C02 free Cement?

nice to see the people with the big bucks willing to spend on carbon neutrality

 

They go over the process here.

Cement made at ambient temperature, using renewable electricity, for a decarbonized future - Sublime Systems

The tl;dr is that they use a modified electrolyzer to dissolve the calcium in the minerals, and precipitate out calcium hydroxide. Which can then be used as a direct replacement for the calcium carbonate from heated limestone used in portland cement.

Even when uses current US grid mix and limestone, it will mean reduced CO2 emissions. Their process is done at ambient temperatures. No burning fossil fuels to reach the high temps in a kiln. That also means the pure CO2 coming off the limestone isn't mixed with the dirty CO2 from the fossil fuels, which makes it cheaper to capture.

Use a non limestone mineral and renewable electricity, and it is carbon free. It is possible to separate out any other valuable compounds and elements that were in the source rock. They mention magnesium.

 

I understand what they are doing, but not completely. Would take more time, The cited PNAS article from 2020 indicates higher energy cost but lower direct CO2 emissions during cement manufacture. It would be interesting to know what Sublime Systems has done since 2023 January.

I suppose they are correct that amine scrubbing to remove CO2 is not yet doing well at industrial scale. This may not justify writing it off though. About mid-page they describe geological settings suitable for CO2 sequestration. Raised a red flag for me because they ignore sequestration in basalt. Those minerals are very widespread globally, and are what the CO2 seq in Iceland uses.

Another technology that is floating around is solar concentration (mirrors around a central tower) to achieve the 1400 oC without fuel burning. Don't know if that has been constructed yet.

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I am happy to see many technologies pursued to lower CO2 emissions during cement manufacture. There is CO2 from energy supply, with research such as above on minimizing. There is CO2 from rocks. The latter is mostly sequestered passively in later decades by standing concrete structures themselves. Not a lot of people know that That sequestration can be prevented if concrete (or concrete demolition waste) are buried deeply enough. It's actually a bad idea. That sequestration can be accelerated if concrete demolition waste is crushed and mixed into agricultural soils. Or any soils really, but ag soils are routinely turned over so it can be added there without more mechanization.

 

"This core innovation, electrochemical calcination, began while we were still in the labs at MIT and has since been further developed at Sublime to be more energy efficient, to eliminate net production of H₂, to allow separation of CO₂ and O₂ gas streams, to extract lime continuously from the reactor, and to use commercial off-the-shelf electrolyzer hardware."

I'm guessing they are doing one of things with the hydrogen mentioned in the discussion of the paper. Using the hydrogen to help fire the cement kiln, or even a fuel cell to provide electricity. Solid oxide fuel cells for standby up and site power are already available.

Well, their goal is use other calcium containing rocks besides limestone to completely avoid the CO2 emissions altogether. I think they just aren't as invested CCS to discuss it all. Limestone is brought up as it could be the only calcium containing, cheapest, less damaging to get, etc. in a region than those other rocks. Then the CO2 from the process is closer to where it needs to be to be captured than from a limekiln.