The biofuel and sustainable aviation fuel industry are not only fairly young industries, but can also be quite cumbersome to navigate with all the unknown terminology and process. This short guide was created to provide a starting point for those interested in understanding some of the basics. 

A few figures

2% of CO2 emissions is generated from the aviation industry – based on 2017 data from – Air Transport Action Group (ATAG)

If aviation was a country it would be the 10th largest emitter globally – World Economic Forum

Air travel will double by 2036 to 7.8 billion passengers per year – IATA

Over 1110 institutions with more than USD $11 trillion in assets under management have committed to divest from fossil fuels. A 22,000% increase since 2014 – 350.org


Biofuel (a.k.a. first generation fuels made of biomass*) is often made from rapeseed, sugar cane, palm oil, soybean, though the full list is significantly longer. The term biofuel is used by some organisations as an all encompassing term that includes any fuel made of biomass, in other words any non-fossil based hydrocarbon fuel. However, if we dig deeper, there are additional terms and categories of biofuel

*Biomass is defined as any organic matter that comes from plants or animals that can be used as a renewable source of energy. There are five (5) main types of biomass: (1) wood and agricultural products, (2) solid waste, (3) landfill gas and biogas, (4) ethanol, and (5) biodiesel. If you’d like to understand biomass types in slightly more detail, please see this biomass PDF made by NEED (National Energy Education Development).

First generation (bio)fuel is another term used for most biofuels that are made from agricultural crop feedstock. Examples: biodiesel or bioethanol made from rapeseed, palm oil, soybean, or sugarcane, again just to name a few.

Alternative fuel or advanced fuel or second and third-generation fuel are synonymous names coined to denote fuels that are aiming to come from more sustainable creation pathways than their first generation biofuel predecessors. Some examples of the products used to create this fuel are: algae, municipal solid waste, cellulosic waste (excess residue from wood, agricultural, and forestry), and used cooking oil.

Some groups, such as ATAG have suggested that the term advanced fuel is sometimes used to distinguish between sustainably sourced versus non-sustainably sourced fuels. However, this differentiation is debated by different stakeholders in the aviation fuel industry.

Though there isn’t 100% agreement on which term(s) to use, most organisations prefer to use the term sustainable aviation fuel because they want to distinguish themselves from other biofuel pathways that have typically been deemed unsustainable in the long-term.

Drop-in fuels is a term used to describe any fuels that can be automatically utilised in existing airport fuelling systems. This means they can be blended with conventional jet fuel and easily processed by conventional jet engines, therefore no major re-engineering or retrofitting of existing aviation technology is required. Once blended the batch is tested before use.

Why blended with petroleum-based fuel? In the aviation industry, safety rank’s first. Therefore, any replacement fuel for fossil fuel must be meticulously tested. Based on current testing standards, commercial flights are only allowed to have up to 50% SAF blended with petroleum-based fuel. But in the future, there is a hope that a version of SAF will be a 100% replacement for petroleum and this has been tested by Boeing.

Not all fuels are created equal:

  • In order for fuels to maintain SAF categorisation they must pass a lifecycle analysis (LCA) proving a net carbon reduction. This analysis factors in other important filters such as:
    • Lifecycle greenhouse emissions
    • Direct and indirect land use change
    • Water needs and supplies
    • High conservation value area and biodiversity
    • Socio-economic conditions of local farmers and local populations (especially in Global South areas)

ASTM approved pathways – ASTM, which stands for American* Society for Testing and Materials, has tested and verified that specific SAF production pathways are safe for commercial aviation and to be blended (at different percentages) with petroleum-based fuel. Though more pathways are being vetted and approved on a rolling basis, prior to January 2020, the four main approved pathways were:

  • HEFA – oleochemical conversion, from used cooking oil, animal fats, algae, and vegetable oils. Max blend proportion allowed: 50%
  • FT – thermochemical conversion, from solid biomass sources. 

Max blend proportion allowed: 50%

  • SIP – biochemical conversion process, from fermented hydroprocessed sugar, also referred to as direct-sugar-to-hydrocarbon fuels. Max blend proportion allowed: 10%
  • ATJ – a mix of thermochemical or biochemical technologies. It involves the fermentation of synthesized gas and catalytic reforming of sugars or carbohydrates

Max blend proportion allowed: 50%

Figure 1-1: Simplified graphic showing different pathways for SAF creation

Source: IRENA, 2017. https://www.irena.org/documentdownloads/publications/irena_biofuels_for_aviation_2017.pdf

*Please note: though the standard is the ‘American’ Society for Testing and Materials , it is an international standards organisation that develops and publishes technical standards for a wide range of materials, products, systems, and services.

If you’d like to discuss any of the information provided above, please get in touch with FGA at sarah@flygreenalliance.org

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