Solids Flow Characterization in Bubbling Fluidized Beds with Induced Horizontal Circulation
Doktorsavhandling, 2025
Bubbling fluidized beds with horizontal solids crossflow can be broadly grouped according to function: 1) those designed for solids looping, as in indirect pyrolysis or gasification, and chemical or calcium looping; and 2) those employed in high-throughput solids processing, such as drying, iron ore reduction, and pharmaceutical manufacturing. Despite their widespread industrial application and the growing demand for their use in new process designs, there remains a lack of a detailed understanding of the flow characteristics in fluidized beds with horizontal solids throughput. This knowledge gap represents a major challenge for optimizing reactor design and advancing industrial implementation.
This thesis aims to develop a mechanistic understanding of how forced horizontal convection of solids in a bubbling fluidized bed influences the solids flow characteristics. The main research objectives are to: (i) assess experimental methods for quantifying the solids circulation rate; (ii) evaluate the efficiencies of different mechanisms to induce horizontal convection of solids; (iii) characterize bed solids transport—specifically, the interrelated effects of solids convection and dispersion—and the resulting fluidization quality; (iv) investigate how the solids crossflow influences overall flow structures; (v) examine the influences of frictional losses on bed solids flow, including the rheological properties of the dense suspension; and (vi) explore how the solids crossflow affects the mixing of a secondary solids phase consisting of large, light particles.
Experiments were conducted in a cold-flow model that comprised a closed-loop system in which solids were circulated horizontally via a solids-conveying module. The apparatus was designed and operated according to Glicksman’s simplified scaling laws. In this cold model, fine bronze particles are fluidized with ambient air to fluid-dynamically resemble conditions representative of industrial-scale thermochemical fuel conversion applications. The industrial unit being modeled features a bed channel with a cross-sectional width of 0.92 m and a transport loop length of 10.35 m, in which coarse, sand-like (Geldart B-type) particles are fluidized with flue gas at approximately 800°C.
Four measurement methods for quantifying solids circulation were evaluated—namely, integral mass accumulation, differential mass accumulation, thermal tracing, and magnetic solids tracing—with the latter proving to be the most precise and robust. Using this method, five solids-conveying configurations based on different fluid-dynamical mechanisms were tested: (a) free solids splashing, which relies on bubble bursts to eject particles; (b) confined solids splashing, whereby turbulent fluidization creates particle transport that is dominated by eddies and bubble buoyancy; (c) slugging, whereby gas slugs drive particle movement in vertical ducts; (d) solids entrainment, which is achieved by elutriation at high gas velocities; and (e) directed gas injection, which imparts lateral momentum through angled nozzles. Conveying solids under a controlled bubbling fluidization regime was found to be the most efficient configuration for promoting horizontal transport of solids in the system.
The horizontal solids flow established in the channel was evaluated under various operational conditions using a combination of experimental and modeling approaches, ranging from reduced-order descriptions to Eulerian–Eulerian computational fluid dynamics (CFD) simulations. A positive linear relationship was observed between the solids dispersion coefficient and the mean solids velocity, both evaluated in the streamwise direction. This was further explained by the CFD simulations, which indicated that at low crossflow rates, the solids flow organizes into coherent, counter-rotating vortices along the bubble paths. In contrast, at high crossflow rates, these structures are disrupted, resulting in less streamlined (stronger mixing) and more elongated (longer characteristic lengths) flow patterns. In addition, microscale dispersion was found to be dominated by bubble- and eddy-induced mixing, rather than by random particle motion or collisions. Rheological analysis revealed that the bed exhibits shear-thinning behavior and that single-phase models for non-circular open-flow channels underestimate the influence of geometry on gas-solids flows.
Analysis of the transport behavior of large, light particles added to the bed as a lean phase showed that, similar to bulk solids, a positive correlation exists between dispersion and convection, with sensitivity strongly dependent on the degree of fluidization. The dispersion of the lean particles was greater than that of the bulk solids; it remained at a similar level for both low and high fluidization velocities. However, compared to the bulk solids, the lean particles were transported at lower convection velocities under high fluidization levels. In contrast, at low fluidization levels, the particles formed a layer above the dense bed and were conveyed in a plug-like manner, exhibiting much higher horizontal velocities than the bulk solids.
This thesis aims to develop a mechanistic understanding of how forced horizontal convection of solids in a bubbling fluidized bed influences the solids flow characteristics. The main research objectives are to: (i) assess experimental methods for quantifying the solids circulation rate; (ii) evaluate the efficiencies of different mechanisms to induce horizontal convection of solids; (iii) characterize bed solids transport—specifically, the interrelated effects of solids convection and dispersion—and the resulting fluidization quality; (iv) investigate how the solids crossflow influences overall flow structures; (v) examine the influences of frictional losses on bed solids flow, including the rheological properties of the dense suspension; and (vi) explore how the solids crossflow affects the mixing of a secondary solids phase consisting of large, light particles.
