Wednesday, July 23, 2008

Don't Pass Gas

Sequestration- or storage- of carbon dioxide has been proposed as a way to deal with emissions of that gas, and reduce the climatic effects of using fossil fuels. I don't dismiss it, but I've been a little leery: too often what looks like a good deal turns out to have unexpected consequences. For example, fossil fuels. No one can argue that the relatively cheap, energy-dense materials haven't been of great benefit to us, but it now looks as if we've tweaked our atmosphere enough to... well, we don't really know yet. The idea of simply shoving our CO2 underground and forgetting about it is appealing, but what if it leaks?

Oddly (and tragically) enough, that is a question that has been answered: on August 21, 1986, Lake Nyos in Cameroon erupted about 1.6 million tonnes (I think that spelling implies metric tons, about 2200 pounds) of carbon dioxide, killing 1700 people and thousands of livestock. A few years later, I argued with a professor over whether pollution was necessarily man-made, and used Lake Nyos as an example. (For the context of atmospheric chemistry, his definition stood) But imagine millions of tons of CO2 erupting near a city. The victims would never know what hit them.

This is why I've been uneasy with the idea of sequestration. Most investigations have been made in land-based sites with sedimentary rocks like sandstones and limestones that have ample pore space and permeability to allow fluids to be pumped into them. But both of these types of rock would tend to be chemically nonreactive with CO2: the gas would just sit there, in human terms, essentially forever. From my perspective, it just looks like an accident waiting to happen.

However, there have been a couple of articles recently that make the idea look a little more appealing to me. A group has looked at the sea floor basalt of the west coast, from Northern California to Southern British Columbia. Basalt has a high proportion of calcium feldspar and magnesium silicates such as olivine and pyroxene. And calcium and magnesium both react nicely with CO2 to form carbonate minerals- calcite and magnesite, respectively. Basalt also contains lots of iron, which will react to form siderite. In other words, if we pump our exhaust into basalt, it will react to form stable minerals. It goes away over time, rather than sitting there as an enormous pressurized pocket waiting to leak.

A similar article out of Queensland University of Technology suggests injecting CO2 into magnesium-rich rock units.

I really like the idea of locking the gas up chemically. I'm really not comfortable with just pumping it into the ground and assuming it will stay put. And these ideas, while promising, do not represent a full answer. I haven't seen any realistic estimates of costs- and costs need to take into account the forgone energy used to pump the CO2 into the ground. If it costs all the energy generated from a ton of coal to sequester the equivalent exhaust, it's a non-starter. Another problem is that conventional methods of burning fossil fuels use regular air, which is 80% nitrogen. Thus the exhaust is 80% N. You need to either use pure oxygen to burn the fuel- expensive- or you need to separate the CO2 from the exhaust stream- which I can't imagine being cheap. Still, I see this as a promising area for further study.

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