CFD Modeling and X-Ray Imaging of Biomass in a Fluidized Bed

Computational modeling of fluidized beds can be used to predict the operation of biomass gasifiers after extensive validation with experimental data. The present work focused on validating computational simulations of a fluidized bed using a multifluid Eulerian–Eulerian model to represent the gas and solid phases as interpenetrating continua. Simulations of a cold-flow glass bead fluidized bed, using two different drag models, were compared with experimental results for model validation. The validated numerical model was then used to complete a parametric study for the coefficient of restitution and particle sphericity, which are unknown properties of biomass. Biomass is not well characterized, and so this study attempts to demonstrate how particle properties affect the hydrodynamics of a fluidized bed. Hydrodynamic results from the simulations were compared with X-ray flow visualization computed tomography studies of a similar bed. It was found that the Gidaspow (blending) model can accurately predict the hydrodynamics of a biomass fluidized bed. The coefficient of restitution of biomass did not affect the hydrodynamics of the bed for the conditions of this study; however, the bed hydrodynamics were more sensitive to particle sphericity variation

Bubble formation from a flexible hole submerged in an inviscid liquid

In the waste water treatment industry, a novel gas sparger based on flexible membranes has been used for the last ten years. The objective of the present work is to study the bubble formation generated from a flexible orifice (membrane). Firstly, the membranes are characterised with regard to their properties: wetting critical surface tension, expanding hole diameter, orifice coefficients, flexibility, critical and elastic pressures. The bubble formation phenomenon in an inviscid liquid at rest is studied experimentally for different membranes and gas flow rates. The variation in the bubble diameter, the bubble centre of gravity and the bubble spread on the membrane are determined as a function of time. An analytic model is proposed to describe the bubble growth and its detachment at a flexible orifice. This theoretical approach, developed by Teresaka & Tsuge (1990) for rigid orifices, is adapted to take into account the membrane features (elastic behaviour and wettability). The predicted bubble diameters at detachment agree with the experimental measurements; however, the model underestimates slightly the bubble formation times. The calculation of the various forces acting on the bubble in the vertical direction indicates that the real forces governing the bubble growth are the buoyancy force, the surface tension force, and near detachment the inertial force

Effects of Mixing Using Side Port Air Injection on a Biomass Fluidized Bed

Fluidized beds are being used in practice to gasify biomass to create producer gas, a flammable gas that can be used for process heating. However, recent literature has identified the need to better understand and characterize biomass fluidization hydrodynamics, and has motivated the combined experimental-numerical effort in this work. A cylindrical reactor is considered and a side port is introduced to inject air and promote mixing within the bed. Comparisons between the computational fluid dynamics (CFD) simulations with experiments indicate that three-dimensional simulations are necessary to capture the fluidization behavior of the more complex geometry. This paper considers the effects of increasing side port air flow on the homogeneity of the bed material in a 10.2 cm diameter fluidized bed filled with 500-600 μmground walnut shell particles. The use of two air injection ports diametrically opposed to each other is also modeled using CFD to determine their effects on fluidization hydrodynamics. Whenever possible, the simulations are compared to experimental data of time-average local gas holdup obtained using X-ray computed tomography. This study will show that increasing the fluidization and side port air flows contribute to a more homogeneous bed. Furthermore, the introduction of two side ports results in a more symmetric gas-solid distribution

Localized fluidization in granular materials: Theoretical and numerical study

We present analytical and numerical results on localized fluidization within a granular layer subjected to a local injection of fluid. As the injection rate increases the three different regimes previously reported in the literature are recovered: homogeneous expansion of the bed, fluidized cavity in which fluidization starts developing above the injection area, and finally the chimney of fluidized grains when the fluidization zone reaches the free surface. The analytical approach is at the continuum scale, based on Darcy's law and Therzaghi's effective stress principle. It provides a good description of the phenomenon as long as the porosity of the granular assembly remains relatively homogeneous, i.e. for small injection rates. The numerical approach is at the particle scale based on the coupled DEM-PFV method. It tackles the more heterogeneous situations which occur at larger injection rates. The results from both methods are in qualitative agreement with data published independently. A more quantitative agreement is achieved by the numerical model. A direct link is evidenced between the occurrence of the different regimes of fluidization and the injection aperture. While narrow apertures let the three different regimes be distinguished clearly, larger apertures tend to produce a single homogeneous fluidization regime. In the former case, it is found that the transition between the cavity regime and the chimney regime for an increasing injection rate coincides with a peak in the evolution of inlet pressure. Finally, the occurrence of the different regimes is defined in terms of the normalized flux and aperture

