"Our greatest responsibility is to be good ancestors."

-Jonas Salk

Monday, November 24, 2008

The peridotite solution

Here's a remarkable sequestration mechanism that seems ideal for our needs. A single wedge, even a single site solution.
The researchers have shown that rock formations called peridotite, which are found in Oman and several other places worldwide, including California and New Guinea, produce calcium carbonate and magnesium carbonate rock when they come into contact with carbon dioxide. The scientists found that such formations in Oman naturally sequester hundreds of thousands of tons of carbon dioxide a year. Based on those findings, the researchers, writing in the current early edition of the Proceedings of the National Academy of Sciences, calculate that the carbon-sequestration rate in rock formations in Oman could be increased to billions of tons a year--more than the carbon emissions in the United States from coal-burning power plants, which come to 1.5 billion tons per year.


The researchers found that the natural peridotite formations in Oman captured carbon dioxide in a network of underground veins. Peridotite contains large amounts of olivine, a mineral composed of magnesium, silicon, and oxygen. As groundwater reacts with the olivine, the water becomes rich in dissolved magnesium and bicarbonate, with the latter effectively increasing the carbon concentration in the water by about 10 times. As this water seeps deeper into the rock and stops reacting with the air, the magnesium, carbon, and oxygen precipitate out of solution and form magnesium carbonate, also called magnesite. Dolomite, which contains calcium, magnesium, carbon, and oxygen, also forms. As the magnesite and dolomite form, they increase the total volume of the rock by about 44 percent, causing cracks to appear throughout it, which creates a network of fractures as small as 50 micrometers across. This opens up the rock and allows water to penetrate further. "It's a little bit like setting a coal seam on fire," says Peter Kelemen, a professor of earth and environmental studies at Columbia University. "You're taking rocks that haven't been exposed to the atmosphere, and you're oxidizing them very fast."

Many a slip twixt cup and lip of course, but (to scramble metaphors) maybe there is a silver bullet after all. I'd love to see this work at scale. Most of the commenters on the linked Technology Review so far tend to disagree, choosing to worry about the local ecosystem. How do In It readers feel, I wonder?

Update: Here's the peer-reviewed article that Tech Review ought to have cited, with thanks to David Benson.

Update: Here's a similar article at Popular Mechanics.


Anonymous said...

To maximize the effect, doesn't it require carbon capture and/or atmospheric drawdown?

Isn't that something that has yet to be made a reality on a meaningful scale?

Michael Tobis said...

This is all new to me but as I currently understand it, the concentration mechanism and the sequestration mechanism are the same. That is, there's enough ambient CO2 to drive the reaction significantly already.

The trick would (it seems to me so far) only be to boost its rate a couple of orders of magnitude mechanically.

It's just as karmically peculiar that this deposit is off the Arabian Peninsula as it is that the wind energy of the US is in West Texas, but there you have it.

I'm no expert on this at present. What interests me at a first pass is whether there are people who actually don't want this to work, and why.

Aaron said...

I like it! Let's start another non-linear geo-engineering experiment! Fracture the deposits with a small nuclear device until [CO2] in the atmosphere drops like a stone. Then, if atmosphereic [CO2] drops too much, we put a small nuclear device in a big coal seam and burn a lot of coal -- fast. This would be better than anything Dr. Strangelove could devise.

David B. Benson said...

thingsbreak --- No carbon capture required, such schemes are entirely air capture, this being enough becasue the reaction of olivine with carbon dioxide is exothermic.

The proposal sited in the main thread here appears to be very inexpensive. Here is a listing of other mineral weathering proposals (including a reference to this one).

Olivine weathering:


See references 7, 8 and 9 in


Peridotite weathering:


Mine tailings:


The proposal to spread ground olivine appears to be about the same cost, initially, as the most optimistic estimates for CCS. That cost can easily be made to go down, possibly by as much as a factor of four.

The in situ peridotite certainly looks to be very inexpensive. My only qualms are the poor controlability and accountability (of how much CO2 was actually removed). Neither of these are actually very serious.

Mitchell said...

I won't believe mineral sequestration is a one-shot solution until someone shows me the numbers. But it looks like it can be part of the solution. So the question is how best to tie it into the global system for financing of mitigation activities.

Magnus said...

you would need to confirm that the reaction rates... wroth a try... however it is no secret that during long time-scales silicates have been regulating the co2 concentration.

Carbonates are also quite easily dissolved so the environment where they precipitate must be saturated in that aspect... hmm will try to look in on this :) but I cant find the PNAS article...

Michael Tobis said...

No, I don't see the article either. Can anyone help?

GRLCowan said...

Olivine weathering rates were reported in Michael A. Velbel's Bond Strength and the Relative Weathering Rates of Simple Orthosilicates.

