Chemical looping combustion (CLC) has emerged as one of the most promising technologies for low-cost CO2 capture technologies for solid fuels. CLC provides the possibility of CO2 capture without the requirement of an air separation unit or an absorption process.
The idea of CLC is to use an oxygen carrier to transport oxygen from air to fuel. In contrast with to conventional combustion, direct contact between fuel and air is avoided.
The chemical looping process consists of two reactors; an air reactor, where the oxygen carrier is oxidized by the oxygen content of air and a fuel reactor, where the oxygen carrier is reduced by the fuel that is converted to mainly CO2 and steam. The process results into two separate flue-gas streams. The stream leaving the air reactor contains nitrogen and some remaining oxygen and the stream leaving the fuel reactor contains mainly CO2 and steam.
Char particles will be formed and exit the fuel reactor together with the oxygen carrier. In order to avoid the char entering the air reactor a carbon stripper is used to separate the char particles from the oxygen carrier and recycle them back into the fuel reactor.
The performance of the oxygen carrier is essential for the efficiency of the CLC process. Metal oxides are the most promising type of oxygen carrier. For coal combustions, natural ores or products containing iron oxides have been tested successfully.
Theoretically, the efficiency penalty for CO2 capture only comes from the compressors that give the CO2 stream the right pressure for subsequent transport and geological storage.
Considerations for commercial viability
Despite CLC has attached scientific and technical research in recent years, there are still a number of issues that require further investigations. Construction and operation of pilot and demonstration plants has to take place, before it is possible to use this technology commercially.