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DAC using mining tailings

The ultimate fate of CO2 in the air is to be turned to rock. It just takes a long, long time.

Volcanic rock is quite basic. Calcium and magnesium oxides and silicates in particular are quite common, and will react spontaneously with CO2 to form mineral carbonates. This process occurs only on the rock surface however - rocks are too dense to allow CO2 to permeate freely within them. It will happen as the rock is slowly beat down during the process of weathering, over the course of thousands to millions of years.

It would be lovely to apply technology to speed up these reactions to counteract our CO2 emissions. The simplest approach is to grind alkaline rock finely, and then dump it in the ocean, shortening the neutralization time to years. To do this at any scale requires a large number of boats, and a global consensus that dumping thousands of tons of rock in the ocean is a safe things to do. This is a real political challenge.

An alternative might be to pipe water to a site, and create a local bubbler to treat the rocks in a single, contained location. This was the subject of a recent paper from researchers in China and the UK, which constructed a multi physics model to help them optimize reaction parameters of a reaction of calcite to form soluble bicarbonate ions when contacted to flue gas. The capex of this system are higher, but it's a lot cheaper to transport water to a mining site than rocks to the ocean, and the politics are simpler. The cost, however, still didn't pencil out because the energy cost of pumping all that water and flue gas is too high.. The best the researchers could do is about $300/ton; the cost of applying the system to DAC would be far higher. But most interestingly, the cost of energy goes down as the capture rate from the particles goes up. Raise the rate of dissolution, and the system can be a lot cheaper.

Two companies are trying to do better on the kinetics - so much better that they believe enhanced weathering can apply to DAC instead. Planetary Hydrogen performs electrolysis on seawater, creating hydrogen and base on one side, and oxygen and acid on the other. Basic minerals like calcite will dissolve much more quickly when immersed in an acidic solution, and while the calcite itself will be neutralized by the acid, the net reaction creates soluble base to be available at the hydrogen-producing electrode. It also creates hydrogen available to sell as a side-product. A similar approach is being attempted by the company Seachange, who is using electrodialysis of reverse osmosis brines rather than electrolysis to create the independent acid and basic streams. This electrodialysis process is less energetically costly than hydrogen production, but requires specialized membranes that have yet to be tested at scale in this application.

Whichever method is used, it's clear that lowering the pH around minerals like calcite matters a lot: Below pH=5.5, reaction rates are proportional to the concentration of acid in the system. Could such a DAC system really compete? If the numbers in the academic paper are to be believed, increasing the carbon capture rate by 10X or more will make the process radically cheaper for flue gas capture. Whether Planetary Hydrogen and Seachange will develop processes for DAC is still uncertain, but I would not be surprised to see their approaches pay off on flue gas in specialized circumstances. Carbon pollution arises in a diversity of situations; it is reasonable to expect that it will be abated by a diversity of solutions.

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