Solids Circulation Flux Research Articles

Solids dispersion in high density circulating fluidized beds

Transient local solid tracer concentration distributions were measured by a novel, simple and accurate measurement system with fast response to obtain axial solids dispersion in the riser of a circulating fluidized bed operated at solids fluxes up to 450 kg/m2s and gas velocities up to 8 m/s. Phosphorescent-coated FCC particles were used as the tracer which were activated by high intensity UV light and injected into the return leg of high-density circulating fluidized bed unit. A radially non-uniform axial dispersion model was utilized to determine axial solids dispersion coefficients, and the results were interpreted with the help of hydrodynamic data obtained in the same column. Comparison of the axial dispersion coefficients at different operating conditions revealed that dispersion decreases when solids circulation flux increases to 450 kg/m2s together with the increase in gas velocity to 8 m/s. Low solids dispersion coefficients (Daz < 0.1) at high solids circulation fluxes and high velocities resulted from the disappearance of the net downward flow of particles at the riser walls, signifying the existence of the dense suspension upflow regime.

Investigation of gas-solid flow characteristics in a novel internal fluidized bed combustor by experiment and CPFD simulation

Based on the coal self-preheating combustion technology, this research proposed a novel internal fluidized bed combustor (IFBC) with an internal separator for stable preheating of fuel. In order to verify feasibility and operation stability of IFBC, cold experiment, electrical capacitance tomography (ECT) and computational particle fluid dynamic (CPFD) simulation were performed in a laboratory-scale IFBC. The effects of superficial air velocity (Ug) and return valve structure on the operation and gas-solid flow characteristics were investigated. The results revealed that the CPFD prediction agreed well with the experiment values. The pressure balance curve presented an “8″ shape distribution, and the particle volume fraction (PVF) showed ‘core-annular’ distribution features. With the increase of Ug, the PVF in the standpipe increased, and the discharge pattern of the return valve changed from continuous discharge to intermittent discharge, and the solid circulation flux showed a trend of increasing first and then decreasing. With the decrease of the outlet opening of return valve (Φ), the gas–solid flow behavior in standpipe experienced a transition from gas leakage, stabilizing material seal, and blocking state. For Ug = 2 m/s, Φ = 50 %, an effective solid seal in the return valve was established and IFBC has a stable circulation and operation.

Estimation of solids circulation rate and char transfer rate from gasifier to combustor in a dual fluidized-bed pilot plant for biomass steam gasification

Operation of a dual fluidized bed, consisting of a riser as combustor and bubbling bed as gasifier, for synthesis gas production from a solid fuel requires determination of the solids circulation rate and char transfer rate. The performance relies on supplying sufficient heat from the combustor to the gasifier by circulation of solids between these two reactors. The flow rate of char is required to track the heat generated in the combustor, which supports endothermic reactions in the gasifier. Direct measurement of these two critical parameters is difficult, with the number of reported techniques capable of working at high temperatures extremely small. An indirect method was developed, using mass and energy balances over the entire system and individual reactors, to estimate the solids circulation rate and char transfer rate. There was general agreement between heat losses estimated from energy-balance calculations and from direct measurement of the outer reactor surface temperature. Under typical gasification conditions, the solids circulation fluxes were estimated to be 45.2 and 55.6kg/(m2s) in two independent tests, which were in good agreement with values obtained using a thermal tracer; char transfer rates were calculated to be 1.2 and 0.6kg/h, which were in reasonable agreement with average biomass feed rates. This method can be applied to dual gasification systems at any temperature or flow rate.

Investigation of gas–solid flow characteristics in the cyclone dipleg of a pressurised circulating fluidised bed by ECT measurement and CPFD simulation

