A question for all you chemist/physicists out there...

[Javaman]

When capturing carbon in gas form from coal plants, is it possible to turn this back into solid form?

[Demeter]

1. To What Purpose?

Anything is possible, if time and money or energy is no object.

[karlrschneider]

2. Do you mean pure carbon...or Carbon Dioxide?

not sure what you're getting at

[TheBaldyMan]

3. You could turn Carbon Dioxide into liquid form

I think this is the method used in Norwegian oilfields for carbon sequestration, the liquid is pumped back into the oil wells under the North Sea.

[rock]

4. Carbon is in solid form (at normal temperatures)

Maybe you could rephrase the question?

[longship]

5. The solution is neither simple, nor easy.

There may be some catalytic action that would do what you describe, but it doesn't solve the problem, it would only postpone it.

The only real solution is to stop burning hydrocarbons. This may seem radical but when one considers the other sources of energy available it isn't. Solar, hydro, wind, geothermal, and other clean energy alternatives are certainly sufficient. There is also nuke, but that is far, far from clean.

We have to wean off of our dependence of automobiles. There isn't a better example of this than Southern California which has a huge network of ever grid-locked freeways. You can't call them expressways because there's absolutely nothing express about them. Yet, for decades, the auto lobby has killed every single mass transit bill they could.

It is absolutely incredible to me that at the same time as all this information on global warming is scaring the shit out of climate scientists, people would be driving around in huge, gas-guzzling SUVs. The disconnect between the science and public opinion is astounding.

Overpopulation. Enough said?

I think we are dooming ourselves. Climate scientists are saying we may have only a decade, but even they do not know for sure. There are several positive feedback mechanisms which could tip things over. I certainly hope that they have accounted for all of them.

This is damned scary because the biological systems on which life depends may be more fragile than we ever knew. What would it take for there to be a collapse of one or more of these biological dependencies.

I am truly worried about mankind's future on this planet. "Not with a bang, but a wimper."

[smb]

15. Actually...

...if competently handled, nuclear power may actually be cleaner than solar (the chemicals used and generated in building semiconductors are nasty, and solar power on a large scale would require more of them than the entire rest of the electronics industry combined).

[longship]

16. Well, there's a more than minor issue of the spent fuel.

In that respect, nuke power is amongst the dirtiest power available. It's not that it's just dangerous, it *stays* dangerous. Many of the daughter products have huge half-lifes.

Burying it in a mountain in an earthquake zone is not the best idea, IMHO.

I don't have an answer on nuke power. It can be helpful, but we have to figure out a better way to get rid of the byproducts. Launching into space just ain't a good option, but that's probably the best place for it.

[Massacure]

18. the longer the half life, the less dangerous a radioactive product is.

Besides, spent fuel is dangerous for 10,000 years. Lead, Arsenic, and Mercury are dangerous forever.

[NNadir]

19. Some caveats to this generally true statement.

Some radioactive substances, including all of the actinides, are subject to decay chains. This means that a radioactive substance is transformed initially to another element that is itself radioactive and will undergo further decays until a stable isotope is obtained. For all of the actinides, including natural uranium and thorium, a stable nuclide is not reached until there have been many decays resulting finally in an isotope of either lead or bismuth.

The specific activity (the amount of nuclear decays per unit mass) of the daughter product is almost different for the decay daughter, and the chemistry is different. In general after isolation of a nearly pure radioactive substance, the concentration of the daughter nucleus will rise to a maximum, after which the daughter will be decaying as fast as formed. The time it will take for this to happen is related to the half-lives of both the parent nuclide and the daughter nuclide. For nuclear calculations, the actual number that is used is the decay constant, which is the natural logarithm of 2 divided by the half-life in whatever units of time are convenient. If we use the letter l to designate the decay constant, the time required is given by the following relationship:

t = ln(l₁/l₂)/(l₁ - l₂)

"ln(x)" refers to taking the natural logarithm of a number x, in this case the ratio of the decay constants.

