Renewable diesel and SAF: Efficiency
Why a SAF plant mostly makes diesel instead
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Hydrotreating turns oils and fats into hydrocarbons identical to fossil diesel and jet fuel, but a standard HEFA plant makes only 15-20% jet fuel, the rest mostly renewable diesel. Routes from solid biomass convert only about 14-18% of the feedstock's mass into distillate fuel.
Efficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| HEFA plant, jet fraction in standard operation | 15 to 20% of product1 | Commercial | Share of liquid output that is jet fuel; most of the rest is renewable diesel |
| HEFA plant, maximum jet fraction | 50% of product1 | Record | With extra hydrocracking and isomerisation, at about 10% lower total yield |
| Gasification and Fischer-Tropsch, distillate yield | 14 to 18% by mass2 | Commercial | Tonnes of distillate fuel per tonne of agricultural residue (14%) or forest residue (18%) |
| Fischer-Tropsch, jet share of product | 40% of product1 | Commercial | Approximate maximum share of jet and middle distillates with current technology |
| Alcohol-to-jet, jet share of product | 70% of product1 | Theoretical limit | Theoretical maximum jet yield, with gasoline as the other product |
| Crude tall oil (pulp mill by-product) to biofuel | 50 to 60% yield1 | Commercial | Fuel output per unit of crude tall oil feed |
Where the energy is lost
| Step | Loss |
|---|---|
| Hydrocracking to shift HEFA output from diesel to jet | about 10% loss of total yield at 50% jet1 |
| Fischer-Tropsch: carbon and energy lost as gas, heat and light by-products in gasification and synthesis | 82-86% of feedstock mass not recovered as distillate (calculated)2 |
Hydrotreated esters and fatty acids (HEFA) is the main route to both renewable diesel (also called HVO, hydrotreated vegetable oil) and sustainable aviation fuel (SAF). Instead of attaching methanol, as biodiesel does, it uses hydrogen at high temperature over a catalyst to strip out the oxygen, leaving straight-chain hydrocarbons that are chemically like fossil diesel. Isomerising and cracking them gives the lighter, branched molecules needed for jet fuel, which is why pushing the jet share higher costs hydrogen and yield.
Fischer-Tropsch and alcohol-to-jet start from solid biomass or sugars rather than oil. They must rebuild fuel molecules from gas or alcohol, so far less of the feedstock ends up as liquid fuel. ASTM has approved eight SAF pathways, but only HEFA is fully commercial.
Sources
- Reaching Zero with Renewables: Biojet fuels IRENA, 2021
- Progress in commercialisation of biojet / SAF (van Dyk, IEA Bioenergy workshop presentation) IEA Bioenergy Task 39 (hosted by DBFZ), 2023