Is SAF Sound?
Even if aviation succeeds at developing clean propulsion technologies at breakneck pace, it is estimated that airlines will need at least 100 billion gallons of sustainable aviation fuel (SAF) annually by 2050 if the industry is to meet its goal of net zero emissions by that year.
But at 16 million gallons per year, current production of SAF is effectively zero in terms of industry requirements.
Therefore, aviation faces immense challenges in scaling up production of what is likely to be the only low-carbon option to bridge the period until hydrogen or electric aircraft enter service in meaningful numbers, which could be anything from 10 to 30 years or more.
“Aviation is a sector where you need to resolve the supply equation as well as the demand equation at the same time,” says Anna Mascolo, president of fuel supplier Shell Aviation.
Another supply question concerns what constitutes SAF.
To ensure that the partially synthetic Jet A/A-1 can be a fleetwide drop-in replacement for fossil kerosene, the proportion of synthetic fuel in the blend is limited to a maximum of 50%, and as little as 10% for some SAFs.
But because low-carbon fuels will be more expansive than fossil jet fuel for the foreseeable future, there is increasing talk of 100% SAF, up from the current 50% blend limit.
Yet it is not as simple as just pouring more biofuel into the tank.
So far, seven SAF blends have been approved by industry standards developer ASTM International.
Many more are in the testing and approval pipeline. The vast majority of fuel produced and consumed so far is a synthetic paraffinic kerosene called HEFA (hydroprocessed esters and fatty acids), produced from animal fats and vegetable oils such as used cooking oil.
HEFA can reduce lifecycle carbon emissions by as much as 80% compared with conventional jet fuel, but it is limited to a 50% blend as a drop-in fuel, so the maximum achievable reduction is 40%. This is because HEFA lacks the aromatic hydrocarbons that are present in fossil jet fuel and which are required for compatibility with fuel seals.
To find you more about the options for SAF and how production may develop, see the forthcoming Engine Yearbook 2022.