Experiments were conducted in a cold-flow model that comprised a closed-loop system in which solids were circulated horizontally via a solids-conveying module. The apparatus was designed and operated according to Glicksman’s simplified scaling laws. In this cold model, fine bronze particles are fluidized with ambient air to fluid-dynamically resemble conditions representative of industrial-scale thermochemical fuel conversion applications. The industrial unit being modeled features a bed channel with a cross-sectional width of 0.92 m and a transport loop length of 10.35 m, in which coarse, sand-like (Geldart B-type) particles are fluidized with flue gas at approximately 800°C.
Four measurement methods for quantifying solids circulation were evaluated—namely, integral mass accumulation, differential mass accumulation, thermal tracing, and magnetic solids tracing—with the latter proving to be the most precise and robust. Using this method, five solids-conveying configurations based on different fluid-dynamical mechanisms were tested: (a) free solids splashing, which relies on bubble bursts to eject particles; (b) confined solids splashing, whereby turbulent fluidization creates particle transport that is dominated by eddies and bubble buoyancy; (c) slugging, whereby gas slugs drive particle movement in vertical ducts; (d) solids entrainment, which is achieved by elutriation at high gas velocities; and (e) directed gas injection, which imparts lateral momentum through angled nozzles. Conveying solids under a controlled bubbling fluidization regime was found to be the most efficient configuration for promoting horizontal transport of solids in the system.
The horizontal solids flow established in the channel was evaluated under various operational conditions using a combination of experimental and modeling approaches, ranging from reduced-order descriptions to Eulerian–Eulerian computational fluid dynamics (CFD) simulations. A positive linear relationship was observed between the solids dispersion coefficient and the mean solids velocity, both evaluated in the streamwise direction. This was further explained by the CFD simulations, which indicated that at low crossflow rates, the solids flow organizes into coherent, counter-rotating vortices along the bubble paths. In contrast, at high crossflow rates, these structures are disrupted, resulting in less streamlined (stronger mixing) and more elongated (longer characteristic lengths) flow patterns. In addition, microscale dispersion was found to be dominated by bubble- and eddy-induced mixing, rather than by random particle motion or collisions. Rheological analysis revealed that the bed exhibits shear-thinning behavior and that single-phase models for non-circular open-flow channels underestimate the influence of geometry on gas-solids flows.
Analysis of the transport behavior of large, light particles added to the bed as a lean phase showed that, similar to bulk solids, a positive correlation exists between dispersion and convection, with sensitivity strongly dependent on the degree of fluidization. The dispersion of the lean particles was greater than that of the bulk solids; it remained at a similar level for both low and high fluidization velocities. However, compared to the bulk solids, the lean particles were transported at lower convection velocities under high fluidization levels. In contrast, at low fluidization levels, the particles formed a layer above the dense bed and were conveyed in a plug-like manner, exhibiting much higher horizontal velocities than the bulk solids.
Bubbling fluidized bed
Solids mixing
Magnetic solids tracing
Lagrangian particle tracking
Frictional loss
Eulerian-Eulerian model
Solids convection
Författare
Munavara Farha
Chalmers, Rymd-, geo- och miljövetenskap, Energiteknik
Assessment of experimental methods for measurements of the horizontal flow of fluidized solids under bubbling conditions
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Characterization of the solids crossflow in a bubbling fluidized bed
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Farha, M., Ström, H., Guío-Pérez, D. C., & Pallarès, D. Spatially-resolved characterization of the flow structure in a bubbling fluidized bed with solids crossflow.
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Horizontally Conveyed Fluidized Beds: A Technological Solution for Poly-Generation
Fluidization is a process where solid particles are suspended and behave almost like a fluid when a gas or liquid flows upward through them. This principle underpins countless modern industrial operations, from chemical manufacturing and pharmaceuticals to food processing and, especially, energy production. In thermochemical conversion, fluidized bed reactors are highly valued for their ability to handle diverse solid fuels (including municipal waste, biomass, plastics, sewage sludge, etc) while ensuring thorough mixing and uniform temperature, which are crucial for efficient combustion, gasification, and pyrolysis. As energy systems shift toward cleaner solutions and circular production chains, new process concepts such as poly-generation are gaining traction. Poly-generation aims to extract not just heat and electricity, but also valuable chemicals, green fuels, and biochar within a single integrated plant. Achieving this in practice, however, requires advanced reactor designs and a deep understanding of how solid particles, such as sand or biomass, actually move and mix within these systems. If particle movement isn’t well controlled, the process can become inefficient, leading to issues such as hot spots or even equipment failure.
