Evaluation of the effect of wall boundary conditions on numerical simulations of circulating fluidized beds

A computational fluid dynamics (CFD) modeling of the gas–solids two-phase flow in a circulating fluidized bed (CFB) riser is carried out. The Eularian–Eularian method with the kinetic theory of granular flow is used to solve the gas–solids two-phase flow in the CFB riser. The wall boundary condition of the riser is defined based on the Johnson and Jackson wall boundary theory (Johnson & Jackson, 1987) with specularity coefficient and particle–wall restitution coefficient. The numerical results show that these two coefficients in the wall boundary condition play a major role in the predicted solids lateral velocity, which affects the solid particle distribution in the CFB riser. And the effect of each of the two coefficients on the solids distribution also depends on the other one. The generality of the CFD model is further validated under different operating conditions of the CFB riser.     

THERMAL BOUNDARY LAYER IN CFB BOILER RISER

Measurement of temperature profiles of gas-solid two-phase flow at different heights in commercial-scale circulating fluidized bed (CFB) boilers was carried out. Experimental results showed that the thickness of thermal boundary layer was generally independent of the distance from the air distributor, except when close to the riser outlet. Through analysis of flow and combustion characteristics in the riser, it was found that the main reasons for the phenomena were: 1) the hydrodynamic boundary layer was thinner than the thermal layer and hardly changed along the CFB boiler height, and 2) both radial and axial mass and heat exchanges were strong in the CFB boiler. Numerical simulation of gas flow in the outlet zone confirmed that the distribution of the thermal boundary layer was dominated by the flow field characteristics.     

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.     

Investigation on gas-solids hydrodynamics and matching enhancement in feedstock injection zone of double-level nozzle riser

With the development of current energy economy, it is necessary to improve the product distribution of fluid catalytic cracking process, which is achieved by a riser reactor with double-level of nozzles. The new riser is constructed by adding a level of secondary nozzle 0.5 m below the main nozzle of traditional riser. This paper investigates the gas-solids flow and oil-catalyst matching feature based on the optical fiber and tracer technologies. According to the distribution of solids holdup, particle velocity and dimensionless jet concentration, the feedstock injection zone can be divided into the upstream flow control region, the main flow control region, and the secondary flow control region in the radial direction. The size of the regions is changed by the jet gas velocity and axial height. There is a poor match of secondary nozzle jet to particles below the main nozzle. The jet gas from secondary nozzles can improve the matching effect of oil-catalyst near the wall and reduce the probability of coking above the main nozzle.     

Rotational asymmetry of reactant concentration and its evolution in a circulating fluidized bed riser

Rotational asymmetric distribution of reactant (ozone) concentration and its evolution along with the gas-solid reactive flow were studied in a 76 mm i.d., 10.2 m high circulating fluidized bed (CFB) riser reactor. The superficial gas velocity ranged from 3 to 5 m/s and the solids circulation rates were 50 and 100 kg/(m² s). Experimental results show that the asymmetry of reactant distribution can extend to a height close to the length of flow developing zone of the CFB riser reactor and then disappears. Based on the hydrodynamics of the gas and solid phases in the solids entrance region, this asymmetry can be attributed to the effect of the solids entrance structure.     

Solids flow diagram of a CFB riser using Geldart B-type powders

Riser operating modes are vital to designing a circulating fluidized bed (CFB) reactor for a required process of either a gas-solid or a gas-catalytic nature. Different operating modes provide different solids’ residence times and mixing behaviors, which define the reactions’ efficiency and yield. The literature demonstrates distinct operating modes resulting from observed differences in slip factors and the range of particle velocities and their associated residence time distribution. The present research uses positron emission particle tracking (PEPT) in a riser of B-type bed material to determine the different operating modes by measuring (i) particle velocities and residence time distribution, (ii) population densities of these particles in the cross-sectional area of the riser, and (iii) solids flow pattern at the bottom of the riser. Data treatment defines four distinct solids hold-up regimes in the riser and proposes a "phase diagram" depicting the existence of the different operating modes (dilute, dense, core-annulus and combined) as a function of the superficial gas velocity and solids circulation flux in the riser. The delineated regimes have good agreement with available literature data and known industrial operations. Comparison with literature data for risers using A-type powders is also fair. The diagram enables CFB designers to better delineate operating characteristics.     