The energy cost of pulverizing it and strewing it so that atmospheric CO2 will consume it within a year turns out to be about an eighth of the energy yield the CO2's formation will have earlier yielded, if it came from coal.

(Or a 16th if the carbonate ions and additional atmospheric CO2 react to form bicarbonate ion.)

--- G.R.L. Cowan (How fire can be tamed)

Magnus said...

Relative rates and kinetics I have no doubt is figured out... but the effect of grinding and then moved to "natural" conditions could be a trickier thing to get right.

Michael Tobis said...

The quantities of oxidizing mineral have to be comparable to the quantities of coal mined; this is certainly not a small operation. (It has one advantage over the coal mining operation because the substance will be used in situ.)

Local disruptions will be nontrivial, which makes for interesting political and ethical issues, but in the end it is better to damage a place than to damage everyplace.

David B. Benson said...

In situ carbonation of peridotite for CO2 storage --- Peter B. Kelemen and Jürg Matter --- PNAS

Martin Vermeer said...

Having read the article (rather superficially), my impression is that to reach the high carbonization rates quoted you have to inject concentrated CO2 or CO2-containing solution.

Alternatively, you can inject water containing 4*10^-4 bar equivalent of CO2 in solution (i.e., in equilibrium with the atmosphere), but then, the reaction rate is limited by not wanting to cool the reaction zone by all that water. The zone keeps its temperature mainly due to the hydration reaction which is also exothermic; but what we're interested in is the carbonation reaction which is limited by the CO2 concentration in solution.

Correct me if I'm missing something. If true, this is interesting but the main problem of concentrating the CO2 remains.

David B. Benson said...

Martin --- You have the right of it. Due to the concentration problem, this in situ technique may not be the most cost effective.

Just considering operating costs, not capital expenditures, there is an ex situ technique which may only cost around $20--25 per tonne of carbon dioxide removed.

Michael Tobis said...

Martin, yes, I think you have it right and I had it wrong on that point.

Michael Tobis said...
This comment has been removed by the author.
Michael Tobis said...

Wait. Actually, I had it right the first time. The original article says:

"The researchers propose a carbon-sequestration strategy that would eliminate the need to transport carbon dioxide, as well as the need to heat up the rock. In this scenario, they would access rock formations in shallow ocean waters off the coast of Oman and elsewhere by drilling into them and fracturing the rock using existing oil-industry techniques. The researchers would drill two holes. Into one, they'd pump cool seawater. Rock temperature increases with depth, so as the water is pumped into the holes, it will get hotter, until it reaches nearly 185 °C. Carbon dioxide naturally dissolved in the water would then precipitate out of the solution. The hot water would eventually make its way through the fractured rock to the second drilled hole, where it would rise to the surface via convection. This seawater would quickly absorb more carbon dioxide, since shallow waters and surf mix well with the atmosphere. Because "the atmosphere transports carbon dioxide all over the world for free," Kelemen says, this approach, if deployed on a grand scale, could be used to lower worldwide levels of carbon dioxide."

David B. Benson said...

Michael Tobis --- Yes, that is the 'slow' method, a thousand times slower than the 'fast' method.

To make the point, one would have to drill, under water, 100,000 holes to the necessary depth which I think is about 3 km.

An under water hole 3 km deep into the crust is an expensive undertaking, but with so many to drill the price might come down. Anyway, wahatever you think such a hole costs, multiply by 100,000.

And oh yes, after some time you'll need to drill another 100,000 replacement holes. And again and again and ...

David B. Benson said...

A quick check suggesta a minimum of three million dollars per hole drilled. So the cost is a minimum of $300 billion every several years.

Actually, that's not bad, even if the actual cost is double that. I'll have to reread the paper.

Michael Tobis said...

That's really very promising. How did you come up with that number?

David B. Benson said...

Michael Tobis --- I went web trawling on the search phrase 'offshore deep drilling costs' and came up, near/at the top, some testimony by some Louiseana offical before the U.S. Congress in 2001 CE. It had a graph showing drilling costs per foot versus depth. Now 3 km is about 15,000 feet, so I just used that figure.

Michael Tobis said...

3 miles is about 15000 ft. 3 km is about 10000 ft. So maybe I can save you a hundred billion or so right off the top.

David B. Benson said...

Good, because the drilling costs are actually much, much higher now. My 3 km holes will cost about 5--6 million each to drill.

Michael Tobis said...

It's an irony that the energy companies themselves are subject to energy costs in their operations budgets. I have heard quite some grumbling from the oil guys a couple of months ago. In other words I think the price of a dry hole has come down again, but admittedly that won't last.

David B. Benson said...

HOwever, after futher consideration of the 'slow' underwatr carbon dioxide removal scheme, I conclude it is far, far less efficient than 'ex situ' enhanced olivine weathering methods.

The best of these seem to cost about $20 per tonne of CO2 removed.