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Straight off I want to say, I never took chemistry classes, so my understanding is very limited. My question is considering the equations below, why is it so hard to reduce greenhouse gases? Is it simply a matter of conversion requiring too much energy?

Can't we remove carbon dioxide & methane from the atmosphere by converting them into water and carbon?

$$\ce{CO2 + CH4 -> 2H2O + 2C}$$

Or by converting them into water and propane?

$$\ce{CO2 + 5CH4 -> 2H2O + 2C3H8}$$

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    $\begingroup$ One problem is that there's a couple hundred times as much CO2 as methane in the atmosphere. Also, there is the energy problem you brought up. Pretty much any reaction to convert atmospheric CO2 or methane to something else will be energetically uphill. Tough problem. $\endgroup$ – airhuff Jul 20 '17 at 1:19
  • $\begingroup$ What about capturing it at the source of emission? $\endgroup$ – Rubellite Fae Jul 20 '17 at 1:21
  • $\begingroup$ But then what do you do with it? There have been lots of ideas, like storing it at the bottom of the ocean, but anything like that will be costly. I think the best solution is to reduce production through conservation and alternative fuels, but we currently burn so much fossil fuels that even that transition is a long-term endeavor. $\endgroup$ – airhuff Jul 20 '17 at 1:35
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    $\begingroup$ Well your understanding is quite in track. Forget the equations you wrote but in principle yes it is possible to reduce CO2, obtaining new fuel or useful chemicals while reducing (or not increasing) the carbon content of the atmosphere. And again yes you are right, this is not really viable as it is an energy consuming process with limited efficiency. As in the case of water splitting to produce H2 as fuel, processes like these have cheap/ green energy production as prerequisite. $\endgroup$ – Alchimista Jul 20 '17 at 1:57
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    $\begingroup$ I see. It's a bit of a catch 22, then. Someone feel free to write up an answer so I can select it. 😉 $\endgroup$ – Rubellite Fae Jul 20 '17 at 2:07
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There are a few problems with what you're proposing:

  1. The reaction requires energy.
  2. Where do get the methane from?
  3. What will you do with the carbon you're generating?

The energy problem can be solved by using surplus energy from renewable source like solar and wind, but, as airhuff also mentiond in a comment, there is much more CO2 in the atmosphere than methane. Also capturing CO2 from the air isn't simple, and finally why would you use a valuable fuel like methane to create carbon or propane?

In any case I think a much more useful reaction is $$\ce{CO2 + 2H2O -> CH4 + 2O2}$$

Water is split into oxygen and hydrogen using electrolysis. The hydrogen reacts with carbon-dioxide to form methane. There are already small plants that do this, but AFAIK the idea hasn't really caught on yet. This may have something to do with the cost and/or the availability of carbon dioxide or surplus energy.

More recently scientists did something even cooler; they captured CO2 directly from the atmosphere and turned it into methanol (CH3OH) using Ruthenium as a catalyst. A drawback is that the reaction works at high temperatures (about 150 degrees C).

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  • $\begingroup$ 2. Industrial farms. 3. If pure carbon, sell for use in graphene or bucky balls, etc??? If propane, sell as propane fuel? "Why use methane to create...?" Because livestock manure and farts contribute to more than 1/3 of current methane emissions in addition to CO2. Also, it's my understanding that when methane hits ozone, even more CO2 (& H2O) is produced. So, if we capture these on-site, industrial farms could immediately & dramatically slow down climate change while also making a buck. "CO2+2H2O⟶CH4+2O2" But, global clean water supplies are dwindling as well. :/ $\endgroup$ – Rubellite Fae Jul 20 '17 at 21:50
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    $\begingroup$ IIRC the price of propane is lower than that of methane so if you can capture and isolate methane it's more economical to use that as a fuel. Perhaps carbon is an option but I think it depends on how high quality you can achieve. $\endgroup$ – THelper Jul 21 '17 at 19:45
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I've been hearing about companies able to extract and concentrate carbon dioxide from ambient air, and was thinking about how it might be sequestered on sufficient scale to remove all excess carbon dioxide from the atmosphere. Pumping the carbon dioxide underground or at the bottom of the ocean would probably work but raises concerns about stability and leakage. It could be reacted with lime or other minerals underground to form stable minerals but the completeness of the reaction would be difficult to monitor. Converting it into methane makes the problem bigger, since then the methane has to be sequestered and it is a more potent greenhouse gas than carbon dioxide.

The carbon dioxide could be converted into carbon, such as by turning organic matter into biochar, and then it would be stable. But why reduce carbon dioxide all the way to carbon ($\ce{CO2 + 2 H2 -> C + 2 H2O}$) when it could be left at oxalic acid ($\ce{2CO2 + H2 -> C2O4H2}$), another stable solid? This could be accomplished electrochemically.

