Hydrodynamic modeling of a circulating fluidized bed

Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB). The numerical simulation of CFBs is very important in the prediction of its flow behavior. From this point of view, in the present study a dynamic two dimensional model is developed considering the hydrodynamic behavior of CFB. In the modeling, the CFB riser is analyzed in two regions: The bottom zone in turbulent fluidization regime is modeled in detail as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase. In the upper zone core-annulus solids flow structure is established. Simulation model takes into account the axial and radial distribution of voidage, velocity and pressure drop for gas and solid phase, and solids volume fraction and particle size distribution for solid phase. The model results are compared with and validated against atmospheric cold bed CFB units' experimental data given in the literature for axial and radial distribution of void fraction, solids volume fraction and particle velocity, total pressure drop along the bed height and radial solids flux. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05-0.418 m, bed height from 5-18 m, mean particle diameter from 67-520 μm, particle density from 1398 to 2620 kg/m³, mass fluxes from 21.3 to 300 kg/m²s and gas superficial velocities from 2.52-9.1 m/s.As a result of sensitivity analysis, the variation in mean particle diameter and superficial velocity, does affect the pressure especially in the core region and it does not affect considerably the pressure in the annulus region. Radial pressure profile is getting flatter in the core region as the mean particle diameter increases. Similar results can be obtained for lower superficial velocities. It has also been found that the contribution to the total pressure drop by gas and solids friction components is negligibly small when compared to the acceleration and solids hydrodynamic head components. At the bottom of the riser, in the core region the acceleration component of the pressure drop in total pressure drop changes from 0.65% to 0.28% from the riser center to the core-annulus interface, respectively; within the annulus region the acceleration component in total pressure drop changes from 0.22% to 0.11% radially from the core-annulus interface to the riser wall. On the other hand, the acceleration component weakens as it moves upwards in the riser decreasing to 1% in both regions at the top of the riser which is an important indicator of the fact that hydrodynamic head of solids is the most important factor in the total pressure drop.     

A model for the pressure balance of a low density circulating fluidized bed

The pressure balance along the solid circulation loop of a circulating fluidized bed equipped with a solid flux regulating device has been modelled and the influence of the pressure balance on the riser behaviour has been predicted.The solid circulation loop has been divided into many sections, where the pressure drop was calculated independently: riser, cyclone, standpipe, control device and return duct. A new theoretical model, that is able to predict the pressure losses in the return path of the solid from the standpipe to the riser, has been built. A new correlation for cyclone pressure loss with very high solid loads has been found on the basis of experimental data.The pressure loss in the riser has been calculated by imposing the closure of the pressure balance, ΣΔP = 0. Once the riser pressure drop had been calculated, the holdup distribution along the riser was obtained by imposing a particular shape of the profile, according to the different fluid-dynamics regimes (fast fluidization or pneumatic transport). In the first case, an exponential decay was imposed and the bottom holdup was adjusted to fit the total pressure drop, in the second case, the height of the dense zone was instead varied.The experimental data was used to develop the sub-models for the various loop sections have been obtained in a 100 mm i.d. riser, 6 m high, CFB. The solid was made of Geldart B group alumina particles. The tests were carried out with a gas velocity that ranged between 2 and 4 m/s and a solid flux that ranged between 20 and 170 kg/m²s. A good agreement was found between the model and experimental data.     

Experimental research on steady-state operation characteristics of gas–solid flow in a 15.5 m dual circulating fluidized bed system

Gas-solid hydrodynamic steady-state operation is the operating basis in a chemical looping dual-reactor system. This study reported the experimental results on the steady-state operation characteristics of gas–solid flow in a 15.5 m high dual circulating fluidized bed(CFB) cold test system. The effects of superficial gas velocity, static bed material height and solid returning modes on the steady-state operation characteristics between the two CFBs were investigated. Results suggest that the solid distributions in the dual CFB test system was mainly determined by the superficial gas velocity and larger solid inventory may help to improve the solid distributions. Besides, cross-returning mode coupled with self-returning is good for steady-state running in the dual-reactor test system.     

