Fluidized Region Research Articles

Co-combustion of coal and sawdust in a three-dimensional industrial scale BFB boiler (BG-043, 67TPH): a numerical approach

ABSTRACT Experiments in large-scale CFBC boilers are expensive and time-consuming, while simulations are economical, time-saving, and provide better control over the physical process. In the current research, mathematical modeling of industrial scale CFBC boiler (BG-043, 67TPH capacity, ACC cement factory Bargarh, Odisha, India) using coal and sawdust blended fuel (90% coal + 10% sawdust) is performed by considering the four distinct (coal, sawdust, sand, and mixture gas) phases using Eulerian–Eulerian multifluid model. Heterogenous chemical reactions kinetics are implemented using user-defined functions. The numerical methodology is validated with the onsite industrial data. The comparison of pressure drop, fluidized bed height, axial velocity profiles, sand, and mixture gas temperature variations, gas compositions, and pollutant emissions (SO2 and NO) using solo coal and sawdust blended fuel are presented in the result section. Hydrodynamics study reveals the recirculation of sand particles in the fluidized bed region of the boiler. The comparison of solo coal and sawdust blended fuel combustion study reveals a 10.61% reduction in pressure drop, 12.13% increase in O2 mass fraction, 9.63% reduction in CO mass fraction, 19.64% reduction in SO2 mass fraction, and 8.67% reduction in NO mass fraction while using the 10% sawdust blends with 90% coal.

A mathematical model for wind-generated particle–fluid flow fields with an application to the helicopter cloud problem

We develop a model for the interaction of a fluid flowing above an otherwise static particle bed, with generally the particles being entrained or detrained into the fluid from the upper surface of the particle bed, and thereby forming a fully two phase fluidized cloud above the particle bed. The flow in this large-scale fluidized region is treated as a two-phase flow, whilst the key processes of entrainment and detrainment from the particle bed are treated by examining the local dynamical force balances on the particles in a thin transition layer at the interface between the fully fluidized region and the static particle bed. This detailed consideration leads to the formation of an additional macroscopic boundary condition at this interface, which closes the two-phase flow problem in the bulk fluidized region above. We then introduce an elementary model of the well-known helicopter brownout problem, and use the theory developed in the first part of the paper to fully analyse this model, both analytically and numerically.

Reactive particle-laden flow in industrial-scale supercritical water gasification reactor: Modeling and study on flow patterns

Coal gasification in supercritical water (SCW) is a highly complicated reactive particle-laden flow process, which couples multiphase flow hydrodynamics, conjugate heat transfers, and complex chemical reactions. A quantitative understanding is the key issue for reactor design and optimization. In this study, the Eulerian-Eulerian model is used to describe the particles-SCW flow in a novel industrial-scale thermally integrated supercritical water gasification (TISCWG) reactor with conjugate heat transfer boundary conditions and porous media model. The sophisticated chemical reactions are using a species transport model with a seven-step lumped kinetics. Four zones of the TISCWG reactor and four typical flow patterns in the gasification chamber are firstly revealed as 1) the bottom accumulation region with the nonhomogeneous solids distribution and back-mixing of particles, 2) the upper developed fluidization region with the SCW and coal particles moving in a plug-flow regime in the center and a downward annular gravity-driven flow near the wall, 3) the jet-affected region strongly affected by the vortexes near the nozzle and presented as a trans-critical jet regime and 4) the dilute region where dilute solids slow down and fall back into the gasification zone. Flow in the gasification chamber is strongly affected by the heat transfer with the preheat zone, as reflected by an annular gravity-driven flow film flowing downwards near the gasification wall. A funnel-shaped structure promotes a more uniform distribution of solids in the accumulation region rather than a cylindrical structure. The effect of flow patterns on coal gasification processes is also studied. Volatile pyrohydrolysis reactions depend on the time-varying solids distribution and the gaseous products at the system outlet with a typical H2 mole fraction of 61.42% and CO2 of 34.98%.

Torque curve bifurcation in T-shaped rotor continuously drilling into granular media

This study explores how a continuous drill of T-shaped rotor interacts with granular media and illustrates an universal torque bifurcation mechanism. Experiments shew that the drilling torque linear increases with depth initially, followed by bifurcation. The bifurcation types are intricately linked to the motion parameters of the rotor, while the depth of bifurcation and saturated torque are directly influenced by rotor geometry. DEM simulations link torque bifurcation to a low-stress fluidized region around the rotor, while the region is enclosed by a fluid–solid interface and influenced by the motion parameters. Moreover, force chain analysis demonstrates the irreversible state change of granular media disturbed by the rotor. Leveraging these insights, we propose two scaling laws quantifying the bifurcation point depth and the saturated torque, then perform qualitative analysis to explain the experimental phenomena. These findings may hold significant promise for practical engineering applications.

