An article about us came up on HN recently and people seemed interested (https://news.ycombinator.com/item?id=19792412), so we thought it would be good to try to answer some of the questions we saw there and try to dive in some more to any questions that follow!

The only inputs to make the fuel are CO2 and water (both from the air) and electricity. The only outputs are fuel and oxygen. One way to think about it is that making fuel is reverse combustion. The process isn’t super efficient (we expect 50-60% overall efficiency at maturity), but it turns out that doesn’t matter as long as the electricity is zero carbon and low cost. If the cost of our equipment is also low, then we believe we can not only make zero carbon fuel, but actually compete on price with fossil fuel.

We’re not the first to make fuel from the air - in fact Google, Audi, Carbon Engineering, Global Thermostat, Climeworks, and labs at universities and national labs have all done it before us. What no one has been able to do so far is do it at a low enough cost to compete with fossil fuel.

The thing that’s new about what we’re doing is that we have gotten rid of all the thermal processes normally used, and instead use a process that uses only electricity (no natural gas, etc) and does it at room temperature. This is a big deal for both capital cost and for being truly carbon zero. We can use inexpensive materials, which keeps our cost low, and can start up and shut down quickly, which allows us to run intermittently, matching the intermittent nature of many renewable energy sources. We can also only run when the power is at the price we want.

Digging in to some more details, we absorb CO2 and water vapor from the air into an aqueous electrolyte. We then react the CO2 in the water with a copper catalyst to directly make alcohols like ethanol, butanol, propanol, etc. Both of these things have been done by many others and the science is known. Normally at this point one would have to use a thermal process (distillation) to get the fuel out of the water, and this is expensive and makes the economics really hard to get right. We don’t have to do this step thermally though, because we have a carbon nanotube membrane that replaces it, extracting the alcohols from water in a single step at room temperature. This makes a huge difference in cost. The last step is that we up-convert the alcohols to gasoline, diesel, and jet fuel. This last step is also well known and we can actually buy this step from others.

The carbon nanotube membrane that makes this all work is the product of 6 years at my previous startup, Mattershift. I was developing it for desalination and water purification. About 3 years ago I realized it could do this job, but it wasn’t clear that a startup could raise money for such an ambitious effort, especially one linked to a political issue (unfortunately) like climate change. When I saw the YC request for startups in carbon removal, I knew that the timing was right, and I founded Prometheus to do it.

Please let me know if you have more questions or feedback. I’ll do my best to answer any questions, but please excuse if I’m not able to go too far into details like our piping and instrumentation design, or other really specific things we wouldn’t want to help competitors with.

Thanks!