Saturday, October 18, 2008

Carbon sequestration- the basics

Our civilization is currently putting dangerous amounts of carbon dioxide into the Earth’s atmosphere. People are finally starting to realize this, and are looking for ways to reduce the impact as painlessly as possible. One way to do this is to bury some of carbon dioxide that we produce, so that it can’t reach the atmosphere or shallow ocean. While this is a valid and sensible thing to try, the process is often poorly understood or misrepresented. So I’ll run through some of the basics.

Firstly, carbon dioxide is poisonous. We all exhale this gas, so those sorts of levels obviously aren’t going to do us too much harm, but percent levels will, and very high levels can displace air, leading to asphyxiation. In 1986, carbon dioxide dissolved in an African lake uncontrollably exolved, and the resulting cloud of CO2 gas killed 1800 people. The annual human production of carbon dioxide is about four million times larger that the flux of CO2 into that lake, so sequestering CO2 poorly could be extremely dangerous to a very large number of people.

CO2 is also acidic when dissolved in water, and can be quite reactive in certain geological environments. So you can’t just stash the stuff any old place, or it might escape and kill somebody.

Secondly, once CO2 is mixed in with other gasses, it requires energy to extract and purify it. So sequestering CO2 from a reaction that produces pure gas will be more efficient that removing CO2 from combustion exhaust, or from the atmosphere.

Thirdly, CO2 is 3.6 times heavier than the carbon was before it was combusted. So transport costs for CO2 will generally be even greater than they were for fuel.

So, what does this tell us?

If we want to sequester carbon efficiently, the best way to do so is sequestering pure CO2 from a factory that is located very close to the sequestration site.

One potential site for storing CO2 is old oil and gas fields. This is sensible, at least at first pass, since the presence of hydrocarbons in a reservoir requires some sort of trapping mechanism. Mineral sequestration is something I mentioned a while back, but it is still in the research phase at this point.

So, if you build a chemical plant (say a plastic factory) at an oil wellhead, injecting the CO2 back into that reservoir is pretty close to the ideal environment for sequestration, as long as the volume of CO2 produced is less than the volume of oil extracted. Since oil has a greater carbon density than CO2, you need to be leaving some of the carbon in some other form (like plastic) for this to work.

Similarly, if you identify as suitable large, safe reservoir for sequestration, building a cement plant or smelter nearby will give fairly pure CO2 which can then be sequestered without too much trouble.

Another sensible carbon sequestration plan would be to build a gas based power plant at the well head for a natural gas field, and then sequester the CO2 extracted from the exhaust. Methane and CO2 both contain one carbon per mole, and they are both gasses, so the CO2 volume produced should be similar to the volume of methane burned. Because CO2 is more compressible, you might even gain some space, but this could be negated if subsidence over the gas field reduces the available volume for sequestration. In this situation, you still need to separate the exhaust gasses, but the transport and sequestration problems are minimized.

So what about CO2 from coal-fired power stations? Well, that CO2 is a combustion product, so separation is required. Unlike gas and oil, coal extraction does not leave a potentially useful reservoir behind- it just makes a big hole in the ground. So safe, large storage areas will need to be found, and the CO2 will then have to be transported to the sequestration site.

Thus, it is likely that sequestering CO2 from coal combustion will be more expensive than sequestering it from other industrial processes, or from natural gas-based electricity production.

This is an important point.

Often, carbon sequestration is only mentioned in the context of “clean coal”. But if we want to sequester CO2 efficiently, we are better off doing it for non-coal CO2 sources first, as they are likely to be easier and more efficient.

As a result, it would be a very serious mistake to allow coal companies to control carbon sequestration technologies, especially if they are funded by the public. Because limiting sequestration to only that CO2 produced from coal combustion makes the process less efficient, and therefore more expensive.


Kim said...

That's a great point about the problems with allowing the coal companies to control the sequestration techniques.

My local coal-bed-methane-producing basin is the site of one carbon sequestration test. I have the impression that the CO2 can be used to help release the methane bound to the coal. (The methane isn't burned here, but the coal is.) I don't know how efficiently it stays in the ground, though (especially because the rock is being hydrofractured to increase its permeability).

C W Magee said...

I think the idea behind coal-bed-methane is that the CO2 replaces the adsorbed methane... That's fine, but you still need to sequester the CO2 from burning the methane.

My biggest worry is that some bug will evolve to exploit the energy available from catalyzing CO2 + coal into carbon monoxide.

Relying on thermodynamically metastable sequestration strikes me as a bit risky, especially when the stable phase is really poisonous.

EliRabett said...

It is a lot cheaper to move fluids in pipes than to truck/RR solids.

Sergio said...

The WRI just came up with a set of Guidelines for CCS.
Anther point to consider is that CO2 is heavier than air, therfore, once deep in a reservoir, it would never reach the surface as a gas. It would only do so, dissolved in brine water and through pressure gradients (old oil wells, fractures etc). The trick here is that CO2 could escape from the injection point via a pipeline rupture, then, it would displace O2 from the air and could cause asphyxiation. Another issue is that the acidic brine in the reservoir could dissolve metals (As, Pb...) migrate upwards and contaminate drinking water supplies.