If we were to return the carbon dioxide in our atmosphere to pre-industrial levels (i.e., $\pu{280 ppm}$), it would require the removal of

$$(\pu{410 ppm} - \pu{280 ppm}) \cdot \frac{\pu{2.13 Gt C}}{\pu{1 ppm CO_{2}}} \cdot \frac{\pu{3.664 Gt CO_{2}}}{\pu{1 Gt C}} = \pu{1.01 Tt}$$

of carbon dioxide. This carbon dioxide could be converted into $\pu{1.04 Tt}$ of oxalic acid, with a volume of

$$\pu{1037797146168 t}\cdot\pu{907185 g/t}\cdot\frac{\pu{1 cm3}}{\pu{1.9 g}}\cdot\frac{\pu{1 m3}}{\pu{1000000 cm3}}\cdot\frac{\pu{1 km3}}{\pu{1000000000 m3}} = \pu{496 km3}$$

of clean, beautiful former coal. If we assume that the shape of this mountain approximates that of a pile of salt (where $\pu{1000 t}$ of salt forms a conical pile $67’1”$ in diameter and $40’$ along its slope having a volume of $\pu{25000 ft3}$), then we can calculate the height of that mountain using the fact that the ratio of the radius of a cone of a given shape to its height ($h^2 = \text{slope}^2 – r^2$) is constant, which is

$$\frac{r}{h} = \frac{33.54’}{21.80’} = 1.54.$$

The equation for a cone is

$$V = \frac{πr^2h}{3},$$

or

$$V = \frac{π(1.54h)^2h}{3} = 2.48h^3.$$

Solving for $h$ where $V = \pu{496 km3}$ gives $h = \pu{5.85 km}$, with $r = \pu{9.00 km}$. For comparison, Mt. Fuji is $\pu{3.78 km}$ high and a radius of about $\pu{22 km}$ at its base. Once paved to prevent it from dissolving in the rain, it could provide a convenient skiing location.

Of course, this still leaves the problems of building the factory to do this and supplying renewable energy to run the factory. This article proposes electrochemically reducing carbon dioxide into stable, storable forms to be sequestered, with a potential to reduce $\pu{967 g}$ of carbon dioxide to oxalic acid per kilowatt-hour of electricity. That comes out to at least:

$$\pu{1037797146168 t oxalic acid} \cdot \frac{\pu{1000000 g}}{\pu{1 t}} \cdot \frac{\pu{1 g CO_{2}}}{\pu{1.023 g oxalic acid}} \cdot \frac{\pu{1 kWh}}{\pu{967 g CO_{2}}} = \pu{1.05 * 10^15 kWh} = \pu{1.05 EWh}$$

of electricity needed to sequester the above excess carbon dioxide from the air (not counting the oceans) as oxalic acid.

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    $\begingroup$ If your calculations are right that is between 5% and 10% of all the world's electricity consumption (or, possibly, 2-3 times that if conversion efficiency is taken into account). We'd need to build a lot of nuclear plants to cover that and we'd need to spend a huge amount of capital for the conversion plants. $\endgroup$ – matt_black Jan 4 at 17:39
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    $\begingroup$ If the scale of the problem wasn't already large enough, it's actually about twice as large as you suggest; a roughly equal amount of anthropogenic $\ce{CO2}$ has been absorbed by the oceans, and must also be removed. Also, I would be wary of making such a massive pile of a highly water-soluble, relatively strong acid, as it would eventually make its way into the ocean. Perhaps it would be best stored as a metal oxalate, e.g. magnesium or calcium oxalate, though of course this significantly increases costs. Very interesting article, though. $\endgroup$ – Nicolau Saker Neto Jan 4 at 21:21
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    $\begingroup$ @NicolauSakerNeto While checking my reference for the ppm/Gt CO2 figure I discovered that I had accidentally used the Gt C figure instead. After reworking my calculations above they are about 3.7 times worse (the ratio of the molecular weights of CO2 to C). I also shouldn't have worked out the energy required in my head, so I showed my work to get the new insanely large figure. It should be large, since it is only a few fold less than the total energy ever derived from fossil fuels. I think oxalic acid solubility would be the least of our problems since it could be generated in a desert, etc. $\endgroup$ – Chris Jenks Jan 4 at 23:27
  • $\begingroup$ Shoot, I briefly thought that $\ce{CO2}$ sequestration as oxalic acid looked feasible with the prior value (relative to a back of the envelope calculation I did a while ago), but $\mathrm{1{\text -}2\ EWh}$ once again squarely puts it in the absolutely bonkers-level of difficulty to implement; that's 6-12 years of all primary energy (electricity + motion + heating) consumed by mankind in 2015. The scale of the problem just escapes comprehension... $\endgroup$ – Nicolau Saker Neto Jan 5 at 0:22
  • $\begingroup$ This is a very monolithic suggestion. Perhaps it could be one of several sequestration techniques since the end product could be sold to producers of cleaning chemicals. But, a literal mountain of any one substance can't be healthy. $\endgroup$ – Rubellite Fae Jun 1 at 16:12

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