Fluidized Bed Riser Research Articles

A Numerical Analysis of the Descending Behaviors of Clusters at the Wall of the Circulating Fluidized Bed Riser

Particle clusters at the wall of the CFB riser have significant effects on the bed-to-wall heat transfer and abrasion, while their descending behaviors are not well understood because the entire descending process is difficult to track with experiments, due to the limitations of measurement technology. In this study, the gas–particle two-phase flow in the CFB riser is simulated using the LES-DSMC method. The entire descending process of the cluster is recognized using a method that involves identifying the continuity of periods in which clusters appear in the successive cells at the wall. Then, the transient velocity, drag force, and particle concentration of the descending cluster as a function of its traveling distance are obtained. The results show that the descending clusters at the wall of the CFB riser are dynamic collections of particles. Their lifetimes are in the range of 0.2~0.5 s. During the descending processes, they are accelerated, and their particle concentrations are continuously decreased. The variation in the particle concentration, velocity, and drag force of different descending clusters indicates that they travel highly similar distances and fluidization velocity has little effect on them.

Modeling Gas–Solid Flows in Circulating Fluidized Bed Risers Using Computational Fluid Dynamics

Modeling Gas–Solid Flows in Circulating Fluidized Bed Risers Using Computational Fluid Dynamics

Center-to-face momentum interpolation and face-to-center flux reconstruction in Euler-Euler simulation of gas-solid flows

In order to resolve the pressure checkerboard field problem with collocated grid, it is essential to employ the momentum interpolation method when formulating the pressure equation, and the flux reconstruction method when updating the cell-centered velocity fields. In this study, we first derive a momentum interpolation method for Euler-Euler simulation of gas-solid flows, which is independent of the time step, the transient term discretization scheme, the under-relaxation factor and the shape of grid; a complete first-order flux reconstruction method is then proposed to update the cell-centered velocities. Their effectiveness are proved by simulating the hydrodynamics of solids settlement, gas-solid fixed bed, bubbling fluidized bed and circulating fluidized bed riser, and then comparing the simulation results to the theoretically known solutions. Their superiority over the standard solver of OpenFOAM® in suppressing the high-frequency oscillations and enhancing the smoothness and accuracy is also proved. Finally, the difficulty in fully eliminating the high-frequency oscillations is attributed to the insufficiency of current methods in handling the situations where the independent variables undergo abrupt change.

Atmospheric Bubbling Fluidized Bed Risers: Effect of Cone Angle on Fluid Dynamics and Heat Transfer

Abstract In this paper, a comparative study of fluid flow behavior and thermal characteristics of sand particles has been carried out numerically and experimentally in bubbling fluidized beds for five-cone angles of the riser wall having 0 deg, 5 deg, 10 deg, 15 deg, and 20 deg. An Eulerian model with a k–ε turbulence model is used to explore the numerical analysis, and the findings are compared to those of the experiments. For the study, the inlet air velocity is fixed at 1.5 m/s with sand particles filled up to 30 cm to maintain bubbling conditions in the risers. The results indicate that increasing the cone angle up to 10 deg while maintaining the amount of bed materials constant leads to a reduction in pressure drop. The expansion of particles along the riser is observed to decrease with the increase in the cone angle up to 10 deg. The radial solid volume fraction profile transforms to a U shape from the W-type profile as the cone angle increases up to 10 deg. Correspondingly, the solid velocity is found to have an inverted U-type and W-shaped profile for the risers. The granular temperature is also found to increase with a decrease in the solid percentage at any location. The average bed temperature, interphase, and bed-to-wall heat transfer coefficient at a location of 10 cm axial height also increase with the cone angle increase up to 10 deg. As a result, the conical riser, when designed with a greater cone angle up to 10 deg, exhibits more efficiency in terms of heat transfer characteristics. The 3D simulation results are in strong concurrence with the experimental results in all investigations.