Study of air-steam gasification of spherocylindrical biomass particles in a fluidized bed by using CFD-DEM coupling with the multi-sphere model

In this work, a reactive CFD-DEM coupling with the multi-sphere model has been established and used to investigate the air-steam gasification of spherocylindrical biomass particles in a fluidized bed. Specifically, biomass particles are assumed to have a spherocylindrical shape while small inert bed materials (i.e., sand particles) have a spherical shape. Both kinds of particles are characterized by the multi-sphere model and each particle is individually tracked. Moreover, particle-particle collisions, heat convection, heat conduction, radiation, and homogeneous and heterogenous reactions during biomass gasification are all considered. After model validation, the effect of biomass particle aspect ratio on five different indicators (e.g., fluidization behavior, gas production, particle thermochemical conversion, particle height, and particle orientation) are systematically explored. Results show that the gas porosity near the fuel injection location is high due to the gas release of the injected biomass. Enlarging the particle aspect ratio can accelerate biomass conversion processes. In addition, more particles prefer to show a nearly vertical orientation in the fluidization region, and this orientation behavior can be enhanced by increasing the particle aspect ratio.

Influence of structural parameters on fluidization effect of typical fluidization unit of metal powder fuel

The metal powder fuel supply system is a critical technology for achieving multiple starts of a solid rocket engine. The supply system has a direct impact on the performance of the engine, which is one of the main problems to be solved. Therefore, the Eulerian continuous medium method is used to research the influence of structural parameters on fluidization effect of typical fluidization unit of metal powder fuel. The influence of the angle between the inlet of fluidization gas and the powder storage tank and the outlet diameter of the fluidization unit on the response characteristics and stability of the powder supply is investigated. The distributions of metal powder in the storage tank, fluidization region and outlet pipe are analyzed. From the results, it is concluded that the fluidization unit with an angle of 40° to 45° takes less time to reach a steady supply state. The outlet mass flow rate oscillates after the outlet diameter is 7 mm, and the oscillation frequency is 12 Hz. When the average mass flux of the powder at the outlet of the powder fluidization unit is 10610.32 kg/m2·s, a suitable tank pressure drop can sustain the powder fluidization unit.

Simulation of FBR-Fenton/GAC process for recalcitrant industrial wastewater treatment with a computational fluid dynamics-kinetic model framework

An integrated computational fluid dynamics (CFD)-kinetic model framework was developed to numerically describe the hydrodynamic and kinetic phenomena in a liquid-solid two phases Fluidized-bed reactor Fenton/granular activated carbon (FBR-Fenton/GAC) system. The model obtained excellent accuracy for predicting chemical oxygen demand (COD) removal in reverse osmosis concentrate (ROC) treatment under different operation conditions. Hydrodynamic evaluation demonstrated that under the quasi-steady state, the GAC particles were uniformly circulated in the bed region with two pairs of counter-rotating recirculation cells, and a clear interface layer formed between the solid and the liquid phases. Superficial liquid velocity highly affected the fluidized bed expansion and solid volume fraction, while its impact on the overall COD removal efficiency was negligible. Chemical evaluation revealed that GAC/H2O2 catalytic reaction enhanced the •OH production in FBR-Fenton/GAC process by 2.7 folds as compared to homogenous Fenton process. Fenton reaction mainly occurred in the upper liquid region and its kinetics for •OH generation significantly diminished by 75% within the first 10 min. GAC/H2O2 reaction took place in the fluidized bed region for continuous •OH generation with a relatively stable rate from 1.21 × 10−6 to 0.60 × 10−6 M/s. Along the ROC treatment with FBR-Fenton/GAC process, the simulated COD degradation rate decreased along the reaction time with 2.05 × 10−6 M/s and 2.93 × 10−7 M/s at 2 min and 60 min, respectively. Faster COD removal was attained in the fluidized bed region due to combining effects of •OH oxidation and GAC adsorption. The overall predicted COD concentration reduced from 122 to 35 mg/L, •OH oxidation and GAC adsorption contributed 59% and 41%, respectively, to the total COD removal.

