Wednesday, October 29, 2008

Carbon Dioxide surface sequestration with alkaline earth silicates: A response to G.R.L. Cowan.

A couple of weeks ago I posted a comment in response to this post by Rod Adams on the Atomic Insights Blog:

The next comment on the thread was from G.R.L. Cowan, who denied that CO2 sequestration was a pipedream, and proposed that excess CO2 can be neatly sequestered by reacting it with crushed alkaline earth silicates, such as olivine and serpentine. There was something of a debate on that thread which I shan’t recapitulate here. I did not participate, as my knowledge of the field was inadequate for me to do so. Nonetheless it was an intriguing proposition, and I have been considering it ever since. I was actually inspired to get in touch with an old friend who I have not contacted for many years to get his input. He holds a doctorate in chemistry, although he hastens to add that his field of specialisation is polymer chemistry and he is unwilling to declare confidently one way or the other on the practicality of this proposal. He did state that he considers the term ‘clean coal’ an oxymoron, and shares my opinion that the ideal carbon sequestration strategy is to leave the coal where it is in the first place, but he also concedes that the chemistry for alkaline earth silicate sequestration does work, although we might question the economics. Here following is the email he sent me in reply, which he has graciously permitted me to quote:

Ahoy Finrod, An interesting ramble which covers many possibilities, but facts are more difficult especially since so much is at its infancy. A perfunctory glance at the net shows the chemistry is well known i.e. it is scientifically established process. However, while the thermodynamics may be favourable, evidently the kinetics are not so much so. In other words, while the overall journey may be downhill, it is uphill for at least part of the way. Getting over the activation hump is the key and there are 2 main ways to achieve this: a) wait for long enough - scatter the stuff around and walk away in expectation it will do its thing eventually. Grinding it up finely is one way to accelerate the process - after all these rocks have been sitting around for millions of years. If not for this activation energy I would expect life could be quite startling as rocks randomly exploded around us, especially if we breathed on them. b) force the issue using e.g. heat/catalysis as discussed in article below (which I selected more or less randomly). This all involves more inputs. So while thermodynamics may rule, kinetics will dictate when this will happen. I am not sufficiently knowledgeable to say if and when this process would become viable, but as we discussed on the phone, there is a hell of a lot of work and machinery involved in locating and crushing all these rocks....and all of this is consuming other resources and generating other byproducts. And are there other side effects of all the dust and carbonates we are generating in the process? CO2 is not our sole enemy.

The ‘article below’ referred to is:

Making rocks
Nature has the best track record for sequestering carbon dioxide from the air into the ground, through the process of weathering. Carbon dioxide is slightly acidic and as it reacts with rocks and soil, it converts into other chemical forms. The only problem in putting nature to work on carbon sequestration is that the process takes too long by human standards. In order to help limit the amount of carbon dioxide in the atmosphere, some geologists are looking to speed the weathering process up through industrial means — converting carbon dioxide into carbonate rocks.“We end up making rocks,” says Klaus Lackner of the Earth Engineering Center at Columbia University. But they have to start with rocks first. To do so, they use magnesium silicates, a class of peridotite rocks that include serpentine and olivine. Exposing magnesium silicate to an aqueous solution of the slightly acidic carbon dioxide forms carbonate and silicate, such as sand. Presto-chango, the carbon dioxide is gone and new carbonates and silicates have replaced the original rock. And the process is exothermic, producing heat. “So its thermodynamics are downhill, it happens spontaneously,” Lackner explains. This is why weathering in nature also occurs over time. So why aren’t we mass-producing carbonate rocks with our abundance of carbon dioxide? Again, time is the limiting factor. The world has an abundance of magnesium silicate rocks, but reacting those rocks with only carbon dioxide is a slow process. “We are trying to take the process and accelerate it for an industrial setting,” Lackner says. In order to speed the reaction up, a stronger acid is also needed and, in some cases, additional heat. The Albany Research Center in Oregon, and Ohio State University, are both working on building cost-efficient methods. Ultimately, achieving large-scale sequestration will mean building power plants at magnesium silicate mines around the world that would convert the olivine and serpentine into carbonates. The newly formed carbonates would then be put back into the mines for permanent disposal.The Ohio group is fine-tuning their high-pressure, high-temperature, three-phase fluidized bed reactor, an apparatus that uses a mixture of acids to dissolve serpentine in an aqueous solution of carbon dioxide. “In 30 minutes we can convert about 25 percent of solid magnesium silicate to carbonate at 1,000 [pounds per square inch] pressure and 80 degrees Celsius,” says Ah-Hyung Alissa Park, lead author on a presentation about this technique at the American Institute of Chemical Engineers in November. “At higher temperatures and pressures the conversion rate goes up.” Still, the science is in its infancy, Lackner says. “It is an example of where we learn more the cleverer and better we will get.”

So there we have it. My mind is open on this subject. I still think it’s quite interesting… although I do question the economics of ameliorating the consequences of coal use through mining and crushing five or six times as much rock as the coal we burn. If there is a way around that issue, someone please let us know.

One question which has occurred to me is just how powerfully is the carbon bound up in the resulting mineral? If it is only bound lightly, could we use these carbonates to recycle the carbon back into liquid fuel using power from nuclear reactors? While the economics of using this technique to continue burning coal might not necessarily work, perhaps it has other uses in an advanced nuclear economy.


Rod Adams said...

FINROD - as a lazy cheapskate, I would not even attempt such a process, especially since there are many paths to the desired result of cost effective power without any emissions.

This is a project that only a scientist could love.

For those who do not know me very well, one of my favored sayings is that a good engineer is a lazy cheapskate.

