The aviation industry has been a major contributor to greenhouse gas emissions, and there is an increased drive to develop and deploy aviation biofuels (biojet) as a sustainable alternative. In particular, there is a growing trend to adopt the use of cheap low-quality biomass feedstocks and wastes, which could reduce the overall cost of production and environmental impact. Sustainable Aviation Fuels (SAFs) are a promising solution to reduce greenhouse gas emissions in the aviation industry. To ensure their successful integration, SAFs must meet specific fuel property specifications. These properties include energy content for efficient aircraft performance, density for optimal fuel load and payload capacity, viscosity to ensure smooth engine operation, flash point for safe handling, freeze point for cold climate operations, low sulphur content to minimise emissions, and specific distillation characteristics for compatibility with aircraft engines ^[1][2][1,2]. Striking a balance between these properties is essential to produce SAFs that not only adhere to safety and operational standards but also contribute to a greener and more sustainable future for aviation. The last few years have witnessed significant developments in this field, in terms of SAF production feedstocks, pathways, capacity, and quality.

Biojet fuels have been gaining traction in the aviation industry as a more sustainable alternative to traditional jet fuel for over a decade now. According to a report by the International Air Transport Association (IATA), in 2022, the production capacity of SAFs including biojet fuels, surpassed 300 million litres (79.2 million gallons) globally [1]. This represents an increase of 300% compared to the previous year, despite the challenges posed by the COVID-19 pandemic. Several countries, including the United States, Canada, Brazil, and France, have established targets for the use of biojet fuels in aviation in the coming years. For example, in the United States, the Federal Aviation Administration (FAA) has set a goal of achieving three billion gallons of SAF production by 2030 [2]. Additionally, the European Union has proposed a plan to increase the use of SAFs in aviation to 6% by 2030 [3].

Furthermore, several airlines have also started using biojet fuels on selected flights. For instance, in 2018, United Airlines began using a blend of biojet fuel and traditional jet fuel on flights between Los Angeles and San Francisco and has taken it further with a national flight filled with passengers entirely on SAFs [4]. In 2021, Delta Air Lines announced a partnership with Airbus and Air BP to develop and test sustainable aviation fuel on flights from North America to Europe ^[4][5][6][4,5,6]. Indeed, since the take-off of the first aircraft using sustainable fuel in 2008, over 490,000 flights have been powered by biojet to date [1]. This drive is promoted by the potential of sustainable aviation fuels to reduce emissions by 80% over the life cycle of aircrafts when compared with conventional jet fuel [1]. In addition to these, Virgin Atlantic has announced the take-off of the first net zero, 100% SAF transatlantic flight, to happen in November 2023 [7].

However, the main challenge lies in selecting an efficient process for the production of aviation biofuels that uses low-quality feedstocks while meeting the target capacity in billions of litres per year. This is because, despite these advances, biojet fuel production still represents a small fraction of the aviation fuel market. According to the IATA, in 2022, biojet fuels accounted for only 0.1% of total aviation fuel consumption [8]. However, the increasing demand for SAFs, driven by environmental concerns and regulatory pressures, is expected to drive further growth in this market in the coming years [3]. In fact, production capacity and usage are steadily increasing due to the global push towards sustainability and the need to reduce greenhouse gas emissions. These pressures mean that the aviation biofuels industry has significant potential for growth in the near future and several emerging technologies are likely to have a significant impact on biojet fuel production in the coming years. These technologies are based on new feedstocks, such as algae and cellulosic materials, which could increase the availability of sustainable biofuels. Additionally, advances in biotechnology, such as synthetic biology and genetic engineering, could enable the creation of more efficient and cost-effective production of biojet fuels using biochemical, thermochemical, and hybrid processes. Each process has its advantages and disadvantages and selecting the most efficient process for a given feedstock and product specification requires careful considerations.

