Tinsel Wing

Monday, August 21, 2006

The carbon return desk

Last week's Nature had a fascinating pair of news features on carbon sequestration. There is, of course, no silver bullet for beating global warming. It's going to take, you should pardon the expression, an energetic attack across several simultaneous fronts. But I had been imagining that sequestering CO2 was no more than a bit of blue-skying. Turns out all of the technology is well established, and it may solve a goodly fraction of the puzzle.

The zine's first piece, The hundred billion tonne challenge, concerns the standard plan: capture the carbon dioxide from coal as it burns, and then inject it into deep aquifers, where it will be stabilized under pressure in dissolved form. Three ongoing industrial scale projects already exist, in Canada, Algeria, and Norway. The Norwegian plant is already economically viable, because of Norway's hefty carbon tax, set at $50 a tonne as compared to the EU's timid $20. A scaled-up project is under construction in Ketzin, Germany, which will stuff 60,000 tonnes of CO2 away over two years.

It had better scale up a lot further. In order to keep from shooting past the doubling mark for carbon dioxide concentrations, we've got 175 gigatonnes of Chevron's favorite gas to make vanish over the next 50 years. The potential storage capacity of deep aquifers, though, is from 1,000 to 10,000 gigatonnes.

For CCS (Carbon Capture and Sequestration) to become a significant chunk of the solution worldwide will take around $80 billion in capital investment. When a coal plant is built from scratch to accommodate the integrated gasification combined cycle (IGCC), you have to pay out 20% more up front. If it has to be retrofitted, the cost gets steeper. Then there are operating costs, the IGCC burns up some energy itself, and you still only recover half the carbon dioxide the coal gives off. The bottom line: IGCC will add about 3 cents per kilowatt hour to the production costs of a coal plant, a bit under double. Enough to make you swallow hard, but when compared to the costs of global warming, not particularly alarming. As a side benefit, the technique also scours such pollutants as sulfur from the plant's stacks.

It's a two-step process.
In IGCC plants, the fuel — coal, fuel oil or biomass — is introduced into a hot gasifier along with oxygen and steam. This produces a fuel gas consisting mainly of carbon monoxide and hydrogen. The carbon monoxide then goes through a second 'shift' reaction with steam, making carbon dioxide and more hydrogen. The carbon dioxide can be relatively easily separated at this point.

The magazine's second article dangles its limbs a little bit out into blue sky territory, but there's some good science behind it. More next time...


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