reducing costs. Regulations and standards are being established to ensure compatibility with existing infrastructure and fleets. Various programs have been introduced to promote SAF as a solution for reducing harmful emissions, while others impose minimum SAF usage requirements for airlines [4,14].
Approaches to SAF policies vary between countries due to multiple factors. Nations differ in their access to raw materials, infrastructure types, production technologies, and technological development levels. They also set different priorities and regulations, which can be more or less stringent. Furthermore, local economic and social conditions play a crucial role. Public pressure and consumer expectations help promote sustainable fuels more effectively. Many countries, organizations and businesses collaborate to achieve common goals in stopping climate change through greenhouse gas reduction and supporting sustainable development [4,10,11].
2.2. ICAO
The International Civil Aviation Organization (ICAO) is responsible for coordinating and regulating international civil aviation. It establishes standards and recommendations for all aspects of air transport, with a significant focus on environmental protection and sustainable development. ICAO strongly supports the advancement of alternative fuel concepts and plays a key role in their promotion. It collaborates with governments, research institutions, and industry to implement sustainable aviation fuels (SAFs) on a large scale. ICAO’s policy involves three main stages [4,7].
The first stage involves promoting SAF. ICAO has introduced several initiatives focused on financial support, tax incentives, and minimizing investment risk to encourage increased production and technological development. Government funding for research and development aims to accelerate innovation in SAF production, particularly in its early phases. Funds are allocated to research institutions, universities, and private research facilities based on program participation. Additionally, governments provide financial grants for infrastructure investments tailored to SAF production and transportation, as well as credit guarantee programs that facilitate loan acquisition for project implementation. Tax incentives allow for deductions on construction costs and other project-related expenses, while performance-based tax credits offer greater benefits for better results. Furthermore, governments may grant business status exemptions, allowing companies to be free from federal income taxes in the United States, and offer accelerated depreciation, enabling larger deductions than actual capital investments due to the speed of the process. Companies may also receive bonds from government authorities and supranational organizations. Tax reductions apply to SAF excise duties (where taxes on commercial aviation fuel consumption are levied), as well as to producers, suppliers, and blending entities. These measures aim to equalize cost differences between fossil fuels and sustainable alternatives. Financial support for farmers and raw material suppliers is also guaranteed to assist them in overcoming the challenges of establishing new crops. Additionally, the use of SAF can yield benefits from reduced emissions, such as exemptions or reductions in carbon taxes and emission-based charges [4,6,7].
The second stage focuses on strengthening demand through government mandates, the introduction of compulsory industry standards, and the establishment of climate targets. Fuel suppliers are required to increase SAF proportions and reduce carbon footprints in fuel deliveries. Adopted standards aim to support the production of larger quantities of alternative fuels. This phase also involves the adaptation and unification of national policies on sustainable fuels, recognizing them as a vital and future-oriented sector worth investing in. Furthermore, governments should actively set emission reduction and fuel production targets while leading by example by using SAF for official travel [4,6,7].
The final stage aims to facilitate the functioning of the SAF market. Governments will support production certification and monitoring system development while providing the industry with necessary information. Certification of raw materials and SAF production processes, in accordance with established and widely accepted sustainability standards, alongside greenhouse gas emission monitoring methods throughout the entire life cycle, fosters SAF market growth and ensures its environmental credibility. Coordination of efforts and information sharing within the leadership will be ensured through collaboration among stakeholders involved in the sustainable fuel supply chain and discussions with industry representatives [4,6,7].
During its 39th Assembly Session in 2016, ICAO established the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). Its main objective is to neutralize carbon dioxide emissions from international aviation. The project stipulates that airlines will offset emissions exceeding baseline levels by purchasing carbon credits or developing and implementing sustainable aviation fuels [6].
SAF plays a significant role in this initiative, as these fuels generate substantially fewer air pollutants than conventional aviation fuels. However, for SAF to qualify under CORSIA, it must meet specific sustainability criteria, including a minimum greenhouse gas reduction of 10% compared to traditional fuels. It is also essential that SAF is monitored and certified to ensure it originates from appropriate, low-emission sources and is produced in a way that minimizes environmental harm, such as deforestation or soil degradation. The plan includes the gradual implementation of requirements. From 2021 to 2026, participation in the program is voluntary for all ICAO member states, while from 2027 onward, it becomes mandatory [6,15,16].
ICAO actively supports the implementation of sustainable aviation fuels, focusing on three key areas. By collaborating with governments, research institutions, and the private sector, it shapes a global framework that accelerates the aviation sector’s transformation toward sustainable development.
2.3. The European Union
The European Union actively supports environmental protection efforts in aviation. It introduces a range of regulations and policies as part of its decarbonization strategy. It is also the creator of programs aimed at reducing harmful emissions. One of its proposed solutions is renewable SAFs, which it actively promotes and supports through various initiatives [5,13,14].
One of the EU’s initiatives is the formulation of the climate policy package called “Fit for 55”, whose primary goal is to reduce greenhouse gas emissions by 55% by 2030 compared to 1990 levels. This is part of the European Green Deal strategy, which aims to achieve climate neutrality by 2050. The “Fit for 55” package includes the ReFuelEU initiative concerning SAF, which plays a key role in this effort. Through its implementation, the goal is to increase the share of sustainable aviation fuels in the market. The planned targets are to reach 2% usage of alternative fuels by 2025 and increase this to 63% by 2050. This is intended to be achieved through progressive mandates requiring airlines to gradually increase the proportion of SAF in their fuel mix, ensuring a significant rise in SAF production and usage by 2050. The introduction of minimum limits is set to begin in 2025. Financial incentives, such as tax breaks and subsidies, will also provide support. Another crucial element is promotion through transparency and openness by implementing certification and standardization, ensuring that sustainable aviation fuels meet high standards [5,7,13,14].
Another measure undertaken by the European Union to limit greenhouse gas emissions is the establishment of the Emissions Trading System (ETS). This program covers several sectors with the highest CO2 emissions in the EU. A total emissions cap is set for each sector within the Union, and businesses receive or purchase allowances accordingly. This means that if a company emits more pollutants than its permitted level, it must buy allowances from another company. Conversely, a company emitting less than its allowed limit can sell the surplus. Companies that remain within their emission limits benefit financially from their environmentally friendly approach, whereas those that harm the environment bear additional costs. This serves as an incentive for companies to implement changes and adopt more eco-friendly solutions. Over time, the number of available allowances decreases, further pressuring companies to invest in sustainable technologies. In this context, using SAF becomes highly beneficial as it helps businesses reduce their emissions, allowing them to avoid extra fees [5,6,14,16].
Furthermore, the European Union strongly supports research and development. It funds international and local projects related to climate protection. It runs the Horizon Europe research and innovation program, which allocates substantial financial resources to scientific research—both for sourcing new raw materials and improving production processes and the necessary infrastructure for development [5,6,7,14].
Grants are also available for the construction or modernization of infrastructure elements dedicated to fuel production, refueling, blending, or other activities related to the SAF production process. This reduces financial risks for investors and encourages them to implement new technologies more quickly [5,6,7,14].
A strong emphasis is placed on forming alliances and partnerships between countries and between the public and private sectors, as this can lead to numerous benefits. The European Union itself engages in cooperative efforts with ICAO to standardize SAF certification rules and regulations. The aim of this initiative is to ensure that all fuels produced worldwide adhere to consistent standards [17].
The European Union undertakes many actions to protect the environment in aviation, introducing numerous rules, regulations, and incentive programs related to sustainable fuels. The ultimate goal is decarbonization and achieving climate neutrality.
2.4. IATA
The International Air Transport Association (IATA) supports the development of sustainable aviation fuels (SAFs), considering them the most effective in achieving a significant reduction in emissions. In 2009, the organization set a goal to reduce total aviation-related emissions by 50% by 2050, compared to 2005 levels. IATA urges governments to provide financial support to businesses and encourages collaboration and partnerships among stakeholders [8,17].
2.5. Other Entities—Governments, Enterprises, Airlines
Governments, airlines, and other enterprises worldwide are involved in the process of minimizing greenhouse gas emissions. Currently, the awareness of the need for change is so high that not only major environmental organizations but also private companies are joining forces to protect the environment.
Portugal, under the “Roadmap for Carbon Neutrality 2050” from 2019, plans to completely eliminate CO2 emissions in the aviation industry by 2050. Spain and France, following their 2020 climate change laws, aim to introduce a certain percentage of SAF into total aviation fuel consumption. Spain has set a 2% target by 2025, while France aims for 5% by 2030. Sweden, as part of the “Sweden’s Fossil-Free Industry Initiative” from 2020, is focusing on achieving a 30% blending rate for sustainable fuels. Malaysia has committed to producing SAF-based products from palm oil, allocating a budget of approximately USD 6.8 million. Brazil, under its 2019 National Biofuels Policy, aims to promote decarbonization by reducing harmful gas emissions and creating a carbon credit market, similar to the EU’s ETS. The UK introduced a Renewable Transport Fuel Obligation in 2018, supporting and rewarding entities producing sustainable fuels. The Netherlands decided in 2019 to build the country’s first SAF production facility. The Indonesian Biofuel and Renewable Energy Working Group launched the “National Action Plan” to increase the share of sustainable fuels in total production, targeting 2% by 2016, 3% by 2020, and 5% by 2025. Indonesia also implemented a carbon credit system under the “Archipelagic Carbon Program”. Japan initiated the “Next-Generation Aviation Fuel Initiative” in 2014 to develop SAF production and distribution systems by 2020. As part of its promotion strategy, the Japanese Ministry of Transport used sustainable aviation fuel for flights during the Tokyo 2020 Olympic and Paralympic Games. The United States actively promotes SAF, mainly through financial incentives. Under the “Renewable Fuel Standard 2”, the U.S. Environmental Protection Agency created a system requiring fossil fuel producers and importers to introduce renewable fuels into their supply chain or purchase credits from companies that produce excess SAF. The California Air Resources Board implemented the “Low Carbon Fuel Standard Policy”, which also includes a credit trading system for low-emission fuel production. The “Sustainable Aviation Fuel Act”, introduced by the U.S. President in 2021, provides tax incentives for SAF [4,6,7,10,18].
