Fluidization Velocity Research Articles

Hydrodynamics in the transport zone of a large-scale circulating fluidized bed boiler

To investigate the hydrodynamics in large-scale CFB boilers, the pressure drops in the transport zone of a 550-MWe CFB boiler were measured by conducting five field tests. The circulating rates G s were calculated based on the pressure drop in the transport zone and then compared with the estimations via the correlations of the elutriation constant in single-particle elutriation theory and those of the saturation-carrying capacity in fast fluidization theory. The Colakyan & Levenspiel correlation of elutriation constant exhibited a good agreement with the experiments within ±14.02% error range. It's reasonable to use it for calculating G s in large-scale CFB boilers because of its wide experimental conditions. The flow regime remained uncertain due to the discrepancies in the saturation-carrying capacity estimated by different correlations. The effect of riser diameter on G s was discussed, including the dimension of D t , ind , beyond which the circulating rate was suggested to be independent of the riser diameter. Comparison of estimated and experimental results of P up . • Five field tests on the circulating rate G s are conducted in a 550 MWe CFB boiler. • Both correlations of K ∞ ∗ and G s ∗ are adopted to estimate the circulating rate. • Correlation of K ∞ ∗ (Colakyan & Levenspiel) shows best agreement with experiments. • Fluidization velocity is found to be higher than transition velocity. • Effect of riser diameter on estimating G s are found to be significant.

Correction to: Calculation of Minimum Particle Mixture Fluidization Velocity in a Wet Biomass Torrefaction Reactor

Correction to: Calculation of Minimum Particle Mixture Fluidization Velocity in a Wet Biomass Torrefaction Reactor

Axial segregation behaviour of a reacting biomass particle in fluidized bed reactors: experimental results and model validation

Axial segregation behaviour of a single biomass particle in a lab-scale bubbling fluidized bed has been investigated from both experimental and modelling perspectives. Experiments were conducted using beech wood particles of different sizes, ranging from 8 to 12 mm under either oxidizing or inert conditions. The fluidized bed reactor was operated at temperatures and fluidization velocity ratios, U/Umf, in the range of 500–650 °C and 1–2, respectively. A one-dimensional model has been developed to predict the axial location of the particle over time, taking into account both dynamic and thermal conversion mechanisms. X-ray imaging techniques allowed to identify endogenous bubbles released during devolatilization and carry out direct measurements of their size. This information was used to propose an expression for the lift force acting on the fuel particle. The model showed very accurate predictions and the segregation behaviour of the fuel particle appeared to be independent of the nature of the fluidizing medium.

Fluidized Bed Membrane Reactor for the Direct Dehydrogenation of Propane: Proof of Concept.

In this work, the fluidized bed membrane reactor (FBMR) technology for the direct dehydrogenation of propane (PDH) was demonstrated at a laboratory scale. Double-skinned PdAg membranes were used to selectively remove H2 during dehydrogenation tests over PtSnK/Al2O3 catalyst under fluidization. The performance of the fluidized bed membrane reactor was experimentally investigated and compared with the conventional fluidized bed reactor (FBR) by varying the superficial gas velocity over the minimum fluidization velocity under fixed operating conditions (i.e., 500 °C, 2 bar and feed composition of 30vol% C3H8-70vol% N2). The results obtained in this work confirmed the potential for improving the PDH performance using the FBMR system. An increase in the initial propane conversion of c.a. 20% was observed, going from 19.5% in the FBR to almost 25% in the FBMR. The hydrogen recovery factor displayed a decrease from 70% to values below 50%, due to the membrane coking under alkene exposure. Despites this, the hydrogen extraction from the reaction environment shifted the thermodynamic equilibrium of the dehydrogenation reaction and achieved an average increase of 43% in propylene yields.

On-line monitoring of flow process in a spout-fluid bed by combining electrical tomography and high-speed CCD camera

To investigate spouted flow characteristics in a rectangular spout-fluid bed with bafflers, a non-intrusive measurement approach, which combines electrical capacitance tomography (ECT), pressure measurements and high-speed CCD imaging technology, is proposed. The ECT sensor has 12 electrodes on the external wall and 12 electrodes inside, forming three imaging regions with 8 electrodes in each region. The flow hydrodynamic characteristics in the spouted bed e.g., pressure fluctuation, particle packing fraction and fountain form, are investigated. The frequency domain of pressure and particle packing fraction signals as well as ECT images indicate that the flow regime in the jet region transits from slugging to spouting in fountain formation process, and when the fluidization velocity in one side increases, the flow regime in the same region transits into slugging bed. The spouting pressure drop is affected by the fluidization condition in the side region, and the high characteristic frequency of pressure signal decreases when the side region condition changes from blockage to fluidization. The fountain height increases linearly with the superficial gas velocity. When the gas velocity is high, the fountain tends to be offset by the influence of the lateral airflow which is caused by the imbalance of the air pressure on two side regions.

