Chemical-Looping Combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2. CLC is considered to be an option with low energy penalty and low cost for CO2 capture. An option for use CLC with solid fuels is the in situ Gasification-CLC (iG-CLC), where the solid fuel gasification and the oxidation of gaseous products, i.e. volatile matter and gasification products, simultaneously take place in the fuel reactor of the CLC system. The objective of this work was to optimize the operating conditions for direct CLC with solid fuels using ilmenite as oxygen carrier. A simplified model for the fuel reactor has been developed, which describes the complex processes happening in the fuel reactor. Thus, the effect of the main operating variables in the iG-CLC process can be analyzed in a simpler way than using a detailed model. The model includes the possibility of using a carbon separation system to recirculate unreacted char particles exiting from the fuel reactor, reducing the by-pass of carbon to the air reactor. Also, the gasification kinetics of a bituminous coal for both H2O and CO2 as gasification agents and the kinetics of the reduction reaction of ilmenite with H-2, CO and CH4 are incorporated to the model. First, the simulated results have been compared to experimental results from tests performed in a continuous 500 W-th CLC plant. Later, model simulations were performed to evaluate the effect of the main operating variables of the fuel reactor (e. g. temperature, solids inventory, efficiency of the carbon separation system, oxygen carrier to fuel ratio, or flow and type of gasification agent) on the combustion and carbon capture efficiencies. The carbon capture was directly related to the extent of gasification, which is promoted by increasing the temperature or the residence time of char particles in the fuel reactor. It is highly beneficial to increase the solids inventory up to 1000 kg/MWth, but further increase does not give a relevant improvement in the carbon capture and it is better to increase the carbon separation efficiency than the solids inventory. With an inventory of 1000 kg/MWth, at 1000 degrees C and a carbon separation efficiency of 90% the carbon capture predicted was 86.0%. (C) 2012 Elsevier Ltd. All rights reserved.