Since 2022, Indian airlines have started blending conventional fuel with sustainable products, initially targeting a 1% SAF content. Other airlines, such as Virgin Atlantic, and Boeing also support the development of sustainable fuels by investing in research, developing technologies, participating in pilot projects, and making their aircraft available for SAF test flights. The French company Global Bioenergies, specializing in renewable fuel and chemical production from biomass, has announced plans to build an SAF production facility by 2027, with an annual capacity of 30,000 tons. TotalEnergies, a global energy company focusing on oil, natural gas, renewable energy, and low-emission fuels, is also actively involved in sustainable aviation fuel development. The company is collaborating with the aviation industry to accelerate SAF adoption and has committed to producing 150,000 tons of SAF annually by 2030, aiming for a 10% share in the global SAF market [4,7].
There is a widely developed, still increasing production of biofuel (HEFA) based on palm oil crops in Indonesia, with a capacity of 3000 million barrels per day as of 2022. Also, Malaysia introduced a policy of blending 5% of such biofuel with conventional diesel back in 2007. Throughout the years, this rate was consistently rising, reaching 10% in 2019 and 20% in 2024, mainly because of higher manufacturing capabilities and lower feedstock prices. Thailand was not producing any kind of SAF as of 2022, but it was announced in 2024 that the construction of a specially SAF-oriented plant at the Bangchak Phra Khanong Refinery in Bangkok had begun, which is expected to be able to produce 1 million barrels per day. In the case of the Philippines, the domestically produced bioethanol is obtained from processing sugarcane. There are also plans from the American company WasteFuels to invest in refining SAF in Manila from municipal and agricultural waste, thus battling the problems with the city’s waste management system [19].
As indicated by studies and analyses conducted in Brazil, hydrothermal liquefaction with hydrogenation (HTL) is one of the flexible technologies with significant potential for reducing carbon dioxide emissions from aviation fuels. It is characterized by high conversion efficiency and can reduce greenhouse gas emissions by up to 85% over the entire life cycle. As noted by R. de Souza Deuber et al., combining the widespread availability of sugarcane or agricultural residues with a modern facility using HTL could strengthen the sustainable production of SAF in Brazil and support the achievement of national and international environmental program goals (RenovaBio, CORSIA). However, a detailed assessment of the potential of such an initiative is still needed [20].
In Colombia, the production of fuel alcohol from sugarcane and the manufacturing of biodiesel are widespread. Among the available feedstocks for biofuel production in the country are oils, fats, palm oil, agricultural residues, and municipal solid waste. Each of these must be certified either under RSB (Roundtable on Sustainable Biomaterials) or ISCC (International Sustainability and Carbon Certification). Taking into account the type of feedstock, its application, and availability in Colombia, the most successful processes have been HEFA, FT, ATJ, and the co-processing of esters and fatty acids in a conventional oil refinery. From the perspective of existing infrastructure, without requiring additional investment, it would be possible to produce fuel via Fischer–Tropsch synthesis and alcohol-to-jet technology at Ecopetrol’s refinery in Cartagena [21].
Governments and airlines are also forming partnerships to take action on a larger scale. In 2010, the U.S. Department of Agriculture, Boeing, and various airlines launched a project to produce 1 billion gallons of biofuels annually. That same year, Qatar University, Qatar Airways, and the Qatar Science and Technology Park initiated a pilot SAF production project, starting with a single barrel per day, later scaling up to a small test facility capable of producing 1.5 million liters per year. This allowed them to optimize production processes before establishing a full-scale plant. The “Carbon Offsetting and Reduction Scheme for International Aviation” (CORSIA) is another example of international cooperation, aiming to ensure that post-2020 aviation growth does not lead to additional emissions. This initiative seeks to offset approximately 80% of the predicted emission increase between 2021 and 2035 through eco-friendly technologies, sustainable aviation fuels, and efficient operational management [4,7,10].
3. Types of Sustainable Aviation Fuels
3.1. General Overview
There are many types of sustainable aviation fuels on the market. There are not only challenges connected with the usage of hydrogen and electricity but also global concerns about climate change influenced by the development of drop-in fuels, which can be burned in existing engines. Widely known ones are Fischer–Tropsch SPK (FT-SPK), hydroprocessed esters and fatty acids such as synthetic paraffinic kerosene (HEFA-SPK), synthesized iso-paraffins from hydroprocessed fermented sugars (SIPs), alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK), catalytic hydrothermolysis jet fuel (CHJ), e-fuels (power-to-liquid), pyrolysis, aqueous phase reforming (APR) and hydrothermal liquefaction [22].
3.2. Fischer–Tropsch (FT)
The FT (Fischer–Tropsch) fuel manufacturing process, first carried out by Franz Fischer and Hans Tropsch in the then-existing German Reich, has been known for around 100 years. It results in the production of synthetic fuels, such as kerosene, gasoline, diesel, jet fuel, and other by-products of the process that are desirable because they can be used in other fields: methane, various hydrocarbons, alcohols, aldehydes, waxes, and more. These types of fuels can be blended with conventional jet fuel in ratios of up to 50/50 [23]. Agricultural and forestry residues, municipal solid waste, or herbaceous energy crops can be used as feedstocks [23]. FT fuel is significantly less widespread than HEFA, but was certified in 2009 by the International Civil Aviation Organization and listed as the first (A1) approved conversion process, also being a standard mentioned in the ASTM D7566 document. It is possible to create a variant called SPK/A, which includes light aromatics, primarily benzene, whose presence prevents fuel leaks and ensures proper sealing within engine system components. SPK/A has also been listed as a certified type since 2015 [24].
3.3. Hydroprocessed Esters and Fatty Acids (HEFAs)
The HEFA fuel production process is relatively simple compared to other types of SAF, which makes it the only kind currently available and produced commercially on a large scale, and the most popular and accessible at a wider selection of airports in the world. It was introduced as a certified type in 2011. Despite the mentioned simplicity of refinement, the production of HEFAs faces challenges in sourcing raw materials. In the case of plant-based ingredients, such as oils from oilseed crops, as well as palm or corn oil, there may be a need to expand agricultural land and increase pressure on landowners to ensure adequate production capabilities. On the other hand, municipal waste, sewage, and animal fats are characterized by inconsistent quality, which translates into variable fuel production efficiency. According to a study by A. Bauen, N. Bitossi, L. German, and others [22], the production cost of HVO ranges from EUR 1100 to 1350 per ton. Converting this type of fuel into the final product (HEFAs) is not expensive, but it is necessary from an operational standpoint, particularly for lowering the freezing point [22]. The mixing ratio of HEFAs with conventional fuel must not exceed 50% [24].
3.4. Synthesized Isoparaffins from Hydroprocessed Fermented Sugars (SIPs)
This type of fuel was certified in 2014. According to ASTM D7566, SIPs can be blended with conventional jet fuel at a maximum ratio of 1/9 [22]. The biggest advantages of this SAF can be seen in its high purity and renewable characteristics. The complexity of the production process however, including the specific feedstocks used (here for example sugar cane and beetroot), and the low efficiency of converting lignocellulosic sugars into fuel, makes SIP the most expensive type of SAF, with production costs reaching around EUR 4000 per ton [22].
3.5. Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ)
ATJ has been a certified fuel type since 2016. The standard allows for ATJ to be blended with conventional jet fuel at a ratio of 50/50 [22]. The ultimate goal is to implement an approved ATJ variant that could fully power turbojet engines. The production methodology of this SAF type is advantageous and worth attention, as it allows the transformation of various types of alcohols, such as bioethanol. It is possible to use some feedstocks similar to those described in the HEFA process—mostly agricultural and forestry residues. Additionally, there is no requirement to build processing facilities directly adjacent to distilleries, which is a significant advantage. However, a challenge remains in the direct use of alcohols to power engines instead of converting them into aviation fuel [22].
3.6. Catalytic Hydrothermolysis Jet Fuel (CHJ)
CHJ is the sixth method approved under the D7566 standard, introduced in 2020. CHJ can be manufactured using a wide variety of different biological feedstocks and involves the conversion of fatty acid esters into sustainable aviation fuel. The raw CHJ composition includes paraffins, isoparaffins, cycloparaffins, aromatics, and organic acids. This method allows for the production of a mixture that is chemically close to conventional petroleum, containing a countless number of components [25]. It is not as widely developed as the FT or HEFA processes.
