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Comment by hannob | Hacker News Reader

This is actually an important point. It doesn't remove the need for CCS though.

I think people advocating for CCS should be careful what they ask for. Reasonable things to do would be:

a) CCS should be confined to areas where no feasible technical alternative exists (e.g. cement) and to negative emissions.

b) No support for CCS as in Enhanced Oil Recovery. This is creating more emissions than it avoids.

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What is the reference to cement supposed to indicate here?

Cement releases CO2 as part of it's chemical curing process. It contributes to about 8% of worldwide emissions.

Another industry where CO2 as a chemical byproduct is vented at a massive scale in the atmosphere is with the steel industry.

Both are good targets for carbon capture at the source.

> Cement releases CO2 as part of it's chemical curing process. It contributes to about 8% of worldwide emissions.

Just to nitpick here, but I think this is crucial to understand: 8% is more plausibly a number for the total emissions of cement, including energy emissions. The chemical emissions are more in the range of 4%.

I'm not sure it would even be practical to capture the carbon emitted during the curing process as this is happening at it's final destination rather than during manufacture.

From what I've read about carbon-negative cement, idea is not to capture the carbon released during curing, but to make concrete that later, over years, chemically reacts and captures the carbon from the air. Because concrete is porous to the depth of 20+ inches, it has a very large surface that can slowly extract more CO2 from air than it initially had released.

AFAIK the usual cement types do not release CO₂ during curing, though a significant amount of CO₂ is released from the ingredients while baking. Sometimes the released CO₂ is reabsorbed when curing (lime).

What type of cement are you talking about?

Reference: http://matse1.matse.illinois.edu/concrete/prin.html

That’s not how CCS or net carbon works. CO2 is basically fungible. You can release it today in Australia and capture it yesterday or tomorrow in Canada to offset, it basically makes no appreciable difference.

I think the biggest reason for capturing at source is the concentration of the gas. Capturing CO2 when it's in atmosphere at 0.04% is much more energy intensive than capturing at source where concentrations could be many orders of magnitude higher.

> Cement releases CO2 as part of it's chemical curing process. It contributes to about 8% of worldwide emissions.

There are 2 sources of CO2 during cement manufacture. One is from the decomposition of lime, the other is from the energy production needed to make the process work. The latter could be avoided by electrical heating, a solar furnace or other renewable heat sources. The decomposition part can be compensated by newer cement chemistries that reabsorb most of the CO2 during the curing process.

So there are alternatives, carbon capture is not the only solution.

Could you point me to any of the newer cement chemistries?

silicate and magnesium oxide based ones.

https://en.wikipedia.org/wiki/Sorel_cement https://www.solidiatech.com/solutions.html

Do you roughly know the relative ratios of emission for the sources?

Cement releases CO2 when lime is made from limestone. The curing of cement absorbs CO2 (less than was released in making the lime).

Our local cement factory (huge, and one of the top polluters in California) burns coal on site as a source of heat. So there's that, too.

Cement also typically covers a large square footage area of natural carbon absorbing material. So on top of the carbon required in the manufacturing/installation of the cement, the amount of trees cleared should be accounted as well since the cement does not absorb carbon.

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