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Fluidization of Biomass Particles: A Review of Experimental Multiphase Flow Aspects
Abstract
Biomass is important in energy conversion processes due to their favourable status with respect to greenhouse gas emissions. However, biomass particles have unusual properties which make them difficult to fluidize and handle. This paper reviews recent research on the hydrodynamics and mixing of biomass particles in fluidized beds. Whereas there has been considerable effort to develop new biomass gasification, combustion, pyrolysis and bio-conversion processes, relatively few authors have characterized the relevant flow characteristics of biomass particles in fluidized beds or investigated measures that could assist in resolving flow issues. The limited work that has been reported on biomass fluidization primarily treats means of achieving fluidization, mixing and segregation. Most of the work has been in low-velocity fluidized beds, although circulating fluidized beds are also important. Further research is needed to provide general understanding of interactions among heterogeneous particles and guidance on conditions that can lead to viable and sustainable processes.
... Some of these steps are based on fluidization technology, like drying [6][7][8][9], combustion [10][11][12], gasification [13][14][15], and pyrolysis [16][17][18]. Generally, the fluidized bed's main advantages include high heat transfer rates, uniform and controllable temperatures, enhanced gas-solid contact, and the ability to handle a wide variation in particulate properties [19]. Specifically for thermochemical conversion processes, the main advantages are the large capacity of processing [20], low emissions of NO x [21], indirect heating with integrated heat exchangers [22], and feeding flexibility and high reaction rates [23]. ...
... Since the biomass is subjected to at least two phase flow in those processes, the main disadvantage of the fluidized bed is the difficulty in fluidizing the biomass particles because of their heterogeneity in shape, size, and specific mass. Biomass particles are so commonly extreme in nature that their flow characteristics are not readily predictable [19]. In this sense, understanding the multiphase flows in fluidized beds through minimum fluidization velocity (u mf ) plays a critical role in the design and optimization of bioenergy production processes. ...
... Determination of the u mf for biomass fluidization has been extensively explored by literature due to the importance of this parameter for practical industrial applications [19,[24][25][26][27][28][29][30]. Besides the essential experimental approach of the research, several empirical and semiempirical equations have been explored or developed to describe u mf for different Geldart groups and under several operating conditions [31], since u mf depends on factors such as particles and fluidizing gas properties, geometry and design aspects of the equipment [32]. ...
Acai berry residues have great potential for sustainable thermochemical conversion processes. Many of these systems, as well as drying technologies, use the fluidized bed technology, which requires the characterization of the minimum fluidization velocity. The experimental work consisted of obtaining pressure drop curves for different bed heights, inlet air temperature, and particles roughness. The theoretical and data simulation approaches encompassed the minimum fluidization velocity prediction by correlations found in literature and neural networks, respectively. The minimum fluidization velocities were determined in the range 0.67–1.56 m/s. The minimum fluidization velocity decreased exponentially with increasing air temperature in the range 50–80 °C, but increased with the increase of the bed height in the range 1.5–4.5 cm. This effect was only observed for particles with fibers, indicating a strong presence of interparticle cohesive forces. Correlations proposed by the literature were not adequate to predict the minimum conditions for fluidization since they are very specific for biomass type and fluidization conditions. Even performing parameter estimation from experimental data, it was observed a strong dependence on the particles properties and bed porosity at minimum conditions. The neural network developed in this work presented good accuracy in predicting the experimental results, and it can be considered a potential viable alternative to estimate the minimum fluidization velocity for different process conditions.
... Hence, the complexities arising in such gasification reactors cannot be predicted based on the information obtained from fluidized beds with one type of particle. 14 On the impact of the particle size distribution (PSD), Yang et al. 15 studied the dynamics and thermal properties of bubbles in a BFB, reporting that an increase in PSD leads to an increase in the bubble volume, aspect ratio, and mass fraction of combustible gases in the bed. In addition, the temperature and thermal conductivity of bubbles increase with the rising velocity, pressure, and density decrease. ...
... Supposing S b is the projected area at the plane P, the bubble volume is thus (14) where z b is the height of the bubble. Since an analytical expression of S b is cumbersome, the volume of the bubble can be approximated as ...
The conversion efficiency, operation, and design of bubbling fluidized bed (BFB) reactors depend on the bed dynamics behavior, which is significantly influenced by the bubble properties. To establish the best operating condition for efficient conversion, this study investigates the dynamics behavior of a BFB reactor using experimental measurements and computational particle-fluid dynamics simulation. The simulations account for particle size distribution and variation of particle properties used in the experiments to eliminate the possible effects on the bed behavior. Compared with a cold bed of similar biomass load and gas velocity, the results show that bubbles propagate with a wider distribution, a smaller size, and a higher frequency in the hot gasifying bed. The bubble diameter and amount of unconverted char particles increase with increasing air flow rate at a constant air−fuel ratio. Although the solid particle distribution over the bed can be uniform with increasing air flow rate, the temperature and gas species distributions lack uniformity due to different degrees of reactions across the bed. An increase in the air flow rate also results in a decrease in the gas residence time, thereby lowering the biomass conversion efficiency in the bed. At the optimum gas residence time, the concentration of hydrogen is maximum, while the concentrations of carbon dioxide and water vapor are minimum in the product gas. For efficient biomass gasification in a bubbling bed, the superficial gas velocity, u 0 , and average bubble diameter, D b , over the bed are related by gD b /u 0 = 3.0, where g = 9.81 m/s 2 is the gravity constant. This proposed model can therefore be used to size BFB reactors or set the operating gas velocity to achieve optimum gasification.
... 3−5 Since a fluidized bed has the advantages of high heat and mass transfer efficiency, wide fuel adaptability, high mixing characteristics, and there are many successful cases of biomass fluidized bed energy conversion at both the laboratory and industrial scales, it is widely used in various fields, such as biopreparation, chemical, oil exploitation, and biomass reaction. 6,7 However, the particle flow characteristics inside the bubbling fluidized bed are complex and widely used, so it has been a hot spot for research. 8,9 On the basis of the increasing maturity of computer technology, computational fluid dynamics (CFD) has also been developed rapidly, and numerical simulation is becoming the main tool to study the particle flow characteristics and flow field distribution in fluidized beds. ...
... For solid-phase particles, the particle motions (translational motion and rotational motion) are solved by the first and second laws of the explicit Euler method, respectively, to track every particle in the fluidized bed by the Lagrangian method: 32 (6) where m p is the particle mass, g is the gravitational acceleration vector, F c is the contact force that accounts for particle− particle and particle−wall collisions, ω p is the angular velocity vector, J p is its moment of inertia, and M c is the net torque generated by tangential forces that causes the rotation of the particle. F fp is the fluid interaction force. ...
The complex multiscale characteristics of particle flow are notoriously difficult to predict. In this study, the evolution process of bubbles and the variation of bed height were investigated by conducting high-speed photographic experiments to verify the reliability of numerical simulations. The gas-solid flow characteristics of bubbling fluidized beds with different particle diameters and inlet flow rates were systematically investigated by coupling computational fluid dynamics (CFD) and discrete element method (DEM). The results show that the fluidization in the fluidized bed will change from bubbling fluidization to turbulent fluidization and finally to slugging fluidization, and the conversion process is related to the particle diameter and inlet flow rate. The characteristic peak is positively correlated with the inlet flow rate, but the frequency corresponding to the characteristic peak is constant. The time required for the Lacey mixing index (LMI) to reach 0.75 decreases with increasing inlet flow rate; at the same diameter, the inlet flow rate is positively correlated with the peak of the average transient velocity; and as the diameter increases, the distribution of the average transient velocity curve changes from "M" to linear. The results of the study can provide theoretical guidance for particle flow characteristics in biomass fluidized beds.
... Solid-fluid interactions are commonly observed in natural transport phenomena and industrial applications. A few examples include sedimentation in rivers [1], volcanic ash dissipation in the atmosphere [2], fixed and fluidized bed reactors [3], and pipeline hydro-transportation of coal [4], limestone [5], and biomass feedstock [6][7][8][9][10]. In all these examples, particle and fluid motions affect each other through drag, shear lift, virtual mass, and other force interactions [11]. ...
... The first quartile of C D,pre data derived from correlations in Table 1 at each Re Q 3 The third quartile of C D,pre data derived from correlations in Table 1 at each Re NRMSE Average Normalized Root Mean Square error N ...
Irregular particles are very common in natural transport phenomena and industrial applications. Although numerous research studies have been conducted on irregular shape particles, there exist several issues like long list of CD correlations with numerous shape factors proposed for wide range of irregular particles. This research aimed at proposing a general CD correlation using a general shape factor applicable to various irregular shapes over a very wide range of Reynolds Number (Re). 60 reference model shapes were produced in SolidWorks® to achieve a very wide range of irregularity. A general shape factor was proposed and applied together with 16 previously proposed CD correlations to the 60 reference model shapes over a range of Re from 0.001 to 300,000 to derive a general CD correlation using nonlinear regression analysis. 60 reference model shapes were 3D printed and their CDs were experimentally measured and compared with estimated values using proposed general correlation.
... Many thermo-chemical processes such as pyrolysis, gasification, and combustion are carried out in fluidized bed reactors (Basu 2006;Khan et al. 2009;Bridgwater 2012). They present several advantages: high mixing capacity, high heat transfer rate, uniform and controllable temperatures, a large contact surface area, and the capacity of dealing with an extensive range of particulate materials including biomass (Cui and Grace 2007). In order to ensure an optimal design and performance of a fluidized bed, key parameters should be well determined: particle size distribution, pressure drop, bed expansion and minimum fluidization velocity (U mf ) as well as the properties of particles (Lin, Wey, and You 2002;Coltters and Rivas 2004;Oliveira, Cardoso, and Ata ıde 2013). ...
... Also, for non-spherical particles, the mean diameter and shape factors (such as sphericity) depend strongly on the evaluation method (Hegel et al. 2014;Olatunde et al. 2016). Cui and Grace (2007) performed a review of the fluidization of biomass particles. They observed very little knowledge existed about the influence of particle size and shape, moisture content, and compressibility on fluidized bed behavior. ...
This paper deals with the influence of different definitions for evaluating the sphericity factor on the prediction of minimum fluidization velocity (U mf) of different agro-industrial/forestry biomass residues and sand. Three biowastes (sawdust, grape marc, and grape stalk) and sand were characterized by sieving, and sphericity was calculated using images obtained by scanning electron microscopy (SEM). Two particle size populations (with mean diameters and density corresponding to B and D Geldart groups) were adopted for each biomass. As for sand, particles with a mean diameter of 0.33 mm were used. Tests were carried out in a lab-scale fluidized bed. Bed pressure drop was measured as a function of the superficial velocity of air at two different initial bed heights, for each type of biomass and mixtures with sand. Ratios of 0.50 and 0.75 (v/v) were used for mixtures. For all cases, U mf values were calculated experimentally and were compared against predicted values obtained from Ergun's equation and other correlations. It was found that the methodology adopted for determining sphericity significantly influences the obtained value of U mf. The lowest relative error between predicted and experimental U mf values was obtained using Riley’s sphericity method. Finally, a new correlation for U mf was proposed.
... Therefore, the biomass, which includes waste from logging and woodworking, as well as agricultural waste (straw, husks of sunflower, rice, millet and other crops, manure and litter) is increasingly applied in the production of heat and electric energy. It is believed that about 10 -13% of the world's thermal and electrical energy is produced using biomass [1,2,3]. A number of thermochemical and biochemical processes is being developed (burning, co-burning with coal, pyrolysis, gasification, torrefaction) using biomass. ...
