Biojetfuel: Biofuel with a future

January 30th, 2015, Published in Articles: EE Publishers, Articles: Energize


The recent announcement that a South African carrier together with a major manufacturer and biofuel companies are to develop biofuel for its fleet of jets may signal the start of an entire new biofuel industry in this country. Aviation biofuels have been used by commercial and military organisations for some time now and this appears to be one of the biofuel applications with a guaranteed future market.

The fuel is to be manufactured from plants grown in this country specifically for the purpose of biofuels, and will be processed locally. The plant is a hybrid nicotine free, seed producing tobacco plant known as Solaris, and trial crops have already been planted. Test flights are expected to commence in 2015.

This is not the first time biofuels have been used for aviation in this country and several test flights have been undertaken successfully. Several thousand flights using biofuels have been undertaken internationally since the fuel was first approved in 2011.

Biofuels for aviation

Unlike the ground or marine transport sector, which can use electric energy from batteries, fuel cells or hydrogen based fuels, commercial scale aviation is tied to the jet engine for propulsion. Battery-powered aircraft are limited to small passenger aircraft, and it is unlikely that electrically powered commercial aircraft will be in service before 2040, and it is not expected their range will exceed
1500 to 2000 km. The design of the jet engine is such that there is no alternative to liquid hydrocarbons as a fuel. Hydrogen can be burned in a turbine engine for aviation. However, there are significant technical challenges in designing a hydrogen-powered aircraft for commercial aviation and in producing enough hydrogen in a sustainable way to supply the industry’s needs. There is research underway using nanotechnology as a potential for storing hydrogen in a convenient and safe way for air transport, but potential commercialisation is a long way off [5].  Sustainable alternative hydrocarbon fuels, which currently are mostly biofuels, are therefore the only renewable energy available for air transport for many years to come [1]. Biofuels therefore have very little competition as an alternative fuel for commercial aviation and it is likely that a strong market will develop for these products in the next years. Projections are that biofuels could supply 80% of the aviation fuel  requirements at peak. When blended with regular aviation fuel, biofuels can reduce the carbon footprint by up to 80%.

Aviation biofuel  requirements

Biofuels used for aviation must meet the following requirements:

Table 1: Fuel readiness level (Frl) table [4].

Table 1: Fuel readiness level (Frl) table [4].

  • They must be able to be mixed with conventional jet fuel, use the same supply infrastructure and do not require adaptation of aircraft or engines. The aviation industry is focused on “drop-in” or fungible alternative fuels – these are fuels that have been shown to be functionally identical to petroleum-derived jet fuel. They are pure hydrocarbons (no oxygen, no ethanol, no water) and perform in an identical manner to petroleum-derived jet fuel [4].
  • They must meet the same specifications as conventional jet fuel, in particular resistance to cold (Jet A: -40°C, Jet A-1: -47°C), and have a high energy content (min. 42,8 MJ/kg) [1].

Because of these reasons automotive bio-ethanol and bio-diesel are not suitable for aviation jet fuel, although they are used in piston engined aircraft. One of the often overlooked requirements for alternative fuels is that it should perform better than the fuel it is replacing, not often the case when looking at things. Biojet seems to meet this requirement, an added unexpected bonus.

Biofuel must also satisfy the following non-technical requirements:

  • Meet sustainability criteria such as lifecycle carbon reductions, limited fresh water requirements, no competition with food production and no deforestation.

Sources of aviation biojetfuel

Fuel readiness level

There are several processes which can yield aviation biofuel, both under development and at commercial status. The commercial aviation alternative fuels initiative (CAAFI) has  defined a set of fuel readiness level tables to describe the state of development of the processes used to produce biojetfuel (Table 1) [4].

Biomass to liquid (BtL)

For the production of biomass-to-liquid (BtL) fuels, solid biomass is converted via thermo-chemical gasification into a syngas composed primarily of carbon monoxide and hydrogen. After purification, this gas is converted into hydrocarbon chains using Fischer-Tropsch synthesis. Then jet fuel can be separated from the resulting hydrocarbon mixture by means of refinery processes.

The large-scale feasibility of BtL technology was successfully demonstrated using coal decades ago. The basic technical feasibility based on wood has also been proven, although the technology is very demanding. This has prevented its commercialisation so far. Large-scale BtL projects are only making slow progress, meaning it is unlikely that BtL will make any noteworthy contribution to the production of biofuels in the near term, although biojetfuel has been ASTM certified as a product of the BtL process.

Agricultural and forestry by-products yield valuable biomass without requiring dedicated land and municipal waste contains biomass that can be diverted from landfills. Biomass could also be obtained from the balance of vegetation in the oil seed plant program, to expand the fuel output.

Gas to liquid (Gtl)

Biomass of very different origin and/or composition is initially converted into a biogas using biochemical processes. Using physical processes, this gas can be used to obtain biomethane (among others). In the subsequent gas-to-liquid process, this bio-methane is converted into carbon monoxide (CO) and hydrogen (H2). Then it is converted into hydrocarbons using what is known as Fischer-Tropsch synthesis – from which it is ultimately turned into jet fuel [2].