Theoretical and numerical studies on the flow multiplicity phenomenon for gas–solids two-phase flows in CFB risers

The dependence of the fully-developed flow profiles on the inlet flow conditions for gas–solids two-phase flows, i.e. the flow multiplicity phenomenon, in circulating fluidized bed (CFB) risers was proposed and discussed in this article. The flow multiplicity phenomenon for gas–solids two-phase flows was first proved mathematically based on the conservation equations of mass and momentum. Then the CFD model using Eulerian–Eulerian approach with k–ε turbulence model for each phase was further adopted to analyze the details of this flow multiplicity phenomenon. It is theoretically and numerically revealed that for gas–solids two-phase flows, the flow profiles in the fully-developed region are always dominated by the flow profiles at the inlet. The solids concentration profile is closely coupled with the velocity profile, and the inlet solids concentration and velocity profiles can largely influence the fully-developed concentration and velocity profiles.     

Euler-Euler CFD modeling of fluidized bed＂ Influence of specularity coefficient on hydrodynamic behavior

Euler-Euler two-fluid model is used to simulate the hydrodynamics of gas-solid flow in a bubbling flu- idized bed with Geldert B particles where the solid property is calculated by applying the kinetic theory of granular flow （KTGF）. Johnson and Jackson wall boundary condition is used for the particle phase, and different amount of slip between particle and wall is given by varying the specularity coefficient （φ） from 0 to 1. The simulated particle velocity, granular temperature and particle volume fraction are compared to investigate the effect of different wall boundary conditions on the hydrodynamic behavior, Some of the results are also compared with the available experimental data from the literature. It was found that the model predictions are sensitive to the specularity coefficient. The hydrodynamic behavior deviated sig- nificantly for φ = 0 and φ = 0.01 with maximum deviation found at φ = 0 i.e. free-slip condition. However, the overall bed height predicted by all the conditions is similar.     

Pressure drop prediction with a modified frictional-kinetic model for alumina in bypass pneumatic conveying system

A new frictional-kinetic model is proposed and modified for pressure drop prediction of alumina in a bypass pneumatic conveying system. This new model is based on the conventional Johnson–Jackson frictional-kinetic model. The critical value of solids volume fraction and maximum packing limit are modified based on the fluidized bulk density and tapped bulk density, respectively. In addition, an offset solid volume fraction is introduced into the frictional pressure model as well as into the radial distribution functions which represents the correction factors to modify the probability of collisions between particles when solid phase becomes excessively dense. For the application of the model, computational fluid dynamics (CFD) simulations were conducted by using kinetic theory, conventional frictional-kinetic model and modified frictional-kinetic model. The simulation results were then compared with the experimental results. It was found that the modified frictional-kinetic model showed the largest improvement on pressure drop prediction results compared with results obtained from applying the kinetic theory and the conventional frictional-kinetic model, especially for denser flows with low air mass flow rates and high solid loading ratios (SLR). In addition, the solids volume investigation of CFD simulations shows a strong comparison to the actual flow conditions in the pipe, as transient slug type flow of alumina is observed.     

Numerical comparison of two modes of gas-solid riser operation: Fluid catalytic cracking vs CFB combustor

Two modes of gas-solid riser operation, i.e., fluid catalytic cracking (FCC) and circulating fluidized bed combustor (CFBC), have been recognized in literature; particularly in the understanding of choking phenomena. This work compares these two modes of operation through computational fluid dynamics (CFD) simulation. In CFD simulations, the different operations are represented by fixing appropriate boundary conditions: solids flux or solids inventory. It is found that the FCC and CFBC modes generally have the same dependence of solids flux on the mean solids volume fraction or solids inventory. However, during the choking transition, the FCC mode of operation needs more time to reach a steady state; thus the FCC system may have insufficient time to respond to valve adjustments or flow state change, leading to the choking. The difference between FCC and CFBC systems is more pronounced for the systems with longer risers. A more detailed investigation of these two modes of riser operation may require a three-dimensional full loop simulation with dynamic valve adjustment.     

Anti-wear beam effects on gas–solid hydrodynamics in a circulating fluidized bed

Anti-wear beams installed on water walls of circulating fluidized bed (CFB) boilers are one of the most effective ways to protect against water-wall erosion. Beam effects from, for example, beam size and superficial gas velocity were investigated on gas–solid hydrodynamics in a CFB test rig using CFD simulations and experimental methods. The downward flow of the wall layer solids is observed to be disrupted by the beam but is then restored some distance further downstream. When falling solids from the wall layer hit the anti-wear beam, the velocity of the falling solids decreases rapidly. A fraction of the solids accumulates on the beam. Below the beams, the falling solids have reduced velocities but upward-moving solids were observed on the wall. The effect of the beam increases with width and superficial gas velocity. Wear occurs mainly above the beam and its variation with width is different above to below the beam. There is an optimum width that, when combined with beam height, results in less erosion.     

