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