Air transport plays a crucial role in modern life. It facilitates global transport, international travel and is critical to the workings of the world. However, the industry is heavily reliant on fossil fuels to operate. It is essential to consider alternative fuel options in which aviation can survive without finite fossil fuels. This poses a challenge as few fuels match up to the energy density of kerosene that are also capable of being integrated into a fuel system aboard an aircraft. The best and most likely candidates for fuel would be liquid hydrogen and biofuels. Following assessment of both possibilities in this paper, the conclusion reached is that biofuels are the more probable candidate for the near future as they would require the least change to existing planes and systems and thus require the least investment.
The aviation industry is currently completely reliant on non-renewable energy – kerosene- as jet fuel. However, as fossil fuel reserves are exhausted, crude oil prices, and subsequently jet fuel prices, are expected to rise in the next couple of decades. It is therefore crucial to look into sustainable and renewable energy sources and alternatives to power the aviation industry.
An issue, however, lies with finding alternative fuel sources to match the energy density provided by fossil fuels. Large, long-range passenger planes such as the Boeing 747, consume huge amounts of energy; provision of this energy is a top priority when searching for suitable renewable fuel alternatives.
A simple back-of-the-envelope calculation rules out solar flight as a plausible option for the future: with current solar power technology, the energy provided wouldn’t even make up 1% of the energy required to fly a Boeing 747 . 
Battery powered flight for large commercial planes can also be ruled out relatively quickly as a viable option for the future. Although there have been developments in battery powered flight, such as the German-based “Lilium Jet” (a fully electric 7-seater aircraft- see Fig A) , these developments are limited to small aircraft as current battery technology is not capable of providing nearly enough energy to power a large passenger plane. Present advanced battery technology such as the Lithium Ion battery can provide an energy density of 0.9 MJ/kg.  Even if this value were to become ten times in size (as it is projected that present battery technology will improve significantly in the future), it would still be orders of magnitude smaller than the energy density provided by kerosene (around 43- 48 MJ/kg).
Figure A: German based Lilium Jet 
Nuclear energy is capable of providing extremely high energy and power densities, however attempting to integrate a nuclear reactor with the infrastructure of a plane would prove an extreme challenge and thus is it very improbable that this type of technology would be attempted.  Additionally, nuclear energy comes from a finite source and thus if nuclear fuel deposits began being used at the rates at which we use fossil fuels, we would run into the same problems as we are with fossil fuels- developing heavy reliance on non-renewable energy.
The only plausible options left for the replacement of jet fuel are planes powered by liquid hydrogen or biofuels. These two options will be assessed in the remainder of this paper and a judgement made on which of the two are most likely to replace jet fuel in the near future.
2. Liquid Hydrogen
2.1. Fuel Properties of Liquid Hydrogen [LH2], and consequent impacts on design considerations
Aviation’s requirement for high energy density fuels makes Liquid Hydrogen an attractive option when considering alternatives to kerosene. Liquid Hydrogen has a high gravimetric energy density, in fact much higher than kerosene. Gravimetric energy (or specific energy) is a measure of energy per unit mass. The specific energy of kerosene is estimated at around 43 to 48 MJ/kg.  On the other hand, the specific energy of LH2 is estimated at around 120 to 142 MJ/kg.  Thus, liquid hydrogen stores approximately 2.9 times more energy than kerosene with respect to weight.
However, with respect to volume, the energy density of kerosene is estimated at around 35 MJ/L , a value approximately 4 times larger than liquid hydrogen which ranges from 8.5 to 10 MJ/L. 
Thus the advantage liquid hydrogen provides in having a greater gravimetric energy density than kerosene is counteracted by its far smaller energy density with respect to volume as it is expected to require a complex fuel system large enough and sophisticated enough to store liquid hydrogen. 
Figure B: A comparison of LH2 and traditional jet fuel kerosene with respect to weight and volume
Furthermore, liquid hydrogen is cryogenic meaning it is only in liquid state at extremely low temperatures due to it having a boiling point of -252.8 °C.  This poses a great technical challenge as the fuel tanks will have to be insulated extremely well from all and any possible sources of heat in order to keep the fuel in liquid state. Rises in temperature are also a concern as when liquid hydrogen changes state from liquid to gas it occupies up to 850 times more volume.  This large factor of expansion could have dangerous implications as the fuel tank could become over pressurized and so to prevent the container from exploding effective venting systems must be implemented.
