Circulating Fluidized Bed Riser Research Articles

Investigating choking phenomena in CFB risers under different operating parameters

Choking phenomena in circulating fluidized bed (CFB) risers pose a significant challenge to industrial operations. To tackle this concern, multiphase particle-in-cell (MP-PIC) simulations are employed to investigate the impact of varying solids flux, bed height, and riser size on solids transport in CFB risers. The research sheds light on the complex interplay of operating parameters in CFB systems. The transition of solids holdup profiles from exponential to S-shaped curves with increasing solids inventory is observed, accurately predicting the choking point. By adjusting geometric dimensions, the choking point distance from the outlet at a fixed length-to-diameter ratio can be efficiently shifted. The findings provide a deeper understanding of saturation carrying capacity (SCC) in CFB risers, which can be leveraged to optimize industrial performance. This research is instrumental in mitigating choking effects and enhancing the design and operation of CFB systems.

Axial behavior of slugging flow in a gas–solid CFB riser using distributed ECVT with 64 electrodes

Understanding particle flow characteristics within circulating fluidized bed (CFB) reactors is crucial for enhancing reaction efficiency. To study the flow characteristics of Geldart D particles with a mean diameter of 3.0 mm and a bulk density of 1640 kg/m3 in a CFB riser with a height of 1090 mm, a 64-electrode electrical capacitance volume tomography (ECVT) sensor with a height-to-diameter ratio (HDR) of 12 is arranged on the CFB riser with an inner diameter of 59 mm. A distributed ECVT architecture under the control of a timing controller is formed to synchronously acquire capacitance data of two 32-electrode ECVT data acquisition systems. So, the three-dimensional axial flow imaging of a CFB riser with a branch pipe is realized with a 14-plane ECVT sensor. The flow rules of slugging flow in the CFB riser under gas velocities of 3.5 m/s, 2.86 m/s, and 2.5 m/s are analyzed from the continuous ECVT images, especially the process of rupture, decomposition, and collapse of particle slugs. The flow parameter characteristics, such as the solid fraction in the riser, the slug rising tail velocity, the slug length, and the spectral characteristics, are investigated. The experimental results demonstrate that the proposed distributed ECVT can effectively realize the collaborative operation of multiple ECVT data acquisition systems and then splice sub-ECT images to achieve complex space imaging.

A CFD study on ultrasound-enhanced CFB riser with calcium oxide and activated coal for CO2 capture application

This numerical study explores techniques for optimizing fluidization in circulating fluidized beds (CFB), which are of significant industrial interest. Various intrusive and non-intrusive approaches have been investigated to mitigate issues such as cluster formation, core-annulus profiles, and solid back-mixing. Specifically, this research focuses on non-intrusive methods, employing ultrasonic waves to enhance solids distribution within a CFB riser by Computational Fluid Dynamics (CFD) simulations. Particulate phases, including calcium oxide and activated carbon particles, were studied alongside a gas phase of air with a 10 % carbon dioxide concentration, commonly encountered in carbon capture processes. A radial ultrasonic device featuring 20 transducers at 20 kHz and 40 kHz was affixed to the CFB to disperse particle flow. Numerical experiments within a 3-D CFD domain employed the URANS-k-ε-KTGF-EMMS mathematical model. Findings reveal that ultrasonic waves significantly influence particle motion, modifying riser flow profiles and enhancing solids distribution. While ultrasounds increase pressure drop within the transducers' region, they decrease it above, reshaping the flow profile for both particle types. Distinct outcomes are observed for calcium oxide and coal particles regarding the radial Péclet number and solids volume fraction dispersion coefficient, suggesting that ultrasounds effectively improve coal particle distribution along the riser's height.

Considering solid shear stress in MP-PIC simulation of a CFB riser

To enable successful multi-phase particle-in-cell (MP-PIC) simulations of circulating fluidized beds (CFBs), both the interphase drag force and interparticle collisions need to be carefully modeled. Particle collisions are usually represented by the solid stress consisting of the normal and shear components, in which the normal stress was found to have a leading role in the numerical stability of MP-PIC simulations, whereas the impact of the shear stress has seldom been reported. In this work, the effects of the solid shear stress are investigated by using two-dimensional simulations of the moderately dense laboratory-scale CFB riser with in-house MP-PIC code implemented on MFIX open-source platform. Cases including only normal solid stress and those considering both normal and shear solid stresses are simulated. The results of solids flux, axial and radial solids profiles are compared with available experimental data. The results show that the solid shear stress plays a minor role on the accuracy of simulation, and the increase of PPP (number particles per parcel) leads to a lower accuracy of simulation.

