CO2 sequestration through accelerated weathering of limestone on ships

No. Not even a little bit.

And if there was, any sequestration would just be used as an excuse to find ways to conduct more CO2 producing activities.

It is exceedingly likely that any sequestration will take substantially more energy than burning fossils fuels produced. I couldn't explain it properly in physics terms but when you burn fuel you are releasing stored energy and when you sequester carbon you are storing the energy.

If we could store energy cheaper than we could use it we'd have a perpetual motion machine, I think? Fairly sure this would be physically impossible. Where this might be wrong is if we found a process to use another energy source (the sun, something that uses the sun, etc) to do it for us, but we haven't go anything that works in that vein either. Trees are actually one of the better options, but to reverse climate change you'd need to reforest the earth AND sequester all the oil we burned.

Burning carbon is effectively debt. If we stopped burning carbon right this second, billions would die, as our whole system depends upon it. But if we don't stop burning it, we increase our future problems.

This is unpleasant to reckon with so most don't. I don't think it makes the problem intractable, but it gets harder the more we delay.

We do need sequestration because simply eliminating all carbon sources wouldn't be enough, we also have to reduce current levels. Also there are some cases where fossil fuel might be the best solution (rocketry?) so we'd need to be able to deal with the waste.

What do you mean exactly? We could plant lots of trees and make charcoal from them and bury it. That scales with the amount of money spent. The problem is that nobody wants to invest a big chunk of gdp into burying coal.

I don't understand the objection. We are un-terraforming Earth now. Terraforming it back is worth is basically no matter the cost, since the alternative is to not have a livable Earth.

The absolute best way to decrease the amount of atmospheric CO2 over time is to keep fossil fuels in the ground. If we don’t do this, no other solution really matters. To date we have been unsuccessful at deciding to do this and in fact there are currently multiple wars going on the extract more at a faster rate.

Yes, if large amounts of energy are available. I haven't done the calculations in a long time, but I thought I came up with a ballpark of 10% of current global marketed energy consumption for a few decades for atmospheric carbon capture. We can place a very firm upper bound of about four times current world marketed energy consumption.

To avoid any concerns about scalability, as well as about energy supply intermittency, I made my estimate using only the oldest process in the chemical industry, lime burning, which predates plastics, oil drilling, steel, iron, bronze, writing, cities, ceramic, and possibly even agriculture. The only difference from the Neolithic method is that you have to retort the lime in a sealed chamber so you can collect the carbon dioxide it absorbed from the atmosphere after the last time you calcined it. This doesn't require an enormous amount of machinery, just very large machinery. Basically, a giant tin can similar to a water tower, maintained at a mildly negative gauge pressure as it's heated up.

My estimate, IIRC, was that, without soda for process intensification, you would need an amount of limestone a few times larger than the amount mined by the cement industry every year. That's fine, though; limestone is about 20% of all sedimentary rock, and sedimentary rock is 73% of Earth's land surface and 8% of the entire crust.

Very roughly, Earth is 6e24 kg, her crust is 6e22 kg, and its limestone is 1e21 kg. By contrast, the 427 ppm of carbon dioxide we need to capture half of is only 3.34 teratonnes, 3e15 kg. Limestone is 44% carbon dioxide by mass, so it absorbs 44% of its mass in carbon dioxide each time through the cycle. This absorption takes about 5 years if it's sitting in a paper bag dry, about a month when you whitewash a wall with it, or a second or so when you catalyze the absorption with a few percent of lye, like in a scuba diving rebreather.

The USGS publishes a lot of information about the world lime market at https://www.usgs.gov/centers/national-minerals-information-c.... From it, we can see that current US lime prices are 20¢/kg, 3.2 billion dollars for a total yearly production of 16 million tonnes. (They say that prices at the plant were as low as US$131/tonne in 02020, so probably the production cost is closer to 13¢/kg, including the cost of mining.) That's probably tonnes of CaO, which is the other 56% of limestone that isn't carbon dioxide. If burning lime in a giant tin can to capture the gas were about as expensive as how it's done today, that's about 24¢ per kg of carbon dioxide, not counting the cost of warehousing the resulting lime until it's absorbed the gas so you can calcine it again. (Remember, you can reuse the same lime every few hours if you dope it with a soda catalyst.)

World lime production is closer to 420 million tonnes per year, mostly of course in China, 26 times US production.

1.8 trillion tonnes of lime is 4300 years of total world production (or 120,000 years of current US production), so we're talking about a significant scale up. It's not just a few times global cement energy production. But it's still only two millionths of the limestone in the crust of the earth, and maybe you'd want to reuse the same lime many times so you don't have to mine it again.

But probably some process involving more sophisticated sorbents like triethanolamine, combined with point source capture, will end up being cheaper in the end. And see my notes in https://news.ycombinator.com/item?id=44461843 about accelerated olivine weathering. The lime approach serves only as an easily computable upper bound on difficulty.

The USGS mineral commodities summaries also have an entry for "stone (crushed)", which is 70% limestone. This is 1.5 billion tonnes per year in the US, which I guess is a billion tonnes of limestone, so 1.8 trillion tonnes of quicklime (3.4 billion tonnes of limestone) is only about 3400 years of US mine production. It only costs about 1–2¢/kg, which is in accordance with what I've seen. No world production figures are given, but we can probably guess that that's another thing China produces 20 or 30 times more of.