To optimize the design and operation of a pressurised circulating fluidised bed (PCFB) system, it is necessary to understand the gas–solid flow characteristics and properly measure the solid circulation flux in the cyclone dipleg and loop-seal of the PCFB. In this research, electrical capacitance tomography (ECT) measurement and computational particle fluid dynamic (CPFD) simulation are combined to investigate the multiphase flow behaviour of a ‘cold’ PCFB, in particular the cyclone dipleg. A dual-plane ECT sensor is used for visualisation and online monitoring of the particle distribution. The particle velocity is measured by cross-correlation of two signals from capacitance electrodes, and the circulation flux is analysed. In addition, 3D full-loop CPFD simulation incorporating an energy-minimization multi-scale (EMMS) drag model is performed on a Barracuda platform, to understand the gas–solid hydrodynamic behaviour, validate and supplement the experimental measurements. The results show that the CPFD prediction agrees well with the ECT measurement in terms of the particle volume fraction distribution, velocity and circulation flux. The time-averaged gas–solid flow behaviour, instantaneous hydrodynamic features and the effects of operating pressure on particle distribution and the flow regime in the PCFB dipleg are unravelled. This is the first investigation of a solid circulation flux measurement in the cyclone dipleg and loop-seal of a PCFB system using a combination of ECT measurements, cross-correlation and CPFD simulation with high-pressure operational conditions.

Influence of Solids Circulation Flux on Coal Gasification Process in a Pressurized High-density Circulating Fluidized Bed

The coal gasification behaviors in the pressurized high-density circulating fluidized bed under various solids circulation fluxes were studied with the CFD method, which combines the two-fluid model and coal gasification reactions represented by the chemical percolation devolatilization and the MGAS models. The numerical method was validated with two experimental cases, and detailed distributions of gas species and temperature in the riser were illustrated to understand the gasification process. To fully understand the influence of solids circulation flux on the gasification behavior, a series of cases were simulated with the solids flux varying gradually from 260 to 1010 kg/m2s, and the composition and quality of syngas were compared between various cases. The higher heating value of syngas firstly increased and then decreased with the increase of solids flux, and it reached the highest value around 480 kg/m2s. The influence of solids flux on gasification process was further analyzed through the contours of temperature, solids concentration, and gas composition in the riser.

Hydrodynamic Study of AR Coupling Effects on Solid Circulation and Gas Leakages in a High-Flux In Situ Gasification Chemical Looping Combustion System

In situ gasification chemical looping combustion (iG-CLC) is a novel and promising coal combustion technology with inherent separation of CO2. Our previous studies demonstrated the feasibility of performing iG-CLC with a high-flux circulating fluidized bed (HFCFB) riser as the fuel reactor (FR) and a counter-flow moving bed (CFMB) as the air reactor (AR). As an extension of that work, this study aims to further investigate the fundamental effects of the AR coupling on the oxygen carrier (OC) circulation and gas leakages with a cold-state experimental device of the proposed iG-CLC system. The system exhibited favorable pressure distribution characteristics and good adaptability of solid circulation flux, demonstrating the positive role of the direct coupling method of the AR in the stabilization and controllability of the whole system. The OC circulation and the gas leakages were mainly determined by the upper and lower pressure gradients of the AR. With the increase in the upper pressure gradient, the OC circulation flux increased initially and later decreased until the circulation collapsed. Besides, the upper pressure gradient exhibited a positive effect on the restraint of gas leakage from the FR to the AR, but a negative effect on the suppression of gas leakage from the AR to the FR. Moreover, the gas leakage of the J-valve to the AR, which is directly related to the solid circulation stability, was exacerbated with the increase of the lower pressure gradient of the AR. In real iG-CLC applications, the pressure gradients should be adjusted flexibly and optimally to guarantee a balanced OC circulation together with an ideal balance of all the gas leakages.

Measurement of solids circulation rate in a high-temperature dual fluidized bed pilot plant

A number of fluidized bed reactor processes operating at high temperature require that solid particles be circulated back and forth between two reactor vessels. Since the circulation rate strongly affects mass and energy balances, and therefore greatly influences hydrodynamics and performance of the system, a reliable technique for its accurate measurement would be helpful in monitoring and modeling the process. However, there are no reported techniques suitable for measuring solid circulation rates at elevated temperatures typical of gasification systems.A novel thermal-tracing technique was developed for measuring the solids circulation rate between two reactors. Particles at room temperatures (cold particles) are injected into a downward-moving packed bed of solids at elevated temperature (hot particles), creating reduced-temperature zones inside the moving bed. The transit time of the cold-particle-clusters between pairs of thermocouples is determined by cross correlation allowing the flux to be estimated. The technique was shown to provide sensitive and reproducible data for a cold model unit with injection of dry ice. The technique was then applied to determine the solids circulation rate between the bubbling bed gasifier and the riser combustor of a pilot scale dual fluidized bed gasification system. A number of conditions are imposed on the data to eliminate unsatisfactory data at high temperatures. Data which satisfy the discrimination criteria are shown to lead to measured solids circulation fluxes up to 133kg/m2-s at temperatures up to 856°C in the gasifier test section. The technique provides high-temperature solids circulation rate information beyond the capability of other techniques.

