A copper based oxygen carrier prepared by impregnation on a highly porous alumina support (14.2 wt% active CuO) has been tested in a 120 kW chemical looping pilot plant. This oxygen carrier has already been under investigation in other pilot plants up to 10 kW fuel power and showed very good performance, i.e. full fuel conversion was achieved. During the experiments, natural gas has been used as fuel and variations of several process parameters like temperature, fuel power, solids inventory and solids circulation rate have been performed. The copper particles showed good performance regarding conversion of CO and H-2 (almost full conversion) but only moderate conversion of CH4 (up to 80%) was achieved. The three process parameters fuel reactor temperature, solids circulation between air and fuel reactor and solids inventory have been identified as significant parameters for fuel conversion, i.e. increasing one of these parameters improves fuel conversion. Continuous analysis of the oxygen carrier particles revealed an initial decay of active CuO content caused by attrition on the external surface of the particles. The CuO content stabilized after 30 h of operation at around 9 wt% and no further decrease was observed. (C) 2015 Elsevier Ltd. All rights reserved.