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Fluidization of Biomass Particles: A Review of Experimental Multiphase Flow Aspects

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Fluidization of Biomass Particles: A Review of Experimental Multiphase Flow Aspects

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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.

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... 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]. ...
Article
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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.
... 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. ...
Article
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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.
... Ataíde, Cardoso and Oliveira [10] adopted biomass weight percentages in the mixtures of 5, 10 and 15%. Cui and Grace [11] assessed studies regarding mixtures with biomass weight content of 2%, 5%, 10% and 15%. ...
... In Eq. (10), d 1 and d 2 represent the mean particle sizes of each component of the mixture. With this calculation of effective average properties, the mixture is treated as a new single component and the Archimedes number can be calculated as a function of effective particle size and effective true density of the mixture, according to Eq. (11). ...
Article
Empirical correlations of fluidization design parameters such as characteristic velocity are based on properties such as particle mean diameter, sphericity and true density. These properties are difficult to measure for particles with morphologies typical of lignocellulosic biomasses. Thus, these correlations generally show limited practical applicability. The aim of the present work was to verify through an innovative multiple linear regression if other physical and physicochemical properties would be more appropriate for characteristic fluidization velocities prediction. The methodology involved evaluating sugarcane straw and bagasse, eucalyptus wood, rice husk and apple tree branches in binary mixtures with sand in the proportions of 20, 40, 60 and 80% of biomass volume fraction. The multiple linear regression analysis indicated that the mixture bulk density and repose angle were the most relevant parameters in the prediction of characteristic fluidization velocities. Two correlations were developed employing these properties and attained suitable results: the adjusted R² coefficient obtained for Umf prediction was 0.772 while for Ucf prediction was 0.855. In 90% of mixtures, the relative error found for Umf was smaller than 24.3% while for Ucf this error was smaller than 22.5%.
... 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 ...
Article
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.
... 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. ...
Article
Full-text available
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 %.
... 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. ...
Article
Full-text available
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.
... 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. ...
Article
Full-text available
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.
... Those new experimental results demonstrate that weak porous flow is (1) able to trigger downward sediment creep far under the criteria for either internal erosion (channelization) or pure avalanching, and (2) that the porous flow intensity also governs the creep deformation rate. This has implications for industrial processes using fluid for fluidizing and manipulating a mixture (18,19). It also has important potential for improving modeling of the inception of erosion or avalanching in nature, as well as the duration of these events. ...
Preprint
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The surface of our planet is covered by a disordered granular layer. When tilted under gravity, the majority of the time it creeps: particles slowly move downward, despite being under the critical angle (or critical stress) for avalanching. This study contributes to the fundamental understanding of granular creep, investigating how a particular mechanism -- a gentle porous flow -- enhances sediment creep under gravitational stress. In a quasi-2D microfluidic apparatus, sediment layer creep experiments were performed under sub-yielding angle and porous flow conditions. Logarithmic decay rates of the deformation are observed, with the rate increasing with both the tilt angle and porous flow rate. We identify a new dimensionless parameter, $P^*$, that accounts multiplicatively for the porous flow and sediment layer slope effects on particle motion; this allows a rescaling of all the widely dispersed creep deformation results on a single curve. This curve presents two very distinct creep regimes whose natures are not fully understood. However, observations of the void size distributions during the creep experiments also show a systematic change of the microstructure between the two regimes.
... The methodology for the design and operation of fluidized bed equipment, using biomasses materials, are based on the knowledge and conventional fluidization technologies developed for coals [25] and underestimating the importance of the properties of the biomasses particles in the fluid dynamics behavior; which is critical to the successful design and operation of the systems. Therefore, it is imperative to research aimed the precise characterization of several types of polydisperse biomass [26], especially in the determination of sphericity which is one of the most important parameters that influence in the fluid dynamics behavior of the particles. In this scenario, the focus of the present work is to determine the main geometric characteristics, such as size distribution, aspect ratio, and sphericity of raw sugarcane bagasse, that is, without any previous process of reduction size (in natura or in its natural state). ...
Article
Sugarcane bagasse is used as energy source in the sugar cane industry. An appropriate characterization of their physical and morphological properties is vitally important for their use in the gasification and combustion processes. This work makes a study of the size and shape distribution of the bagasse generated in a sugarcane mill based upon two important factors that influence their physical properties, such as aspect ratio (AR) and sphericity. The distribution of the grain sizes has been determined for the analyzed samples; the shapes of the particles has been determined by a manual method and through an image-analysis method, using the ImageJ software. It has been observed that sugarcane bagasse has a larger length than the other analyzed biomasses, with an almost similar width. The aspect ratio was between 2.14 and 5.5 for all the studied size ranges (9.5–0.15 mm). For particles with diameters smaller than 0.3 mm, the aspect ratio has a similar behavior to the other biomasses, with a reported value of 2.5. The determined sphericity was between 0.27 and 0.558, with a mean value of 0.397.
