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Heirloom Carbon: DAC with rocks

Heirloom Carbon Technologies, a California-based startup, is taking a unique approach to direct air capture (DAC) by eliminating the fans usually required to force air over the sorbent systems. This approach, if it works, offers a radically lower capex, and far lower energy consumption than competitors such as Carbon Engineering.

The Heirloom process uses 5 mm layers of solid calcium hydroxide as the sorbent. The calcium oxide is stacked on plastic substrates and left in ambient air for several days, over which time they achieve about 85% conversion to the carbonate. The solids are collected and calcined to drive off CO2, then rehydrated to restore the calcium hydroxide for re-use.

Can this work? Heirloom claims a much faster absorption time for CO2 than other researchers, and I am sure they have established this at small scales. However at large scales, with no forced air, exchange of decarbonated air with ambient air will be slower, extending the system cycle time. Further, the larger the scale of the system the greater the cost of transport, collection, and layer maintenance. I have seen similar designs in academia that do not scale well, but Heirloom is much better funded than any academic group, and may have solutions to these problems that are not public yet.

The tradeoff for Heirloom is that their sorbent costs are 100X lower than a competitor like Climeworks, with a cycle time perhaps only 10X slower. This translates into a large capex advantage on a sorbent basis. However, Heirloom's powder cannot be packed space-efficiently, so the cost of support infrastructure will be higher. If this scales, Heirloom could be very interesting.

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