Carbyon is a company which is developing a device to capture CO₂ from the air, to 'clean up our atmosphere'. This is certainly a very noble aim, and it would be a big relief to mankind if it could be realised.

Carbyon explains how its system works on a nice looking website, so I will not repeat it here (link Carbyon). The principle is absorption and subsequent desorption of CO₂ using a suitable absorbent, which is applied to a thin fibre membrane. The website gives the main characteristics with numbers. At Reden we think that numbers are essential in any discussion about the advantages of new techniques. Let’s look at the numbers Carbyon is presenting, and use them to answer three important questions.

1. Can it be done?
2. Can it be done on a large enough scale?
3. How much does it cost?

Can it be done?

There is no reason to doubt that CO₂ can be absorbed by the right absorbent, which then releases it upon heating. It may be assumed that the principles of Carbyon’s device (chemistry, chemical engineering, machine construction) have been proven. Carbyon is building a pilot installation in Eindhoven.

One check is the overall effectiveness. Carbyon states that the energy consumption is 1000 kWh of electricity per ton CO₂ removed from the air. But how much CO₂ is released when producing 1000 kWh of electricity? In 2020, the production of electricity in the Netherlands had an emission of 290 kg of CO₂ per 1000 kWh¹. Before being transported, the CO­₂ must be compressed. This requires another 90 kWh per ton², which adds another 26 kg of CO₂. The net CO₂ removal for the initial 1000 kWh is thus 1000 – 290 – 26 = 684 kg, and the energy used is 1090 kWh. The net removal cost now becomes 1594 kWh per ton of CO₂. This is less efficient than Carbyon suggests, but there is net CO₂ removal. It is, therefore, effective to generate extra electricity using fossil fuels (though, perhaps, not lignite), and use the electricity to capture CO₂. There is currently not enough green electricity for current users, so any extra electricity has to derive from non-renewables³.

Answer: yes, it can be done.

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[1] Rendementen, CO2-emissie elektriciteitsproductie, 2020 (cbs.nl)

[2] Jackson, Steve & Brodal, Eivind. (2018). A comparison of the energy consumption for CO 2 compression process alternatives. IOP Conference Series: Earth and Environmental Science. 167. 012031. 10.1088/1755-1315/167/1/012031.

[3] this point is often missed by whoever considers electric cars as ‘zero emission transportation.’ The real argument in favour of electric cars is that they are energy efficient. They still consume electricity that could be used for other things, and still causes emessions at 290 kg CO₂ per 1000 kWh.

Can it be done on a large enough scale?

In order to solve the CO₂ problem, we must not only absorb all future carbondioxide excess emissions (i.e. the part that nature cannot absorb), but also the accumulated excess CO₂ in the atmosphere.

The graph below shows that the rate of increase in 2020 was roughly 2.6 ppm per year. Removing ‘just’ this increase would not result in ‘cleaning up our atmosphere.’ If we want the level to return to 320 ppm (1965 level) by 2050, we have to remove the excess 92 ppm in 27 years (3.4 ppm per year) as well. The required capture rate is, therefore, 6 ppm per year.

How much CO₂ is this?

The imbalance in the global carbon cycle is 19 Gton of CO₂ per year (see figure below), and this is the 2.6 ppm yearly increase. This means that 6 ppm is 43.8 Gton. The figure also shows that the fossil contribution is 35 Gton/year (average 2012-2021); 35 - 19 = 16 Gton/year of this is absorbed by oceans and the biosphere.

Removal of this amount at the net energy requirement of 1594 kWh per ton, would require 43.8·10⁹·1594 kWh = 7.0·10¹³ kWh (still per year!) The total global yearly energy consumption is 580 million terajoules¹  =  1.6·10¹⁴ kWh. The amount needed for CO₂ removal is, therefore, 43% of the current yearly energy consumption.

In other words: to ‘clean up our atmosphere’ using the Carbyon system, the energy production (coal, oil, gas, nuclear, renewable) would have to be increased by 43%. Also, the energy would have to be converted to electricity, for which many extra powerplants would need to be built², well before 2050, as the current energy usage is mostly non-electric. Is this a realistic scenario?

Answer: almost certainly not.

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[1] Global Energy Consumption (theworldcounts.com)

[2] In the Netherlands, 17% of the primary energy reaches the user as electricity. Assuming this is roughly the same everywhere, the electricity production would have to increase for 17% of current energy use to 17%+43%=60% of current energy use. This is an increase in electricity use of 43/17=250% to reach the CO₂ reduction goal by 2050. (i.e for each two power plants, we would need to build five new ones extra).

Question 3 is now irrelevant. The cost of the many new powerplants, the increase of mining and transport capacity, and the Carbyon machines could be estimated, but the answer would only re-inforce the conclusion that the scenario is unrealistic. The Carbyon system may be very clever, it can extract CO₂ from the air, but not efficiently enough to claim that the system can ‘clean up our atmosphere’.

From a thermodynamic point of view, it is better to capture CO₂ where it is produced, before it is diluted (see footnote below). Perhaps Carbyon can be used to stop the increase in the CO₂ level if it can be applied at the fossil fuel combustion sites, and then the absorption by plants on land and by the oceans can finally begin to cope with the net CO₂ emissions.

Comparison with Thermodynamic minimum.

The reason that extracting CO₂ from air will always cost a lot of energy, is because it is present in a very low concentration. If two ideal gasses (air without CO₂ and CO₂) are mixed, the Gibbs free energy change is:

x₁ and x₂ are the mole fractions of the two gases. For 400 ppm CO₂ in air, this is -353 J/m³, or 136 kWh/ton of CO₂. This is 13.6 % of what Carbyon needs. Less energy is needed if the concentration is reduced to, say 200 ppm per pass through the system, but that would mean much more air must be processed. It maybe possible to improve this, but removing CO₂ from air will always cost a lot of energy.

If CO₂ is captured from a combustion process exhaust, the CO₂ concentration is higher, so less energy per ton CO₂ is required (assume 21% (all oxygen used up), then ΔG_mix /V = -51396 J/m³, so the energy per m³ is a factor 146 higher, but the amount of CO₂ a factor 525 higher. The cost per ton of CO₂ reduces from 136 kWh to only 38 kWh.