From a health perspective, the most important example of this case has little to do with nuclear power but is related to the properties of uranium containing geologic formations, including soils. Because sufficient time has passed in uranium formations for them to be in radioactive equilibrium with all of their decay daughters, all of the daughters are present in concentrations related to their half-lives in comparison to the half-life of the parent uranium. One decay daughter in the uranium series, arising from the decay of U-238, is the gaseous element radon, which is highly radioactive, having a half-life of a few days. While the equilibrium concentration of this element is small, it is very mobile and diffuses easily, particularly in soils which have large surface area to mass ratios. Some of this radon diffuses into structures built by human beings, in places like basements. As a result people can be exposed to fair amounts of radiation. In fact radon from the decay of uranium is one of the major sources of background radiation to which all humanity has been exposed since the beginning of time. The average exposure of a citizen of the US is said to be about 360 millirems per year, 200 of which derive from radon. After cigarette smoking and air pollution, exposure to natural radon is thought to be the third largest contributor to lung cancer.

http://extranet.urmc.rochester.edu/radiationSafety/WebTraining/Modules/Natbkg.html#Natural%20Background

Most of the external cost now attached to the use of nuclear power comes from the assumption that the uranium will eventually decay to radon in whatever form so called "nuclear wastes" are stored. This assumption is based on the once through uranium cycle. It is easy to show that continuous recycling will actually result in a decrease in exposure to radiation in less than 1000 years of use.

However in the "once through" cycle, although the risk of most fission products falls below the risk of uranium ores within a few hundred years, the risk associated with the actinides can remain slightly higher than uranium ores for much longer periods. For the short term, the first several hundred years the risk of actinides actually rises because of equilibrium effects, owing to the decay of plutonium-241, which is always formed in nuclear reactors, into americium-241, an effect controlled by equilibrium effects. This state is achieved after about 200 years. After 1000 years the risk of actinides is dominated by neptunium-237, the decay daughter of americium-241.

This is a strong argument for abandoning the "once-through" fuel cycle and fissioning all of the actinides that are mined. Nuclear engineers around the world are developing programs to do exactly just this. I have no doubt that this is the approach that will ultimately embraced by humanity should humanity survive global climate change. The use of continuous recycling will destroy all of the uranium before it has an opportunity to decay into radon thus eliminating the vast majority of the radon risk.

None of these issues have any bearing on the fact that all fossil fuels are more dangerous by many orders of magnitude than nuclear fuels, fossil fuels all being unacceptably dangerous, and nuclear fuels having almost no impact in a relative sense.

[ewoden]

6. Simple,

C)2 solubilization into water. Precipitation as CaC03

[Poiuyt]

7. Pure carbon doesn't exist as a gas

Theoretically, you could vaporize it if the temperatures were high enough, but I think that everything else would be vaporized first. Remember, if you heat elemental carbon hot enough (with pressure) you get diamonds.

[phantom power]

8. For one example, plants do it all the time.

In fact, the fossil fuels we burn are CO2 turned into solids or liquids, via plant metabolism.

But industrially speaking, it can certainly be done. One possibility would be to combine it with hydrogen to form fuels like DME. You need an energy source to crack water into H2, extract the CO2, and combine them both into DME. If you want to do it on a scale necessary to compete (and I don't mean equal, just get into the ballpark) with our current rate of fossil fuel use, it will take truly huge amounts of energy. On the order of a thousand nuclear reactors, or the energy equivalent.

Other avenues include biodiesel and ethanol. These fuels are essentially CO2 turned into liquid fuels, with plants doing the initial capture. Scaling up to current levels of usage will require the creation of an entire new industrial infrastructure for distillation, refineries, etc, to say nothing of the agricultural resources required.

[NNadir]

12. All of these systems have energy inputs though.

[phantom power]

13. Yes, huge energy inputs.

[intaglio]

9. OK take a look at it logically

Carbon from coal/oil/gas burns to make Carbon compounds and energy (heat)
put that in a diagram and you have
C + X => CX + Δh the symbol Δh just means heat, so to pull the carbon compound apart you would need to put the same energy back into the gasses produced by burning but things get messy. Part of this is because the bulk product is CO2 which is hugely stable. There are tricks; plants have a neat one called photosynthesis but as of this time we cannot copy that on an industrial scale.