... Therefore, Puncochar's method for the determination of the Umf must be clarified in relation to polydisperse beds. The following algorithm for Umf determination is proposed: (1) pulsations of the ΔP are measured at several values of the gas velocity; (2) the values of the σ of the ΔP pulsation are determined for two values of the gas velocity U; (3) the method of numerical differentiation determines the values of the derivative [Δ(σi-σi-1)/(Ui-Ui-1)]; (4) two gas velocity values are selected for which (dσ/dU) = const is true; (5) the values of σ corresponding to the gas velocities mentioned in (4) are applied on the dependence σ=f(U). A straight line is drawn through these two points until it intersects with the abscissa axis; (6) the U, which corresponding to this intersection is the velocity of complete fluidization of the multicomponent bed at room temperature. ...
The analysis of experimental method for the evaluation of Umf in gas fluidized beds was carried out. It was based on pressure fluctuation measurements in which Umf is determined by the relationship between the standard deviation of pressure measurements and fluid velocity. Unlike previous works, this method was used to determine the minimum fluidization velocity of polydisperse beds of biomass (milled litter), and for beds of mixture of coal ash or sand particles and straw pellets. The experiments were carried out at room temperature in a transparent apparatus with a diameter of 172 mm. It was found that for some mixtures it is impossible to determine Umf from the curve of the pressure drop in the bed on fluid velocity. On the other hand, the method of determining Umf from the relationship between standard deviation of pressure measurements and fluid velocity allows this to be done for all studied beds.
... Examples include the removal of hardness from natural hard ground waters [12], the recovery of CO 2 in direct air capture processes [13], the improvement of pellet characteristics for reuse potentials [14], reduced sludge production [15], groundwater softening in circulating pellet fluidised bed reactors usage in thermal power plants [16], and organic micropollutant removal from groundwater [17]. There is also a growing interest in fluidisation of biomass particles [18], fluidised bed reactors used in wastewater treatment [19], and other liquid-solid fluidisation techniques with many applications in engineering [1,20,21]. In pellet softening [22], drinking water is treated in an up-flow fluidised cylindrical bed reactor (with flow velocity of 60−120 m/h). ...
... Based on graphical explorations (Figs. 1, 2, and remaining figures in the Supplementary Material), the Ergun model [58] shows Table 3 Fitting parameters for Eqs. (18) overprediction, in particular for higher velocities and larger grains. The Carman-Kozeny model [59] shows overprediction for smaller grains at low and intermediate velocities. ...
In full-scale drinking water production plants in the Netherlands, central softening is widely used for reasons related to public health, client comfort, and economic and environmental benefits. Almost 500 million cubic meters of water is softened annually through seeded crystallisation in fluidised bed reactors. The societal call for a circular economy has put pressure on this treatment process to become more sustainable. By optimising relevant process conditions, the consumption of chemicals can be reduced, and raw materials reused. Optimal process conditions are feasible if the specific crystallisation surface area in the fluidised bed is large enough to support the performance of the seeded crystallisation process. To determine the specific surface area, crucial variables including voidage and particle size must be known. Numerous models can be found in the literature to estimate the voidage in liquid-solid fluidisation processes. Many of these models are based on semi-empirical porous-media-based drag relations like Ergun or semi-empirical terminal-settling based models such as Richardson-Zaki and fitted for monodisperse, almost perfectly round particles. In this study, we present new voidage prediction models based on accurate data obtained from elaborate pilot plant experiments and non-linear symbolic regression methods. The models were compared with the most popular voidage prediction models using different statistical methods. An explicit model for voidage estimation based on the dimensionless Reynolds and Froude numbers is presented here that can be used for a wide range of particle sizes, fluid velocities and temperatures and that can therefore be directly used in water treatment processes such as drinking water pellet softening. The advantage of this model is that there is no need for applying numerical solutions; therefore, it can be explicitly implemented. The prediction errors for classical models from the literature lie between 2.7 % and 11.4 %. With our new model, the voidage prediction error is reduced to 1.9 %.
... The furnace contains stacked glass beads at the bottom to facilitate combustion. Glass beads are inert bed material, due to their excellent fluidization and heat transfer properties, high sphericity, and breaking resistance, they improve the mixing of solids, heat transfer, and reaction rate [12,6] in char combustion. Char particles are fed into the boiler at a constant rate through the 2 mm × 2 mm char inlet on the left wall. ...
This work presents a design under uncertainty approach for a char combustion process in a limited-data setting, where simulations of the fluid-solid coupled system are computationally expensive. We integrate a polynomial dimensional decomposition (PDD) surrogate model into the design optimization and induce computational efficiency in three key areas. First, we transform the input random variables to have fixed probability measures, which eliminates the need to recalculate the PDD's basis functions associated with these probability quantities. Second, using the limited available data from a physics-based high-fidelity solver, we estimate the PDD coefficients via sparsity-promoting diffeomorphic modulation under observable response preserving homotopy regression. Third, we propose a single-pass surrogate model training process that avoids the need to generate new training data and update the PDD coefficients during the derivative-free optimization process. The results provide insights for optimizing process parameters to ensure consistently high energy production from char combustion.
... This is the moment when turbulent regime occurs with no big bubbles in the bed [3,5]. In the particulate phase of the bed, the bed is becoming more homogenous, smaller voids exist in the form of the channels and jets, and particles form clusters [3,4,[6][7][8]. The parameters responsible for the transition between the regimes are the fluidization velocity, pressure drop across the bed, the bed height and nature of the particles [9,10]. The minimum fluidization velocity is important as it helps in understanding the kinetics of the reaction and in the design, scale up and modeling of fluidized reactors [7,10]. ...
... Sedimenting multiphase flows occur in a wide range of society critical applications, such as the upgrading of biomass into biofuels in circulating fluidized bed reactors (Cui & Grace, 2007;Mettler et al., 2012), transport of contaminants within ground water (Miller et al., 1998) or the atmosphere (Ravishankara, 1997;Arganat & Perminov, 2020) and several other environmentallyrelevant systems, such as volcanic processes (Lube et al., 2020) and avalanches (Gardezi et al., 2022;Cicoira et al., 2022;Sovilla et al., 2018;Zhuang et al., 2023). All of these inherently multiphase systems represent problems of great societal interest that are also historically challenging to study, both computationally and experimentally. ...
Sedimenting flows occur in a range of society-critical systems, such as circulating fluidized bed reactors and pyroclastic density currents (PDCs), the most hazardous volcanic process. In these systems, mass loading is sufficiently high () and momentum coupling between the phases gives rise to mesoscale behavior, such as formation of coherent structures. This heterogeneity has been demonstrated to generate and sustain turbulence in the carrier phase and directly impact large-scale quantities of interest, such as settling time. While contemporary work has explored the physical processes underpinning these multiphase phenomenon for monodisperse assemblies of particles, polydispersity flow behavior has been largely understudied. Since all real-world flows are polydisperse, understanding the role of polydispersity in gas-solid systems is critical for informing closures that are accurate and robust. This work characterizes the sedimentation behavior of two polydispersed gas-solid flows, with statistical and physical properties of the particulate phase sampled from historical PDC ejecta. Highly resolved data for both polydisperse distributions of particles at two volume fractions (1% and 10%) is collected using an Euler-Lagrange framework and is compared with monodisperse configurations of particles with diameters equivalent to the arithmetic mean of the polydisperse configurations. From this data, we find that polydispersity has an important impact on cluster formation and structure and that this is most pronounced for more dilute flows. At higher volume fraction, the effect of polydispersity is less pronounced. We also propose a new metric for predicting the degree of clustering, termed `surface loading', and a model for the coefficient of drag that produces accurate predictions for settling velocity given the high-fidelity data presented.
... CFB boilers can burn even the worst grade of available fuels without any major performance penalty (Basu and Fraser, 1991). Thus, with the increase in waste production, CFBC is playing an important role in (co-)firing secondary fuels that would be disposed of otherwise, e.g., biomass (Cui and Grace, 2007), sludge (Van de Velden et al., 2007), petroleum coke (Chen and Lu, 2007), or even municipal solid waste (Wheeler et al., 1995), in addition to high-ash or high-sulfur (S) coals that would not routinely be considered in pulverized-coal combustion (Kitto and Stultz, 2005). ...
The fuel, bed ash, and fly ash were sampled from a circulating fluidized bed combustion (CFBC) unit at two times. The first sampling was a high-sulfur (S) coal-only run, and the second sampling coincided with an experimental burn of up to 10% switchgrass (Panicum virgatum) pressed pellets mixed with a highS coal. The latter blend had a higher moisture content and a lower heating value than the coal-only fuel. Given the time between the samplings and the special needs for the experimental run, unavoidable changes in the coal and limestone complicate comparisons of the bed ash and fly ash chemistry between the sampling times. The bed ash is dominated by CaO and SO 3 , and the fly ash has a higher CaO content than would be expected for a pulverized-coal burn of the same coal. The fly ash chemistry bears a superficial resemblance to class C fly ashes, but given the different combustion conditions and consequent differences in the ash mineralogy, the fly ash should not be considered to be a class C ash. The bed ash mineral assemblages consist of anhydrite, mullite, portlandite, and anorthite, while the fly ash has less portlandite and more anorthite than the bed ash.
... Biomass combustion has attracted much attention in recent years because of its renewable, low-cost, and low-pollution advantages. 1 As a typical fluidization equipment with the advantages of sufficient gassolid contact, high heat and mass transfer efficiency, and suitable for handling irregularly shaped particles, the spouted bed is widely used in the field of biomass combustion. 2 However, in actual industrial processes, due to the large size and low density of biomass particles, small and heavy inert particles are often added to assist the fluidization of biomass particles. 3 In this context, the knowledge of the flow mechanisms of binary mixtures involving biomass particles inside a spouted bed is of great importance for the optimal design of related industrial processes. ...
Spouted bed is widely used in biomass combustion and other industrial production due to the advantages of good heat transfer performance and sufficient gas–solid mixing. In order to achieve higher heat and mass transfer performance and conversion efficiency, inert particles are often added to assist in the fluidization of biomass particles. However, the stacking patterns of different particles in a binary particle system can have some effects on particle flow, distribution, and bed stability. Therefore, in this study, the computational fluid dynamics–discrete element method was used to analyze the particle fluidization characteristics under four different particle stacking modes in a spouted bed. The results show that the average bed height of larger spherocylindrical particles is prioritized in binary particle systems. The void fraction of spherocylindrical particles tends to increase in the near-wall region, whereas spherical particles tend to decrease. When the binary particles are mixed at the initial moment, the change rule of vertical velocity of the two particles remains consistent. In addition, the vertical velocities of two kinds of particles when layered stacking is used are gradually close to each other only after a period of time. In addition, the orientation angle of the spherocylindrical particles in the spouted bed tends to be horizontal for both the single-component spherocylindrical particle system and the wall effect attenuates this phenomenon.
... Solid-gas multiphase flow is a complex phenomenon involving both gas and solid particles, encountered in many processes such as combustion (De Mello et al., 2013;Li & Shen, 2022), fluidized bed reactors (Cui & Grace, 2007;Hashim et al., 2020), pneumatic conveying (Aboudaoud et al., 2023;Chu & Yu, 2008;Rautiainen et al., 1999), spraying (Mezhericher et al., 2012;Zhang et al., 2021;Chen et al., 2022), and electro winning (Pourahmadi & Talebi, 2020). Among these processes involving solidgas multiphase flows, fluidized bed reactors have excellent mixing and heat transfer characteristics, and they are therefore widely used in petrochemicals . ...