... The predicted minimum fluidization velocities, based on Eq. (15), were consistently in good agreement with the empirical correlation of the superficial velocity and terminal settling velocity of the fluid in this study and proved that the system had undergone a proper fluidization system. Extensive studies have been carried out to predict the minimum fluidization velocity of inert particles, including sand, glass beads and alumina [36e38, 57,58,60,61]. Table 7 shows a comparison of some of the values of U mf developed by several researchers. Most of them corrected their experimental data to develop empirical correlations in different operating conditions. ...
Article
Full-text available
A mathematical model of minimum fluidization velocity (U mf ) was developed based on the hydrodynamic characteristics of the fluidized bed reactors (FBR) with immobilised cells attached to activated carbon at thermophilic biohydrogen fermentation. The maximum hydrogen productivity rate of 7.8 mmolH 2 /L.h and hydrogen yield of 2.2 molH 2 /mol of sugar consumed was obtained when the HRT was shortened from 48 h to 6 h. The presence of the immobilised cells enriched the biomass composition in the FBR from 4.9 to 7.1 g VSS/L and maximum energy generated was 58.7 KJ H 2 /L.d. The FBR had to be operated at a high U mf of 0.05–0.44 cm/s and a low terminal velocity of 2.11 cm/s to prevent the immobilised cells from washed out from the FBR, hence achieved an adequate fluidization system. A screening of the microbial population by DGGE revealed that the T. thermosaccharolyticum sp. was dominant for all the HRTs, thereby indicating that this bacterium is resilient towards environmental disturbances.
... Instead, Atienza-Martìnez et al. (2013) processed dry sewage sludge (SWS) in the 250-500 μm size range with a bed consisting of torrefied SWS particles from previous experiments in order to reduce problems associated to start-up. However, it is worth noting that, although the test configurations proposed by the abovementioned Authors exploit the benefit of avoiding the separation of the torrefied product from heat carrier solids, the well-known difficulty to fluidize biomass particles alone (Cui and Grace, 2007) may restrict the use of such technology to a few biomass feedstocks. ...
Article
An experimental study was carried out to evaluate an alternative disposal route for waste orange peels, which includes their upgrade into a solid biofuel and, possibly, the production of valuable liquid bio-chemicals via fluidized bed torrefaction. The influence of the torrefaction temperature (200 and 250°C), the reaction time (5 and 15 min) and the use of different bed materials (Ticino sand and γ-alumina spheres) on the distribution and composition of the main output products (torrefied solids, condensable volatiles and permanent gases) was investigated. Results show that the torrefaction at 250°C transforms orange peels from low-value residue to high-quality energy carrier by increasing their energy content (by a factor of 1.5) and reducing the O/C ratio (up to 67%). The temperature strongly influences the process performance and is the key process variable. Conversely, almost no effect were detected by changing the reaction time and the kind of granular solids on both the distribution and the composition of the main torrefaction products. Although γ-alumina does not exhibit a remarkable catalytic activity in the investigated temperature range, its use instead of sand is recommended as its lower density makes the bed smoother to be fluidized.
... This is due to the effect of temperature on the activity of the yeast and in turn the rate of glucose conversion. This result is agreed with the results reported by Heping [13] and Baur [14]. yeast / sugar ratio=1 yeast /sugar ratio=2 yeast/ sugar ratio=5 yeast/sugar ratio=10 ...
... Due to some peculiar characteristics (stickiness, strongly irregular shape or also large size), materials of this kind are often difficult to fluidize alone so that an inert solid, usually made of denser and smaller particles, is added to them both to facilitate their suspension and to ensure high rates of heat exchange and good overall process performance. Silica sand, alumina, or calcite are some of the inert solids in common use [6]; in fluidized bed processing, they are the most abundant components of the particle system, while the biomass usually represents a lower percentage of the total mass of the bed. In this way, the thermal effects associated to the combustion or gasification process are not as strong as to produce hotspots in the equipment, while the large mass of the inert component ensures the thermal stability of the fluidized system. ...
