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(WK-intern) – Die Auswirkungen des USrael-Iran-Krieges auf den Ölmarkt haben die Aufmerksamkeit erneut auf die Pläne der EU zur heimischen Produktion fossilfreier Flugkraftstoffe gelenkt.

Doch die EU-Vorschriften für synthetische Flugkraftstoffe bergen das Risiko, die Entwicklung hin zu Produktionswegen zu lenken, die sowohl teurer als auch energieintensiver sind als nötig – was die Erreichung der Klimaziele erschwert.

Dies geht aus einer aktuellen Studie der Technischen Hochschule Chalmers in Schweden hervor, die verschiedene Methoden zur Herstellung von synthetischem Methanol analysiert hat.

Im vergangenen Jahr wurden Vorschriften eingeführt, die an EU-Flughäfen eine Mindestbeimischung von 2 Prozent nachhaltiger Flugkraftstoffe vorschreiben. Diese Beimischungspflicht wird schrittweise ansteigen und bis zum Jahr 2050 mindestens 70 Prozent erreichen. Bis dahin muss die Hälfte der nachhaltigen Flugkraftstoffe aus einer Kategorie bestehen, die als RFNBO bezeichnet wird: „Renewable Fuel of Non-Biological Origin“ (Erneuerbarer Kraftstoff nicht-biologischen Ursprungs). Dabei handelt es sich um synthetische Kraftstoffe – auch als Elektrofuels bekannt –, die aus erneuerbarem Wasserstoff und abgeschiedenem Kohlendioxid hergestellt werden.

Forscher der Technischen Hochschule Chalmers zeigen nun auf, dass die RFNBO-Vorschriften einen „Umweg“ bei der Herstellung synthetischer Kraftstoffe begünstigen, was das Risiko birgt, sowohl die Kosten als auch den Energieverbrauch in die Höhe zu treiben.

„Regulierungen beeinflussen nicht nur die Investitionen der Industrie in Technologien, sondern auch, welche Prioritäten bei Forschung und Entwicklung gesetzt werden. Anstatt Innovationen hin zu den effizientesten Lösungen voranzutreiben, laufen wir Gefahr, uns auf weniger ressourceneffiziente Produktionsmethoden festzulegen“, sagt Henrik Thunman, Professor für Energietechnik an der Chalmers und Koautor des wissenschaftlichen Artikels.

Weltweit werden Tausende neuer Anlagen benötigt, um die wachsende Nachfrage nach nachhaltigen Flugkraftstoffen in den kommenden Jahrzehnten zu decken. Dies erfordert sehr hohe Investitionen in Anlagen mit langen Betriebslaufzeiten.
Große Unterschiede zwischen alternativen Wegen für dasselbe Produkt unter Verwendung desselben Rohstoffs

Das Forschungsteam der Chalmers hat die Herstellung von synthetischem Methanol untersucht; dies ist ein Beispiel für ein Kraftstoffmolekül, das in nachhaltigen Flugkraftstoff umgewandelt werden kann. Es dient als repräsentativer Fall für die Analyse, wie sich unterschiedliche Produktionswege auf den Ressourcenverbrauch bei der Herstellung solcher Kraftstoffmoleküle auswirken.

Diese energiereichen Moleküle lassen sich durch die Kombination von Kohlenstoffatomen und Wasserstoff in chemischen Prozessen herstellen. In der Studie verglichen die Forscher drei verschiedene Produktionswege für Methanol, bei denen die Kohlenstoffatome aus Biomasse stammen – sogenannter biogener Kohlenstoff. Zwei der Methoden basieren auf der Verbrennung von Biomasse, wobei Kohlendioxid aus den Rauchgasen abgeschieden und anschließend mit Wasserstoff vermischt wird, der separat unter Einsatz von Strom erzeugt wurde. Die dritte Methode basiert auf der Vergasung, bei der erhitzte Biomasse direkt in Synthesegas umgewandelt wird, das sowohl Kohlenstoff als auch Wasserstoff enthält.

