Will photosynthesis based carbon sequestration always be more efficient than other chemical means - in terms of rate?

Question

There seem to be many ideas for carbon sequestration. The problem with anything electrochemical, is that unless it is solar-based, you're putting carbon into the atmosphere at probably a great rate than you're taking it out.

One interesting one seems to have been dumping limestone into the oceans, but calculations indicate that this will be too slow in terms of reaction mixing time, given our current CO2 emission rate.

One wonders if you could build an extrochemical device based on solar power - to pass carbon dioxide through water, and create an ionic reaction that would lock up the carbon. But that would just be a less efficient version of photosynthesis.

My question is: Will photosynthesis based carbon sequestration always be more efficient than other chemical means - in terms of rate?

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Edit: A helpful question has been raised about what efficient means. It could be land use or energy consumption or rate. I meant rate.

Answer 1

Rate.. no.

Brute Force

We can treat this as an industrial chemistry question. The brute-force method for carbon sequesteration is simply:

1. Extract CO2 from the air (using the acid/base chemistry, Sodium Hydroxide will scrub it as fast as you can supply air)
2. Reduce CO2 to elemental carbon and oxygen. The exact details are not actually important - that's one for the industrial chemists - just the concept that it would take a LOT of energy. Basically as much energy as we currently get from fossil fuels, and then some, to extract and reduce at the same rate.
3. Store the carbon somewhere safe, where it isn't going to catch fire. As long as it doesn't, it's stable on geological timescales.

The rate of this process is dependent on how much energy you are prepared to throw at it. A fleet of several thousand nuclear reactors running 24/7 on this problem would do it.

Burial

Well, brute force sounds a bit expensive.. so we can try the approach mentioned by Jack R Woods. In this case, we capture CO2 (this still takes energy, although not much in theory), and pump it into thick basalt formations such as the ones in Iceland. Many flood-basalt igneous provinces have the combination of low-silica rocks and high porosity that are ideal for this kind of sequestration. The rate is limited by how fast you can capture CO2 out of the air. You could imagine using a Solar Updraft Tower - this would give an air flow that you could extract CO2 from, and provide power for the process. If the reaction goes as planned, it's safe on geological timescales.

The problem is, as ever, getting enough resources to build these things on the scale required.

Answer 2

Photosynthesis is up to about 2% efficient while solar panels are above 20% efficient. So just based on the energy available per unit area of sunlit land or sea, chemical carbon sequestration holds more promise, especially since the area need not be arable. Another advantage of chemical means is that carbon dioxide need not be reduced as far as hydrocarbon, elemental carbon or even carbohydrate for it to be a stable solid. Captured carbon dioxide could be chemically reduced to oxalic acid - a molecule made from two carbon dioxide molecules and just one hydrogen molecule - and stored as a mountain of solid. Since the carbon is almost as oxidized as oxalic acid as it was as carbon dioxide, this minimizes the energy the process requires.

Here I describe how such a project might look. To sequester all the excess carbon dioxide in the atmosphere as a pile of oxalic acid would create a mountain about the size of Mt. Fuji and would require at least one exawatt-hour of electricity.