Atmospheric spouted bed combustion: The role of hydrodynamics in emissions generation and control

Recent studies have reaffirmed the utility of spouted beds as potential combustion devices for a variety of fuels: solid, liquid and gaseous fuels (Arbib and Levy, 1982; Weinberg et al., 1988; Zhao et al., 1987; Altwicker et al., 1989; Altwicker and Lin, 1991; Altwicker et al., 1993; Konduri et al., 1994). In this study, the role of annulus residence time on the performance of such a spouted bed combustor is investigated. The experiments presented here show that fluid hydrodynamics in the annulus of a spouted bed combustor can significantly influence the overall performance; changing the residence time in the annulus significantly affects the concentrations of products due to incomplete combustion at the exit of the combustor. The annular residence time is varied by changing the shape of the bottom of the reactor, by introducing a draft tube and by changing the bed heighn. Propane is used as the fuel and sand as the bed medium. Inferences from the combustion results are supported by pressure and temperature profiles. A streamtube model (Lim and Mathur, 1976) was used to obtain an estimate of the residence time in the annulus. It is concluded that the flame processes taking place at the top of the annulus play a critical role in the emission generation and that more information is needed to understand the annulus-flame.     

A study on mixing mechanism in air spouted beds

The mixing mechanism of solid particles in the air spouted bed was studied by employing an impulse response technique. The particles, german millet and barley, were spouted by air in columns of diameter of 8.4 cm and 12.6 cm. In the proposed theory, it was assumed that the mixing of the particles in the spouted bed occurs when they circulate through spout, fountain and annulus. Also a theoretical model was derived by assuming that the particle flow in the annulus is a combination of many annular plug flows while the flow in the spout as well as in the fountain is a mixed flow The residence time distribution of the particles in the bed was measured by injecting a portion of colored particles into the feed line and analyzing the concentration of the colored particles in the discharge line. The experimental results and the proposed theory were most satisfactorily agreed when the null residence time in the spout and in the fountain was assumed in the theoretical model.     

喷动床中气体停留时间分布的研究

In a spouted bed of 80mm in ID and 1700mm in height, the gas residence time distributions at different radial positions in both spout and annular area were measured with five different kinds of particles as spouting material, air as spouting gas, and hydrogen as tracer. The effects of superficial gas velocity, operating pressure, particle size and its category on gas residence time distribution were discussed. It was found that the gas velocity profile in spout was more uniform than that in annulus. It could be concluded that the gas flow in the spout could be treated as a plug-flow, while that in the annulus inhibited a strong non-ideal flow behavior. Increasing the superficial gas velocity and decreasing the operating pressure, the particle density and its size gave rise to spouting disturbance, thus the measured tracer concentrations vs. time curves fluctuated. The variances of residence time distribution curves could be taken as a measure of the gas fluctuation degree.     

Flow in the annulus of a bed of fine particles spouted with water

The flow in the annulus of a water spouted bed of glass particles (275 to 774 μm) was studied experimentally in a cylindrical half-column 50.8 mm in diameter. An axisymmetric model, which assumes Darcy flow in the annulus and uses the experimental spout pressure distribution, predicts the flow and pressure fields in the annulus. The substantial differences between this flowfield and that for coarse particle beds that are observed are caused by differences in the normalized interfacial pressure profile. The model predicts that both the fluid velocity and the normalized fluid velocity at the top of the annulus decrease as the particle size and bed height are reduced. Particles are observed to enter the spout primarily near the spout inlet in agreement with predictions of the axial spout voidage distribution. The residence time distribution of the fluid in the annulus is relatively broad and the measured residence times are about 25% higher than those calculated.     