Experimental investigation of three-dimensional gas-solid flow in CFB cyclone separator using electrical capacitance volume tomography

The anisotropic three-dimensional flow field inside cyclone separators makes it challenging to capture the gas-solid flow characteristics, and the internal behavior of the cyclone separator remains not fully understood. The gas-solid flow characteristics inside a cyclone separator of the circulating fluidized bed (CFB) are investigated using electrical capacitance volume tomography (ECVT). The solid fraction, average velocity, and frequency characteristics of particles were extracted from ECVT images. The experimental results show that the number of turns for the dust strands spirals gradually increases as the inlet gas velocity Uf increases. The axial velocity of the particles in the cylindrical part of the cyclone separator is always greater than in the conical part. In addition, the difference in inlet gas velocity and particle size results in different particle flow characteristics in the CFB riser. It leads to the differences in the particle circulation rate and flow pattern in the cyclone separator.

Particle-scale and sub-grid drag models coupled CFD for simulating the CO methanation in a CFB riser

The diffusion and chemical reactions inside the catalyst particles and the heterogeneous flow structure in the computational cells are key factors to affect the accuracy of the coarse-grid simulation in circulating fluidized bed (CFB) methanation reactors. In this work, a particle-scale model is developed to calculate the effective reaction rate considering the transient diffusion and chemical reactions in the particle scale, i.e., the scale of the single catalyst particle. A modified sub-grid drag model is proposed to consider the effects of the meso-scale and chemical reactions on the heterogeneous gas-solid interaction, where the meso-scale is between the single particle and the whole reactor and featured with the particle cluster. Subsequently, a coupled model is developed by integrating the particle-scale and modified sub-grid drag models into CFD. Moreover, the coupled model is validated to achieve accurate predictions on the CO methanation process in a CFB riser. Notably, the coupled model can be performed with a coarse grid (∼58 times particle diameter) and a large time step (0.005 s) to accelerate the simulation. By simply changing the reaction kinetics, different gas-solid catalytic reaction systems can be simulated by using the coupled model.

Solid Phase Macromixing Study in a Pilot-Scale Geldart Group B Circulating Fluidized Bed Riser Using Single Particle RTD and RPT Measurements

Solid flow in a Geldart’s group B circulating fluidized bed (CFB) riser is complex, and it exhibits backflow and recirculation in the riser. A single radioactive tracer particle is used to measure the overall and sectional residence time distribution in a CFB riser at a gas velocity of 7.6–9.2 m/s and a solid flux of 100–200 kg/m2s. At the same time, radioactive particle tracking (RPT) data are used to measure the trajectories of the tracer particle and its length distribution at the bottom and middle sections of the riser. Both residence time distribution (RTD) and trajectory length distribution data obtained from RPT and RTD experiments are processed and compared. Results show that the bottom section has higher back mixing than the middle section. The results also show that back mixing in both the sections reduces with an increase in the gas inlet velocity and reduces marginally with an increase in the solid flux. Results confirm that RPT and RTD data are highly correlated and can be used with the same accuracy to quantify the macromixing behavior of any process vessel/reactor.

A novel methodology to construct compartment models for a circulating fluidized bed riser

During the integrated modeling of a chemical process, there always exists a conflict between model accuracy and computational speed for non-ideal reactors. In view of this, a novel, easy to use, methodology to construct compartment or ideal reactor network models is proposed for a circulating fluidized bed (CFB) riser based on time-series. The pressure drop time-series is sampled during the Multi-Phase Particle-in-Cell (MP-PIC) simulation of the riser. Detailed procedures for mapping the time-series to a directed weighted complex network and the determination of model parameters are presented. Effects of the sampling frequency and division scheme for the time-series on compartment models are also investigated. Findings indicate that the MP-PIC simulation is reliable to record the time-series of pressure drop in the CFB riser. The non-ideal flow in the riser can be equivalently reproduced by a compartment model consisting of eight CSTRs with various sizes and backflow rates. The appropriate sampling frequency for the time-series should be greater than 200 Hz. The optimum division scheme for the time-series is with the predefined threshold value of K = [4025, 4150, 4275, 4400, 4525, 4650, 4775]. It is believed that the proposed methodology based on the time-series can be generalized to develop compartment models for non-ideal flow. It is also expected that compartment models can be readily integrated into a process simulator, e.g., Aspen Plus, to resolve the conflict between model accuracy and computational speed in process simulation, on-line analysis and real-time optimization.