The extremely low storage temperature of liquid hydrogen also causes a technical challenge in terms of finding suitable materials for the fuel tanks. Metals generally become quite brittle at low temperatures. The graph (see Fig. C) below shows how the critical stress intensity factor – a function of maximum applied stress before ultimate failure – changes with respect to temperature for steel. It can be seen that as the temperature decreases, the critical stress intensity factor decreases significantly, and the steel tends towards brittle fracture (the rapid breaking of a material under stress). This observation of behaviour in steel is common in metals. Thus, in order to accommodate liquid hydrogen fuel tanks, the typical metal alloy materials often used in aircraft may no longer be suitable.
Figure. C: Graph showing the change of critical stress intensity factor with respect to temperature of steel 
Accordingly, it can be seen how the use of liquid hydrogen as aviation fuel will require major changes to typical commercial aircraft infrastructure. Designs for a liquid hydrogen-powered plane will need to account for the smaller energy to volume ratio through more space to store fuel. The cryogenic nature of the fuel will also require considerations to be made with regards to the insulation and pressure of tanks.
2.2. Aircraft Design Propositions
The space required to store liquid hydrogen results in unusual designs for hydrogen-fuelled aircraft. Most designs proposed see fuel tanks integrated as part of the fuselage (the main body) of the plane as opposed to the wings (the majority of modern commercial planes store fuel in the wings). For large passenger planes if liquid hydrogen were simply to be stored in the wings in a traditional manner, the wings would have to be bigger in order to accommodate the space required. In doing so a very large part of the weight of the plane would be consumed by the fuel which would be unsuitable. 
2.2.1. Integral tank configurations (integrated with the fuselage)
Integral tank configurations refer to configurations where the tank is integrated with the fuselage. Integral tank configurations provide advantages in potentially increasing the structural integrity of the fuselage because the elimination of wing tanks decreases shear stress as well as associated bending moment forces.  Some proposed integral tank configurations are shown below in Figure D:
Figure D: Showing three designs for an integrated tank configuration A,B and C 
Design A shown in Fig. D with a single tank at the rear of the fuselage lends itself to the lightest tank structure however poses challenges in terms of the aircraft centre of gravity management. 
Design B (see Fig. D) with two tanks in front and behind the passenger cabin provide a second option which removes the issue of centre of gravity management. However, this would pose issues in terms of access between the cockpit and passenger cabin. A possible solution to this could be a catwalk between the two (see below in Fig. E).
Figure E: Catwalk to allow access between cockpit and passenger cabin 
The length of the fuselage would also increase rather significantly in design B.
The third option -design C- sees multiple tanks stored above the passenger cabin and one in the rear of the fuselage (see Fig. F) This option does not compromise access between cockpit and passenger cabin and would not increase the length of the fuselage as much. Nevertheless, storage above the passenger cabin results in a larger wetted area (the surface area that is in contact with the external airflow) and subsequently a significant drag increase.  This would prove a notable disadvantage as energy consumption due to fuel storage would increase an estimated amount of 5% (from 9% to 14%). 
Another option proposes all storage of fuel to be at the top of the fuselage (see Fig. G) :
Figure G: design with all fuel stored at top of the fuselage 
Storing fuel above the cabin is advantageous at is allows the opportunity to increase the liquid hydrogen carrying capacity , and also allows minimal interference with the traditional design of the cabin. However, again, storage above the cabin would result in a larger wetted area and thus a drag increase. 
All the design options discussed have their respective advantages and disadvantages. Considering a hydrogen-fuelled aircraft would already require significant investment, the most likely option for the future seems the one which requires minimal change to existing designs/can be integrated most easily with existing infrastructure, as this would minimise investment.
Research conducted by Dieter Scholz and Leon Dib  concluded that a modification of an existing fuselage such as the airbus A320, by integrating tanks along the fuselage i.e. front and aft of the passenger cabin, would provide the most economically preferable, and thus likely, solution.
In fact, the first aircraft in the world to fly using liquid hydrogen, the TU-155 which successfully flew in 1988, used integrated infrastructure with a tank placed along the fuselage
2.2.2. Blended Wing Body Design
Although storing liquid hydrogen fuel in the wings, in the traditional manner of kerosene- fuelled aircraft, is not feasible, a blended wing body option seems a possible option for future aircraft configuration.