Effect of bed particle size on heat transfer between fluidized bed of group b particles and vertical rifled tubes

The effect of bed particle size on the local heat transfer coefficient between a fluidized bed and vertical rifled tubes (38mm-O.D.) has been determined in a large-scale circulating fluidized bed (CFB) reactor. Bed particles with different Sauter mean particle diameter within the range of 0.219–0.411mm and particle density in the range of 2650–2750kg/m3 were used as bed material in this heat transfer study. A gas fluidized bed furnace with 27.6×10.6m cross-section above refractory line and 48m in height was used. Air coal firing conditions at the membrane wall in the form of water tubes welded with lateral fins corresponded to a suspension density covering the range of 1.36–6.22kg/m3, furnace temperatures in the range of 1080–1164K, a superficial gas velocity varied from 2.99 to 5.11m/s and solids circulation flux covered a range of 23.3–26.2kg/(m2s). For these operating conditions, the heat transfer analysis of CFB reactor with detailed analysis of bed-to-wall heat transfer coefficient along furnace height was investigated. In this work, the overall heat transfer coefficient was estimated using a mechanistic heat transfer model based on cluster renewal approach. The experimental results show that: (i) higher heat transfer coefficients along furnace height were found under finer bed particles size dp<0.241mm, (ii) heat transfer data confirms strong dependency of the overall heat transfer coefficient on suspension density and also hydrodynamic conditions within CFB furnace, (iii) for small bed particles, dp<0.233mm, the particle convection component plays dominant role in heat transfer mechanism, (iv) for large bed particles, dp>0.366mm, the effect of particle size on relative contribution of radiation from dispersed phase become essential with particle diameter increasing, and (v) for all bed particles with diameters in the range of 0.240–0.411mm, the gas convection heat transfer coefficient between the fluidized bed (Geldart B particles) and the rifled tubes increased as the bed particles size increased.

Numerical investigation of solid circulation flux in an internally circulating fluidized bed with different gas distributor designs

Numerical simulations using two-fluid model incorporating the kinetic theory of granular flow (KTGF) are conducted to study the behavior of bed particles in a two dimensional baffle type internally circulating fluidized bed (ICFB). Complete knowledge of the solid circulation flux in an ICFB is very important as the bed particles are continuously circulating between different chambers above and below the central baffle. The effects of the gas distributor types, superficial fluidizing velocity and chambers height elevation difference are evaluated through computer simulations and the results are quantified in terms of solid circulation flux. By increasing the gas velocity (Uf) in the RC chamber, solid circulation flux from the heat exchange chamber (HEC) to the reaction chamber (RC) through the slot under the baffle is increased regardless of the distributor design. From further investigations, the solid circulation flux and particles radial velocity through the slot in the tubular gas distributor ICFB are found to be considerably increased as compared to the other configurations in our case. The elevated RC distributor is found to have a little impact on the solid circulation flux as a function of Uf. By varying RC distributor elevation while keeping HEC constant, the circulation flux is also changing and the highest flux is observed at an elevation level of 90mm.

Hydrodynamics of Biomass Gasification in a Dual Chamber Circulating Fluidized Bed Reactor

This paper presents work on hydrodynamics of several types of biomass mixture in a dual chamber circulating fluidized bed. In designing the CFBreactornecessary to know the distribution of solid particles radially and axially influenced by fluidizing gas velocity, particle size, solid circulation flux, reactor diameter and height of the reactor. These factors will affect pressure drop along the riser of the reaction chamber. Pressure drop is an important factor in the study of hydrodynamics of particle flow. The pressure drop was measured using mathematical model compared to experimental results done on a cold mode. Since it was found that both results were consistent which means that the model can be used to predict the operating parameters of CFB design.

