Mixing of Large Solids in Fluidized Beds

Fluidization is a technology that is widely used in systems in which particulate solids are to be transported, mixed, and/or reacted with gases. In fluidized bed applications, the lateral mixing rate of the solids and the heat and mass transfer with their surroundings play important roles in process performance. These transport mechanisms are affected by the solids axial mixing, as particles immersed in the dense bed will experience higher levels of heat transfer, lower mass transfer, and lower rates of lateral mixing than they would if floating on the bed surface. However, there is a lack of knowledge regarding the effects of the solids properties and operating conditions on the solids mixing. As a consequence, there is a lack of predictive tools that can be used for optimizing the design and operation of fluidized beds.This work focuses on advancing the current understanding of the mixing of large solids (typically fuels) in fluidized beds, with the aims of promoting the design of new applications and improving the scale-up and operation of commercial units. While a generic approach is adopted in terms of considering a wide range of solid particle properties (size and density), the focus is on biomass particles, for which thermochemical conversion fluidized beds are especially suited, due to their: high-level fuel flexibility (being able to convert efficiently low-grade fuels); ability to control emissions with in-bed methods; and inherent capability to capture CO2 with looping dual fluidized bed systems. This work combines semiempirical modeling with experiments that apply magnetic particle tracking in a fluid-dynamically downscaled bed, enabling the closure as well as the validation of the model. By deriving a mechanistic description of the motion of a spherical object, the model identifies key parameters that are crucial for describing the mixing. Among these, the effective drag of the bed emulsion acting on the fuel particle is further studied in dedicated experiments with falling and rising tracers in various types of beds at minimum fluidization. The stress patterns observed in these rheological experiments reveal a non-Newtonian behavior of the drag between the bed emulsion and immersed larger objects. This is then implemented in the model for further upgrading of the mechanistic description. The model is shown to describe ably both axial mixing and the lateral mixing of different fuel types under conditions applicable to industrial-scale hot units.The combination of modeling and experimental work shows that while axial mixing is fostered by increasing the fluidization velocity, bed height, distributor pressure drop, or fuel particle density and decreasing the fuel particle size, only a higher fluidization velocity exerts a clear influence on the lateral dispersion. This can be explained in terms of the influence of the fluidization velocity on the width of recirculation cells, which are found to play a major role in the lateral mixing of fuel particles and warrant further study

Axial Mixing of Large Solids in Fluidised Beds – Modelling and Experiments

Fluidisation is a technology commonly found wherever particulate solids are to be transported, mixed and/or reacted with a gas. At present, it is a widespread technology with applications ranging from the production of carbon nanotubes in the manufacturing industry to the conversion of solid fuels in the heat and power sector. As for the latter, fluidised beds are well received for their fuel flexibility (being able to efficiently convert low-grade fuels) and for their ability to control emissions with in-bed methods. In most applications, like solid fuel conversion, the heat and mass transfer between the gas and the solids (e.g.\ua0fuel particles) play an important role in the process performance. In turn, these transfer mechanisms are affected by the axial solids mixing, as solids immersed in the dense bed will experience higher heat transfer and lower mass transfer than otherwise. This work focuses on the axial mixing of large solids in fluidised beds with the aim to advance current knowledge on in-bed mixing with an emphasis on biomass particles. As the latter typically have a high content of moisture, volatile and ash and are larger and lighter than conventional fuels like e.g. coal or lignite, they are even more prone to segregate axially in the bed in a flotsam fashion. Yet, the effect of fuel density and size as well as the effect of fluidisation conditions on the axial mixing of fuel has not been fully understood. To enhance the understanding of solids mixing, this work combines a one-dimensional semi-empirical model with experiments applying magnetic particle tracking (MPT) in a fluid-dynamically down-scaled fluidised bed. The model is used to identify governing mechanisms and the respective key parameters to be studied with dedicated experiments which, in their turn, contribute to the continuous upgrading of the model.The key parameters in the axial mixing of larger solids in a fluidised bed are found to be: i) the apparent viscosity of the emulsion, for which MPT measurements confirmed its Newtonian character, and ii) the bubble flow, which experiments revealed to have a higher upwards velocity and fuel-to-bubble velocity ratio than shown in previous literature not accounting for hot conditions

