In order to burn a hydrocarbon fuel efficiently using conventional cycles, very high temperatures are required. Chemical looping combustion offers an alternative cycle for large scale power production. In chemical looping combustion a carrier molecule is used to transport oxygen between two redox reactions, one where the carrier is oxidised and another where it is reduced by reaction with a fuel. Separation of the oxygen carrier from fuel ash can be aided by means of phase difference and this is a key advantage of fluid phase chemical looping combustion where the carrier medium proposed is sodium, potassium or zinc. The principle exploited in fluid phase chemical looping combustion is the recirculation of both energy and entropy. High thermal efficiencies, circa 75% at 35 bar are theoretically achievable taking into account component efficiencies, with separation of nitrogen and carbon dioxide, in combination with the water shift gas reaction, as an inherent part of the cycle if air is used as the oxygen source.