From Old Waste to New Wings
Agricultural residues and waste wood could help fuel planes. Stavroula Zervopoulou outlines how the Circular Fuels project is scaling sustainable aviation fuels in the EU.
Stavroula Zervopoulou has an academic background in mineral resources engineering, energy economics, and petroleum engineering. She is currently a research project assistant at Vienna University of Technology and serves on the executive board of the EU-funded Horizon Circular Fuels project. The initiative aims to turn waste biomass into sustainable aviation fuels (SAF) using solar-powered fast pyrolysis. In this conversation, she discusses how advanced biofuels can be scaled, the challenges of supply chains, and why circular economy thinking is key to decarbonising aviation.
Could you briefly introduce the Circular Fuels project and your role within it?
The Circular Fuels project aims to produce sustainable aviation fuels from waste biomass by coupling fast pyrolysis with solar energy. It is not only about producing fuels through renewable energy innovation, but also about demonstrating that this technology works in practice and can be profitable. The main objective is to show the effectiveness of an innovative approach that combines solar energy with fast pyrolysis of biomass.
As for my role, I am mainly working on scale-up perspectives, exploitation, and policy recommendations. This part of the project looks at how to take the technology from the research stage to a larger scale and how to connect the technical results with policy and market realities. Vienna University of Technology leads this work package, and my job is to support the development of strategies that ensure the project’s outputs can be translated into real-world solutions.
What kinds of agricultural residues and waste wood are actually used?
We are working with two main categories of feedstocks. The first is waste wood. This includes demolition wood grade B, which is non-hazardous wood that has been treated with coatings like paint or varnish. It also includes pinewood sawdust, a byproduct of wood processing that is commonly available.
The second category is agricultural residues. These are materials such as straw from wheat and crops. All four feedstocks we are focusing on are low-cost, bio-based materials. Importantly, they do not compete with food production. Both categories are recognised in the Renewable Energy Directive II (RED II), specifically in Annex IX, which lists sustainable feedstocks for advanced biofuel production.
Under the updated RED III, there is even greater emphasis on using waste and residue-based feedstocks, especially to meet stricter targets for renewable energy in transport. This includes specific sub-targets for advanced biofuels and renewable fuels of non-biological origin. What is important is that RED III not only promotes these materials but also reinforces the sustainability criteria. That ensures the feedstocks we use do not compete with food crops or lead to negative environmental impacts. The project is fully aligned with this push for sustainable, circular, low-indirect land use change feedstocks that contribute to renewable energy goals in aviation.
Where does the feedstock come from?
At first, we focused our research on three countries: France, the Czech Republic, and Denmark. These were our starting points for assessing feedstock availability and regional potential. As the project progressed, we expanded to include all 27 EU Member States. This gave us a much broader perspective on availability across Europe and helped us identify potential synergies between different regions.
To be more precise, the assessment was done first at the national level and then refined down to the NUTS-3 regional level. By going all the way down to this level, we were able to pinpoint the most important locations for bio-oil production plants. This matters because feedstock availability is one of the most critical factors in deciding where production facilities should be located.
How much sustainable aviation fuel would Europe need to replace fossil kerosene? Is it realistic, and if so, by when?
This is a very important question. According to the RefuelEU Aviation Regulation, the EU has set clear targets for integrating SAF. It starts with 2% of fuel requirements by 2025, but the ambition scales quickly, going up to 70% by 2050.
To put this into perspective, aviation fuel demand at EU airports is projected to be about 46 million tonnes by 2030. Meeting 70% of that with SAF by 2050 would mean producing around 35 million tonnes annually. As of 2030, there is also a sub-mandate for synthetic fuels such as e-kerosene and methanol, within the overall SAF share.
Our project is contributing to this vision by examining whether the solar-driven fuels we are developing from waste-based feedstocks can meet the technical and chemical requirements to be classified as drop-in fuels, or at least to what extent they can be blended into conventional fossil jet fuels. Beyond just fuel quality, we are also exploring how to build efficient supply chain models, from sourcing feedstocks to converting them into SAF, with a strong focus on both scalability and sustainability.
What is the current state of advanced biofuels in Europe?
The most recent data we can refer to is from 2020. According to the Advanced Fuel Project, the share of renewables in transport, which includes biofuels, green electricity, hydrogen and so on, rose from 1.5% in 2004 to 8.3% by 2018. Zooming in on biofuels, total consumption reached 17 million tonnes of oil equivalent (Mtoe) in 2018, up from 15.4 Mtoe the year before.
This is an increase of about 2 to 3 Mtoe equivalent per year, showing a steady upward trend. In terms of energy content, biodiesel was the majority at 82%, bioethanol was about 17%, and biomethane fuel was less than 1%.
When it comes to advanced biofuels, blending only makes up about 1.2%. Most of this came from fatty acid methyl esters for biodiesel produced from waste fats and oils, with only a small share from agricultural and forest byproducts such as used cooking oil, tall oil, and cellulosic feedstock oil. So, the share of advanced biofuels is still relatively small, but the foundation exists. Scaling this up will be critical if Europe wants to meet climate and transport decarbonisation targets.
Farmers sometimes use crop residues to enrich soil or feed animals. Are they really willing to give them up?