I also tend to think of scientists as people who love studying obstacles so much that they ignore the well marked paths around the obstacle to their advertised destination. For them, the study and the journey are far more important than arriving where you want to be with as little expenditure of effort as possible.

Part of that predisposition is that professional scientific income is dependent on long journey's that expend a lot of excess effort. If the customers recognize how easy it is to get to the destination, why would they hire a scientist?

Please understand that I make a distinction between science and professional scientists.

Joffan said...

This particular project, while ingenious, loses me on mining 6-8 times as much rock as the coal burnt, and again on the "spraying fine rock dust into the air" business.

Even the idea of deep injection of CO2 into a fractured bed of suitable rock would almost certainly be massively overtaken by the scale of CO2 that would need sequestration, and the difficulty of creating (or rarity of finding) suitably intense fracturings.

Not an idea to ignore, by any means, but not practical on a scale basis I think.

Finrod said...

Well, I'm prepared to extend the benefit of the doubt a certain distance, but I have to say that my own conclusion pretty much reflects that of G.R.L.'s other critics. It seems like an enormous effort to go to, and if we are currently blowing the tops off mountains to feed coal-fired furnaces, things will only go downhill with all that olivine and whathaveyou needed. I suspect that the proponents of this strategy might shudder at the EROEI calculations when they are done. I also suspect that capturing CO2 in that manner may make it unreasonably difficult to cycle it back into the liquid fuel stream once we get to using nuclear power to synthesize petroleum or DME.

In short, while I'll listen to what G.R.L. and fellow-travellers have to say, I largely regard the scheme as a strategy for shipwrecking the nuclear renaissance with a siren-call falsely promising an easy solution for ‘clean-coal’.

Charles Barton said...

Finrod, My assessment is that even if sequestration is technical possible, it will require so much energy from the coal burning process to accomplish it, that it would not be worth while. This would be true if you sequestered cO2 under the surface or used a surface capture technology. I have not done the surface calculations, but the energy cost of subsurface sequestration might well run over 75% of the electrical energy produced by burning coal.

GRLCowan said...

I have too many points to make for any of them to get made. Each must lead.

Maybe this is a way in. Two assumptions: (1) Excess CO2 in the atmosphere is a real problem. (2) Most who claim to be concerned about it are not genuinely so.

They want to get us off coal because natural gas costs more, i.e., supports more of them, usually through taxation. They don't give a flying toss about excess CO2 that's already in our air.

So patient explaining and reexplaining of the olivine proposal -- which is closer to being Dr. R.D. Schuiling's than it is to being mine -- is a nice way to make these people identify themselves. Heard it too often, yet couldn't relay the vital information to someone who expressed, and may have felt, curiosity.

Why don't we build 300-GW nuclear electricity stations to smash the fossil fuel industry in a few big, crunching blows? Government wouldn't allow it -- they tax that industry's customers, sin tax, and a clean substitute can't be sin-taxed -- and it wouldn't work anyway. Electricity needs a grid, and the grid isn't proffering any three-pronged plugs of the required size.

But a sequestration plant to take down 300 coal GWe worth of CO2 could be built; its grid is the trade winds.

It would not require 300 GWe of new, special coal plant to be built, and that means it is not a "clean-coal" proposal. It cleans up after yesteryear's coal burning, cars, butane lighters.

I did look at the EROEI, more specifically, the extra EI this incurs on top of past energy inputs for a unit ER of coal electricity. Looks to be about an eighth. So that 300-GW plant would need a dedicated 40-GWe nuke.

The hobbyhorse that is actually mine, not Schuiling's, is of course cars that go 1 Mm on a single front-bumperful of fuel, and so cause the world's motorists to thrust the mentioned giant plug at nuclear power plant builders, again, without requiring a grid.

--- G.R.L. Cowan, H2 energy fan 'til ~1996

Finrod said...

OK, G.R.L. My apologies for taking so long to get back to you. I take it from your comment and looking at some of the stuff you linked to that you have no desire to use alkaline earth silicate sequestration as an excuse to continue coal use, but rather see it as a remediation effort for purging excess CO2 from the atmosphere in a post-carbon world. Very well.

Earth’s atmosphere masses about 5.3 x 10^18 kg. The CO2 level in pre-industrial times is held to be (from memory) ~280ppm. Let’s assume we hit 450ppm before we stop adding CO2 and begin the amelioration effort. That’s 170ppm we want to sequester. I make that out to be ~1.2 trillion tonnes of CO2 to be disposed of. Multiply by 1.6 to get the mass of olivine required, and we have ~2 trillion tonnes of olivine to be mined, crushed and dispersed (or cooked up in pressure cookers).
“I did look at the EROEI, more specifically, the extra EI this incurs on top of past energy inputs for a unit ER of coal electricity. Looks to be about an eighth. So that 300-GW plant would need a dedicated 40-GWe nuke.”
So to fix up the atmospheric CO2 waste using this method would require the generation of an eighth of all the energy generated by carbon combustion which produced the problem in the first place. That sounds onerous, but admittedly, if our fondest hopes for a wealthy nuclear economy are realised (including a comfortably higher level of per-capita energy consumption than that currently enjoyed in developed nations for all the world’s people), it may come to be seen as a relatively small burden on top of business-as-usual. We may be able to double, or even triple the rate of sequestration by natural carbon sinks, reducing the healing time for the atmosphere from centuries to decades.

This actually sounds like it might be worthwhile, but I doubt there’d be much chance of getting it up and running before the aforementioned vastly wealthy global nuclear economy is in place. Still, it’s worth filing away for that eventuality, I suppose.