Article
The paper shows how the equations developed for modeling the fluidization process of two-solid mixtures, originally tested on mixtures of spherical particles, can successfully be extended to segregating fluidization of beds of irregular solids. The influence of particle shape on bed voidage at varying component concentration is analysed and it is demonstrated, in particular, that the relationship that calculates the final fluidization velocity of any binary mixture maintains its predictive ability also for solids of practical interest like in the case of beds of biomass and inert particles. As regards the mixtures of olive pits and sand addressed in this study, that makes possible a comparison among the mixing states of these systems at varying average size of the inert component, each at its own uff, a fluid dynamic condition at which the equilibrium between the mixing and segregation tendencies in the dense phase of the bed is fully developed. In this way, an appropriate choice of the particle size of the inert material can be made for the purpose of promoting component mixing and even biomass distribution along the bed height.
... At particle scale, the porosity and micro-structure of biomass directly influence the species and heat transfers (Ciesielski et al., 2017;Pecha et al., 2018), and momentum transfer between the gas and biomass particles. The low bulk density of biomass affects the performance of feeding systems, when gravity-based feeding and handling systems are used (Williams et al., 2016), the size and shape of biomass particles will directly influence the fluidization behavior (Cui and Grace, 2007). ...
Article
Efficient utilization of biomass is challenging due to the complexity of the feedstock at molecular, particle, and reactor scales. Direct coupling of particle scale model and reactor scale simulation is computationally infeasible. In this research, the results from a particle resolved simulation were used to calibrate the heat transfer and reaction kinetics of particles. A hybrid drag model was used to consider the different fluidization properties of sands and biomass particles. The computation cost was reduced using a coarse-grained Discrete Element Method (DEM). The simulated conversion rate, char yield, tar yield, and pyrolysis gas yield compare well with experiment. The residence time and axial distributions of biomass were also analyzed. This novel multi-scale method provides an efficient and accurate tool for the modeling of biomass pyrolysis reactors.
... However, all the aforementioned studies considered particle shape as spherical. In practice such as biomass particles in gasification or combustion processes [18], tablets coating in fluidized beds [19], food processing [20], and bulk solids handling [21], particle shapes are complex, either regular or irregular. Particle shape can affect particle behaviour significantly, as demonstrated in mono-sized fluidizations [22][23][24][25][26][27][28]. ...
Article
Shape is one of the most important properties of particles, and can affect particle flow behavior significantly in particulate systems. In the past, extensive studies have been conducted on the effect of particle size and density on the mixing quality of particle mixtures in gas-fluidized beds, but little is known regarding the influence of particle shape. In this work, CFD-DEM approach is used to investigate the mixing of binary mixtures composed of ellipsoids and spheres. A modified drag model suitable for multicomponent mixtures of nonspherical particles is proposed first, and its validity is then verified by comparison to experimental data. The simulation results show that for the cases considered, adding a second component of ellipsoids to spheres results in reducing the minimum fluidization velocity of the mixtures; however, oblate particles in decreasing the minimum fluidization velocity is more significant than prolate particles. The mixing index of all binary mixtures generally increases with increasing gas superficial velocity. The results also show that with the added component varying from spheres to oblate or prolate particles, segregation happens in the bed and becomes more severe with aspect ratio. It is found that the effect of particle shape on the drag force is responsible for the occurrence of particle segregation.
... Two-component beds used in process industry are frequently made of solids of different nature and size that form mixtures of various compositions [1][2][3][4]. Phenomena of segregation may occur during their fluidization and have negative effects on the efficiency of the operation. ...
Article
Slow defluidization of a two-solid mixture brings to a fixed bed with a characteristic concentration profile of the two species along the bed height, which is related to the initial and the final fluidization velocities of the mixture. The paper shows that the one-parameter model devised for describing the progress of binary fluidization in fundamental terms can be used for calculating the defluidized composition profile. It is discussed how the model equations cover the behaviour of systems that achieve the fluidized condition both through the mechanism of ‘top fluidization’ and that of ‘bottom fluidization’, a condition typical of beds in which the differences in component density and size act simultaneously on the segregation tendency and cause the denser material to be the fluidized component, if sufficiently smaller. Changing the fitting procedure used to assign a value to the parameter of the model is demonstrated to improve its predictive ability, although a limit to it is set from the singularity of the fluid dynamics which characterizes the distributor region at the base of the fluidization column.
... Dual circulating fluidized bed (DCFB) gasification, as illustrated in Figure 2, is a better design in overcoming this challenge. A supporting bedding material such as sand particles is used, as the wood chips/pellets are hard to fluidize [10]. DCFB system separates the combustion reactions from the gasification and pyrolysis reactions. ...
