Residual streams from forests, agriculture, and households now have numerous applications, ranging from green gas and heat to bio-based chemistry and materials. Two of these yield climate benefits in fundamentally different ways. With biochar, the material is heated without oxygen, resulting in a carbon-rich product that is applied to the soil, where the carbon remains for a long time instead of being released into the air as CO2. With sustainable aviation fuel, usually referred to as SAF in the industry, it is converted into a liquid fuel that replaces fossil kerosene. The first route removes carbon from the cycle; the second prevents the addition of new fossil carbon. It is precisely this difference that makes the two difficult to compare side by side.
Yet that is precisely what three researchers did in the open-access journal iScience, in an article published on August 21, 2026. Hassan El-Houjeiri of the KAPSARC research institute in Riyadh, Adam Brandt of Stanford University, and Mohammad Masnadi of the University of Pittsburgh deliberately focused on these two routes. Biomass is a scarce commodity with limitations regarding moisture content, energy density, and supply, they argue, and therefore its value is best realized in sectors where hardly any alternatives exist. Aviation is the prime example of this. Moreover, there are enough published cost studies on both routes to build a comparison. Other applications of residual streams fall outside their analysis.
What was missing until now was a way to make those studies comparable, the authors write. They converted everything to 2024 dollars and applied the same yardstick: what does it cost to remove one ton of CO2 from the air or keep it out? The research received no external funding, and the authors report no conflicts of interest.
What the comparison yields
For biochar, the average comes out at approximately 177 dollars per ton of CO2. For aviation fuel from residual and waste streams, that figure is around 448 dollars, so more than twice as high. For biochar, the most known cases range between 111 and 497 dollars. For SAF, the spread is much wider, from 98 to 948 dollars, and this is because very different techniques fall under that single umbrella.
These techniques are worth examining, as they differ significantly. The only route currently operating commercially on a large scale uses waste fats and oils and is called HEFA. This comes in at around 362 dollars per ton of CO2. Gasification followed by Fischer-Tropsch synthesis comes close to that at 342 dollars. Clearly more expensive are the routes that produce aviation fuel from alcohol, at 938 dollars, and the route via rapid pyrolysis, at 652 dollars. Precisely these more expensive routes are necessary to ever achieve large volumes, because there is simply not enough waste fat in the world. The researchers do note, however, that they could only find three and two public cases for those two, respectively—too few to base firm conclusions on.
The substantiation for biochar is more robust. The researchers used seventeen cost studies and also examined nineteen installations included in the Puro.earth certification register. Such installations are required to have verified how much CO2 they actually sequester, after deducting the emissions from collection and processing itself. This turned out to lie between 0,9 and 3,2 tonnes of CO2 per ton of biochar. Scale makes a significant difference here: an installation producing ten times as much does so at nearly sixty percent lower costs per ton. That gain has its limits, however, because beyond a certain scale, raw materials must be sourced from increasingly further afield. The researchers were unable to substantiate exactly where that tipping point lies with figures.
Not a choice, but an order
The main message of the study is not that biochar wins. The authors state in so many words that their outcome is not a prescription for where residual streams should go. That depends on what is available locally, who else lays claim to it, what subsidies are available, and what infrastructure is already in place. Moreover, they point out that biochar itself is part of a broader family of options for capturing carbon from biomass, and that some of these can prove cheaper or more sustainable in certain situations. Furthermore, the additional benefits of both routes, such as the soil activity of biochar or the contribution of SAF to one's own fuel supply, were not included in the cost comparison.
What the authors do point out is a difference in timing. Biochar is a mature technology that is already operational, accounting for an estimated global production capacity of more than 350 kilotons per year in 2023, spread across six continents. For the period up to approximately 2030, that is the route that can scale up rapidly. Aviation fuel from woody residues and household waste would only really start to matter in the 2030s and 2040s, and then only if the first factories of that type prove to operate reliably. That is not a given. The researchers point to a series of projects that were delayed, restructured, or failed altogether.
The current figures underscore how nascent that market still is. In 2025, around 2 million tonnes of SAF were produced worldwide, approximately 0,7 percent of all kerosene consumed by aircraft. In its forecast for 2050, aviation umbrella organization IATA assumes that waste fats and oils will supply slightly more than fifteen percent of SAF by then, agroforestry residues about a quarter, and household residual waste about seven percent.
For policymakers, there is another catch. Blending obligations, tax benefits, and purchasing agreements with airlines can push biomass towards fuel, even if sequestration per ton of CO2 is cheaper. Moreover, the two certification systems operate independently of each other. Those who sequester CO2 receive certificates at the project level, while those who supply fuel use aviation life cycle standards. As soon as the same raw material is listed in both systems, double counting is a real risk.
The researchers themselves call their study a snapshot and not a definitive ranking of biomass routes. Better operational data from SAF plants running on woody residues could still shift the picture.
Source and report: sciencedirect.com
Photo: Madelaine – Adobe Stock
