Circulating fluidized bed (CFB) has been widely applied to many chemical engineering processes. Although significant developments have been made in understanding the performance using the complex CFB technology during the last decades, the detailed inner information cannot be obtained by experiments because of complicated flow pattern in the system and backward measuring equipment. Numerical simulation has become the primary method to accelerate the development of complex CFB technology, reduce the cost of design and operating time, as well as reduce the technical risks. This thesis aims to provide more detailed in-furnace phenomena of complex CFB systems, including the hydrodynamic behaviours and chemical reactions based on the numerical simulation method. The promising chemical looping combustion (CLC) technology, as an example of complex CFB systems, will be focused on in this thesis. Meanwhile, the non-uniformity phenomenon in complex CFB units is comprehensively investigated in two symmetrical CFB configurations connected in parallel and series. Sequentially, an integrated method to dynamically combine CFD modelling and the process simulation is developed as a solution to improve the CFB performance. Specifically, it covers the following five aspects: 1. The hydrodynamic characteristics in a full-loop dual CFB CLC unit are comprehensively investigated based on the Eulerian multi-fluid model to give more detailed information about the flow behaviours. 2. The hydrodynamic characteristics in a unique counter-current moving bed full-loop CLC unit are comprehensively investigated based on the Eulerian multi-fluid model to study the unique configuration and in-furnace fluidization. 3. The reaction characteristics in the unique counter-current moving bed full-loop CLC unit are firstly attempt based on the hybrid Eulerian- Eulerian-Lagrangian model to study the in-furnace reaction details. 4. The non-uniformity characteristics of the multiphase flow in two complex CFB units connected in parallel and series, respectively, are studied based on the Eulerian multi-fluid model. 5. A novel direct integrated method to dynamically combine CFD modelling and the process simulation is developed. A case study of real-time regulation of boundary and operating conditions of reactors in complex CFBs is realized. These studies contribute to the deep understanding and further optimization of complex CFB systems.