Residence time distribution of gas in spouted beds

Stimulus-response experiments have been carried out, using helium gas as tracer, to measure the residence time distribution of gas in spouted beds. The data obtained, under a variety of experimental conditions, are compared against predictions from a two-region gas flow model. The model postulates plug flow of gas in the spout and axially-dispersed plug-flow in the annulus, the extent of axial dispersion being accounted for by a coefficient, D, which is an adjustable parameter of the model. The values of D thus determined were found to increase with gas flow rate, to be relatively insensitive to bed path, and to be much larger for beds of wheat than of polystyrene.     

Drying characteristics of a draft tube spouted bed

Using the results of earlier work on the flow pattern of gas and solids in a draft-tube spouted bed, the drying characteristics of such a spouted bed are discussed. Paddy (unpeeled rice) was dried in a 30 cm bed with a 5.5 cm draft tube and temperatures and moisture content of solid and gas phase were measured as a function of drying time at various points in the system. It appears that the (constant) drying rate is determined by the heat transfer in the recirculation zone (just above the air inlet) and in the draft tube. After the short and rapid heating in the tube, the temperature and moisture gradients in the particle equilibrate in the annulus. It is concluded that a draft-tube spouted bed is a promising dryer design for heat-sensitive particles with slow intra-particle mass transfer coefficients.     

An axisymmetric model of flow in the annulus of a spouted bed of coarse particles. Model,experimental verification and residence time distribution

An axisymmetric model of flow in the annulus of a spouted bed of coarse particles which takes into account non-Darcy flow and utilizes a new and general axial pressure boundary condition at the spout-annulus interface is presented. The pressure boundary condition and the annulus pressure distribution are experimentally verified and the model used to calculate the residence time distribution and the flowrate at the top of the annulus. In addition, the average axial velocity profile in the annulus is calculated and compared with predictions of several one-dimensional flow models.     

Study of the hydrodynamics within a draft tube spouted bed system

Two dimensional Darcy and Ergun models are used to describe the hydrodynamics within the annulus of a spouted bed equipped with a draft tube. Experimental pressure and stream function data for water spouting are shown to be in good agreement with theoretical predictions. Verification of a general spout-annulus interfacial boundary condition is also established and it is shown that the entry region below the draft tube functions as a classical spouted bed. In addition, an experimental procedure for determining the location of the spout-annulus interface is presented. Fluid residence time distributions are calculated for a few cases of practical interest.     

Spouting with a porous draft-tube

Particle motion and gas distribution have been measured in 0.3 m spouted beds with porous and solid wall draft-tubes. Particle velocity in the annulus varied with gas rate and bed geometry. Narrow particle residence time distributions were observed under all conditions. Annular gas flow with the porous tubes exhibited a characteristic maximum in the lower part of the bed. A computer model has successfully simulated this phenomenon. The porous tube had a higher annular airflow than the solid tube at low separation distances, and a lower pressure drop for a given annular flow. A composite porous and solid wall tube offers improved performance.     

Fluid and particle flow characteristics in a draft tube spouted bed with modified fluid outlet

Glass particles, 0.54-1.39 mm in diameter, are spouted with air in a half-cylindrical draft tube spouted bed with modified fluid outlet to investigate the flow characteristics of the fluid and particles in the annulus. Using the measured pressure distribution and particle velocity and pathline in the annulus, the fluid streamlines, fluid and particle residence time distributions are obtained. Effects of the spoul inlet fluid flowrate, distance between the draft lube and spout inlet and the modified fluid outlet on the flow fields are also discussed.     

Pressure gradients,voidage and gas flow in the annulus of spouted beds

Parallel measurements of pressure gradients with a differential pressure probe and voidage profiles with a fibre optic system have been carried out to study gas flow distributions in the annulus of spouted beds. The observation of Grbavcic et al. (1976) that for a given fluid‐solid combination and column geometry the annulus pressure gradient at any bed level is independent of bed depth was corroborated again. Calibration curves of pressure drops versus superficial gas velocities for beds of voidage higher than the loose‐packed voidage were obtained by applying the Ergun (1952) equation, making it possible to estimate superficial gas velocities in the annulus using the static pressure gradient method. The local superficial gas velocity in the annulus was found to be higher in a deep bed than in a shallow bed of the same material, contrary to the conclusion (Grbavcic et al., 1976) that, for a given fluid‐solid combination and column geometry, the annulus fluid velocity at any level is independent of bed depth. Theoretical models and equations which do not account for the conical geometry near the bottom were found to underpredict superficial gas velocities in the annulus. Increasing the spouting gas flow was found to increase the net gas flow through the annulus.     

