In over 400 h of running experiments for chemical looping combustion of solid fuels, more than ten different oxygen carriers and six different fuel types were tested at the chemical looping combustion research infrastructure at TU Wien. For preliminary testing of oxygen carriers, a laboratory unit was designed and operated extensively as a simple fluidized bed to investigate oxidation and reduction reactions independently. An innovative sampling device allows the extraction of the oxygen carrier material at any time during the experiment for the detection of the oxidation state. Based on results from the laboratory unit, a characterization scheme was developed to evaluate oxygen carriers for their suitability for CLC. The classification of fluidized bed properties and oxygen carrier properties enabled a holistic assessment of the oxygen carrier. The characterization scheme of the oxygen carrier was successfully confirmed by experiments in the 80 kWth pilot plant at TU Wien. The innovative design of this pilot plant allowed steady state operation under autothermal conditions. The lower part of the fuel reactor was realized as bubbling fluidized bed to increase the residence time of fuel, char and to minimize carbon slip. The upper part of the fuel reactor was designed as counter current column with restrictions along the height to improve gas/solid contact. Depending on operation conditions and the type of oxygen carrier, a carbon capture rate of up to 99% was reached. With natural ores, CO2 selectivities up to 95% were achieved and by usage of the synthetic oxygen carrier C28, a nearly full conversion (99.7%) was attained. A cold flow model study addressed possible improvements of the reactor design to further increase the efficiency and conversion rates. Furthermore, an experimental process chain setup included the direct utilization of exhaust gas of the fuel reactor by methanation. A methane yield as high as 96 % could be achieved out of the carbon transported to the methanation unit.