[eppur_se_muova]

10. Yes, by expending huge amounts of energy.

To convert CO2 back to solid carbon, you have to "unburn" it, that is, remove the oxygen. If you had the energy to do this, you wouldn't need to be making CO2 in the first place.

If you trap the CO2 as carbonate (CO3=), you need some source of oxide or hydroxide to do this. The obvious candidate is CaO, but this does not occur naturally. It has to be made from CaCO3 by heating in a furnace, giving off CO2 -- so you'd just be going around in a circle. There are no basic oxides available in significant quantity on the surface of the earth. Some deep subsurface rocks may be more alkaline, and I believe it is exactly this type of rock that some projects are aimed at exploiting to "sequester" carbon. I would be very surprised if the amount which could be located and accessed would be anywhere near adequate, at least in the short term. Bear in mind that drilling to access these deposits of alkaline rock would be a project comparable to drilling to extract oil.

As others have pointed out, the best way we have to trap CO2 is to have plants convert it back to carbohydrates (sugars and cellulose) and oils (vegetable oils, not pure hydrocarbon oils but pretty close). That's why stopping the deforestation of the tropics -- mostly done by burning, which dumps massive amounts of CO2 all at once -- is so critical. It's also why the idea of growing biofuels is so appealing -- the plants trap CO2 as fuel, which is released when the fuel is burned -- thus switching that sector of the energy economy onto a 'carbon-neutral' footing.

[NNadir]

11. The idea is a perpetual motion machine and is prohibited by the second law of thermodynamics.

The material you have will be exactly the same form with which you started, carbon. Meanwhile you will have gone through several exothermic steps rejecting heat to the atmosphere. It follows that this heat must come from somewhere. Therefore the process as a whole will require energy, not release it.

In practice you will also have to separate the carbon dioxide from other gases. This will incur a thermodynamic cost, since a pure substance has a lower entropy (a measure of disorder) than a mixture (exhaust containing carbon dioxide, argon, unconsumed oxygen and nitrogen).

Note that the process is not impossible. You can reduce carbon dioxide to carbon, but if you start with carbon to make carbon dioxide you will always have to add energy and will not be able to obtain energy from the process.

It may be possible, if one has access to hydrogen - generally a form of stored energy - to have a defacto sequestration if one hydrogenates the carbon dioxide with an appropriate catalyst to make monomers used to make plastics, propylene for instance. Then the plastic, while in use or while landfilled (for as long as it is stable) will represent sequestered carbon. Generally though, these processes will require energy inputs.

I advocate the hydrogenation of carbon dioxide to make the ultraclean motor fuel dimethyl ether. However the hydrogen - which I think is best obtained from nuclear energy - is the energy input in question.

Because carbon dioxide is a gas, most "sequestration" schemes are short term fixes of little long term merit. It's merely sweeping dust under the rug, another case of shuffling responsibility and cost off on to future generations.

It's a good question you asked by the way.

[Javaman]

14. Thanks for answering my question. :)

I think what I'm after is this; if there was a way to recombine C02 with a another element into a solid, more than likely not carbon (especially after reading your post), I would think it would be easier to manage and therefore, easier to store and or reused, in this new form, as something else entirely.

Since I'm no chemist, and certainly don't pretend to be one, I'm just kind of poking around and trying to maybe inspire a new way of thinking.

My rational: if there was a way to reuse what ever is captured in the form of a solid, that could generate a new type of industry based upon it.

I just think there has to be a better way then pumping underground. Especially after reading a few articles regarding the fact that C02 may cause other issues regarding decay of the underground caverns in which it will be stored.

[NNadir]

17. My opinion is that storing carbon dioxide in any form is not realistic.

It's not going to happen. It's 27 billion tons a year. It's another one of those things that people want to believe is possible, in spite of clear evidence that it isn't possible.

The only way to stop adding carbon dioxide from being added to the air and water is to stop burning fossil fuels by banning them.