Audi has taken another big step in the development of new, CO2 neutral fuels – after it has successfully produced a synthetic diesel.

A pilot plant in Dresden has started production of the synthetic fuel – called Audi e-diesel.

After a commissioning phase of just four months, the research facility in Dresden started producing its first batches of high‑quality diesel fuel a few days ago. To demonstrate its suitability for everyday use, Federal Minister of Education and Research Prof. Dr. Johanna Wanka put the first five litres into her official car, an Audi A8 3.0 TDI clean diesel quattro*, this Tuesday.

“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,” declared Wanka.

The Dresden energy technology corporation Sunfire 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 carbon dioxide. 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.

Production of Audi e‑diesel involves various steps: First, water is heated up to form steam which 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 stabilise 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 very high at around 70%. Similarly to a fossil 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.

As lab tests conducted at Audi have shown, it is suitable for admixing with fossil diesel or, prospectively, for use as a fuel in its own right. The Federal Ministry of Education and Research is supporting the Sunfire project, which started in May 2012. Construction work on the facility in Dresden‑Reick kicked off in July 2013 and the plant was commissioned on 14th November, 2014. The plant is set to produce over 3,000 litres (792.5 US gal) of Audi e‑diesel over the coming months. Audi is Sunfire’s exclusive partner in the automotive sector.