A vital process for converting heavy petroleum productions is Fluid Catalytic Cracking (FCC). As a major source of CO2 emissions, the regenerator reactor in the FCC unit accounts for about 20-35% of the refinery's total emissions. A common method for reducing CO2 emissions from the FCC regenerator is oxy-combustion, which has different advantages with regard to reducing energy penalties and associated costs. In this study, a computational fluid dynamic (CFD) study was used to examine the hydrodynamic characteristics of solid particles and gas inside the FCC regenerator, allowing CO2 to be captured more efficiently. Utilizing Ansys Fluent platform, the Eulerian-Eulerian model was applied with granular flow kinetic theory. In the simulations, different mesh sizes were tested, and the hydrodynamics of the oxy-combustion regenerator were evaluated by adjusting CO2 flow rates to achieve similar fluidization behaviors. The CFD results indicated that the conventional drag model accurately predicted the density phases within the bed. In oxy-combustion, CO2, due to its density, naturally creates a smaller dense phase compared to air-combustion. Moreover, optimizing the fluidizing gas velocities resulted in enhanced particle mixing, resulting in a distributed flow with vortices within the dense phases due to a reduction in gas velocity. To improve the environmental performance of the FCC unit, this research provides valuable insight into the hydrodynamics of solid catalysts used in the oxy-combustion process. Article History
... Non-spherical particle-laden turbulent flows are ubiquitous in both natural and industrial processes, such as pollen transmission (Sabban & van Hout 2011), papermaking processes (Lundell, Söderberg & Alfredsson 2011), biomass combustion (Cui & Grace 2007) and drug delivery industries (Kleinstreuer & Feng 2013), to name a few. In practical applications, the particles' shape has a significant effect on their orientation and rotation, thereby shaping the dynamics of particles and the particle-fluid interactions. ...
Rotation and orientation of non-spherical particles in a fluid flow depend on the hydrodynamic torque they experience. However, little is known about the effect of the fluid inertial torque on the dynamics of tiny inertial spheroids in turbulent channel flows, as only Jeffery torque has been considered in previous studies by point-particle direct numerical simulations. In this study, we investigate the rotation and orientation of tiny spheroids with both fluid inertial torque and Jeffery torque in a turbulent channel flow. By comparing with the case in the absence of fluid inertial torque, we find that the rotational and orientational dynamics of spheroids is significantly affected by the fluid inertial torque when the Stokes number, which is non-dimensionalized by fluid viscous time scale, is larger than the critical value , indicating that the fluid inertial torque is non-negligible for most particle cases considered in earlier studies. In contrast to the earlier findings considering only Jeffery torque (Challabotla et al. , J. Fluid Mech. , vol. 776, 2015, p. R2), we find that prolate (oblate) spheroids with a large Stokes number tend to tumble (spin) in the streamwise–wall-normal plane in a thinner region near the wall due to the presence of the fluid inertial torque. Approaching the channel centre, the flow shear gradually vanishes, but the velocity difference between local fluid and particles is still pronounced and increasing as particle inertia grows. As a result, in the core region, fluid inertial torque is dominant and drives the particles to align with its broad side normal to the streamwise direction rather than a random orientation observed in earlier studies without fluid inertial torque. Meanwhile, the presence of fluid inertial torque enhances the tumbling rates of spheroids in the core region. In addition, the effect of fluid inertial force on the dynamics of spheroids is also examined in this study, but the results indicate the effect of fluid inertial force is weak. Our findings imply the importance of fluid inertial torque in modelling the dynamics of inertial non-spherical particles in turbulent channel flows.
... The experimental study plays a crucial role in revealing the mechanisms of multi-scale fluidized bed furnaces, e.g., hydrodynamic, chemical conversion, and formation and emission of pollutants [29][30][31][32]. Leckner et al. [33][34][35] investigated the characterization of fluidization regimes using time-series analysis of pressure fluctuations and found this method effectively revealed the structure of cold particle-fluid flow at the macro level. ...
Biomass is an environmentally friendly renewable energy source and carbon-neutral fuel alternative. Direct combustion/gasification of biomass in the dense particle-fluid system is an important pathway to biomass energy utilization. To efficiently utilize biomass for energy conversion, a full understanding of biomass thermal conversion in lab/industrial-scale equipment is essential. This thesis aims to gain a deeper understanding of the physical and chemical mechanisms of biomass combustion/gasification in fluidized bed (FB) furnaces using computational fluid dynamics (CFD) simulations.
A three-dimensional reactive CFD model based on the Eulerian-Lagrangian method is developed to investigate the hydrodynamics, heat transfer, and gasification/combustion characteristics of biomass in multiple-scale FB furnaces. The CFD model considered here is based on the multi-phase particle-in-cell (MP-PIC)collision model and the coarse grain method (CGM). CGM is computationally efficient; however, it can cause numerical instability if the clustered parcels pass through small computational cells, resulting in the overloading of solid particles in the cells. To address this issue, a distribution kernel method (DKM) is proposed.
This method is to spread the solid volume and source terms of the parcel to the surrounding domain. The numerical stiffness problem caused by the CGM clustering can be remedied using DKM. Validation of the model is performed using experimental data from various lab-scale reactors. The validated model is employed
to investigate further the heat transfer and biomass combustion/gasification process.
Biomass pyrolysis produces a large variety of species in the products, which poses great challenges to the modeling of biomass gasification. A conventional single-step pyrolysis model is widely employed in FB simulations due to its low computational cost. However, the prediction of pyrolysis products of this model under varying operating temperatures needs to be improved. To address this issue, an empirical pyrolysis model based on element conservation law is developed. The empirical parameters are based on a number of experiments from the literature. The simulation results agree well with the experimental data under different operating conditions. The pyrolysis model improves the sensitivity of gasification product yields to operating temperature. Furthermore, the mixture characteristics of the biomass and sand particles and the effect of the operating conditions on the yields of gasification products are analyzed.
The validated CFD model is employed to investigate the fluidization, combustion, and emission processes in industrial-scale FB furnaces. A major challenge in the CFD simulation of industrial-scale FB furnaces is the enormous computational time and memory required to track quadrillions of particles in the systems. The
CFD model coupling MP-PIC and CGM greatly reduces the computational cost, and the DKM overcomes the unavoidable particle overloading issue due to the refined mesh in complex geometry. The CFD predictions agree well with onsite temperature experiments in the furnace. The CFD results are used to understand the granular flow and the impact of operating conditions on the physical and chemical processes in biomass FB-fired furnaces.
... The operation of the BFBGs is more economical compared to CFB applications. Besides, BFBG can use a wider range of materials as feedstocks [18]. Before being used in the different applications previously described, the syngas is cooled and filtered. ...
Bubbling Fluidized Bed Gasifier (BFBG) technology is an efficient and economical way of producing syngas from various feedstocks, such as coal, biomass, and municipal waste. However, the prediction of the gasification process inside the BFBG is quite complex due to many factors, including multiphase flow hydrodynamics. This study analyzed the hydrodynamics of a bench-scale top-fed bubbling fluidized bed coal gasifier with sand or glass beads used as bed materials at different bed aspect ratios. Two separate test rigs were built with the same dimensions for cold flow (without reaction) and hot flow (with reaction) studies, respectively. The cold flow test rig was used to investigate the hydrodynamics of BFBG fluidization. Bed pressure drop, minimum fluidization velocity, and mixing were analyzed in the test room conditions. Following that, gasification tests were carried out in the hot flow BFBG test rig with a novel feeding system using the optimum hydrodynamical parameters determined from cold flow analyses. Results showed that syngas was successfully produced at an adequate composition. This study contributes to a better understanding of the fluidization hydrodynamics of the binary coal and bed material mixtures in a top-fed BFBG for a more optimum gasification process and easier operation of the BFBG.
... Mercury can cause mutations and genetic damage, while copper, lead, chromium can cause brain and bone damage (Wang and Chen, 2009). Biosorption is an innovative technology that employs inactive and dead biomass for the removal and recovery of metals from aqueous solutions (Romera et al., 2007;Cui and Grace, 2007). Various biomasses such as bacteria (Ridha, 2011), sludge (Ali, 2011), yeast (Sulaymon et al., 2010), algae (Kratochvil, 1997), fungi (Brady et al., 1999) and plants (Melcakova and Ruzovic, 2010) have been used to adsorb metal ions from the environment. ...
This paper aims to study the biosorption for removal of lead, cadmium, copper and arsenic ions using algae as a biosorbent. A series of experiments were carried out to obtain the breakthrough data in a fluidized bed reactor. The minimum fluidization velocities of beds were found to be 2.27 and 3.64 mm/s for mish sizes of 0.4-0.6 and 0.6-1 mm diameters, respectively. An ideal plug flow model has been adopted to characterize the fluidized bed reactor. This model has been solved numerically using MATLAB version 6.5. The results showed a well fitting with the experimental data. Different operating conditions were varied: static bed height, superficial velocity and particle diameter. The breakthrough curves were plotted for each metal. Pb2+ showed the largest breakthrough time compared with others, while Cd2+ had the lowest value
... In fact, they commonly have a wide distribution in size, large average size and numerous shapes (splinters, slivers, fibers, chips, flakes, stalks, and so on). Moreover, these particles are generally heterogeneous, compressible and flexible, which contribute to their distinct performance compared to regular particles when operating in CSBs [46][47][48][49]. In different studies, the good efficiency of the spouted reactor with different sizes and wide distributions of particles has been verified [40,49]. ...
This study aimed to model the minimum spouting velocity (Ums) of vegetable biomasses in conical spouted beds including five biomasses. A statistical analysis of the literature correlations corroborated the lack of accurate models, since the average absolute relative errors (AAREs) exceeding 26%. Therefore, a new simple correlation was developed based on reliable data and the least square fitting method (LSFM) that improves the predictions for Ums. Furthermore, three predictive methods were also designed based on intelligent approaches. The model developed based on the Gaussian Process Regression (GPR) provides the most accurate results with excellent AARE and R2 values of 5.42% and 97.02%, respectively, in the testing step. The novel models were found to be applicable for vegetable biomasses of different shapes, as they excellently described the trends of Ums under different conditions. Finally, the parameters of greatest influences on the performance of the models were discovered through a sensitivity analysis.
... Gas-solid fluidized beds are advantageous in many processes involving heat and/or mass transfer between phases. They provide efficient mixing, which results in excellent gas-solid contact and relatively uniform temperature/concentration profiles within the bed (Cui and Grace, 2007;Gidaspow, 1994). Indeed, the need for cleaner and sustainable energy source has led to the development of biomass gasifiers which employ fluidized bed technology. ...
In this work, the business Computational Fluid Dynamics (CFD) is used for the numerical simulations of an air-solid fluidized bed container in the solid phase involving a multi-fluid Eulerian multi-phase model and the Kinetic Theory of Granular Flow (KTGF). The height of the fluidized bed setup is 1.5 m while its diameter is 0.2 m. With it, a series of experimentation were obtained using Helium tracer to determine the Residence Time Distribution (RTD) at various normalized velocities, that is, mixing of air-solids at different degrees. 2 and 3 dimensions of the fluidized bed container are simulated. The main purpose of this study is to understand the hydrodynamic behavior of air-solid fluidized bed container through a framework of Eulerian multi-phase model and to analyze the hydrodynamic behavior of the air-solids mixing. As a first approach, the CFD model is validated using the experimental results of the residence time study. The numerical results of RTD corresponded well with the experimental findings. This shows that the CFD modeling might be used to indicate the performance of a fluidized bed reactor.
