Wind Scrubbers Again

Sometime last year I remember reading an article about "wind scrubbers," which would take CO2 directly out of the atmosphere with the aim of combatting global warming. Now the idea is being rehashed again (via digg). There's a major flaw in this whole approach: CO2 in the atmosphere just isn't that concentrated, so you need a lot of these things ($$$) to have any real impact.

A much better solution, IMHO, is to build biomass power plants with CO2 scrubbers. Plants made from atmospheric CO2 go in. Energy comes out, with the carbon staying in the scrubbers. The energy you get as a side effect might even be enough to make more scrubbers!

There is going to be a lot more interest in this sort of technology if certain theories of oil formation turn out to be correct, so I really hope someone steals my idea and the whole idea of "wind scrubbers" falls by the wayside. We don't need artificial trees, folks, when we have plenty of real ones to chop down and burn in scrubber-equipped boilers!

Update: Engineer-Poet posted in the comments that he's been thinking about the same sort of thing. He says that grass is better than trees for pulling carbon out of the atmosphere. Yard trimmings anyone? So if we're going to plant specifically for this, we should use grass. In fact, biomass and gasification plants in Sweden do use grass. On the other hand, there's plenty of waste wood available already, so it would make sense to use it as well, even if we aren't planting trees specifically for this purpose.

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I was toying with the idea

I was toying with the idea of using trees to sequester carbon dioxide a few weeks ago. If my calculations are correct (and there's considerable room for error), you'd need something like 1.2 million square miles (or about 20% of the United States' area) of heavily forested area growing constantly to sequester all the carbon dioxide emitted in the US.

What form would the energy come out in? Hydrogen? Obviously the carbon doesn't stay in the scrubbers if it's coming out as some form of hydrocarbon. Not that recycling carbon and getting free energy out of it is such a bad deal.

...you’d need ... about

...you’d need ... about 20% of the United States’ area ... heavily forested ...

Today about one-third of the nation is forested.

You've made me curioius, now. Your calculation suggests that US forests are roughly large enough to sequester all US CO2. I'll try to hunt around and see if I can find more on the topic.

Of course, if your total emissions don't include animal respiration, you might be missing a significant source.

Mark, I think he means in

Mark,

I think he means in addition to the status quo, which would mean putting forests up to 53% of US land.

Animal respiration should be

Animal respiration should be cancelled out by the growth of the plants that they eat. My calculations were based on something I saw somewhere that said that the lumber industry can produce 12 tons of lumber per acre per year. That may be a bad assumption, in that you might get much less without heavy cultivation. Of course, it doesn't take into account parts of the tree that aren't good for lumber (bark, small branches, etc.), either.

I also wasn't sure about the CO2->wood conversion factor. I think I assumed it took one ton of CO2 to make two tons of wood. I'll try to reconstruct my calculations later.

Brian: I meant in total.

Brian:
I meant in total.

PBS had a global warming

PBS had a global warming special on the other night hosted by global warm expert Alanis Morrisette. She talked about CO2 scrubbers. These things are as big as a ten story building and you would need lots of them, like a forest or a CO2 Scrubber Farm. I didn't learn how the CO2 being released from the tundra that is melting would affect the numbers needed or what level of CO2 is needed in the atmosphere. But, I learned it is a problem, today. There was no discussion of the high levels of CO2 that have been discovered in ice cores that dwarf today's levels. IMO, it is the height of arrogance to think humans have that big of an effect as do vocanoes etc.
Mover Mike

What form would the energy

What form would the energy come out in? Hydrogen? Obviously the carbon doesn’t stay in the scrubbers if it’s coming out as some form of hydrocarbon. Not that recycling carbon and getting free energy out of it is such a bad deal.

I was thinking electricity. There are a bunch of ways to do this. You can burn wood or grass pellets in a boiler to run a steam turbine, or you can gasify it and run a gas turbine, possibly with a boiler on the backend to increase efficiency. You run the waste gases through a scrubber, then when the scrubber is used up you do whatever you would have done with a wind scrubber. Dump it in a landfill? The ocean? You can't extract energy from the scrubber itself, if that's what you were thinking. You'd have to add energy to get the carbon out of it, which is why it works in the first place.

