A novel gasification technology, allowing for the efficient conversion of residual materials into high-calorific syngas, is the chemical looping gasification (CLG) process. Its major advantage is that the oxygen required for efficient feedstock conversion is supplied through the cyclic reduction and oxidation of a circulating oxygen carrier. Hence, CLG requires stable operation of two coupled fluidized bed reactors, between which controlled solid circulation occurs. The viability of the CLG technology was demonstrated using the 1 MWth modular pilot plant within three dedicated multi-day 24 h/day test campaigns, investigating three different biomass feedstocks. During CLG operation a significant carbon reservoir is built up inside the FR and subsequently gasified. However, carbon conversion inside the fuel reactor (FR) was found to be incomplete in the range of 60 % to 95 % of feedstock carbon, as some carbon is lost via the unwanted carbon slip towards the air reactor (AR) and some is lost through the FR cyclone. The observed carbon slip is strongly correlated to the FR temperature which influences the reaction kinetics of char gasification and initial pyrolysis. At very low FR temperature, the carbon slip is in the range of the fixed carbon determined via proximate analysis, indicating that feedstock residence time inside the FR is too low for significant char gasification to occur. During the chemical looping conference, the influence of temperature and feedstock properties on pyrolysis, fixed carbon gasification, and the minimization of carbon slip will be discussed based on 1 MWth experimental data and lab data.