Particle circulation loops in solar energy capture and storage: Gas–solid flow and heat transfer considerations

A novel application of powders relies on their use as heat transfer medium for heat capture, conveying and storage. The use of powders as heat transfer fluid in concentrated solar systems is discussed with respect to current technologies. The specific application reported upon is the use of powder loops in Solar Power Tower plants. In the proposed receiver technology, SiC powder is conveyed as a dense particle suspension through a multi-tube solar receiver in a bubbling fluidization mode, the upwards flow being established by pressurizing the powder feed. Tests were conducted with a single-tube receiver unit at the 1MW solar furnace of CNRS (Odeillo Font-Romeu, F). The measured wall-to-suspension heat transfer coefficient is a function of operating temperature, applied air velocity and imposed solid circulation flux: values increased with increasing solids flux from ∼430 to 1120W/m2K. Empirical approaches and a heat transfer model were applied to compare experimental and predicted values of the heat transfer coefficient, with a fair agreement obtained. The research moreover provides initial data concerning the overall economy of the system. The high temperature of the circulating powder leads to an increased power cycle efficiency, an increased storage density, reduced thermal power requirements, reduced heliostat field size, reduced parasitic power consumption and increased plant capacity factor.

Operation Diagram of Circulating Fluidized Beds (CFBs)

CFBs are widely used in the chemical, mineral, environmental and energy process industries. Several authors stressed the need for a clear identification of the different operation regimes in the riser of a CFB, to ensure a better comprehension of the hydrodynamic context, and thus better define the operation and design parameters. First approaches to develop a “work map” of the riser operation, were presented by e.g. Grace[1], Yerushalmi and Avidan[2], Bai et al.[3]. It was further developed by Chan et al.[4] and Mahmoudi et al.[5,6] for both Geldart A- and B-type powders, in terms of the operating gas velocity (U) and the solids circulation flux (G), which jointly delineate different regimes, called respectively Dilute Riser Flow (DRF), Core-Annulus Flow (CAF) (possibly with a bottom Turbulent Fluidized Bed, TFBB), and Dense Riser Upflow (DRU). For a given powder and its associated transport velocity, UTR, the combination of U and G will determine the flow regime encountered. Experiments in CFB risers of 0.05 (2.5 m high), 0.1 and 0.15 m I.D. (both 6.5 m high), have demonstrated that common riser operations can be hampered by a specific (U,G) range where choking occurs. Angular sand, rounded sand, and spent FCC (all A-type powders) were used as bed material. Gas velocities were varied between 2 and 10 m/s, for solids circulation fluxes between 10 and 260kg/m2s. Choking is understood as the phenomenon where a small change in gas or solids flow rate prompts a large change in the pressure drop and/or solids holdup during the gas-solid flow: the stable riser upflow regime is no longer maintained when G-values exceed a certain limit for a given gas velocity. Experimental results were empirically correlated, and proved to be about 30% lower than predicted by the correlation of Bi and Fan[7], but largely exceeding other predictions. Introducing the findings into the available operation diagram [5,6], adds a region where stable riser operation is impossible. The adapted diagram enables CFB designers to better delineate the operating characteristics.

The Voidage in a CFB Riser as Function of Solids Flux and Gas Velocity

Circulating Fluidised Beds (CFBs) are widely applied in the process industry, for mostly gas-solid and gas-catalytic reactions. The riser is the key component of the CFB being the process reactor. The important design parameters are the operating gas velocity (U) and the solids concentration flux (G). The CFB operation starts at moderate to high superficial gas velocities. Its voidage exceeds ∼ 0.9 and is a function of the solids circulation flux. Different flow modes have been presented in literature, and result in an operation diagram where G and U delineate specific operations, from dilute riser flow, through core-annulus flow, to dense riser upflow (mostly at any U, G exceeding 80 to 120 kg m-2s-1). Increasing G whilst maintaining the gas velocity will cause an increase in suspension concentration. The riser flow can hence be characterized by its apparent voidage, ɛ. In the core-annulus operation, clusters of particles reflux near the wall, thus influencing the local radial voidage in the cross section of the riser, and also extending over a given distance, δ, from the wall to the core .Through measurements in CFBs of 0.1 and 0.14 m I.D., the research has been able to determine the average axial and radial voidages of the dense phase within the different regimes, whilst also determining the thickness of the annulus (in CAF-mode). Experimental results will be illustrated and compared with previous empirical equations, shown to have a limited accuracy only both for ɛ, and for the thickness of the annulus in CAF operation. Within the operating conditions tested, results demonstrate that the annulus thickness is about 15 to 20% of the riser diameter in CAF, and that the voidage in the riser is a function of U and G, with riser diameter and distance along the riser length as secondary parameters.

