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Glossary · Energy industry & energy transition

Fischer-Tropsch synthesis

Fischer-Tropsch synthesis power-to-liquid E-Fuels hydrogen CO2 synthetic fuels

Fischer-Tropsch synthesis is a chemical process that produces liquid hydrocarbons from synthesis gas. Synthesis gas consists essentially of carbon monoxide (CO) and hydrogen (H₂). On a metal catalyst, usually based on cobalt or iron, hydrocarbon chains of varying length form from it at elevated temperature and pressure. Water is formed as a by-product. In simplified terms, the chain grows link by link: each CO molecule supplies one carbon atom, and the hydrogen saturates the chain and binds the oxygen to form water. The process was developed in the 1920s by Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Institute for Coal Research in Mülheim an der Ruhr. It was originally used to obtain liquid fuels from coal.

The reaction does not yet yield a finished fuel of standard quality. It produces a broad product mixture: from methane and liquefied petroleum gas through naphtha, kerosene and diesel fractions to solid waxes. The composition of this mixture depends on the catalyst, temperature, pressure, gas ratio and reactor type. A single desired product cannot be made selectively, because the chain lengths are statistically distributed. The crude product, often called syncrude, is therefore separated and upgraded. Long wax chains are, for example, broken down by hydrocracking and isomerised. Fischer-Tropsch therefore does not describe the entire production pathway of a synthetic fuel, but a central conversion step within a longer process chain.

Operating modes and catalysts

In the technical literature, a distinction is usually made between low-temperature and high-temperature Fischer-Tropsch (LTFT and HTFT). The low-temperature variant operates at roughly 200 to 260 °C with cobalt or iron catalysts. It mainly yields long-chain paraffins and waxes, from which diesel and kerosene can be obtained. The high-temperature variant runs at about 300 to 350 °C with iron catalysts and tends to produce short-chain olefins and gasoline fractions. Pressures are typically in the range of a few megapascals, i.e. a few tens of bar. The exact values differ depending on the source and the plant and should be understood as ranges. The reaction is strongly exothermic. Heat removal is therefore a central task in reactor design. The waste heat can partly be used elsewhere in the process.

Cobalt is regarded as more active and more stable and is usually preferred for the low-temperature route. Iron is cheaper and additionally catalyses the water-gas shift reaction. It therefore also copes with synthesis gas that contains little hydrogen relative to CO, as arises for example in coal gasification. For cobalt-based synthesis, the required H₂/CO ratio is close to 2.

Synthesis gas, hydrogen and carbon source

Historically and in today's large-scale plants, synthesis gas is obtained predominantly from coal (coal-to-liquid) or from natural gas (gas-to-liquid). Such fuels are liquid, low in sulphur and easy to store, but not climate-neutral. Their carbon comes from fossil sources and enters the atmosphere as CO₂ on combustion. The gasification of biomass or waste can also yield synthesis gas.

In the context of power-to-liquid, the required hydrogen is produced by electrolysis from water and electricity. The carbon comes from captured CO₂, for example from biogenic processes, industrial point sources or direct capture from the air (direct air capture). Because classical Fischer-Tropsch synthesis processes CO and not CO₂, the CO₂ must first be converted, for example via the reverse water-gas shift reaction. In this reaction, CO₂ reacts with hydrogen to form CO and water. Alternatively, co-electrolysis of steam and CO₂ is an option. Only this combination makes Fischer-Tropsch synthesis a building block for e-fuels. The climate impact therefore does not depend on the name of the process, but on the origin of the electricity, hydrogen and carbon.

Distinction from e-fuels and power-to-liquid

Fischer-Tropsch synthesis, power-to-liquid and e-fuels are often equated, but they describe different levels. Fischer-Tropsch is a specific synthesis process. Power-to-liquid is a process route that converts electrical energy into liquid energy carriers. E-fuels is a collective term for electricity-based fuels. Not every e-fuel is produced via Fischer-Tropsch: methanol can be synthesised directly from hydrogen and CO₂, and synthetic methane is produced via methanation. Kerosene can also be produced via a methanol intermediate step. Ammonia is not a hydrocarbon and not a Fischer-Tropsch product. Conversely, not every Fischer-Tropsch product is an e-fuel: if the synthesis gas comes from coal or natural gas, the fuel is of fossil origin.

Use in aviation

The most important target product of the electricity-based route is currently synthetic kerosene. Fischer-Tropsch kerosene (FT-SPK) has been approved since 2009 as Annex A1 of the aviation fuel standard ASTM D7566. To date it may be blended with fossil jet fuel at no more than 50 percent. The reason for the limit is not the origin of the fuel but its composition: pure FT-SPK contains hardly any aromatics, and these are important, among other things, for the seals in today's fuel systems. A variant with aromatics (FT-SPK/A) has been approved since 2015 as Annex A4. The blend can be used via existing tanks, pipelines and aircraft. This is a practical advantage, but not an efficiency argument.

