2.4: DAC + minerals

Electrochemically enhanced mineral weathering

Here is a paper about this kind of “electrogeochemistry” approach, and a more recent evaluation by the same team. They estimated that “electrogeochemical methods could, on average, increase energy generation and carbon removal by more than 50 times relative to BECCS [bioenergy with carbon capture and storage], at equivalent or lower cost”. Here is a talk on this.

I wish that the world market for H2 were large enough to make this fully commercially viable, but — although hydrogen has major commercial and decarbonization potential in a number of sectors that are otherwise difficult to decarbonize — it seems like we can’t purely drive the emergence of this technology today on the demand for hydrogen. Also without some market incentives like a carbon credit producing hydrogen using this method is currently not competitive with conventional sources like methane reforming.
This is kind of an intellectual hybrid between the direct air capture and rock weathering approaches, in the sense that it requires an energy input. It solves several key problems with DAC and mineral weathering by combining them. Unlike DAC, it produces hydrogen, an economically useful fuel, and unlike DAC, it doesn’t require transporting captured carbon to some underground permanent storage, instead having carbon capture characteristics closer to those of olivine weathering. Meanwhile, it creates a more optimal chemical environment for enhanced mineral weathering.

Interestingly, Y Combinator proposes an enhanced version of rock weathering as a frontier research area for carbon capture — they propose to electrochemically generate hydrogen fuel from seawater using renewable energy, in the process enhancing the rate of mineral weathering and its associated CO2 capture and ocean de-acidification! In the proposed scheme:

“Electro-geo-chemistry uses an electrochemical process to increase the rate of geochemical CO2 removal. This approach also produces energy in the form of hydrogen gas (H2). It uses saline water electrolysis in the presence of minerals to generate H2 while at the same time creating a highly reactive solution that acts like a chemical sponge, absorbing and converting CO2 into dissolved mineral bicarbonate. Adding this bicarbonate to the ocean not only provides long term carbon storage, but it also helps counteract ocean acidification. Thus, when powered by renewable electricity, this electro-geo-chemistry can be used to produce a non-fossil transportation fuel, H2, while simultaneously removing CO2 from the atmosphere and countering ocean acidification. The global abundances of the required materials and energy for this negative-emissions H2 process suggest that it can be done at very large scales.”

Mineral looping

Instead of combining DAC and minerals in a way that simply accelerates natural weathering another option is to use minerals that naturally bind with CO₂ in the atmosphere as the actual DAC component and then heat these up to release a pure stream of CO₂, just like what occurs with normal DAC set ups.

The advantage of using minerals to do this is that, if you pick the right ones, they can be abundant and hence cheap. This is in comparison to many current sorbents which can contribute up to 80% of the cost of DAC.

The basic idea with these types of solutions is that you take some feedstock, you heat it up to release a stream of CO₂, you then take the remaining mineral, get it into a state where it will react again with CO₂ in the atmosphere, expose it to the air, and repeat.

One option is to do this with magnesite as this paper talks about:

​​Here we detail a land-based enhanced weathering cycle utilizing magnesite (MgCO3) feedstock to repeatedly capture CO2 from the atmosphere. In this process, MgCO3 is calcined, producing caustic magnesia (MgO) and high-purity CO2. This MgO is spread over land to carbonate for a year by reacting with atmospheric CO2. The carbonate minerals are then recollected and re-calcined. The reproduced MgO is spread over land to carbonate again.

This same kind of approach is being used by Heirloom but instead of magnesite they use calcium carbonate as their feedstock.

Heirloom mineral looping method
You basically have the same process but you need to hydrate the calcium oxide after you have stripped out the CO₂.

According to their white paper “The fastest rate consistently achieved by our process is equivalent to 630g CO2 removed per 1m2 of exposed contactor area every 2.5 days, reaching 85% carbonation of the Ca(OH)2 utilized”. To capture 1Gt of CO₂ a year with these values you would need roughly 1e10 m², or 10,000km², of exposed contactor area. On the face of it this is a lot, three times the size of Rhode island. However, as Heirloom notes there is no need for these contactors to be spread out individually, instead they can be stacked on top of each other which can rapidly reduce the ground level surface area needed.

If we take the depth of the contactor to be 5mm then you would need 55,000,000 m³ of calcium carbonate. Given that calcium carbonate has a density of around 2,930 kg/m3 this would give us a mass of around 160,000,000t, if costs are between $10 - 50 per metric ton this gives us a price range of $1.6-8bn for the raw calcium carbonate needed to remove 1Gt a year. Actually getting this quantity though shouldn’t be a problem as CaCO3 is one of the most abundant compounds in the earth and is already mined extensively.

In terms of energy the white paper says:

The decomposition of CaCO3 alone requires 1.78 GJ per metric ton of CaCO3 decomposed, or roughly 4 GJ per metric ton of CO2 captured. Since the reaction occurs at 900 ºC, there is also energy associated with elevating the material to temperature. Heirloom targets a total energy requirement of less than 1,500 kWh/tCO2 , or 5.4 GJ/tCO

This is not an unreasonable target, it would put it just less than 3 times higher than McKay’s estimate for the best we’re going to do of 90kJ/mol of CO₂ (0.5682 kWh/kg CO₂). However, to remove 1Gt of CO₂ it would still require almost 5 times the UKs annual energy consumption from electricity.

Ultimately this is still going to be an expensive operation but hopefully the use of an abundant material for the feedstock can allow the approach to scale more easily.