Alternative aviation fuel has not yet actually been produced via the GtL process. However, GtL technology has already been applied for years in conventional refineries and has met the international ASTM standard since 2009. Since biomethane and fossil methane are chemically identical and the technology has been used successfully for natural gas on an industrial scale, straightforward production is considered possible. Unfortunately, there are high costs associated with the production of alternative aviation fuels using the GtL method. This method is considered to be at fuel approval stage (FrL 7).

Alcohol to Jet fuel (AtJ)

To produce alcohol-based biofuel (Alcohol to Jet, AtJ), hydrocarbon chains are produced from the alcohols with the aid of thermo-chemical reactions, and then the jet fraction is separated in a final step. In doing so, the required alcohols can be produced in a number of ways: One approach, for example, is to convert carbon monoxide into alcohol using micro-organisms. In another method, a sugar-containing solution is initially obtained from biomass and then the solution is subsequently converted into alcohol in a fermentation process. It is also possible to leave out the alcohol phase entirely: One example is the direct sugar to hydrocarbons (DSHC) method in which micro-organisms are used to process sugar molecules so that they can subsequently be converted directly into long chain hydrocarbons via hydrogenation.

A product known as Farnesane is being produced by this process in a refinery specialising in biofuels in Brazil. The fuel has been certified and has been successfully used blended with jet fuel on several test flights [3].

Some companies are already developing the production of biojet on an AtJ basis beyond the demonstration phase. Given the limited number of production sites, so far there have still been no major breakthroughs made regarding the large-scale deployment of such technology. Alternative aviation fuel from AtJ production processes is in the testing phase with regard to the certification processes; certification is expected by the middle of 2014. Due to the early stage of development, AtJ methods are considered to be at certification stage (Frl 7).

Sugar cane is a prime feedstock for the production of ethanol, and could also be used as a feedstock for this process, although it violates the no-competition with food production requirement.

Hydro-processed esters and fatty acids (HEFA)

To produce biofuels using this process, any form of native fat or oil can be used. Apart from waste fats left over from the food industry, vegetable oils and fatty acids from oil and fat refining processes are the most common forms used. Reports on the first flights undertaken using recycled cooking oil raised some eyebrows. This process is ideally suited to vegetable oils produced specifically for fuel purposes.

Production consists of three stages:

  • In the first stage of production, the raw feedstock is purified and filtered
  • In the second stageoils and fats are hydrogenated in a catalytic process which converts the free fatty acids and triglycerides to straight chain diesel range waxy parafins [6].
  • In the third stage they are refined, in a very similar process as is used with fossil fuels. This process of cracking and isomerisation produces synthetic paraffinic kerosenes, similar to those found in existing jet fuel [6].

The relevant production process is already fully developed and has been certified by the international standardisation organisation ASTM since 2011. Increased amounts of HEFA jet fuel are already being used for testing purposes in scheduled passenger flights. Therefore, the production of biojet on a HEFA basis is considered to be at commercial production capability stage, (FrL 9) and there are several refineries around the world producing jet fuel by this method.

The HEFA process is considered at this stage to be the most promising for the production of alternative jet fuels. [2] There are currently several large-scale bio-refineries in the world that specialise in the production of certified fuels from vegetable oil – including  alternative aviation fuel [2].

In addition to waste oils and fats, oil seed crops are specially grown for the purpose of producing biojetfuel. Some examples include. Camelina, jatropha, halophytes and various other non-food seed crops that yield oil, grow in areas unsuitable for food crops.

The South African project is based on a hybrid nicotine free seed oil tobacco plant, known as solaris. Project Solaris began in 2012 with two hectares of crop, rising to 11 hectares in 2013, before expanding to the current 50 hectares. The partners aim to expand the project to 30 000 hectares by 2020, leading to the production of 140 000 t of jet fuel, the creation of 50 000 direct jobs and a reduction of 267 kt of CO2 emissions. They envisage 250 000 hectares of solaris under cultivation by 2025 [7].

Hurdles to overcome biojet fuels have already passed certification stage, and demonstration in use. Tests have shown improved performance and fuel economy when using biojet fuels. The remaining problem however is one of price, as biojet fuels are more expensive than conventional jet fuel. Conventional fuel is also subject to market prices, and current low crude oil prices indicate the volatility of the market. However, the practice at the moment is to blend biojet with conventional fuels, in fairly low proportions.


[1]    IATA: “Factsheet: alternative fuels”,
[2]    Aireg: “The future of climate friendly aviation”,
[3]    Amyris: “Amyris renewable jet fuel receives regulatory approval in Brazil”, Press release Amrysis, 16  Dec 2014,
[4]    CAAFI: “Fuel readiness tools”,
[5]    MASBI: “Introduction to biofuels”
[6]    J Sandquist: “Aviation and biofuels”, Sintef Energy,
[7]    SAinfo reporter: “SA Airways to test tobacco biofuel in 2015”,

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