The point is that there's no reason whatsoever to use CO2 scrubbers on something with as low of a CO2 concentration as the atmosphere. It's pretty easy to find or make places with much higher concentrations, and we can get energy in the process.

You know, another thing that's occurred to me is that it doesn't make a whole hell of a lot of sense to recycle paper if you're worried about atmospheric carbon. Paper is sequestered carbon, so we should be making as much virgin paper as possible. When we're done with it, it should be buried someplace it won't decompose for a very long time, or we should burn it in something with a CO2 scrubber. Greenpeace doesn't seem to get this.

Back in July, I proposed an

Back in July, I proposed an energy cycle built around exactly this reversal of the fossil system of earth -> energy extraction -> atmosphere.  We could sequester one Heaping Shitload (tm) of carbon by dumping CO2 down depleted oil and gas wells and into deep aquifers.

The best biomass source for this appears to be grass, not wood (greater productivity).  I took a look at that in Grass power revisited.

Recycling paper might actually make sense if uncut forests accumulate more carbon than pulpwood plantations.  Unless it gets into a landfill, paper tends to decompose or be burned fairly rapidly.

My comments don't actually

My comments don't actually reflect any believe in anthropocentric CC.

Brandon Berg,

Well, that assumes you are using conventional trees. You could engineer trees to be better carbon uptakers after all, and thus double, triple, etc. your results.

Brian W. Doss,

Well, that is entirely possible if you can engineer trees that will survive desert-like conditions (I know some people working on that right now).

Recycling paper might

Recycling paper might actually make sense if uncut forests accumulate more carbon than pulpwood plantations. Unless it gets into a landfill, paper tends to decompose or be burned fairly rapidly.

EP:

Landfill is exactly where I was thinking of putting it, or burning it in something with a scrubber. Your uncut wood comparison doesn't make much sense because a pulpwood plantation would not necessarily be displacing forest. Recycling makes sense if it's cheaper; we can divert land that would be used for pulpwood plantations toward growing grass for IGCC plants or whatever, and you did mention that grass is better than wood for taking CO2 out of the air.

Landfills have difficulties

Landfills have difficulties too:
1. they're getting expensive;
2. they represent a loss of whatever energy is in the refuse; and
3. leakage of methane from the decomposing matter can be a bigger greenhouse problem than the CO2 you'd get from burning it.

This points back to waste-to-energy with CO2 capture.  Scrubbers are half-measures; they're designed to be tacked onto existing fossil-burners as afterthoughts, rather than being part of an optimized design.  The typical scheme involves capturing CO2 at roughly atmospheric pressure and then compressing it, which is the opposite of what you want to do with a heat engine; these mechanisms are correspondingly energy-hungry.  As natural gas and even coal prices are rising steeply, devices which need more of them to function aren't going to be popular.  The energy value of waste paper isn't going to be any easier to ignore, or attractive to waste.

The ideal system would burn the fuel (paper, whatever) at high pressure so that the CO2 can be condensed at ambient temperature and piped off for disposal.  It requires a high-pressure oxygen supply, but this is already part and parcel of e.g. oxygen-blown IGCC plants; off the shelf.  And ideally, it would be much more efficient than a steam plant.

Unfortunately, such a system doesn't bear much resemblance to atmospheric-furnace boilers, pressurized fluid beds, or IGCC plants blown by either air or oxygen.  Getting energy out of the fuel without letting the combustion gases blow down to atmospheric is going to need new designs and new technology.  Solid-oxide fuel cells are one possibility, zinc reduction followed by zinc-air fuel cells is another (though probably lossier).

The contrast between scrubbers and the optimal carbon-sequestering power system is like catalytic converters vs. hydrogen fuel cells; the latter are so much more perfect, but the former are all that we can make at commodity prices.