The design of an efficient and scale-up ready reactor system is, together with development of a high performance oxygen carrier, one of the most important research topics in chemical looping combustion. The dual circulating fluidized bed (DCFB) concept is a reactor concept consisting of two interconnected circulating fluidized beds, air and fuel reactor, where the oxygen carrier is the bed material. In the present study a so-called cold flow model is used to investigate the fluid dynamic behavior of a next scale design based on the DCFB concept (10 MW fuel power input) in order to optimize the system design. Four different designs of the fuel reactor are investigated with focus on the solid distribution and general operating parameters and their influence on the operating range. For that purpose, several parameters like e.g. the amount of fluidization gas for both reactors, the total solid inventory, and the fuel power, are variated. Pressure profiles are used to get a comprehensive overview of the fluid dynamic behavior of the different reactor designs. Comparison of the fuel reactor designs shows that operation of the air reactor is not affected by changes of the fuel reactor design. In contrast significant changes, especially in the solid distribution, are recognizable for the different fuel reactor designs. These changes concern mainly the adapted sections of the fuel reactor. Two of the proposed fuel reactor designs are limited in their operating range due to unstable operating conditions, caused by fluid dynamic effects. At such unstable operating conditions the solid inventory distribution between both reactors fluctuates. This behavior is caused by the geometry of the fuel reactor. Further, due to variations in the geometry of the fuel reactor effects leading to the unstable operating conditions are identified. However, unstable operating conditions are caused by the shape of the conical form of the bottom part of the fuel reactor. (C) 2017 Elsevier Ltd. All rights reserved.