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Temperature Influence on Fluid Dynamics at the Transition from Bubbling to Turbulent Fluidization for Geldart's Group B Particles

Authors: 
Wytrwat, T., Hamburg University of Technology
Hartge, E. U., Hamburg University of Technology
Heinrich, S., Hamburg University of Technology
For upscaling of fluidized bed processes, knowledge about fluid dynamic behavior is indispensable. Especially, Chemical Looping Combustion (CLC), a modern process for CO2 capture, shows high complexity in its fluid dynamics because it is designed as a system of different interconnected fluidized bed reactors. An oxygen carrier is circulating between two main reactors, called the air reactor and the fuel reactor. To guarantee a complete combustion of biomass or char particles in the fuel reactor, the residence time of these particles must be high enough and reaction should take place in the dense phase of the fluidized bed. Thus, fuel reactors are often designed as bubbling or turbulent fluidized beds. Fluid dynamics of the turbulent regime and its transition from the bubbling state are widely investigated in literature for Geldart’s group A particles, but not for group B particles. Besides, most studies focus on ambient conditions, however combustion processes are operated at high temperatures.

Generally, oxygen carriers used for CLC are in the range of group B according to Geldart’s classification. Thus, a thorough investigation of the turbulent fluidized bed regime for this kind of particles, starting at superficial gas velocities in the bubbling state and ending at fast fluidization is the aim of this work. Temperature influences on fluid dynamics are investigated in a range between ambient conditions and temperatures of 800°C.

Three fluidized bed plants in pilot scale are used for this investigation. Two of them are heated electrically to reach the temperatures mentioned above. The first one has a diameter of 100 mm and the second one a diameter of 150 mm with heights of 17 m and 9 m, respectively. The third plant, having a diameter of 400 mm and a height of 17 m, is operated at ambient conditions.

In addition, the measurements are compared to results carried out in two laboratory scale plants with diameters of 50 mm and 100 mm.

A superficial gas velocity range between 0.2 m/s and 3 m/s is investigated. Aspect ratios (static bed height divided by bed diameter) are varied between 1 and 4. Two quartz sand fractions of different sizes (Sauter mean diameter: 188 μm and 348 μm) and ilmenite (Sauter mean diameter: 146 μm) are used as bed material.

Pressure signals are recorded and analyzed related to their standard deviation and frequency spectra to get information about the transition from bubbling to turbulent fluidization and thus the flow structure. As local online measurement technique, capacitance probes are used for the determination of local solids concentrations and local velocities of bubbles/voids in the dense zone of the bed.

Static bed height and plant diameter are found to have a significant influence on the transition velocity from bubbling to turbulent fluidization for the range investigated. Higher static bed heights lead to higher transition velocities. With increasing temperature of the fluidizing gas, its viscosity and density change. Due to this change, in contrast to group A particles according to Geldart’s classification, for group B particles an increase in the transition velocity with increasing temperature can be observed. Finally, a correlation for the determination of the transition velocity from bubbling to turbulent fluidization is introduced by taking into account the static bed height, bed diameter and temperature.