Recent progress in the operational flexibility of 1 MW circulating fluidized bed combustion

In this study, various experimental tests were carried out with a 1 MWth circulating fluidized bed pilot plant at the Technical University of Darmstadt to investigate the combustion and co-combustion behaviour of different fuels under load change conditions in air and oxyfuel mode. The combustion tests were carried out with low-calorific lignite and hard coal as reference cases for co-combustion. In the co-combustion experimental tests, straw pellets, and refuse-derived fuels (high and low calorific) were used as co-fuels for coal. In addition, a comprehensive dynamic 1.5-D process simulation model was established for a 1 MWth circulating fluidized bed pilot plant. The model created with the “APROS” software precisely reproduces both the gas/solids and the water/steam side of the 1 MWth pilot plant. The dynamic process simulation model was first validated at various steady-state operation points and, in a second step, at various dynamic load changes. The numerical results, such as the pressure drop and temperature profiles along the fluidized bed riser as well as the flue gas concentrations at the cyclone outlet, were compared with the measurements and showed agreement during the long-term tests of the combustion and co-combustion processes.

Hybrid machine learning modeling of nitrogen removal from wastewater using gas-liquid-solid circulating fluidized bed riser

Municipal wastewater generally contains high amounts of nitrogen (N), approximately 23–28 mg/L of total Kjeldahl Nitrogen (TKN), which causes eutrophication and pollution of groundwater. This article reports the generation of empirical models for estimating the nitrogen removal efficacy of two outputs (NH4-N and NO3-N) from the anoxic riser of a highly efficient municipal wastewater treatment system involving a gas-liquid-solid circulating fluidized bed (GLSCFB) bioreactor. To identify the important variables and discard irrelevant ones, the ReliefF algorithm is applied initially. Out of 10 independent variables, this method indicates that only three, such as flow rate, total Kjeldahl Nitrogen (TKN), and downer effluent of NO3-N, are meaningful for anoxic nitrogen removal. Then, a hybrid Bayesian Optimization Algorithm and Support Vector Regression (BOA-SVR) is implemented, considering only the essential variables. In this regard, real-life laboratory data are utilized to develop the models. The BOA approach and k-fold cross-validation technique are explicitly applied to optimize the model's hyperparameters and avoid overfitting. The established models provided sufficiently accurate predictions via their comparisons to the experimental observations. Besides, the BOA-SVR model outperforms the classical multiple linear regression (MLR). The BOA-SVR models' forecasted results for both the predicted amount of NH4-N and NO3-N are favorably compared with the laboratory trials (R2> 99 %). A comprehensive evaluation is further conducted to validate the performance of this model by calculating the root mean square error, mean absolute percentage error, fractional bias, and computational efficiency. Gaussian white noise is used to add three distinct noise levels (20 %, 40 %, and 60 %) to the testing data to evaluate the model's robustness. Moreover, the plots of relative deviations and residuals were dispersed across the zero-reference line with moderate fluctuations. Sensitivity analysis indicated the relative importance of the three independent variables on the anoxic nitrogen removal in the order of influent TKN > downer effluent NO3-N >flowrate. Overall, the developed tool has substantial potential in modeling other convoluted processes.

Steady-state multiscale CFD simulation of a circulating fluidized bed riser

Compared to transient simulation, steady-state simulation of circulating fluidized bed risers is more efficient, but is also harder to perform due to the complex scale-dependency of dense gas-solid flows. In this work, steady-state computational fluid dynamics (CFD) simulation of a riser is performed using the steady energy-minimization multi-scale (EMMS) drag. It is found that the steady state corresponds to an extremely large scale of length and time, thus the grid size required in steady-state simulation is larger than that in transient one. The time-averaged two-fluid model (TFM) coupled with the steady-state EMMS/1M drag model enables a good prediction of the S-shaped, axial solids distribution and the choking transition, whereas the two-phase turbulence and solids stress models are important in predicting the radially core-annular distribution of solids. So far as we know, this is the first time that one can predict the choking transition in a steady-state CFD simulation. Further improvement may need an EMMS modeling of the time-averaged solid stresses.

