Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO2 capture and sequestration, which is able to mitigate greenhouse gases (GHGs) emission. In this study, to design a 0.5MWth pressurized chemical looping combustor for natural gas and syngas the effects of solid residences time on CO2 selectivity were investigated in a novel semi-continuous CLC reactor using Ni-based oxygen carrier particle. The semi-continuous chemical looping combustor was designed to simulate the fuel reactor of the continuous chemical looping combustor. It consists of an upper hopper, a screw conveyor, a fluidized bed reactor, and a lower hopper. Solid circulation rate (Gs) was controlled by adjusting the rotational speed of the screw conveyor. The measured solid circulation rate increased linearly as the rotational speed of the screw increased and showed almost the same values regardless of temperature and fluidization velocity up to 800 °C and 4 Umf, respectively. The solid circulation rate required to achieve 100% CH4 conversion was varied to change Gs-fuel ratio (oxygen carrier feeding rate/fuel feeding rate, kg/ Nm3). The measured CO2 selectivity was greater than 98% when the Gs-fuel ratio was higher than 78 kg/Nm3.