Chemical Looping. Combustion Processes of Carbon Compounds

One of the major challenges facing the environment is the fight against global warming, caused by increasing emissions of greenhouse gases into the atmosphere, particularly carbon dioxide. At world conferences on the environment, industrialized countries undertook to control their carbon dioxide emissions under the Kyoto Protocol, signed in 1997 and ratified in 2005. Since then, considerable efforts have been made to replace carbon-based energies with renewable energies. However, our dependence on fossil fuels is so great, particularly in the energy, industry, transport and residential sectors, that it seems impossible to free ourselves from them in the near future.

In response to the environmental emergency, CO₂ capture and storage technologies have been intensively developed in recent years to equip plants with high carbon dioxide emissions, such as power plants, steelworks, cement plants, oil refineries and so on. The aim being to capture the CO₂ produced, transport it and store it outside the atmosphere in deep saline aquifers, untapped coal seams or similar. It should be noted that a proportion of CO₂ may be exported to plants that consume it, for urea production for example, but this remains a derisory share compared with the total quantity of CO₂ emitted.

Thus, technologies to facilitate the capture of massive CO₂ emissions from industries are currently being developed. They are listed in three main categories, named CO₂ capture processes:

1. In post-combustion, during which the CO₂ is separated from the other gases contained in the combustion products (flue gases);

2. In oxycombustion using pure oxygen to produce fumes composed solely of CO₂ and water vapor, easily separated by condensation ;

3. In pre-combustion, which consists in extracting CO₂ before combustion; to do this, the fuel is transformed into a mixture of H₂ and CO₂, from which carbon dioxide is isolated, enabling hydrogen to achieve "decarbonized" combustion.

The "Chemical Looping" processes discussed in this article enable CO₂ to be captured in oxycombustion. The combustion carried out can be complete or incomplete, associated with a steam reforming unit or a water dissociation reaction; a gasification stage can also be integrated in the case of solid fuel processing. A number of processes have been developed around this technology, which operate in slightly different ways depending on the intended application.

Chemical Looping" technology facilitates the capture of CO₂ during the conversion of carbonaceous fuels into valuable products such as electricity, dihydrogen, and so on. The special feature of the "chemical loop" concept...