2.1 Synthetic Paraffinic Kerosene Produced from Hydroprocessed Esters and Fatty Acids (HEFA-SPK)

The HEFA route is the most mature alternative fuel pathway (currently at TRL 8) and it is certified by ASTM International as HEFA-SPK (14). HEFA is produced through hydroprocessing of vegetable oils and animal fats. Hydrogen is used to convert unsaturated compounds such as alkenes and aromatics into paraffins and cycloalkanes, which are more stable and less reactive. The process is the same as for hydrotreated vegetable oil (HVO) production but includes an additional isomerisation step that lowers the fuel freezing point. The energy conversion efficiency of oils and fats into HEFA-SPK (and other byproducts) is about 76%, the highest efficiency of bio-jet fuel routes (17). The conversion energy efficiency is calculated as the ratio of the total energy input (feedstock, electricity, natural gas and hydrogen) to the total energy content of the liquid products (in general jet, diesel, gasoline, heavy fuel oil and naphtha). Gaseous products (for example, methane) are excluded from the denominator.

Because of its maturity and simplicity compared to other routes, HEFA is the only alternative fuel in commercial use. Depending on the plant size and deployment stage, the production cost of HVO ranges between €1100 and €1350 per tonne. Upgrading to HEFA incurs a relatively small additional cost, associated with the isomerisation step. The main limitation of this route is feedstock availability. UCO and tallow represent a relatively small resource globally, and the supply of virgin vegetable oil is constrained by land availability and sustainability concerns. Novel crops are being investigated in terms of potential and sustainability, such as camelina, carinata and oil-bearing algae. Fermentation of sugars to lipids is also being considered to produce feedstock for HEFA plants (see later subsection).

2.2 Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK)

The AtJ process turns alcohols into jet fuel through the following reactions: dehydration, oligomerisation, hydrogenation, isomerisation and distillation. The alcohol used can be produced through conventional processes involving fermentation of sugar or starch crops such as sugarcane, corn and wheat, or through advanced routes from lignocellulosic feedstocks, such as woody and grassy feedstocks and wastes. Currently, most developers are focused on upgrading conventional alcohols, but there are larger demonstration plants planned using advanced routes to alcohols that may be operational by 2020. AtJ-SPK blends up to 50% v/v are certified by ASTM International since 2016, though the technology is currently at TRL 6–7 (14). Certain AtJ routes, depending on the catalytic process used, produce a jet fuel containing aromatics, and efforts are underway for certification of 100% use of jet fuel derived from these routes.

AtJ routes are attractive as they can convert various types of alcohols (such as ethanol, methanol and isobutanol) from a wide range of sources into jet fuel as well as other hydrocarbons. Additionally, the AtJ route offers logistical flexibility because the alcohol catalysis plant does not need to be co-located with alcohol production, and alcohols can be conveniently transported and stored. The main weaknesses of this pathway may be the selectivity of jet fuel production. An issue to consider in relation to this route is the opportunity cost of using the alcohols directly in transport applications (for example road and marine) as opposed to converting them to jet fuel, at the cost of additional capital expenditure and some efficiency loss. Jet fuel costs produced via this route could be 20–40% higher than the ethanol feedstock on an energy basis, with the lower end of the range being for high ethanol input prices and the higher end of the range for lower ethanol input price.

2.3 Synthesised Isoparaffins Produced from Hydroprocessed Fermented Sugars (HFS-SIP)

Genetically modified microorganisms can be used to convert sugar into hydrocarbons or lipids. These routes are known as direct sugars to hydrocarbons (DSHC) routes, and there are three main routes under development whose products can be further processed into jet fuel: heterotrophic algae or yeast converting sugars into lipids within their cells; genetically modified yeasts which consume sugars and excrete long-chain liquid alkenes (such as farnesene); genetically modified bacteria consuming sugars and excreting short-chain gaseous alkenes (such as isobutene). Currently biological routes almost exclusively use conventional sugar feedstocks, although pilot projects are testing cellulosic sugars. DSHC routes using conventional sugar feedstocks are at TRL 7–8, while the same processes based on cellulosic feedstocks are at TRL 5. A specific route based on the production of farnesane from sugar is certified as hydroprocessing of fermented sugars (synthetic iso-paraffinic fuels (HFS-SIP)) and can be blended with fossil kerosene up to a maximum of 10% (14).