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

A semiempirical model for the axial mixing of fuel particles in the dense region of a fluidized bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamically downscaled fluidized bed (K\uf6hler et al. Powder Technol., 2017, 316, 492-499) that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solid 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 upward 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 hot large-scale conditions. When applying values measured in a fluid-dynamically downscaled fluidized bed (K\uf6hler et al. Powder Technol., 2017, 316, 492-499), the modeled axial mixing of fuel tracers shows good agreement with the experimental data.\ua0\ua9 2020 American Chemical Society

Rheological effects of a gas fluidized bed emulsion on falling and rising spheres

To enable the mechanistic description of the mixing of larger particles in gas-fluidized beds in models (e.g. fuel particles in combustors), knowledge about the rheology of the bed emulsion is required. Here, it is crucial to determine the drag on large fuel-alike particles. This work presents the experimental work on the fate of 13 different solid spheres falling or rising through a bed of air and glass beads at minimum fluidization. The trajectories of the tracer are highly resolved (sampling rate of 200 Hz) by means of magnetic particle tracking, this previously unmet accuracy allows disclosing the complex rheological behavior of gas-solids fluidized bed emulsions in terms of drag on immersed objects. The trajectories reveal that none of the tracers reach terminal velocity during their fall and rise through the bed. The shear stress is obtained through the drag force by solving the equation of motion for the tracer. The data reveal particularities of the bed rheology and clear differences of its effect on rising and falling particles. When studying the shear stress over the characteristic shear rate of each tracer, it can be seen that the stress of the bed on the tracers is dominated by a yield stress, with a somewhat smaller contribution of the shear stress. For rising tracers this last contribution is almost negligible. The falling tracers show strong interaction with the bed emulsion, resulting in a fluctuating shear stress, which increases with tracer size and density. The stagnation of some tracers at low shear rates reveals a viscoplastic behavior of the bed emulsion, exhibiting a typical yield stress that showing a clear dependence on the tracer diameter and buoyant density. The concept of yield gravity is used in order to introduce a normalized shear stress which provides additional verification of the experimental observations in relation to the influence of tracer size and relative density on the shear stress

Effective drag on spheres immersed in a fluidized bed at minimum fluidization—Influence of bulk solids properties

The aims of this work are to elucidate the effects that bulk solids properties have on the effective drag experienced by large spheres immersed in an emulsion of group-B solids under minimum fluidization conditions and to analyze the ways in which the different suspensions react towards different applied shear rates. To investigate this, magnetic particle tracking was applied to resolve the trajectory of falling-sphere measurements in which the size, density, and sphericity of the bulk solids were varied as well as the size and density of the spherical tracers. The resulting experimental scope included both rising and sinking tracers as well as full segregation and in-bed stagnation of the tracers. The set-up provided highly resolved tracer trajectories, from which the drag experienced by the sphere can be calculated. For sinking tracers, the results showed that an increase in bulk solids size, angularity, and density reduced the terminal velocity of the sphere. This effect correlated well with the bed expansion and Hausner ratio, indicating that a reduced void space among the bulk solids is the main reason for the increase in motion resistance. At lower shear rates, namely, during the de-acceleration towards the stagnant state, beds of larger, more angular, or denser bulk solids yield lower levels of shear stress. The angle of repose of the bulk solids correlated with the rate at which the emulsion thins with increasing shear rate. For rising tracers, shear stress did not show any significant dependency on the properties of the bulk solids

Determination of the Apparent Viscosity of Dense Gas-Solids Emulsion by Magnetic Particle Tracking

When designing fluidised bed units a key to ensure efficient conversion is proper control of the mixing of the fuel in both lateral and axial directions in the bed. In order to mechanistically describe the mixing of fuel particles in a fluidised bed, there is a need to determine the apparent viscosity of thegas-solids emulsion, which determines the drag on the fuel particles. In this work the apparent viscosity of a bed of spherical glass beads and air at minimum fluidisation was determined by means of the falling sphere method. Hereto the drag of the bed on a single immersed object was obtained by measuring the velocity of a negatively buoyant tracer with magneticparticle tracking (MPT). MPT allows for highly temporally and spatially resolved trajectories (10-3 s and 10-3 m, respectively) in all 3-dimensions. The bed consisted of glass beads with a narrow size distribution (215 to 250 μm) and tracers with a size from 5 to 20 mm and densities from 4340 to 7500kg/m3 were used. Hence, the literature, which typically covers data for velocities lying within or just above the Stoke flow regime (0.002 < Re < 2.0) could be expanded to Re numbers (53 to 152) well within the transition flow regime. The drag and apparent viscosity was compared to different fluidmodels and agreed well with the Newtonian model, when taking into account possible effects of the bed walls. Comparing the drag coefficient of data of free falling spheres and data of spheres falling with controlled velocities, the latter showed a dependence on the product of tracer diameter andfalling velocity, dput, while the former was constant over dput. This indicates the method with controlled falling velocities to be intrusive and influencing the result of the apparent viscosity of the bed. Using the free falling sphere method this work obtained an apparent viscosity of 0.24 Pa s, which isconsistent with values found in earlier literature for an emulsion of air and sand of similar size and density

