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I am a student from a school and recently I came across the reaction

$\ce{2Na2O2 + 2CO2 -> 2Na2CO3 + O2}$

Can't we use this reaction to reduce $\ce{CO2}$ in the atmosphere by placing some $\ce{Na2O2}$ filters in places where there is high level $\ce{CO2}$ emission? I then realized that the filter would have to be loaded with lot of $\ce{Na2O2}$ to achieve this. So why not adsorb as much $\ce{CO2}$ as much as possible using a good adsorbent for $\ce{CO2}$. I am not sure if there are other methods better than these but I would like to know if there is any?

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  • $\begingroup$ Please revise the question to show the correct chemistry. The equation you gave does not balance. Is it supposed to be 2 Na2O2 + 2 CO2 -> 2 Na2CO3 + O2? $\endgroup$ – iad22agp Nov 7 '14 at 18:08
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    $\begingroup$ In any case, the amount of energy (i.e. carbon footprint) it takes to make sodium peroxide is likely greater than the carbon footprint of the CO2 you would capture. $\endgroup$ – iad22agp Nov 7 '14 at 18:10
  • $\begingroup$ I think the sodium peroxide chemistry is used to get oxygen/remove CO2 in spacecraft, but I've never heard of it being used for large-scale carbon capture. This sort of thing seems more suited, perhaps? $\endgroup$ – Michael DM Dryden Nov 8 '14 at 7:34
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The problem with carbon capture is not so much that we lack the technology to do it, it's more that we lack the technology to do it in a way that is economically feasible.

Take your example - according to the US Energy Information Adminstration, it takes about 1.09 lbs of coal to generate 1 kWh of electricity. A typical coal-fired plant might produce an average of 500 MW throughout each day (this is a rough estimate), which works out to roughly 12,000 MWh, or 12,000,000 kWh. So that's something like 12 million pounds of coal each day.

If we assume that coal is 100% carbon (close but not quite), then it's about 37.8 mole C per pound of coal. That's 453.6 million moles of carbon each day being converted to $\ce{CO2}$, and since the combustion of carbon follows:

$\ce{C + O2 -> CO2}$

that means a single medium sized-coal plant produces roughly 454 million moles of $\ce{CO2}$ each day. By now you are probably getting a sense of the scale of this problem. Since sodium peroxide and carbon dioxide have a 1:1 ratio in the absorption equation:

$\ce{2Na2O2 + 2CO2 -> 2Na2CO3 + O2}$

you will need an equal number of moles of sodium peroxide. That works out to about 78 million pounds per day.

Alibaba lists sodium peroxide at around \$400-500 per ton, so that means it would cost you (at a minimum) \$15.6 million dollars per day to scrub the $\ce{CO2}$ emissions from one medium-sized power plant. The real costs would be much higher because of the additional equipment and operating expenses.

Electricity currently sells for about \$0.10 to \$0.20 per kWh in the US, and around half of that is usually for production, with the rest being delivery. That means our 500 MW coal plant is going to be bringing in something like (at the high side) \$1.2 M each day. As you can see, the costs for scrubbing with sodium peroxide far exceed the potential revenue for the plant.

Now, there are other options:

One is to reuse the sodium peroxide, although the costs of regenerating it might exceed just buying more, and you would still need to find a place to store the $\ce{CO2}$.

Another is to use a cheaper/more effective absorbent - according to Wikipedia, amine-gas scrubbing is the most promising technology so far, although I am sure it faces the same kinds of cost scaling problems.

Another more popular approach is carbon capture, where carbon dioxide is basically just pumped into underground reservoirs in the hopes that it will stay there. While this is probably going to turn out to be the most economical approach, it has the risk of releasing large amounts of $\ce{CO2}$ accidentally, as well as having unknown effects on the local geological formations.

Yet another option is to use photosynthesis to scrub carbon dioxide. This has the advantage of being very cheap in comparison to other methods, but the downside is that the rate at which plants and algae can consume $\ce{CO2}$ is much slower than the rate at which we produce it.

It's a big problem, and all of the proposed solutions so far have been very expensive to implement - which is the main reason the fossil fuels industry opposes them.

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  • $\begingroup$ This is a great analysis. Presumably there are energy costs associated with producing the 78 million pounds of sodium peroxide per day, energy that comes from a coal plant... $\endgroup$ – jerepierre Nov 10 '14 at 23:45
  • $\begingroup$ @jerepierre - yes, definitely, and that would have to be considered if the economics weren't the limiting factor. There are three important considerations: 1) what is the net energy change for the overall process; 2) what is the net cost per unit of energy; and 3) what is the net change in CO2. $\endgroup$ – thomij Nov 11 '14 at 21:56

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