Role of inter-particle collision on solids acceleration in riser

Collision among particles plays a significant role in governing the structure of gas-solids flow in a riser,especially in the dense and acceleration region.The inter-particle collision is the major cause not only for the kinetic energy dissipation(in terms of additional pressure drop beyond the solids hold-up) but also for the control of solids acceleration(in terms of a balancing force to prevent a free acceleration of solids).A neglect of the balancing force of inter-particle collision against the hydrody...     

Particle size distribution in CPFD modeling of gas-solid flows in a CFB riser

A computational particle fluid dynamics(CPFD) numerical method to model gas-solid flows in a circulating fluidized bed(CFB) riser was used to assess the effects of particle size distribution(PSD) on solids distribution and flow.We investigated a binary PSD and a polydisperse PSD case.Our simulations were compared with measured solids concentrations and velocity profiles from experiments,as well as with a published Eulerian-Eulerian simulation.Overall flow patterns were similar for both simulation cases,as confirmed by experimental measurements.However,our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser,as seen in other numerical studies.Above this bottom region,distributions of particle volume fraction and particle vertical velocity were consistent with our experiments,and the simulated average particle diameter decreased as a power function with riser height.Interactions between particles and walls also were successfully modeled,with accurate predictions for the lateral profiles of particle vertical velocity.It was easy to implement PSD into the CPFD numerical model,and it required fewer computational resources compared with other models,especially when particles with a polydisperse PSD were present in the heterogeneous flow.     

Non-uniform distribution of gas–solid flow through six parallel cyclones in a CFB system: An experimental study

In large-scale circulating fluidized bed (CFB) boilers, it is common to use multiple cyclones in parallel for the capture of solids, assuming that gas–solid flow to be the same in the cyclones. This article presents a study investigating gas–solid flow through six parallel cyclones in a CFB cold test rig. The six cyclones were located asymmetrically on the left and right walls of the riser. Solid volume fraction and particle velocity profiles at the riser outlets and in the horizontal ducts were measured using a fiber optical probe. Cyclone pressure drop and solid circulating rate were measured for each individual cyclone. Measurements showed good agreement as to the non-uniform distribution of the gas–solid flow, which occurred mainly across the three cyclones on one side: the middle cyclones on both sides had higher particle velocities. Conversely, the solid volume fractions, solid fluxes and solid circulating rates of the middle cyclones were lower than those of the other four cyclones. The apparent reason for the flow non-uniformity among the cyclones is the significant flow non-uniformity at the riser outlets. Under typical operating conditions, the solid volume fractions at the riser outlets had a deviation of up to 26% whereas the solid circulating rates at the stand pipes, 7%. These results are consistent with most other studies in the literature.     

Particle size distribution in CPFD modeling of gas–solid flows in a CFB riser

A computational particle fluid dynamics (CPFD) numerical method to model gas–solid flows in a circulating fluidized bed (CFB) riser was used to assess the effects of particle size distribution (PSD) on solids distribution and flow. We investigated a binary PSD and a polydisperse PSD case. Our simulations were compared with measured solids concentrations and velocity profiles from experiments, as well as with a published Eulerian-Eulerian simulation. Overall flow patterns were similar for both simulation cases, as confirmed by experimental measurements. However, our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser, as seen in other numerical studies. Above this bottom region, distributions of particle volume fraction and particle vertical velocity were consistent with our experiments, and the simulated average particle diameter decreased as a power function with riser height. Interactions between particles and walls also were successfully modeled, with accurate predictions for the lateral profiles of particle vertical velocity. It was easy to implement PSD into the CPFD numerical model, and it required fewer computational resources compared with other models, especially when particles with a polydisperse PSD were present in the heterogeneous flow.     

Numerical study on mitigating severe slugging in pipeline/riser system with wavy pipe

The gas/liquid two-phase flow in pipeline/wavy-pipe/riser systems was investigated numerically with CFD. A CFD model of the pipeline/wavy-pipe/riser system was obtained by adding a wavy pipe to the model of the pipeline/riser system verified by the experimental data previously. The effects of the geometrical parameters and location of the wavy pipe on its performance of slug mitigation and flow characteristics in pipeline/wavy-pipe/riser systems were examined through the CFD models. With the increase of the amplitude or length of the wavy pipe, the slug in the pipeline/riser system becomes shorter. The optimum location of the wavy pipe in the pipeline exists for a pipeline/riser system and a wavy pipe at given operating conditions. The CFD modelling provides a feasible and flexible way to investigate the effectiveness of the wavy pipes on mitigating severe slugging in pipeline/riser systems.     

