Audi has begun production of a synthetic diesel fuel made from water, carbon dioxide, and hydrogen. Unlike fossil fuels, which release additional carbon into the atmosphere, Audi’s “e-diesel,” which is being produced at a plant in Dresden in conjunction with the German alternative energy company Sunfire, has a net-zero carbon footprint because it is made with carbon dioxide taken from the air.
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 very high at around 70 percent. 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.
Audi has been active in the development of CO2‑neutral fuels – Audi e‑fuels – since 2009. Audi is also conducting joint research into the synthetic manufacture of Audi e‑gasoline with Global Bioenergies, of France. In a further project, Audi has joined forces with the U.S. company Joule, which uses microorganisms to produce the synthetic fuels Audi e‑diesel and Audi e‑ethanol.means it is readily ignitable.
While this is exciting it is still a tiny drop in the ocean as the production is extremely minimal. The Dresden plant will produce a mere 3,000 liters (794 gallons) of the carbon-neutral fuel over the next few months