That is a complex question. Willingness can vary depending on regional practices, economic incentives, and environmental awareness. It is not something we can answer with certainty right now, but it is an issue we take seriously.
We have assessed the sustainable rate of removal for agricultural residues, especially wheat and rye straw, because excessive removal can reduce long-term soil productivity. This project aims to provide, at a high level, an innovative technological approach to the production of SAF and assess it using a systemic multicriteria cradle-to-grave LCA approach to monitor such burden shifting, and promote the most environmentally benign technological solutions. One of the byproducts of pyrolysis is biochar, which is mainly used in soil. It can improve nutrient availability, increase water retention, and act as a carbon sink on agricultural land.
While we cannot predict how farmers will respond, our project provides evidence on how environmentally friendly by products like biochar can return benefits to agriculture.
So, while we cannot predict how farmers will respond, our project provides evidence on how environmentally friendly byproducts like biochar can return benefits to agriculture. This helps close the loop in line with bioeconomy and circular economy principles, making agriculture, fuel production, and climate goals work in harmony. By providing clear data on environmental performance and economic feasibility, we can support better decision-making for policymakers, stakeholders, and farmers alike.
There’s been concern about importing waste wood. How does the project approach this?
Our focus is on using the types of waste wood that are already produced within Europe. Demolition wood and pinewood sawdust exist in all EU member states, though in different amounts. The challenge is not about importing waste from outside the EU, but about coordinating supply chains within Europe.
Circular Fuels is studying how EU countries can work together to make the most of existing waste resources and build a system that is both economically viable and environmentally sustainable.
Scaling advanced biofuels is often cited as the biggest challenge. How do you tackle this?
This is one of the core challenges of the project. We have gathered comprehensive historical data covering the past 20 years for the feedstocks we plan to use. We have also forecasted this data into future scenarios for 2030 and 2050, and mapped everything down to the regional level.
But data alone is not enough. We are evaluating the scalability of production and the deployment potential in selected EU locations. This includes identifying and addressing key technical and logistical challenges associated with scaling up the chosen technology from its current technology readiness level to demonstration and eventually commercial plants.
Circular Fuels is not just working on new conversion technologies, but also on the entire system from sourcing and transport to final fuel production.
Circular Fuels takes a whole value chain approach. We are not just working on new conversion technologies, but also on the entire system from sourcing and transport to final fuel production. We are developing efficient, regionally tailored supply chain models, guided by life cycle assessments and cost analyses. This ensures that scaling is strategic and sustainable.
If aviation switched to SAF, how much emissions could be reduced?
Sustainable aviation fuels are crucial for cutting greenhouse gas emissions, especially CO2. Studies so far suggest that advanced biofuels could offer 60 to 70% emission savings compared to their fossil counterparts, when assessed over their full life cycle. Aviation accounts for about 14.4% of the EU’s transport emissions, so the potential impact is significant.
In our project, we are carrying out detailed life cycle assessments to provide precise numbers by the end. These cover all stages from biomass collection and transport, pretreatment, upgrading, final fuel production, and by-products use over a defined time frame. The aim is to provide solid data on the real impact SAF can have. Sustainable aviation fuels are crucial for cutting greenhouse gas emissions, especially CO2. Studies so far suggest that advanced biofuels could offer 60 to 70% emission savings compared to their fossil counterparts, when assessed over their full life cycle. Aviation accounts for about 14.4% of the EU’s transport emissions, so the potential impact is significant.
In our project, we are carrying out detailed life cycle assessments to provide precise numbers by the end. These cover all stages from biomass collection and transport, pretreatment, upgrading, final fuel production, and by-products use over a defined time frame. The aim is to provide solid data on the real impact SAF can have.
What about contrails, those white trails planes leave behind that also warm the climate?
Yes, this is an insightful question. We are actively studying it. The project has specific work packages focused on how SAF behaves during combustion in jet engines, including contrail formation. This involves experimental testing and advanced modelling techniques to assess emissions.
It is too early to quantify the reduction now, but the results will be integrated into our life cycle assessments. By the project’s conclusion, we will have science-based answers to this issue.
And beyond emissions, could SAF also improve air quality for airport workers?
SAF could reduce emissions of sulphur oxides and particulate matter, depending on the feedstock and process. While the main benefit is on system-wide environmental impact, there may also be positive effects for people working at airports who are exposed to fumes.
For the big picture, how do you balance sustainability with scale?
We rely mainly on waste biomass like wheat and rye straw, with careful management to avoid over-removal and protect soil health and carbon stocks. Byproducts like biochar are reintegrated into agriculture to recycle nutrients and reduce reliance on synthetic fertilisers.
To reduce emissions further, the project uses solar energy to heat the pyrolysis process and photovoltaic systems to generate steam and hydrogen, replacing fossil-based inputs with clean energy.
We also use systemic, multi-criteria cradle-to-grave life cycle assessment using the holistic ReCiPe framework to monitor potential burden shifting across impact categories and identify the most environmentally favourable technological solutions, ensuring that the project advances climate and sustainability objectives on multiple fronts. In short, the aim is to scale advanced biofuels without creating new problems, ensuring that aviation decarbonisation is both sustainable and circular.