CO2 capture to limit climate change: dream or reality?

True about statement #1…

The var­i­ous cli­mate change mit­i­ga­tion sce­nar­ios incor­po­rate CO₂ cap­ture, stor­age and util­i­sa­tion (CCUS) technologies.

In its 2050 car­bon neu­tral­i­ty sce­nario¹ projects that 7.6 Gt of CO₂ will be cap­tured each year by 2050. As for the Inter­gov­ern­men­tal Pan­el on Cli­mate Change (IPCC), in its sixth report pub­lished in April 2022², it empha­sizes: « Mit­i­ga­tion strate­gies to lim­it warm­ing to 1.5°C or 2°C include the tran­si­tion to fos­sil fuels asso­ci­at­ed with CO2 cap­ture and stor­age […], and the deploy­ment of CO₂ cap­ture meth­ods [edi­tor’s note: such as direct atmos­pher­ic cap­ture] to off­set resid­ual emis­sions. »

How­ev­er, it must be kept in mind that CO₂ cap­ture is only part of the solu­tion: 37.1 Gt of fos­sil CO₂ were emit­ted in 2021³. A range of mea­sures must be imple­ment­ed to mit­i­gate cli­mate change: agri­cul­ture, forestry and the oceans all play, in their own way, a very impor­tant role in the glob­al car­bon balance.

False about statement #1…

The tar­gets are well above cur­rent CO₂ cap­ture and recov­ery capacities.

The IEA esti­mates that 4 Gt of CO₂/year must be cap­tured by 2035 and 7.6 Gt by 2050. To date, only 35 com­mer­cial facil­i­ties are using CCUS. These are pilot projects and their cap­ture capac­i­ties are extreme­ly small: less than 0.05 Gt of CO₂ per year⁴.

The IEA esti­mates that 4 Gt of CO₂/year must be cap­tured by 2035 and 7.6 Gt by 2050.

The indus­try has yet to be devel­oped, and the IEA’s tar­gets are out of reach due to the invest­ments required, par­tic­u­lar­ly in high-emit­ting coun­tries like Chi­na, India and the Unit­ed States. The ramp-up will require the instal­la­tion of new facil­i­ties, as well as the very cost­ly ren­o­va­tion of exist­ing units (retro­fitting).

Uncertain about statement #1…

The amount of water and ener­gy required for CCUS could lim­it the process.

Let’s look at the impact of installing a CO₂ cap­ture unit on an exist­ing coal plant⁵. Assum­ing an elec­tri­cal effi­cien­cy of 46% for a pro­duc­tion capac­i­ty of 500 MWe, the effi­cien­cy loss in the ide­al case would be at least 3.8%. In real­i­ty, this loss is more like 10%, or an effi­cien­cy after cap­ture of 36%. Two-thirds of the ener­gy con­sumed is for cap­ture, and the remain­ing third for com­pres­sion of the CO₂. To this must be added the ener­gy required for trans­port and injec­tion: keep­ing the CO₂ in a super­crit­i­cal or liq­ue­fied state requires a lot of energy.

As for water, cur­rent process­es gen­er­ate a lot of heat. The water need­ed to cool the instal­la­tion will increase the con­sump­tion of this resource.

True about statement #2…

Many sec­tors are already using CCUS technologies.

CO₂ cap­ture tech­nolo­gies have been around since the 1970s and are used for enhanced oil recov­ery and nat­ur­al gas pro­cess­ing. How­ev­er, in this con­text, the CO₂ is not recov­ered but released into the atmos­phere. 35 com­mer­cial indus­tri­al facil­i­ties use CCSUs to cap­ture and recov­er CO₂, for exam­ple by treat­ing flue gas­es with amine chemistry.

False about statement #2…

The recov­ery or stor­age chan­nels have yet to be developed.

To date, of the 440 Mt of CO₂ cap­tured each year, 230 Mt of CO₂ are used main­ly for the pro­duc­tion of urea for fer­til­iza­tion (130 mil­lion tons) and enhanced oil recov­ery (80 mil­lion tons)⁶. We can­not imag­ine recov­er­ing bil­lions of tons of CO₂ in this way.

