A few numbers on Direct Air Capture (DAC)

In October 2021, Fatih Birol, Executive Director of the International Energy Agency wrote in the Financial Times about fugitive emissions of methane from the oil and gas industry. Birol stated that fossil fuel operations worldwide emitted 120m tonnes of methane in 2020. https://www.ft.com/content/dc446e80-ff4a-44c8-ae08-da1b582c63d5

The magnitude of this intrigues me, especially in the light of all the current talk about Direct Air Capture (DAC) of CO2.

Suppose you set-out to remove enough CO2 from the atmosphere each year with your DAC machine to compensate for those fugitive methane emissions. How much air would you have to process?

Assuming a Global Warming Potential of 86 (the now widely-accepted 20-year figure); 120 Mt of methane corresponds to 10,320 Mt of CO2, that would have to be removed from the atmosphere every year.

CO2 makes up 0.04% of air (400 ppm), which corresponds to 0.018 kg CO2/kmol of air. So to capture 10,320 Mt CO2 you would have to process 5.7E14 kmol of air. (Assuming a 100% capture rate.)

Now, air has a molar mass of 29 kg/kmol. So you would have to process 5.7E14 x 29 = 1.7E16 kg air per year. Since the density of air is about 1.1 kg/m3 at atmospheric pressure and temperature, this corresponds to about 1.5E16 m3 per year or 479E6 m3 per second.

Putting that in simpler language: you would have to process 479 million cubic metres of air per second – ie about half a cubic kilometre per second.

A Rolls-Royce Trent 900 jet engine, (one of the world’s largest jet engines, which is used to power the Airbus A380 airliner) has an air mass flow rate of 1,245 kg/s… around 1100 m^3/s at sea level. So to shift 479E6 m3/s, you would only need 435,000 of the world’s largest jet engines!

I would be interested to see the DAC machine that could do that..

David Cebon

p.s. (28/8/2022): I thought it would be helpful to add some more usable numbers, derived from the calcs above:

  • To remove 1 Mt of CO2 from air per year you would need to move 479E6/10,320 = 46,400 m3 of air per second at atmospheric pressure.
  • Each 1Mt of CO2 removal per year would therefore require 46,400/1,100 = 42.2 jet engines.
  • You can calculate the total number of jet engines you would need, by multiplying 42.2 x the number of Mt of CO2 you want to remove from the atmosphere each year.
  • For example, global CO2e emissions in 2022 (calculated on a conservative 100-year global warming basis) were about 37 billion tonnes = 37,000 Mt. It could be argued that a successful ‘Net Zero’ emissions reduction strategy might leave behind 5% of these emissions to be disposed-of via DAC, ie 0.05×37,000 =1,850 Mt. To remove this much CO2 from the air, you would need 42.2 x 1850 = 78,000 of the world’s largest jet engines. This is a slightly more practical number than 435,000, but is still mind bogglingly-large.

p.p.s. For more information about DAC, I recommend this article by Paul Martin of the Hydrogen Science Coalition: Why Direct Air Capture Sucks (and not in a good way!) | LinkedIn