... Particulate suspension flow occurs in diverse scientific and technological applications. In the automotive industry, particle-fluid interactions and flow of particulate matter in thermal multiphase fluidic environments affect the design of combustion chambers and cyclone separators, as nano-to micron-size particles enhance the heat transfer in fluidic systems [4,5,6]. In industrial processes such as inkjet printing [7,8], additive manufacturing of ceramics and cement [9], and cosmetics [10], the impact of a liquid drop containing solid particles on solid surfaces affects the deposition and splashing dynamics, in which particle-to-droplet size ratio and particle's wettability play critical roles [11]. ...
Modeling and direct numerical simulation of particle-laden flows have a tremendous variety of applications in science and engineering across a vast spectrum of scales from pollution dispersion in the atmosphere, to fluidization in the combustion process, to aerosol deposition in spray medication, along with many others. Due to their strongly nonlinear and multiscale nature, the above complex phenomena still raise a very steep challenge to the most computational methods. In this review, we provide comprehensive coverage of multibody hydrodynamic (MBH) problems focusing on particulate suspensions in complex fluidic systems that have been simulated using hybrid Eulerian-Lagrangian particulate flow models. Among these hybrid models, the Immersed Boundary-Lattice Boltzmann Method (IB-LBM) provides mathematically simple and computationally-efficient algorithms for solid-fluid hydrodynamic interactions in MBH simulations. This paper elaborates on the mathematical framework, applicability, and limitations of various 'simple to complex' representations of close-contact interparticle interactions and collision methods, including short-range inter-particle and particle-wall steric interactions, spring and lubrication forces, normal and oblique collisions, and mesoscale molecular models for deformable particle collisions based on hard-sphere and soft-sphere models in MBH models to simulate settling or flow of nonuniform particles of different geometric shapes and sizes in diverse fluidic systems.
... The U mf is the superficial gas velocity to induce motion of particles and at which the drag force of the upward moving gas is equal to the apparent weight of the particle bed. The U mf value is important because it indicates the onset of fluidization and phenomena like the extent of segregation in bubbling fluidized beds [19][20][21][22][23][24][25]. Therefore, the knowledge of minimum fluidization velocity is of great importance for successful design and operation of gas-solid fluidized beds. ...
Fine or ultrafine powders (Geldart Group C) have attracted increasing attention in both industrial and academic scopes due to their very small primary particle size and large specific surface area. These Group C particles are usually regarded as cohesive and non-fluidizable because of strong interparticle forces. With the application of nanoparticle modulation technique, a new type of Group C⁺ particles with reduced cohesiveness has been proved to be able to fluidize. The minimum fluidization velocity (Umf) is one of the most important parameters associated with a fluidized bed system, and it is generally defined as the superficial gas velocity at which the drag force of the upward moving gas counterbalances the particle weight in the bed. However, the incipient fluidization phenomenon for Group C⁺ particles showed considerable discrepancies with this definition because of their non-negligible cohesive forces. A number of Umf values available in literatures for Group C⁺, A, and B particles were collected and reviewed in this study to provide a comprehensive scope on the Umf by particle properties. More importantly, the effects of particle cohesion and gravity on the Umf were qualitatively analyzed. The following conclusions were highlighted: the Umf for Group C⁺ particles was controlled by the particle cohesive force, and exponentially increased with the cohesion index (σ*); while the Umf for Group A and B particles was governed by the gravitational forces, and showed a power function with their gravitational forces, not involved in the interparticle forces. This study endeavors to advance the understanding of the Umf in the fluidization of each group powders, nonetheless, more studies are still needed to enhance and deepen the current knowledge of the fluidization processes.
... The particle-fluid interaction commonly occurs in nature and industry; it is also a factor responsible for environmental pollution such as sand particle sedimentation in rivers and during mining and coal particle pipeline transportation and microplastic particle settlement in marine environments [1][2][3][4][5][6]. When a single particle of an arbitrary shape settles through a static and viscous fluid, the settling velocity continues to increase until the terminal velocity is reached. ...
In this study, a series of single-size super-ellipsoidal particles with a wide variety of shapes were generated and 3D printed. Two different drag coefficients (CD and CDA) were obtained by simultaneously measuring the windward surface area and the terminal velocity of printed particles in settling experiments. By use of power function, the influence of various shape factors introduced in previous drag correlations was analyzed and a new drag correlation suitable for both spherical and super-ellipsoidal particles was proposed. Comparative study showed that the present model allowed more effective prediction of the drag coefficient than the other 14 models. Furthermore, the approximate calculation formula of CDA was constructed from CD. Because the shape factors of super-ellipsoidal particles can be calculated accurately, the obtained experimental results in this work can be used as a benchmark database for calibrating drag correlations of nonspherical particles.
... The minimum fluidization velocity (Umf), which is defined as the superficial gas velocity at which the drag force of the upward moving gas is equal to the apparent weight of the particle bed [1,2], seems to be amore tractable parameter in fluidization. The Umf value is important because it dictates the onset of fluidization [1,3] and phenomena like the extent of segregation in bubbling fluidized beds [4][5][6][7][8]. ...
The attractiveness of the application of fluidization in various technological operations comes from the fact that it provides features that can be viewed as: intensive mixing of solid particles in a fluidized bed, easy supply and drainage of material, a large contact surface between the gas and solid particles and nearly constant temperature all over the bed. Knowledge of the minimum fluidization velocity is fundamental to optimizing the performance of fluidized beds composed of mixtures. The present work aimed to determine the minimum fluidization velocity of binary mixtures using the characteristic diagram of pressure drop in the bed and to develop an experimental correlation for the minimum fluidization velocity of zeolite and polyethilene binary mixtures. In the case of two-component mixture fluidization, the transition to fluidized condition occurs gradually because the finer particles pass into a fluidized state at lower fluid velocities. Heavier particles transit more slowly to the fluidization state. The transitional area size depends on the physical characteristics of particles and increases with an increasing difference in the physical properties of the particles. The fluidization curve which is obtained at the transition to the fluidized state of two component mixtures is presented.
... The work of Fiorentino et al. (1997 a), , and Bruni et al. (2002) showed that the gas evolved from a devolatilising wood particle in fluidised beds pushes the bed particles away from the wood particle. Cui and Grace (2007) pointed out that characterising the hydrodynamic interaction between the biomass particles and the bed particles, and how this changed with bed temperature, was a key area in which more research was required. The rate of devolatilisation depends on both internal and external heat transfer. ...
This dissertation concerns solid spheres, with diameters ~ 6 - 10 mm, densities between ~700 - 1500 kg m-3 and emitting gas at various peripheral velocities, Ud, and their tendency to float or sink when introduced into gas-fluidised beds of Geldart Group B particles. This is relevant, for example, to the fluidised bed combustion of biomass, and the apparent tendency of the fuel to devolatilise predominantly near the upper surface of the bed with the attendant undesirable complications of unconverted volatile matter (VM) entering the freeboard. Inert spheres (viz. where Ud = 0) in a bubbling fluidised bed can sink, even if less dense than the fluidised medium, owing to the additional weight of bed particles which tend to settle on top of them forming a defluidised hood. A 2-D fluidised bed, at room temperature, was used to investigate the structure of the fluidised bed in the vicinity of a cylinder emitting gas, as a mimic of a 3-D system. It was found that if Ud is more than 0.7, then the gas emitted can fluidise the bed particles in the entire defluidised hood. Consequently, it was inferred for a 3-D system that gas emitting spheres are not burdened by a defluidised hood and will rise to the surface more rapidly than inert spheres, which are burdened. The hypothesis that a gas-emitting sphere forms a pocket of high pressure around its underside sufficient to enable it to hover above the surface of the fluidised bed, was investigated, in a mechanism akin to the Leidenfrost effect exhibited by liquid drops on a hot plate. Experimentation showed that this hypothesis could be rejected. In fact, by observing the structure of the bed and measuring the pressure around a gas-emitting cylinder close to the surface of a 2-D fluidised bed, it was found that the emission of gas from a freely-floating sphere decreases the net upthrust of the bed on its underside thereby causing the sphere to sink lower into the bed than buoyancy alone would suggest. However, it was also discovered that the emission of gas from a sphere sunk deep within a fluidised bed caused the net upthrust from the bed to increase, causing the sphere to rise more rapidly to the surface than an inert sphere. This suggests that there exists a stable depth at which gas-emitting spheres reach dynamic equilibrium just beneath the surface of the bed where the bed’s upthrust matches the weight of the sphere. An interesting aside of investigating the Leidenfrost mechanism was that, as far as Geldart Group B solids are concerned, experiments showed the two-phase theory of fluidisation holds exactly. To simulate spheres of devolatilising biomass, spheres of dry ice, sublimating in a hot fluidised bed were used, because dry ice emits a single, readily detectable gas. The spheres of dry ice were, however, much denser than any biomass fuel and so only segregated once the rate of sublimation was very high. The external heat transfer coefficient for the spheres of dry ice was measured at a variety of bed temperatures and bed particle sizes. Unlike inert particles, gas emitted by the dry ice particles caused the heat transfer coefficient to a) decrease as the bed material size was decreased and b) decrease as the bed temperature increased. For the first time, a heat transfer model, which accounted for the change in structure of the bed material near the gas-emitting particle, was developed to predict the rate of gas emitted from the dry ice particles and gave good agreement with the experimental results. A novel method for finding the peripheral velocity of VM, emitted by spheres of biomass during devolatilisation in a fluidised bed, was developed and validated experimentally. The mean molar mass and composition of the VM was measured, with the result that measuring the concentration of the combustion products of the VM alone could be used to find the molar flowrate of the VM. Using this method, values of Ud, for spheres of beech, devolatilising in a hot fluidised bed, were measured and, simultaneously, the depth of the spheres in the bed was determined using X-radiography. The simultaneous measurements of gas velocity and depth allowed the behaviour of freely floating, devolatilising spheres to be compared with the calculations obtained with the 2-D fluidised bed. The spheres of beech remained just beneath the surface of the bed throughout devolatilisation and were less influenced by the mixing motions of the bed than inert spheres, even when the fluidisation velocity was increased. The devolatilising beech behaved much as anticipated by the results of the 2-D bed experiments. Tentatively, a dimensionless plot was made which, brings the variables Ud, the incipient fluidisation velocity Umf, the densities of the gas-emitting particles and the bed material, and the depth at which a particle will neither rise nor sink in the bed, together. The plot shows under what conditions a gas-emitting particle is likely to have a sinking or rising tendency in a fluidised bed. The plot is a tool for predicting if segregation of a particular fuel particle is likely to occur in any bubbling fluidised bed. Overall, this dissertation concludes that the emission of VM from a devolatilising particle of biomass not only draws the particle to the surface of the bed but acts to keep it there, even at low rates of gas emission. To eliminate the segregation of biomass during combustion in a bubbling fluidised bed, the biomass must be denser than the emulsion phase of the fluidised bed and the velocity of VM leaving the biomass must be as low as possible. An impracticable degree of pre-processing of the biomass would be required to achieve these conditions.
... In applications of dense gas-solid flows such as in fluidized bed (FB) combustor, gasifier, and incinerator, the shape as well as the size of solid materials in the beds are non-uniform, and large non-spherical solid objects such as fuel pellets and incombustible wastes co-exist with small bed particles. In the case of FB gasifier using biomass fuels, for example, it is common to use elongated pellets as a fuel that are largely different in shape and size from inert bed particles such as silica sand installed in the bed to enhance fluidization and heat transfer (Cui and Grace, 2007). The difference of shape and size (as well as density) is expected to induce the complex and diverse dynamics of large objects and may cause undesired problems such as segregation and tunneling in industrial operations. ...