Article
Full-text available
Gasification in fluidized beds is an outstanding technology in biomass to energy conversion. The multiphase particle-in-cell modelling has reduced the computational time related to CFD simulations of dense gas-solid systems like fluidized bed gasification. Barracuda VR commercial CFD package was used to analyse the effect of reactor temperature in steam gasification of biomass. The product gas composition, lower heating value and the cold gas efficiency were compared for steam at 873K, 973K and 1073K. The steam-to-biomass ratio was maintained at a constant value of 0.45. The hydrogen content of the product gas changed from 36% to 57% as the temperature was increased from 873K to 1073K whereas the carbon monoxide content changed from 33% to 13%. The lower heating value and the cold gas efficiency changed from 10.4 MJ/kg to 10.1 MJ/kg and 76.6% to 73.2% respectively within the same temperature range. The formation of tar was not modelled and the gas composition showed high sensitivity towards the reactor temperature
... In fact, divergence from the general behavior may also occur such as channeling, jetting and slugging [13]. Using a second (inert) material (silica sand, alumina or calcite, for instance) might facilitate fluidization of highly heterogeneous particles such as biomass, by reducing the friction forces between particles [14]. Although some segregation may occur due to the particle size and density differences between materials [15], binary particle systems are extensively used in industrial applications favoring fluiddynamic performances and conversion. ...
Conference Paper
The present work reports on the experimental results from the fluid-dynamic characterization of different Colombian agroindustry residues combined in a binary system with a bed material. Oil palm shell, fiber and rachis, sugar cane bagasse and top, rice husk, coffee husk and poultry litter were studied. Experiments were carried out using mixtures of 3, 5, 7, 10% w/w of biomass in silica sand. A cold flow unit of 13,5 cm diameter and 67,0 cm height was used, pressure drop, superficial gas velocity and bed height were simultaneously measured. This study focus on determining the minimum fluidization velocity of the mixtures that exhibited bubbling behavior, using correlations based on pressure drop and dimensional analysis principles. More than 20 different correlations were compared and five of them were analyzed in detail. The methodology proposed by Rao and Bheemarasetti is also studied and deviations of predicted and experimental minimum fluidization velocities for each mixture are shown. The results showed, on the one hand, that the minimum fluidization velocity of the mixtures can be satisfactorily predicted with a mean relative error of 7,3%, lower than in other studies using similar correlations. It was found that the correlation proposed by Grace accurately predicts the performance of small particles type A or near to the AB boundary, the one deduced by Wen and Yu agrees with the observations made for B particles and, type D can be well described with Chang correlation. On the other hand, an empirical factor close to the predicted by Rao equation was obtained, it allowed the estimation of the minimum fluidization velocity for the 3% mixtures with a mean relative error of 11,4%. Nevertheless, large errors were observed for the remaining mixtures. This study broadens the understanding of fluidizing operations using residual biomass particles and contributes in this way to the optimal design of fluidized beds for thermochemical conversion of such materials.
... Innovación para un nuevo país Poca o nula información técnica es reportada en la literatura que permita estimar el comportamiento de estas biomasas residuales en reactores de lecho fluidizado [4]. Conocer sus características físicas y fluidodinámicas resulta esencial para determinar la viabilidad técnica del proceso, para el modelamiento del mismo y para el diseño de los reactores [5]. ...
Conference Paper
En el presente documento se presentan los resultados de la caracterización física y fluidodinámica de cuesco, fibra, raquis de palma de aceite, bagazo y cogollo de caña, cascarilla de arroz, cascarilla de café y pollinaza, como las biomasas residuales actualmente disponibles y con el mayor potencial energético técnico en Colombia. Este estudio tiene como objetivo emitir conceptos sobre la idoneidad de dichas biomasas para su aprovechamiento termoquímico en reactores de lecho fluidizado. Las propiedades físicas analizadas fueron el contenido de humedad, el tamaño de partícula, la esfericidad, la densidad de partícula, la densidad a granel y la clasificación de tipo de partícula según el diagrama de Geldart. La caracterización fluidodinámica incluyó la determinación de la velocidad mínima de fluidización y la descripción de los fenómenos destacables durante la fluidización con aire a condiciones ambiente. Los resultados permiten observar la gran variabilidad en las propiedades para diferentes biomasas y la falta de normas apropiadas para la caracterización de estos materiales. Se concluye que las muestras con baja densidad a granel, baja esfericidad y un alto diámetro medio de partícula tienden a formar flujos tipo pistón y aglomerados que inhiben la fluidización. Aquellas con alta esfericidad, mayor densidad de partícula y menor tamaño medio de partícula exhiben comportamientos más regulares, que permiten la operación en regímenes de lecho burbujeante.