An image processing method for feature extraction and dynamic tracking of particle clusters in CFBs

A new image processing method based on the high-speed camera is proposed to identify, locate, and track clusters. The instantaneous characteristic parameters of particle clusters in the riser of the circulating fluidized bed (CFB) can be acquired, such as solids holdup, vertical velocity, lateral displacement, aspect ratio and near-circularity. Experiments were carried out with glass bead particles, river sand particles and FCC particles. The time series of images of gas–solid flow in a CFB riser with a 100 mm × 25 mm cross-section and 3.2 m in length were obtained using high-speed cameras. The k-means++ clustering algorithm is utilized to identify the clusters, centroid is applied to locate the clusters, and the cross-correlation algorithm is employed to track the specific clusters and number them to get the instantaneous characteristic parameters. The results illustrate that the shapes of clusters in the center area are closest to circle, moving upwards at a uniform speed, while the clusters in the side-wall area are mostly elongated or long chain-like, moving slowly downwards. In the transition area, the clusters are more complex, moving upwards at a constant speed, and having large lateral displacement. The results show that the image processing method used in this study is successful in acquiring the dynamic and structural parameters of the clusters simultaneously.

CFD simulation of hydrodynamics and heat transfer characteristics in gas–solid circulating fluidized bed riser under fast pyrolysis flow condition

• Proper heat is needed for Circulating fluidized bed riser for fast pyrolysis. • A 3D EE-TFM model was developed and validated with experiment. • Hydrodynamics and heat transfer characteristics were examined for CFB riser. • Demonstrate the gas–solid heat transfer affected under different hydrodynamics. Pyrolysis process through circulating fluidized bed (CFB) is a promising technology to produce synthetic fuel and other products from biomass feedstocks. Computational fluid dynamics (CFD) computing means provide valuable insights to better understand gas–solid flow hydrodynamics, troubleshoot performance issues and optimize reactor operations. In this study, gas–solid flow hydrodynamics and heat transfer characteristics of a CFB riser for fast pyrolysis are investigated using a three-dimensional (3D) Eulerian-Eulerian CFD model. The main observations are discussed to provide insights on the factors affecting CFB riser performance. The model parameters, specularity and particle–particle restitution coefficients, were considered and tuned to accurately predict of gas–solid flow hydrodynamics and heat distribution with respect to different gas velocities and solid circulation rates. The results have shown that the CFD model predicted well the flow hydrodynamics and both specularity and particle–particle restitution coefficients are critical parameters as they affect particle behavior and temperature distribution fields. The lower specularity coefficient (Φ − 0.00001) was fairly able to predict the axial solid holdup profile into CFB riser. However, the lower value for particle–particle restitution coefficient ( e s s - 0.8) significantly overpredict the bottom dense region. The results shows that the increase of operating velocity promotes the mixing behaviors and heat transfer performance. In this work the suitable gas and solid circulation flow rates are U g = 4.5 m/s and G s = 81.23 kg/m 2 s.

ECVT study of 3D flow characteristics at L-shaped exit of CFB riser

Riser exit of the circulating fluidized bed (CFB) has a complex structure, which makes it difficult to capture the characteristics of gas-solid flow. Besides, the axial behavior of CFB exit of the riser has not been entirely understood. In this paper, electrical capacitance volume tomography (ECVT) was used to study the characteristics of gas-solid flow in the L-shaped exit of the CFB riser, and a 32-electrode ECVT data acquisition system was developed with an imaging speed of 100 frames per second. As a result, the solid concentration, the characteristics of solid frequency, the solid velocity, and the solid flux were extracted from the ECVT images. When the gas velocity Uf was adjusted from 2.29 m/s to 0.83 m/s, the particles in the L-shaped exit of the CFB riser changed from dispersed flow to aggregative flow, and the solid flux decreased. Finally, the concentration distribution of solid was simulated in the CFB according to the Eulerian-Eulerian multiphase flow model. In the exit of the CFB riser, the experimental ECVT results and the numerical simulation results demonstrated good agreement in terms of particle flow patterns.

Hybrid CFD-neural networks technique to predict circulating fluidized bed reactor riser hydrodynamics

Circulating fluidized bed (CFB)-based co-pyrolysis is a promising technology for producing synthetic fuel and chemical co-products from biomass and waste feedstocks. Computational fluid dynamics (CFD) tools provide valuable insights for understanding gas-solid flow hydrodynamics, troubleshooting performance issues, and optimizing reactor operations. CFD simulations are computationally expensive and time-consuming; therefore, in this study, we developed an artificial neural network (ANN)-based machine learning model from a wide CFD simulation campaign. The CFB riser axial solid holdup profile was predicted using a two-fluid model and validated using the experimental data. Finally, a dataset connecting the input features of the model parameter-based simulations with their axial solid holdup was constructed for training the ANN. The performance of the developed model was later compared with the experimental data. The developed ANN model exhibited low mean square error values of order of 10−3 for both validation datasets to predict axial solid holdup. The ANN model performed well with an interpolated solid circulation rate of 45 kg/m2s and 62 kg/m2s and an extrapolated solid circulation rate of 30 kg/m2s and 80 kg/m2s.