Experimental and numerical study on multiphase fluid dynamics of coal and blends with biomass (sugarcane bagasse) in a lab-scale full-loop circulating fluidized bed gasifier

ABSTRACT The current research presents a 3-D hydrodynamic simulation of the fluidized bed riser with a validated CFD model at different superficial gas velocities and inventory composition (coal of 2 kg is blended with sugarcane bagasse weights 0.2 kg and 0.8 kg). The experiments are performed by varying superficial gas velocity (0.5 Umf to 3 Umf), coal particle size (460 μm and 460 μm), and coal bed inventory size (1.5 kg and 2 kg) in a pilot-scale circulating fluidized bed riser of 100 mm diameter and 3000 mm height. The CFD model results are in good agreement with experimental pressure drop and solid volume fraction. The effect of drag and lift force in fluidization hydrodynamics has been studied. The validated CFD model is then extended to predict the hydrodynamics of the three-fluid, coal-sugarcane bagasse-air system. A mixing factor is defined to quantify the mixing behavior of coal and sugarcane bagasse. It is concluded that a 200 gm sugarcane bagasse blend has good mixing than the 800 gm blends. For efficient mixing, coal and sugarcane bagasse fluidized beds should operate in the 2 Umf to 4 Umf superficial gas velocity range. The results of axial and radial distribution of coal, sugarcane bagasse and air volume fraction, pressure drop, slip velocity, and fluidized bed heights are presented.

Radar-based measurement of solids back-mixing in the freeboard of a circulating fluidized bed

This work investigates solids back-mixing in the freeboard of a circulating fluidized bed based on THz-radar measurements of the concentrations and velocity distributions of the solid particles along the height of the riser. These data allow height-resolved closure of the solids mass balance and, thereby, quantification and further insight of the two main mechanisms for the back-mixing of the solids entrained from the bottom region of the fluidized bed: (i) solids disengagement and backmixing within the core region of the riser cross-section,; and (ii) solids lateral transfer of from the core region to the wall layers. The experiments were carried out in a circulating fluidized bed riser (3.1 m in height and 0.45 m2 in cross-section), which was operated with Geldart B solids fluidized with air at room temperature and for different gas velocities. The experimentally-derived data are expressed in terms of the disengagement rate and a lateral core-to-wall layer mass transfer coefficient. From the results, it is estimated that the presence of disengagement-based solids back-mixing is significant all along the 3-m riser. The disengagement rate shows a non-linear dependency on the solids concentration, with the lateral solids transfer to the walls (which follows a linear dependency on the solids concentration) eventually becoming the dominant form of back-mixing at upper heights.

Anisotropic turbulence of dense and dilute phases of particles in fluidized risers characterized using four-way coupled second-order moment method

The gas-particle flow in a fluidized bed is characterized as a “core-annulus” pattern with transient clusters moving along the circulating fluidized bed riser. These clusters exist as regions with a higher concentration of particles and are surrounded by dispersed particles at lower concentrations. In this study, fluid dynamics of both the clusters and dispersed particles suspended in the risers were investigated using the four-way coupled second-order moment method of the fluid–particle Eulerian–Eulerian two-fluid model. In the simulation, the anisotropic turbulence of either phase in the clusters and dispersed particles was studied in the risers. According to the turbulence intensities of both phases, the gas-particle flow in the fluidized riser was classified into six zones, the wall effect, cluster edge, cluster rising, cluster falling, and transient zone between the cluster rising and falling zones. As the volume fraction of the particles increased, the anisotropic ratios of the turbulence intensities in both phases increased in the dispersed zone and decreased in the cluster zone, respectively.

Quantum machine learning – A novel approach for hydrodynamics analysis and modeling of liquid-solid circulating fluidized bed risers