3.7. E-Fuels (Power-to-Liquid)
Electrofuels or power-to-liquid fuels, known as e-fuels or PtL for short, respectively, are some of the newest technologies developed on the aviation fuel market. They are generated using renewable electricity. According to information found by J. Peacock, R. Cooper and G. Richardson, electrofuels are able to reduce emissions by 99% in comparison to conventional fossil fuels and by 100% with a decarbonized supply chain, and they can be seen as innovative, but their production is rather limited and troublesome. The greatest challenges are the requirement of power sources such as solar cells or wind turbines, the high costs of processing and introducing the producing technologies, and low manufacturing capacities [26].
As Gonzalez-Garay et al. have studied, the production of sustainable aviation fuels with renewable energy sources, electrolysis, and the Fischer–Tropsch process would have an impact on increasing flight ticket prices, with increases of up to 100% [27]. C. Graves, S. Ebbesen, M. Mogensen, and K. Lackner have analyzed the production costs of power-to-liquid fuels made with renewable and nuclear energy, obtaining values of approx. EUR 1 per kilogram, assuming electrical energy costs of around 0.04–0.05 EUR/kWh [28].
In the case of one of the key applications of PtL—namely, the production of synthetic fuels—it becomes possible to reduce greenhouse gas emissions in transportation sectors where full electrification and the resulting transformation of propulsion systems pose significant challenges. This technology allows for the maximum utilization of renewable energy sources by expanding existing generation systems, which is particularly valuable in developed countries focused on a broadly understood sustainable approach to energy management and the provision of various forms of fuel for transport. PtL production and energy storage in this form also offer a solution to the variable and unstable nature of renewable energy sources, which are dependent, for example, on sunlight or wind strength and direction [29].
Among important intermediate products, methanol—referred to in the literature as e-MeOH—deserves a mention. The fuel derived from it can be freely blended with aviation kerosene and produced in any location with an abundant supply of renewable energy. The use of so-called green hydrogen and CO2 captured from the air enables GHG emissions to be reduced by as much as 89% to 94% compared to conventional fuels [30].
The model production of SAF (sustainable aviation fuel) using PtL technologies is currently being implemented in various parts of the world, including countries such as the Netherlands, Germany, Norway, Singapore, Uruguay, the United States, and Canada [29].
Moreover, in other industries and sectors, different variants of technologies for producing energy-rich products can be distinguished: gases such as hydrogen or methane (power-to-gas, PtG), heat that is stored and used for heating buildings or in industrial processes (power-to-heat, PtH), and chemical compounds applicable in industrial settings (power-to-chemicals, PtC) [29].
From the perspective of Technology Readiness Levels (TRLs), each pathway for producing PtL-type fuels demonstrates a high level of technological maturity. The demonstration facilities being built in several European countries (as previously mentioned) provide practical knowledge, especially for investors and end users. The highest TRL values are recorded for low- and high-temperature methanol processing, which are 8 and in the range of 7 to 8, respectively. Fischer–Tropsch processes at low temperatures also score highly (a TRL of 7). The lowest ranking is given to high-temperature Fischer–Tropsch processing (a TRL in the range of 5–6) [31].
In the case of electrolysis itself, the use of polymer membranes has enabled the technology to reach market readiness (TRL 9). For water electrolysis, the TRL ranges from 7 to 8, while for co-electrolysis, it ranges from 5 to 6. Among CO2 capture methods, the most mature technology is biogas upgrading (TRL 9), followed by vacuum temperature swing adsorption (TRL 7–8) and modern electrodialysis processes (TRL 6) [31].
The economic viability of PtL has shown an upward trend in recent years, primarily due to decreasing costs of system components and the cost of electricity generation itself. Despite numerous projects and R&D efforts, the still relatively high production costs compared to conventional jet fuel continue to limit the widespread adoption and commercialization of power-to-liquid [31]. However, efficiency improvements and cost reductions are being achieved. In 2023, D. Huber, F. Birkelbach, and R. Hofmann from the Vienna Institute of Energy Systems and Thermodynamics described PtL fuel production using a 1 MW electrolyzer within a heat exchanger network synthesis (HENS) process, which resulted in a cost of EUR 1.83 per kilogram of fuel. The maximum production efficiency reached 61.84% [32].
3.8. Hydrogen
Among fuels with a composition similar to conventional hydrocarbon mixtures, hydrogen has also been considered for years. Although its use contributes to the reduction of noise and greenhouse gas emissions, as well as to increased propulsion system efficiency, it also presents many challenges for engineers. Due to its low energy density per unit volume—approximately three times lower than that of aviation kerosene—hydrogen must be stored in liquid form (or gaseous, which is also possible) in special cryogenic, insulated tanks. This solution not only requires enormous amounts of energy to produce, cool, and compress hydrogen into these tanks at very high pressure, but also necessitates significant modifications to the airframe design. These lead to higher capital expenditures (CAPEX) due to the need for thermal insulation [22].
The most widespread approach involves using proton exchange membrane fuel cells (PEMFCs) or solid oxide fuel cells (SOFCs) powered by gaseous hydrogen, which convert chemical energy into electrical energy. These fuel cells can be used to generate electricity for powering engines and/or avionics and other electrical components. Existing examples include relatively small aircraft: the HyFlyer developed by the American company ZeroAvia, and the German HY4. These are six- and four-seat short-range aircraft, respectively, utilizing electric propeller propulsion based on PEMFC technology [22].
In addition, the use of hydrogen as a component of a fuel mixture is being considered to indirectly power existing turbofan engines. In many research centers around the world, the combustion characteristics of aviation fuel with hydrogen content are being studied and analyzed in terms of various benefits, including environmental and economic ones. According to a study by A. Bauen et al., published in 2020, hydrogen-powered propulsion systems (either through direct combustion or fuel cells) would be suitable for short- or possibly medium-range operations, while providing significant CO2 emission reductions when using green hydrogen [22].
Current TRLs for hydrogen production are as follows: alkaline electrolyzers—9; polymer–electrolyte membrane (PEM) electrolyzers—also 9; and high-temperature electrolyzers (SOEC)—7 or 8. A TRL is even defined for stationary hydrogen storage, which stands at level 9 [31].
Proton exchange membranes (PEMs) represent one of the most promising technologies for green hydrogen production through electrolysis. The German Fraunhofer Institute for Solar Energy Systems ISE is conducting research in this field and, in February 2025, announced the development of ultra-thin, porous titanium layers produced using screen printing technology. The use of these layers can reduce the amount of iridium traditionally required. Optimizing the surface properties of these thin layers is a key factor in reducing costs and increasing the efficiency of PEM electrolysis. This optimization involves ensuring that the microporous titanium layers (MPLs) are as well-matched as possible to standard catalyst layers. This allows for a reduction in their thickness (down to approximately 20 μm), surface roughness (by 46%), material usage, and overall cost [33].
4. SAF Production Methods
4.1. Certification
There are various methods for producing sustainable aviation fuel (SAF). However, for such a product to be deployed on a large scale within international aviation, it must meet a comprehensive set of criteria, and the production process must undergo certification. Currently, 11 production pathways are fully approved, with an additional 11 under evaluation. These criteria are defined by international bodies such as the International Civil Aviation Organization (ICAO), and the fuel must also comply with ASTM (American Society for Testing and Materials) standards. These are internationally recognized specifications developed by ASTM International, which establishes testing methods, technical standards, and performance requirements for a wide range of materials, products, systems, and services—including those within the aviation industry.
In the context of SAF, beyond meeting the same requirements as conventional jet fuel, producers must comply with ASTM D7566, the specification that defines the technical requirements and properties of aviation fuel (including composition, volatility, fluidity, and density). Any production pathway seeking approval must meet these specifications to be authorized for use in commercial aviation [6,7,11,15,34,35].
To obtain certification, the fuel must demonstrate a significant reduction in greenhouse gas emissions compared to conventional jet fuel and contribute to the reduction of the carbon footprint across the full life cycle. Feedstocks must be derived from renewable resources, which do not compete with food production, nor contribute to deforestation. Additionally, freshwater consumption during the production process should be kept to a minimum. The chemical processes used in SAF production must comply with all applicable safety, quality, and technical fuel standards (ASTM D7566). Furthermore, the production method must adhere to sustainability principles and be certified in accordance with frameworks such as the Roundtable on Sustainable Biomaterials (RSB) or the International Sustainability and Carbon Certification (ISCC) schemes.
SAF must also undergo extensive testing—both in laboratory settings and under real-world operational conditions—to validate its performance and ensure full compliance with aviation requirements [4,6,7,35].
Depending on the region and the local regulatory environment, additional criteria may also apply, such as compliance with national environmental protection laws, life cycle carbon footprint thresholds, as well as considerations related to water footprint, biodiversity, and ecosystem impacts [4].
4.2. Fischer–Tropsch Hydroprocessed Synthesized Paraffinic Kerosene (FT-SPK)
FT-SPK aviation fuel is obtained through gasification—a thermochemical process that fully converts biomass or other solid feedstocks, such as residual or organic waste, into gas. Gasifying high-energy-content feedstocks is highly efficient and yields synthesis gas (syngas), a mixture of hydrogen (H2) and carbon monoxide (CO) with significant energy potential, which is essential for subsequent Fischer–Tropsch synthesis [4,6,7,8,11,36].
The next stage involves syngas purification and conditioning. Contaminants such as sulfur compounds, nitrogen oxides, hydrogen chloride, and particulates are removed to prevent catalyst poisoning in downstream fuel production processes. Subsequently, the syngas is enriched with hydrogen to optimize the hydrogen-to-carbon monoxide ratio [4,7,8,11,16,37,38].