Figure J: Airbus ZEROe proposed blended wing body aircraft design 
A blended wing body offers opportunity for better aerodynamic economy, better payload efficiency and larger airframe volume.  The larger airframe volume is particularly useful as it allows more options for the storage and distribution of liquid hydrogen.  In the design created by Airbus (see image on right), hydrogen tanks are proposed to be stored underneath/along the wings (made possible by the greater volume of the airframe). NASA has also been researching into blended wing body aircraft options due to its potential environmental benefits most notably reduced noise pollution. 
Nevertheless, whilst a blended wing body configuration is certainly an interesting option, aircraft configuration with integral tanks are a far more realistic and feasible option for the near future. This is because an integrated design has far more similarity to the traditional commercial aircraft we are used to seeing, thus, it seems the most economically preferable option for a liquid hydrogen-fuelled aircraft.
2.2.3. Fuel Tank Design
As discussed previously, the cryogenic nature of liquid hydrogen has implications for the storage tank design.
The most likely option for tank shape is a cylindrical tank shape as it provides good volumetric efficiency (important due to the large volumes required to store LH2) and the easiest shape which can be integrated with the fuselage.  The ends of the tanks would have a capped shape so pressure in the tank is more equally distributed. 
Figure K: Image to show a cylindrical tank with a capped end 
The insulation of the tank is also extremely important; effectively insulated tanks are required for reducing boil-off rate of liquid hydrogen which has important implications for the pressure of the tank, safety and minimising costs. Three main insulation methods are frequently highlighted  : multi-layer insulation, vacuum insulation and foam insulation. Multi-layer insulation consists of many layers of thin sheets of insulating materials, however, it is only effective at certain pressures.  Vacuum insulation involves maintaining a vacuum within the tank walls using a pump, however it requires heavy structures.  Foam insulation (see Fig. L) involves two thin metal layers and in between them a layer of foam. Foam insulation seems the most likely option as it provides good levels of insulation  , and in comparison to multi- layer and vacuum insulation has much smaller chances of catastrophic failure.  Foam insulation is also advantageous as it is cheaper than multi-layer and vacuum insulation. 
Figure L: Diagram to illustrate foam insulation 
2.2.4. Engine considerations
The combustion of liquid hydrogen in a conventional jet engine would require some adjustments to the engine and combustion process. This is needed to achieve optimal efficiency and to prevent the formation of excessive nitrous oxide compounds due to the higher temperatures associated with combustion of hydrogen. 
Having to redesign the jet engine proves a disadvantage because it is another aspect of investment.
However, the combustion of liquid hydrogen does provide numerous environmental advantages in comparison with combustion of kerosene.
Figure M: Image to show the emissions produced by fuel masses of kerosene and hydrogen with equal energy content 
Notably combustion of hydrogen does not produce carbon dioxide, soot, sulphur dioxide, carbon monoxide and unburnt hydrocarbon compounds. Despite the production of nitrous oxide compounds, these emissions can be limited and minimised by “lean combustion”. 
In comparison with kerosene, hydrogen combustion results in the production of more than 2.5 times the amount of water vapour. This is not favourable as water vapour can act as an effective greenhouse gas at high altitudes. Nevertheless, this disadvantage is less significant because the atmospheric residence time of water vapour is about half a year whilst residence time of carbon dioxide is approximately 100 years. Furthermore, at usual subsonic cruise levels it seems that the production of water vapour is negligible. 
2.3. Summary on LH2
The fuel properties of liquid hydrogen mean that a plane powered by LH2 would require entirely new infrastructure in terms of the airframe, engine and tank/fuel storage. Thus, this constitutes quite a radical redesign of aircraft and structures we are used to seeing now.
Beyond just the aircraft, liquid hydrogen planes would require new infrastructure with regards to the fuel supply chain; on the ground storage of liquid hydrogen and refuelling systems in airports would also need to be redesigned. All of this makes liquid hydrogen a less likely option for the future as it would require massive investment.
Biofuels are a promising option for the replacement of fossil fuels in aviation because they would require minimal to no change to the aircraft if used. Determining whether a biofuel is a viable option for replacing kerosene requires assessment of its fuel properties, its compatibility with jet engines as well as production costs and feedstock availability.
Bio alcohols such as methanol and ethanol – which have had some usage in road vehicles – are quickly ruled out as options. Energy density values are simply too low; they do not provide near enough energy to fuel a large passenger airliner. 