The Liquid-Solid Circulating Fluidized Bed (LSCFB) system is commonly employed in chemical and biochemical applications, serving as a reactor where solids act as catalysts or biofilm carriers. Mass and heat transfer heavily rely on the distribution of solids holdups, which are influenced by various operating conditions, including superficial liquid velocities and auxiliary liquid velocities. These operating conditions play a key role in controlling the hydrodynamics of solid distribution. Quantum machine learning (QML)-based modeling techniques are employed to investigate the solids holdup distribution in a Liquid-Solid Circulating Fluidized Bed (LSCFB) system. A novel application of quantum machine learning for the hydrodynamics analysis and modeling of the LSCFB riser. This work aims to lay the groundwork for investigating the applicability of QML for understanding and predicting the hydrodynamic behavior of CFB risers by leveraging the unique properties of quantum systems during the present Noisy Intermediate-Scale Quantum (NISQ) era. The unavailability of full-scale quantum infrastructure limits proper exploitation of different quantum computing opportunities including embedding. In this study glass beads particles with 500 µm used as the solid phase and water as liquid phase. At various axial locations (H = 1.0, 2.0, 3.0, and 3.8 m above the distributor) and with varying superficial liquid velocities, radial nonuniformity of solids holdup is observed. The derived models account for the impact of operating parameters, such as auxiliary and primary liquid velocities and superficial particulate velocity, on the radial phase distribution at various axial positions in the riser. By comparing the predicted solids holdup data with experimental data obtained from a pilot-scale LSCFB reactor, the accuracy of the models is determined. The models exhibit reasonable correlations with the experimental data and consistent phase distribution trends. The correlation coefficients between predicted outputs and experimental data for sequential and nonsequential QML models are 0.95 and 0.96, respectively.

Hydrodynamic characteristics of central pulse gas-liquid-solid circulating fluidized bed: Effect of gas and solid parameters

As an important multiphase reactor, gas-liquid-solid circulating fluidized bed is widely used in petroleum, chemistry, metallurgy, pharmacy and environment fields. In this work, a gas-liquid-solid circulating fluidized bed with central pulsating liquid flow was proposed and investigated to further improve the reaction rate of the reactor. The Eulerian-Eulerian- Eulerian method coupled with kinetic theory of granular flow is used to simulate the flow characteristics in the fluidized bed riser. At the same time, experiments under some operating conditions are carried out to verify the simulation results. The result shows that the relative motion of solids and liquid phase in the novel reactor is significantly increased compared with the conventional circulating fluidized bed. The continuous variation of the central pulse flow improved the pressure gradient fluctuation and mass transfer intensity. The radial/axial liquid-solid relative velocity regression formulas were obtained based on the numerical calculation results.

Solids separation efficiency at the outlet of a circulating fluidized bed riser

In a circulating fluidized bed (CFB) fuel converter the outlet geometry plays a decisive role for the solids flow in the circulating loop, thus affecting the mass and heat balances and the conversion process. A Dense Discrete Particle Model (DDPM) framework is used to investigate the solids flow, with model validation against experimental data from a cold model, scaled to represent the fluid dynamics of a commercial CFB boiler operated under hot conditions. Riser outlets with different configurations (L-shape and T-shape) are studied. The results show that the solids separation efficiency of the outlet (and, hence, of the riser) can be related to the Stokes number (Stk, which varied within 0.034–1.24). Decreased outlet area at a given riser cross-section and/or increased distance of the exit window from the top of the riser yield a higher solids separation efficiency. The smaller the Stk, the lower the solids separation in general, and the less it will be affected by the geometrical configuration. The solids flux value did not show any major impact on the solids separation efficiency attained. Finally, an expression for the solids separation efficiency of the riser outlet is derived, covering a range of efficiencies within 0.3–0.9 and yielding an average error of <20% compared to experimental data from literature.

Solid Flow Mapping at the Bottom Section of a Pilot-Plant Scale Riser with the Help of a Radioactive Particle Tracking Technique

Solid phase velocity measurements are performed at the bottom section of a pilot plant scale cold-flow circulating fluidized bed riser with Geldart group B solids using a radioactive particle tracking technique. Experiments are performed at different gas velocities and solid fluxes to evaluate the effect of gas velocity and solid flux on solid phase velocity in the bottom section of the riser. Different flow quantities such as mean velocity, root-mean-square velocities, and granular temperature are calculated and presented at different heights. Solid motion is seen to occur in the axial direction primarily. It is observed that axial velocity fluctuation increases with the height where radial solid velocity fluctuation decreases. Both gas–solid and solid–solid interactions are vital in determining the solid flow field. Gas–solid interaction is critical in the axial direction, while solid–solid interaction is important in the radial direction.