The core of the process is the Fischer–Tropsch synthesis, where hydrogen and carbon monoxide react over a catalyst to form long-chain hydrocarbons, which constitute the base of synthetic fuels. Cobalt and iron are the most commonly used catalysts. The target product range is typically C10–C20 hydrocarbons due to their suitability for further refining [4,6,7,11,16,36,38,39].
The main product of this process is synthetic kerosene. Other co-products include saturated aliphatic hydrocarbons (paraffins), heavier fractions suitable for conversion into high-quality diesel, and the heaviest fractions—waxes—which may be used in the chemical industry or further processed. Water is a by-product of this reaction, formed from the hydrogen and oxygen atoms [11].
Once the desired hydrocarbons are obtained, they undergo hydrorefining to enhance their chemical composition. In this process, hydrocarbon feedstocks are treated with hydrogen gas under high pressure and temperature in the presence of a catalyst. Hydrogenation eliminates sulfur, nitrogen, and oxygen compounds, thereby improving the physical properties and chemical stability of the fuel [8,11,36,39].
Finally, the hydrocarbon mixture is distilled to separate the components by boiling point. The resulting fractions are blended in specific ratios to produce synthetic paraffinic kerosene (SPK), which is then mixed with conventional Jet-A1 fuel. The synthetic component usually constitutes between 10% and 50% of the blend [11,36].
4.2.1. Synthesized Kerosene with Aromatics Derived by Alkylation of Light Aromatics from Non-Petroleum Sources (FT-SKA)
FT-SKA is a variant of the FT-SPK process. While the core production stages—syngas generation, Fischer–Tropsch synthesis, and hydrorefining—remain unchanged, the final product differs due to the addition of aromatic hydrocarbons [40].
The key differentiator is the blending of Fischer–Tropsch-derived synthetic kerosene with aromatics (e.g., benzene, toluene, xylenes) derived from biomass [4,6,11,38].
4.2.2. Co-Hydroprocessing of Fischer–Tropsch Hydrocarbons in a Conventional Petroleum Refinery
Another variation is the co-hydroprocessing of Fischer–Tropsch hydrocarbons in conventional petroleum refineries. Unlike standalone FT-SPK production, this approach integrates Fischer–Tropsch products into existing refinery streams, allowing simultaneous processing with crude oil. This method leverages existing infrastructure, reduces capital costs, and limits harmful emissions [4,6,11].
4.3. Synthesized Paraffinic Kerosene from Hydroprocessed Esters and Fatty Acids (HEFA-SPK)
The HEFA-SPK process is obtained through the hydroprocessing of oils, fats, or waste materials, including plant-based oils and animal fats, which allows for converting such materials into aviation fuel.
The process includes the following steps:
- Distillation into fractions;
4.3.1. Co-Hydroprocessing of Esters and Fatty Acids in a Conventional Petroleum Refinery
This advanced process co-processes biological feedstocks—plant oils and animal fats—with petroleum in existing refinery infrastructure [4,11,41].
After purification, the following occurs:
4.3.2. Co-Processing of HEFA
This approach is similar to the standard HEFA process but differs in that biological feedstocks are processed simultaneously with crude oil within existing refinery systems [4].
While HEFA, hydroprocessing, and HEFA co-processing are closely related, they differ in configuration. HEFA uses dedicated systems to convert fats and oils, while co-hydroprocessing involves simultaneous treatment with petroleum in an integrated stream. Co-processing, on the other hand, refers to separate yet parallel streams of biological and fossil feedstock processed and later combined [4].
4.3.3. Synthesized Paraffinic Kerosene from Hydrocarbon—Hydroprocessed Esters and Fatty Acids (HC-HEFA-SPK)
4.4. Synthesized Iso-Paraffins from Hydroprocessed Fermented Sugars (SIP)
SIP is based on the conversion of sugars derived from the processing of genetically modified microorganisms such as algae, fungi, yeast, and bacteria. The latter two groups, through the consumption of sugar, excrete long-chain liquid alkenes such as farnesene or short-chain gaseous alkenes such as isobutene, enabling further conversion into sustainable aviation fuel [4,6,11,36,38].
The received isobutanol undergoes the following processes:
4.5. Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK)
Alcohol-to-jet involves converting alcohol into synthetic paraffinic kerosene [4,6,11]. The steps are as follows:
- Fermentation of sugar-rich materials (such as sugarcane, corn, and wheat). Alcohol can also be obtained from processing wood (lignocellulose), or even waste, to produce a mash containing ethanol or isobutanol.
- Distillation and blending with Jet-A1 in 10–50% proportions [38].
Synthetic Paraffinic Kerosene with Aromatics (ATJ-SKA)
ATJ-SKA is a variant of the ATJ process, distinguished by the addition of aromatic compounds—benzene, toluene, and xylenes—after base hydrocarbon formation. These aromatics enhance energy density, lubricity, and chemical stability of the fuel [11].
4.6. Catalytic Hydrothermolysis Jet Fuel (CHJ)
The feedstocks used in this method include vegetable oils, animal fats, and used cooking fats—like those employed in the HEFA process. The process begins with a so-called preconditioning stage, aimed at altering the structure of fatty acid molecules and increasing the efficiency of the overall thermolysis process [11,43]. Along the way, several intermediate processes occur, such as the following:
- Cracking (breaking down long-chain hydrocarbons into shorter molecules);
- Hydrolysis (breaking of chemical bonds in a molecule through interaction with water);
- Decarboxylation (removing COOH groups);
- Dehydration (removing water);
- Isomerization;
- Recombination (forming new, more stable hydrocarbon molecules);
- Aromatization [4].
Water is essential for CHJ production due to the presence of hydrogen, which is used for triglyceride hydrolysis and hydrocarbon cracking. This latter intermediate process is crucial, as it determines the carbon chain length and the final structure of the resulting fuel (i.e., the hydrocarbon-based compound). Subsequently final distillation and blending with Jet A-1 in ratios of approximately 10–50% [43,44].
4.7. E-Fuels (Power-to-Liquid)
The first step is to produce hydrogen using the electrolysis process and to capture carbon dioxide from the atmosphere thanks to low-temperature direct air capture technology. Both H2 and CO2 can then be reformed into syngas, which is later used for final hydrocarbon synthesis using the Fischer–Tropsch method. PtL fuel production is closely linked to relevant CO2 sources, such as air, industrial areas, or power plants, and to access to renewable electricity sources, for example wind turbines, photovoltaics or geothermal energy. Electricity here is used for transforming H2O into H2 during electrolysis, which can be realized with alkaline electrolysis (AEL), a proton exchange membrane (PEM), or high-temperature solid oxide electrolysis (SOEL). Considering greenhouse gas (GHG) reduction as the main goal of using e-fuels, the production process is based on capturing carbon dioxide, for example by using the popular method of direct air capture (DAC), briefly mentioned before. Closing the CO2 cycle in the environment is possible only by extracting it from the atmosphere or from so-called renewable sustainable sources: biomass combustion, fermentation of organic residues, or bioethanol production. To carry out such transformations successfully, it is also important to ensure that the water supply purity is high enough [45].
The captured CO2 is converted into CO in order to produce synthetic gas, shortly known as syngas, which is a mixture consisting above all of carbon monoxide and hydrogen. After that, the synthesis—the process of transforming gases or solids into liquids—takes place. Among often-used processes of creating PtL, Fischer–Tropsch synthesis is the most typical, in which diverse hydrocarbons are products of such reactions. The main by-product is water [45].
To provide the fuel with adequate parameters, some conditioning processes after the synthesis may be required. The composition of the resulting mixture requires distillation to separate short- and long-chain hydrocarbons from each other. To convert the waxes, i.e., long-chain hydrocarbons, into the desired product, cracking or hydrocracking is used [45].
5. Economic Profitability
Sustainable aviation fuels (SAFs) are a relatively new technology that is gradually being introduced to the market. Currently, SAF production is not yet happening on a mass scale, and in most cases, it is used as an additive to conventional jet fuel rather than a standalone product. As a result, the production and distribution costs of SAF are significantly higher than those of traditional fuel, which also affects purchase prices [6,13].
The prices of different SAF types vary significantly (Figure 1). This is due to differences in processing and production methods, which require more or less advanced technologies. The type of feedstock used for fuel production also plays a role, as some raw materials are harder to obtain, exist in limited quantities, or require higher labor input and greater consumption of natural resources. All SAF variants are currently more expensive than Jet A-1. However, when comparing ATJ-SPK, HEFA-SPK, FT-SPK, and HC-HEFA-SPK over the years, a downward trend in prices can be observed. This may be attributed to various programs promoting aviation decarbonization, as well as growing pressure from aviation organizations and governments to reduce emissions from air transport. In addition to tax incentives and subsidies for companies implementing SAF, penalties for non-compliance with sustainability regulations have been introduced. These measures motivate the industry to adapt to new standards, increase production, and ultimately reduce costs.
The economic viability of SAF is expected to improve over time as environmental initiatives progress. Currently, high production costs are a natural consequence of limited manufacturing capacity. However, in the future, as SAF’s potential expands, technological advancements continue, and political support strengthens, large-scale production will become possible. This will lead to lower manufacturing and acquisition costs, making SAF a more economically competitive alternative to conventional fuels.
