Fuels from thin air: Prometheus joins the chase to make captured CO₂ into net zero hydrocarbon fuels

One more company to focus on in our tour of CO2 usage. It’s another electrofuels play called Prometheus Fuels using atmospheric CO2 using water, electricity, and nanotube membranes to produce commercially viable fuels. The project was one of two selected for investment in March of 2019 by Y Combinator, a prominent Silicon Valley business incubator, after requesting proposals which address carbon removal.

In June 2020, BMW announced an investment of $12.5 million into Prometheus. A section of the official BMW website declared that “By perfecting existing chemical reactions and processes, Prometheus will make drop-in replacement fuels that are guilt-free. Prometheus will help to fuel the power of choice.” Now the Norwegian fund Tjuvholmen Ventures has also made an investment of undisclosed size.

What is it?
Prometheus removes CO2 from the air and turns it into zero-net carbon gasoline and jet fuel at a price that will compete with conventional fossil fuels used in transportation today. Because only renewable energy sources are used in its production, you get net zero carbon fuels.

How does it work?

BMW reports:

The process uses a solution of liquid water and CO2 that is exposed to an electrified copper plate. This catalyzes a reaction and produces fuel alcohols (mostly ethanol). Closely packed filters made from cylindrical carbon nanotubes embedded in plastic allow ethanol through while blocking water molecules. From there, the more concentrated solution of approximately 95% ethanol can be catalyzed with zeolite to join into more complex hydrocarbons, including gasoline, diesel, or jet fuel. This technique works at room temperature, while traditional methods of extraction require heat to distill it from a solution.

Founder Rob McGinnis speculated that even though the theoretical efficiency of Prometheus’ system was only 50–60%, their less energy-intensive process could nevertheless considerably lower overall cost and be competitive with fossil fuels.

Why a big deal?
As BMW notes, “The modularity of the approach will enable micro-cells of gasoline production where there is a surplus of renewable energy available.”

What’s difficult here? For one, the CO2 processing — it’s aiming to use atmospheric CO2 not point source. That’s tougher. So, the company has pioneered a DAC technology, that’s short for Direct Air Capture. You have to process about 1600 pounds of air to capture a pound of CO2, so having a very passive process is a must.

As BMW noted that the “salvaged CO2 encounters renewable electricity in an electrochemical stack called the Faraday Reactor. The electricity “charges” the carbon with hydrogen molecules from the water to create long-chain alcohols, releasing pure oxygen.

Well, think of it this way, chemists:

8 CO2 + 9 H2O —>  C8H18O +  13 O2 (specifically, the formula is probably something like C7H14CH3OH, but you get the idea. And you’ll get a pound of fuels for every 4.3 pounds of water and CO2, more or less.

That’s an illustration of a long-chain fatty alcohol — in this case, 1-octanol. It appears that Prometheus is making a spread of alcohols, and long chain alcohols can range from 4 to more than 30 carbon atoms, so there’s a lot of them they might make. But they all have the basic formula CnH2nO, so our octanol example above is useful for any and all of the alcohols produced.

In the next step, the alcohols are harvested using the Maxwell Core, a special type of nanotube membrane exclusively owned by Prometheus. In a final catalyst step, the alcohols are combined and water is recovered.

Well, that feels like hydrotreating, doesn’t it?

C8H18O +  H2 —> C8H18 + H2O

There you have a nice octane molecule, and some water. For every six pounds of alcohols and hydrogen you get five pounds of fuel.

Progress towards scale
The company says it will sell its first commercial fuel in California at end of 2020, which has now passed, so we’re waiting on the usual COVID delays to work themselves out, if Prometheus hasn’t already placed some fuels quietly into the marketplace. As the Zen Master said, we’ll see.

Source: Biofuels Digest, 2021-03-03.