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:

http://atomicinsights.blogspot.com/2008/10/comparing-scale-of-used-nuclear-fuel-to.html

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.