6. Analysis of Emissions from Engines Powered by Sustainable Aviation Fuels
Based on the collected data, an analysis was conducted regarding the emission of harmful chemical compounds resulting from the combustion process. A total of 11 sustainable aviation fuels (SAFs, Figure 2) were evaluated, along with conventional Jet A-1 fuel used as a reference baseline. The following observations were made.
Carbon dioxide (CO2) plays a dominant role in the total emission of harmful compounds (Figure 2). Its magnitude is approximately two orders of magnitude higher than that of nitrogen oxides (NOx), which are the second most prevalent emissions. For conventional Jet A-1 fuel, CO2 emissions are approximately 3160 g CO2/kg of fuel. The sustainable aviation fuels show no significant variation in this respect; the values recorded are either identical or very close. HEFA-SPK, HEFA co-processing, HEFA hydroconversion in a petroleum refinery, ATJ-SPK, ATJ-SKA, and CHJ all exhibit the same emission levels. Other fuels show the following emissions: SIP—3150–3160 g CO2/kg; FT-SPK, FT-SKA, and FT hydroconversion—3090–3110 g CO2/kg. The only fuel to demonstrate a noticeable CO2 emission reduction is HC-HEFA-SPK, which, due to the use of algae as a feedstock, achieves a 6% decrease, resulting in approximately 2960 g CO2/kg of fuel.
The second most emitted compounds during combustion are nitrogen oxides (NOx). Most analyzed SAFs show emission levels similar to Jet A-1, which emits around 15 g NOx/kg of fuel (Figure 3 and Figure 4). This is true for FT-SPK, HEFA-SPK, SIP, ATJ-SPK, and CHJ. These similarities are due to the comparable chemical compositions of the fuels, which yield similar combustion properties under specified conditions. However, some fuels outperform Jet A-1 in this regard, primarily due to lower combustion temperatures, purer composition, and more stable and controlled combustion. These include HEFA hydroconversion in a refinery—12–15 g NOx/kg; ATJ-SKA—10–14 g NOx/kg; HEFA co-processing—10–12 g NOx/kg (a reduction of 10–20%); and HC-HEFA-SPK—8.4 g NOx/kg (a reduction of 44%). The significant reduction of NOx in HC-HEFA-SPK results from the use of algae, which naturally contain minimal nitrogen compounds compared to other plant-based feedstocks. Consequently, fuels derived from algae also contain minimal nitrogen, leading to lower NOx emissions. On the other hand, certain variants of the Fischer–Tropsch (FT) process exhibit higher NOx emissions than Jet A-1, mainly due to the presence of heavy components such as aromatics, which are harder to combust fully and contribute to localized high-temperature zones in combustion chambers, thus increasing NOx production [47].
Carbon monoxide (CO) emissions are significantly lower in magnitude compared to CO2. Jet A-1 produces about 0.6 g CO/kg of fuel (Figure 3 and Figure 4), a relatively negligible amount. Differences among fuels are also minor and do not significantly impact the overall emission balance. The same value (0.6 g CO/kg) was measured for FT-SPK, HEFA-SPK, HC-HEFA-SPK, SIP, ATJ-SPK, and CHJ. HEFA co-processing yielded similar results: 0.5–0.7 g CO/kg. Reduced emissions were observed for FT-SKA (20–40% reduction), FT hydroconversion (10–20% reduction), and HEFA hydroconversion (5–10% reduction), largely due to cleaner chemical compositions and more efficient combustion. Fuels containing aromatics tend to emit more CO, as seen in ATJ-SKA, where emissions increase by 10–20% relative to Jet A-1.
Sulfur oxides (SOx) are emitted in minimal quantities. Jet A-1 stands out with significantly higher SOx emissions compared to the SAFs, with only HEFA co-processing reaching a similar level of approximately 0.8 g SOx/kg (Figure 3 and Figure 4). SIP and FT-SPK demonstrate much lower emissions at around 0.064 g SOx/kg, marking a 92% reduction compared to Jet A-1. Emissions from FT hydroconversion depend on the blending ratio with crude oil and range from 0 to 0.2 g SOx/kg. Emissions from other SAFs are nearly zero due to the absence of sulfur in biological feedstocks and clean alcohol sources. Minor increases may occur due to aromatics in FT-SKA and ATJ-SKA, though these remain below 0.01 g SOx/kg.
Particulate matter (PM) emissions are generally reduced in SAFs. Most exhibit PM levels no higher than Jet A-1 (0.01–0.03 g PM/kg, Figure 3 and Figure 4). An exception is HEFA hydroconversion, which may exceed this range slightly (0.03–0.05 g PM/kg) due to the presence of heavier hydrocarbons and impurities from crude oil co-processing. ATJ-SPK records the lowest PM emission of all fuels at around 0.0003 g PM/kg, thanks to its highly refined production process that eliminates soot precursors. However, its derivative, ATJ-SKA, performs worse due to aromatic content, emitting 0.0092–0.0276 g PM/kg. FT-SPK also shows excellent performance with emissions of 0.0005–0.0030 g PM/kg, resulting from its clean composition and absence of soot-promoting additives, unlike FT-SKA, which emits 0.0025–0.0150 g PM/kg (a 50–75% reduction). FT hydroconversion fares slightly worse, with emissions of 0.004–0.180 g PM/kg (40–60% reduction) due to heavier fractions and crude-related impurities. Other SAFs maintain low PM levels: HEFA co-processing—0.002–0.004 g PM/kg; HEFA-SPK and HC-HEFA-SPK—0.004–0.006 g PM/kg; SIP—0.003–0.009 g PM/kg; and CHJ—0.0075–0.0093 g PM/kg.
In terms of unburned hydrocarbons (HCs), only one SAF performs better than Jet A-1. Most fuels, including FT-SPK, HEFA-SPK, ATJ-SPK, SIP, and CHJ, emit similar levels of 0.1 g HC/kg (Figure 3 and Figure 4). HEFA co-processing, HEFA hydroconversion, and FT hydroconversion tend to emit more (0.2–0.4 g HC/kg), due to the presence of heavier hydrocarbon fractions. Similarly, FT-SKA and ATJ-SKA, owing to their aromatic content, exceed their base versions in HC emissions. HC-HEFA-SPK achieves a 32% HC reduction compared to Jet A-1, attributed to the use of algae, which lack heavy aromatic hydrocarbons and other HC precursors. Algae primarily contain lipids, which convert into simpler, more combustible hydrocarbons, resulting in more efficient combustion and lower HC emissions [36,47].
Volatile Organic Compound (VOC) emissions are similar across all fuels. SAFs emit 0.43–0.49 g VOC/kg, while Jet A-1 emits 0.47 g/kg. VOCs consist mainly of ethylene, acetylene, and aldehydes such as formaldehyde and acetaldehyde. Aromatic and heavy fractions may slightly increase VOC content in certain SAFs.
In conclusion, carbon dioxide is by far the largest contributor to total emissions during aviation fuel combustion, with values over 200 times higher than the next most prevalent compound—NOx. The sheer magnitude of CO2 emissions means they largely determine whether a fuel can be considered environmentally superior to Jet A-1. Other pollutants represent only a small percentage of total emissions, and while reducing them is desirable due to their toxicity, it is not sufficient alone to shift the environmental performance of a fuel.
Most analyzed SAFs exhibit total emissions comparable to Jet A-1 (Figure 2), differing by only a few dozen grams per kilogram of fuel. HC-HEFA-SPK is the only product that significantly deviates from the norm, with total emissions around 200 g/kg lower than Jet A-1—a promising improvement. This is largely due to the use of algae, which, being lipid-rich, enable the production of hydrocarbons with higher hydrogen-to-carbon ratios. This leads to increased water (H2O) production and reduced CO2 emissions. Furthermore, algae-based fuels are chemically clean, free of heavy fractions and impurities, which enhances combustion efficiency and reduces overall pollutant emissions [12,36,47].
Notable improvements are also observed in Fischer–Tropsch-derived fuels, whose CO2 reductions, while not as significant as HC-HEFA-SPK, show potential for further optimization (Figure 2). For other pollutants, Jet A-1 consistently exhibits the highest emissions, though SAFs show slightly higher relative reductions than for CO2 (Figure 3). Again, HC-HEFA-SPK outperforms all others, with emissions below 10 g/kg, compared to approximately 20 g/kg for Jet A-1—a 50% reduction. This is mainly due to reductions in NOx, PM, SOx, and HC emissions (Figure 4). Other strong performers include ATJ-SKA and HEFA co-processing, both emitting less than 15 g/kg. However, FT-derived fuels perform less favorably in this aspect, with emissions approaching Jet A-1 levels, largely due to higher NOx and SOx values (Figure 4).
Recent research on non-CO2 emissions demonstrates the beneficial impact of sustainable aviation fuels (SAFs) in reducing the formation of these compounds. The results of a new chase-type experiment (GRIM-SAF, United Kingdom, 2024), which involved both ground-based and in-flight testing using jet engines powered by 100% SAF, showed a significant reduction in soot and ultrafine particulate matter emissions compared to conventional Jet-A1 fuel. This directly contributes to the mitigation of contrail formation and, consequently, to the reduction of radiative forcing in the upper atmosphere [56,57].
In parallel, the ECLIF3 project, conducted by the German Aerospace Center (DLR), confirmed that the fuel type has no significant impact on NOx emissions. Instead, the generation of nitrogen oxides is primarily determined by engine operating parameters, such as combustion temperature, pressure, and fuel flow rate [58,59].
