Audi has taken another big step in the development of new, carbon dioxide (CO2)-neutral fuels. A pilot plant in Dresden, Germany has started production of the synthetic fuel Audi e-diesel.

The Sunfire project, which is supported by Germany’s Federal Ministry of Education and Research, can produce more than 3,000 litres over the coming months.

After a commissioning phase of just four months, the research facility in Dresden started producing its first batches of high-quality diesel fuel in late April.

“This synthetic diesel, made using CO2, is a huge success for our sustainability research. If we can make widespread use of CO2 as a raw material, we will make a crucial contribution to climate protection and the efficient use of resources, and put the fundamentals of the “green economy” in place,” said Federal Minister of Education and Research Prof. Dr. Johanna Wanka.

The Dresden energy technology corporation Sunfire GmbH is Audi’s project partner and the plant operator. It operates according to the power-to-liquid (PtL) principle and uses green power to produce a liquid fuel. The only raw materials needed are water and CO2. The CO2 used is currently supplied by a biogas facility. In addition, initially a portion of the CO2 needed is extracted from the ambient air by means of direct air capturing, a technology of Audi’s Zurich-based partner Climeworks.

Reiner Mangold, head of sustainable product development at Audi, sees Audi e-diesel and Audi e-fuels in general as an important component that complements electric mobility. “In developing Audi e-diesel we are promoting another fuel based on CO2 that will allow long-distance mobility with virtually no impact on the climate. Using CO2 as a raw material represents an opportunity not just for the automotive industry in Germany, but also to transfer the principle to other sectors and countries.”

Production of Audi e-diesel involves various steps: First, water heated up to form steam is broken down into hydrogen and oxygen by means of high-temperature electrolysis. This process, involving a temperature in excess of 800 degrees Celsius, is more efficient than conventional techniques because of heat recovery, for example. Another special feature of high-temperature electrolysis is that it can be used dynamically, to stabilize the grid when production of green power peaks.

In two further steps, the hydrogen reacts with the CO2 in synthesis reactors, again under pressure and at high temperature. The reaction product is a liquid made from long-chain hydrocarbon compounds, known as blue crude. The efficiency of the overall process – from renewable power to liquid hydrocarbon – is around 70%. Similar to crude oil, blue crude can be refined to yield the end product Audi e-diesel. This synthetic fuel is free from sulfur and aromatic hydrocarbons, and its high cetane number means it is readily ignitable.

It is suitable for using as a blend with fossil diesel or as 100% synthetic diesel.

Audi has been active in the development of CO2-neutral fuels – Audi e-fuels – since 2009. The Audi e-gas plant in Werlte, Lower Saxony, Germany already produces Audi e-gas (synthetic methane) in a comparable manner. Audi is also conducting joint research into the synthetic manufacture of Audi e-gasoline with Global Bioenergies of France. Another project is with U.S.-based Joule, which uses microorganisms to produce the synthetic fuels Audi e-diesel and Audi e-ethanol.