Magnetic tracking of a fuel particle in a fluid-dynamically down-scaled fluidised bed

The mixing of a fuel particle in a fluid-dynamically down-scaled bubbling fluidised bed was studied using magnetic particle tracking. Both the resulting steady-state fuel distributions and the underlying mixing dynamics (fuel velocity field) were investigated. The experimental set-up applied resembles the mixing of an anthracite coal particle in a bed with a cross-section of 0.85\ua0 7\ua00.85\ua0m2 operated at 900\ua0\ub0C with fluidisation velocities in the range of 0.16–0.45\ua0m/s and bed heights in the range of 0.25–0.35\ua0m. Four different gas distributors with variable pressure drops and orifice configurations were investigated. For the cases studied, 7.5\ua0min of sampling time at a sampling frequency of 20\ua0Hz was found to be sufficient to resolve the spatial distribution of the tracer. However, to provide a reliable estimate of the mixing dynamics, a sampling frequency of at least 100\ua0Hz was required, together with a sampling time of approximately 20\ua0s. Results on axial mixing showed improved mixing with increasing fluidisation velocity and bed height. The lateral dispersion coefficients were in the order of 10−\ua03–10−\ua02\ua0m2/s (on an up-scaled basis), increased with fluidisation velocity, and were only moderately influenced by the configuration of the gas distributor

Modelling axial mixing of char - application to the dense bottom bed in CFB boilers

This work presents a semi-empirical two-phase model for the axial mixing of a spherical tracer particle, aiming to represent a char particle, with application to conditions relevant for the dense bottom bed flow in CFB boilers. The velocity fields of both the bubble and emulsion phases are modelled with validated expressions from literature, while the velocity of the tracer particle in the emulsion phase is obtained from the equation of motion. A correlation for the drag force created by the bed and acting on the tracer particle is found with the help of experimental data from fluid-dynamically down-scaled tests (K\uf6hler et al, 2017). The model is used to predict trends in axial mixing with operational parameters (fluidization velocity, dense bed height) and tracer properties (size and density), which agree well with experimental findings in literature

Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking

A magnetic particle tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74 70.74 m^2 fluidized bed reactor operating at 800\ub0C. This work uses a tracer particle of 10 mm in diameter, corresponding to a fuel particle of 44 mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10^-3 m and a time resolution of 10^-3 s.For the operational range investigated, three fuel segregation regimes can be identified from the MPT measurements: 1) A flotsam regime which occurs at low fluidization velocities and for low density tracer particles, 2) A transition regime over which an increase in fluidization velocity results in the presence of fuel particles at the bed surface decreases rapidly, and 3) A fully developed mixing regime in which the presence of tracer particle at the bed surface and the splash zone remains constant with fluidization velocity. The transition velocities between the regimes depend on bed height and density of the tracer particle

3-dimensional particle tracking in a fluid-dynamically downscaled fluidized bed using magneto resistive sensors

This paper presents a measurement technique for continuous tracking of particles in 3-dimensional bubbling fluidized beds operated according to scaling laws. By applying Glicksman’s full set of scaling laws to both bulk solids and tracer particle the bed is assumed to be fluid-dynamically similar to a combustor operated at 900 \ub0C with the tracer particle corresponding to a fuel particle with properties similar to anthracite coal. Two different gas distributors with varying pressure drop are used to investigate the influence of bed design on fuel mixing.Flow structures formed around rising gas bubbles, so called bubble paths, are identified and the tracer particle traverses the entire bed for a gas distributor yielding a high pressure drop. For a gas distributor yielding a low pressure drop flow structures are less pronounced and the tracer particle is not circulating the entire bed