A Four-Lump Kinetic Model for Atmospheric Residue Conversion in the Fluid Catalytic Cracking Unit: Effect of the Inlet Gas Oil Temperature and Catalyst-Oil Weight Ratio on the Catalytic Reaction Behavior

The effects of the inlet gas oil temperature and catalyst-to-oil weight ratio on the catalytic cracking behaviors and the distribution of the product and the yield of the riser outlet main products as unconverted gas oil, gasoline, and coke were investigated on a real industrial FCC Riser reactor with a model LRC-99 catalyst and atmospheric residue derived from Niger’s crude oil of Agadem bloc was used as the feedstock. A four-lumped kinetic mathematical model is developed to predict the feedstock conversion and the product distribution by using the deactivation function coke-on- catalyst approach. The model was validated with a good agreement against real industrial fluidized bed riser reactor plant data from Niger and industrial riser data from literature sources. The results show that the inlet gas oil temperature and catalyst-to-oil weight ratio obviously affect the cracking reactions behavior and govern the reaction rates.

Directly irradiated fluidized bed autothermal reactor (DIFBAR): Hydrodynamics, thermal behaviour and preliminary reactive tests

The advancements of the Concentrating Solar Thermal (CST) technologies in the heat and power sector are pushing researchers to find new ways to exploit solar energy. Solar-driven thermochemical processes can open new scenarios and set a milestone towards a greener industry. This paper presents the development of a Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR), that exploits fluidization technology and the principle of an autothermal reactor to carry out solar chemical processes with high efficiency. The viability of recovering the sensible energy of the solid products to preheat the reactants is studied in an experimental prototype that couples a cavity receiver/reactor and a countercurrent double-pipe heat exchanger. The prototype is operated as a circulating fluidized bed: the inner tube of the heat exchanger is a fluidized bed riser, the receiver works as a gas–solid separator, the outer tube (annulus) of the heat exchanger is an overflow standpipe and a buffer tank (reservoir) connects the annulus to the riser, closing the loop. The reservoir can also be operated as a fluidized bed reactor like in a dual-fluidized bed system. The first experimental results are reported using a Geldart B sand as bed inventory. A hydrodynamic study has been carried out to verify proper control of the system. Solid circulation rates have been determined and match the design target of 1.4 g/s. Pressure measurements have been used to control bed levels and the flow of the sand through the standpipe. The effects of gas velocities and outlet pressure drops are reported. Internal gas flow patterns have been determined by a gas tracing technique. Undesired gas by-passing streams are very small and can be zeroed by regulating the operating conditions. High temperature experiments have been conducted with a high-flux solar simulator under inert and reactive conditions, to prove the operating principle of the reactor. Steady state temperature profiles have been analyzed to assess the performance of the heat exchanger: the heat transfer coefficient ranges between 340 and 490 W/(m2 K). The operability of a solar-driven chemical process has been proved by calcining a batch of magnesium carbonate (MgCO3) particles, added to the inventory.

Investigations on biomass gasification of compact-fast dual fluidized bed calcium looping

In this study, a 10 kWth hot rig of dual fluidized bed is developed to investigate the performance of the calcium looping process for biomass hydrogen-rich gasification and carbon dioxide capture. The system consists of a compact fluidized bed and a fast fluidized bed riser. The compact fluidized bed, which is comprised of a lower bubbling fluidized bed and an upper riser, is used as the gasifier, and the fast fluidized bed riser is used as the regenerator. The solid from the regenerator goes upward from the bottom of the upper riser gasifier, and then downward to the lower bubble fluidized bed gasifier, bringing heat from the generator to the gasifier, and is finally circulated to the regenerator form the bottom of the bubble fluidized bed, taking carbon dioxide from the gasifier to the regenerator. Dolomite and quartz sand are used as bed materials. The effects of variables such as temperature, bed material, and solid circulating flux on product gas compositions, yield, calorific value, tar content, and cold gas efficiency were investigated. The dolomite bed material has a more obvious improvement in hydrogen production as compared with quartz sand as bed material. The maximum H2 production is 0.46 Nm3/kgFuel, the purity reaches 70.88%, the cold gas efficiency is 45.93%, and the low calorific value is 13.26 MJ/Nm3. The high temperature of the upper riser favors tar conversion. When the riser temperature was 800 °C, the tar content decreased by 4.46 g/Nm3, and the total gas production increased by 0.03 Nm3/kgFuel. In addition, the increase of solid circulating flux can improve the concentration and the yield of hydrogen, and decrease the content of tar.

