For combustion with CO2 capture, chemical-looping combustion has the advantage that no energy is lost for the separation of CO2. In chemical-looping combustion oxygen is transferred from the combustion air to the gaseous fuel by means of an oxygen carrier. The fuel and the combustion air are never mixed, and the gases from the oxidation of the fuel, CO2 and H2O, leave the system as a separate stream. The H2O can easily be removed by condensation and pure CO2 is obtained without any loss of energy for separation. This makes chemical-looping combustion a most interesting alternative to other CO2 separation schemes, which have the drawback of a large energy consumption. A design of a boiler with chemical-looping combustion is proposed. The system involves two interconnected fluidized beds, a high-velocity riser and a low-velocity bed. Metal oxide particles are used as oxygen carrier. The reactivities needed for oxygen carriers to be suitable for such a process are estimated and compared to available experimental data for particles of Fe2O3 and NiO. The data available on oxygen carriers, although limited, indicate that the process outlined should be feasible.