However, at present, potential DSHC developers are targeting the chemical, pharmaceutical, food and feed markets, which are generally higher value than bulk transport fuels. This in turn helps to prove the technology and reach the scale and lower production costs that may be required for fuels. The complexity and low efficiency of converting lignocellulosic sugars into fuels through DSHC translates into high feedstock cost and high energy consumption, which makes DSHC the most expensive alternative fuel route. HFS-SIP costs have been projected to remain high at above €4000 per tonne (19).

2.4 Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK)

The gasification with Fischer-Tropsch (Gas+FT) synthesis process transforms lignocellulosic biomass or solid waste into fuels, such as naphtha, gasoline, diesel and jet fuel, as well as other valuable coproducts. The process consists of the following key steps: feedstock pretreatment (sorting, sizing and drying), gasification, syngas clean-up and conditioning, FT catalysis, distillation and hydrocracking. And may involve additional steps such as isomerisation and catalytic reforming depending on the type of fuel produced. The jet fuel produced through the Gas+FT route is certified as FT-SPK and can be blended with fossil kerosene up to 50% (14). While a commercially mature route exists for coal and natural gas-to-liquid routes, the bio-based route is only now approaching TRL 7–8.

While the individual components of a biomass gasification to FT fuel route are commercially demonstrated in different applications such as biomass gasification to heat and power applications and coal-to-liquid plants, the integrated application of biomass gasification to FT fuel has yet to be demonstrated at scale. Challenges faced by this route are the economic viability of scaling down processes to scales suitable for biomass and waste-based systems, the design of processes and catalysts better suited to relatively small scale systems, including catalyst selectivity, the design of systems that can cope with biomass and syngas heterogeneity and the overall efficiency of integrated systems (20). An option for this route could be to produce FT waxes that could then be co-processed at oil refineries.

2.5 Pyrolysis and Upgrading

Pyrolysis transforms lignocellulosic biomass or solid waste into an intermediate bio-crude oil, which can then be upgraded to fuels. The fast pyrolysis to bio-crude oil process is at TRL 8, with several first commercial facilities selling the pyrolysis oil for heating applications. However, refinery upgrading of pyrolysis oils to a finished fuel product is only at the early demonstration stage (TRL 6), with batch production in limited trial runs. The dedicated upgrading of pyrolysis oil via hydro-deoxygenation (HDO) is currently at TRL 3–4, with pilot activities such as the Horizon 2020 4REFINERY project (21). Therefore, the overall route from pyrolysis to jet fuel is at most at TRL 6. KiOR, USA, had embarked on the ASTM International certification process for bio-kerosene from fast pyrolysis but the company filed bankruptcy (22). By 2019, the catalytic pyrolysis process (IH²), developed by Shell, the Netherlands, was in Testing Phase 1 of the ASTM International’s ASTM D4054-19 qualification procedure (14).

A range of pyrolysis-type technologies are possible that can process a wide range of feedstocks (even low-quality wet feedstocks). Bio-crude oil could be transported to centralised dedicated or fossil refinery facilities for upgrading to fuels. The challenges with crude pyrolysis oil are its high water, acidity and oxygen content, as well as viscosity and chemical instability, though the quality of the oil is heavily dependent on the pyrolysis process (20). Transport of pyrolysis oil may require some pre-processing and specialist infrastructure. To date there is no commercial process for upgrading pyrolysis oil to finished fuel in dedicated plants. However, research into materials and catalysts for such systems is ongoing (23).

2.9 Demand and Supply Scenarios for Drop-in Kerosene Fuels

Today global use of aviation fuel for commercial international and domestic aviation is around 280 million tonne year^–1 (33), however less than 0.1% of this is currently alternative or low-carbon fuel (34).

