Insight Focus
Hydrogen plays a key role in sustainable aviation fuel production. The more oxygen in feedstocks, the more hydrogen is needed. Identifying hydrogen sources will shape SAF development.
Hydrogen’s Role in Producing Sustainable Jet Fuel
In our last article, we explored hydrogen as a fuel for flight, highlighting the challenges it poses due to its difficulty and high cost of handling. But is there another potential use for hydrogen? Indeed—here, we reveal its role as an intermediary compound in producing jet fuel from biomass.
Remember, the starting point for jet fuel is crude oil, which is primarily composed of carbon and hydrogen. The fuel is mostly extracted through physical separation (distillation). The starting point for sustainable jet fuel, however, is a mishmash of carbon, hydrogen and oxygen, typically from carbohydrates, glycerides and lipids. So, the oxygen needs to be removed to transform it into jet fuel.
This is where hydrogen comes in. Reactions involving hydrogen are known as reduction reactions, with hydrogen acting as the reducing agent. By replacing oxygen with hydrogen on a carbon backbone, the molecular weight is reduced, and the available energy for future combustion reactions is increased.
Hydrogen’s Contribution to SAF Production Processes
HEFA
In the HEFA process, we start off with a lipid — a long-chain hydrocarbon with a brace of oxygen atoms at one end. The process of converting it into a hydrocarbon requires a relatively light touch, replacing a small portion of the core chemical with hydrogen, which is easy and light on energy demands.
This relatively simple and energy-easy chemical reaction explains why the HEFA process is currently so popular. The feedstock is 85% of the way towards a hydrocarbon, and a small amount of hydrogen gets us to the finish line. It’s easy to get from used cooking oil (UCO) and vegetable oils to jet fuel, but hydrogen is essential.
Fischer Tropsch
Other pathways to SAF are more complex and energy intensive. For example, the Fischer Tropsch pathway requires a series of complex reactions, starting with converting feedstocks into syngas—a mixture of carbon monoxide and hydrogen—which is then transformed into hydrocarbons over a catalyst. This complexity reduces efficiency, increasing energy consumption and driving up costs.
Alcohol to Jet
Similarly, the alcohol-to-jet (AtJ) process requires fermentation of carbohydrates to ethanol, dehydration, oligomerisation and then cracking. This multistage process is highly energy-intensive, requiring significant energy input throughout.
Therefore, hydrogen is essential across the main pathways to jet fuel from sustainable feedstocks. The more oxygenated the feedstock, the greater the hydrogen requirement. Carbon dioxide, for example, demands the most hydrogen to produce PtL or E-Jet, while used cooking oil needs the least.
Regardless of the starting point, hydrogen is always required. Vegetable oils demand the least, carbohydrates more, and carbon dioxide the most. This manifests itself in the availability of the different types of SAF—the closer the feedstock is to a hydrocarbon, the less hydrogen is required, making it more accessible.
In the next article, we look at where hydrogen comes from – green, blue, brown – choose your colour, and explore why it matters in the context of sustainable aviation fuel.
