A complete chemical looping combustion (CLC) system, including both a fuel reactor (FR) and an air reactor (AR), is simulated using a multiphase computational fluid dynamics (CFD) model. The AR model comprises a bubbling fluidized bed and a high velocity riser. The FR and loop seal are both bubbling fluidized beds. The oxygen carrier material is selected as NiO. The models of units in the CLC system are implemented in CFD simulations to correlate the exchange of mass, momentum, and heat. Simulated concentrations of gas species are compared to experiments measured in a chemical looping combustor with NiO-based oxygen carriers and CH, as fuel. The instantaneous and time-averaged distributions of the solid concentration and concentration of gas species in the AR, FR, and loop seal are predicted. The weight fractions of CH, in the FR and O-2 in the AR decrease along the bed height. The weight fraction of CO2 in the FR increases along the bed height.