Analytical and numerical considerations of the minimum fluidization velocity of the molybdenite particles

Analytical and numerical considerations of the minimum fluidization velocity of the molybdenite particles

Enhanced fluidization of solid particles in an oscillating acoustic field

Compared with an ordinary fluidized bed, the fluidization quality of solid particles can be effectively improved by vibration induced by appropriate acoustic fields. The effects of sound on the hydrodynamic behavior of fluidized bed have been investigated under the application of acoustic fields of different intensities (110–130 dB) and frequencies (50–500 Hz). The obtained results show that the perturbation effect of the sound field on fixed-bed pressure drop becomes more significant with increasing sound pressure level, exerting a larger pressure than present under ordinary conditions, due to the change in particle arrangement induced by the acoustic field. Except for a particular frequency, the minimum fluidization velocity in the bed decreases gradually with the increase in the ratio of bed height to bed diameter. The rising velocity of the bubble and the average overflow velocity of residual gas in collapse tests are reduced by the acoustic field.

Tannery Sludge Gasification in a Fluidized Bed for Its Energetic Valorization

The present article deals with the valorization of the organic content of tannery sludges to produce energy vectors. In this scenario, gasification is a viable option to obtain a flexible gaseous stream (syngas) of interesting energetic value, under operating conditions that do not favor the oxidation of Cr(III) (typically found in tannery sludges) to the more harmful Cr(VI) state. To this end, an industrial tannery sludge was characterized through proximate/ultimate analyses and determination of the heating value, showing its capability to act as a solid fuel in a gasification process, and metal analyses, showing that its Cr(VI) content was below the detection limit (2 ppm). The material was subjected to gasification tests in a lab-scale fluidized bed (FB) reactor. The reactor, with a 41 mm inside diameter and a 1 m height, was electrically kept at an operating temperature of 850 °C. The fluidization velocity was 0.30 m/s at 850 °C, i.e., 7.5 times the value of the minimum fluidization velocity. The gasifying stream was composed by O2 (3% vol.) diluted in N2. The adopted oxidant equivalence ratio (ER) levels were 0.15 and 0.24, to ensure substoichiometric (i.e., reducing) conditions in the FB atmosphere. Under the most reducing operating conditions, it was possible to produce syngas with a lower heating value of 12.0 MJ/N m3 (dry and N2-free basis). It contained, under these conditions, about 42% H2, 36% CO, and 4% CH4, plus 16% CO2 and other components. The tar produced from the process, fully characterized by gas chromatography–mass spectrometry, showed a favorably low concentration of about 25 g/N m3. FB bottom and fly ashes were analyzed for their carbon and metal contents. In bottom ash, the total Cr concentration resulted in the range of 8–12 g/kg, with a Cr(VI) concentration between 8 and 10 ppm. In the elutriated stream, the total Cr concentration was about 55 g/kg, with a Cr(VI) concentration between 4 and 7 ppm. The Cr(VI) concentration was higher when higher values of the ER were used, but it resulted in 3–4 orders of magnitude lower than the total Cr concentration, showing the appropriateness of the process to produce syngas with very limited oxidation of chromium in the solid residues.

Negative CO2 emission from oxy-fuel combustion in CFB boilers

Oxy-combustion by flue-gas recirculation for CO2 capture is applied to an existing, already CO2-neutral, biomass-fired circulating fluidized bed (CFB) boiler, thus resulting in negative CO2 emissions. The required oxygen concentration is determined by a heat balance, but in an existing plant the volume flow is then reduced as well as the fluidization velocity, which affects the heat transfer. Methods to resolve this problem are investigated. The oxygen is usually proposed to be supplied by air separation; a method that consumes a considerable share of the energy produced in the plant. Here, it is instead suggested to use oxygen produced together with hydrogen in electrolysis by excess wind and solar power.