Hydrodynamic Characterization of 3D Fluidized Beds Using Noninvasive Techniques

Fluidized beds are useful processing systems that are employed by many industries for their relatively unique operating properties. Low pressure drops, uniform temperature distributions, and high heat/mass transfer rates occur through the action of vertical gas injection into a column of solid particles. Although these properties give fluidized beds great advantages over other processing systems, the hydrodynamic characterization of fluidized beds is important for the efficient processing of many consumer products. However, fluidized bed hydrodynamics are difficult to visualize and quantify because most fluidized beds are opaque. Traditionally, the monitoring of local fluidized bed hydrodynamics has been done with intrusive probes that disturb local structure and the collection of data over large areas is time consuming. X-ray computed tomography (CT), as a noninvasive technique, can quantify local time-average phase fractions in highly dynamic multiphase systems without disturbing local structure. Using X-ray visualization techniques, methods have been developed in this study to: 1) test the repeatability of calculating local time-average gas holdup values using X-ray CTs; 2) find the fluidization uniformity of a non-reactive cold-flow fluidized bed; 3) compare local time-average gas holdup values in various bed materials, diameters, and operating conditions; and 4) compare annular hydrodynamic structures within the beds. Tests for the first two objectives were completed using a 15.2 cm ID reactor, while varying between two bed materials (crushed walnut shell and glass beads) of the same size and two gas flow rates. The third objective used a 10.2 cm and 15.2 cm ID reactor, varied between three bed materials (ground corncob, crushed walnut shell, and glass beads) of the same size, and over four and five relative superficial gas velocities and side-air injection gas flow rates respectively. The fourth objectives mirrored the third, however, did not use side-air injection. Observations show that local time-average gas holdup values can be calculated through the use of multiple X-ray CTs. The method of calculation is shown to be highly repeatable over the various flow rates, bed materials used, and ambient environmental conditions. Axisymmetric fluidization uniformity of the bed is also confirmed using the same method, while some differences are observed with varying materials and flow rates. Uniformity is observed to increase with bed height and increased gas flow rates, due to increased dispersion of gas into the bed and mixing rates respectively. Local time-average gas holdup is observed to differ somewhat between reactors. However, the overall results show that the hydrodynamic structures, i.e. aeration jets, bubble coalescence zones, bubble rise zones, particle shearing zones, and the side-air injection plume, within the fluidized beds for each reactor are very similar. These structures coupled with axisymmetric fluidization uniformity indicate that gas flow and material circulation tend to be annular in shape. Moreover, changes in the shape, size, number, and location occur with changes in superficial gas velocity, bed diameter, and bed material density. It is also suspected that the aeration scheme of the bed and the bed material properties i.e. shape factor, coefficients of restitution, and porosity play a role in the development of these structures. The aeration jets are similar in length in all beds regardless of material density or bed diameter. They also tend to decrease in height and become increasingly wall leaning as superficial gas velocity increases. The coalescence of bubbles tends to occur in regular locations near the reactor wall just above the aeration jets within all beds regardless of material density, bed diameter, and gas flow rates. The rise paths of bubbles through all beds emanate from the coalescence zones with relatively small widths and increasing in width as bed height increases. Particle shear zones occur in differing size, shape, number outside of all other hydrodynamic structures while migrating around the bed with changing material density, bed diameter, and superficial gas velocity. The diffusion of gas into the fluidized bed from the side-air injection plume in each bed is similar, due to advection dominance within the plume. Gas dispersion does not seem to occur by similar means between materials though, because crushed corncob and ground walnut shell are natural systems and have a higher porosity and lower density than glass beads. The natural materials also have non-uniform shape factors causing behavior differences with the fluidization gas. The time-average bed height between bed diameters is different for each material density and gas flow rate, where the height in the 10.2 cm diameter reactor is observed to be greater on average in all tests than in the 15.2 cm reactor, due to wall effects. Lastly, the techniques used for analysis in this study are valuable to computational fluid dynamicists for direct comparison to simulation and models of fluidized beds