Numerical Simulation of Gas-Solid Flow in CFB Riser Using Different Approaches of Kinetic Theory of Granular Flows

In this paper is applied the two-fluid model to simulate the gas-solid flow in a riser of a circulating fluidized bed (CFB). The phases are modeled as a continuum medium computing the solid’s pressure and dynamic viscosity by the kinetic theory of granular flows (KTGF). The numerical simulations are performed with the MFIX (Multiphase Flow with Interface eXchanges) computational fluid dynamics (CFD) code developed in the National Energy Technology Laboratory (NETL). The main aim of this work is to perform a comparative analysis of the simulation results obtained from three different versions of the KTGF. In the work are presented time-averaged results comprehending the radial profiles of the axial velocities of gas and solid phases, the solid volumetric fraction and the solid mass flow. These results are compared with the available experimental data showing a different behavior for each version of the KTGF. For a more complete comparative analysis also are presented time-averaged and transient snapshots of the gas volumetric fraction in two risers sections disposed at two different heights of the column. In all the simulations is used a uniform two-dimensional computational mesh and the Superbee second order scheme for the discretization of the advective terms. The numerical results show that the procedure for the computation of the solid phase pressure and viscosity influences in a significant way the behavior of the gas-solid flow in a riser. These differences are obtained even when it is considered only the KTGF with slightly variations for the constitutive equations computations.

Three-dimensional axial dispersion of gas-solid fluidized beds using 8-plane electrical capacitance volume tomography

Due to the complex flow characteristics and opaqueness of gas-solid fluidized beds, the axial behavior of circulating fluidized bed (CFB) risers is not entirely understood. A 32-electrode electrical capacitance volume tomography (ECVT) system with eight planes was developed to investigate the three-dimensional axial dispersion of gas-solid flow in a CFB riser. To maximize the imaging length in the axial direction, the 32-electrode ECVT sensor was designed with eight planes, with each plane containing four electrodes. The ratio of the axial imaging length to the sensor inner diameter was 8.3. We developed a 32-channel ECVT data acquisition system with an imaging speed of 120 frames per second to record the data of the ECVT sensor. Experimental results show that this ECVT system can effectively collect the flow details in the axial direction. The flow changed from slug flow to bubble flow when the fluidizing gas velocity Uf was adjusted from 2.34 m/s to 0.56 m/s. The distribution of the solid fraction, the velocity of the slug/bubble in the axial direction and the slug length were extracted from the 8-plane ECVT images to reveal the flow characteristics of the CFB riser.

Application of Wray–Agarwal Turbulence Model for Numerical Simulation of Gas-Solid Flows in Circulating Fluidized Bed Risers

Abstract In recent decades, increasing attention has been paid on accurate modeling of circulating fluidized bed (CFB) risers to provide valuable guidance to design, optimization, and operation of reactors. Turbulence model plays an important role in the accurate prediction of complex gas-solid flows. Recently developed Wray–Agarwal (WA) model is a one-equation turbulence model with the advantages of high computational efficiency and competitive accuracy with two-equation models. In this paper for the first time, the Eulerian–Eulerian approach coupled with different turbulence models including WA model, standard κ–ɛ model, and shear stress transport (SST) κ–ω model is employed to simulate two-phase flows of gas phase and solid phase in two CFB risers, in order to assess accuracy and efficiency of WA model compared to other well-known two-equation models. Predicted gas-solid flow dynamic characteristics including the gas-solid volume fraction distributions in radial and axial directions, pressure profiles, and solid mass flux distributions are compared with data obtained from an experiment in detail. The results demonstrate that the WA model is very promising for accurate and efficient simulation of gas-solid multiphase flows.

Numerical investigations on gas–solid flow in circulating fluidized bed risers using a new cluster-based drag model

A cluster-based drag model is proposed for the gas–solid circulating fluidized bed (CFB) riser by including the cluster information collected from image processing and wavelet analysis into the calculation of system drag. The performance of the proposed drag model is compared with some commonly used drag models. A good agreement with the experimental data is achieved by the proposed cluster-based drag model. Error analysis of the proposed cluster-based drag model based on the local distributions of solids holdup and particle velocity is conducted. The clustering phenomenon in the low-density and high-density CFB risers and the effect of the cluster size on the simulation accuracy are also numerically studied by the proposed drag model.