Numerical simulation of two-dimensional spouted bed with draft plates by discrete element method

A discrete element method (DEM)-computational fluid dynamics (CFD) two-way coupling method was employed to simulate the hydrodynamics in a two-dimensional spouted bed with draft plates. The motion of particles was modeled by the DEM and the gas flow was modeled by the Navier-Stokes equation. The interactions between gas and particles were considered using a two-way coupling method. The motion of particles in the spouted bed with complex geometry was solved by combining DEM and boundary element method (BEM). The minimal spouted velocity was obtained by the BEM-DEM-CFD simulation and the variation of the flow pattern in the bed with different superficial gas velocity was studied. The relationship between the pressure drop of the spouted bed and the superficial gas velocity was achieved from the simulations. The radial profile of the averaged vertical velocities of particles and the profile of the averaged void fraction in the spout and the annulus were statistically analyzed. The flow characteristics of the gas-solid system in the two-dimensional spouted bed were clearly described by the simulation results.     

The characteristics of fluid flow in beds of small glass particles spouted with water

The characteristics of beds of small glass particles 0.28, 0.46 and 0.77mm in diameter spouted with water were studied in a half-cylindrical column 51mm in diameter with inlet tube diameter of 3.2mm. The minimum spouting velocity, bed pressure drop at minimum spouting and spout diameter were measured. Assuming Darcy flow, the fluid flow in the annulus is modeled and shown to represent the streamlines quite well. The residence time of the fluid in the annulus is calcuated from the model and compared with experimental data.     

Influence of Longitudinal Vortex Generator Configuration on the Hydrodynamics in a Novel Spouted Bed

The gas‐solid flow in a cylindrical spouted bed with a pair of spherical longitudinal vortex generators (LVGs) was numerically investigated by a two‐fluid model with kinetic theory for granular flow. Simulations and analyses were conducted on five types of spouted beds: a conventional spouted bed without disturbance units as well as spouted beds with a pair of LVGs in which the radius of spheres installed on the LVGs had four different dimensions. Results of the computational fluid dynamics demonstrate that the fountain height decreases with larger radius, and the influence range of the longitudinal vortex increases with the greater radius, both for the gas phase and particle phase. The turbulent kinetic energy of the gas phase along the radial and axial directions in the spouted bed was also promoted significantly by the longitudinal vortex and increased with larger radius, which is due to the higher LVG volume.     

Numerical Simulations of Flow Behaviour of Agglomerates of Nano-Size Particles in Bubbling and Spouted Beds with an Agglomerate-Based Approach

Numerical simulations are performed on the behaviour of agglomerates of nanoparticles in bubbling gas fluidized beds and spouted beds—systems that are widely used in handling and processing particulate solids in various industrial sectors including biomaterials, foods and pharmaceuticals. An Eulerian two-fluid approach is used and the cohesive force between particles is considered. An empirical expression for the solids pressure of agglomerates is used (Jung and Gidaspow, 2002). The interaction between gas and agglomerates is considered with an agglomerate-based approach. Simulated results show that the fluidized bed has a very high expansion ratio with no distinct bubbles in the bubbling fluidized bed. In the spouted bed, however, the concentration of agglomerates is nearly homogeneous in both the dilute and transitional zones. The velocity of agglomerates is found to be higher than that in the annulus region, whereas the concentration distribution shows an opposite trend with a nearly closing packing of agglomerates in the annulus region. A high spouting gas velocity is shown to be required to fluidize agglomerates in the spouted bed. Comparisons of the modelling results are also made with limited experimental results.     

