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.