Flow Hydrodynamics in Pyridine Synthesis Coupled Reactor Feed Injection Zones

The flow hydrodynamics in the pyridine synthesis coupled reactor feed injection zones was investigated by cold mode experiments. The results showed that the feed injection zones can be subdivided into three regions, which are the dense bed region, feed injection region, and fast fluidized bed region. A dense bed is formed in the bottom of the feed injection zones, which is to be minimized as to avoid over-reaction. To determine the height of the dense bed, the catalyst concentration was used to determine the boundary. The results showed that the height of the dense bed is affected by operating conditions like superficial gas velocity, catalyst circulation rates, and the axial positions of the nozzles. The height of the dense bed increased with the increase in the catalyst circulation rate, the decrease in the feedstock to primary gas ratio, and the increase in the axial position of the nozzles. Based on experimental results and jet trajectory function proposed in previous reports, an empirical model is proposed to model the height of the dense bed. The prediction results agreed well with experimental results with an average deviation of 3.9%. Based on prediction results, the optimal operating conditions are suggested as to minimize the height of the dense bed.

CFD-DEM simulation of air-blown gasification of biomass in a bubbling fluidized bed gasifier: Effects of equivalence ratio and fluidization number

In this study, air-blown gasification of biomass in a bubbling fluidized bed (BFB) gasifier was numerically investigated. A discrete element model coupled with computational fluid dynamics (CFD-DEM), in which gas is considered as continuum and particle is considered as discrete phase, was employed. The flow characteristics of the BFB were discussed and compared with the reported experimental data. Then, the gasification performance of the BFB was discussed for cases with varying fluidization number (FN) values at a constant equivalence ratio (ER). In a parametric study, calculations were performed for various FN values, and the ER value was fixed as 0.27. The formation of CO2 commenced inside the fluidized bed region for FN values exceeding 3.6. When the FN value increased, slight decreases were observed in the CO and H2 concentrations as well as CCE. Thus, an increase in FN beyond a certain value was not recommended. Another parametric study was performed for various combinations of FN & ER values. The CGE increased for FN & ER values between 2.49 & 0.21 and 3.56 & 0.3, whereas it decreased for FN & ER values exceeding 3.56 & 0.3. Therefore, the optimum FN & ER values were considered as 3.56 & 0.3.

Study on the influence of different carrier gases on the fluidization properties of glass bead and FCC powders

The powder flow was influenced by carrier gas in the unit operation, such as pneumatic conveying and hopper discharge, in the pressurized entrained flow gasification technology. Carrier gases play a vital role in fluidization/ defluidization of powders. In this paper, we investigate the fluidization/ defluidization of absorbent Fluid Catalytic Cracking (FCC) and non-absorbent glass bead with two different carrier gases, N2 and CO2. The flowability of aerated powders with different carrier gases were obtained in the fixed bed region whose superficial gas velocity was lower than minimum fluidization gas velocity and fluidization region. The powder flowability was characterized by basic flow energy, flowability rate index and aeration energy with the help of FT4 powder Rheometer. The influence of different carrier gases on fluidization/ defluidization of powder bed was further discussed. Transition line of A/C powders with different carrier gases was revealed. Effect of gas type and interparticle force were calculated, compared with the flowability of aerated powder bed. In addition, combined with bed stress analysis, different bed fluidization mechanism with different gas velocities were revealed. Our findings might help people understand the influence of carrier gases on the fluidization/ defluidization of powders in the relative chemical unit operations.

Using water injection to remove pile base resistance during installation

Water injection is currently used to reduce pile installation resistance in a variety of soil conditions. However, the mechanisms governing load reduction remain unclear, and there are uncertainties over the effect of water injection on the long-term behaviour of the installed pile. This paper offers some insights based on centrifuge model tests, with the aim of clarifying the installation mechanism. A novel approach to scaling was required, and the particle size was deemed to scale with enhanced gravity. A three-stage variable permeability mechanism is proposed and validated against a series of instrumented pile installations. Various pile installations were completed using different flow rates and injection geometries to investigate the differing injection effects. Base resistance was eliminated by forming a fluidized region around the pile toe, extending to two pile radii. However, fluidization could not be maintained beyond a critical depth of installation, and base resistance increased thereafter.

