Audi in new e-fuels project: synthetic diesel from water, air-captured CO2 and green electricity; “Blue Crude”

14 November 2014

Audi is active in the development of CO₂-neutral, synthetic fuels; the company already has projects underway with Joule in the US for the development and testing of synthetic ethanol and synthetic diesel (earlier post); has an e-gas project underway in Werlte, Germany (earlier post); and has a new partnership with Global Bioenergies on bio-isooctane (bio-gasoline) (earlier post).

Audi’s latest e-fuels project is participation in a a pilot plant project in Dresden that produces diesel fuel from water, CO₂ and green electricity. Audi and project partners including Climeworks and sunfire (earlier post) opened the plant today. The project combines two innovative technologies in this project, which is funded in part by the German Federal Ministry for Education and Research and was preceded by a two-year research and preparation phase: direct capture of CO₂ from ambient air and a power‑to‑liquid process for the production of synthetic fuel. Audi is the exclusive partner in the automotive industry.

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Other partners in the project consortium include Lufthansa; Fraunhofer ICT; Universität Stuttgart; Forschungszentrum Jülich; GEWI AG; CVT Chemical Engineering; and HGM.

The sunfire plant, which operates according to the “power-to-liquid” (PtL) principle, requires carbon dioxide, water and electricity as raw materials. The carbon dioxide is extracted directly from the ambient air using direct air capture (DAC)—a technology developed by Swiss partner Climeworks.

The Climeworks CO₂ capture technology is based on a cyclic adsorption / desorption process on a novel sorbent. During adsorption, atmospheric CO₂ is chemically bound to the sorbent’s surface. Once the sorbent is saturated, the CO₂ is driven off the sorbent by heating it to 95 °C, thereby delivering high-purity gaseous CO₂. The CO₂-free sorbent can be re-used for many adsorption/desorption cycles.

Around 90% of the energy demand can be supplied by low-temperature heat; the remaining energy is required in the form of electricity for pumping and control purposes. The patent-pending technology has been developed in collaboration with the Professorship of Renewable Energy Carriers at ETH Zurich. The optimization of the sorbent and scale-up of the sorbent synthesis process is carried out in collaboration with the Swiss Laboratories for Materials Science and Technology (Empa).

In a separate process, a solid oxide electrolysis (SOEC) unit powered with green electricity splits water into hydrogen and oxygen. (sunfire acquired staxera, a developer and manufacturer of SOFC high-temperature fuel cells sited in Dresden in 2011.) The hydrogen is then reacted with the carbon dioxide in two chemical processes conducted at 220 ˚C and a pressure of 25 bar to produce a hydrocarbon liquid called Blue Crude. The process is up to 70% efficient.

Top: Two cartoons of the basic sunfire concept and process. PtL = Power-to-Liquids. Bottom: 3D model of sunfire SOEC (left) and the demo plant (right). Source: sunfire. Click to enlarge.

As currently built, the pilot plant on the sunfire grounds in Dresden-Reick can produce approximately 160 liters of Blue Crude per day. Nearly 80% of that can be converted into synthetic diesel. This fuel—Audi e‑diesel—is free of sulfur and aromatics, and features a high cetane number. Its chemical properties allow it to be blended in any ratio with fossil diesel—i.e., it can be used as a drop-in fuel.

The Audi e‑gas plant in Werlte, Lower Saxony, already produces synthetic methane (Audi e‑gas) in a comparable manner; drivers of the Audi A3 Sportback g‑tron can fill up on it using a special fuel card. (Earlier post.) Audi is also conducting joint research into the synthetic manufacture of Audi e‑gasoline with Global Bioenergies. And a joint project with Joule is striving to produce the synthetic fuels Audi e-diesel and Audi e‑ethanol with the help of microorganisms.

With this latest collaboration, Audi said, it and its partners are demonstrating that industrialization of e‑fuels is possible. The pilot plant was officially brought on stream today in the presence of Prof. Dr. Johanna Wanka, German Federal Minister for Education and Research, and Dr. Hagen Seifert, Head of Environmental Assessments, Renewable Energies and New Materials at Audi AG.