A comprehensive three-dimensional numerical model including the kinetic theory of granular flow and complicated reactions was developed to simulate the chemical looping combustion process in the fuel reactor. The standard kappa-epsilon model was used to simulate the gas-phase turbulence and the kinetic theory of granular flow to simulate the solid phase. The shrinking core model (SCM) with the reaction controlled by the chemical reaction in the grain was applied. The fuel reactor was designed as a bubbling fluidized-bed reactor with a height of 1.2 m and diameter 0.1 m. A Cu-based oxygen carrier was prepared with 14 wt% CuO on 86 wt% inert Al2O3. The inlet fuel gas was coal gas containing 44.5 vol% CO, 22.2 vol% H-2, 22.2 vol% H2O, and 11.1 vol% CO2. The flow patterns, distributions of gas components, and conversions of gas reactants were obtained. The effects of the operating conditions (the initial bed height, bed temperature, and operating pressure) on fuel conversion were analyzed. The results show that the fuel conversion with the same inlet gas velocity would go up by modestly increasing the initial bed height and the temperature but would slightly decrease with an increase in the operating pressure. The high conversion of coal gas with a low solid inventory could be reached in a proper operating condition.