高颗粒通量循环流化床的构型作用

In order to achieve high solids circulation rate (G_s),an idea of coupling a moving bed to the bottom section of the riser of a circulating fluidized bed (CFB) was proposed and tested.The results from the preliminary study demonstrated that the solids circulation rate in the new-structure bed approached 370 kg·m^-2·s^-1 at superficial gas velocities around 10.5 m·s^-1 for sand particles with an average Sauter mean size of 378 μm.This study was devoted to further justifying the effects of the coupled moving bed by performing comparative studies in two CFBs with conventional configurations.It was shown that the pressure at the riser bottom and the realized solid circulation rate were only about 15 kPa and 230 kg·m^-2·s^-1 in the two conventionally configured CFBs,obviously lower than 25 kPa and 370 kg·m^-2·s^-1 in the moving bed coupled CFB.These verified that the coupled moving bed increased the force driving particles form the particle recycling side into the riser.The study further tested the effect of a few specially designed riser exit configurations,revealing that a smooth riser exit could facilitate solids circulation to increase the solids circulation rate.     

Experimental validation of the gas-solid flow in the CFB riser

In this paper, an experimental study is performed to investigate the flow structure in a circulating fluidized bed (CFB). The typical core-annulus structure and small amount of back-mixing of particles near the wall of the riser were observed. The axial solid concentration distributions contain a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. Furthermore, the solid concentration decreases with the increase of the superficial gas velocity, and increases with the increment of the circulation rate at the same height position. The total pressure drop of the main bed represents a linear relationship with the solid flux rate. In the dense phase zone, the solid concentration increases linearly with the augmentation of the solid flux, however, the change of the solid concentration is slight, even unchangeable at the up zones. In addition, based on the Energy-Minimization Multi-Scale (EMMS) method, a revised drag force model is proposed, which is coupled in the Eulerian two-fluid model for simulating the flow structure in the riser. Numerical results are consistent with the experimental data, which indicate the revised drag force model is very successful in simulating flow structure of the dense gas-solid two-phase flow.     

循环流化床提升管中颗粒速度的径向分布及其沿轴向的发展

采用5光纤速度探头对f100mm?5.1m循环床提升管8个高度截面上11个径向位置的局部颗粒速度进行了实验测试,并采用径向不均匀指数(RNI)对颗粒速度径向分布的不均匀性及其沿轴向的变化进行了定量描述。研究结果表明:在高气速、高颗粒循环量操作时,操作条件对颗粒上升速度和下降速度的径向分布的影响在加速段和充分发展段呈现出不同的规律;颗粒上升速度和下降速度沿轴向的变化在核心区和边壁区也表现出不同的趋势。当颗粒循环速率大于200 kgm-2s-1时,颗粒的加速段长度大大延长,以至于大于提升管的高度(15.1m)。颗粒速度径向分布的不均匀性沿轴向是逐渐增大的,并且与截面平均颗粒速度存在很强的相关性。     

Two-dimensional biomass combustion modeling of CFB

In this study, a 2D model for a CFB biomass combustor has been developed which integrates and simultaneously predicts the hydrodynamics, heat transfer and combustion aspects. Combustor hydrodynamic is modeled taking into account previous work. Simulation model calculates the axial and radial distribution of voidage, velocity, particle size distribution, pressure drop, gas emissions and temperature at each time interval for gas and solid phase both for bottom and upper zones. The model results are compared with and validated against experimental data both for small-size and industrial-size biomass combustors which uses different types of biomass fuels given in the literature. As a result of sensitivity analysis, it is observed that: major portion of the combustion will take place in the upper zone, the air staging could improve combustion, for industrial-size CFB biomass combustors and the decrease of NO_x adversely results in high CO emissions as air ratio decreases. Unexpected results concerning the emissions is caused by using data of different sized CFBs and is clearly an indicator of the necessity to compare the model results with various sized CFBs as far as possible.     

下行气固两相流充分发展段的颗粒浓度

在较宽的操作条件范围内采用光纤颗粒浓度探头测定了下行管(φ100 mm×9.5 m)充分发展段内的真实颗粒浓度,并结合文献上的大量实验数据,系统研究了操作条件、颗粒直径和床层直径对下行气固两相流充分发展段内真实颗粒浓度的影响.结果表明,当操作气速一定时,充分发展段内的颗粒浓度随着颗粒循环速率的增大而线性增加.颗粒直径对下行床充分发展段内颗粒浓度的影响随操作气速的增加而逐渐减弱.床层直径对下行床充分发展段内的颗粒浓度基本上没有影响.所提出的预测关联式能很好地拟合本文及文献上的实验数据.     