Impact of polydispersity on the flow dynamics in the riser of a circulating fluidized bed

Circulating fluidized bed (CFB) has been commonly operated with polydisperse particles with a wide particle size distribution (PSD), which significantly affects solid transportation and the resulting thermochemical performance. Dense gas-solid flows in a pilot-scale CFB riser are simulated via the multiphase particle-in-cell (MP-PIC) coupled with a heterogeneous drag model. Gas-solid hydrodynamics together with the particle-scale information with a wide PSD is discussed. The results show that in the riser, the non-uniformly spatial distribution of solid phase appears with the presence of a core-annulus structure and a dilute-upper dense-bottom profile. The wide PSD gives rise to the axial segregation phenomenon with the accumulation of large particles in the bottom region. Enlarging the superficial gas velocity and PSD width respectively decreases and increases the segregation intensity of solid phase. As compared with gas velocity, the PSD width performs limited impacts on solid concentration, particle-scale information along the axial direction, and solid residence time. The horizontal solid dispersion coefficient is at the scale of 10−3 m2/s, while the vertical dispersion is at the scale of 1 m2/s. For a wide PSD, particles with larger sizes have larger slip velocities but smaller dispersion coefficients in the riser.

Computational fluid dynamic model of nonisothermal circulating fluidized bed riser for CO2 capture

The concern about the environment is the driving force to reduce the CO2 emission. The main source of CO2 emission is from the flue gas from combustion process. The K2CO3 solid sorbent in circulating fluidized bed reactor (CFB) is nominated as an efficient method for capturing CO2 in flue gas. The CFB reactor consists of two reactors including circulating fluidized bed riser (CFBR) and circulating fluidized bed downer (CFBD). The study focused on the CFBR as it was used in adsorption process. However, CO2 capture capacity is limited due to the equilibrium at the specific system temperature. This study developed the computational fluid dynamics model of the nonisothermal circulating fluidized bed riser which included effect of the cooling water tube. The simulation procedure found the suitable grid number and simulation time of 420,000 cells and 100 s, respectively. The effect of cooling water temperature on the system hydrodynamics and temperature profiles was investigated. The results showed that the CO2 capture capacity increases with the decreasing temperature of cooling water temperature.

Experiment and multiphase CFD simulation of gas-solid flow in a CFB reactor at various operating conditions: Assessing the performance of 2D and 3D simulations

Accurate prediction of gas-solid flow hydrodynamics is key for the design, optimization, and scale-up of a circulating fluidized bed (CFB) reactor. Computational fluid dynamics (CFD) simulation with two-dimensional (2D) domain has been routinely used, considering the computational costs involved in three-dimensional (3D) simulations. This work evaluated the prediction capability of 2D and 3D gas-solid flow simulation in the lab-scale CFB riser section. The difference between 2D and 3D CFD simulation predictions was assessed and discussed in detail, considering several flow variables (superficial gas velocity, solid circulation rate, and secondary air injection). The transient Eulerian-Eulerian multiphase model was used. CFD simulation results were validated through an in-house experiment. The comparison between the experimental data and both computational domains shows that the 3D simulation can accurately predict the axial solid holdup profile. The CFD simulation comparison considering several flow conditions clearly indicated the limitation of the 2D simulation to accurately predict key hydrodynamic features, such as high solid holdup near the riser exit and riser bottom dense region. The accuracy of 2D and 3D simulations was further assessed using root-mean-square error calculation. Results indicated that the 3D simulation predicts flow behavior with higher accuracy than the 2D simulation.

Validation of a filtered drag model for solid residence time distribution (RTD) prediction in a pilot-scale FCC riser

Computational fluid dynamics (CFD) is a powerful tool for prediction and analysis of complex multiphase flow hydrodynamics and residence time distribution (RTD) in chemical reactors. This study presented the validation and application of a filtered drag model for solid RTD prediction in a pilot-scale fluid catalytic cracking (FCC) circulating fluidized bed (CFB) riser with Geldart A particles. First, the filtered drag model implemented in the open-source MFiX-TFM solver was validated for flow hydrodynamics simulation in a FCC CFB riser. After that, the model was further employed to validate its ability for solid RTD prediction in the same riser by comparing the simulation results with the experimental data. The simulation with the filtered drag model well reproduced the tracer response experiment which is more accurate than that with the Gidaspow drag model. Simulations with both pulse and step tracer injection methods were compared, which reveals the limitation of solid RTD measurement using a pulse tracer injection method in the experiment.