The current global capacity for HEFA production from dedicated hydroprocessing and co-processing in refineries is around 5 million tonne year^–1 (35). With incentives for the use of alternative fuel in the road transport sector substantially stronger than in the aviation sector, the majority of the output from hydroprocessing plants today goes to substituting diesel in the road transport sector, as opposed to producing HEFA for aviation. Therefore, in 2018 less than 0.1 million tonne of HEFA aviation biofuel was actually produced (34). Nevertheless, hydroprocessing outputs require relatively minor treatment to produce aviation HEFA, meaning that HEFA production could scale-up fairly rapidly if policy were to make the use of alternative fuels in the aviation sector competitive with their use in the road transport sector.

Production capacity of sustainable aviation fuel (SAF) from all other routes is substantially lower (less than 0.1 million tonne in total), but plants are planned or being built that will progress the commercialisation of these routes (shown in Figure 3).

For example, Fulcrum BioEnergy, USA, is building a 31,000 tonne year^–1 jet fuel plant based on gasification of municipal solid waste and FT synthesis (36); Lanzatech, USA, in collaboration with Virgin Atlantic are planning an AtJ plant in the UK (37); and Velocys, UK, in collaboration with British Airways, UK and Shell have provided funding to support development of a plant based on gasification of municipal waste and FT synthesis also in the UK (38).

As HEFA is currently the only SAF production technology at commercial scale, it is likely to dominate global SAF production capacity over the next decade or so. However, production of HEFA relies on the use of oils and fats as feedstock, and concerns around the sustainability of oil crops means that HEFA production is likely to be increasingly limited to the use of waste fats and oils unless other sustainable sources of oils are developed. Estimations from a number of sources suggest that around 20 million tonne year^–1 of UCO and tallow could be collected globally (total arisings will be higher, but not all can be collected and used). E4tech Ltd, UK, carried out analysis based on Ecofys Ltd, UK, 2014 (39) and World Bank, USA, data on population (40). Even assuming that virgin vegetable oil currently used for fatty acid methyl ester (FAME) production (24.4 million tonne in 2017) was diverted into HVO or HEFA production instead, the total available feedstock would still be fairly limited compared to aviation fuel consumption.

Therefore, in the longer-term SAFs are likely to be produced from a range of lignocellulosic waste biomass sources, lignocellulosic or oil crops with a low risk of causing direct and indirect negative environmental and social impacts and renewable electricity (Figure 4).

However, the technologies to process lignocellulosic feedstocks into SAF are still at an early stage of development and commercialisation. Ramping-up from the demonstration-scale or first-of-a-kind commercial FT and AtJ plants, currently planned or under construction, to the construction of multiple commercial-scale plants will happen over a period of at least 10 years. Other biofuel routes and PtL routes are likely to take longer to achieve multiple commercial scale plant output, as they are at earlier stages of development and demonstration, there are fewer companies currently developing them and production costs are high.

Despite the current low production volumes, the opportunity for SAF production is large, and the imperative is strong if decarbonisation targets are to be met. The International Energy Agency (IEA, France) 2°C scenario (2DS) anticipated that even with substantial improvements in aviation efficiency and modal switching to high-speed rail for some journeys, there would still be a requirement for around 150 million tonne year^–1 of SAF in 2060 from international aviation alone (45). With the introduction of the CORSIA mechanism over the next decade, and an increasing number of governments considering the introduction of SAF blend mandates or other policy measures to promote the uptake of SAF, growth is likely to accelerate over the coming years.

3.1 Hydrogen Turbofan

In 2000, the European Commission commissioned a study to Airbus called ‘Cryoplane’ (48), one of the objectives being to explore the conceptual design of an aircraft equipped with hydrogen-fuelled turbo-engines and cryogenic tanks to store LH₂. The study found that energy consumption increases by 10% compared to a reference kerosene aircraft, due to the additional weight of the hydrogen tanks (48). More recent studies (49, 50) argue that the Cryoplane project adopted a ‘minimal change’ approach to wing planform and engine design for the hydrogen aircraft. They show that when airframe and engine design are optimised for a hydrogen-fuelled aircraft then an energy saving up to 12% is achievable on long-haul aircraft compared to a kerosene benchmark. However, short-haul flights are penalised in terms of energy consumption when switching to hydrogen.

