Dual-dense gas-solid circulating fluidized bed reactor.

[Display omitted] • Dual-bed circulating fluidization is divided into dilute-dense and dual-dense ones. • For dual-dense one, particles are in dense fluidization regime in ascending bed. • Three types of internal circulating fluidized beds (ICFB) were reviewed in detail. • Various factors affecting particle circulation and gas bypassing were analyzed. • Numerical simulation and potential applications of ICFB were summarized. Circulating fluidized bed reactors are widely employed to deal with the gas–solid reactions in the energy industry. They could be classified into two categories as per the combination of fluidization regimes. The traditional circulating fluidized bed belongs to the dilute-dense one. The dual-dense one denotes that particles are densely fluidized in the ascending bed (riser) and in dense fluidization or moving-bed state in the descending bed (downcomer). Yet, the gas velocity in the ascending bed is greater than that in the descending bed. Compared with the dilute-dense one, the dual-dense gas–solid circulating fluidized bed is much more compact in structure and lower in construction cost. However, the concept of dual-dense gas–solid circulating fluidization has not been well realized by nowadays scholars, severely hindering its commercial application. To deepen/broaden people's understanding, a detailed review was carried out on the dual-dense gas–solid circulating fluidization system. The related research involved three types of internal circulating fluidized beds (ICFB): ICFB without internals, ICFB with a flat baffle, and ICFB with a central draft tube. The following issues were deeply analyzed and discussed: 1) mechanism and benefits of dual-dense gas–solid circulating fluidization, 2) effects of various structural and operating parameters on the particle circulation and bidirectional gas bypassing, 3) prediction of particle circulation based on the orifice flow theory, 4) numerical simulation of gas–solid flow behavior, and 5) potential applications already explored in the literature. More importantly, the existing controversies and unsolved issues in this area were identified. [ABSTRACT FROM AUTHOR]