2. Biojet Fuel Technologies

Jet A1 is the conventional jet fuel, which has become the benchmark for SAF in terms of molecular compositions and overall fuel properties [9]. Typically, Jet AI consists of C₈–C₁₆ hydrocarbons, with approximate compositions of 26.8% n-paraffins, 39.7% iso-paraffins, 20.1% cycloparaffins, and 13.4% aromatics ^[9][10][9,10]. Of these, iso-alkanes (for specific energy, good thermal stability, and low freezing points) and cycloalkanes (for density and seal-swelling capacity requirements) are most desirable to provide the specific energy and energy density for thermal stability, low particular emissions, increased range, higher payload capacity, or fuel savings [10]. While aromatics are relatively less energy dense than the various alkane components of jet fuel, they are currently required to ensure proper swelling of airplanes’ nitrile seals, which is essential to minimise fuel leakage [11]. Crucial to fuel properties of standard are the H/C ratio of 2, lower heating value (LHV) of 43.2 MJ/kg, and its oxygen content of zero ^[9][10][11][9,10,11]. These molecular functionalities of aviation fuels such as Jet A are of paramount importance as they directly influence the fuels’ performance, safety, and environmental impact. The compatibility of the fuels’ molecular structure with aircraft engines and infrastructure is crucial to ensure smooth combustion, efficient energy release, and safe aircraft operations. Additionally, adherence to aviation fuel specifications, guided by molecular functionalities, is essential to meet stringent industry standards, ensuring uniformity and reliability across the aviation sector. The energy content, combustion characteristics, volatility, flash point, freezing point, and emissions profile of aviation fuels are all intricately linked to the molecular compositions of the fuel ^[9][11][9,11]. Therefore, an important research area is the production and optimisation of the molecular compositions of SAF to be similar to those of conventional fuels, allowing for enhanced fuel efficiency, reduced emissions, and a lower carbon footprint compared to conventional fossil-based aviation fuels. Hence, a deep understanding and consideration of molecular functionalities in synthetic candidates of SAF play vital role in their eventual approval for use in aircrafts to advance sustainable aviation practices, improve aircraft performance, and reduce environmental impacts [10]. The ASTM D7566 [9], which contains the stringent specifications required for SAF, had its first pathway approved in 2009. This is the standard specification for biojet fuel containing hydrocarbons derived from biomass, wastes, and hydroprocessed plant oils and animal fats. The sustainable aviation fuel used today contains a maximum of 50% blend from biofuel mixed with fossil jet fuel for use in conventional engines [12]. A number of collaborations are ongoing between airline companies that are keen on the deployment of biojet fuels in their fleets and fuel production companies as shown in Table 1 ^{[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]}[12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Biojet fuel production technologies have gained significant attention in recent years with the main goal of reducing carbon emissions and increasing the sustainability of aviation operations. Biojet fuels, made from renewable sources such as plant oils, agricultural waste, and algae, are one promising option [28]. A study published in the journal Environmental Science and Technology in 2020 found that the use of biojet fuel produced from forestry residues and waste cooking oil could reduce life cycle greenhouse gas emissions by up to 165% [29]. There have been several successful biojet fuel flights in recent years ^[1][30][1,30]. The advancement of biojet fuel as a sustainable alternative to petroleum jet fuel has been bolstered by incorporating life cycle analysis (LCA) studies. LCA provides a comprehensive assessment of the environmental impact of alternative fuels production, encompassing all stages from feedstock cultivation to fuel distribution. By quantifying greenhouse gas emissions, energy consumption, land, and water use, LCA offers a data-driven comparison of Biojet’s lower carbon footprint compared to petroleum jet fuel. These analyses reveal the potential environmental benefits of biojet and SAFs, highlighting their role in mitigating greenhouse gas emissions and reducing overall environmental impact. Moreover, LCA can shed light on the social and economic implications of biojet fuel and SAF production, such as job creation and economic development [22]. Indeed, the definition of SAF stemmed from its three life cycle benefits, including the requirements to: “(i) achieve net reduction in greenhouse gases (GHG) emissions; (ii) respect the areas of high importance for biodiversity, conservation and benefits for people from ecosystems, in accordance with international and national regulations; and (iii) contribute to local social and economic development, and competition with food and water should be avoided” [31]. Many companies and research institutions are actively working on developing biojet fuel technologies. For example, Boeing and the Air Force Research Laboratory have partnered to develop a plant-based biojet fuel made from Camelina, a type of oilseed. Researchers at the University of Bristol in the UK are working on a process to produce biojet fuel from seawater-tolerant plants [32]. Despite the promising progress in biojet fuel technologies, there are still some challenges that need to be addressed. One of the biggest challenges is the cost of production, which is currently higher than that of traditional jet fuel. However, with continued research and development, it is hoped that the cost of production will decrease over time. There is a growing consensus that biojet fuels are a promising option for reducing carbon emissions in the aviation industry. The progress made in this field has been encouraging, with successful biojet fuel flights and ongoing research and development. However, there is still work to be done to address the challenges facing this technology, such as cost of production. Several biojet fuel production pathways have been developed and approved by the ASTM, each with its own advantages and disadvantages. The nine ASTM-approved biojet fuel production pathways are presented in Table 1, and they offer different methods for converting biomass and biomass-derived feedstocks into aviation fuel. The Fischer–Tropsch process (FT), also known as Gas-to-Jet, can convert various biomass feedstocks to synthetic gas (syngas) that can be catalytically reformed to hydrocarbons including biojet fuel. The Fischer–Tropsch Synthetic Paraffinic Kerosene with Aromatics (FT-SPK/A) is similar to the FT process but the pathway produces sustainable aviation fuel containing aromatics. Hydroprocessed Esters and Fatty Acids (HEFA) and Catalytic Hydrothermolysis (CH) are similar to traditional petroleum refining and can use various oils and fats (lipids) as feedstocks. Hydroprocessed Hydrocarbons Hydroprocessed Esters and Fatty Acids (HH-SPK or HC-HEFA) is similar to HEFA but uses algae as feedstock. Synthesised Iso-Paraffins (SIP) (formerly known as Direct Sugar to Hydrocarbon (DSHC)) is a pathway that directly converts sugars into hydrocarbon molecules, while Alcohol to Jet (ATJ), HC-HEFA, CH, and Gas-to-Jet have the potential to use non-food biomass feedstocks such as algae and aquatic plants. The choice of pathway will depend on factors such as feedstock type and availability, number of processing steps, conversion efficiencies, and sustainability credentials including economic viability, environmental impact, and life cycle analysis. FT, HEFA, and CH pathways exclusively rely on thermochemical processes, whereas SIP and ATJ employ a combination of biological or enzymatic conversion of feedstocks into platform molecules (alcohols or farnesene) before undergoing subsequent thermochemical conversion steps. Another sustainable aviation fuel production pathway that has also been broadly recognised but is yet to be ASTM approved is Power-to-Liquids (PtL). This pathway is part of the power-to-X (PtX) group of technologies that use green hydrogen from the electrolysis of water to produce SAFs from carbon dioxide [33]. However, the pathway only becomes truly bio-based if it involves biogenic CO₂. In general, this pathway is forecast to contribute 1.4 billion litres of sustainable aviation fuel by 2040 [33]. Table 2 shows the main commercially relevant biojet fuel technologies, their owners/operators, their capacities, and current statuses.