Regard­ing stor­age, some pilot projects are already exper­i­ment­ing with injec­tion into for­mer gas fields. But the devel­op­ment of this sec­tor will have to deal with ques­tions of social accept­abil­i­ty. It is impor­tant to store as close as pos­si­ble to the cap­ture site to lim­it costs: for exam­ple, in France, in the Paris Basin east of Paris or in the vicin­i­ty of Pau.

Uncertain about statement #2…

The val­ue of direct air cap­ture (DAC), a process that cov­ers dif­fuse emis­sions (heat­ing, cars, etc.), remains a sub­ject of debate.

There are cur­rent­ly 18 DAC facil­i­ties in the world (Europe, USA and Cana­da) cap­tur­ing almost 0.01 Mt of CO₂ each year. These small facil­i­ties cap­ture CO₂ for direct use, such as in the man­u­fac­ture of soft drinks. Only two of them store CO₂ in geo­log­i­cal for­ma­tions⁷.

The glob­al con­cen­tra­tion of CO₂ in the atmos­phere has increased from 277 ppm before the indus­tri­al era to 417 ppm in 2022⁸. With DAC, we would have to treat gigan­tic vol­umes of air to reach pre-indus­tri­al era con­cen­tra­tions! I believe that the nec­es­sary invest­ments, oper­at­ing costs and ener­gy required make DAC a fin­ish­ing option: the pri­or­i­ty is to cap­ture CO₂ from the most emit­ting industries.

True about statement #3…

The long-term per­me­abil­i­ty of deep saline aquifers is unknown.

Deep saline aquifers are spe­cialised reser­voirs for CO₂: they are even­ly dis­trib­uted over the sur­face of the globe and there­fore lim­it the trans­port of CO₂. More­over, they offer a very large stor­age poten­tial, rang­ing from 400 to 10,000 Gt⁹. In these reser­voirs, the CO₂ would be dis­solved in water for stor­age, but uncer­tain­ties remain as to the sta­bil­i­ty of the reser­voir, par­tic­u­lar­ly in terms of geo­chem­i­cal risks. Indeed, the addi­tion of CO₂ will acid­i­fy the water, which could cause chem­i­cal reac­tions with the host rock and weak­en the reservoir.

False about statement #3…

Old­er hydro­car­bon reser­voirs appear to be more sta­ble over time.

Old­er hydro­car­bon reser­voirs are among the oth­er reser­voirs being stud­ied for CO₂ stor­age. While they have the dis­ad­van­tage of hav­ing a less inter­est­ing geo­graph­i­cal dis­tri­b­u­tion, they have been proven to be water­tight for mil­lions of years as hydro­car­bon reser­voirs (gas, coal, or oil). The geo­me­chan­i­cal and geo­chem­i­cal risks asso­ci­at­ed with CO₂ injec­tion must nev­er­the­less be bet­ter under­stood: this is cur­rent­ly the sub­ject of demon­stra­tors in the Unit­ed States, Cana­da, Alge­ria and Norway.

Uncertain about statement #3…

New ways of recov­ery could make it pos­si­ble to do away with CO₂ stor­age in part.

As we have already men­tioned, it is also pos­si­ble to recov­er cap­tured CO₂. But the chan­nels remain lim­it­ed today: it is cer­tain that we can­not increase fer­tilis­er pro­duc­tion indef­i­nite­ly! How­ev­er, many well-known process­es could enable CO₂ to be recov­ered in oth­er ways, some of which have already been devel­oped on an indus­tri­al scale in the past. For exam­ple, thanks to a reac­tion between hydro­gen and CO₂, many syn­thet­ic fuels can be pro­duced. CO₂ can also be used to pro­duce plas­tics or min­er­al car­bon­ates for build­ing mate­ri­als. The prob­lem is that these process­es are, at this stage, more expen­sive than those based on fos­sil mate­ri­als such as oil.

Anaïs Marechal