Chemical looping combustion is a technique that utilizes metal oxides instead of air in the process of burning fuel. These systems typically use a fluidized bed to circulate metal oxides into a reaction chamber, where they provide the oxygen necessary for combustion. The metal oxides are then circulated back into a different area where more oxygen can be added. Since only pure oxygen is used in the combustion process, no nitrogen oxide is produced. The flue gasses from a chemical looping combustion process typically consist mainly of carbon dioxide (CO2) and water vapor, so this method can be useful to sequester carbon in an economical way.
In a chemical looping combustion process, there are typically two different reactors that are interconnected in some way. One of these reactors contains fuel of some type that requires oxygen to burn. Metal oxides are circulated, typically through a fluidized bed, into this reactor. The metal oxides are then used to oxidize the fuel. Once there is oxygen present in the fuel, it can be combusted.
After the metal oxides are stripped of oxygen in the first reactor, they are circulated back into an air reactor. When the reduced metal passes into this chamber, it is exposed to air, at which point it can take in more oxygen atoms and become oxidized again. The process is then looped, as the newly oxidized metal is circulated back into the fuel reactor and used to facilitate another combustion cycle.
When any type of fuel is combusted in the presence of air, a number of byproducts are formed. Each of the different elements present in the air and fuel can interact in many different ways, creating a number of chemical combinations. Carbon dioxide is one main component of most flue gasses, but due to the presence of nitrogen in the air, most combustion processes result in the production of nitrogen oxide as well. Oxy-fuel combustion refers to a process where oxygen is separated in some way from the air, which simplifies the chemical reactions that occur during combustion. Chemical looping combustion is one type of oxy-fuel combustion process.
The primary benefit of chemical looping combustion is that the contents of the flue gasses can be simplified. It can be difficult and costly to separate carbon dioxide from nitrogen oxide, or to sequester carbon directly from the air. The process of separating water vapor from carbon dioxide is significantly easier, so the flue gasses created by chemical looping combustion contain carbon dioxide that is less expensive to sequester.