... Examples include the removal of hardness from natural hard ground waters (Mahvi et al., 2005), the recovery of CO2 in direct air capture processes (Burhenne et al., 2017), the improvement of pellet characteristics for re-use potentials , reduced sludge production (Mercer et al., 2005), groundwater softening in circulating pellet fluidised bed reactors usage in thermal power plants (Hu et al., 2019) and organic micropollutant removal from groundwater (Maeng et al., 2016). There is also a growing interest in fluidisation of biomass particles (Cui and Grace, 2007), fluidised bed reactors used in wastewater treatment (Akkoyunlu, 2003) and other liquid-solid fluidisation (LSF) techniques with many applications in engineering (Edzwald, 2011;Crittenden et al., 2012;Epstein, 2003). ...
In drinking water treatment plants, multiphase flows are a frequent phenomenon. Examples of such flows are pellet-softening and filter backwashing where liquid-solid fluidisation is applied. A better grasp of these fluidisation processes is needed to be able to determine optimal hydraulic states. In this research, models were developed, and experiments performed to gain such hydraulic knowledge. As a result, treatment processes can be made more flexible. In a rapidly changing environment, drinking water production must be flexible to ensure robustness and to tackle challenges related to sustainability and long-term changes. In the hydraulic models, the voidage in the fluidised bed and the particle size of the suspended granules are crucial variables. Voidage prediction is challenging as the fluidised bed is a dynamic environment showing highly heterogeneous behaviour that is hard to describe with an effective model. And particle size causes a conundrum due to the irregular shapes of the applied granules. Through the combination of hydraulic dimensionless Reynolds and Froude numbers, an accurate voidage prediction model has now been developed. With a straightforward pseudo-3D image analysis for non-spherical particles measuring particle mass and density, the dimensioned shapes of, for instance, ellipsoids can be determined. Particle shape factors included in models are not constant as is commonly believed, but dynamic. Applying advanced computational fluid dynamics simulations confirmed significant heterogeneous particle-fluid patterns in fluidised beds. Comprehensive sedimentation experiments showed that the average drag coefficient and terminal setting velocity of individual grains can be estimated reasonably well, but with a significant degree of data spread around the mean values. For engineering purposes, this is relevant information which should be taken into consideration. A new soft-sensor was designed to determine the voidage gradient and particle size profile in a fluidised bed. The expansion degree of highly erratic, polydisperse and porous granular activated carbon grains can be predicted with a model, but in full-scale processes the grains are subject to change, and therefore it is most likely that the prediction accuracy will deteriorate rapidly. For reliable drinking water quality, smart models provide solutions to complex challenges, but they are only effective when they are calibrated and validated in advanced pilot plants and are applied in full-scale processes with diligence and commitment on the part of multidisciplinary teams.
... The utilization of both plastic particles and biomass in a fluidized bed system has become increasingly popular and is investigated in many studies due to their great potential for sustainable energy conversion processes such as combustion, gasification, and pyrolysis, to reduce greenhouse gas emissions and abundant supply of the raw material [1][2][3] . Most of these recycled raw materials are physically processed into granular materials of non-spherical shape. ...
The binary fluidization of Geldart D type nonspherical wood particles and spherical low density polyethylene (LDPE) particles was investigated in a laboratory‐scale bed. The experiment was performed for varying static bed height, wood particles count, as well as superficial gas velocity. The LDPE velocity field were quantified using particle image velocimetry (PIV). The wood particles orientation and velocity are measured using particle tracking velocimetry (PTV). A machine learning pixel‐wise classification model was trained and applied to acquire wood and LDPE particle masks for PIV and PTV processing, respectively. The results show significant differences in the fluidization behavior between LDPE only case and binary fluidization case. The effects of wood particles on the slugging frequency, mean, and variation of bed height, and characteristics of the particle velocities/orientations were quantified and compared. This comprehensive experimental dataset serves as a benchmark for validating numerical models.
... There are technical challenges in the co-combustion of biomass with coal in existing coal-fired power plants. The most common challenges are low energy density causing flame instabilities in the combustion chambers [24], poor grindability resulting in higher grinding energy requirements, slagging and fouling problems in boilers [40], low biomass flowability and fluidization properties [19] leading to difficulties in feeding biomass into combustors. These facts create difficulties for utilization of biomass directly. ...
Combustion behavior of rice straw torrefied at 300 °C for different residence time and their blends with sub-bituminous coal was investigated. The torrefied product and its blends were characterized for fuel properties and Hardgrove Grindability Index (HGI). Also, the torrefied product is characterized fordensity, proximate and ultimate analysis, energy yield, and structural analysis by Raman spectroscopy. The calorific value data of blend shows its value is equivalent to the value of sub-bituminous coal, i.e., 17.21 MJ/kg. On the other hand, HGI of torrefied product is 40–45 and that of coal is 80. On blending, the value of HGI up to a certain ratio shows the synergetic effect while higher content of the torrefied product in blend demonstrated non-additivity behavior and it is dominated by coal as a consequence of density difference between torrefied product and coal. Characteristic combustion parameter for blends reveals the synergetic behavior. It is found that the blend of torrefied products and sub-bituminous coal at a ratio of 10:90 and 20:80 had ignition and burnout temperature almost close to coal sample. The ignition and burnout temperature of 10:90 of torrefied product at 300 °C for 60 and 120 min are 295 °C and 507 °C, 301 °C and 505 °C whereas that of sub-bituminous coal is 325 and 515 °C. The change in the fuel properties of the blend suggests there is a certain degree of interaction that occurred during combustion.
... Therefore, biomass is accepted an advantageous and sustainable option to produce hydrogen gas through gasification technologies [17,18]. Fluidized bed technology is widely used in gasification because of its various advantages including high heat transfer, uniform and controllable temperature, favorable gas-solid contact and fuel flexibility [19][20]. Fluidized bed gasification (FBG) of biomass by using air-steam mixture as the gasifying agent is an economically available potential solution to produce syngas with high hydrogen content. ...
In this study, hydrogen production performances of groundnut shells in a circulating fluidized bed gasifier is evaluated by employing a previously developed and validated model. Basically, we simulate a circulating fluidized bed gasification system that is connected to a water-gas shift reactor, for hydrogen purification with the gasifier temperature of 1150 K. We find that the amount of hydrogen gas produced from circulating fluidized bed gasification of groundnut shells increases from 49.25 kmol to 68.83 kmol (per 1000 kg of raw groundnut shells) when the gasifier is integrated with water-gas shift reactor. We observe that it is possible to obtain a high yield of hydrogen gas from the gasification of groundnut shells. Therefore, we conclude that the groundnut shell is a remarkable feedstock for bioenergy.
... Unlike coal, biomass particles are very irregular in size and shape ranging from grains to fibers, hulls, flakes or sticks, and are usually porous and pliable [13]. A bulk of particles often have no homogeneous sizes and shapes: size distribution in biomass is rather flat and bidisperse than monodisperse [14] and the shape varies throughout the size distribution. ...
The present study explores the relevance of the physical properties of biomass particles on the determination of the minimum fluidization velocity ( U mf ) of binary mixtures. Fluidization experiments were performed in a cold flow unit with diverse biomasses mixed with sand in different mass fractions. Gas velocity and pressure drop across the bed were used to determine U mf . Different correlations reported in the literature were evaluated on their ability to accurately predict U mf of the mixtures. Results showed satisfactory predictions when appropriately identifying correlations according to the corresponding Geldart groups for the biomass particles. This perspective opens new possibilities toward the generalization of correlation factors and helps in improving the accuracy of the prediction for highly heterogeneous mixtures. The methodology also allows the analysis of mixtures for which the experimental approach is difficult, such as those including char particle, with the only requirement of carefully measuring the physical properties of the particles.
... The fluidized bed gasification were also analyzed technology is preferred because of its different preferences which incorporate the variety of fuel used, great heat transfer rate, even and wellregulated temperature of bed, encouraging gas-solid mixing, etc [9]. The dynamic displaying of a bubbling fluidized bed biomass gasifier was studied and this model has been validated to measurements from transient tests in a small-scale fluidized bed biomass gasifier. ...
The assessment of the performance of various biomass fuel sources in gasifier is a significant zone of study for the execution of this innovation in different applications, pertinent to remote villages. This paper reveals the prediction of the composition of producer gas in a fluidized bed gasifier using Sugarcane Bagasse as feedstock where air to be used as gasifying agent. The model dependent on equilibrium constant and material balance has been created to assess the syngas compositions. The developed model has been used to predict the composition of constituent gases by varying equivalence ratio (ER) between 0.1 and 0.35 and keeping the moisture content (MC) constant. The outcomes show that the mole portion of CO from sugarcane bagasse is higher as compared to rice husk and coir pith and the composition of H2 and CH4 where absorbed very close to rice husk. Hence it recommended that the sugarcane bagasse and coir pith shall replace the rice husk and coir pith in fluidized bed gasifier for rural applications.
... Many authors have investigated drag reduction with the introduction of flexible polymers [3], polymers [4], surfactants [5]. solid particles [6] and [7], complexes of two or more additives [8] and microbubbles [9]. Among all these drag reducing agents (DRAs), many factors such as the chemical structure, concentration, flowrates, morphology, molecular weights, intrinsic viscosity of the DRAs have all been investigated at various flow parameters e.g pipe diameters and fluid velocity [10] [11] and [12]. ...
One of the economical important challenges in pipeline transportation is reducing the drag; e.g reducing the pressure drop along the pipe as much as possible. Although many efforts have been made, a universal mechanism and environmentally friendly approach to reduce the drag is still setbacks. The present work introduces an environmentally friendly method to reduce drag using microbubble, polyethylene oxide (PEO) with concentrations (100, 400,1100,1900 and 2500 ppm) and nanopaticles (carbon nano tube CNT) with concentrations (100, 400,1100,1900 and 2500 ppm). The pipeline loop was used to individually study the materials, their complexes as well as the combination of either of these with micro bubbles. The experiments were conducted in a pipeline loop to mimic the practice in the petroleum industries. The result showed that the drag reduction increases as flow and additive concentraons increases and the maximum reduction was 40% for PEO and 58% for CNT. The results also show that the use of blend additives has greater effect than the individual materials and combining the polymer, CNT suspended, and the microbubbles gave 80% drag reduction.
In this paper, the interaction between cylindrical particles and shear-thinning non-Newtonian fluids in a linear shear flow is investigated using particle-resolved direct numerical simulation. The Carreau model is used to represent the rheological properties of shear-thinning fluids, and the numerical method is validated against previously published data. Then, the effects of Reynolds number (Re), aspect ratio (Ar), power-law index (n), Carreau number (Cu), and incident angle (α) on drag coefficient (CD), lift coefficient (CL), and torque coefficient (CT) of cylindrical particles are investigated. The numerical results show that the flow field structure and pressure distribution around the cylindrical particle in a shear flow are different from those in a uniform flow, and the particles in a shear flow generate extra CL and CT. Furthermore, comparing with Newtonian fluids, the shear-thinning properties of the non-Newtonian fluid change the viscosity distribution and significantly decrease the CD, CL, and CT of the particles. The variation laws and influencing mechanisms of CD, CL, and CT under different working conditions are discussed by dividing the total coefficients into pressure and viscous shear contributions. Predictive correlations of CD, CL, and CT are established by considering the effects of Re, Ar, n, Cu, and α. The findings indicate that both the shear flow mode and shear-thinning properties must be considered when evaluating relevant particle–fluid interactions, which provides important guidance for predicting and controlling the orientation and distribution of cylindrical particles in shear-thinning fluids. Meanwhile, the predictive correlations can be used for large-scale simulations of multiphase coupling.
