Investigation on Conveying Mechanisms for Forcing Solids Horizontal Circulation Under Bubbling Fluidized Bed Conditions

The solids circulation rate is one key fluid-dynamic parameter of dual fluidized bed (DFB) systems, as it profoundly impacts the mass and energy transport between reactors and, thus, the overall system performance. Therefore, efficiently forcing a controllable flow of solids is a strongly desired feature in the design of DFB systems. The solids circulation in DFB systems with circulating fluidized beds (CFB) is typically attained by the solids elutriation from the riser. In contrast, DFB systems with interconnected bubbling beds lack knowledge of how efficiently fluidized solids can be transported between the reactors.

This work investigates different solids conveying mechanisms to induce a controlled horizontal circulation of fluidized solids at bubbling conditions. A closed horizontal loop configuration was used in the present study to circulate Geldart B solids in a bubbling fluidized bed. The experimental setup comprises a conveying section inducing the solids circulation and a transport section feeding back the bubbling fluidized solids to the conveying section. The design and operation of the unit follow the Glicksman scaling laws, and the closed loop with a characteristic length of 1.24 m operated at ambient conditions scales up to 10.35 m at hot conditions typical for solid fuel conversion (gasification, in this case). The resulting solids horizontal circulation rate is measured using magnetic solids tracing. Furthermore, the differential bed pressure induced in the conveying section is monitored, which allows the calculation of an energy-based efficiency of the conveying zone.

The experimental test matrix contains variations of bed height, gas flow rate provided to the solids conveying zone, and geometric configuration of the conveying zone. Regarding the latter, five different configurations were tested, each exploiting distinct features of the gas-solids flow to induce a horizontal solids flow: directed gas injection, free solids splashing, confined solids splashing, slugging, and solids entrainment.