循环型高通量输送床的建立与控制

密相输送床气化和双流化床气化是基于循环型流化床反应器发展起来的两种新型煤和生物质气化技术,根据这两种技术对流动的要求,提出了在循环流化床的下行床底部耦合一段移动床,为输送床内的流动提供足够高的驱动压力而提高颗粒循环量的技术思想。在根据该思想而建立的直径90 mm的输送床实验装置上的实验研究表明,利用所提出的床型构造可在表观气速9.6 m•s^-1下实现400 kg•m^-2•s^-1的颗粒循环量。输送床的一次风速和移动床松动风速是影响颗粒循环量和输送床内颗粒浓度的主要因素,但循环量随输送床一次风速的增大而增加的走势弱于普通循环流化床。移动床松动风速在小于颗粒最小流化速度的范围内轻微变动即可显著改变颗粒循环量和输送床内颗粒浓度。在保持输送床总气速不变的前提下,通过二次风可在40%的比例范围内调节颗粒循环量,且调节作用随二次风位置的增高而减弱。     

下行气固两相流与管内壁间的摩擦压降

在较宽的操作条件范围内系统测试了下行床床层压力降,获得气固两相流与管内壁间的摩擦压降,提出了下行气固两相流与管壁间摩擦压降的计算模型。结果表明,在下行床的充分发展段,气固两相流与管壁间的摩擦导致表观颗粒浓度显著小于真实颗粒浓度;当表观气速大于8 m·s^-1时,气固两相流与管壁间的摩擦压降接近甚至超过气固两相流重力产生的静压降。在采用压差法测试下行床中的平均颗粒浓度时,如忽略气固两相流与管壁间的摩擦,则可能导致显著的偏差。下行气固两相流与管内壁间的摩擦压降主要来自于颗粒与管壁间的摩擦。颗粒直径对气固两相流与管壁间摩擦压降的影响随着操作气速的提高逐渐减弱。采用提出的摩擦压降模型对表观颗粒浓度进行修正后,预测值与实验值吻合较好。     

A study of the pressure balance around the loop of a circulating fluidized bed

Pressure measurements around the loop of a circulating fluidized bed with 152 mm ID riser and L-valve fecuer were analysed to determine the effect of operating parameters (superficial gas velocity in the range 2.2 - 4.0 m/s, solids circulation flux in the range 5 - 50 kg/m² · s and solids inventory, in the range 80 - 180 kg) on the components of the pressure balance. The riser pressure drop, and hence, riser solids holdup were not affected by changes in the inventory of solids in the system, provided riser superficial gas velocity and solid circulation flux were held constant. The mean suspension concentration in the riser was found to be directly proportional to the ratio of solids flux to superficial gas velocity (G / U) in the riser.     

含内构件的循环流化床的动态压力特性

针对含内构件的循环流化床,以石英砂为物料,使用动态压力传感器测量了含内构件的流化床中气固两相流的动态压力,分析了床内的瞬时压力特性. 结果表明,在进出口总压降中,文丘里压降最大,占主床压降的60%以上. 表观气速和固体颗粒循环流率共同影响循环流化床内的压力特性. 压力瞬时波动功率谱分析表明,压力波动对应一个主频,表观气速越小、颗粒循环流率越大时,压力波动越大,且循环流化床底部压力波动比上部大. 加入内构件能有效引导气流,使流动更均匀.     