... Given the strong interaction of gas and solids in fluidized bed, fluidization properties of the materials are important for the design and modelling of the system [3]. Some researchers already indicated the need for studying not only chemical but also fluiddynamical characteristics of the biomasses in order to better predict their behavior [4]. Biomass particles have in general rather low densities and tend to have irregular forms and sizes, therefore, fluidization is promoted by mixing the biomass with a bed material, of smaller particle size and higher density, which additionally acts as a heat transfer medium inside the reactor and also outside, for the case of circulating fluidization. ...
Conference Paper
In Colombia a wide range of biomass is available and a high rate of residual biomass is generated from the main economic activities. The implementation of efficient technologies for energy conversion processes depends in large extent on the adequate study and selection of the biomass. In Colombia, only little experience have been made in fluidization of biomass, the implementation of the technology requires accurate information that allows a feasibility evaluation of the process for each type of biomass. In the present work, an initial stage has been focused on the availability, accessibility and technical suitability of a number of residual materials that can be found in the country; environmental and social aspects have been considered in some extent depending on the available information. Biomass of four different types was taken under consideration, agriculture, livestock, forestry and urban waste. This first screening has resulted in a short list of materials of interest for which the calculation of the theoretical and technical energy potential was done. Twelve residual biomasses are identified to be technically suitable and available in Colombia for their use in thermochemical conversion processes in fluidized beds. The results make possible a second screening of materials, selected materials will be analyzed for fluid-dynamical and thermochemical characterization.
... Many studies and applications have shown that circulating fluidized beds have become an important reactor for the combustion and utilization of petroleum coke (Chen and Lu 2007). The process of biomass conversion and utilization is also often carried out in a fluidized bed (Cui and Grace 2007). However, fluidization of biomass particles is a difficult task because the non-spherical biomass particles are generally large and have a low density. ...
Article
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Fluidized beds are widely used in many industrial fields such as petroleum, chemical and energy. In actual industrial processes, spherical inert particles are typically added to the fluidized bed to promote fluidization of non-spherical particles. Understanding mixing behaviors of binary mixtures in a fluidized bed has specific significance for the design and optimization of related industrial processes. In this study, the computational fluid dynamic–discrete element method with the consideration of rolling friction was applied to evaluate the mixing behaviors of binary mixtures comprising spherocylindrical particles and spherical particles in a fluidized bed. The simulation results indicate that the differences between rotational particle velocities were higher than those of translational particle velocities for spherical and non-spherical particles when well mixed. Moreover, as the volume fraction of the spherocylindrical particles increases, translational and rotational granular temperatures gradually increase. In addition, the addition of the spherical particles makes the spherocylindrical particles preferably distributed in a vertical orientation.
... However, fluidizing biomass particles is challenging, due to size and density distribution and nonspherical shape of biomass particles. 1 To achieve an efficient gassolids contact, the biomass particles are usually co-fluidized with inert materials such as sands. The mass fraction of sand particles is usually higher than 90%. ...
Article
Full-text available
The bubbling fluidized bed reactor is widely used in fast pyrolysis of biomass. Discrete simulation of this reactor is challenging due to many sand particles and lack of accurate drag corrections accounting for the interaction of two different solid particles with different properties. In this research, the computational cost is reduced by using the Coarse‐Grained Computational Fluid Dynamic Discrete Element Method (CFD‐DEM), where many sand particles are lumped into a larger numerical parcel. The Syamlal‐O'Brien drag model is used for sand, while Ganser correction coupled with Gidaspow model is used for the non‐spherical biomass particles. This hybrid approach shows superior behavior over other drag models using pressure drops as a benchmark. The predicted bed height and pressure fluctuating frequencies compare well with experiment. The mixing of biomass is close to perfect if the superficial velocity is larger than four times the minimum fluidization velocity. This article is protected by copyright. All rights reserved.
... One of the major issues that arise during model development for solid particles is the incorporation of non-spherical particle shape especially with large aspect ratio ( Gera et al., 2002 ). In recent years, several investigations of spherical particles have been carried out, however the work on the influence of particle properties such as size, shape, etc., remains limited ( Zhong et al., 2008;Fotovat et al., 2014;Cui and Grace, 2007;Zhong et al., 2016 ). One commonly encountered particle in the industry is of cylindrical shape. ...