Hydrodynamics of lime-based pellets in a Dual Fluidized Bed and the effect of temperature

Abstract The looping of dry CO 2 sorbents in Dual Fluidized Beds (DFBs) plays a prominent role in CO 2 capture methods, and understanding the hydrodynamics of DFBs is of great importance. Geldart B lime-based pellets were investigated in a DFB with a constant bubbling fluidized bed velocity of 0.17 m/s, a riser velocity varied between 1.3 and 5.3 m/s, a loop-seal aeration velocity ranging from 0.08 to 1.08 m/s and a temperature from ambient to 550 °C. The solids circulation flux ( G s ) increased with increasing superficial riser velocities, as well as with increasing loop-seal aeration. Experiments at elevated temperatures revealed that increasing the temperature decreased G s and the solids volume fraction in the bottom section of the riser. The riser underwent transition from fast fluidization to pneumatic transport by increasing either the superficial riser velocity or the temperature. The interdependence between the riser solids fraction, G s and particle drag was explored by means of the slip factor. A new slip factor correlation is proposed which makes use of the drag coefficient and terminal Froude number.

Gas leakage between reactors in a dual fluidized bed system

The leakage of gas between two interconnected gas–solid fluidized beds is investigated for 48 operating conditions in a lab-scale dual fluidized bed (DFB) consisting of a riser and a bubbling fluidized bed (BFB). Helium is used as the tracer gas with a spent FCC catalyst bed in ambient conditions. Gas leakage occurs from the riser to the BFB via the downcomer; and from the BFB to the riser via the loop-seal. Gas leakage from the riser to the BFB varied between 0.1 and 0.6%, translating to 1 and 7% of the BFB being contaminated with riser gas. This leakage is dependent on pressure difference across the downcomer and the solids circulation flux. Leakage in the downcomer is proposed to be caused by the riser gas being dragged down by the falling particles in the upper section of the downcomer.

The convection heat transfer coefficient in a Circulating Fluidized Bed (CFB)

Circulating Fluidized Beds are increasingly used in gas–solid and gas–catalytic reactions. A recent development involves their use in physical gas–solid processes such as drying, VOC adsorption or solar energy capture and storage. The heat transfer from the wall of the CFB to the flowing gas–solid suspension is the major design parameter, and was studied for different powders at different operating conditions as determined by the gas velocity and solids circulation flux. Measured values of the heat transfer coefficients are discussed, and compared with empirical predictions of Molodtsof–Muzyka, and Gorliz–Grace. Whereas Gorliz–Grace predicts heat transfer coefficients correctly within a narrow range of operating conditions only, the Molodtsof–Muzyka approach can be simplified into a linear relationship.

LES-DEM investigation of gas–solid flow dynamics in an internally circulating fluidized bed

Gas–solid flow dynamics is numerically simulated in an internally circulating fluidized bed (ICFB) featuring a centrally located baffle plate and uneven aeration. Its detailed features are derived using the computational fluid dynamics-discrete element method (CFD-DEM). The gas motion is resolved by means of large eddy simulation (LES) while the dynamics of solid phase is handled by the soft-sphere model. The internal circulation of the solids (spherical particles, with diameter of 1.2mm and density 1000kg/m3) is modeled to clarify the flow characteristics of the bed for different design or operating parameters, such as superficial gas velocity, gap height, inclination angle of the gas distributor, baffle plate or side wall of the bed. Both the gas–solid flow and the underlying mechanisms of solids circulation are addressed together with the simulation results. Meanwhile, the operational control behavior of different parameters is quantified by estimating both the solids circulation flux (SCF) and the gas bypassing flux (GBF). The computational results indicate that the performance of the bed can easily be regulated by adjusting the aeration to each chamber of the bed and that within the range tested, increasing gap height enhances solids circulation and suppresses gas bypassing. There is an optimal value for the inclination angle of both side wall and baffle plate. However, increasing the inclination angle of the gas distributor continuously suppresses solids circulation. Furthermore, the residence time of solids in each chamber is figured out and its variations with these design and operating parameters are examined.