Measurement of particle concentration in a Wurster fluidized bed by electrical capacitance tomography sensors

It is essential to measure and monitor the particle flow characteristics in a Wurster fluidized bed to understand and optimize the coating processes. In this article, two electrical capacitance tomography (ECT) sensors are used to measure the particle concentration in different regions in a Wurster fluidized bed for the “cold” particle flows. One ECT sensor has a 12‐4 internal‐external electrodes and another has eight electrodes. The 12‐4‐electrode ECT sensor is used to measure the particle concentration in the annular fluidization region (outside of the Wurster tube) and the eight‐electrode ECT sensor is used to measure the particle flow in the central region (inside the Wurster tube). The effect of particle type, particle moisture, fluidization velocity, and geometrical parameters on the Wurster fluidization process is studied based on the two ECT measurements. The radial particle concentration profiles in the annular fluidization and central flow regions with different operation parameters are given. Fast Fourier Transform analysis of the particle concentration in the Wurster tube is performed with different superficial air velocities. The optimum operating ranges of the Wurster fluidization process for different particles are given. In the end of the article, computational fluids dynamics simulation results are given and used to compare with the measurement results by ECT for a typical Wurster fluidized bed. © 2014 American Institute of Chemical Engineers AIChE J 60: 4051–4064, 2014

Study of Hydrodynamics and Dry Beneficiation Characteristics of Coal in the Riser of a Circulating Fluidized Bed Reactor

Dry beneficiation is an effective way of reducing the ash content of coal especially in the water scarce region. Fast fluidization region was used to carry out some exploratory investigations with raw coal to reduce the ash below a 12% level from a feed ash of 43%. As the coal was fluidized in the riser of a fast fluidized bed, the heavier and coarse particles generally settled at the bottom in a dense zone of the riser while the finer and lighter particles were entrained to the top dilute zone and, using a cyclone, were fed back to the bottom of the circulating fluidized bed (CFB) riser column. Most of the low-ash fine-size coals being lighter moved up to the riser, where most of the beneficiation occurred.

Segregation of particles in a tapered fluidized bed

Abstract Tapered fluidized beds are widely used in industrial operations to fluidize a wide range of particle sizes, and are thought to induce relatively strong particle mixing. Like their straight-sided counterparts, tapered fluidized beds are often considered as a homogeneous emulsion phase through which bubbles propagate. However, it has been shown that gas flow through the dense-phase of tapered beds is heterogeneous, generating a central fluidized core and unfluidized peripheral regions. When polydisperse particle mixtures are fluidized in a tapered bed, the structure becomes much more complex once the particles reach the minimum fluidization velocity (Umf), and a variety of segregation structures are generated. The aim of this study is to investigate how the two different types of structure (i.e. flow structure owing to the tapered shape of the bed and the segregation structures) interact and affect each other. Experiments were performed in a tapered (θ = 15°) planar bed using bidisperse mixtures of ballotini. The growth and extent of flow and segregation structures were measured, as well as the fabrics observed under different conditions. Under most conditions, the structure of the tapered bed is unaffected by the bed composition and segregation structures that form. An exception is at flow-rates just in excess of Umf when vertical columns of particles form, completely displacing larger-scale flow structures. The time scale for particle turnover in the central fluidized region is much shorter than that of particles captured within the peripheral regions. However, at sufficiently high gas flow-rates, uniform mixing can take place across the entire width of the bed.

Flow-induced waterway in a heterogeneous granular material

In a heterogeneous granular material, viscous flow concentrates to regions with lower particle number density, or higher permeability region, denoted here by “macroscopic cavity”. This in turn enhances the normal stress toward the fluid region on the upstream boundary, which destroys the boundary if the local stress exceeds a certain magnitude. The latter may further enhance the concentration of flow into the cavity region, which is repeated to form a large scale fluidized region toward upstream direction. These processes have been elucidated in our previous experiment using glass beads layer confined between two parallel plane walls. In this paper, a numerical simulation taking into account of the global flow field on the basis of the generalized Darcyʼs equation as well as the local stick-slip equation on the basis of the Newtonʼs equation of motion is performed. Our numerical simulation can successfully reproduce our experimental findings by imposing a suitable pressure gradient and frictional coefficient. The present numerical method can be applied to more general distribution of cavities, including three-dimensional ones, which may predict the formation of long underground waterways or creation of the passage of blood flows (angiogenesis).

