Jet fuel is made of kerosene - a mixture of hydrocarbon chains containing between ten and sixteen carbon atoms per molecule. Traditional jet fuel is made by distilling crude oil, making the aviation industry one of the biggest emitters worldwide. The IEA estimated that aviation was responsible for 2.5% of total carbon emissions worldwide in 2023.
The sector is only expected to grow, making reducing emissions a pressing concern. Replacing kerosene with batteries or hydrogen, as has been done with road transportation, is challenging: batteries are heavy and hydrogen is less energy dense than kerosene, meaning more fuel is needed to travel the same distance. The heavier the aircraft, the more energy is needed for take off. This limits the use cases for both batteries and hydrogen.
Sustainable Aviation Fuels (SAF) is synthetic kerosene - kerosene that’s been synthesised without using crude oil or other fossil fuels. SAF is a drop in replacement to traditional kerosene. But what exactly makes synthetic kerosene sustainable?
For fossil free kerosene to be classified as SAF, it must demonstrate a measurable reduction in lifecycle carbon emissions. After all, burning through one tonne of kerosene in a jet engine is going to produce the amount of carbon dioxide, no matter how the kerosene was made.
As with many fossil fuels, the majority of emissions from traditional, fossil kerosene are the result of the process of bringing crude oil to the Earth’s surface, transportation and refining. These are calculated and compared to SAF alternatives using Lifecycle Assessments: an assessment of the total emissions from extraction of raw materials to disposal of the finished product.
The UK’s SAF Mandate estimates that 89 grams of CO2e are emitted for every MJ of energy provided by fossil derived kerosene over the full product life cycle. The SAF Mandate requires producers to demonstrate a lifecycle emissions reduction of 40% for synthetic kerosene to be classified as SAF. The EU’s ReFuelEU has even more stringent criteria of 70% lifecycle emissions reduction. Both sets of legislation impose financial penalties for failing to fuel with the specified minimum percentage of SAF. The minimum rises annually. It is forecasted that these groundbreaking pieces of legislation, combined with generous subsidies in the US, will lead to maturation of today’s nascent SAF production industry.
The exact lifecycle emissions reductions of a given batch of SAF depend on how it was produced. Broadly speaking, there are three categories of SAF. We’ve provided an overview of each below, starting with the most mature.
Esters and fatty acids are complex chains of oxygen bearing hydrocarbons found in oils and fats. Hydrogen can be used to remove the oxygen to create straight chain hydrocarbons that can be cracked into shorter chains to make kerosene. The first process is called hydrogenation, leading to this generation of SAF being known as Hydrotreated Esters and Fatty Acids or more commonly HEFA.
In order to achieve the life cycle carbon reductions necessary to produce SAF, producers must source waste oils and fats. This has the bizarre unintended consequence that the price of used cooking oils in many Asian markets is higher than the price of virgin cooking oil.
As the supply of waste oils and fats is limited, both ReFuelEU and the SAF Mandate place caps on the volumes of HEFA that they will count towards their mandated minimums.
Plant based biomaterials, such as corn and sugar cane, are made up of complex chains of hydrocarbons that can also be converted into kerosene. This can be done through fermentation to make alcohol that can then be synthesised into kerosene or by processing solid biomaterials at high temperatures and pressures to produce light chains of hydrocarbons as kerosene precursors.
As with bio-diesel and other plant derived fuels, there are concerns that food crops could be diverted or that ecological damage will result from a high concentration of cash crops grown as feedstocks. The UK’s SAF mandate includes very strict feedstock selection criteria to guard against these possibilities, but not all jurisdictions have such regulatory protections.
Power to Liquid (PtL) processes use electrical energy to combine molecules of carbon and hydrogen, usually using a Fischer-Tropsch process, into long chain hydrocarbon molecules. The long chains can be broken up and recombined to form a liquid that is chemically identical to traditional jet. Hydrogen and carbon molecules are sourced by electrolysing water and carbon dioxide to make hydrogen and carbon monoxide gas.
As power is such an important input to this process, it is critical that the source of power is carbon free. Depending on the jurisdiction, power can be sourced from wind, solar, hydropower and nuclear generators. In a rapidly decarbonising world, competition for renewable power can be fierce. Access to abundant renewable generation as well as markets for the fuel produced will be key factors in determining where this technology is deployed.
If your business needs help navigating the complex world of SAF, contact us today to arrange a free consultation to find out how Innovative Energy Consultants can help.
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