LES–DEM investigation of an internally circulating fluidized bed: Effects of gas and solid properties

Gas–solid flow dynamics of a three-dimensional baffle-type internally circulating fluidized bed (ICFB) is numerically investigated in this paper. Detailed multiphase flow characteristics are illustrated using a computational fluid dynamics–discrete element method (CFD–DEM) in which the gas flow field is modeled using large eddy simulation (LES) while solid kinematics is handled by soft-sphere model. The mechanisms of the internal circulation are addressed together with instantaneous and time-averaged results and solid rotation plays an important role as that of its translational motion. Simulations are also carried out to examine the influences of gas–solid properties (bed pressure, solid friction coefficient, restitution coefficient, Young modulus, diameter and density) on the bed circulation performance, which is quantified by the solid circulation flux (SCF) and gas bypassing flux (GBF). The results indicate that the behaviors of solid circulation and the corresponding gas bypassing exhibit great resemblance, especially when the solid properties change. The gas property, bed pressure, seems to show significant influence on the gas bypassing flux. Meanwhile, both solid circulation and gas bypassing are affected by the solid properties to a larger extent. They are enhanced with increasing solid density, restitution coefficient and gas pressure, while weaken as the solid friction coefficient, diameter and gas temperature increases. However, solid Young modulus shows inconspicuous influence on the circulation performances of the bed. Furthermore, additional normalization of the simulation data finds that the minimum fluidization velocity (Umf) of the system is a key parameter to the operational control of the ICFB.

Circulating fluidized bed heat recovery/storage and its potential to use coated phase-change-material (PCM) particles

Within the thermal energy capture and/or storage systems currently available or investigated, PCMs are the sole latent heat stores. Despite their low thermal conductivity, that limits charging and discharging times, the higher energy storage capacity per unit weight in comparison with sensible heat stores, makes them increasingly attractive for high temperature applications, resulting in reduced storage volumes and required circulation rates within the heat collector. The present paper introduces these PCMs, and their potential application in high temperature energy capture and storage, using a circulating fluidized bed (CFB) as transfer/storage mode. Thermal considerations determine the optimum size range for the applied particles (<400μm). The heat transfer from the wall of the CFB to the flowing gas–solid suspension is a major design parameter of the collector, and studied for different operating conditions as determined by the gas velocity and solids circulation flux. Measured values of the heat transfer coefficients are discussed, and compared with empirical predictions of Molodtsof–Muzyka, and Gorliz–Grace. Fair agreement is obtained only when the empirical parameters are carefully predicted. The application of a packet renewal mechanism at the wall is also investigated, with a fair prediction of the heat transfer coefficient in terms of the expected solid contact time at the wall.

Heat transfer to the riser-wall of a circulating fluidised bed (CFB)

The circulating fluidized bed is of increasing importance for gas–solid and gas–catalytic reactions, for drying, and recently its use in solar energy capture and storage has been advocated. In all applications, the supply or withdrawal of heat is a major issue, and the heat transfer coefficient from the gas–solid suspension to the heat transfer surface needs to be determined as design parameter. The present paper investigates the heat transfer coefficient for different operating gas velocity and solids circulation flux, whilst covering the different hydrodynamic solid flow regimes of dilute, core–annulus or dense mode. Measured values of the wall-to-bed heat transfer coefficients are compared with empirical predictions of both Molodstof and Muzyka, and Golriz and Grace. The application of a packet renewal mechanism at the wall is also investigated, and introducing the predicted solid contact time at the wall provides a very fair estimate of the heat transfer coefficient.