Thermal design of dual circulating fluidized bed reactors for carbon dioxide (CO2) capture was carried out. To handle large heat duties for regeneration, a thin rectangular reactor was proposed. For feasible thermal design, the effect of varying reactor thickness on the gas-solid flow and heat transfer of the thin rectangular fluidized bed was investigated. Reactor thickness of 10, 30, and 60 mm was tested. Numerical simulations were conducted to analyze the pressure difference, solid particle hold-up distribution, particle velocity, granular temperature, and heat transfer in detail. According to our results, when the reactor is between 10 mm and 30 mm thick, a large solid hold-up occurs adjacent to the narrow wall. This causes a large pressure difference due to the wall effect. Furthermore, the particle velocities were analyzed to evaluate that there is the two-dimensional (2D) particle mixing behaviors. On the other hand, in the case of reactors with a thickness of 60 mm, tuning flows occur adjacent to the narrow wall. This reduced the pressure difference and the three-dimensional (3D) particle mixing behaviors. This difference in particle behavior affected heat transfer. In the case of reactor thicknesses between 10 mm and 30 mm, the heat transfer increased with the reactor thickness. In particular, the heat transfer at the narrow wall of the reactor with a thickness of 10 mm was extremely low due to the low particle mixing. On the other hand, there was more heat transfer with a thickness at the 60 mm wall, despite the low solid hold-up.