In the Pits: Time to Refuel in the Race to Net Zero

Night has fallen on the most consequential race humanity has ever run. The pit lane glows under floodlights as engines scream into view — and with them come the hopes we’ve placed on sustainable fuels to save the world.

First over the line: Team Road Mode. A diesel-class endurance machine: scarred by two centuries of hauling freight, food, medicine. Industrial armor. Zero glamour. All impact.

Right behind: Team SAF Staff. Every aerodynamic curve polished — the hypercar we cheer for because we want the sky to be saved first. The drumbeat of airguns. The whoosh of jacks. The world leans in. A single silver can is lifted.Silence.

This is the last can of truly sustainable HEFA fuel. Used cooking oil (UCO) and tallow: the rarest energy miracle we have — waste-based, low-carbon… and mathematically scarce. They can meet only: ~20% of global road diesel needs, and a tiny fraction of aviation demand

One can. Two sectors. Scientific Reality vs. Marketing Appeal. Where does it go? A new study out of the University of Sherbrooke has answered with brutal clarity: If the goal is maximum CO₂ avoided now, put the can in the truck — not the plane. Why would the glamorous climate hero lose? To understand that, we move to the pit wall.

The core of the researchers’ argument lies in the technical differences between producing Renewable Diesel and HEFA-tJ . Although both use the same feedstocks—such as used cooking oil, animal fats (tallow), and vegetable oils—the refining processes differ significantly in their efficiency.

1. The Yield Gap – Where Team SAF Starts Losing Seconds

From the very first lap, the difference between RD and SAF shows up on the stopwatch. Although both start with the same low-CI feedstocks, the chemistry sends their race lines in opposite directions. A HEFA refinery can be configured to favor either diesel or jet fuel. When configured for HEFA-road (RD), the process is highly efficient, achieving a selectivity for high-value fuels of approximately 78% to 81%. However, producing SAF (HEFA-tJ) requires a hydrocracker to break down heavier molecules into the jet fuel range. This extra step generates higher rates of naphtha and refinery gas, which are lower-value byproducts.

Consequently, the overall yield for SAF is notably lower. The sources estimate a yield of approximately 79% for SAF versus 85% for RD. While a 6% difference may seem minor, when applied to a standard 900 kt/y plant, it results in several hundred thousand tons less GHG reduction per project. In essence, the process of making SAF “wastes” a portion of the sustainable feedstock by converting it into byproducts that do not contribute to flight decarbonization.

2. The Carbon Intensity Penalty — More Drag, Less Distance

Yield isn’t just a technical footnote — it decides how clean each megajoule looks crossing the finish line. The researchers found that shifting production to SAF can actually increase the carbon intensity of the resulting fuel. Because yield is the denominator in CI calculations, the yield losses in the SAF pathway automatically inflate the carbon footprint of every megajoule of fuel produced.

Furthermore, the most sustainable feedstocks—UCO and tallow—are extremely limited, covering only about 20% of current road fuel demand. To meet ambitious SAF targets like the U.S. SAF Grand Challenge or EU ReFuelEU, producers must turn to vegetable oils like soybean, rapeseed, or palm oil. These “high-CI” feedstocks provide much lower environmental benefits. For example, using rapeseed oil for SAF offers only a 14–28% reduction in emissions compared to fossil jet fuel, whereas using tallow or UCO can reach over 65–80%.

3. Social Cost of Carbon — Paying More to Finish Behind

Now comes the pit-wall metric that decides real-world winners: the social cost of CO₂ avoided. It reveals who’s actually scoring climate points per dollar spent. The study introduces a critical metric: the social cost of GHG reduction (USD per ton of CO2 avoided). This combines the “premium” (the extra cost of green fuel over fossil fuel) with the actual carbon savings. Because SAF is two to three times more expensive to produce than conventional jet fuel and suffers from yield losses, its social cost is staggering.

According to the researchers’ model, the social cost for SAF typically ranges from $700 to $1,100 per ton of GHG avoided. In contrast, the social cost for road fuels is significantly lower, often ranging between∗∗200 and $650 per ton. Society is essentially paying a massive premium to reduce fewer emissions in the air than it could on the ground.

“If So, Then What?” — When the Pit Wall Changes Strategy

If Team SAF is burning fuel just to stay in the race, where should the next gallon go? The researchers propose a smarter race plan — one that ranks every option by global tonnage per dollar.

1. Go After the Methane Volcanoes First

Rather than spending billions on new SAF refineries with low GHG efficiency, the researchers suggest focusing on methane capture at palm oil mills (POME). Indonesia and Malaysia produce 68 Mt of palm oil annually; capturing the methane emitted during its production is an order of magnitude less capital-intensive than building SAF plants. Integrating this technology could avoid over 1 million tons of CO2 per year—far more than a new SAF plant using the same resources.

2. Let Aviation Buy the Lap Time (Book & Claim)

The aviation industry faces immense pressure to decarbonize, but it shouldn’t necessarily do so by burning inefficient fuels in its own engines today. The researchers advocate for “book and claim” systems that allow airlines to invest in high-efficiency carbon reductions in other sectors, such as road diesel or methane capture, and claim those offsets. This ensures that aviation’s capital is directed toward the most efficient global GHG reductions rather than being trapped in high-cost, low-yield SAF production.

3. Reward Regions That Actually Race Clean

Current sustainability frameworks (like CORSIA) often use country-level averages to calculate indirect Land Use Change (iLUC). The researchers demonstrate that this masks massive regional disparities. For instance, some regions in Indonesia produce palm oil with very low carbon footprints (less than 20 g/MJ), while others are over 200 g/MJ due to peatland degradation. Policies should favor traceable, regional data, allowing the market to reward producers who actually implement sustainable practices.

4. Stop Red-Lining HEFA — Invest in the Next Engines

The HEFA pathway is mature, meaning its potential for cost reduction is low because it is limited by feedstock prices. To achieve real breakthroughs, policy support must shift toward emerging technologies like co-pyrolysis of lipids with waste plastics or biomass. While “risky,” these technologies could allow for the use of much cheaper, low-CI feedstocks that HEFA cannot process. Investing in these now prevents the market from being saturated by inefficient HEFA-tJ production.

5. Tune Up the Fleet Already on the Track

Rather than letting biodiesel plants sit idle or converting them to RD/SAF at high cost, policy should incentivize the adoption of supercritical or flexible pretreatment technologies. This allows existing, lower-capital-cost BD plants to process cheaper, high-acid feedstocks like yellow grease or tallow, achieving low social costs (as low as 30−70/t avoided in some scenarios).

Checkered Flag: Fuel the Leader — Then Lift the Trophy

The conclusion from the researchers lands like a headset slam on pit wall: “Raising production to a new market (SAF) is potentially not a sound orientation”— if the mission is maximum global GHG reduction now. Instead, they contend we should focus on greening the existing road fuel supply and investing in methane capture, which are less capital-intensive and considerably faster to deploy.

To visualize this, consider a leaky irrigation system. In this analogy, the sustainable feedstock is our limited water supply. Currently, policy is attempting to pump that water through the “Aviation” pipe, which is full of holes (yield losses) and requires an expensive, high-pressure pump (capital investment). Meanwhile, the “Road Diesel” pipe is already in place and much more watertight. If our goal is to grow the most crops (reduce the most CO2), it is far more sensible to fix the existing road pipe and ensure every drop of water reaches the field, rather than building a glitzy, inefficient new system for the orchard on the hill.

Only once we have plenty of water and the ground is saturated should we invest in the complex engineering required to reach the most difficult heights.