Some activities, such as natural gas treatment or ammonia and hydrogen production, already separate CO2 from the gas streams when concentrations exceed a prescribed threshold. In those examples, however, the aim is to purify other gases, and the CO2 is often discharged to the atmosphere.

Today’s challenge is to develop more efficient and larger-scale technologies that will permit storage of CO2. As for transport, there are two options – by pipeline or by ship – the choice of which depends on the distance between the emission source and the storage site.

The three capture techniques

According to the type of installation, CO2 capture may take place at three different stages, termed post-combustion, precombustion, or oxyfuel combustion decarbonization. Each of these techniques is at a different stage of maturity and offers its own advantages and drawbacks (cost, energy consumption, etc.).

• Post-combustion decarbonization is the most mature, but also the most costly of the three techniques, and is appropriate for existing installations. It involves separating the CO2 contained in combustion gases, usually by means of a liquid solvent such as mono ethanol amine (MEA).
  
• Pre-combustion decarbonization yields two separate concentrated streams of hydrogen and CO2, thereby facilitating CO2 capture. The process consists of treating the fuel either with steam and air (steam reforming) or with oxygen (partial oxidation) to produce a synthesis gas that contains mainly carbon monoxide (CO) and hydrogen, a potential energy carrier that generates no CO2 emissions. A second step converts the CO in the presence of water (H2O) then separates the resulting CO2 for capture and storage.
  
• Oxyfuel combustion decarbonization is still in the pilot phase. This technique yields a combustion gas highly concentrated in CO2 (between 80% and 90% by volume) and could constitute a suitable retrofit technology for existing installations. The process uses high-purity oxygen instead of air for combustion, the main difficulty being to extract the oxygen from the air. Due to the high cost of this separation step, a “chemical looping” process is being investigated in which the oxygen supply is derived from a reaction involving a metal oxide, using metal particles such as iron filings, which would serve as the oxygen carrier from air to fuel.

                             
                                                                        Click to enlarge diagram