Alle drei Produktionswege sind technisch machbar, und sowohl der Rohstoff als auch das Endprodukt können identisch sein. Sie unterscheiden sich jedoch deutlich hinsichtlich Energieeinsatz, Kosten und Strombedarf.
Der direkte Produktionsweg wird durch EU-Vorschriften benachteiligt

„Der Vergasungsweg erwies sich in unserer Analyse als die ressourceneffizienteste Option – mit bis zu 46 Prozent geringeren Produktionskosten und einem um 30 Prozent niedrigeren Strombedarf als die beiden verbrennungsbasierten Alternativen. Dieser Unterschied zeigt, wie groß die Energieverluste sein können, wenn Biomasse zunächst zu Kohlendioxid verbrannt wird, das anschließend unter Einsatz großer Mengen an Strom und Wasserstoff wieder zu Kraftstoffmolekülen aufgebaut werden muss“, sagt Johanna Beiron, Forscherin für Physikalische Ressourcentheorie an der Chalmers University und Erstautorin des Artikels.

Dennoch wird die Verbrennung durch den regulatorischen Rahmen der EU weitaus stärker begünstigt als die Vergasung. Die Kategorie der RFNBOs – deren Anteil an allen Flugkraftstoffen in der EU voraussichtlich von heute fast null auf 35 Prozent im Jahr 2050 ansteigen wird – umfasst sämtliche Kraftstoffe aus den verbrennungsbasierten Alternativen, schließt jedoch etwa die Hälfte der über Vergasung erzeugten Kraftstoffe aus.

Der Grund dafür ist, dass RFNBO-Kraftstoffe nicht unter Verwendung von Energie und Kohlenstoffatomen erzeugt werden dürfen, die direkt aus Biomasse stammen – wie dies bei der vergasungsbasierten Produktion größtenteils der Fall ist. Im Gegensatz dazu ist die Verwendung von Kohlenstoffatomen aus Biomasse bei verbrennungsbasierten Verfahren zulässig, sofern dies durch die Abscheidung des Kohlendioxids erfolgt, das bei der Nutzung von Biomasse für andere energetische Zwecke entsteht. Ein Beispiel hierfür ist die Verbrennung von Reststoffen aus der Forstwirtschaft in Kraft-Wärme-Kopplungsanlagen.

EU rules could make fossil-free aviation fuels unnecessarily expensive and energy-intensive

EU regulations risk leading to inefficient use of biomass for production of sustainable aviation fuel, according to a study from Chalmers University of Technology. Image: CC BY Gustavo Ramirez/CC0

The effects of the US-Israel-Iran war on the oil market have brought renewed attention to the EU’s plans for domestic production of fossil-free aviation fuels.

But EU rules for synthetic aviation fuels risk steering development towards production pathways that are both more expensive and more energy-intensive than necessary – making it harder to meet climate targets. This is shown in a recent study from Chalmers University of Technology, Sweden, that has analysed different methods for producing synthetic methanol.

Last year, rules were introduced requiring a minimum blend of 2 percent sustainable aviation fuel at EU airports. This blending requirement will increase gradually, reaching at least 70 percent by 2050. By then, half of the sustainable aviation fuel must consist of a category known as RFNBO: Renewable Fuel of Non-Biological Origin. These are synthetic fuels, also known as electrofuels, produced from renewable hydrogen and captured carbon dioxide.

Researchers at Chalmers University of Technology now show that the RFNBO rules favour a “detour” in the production of synthetic fuels, which risks increasing both costs and energy use.

“Regulations influence not only industry’s investments in technology, but also which research and development priorities are pursued. Instead of driving innovation towards the most efficient solutions, we risk locking ourselves into less resource-efficient production methods,” says Henrik Thunman, Professor of Energy Technology at Chalmers and co-author of the scientific article.

Thousands of new plants will be needed globally to meet the growing demand for sustainable aviation fuels in the coming decades. This will require very large investments in facilities with long operating lifetimes.

