True biomass waste is not the oil a plant produces, which can be fairly easily converted into fuel, nor the sugar it produces, which can be fermented into ethanol. Certain amounts of oil- or ethanol-based fuels can be used, but at scale the issue is land use, and we also need that land for growing food. True waste is the cellulose and/or lignin in biomass, as well as the CO₂ emissions that result from processing the biomass.

This distinction is important. If the industry is to scale sustainably, it cannot rely on feedstocks that compete with food or drive additional land use change. The long-term solution must focus on carbon that already exists as residue or emission.

Cellulose is tough to break down, requiring a community of microbes or a number of process steps. The easiest product for it to break down into is methane (CH₄), as, thermodynamically, this is the most stable end point. Methane is also a non-toxic gas which does not inhibit the microbes involved in the process. This is biogas, and it is a great feedstock for converting into aviation fuel via gas-to-liquids. The biomethane in the biogas can be converted into carbon monoxide (CO) and hydrogen (H₂), using a process called steam methane reforming, with some of the byproduct CO₂ added and also converted to increase the greenhouse gas (GHG) emissions saving.

Wastes higher in cellulose, such as food waste or agricultural waste, can therefore be digested into biogas, along with any sugars or starches present. The biomethane can then be reformed into syngas, with some of the byproduct CO₂ added back in and converted to reduce the overall carbon intensity.

Biogas is already produced at scale in many regions, including China, Germany, the US, India and several other European countries. The opportunity is to redirect this feedstock away from heat and power applications, where it is often used today, and instead into liquid fuel production, where the decarbonisation challenge is harder.

Lignin and gasification
High-lignin waste feedstocks such as wood waste, sugar cane bagasse, corn stover and rice straw are much tougher to process. Here, the most likely route is to partially combust them via gasification to produce a mix of hydrogen, CO and CO₂. Gasification is difficult to control and presents challenges such as tar formation. However, if the resulting syngas is properly cleaned, it can be used to make aviation fuel. In some cases, additional green hydrogen can be added to convert more of the carbon and improve the carbon intensity.

Waste-based fuels and chemicals derived from high lignin feedstocks are a key part of the mix. The technologies are complex and the feedstocks variable, but the carbon is available and abundant. In some cases, there are additional benefits, such as reducing air pollution from traditional rice straw burning. As demand for sustainable aviation fuel (SAF) grows, these routes will need to form part of the solution.

Gasification of plastic or municipal solid waste is another option, although it is more challenging due to the variability and complexity of the feedstock. As a result, total combustion is often preferred, as seen in energy-from-waste plants. While gasification of these waste streams can reduce landfill, if the carbon ultimately becomes fuel and is then released to the atmosphere when burned, it will not reduce emissions and could even increase them. A more sensible approach may be to use the CO₂-rich syngas to produce plastic precursors such as naphtha, and from this manufacture ethylene and propylene before returning them to plastics, thereby encouraging circularity.

CO₂ and power-to-liquids
Finally, there is CO₂, which can be derived from fossil sources or biogenic sources. Biogenic sources are ideal, as circularity can be claimed. However, if fossil CO₂ comes from hard-to-abate sectors that would have emitted anyway over the next few decades and cannot go underground, distinguishing between fossil and biogenic CO₂ will make no difference to the atmosphere in that period.

Capturing CO₂ directly from the air through direct air capture (DAC) is theoretically possible. However, it is likely to remain extremely expensive because of the vast volume of air that must be processed continuously. The amount of air that needs to be handled per unit of CO₂ captured is fixed by thermodynamics, specifically by the entropy change involved in concentrating CO₂ from around 420 parts per million to near pure (one million). In some locations, wind may be used through a chimney effect rather than mechanical fans. Even so, the material requirements, energy demand and land area involved mean that DAC will remain a significant technical and economic challenge.

For power-to-liquids (PtL), biogenic CO₂ is likely to be the primary focus for now, with fossil CO₂ from hard-to-abate sectors used where permitted. This CO₂ is converted into fuel using large quantities of green hydrogen, which in turn requires substantial renewable electricity. The key advantage compared with biomass-based fuels is the lower land use requirement. However, production must be located in regions with abundant, low-cost renewable electricity, typically sunny or windy areas. In many of these regions, available CO₂ is more likely to be fossil-derived due to limited biomass growth.

Regardless of the challenges, PtL (or e-fuels) is widely recognised as critical for the future of SAF due to the limits of the other feedstocks, and regulation is already underway to support this transition. For example, the EU requires 50% of the 2050 blending requirement under ReFuelEU to be met via PtL.

Enabling conversion across feedstock routes
The scale of demand means no single feedstock can deliver the required volumes. To meet the huge demand for sustainable aviation fuel, we will need them all.

OXCCU is developing an iron-based Fischer-Tropsch catalyst able to convert a wide range of syngas compositions, including CO₂, CO and hydrogen, directly into liquid hydrocarbons in the aviation fuel range. Our feedstock, CO₂-rich syngas, can be derived from biogas reforming, solid carbon waste gasification, or from CO₂ and green hydrogen. This approach consolidates the traditional production process from a two-step reverse water gas shift and Fischer-Tropsch reaction into a single catalytic conversion. Fewer steps reduce energy input and improve overall yield.

In practical terms, the same catalytic platform can be applied whether the carbon originates from CO₂, biogas or gasified solid carbon waste. In all cases, the objective remains consistent: reducing both cost and carbon intensity. If aviation is to decarbonise at scale, it cannot depend on a single carbon source. It must use waste carbon in all its forms. The focus now is on deploying technologies capable of converting that waste carbon efficiently and economically into aviation fuel.

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.

• Further reading: ‘IATA SAF Registry goes live’. Discover more about the International Air Transport Association’s Sustainable Aviation Fuel Registry, designed to enable a global market for SAF that will accelerate the transition to net zero by 2050.
• ‘Heathrow ramps up SAF ambition as UK government moves to de-risk supply’. Heathrow Airport has unveiled an enhanced incentive scheme targeting 5.6% sustainable aviation fuel use in 2026, as the UK government advances plans for a contracts-for-difference-style revenue certainty mechanism (RCM).