Chemical-looping combustion (CLC) is a novel combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the combustion of gaseous fuel using a metal oxide for oxygen transfer. The system consists of two reactors, a fuel reactor and an air reactor, and an oxygen carrier in the form of a metal oxide that transports oxygen from the air to the fuel. Direct contact between fuel and combustion air is avoided, and the products from combustion are kept separated from the rest of the flue gases. A dual-fluidized bed reactor system representing a 10-kW CLC prototype was designed. A scaled flow model was built and investigated. Gas velocities and designs were varied, while solids circulation rate and gas leakage between the reactors as well as static pressure balance and residence time distribution of gas and particles were measured. Results show that the solids circulation rates were sufficient and the gas leakage could be decreased to very low values.