Efficient design of heat exchange for CFB reactors in CO₂ capture system regarding geometry-induced secondary flow

CO₂ capture technology using temperature-swing adsorption faces many obstacles for commercialization. Circulating fluidized bed systems have high capture performance, but an energy-efficient design is essential to reduce the CO₂ capture cost. In this study, thermal-fluid characteristics of circulating fluidized bed with two-phase flow are analyzed, and thermal design is implemented. First, a laboratory-scale circulating fluidized bed system was used to investigate the relationship between hydrodynamics and heat transfer in experiments and numerical simulations. Numerical method predicted well the pressure fluctuations and local bed-to-wall heat transfer. Particle behaviors affected greatly the heat transfer. Accordingly, to control solid particle flow, the effect of reactor geometry on hydrodynamics and heat transfer was analyzed. Compared with the conventional circular reactor, the rectangular reactor had a secondary flow of solid particles in the horizontal section. Flow toward the narrow wall, and rotational flow near the corners, were identified. Intensity of the secondary flow depended on the aspect ratio. It was firstly reported that the geometry-induced secondary flow affected significantly heat transfer. Due to the strong secondary flow in the rectangular reactor, the local heat transfer was more than doubled. Ultimately, thermal design was carried out by analyzing the minimum temperature difference (MTD) according to the reactor geometry. The rectangular reactor showed an MTD of 67% compared to the circular reactor and 45% compared to the square reactor. Because MTD decreased and the coolant amount increased simultaneous with increasing aspect ratio, the rectangular reactor with an aspect ratio of 4:1 was the most energy-efficient design. Finally, through thermal design, the pilot-scale rectangular reactor is designed and under-constructed in order to evaluate the CO₂ capture system performance and the potential for commercialization.