Article
Hydrodynamics of rod-like particles, found extensively in the chemical and process industry, is investigated here, using direct numerical simulations. A recently developed formulation of the smoothed profile method for rigid bodies is first validated for rod-like particles (rods) and then used to characterize the concentration effects on the static and dynamic properties of rods in the Stokes regime. Normalized drag and lift coefficients of a single rod of aspect ratio 3 and 4, followed the well known sin²θ and sinθcosθ curves against the incident angle (θ), respectively. We found significant inhomogeneity in the microstructure of the settling rods. These inhomogeneties, cause the formation of clusters, even at low volume fractions. These clusters move as a large lump, inducing pronounced hydrodynamic interactions, which have significant effects on the settling system. The average settling velocity normalized by the terminal velocity of an isolated rod, shows a non-monotonic behavior; increasing at low concentration because of the cluster formation in which rod packets settle faster than an isolated rod, and decreasing at high concentration because of many particle interactions. Furthermore, velocity fluctuations are larger than those of spherical particles because of the cluster formation. Histogram of particles’ orientation angle shows that the majority of the particles are vertically aligned.
... 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]. ...
Article
Full-text available
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.
... 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]. ...
Article
Full-text available
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. ...
Thesis
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.
... 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). ...
Thesis
Full-text available
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. ...
Preprint
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The binary fluidization of Geldart-D type non-spherical wood particles and spherical 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. ...
Article
Full-text available
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. ...
... 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. ...
Article
Full-text available
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. ...
Article
Full-text available
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.
... This obviously causes the segregation of bed material and fuel particles. In 2007, Cui [20] reviewed present work related to the mixing behaviour in fluidised bed. He stated that, so far, still little knowledge is available for inert particles, let alone for reacting particles, for example, during the pyrolysis. ...
Article
Full-text available
Differences in the densities of bed material and—especially biogenic—solid fuels prevent an ideal mixture within bubbling fluidised bed (BFB) combustors. So, the presence of fuel particles is usually observed mainly near the surface of the fluidised bed. During their thermal conversion, this leads to a release of unburnt pyrolysis products to the freeboard of the combustion chamber. Within the further oxidation, these species will not transfer their heat-of-reaction to the inert bed material in the way of a convective heat transfer, but rather increase the gas phase temperature providing probably some additional radiative heat transfer to the dense bed. In this case, the so-called heat release efficiency to the fluidised bed, being the ratio of transferred heat to the fuel input, will be reduced. This paper presents a methodology to quantify this heat release efficiency with lab-scale experiments and the observed effects of common operating parameters like bed temperature, fluidisation ratio and fuel-to-air ratio. Experimental results show that the air-to-fuel ratio dominates the heat release efficiency, while bed temperature and fluidisation ratio have minor influences.
Chapter
This chapter contains sections titled: Introduction Fast Pyrolysis Bio‐Oil Upgrading Gasification Fischer‐Tropsch Synthesis Summary
Article
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.
Article
A three-dimensional CFD model was developed to simulate the hydrodynamics in a pilot-scale dual fluidized bed system for biomass gasification. The system includes a fast fluidized bed (FFB) for char combustion, a cyclone separator, two loop-seals, and a bubbling fluidized bed (BFB) for biomass gasification. A comparison of a hybrid EMMS drag model (i.e. the EMMS/matrix drag model for FFB, and the EMMS/bubbling drag model for BFB) proposed in this paper and the Gidaspow drag model was carried out at air and steam gasification conditions. The solid circulation rate, solid inventory distribution, and the pressure distribution predicted by the hybrid EMMS drag model were in better agreement with experimental data. The effect of solid inventory on hydrodynamics was evaluated by using the hybrid EMMS drag model. The results indicate that the solid circulation rate and its fluctuation increased with increasing solid inventory. In the simulated system, the operating stability was reduced when the solid inventory was increased to 200 kg due to “choking” phenomenon.
Article
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.
Article
Heat transfer behaviors in a pulsed fluidized bed are numerically investigated by means of a combined approach of computational fluid dynamics (CFD) and discrete element method (DEM). The effect of pulsation flow on bed heat transfer characteristics is evaluated. It is found that the pulsed gas flow leads to an intense oscillating flow behavior in the bed, enhancing the the cooling rate of hot particles. Three kinds of heat transfer mechanisms are examined. The results reveal that the particle-fluid convection is dominated. Meanwhile, heat transfer mechanisms in a polydisperse system are also discussed. It is observed that the addition of fine particles promotes the total particle-fluid convection and the fluctuating degree with time becomes evident.
Chapter
The emission of greenhouse gases in the environment in order to satisfy the demand for electricity and fuel has raised severe climate change issues in various parts of the world. Thus, switching from conventional to renewable power sources has become necessary. Biomass a renewable energy source has the potential of becoming an alternative to the conventional energy sources. Gasification is a thermochemical process that converts waste biomass into a gaseous product known as a syngas and provides environment-friendly waste disposal. Synthesis gas produced through biomass gasification process can be further utilized for power generation or various thermal applications. This chapter discusses various conventional gasification systems existing for biomass gasification along with new technological development. It also delivers an assessment of the impacts of fundamental and interrelating process parameters such as reactor temperature, equivalence ratio, biomass particle size, bed material, etc. on gasification process. Further, a section on various producer gas cleaning technologies to make syngas suitable for power generation applications is also included in this chapter.