Gaseous fluidization of short fibers and powders, influence of temperature and pressure

The fluidization of short Hi Nicalon S fibers was investigated before considering CVD coatings with controlled composition and thicknesses. The fluidization of a short fiber batch alone being delicate and not reproducible, the study consisted in comparing the hydrodynamic behavior of a powder alone, to that of a short fiber/powder mixture. The fluidization behavior of the particle beds was studied by following their pressure drop as a function of pressure and temperature. The minimum fluidization velocity (Umf) was evaluated at room temperature (Tamb) at different reduced pressures, and at 200 mbar as a function of temperature. The results are in good agreement with the theoretical equations. At low pressures and Tamb, Umf increases rapidly. In contrast, Umf varies only slightly with temperature. While the pressure does not seem to affect the fluidization behavior of the powder alone, it displays a clear optimum at 200 mbar for the short fiber/powder mixture.

Mixing evolution behavior of raw and gasified biomass pellets in a fluidized bed reactor

Due to a large particle size and a small specific surface, a homogeneous mixing of biomass pellets with bed materials during gasification plays a critical role in the devolatilization and carbon conversion. In this work, the mixing evolution behavior of biomass pellets at different gasification stages is investigated for the first time. Two bubbling fluidized beds are established to perform the preparation of biomass samples undergoing different gasification times and visualized mixing experiments, respectively. An image processing technique is introduced for the determination of the real-time distribution of biomass pellets. The vertical and lateral migration paths of biomass pellets at different gasification stages are revealed. The improvement of binary mixing by adjusting the operating conditions as well as the adaptability to different biomass loadings are discussed. A convolutional neural network is developed to validate the influence of fluidization velocity on the resulting flow and classify the fluidization behavior.

Computational Fluid Dynamics (CFD) modelling of the effect of distributor plate geometry in the bubbling fluidized bed reactor

Air distribution is the primary factor that influences the performance of fluidized beds. To understand the hydrodynamics and operation of a fluidized bed, an assessment of the airflow and its distribution is essential. The design and operation of the fluidized bed are complex therefore numerical models have been proposed to predict the behaviour of fluidized beds. Although fluidisation has been studied fundamentally, systematic investigation of air maldistribution due to fluidised bed reactor design and geometry has not been characterised. The effect of four distributor plates (Plate A, B, C and D with 8.67%, 6.12%,4,2% and 2.4% open area, respectively) and the two fluidised bed reactors (straight inlet and bend inlet reactors) on the fluidisation hydrodynamics have been investigated. OpenFOAM™ v1712 was used to develop a computational fluid dynamic (CFD11CFD- computational Fluid dynamics) model to predict the behaviour of a bubbling fluidised bed. The determination of these characteristics will allow for an improved CFD model and optimisation of an industrial fluidised bed granulator. The model was validated with experimental data. Plate pressure drops of 76 Pa,121 Pa,296 Pa and 886 Pa were achieved with plates A, B, C and D respectively. High bed pressure drops were observed for plate It was therefore concluded that the pitch and open area percentage of the distributor has an effect on both the plate and bed pressure drop. Particles with 1.3 mm diameter fluidised quicker with minimum fluidisation velocity range between 0.3 and 0.8 m/s. There was an agreement between the model and experiments for plate and bed pressure drop and bed expansion. Air maldistribution was observed for bend inlet reactor.

Bubble hydrodynamics of adding fine particles in 2D pulsed fluidized bed

The bubbling behavior of powders in the range of 30–2600 µm is well understood. However, the bubbling behavior in binary-medium pulsed fluidized beds is complex and has received little attention. Therefore, Geldart C and B magnetite powders were mixed to form a dense medium. The effects of C particles and frequency on the minimum fluidization gas velocity, bubble area, number, shape factor, and aspect ratio were studied. The results show that the introduction of pulsating energy can increase the bubble area but decrease the number of bubbles. Under an appropriate content of fine particles, the pulsating energy increases the bubble shape factor and reduces the aspect ratio. A bubble size growth model was developed. A pulsed fluidized bed mixed-medium minimum fluidization velocity model was developed with 10% error. A pulsed fluidized bed bubble size model was developed with a prediction error of 10% or less.

