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Comment by schiffern | Hacker News Reader

>Grinding rock is incredibly energy inefficient. I would suspect this entire process is carbon-positive despite the atmospheric capture.

Grinding the olivine to this study's grain size of 83 microns[0] will consume roughly ~14 kWh/tonne[1]. One tonne of olivine absorbs about 1.25 tonne of CO2, so that's 11 kWh/tonne for grinding, which is indeed likely to be the majority energy consumer.

Clearly the solution is... build roads from olivine! :D Collectively, roadways are "naturally" subject to a huge amount of mechanical weathering, which currently mostly just generates asphalt microplastics.

No doubt the cost/geotechnics doesn't work out, but it's an amusing thought nonetheless. Maybe it could even be made workable, somehow...

[0] https://iopscience.iop.org/article/10.1088/2515-7620/acfd89/...

[1] https://worksinprogress.co/issue/olivine-weathering/

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> Grinding the olivine to this study's grain size of 83 microns[0] will consume roughly ~14 kWh/tonne[1]. One tonne of olivine absorbs about 1.25 tonne of CO2, so that's 11 kWh/tonne for grinding

11kWh per tonne of CO2 sequestered is 2 orders of magnitude less energy per tonne than other DAC methods [1]. Assuming it is scalable (i.e. there is enough open land near the olivine deposits), then it is far cheaper.

1. https://www.mcc-berlin.net/en/news/information/information-d...

The "gotcha" is that at 83 microns, this study showed only 0.15% of the theoretical CO2 capacity absorbed in the first year. Worse, the authors expect this rate to taper significantly in the second and subsequent years.

This is why most EW proposals specify a smaller grain size, but that also has the effect of dramatically increasing the grinding energy required. See [1] (included again below) for a more complete and nuanced discussion of these tradeoffs.

[1] https://worksinprogress.co/issue/olivine-weathering/

I think the second "gotcha" is that even at 83 microns, that particle size is likely unsafe. That's about the size of gypsum dust used in drywall and you can't cut or sand or do anything without PPE for long without serious health consequences. Make it any smaller and it'll be well into the asbestos size range, which would probably be carcinogenic based on the mechanical damage it causes alone (a lot of nanoparticles are toxic for this reason). That's safe in a controlled industrial environment with everyone using fitted respirators but spread out on fields exposed to wind, near residential areas? Dumping that much fine dust everywhere would probably create a similar hazard to the dried out beds of the Great Salt Lake or the Salton Sea or Blackrock Desert* except everywhere there is farmland.

It'd have to be combined with a binder to form much larger particle sizes so that the natural weathering safely exposes the olivine slowly over decades. I suspect that once one does the math on the whole lifecycle, it's net carbon positive only after decades if it is to be done safely.

* People who have been to Burning Man can testify to just how much trouble dust that small causes, not to mention the frequent dust storms with zero visibility.

> I think the second "gotcha" is that even at 83 microns, that particle size is likely unsafe.

It’s a bit dark, but wouldn’t a death toll improve the situation from an emissions perspective?

If you execute all the staff at every power plant in the world and then keep executing the subsequent replacement cohorts, every power plant would have to shut down and we would dramatically reduce global CO2 production. Similarly, if you position machine guns at major roads and keep shooting every car that comes through, you will eventually reduce the environmental toll of driving cars.

> The "gotcha" is that at 83 microns, this study showed only 0.15% of the theoretical CO2 capacity absorbed in the first year

According to the study [1] (and the article), that's because the experiment was purposely conducted in a dry climate (inland northern CA) and during a drought to determine a lower bound on the CO2 absorption capacity. More well suited (wetter) climates are expected to result in more CO2 absorption.

"Given that climate change impacts such as heatwaves and droughts are already widespread, knowledge of the robustness of enhanced weathering under extreme conditions is essential to understanding its future efficacy. Here we show that enhanced weathering maintains modest carbon dioxide (CO2) removal in a multi-acre field trial under an extreme drought in California, one of the largest agricultural producers globally."

1. https://iopscience.iop.org/article/10.1088/2515-7620/acfd89

Showing yet again that there is no free lunch, and we continue to eat the earth's future with abandon.

Seems like US average is 0.855 pounds of CO2 emissions per kWh so this process looks like a significant win. Also grinding a ton of rocks doesn't use much energy if its just 14 kwh...

https://www.eia.gov/tools/faqs/faq.php?id=74&t=11

Hard to follow all the units, but, just for grinding, using these numbers:

grinding: ~14 kWh/tonne olivine

absorption: 1.25 tonne of CO2 per tonne of olivine

emissions: 0.855 lbs CO2 / kWh

emissions: 5.4 kg CO2 per 1.25 tonne CO2 (1250 kg) absorbed

> emissions: 0.855 lbs CO2 / kWh

Why do there have to be CO2 emissions?

There don't. The problem is a lack of remote industrial equipment that are EVs powered by renewable electricity sources rather than diesel fuel.

That's not really that much of a problem to be honest.

If this is to be an industrial mining scale operation that goes in with a 600 million tonne per annum target (similar scale to iron ore for steel mining operations) then the company building out the capital assets will setup EV's and solar on the grounds of both economy and PR.

You can see this with Rio Tinto Iron, Fortescue Metals, etc.

You could solar power it, or whatnot. I was just trying to contextualize the numbers others had written.

> Clearly the solution is... build roads from olivine! :D Collectively, roadways are "naturally" subject to a huge amount of mechanical weathering, which currently mostly just generates asphalt microplastics.

Nah, asphalt microparticles are in the end just very viscose oil and rock dust from the filler. The microplastics from roads come from tire wear and brake dust.

>asphalt microparticles are in the end just very viscose oil

Bitumen is still a plastic. In fact there are also increasing efforts to use post-consumer recycled plastic as an additive.

Microplastics from road transport mainly come from tire dust, but also include road markings[0] (which are plastic mixed with glass microbeads), bitumen from asphalt[1], and yes a bit of brake dust.

[0] https://resourcelab.dk/plastics/pollution/oceans/2018/10/11/...

[1] https://www.sciencedirect.com/science/article/pii/S004896972...

Or let oceans do some of the work: https://www.withouthotair.com/c31/page_246.shtml

> Clearly the solution is... build roads from olivine! :D

Haha. I'm glad you didn't include "solar" or "freakin'". ;D

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