This thesis explores the movement of solid particles in a horizontally conveyed bubbling fluidized bed: a reactor design that has received less attention but offers promising potential for better control over solids residence time in the system. Several measurement techniques for tracking particle circulation are tested, including a newly developed magnetic tracing method that proves especially accurate. Different mechanisms for driving solids flow are also evaluated, including particle splashing from bubble bursts, gas jets, and high-velocity entrainment, to determine which approach moves solids most effectively. Compared to the conventional riser configuration, which relies on high-velocity gas to lift solids vertically, the studied system can deliver comparable solids circulation while requiring far less energy to operate. Experiments combined with advanced modeling techniques were used to reveal how particles move and spread, and how characteristic flow structures form as solids are pushed through the bed. The study highlights how the quality of fluidization directly determines the extent of stagnant regions in the bed, which, at an industrial scale, can cause severe temperature maldistribution and incomplete fuel conversion. Further, this work provides a method to estimate frictional losses based on the established solids flow rate, offering a practical foundation for effective control of energy consumption and equipment longevity.
After establishing a detailed understanding of how the main bed material moves, the research also examines what happens when larger, lighter particles resembling biomass fuel are introduced. The findings show that these particles can behave quite differently from the bulk solids: under high fluidization, they mix efficiently with the rest of the bed, but at lower gas velocities, they tend to accumulate near the surface and travel faster across the reactor. This behavior highlights the need to tailor operating conditions to process requirements, enabling the system to switch between thorough mixing (for complete conversion) and rapid particle separation (when needed for specific products or fuels).
By clarifying these fundamental processes, this research provides practical guidance for designing and upgrading existing fluidized bed systems, enabling more efficient operation and greater flexibility in adapting to alternative renewable fuels.
Fluidization is a process where solid particles are suspended and behave almost like a fluid when a gas or liquid flows upward through them. This principle underpins countless modern industrial operations, from chemical manufacturing and pharmaceuticals to food processing and, especially, energy production. In thermochemical conversion, fluidized bed reactors are highly valued for their ability to handle diverse solid fuels (including municipal waste, biomass, plastics, sewage sludge, etc) while ensuring thorough mixing and uniform temperature, which are crucial for efficient combustion, gasification, and pyrolysis. As energy systems shift toward cleaner solutions and circular production chains, new process concepts such as poly-generation are gaining traction. Poly-generation aims to extract not just heat and electricity, but also valuable chemicals, green fuels, and biochar within a single integrated plant. Achieving this in practice, however, requires advanced reactor designs and a deep understanding of how solid particles, such as sand or biomass, actually move and mix within these systems. If particle movement isn’t well controlled, the process can become inefficient, leading to issues such as hot spots or even equipment failure.
This thesis explores the movement of solid particles in a horizontally conveyed bubbling fluidized bed: a reactor design that has received less attention but offers promising potential for better control over solids residence time in the system. Several measurement techniques for tracking particle circulation are tested, including a newly developed magnetic tracing method that proves especially accurate. Different mechanisms for driving solids flow are also evaluated, including particle splashing from bubble bursts, gas jets, and high-velocity entrainment, to determine which approach moves solids most effectively. Compared to the conventional riser configuration, which relies on high-velocity gas to lift solids vertically, the studied system can deliver comparable solids circulation while requiring far less energy to operate. Experiments combined with advanced modeling techniques were used to reveal how particles move and spread, and how characteristic flow structures form as solids are pushed through the bed. The study highlights how the quality of fluidization directly determines the extent of stagnant regions in the bed, which, at an industrial scale, can cause severe temperature maldistribution and incomplete fuel conversion. Further, this work provides a method to estimate frictional losses based on the established solids flow rate, offering a practical foundation for effective control of energy consumption and equipment longevity.
After establishing a detailed understanding of how the main bed material moves, the research also examines what happens when larger, lighter particles resembling biomass fuel are introduced. The findings show that these particles can behave quite differently from the bulk solids: under high fluidization, they mix efficiently with the rest of the bed, but at lower gas velocities, they tend to accumulate near the surface and travel faster across the reactor. This behavior highlights the need to tailor operating conditions to process requirements, enabling the system to switch between thorough mixing (for complete conversion) and rapid particle separation (when needed for specific products or fuels).
By clarifying these fundamental processes, this research provides practical guidance for designing and upgrading existing fluidized bed systems, enabling more efficient operation and greater flexibility in adapting to alternative renewable fuels.
Termokemisk samproduktion i kraftvärmesektorn
Energimyndigheten (51182-1), 2020-11-02 -- 2024-06-28.
Drivkrafter
Hållbar utveckling
Ämneskategorier (SSIF 2025)
Strömningsmekanik
Energiteknik
Styrkeområden
Energi
Infrastruktur
Chalmers kraftcentral
DOI
10.63959/chalmers.dt/5801
ISBN
978-91-8103-344-1
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5801
Utgivare
Chalmers
Hörsalsvägen 4, Lecture Hall - HA1 (Room 110)
Opponent: Prof. Ruud van Ommen, Full Professor of Chemical Engineering at Delft University of Technology (TU Delft), The Netherlands