Hydrodynamic study of a dual fluidized bed system at room and elevated temperatures

Gasification with in-situ CO₂ looping cycle is a promising technology to produce energy while reducing CO₂ emissions. An alternative reactor concept to carry out the integrated process is a Dual Fluidized Bed (DFB). In this reactor, a solid sorbent is continuously circulated between two vessels to undergo multiple carbonation and sorbent regeneration. A comprehensive understanding of the fluidization characteristics of lime-based sorbents (commonly used for CO₂ capture), solids transport between the vessels, and operational analysis of DFB are important for the design and scale-up of the integrated process. In this study, the hydrodynamic behaviour of limestone of mean diameter 438 µm in a DFB consisting of a riser interconnected via a loop-seal and a downcomer to a bubbling fluidized bed (BFB) was investigated at temperatures up to 250°C. The effects of operating parameters influencing the performance and stability of the system such as riser superficial gas velocity (2.5-6.5 m/s), aeration velocity (1 to 7Umf) and solids circulation flux (25-139 kg/m²･s) on the pressure profiles and cross-sectional average solids holdup in the riser were determined at room temperature. Further experimental work was conducted in the same DFB facility, and employing the same limestone particles to study the effect of scale on hydrodynamics, while increasing the bed temperature. Gas leakage between the coupled fluidized beds and its relationship with operating conditions, was also measured based on a gas tracer technique. Stable operation of the DFB was obtained under the conditions evaluated. The cross-sectional solids holdup in the riser was found to increase with increasing solids mass flux and decreasing riser gas velocity. The DFB riser operated within the fast fluidization and the dilute-phase transport regimes with the transition point characterized by the presence of accumulative choking. Analysis of the pressure profiles revealed that the pressure head in the loop-seal is an important parameter to obtain high solids circulation flux. Furthermore, the pressure in the BFB was found to influence the rate of leakage of riser product gas into this reactor. Smoother fluidization was obtained at elevated temperatures. The results showed that solids holdup in riser decreased with increasing temperature.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat