Making diesel… from water and carbon dioxide!


German car manufacturer Audi has recently announced that, working with Sunfire, they have been producing a synthetic diesel which has been of great interest due to the fact that the raw materials it utilises are simply water and… carbon dioxide! A fuel that can be used as a fossil fuel while using harmful waste CO2 sounds like it’s either a massive world changing development, or maybe like it’s too good to be true. Either way it’s a fascinating idea! So, what’s the science of this process, and will it really be a game-changer for the automotive industry?

How does it work?

Water is first vapourised into steam, which can be separated into its two components (hydrogen and water) when it undergoes a process called electrolysis at a temperature of 800°C. It is the hydrogen which is of interest in the next part of the process - when reacted with CO2, a mixture of long-chain hydrocarbons (compounds made of carbon and hydrogen) is formed, and Audi have attractively named this “blue crude”. As with fossil fuel crude, this undergoes further treatment in order to create a useful produgct. This is comparable to operations in an oil refinery, where distillation (separating hydrocarbons of different chain lengths) and cracking (splitting long chain hydrocarbons into shorter, more useful ones) take place. The outcome of this is a synthetic diesel, branded as e-diesel by Audi, which contains less contaminants than conventional diesel.

More about the process

Using CO2 as a reactant sounds awesome, but where does it come from? Audi have been using CO2 produced in a biogas reactor, and supplementing it with CO2 captured directly from ambient air. The latter would be attractive, but this is still a developing technology, lagging behind carbon capture techniques at the source of emissions. e-diesel is considered carbon-neutral since CO2 emitted when it is burnt in engines is offset by the amount put in as a reactant. These claims are unlikely to take into account transportation, and powering the plant equipment. But Audi have been taking care to reduce these embodied emissions by using renewable energy to power their plant. Finally, we come to water. As discussed in my article on fracking, using water to produce fuel is contentious. However, it’s not necessarily a deal breaker for this process at this stage, since it’s very comparable to the amount of water used to produce conventional diesel and petrol.


(Source: Audi)

Uses of e-diesel

Audi’s synthetic diesel certainly has potential as an automotive fuel – it can be used on its own or mixed with conventional diesel for use in cars. In fact, Germany’s Federal Minister of Education and Research - Prof. Dr. Johanna Wanka, has been using some of the e-diesel in her car (an Audi model, of course!). But at this stage, Audi are intending to produce just 3,000 litres over the next few months, hardly a big threat to conventional liquid fuels! That said, this is a pilot-scale operation which has only just started production, following a very short commission phase of 4 months, so it could potentially increase its output dramatically should there be enough demand and investment. It’s unlikely to change the industry dramatically in the near future, but is still a really cool development and perhaps a step in the right direction.

And this is a new idea… right?

Actually, this isn’t necessarily a brand new idea. Audi’s process uses principles from the Fischer-Tropsch process, which has been used for decades, although this is a little different (mainly in that CO2 is used as an input material, rather than carbon monoxide - meaning there are extra steps involved). There are countless synthetic diesel projects with similar aims too. A particularly interesting one, made by Joule Unlimited, makes similar upfront claims to Audi about using water, CO2 and sunlight. This is a fundamentally different process, using genetically modified bacteria to carry out the reaction, but still super cool and interesting to read about! Certainly, despite all the work done before, Audi’s operation seems to be the first “carbon neutral” diesel produced viably at this scale, which can be put into cars immediately.

#EmilyVincent #Chemistry #Environmental

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