Modifications to the turbo-engine are required when using hydrogen due to a different composition of combustion gases and variations between the properties of hydrogen and kerosene (for example calorific value and volumetric density). Modifications affect several engine parts, such as burners, fuel ducts, cooling system and turbine blades (47). Adoption of hydrogen as an aviation fuel will also require redesign of the fuel supply chain, including on-the-ground storage and refuelling.

3.2 Hydrogen Fuel Cell Aircraft

Hydrogen can also be used in fuel cells (FCs), and both the proton exchange membrane fuel cell (PEMFC) and solid oxide fuel cell (SOFC) are options being considered for use in aviation. Hydrogen FCs convert chemical energy into electrical energy that could power on-board electrical equipment, or an electric propulsion system.

FCs could be used on-board in parallel to or in place of auxiliary power units (APUs). Traditional APUs consist of a small gas turbine supplying power for electrical and pneumatic loads when the aircraft is stationary as well as back-up power while cruising. FCs could see a gradual integration in aircraft APUs through powering systems currently powered by batteries, such as emergency door systems (47). A report by The Boeing Company suggested hydrogen SOFC-powered APUs for all non-propulsion loads in the aircraft would reduce fuel consumption for on-board energy by 40% during cruising compared with traditional APUs (47). However, it is important to bear in mind that auxiliary units account for a small portion of the total energy consumption of an aircraft.

There have been several projects to develop hydrogen FC aircraft, focusing on small low-speed aircraft. The HyFlyer project, led by ZeroAvia, USA, aimed to decarbonise medium range, six-seater aircraft by replacing the conventional propeller powertrain with a compressed (5000 psi) hydrogen PEMFC system (51). ZeroAvia flight tested its prototype powertrain, using a Piper PA-46 Light Sport Aircraft (Piper Aircraft, USA) (52, 53). National Aeronautics and Space Administration (NASA), USA, funded a project by the Center for Cryogenic High-Efficiency Electrical Technologies for Aircraft (CHEETA) to develop an aircraft that uses a LH₂ PEMFC system to power fully electric fans. One of the aims of the project was to demonstrate the potential of cryogenic hydrogen for larger aircraft (54). A research consortium led by The German Aerospace Center (DLR) developed HY4, a four-seater hydrogen FC aircraft (55), which completed its first flight in 2016 (56). The powertrain consists of a PEMFC coupled to a single 80 kW electric motor and supported by a battery. About 10 kg of hydrogen is stored in gaseous form in a tank at 437 bar. HY4 has a maximum weight of 1.5 tonne and can fly at 145 km h^–1 for about 1000 km.

4.1 Hybrid Electric Aircraft

In hybrid-electric systems, where an electric motor and a turbofan are configured in series or parallel, an electric battery can supply power to optimise overall flight energy consumption and emissions. The electric motor runs together with the turbofan when high thrust is needed, or alone when low thrust is needed such as during cruising. This mechanism enables downsizing of turboengines and increased fuel economy (58).

Large industry players have worked on demonstrating the hybrid-electric architecture for future application in the large commercial aircraft segment. In 2017, Airbus, Siemens AG, Germany, and Rolls-Royce, UK, established a collaboration to develop the E-Fan X, a hybrid-electric aircraft demonstrator (59). They planned to replace one of the four jet engines in a BAE 146/RJ100 airliner with a 2 MW electric motor powered by a Rolls-Royce AE2100 gas turbine power-generation system and a lithium-ion battery pack (60, 61). Boeing and NASA partnered in a study called Subsonic Ultra Green Aircraft Research (SUGAR), to develop a hybrid-electric aircraft named ‘Volt’ (62) equipped with twin-engines. The engines were designed to burn fuel when the power requirement is high (such as during take-off), and to use electricity to supplement or replace power from the turbo engines while cruising. The EU Horizon 2020 Modular Approach to Hybrid Electric Propulsion Architecture (MAHEPA) project was set up as a collaboration between small and medium-sized enterprises (SMEs) and academic parties including Pipistrel Vertical Solutions, Slovenia, DLR and Delft University of Technology (TU Delft), The Netherlands. The team worked on developing two four-seater aircraft with the objective, among others, of collecting real-world data on hybrid-electric flights. The configuration of the first prototype being built by MAHEPA is a series hybrid-electric powertrain based on a reciprocating internal combustion engine connected to the propeller (63). The second prototype is a four-seater aircraft based on a FC hybrid powertrain.

