Numbers on Ocean Alkalinity Enhancement inputs & effectiveness?

[Jasper Sky]

Dear all,

I'm an economist working on a survey of CDR methods for a multilateral development bank. My brief is to figure out which, if any, CDR and CCS/CCUS methods the bank should get interested in backing. I'm not a physical chemist and I'm a noob to EW and OAE, so I need a little help (in plain English rather than in impenetrable chem-anorak jargon) to get a sense of these various methods' real-world plausibility. We need cheap and hugely scalable carbon drawdown solutions. I'd be very grateful for numbers, input, and links to the best not-too-jargony scientific review articles (or better: well-written science-journalism pieces) that can help a noob understand this stuff.

I'm particularly interested in ocean-based CDR. It seems to me that if some form of ocean-based CDR can be made to work at reasonable efficiency in terms of tons of material input and kWh per ton of energy input per ton of CO2 sequestered, then that would be a silver bullet, because the ocean is huge, innit? 

One question I have is in relation to OAE. I'm imagining a nearly unpopulated mountainous coastal region, e.g. in northwest Namibia or Iceland, where the land is made of basalt or olivine or peridotite or serpentine. If I want to achieve 1,000,000 tons of CO2 drawdown there in some reasonably brief period of time, e.g. a year, what do I have to do? 

- How many tons of e.g. basalt do I have to quarry and comminute (crush and grind) to a fine powder to achieve 100 tons of CO2 drawdown via OAE?

- How finely should I comminute the basalt? What's the optimal grain size? 

- The optimal size is presumably "as small as possible, without wasting energy," and so it will be a function of the electrical energy input required to operate the crushers and grinders to get any particular particle size, on the one hand, and the CO2 sequestration effectiveness per gram of material, on the other hand. Right? So, what's the optimal grain size, and how many kWh per ton of basalt (or olivine, or whatever) should I expect to expend on comminution operations?

- Once I have finely ground basalt powder -- what do I do with it? If I'm at or near the coast, I'm envisioning loading the powder onto a u-shaped conveyor belt and sending it onto a barge anchored no more than a couple of hundred meters away. The barge then gets towed out to sea and... what? The basalt powder simply gets dumped in one place, perhaps over a fast-moving marine current? 

- Why have I seen articles suggesting that ground olivine should be distributed onto beaches (as distinct from out in the open ocean)? What's the advantage of that? Also, here's an article saying it won't really work in practice anyway. Comments?

- I've read (okay: skimmed) some stuff that in dense chem-anorak jargon, nearly impenetrable to an economist, seemed to suggest that spreading ultramafic rock dust in seawater may not work to sequester carbon after all (or works only 1/5 as well as had bee hoped). Comments?

- It seems that pressing large amounts of CO2 down wells into basalt formations also may not work as well as had been hoped (by the likes of ClimeWorks), either, because the rock gets supersaturated...  Comments?

- If simply dumping ultramafic rock flour at sea won't actually work, is there something else that I could do with that rock flour that would work to sequester carbon? Like, draw seawater into a solar-powered seawater desalination plant, mix the desalinated water into the rock dust in a big tube with some parabolic mirrors aimed at it to heat it up, and then pump it out to sea? Or something? 

Not to put too fine a point on it, but I'm looking for a silver bullet. There's no shortage of basalt out there. If processing, say, a couple thousand billion tons of basalt into rock dust and then doing some simple procedure on it could absorb a thousand billion tons of CO2, then I want to know about it. 

I recognize that a huge energy input will be required -- from vast solar fields set on top of hundreds of coastal basalt province locations in remote areas in places like Namibia, Iceland, Mauritania, or Australia, perhaps. What I'm thinking is that once a plausible set of very commonly available, cheap material inputs has been identified and a processing method that sufficiently speeds the carbonate formation reaction has been designed and tested to make the process fit-for-purpose as a carbon sequestration method, then it's just a matter of scaling the thing. Then we go find a couple of hundred sites where there are big coastal basalt formations in remote, unpopulated regions, set up huge fields of very cheap solar PV panels, all financed by low-interest concessional loans and a carbon market mechanism, and away we go. Silver bullet. 

Let me know - either based on empirical results, or first-principle theory - what the design of that OAE or EW silver bullet looks like. Please provide numbers -- I may be a mere economist, not a physical chemist, but I did study applied maths and physics as an undergrad, so I'm numerate. I really only believe stories that come with hard numbers, tbh. 

Kind thanks, dear chem-anoraks!

[Chris Vivian]

Jasper,

 

Firstly, you might want to take a look at section 5.13 of the GESAMP report published in 2019 that summarises the issues for OAE http://www.gesamp.org/site/assets/files/1996/rs98e-1.pdf.

 

Some comments on some of your questions:

 

1. Regarding the source of material for OAE you may not be aware that the largest deposit of peridotite is in Oman and there are plans to exploit it – see https://4401.earth/ and https://www.project-hajar.com/. Also, there are some 100 billion tonnes of waste rock from mining produced each year, some of which is likely to be suitable for OAE purposes. Being already mined and ground is a distinct advantage over having to mine fresh rock. Also, Planetary Technologies process utilises mine waste for its process producing magnesium hydroxide for OAE - https://www.planetarytech.com/technology/.  

 

2. On the issue of ground olivine being distributed onto beaches (as distinct from out in the open ocean), the reason for that is that coarse ground olivine is put on beaches or below the Low Water level so that wave action grinds the material to a finer size. There is another article olivine addition may thus be a less efficient CDR method than previously believed – Fuhr et al. (2022) https://www.frontiersin.org/articles/10.3389/fclim.2022.831587/full. Vesta should have more up to date results from their field testing soon to answer this question.

 

3. You said “I've read … seemed to suggest that spreading ultramafic rock dust in seawater may not work to sequester carbon after all (or works only 1/5 as well as had bene hoped)”. That paper only relates to the use of olivine, a mineral not an ultramafic rock. So, you cannot necessarily generalise that to all ultramafic rock materials. Also, carbonate rocks can be used for OAE and they have different characteristics. You might want to be aware of this review paper https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000533.

 

4. You also said “It seems that pressing large amounts of CO2 down wells into basalt formations also may not work as well as had been hoped (by the likes of ClimeWorks), either, because the rock gets supersaturated…”. I would not take a single paper on this subject as the definitive answer on this issue. A lot will depend of how fractured the rock is and this can vary a lot.  Also, other rocks than basalt are likely to have potential e.g., peridotite or serpentine.

 

5. On your question “…is there something else that I could do with that rock flour that would work to sequester carbon?”, dissolution of carbonate minerals can be achieved by reacting them with waste flue gas CO2 and seawater.  This raises seawater pCO2 and lowers pH and CaCO3(aq) saturation state such that when contacted with solid calcium carbonate, reaction with CO2 spontaneously occurs. The resulting alkalinity is discharged to the ocean. See the GESAMP report for further information.

 

6. I don’t think there are any silver bullets that will solve the climate crisis by themselves. I think it is much more likely to need a portfolio of techniques – see the Pacala and Socolow (2004) paper at http://science.sciencemag.org/content/305/5686/968.abstract.

 

Best wishes

 

Chris.

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