art with code

2012-11-18

Carbonated air

[copypasta from G+]

The problem is one of controlling the amount of carbon in the atmosphere. Which would be a very handy technology to have. Bye bye ice ages, etc.

There was a bunch of carbon under the ground. People dug it up and put it into the air by coupling it with oxygen, creating an airborne CO2 molecule and releasing a decent amount of heat in the process. The amount of carbon moved from underground into the atmosphere is around 10 GT per year, and it's rising as more and more people want to have heat and heat-byproducts like electricity. A small portion (~2.5%) of CO2 is created in the production of concrete, where the shells of long-dead marine organisms are decomposed from CaCO3 to CaO + CO2.

To break the carbon away from the CO2 molecules, you'd probably have to expend more energy than the act of putting them together released. The other option is to move the CO2 out of the atmosphere.

To move the CO2 out of the atmosphere, we have to separate it from the rest of the air and move it into a place where it can't escape from. To successfully do this, we have to move at least as much carbon out of the atmosphere as we're putting in there.

Trees are one option. A tree is mostly solidified air and water. It takes CO2 from the atmosphere and with the power of sunshine turns it into cellulose, growing a little bit in the process. One square kilometer of forest generates around 300 cubic meters of tree biomass per year, which contains around 75 tons of carbon. To capture 10 GT of carbon per year, we'd have to plant 133 million square kilometers of new forest and bury all the new growth back underground. The land area of the world is 148 million square kilometers.

Another option is chemical weathering to bind the CO2 into silicate rocks. This happens naturally and triggers ice ages. But it's kinda slow and requires exposing a whole lot of rock.

These guys http://pubs.acs.org/doi/full/10.1021/es0701816 propose a method to do a sort of oceanic acid switch. Put the CO2 into the ocean and take HCl out so that the ocean acidity doesn't change. Then get rid of the HCl by using it to weather silicate rocks, a reaction that's much faster than the CO2 weathering. The problem here is that it takes 100-400 kJ per 12 grams of carbon. To take out 10 GT of carbon per year would use 2.5-10 TW of energy (or 15-60% of the world's annual energy production. Which might be a useful number for an atmospheric carbon tax.)

Anyway, the stuff is up there and the current biosphere can't use it up fast enough. Industrial-level use of carbon put it there, and it's going to take an industrial-level solution to get it back.

The really nice solution would be a chemical reaction that releases energy and binds CO2 into a heavier-than-air compound that's easy to store. Like http://pubs.acs.org/doi/abs/10.1021/jp205499e . Then you could take the CO2 exhaust, react it further to generate more energy, and put the solid exhaust into a pile. The problem with the Li3N + CO2 reaction is that lithium is pretty rare. Total worldwide production is measured in thousands of tons, compared to the billions of tons required for getting rid of CO2.

[new words]

You could also react CO2 with something else to produce a solid exhaust. Photosynthesis turns CO2 and water into sugar and oxygen. Another possibility might be burning magnesium in CO2, producing MgO and carbon soot, CO2 + 2Mg => 2MgO + C.  The resulting MgO turns to MgCl+ H2O in HCl, which can be electrolysed to separate out the magnesium. The captured magnesium can be used to burn the next batch of CO2. Anyway, the energy required to electrolyse the magnesium is likely going to be more than the energy released in burning the C and Mg in the first place. Say, the electrolysis might require around 18 MJ per kg of Mg. You need 2Mg @ 24 g/mol to burn 1C @ 12g/mol, or about 40 Gt of Mg to burn 10 Gt of C. At that kind of energy use, you'd need around 22 terawatts for the electrolysis. Global energy production is around 15 TW, natch.

As for usable liquid exhausts, CO2 reacts with hydrogen to produce methanol. Methanol is a fuel and can be burned to produce CO2. This is the basis of Methanol Economy.

Other alternatives include capturing CO2 at the factory pipe, using a portion of the generated power to store the CO2 in a tank. Where the tank may be the bottom of the ocean or a drilled gas deposit, as you're going to need a lot of volume. 10 GT of carbon means 36 GT of CO2. Stored as uncompressed gas, 36 GT of CO2 would take 18 thousand cubic kilometers of space. If you freeze the CO2 solid, it'd still take up 23 cubic kilometers. As liquid, around double that. The biggest LNG storage tank in the world is 200,000 cubic meters. You'd need to build 115,000 of those every year to fit 23 cubic kilometers of solid CO2.

Deep ocean waters contain something like 38,000 gigatons of CO2. You could pump all the fossil carbon in the world – around 1,000 gigatons – there and cause just a 3% increase. That's kind of a silly way to go about it though. Carbon is valuable. Today, 90% of the world's energy production is from carbon oxidation (France is the major anomaly here, they're producing 80% of their electricity with nuclear power.) Likewise, control over atmospheric carbon is valuable. Ice age coming? Crank up the CO2. Too much heat in the atmosphere? Sequester some away.

Dealing with carbon is going to require a lot of energy though. You'd want to build a lot of wind, solar or nuclear to produce enough energy to power the carbon capture mechanisms. In the long term, hydrocarbons might work as a battery technology for continuable energy production. Continuable? Why yes, there's a limited amount of carbon readily available. Putting it all into the atmosphere or into the oceans isn't really going to help you keep burning it. Putting the carbon into trees and relying on tree-driven solar power to turn it back into burnable carbon requires lots of trees. In 2010, we moving 9 Gt of carbon into the atmosphere. The current estimated worldwide reserves of carbon are around 800 Gt. The carbon gasification growth rate between 1990 and 2010 was 2% annually. At that rate, all the carbon reserves known to energy companies would be up in the atmosphere and down in the oceans by 2060.

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