Big differences between alternative pathways for the same product using the same raw material

The research team at Chalmers has studied the production of synthetic methanol, which is an example of a fuel molecule that can be converted into sustainable aviation fuel. It provides a representative case for analysing how different production pathways affect resource use in the production of such fuel molecules.

These energy-rich molecules can be produced by combining carbon atoms and hydrogen in chemical processes. In the study, the researchers compared three different production pathways for methanol in which the carbon atoms come from biomass – so-called biogenic carbon. Two of the methods are based on biomass combustion, where carbon dioxide is captured from flue gases and then mixed with hydrogen produced separately using electricity. The third is based on gasification, where heated biomass is converted directly into synthesis gas, which contains both carbon and hydrogen.

All three production pathways are technically feasible, and both the raw material and the final product can be the same. However, they differ clearly in terms of energy use, cost and electricity demand.

The direct production pathway is disadvantaged by EU regulation

“The gasification pathway proved to be the most resource-efficient option in our analysis, with up to 46 percent lower production cost and 30 percent lower electricity demand than the two combustion-based alternatives. The difference shows how large the energy losses can be when biomass is first combusted into carbon dioxide, which is then rebuilt into fuel molecules using large amounts of electricity and hydrogen,” says Johanna Beiron, researcher in Physical Resource Theory at Chalmers and first author of the article.

Despite this, combustion is favoured much more strongly than gasification by the EU regulatory framework. The RFNBO category – which is expected to expand from close to zero today to 35 percent of all aviation fuel in the EU by 2050 – includes all fuel from the combustion-based alternatives, but excludes around half of the fuel produced via gasification.

The reason is that RFNBO fuels may not be produced using energy and carbon atoms that come directly from biomass, as they largely do in gasification-based production. In contrast, it is permitted to use carbon atoms from biomass in combustion-based routes, provided this is done by capturing the carbon dioxide formed when biomass is used for other energy purposes. One example is the combustion of residual material from the forest industry in combined heat and power plants.

But such residual material can be used more resource-efficiently through gasification.

“One of the combustion-based alternatives we analysed was the process in combined heat and power plants,” says Johanna Beiron. “It has lower cost and energy efficiency than gasification, even when we include the additional electricity needed to replace, for example, the district heating that the combustion process can contribute.”

Regulation risks working against its own objectives

One purpose of the RFNBO classification is to stimulate increased generation of renewable electricity for the production of green hydrogen, and to reduce dependence on biomass, which is a limited resource.

But the carbon atoms for synthetic aviation fuel must come from somewhere. Biomass is expected to be the least costly fossil-free carbon source for RFNBO production, and the researchers expect that today’s regulatory framework will result in a very high demand for carbon dioxide from biomass combustion. Instead of reducing the need for biomass, the EU regulations risk driving a less energy-efficient use of the limited biomass resource.

“The regulatory framework does not account sufficiently for how efficiently different systems use energy and resources,” says Henrik Thunman. “The study therefore highlights a structural issue in EU energy and industrial policy: regulation risks working against its own objectives when definitions of sustainable fuels are not aligned with fundamental energy principles or with the Union’s broader ambitions for resource efficiency.”

Adjusted regulation may be needed to enable effective transition

The researchers hope that their results will contribute to greater knowledge about the technologies and systems that are available.

“It is surprising that EU rules do not provide clearer incentives for the most efficient alternatives,” says Johanna Beiron. “The current regulatory framework risks causing lock-in to combustion-based energy systems, even though technically mature processes already exist that would provide both lower energy use and lower cost – such as gasification and electrification of district heating.”

“Our study shows that some parts of the regulatory framework probably need to be adjusted if the EU is to achieve its long-term goals,” says Henrik Thunman. “Better coordination is needed between climate targets, resource efficiency and industrial feasibility in order to address the uncertainty that currently exists. This uncertainty makes it difficult to make rational investment decisions for the large-scale expansion of sustainable aviation fuels in the coming years.”

