At the California plant, workers heat limestone to 1,650 degrees Fahrenheit in a kiln powered by renewable electricity. Carbon dioxide is released from the limestone and pumped into a storage tank.

The leftover calcium oxide, which looks like flour, is then doused with water and spread onto large trays, which are carried by robots onto tower-high racks and exposed to open air. Over three days, the white powder absorbs carbon dioxide and turns into limestone again. Then it’s back to the kiln and the cycle repeats.

“That’s the beauty of this, it’s just rocks on trays,” Mr. Samala, who co-founded Heirloom in 2020, said. The hard part, he added, was years of tweaking variables like particle size, tray spacing and moisture to speed up absorption.

The carbon dioxide still needs to be dealt with. In California, Heirloom works with CarbonCure, a company that mixes the gas into concrete, where it mineralizes and can no longer escape into the air. In future projects, Heirloom also plans to pump carbon dioxide into underground storage wells, burying it.

So they're using the "limestone -> quicklime -> slaked lime -> limestone" cycle. The kiln must be powered by renewables (otherwise the process is pointless), but it's a perfectly reasonable capture method.

Storage is slightly less straightforward. Concrete naturally absorbs carbon dioxide over decades, mixing carbon dioxide in during production is just accelerating the inevitable.

Additionally, the reason concrete can absorb carbon dioxide is that cement contains quicklime, which is mainly produced by... you guessed it, heating limestone to release the carbon dioxide! The concrete won't absorb more carbon dioxide than was released during its production, so making excess concrete is not a solution to CO2 capture. However, if the concrete was going to be produced anyway (and we produce a lot), I suppose it's slightly better to have it absorb carbon dioxide sooner rather than later.

Pumping carbon dioxide into underground storage wells a more scalable solution, provided that the local geology (olivine?) can absorb the carbon dioxide.

An alternative not discussed in the article is to reduce the carbon dioxide into various feedstock chemicals that we currently derive from fossil fuels. Again, this would need to be powered by renewables otherwise the process is pointless.