Hydrodynamics of lime-based pellets in a Dual Fluidized Bed and the effect of temperature

• A hot Dual Fluidized Bed system was successfully operated using Geldart B pellets.
• Solid circulation increases with riser velocity and decreases with temperature.
• Solids hold-up in the bottom section of the riser decreases with temperature.
• The interdependence between the solids hold-up, -circulation and drag was explored.
• Solids hold up characterized by a new slip factor correlation based on the drag equation is proposed.

The looping of dry CO2 sorbents in Dual Fluidized Beds (DFBs) plays a prominent role in CO2 capture methods, and understanding the hydrodynamics of DFBs is of great importance. Geldart B lime-based pellets were investigated in a DFB with a constant bubbling fluidized bed velocity of 0.17 m/s, a riser velocity varied between 1.3 and 5.3 m/s, a loop-seal aeration velocity ranging from 0.08 to 1.08 m/s and a temperature from ambient to 550 °C. The solids circulation flux (Gs) increased with increasing superficial riser velocities, as well as with increasing loop-seal aeration. Experiments at elevated temperatures revealed that increasing the temperature decreased Gs and the solids volume fraction in the bottom section of the riser. The riser underwent transition from fast fluidization to pneumatic transport by increasing either the superficial riser velocity or the temperature. The interdependence between the riser solids fraction, Gs and particle drag was explored by means of the slip factor. A new slip factor correlation is proposed which makes use of the drag coefficient and terminal Froude number.