Clean Fuel For Climate-neutral Heavy Vehicles

HyFiT fuels can reduce the carbon footprint of heavy goods transport and cut harmful emissions

Synthetic fuels could make heavy goods traffic climate-friendly in the future. A team from the Max Planck Institute for Chemical Energy Conversion, RWTH Aachen University, and ETH Zurich is now presenting a synthesis route that can be used to produce a fuel, known as HyFiT fuel, made from biomass or CO₂ that can be used to refuel conventional combustion engines. Here, the researchers are combining processes that the chemical industry already uses in many areas. If the fuel is produced from biomass, under certain conditions, the amount of CO₂ emitted throughout the entire production and utilisation chain is just as low as that of a battery-powered lorry. The overall life cycle assessment of synthetic fuels is also comparable to that of battery operation. In addition, the HyFiT fuel can be customised so that its combustion produces less particulate matter and nitric oxides than a diesel that meets the future Euro 7 standard.

Various technologies are in the running to enable lorries to transport goods in a climate-friendly way in the future. These include not only an electric drive with battery but also synthetic fuels. Electric lorries have a particularly small carbon footprint depending on the electricity mix, but the batteries are large and heavy, especially if one charge is to last for more than 500 kilometres - so in this respect, synthetic fuels have a clear advantage. Another argument in their favour is that they can be used in today's vehicles and with the existing refuelling infrastructure. This is particularly relevant in countries where there will be no charging infrastructure in the foreseeable future.

The question remains as to which substances are best suited for climate-friendly refuelling and how they can be produced most efficiently, but answers to this question have now been provided by the team from Germany and Switzerland in a comprehensive study published in the specialist journal Nature Energy. This begins with the researchers presenting a synthesis route that allows the targeted production of different mixtures of synthetic diesel, i.e. pure hydrocarbons, so-called alkanes, and long-chain alcohols. The team then tested which mixture burns most cleanly and is best suited to today's engines.

High cetane number, low pollutants and compatibility with today's engines

When it comes to combustion properties, it is important that the fuel has the highest possible cetane number, on the one hand, and that it releases less particulate matter and nitric oxide than conventional diesel on the other. While the cetane number is particularly high in synthetic diesel, the combustion of long-chain alcohols produces particularly low levels of particulate matter and nitric oxides, but the cetane number of alcohols is too low for use in engines. At the same time, it must be ensured that the fuels do not damage the materials of today's engines, especially the plastics used in them. And in extensive tests, the team actually discovered that an optimum compromise can be found between a high cetane number, pollutant-free combustion, and compatibility with today's engines, namely with an alcohol content of 20 per cent of the mass.

"HyFiT fuels can be used in current engines, resulting in tailpipe emissions that are far below the upcoming Euro 7 regulations," says Walter Leitner, Director at the Max Planck Institute for Chemical Energy Conversion and Professor at RWTH Aachen University. The Euro 7 emissions standard will apply from the end of 2026, but will not change significantly compared to the current Euro 6 standard.

Two common procedures in one process

The Mülheim team can programme the optimum alcohol content of 20 per cent into the fuel production process. "We rely on established chemical production processes, but we combine them in new ways," says Andreas Vorholt, Group Leader at the Max Planck Institute for Chemical Energy Conversion in Mülheim an der Ruhr. On the one hand, the researchers use the Fischer-Tropsch synthesis, which Franz Fischer and Hans Tropsch invented in the 1920s at the predecessor institute of today's Max-Planck-Institut für Kohlenforschung in Mülheim. In this process, a mixture of carbon monoxide and hydrogen is first produced from carbon-containing starting materials such as coal, biomass or CO₂, which then reacts with suitable catalysts to form hydrocarbons.

The chemists working with Andreas Vorholt and Walter Leitner use both biomass and CO₂ as starting materials. They combined the Fischer-Tropsch synthesis with another standard reaction in chemical production, namely hydroformylation. Long-chain alcohols are formed from alkenes (also known as unsaturated hydrocarbons), which can likewise be produced in the Fischer-Tropsch synthesis, in a reaction with carbon monoxide and hydrogen. The special thing about the Mülheim approach is that the researchers have found a way to carry out both reactions in a single process - this significantly simplifies large-scale production, as the intermediate products do not have to be isolated and purified. The ratio of alkanes and alcohols produced depends on the catalysts and the process control. Deriving the name from the underlying chemical processes, the team calls the fuels HyFiT fuels.

Similarly climate-friendly: Fuels from biomass and battery operation

Naturally, the team paid particular attention to how climate-friendly the synthetic fuels are compared to conventional diesel and battery operation. The most important factor here is whether the fuel is obtained from biomass or CO₂. Recycling CO₂, the cause of climate change, into a fuel may sound particularly appealing, but it requires a lot of electricity to split the hydrogen required for the Fischer-Tropsch process from water. And if the electricity is not green, the carbon footprint of the synthetic fuel made from CO₂ does not look good. Even with the electricity mix that the EU is aiming to achieve by 2040, it performs worse than fossil diesel in terms of CO₂ emissions. And it can only compete with battery operation if the electricity for its production comes entirely from wind power.

The carbon footprint looks much better for fuel from biomass, as it provides a large proportion of the hydrogen for Fischer-Tropsch synthesis. If everything, from its production to its operation, is optimised to avoid CO₂, then this fuel is already as climate-friendly as battery operation with today's electricity mix. However, if farmers grow biomass exclusively for fuel production, this means they can produce less food. "But our process offers the opportunity to achieve an optimal compromise between climate-friendliness and land use," says Andreas Vorholt. Where this takes place also depends on the power source. In windy or sunny locations, for example, climate-friendly synthetic fuel could also be produced from CO₂.

Synthetic fuels for heavy goods transport in particular

The carbon footprint and harmful emissions, however, are not the only factors that determine how environmentally friendly a lorry is. Others include the consumption of water, minerals, metals and land, as well as damage to the ozone layer. The researchers analysed a total of 16 such environmental categories. "HyFiT fuels have a very favourable environmental profile, even compared to battery-based technologies," says Walter Leitner. And the greater the range of a battery-powered lorry, the better the life cycle assessment of synthetic fuels in comparison, although those made from biomass require a relatively large amount of space.

"The results from our team show that HyFiT fuels are a viable option for the energy transition," says Walter Leitner. The team used renewable energies and raw materials as well as production processes that can be adopted by today's petrochemical industry. "The study shows the potential of fuel design for the decarbonisation of heavy goods transport and is also relevant beyond the energy transition in Germany," says Leitner. Yet it remains to be seen how competitive synthetic fuels are economically. According to the study, fuels made from CO₂ in particular need political support in order to survive on the market in the near future. They should therefore be used in road transport primarily where liquid fuels are foreseeably the most sensible option, namely in heavy goods traffic over long distances.