Heat transfer in a novel type spouted bed

A new method for spouted bed drying of coarse and even fine particulate solids involves introducing the gas tangentially and maintaining the uniform particle recirculation by a conveyor screw. Some flow characteristics are compared with those of a conventional spouted bed. The particle-fluid heat transfer was investigated in the annulus and spout respectively.     

二维导流管喷动床离散颗粒动力学模拟

采用离散单元法(DEM)-计算流体力学(CFD)双向耦合数值方法对二维导流管喷动床进行了模拟,颗粒的运动通过DEM模型描述,而气体的运动用Navier-Stokes方程进行求解,气体和固体颗粒之间的相互作用通过曳力形式传递。文中将DEM和边界元方法(BEM)结合起来解决颗粒在具有复杂边界设备内的运动。通过采用BEM+DEM-CFD相结合的方法进行模拟计算,得到了喷动床的最小喷动速度,研究了不同表观气速下床内的流型,得到了二维导流管喷动床的床层压降与表观气速的关系,统计分析了喷射区、环隙区内颗粒的运动速度和相应的空隙率,全面地描述了二维导流管喷动床内的气固流动特征。     

Grain Drying in a Paraboloid-Based Spouted Bed with and without Draft Tube

In this study, grain drying in a spherical-based spouted bed (SBSB), a cone-based spouted bed (CBSB), and a paraboloid-based spouted bed (PBSB) with and without draft tube was investigated. Spouted-bed bases with the same volume in different shapes—spherical, cone, and paraboloid—were used for the drying experiments to investigate the effect of the spouted-bed base shape on drying. The drying experiments were carried out with perforated and solid draft tubes. The effects of the distance between the gas inlet nozzle and the bottom of the draft tube (entrainment zone height) and the draft tube diameter as geometric parameters on drying were also investigated. It was seen that the geometrical shape of the contactor base influenced the drying time. The highest drying rate was achieved for drying in a paraboloid-based spouted bed. The results also showed that using a draft tube caused a significant increase in drying time. Because the perforated draft tube allows a higher gas flow rate through the annulus, it decreases the drying time when compared with the solid draft tube. Drying time decreased slightly with the decreasing height of the entrainment zone but draft tube diameter did not have a considerable effect on drying.     

Simulations of flow behavior of gas and particles in spouted bed with a porous draft tube

Flow behaviors of particles in a two-dimensional spouted bed with a porous draft plates are studied using a fluid dynamic computation with kinetic-frictional stresses models. Gas flow through the porous draft tube is simulated by Brinkman-Darcy-Forchheimer formulation. The distributions of concentration and velocities of particles are predicted. Simulated results predict the solid circulation rate and gas flux rate measured in the spouted bed with a porous draft tube (Ishikura et al., 2003). The solid circulation rate in the spouted bed with a porous draft tube is larger than that with a non-porous draft tube. The predicted bed pressure drop with the porous draft tube is high in comparison to the spouted bed with non-porous draft tube. The effect of the porosity of the porous draft tube on distributions of gas flux rate through the annulus and solid circulation rate are discussed.     

CFD simulation of gas–solid flow in a spouted bed with a non-porous draft tube

A multi-fluid Eulerian–Eulerian approach incorporating the kinetic theory of granular flow was used to simulate a spouted bed containing non-porous draft tube. Drag function and coefficient of restitution were investigated. Solid and gas velocity vector, gas flow rate in annulus and spout regions and longitudinal pressure distribution were evaluated. In addition, the effects of the entrainment height and the draft tube diameter were studied. Simulation indicates the formation of three regions namely, annulus, spout and fountain; similar to a conventional spouted bed. Current model predicts acceptable results in both spout and annulus regions. Simulation results indicate that the model can be employed for both mono-size and multi-size particles reasonably. This paper provides useful basis for further works on understanding gas–solid flow mechanism in spouted beds containing a non-porous draft tube.