Biodiesel – which has also been used in road vehicles – is ruled out as an option. Although it is more suitable in terms of energy provision than bio alcohols, bio diesel does not perform well in the typical low temperatures experienced at high altitudes. Biodiesels at low temperatures become cloudy due to the formation of micro crystals which is unsuitable as it poses the danger of blocking fuel lines. 
The best two remaining options are Fischer-Tropsch biofuels and hydro-processed bio fuels which are assessed below.
3.1. Overview of the two main biofuel options for commercial aviation
3.1.1. Fischer-Tropsch fuels
Fischer-Tropsch – abbreviated to FT fuels – are liquid hydrocarbons produced by the catalytic conversion of synthetic gas. Kerosene is one of the fuels which can be produced synthetically by the Fischer-Tropsch process. Manufacturing FT Kerosene usually involves three steps: synthetic gas (composed of hydrogen and carbon monoxide) is generated by converting feedstock, syngas is then catalytically converted into hydrocarbons to create “synthetic crude”, and the synthetic crude is upgraded through hydrocracking and isomerization so it can be fractioned into desired fuels – one of these being FT Kerosene. 
Figure N: The reaction of hydrogen and carbon monoxide to produce hydrocarbons and water, under a cobalt or iron catalyst and high temperatures and pressure 
When compared to regular kerosene, FT kerosene has a slightly lower energy density which could affect the maximum range of the aircraft (the maximum distance which can be travelled between take-off and landing). However, this disadvantage is relatively minimal , and could even be offset by the slightly higher gravimetric heating value of FT kerosene (the amount of energy released with respect to weight). 
Other advantages of FT kerosene include environmental advantages: feedstock for the syngas is derived from biomass such as municipal solid waste, forestry and grasses  providing a
sustainable source, and FT kerosene is sulphur free so harmful sulphur compound emissions are eliminated. 
Furthermore, FT kerosene is advantageous in that it is operable at very low temperatures; during flight at high altitudes fuel can approach low temperatures and thus this is an important property. FT kerosene can have a freezing point as low as −59°C which is even lower than that of conventional jet fuel (-47°C). 
Nevertheless, use of FT kerosene has disadvantages when compared with kerosene. One is the effect on elastomer swell: fuel systems usually swell when exposed to fuel, a desired effect as it creates a seal and prevents fuel leakage. FT kerosene on its own causes very little swelling (< 2%) in comparison with conventional kerosene which causes around 16% swelling when compared with the same seal material.  But, swelling of FT kerosene can be improved through use of seal swell additives (chemical substances which can be mixed in with the fuel) so this issue is minimised.
Another disadvantage of FT kerosene is its poor lubricity. Fuel lubricity is an important property as it allows for better operation of components of the fuel system such as the fuel tank and fuel pump. Due to the lack of sulphur content in FT kerosene as well as its low aromatic compounds content (aromatics are a type of hydrocarbon usually found in conventional crude oil), FT kerosene has a poor lubricity. This issue, however, can also be reduced through the adding lubricant additives and aromatic fractions to the fuel. 
Overall, FT kerosene is an effective replacement for jet fuel due to its physical and chemical similarities, and so it is broadly compatible with already existing jet engines and fuel handling systems. The main drawback with FT kerosene is the cost of FT process which is relatively expensive in comparison with the production of other biofuels. 
3.1.2. Hydro-processed renewable jet fuels
Hydro-processed renewable jet fuels are fuels made from the treatment of fats and oils. Generally, the process consists of two steps. First is the hydrodeoxygenation step where oxygen is removed from the feedstock using hydrogen. Then the product of this step is refined through cracking and isomerization to get the desired fuel.
Figure O: A simplified graphic to show the two mains steps of hydro processing 
Jet fuels produced through hydro-processing are high in energy and suitable for use in conventional jet engines without requirement for modification of any part of the aircraft.  They also have a good thermal stability (the ability for fuel to resist deposit formation during combustion). 
A major advantage in the use of hydro-processed jet fuel is the reduction in greenhouse gas emission such as nitrous oxides and sulphur oxide compounds. 
Also, feedstock options for hydro-processed fuels are plentiful: algal oil from algae, jatropha oil, bio oil, camelina oil and animal fats and more. 
A drawback of hydro-processed jet fuels is their low lubricity due to the lack of aromatic and sulphur compounds. However, similar to FT kerosene use of lubricant additives and addition of aromatic fractions can minimise this problem. 