A review of multiscale CFD for gas–solid CFB modeling

Meso-scale structure is of critical importance to circulating fluidized bed (CFB) applications. Computational fluid dynamics (CFD) with consideration of meso-scale structures can help understand the structure-oriented coupling between flow, heat/mass transfer and reactions. This article is to review our recent progress on the so-called multiscale CFD (MSCFD), which characterizes the sub-grid meso-scale structure with stability criteria in addition to conservation equations. It is found that the mesh-independent solution of fine-grid two-fluid model (TFM) without sub-grid structures is inexact, in the sense that it overestimates the drag coefficient and fails to capture the characteristic S-shaped axial profile of voidage in a CFB riser. By comparison, MSCFD approach in terms of EMMS/matrix seems to reach a mesh-independent solution of the sub-grid structure, and succeeds in predicting the axial profile and flow regime transitions. Further application of MSCFD finds that neglect of geometric factors is one of the major reasons that cause disputes in understanding the flow regime transitions in a CFB. The operating diagram should, accordingly, include geometric factors besides commonly believed operating parameters for the intrinsic flow regime diagram. Recent extension of MSCFD to mass transfer finds that Reynolds number is insufficient for correlating the overall Sherwood number in a CFB. This is believed the main reason why the conventional correlations of Sherwood number scatter by several orders of magnitude. Certain jump change of state of motion around Reynolds number of 50–100 can be expected to clarify the abrupt decay of Sherwood number in both classical- and circulating-fluidized beds. Finally, we expect that the real-size, 3-D, full-loop, time-dependent multiscale simulation of CFB is an emerging paradigm that will realize virtual experiment of CFBs.     

STUDY ON THE OVERALL PRESSURE BALANCE OF A DOWNFLOW CIRCULATING FLUIDIZED BED SYSTEM

A pressure balance model for a circulating fluidized bed unit that incorporates a downer has been proposed. The model predictions were validated with the experimental data obtained from a special cold-model circulating fluidized bed. Comparison of the operation stability between a CFB downer and a CFB riser has been carried out. Only one critical gas velocity exists in the CFB-riser for a given riser solids flux, while there can be many critical gas velocities for the operation of a CFB downer. Therefore, it is possible to achieve high solids concentration in a CFB downer if appropriate operating conditions are used.     

CFD study: Effect of pulsating flow on gas–solid hydrodynamics in FCC riser

Gas–solid flow in a fluid catalytic cracking (FCC) riser exhibits poor mixing in the form of a core–annulus flow pattern and a dense bottom/dilute top distribution of solids. To enhance gas–solid mixing, studies on dense fluidized beds have suggested using a pulsating flow of gas. The present study investigates the effect of pulsating flow on gas–solid hydrodynamics inside the FCC riser employing computational fluid dynamics. Two flow conditions are investigated: a cold flow of air-FCC catalyst in a pilot-scale riser and a reactive flow in an industrial-scale FCC riser. In the cold-flow riser, pulsating flows cause the slug flow of solids and thus increase the average solid accumulation in the flow domain and solid segregation towards the wall. In the industrial FCC riser, pulsating flows produce radial profiles that are more homogeneous. Pulsating flows further improve the conversion and yield in the initial few metres of height. At 7 m, the conversion from pulsating flow is 59%, compared with 44% in without pulsating flow. The results and analysis presented here will help optimize flow conditions in the circulating fluidized bed riser, in not only FCC but also applications such as fast pyrolysis and combustion.     

Flow dynamics and contact efficiency in a novel fast-turbulent fluidized bed with ring-feeder internals

The flow dynamics in a novel fast-turbulent fluidized bed(FTFB) with middle-upper expanding structure and two different ring-feeder internals(mixed and vortex ring-feeder) were studied to achieve a reduction in gasoline olefin production.Compared with a conventional circulating fluidized bed,the novel FTFB displayed unique characteristics and advantages.A higher solids holdup and more uniform solids holdup distribution existed in the diameter-expanding region,especially for the FTFB with vortex ring-feeder structure.A probability density distribution analysis indicated that the novel fluidized bed could reduce gas-solids segregation and enhance gas-solids interaction.A constant carbon dioxide tracer system was used to simulate the reactant gas distribution.The gas-solids contact efficiency was defined according to the solid dispersibility and the amount of gas covering the solid surface.Novel FTFB risers,especially those with vortex ring-feeders,have a much higher gas-solids contact efficiency than that of traditional risers.