Significance for the power system

Fischer-Tropsch synthesis is relevant for the power system because it can convert electricity into storable, transportable liquid fuels with high energy density. This is of interest above all where direct electrification is technically difficult or economically unfavourable, for example in long-haul flights, in parts of maritime shipping and in the chemical industry as a raw material source. The chain, however, involves considerable losses: electrolysis, CO₂ capture, synthesis gas production, synthesis, upgrading and finally combustion in the engine or turbine. A passenger car running on Fischer-Tropsch fuel requires, per kilometre, a multiple of the electricity of a battery-electric vehicle. This difference determines how much renewable generation, electrolysis capacity and CO₂ supply would have to be built up. Fischer-Tropsch synthesis is therefore not a general substitute for efficiency, electrification or grid expansion.

Load, flexibility and siting issues

A power-to-liquid plant with Fischer-Tropsch synthesis is a large industrial electricity consumer. Electrolysers can be operated comparatively flexibly. The Fischer-Tropsch reactor, by contrast, works most stably and most economically in continuous operation that is as steady as possible. Rapid load changes affect temperature control, product distribution and the service life of the catalyst. Between fluctuating hydrogen production and continuous synthesis, hydrogen storage, buffers and coordinated plant control are therefore needed. How far the reactor itself can be operated dynamically is the subject of ongoing research.

A plant close to inexpensive renewable generation can use electricity that would otherwise go unused because of grid congestion. This does not, however, automatically make it beneficial to the grid. Whether it incentivises additional renewable generation or aggravates existing scarcities is determined by electricity prices, grid charges, support conditions, and rules on origin and electricity sourcing. In the EU, these include the additionality criterion and requirements on the temporal and geographical correlation between electricity sourcing and hydrogen production. For industrial plants, occasional surplus electricity is usually not enough: the less often they run, the higher the capital costs per litre. Power-to-liquid is thus not a mere valve for power peaks, but a demand block in its own right. Water and CO₂ are also siting factors. Biogenic CO₂ is available only to a limited extent. Fossil process CO₂ tends to shift emissions rather than avoid them. Direct air capture requires additional energy and capital.

Legal framework in the EU

In the EU, it is above all Regulation (EU) 2023/2405 (ReFuelEU Aviation) that creates a market for electricity-based kerosene. The regulation applies directly and obliges fuel suppliers at EU airports. According to Annex I, the following minimum shares of sustainable aviation fuels (SAF) and of the synthetic aviation fuels contained therein apply:

  • from 2025: 2 % SAF;

  • from 2030: 6 % SAF. Of this, in the years 2030 and 2031 an average of 1.2 % must be synthetic, but at least 0.7 % in each individual year. For 2032 to 2034, the average synthetic share is 2 %;

  • from 2035: 20 % SAF, of which at least 5 % synthetic; from 2040: 34 % and 10 %; from 2045: 42 % and 15 %;

  • from 2050: 70 % SAF, of which at least 35 % synthetic aviation fuels.

Synthetic aviation fuels within the meaning of the regulation are renewable fuels of non-biological origin. They must meet the sustainability and greenhouse gas criteria of the Renewable Energy Directive. The regulation states that such fuels, produced from renewable electricity and carbon captured directly from the air, can achieve emission savings of up to 100 percent compared with fossil kerosene. The quota does not prescribe a particular synthesis process. Fischer-Tropsch competes here with other routes, for example via methanol. Individual Member States may have additional national requirements. Whether these remain in place alongside the EU regulation must be examined case by case.

Common misconceptions

First, Fischer-Tropsch fuels are not climate-neutral merely because of their similarity to diesel or kerosene. CO₂ is still emitted at the tailpipe or turbine, and in flying, non-CO₂ effects such as contrails are added. The cycle becomes nearly climate-neutral in terms of the carbon balance only if the carbon comes from the atmosphere or from a short-term biogenic cycle and the entire chain is operated with low-emission, ideally additional electricity. Second, compatibility with existing infrastructure removes neither the high electricity demand nor the question of which applications should receive the scarce fuel. Third, Fischer-Tropsch is not a new technology: coal-to-liquid and gas-to-liquid plants have been running on an industrial scale for decades. What is new and still little proven is the coupling with electrolysis and CO₂ capture on a large scale.

Economic assessment

Costs depend strongly on the electricity price, electrolyser utilisation, CO₂ supply, plant size and financing. Regions with abundant wind and solar power, water, land and ports can have advantages. This can give rise to international trade, which at the same time raises questions of security of supply and certifiability. The fuel is only as climate-compatible as the demonstrable production chain. Used precisely, Fischer-Tropsch denotes the synthesis of hydrocarbons from synthesis gas. It becomes significant for the energy economy only through the question of where this synthesis gas comes from and what the products are used for.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:47

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