Experimental and numerical study on the development process and flow characteristics of powder fuel jet in the powder fuel scramjet

The injection process of powder fuel directly affects the scramjet operating performance. This study analyzes gas-particle two-phase flow during powder fuel injection. The macroscopic behavior of powder fuel jets under different injection pressure conditions is characterized using the high-speed shadow imaging technology. Simultaneously, the coupled model of computational fluid dynamics-discrete element method is employed to determine the experimental conditions, and the flow field structure and gas-particle interactions are analyzed. The results show that the development of powder fuel jets mainly occurs in three stages: the low-speed development stage, the high-speed development stage, and the stable stages. A long transition period exists between different stages, and the transition time decreases with increasing injection pressure. In the low-speed development stage, the powder fuel jet initially develops slowly and then develops rapidly. In the high-speed development stage, the jet initially develops rapidly and then develops slowly. The jet in the stable stage is classified into the powder particle, fluidized gas, and free flow regions. The distribution range of the particles decreases with increasing injection pressure. The fluidized gas velocity and drag force curve on the axis experience many fluctuations, and the particle velocity is almost always accelerated. Fluidized gas velocity, drag force, particle velocity, and particle acceleration distance increase with the injection pressure.

Investigation of wet particle drying process in a fluidized bed dryer by CFD simulation and experimental measurement

Wet particle drying process is complex due to the change of particle moisture, which has big effect on the gas–solid properties, flow dynamics and mass and heat transfer behavior. In this research, a two-fluid model (TFM) coupled with the mass and heat transfer was used to simulate the particle drying process in a fluidized bed dryer. Meanwhile, online electrical capacitance tomography (ECT) was used to measure the solids concentration and off-line moisture meter was employed to measure the particle moisture. Two periods, i.e., constant-rate period and falling-rate period, were captured in the experimental drying curve. The continuous drying curve was obtained by the 2D CFD simulation. The drying rate increases sharply in the pre-warming period, reaches high in the constant-rate period, and decreases markedly in the falling-rate period at the beginning and then decreases slowly. The particle temperature increases gradually until it reaches the air temperature and keeps constant. In order to capture detailed flow dynamics in the drying process, 3D CFD simulations at certain moisture contents were performed. It is found that the simulated cross-sectional particle volume fraction shows core-annular structure and is verified by the on-line ECT measurement. An internal circulation is formed along the reactor height. The mass and heat transfer mainly happens at the lower part of the dryer and at the contact region between the bubbles and the solids phase. The minimum fluidization velocity decreases, and the bubble rising velocity increases when the particle moisture level decreases.

Numerical Analysis and Experimental Verification on Flow Characteristics of a Pressurized Fluidized Bed System

Abstract The pressurized fluidized bed technology is recently shown to be a most promising technology for energy conversion and carbon capture. In the present work, two-phase flow characteristics of a laboratory-scale bubbling fluidized bed (BFB) system have been examined numerically under pressurized cold flow conditions. The BFB hydrodynamics is presented by axial pressure distributions and radial particle volume fractions. Moreover, the influence of drag models on minimum fluidization and bubble formation is studied using two different drag models, namely Wen-Yu/Ergun and EMMS–Yang, at both atmospheric and elevated pressure conditions. The numerical results are validated with a series of pressure drop and axial pressure measurements conducted at three different pressure conditions. The experimentally observed minimum fluidization velocity results are also compared with the theoretical correlations found in the literature. Good agreement has been found between experiments and numerical predictions.

Numerical simulation study on mixing characteristics of binary Geldart-D particles in a pressurized fluidized bed

Understanding the mixing behaviors of different particles is significant for improving the mass and heat transfer performance of a pressurized fluidized bed. A numerical simulation study on the mixing behavior of binary Geldart-D particles in a pressurized bubbling fluidized bed is performed based on the CFD-DEM method. The mixing characteristics and bubble behavior of binary Geldart-D particles with different physical properties under elevated pressure are investigated in the current study. Increasing the fluidization gas velocity u g and operating pressure P could promote the mixing of the particles. Raising the operating pressure P would cause a smaller bubble size and large bubble quantity at the same fluidization number u g /u mg . Higher operating pressure P and fluidization number u g /u mg could create a stable and uniform distribution of jetsam in the fluidized bed. • Mixing behaviors in the pressurized fluidized bed are investigated by CFD-DEM. • Minimum fluidization velocity decrease as the increase of operating pressure. • Bubble size decreases as raising operating pressure of fluidized bed. • High operating pressure and gas velocity promote the mixing rate of particles.