More about: the research

The study Locked in on RFNBOs – Will EU mandates for drop-in synthetic aviation fuels lead to decreased energy- and cost-efficiency? is published in the journal Fuel. It was carried out by Johanna Beiron, Simon Harvey and Henrik Thunman at Chalmers University of Technology in Sweden.

The research is part of the project FUTNERC, Transformative change towards net negative emissions in Swedish refinery and petrochemical industries. It is a five-year research project funded 50 percent by the Swedish Energy Agency and 25 percent each by the companies VaroPreem and Borealis. The project aims to accelerate the transformation of the chemical industry to achieve net zero greenhouse gas emissions from refineries and chemical plants by 2050.

More about: the three methods for producing synthetic methanol

The researchers chose to study the production of methanol because it provides a clear example of how the overall efficiency of fuel production is affected by whether carbon is used directly in the form of carbon-containing gas from biomass, or first converted into carbon dioxide and then rebuilt with the help of hydrogen. The study compares three established, but not yet commercially used, production pathways based on renewable hydrogen and biomass in the form of residual material from the forest industry.

1. Combustion with carbon capture

Biomass is combusted and carbon dioxide is captured from the flue gases. Hydrogen, produced separately from water and electricity, is then added. The carbon dioxide reacts with hydrogen in a catalytic process to form methanol.

• High production cost: 1055 euros per tonne of methanol
• High electricity consumption: 1.8 megawatts of electricity per megawatt of methanol
• Low energy efficiency: approximately 37 percent

2. Combustion with carbon capture and simultaneous energy production

This pathway is similar to the first, but the combustion is also used to produce electricity or heat, for example for district heating systems. The study also includes the additional electricity required to replace this form of energy.

• Highest cost of the three alternatives: 1495 euros per tonne of methanol
• High electricity consumption: 1.6 megawatts of electricity per megawatt of methanol
• Equally low energy efficiency: approximately 37 percent

3. Biomass gasification

Biomass is converted through gasification into a synthesis gas consisting of carbon monoxide and hydrogen, which is then used directly in methanol synthesis. Gasification also produces some carbon dioxide, which can also be converted into methanol together with limited amounts of added hydrogen.

• Lowest cost of the three alternatives: 820 euros per tonne of methanol
• Lowest electricity consumption: 1.2 megawatts of electricity per megawatt of methanol
• Highest energy efficiency: approximately 46 percent

More about: the EU regulatory framework

The EU regulation ReFuelEU Aviation introduces binding requirements for a growing share of sustainable aviation fuel to be blended into aviation fuel sold at EU airports. The first requirements took effect in 2025 and will gradually become stricter in order to drive new production of sustainable fuels.

By 2050, at least half of all sustainable aviation fuel must consist of RFNBOs: Renewable Fuels of Non-Biological Origin. The other half consists of four other categories of sustainable aviation fuel.

For more information, please contact:

• Johanna Beiron, Doctor, Physical Resource Theory, Department of Environmental and Energy Sciences, Chalmers University of Technology, Sweden, beiron@chalmers.se
• Simon Harvey, Professor, Energy Technology, Department of Environmental and Energy Sciences, Chalmers University of Technology, Sweden, +46 31 772 85 31, simon.harvey@chalmers.se
• Henrik Thunman, Professor, Energy Technology, Department of Environmental and Energy Sciences, Chalmers University of Technology, Sweden, +46 31 772 14 51, henrik.thunman@chalmers.se

All the researchers speak Swedish and English, and Simon Harvey also speaks French and Italian.

PR: Chalmers University of Technology, Sweden

PB: EU-Vorschriften bergen das Risiko, zu einer ineffizienten Nutzung von Biomasse für die Herstellung nachhaltiger Flugkraftstoffe zu führen – dies geht aus einer Studie der Technischen Hochschule Chalmers hervor. Bild: CC BY Gustavo Ramirez/CC0