Abstract
A semiempirical model for the mixing of fuel particles in a fluidized bed is presented and validated against experimental data from the literature regarding lateral fuel mixing. The model of fuel particle mixing categorizes the fluidized bed into three mixing zones: a rising bubble wake solid zone, an emulsion zone with sinking bulk solids, and a splash zone located above the dense bed. In the emulsion zone, the axial motion of the fuel particle is described by a force balance, applying a viscoplastic stress model, i.e., with a dominant yield stress and only a minor contribution of the shear stress, using an empirical expression from the literature. In the lateral direction, the model is divided into so-called ‘recirculation cells’, which are crucial for the lateral mixing.Comparisons of the modeled and measured lateral dispersion coefficients of different fuel types measured in three different large-scale fluidized bed units under both hot and cold conditions (covering a broad range of coefficients: 10−4–10−1 m2/s) reveal satisfactory agreement. The validated model was used to investigate how the lateral mixing of fuel particles depends on the excess gas velocity, the bed height, and the lateral distribution of bubbles over the bed cross-section (which is typically uneven in industrial FB furnaces), as well as the size and density of the fuel particles.
Highlights
Fluidized bed (FB) units are applied as industrial chemical reactors due to their ability to convert low-quality solid fuels with high effi ciency, while maintaining low levels of emissions
Fuel 305 (2021) 121424 well as in 3-dimensional (3D) beds [23,24,25,26,27,28] using a large variety of experimental techniques to track solids tracers. These studies have enhanced our understanding of the axial mixing of fuel-like particles, revealing fuel mixing patterns that are strongly coupled to the bubble flow. While all of these studies have been performed in cold laboratoryscale units, the application of Magnetic Particle Tracking (MPT) in a fluid-dynamically downscaled FB by the authors of this work [29,30] has enabled the simulation of the hot large-scale conditions that are prevalent in FBs used for fuel conversion
The model is used to analyze how the lateral mixing of fuel particles depends on the excess gas velocity, the bed height, and the lateral distribution of bubbles over the bed cross-section
Summary
Fluidized bed (FB) units are applied as industrial chemical reactors due to their ability to convert low-quality solid fuels with high effi ciency, while maintaining low levels of emissions. These studies have enhanced our understanding of the axial mixing of fuel-like particles, revealing fuel mixing patterns that are strongly coupled to the bubble flow While all of these studies have been performed in cold laboratoryscale units, the application of Magnetic Particle Tracking (MPT) in a fluid-dynamically downscaled FB by the authors of this work [29,30] has enabled the simulation of the hot large-scale conditions that are prevalent in FBs used for fuel conversion. The larger and lighter fuel particles are more likely to segregate axially and float on top of the bed surface [35,36,37,38], depending on the gas velocity and the extension of the splash zone above the dense bed Such segregation usually enhances lateral mixing, it still is typically identified as a limiting factor for the performance of large-scale fluidized bed units [31]. The work has been carried out in collaboration with a major boiler manufacturer, in which a holistic semiempirical model of FB combustion, including fuel mixing, is used for design purposes of industrial FB applications
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