4.2 Full-Electric Aircraft

Full-electric propulsion (battery as the only energy storage) could lead to zero onboard emissions and very high levels of energy efficiency and noise reduction. For these reasons policymakers are starting to show interest in electric planes. Norway, for example, has announced that all of its short-haul flights will be electric by 2040 (64).

At the time of writing, there were more than 150 electric aircraft development programmes around the world, although the majority of them focused on the urban air taxi, also known as passenger drone, and general aviation (defined as civil non-commercial aviation, i.e. small aircraft for private transport and recreational activities) (57). The general aviation segment is seen as a ‘test bench’ for further development. With lighter weight and short range, the technical requirements of the general aviation segment are more suited currently to a higher degree of electrification.

One of the innovations, enabled by full-electric propulsion, which is expected to deliver the benefits of full electrification is ‘distributed electric propulsion’. This propulsion strategy is based on the optimal placing of multiple electrically driven propellers across the aircraft wetted surface. An example of distributed propulsion is the Lilium Jet (Lilium GmbH, Germany): a full-electric five-seater aircraft, with 36 fans distributed to enable vertical take-off and landing (VTOL). With a range of 300 km, the Lilium Jet was designed for intracity and regional commuting. In 2019, Lilium GmbH announced the aim of launching its air taxi service in several cities by 2025 (65, 66).

Several initiatives, involving tech and aerospace actors, have been set up to develop novel aircraft designs using full-electric powertrains aimed at the air taxi market. For example, Kitty Hawk, USA, backed by Google, USA, worked with Boeing to develop a two-seater with a 100 km range using 12 lifting rotors, which was expected to be used by Air New Zealand for air taxi (67). Uber Technologies Inc, USA, the ride-hailing app company, has been linked with at least five aircraft manufacturers developing VTOL technology (68). One of these manufacturers is Aurora Flight Sciences, USA, a subsidiary of the aerospace major Boeing. Airbus also began an air taxi project called Vahana (69).

Another player, Eviation Aircraft Ltd, Israel, has produced a full-electric prototype (Alice) designed to take up to nine passengers, with a range of 650 miles, and capable of flying at 240 knots at 10,000 feet. It utilises Honeywell’s fly-by-wire avionics, three electric motors producing around 900 kW of power, and Li-ion batteries supplying 900 kWh of energy, with a recharge ratio of 2:1, meaning 30 min of charging are needed for every hour in the air (70).

Despite very promising benefits, full-electric propulsion is confronted with a fundamental limitation with regard to energy storage in the form of battery energy density. Current state of the art Li-ion battery has an energy density of 0.9 MJ kg^–1, which theoretically could go up to 1.4 MJ kg^–1, but this is still an order of magnitude smaller than jet fuel’s 43 MJ kg^–1. One promising novel battery chemistry, Li-O₂ is claimed to have a theoretical gravimetric density of 12 MJ kg^–1, still far short of kerosene (71). A further limitation is posed by the power-to-weight ratio of electric propulsion systems which has been historically lower than turbofans, though significant advances have been made in motor power density (72).

Electrification of aviation requires significant developments in battery energy and power density, as well as in other areas as airframe design, motor design, power electronics, cooling, heat recovery and power systems integration. Issues such as battery safety, charging and power infrastructure also need consideration for an increased electrification of aviation.