Insight Focus
Aside from crude oil, HEFA is still the preferred pathway for generating SAF. But there are a few drawbacks. The good news is that SAF production using HEFA processes is a start on the road to sustainability.
Sustainable Aviation Fuel Production Still Lags
There are currently eight (more coming) technical pathways and a huge array of feedstocks, which can essentially be almost anything carbonaceous to start with. There are also a raft of catalysts and process chemicals like green hydrogen (that mythical totem!) and somehow, the chemists have found a catalyst to drive through ways of making jet fuel.
Bravo. Smart, indeed.
So how come so few are using this technology to elbow conventional jet fuel into the history books? Why is it that there are more articles written about how to manage the arrival of SAF to airports than contracts for delivery? We are waiting in the aisle, but where is the bride?
Which Pathway Wins?
As we’ve seen in previous articles, each of the technical pathways presents differing degrees of chemical difficulty and complexity. The easiest way to make jet fuel in terms of process and energy requirements is of course from crude oil – the jet fuel is already there, for the most part, and separation is straightforward, via distillation.
The next best way, and so the route that is most popular for SAF at the moment, is the HEFA process, Annex A2. Why? Because the feedstock is vegetable oil, often recycled, which is generally three longish paraffinic hydrocarbons with a carboxy (carbon-oxygen) hub holding them all together.
In other words, it’s nearly all hydrocarbon. A little heating, and a dash of hydrogen and the molecule breaks up to give neat, pure hydrocarbons, in the diesel range, and releases a bit of CO2.
The Trouble with HEFA
This is good news to kickstart the adoption of SAF. But there are problems down the road. First, the HEFA process is better designed to make Renewable Diesel (RD) than SAF because less processing is involved. HEFA plants nearly always, make a higher proportion of RD than SAF for this very reason, provided the economics are favourable.
Secondly, the feedstocks have other uses already, many linked to the production of food for animals (and humans) thereby creating a need for alternative feedstock sourcing in the food chain. This may lead to greater expense and, in the worst case, shortages, through land use change.
Thirdly, the HEFA process doesn’t score most highly in terms of sustainability. The power to liquid (PtL) processes remove CO2 directly from the atmosphere and other processes take methane or biogas from agricultural production. Thereby, they not only recycle carbon but also eliminate the release of the more harmful greenhouse gas methane.
Despite these shortcomings, HEFA is gaining traction: if we look at the current state of production units in the US, we can see the widespread adoption of the SAF-RN process across the US, with combined SAF and RN plants offering the greatest output.
So HEFA may not be perfect, but it’s a start.
Next time we’ll look at the adoption of Fischer Tropsch pathways, and some more complex, if exciting, conversion processes.