Experimental study on heat transfer and hydrodynamics of a concentric double-pipe pulsed fluidized bed using microencapsulated phase change material and sand

This paper presents an experimental study on the heat transfer and hydrodynamic of a concentric double-pipe gas-solid pulsed fluidized bed filled in outer tube with microencapsulated phase change material and inner tube with sand. The effect of flow velocity, frequency and bed height are investigated on the heat transfer. Experiments are carried out at frequency in the range of 1 to 5Hz with the bed height of 15, 18.5and 22 cm and the ratio of air velocity to minimum fluidization velocity ( U / U mf ) of 1, 1.1, 1.2, 1.3, 1.4, 1.5. Results show that heat transfer enhances as the frequency, bed height and U / U mf increase. Besides, it is found that pulsed flow has higher heat transfer enhancement than continuous flow. For example, at U / U mf of 1.5, the heat transfer coefficient of the pulsed flow increases by 7.5% than that of the continuous flow at the bed height of 22 cm. • Hydrodynamics and heat transfer in a concentric double-pipe pulsed bed are studied. • Pulsed flow in bed has higher heat transfer coefficient than continuous flow. • Heat transfer coefficient increases as U / U mf , frequency, and bed height increase. • Heat transfer becomes higher as the fluidity increases.

Recent Development in Hydrodynamic and Heat Transfer Characteristics in the Three-phase Fluidized-bed System

Gas–liquid-solid fluidized beds are broadly utilized in the petrochemical, pharmaceutical, refining, food, biotechnology, and environmental industries. Due to complex phenomena, such as the particle-particle, liquid-particle, particle-bubble interactions, complex hydrodynamics, and heat transfer of three-phase (gas-liquid-solid) fluidized beds, they are incompletely understood. The ability to accurately predict the essential characteristics of the fluidized-bed system, such as hydrodynamics, individual phase mixing, and heat transfer parameters, is necessary for its successful design and operation. This paper investigates the pressure drop, minimum fluidization velocity, phase holdup, heat-transfer coefficient of a fluidized bed reactor, heat transfer studies, CFD simulation, and the effect of these parameters on the extent of fluidization. Many variables (fluid flow rate, particle density and size, fluid inlet, and bed height) affect the fluidizing quality and performance of the fluidization process. The hydrodynamics parameters, mixing of phases, and the behavior of heat transfer with various modes of fluidization were investigated to predict hydrodynamics parameters. Several publications have demonstrated the utility of (CFD) in explaining the hydrodynamics, heat, and mass transfer of fluidized beds. Principles of measurement, details of the experimental configurations, and the applied techniques by various researchers are also presented. Feng's model was statistically validated using experimental data that was both time-averaged and time-dependent. Furthermore, this model successfully predicted the instantaneous flow structures, which should provide strategies for the best design, scale-up, and operation in fluidized bed columns. The divergence between the simulated and observed values can be reduced by better understanding the fluidized bed's nature.

Performance analysis of a 2D numerical model in estimating minimum fluidization velocity for fluidized beds

The minimum fluidization velocity of a fluid–solid particle fluidized bed is the primary focus of this paper. The computationally economic Eulerian Granular model has been used to analyze fluidization for both gas–solid particle and liquid-solid particle fluidized beds. The conventional approach of finding minimum fluidization velocity (umf) is either with a pressure drop across the particle bed or the change in bed height. However, these parameters are often unstable and cannot be used to generalize the degree of fluidization accurately. In this paper, the dominant factor of unstable pressure drop estimation in the 2D Two-Fluid Model (TFM) and a key non-dimensional Euler number has been investigated in determining minimum fluidization velocity for different quasi-2D fluidized beds for different bed sizes, particle sizes, and particle numbers. Averaging assumptions and limitations of these numerical models are discussed in detail for four different fluidized bed cases. A comparative study of the drag model shows little to no influence in unstable pressure drop estimation near fluidization velocity, and all drag models perform similarly. It is observed that particle-particle collision is not the dominant reason for unstable pressure drop near minimum fluidization. Instead, wall effects on the particle bed including frictional losses and wall-particle collision play a key role in unstable pressure drop calculation for the quasi-2D fluidized beds. Pressure drop characteristics alone do not suffice to obtain minimum fluidization velocity with 2D TFM using existing models. Thus, a different approach has been proposed to investigate minimum fluidization involving the Euler number, which has shown promising performance in determining minimum fluidization velocity and characterizing fluidization with 2D TFM. Results show consistency in Euler number characteristics for all different fluidized bed cases considered in this paper. This can revitalize computationally economic 2D Eulerian simulations, increase the range of possible applications, and provide guidance to the future development of computationally efficient and more accurate numerical models, and empirical correlations for minimum fluidization velocity.