Fluidized bed technology has a 100‐year history of delivering energy solutions to the world. Examples include fluid catalytic cracking, coal combustion and gasification, and fluid coking. Moving forward, fluidization technology has the potential to underpin the development of entirely new sustainable processes in the energy transition and the circular economy and many of these will be advanced by small‐and‐medium enterprises (SMEs) and start‐ups. Focused, low‐cost, and time‐bound research outcomes will be needed to support these SMEs as they bring their new technologies to market as quickly as possible. This paper first summarizes some of the fluidized bed technologies that will play a key role in the energy transition and then considers how the strategic concept of discovery driven growth can lead to focused, rapid, and low‐cost information. The experimental data can then be used to develop hybrid models using machine learning methods that will be more robust, accurate, and reliable models. With focused, interdisciplinary research, fluidization models may be developed that would allow fluidized beds to go directly from lab or pilot scale directly to commercial. This would reduce development costs and timelines dramatically, hence bringing these new technologies to market more quickly. Early commercialization will allow the environmental benefits to begin to accrue earlier and will improve returns on investment.
From colloid suspension to particle aggregation in protoplanetary formation, electrostatic attraction and repulsion between particles is a key mechanism behind the aggregation and clustering of particles. Although most studies have focused on canonical spherical particles, it remains unclear how nonspherical and rough dielectric particles interact and whether the complicated interplay between roughness and charge distribution affects their force couplings. Here a boundary-element method model was leveraged to study electrostatic interactions between charged dielectric particles with modular, axisymmetric surface features. Charge accumulation at convex surface asperities decreases the strength of electrostatic interactions between particles, and the sensitivity of the electrostatic force to the particle surface roughness and orientation is especially apparent at small particle separations. Surface interactions between the particle near-contact regions were isolated to determine the degree that near-contact interactions dictate the relationship between the net electrostatic force and the particle roughness and orientation. A correction factor ΔF is introduced to recover higher order dielectric effects from a low order analytical model. Finally, implications of surface charge asymmetries produced for different particle orientations and surface roughnesses on the long-standing problem of triboelectrification are discussed.
In this paper, the most influential parameters (numerical and operating parameters) affecting the performance of fluidized bed reactors are studied. The investigated parameters are constitutive and numerical parameters, minimum fluidization velocity, operating pressure, temperature, gas distributor, and particle size distribution (PSD). Furthermore, the recent methods for solving population balance equations coupled with computational fluid dynamic (CFD-PBM) are discussed. The direct quadrature method of moments (DQMOM) was found to be the most efficient method for CFD-PBM coupled simulations. It must be pointed out that due to the computational cost there is limitation on the application of CFD in practical reactor with detailed mass/heat transfer and reaction mechanisms, especially under industrial operating condition.
Recently, biomass gasification process has gained lots of attention because of sustainable energy sources utilization. Being renewable energy source, biomass can serve as a viable replacement for fossil fuels. The process gasification is the conversion of organic substance via thermochemical process where syngas is produced along with the solid product termed as char. Such process is also well known for the generation of heat and power and synthesize the second-generation biofuels and hydrogen production. Unfortunately, tar formation in gasifiers during biomass gasification remains a main problem to commercialization. In the current review we focus at recent advancements in catalytic biomass gasification about supercritical water catalytic gasification (SCWG), catalytic steam gasification and catalytic CO2 gasification, and discuss on gasification process, parametric impact, biomass-pretreatment and catalytic deactivate mechanism in order to overcome the challenges and improve the catalytic yield. Future direction and critical prospective of catalytic biomass gasification are also discussed in this review.
Biomass particles have a wide variety of size and shape, and their characterization is fundamental for energy generation design, production of biofuels, simulation, and sizing of processes involving two-phase flow, such as fluidized bed, fixed bed, and aerodynamic transport. In order to find design data for projects involving biomass, this study characterized sugarcane bagasse, sugarcane straw, bamboo, and cassava plant branches regarding size and shape, using two methods of particle separation: sieving and elutriation, in addition to image analysis. An experimental bench was built for the separation of biomass by elutriation. Each biomass was separated by sieving in the range 388 to 2360 µm openings and drag velocities from 0.6 to 6 m/s. The size distribution (mass, major and minor axis length, area and volume) and the calculation of sphericity resulted in different patterns according to the method used: sieving or elutriation. Biomass portions of smaller physical size, or smaller aerodynamic size, concentrate a higher ash content than larger portions. However, some minerals, such as Fe, Si, and Cl, were more concentrated in larger particles, characteristic details that may be useful for controlling the composition of biomass ash to prevent corrosion or incrustation in thermal process equipment.
In dense gas-solid flows such as that in fluidized bed gasifier, large non-spherical solid objects co-exist with small bed particles. During the fluidization, the large non-spherical objects experience complex forces depending on their shape, size, and orientation and a diverse and complex dynamics emerges. Here, we present a numerical model for dense gas-solid flows with large non-spherical objects. The proposed model named FPM-SE is based on the combination of Fictitious Particle Method (FPM) for large size differences and Super-ellipsoid (SE) model for various shapes. We also perform experiments using a Lagrangian sensor system to investigate the influence of object shape on its sinking/floating motions in a gas-fluidized bed. We observe that non-sphericity facilitates the floating of large objects. Not only FPM-SE reasonably capture the experimental observations but it also demonstrate a typical sinking–staying in the bottom–floating behavior with an orientational change of large non-spherical objects.
As a by-product in the vinegar brewing process, vinegar residue always has a high moisture content, which is detrimental to the storage and recycle process. The vibrated fluidized bed can be used to dry the vinegar residue. In present work, inert particles were added to a vibrated fluidized bed to improve the fluidization of vinegar residue. Experimental studies were carried out to investigate the fluidization behaviors of the binary mixtures. Flow pattern maps indicated that there was an upper limit to the vinegar residue mass concentration cw at which stable fluidization could be achieved. The minimum fluidization velocity umf of the binary mixture increased as the vinegar residue mass concentration cw increased and decreased with the increase of the vibration intensity Λ. As increasing vibration intensity Λ or decreasing vinegar residue mass concentration cw, the drying rate of vinegar residue increased.
It is necessary to investigate and understand the gas-solid flow characteristics for the design and optimized operation for chemical looping combustion reactors and other chemical process which involves dual fluidized bed (DFB) system. In this research, computational fluid dynamics simulation was performed in a lab-scale cold three-dimensional full-loop dual fluidized bed model based on the Particel-In-Cell (MP-PIC) method to understand its gas-solid flow characteristics. The simulated results were analyzed and validated with the pressure and electrical capacitance tomography (ECT) measurements. It has been shown that Wen-Yu/Ergun drag model is feasible for the prediction of the gas-solid flow dynamics of this DFB system. The simulated particle volume fraction and ECT results have a good agreement. In the bubbling fluidized bed (BFB), four flow zones are identified based on the bubble and particle behaviors. In each zone, bubbles are mainly concentrated in the middle region. Moreover, the fluidizing air in the chute functions as a secondary air of the BFB to facilitate the particle movement. The BFB and the lower U valve are working as one component and function as a particle buffer region. Therefore, the balance of the whole system is determined by the time when the riser reaches steady state. The higher the air inlet velocity of the riser, the sooner the system reaches steady operation. In addition, the start-up characteristics among the riser, the BFB and U-valve are investigated based on the simulation results. The results indicated that there is a start-up time lag between the riser and bubbling fluidized bed when they each reach steady state.
This chapter will discuss new software, Particles and Geometric Shapes Analyzer (APOGEO), aiming the determination of aspect ratio and sphericity of solid particles by image processing technique without any manual work. This software can quantify the major and minor axes correlating two or three dimensions of particles (e.g., biomass, mineral, pharmaceutical, and food products) to obtain their shape. The particles can be associated with different geometries, such as rectangular parallelepiped, cylinder, oblate and prolate spheroids, and irregular. The results are presented in histograms and tables, but also can be saved in a spreadsheet.
This article discusses the use of population balance techniques in the understanding of particulate processes. Population Balance Equations (PEEs) describe how properties of a group of particles change with time and position. These "properties" are frequently some measure of particle size, so PEEs are most often used to describe how particle size distributions (PSDs) change during the processing of particulate materials. After a brief review of how PEEs are formulated and solved, three case studies, from the author's research, are considered. Crystallisation: Aggregation During Precipitation From Solution. This case study reports a decade of work on the aggregation of calcium oxalate monohydrate crystals in supersaturated solutions. Our knowledge of the dependence of growth and aggregation rates on solution composition is reported and physical models explaining the dependencies are discussed. The main conclusion for this study is, apart from a description of COM, that substantial microscopic lessons may be drawn from observations made at a mesoscopic (i.e. many particle) scale. Fluidisation: Spray Coating Of Grass Seeds The objective of this work was a description of how seed particles increase in size as new material is sprayed on. This study is unusual in that the PBE was solved analytically and gave rise to a model with no adjustable parameters. At first sight the model is capable of very high fidelity predictions; it is not until each particle is considered to have two properties, and not just one, in size, that failings in the model become apparent. Granulation: Extracting Kinetics This study reports a substantial experimental and numerical investigation of the rates at which size enlargement occurs in high-shear granulation equipment. It is shown that breakage is extremely important and that its rate can be quantified. It is also shown that it is not possible to uncouple the-effects of particle size, age and other properties without careful experimental design. The main conclusions of this work are: • It is possible to pose and solve PEEs for a variety of problems • It is possible to extract physical information about particle rate processes by means of PEEs • There are indications that this type of work must move on from representing particles by a single property, i.e. their size, and recognise that most particles have a variety of properties that affect their behaviour.
This paper presents the results from biomass gasification tests in a pilot-scale (6.5-m tall × 0.1-m diameter) air-blown circulating fluidized bed gasifier, and compares them with model predictions. The operating temperature was maintained in the range 700–850°C, while the sawdust feed rate varied from 16 to 45kg/h. Temperature, air ratio, suspension density, fly ash re-injection and steam injection were found to influence the composition and heating value of the product gas. Tar yield from the biomass gasification decreased exponentially with increasing operating temperature for the range studied. A non-stoichiometric equilibrium model based on direct minimization of Gibbs free energy was developed to predict the performance of the gasifier. Experimental evidence indicated that the pilot gasifier deviated from chemical equilibrium due to kinetic limitations. A phenomenological model adapted from the pure equilibrium model, incorporating experimental results regarding unconverted carbon and methane to account for non-equilibrium factors, predicts product gas compositions, heating value and cold gas efficiency in good agreement with the experimental data.
Flow past and within an isolated permeable spheroid directed along its axis of symmetry is studied. The flow velocity field is solved using the Stokes creeping flow equations governing the fluid motion outside the spheroid, and the Darcy equation within the spheroid. Expressions for the hydrodynamic resistance experienced by oblate and prolate spheroids are derived and analyzed. The limiting cases of permeable circular disks and elongated rods are examined. It is shown that the spheroid’s resistance varies significantly with its aspect ratio and permeability, expressed via the Brinkman parameter.
