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CO2 capture and storage (CCS) technology consists in capturing CO2 from its source of production and storing it in an underground facility. It is of interest to industrial players since it would enable them to significantly reduce their CO2 emissions. But this promising solution still has to demonstrate that its implementation on an industrial scale is feasible at a reasonable cost.
 

Captage et stockage du CO2

 

CCS: a necessity for the climate

Global greenhouse gas reduction scenarios highlight the role of CCS in the energy mix. 

The International Energy Agency estimates that the 2°C objective requires a contribution from CCS of around 14% by 2060, a contribution that rises to 32% in the “Beyond 2°C” (B2DS) scenario. 

To achieve the 2°C objective, more than 100 billion tonnes of CO2 would have to be stored and thousands of CCS facilities rolled out by 2050. Today, only 17 large-scale facilities are operational, injecting some 40 million tonnes of vper year. 

Industrial players are taking an interest in the technology as the only solution likely to enable them to significantly reduce the carbon footprint of their activities. In 2016, 13 major oil companies created a one billion dollar fund, the Oil & Gas Climate Initiative, aimed at developing technologies to reduce CO2 emissions, including CCS, which is considered to be indispensable.

What sources of CO2 emissions can be captured and stored?

CCS concerns industrial sources where emissions are geographically concentrated. The rapid development of renewable energies limits the scope of CCS for electricity production, which has already embraced the decarbonization process. However, many heavy industries (steel-making, cement plants, refining, chemicals and petrochemicals) do not have yet access to substitution technologies enabling them to reduce their CO2 emissions to any significant degree. 

The challenges of CCS

CCS is not a new technology: CO2 capture and separation techniques have been applied in industry for decades, and CO2 injection has been used since the 1970s for enhanced oil recovery. But for large-scale roll out to be possible, a number of challenges still need to be addressed in order to:
 

  • reduce the costs of capture, the most expensive step in the CCS chain;
     
  • demonstrate the possibility of storing very large quantities of CO2 in deep saline aquifers, and, in particular, remove uncertainties concerning the behavior of CO2 over long periods (a few thousand years) in geological storage structures;
     
  • construct an industry comparable in size to that of the oil industry: capture facilities with a cumulative size comparative to that of the global refining industry, transport networks comparable to those used for natural gas, and storage infrastructures comparable to those found on the biggest oil fields. 

CO2 capture

This step can account for up to 70% of the process in terms of cost and represents a major technological and economic challenge. There are two principal methods of capturing CO2:
 

  • Capture from "post-combustion" flue gases consists in recovering the CO2 by cleaning the flue gases emitted by combustion with a solvent. CO2 capture techniques exist that are well known and employed in the treatment of natural gas, whose CO2 concentrations are regulated. The facilities required for cleaning combustion flue gases are extremely expensive and consume considerable amounts of energy. Innovative options for capturing flue gas CO2 are being considered with a view to minimizing energy consumption and reducing the size of facilities and investments.
     
  • "Oxycombustion” capture consists in burning carbon-based fuels in the presence of pure oxygen instead of air, making it possible to obtain flue gases with a heavier CO2 concentration (around 90%). The latter is then easier to separate from the steam it is mixed with. The main problem relates to the cost of producing pure oxygen, which is generally obtained via cryogenic distillation of air. An alternative, cheaper method of producing oxygen is under consideration: "Chemical Looping Combustion" (CLC).
     

The "Chemical Looping Combustion" process
This process is based on the use of metal oxides for the purposes of combustion in the absence of nitrogen. When metal particles become oxidized, oxygen can be transferred to the combustion zone, thereby separating oxygen from nitrogen. This principle should make it possible to significantly reduce capture costs provided the associated technological challenges are overcome.
The technology is being developed by IFPEN in partnership with Total.

Schéma du procédé du "Chemical Looping Combustion"
The "Chemical Looping Combustion" process

  
At present, existing facilities are only suitable for post-combustion flue gas capture. Chemical Looping requires design changes to facilities, representing a substantial financial investment but making it possible to obtain a very low energy penalty for CO2 capture.

CO2 capture and separation have been applied in industry for decades.

   

CO2 transport

The CO2 must then be transported to a storage facility, sometimes hundreds of kilometers away. CO2 transport is not particularly difficult and is already widely practiced on an industrial scale by both boat and gas pipeline. For the needs of the oil industry, it is transported in gas pipelines in supercritical state (at ambient temperature and at pressures over 73 bars), which requires the appropriate compression and injection facilities. The USA has a network of 4,000 km of such pipelines. The pooling of CO2 transport and collection infrastructures in major industrial zones, particularly ports, is being considered.

