A newly designed continuous lab-scale rotating bed reactor for chemical looping combustion using CuO/Al2O3 oxygen carrier spheres and methane as fuel gives around 90% CH4 conversion and >90% CO2 capture efficiency based on converted methane at 800 degrees C. However, from a series of experiments using a broad range of operating conditions potential CO2 purities only in the range 20-65% were yielded, mostly due to nitrogen slip from the air side of the reactor into the effluent CO2 stream. A mathematical model was developed intending to understand the air-mixing phenomena. The model clearly reflects the gas slippage tendencies observed when varying the process conditions such as rotation frequency, gas flow and the flow if inert gas in the two sectors dividing the air and fuel side of the reactor. Based on the results, it is believed that significant improvements can be made to reduce gas mixing in future modified and scaled-up reactor versions. (C) 2013 Elsevier Ltd. All rights reserved.