Fluidization, Mixing and Segregation of a Biomass-Sand Mixture in a Fluidized Bed

Fluidization, mixing and segregation of a biomass-sand mixture in a 3D gas-fluidized bed have been investigated by means of visual observation, pressure fluctuation analysis and the bed-frozen method. Three types of mixtures are considered, in which biomass is a thin long stalk, and sand belongs to the Geldart B category. Experiments are carried out in a segmented fluidized bed equipped with multiple pressure transducers. Three initial packing conditions and two experiment procedures are used. The fluidization velocity varies to cover a wide range. Results show that in the local fluidization region, the mixing and segregation patterns are sensitive to the initial packing condition. In the case of a fully segregated state with biomass at the bottom, the bed inversion can be significantly observed due to the great segregation tendency of biomass. Further analyses indicate that the mixing ratio exerts a subtle influence on the competition between mixing and segregation by disturbing the coalescence and break-up of the bubble. In addition, the pressure fluctuation signal proves to be helpful in understanding the dynamic features of the phenomenology.

Batch separation of shredded bulky waste by gas–solid fluidized bed at laboratory scale

A gas–solid fluidized bed separator using various bed materials was used to separate shredded municipal bulky waste (SBW). Using 290 μm glass beads as the bed material, the apparent density of the fluidized bed was 1.5 g/cm 3 and the SBW could be separated into combustibles such as wood, paper and plastics and incombustibles such as metals and glass. The overall efficiency (Newton’s efficiency) of the separation was calculated to be 0.93. In order to obtain high efficiency, the superficial velocity must be adjusted so that the fluidized bed is agitated moderately and at the same time there is no weak fluidized region. Using a mixture of particles of nylon shot and 68 μm glass beads, the apparent density of the fluidized mixture bed could be varied between 0.63 and 0.99 g/cm 3 by changing the mixing ratio of the two materials. In the case of a mixing ratio of 20% for glass beads, an apparent density of 0.65 g/cm 3 was produced, in which wood and paper components were recovered while plastics remained in the bed to give a final overall efficiency of 0.88.

Minimum Bubbling Velocity and Homogeneous Fluidization Region under Reduced Pressure for Group-A Powders

The effects of the reduced pressure on the minimum bubbling velocity and on the homogeneous fluidization region were investigated for three kinds of glass bead powders that belonged to group-A in Geldart’s classification. The minimum bubbling velocity as well as the minimum fluidization velocity increased with a decrease in the pressure. It was found that the Reynolds number at the minimum bubbling velocity was related to the Knudsen number. The effects of the particle diameter and reduced pressure on the homogeneous fluidization region were experimentally examined. The homogeneous fluidization region became larger as the particle diameter decreased, and it became smaller as the pressure decreased.

Bubbling and Bed Expansion Behavior Under Vibration in a Gas-Solid Fluidized Bed

The effect of vibration on the flow patterns and fluidization characteristics including the minimum fluidization velocity (u mf ), the void fraction (e mf ) at u mf and the bed expansion ratio were examined. The powders used were spherical glass beads and their diameters were 6, 20, 30, 60 and 100 μm. For group A powders, the manner in which the vibration affects the bubble formation was examined from the bed expansion ratio and the index of n/4.65. The area of the homogeneous fluidization region was also observed. The homogeneous fluidization region was broadened at a certain vibration strength, where the value of n/4.65 was a minimum. The bubble formation was observed even for 20 μm powder (group C), at large vibration strengths and at high gas velocities. Under such conditions, the bed expansion ratio increased suddenly due to bubble formation. The bubbles broke the irregular bed structure, including various properties of agglomerates. Although the channel breakage was dominant flow pattern for group C powders, the bubbles also played an important role in the improvement of the fluidization.

Characteristics of Heat Transfer around a Horizontal Fine Wire in a Circulating Fluidized Bed.

Experimental study on the heat transfer mechanism from a horizontal fine platinum wire to particles was carried out in a riser of a circulating fluidized bed(CFB). As an experimental apparatus, the riser duct consisted of 33.7mm squared cross sectional area and 1620mm in a height. The test section was installed at the suitable height (0.9 m) from a distributor. Both the particle diameters (dp) and the wire diameters (dw) were 100, 200 and 400 μm. As the results, it was found the significant relationship between the particle behavior and the characteristics of heat transfer around a horizontal fine wire in CFB was found. It was found out that the heat transfer coefficient for the case of dp<dw was larger than that for the case of dp>dw in the turbulent fluidization region. It was also found that there were some relationships between the gas velocity and the characteristics of the heat transfer around a horizontal fine wire in the dilute fluidization region.