CO2 storage

Once captured and transported, the CO2 must be injected and stored underground. Deep saline aquifers have been identified as the only geological structures presenting adequate capacities to store large quantities of CO2.

 

Geological storage structures

Two types of structure are being considered:
  

  • Deep saline aquifers. These non-potable salt water reservoirs, located deep underground, represent the greatest potential when it comes to storage capacity (400 to 10,000 Gt). They are better distributed around the world than oil and gas fields but their structure and their capacity to trap CO2 over long periods are not as well understood. A major research drive is therefore necessary to assess their geological storage potential and their capacity to confine CO2 over the long term.
     
  • Depleted oil and gas fields. This option is interesting since the structures have acted as sealed oil and gas traps for millions of years and the geological environment is relatively well understood. However, their capacities are limited, and often located a long way from industrial facilities. The CO2 will thus have to be transported over long distances. This solution will doubtless be the first to be implemented but it will not be sufficient.
    Storage operations in oil fields are already under way within the context of full-scale trials.
     

Storage duration and reliability

CO2 storage needs to cover not only the length of time fossil fuels will remain available (1 to 2 centuries) but also the oceanic cycle (around half a millennium). It is vital to take into account the carbon cycle, which is governed by two exchanges: the exchange between the atmosphere and the ocean and the exchange between the biosphere and the atmosphere. While exchanges with the biosphere take place over periods of time measured in decades, the ocean cycle extends over several centuries. In order to stabilize CO2 concentrations in the atmosphere, it is therefore necessary to store CO2 in underground facilities for periods compatible with the oceanic cycle. As a precautionary measure, solutions are being considered that make storage possible over periods of up to thousands of years.

Storage reliability over a long period represents a major challenge. This solution must demonstrate that it is an effective way of tackling climate change (adequate CO2 retention time) and that it does not cause any local environmental damage. Only those sites that present all the safety guarantees will be selected. To do so requires reliable tools to model the future of the CO2 stored, as well as powerful management and surveillance tools that will detect any CO2 that might be leaking so that corrective measures can be put in place. 
  
 

CCUS (carbon capture and utilization): towards CO2 conversion?

CO2 could be converted for use as a raw material in different industries (chemicals, agrifoods, etc.) or for enhanced oil and gas recovery. Chemical and biological conversion processes are currently still at the laboratory or pilot stage. Their profitability and environmental footprint still have to be validated, taking into account a number of criteria: the value of the COmarket, the volume of COthat can be treated, the concentration and purity of CO2, the possibility of using non-greenhouse gas-emitting and low-cost energy to transform CO2, the proximity of COemitting sites and potential conversion sites, life cycle analyses of the converted products, etc.

Chemical and biological conversion processes are currently still at the laboratory or pilot stage.

Current uses of CO2 :
• for enhanced oil recovery (EOR); 
• in various industries (for the production of sparkling drinks, supercritical CO2 or refrigerants, etc.), for applications that generally require gas that is almost pure.
• Similarly, the biological conversion of CO2, using microalgae, is already at the commercial stage for some applications, including the production of substances with a high added value aimed at the cosmetics and pharmaceutical industries.
• The chemical conversion of CO2 is also used in industry for the production of urea or salicylic acid.

Numerous industrial players are interested in CO2 conversion and are examining different options: mineralization processes (Skyonic, Calera, etc.), polycarbonate production (Bayer, Novomer, etc.), methanol via direct hydrogenation (Carbon Recycling International, Mitsui Chemicals Inc., etc.).

CO2 conversion in Power-To-Gas
Power-To-gas is being considered as an option to convert surplus electricity from intermittent renewable energies (wind and solar) into gas. 
• Surplus electricity is converted into hydrogen via the electrolysis of water. 
• The CO2 is then used for the production of synthetic methane via the methanation reaction (conversion of carbon dioxide in the presence of hydrogen). 
• This synthetic methane can be injected into existing natural gas networks. 
But the economic viability of the chain is far from established.

While the use of CO2 may be of economic interest in the future, questions still need to be asked concerning the potential benefits from a climate point of view. The fact is that, with the exception of CO2 mineralization, products resulting from CO2 conversion only store it temporarily before releasing it back into the atmosphere. However, some of these products have a beneficial effect - albeit to a limited extent -, related to the substitution of a product using fossil carbon. The CO2 used still needs to be biological, and thus already neutral from a climate point of view.