"Chemical Looping". Oxygen Carrier Metals

The special feature of "Chemical Looping" processes is to achieve combustion of carbonaceous compounds by facilitating capture of the CO ₂ emitted thanks to an oxidizing material commonly known as an oxygen carrier.

The oxygen carrier is generally a metal oxide reinforced with a carrier material. It is powdered and circulates in a loop between a – combustion reactor, where it is reduced by burning the – fuel, and a regeneration reactor, where it is oxidized to create a new combustion cycle. A major economic challenge is to achieve numerous redox cycles with the same material. However, solid oxygen carriers degrade rapidly. A great deal of research has been carried out over the last few decades in an attempt to develop long-lasting oxygen-carrying materials. Over a thousand materials have been tested. It is not easy to obtain materials that are resistant in terms of both mechanical strength and reactivity, and this is still one of the main challenges to be met before "Chemical Looping" processes can be commercialized.

This article is a continuation of the "Chemical Looping" article [AF6937] , which presents general aspects of "Chemical Looping" processes.

The first part of this article first reviews the criteria generally used to select an oxygen carrier, then describes the composition, morphology and structure of oxygen-carrying materials, as well as the degradations they can undergo during redox cycles, and suggests ways of avoiding or minimizing them.

In a second section, we present the various methods used to produce and shape oxygen carrier particles, and describe the characteristics of the materials obtained in each case.

Finally, the third part of the article provides a summary of the many – natural and synthetic – materials tested as oxygen carriers, giving their performance and limitations in Chemical Looping processes.