Distribution of large biomass particles in a sand‐biomass fluidized bed: Experiments and modeling

The axial distribution of large biomass particles in bubbling fluidized beds comprised of sand and biomass is investigated in this study. The global and local pressure drop profiles are analyzed in mixtures fluidized at superficial gas velocities ranging from 0.2 to 1 m/s. In addition, the radioactive particle tracking technique is used to track the trajectory of a tracer mimicking the behavior of biomass particles in systems consisting of 2, 8, and 16% of biomass mass ratio. The effects of superficial gas velocity and the mixture composition on the mixing/segregation of the bed components are explored by analyzing the circulatory motion of the active tracer. Contrary to low fluidization velocity (U = 0.36 m/s), biomass circulation and distribution are enhanced at U = 0.64 m/s with increasing the load of biomass particles. The axial profile of volume fraction of biomass along the bed is modeled on the basis of the experimental findings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 869–880, 2014     

Modelling and simulation of a gas–solids dispersion flow in a high-flux circulating fluidized bed (HFCFB) riser

Radial solids velocity profiles were computed on seven axial levels in the riser of a high-flux circulating fluidized bed (HFCFB) using a two-phase 3-D computational fluid dynamics model. The computed solids velocities were compared with experimental data on a riser with an internal diameter of 76 mm and a height of 10 m, at a high solids flux of 300 kg m⁻² s⁻¹ and a superficial velocity of 8 m s⁻¹. Several hundreds of experimental and numerical studies on CFBs have been carried out at low fluxes of less than 200 kg m⁻² s⁻¹, whereas only a few limited useful studies have dealt with high solids flux. The k– two-phase turbulence model was used to describe the gas–solids flow in an HFCFB. The model predicts a core–annulus flow in the dilute and developed flow regions similar to that found experimentally, but in the region of highest solids concentration it is somewhat overpredicted at the level close to the inlet.     

Direct Measurements of Instantaneous Solid Flux in a CFB Riser using a Novel Multifunctional Optical Fiber Probe

With a novel optical fiber probe that can measure instantaneous local particle velocity and solid concentration simultaneously, extensive experiments were conducted to study transient flow structures in a 15.1‐m long circulating fluidized bed (CFB) riser of 100 mm in diameter. This study analyzed the radial and axial distributions of solid concentration, particle velocity, and their variations with nine operating conditions and at six axial levels. Instantaneous local solid concentration and particle velocity were found to be well correlated at most of the radial positions. The detailed time evolution, axial and radial distribution of instantaneous solid flux, and the variation of solid flux with operating conditions were also investigated. The radial solid flux profile showed a flat shape with a maximum at near wall area under most operating conditions. The instantaneous solid flux was found to have a strong fluctuation at a radial position of r/R = 0.8 ~ 0.9.     

Particle Holdup and Average Residence Time in the Cyclone of a CFB Boiler

Particle holdup and the average residence time in the cyclone of a Circulating Fluidized Bed (CFB) boiler are important information for describing events post‐combustion in the cyclone that often lead to a noticeable increase in the temperature of the flue gas. The existing results for the variation of particle average residence time with fluidizing gas velocity are contradictory since they were obtained under different operation conditions. A cold CFB apparatus made of plexiglass was established with a riser of 5 m in height and 0.2 m in diameter and equipped with a standard Lapple cyclone. The particle holdup was directly measured by the mass in the cyclone when the system was shut down. The solid concentration at the cyclone inlet was kept in the range generally used in CFB boilers. The experimental results showed that the particle holdup in the cyclone was equal to ca. 10–40 % of the corresponding bed material in the riser and that it increases monotonously with both the fluidizing gas velocity and the initial static bed height, and approximately linearly with the solid circulation rate. In addition, within the experimental conditions, the cyclone pressure drop increases monotonously with particle holdup. It was found that the average residence time of the particles either increased or decreased linearly with the fluidizing gas velocity, depending on the initial static bed height. Nevertheless, both variation rates were very small. In a view of engineering applications, the average residence time of the particles in the cyclone is insignificantly affected by the fluidizing gas velocity, initial bed inventory and solid circulation rate, within the range of experimental conditions examined.     

