Sunday, April 12, 2009

Carbon Sequestration Insanity

On to another topic of far greater global importance...

It is astounding to me that people who would seem to have a modicum of scientific sense can even vaguely entertain the various concepts of "carbon sequestration" as being in any way valid approaches to "solving" the problem of carbon releases from burning fossil fuels. The problems of scale here are huge, and shold be obvious to anyone who sits down for a quiet moment to think about the global carbon cycle.

Let's just look at the U.S. piece of the pie here. Our annual carbon releases from burning petroleum and coal run around 2 x 1015 gC/yr. This is an enormous amount of matter. It averages 7,000,000 gC per capita each year -- 7 metric tonnes of carbon for each of us to be somehow captured out of the atmosphere and trapped permanently in solid (or aqueous solution) form. We can't capture it as pure carbon; that would use up pretty much all the energy that we got from burning the fuel in the first place. All the proposed schemes capture it either as CO2 or biomass. Both of these weigh considerably more per mole of C. Of course, CO2 is not stable in solid or aqueous form at the surface of the earth. There are proposed elaborate schemes to transfer captured CO2 directly into the crust or deep ocean for long-term storage; the practicality of these schemes on the massive scale required is highly dubious. Dubious as well are the conclusions that this "stored" CO2 would remain in place for thousands of years without adverse geological, ecological, or oceanographic consequences. Additionally, direct CO2 capture is only applicable to large-scale point sources (i.e. big power plants); these only represent about half of the problem.

Realistically we're probably talking about making calcium carbonate (CaCO3) out of the CO2 and then disposing of this product somehow. This compound weigh about 8 times as much as does the carbon it contains. So now we're talking about 50 metric tonnes of solid waste per person in the U.S. to be disposed of. This is about 60 times greater than the per capita creation of municipal solid waste in this country. Imagine a solid waste disposal problem 60 times bigger than what we already face! Or, picture for every coal train headed towards a U.S. power plant, and every oil tanker headed ito a U.S. port, eight trains and tankers loaded with sequestered carbon headed the other direction -- headed to where?

An additional issue with trapping CO2 this way is the rest of the reaction. Where does all this calcium come from? Fortunately Ca is a very common element on earth, though much of the solid deposits are carbonates and alumiosilicates that are not really useful for carbon trapping. Some proposals involve direct catalyzed reactions with CaO (as well as other rock-forming metal oxides) to produce stable carbonates. Calcium oxide is in theory an abundant mineral in the crust; however it occurs mostly intermingled molecularly with the other common metal oxides of the earth, rarely in pure form. Does this mean that for every barrel of oil or ton of coal we extract, we will now be required to also mine 10 times as much ordinary rock to crush up to yield the metal oxides? Is increasing our rate of strip mining by an order of magnitude the direction we want to take here? The simplest source of calcium for making carbonates would probably be the chloride and sulfate salts. These are reasonably common; seawater effectively contains vast reserves of CaCl2 in aqueous solution. The reaction is simple: just bubble the CO2 through the solution, and the calcium carbonate forms without the need for a catalyst or any additional energy inputs. Still, gathering and concentrating all these calcium salts will require massive energy and infrastructure. Once you manage this, you then run against the next problem. Reactions for trapping CO2 that way generally look like this:

CO2 + CaCl2 + H2O --> CaCO3 + 2 HCl

Oops... for every molecule of CO2 you trap, you produce 2 molecules of hydrochloric acid. Calcium sulfate will produce sulfuric acid instead. Any reaction that produces a stable carbonate from CO2 and a mineral salt will produce an acid as well. This means that those 60 metric tonnes of calcium carbonate we make for each person each year will be paired with about 100,000 liters (nearly 30,000 gallons) of concentrated hydrochloric acid also needing disposal. Nationwide this will be the equivalent of about 30 trillion liters of concentrated HCl -- 30 cubic kilometers of one of the most corrosive substances known. Exactly what will we do with this?

As an additional note, if we just passively allow or actively or encourage the shallow ocean to fix all this CO2 instead of doing it ourselves in our own facilities, we are doing the exact same reaction, just effectively dumping all these waste products directly in the ocean. The atoms won't just go away, and the stoichiometries are unavoidable.

What about biomass? Terrestrial net primary production (NPP, the amount of carbon taken from the atmospere and turned into biomass by living plants) averages roughly 500 gC m-2yr-1 -- higher in some places, lower in others, but within a factor of 2 of this value for most land areas. This means that our U.S. annual carbon releases are equivalent to the NPP of 4 x 1015 m2 of land surface. This is nearly half of the 9.1 x 1015 m2 land area of the entire U.S. including Alaska. So what we are talking about here is somehow taking half of all the NPP of the entirety of the U.S. and capturing it forever so that it will never be returned to the atmosphere. Anyone who has ever looked out a window and examined the real world should know immediately that this is ludicrous. Biomass capture could never be, in any practical sense, more than a small drop from a huge bucket of global carbon.

Some engineers who do realize these insurmountable obstacles instead turn to "geoengineering" solutions. Many of these take the form of seizing control of the global marine ecosystem, turning it upside down (in some cases literally), and converting the global ocean into a giant, intensively managed carbon sink. Others do the same to the stratosphere. Let's set aside the obvious problem of the fact that in a world where we can't even keep our highways paved we will never find the resources for this sort of massive global infrastructure undertaking. Let's focus on the more obvious matter: anyone with the smallest bit of environmental scruples should be revolted, horrified, and up in arms against this sort of "solution."

There is only on solution to the global carbon problem: stop burning so many fossil fuels. This will happen eventually. If it is not done willingly (and it never has been before, on the long-term and large-scale), it will happen when it is forced by economic and environmental constraints and harsh realities.

P.S. If you find errors in my numbers please let me know and I will fix them! Though there might be quantitative slip-ups, the qualitative issues will not change.


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