Mixing of Large Solids in Fluidized Beds
Doctoral thesis, 2021

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.

fuel mixing

fluidized beds

magnetic particle tracking


semiempirical modelling

fluid-dynamic downscaling

Online (Zoom)
Opponent: Naoko Ellis, Professor in Chemical and Biological Engineering, University of British Columbia, Canada


Anna Köhler

Chalmers, Space, Earth and Environment, Energy Technology

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

Energy & Fuels,; Vol. 34(2020)p. 3294-3304

Journal article

Modeling the motion of fuel particles in a fluidized bed

Fuel,; Vol. 305(2021)

Journal article

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

Fuel Processing Technology,; Vol. 162(2017)p. 147-156

Journal article

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

Powder Technology,; Vol. 393(2021)p. 510-518

Journal article

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

Canadian Journal of Chemical Engineering,; Vol. 101(2023)p. 210-226

Journal article

In light of the current climate emergency, with human activities causing the depletion of resources, accumulation of waste, and global temperature increase, the need for drastic reductions of human-caused carbon dioxide (CO2) emissions through transitioning energy and land use to greater sustainability is unavoidable if we want to stay within the 1.5°C limit for global warming. The heat and power and the industry sectors account for 65% of the global greenhouse gas (GHG) emissions, with the vast majority of CO2 originating from thermal conversion of fossil fuels, such as coal through combustion, to provide energy for the production of heat and electricity, the cracking of crude oil into lighter petrochemicals, and the production of cement, steel and fertilizers.

In the last decades, tremendous efforts have been made to reduce the CO2 emissions of these processes, which can be achieved by either capturing the CO2 in the conversion process to hinder it from reaching the atmosphere, or by a shift from fossil to carbon-neutral fuels, such as sustainably managed biomass or bio-based wastes. While the capturing of CO2 is comprised of high additional energy costs and the shift of fuel is challenging and associated with costly fuel preparation for most conversion processes, fluidization is a technology that has a high potential to provide process solutions without major energy penalties.

Fluidization is the dynamic state that develops as gas is passed through a bed of small solid particles (e.g. sand, catalytic powder). As the particles are lifted by the gas, they form a state, in which they behave like a liquid and can be transported, mixed with other particles (e.g. the fuel), and heated more easily. These features make it possible to convert difficult fuels, such as wet biomass and waste, which come in a great variety of sizes and shapes, without expensive fuel preparation. When used with certain bed materials, fluidized beds can be used to capture CO2 inherently in the process, as applied to heat and power plants, and in energy-intensive cement production.

In fluidized bed processes, the mixing of the fuel particles in the bed of small solids is of major importance, as it determines the contact of the fuel with its reacting gasses and the heat supply to the fuel particle. By this, the mixing affects the rate of conversion and the amount of energy released, and therefore, determines the performance of the whole process. Thus, when designing fluidized bed processes or develop new applications, a good understanding of the mixing of the fuel particles in the bed, i.e., the parameters affecting the mixing and the underlying mechanisms is crucial.

This thesis presents a combination of a mathematical model and experimental input data, which together provide a mechanistic description of the motion of a fuel particle in a fluidized bed. Validating the model using experimental data from industrial-scale beds, showed that the model can predict the mixing of the fuel particles in conditions relevant for commercial-sized units well. The model was used to study the influence of operational conditions, and properties of bed material and fuel particles on the mixing of the fuel.

With the applicability of the model for industrial-scale fluidized beds, the findings of this work can be used to improve the design process of existing as well as future fluidized bed applications. The knowledge obtained can enhance the shift to more carbon-neutral fuels and the development of processes used for capturing CO2.

Gasification of Biomass

Chalmers, 2019-01-01 -- .

Subject Categories

Mechanical Engineering

Energy Engineering

Chemical Engineering



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4976



Online (Zoom)


Opponent: Naoko Ellis, Professor in Chemical and Biological Engineering, University of British Columbia, Canada

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