However, reports from institutions such as ICAO, RSB (Roundtable on Sustainable Biomaterials), and MIT (Massachusetts Institute of Technology) emphasize that a meaningful reduction in the effects of non-CO2 emissions can be achieved when SAF is blended at a minimum ratio of 35%, provided that the chemical composition of the fuel is carefully controlled—particularly by minimizing the content of aromatics, sulfur, and heavy hydrocarbons [60,61,62].
Such measures are expected to reduce both global and local climate impacts of aviation, including deteriorated air quality around airports, the formation of acid rain (leading to water acidification, soil degradation, and ecosystem disruption), photochemical smog, and disturbances in the Earth’s radiative balance—resulting in either warming or localized cooling effects [60,61,62].
In summary, the SAFs discussed do not perform worse than Jet A-1; however, their emission reductions are generally insufficient to significantly mitigate aviation’s environmental impact. From a combustion perspective, SAFs do not yet demonstrate a distinct environmental advantage over conventional fuels. Although HC-HEFA-SPK shows the most promise, its emission reductions remain inadequate to qualify it as a fully effective alternative. Further reduction of combustion-derived CO2 remains essential to achieve meaningful environmental benefits in aviation.
7. Life Cycle Assessment
For the purpose of conducting a life cycle assessment (LCA), evaluating the climate impact of sustainable aviation fuels, and comparing them with conventional Jet A-1 fuel, greenhouse gas emission data were used. These data cover the entire life cycle as well as specific stages, including feedstock acquisition, processing and production, transportation and distribution, and end-of-life management (recycling or disposal). Emissions are expressed in [g CO2e/MJ—grams of carbon dioxide equivalent per megajoule of energy].
This unit represents the amount of greenhouse gases, converted to their carbon dioxide equivalent, emitted per megajoule of energy produced from the respective fuel.
7.1. Raw Material Acquisition
The biggest challenge involves plant-based raw materials, whose cultivation is a demanding process. It requires significant natural resources such as water and, in many cases, agricultural land, along with nitrogen, phosphorus, potassium, or calcium fertilizers, which, in excessive amounts, negatively impact the environment. Additionally, electricity and fossil fuels are consumed to power agricultural machinery, operate necessary systems, and maintain infrastructure. All these components generate emissions that contribute to the LCA of a given fuel. However, it is assumed that plants absorb amounts of CO2 during growth comparable to those produced during fuel combustion, thus reducing the overall impact [39].
Obtaining waste materials is much simpler and less burdensome. Waste can be categorized as food, municipal, or industrial waste, representing unused residues, by-products, or seemingly useless materials. Since they are classified as waste—meaning their generation and associated emissions resulted from the production of another good—they are not included in the LCA analysis. However, it is essential to consider how much energy is used for collection and preliminary processing, as well as the emissions resulting solely from these processes [12,36,39].
For non-renewable energy sources like coal and natural gas, the greatest threat comes from methane (CH4). This compound is released during resource extraction, and even the smallest leaks can significantly impact the environment, as methane has a greenhouse potential about 30 times greater than CO2. Additional emissions stem from fossil fuel use in mining equipment, pumps, compressors, and overall mine infrastructure operation [63,64].
Regarding raw material acquisition, average emission values for various fuels differ significantly (Figure 5). For some fuels—such as FT, HEFA-SPK, HC-HEFA-SPK, ATJ-SPK, ATJ-SKA, co-processing HEFA, and HEFA hydroconversion in refineries—identical values were assumed since process differences emerge only at the production stage. Few products outperform Jet A-1, whose crude oil extraction emissions amount to approximately 8 g CO2e/MJ. More favorable results are observed for CHJ and fuels produced via Fischer–Tropsch synthesis—7 g CO2e/MJ. The low emissions are primarily due to the use of plant-based, forest, wood, and used oil waste (Figure 6). These resources do not require additional natural resource input or fossil fuel energy consumption for cultivation or production, only for collection and preliminary preparation for further processing. Additionally, they help address excessive waste accumulation and landfill storage issues. A closed-loop approach can be implemented, where by-products become raw materials.
The average emissions from raw material acquisition for other fuels range from 14 to 21 g CO2e/MJ (Figure 5). Most of these raw materials require cultivation, and some need additional processing into oil. The most significant environmental impact components include fertilizers and substances often released into the atmosphere, particularly nitrogen (N), phosphorus pentoxide (P2O5), potassium oxide (K2O), calcium carbonate (CaCO3), and pesticides. Nitrogen application in cultivation can reach up to 20 g per kilogram of raw material. Fossil fuels powering agricultural machinery also play a major role, with consumption ranging from approximately 0.001 to 0.3 MJ/kg of raw material [68].
Various oils exhibit higher emissions than the plants themselves (Figure 6) due to harmful compounds released into the atmosphere during extraction. Rapeseed and corn oils (HEFA-SPK) are among the most emission-intensive materials. Similar values are observed for camelina, soybean, and palm oils (HEFA-SPK), as well as soybean oil (HC-HEFA-SPK), all exceeding 20 g CO2e/MJ. Above this threshold is also animal tallow (co-processing HEFA, HEFA hydroconversion in refineries), due to the energy-intensive process required for heating, melting, and collecting fat. Considering the average fuel emissions (Figure 5), HEFA-SPK reaches the highest value due to the use of highly emissive raw materials.
Despite the lack of specific numerical data, algae are a raw material worth attention. They do not require fertile soil, avoiding competition with other crops, and can grow under challenging conditions—on marginal agricultural land or in saltwater, freshwater, treated wastewater, and other non-consumable waters. Algae require no fertilizers, relying solely on sunlight and basic nutrients. Additionally, during growth, they purify air by absorbing large amounts of CO2 and clean water, making it reusable in the next production cycle. The entire cycle from culture inoculation to harvest is very short, and processing is straightforward. Moreover, the biomass remaining after oil extraction can be used as fertilizer or animal feed. These numerous advantages make algae a promising raw material for sustainable aviation fuel production [12,36,47].
7.2. Fuel Production
The production of finished fuel requires more than just raw materials. Other substances are also necessary in the manufacturing processes, such as elemental molecules like hydrogen and oxygen, additives in the form of antioxidants and hydrocarbon-based aromas, as well as catalysts including iron, cobalt, nickel–molybdenum, and platinum, along with natural gas [9,55,65,66,67].
Some fuels, despite being produced from the same raw materials and undergoing identical processing technologies, function as different products. This is due to the application of various substances added during the blending process. As a result, fuels with similar compositions can exhibit completely different properties and performance characteristics. A prime example of this is FT-SPK and FT-SKA. Both products are derived from the Fischer–Tropsch process, but FT-SKA contains additional aromatic compounds. Based on emission results from different post-production stages, discrepancies are observed [9,65,66].
In some cases, the entire production process takes place in an oil refinery. Again, the raw materials and technology remain the same, but after undergoing primary transformations, the blend is further co-processed with crude oil. This leads to contamination with heavy hydrocarbon fractions and alters the final composition. Generally speaking, such practices are cost-effective as they utilize existing infrastructure and reduce investment costs. However, to prevent contamination of SAF with undesirable substances, alternative solutions should be considered [66,67].
A significant advantage of sustainable aviation fuels (SAFs) is that their production generates excess heat, which is converted into electricity. The efficiency of these processes allows for the recovery of more energy than is required for their operation, resulting in a surplus. This surplus energy can be utilized in other industrial sectors or fed into the power grid [66,67].
Each fuel type has distinct production processes and manufacturing technologies. Different transformations and chemical reactions lead to varying emission levels (Figure 7). The highest amount of released substances is associated with ATJ-SPK, at approximately 25.5 g CO2e/MJ. The production of this SAF is complex, multi-stage, and requires significant hydrogen input, with each step contributing to additional energy consumption and emissions. Additionally, the efficiency of this method is lower than, for example, HEFA-SPK, meaning that larger quantities of raw materials must be processed per unit of fuel, thereby intensifying machine operations. High production emissions are also observed for SIP and HEFA-SPK, amounting to approximately 24.8 and 21.5 g CO2e/MJ, respectively (Figure 7). SIP exhibits very low process efficiency, producing only 0.17 tons of fuel per ton of raw material. Increased emissions result primarily from the extensive operation of equipment required to process large quantities of raw materials. Meanwhile, the high emission levels of HEFA-SPK are due to the specifics of the process, which requires high temperatures and pressures, as well as substantial hydrogen usage. Co-processing HEFA within an oil refinery, but in separate streams, can nearly halve the release of harmful substances. This reduction is attributed to utilizing existing large-scale infrastructure, which employs advanced hydrogen production and energy recovery technologies. Co-processing also allows emissions to be distributed across multiple products, reducing individual impact.
Other fuels, such as FT-SPK and CHJ, demonstrate low production emissions—2 and 4.9 g CO2e/MJ, respectively (Figure 7). These values are significantly lower than the 9.8 g CO2e/MJ assigned to Jet A-1. These processes are much simpler and shorter than, for instance, HEFA-SPK, as they do not require multiple stages of raw material transformation. Additionally, hydrogen demand is lower than in the production of the previously mentioned fuels, and in the case of FT-SPK, a significant portion of hydrogen is generated internally through syngas conversion.
