Alternative jet fuel is chemically similar enough to conventional kerosene to be used in existing aircraft without modification. The obstacle is that very little of it exists.
The saving is in the feedstock, not the flame
Burning alternative jet fuel releases carbon much as kerosene does. The reduction is claimed across the lifecycle, comparing where the carbon came from.
Fuel made from waste oils or crop residues uses carbon that was recently in the atmosphere, whereas kerosene releases carbon that has been underground for a very long time.
The size of the saving therefore depends entirely on the feedstock and the energy used to process it, and it varies widely between production routes.
Waste feedstocks are already spoken for
Used cooking oil and animal fats are the cheapest inputs and produce the largest savings, which is why they dominate current production.
Their supply is fixed by how much cooking and meat processing occurs, and much of it already goes to road biodiesel and other industrial uses.
Competition between those markets means aviation cannot simply expand its share, and collection systems for the remaining volumes are fragmented.
Crop-based routes carry a land question
Purpose-grown oil crops can be converted to jet fuel, but growing them uses land that could produce food or hold carbon as forest.
Where cultivation displaces existing land use, the emissions from that displacement can erode or exceed the saving, which is the indirect land use problem.
Sustainability criteria exclude the worst cases, and the exclusions further limit the feedstock pool that qualifies.
Synthetic fuel needs enormous clean power
Fuel can be made from captured carbon dioxide and hydrogen, which sidesteps land and waste constraints entirely and is the only route that scales indefinitely.
It requires very large quantities of low-carbon electricity, since both hydrogen production and the synthesis step are energy-intensive.
Until clean power is abundant and cheap, that electricity displaces other decarbonisation uses, and the fuel remains far more expensive than kerosene.
What the blends mean in practice
Alternative fuel is blended with conventional kerosene rather than used neat, and the permitted blend ratio is capped by certification.
Fuel is not tracked to individual aircraft, so airline programmes work by purchasing a quantity delivered into the network and allocating the claim, which is a book-and-claim arrangement rather than a physical one.
The honest position is that these fuels are a real but currently marginal contribution, and the volumes are far from what would be needed to change aviation's overall footprint.