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Can we sweep our CO2 emissions under the rug?

Why is it so difficult to capture CO2 directly from the atmosphere?

Didier Dalmazzone, Professor of Chemistry and Processes at ENSTA Paris (IP Paris)
On May 26th, 2021 |
4 min reading time
Ddidier Dalmazzone
Didier Dalmazzone
Professor of Chemistry and Processes at ENSTA Paris (IP Paris)
Key takeaways
  • The atmospheric concentration of CO2 has increased from 300 parts per million (ppM) in 1950 to more than 400 ppM today.
  • Even if huge amounts of atmospheric CO2 are already captured in nature, it is not enough to diminish these to the levels required to fight against global warming.
  • The direct air capture of CO2 (DAC) is presumably an efficient solution to face the problem of diffuse emissions.
  • However, the weak concentration of CO2 in the atmosphere – 400 ppM in the air – is a major challenge. With existing technologies, we would need to treat 1.25 million cubic meters of air to capture one ton of CO2.
  • Technological solutions are under development to overcome this challenge.

Atmos­pher­ic CO2

No one can rea­son­ably ignore it today: car­bon diox­ide (CO2) is one of the main fac­tors respon­si­ble for the green­house effect, the phe­nom­e­non that con­tributes to glob­al warm­ing by redi­rect­ing reflect­ed radi­a­tion towards the low­er lay­ers of the atmos­phere and the ground. Though the green­house effect is essen­tial to main­tain a tem­per­a­ture suit­able to the devel­op­ment of life on Earth, its excess threat­ens our cli­mate with seri­ous dis­rup­tions in the short to medi­um term.

The evo­lu­tion of atmos­pher­ic CO2 con­cen­tra­tions shows an alarm­ing increase from the begin­ning of the indus­tri­al era and, more par­tic­u­lar­ly, a real boom since the mid-twen­ti­eth cen­tu­ry. It has increased from 300 parts per mil­lion (ppM) in 1950 to more than 400 ppM today. Accord­ing to most recent esti­mates by the experts of the Inter­gov­ern­men­tal Pan­el on Cli­mate Change (IPCC), a dras­tic and rapid reduc­tion of CO2 emis­sions is vital to keep glob­al warm­ing with­in accept­able lim­its. We must quite sim­ply reduce these emis­sions from 50 bil­lion tons per year to zero by 2050 (sce­nario +1,5 °C) or 2075 (sce­nario +2 °C). We will over­come this chal­lenge by com­bin­ing a range of solutions.

Direct Air Cap­ture (DAC) of CO2

In nature, espe­cial­ly through pho­to­syn­the­sis, huge amounts of atmos­pher­ic CO2 are cap­tured and then very sus­tain­ably stored in plants and the ani­mals which eat them. Over time, these will in turn even­tu­al­ly become coal, oil, and gas. This nat­ur­al cap­ture of CO2 is not the sub­ject here, even though bio­mass con­ver­sion is a promis­ing solu­tion to reduce the con­cen­tra­tion of green­house gas in the atmosphere.

Among the oth­er solu­tions, indus­tri­al cap­ture of CO2 and its long-term stor­age – its “seques­tra­tion” – could rep­re­sent up to 20% of emis­sion reduc­tions. Until very recent­ly, the cap­ture of CO2 was only con­sid­ered in efflu­ents of indus­tries emit­ting high lev­els of CO2: coal-fired or heavy fuel oil pow­er plants, cement and steel fac­to­ries, oil refin­ing, ammo­nia pro­duc­tion, etc. Giv­en the high con­cen­tra­tion of CO2 in these efflu­ents, their cap­ture is rel­a­tive­ly “easy”, and car­bon cap­ture tech­nolo­gies have exist­ed for a long time. How­ev­er, these con­cen­trat­ed emis­sions only rep­re­sent about 50% of the total emis­sions, the oth­er half includes dif­fuse emis­sions due to trans­porta­tion, con­struc­tion or small industries.

Direct Air Cap­ture (DAC) of atmos­pher­ic CO2 could offer an effi­cient solu­tion to deal with the prob­lem of dif­fuse emis­sions. How­ev­er, the rel­a­tive­ly low con­cen­tra­tion of CO2 in the air is a major dif­fi­cul­ty. With 400 ppM in air, and assum­ing a cap­ture rate of 100%, we would indeed need to treat 1.25 mil­lion cubic meters of air to cap­ture one ton of CO2. Let’s not for­get: the chal­lenge is to cap­ture hun­dreds of mil­lions, even bil­lions of tons of CO2! That is prob­a­bly one of the rea­sons why devel­op­ment plans of DAC have only very recent­ly appeared. Oth­er rea­sons include the dif­fi­cul­ty to find an out­let for the cap­tured CO2 and an eco­nom­ic mod­el to jus­ti­fy the required invest­ments, as well as the very high ener­gy cost of these processes.