The raw materials that contribute the highest emissions during SAF production include corn grain (processed into both isobutanol and ethanol in ATJ-SPK), green energy crops (ATJ-SPK), sugar beets (SIP), and rapeseed, soybean, and camelina oil (HEFA-SPK) (Figure 8). The released substances exceed 25 g CO2e/MJ and can reach up to 37.35 g CO2e/MJ. Conversely, the lowest emissions are mainly associated with waste-based products such as plant, forestry, and wood residues (FT-SPK) and UCO (CHJ), as well as green energy crops (FT-SPK) and soybean oil (CHJ). These do not exceed 6 g CO2e/MJ. Different raw materials translate to varying average performance levels of sustainable fuels.
SAF production processes generate by-products, most of which are not harmful and can be repurposed for other uses. Biomass gasification results in biochar, a solid carbon material that can be used as a soil-enriching fertilizer. Glycerin, derived from processing vegetable oils and animal fats, finds applications in the cosmetics and pharmaceutical industries, as well as an additive in animal feed. Process water is treated and reused in technological processes. Unused alcohols (isobutanol and ethanol) undergo further distillation and are utilized in the chemical industry. Residual gases (CO2, CO, H2, CH4), light fractions (C1–C4), unreacted components, and petroleum-based waste can be processed and used in industrial operations within the production facility for the generation of other fuels.
In addition to useful substances, dust and sediments containing hydrocarbons, heavy metals, and solid particles are also produced. These are collected and either incinerated in special energy recovery furnaces or processed, stabilized, and stored in landfills.
7.3. Transport and Distribution
The concept of transport and distribution encompasses several stages during which cargo is moved. Initially, raw materials pre-processed for refinement are transported to production plants or refineries. Then, the finished products are delivered to fuel terminals operated by distribution companies, from where they are sent to airports.
SAF is most commonly transported in specially adapted road tankers or, for long-distance exports, by rail and ships. Pipelines are rarely used to transport sustainable fuels from storage bases to airports because they require a dedicated channel to maintain the purity of SAF and prevent contamination from residual conventional fuels. Emissions at this stage mainly result from the combustion of fossil fuels used in transportation vehicles. However, by maximizing the load per trip, these emissions can be effectively reduced [44].
Jet A-1 emits significantly fewer harmful substances during raw material transportation compared to other fuels (Figure 9). This is because crude oil, even over long distances, is transported via pipelines, which are highly efficient, allowing the simultaneous and continuous transport of large quantities of the product. Pipeline transport of crude oil consumes far less energy than road transport.
The transportation of SAF feedstocks does not generate significant emissions, with values remaining within 4 g CO2e/MJ (Figure 9). Looking at individual raw materials, the emissions do not exceed 5 g CO2e/MJ (Figure 10). Small variations between them may result from the distances the cargo must travel and their physical form, as liquids and gases can be transported more efficiently than bulk materials.
For produced fuels, emissions during transportation are even lower than for raw materials (Figure 11). The differences between various SAF types and traditional fuels are negligible. Sustainable fuels release harmful substances within the range of 0.3 to 0.6 g CO2e/MJ, while Jet A-1 emits less than 0.2 g CO2e/MJ. Compared to other stages of the fuel life cycle, these values are insignificantly small. Achieving such low values is facilitated by the ability to transport large amounts of fuel in a relatively small volume and by maximizing the load capacity per trip, which increases the amount of fuel transported per journey while reducing the number of trips required. Another contributing factor is the short distances between SAF production plants and airports, as they are intentionally built close to each other.
7.4. Usage
The usage phase involves the combustion of fuel in aircraft engines. When analyzing the entire life cycle of SAF, emissions from this stage are largely neutral. This is due to the fact that the raw materials used to produce these fuels are predominantly organic. It is assumed that during their growth and development, these resources absorb amounts of CO2 comparable to those emitted during combustion (approximately 2.5 kg of carbon dioxide per kilogram of biomass through photosynthesis). As a result, they counteract their negative impact, achieving carbon neutrality.
7.5. Waste Disposal and Recycling
The final phase of the SAF life cycle is the disposal of waste generated from its use and the recycling of remaining fuel that can be reused or processed.
Unused fuel can be processed in an environmentally friendly manner. Any product that does not meet quality standards (ASTM D7566) can be regenerated at a refinery through re-hydrorefining, purification from contaminants, chemical stabilization, and blending with fresh fuel. Unwanted particles removed from the fuel are incinerated. This approach exemplifies an eco-friendly and efficient recycling solution. Due to the simple chemical composition of most SAFs, they are more recyclable than conventional fuels. As a result, their further processing in refineries is less emission-intensive than Jet A-1 [40].
Sustainable aviation fuel residues can also be disposed of. Leftover fuel mixtures are burned in specially designed industrial furnaces, enabling energy recovery that can later power new technological processes. The amount of CO2 produced during this operation is proportional to the emissions from the usage phase, as essentially the same process is being carried out—just on a smaller scale and under controlled conditions. A significant portion of CO2 can be captured using Carbon Capture and Utilization (CCU) technology and repurposed for new fuel production. Meanwhile, aromatic emissions can be reduced with filtration systems and gas purification methods. After combustion, only minimal amounts of ash and residues remain, which are treated as hazardous waste and stored in landfills. Although this method allows for energy recovery, it does not enable complete material recycling or the avoidance of harmful waste [70].
In real-world conditions, aviation fuel is rarely recycled or disposed of. Efforts are made to minimize product waste, and fuel resource management is handled with great care. SAF storage locations and aircraft fuel tanks are closely monitored for cleanliness to protect the fuel from contamination. Such practices allow for unused fuel from flights to be drained back into airport storage tanks and reused for refueling other aircraft.
The emissions from the recycling and disposal phases of all discussed fuels can be considered minimal, despite the lack of precise data. SAF has a slightly lower environmental impact than Jet A-1. These products are rarely sent to refineries or incinerators for processing, and when they are, the quantities involved are negligible. Thanks to the closed carbon cycle, CO2 is captured and utilized in other processes, limiting its release into the atmosphere and reducing environmental harm.
7.6. Environmental Impact Across the Entire Life Cycle
Life cycle analysis encompasses all stages of a fuel’s existence, from the cultivation or collection of waste materials to its combustion during use or subsequent recycling and disposal. At each phase, various processes are conducted on raw materials or the final product to prepare it for its intended function. These processes require energy consumption and result in emissions of substances harmful to the environment. However, some raw materials and processing methods can have positive environmental effects, mitigating their negative impact. The absorption of CO2 during plant growth supports carbon neutrality by offsetting some of the released chemical compounds, while fuel production processes allow for partial energy recovery, crediting SAF with negative emissions. By systematically gathering data on each stage and then summing and adjusting all factors, results can be obtained that characterize the entire life cycle. These findings help assess the overall harm and benefits of using sustainable aviation fuels and determine which products are the most environmentally friendly and which require improvement [4,11,16,46,65].
Analyzing the entire life cycle of sustainable aviation fuels, all SAF types exhibit lower total emissions than conventional fuel (89 g CO2e/MJ) (Figure 12). Although at certain life cycle stages, SAF releases more emissions than Jet A-1, the reductions from CO2 absorption during photosynthesis and energy recovery in production lead to surprising overall results.
The lowest average emissions among all fuels are achieved by FT-SPK, amounting to just 8.97 g CO2e/MJ (Figure 12). When considering specific feedstocks used for its production (Figure 13), municipal solid waste is the most favorable option—only 5.2 g CO2e/MJ. A very similar average result, 12.35 g CO2e/MJ, is recorded for CHJ. Despite its slightly higher average emissions, CHJ produced from waste corn oil has the lowest emissions throughout its life cycle among all fuel variants and feedstocks. Both FT-SPK and CHJ have consistently achieved some of the lowest emissions at each stage, leading to similarly favorable final results.
Slightly higher but still satisfactory emissions are observed for co-processed HEFA (28.18 g CO2e/MJ), SIP (29.03 g CO2e/MJ), HEFA-SPK (36.51 g CO2e/MJ), and ATJ-SPK (41.79 g CO2e/MJ) (Figure 12). Used cooking oil (UCO) and animal tallow, both waste products, rank among the least emissive options (Figure 13). Plant-based materials such as sugarcane, sugar beet, and plant residues are positioned in the middle range between the least and most emissive resources. Oils, forestry residues (ATJ-SPK), green energy crops (ATJ-SPK), and corn grain (ATJ-SPK) contribute the highest atmospheric pollution. The highest emissions among all SAF variants are recorded for HC-HEFA-SPK, reaching 65.22 g CO2e/MJ (Figure 12). Due to insufficient data, it is difficult to determine which phase of its production pathway has the most significant impact on its final emission value.
In conclusion, sustainable aviation fuels significantly reduce emissions and environmental impact throughout their life cycle. Their performance is considerably better than conventional fuels, leaving a much smaller carbon footprint. FT-SPK and CHJ reduce emissions by approximately 90% and 86%, respectively.
CO2 emissions can be effectively reduced through the operation of a closed carbon cycle in the environment. The compounds generated during the production and utilization of SAF are balanced by the uptake of atmospheric CO2 during the growth and processing of feedstocks. In contrast, fossil fuels are not considered part of a closed carbon cycle because their combustion releases carbon that has been geologically stored for millions of years, thus increasing the net atmospheric CO2 concentration.
Furthermore, by increasing the proportion of SAF in blends with conventional Jet-A1 fuel, it is possible to significantly reduce non-carbonaceous emissions as well.