3.2. Current usage of biofuels in aviation
Biofuels in aviation have already had significant investment and current sustainable fuel replacement options are widely known as sustainable aviation fuels (SAF). Usage of SAF now is limited as a “drop-in” option. Essentially the SAF is mixed with conventional kerosene. Internationally approved SAF options (i.e. SAFs which are approved for drop in use in aircraft now) are shown in Figure P below:
Figure P: Table of internationally approved SAF including Fischer-Tropsch kerosene and hydro processed fuels 
The main limitation as of now is the “blend limit” on how much SAF can be blended with conventional jet fuel. The current maximum approved blend limit is 50%, however, the expectation for the future is that this will increase towards 100% so aircraft can eventually run on 100% SAF.
Recently, Airbus, German research centre DLR, and a SAF producer company Neste launched a project “Emission and Climate Impact of alternative fuels” to assess the possibility of 100% SAF usage on large passenger commercial planes.
The findings have been promising thus far, with the first test flight of an A350 (a long-range large passenger airliner) fuelled with 100% SAF – specifically HEFA-SPK [Hydro-processed Esters and Fatty Acids-Synthetic Paraffinic Kerosene]- occurring successfully in March 2021. 
3.3. Current costs of biofuels and comparison with fossil jet kerosene
Figure R: Chart to show the current market price estimates of HEFA fuels in comparison with fossil jet kerosene prices in USD per litre 
Hydro-processed renewable jet fuel (HEFA) is currently the cheapest biofuel route.  Comparing this biofuel with fossil jet kerosene, however, shows that is it far from being economically competitive with fossil fuels. As presented in the above chart the market price
of fossil kerosene (from 2019-2020 data) averages at around 0.25 US dollars per litre. In stark comparison, the market price of HEFA ranges from 0.85-1.50 US dollars. Hence it can be seen why presently kerosene is a far more economically logical option.
However, crude oil prices are projected to increase throughout the coming decades. The UK Government’s Department for Energy and Climate Change forecasts an increase to 0.99 US dollars per litre by 2030 in a “central scenario”.  Although there are some assumptions made in these projections related to uncertainty about changes in worldwide supply and demand, it can be reasonably estimated that rapid population growth, increase in global demand and the fact that fossil fuel reserves are quickly depleting will lead to an increase in market price of crude oil.
Therefore, it is a matter of time before biofuels, such as HEFA-SPK, begin to compete with kerosene on an economic level, and airline companies have a financial incentive to invest in biofuels.
3.4. Summary on Biofuels
Biofuels are a suitable and very likely option for use as jet fuel in the future. They require minimal to no change to the aircraft, as well as to the airport infrastructure involved in storing and supplying fuel, and thus they require least investment. Amongst biofuels, FT kerosene and hydro-processed renewable jet fuels are the best candidates to replace conventional jet fuel currently in use.
At least for the immediate future, hydro-processed renewable jet fuels seem a more likely option over FT kerosene, as hydro-processed jet fuels are the most commercially mature fuel route, currently at Technology Readiness Level 8  (TRLs are a system of ranking the maturity/readiness of technology developed by NASA). Hydro-processed jet fuels are also the current SAF of interest in researching the viability of increasing the SAF blend limit to 100%. 
As of now, the main limitation of biofuels is that the production cost of SAF is significantly higher than that of fossil kerosene prices.  But as kerosene prices are expected to inflate, the economic advantage crude oil kerosene has over biofuels will decline until biofuels become economically competitive with kerosene. Furthermore, the current political climate could propel governments towards more environmentally friendly policies and with regards to the aviation industry, SAF will be viewed favourably.
Liquid hydrogen being an appealing fuel coming from a renewable source of energy, means LH2 powered planes certainly provide an exciting prospect for the future. The use of LH2 in commercial aviation, however, would require radical changes and huge investments meaning it is not extremely likely to become a replacement for kerosene as jet fuel at least in the short term.
By far the most likely and suitable option for the short term is the use of biofuels, particularly hydro-processed renewable jet fuels. These fuels have already had significant investment and would require minimal change in the case of a transition from traditional jet fuel to SAF. Furthermore, SAF technology is already fairly mature and has received/still is receiving a lot of funding for development and research.
Therefore, it seems foreseeable that as biofuels become more economically competitive with kerosene, once kerosene prices begin to greatly inflate, biofuels will pave the way in the near future for commercial flight without fossil fuels.
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