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

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.