Thursday, March 25, 2010

Some Chemistry, Please!

Joe Romm has been taking on the Calera process for (allegedly) making limestone out of carbon dioxide emissions and seawater. Here's the latest.

As a chemist, I've been suspicious of the Calera process and some other bright ideas out there for sequestering carbon dioxide. Unfortunately, Romm's recent posts don't help me to understand them. I know that what I'd like to see gets pretty deep in the weeds, but that's the only way these processes can actually be evaluated.

Romm's posts have been summaries of what others (Eli Kintisch, Ken Caldeira) have found about the process. But they emphasize some things that aren't all that important as far as I can see and leave out others that are.

Not so important: Romm's and Kintisch's concerns about "caustic solutions." I'm tired of seeing the dangers of "chemicals" (cue scary music) overrated. Practically everyone has that "caustic" bicarbonate in their house. It's called baking soda, and some even brush their teeth with it neat. And caustic.

What is scary about bicarbonate, to a chemist, is its ability to participate in an enormous number of equilibria with various metal ions that might be in the rock and that could make them more, not less, soluble. Bicarbonate is used in mining uranium, for example, by dissolving it out of underground formations and pumping the uranium-bearing solution to the surface. This is the issue that needs to be addressed.

And then there's thermodynamics. Hard to popularize, but it provides the limits on what one can do with this sort of process. If you calculate the free energy of a reaction, and the result says it's not going to happen under your conditions, it's not going to happen. Period. Nothing about thermodynamics (and those solubility constants are part of it) in Romm's posts or in anything I've seen from Calera.

Another problem: the relevant ions are very dilute in seawater. At least two potential problems flow from this. First, a lot of seawater is going to have to be processed, and the more material you handle, the higher the energy requirements. Second, those pesky solubility equilibria. The more water you have, the more material you can dissolve.

So I'd like to see a full (with solubility and thermodynamic considerations) explanation of what it is Calera claims to be doing. Not just what others may have concluded from something they're not fully telling us either.

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