Article
Fluidization behavior of biomass and glass beads binary mixtures in a bubbling fluidized bed was experimentally investigated. Mixtures containing different mass fraction of Loblolly Pine white wood and glass beads were fluidized at different fluidization velocities. The particle properties were characterized in a QICPIC that uses a dynamic image processing method to measure both particle size and sphericity. The minimum fluidization velocity was determined using the pressure drop method. An image processing method was developed to capture the dynamic expanded bed height at a very high frequency. The effect of biomass mass fraction and inlet gas velocity on mixing and segregation behavior was studied and analyzed through pressure drop measurements. Pressure drop fluctuations and expanded bed height fluctuations via Fast Fourier transform were analyzed and compared. The complete and accurate experimental data reported in this study could provide a benchmark dataset for various CFD models validation, calibration and identification.
Article
Non‐spherical particles are commonly found when processing biomass or municipal solid waste (MSW). In this study, cylindrical particles are used as generic non‐spherical particles and are co‐fluidized with small spherical particles. X‐ray particle tracking velocimetry (XPTV) is used to track the 3D particle position and velocity of a single tagged cylindrical particle over a long time period in the binary fluidized bed. The effects of superficial gas velocity (uf), cylindrical particle mass fraction (α), particle sphericity (Φ), and bed material size on the cylindrical tracer particle location and velocity are investigated. Overall, the cylindrical particles are found in the near‐wall region more often than in the bed center region. Increasing the superficial gas velocity uf provide a slight improvement in the uniformity of the vertical and horizontal distributions. Increasing the cylindrical particle mass fraction α causes the bed mixing conditions to transition from complete mixing into partial mixing. This article is protected by copyright. All rights reserved.
Article
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Aiming to better understand the biomass pyrolysis and gasification processes, a detailed experimental study of the mixing characteristics is conducted in a fluidized bed with binary mixtures. Rapeseed is used as biomass, and silica sand or resin as inert material. The effect of mixture composition, initial packing manner, and superficial gas velocity on the concentration distribution is investigated in a rectangular fluidized bed by means of photography and sampling methods. The results show that the mixture composition plays an important role in the axial solids profile of binary mixtures. The mixing behavior of binary mixture is dominated by the bubble movement. The axial distribution of binary mixtures becomes uniform with increasing superficial gas velocity, whilst no obvious effect of initial packing manner is observed in this study.
Article
This work presents hydrodynamic studies on the effect of temperature and bed particle variation on minimum fluidisation velocity. A lab-scale bubbling fluidised bed made of stainless steel, with column height of 1 m and internal diameter of 0.15 m was used. Five Geldart-B-type alumina bed materials of mean particle sizes 75, 177, 250, 320 and 500 µm were used and the bed temperature was varied between 50 ℃ and 600 ℃. The hydrodynamic results showed that minimum fluidisation velocity varied directly with mean particle size and inversely with temperature. The pressure drop was found to increase with bed temperature for all the sizes of bed particles. The minimum fluidisation velocity is a key parameter in the design of fluidised bed systems and as such its prediction lies with good choice of correlation. The experimental values were compared with predicted minimum fluidisation velocity from six correlations, with the overall objective of verifying the most suitable correlation. Of the correlations used, the Hartman et al. correlation showed very good fitting with experimental data for beds of 75 and 177 µm particles, and the Mohanta et al. correlation showed good fitting with 250–500 µm bed particles. The models exhibited very good coefficient of multiple determination (R²), very low fitting errors and low geometric standard deviations across the range of particle sizes and temperature variation. However, the lack of understanding of the parameters such as voidage, particle sphericity, density and viscosity would need more attention, particularly in view of the sensitivity of the minimum fluidisation velocity predictions to these parameters. Further studies are therefore recommended for the verification of the correlations in this regard.
Article
Particle mixing and segregation behavior in an Air Dense Medium Fluidized Bed with binary mixtures of solid particles were investigated for dry coal beneficiation. Magnetite mixed with coal/gangue/sand particles belonging to Geldart B/D group were tested individually for the bed density adjustment. The effects of operating parameters including particle density ratio, particle size ratio, mixture composition, superficial gas velocity, and fluidized bed height on the mixing and segregation pattern were examined. The results demonstrated that the segregation becomes more severe with increasing density difference of binary mixtures. An increase in particle size ratio may also lead to partial segregation. Mixing and segregation of binary systems are almost independent of lower excess gas velocity and initial bed height when it is over 15 cm. Moreover, a mixing index was employed to evaluate the mixing and segregation performance, and the criteria for good mixing to achieve the bed density adjustment were identified.
