Modeling Axial Mixing of Fuel Particles in the Dense Region of a Fluidized Bed

Anna Köhler,Filip Johnsson,David Pallarès

doi:10.1021/acs.energyfuels.9b04194

Anna Köhler, Filip Johnsson + Show 1 more

Open Access

https://doi.org/10.1021/acs.energyfuels.9b04194

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Abstract

A semi-empirical model for the axial mixing of fuel particles in the dense region of a fluidised bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamical, down-scaled fluidised bed that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solids zone; a zone with sinking emulsion solids; and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upwards velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to h...

Highlights

Fluidized bed (FB) units have been used commercially for the combustion and gasification of solid fuels since the early 1970s.2 Solid fuel conversion uses the two existing types of FB units: bubbling fluidized beds (BFBs) and circulating fluidized beds (CFBs)
The axial-segregating tendency of the fuel, which, as mentioned in Section 1, is a critical phenomenon in the design of FB units for solid fuel conversion, can be represented as the fraction of time spent by the particle on and above the dense bed surface
We show that the axial mixing of a fuel particle in the bottom region of a fluidized bed can be mathematically described in semiempirical modeling, giving good agreement with actual measurements

Summary

Introduction

Fluidized bed (FB) units have been used commercially for the combustion and gasification of solid fuels since the early 1970s.2 Solid fuel conversion uses the two existing types of FB units: bubbling fluidized beds (BFBs) and circulating fluidized beds (CFBs). BFB boilers are preferably used for lower thermal capacity and typically fed with local fuels with higher transport costs in terms of energy density (such as waste and biomass). Despite their obvious design and operational differences, BFB and CFB units for solid fuel conversion use Geldart B group solids[3] as the bed material, yielding the formation of bubbles at the bottom grid of the reactor, which travel upward through a dense bed. As BFB and CFB units for large-scale solid fuel conversion involve similar underlying mechanisms for the mixing of the solids (both bed materials and fuel particles) in the bottom region, the present work is of relevance for both types of units

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