CPFD simulation of a pilot-scale CFB riser for sugar cracking to glycolaldehyde and other oxygenates: Coupling hydrodynamics and reaction kinetics

Cracking of sugars to glycolaldehyde and other value-added oxygenates has shown potential in lab-scale fluidized beds with high yield and selectivity towards glycolaldehyde. In this work, a homogeneous gas-phase kinetics model for sugar cracking available in literature is adopted and implemented into the Computational Particle Fluid Dynamics (CPFD) framework in order to simulate sugar cracking in a pilot-scale circulating fluidized bed riser operated at conditions relevant for continuous production of glycolaldehyde in the industry. An Eulerian-Lagrangian approach is applied to model the three-phase flow where liquid feed droplets are injected into the hot gas–solid fluidized bed. A modified gas-to-liquid ratio dependent Rosin-Rammler droplet size distribution is proposed to accurately represent the gas–liquid jet in the simulation. Simulated temperature and pressure distributions are in good agreement with measured ones. Prediction of the yield of glycolaldehyde at 60.9 wt%-C and other oxygenates using the the CPFD model corresponds closely to the results obtained from the same kinetic model implemented in an isothermal plug flow reactor model. This suggests that the CFB riser behaves essentially like a plug flow reactor, as hydrodynamic and thermal deviations from plug flow conditions do not have a considerable effect on the predicted yields. Comparing the model predictions to the measured yield of glycolaldehyde, a 50% overprediction is observed, suggesting that the hydrodynamic and cracking kinetics submodels need to be more closely coupled to more accurately predict the product yields, by means of e.g. heterogeneous cracking reactions.

Numerical study on flow dynamics in the supercritical water circulating fluidized bed riser

There are a few reports about supercritical water circulating fluidized beds (SCWCFBs), and simulations were conducted via a two-fluid model to investigate flow dynamics in a riser of SCWCFB across different flow velocities, solid circulation rates, pressures, and temperatures. The investigated characteristics include the void fraction distributions, fluid velocities, particle velocities, and drag forces. The results show that the flow characteristics in the SCWCFB riser are similar to those in the traditional gas–solid riser. In the supercritical water fluidized bed riser, the void fractions are mainly in the range of 0.85–0.95. They are low at the bottom but high at the top and low near the wall but high at the center. Particle velocities are mainly distributed between 0 and 1 m s−1. They are upward near the axis of the riser and downward near the walls, showing the annular-core flow structures. Fluid paths are tortuous at low fluid velocities but straight at high fluid velocities. Particle drag forces are mainly around 0.7–0.8 times particle weights. The effects of pressures on flow dynamics in the SCWCFB riser can be ascribed to the changes in density and viscosity of supercritical water, affecting the ability of fluid to carry particles.

Evaluation of mixing of a secondary solid phase in a circulating fluidized bed riser

Mixing of the secondary solids phase, i.e. fuel or sorbent is crucial in fluidized bed reactors due to its effect on the performance of the reactor. Fluctuations in the gas-particle flow affect mixing, thus modeling this is essential for the analysis of mixing. However, simplified mixing models are used for large circulating fluidized bed (CFB) reactors, which causes uncertainties. A novel averaging method is presented to obtain representative results of transient secondary solid phase mixing in CFBs, acknowledging the effect of fluctuations. A parametric study is performed to investigate the effect of particle size, density, and inlet velocity on mixing and for insight into the mixing process. Additionally, the presented approach is used to compare different steady-state mixing methods to evaluate their applicability for modeling mixing in CFB reactors. Initial momentum and elutriation tendency of fuel were found to affect the mixing behavior, with two distinct routes identified.