In terms of tech­nol­o­gy, exist­ing projects rely on trust­ed solu­tions, based on the chem­i­cal reac­tiv­i­ty of CO2 (an acidic gas) with basic reagents. The first pro­to­types devel­oped at the turn of the cen­tu­ry did not offer any major inno­va­tions. But of note was the demon­stra­tor pre­sent­ed in 2008 by Cal­gary Uni­ver­si­ty made from an absorp­tion col­umn using a sodi­um hydrox­ide solu­tion, with a cap­ture capac­i­ty of 20 tons of CO2 per year. Since then, tech­nolo­gies have evolved and sev­er­al indus­tri­al actors seem to be mov­ing towards the large-scale devel­op­ment of DAC.

The wet process used in the begin­ning (bub­bling of cap­tured air in a solu­tion of sodi­um or potas­si­um hydrox­ide) is now rivalled by dry process­es, using for exam­ple mem­branes impreg­nat­ed with basic reagents. This process is pro­posed by the Swiss start-up Clime­works, from the fed­er­al Ecole Poly­tech­nique in Zurich. The com­pa­ny has 14 oper­a­tional or planned facil­i­ties thus far, among which the biggest com­mer­cial DAC fac­to­ry in the world. The ORCA project, under con­struc­tion in Ice­land, will be able to cap­ture 4,000 tons of atmos­pher­ic CO2 per year. But even if progress seems to speed up with grow­ing aware­ness of the issues at stake, we are still very far from the medi­um-term objectives.

Asso­ci­at­ed costs

What­ev­er the reagent used to cap­ture CO2 is, one of the main issues relat­ed to DAC is the ener­gy required for extrac­tion. Ener­gy is essen­tial to obtain pure CO2, both to store it in geo­log­i­cal reser­voirs or to make use of it as an indus­tri­al raw mate­r­i­al. This is because even though CO2 reacts quick­ly with basic reagents, the reverse reac­tion requires very high tem­per­a­tures – above 100°C. Hence, whilst this regen­er­a­tion process makes it pos­si­ble to recov­er the basic reagent that can rein­ject­ed into the cap­ture cycle, it is ener­gy inten­sive. Also, this step also results in loss of reagent. Final­ly, ener­gy is required for pack­ag­ing of cap­tured CO2 – name­ly to com­press it into a super­crit­i­cal state at over 80 bars of pressure. 

Beyond the eco­nom­ic aspect, these ener­gy costs have a para­dox­i­cal effect: the cap­ture process itself has an unde­sir­able car­bon foot­print. Thus, the quan­ti­ty of CO2 released by this cap­ture process can amount to 30% of the car­bon that it elim­i­nates. To over­come these bar­ri­ers, more inno­vat­ing process­es are being explored, such as “Elec­tro-Swing-Absorp­tion (ESA)1; a process based on an elec­tro­chem­i­cal bat­tery which uses polyan­thraquinone as an elec­trode mate­r­i­al. It is a poly­mer capa­ble of seques­ter­ing CO2 when sub­ject­ed to an elec­tri­cal poten­tial dur­ing charge. Dur­ing the reverse process, the dis­charge of the bat­tery releas­es the CO2 while pro­vid­ing a usable elec­tri­cal cur­rent. Still in the research stages, this process was the sub­ject of tech­no-eco­nom­ic stud­ies to eval­u­ate the cost of large-scale cap­ture in a range of $50 to $100 per ton of CO2. In com­par­i­son, the price of the ton of CO2 on the Euro­pean emis­sion rights mar­ket, which has strong­ly risen in recent months, cur­rent­ly varies around 55€ ($66) per ton.

1Devel­oped at MIT by Sahag Voskian and T. Alan Hat­ton.


Ddidier Dalmazzone

Didier Dalmazzone

Professor of Chemistry and Processes at ENSTA Paris (IP Paris)

Didier Dalmazzone is a member of the Management Committee of the Interdisciplinary Centre Energy for Climate of the Institut Polytechnique de Paris. He is in charge of the Energy Production and Management course in the 3rd year of the ENSTA Paris engineering curriculum, and is also in charge of the Master's Degree in Energy at IP Paris. His research activities on processes for the energy transition concern the hydrogen sector, CO2 capture and refrigeration.

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