Article
A method for estimating minimum fluidisation velocities in a well-mixed, bi-disperse fluidised bed of spherical particles is described where a drag model is combined with a particle packing model. The method described does not require empirical input about a specific particle mixture, and so these minimum fluidisation velocities can be estimated over wide ranges of size and density ratios. The treatment is fully non-dimensionalised. It is shown that two minimum fluidisation velocities may be defined for a well mixed bi-disperse bed: the gas speed at which fluidisation initiates determined from considering the bed as a whole, and a higher one corresponding to the balance of forces on an individual particle. The differences between bi- and mono-disperse beds are the change in particle volume fraction owing to packing, the difference in drag around individual particles compared with the average drag, and the action of the hydrostatic pressure gradient. The latter two effects tend to increase the difference between the two limits of minimum fluidisation velocity, while packing decreases it and intensifies the dependence on mass fraction of the minimum fluidisation velocities. The influence of inertia is determined from particle properties through an Archimedes number. Though the inertial effects are not large for a wide range of particles, they can start to dominate other influences on the minimum fluidisation velocities as particle diameter increases.
Article
In this study, computational particle fluid dynamics were applied to investigate the segregation characteristics of char particles in a bubbling fluidized bed. We examined the bubble behavior and the consequent segregation of char particles with respect to the nozzle arrangement of the distributor in a bubbling fluidized bed. The segregation characteristics of a mixture of char and sand particles were analyzed in terms of pressure fluctuation, bubble diameter, the number of bubbles, and the mixing index. It was observed that an increasing number of nozzles resulted in a decreased bubble diameter, which led to better segregation between char and sand particles. The mixing index of char and sand particles decreased from 0.75 to 0.42 from nnozzle = 8 to nnozzle = 17, then increased to 0.50 at nnozzle = 20. When the number of nozzles was more than nnozzle = 17, the bubble diameter increased, and the number of bubbles decreased because the bubbles coalesced with surrounding bubbles.
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A CPFD hydrodynamic model was developed for a circulating fluidized bed system and the simulation results were validated against experimental data based on particle circulation rate. Sensitivity of the computational mesh was primarily tested and extended grid refinement was needed at the loopseal to match the particle circulation rate with experimental data. The particle circulation rate was independent of the range of number of computational particles used in this study. A 10% reduction of the particle circulation rate was observed as the particle-wall interaction parameter was changed from 0.85 to 0.55 and 17% increment when the close-packed volume fraction was changed from 0.56 to 0.62. The pressure constant in the particle stress model showed the greatest impact for the circulation rate with 57% increment as the constant was changed from 2.5 to 5. The highest absolute variation in the pressure was observed at the loop seal and pressure values were under predicted in all sections. • Highlights • CPFD simulations are efficient in analyzing fluidized bed systems. • Manipulating of particle circulation rate is important in circulating fluidized bed. • Pressure constant in particle stress model is the most influential factor. • Uncertainties should be minimized prior to optimization of model parameters.
Chapter
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.
Article
Recently, biomass has gained attention as a renewable energy resource due to low emissions and ample availability. Thermochemical conversion technologies to handle biomass are being developed rapidly. Due to complicated multiscale nature of biomass thermochemical conversion, numerical methods are playing important role in understanding detailed underlying mechanisms governing the processes. In this context, computational fluid dynamics (CFD)is playing an important role both as a research as well as design tool. There have been many publications in recent years using CFD to simulate systems involving biomass feedstock. In this work, we have tried to briefly summarize the CFD simulations of biomass systems especially after the year 2010 with focus on the drag models being used under the framework of Eulerian methodology. Drag modeling of biomass systems requires special attention due to non-spherical nature of particles both before the initiation of conversion process as well as during the process of thermochemical conversion. It is, therefore, this aspect that we have tried to highlight in this work with a brief review of recent relevant literature as well as a summary of some popular spherical and non-spherical drag models that can be or are being used to model systems containing biomass particles.
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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.
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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.
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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.
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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.
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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.
Article
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.
Article
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.
Article
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.
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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.
Article
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.
Chapter
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.
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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.
Conference Paper
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.
Article
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.
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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.
Article
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.
Article
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
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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.
Article
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.
Article
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.
Article
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].
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Conference Paper
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.
Article
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.
Article
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.
Article
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.