The environmental benefits resulting from the use of sustainable aviation fuels—and consequently the reduction of harmful emissions—include the following:
- Reduced contribution to global warming and a slowdown in climate change;
- Mitigation of contrail and cirrus cloud formation, which amplify the greenhouse effect;
- Improvement of air quality through the reduction of photochemical smog and atmospheric acidification;
- Decreased degradation of ecosystems, vegetation, soils, and water bodies;
- Reduced human exposure to pollutants associated with respiratory and cardiovascular diseases.
In addition, utilizing waste materials as feedstocks contributes to overall waste reduction, supports the circular economy, and alleviates pressure on the environment.
SAF already delivers substantial ecological benefits and holds potential for further development and refinement of existing methods and technologies. One promising approach is the transition to renewable energy sources, which could further reduce greenhouse gas emissions. Key phases requiring improvement include fuel combustion during use, raw material acquisition, and fuel production, as these stages contribute the most to total emissions.
8. Conclusions and Future Outlook
Sustainable aviation fuels (SAFs) represent one of the most promising solutions for decarbonizing the aviation sector. This review has shown that certified pathways such as HEFA and FT-SPK are technologically mature, while emerging options like power-to-liquid (PtL) and alcohol-to-jet (ATJ) hold significant long-term potential. The environmental benefits of SAF—particularly in terms of life cycle greenhouse gas reductions and decreased pollutant emissions—are well documented.
However, large-scale deployment remains constrained by several challenges: high production costs, limited feedstock availability, insufficient standardization of life cycle assessment (LCA) metrics, and underdeveloped infrastructure. Policy support is fragmented across regions, and major investment risks persist due to uncertain regulatory frameworks and low fuel demand elasticity.
To address these challenges, the following actionable recommendations are proposed:
For policymakers:
- Introduce long-term, stable incentive frameworks (e.g., tax credits, carbon pricing, production subsidies) to de-risk SAF investments.
- Harmonize certification criteria and LCA methodologies across jurisdictions (e.g., ICAO, EU ETS, CORSIA) to avoid regulatory fragmentation.
- Mandate increasing SAF blending quotas with clear compliance timelines and enforceable targets.
- Prioritize public investment in green hydrogen and CO2 capture infrastructure, enabling scalable PtL deployment.
For the aviation and energy industries:
- Accelerate joint ventures and consortia to develop large-scale SAF production facilities, especially in regions with renewable energy surpluses.
- Integrate SAF supply chains into airport logistics and refueling systems through standardized blending and distribution protocols.
- Diversify feedstock portfolios by investing in non-food biomass, algae, and municipal waste streams to reduce competition with other sectors.
For researchers and academia:
- Advance techno-economic models for SAF scale-up, including PtL systems powered by intermittent renewables.
- Develop standardized LCA frameworks to assess not only CO2 emissions but also non-CO2 effects and land-use impacts.
- Explore catalyst innovation and process intensification to reduce energy input and costs in SAF synthesis pathways.
As a summary of some of the information collected in the article, Table 1 was prepared.
Table 1.
A general overview of the key characteristics of the described fuels [15,20,21,30,38,45,46,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92].
| Fuel Type | Feedstock | TRL | Production Cost | LCA Emissions | Certification Status | Process Efficiency |
|---|---|---|---|---|---|---|
| HEFA-SPK | bio-oils (palm and corn oil), animal fats, waste materials [21] | 9 [74] | 1028 EUR/tonne [80] | 36.51 g CO2e/MJ | ASTM D7566 Annex A2 (blending rate up to 50%) [20] | 83% [87] |
| FT-SPK | residual or organic waste, biomass [21,30] | 6–8 [74] | 1320 EUR/tonne [80] | 8.97 g CO2e/MJ | ASTM D7566 Annex A1 (blending rate up to 50%) [20] | 20% [38] |
| ATJ-SPK | sugarcane, corn, wheat, lignocellulose [30] | 5–8 [77] | 1011 EUR/tonne [80] | 41.79 g CO2e/MJ | ASTM D7566 Annex A5 (blending rate up to 50%) [20] | 60–75% [87] |
| SIP | algae, fungi, yeast, bacteria [15] | 7–8 [77] | 3990 EUR/1000 L (minimum jet sales price) [46] | 29.03 g CO2e/MJ | ASTM D7566 Annex A3 (blending rate up to 10%) [20] | - |
| CHJ | bio-oils, animal fats, used cooking fats [21] | 5–8 [77] | 1300 EUR/1000 L (minimum jet sales price) [46] | 12.35 g CO2e/MJ | ASTM D7566 Annex A6 (blending rate up to 50%) [20] | - |
| PtL | e.g., electricity from renewable sources, water, CO2 captured from atmosphere [45] | 4–7 [77] | 2670 EUR/tonne (Co-SOEC) [80] | - | - | 24% [87,90] |
| HTL | wet biomass [74] | 5–7 [77] | 1086 EUR/tonne [81] | - | - | 38–64% [88] |
| Pyrolysis | biomass, forestry residues [75] | 4–7 [77] | 481 EUR/1000 L (at a 75 kg/h feed rate) [82] | - | - | 27–28% [87] |
| Hydrogen—SMR | natural gas [76] | 9 [76] | 1380–5520 EUR/tonne [83] | - | - | 65–75% [89] |
| Hydrogen—SMR + CCS | natural gas, CO2 captured from the atmosphere (Carbon Capture and Storage) [76] | 7–8 [76] | 1840–6440 EUR/tonne [83] | - | - | 85–90% [90] |
| Hydrogen—electrolysis | water, electricity from renewable sources [76] | 6–8 (PEM), 5 (SOEC), 9 (AEL) [78] | 4160 EUR/tonne (PEM), 3200–5200 EUR/tonne (AEL) [84] | - | - | 70–90% (PEM), 90–100% (SOEC), 60–80% (AEL) [91] |
| Hydrogen—methane pyrolysis | methane [76] | 3–5 [78] | 2600–3200 EUR/tonne [85] | - | - | 58% [92] |
| Hydrogen—APR (aqueous phase reforming) | glycerol, lignocellulose [76] | 4–5 [79] | 3550 EUR/tonne [86] | - | - | 50–70% [92] |
Overall, the transition toward widespread SAF adoption will require coordinated action between governments, industry, and academia. Only through such integrated efforts can the aviation sector meet its climate targets while maintaining economic resilience and technological competitiveness. An overview of key characteristics of the described fuels in this study is provided in Table 1.
Author Contributions
Conceptualization, R.J. and W.K.; methodology, W.K.; formal analysis, R.J.; investigation, M.S.; resources, A.Z.; data curation, W.K.; writing—original draft preparation, J.N.; writing—review and editing, R.J.; visualization, W.K.; supervision, A.Z.; project administration, M.S.; funding acquisition, R.J. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Conflicts of Interest
The authors declare no conflicts of interest.
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Figure 3.
Total emissions during the combustion process of aviation fuels, excluding CO2 (own elaboration based on [11,35,36,48,49,50,51,52,53,54]).
Figure 4.
Emissions of individual chemical compounds during the combustion process of aviation fuels, excluding CO2 (own elaboration based on [11,35,36,48,49,50,51,52,53,54,55]).
Figure 5.
Average emissions from raw material extraction for SAFs (own elaboration based on [9,23,55,65,66,67]).
Figure 6.
Emissions from the extraction of individual raw materials for SAFs (own elaboration based on [9,23,55,65,66,67]).
Figure 8.
Emissions from SAF production from individual raw materials (own elaboration based on [9,55,65,66,67]).
Figure 9.
Average emissions from raw material transport for SAFs (own elaboration based on [48,55,66,69]).
Figure 10.
Emissions from the transport of individual raw materials for SAFs (own elaboration based on [48,55,66,69]).
Figure 11.
Average emissions from SAF transport (own elaboration based on [55]).
Figure 11.
Average emissions from SAF transport (own elaboration based on [55]).
Figure 12.
Average LCA emissions of SAFs (own elaboration based on [1,11,33,46,52,55,66,69,71,72,73]).
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Klimczyk, W.; Jasiński, R.; Niklas, J.; Siedlecki, M.; Ziółkowski, A. Sustainable Aviation Fuels: A Comprehensive Review of Production Pathways, Environmental Impacts, Lifecycle Assessment, and Certification Frameworks. Energies 2025, 18, 3705. https://doi.org/10.3390/en18143705
AMA Style
Klimczyk W, Jasiński R, Niklas J, Siedlecki M, Ziółkowski A. Sustainable Aviation Fuels: A Comprehensive Review of Production Pathways, Environmental Impacts, Lifecycle Assessment, and Certification Frameworks. Energies. 2025; 18(14):3705. https://doi.org/10.3390/en18143705
Chicago/Turabian StyleKlimczyk, Weronika, Remigiusz Jasiński, Jakub Niklas, Maciej Siedlecki, and Andrzej Ziółkowski. 2025. "Sustainable Aviation Fuels: A Comprehensive Review of Production Pathways, Environmental Impacts, Lifecycle Assessment, and Certification Frameworks" Energies 18, no. 14: 3705. https://doi.org/10.3390/en18143705
APA StyleKlimczyk, W., Jasiński, R., Niklas, J., Siedlecki, M., & Ziółkowski, A. (2025). Sustainable Aviation Fuels: A Comprehensive Review of Production Pathways, Environmental Impacts, Lifecycle Assessment, and Certification Frameworks. Energies, 18(14), 3705. https://doi.org/10.3390/en18143705
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