Explicit equations are developed for the drag coefficient and for the terminal velocity of falling spherical and nonspherical particles. The goodness of fit of these equations to the reported experimental data is evaluated and is compared with that of other recently proposed equations.Accurate design charts for CD and ut are prepared and displayed for all particle sphericities.
Suspensions of semi-bleached kraft pulp at low and medium mass concentrations were sheared in a rotary shear tester at angular velocities up to 524 rad/s (5000 rpm). The use of baffled housings, with rotors having lugs, imposed shear within the body of the suspensions and prevented slip at the vessel walls. The torque was measured as a function of angular velocity, and high speed photographs were taken through a transparent plate to aid in interpretation of the findings. The torque vs. angular velocity curves and flow phenomena were found to depend on the mass concentration of the suspension, the gap width between the rotor and housing, and the air content of the suspension. With little gas present, the flow initially involved tangential flow in an active cavity whose radial extent increased with increasing angular velocity. When the flow reached the outer baffles, a marked flow transition occurred, probably explaining what has been called “fluidization” by Gullichsen and Harkonen (1981). As the gas content of the suspension increased, the gas congregated around the rotor, impeding the transfer of momentum to the suspension. The apparent viscosity of a 10% by mass suspension was of the order of 16 Pa · s under the test conditions.
The objective of this paper is to discuss the use of coffee grounds in the Brazilian soluble coffee industry. This residue is used as a fuel in the boilers of the same industry; so, data about their utilization are presented and analysed, discussing the actual technology and the advantages of improving the drying of the biomass with the exhaust combustion gases. After that, an experimental study is reported on the characteristics of this material, which are important for the combustion process, including the transport, storage and drying, the mean diameter of the particles, talus angle, apparent and real density, sphericity, surface area, terminal velocity, spontaneous ignition temperature and heat of combustion.
The hydrodynamical behaviour of fluidized beds at reduced pressures is studied. The minimum fluidization velocity is strongly influenced by pressure, decreasing as pressure increases. The well-known equation of Wen and Yu and two equations proposed for the calculation of umf at reduced pressures are tested; the comparison with experimental data shows that they can be applied only over restricted pressure ranges. The relationship between flow rate, pressure and pressure drop at conditions ranging from high vacuum to high pressure is analysed for the different flow regimes (molecular, slip, laminar, transition and turbulent). A generalized equation is found which predicts fairly well the value of umf at vacuum conditions, atmospheric pressure and high pressures.
This second article in a series presents further experimental data on the fluidization with air at ambient conditions of binary mixtures of a biomass and a second solid, admixed to make the biomass fluidizable. The latter included sawdust, straw, wood chips, and thistle, while sand, dolomite, and a commercial catalyst (FCC) were employed as second solids.
Measurements were made of incipient fluidization velocities for 48 gas/solid systems including many angular and/or porous powders, and 5 gases at ambient conditions. To correlate the data, it was essential to know the envelope or particle density of each powder, and this is notoriously difficult to determine for porous materials. However, the authors have been able to develop an experimental technique which is reliable and simple to use, and this enabled us to compare their data with a number of commonly used correlations for U//m//f.
Coffee husk is one of the most commonly available agricultural residues in the mountainous regions of Cuba that can be converted into different types of fuel and chemical feedstocks through a variety of thermochemical conversion processes. This study provides information on moisture content, bulk density, particle size, minimum fluidization velocity, and terminal velocity of this biomass, which has been used for the design of different technologies of thermolysis.
Agricultural and forest residues have a density, shape, and size which make their fluidization impossible. Thus, for their proper thermochemical processing, i.e., gasification, pyrolysis, and combustion, in a fluidized bed, a second solid must be added, which serves to facilitate their fluidization. The fluidization of mixtures of biomass with a second solid is studied in this present work. The biomass used herein were cereal straw of three different sizes, pine thinnings, sawdust, wood chips of various sizes, and ground thistle from energy plantations. The second solids employed in these mixtures were beach and silica sands of different sizes and densities, dolomite, and a commercial catalytic cracker. Definitions of the minimum fluidization (umf) and maximum fluidization (umf) velocities for these mixtures are discussed in this first part. These velocites were determined in columns of 14 and 30 cm in diameter. Several problems and characteristics which were exhibited during fluidization of these mixtures are described, and experimental data on Umf and Ucf for mixtures of sand and pine sawdust are presented.
The mixing and segregation of spherical and flat-shaped particles of up to 15 mm volume equivalent diameter in beds of alumina have been studied by X-rays. Visual observations enabled the mechanism to be more fully understood and the analysis of 40 photographs (from 2.5 s and from 40 min. periods) enabled “instantaneous” and “time-average” segregation patterns to be determined. The effect of shape was shown to be relatively minor except where the large flat particles are, from density considerations, inherently flotsam. At low excess gas velocities, these particles remain on or near the distributor. A comparison of the present “time-averaged” segregation patterns and ones obtained previously by bed-sectioning showed good agreement between the two techniques.
Pressure measurements were conducted in a two-dimensional hot atmospheric bubbling fluidized bed reactor in the laboratory of Energy and Process Engineering at Tampere University of Technology. A set of six fast pressure transducers was used to detect the rapid pressure fluctuations inside the bubbling bed of the reactor. These pressure transducers were placed both vertically and horizontally into the reactor. From these measurements it was found that the vertical pressure fluctuation took place at the same time at different levels of the bed. Also the same fluctuation could be seen under the air distributor. The horizontal pressure fluctuation was found to vary both by place and time. At the bottom part of the bed the highest pressure peaks was found at centre of the bed. Most of the time there was a pressure gradient the highest pressure being in the centre of the bed. This gradient creates horizontal flow of gases from middle to the sides. The velocity of this flow varies with the size of the pressure gradient. The opposite effect can be found in the upper part of the bed. The highest pressure was no more in the middle part of the bed. Instead, it was found to be between the centre of the bed and left and right walls. The pressure was low at the walls but also rather low at the middle of the bed. There must be flow towards the walls and to the centre axis. These pressure fluctuations can provide an explanation for the well-known “wandering plume” effect. They can also give a tool to better describe the mixing inside a bubbling fluidized bed. This kind of tool is needed when biomass combustion is modelled in bubbling fluidized bed.
The drag forces on biocoated particles, commonly encountered in biological fluidized beds, are greatly augmented due to increased skin drag, caused by the rough surface, and form drag, caused by the nonspherical shape of the particle. Settling velocities of biocoated particles are approximately 55-60% of an equivalent smooth, spherical particle. For nonspherical particles, the drag increases slightly more, decreasing the settling velocity another 5%. Drag coefficient correlations for biological particles should be determined for specific applications due to the observed differences in the composition of the biofilm and its impact on the particle's drag coefficient.
To develop a technique for processing high-consistency (>- 7%) fiber suspensions (HCFSs) in paper mills, it is necessary to cause the HCFSs to reach fluidization. Experiments were carried out with seven kinds of HCFSs. They could be fluidized when they were exposed to sufficient shear stresses in high shear stress fields. Formulas of the critical shear stress Td above which the seven kinds of HCFSs reached fluidization were obtained. Research showed that the Td of a turbulent generator of HCFS depends on the rotational speed N at a given shear stress field. Therefore, there is a critical rotational speed Nd once the HCFS is in a turbulent state, i.e., fluidized. A fluidized HCFS can be pumped, stored, screened, cleaned, mixed, and beaten with higher effectiveness, lower energy consumption, and less pollution.
The steady-state free-fall conditions of isolated groups of ordered packed spheres moving through Newtonian fluids have been studied experimentally. Measurements of the drag coefficients are reported in this paper for six different geometrical shapes, including isometric, axisymmetric, orthotropic, plane and elongated conglomerates of spheres. From these measurements, a new and accurate empirical correlation for the drag coefficient, CD, of variously shaped particles has been developed. This correlation has been formulated in terms of the Reynolds number based on the particle nominal diameter, Re, the ratio of the surface-equivalent-sphere to the nominal diameters, dA/dn, and the particle circularity, c. The predictions have been tested against both the experimental data for CD collected in this study and the ones reported in previous works for cubes, rectangular parallelepipeds, tetrahedrons, cylinders and other shapes. A good agreement has been observed for the variously shaped agglomerates of spheres as well as for the regularly shape particles, over the ranges 0.15<Re<1500, 0.80<dA/dn<1.50 and 0.4<c<1.0.
L'etude experimentale fait apparaitre deux regimes hydrodynamiques differents selon la valeur de la vitesse du gaz qui aboutissent pour chacun d'entre eux a caracteristiques differentes pour le transfert de chaleur et le transfert de matiere. Le systeme etudie est constitue de spheres d'alumine poreuses dans un lit fluidise contenant des particules de sable
An experimental study was carried out to estimate the residence time distribution of low density particles injected into a fluidized bed reactor containing sand particles. Tracer experiments were performed at room temperature, using a pilot plant pyrolysis reactor. A novel technique was used to detect and measure the flow of solid tracer particles having different physical characteristics entering and leaving the fluid bed. The experimental results demonstrated that the degree of particle entrainment was a function of the fluidizing gas velocity, the particle size and the particle density. Solid mixing, segregation and entrainment were also studied as functions of physical and operating parameters. Various models were tested to characterize the non-ideal solids flow patterns within the fluid bed. A circulation model appeared to give a good description of the physical mechanism involved and to provide the best agreement with the experimental results.
A novel biomass gasification (first stage of hydrogen production from biomass) process using a supercritical water fluidized bed was proposed and the fundamental design of the process was conducted. The flow rate was determined by evaluating the minimum fluidization velocity and terminal velocity of alumina particles enabling fluidization with the thermodynamic properties of supercritical water. Three cases were examined: a bubbling fluidized bed in which water was used mainly as the fluidized medium and biomass were added for gasification, a bubbling fluidized bed fluidized by biomass slurry feed alone, and a fast fluidized bed fluidized by biomass slurry feed alone. According to calculations of the residence time and thermal efficiency assuming heat recovery with a heat exchanger efficiency of 0.75, the bubbling fluidized bed fluidized by biomass slurry alone was appropriate for continuous biomass gasification using a fluidized bed.
In this work, a selection of widely used correlations have been critically evaluated for estimating the drag coefficient of non-spherical particles in incompressible viscous fluids. Experimental results have been culled from 19 independent studies embracing wide ranging particle shapes including cylinders, needles, cones, prisms, discs, rectangular, parallelepiped and cubes. The resulting data base consisting of 1900 data points encompasses wide ranges of physical and kinematics conditions as: sphericity, 0.09 to 1 and the Reynolds number ranging from 10−4 to 5×105. In particular, the performance of five methods has been critically examined. The best method appears to be that of Ganser which uses the equal volume sphere diameter and the sphericity of particle. The resulting overall mean error is about 16%, though maximum error can be as large as ∼100%. In general, the lower the sphericity, the poorer is the prediction.
Experiments were carried out to study effect of vacuum conditions over hydrodynamics of fluidization in large particle beds. Fluidization is one of the most attractive industrial processes due to its very high rates of heat and mass transfer provided by a good contact between particles and fluid.A series of experiments was carried out to analyze the hydrodynamics of a vacuum fluidized bed. A fluidization column, made of glass, with an i.d. of 100 mm was employed and the vacuum conditions were established through a vacuum pump. The tests were run with 10 different types of large particles (2250–7730 μm) under various temperatures and vacuum pressures.Experimental results show that the minimum fluidization velocity increases, decreasing the operation pressure. This tendency fully confirms the results obtained by Llop et al. [Chem. Eng. Sci. 51 (1996) 5149] under vacuum conditions. Using the experimental results two equations, one for sharp particles the other one for round particles, were developed to predict the minimum fluidization velocity and the results were compared with the other equations proposed in the technical literature. On the other hand, the experimentally obtained values of bed voidage under minimum fluidization conditions were found to be insensitive to operation pressure as also observed by Chitester et al. [Chem. Eng. Sci. 39 (1984) 253].
