Hydrodynamic Characterization of Different Geldart B Solids in Dense Gas-Fluidized Beds By Capacitance Probes

Fluidization technology presents valuable options for efficient thermochemical processing of solid fuels. In this framework, bed hydrodynamics strongly influence the performance of the reaction as they determine mixing/segregation patterns of the fluidizing gas, and of the fuel in the inert fluidizing medium. Knowledge about the last-mentioned aspects is a key prerequisite for successful design and operation of such converters. However, there are still broad areas of uncertainties regarding fluidized beds hydrodynamics, mixing, heat and mass transfer phenomena.

In the present study, the hydrodynamics of bubbling fluidized beds operated at ambient temperature and 500°C are investigated by capacitance probes at different gas superficial velocities. The probes are fitted to a medium-sized laboratory scale unit with the sensing volume placed at multiple radial/axial column coordinates. Statistical analysis of the time-series of the local bed voidage generates probability density functions that display multimodal patterns. A bimodal character can be ascribed to the emulsion phase when the gas superficial velocity is increased. It is inferred that two phases with different porosity co-exist in the emulsion phase: a lower voidage LV-phase and a higher voidage HV-phase.

Three granular materials belonging to group B of Geldart classification are investigated, which differ for particle diameter, density and sphericity. Quantitative and qualitative differences in terms of hydrodynamic behaviour are highlighted and related to the above-mentioned solids properties.

The influence of the bed hydrodynamics on the gas mass transfer around freely moving coarse active particles is also assessed. Based on the results obtained by the above-described experimental campaign, the use of a Frössling-type equation is critically reviewed in the light of the observed expansion patterns of the emulsion phase.