As the title says, Audi is in the process of developing synthetic diesel which will be nearly be free of carbon. To help develop the diesel, Audi is in the planning phase of building a plant in laufenburg in Germany to move forward with their plans. They are also working in partnership with Ineratec GmbH and Energiedienst to help develop the diesel. You can read more here:
https://www.carthrottle.com/post/audi-plans-to-produce-nearly-carbon-neutral-synthetic-diesel/
http://www.express.co.uk/life-style/cars/879107/Audi-diesel-cars-fuel-ediesel-clean-fuel
Cheers
Steve