The residence time distribution and mixing of the gas phase in the riser of a circulating fluidized bed

CFBs are increasingly used for both gas-catalytic and gas-solid reactions. The conversion is a function of the gas hydrodynamics, subject of the present research.Available literature on the gas mixing in the riser of a CFB shows contradictory results: some investigators neglected back-mixing of gas, whereas others report a considerable amount of back-mixing in CFB risers. The present paper reports experimental findings obtained in a 0.1 m I.D. riser, for a wide range of combined superficial gas velocity (U) and solid circulation flux (G). The gas flow mode (plug vs. mixed) is strongly affected by the operating conditions, however with a dominant mode within a specific (U, G)-range. Sand was used as bed material. The superficial gas velocity was varied from 5.5 to 8.3 m/s, the solids circulation flux was between 40 and 170 kg/m² s. A tracer pulse response technique was used with a pulse of propane injected at the bottom and detected at the riser exit. The cumulative response curves, F(t), define (i) an average residence time (t₅₀) obtained for F(t) = 0.5; and (ii) the slope of the curves (a steeper one corresponding with more pronounced plug flow) and expressed in terms of a span, σ. These parameters (t₅₀ and σ) define the gas flow mode. A quantitative comparison of experimental results with literature RTD-models is inconclusive although the occurrence of both mixed flow and plug flow is evident, and (U, G)-dependent. The experimental results are expressed in empirical design equations, and the comparison of predicted and experimental results is fair: low values of σ determine the plug flow regimes, whereas back-mixing is more pronounced at higher value of σ. Experiments with similar systems might favor plug flow or mixing as function of the combined (U, G)-values. The introduction of the RTD-function in reaction rate equations can improve the prediction of the gas-conversion in a riser-reactor.     

Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions

A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas-solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian-Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas-solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas-solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes.     

A new correlation for predicting solids concentration in the fully developed zone of circulating fluidized bed risers

The present work focuses on developing a new comprehensive correlation for better prediction of the solids concentration in the fully developed region of co-current upward gas-solid flow in circulating fluidized bed (CFB) risers. Systematic experiments were carried out in two risers (15.1 m and 10.5 m high with the same 0.1 m i.d.) with FCC and sand particles. The results obtained from about 200 sets of operating conditions show that the average solids concentration in the fully developed region is more than just a function of the corresponding terminal solids concentration, as most previous correlations are based on. Operating conditions, particle properties and riser diameters also have significant effects on the solids concentrations in the fully developed region of CFB risers. Based on our experimental data and those reported in the open literature from CFB risers up to 0.4 m in diameter and 27 m in height with superficial gas velocities and solids circulation rates up to 11.5 m/s and 685 kg/m²·s, a new empirical correlation for predicting the average solids concentrations in the fully developed region of CFB risers is proposed. The correlation works well for a wide range of operating conditions, particle properties and riser diameters.     

Local Flow and Heat Transfer Behaviours in V-L-S Fluidized Bed Evaporator

The fluidized bed evaporator is a subject of considerable interest and can be employed in the process industries. However, the V-L-S flow behavior and heat transfer characteristics are still not well understood. In this work, a vertical fluidized bed evaporator with external natural-circulation flow boiling was established to investigate the local V-L-S flow behaviour and heat transfer characteristics and to reveal the influence of flow on heat transfer performance. It consists of a heated tube with inner diameter of 3.8 × 10⁻² m and height of 1.4 m and a circulating tube with the same inner diameter and height of 0.92 m. Both tubes are made of quartz glass plated with transparent electrically heating film. The solid particles added to the evaporator are glass, ceramic, Teflon™ and poly-formaldehyde beads or cylinders with the diameter ranging from 1.8 × 10⁻³ to 4 × 10⁻³ m. The particle volume changes from 0 to 1 × 10⁻³ m³ and heat flux varies from 5 × 10³ to 1.2 × 10⁴ W m⁻². The local velocity and holdup of solid particles, flow region transition, length of the V-L-S flow boiling, fluid circulating velocity and pressure drop in the V-L-S fluidized bed evaporator were visually investigated with CCD measuring technique. The main results on flow are as following. Axial solid holdup in heated tube of the evaporator decreases obviously for middle-density particle systems against the direction of the gravity. Three flow regions can be found in heated tube of the evaporator: L-S region, transition region and V-L-S region, and in transition region, the radial profile of solid holdups is relatively uniform. The increase of particle volume enlarges the length of V-L-S region and pressure drop, while decreases the circulating velocity of fluid mixture. The average heat transfer film coefficients of V-L-S flow boiling were estimated and a dimensionless correlation was obtained based on 120 sets of experimental data with a maximum relative deviation of 10.8%. The axial variation of the heat transfer coefficients has close relation to the axial distribution of the solid holdups.