So, the cost of quicklime is almost all (>80%) calcination, and I believe that almost all of that number is energy. We can put an upper bound on its energy consumption based on that 13-cent cost in 02020: coal is often the cheapest source of the heat needed for calcination, and in 02020, it reached a low around US$60/tonne ( https://fred.stlouisfed.org/series/PCOALAUUSDA). So we know that producing a kg of quicklime can't require more than about 2 kg of coal, which provides 33MJ/kg or less (0.7¢/kWh or US$1.80/GJ), so 70MJ per kg of quicklime or of carbon dioxide. That's probably not a very tight upper bound, but I doubt it's high by more than a factor of 3.

Removing 1.4 teratonnes of carbon dioxide over 40 years is 1.1 million kg per second. At 70MJ/kg this multiplies out to 78 terawatts, roughly four times world marketed energy consumption.

Today, devoting a couple of terawatts to the problem would be unreasonably expensive, and tens of terawatts would require expanding world energy production considerably. At 24¢ per kg of carbon dioxide, removing 1.6 trillion tonnes of it would cost 400 trillion dollars, four years of world GDP (say, 10% of world GDP over 40 years). But that's just because the rollout of photovoltaic energy has just begun; the majority of the 18 terawatts or so of world marketed energy consumption is still supplied by fossil fuels, although they are clearly no longer cost-competitive with PV. PV manufacturing is still scaling up, though, and presumably PV energy production will exceed current world marketed energy consumption in a few years, and then continue to increase as new uses are found for the newly much cheaper energy.

78 terawatts is 0.04% of the 174 petawatts of terrestrial insolation.

It's about 10 times cheaper to not burn fossil fuels than to sequester

There's this thing setting off a large nuke https://youtu.be/aGPKpx6pMko

Could there be any possible negative effects from an 81 gigaton nuke exploding though? A 100 megaton Tsar Bomba would wipe out all of New York, a thousands times more than that might have some adverse impact.

This feels like the difference between science and engineering. Although to be fair to you, this paper really feels like somewhere on that spectrum and not fully on the side of science. It's similar to how every battery advancement doesn't always make it to manufacturing.

To address the things that you thought of off the top of your head: * Anything done on land will be less carbon intensive as we transition to renewables. * Trains are incredibly efficient at transport, and depending on the quarry might already be established. * Boats are so good at shipping heavy weights because of those hydrodynamics, so as long as some reduced returns are okay (reduced cargo space), there's plenty of space on those cargo ships. The real limiter is stability of steering in canals and depth of ports/canals.

Dirk Pässler (carbon-drawdown.de) did some back-of-the-napkin calculations for their experiments (enhanced weathering of basalt dust on cropland):

> In the end we calculated the actual emissions to be 44,6 tons CO₂, due to the fact that we drove more truck kilometers than we had planned. This is less than 10% of 1217 t basalt’s CDR potential of 509 t CO₂. In summary: The mining and transport emissions are one order of magnitude smaller than the CDR effect, even with 300-500 km transport distances. This will only get better in the future with more green energy, electric trucks and optimized logistics.

https://www.carbon-drawdown.de/blog/2022-12-14-how-cdr-with-...

Atmospheric CO2 cannot be converted back into hydrocarbons without expending more energy than the fuel originally released in combustion. Atmospheric CO2 can be converted into stable minerals that no longer drive global warming, at an energy cost less than the fuel released when it first burned.

See this chapter from the IPCC Special Report on Carbon Dioxide Capture and Storage:

https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapte...

In particular, section 7.2.2, "Chemistry of mineral carbonation." The carbonation of magnesium and calcium silicates is thermodynamically spontaneous but kinetically hindered. The kinetic hindrance is why an additional energy input is needed to draw down atmospheric CO2 in less than geological time: the mineral's accessible surface area must increase dramatically for fast silicate weathering. The thermodynamic spontaneity is why the additional energy input can be small compared to the original energy embodied in the fuels that generated the CO2.

All that and who will pay for it? Politicians? They use taxpayers money. Companies? They will pass on the bill to the customers. So in the end same group will experience higher prices. I can't wait for all the poor becoming poorer.

Simple solution to all those. Have sustainably powered collection craft that slowly follow a set path, and have them release the limestone into the sea to build up artificial reefs. The reef and the resulting ecosystem can help improve fish stocks in the ocean and encourage diversity.

That's a lot of power, you'd need a lot of solar/wind.

"Then just use batt..."

Yeah and then you have grid loss, more carbon emissions and a bunch of other issues.

I don't understand how people can propose these ridiculous schemes with a straight face. I suspect it is because "climate change" is a good source of grant money right now, so you have to get creative to come up with a new approach.

They propose the equivalent of twelve containers carrying stone, out of ten thousand on the ship - 0,1% extra weight.

In general, green-washing never truly confesses the scale of the problem. The data driven conclusion is exhausting every mine on every landmass would only buy another 5 to 7 years at most. Geoengineering is currently mostly BS idealism.

The planet will be fine again in 30 thousand years or so, after the current population of psychotic primates go extinct. It is ultimately a self-correcting problem regardless of what humans collectively choose to do. =3