In the past, general formulas for calculating the drag coefficient for both spherical and nonspherical particles have involved a dependence on a Reynolds number (Re) and one or more shape descriptors resulting in complicated functions of at least two variables. This paper shows that both the Stokes' shape factor (K1) and the Newton's shape factor or scruple (K2) are important for the prediction of drag. However, instead of requiring a function of three variables (Re, K1, K2) to predict drag, a function only of the generalized Reynolds number ReK1K2 is needed. The shape factors K1 and K2 are then modeled as functions of the geometric shape descriptors' sphericity and the projected area in the direction of motion. Goodness of fit to experimental data is calculated and compared with a recently determined formula.
The total bed pressure drop, the pressure-drop profile, bed expansion and bed voidage have been measured for a variety of binary particle mixtures over a wide range of gas velocities.Apparent minimum fluidisation velosities have been defined for segregating systems, and the addition of dense particles of lower minimum fluidisation velocity can cause a decrease in apparent minimum fluidisation velocity of the mixture in a very similar fashion to the addition of finer particles to larger ones of the same density.The measured umf s are compared with presently derived simplified theoretical equations and with equations from the literature. It is clearly shown that because of the sensitivity of umf determination to voidage, such relationships cannot be used with confidence. However, the empirical equation of Cheung on average follows the shape of the experimental curves well, includig those for binary systems of different density, provided the bed is in a well-mixed condition.Bed pressure-drop profiles are related to the mixing/segregation state and to the amount of fluidisation of the bed and may offer a simple indirect method of determining these conditions in practice.
This paper focuses on the determination of the solids circulation of a CFB gasification system with a dual fluidized bed concept, and the distribution of the solid hold up under different fluidization conditions. A mathematical model of the riser was designed and implemented in a model of a dual fluidized bed system. This model contains routines for calculation of each section of the dual fluidized bed system. The behaviour of the system was analysed regarding changes in solid inventory and variations of geometry. A diagram is presented which allows an illustration of the influence of changes in the dual fluidized bed system configuration on the resulting stable operation points. Analysis concerning the effect of counter pressure on the combustion and gasification side confirms the role of the seal loop in stabilizing the operation of the gasification system.
The interaction between fuel particles and incipiently bubbling gas fluidized beds during devolatilization has been investigated by X-ray imaging. The fuel consisted of a ligneous biomass (Robinia pseudoacacia) reduced into millimeter-sized particles and doped with lead nitrate in order to make particles visible upon X-ray irradiation. A purposely designed single-particle-injector was used to impulsively introduce fuel particles one at a time at a given depth into the fluidized bed.Experiments highlighted three main features of the phenomenology, namely: (a) the formation of (endogenous) volatile matter bubbles around devolatilizing fuel particles; (b) the uprise of endogenous bubbles; and (c) the uprise of fuel particles closely associated to endogenous bubble motion. Bubble and particle trajectories and bubble cross sections as functions of time were worked out in order to assess fuel particle segregation times and endogenous bubble growth rate.The choice of operating under incipient bubbling conditions enabled thorough assessment of interactive processes establishing between gas-emitting particles and the fluidized suspension. The formation, growth and motion of endogenous volatile bubbles and the associated motion of the fuel particle could be characterized without the perturbation caused by exogenous gas bubbles (i.e. bubbles formed under freely bubbling conditions). This represents a first step towards the characterization of the interaction between gas-emitting particles and freely bubbling beds.
Coffee husk is one of the most commonly available agricultural residues in the mountainous regions of Cuba that can be converted into different types of fuel and chemical feedstocks through a variety of thermochemical conversion processes. This study provides information on moisture content, bulk density, particle size, minimum fluidization velocity, and terminal velocity of this biomass, which has been used for the design of different technologies of thermolysis.
In this paper, we apply conventional and chaotic time series analyses to the interpretation of pressure-drop measurements from a 250 kW, pilot-scale, bubbling fluidized bed combustor. Our results demonstrate that such analyses can be useful for discriminating different types of fluidization in a practical combustor and offer a basis for improving fluidized bed monitoring and control. A new comparative index that reflects the multivariate structure in pressure-drop measurements is proposed for detecting variations in the fluidized state. We also propose standards for acquiring fluidized bed pressure-drop measurements in pilot and commercial-scale facilities as well as three potential commercial uses. Recent findings regarding the effect of pressure tap location and design on the measured signal fidelity are discussed.
Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.©Her Majesty the Queen in Right of Canada, represented by the Minister of Natural Resources, 2002.
The existing methods and equations for predicting the values of minimum fluidization velocities have proved unsuccessful for the sand/straw binary mixtures. In this work these values have been determined visually for different experimental conditions. The velocity values obtained have been correlated with the straw and sand sizes and the proportions of both solids in the bed. The maximum amounts of straw in the mixtures to fluidize in different conditions have also been determined.
CFB biomass gasification has been studied by experimentation with ECN's pilot facility and a cold-flow model of this plant. Data obtained by normal operation of this plant and the results of some special experiments have provided new insight into the behavior of circulating fluidized bed reactors and CFB biomass gasifiers in particular. It has been found, for instance, that char gasification reactions and reform reactions of lower hydrocarbons do not proceed in an atmospheric CFB. Moreover, it appears that the axial and radial mixing of gas and solids is not ideal and that the coupling between the prevailing solids circulation rate, the gas velocity (profile), and the solids hold-up does not provide optimal reactor conditions. As a consequence, problems are observed in practice, such as insufficient thermal efficiency and an excessive concentration of contaminants (e.g., tars) in the product gas of CFB gasifiers. A two-dimensional engineering reactor model has been developed, partially on the basis of the results obtained from measurements with the pilot plant and the cold-flow model. The predictions of this model are in good agreement with the observed overall performance of the plant, as well as the measured axial and radial gas concentration profiles inside the riser. A novel multistage fluidized bed reactor concept has been tested (at laboratory and pilot scale) to investigate whether the problems encountered in conventional CFB gasifiers could be resolved. In this novel reactor, a separate char combustion zone is created that results in a significant increase of the thermal efficiency.
When a rotating cone with supply openings near the bottom is partly inserted into a fluidized bed, solid particles are taken up and conveyed spirally over the inner surface. This principle for particle transport was used in a novel reactor for the flash pyrolysis of biomass with several distinct advantages. Particle velocities (up to 5 m/s) were measured, as well as the solids flow rate (up to 1.6 kg/s), as a function of the cone rotational velocity, cone size and cone top angle, the fluidization velocity, and the depth of the cone in the fluidized bed. Three hydrodynamic regimes were recognized, each with its own, specific flow characteristics influencing the particle residence time (distribution) on the cone wall and the solids flow rate. The transition between the regimes could be predicted using a force balance for a single particle. The particle velocity and residence time were modeled successfully for one of the regimes.
Terminal velocity of porous spheres was experimentally measured for a Reynolds number range of 0.2 to 120 for a
normalized sphere radius, β = R/R of 15.6 to 33, where R and k are the sphere radius and permeability, respectively. The drag coefficient for 15 < β < 33 was found to be CD = 24Ω/Re [1 + 0.1315 Re(0.82 - 0.05w)] for 0.1 < Re ≤ 7 and CD = 24Ω/Re [1 + 0.0853 Re(1.093 - 0.105w)] for 7 < Re < 120 with w = log10Re where Re is the sphere Reynolds number and Ω=2β2 [1 - (tanh β/β)] / 2β2 + 3[1 - tanh β/β)]
At high Reynolds numbers, it was found that the porous sphere terminal velocity was less affected by the container walls than for the case of an impermeable sphere. However, at very low Reynolds numbers, the wall effects were found to be similar for both the permeable and the impermeable spheres.
On a mesuré expérimentalement la vitesse de chute libre de sphères poreuses, pour des nombres de Reynolds variant entre 0.2 et 120, et pour un rayon normalisé de sphère( β = R/k) de 15.6 à 33; R et k sont respectivement le rayon et la perméabilité de la sphère. On a trouvé que le coefficient de frottement, dans le cas où 15 < β < 33, était: CD = 24Ω/Re [1 + 0.1315 Re(0.82 - 0.05 w)] lorsque 0.1 < Re ≤ 7 et CD = 24Ω/Re [1 + 0.0853 Re(1.093-0.105 w)] lorsque 7 < Re < 120 w = log10ReRe est le nombre de Reynolds de la sphère et Ω = 2β2 [1 - (tanh β/β)]/2β2 + 3[1 - (tanh β/β)]
On a trouvé que, pour des nombres de Reynolds élevés, la vitesse limite de la sphère poreuse était moins affectée par les parois du contenant que lorsqil agissait d'une sphère imperméable; toutefois, pour des nombres de Reynolds faibles, les effets des parois étaient les měmes, dans le cas de sphères perméables et imperméables.
The present work reports studies on the fluidization of mixtures of biomass and sands. The biomass materials used are rice husk, sawdust and groundnut shell powder, and the sands employed are of two different densities and particle sizes. Experiments are carried out in a 5 cm ID fluidized bed column to determine the minimum fluidization velocities. The percentage of biomass materials in the mixtures studied are 2, 5, 10 and 15% by weight. Equations are developed for predicting the Umf values of these mixtures. The equations are also tested for their validity against the data in current literature on Umf values of mixtures of biomass and sands and also mixtures of particles of different sizes only. It is found that the proposed equations quite satisfactorily predict the Umf values for mixtures of different particle densities and sizes.
Circulating fluidized beds often apply secondary gas injections and diffusers in the riser. These strongly affect the fluid dynamics of the gas–solid flow in the system. This work is performed to study these effects in a cold flow model of an biomass gasifier. It is shown that in the diffuser there is a bulb of the suspension flow, which enhances the internal solids recirculation by a factor of 3.5. Thus, the solids hold-up and the pressure drop in the diffuser are significantly increased. The study on the effect of gas injection confirms that the solids circulation rate is more enhanced by gas injections in the lower part of the riser than in the upper part. From the investigated operating parameters, the gas flow rates and the particle diameter have the strongest effects on solids circulation and mass distribution in the riser. The effect of riser geometry properties, besides the cross-section areas and the total height, was found to be small.
The fluidized behavior of binary mixtures of moist sawdust and glass spheres has been investigated. The sawdust alone was observed to fluidize poorly, with extensive channelling occurring. The addition of 0.322 and 0.516 mm glass spheres to the fluidized bed of sawdust improved the fluidization characteristics. The mixtures of sawdust and 0.322 mm spheres were completely mixed when fluidized. Mixtures of sawdust and 0.516 mm spheres were either partially or completely mixed, depending upon gas velocity in the fluidized bed. As the moisture content of the sawdust was increased, the minimum fluidization velocity of the binary mixture also increased. There was an upper limit to the moisture content of the sawdust at which fluidization could be achieved. When the moisture content of the sawdust exceeded 33 and 54 wt% on a dry basis, agglomeration and channelling occurred in the mixtures of sawdust and glass spheres, with sizes 0.322 and 0.516 mm, respectively. The moisture likely contributes to interparticle liquid bridging forces. Binary mixtures of larger 0.777 and 1.042 mm glass spheres and up to 82% moisture sawdust did not readily agglomerate, but the two components completely segregated during fluidization.