The dual fluidized bed (DFB) steam gasification principle can be used to convert biomass in a valuable product gas. However, in such systems various particle species are present: fresh fuel, converted fuel or bed material particles and additives. These particle species show large differences in size and density and, therefore, potential for segregation processes arise. Segregation can have a negative impact on the operation behavior of DFB reactors and thus, the particle movement has to be known. Between the two fluidized beds, the gasification and combustion reactor, of a DFB system a bed material recirculation stream is circulating. It determines on the one hand how much converted fuel is transported to the combustion reactor for bed material heat-up. On the other hand it transports this energy back to the gasification reactor. The first issue is addressed by how much converted fuel is transported to the combustion reactor and how it is mixed in the gasification reactor. ^The second issue depends on the mixing of the hot stream into the bed of the gasification reactor. In this work two approaches were used to investigate particle mixing inside of a dual fluidized bed reactor: computational fluid dynamics simulations and cold flow modeling. In the cold flow model the influence of various operation parameters on the char concentration in the recirculating stream between the two fluidized beds of the DFB system was investigated. Computer simulations were used to investigate the mixing of hot bed material and of the fuel in the bed as well as to investigate the influence of the design of the reactor on mixing. For the simulations the commercial code CPFD Barracuda was chosen. The cold flow model of the commercial plant as well as the commercial plant itself were simulated. ^It was shown that the char concentration in the recirculating stream is strongly dependent on various operation parameters like fluidization velocity, recirculation rate, char concentration in the system or bed height. Mixing gets better with increasing fluidization velocity and recirculation rate. However, increasing char concentration in the system and bed height can worsen the mixing behavior. The choice of the drag law has a massive influence on the predicted bed material recirculation rate. From the simulations of the Güssing cold flow model and commercial plant no general recommendation can be given which drag law to choose in general. In the present study the EMMS drag model was able to best predict the recirculation rate and pressures inside the system for the commercial plant but also showed some limitations for predicting the behavior of the cold flow model of the commercial plant. ^A simulation model was built which was able to predict the bed material recirculation rate, pressures, temperatures and gas compositions inside the hot system with good accuracy. The simulations showed that the hot bed material coming from the combustion reactor is not ideally mixed. Furthermore, for a plant of the size of Güssing one fuel feed point is enough according to the simulations. Plants larger than the size of Güssing the design could benefit from optimization. It was shown with cold flow model experiments that poorly fluidized zones could occur in large DFB plants and that implementing an additional fluidization for these zones could significantly enhance particle mixing. The experimental and simulation results were able to broaden the knowledge about the mixing and segregation processes inside DFB systems. Furthermore, the results indicate that for larger DFB plants further scale effects concerning mixing could occur due to the larger dimensions. ^Therefore, adaptations regarding the design or the number of fuel feeding points could be necessary.