A new category of SAF is emerging, called e-fuels or power-to-liquids (PtL) which use CO₂ and hydrogen as the feedstock. Producing green hydrogen requires a significant amount of renewable energy and a clean source of CO₂. Nevertheless, the sector is gaining momentum, supported by new regulatory frameworks, which value the limited impact on land use and the potential for scale. As such, it is anticipated that costs will decrease over time.

Typical SAF 
Today, most SAF is derived from vegetable oil, commonly known as ‘HVO’ fuel. Hydrotreated vegetable oil (HVO) is a first-generation biofuel made from vegetable oil via the HEFA (hydrotreated esters and fatty acids) process. First the oxygen is removed by hydrodeoxygenation, then the straight paraffinic molecules are cracked and isomerised to branched jet fuel molecules with the right chain length. The fuel’s appeal stems from its biological origin, and the fact it is available today via a mature process.

This approach recently gained public traction as the first transatlantic Virgin Atlantic flight took off from London, powered by 88% HEFA-based fuel. However, there are strict limitations on growth due to land use. Growing crops for vegetable oil on land which could otherwise grow food puts strain on available land resources, and there are limits on the availability of used cooking oil and waste fats, prompting concerns about its scalability.

Another option gaining traction is converting corn ethanol into jet fuel, especially in the US where ethanol is readily available. This alcohol-to-jet (ATJ) process requires a number of catalytic steps: first fermentation of corn to ethanol then alcohol dehydration, olefin oligomerisation, and hydrogenation. This product has similar drawbacks in terms of land-use competition with food production. This is leading some companies to look at ways of making ethanol from waste, but yields are significantly lower.

The next route, waste-to-jet fuel, is the broadest as it includes using a wide variety of waste feedstocks, including municipal solid waste, plastic waste, food waste, farm waste, sewage waste and wood waste – essentially transforming discarded carbon-based materials into SAF. The technologies to do it are very broad and different, although generally are either a type of pyrolysis (thermal decomposition) or gasification (converting fuel to gas without combustion).

Many of the technologies still have challenges in terms of aggregating the waste, sorting the waste and, in the case of gasification, clean-up before further processing and, in the case of pyrolysis, dealing with a challenging, unstable and potentially toxic crude oil product.

A third biological option for wet, high-cellulose waste is anaerobic digestion (AD) to make biogas. Biogas already has some scale and instead of being burnt for power or heat, it can be reformed to syngas and then converted into fuel. The challenge here is less the technology, as it is mature, but the amount of available biogas as it has significant demand elsewhere.

Alternative solutions 
Given the challenges above, there is increasing interest in e-fuels that use CO₂ as the feedstock, along with green hydrogen. This production pathway is emerging as critical for the future of SAF, and regulation is already underway to support this transition. The EU requires 44% of the 2050 blending requirement under ReFuelEU to be via PtL. The UK and US are following suit and looking at setting their own targets.

The clear advantage versus the other options is scale; the feedstocks required, CO₂ and hydrogen are theoretically unlimited given access to a lot of green energy. The challenge for e-fuels currently is cost.

First, the green hydrogen cost is key, and this depends on the green electricity price, as well as the cost of reliable low-cost electrolysers. The good news is both these are coming down rapidly.

Second, you need low-cost, pure CO₂. If the CO₂ is biogenic or captured from the air (DAC) the process is circular, but in the short to medium term, fossil or mineral CO₂ can also be used. Using fossil or mineral CO₂ in the process is not circular, but as you’re getting two uses out of some of the carbon that has been dug up before it ends up in the atmosphere, it still reduces emissions significantly. The good news is the CO₂ capture industry is increasingly mature and low-cost captured CO₂ sources are becoming more common.

Finally, the industry needs to develop a simple process to convert the CO₂ and hydrogen with high yield into SAF.

OXCCU is scaling up a simplified process to convert CO₂ and hydrogen. We have shown that it’s possible to make jet fuel-range hydrocarbons in a single step with high conversion and selectivity.

Ultimately the prize is there for PtL, as unlike the routes which depend on crop-based or waste-based feedback, PtL is dependent only on CO₂, green electricity and water, giving it the potential scale needed for SAF.

The views and opinions expressed in this article are strictly those of the author only and are not necessarily given or endorsed by or on behalf of the Energy Institute.