Simulation of Particle Mixing and Separation in Multi-Component Fluidized Bed Using Eulerian-Eulerian Method: A Review

In practical engineering applications, the mixing and separation behavior of multi-component particles is importance to the fluidized bed operation. The development of many practical processes is inseparable from the knowledge of particle mixing and separation, such as material processing of ash-soluble coal gasification, multi-phase flow in boilers, and petrochemical catalytic processes. In recent years, due to the obvious advantages of the Eulerian–Eulerian model, many researchers at home and abroad have used it to study the mixing and separation behavior of particles. The paper reviews the use of Eulerian–Eulerian model to study the mixing and separation of multi-component particles in fluidized beds. The Eulerian–Eulerian model describes the gas-phase and each of the individual particles as continuums. The mechanism of particle mixing and separation, the influence of different factors on the particle mixing and separation including differences in particle size and density, the differences in apparent air velocity, the differences in model factors are discussed. Finally, an outlook for the use of Eulerian–Eulerian model to study the mixing and separation behavior of three component particles and related research on the drag model between particles

Investigation of Void Fraction Schemes for Use with CFD-DEM Simulations of Fluidized Beds

© 2018 American Chemical Society. This paper investigates the spatial resolution of computational fluid dynamics-discrete element method (CFD-DEM) simulations of a bubbling fluidized bed for seven different void fraction schemes. Fluid grids with cell sizes of 3.5, 1.6, and 1.3 particle diameters were compared. The particle velocity maps from all of the void fraction schemes were in good qualitative agreement with the experimental data collected using magnetic resonance imaging (MRI). Refining the fluid grid improved the quantitative agreement due to a more accurate representation of flow near the gas distributor. The approach proposed by Khawaja et al. [ J. Comput. Multiphase Flows 2012, 4, 183-192 ] provided the closest match to the exact void fraction though only the particle centered method differed significantly. These results indicate that the fluid grid used for CFD-DEM simulations must be sufficiently fine to represent the inlet flow realistically and that a void fraction scheme such as that proposed by Khawaja be used

Sand-assisted fluidization of large cylindrical and spherical biomass particles: Experiments and simulation

In this study, bubbling fluidization of a sand fluidized bed with different biomass loadings are investigated by means of the experiments and numerical simulation. The radioactive particle tracking (RPT) technique is employed to explore the impact of the particle shape factor on the biomass distribution and velocity profiles when it is fluidized in a 152 mm diameter bed with a 228 mm static height. Using a pair of fiber optic sensors, the bubbling characteristics of these mixtures at the upper half of the dense bed are determined at superficial gas velocities ranging from U=0.2 m/s to U=1.0 m/s. The experimental results show that despite cycling with a similar frequency, spherical biomass particles rise faster and sink slower than the cylindrical biomass particles. Furthermore, bubbles are more prone to break in the presence of biomass particles with lower sphericity. In the separate series of experiments, the reliability of the “frozen bed” technique to quantify the axial distribution of biomass particles is assessed by the RPT results. Using NEPTUNE_CFD software, three-dimensional numerical simulations are carried out via an Eulerian n-fluid approach. Validation of the simulation results with the experiments demonstrates that, in general, simulation satisfactorily reproduces the key fluidization and mixing features of biomass particles such as the global and local time-average distribution and velocity profiles

Development of a Simulation Model for Fluidized Bed Mild Gasifier

A mild gasification method has been developed to provide an innovative clean coal technology. The objective of this study is to developed a numerical model to investigate the thermal-flow and gasification process inside a specially designed fluidized-bed mild gasifier using the commercial CFD solver ANSYS/FLUENT. Eulerain-Eulerian method is employed to calculate both the primary phase (air) and secondary phase (coal particles). The Navier-Stokes equations and seven species transport equations are solved with three heterogeneous (gas-solid), two homogeneous (gas-gas) global gasification reactions. Development of the model starts from simulating single-phase turbulent flow and heat transfer to understand the thermal-flow behavior followed by five global gasification reactions, progressively with adding one equation at a time. Finally, the particles are introduced with heterogeneous reactions. The simulation model has been successfully developed. The results are reasonable but require future experimental data for verification