Gas-solid Fluidization Research Articles

Comparative Analysis of Combustion Characteristics of a CFB Boiler during the Changes Process between High-Rated Loads and Low-Rated Loads

In order to alleviate problems such as large fluctuations in grid load caused by the high proportion of renewable energy, circulating fluidized bed (CFB) power plants undertake the task of rated load regulation. This study discussed the combustion characteristics of a 100 KW CFB boiler during the operation process of varying loads and analyzes the combustion characteristics, load regulation rate and emissions variation law during the operation process of high- and low-rated load intervals. The experimental results showed that under the condition of a high-rated load, the average temperature of each area in the furnace was proportional to the size of the load. Under low-rated load conditions, the temperature change increased first and then decreased with the reduction in load. In the 30% load stage, the lowest temperature in the riser was 740 °C, while the temperature in the loop seal was even as low as 650 °C. The concentrations of O2, CO and unburned carbon mainly depended on the combustion reaction intensity under each load condition, which showed a higher trend at low load (30%). In terms of NOx emission, it was proportional to the load in the high-rated load range. However, the NOx generation at the 30% load was about 30 mg/Nm3(@6%O2) higher than the 50% load. In addition, the regulation load rate (2.5%/min) between high-load conditions exhibited significantly greater than that between low-load conditions (0.78%/min). Therefore, the low-load operation will face problems such as low furnace temperature, uneven gas–solid fluidization, and difficult control of pollutant generation, which need to be paid attention to during operation.

Passivation coating on titanium powder surface via SnCl4-Ti gas–solid fluidization reaction

Titanium powder is easily oxidized in the forming and sintering process due to its active chemistry. In this study, a passivation coating is formed on Ti powder surface via SnCl4-Ti gas–solid fluidization reaction using fluidized bed. It is found that at 200–350 ℃, Ti-Sn-Cl-O passivation layer is formed on Ti powder surface on account of the reaction among SnCl4, oxide film on powder surface, and Ti matrix. Upon further increasing the temperature to above 500℃, deoxygenation reaction occurs accompanied by the formation of gaseous TiClm and TiOxCly escaping from Ti powder matrix, resulting in the decreasing of surface oxygen content. Besides, elemental Sn and Ti-Sn compounds are formed on Ti powder surface, and part of Sn is diffused into titanium matrix. The passivation behavior is mainly consisted of oxygen film destruction and oxygen removal with the fluidization temperature increasing. Overall, the suitable temperature is about 350℃ to form a passivation coating on Ti powder surface, which can achieve oxidation isolation and deoxygenation in subsequent forming and sintering process.

Homogeneous drag models in gas–solid fluidization: Big data analytics and conventional correlation

AbstractThe drag force model is vital for capturing gas–solid flow dynamics in many simulation approaches. Most of the homogeneous drag models in the literature are expressed as a function of phase fraction (ε) and particle Reynolds number (Res). In this work, we use a “big data” approach to analyze ~108 data points for drag coefficient (Fd) for Geldart Group A particles at atmospheric pressure and find that the contribution of Res on Fd is much less than ε based on the Maximal information coefficient analysis. Thus, these drag models are separately reduced to machine learning and conventional expressions only related to ε. The reduced models achieve almost the same predictive performance as the originals in bubbling, turbulent, and jet fluidizations. Moreover, the reduced models provide better numerical stability for coarse grid simulations. These findings provide new insights into the drag coefficient for Geldart Group A particles under full fluidization conditions.

Development of gas-solid fluidization: particulate and aggregative

Development of gas-solid fluidization: particulate and aggregative

Apparent viscosity of high-density fluidized bed and synergistic effect of density and apparent viscosity on particle separation

ABSTRACT For gas-solid fluidization separation, mineral particles are separated under the joint action of bed density and apparent viscosity. In present study, the apparent viscosity of high-density binary mixture media was investigated by the falling ball method. Moreover, the significant effects of bed density and apparent viscosity on simulated mineral pellets and their interactions were investigated. The apparent viscosity model of high-density gas-solid fluidized beds was modified, and it was found that the Pearson correlation coefficient is 0.86. The results show that the apparent viscosity of high-density gas-solid fluidized bed decreases with increasing fluidization gas velocity. When the fluidization number is higher than 1.3, the apparent viscosity is basically stable in the range of 3.96–5.21 Pa·s. Under low apparent viscosity, when the pellet diameter is 20 mm and the density difference ∆ρ is greater than 0.27 g/cm3, gravity is the dominant factor affecting the settling behavior of the pellet. Under high apparent viscosity, the effect of viscous resistance on the settling behavior of pellets is dominant when the ∆ρ is less than 0.27 g/cm3. Based on the performed analyses, the low-viscosity medium can be selected for the separation of large mineral particles. Furthermore, for high-viscosity medium, the influence of bubbles on particle sedimentation can be effectively alleviated and it can be selected for the separation of small particles.

Numerical simulations and validation of gas–solid flows in a fluidized‐bed roaster based on the CFD‐DPM model

AbstractIn view of the complex gas–solid flow characteristics in a fluidized‐bed roaster, the discrete phase model (DPM) provided by ANSYS software was used to numerically analyze the model using a coupled algorithm. The asymmetric flow phenomenon in the transition section at the top of the furnace was found to be unfavourable to the gas–solid flow, and an inverted U‐shaped furnace structure was proposed to optimize the transition section at the top of the furnace. A large cold experimental setup was built to verify the model. The results showed that the air velocity is mainly in the axial upward direction; under the action of the gas, the solid particles and the air velocity are basically in the same direction. The main furnace and subfurnace connection section changes the movement of the gas–solid mixture, and its unreasonable structure leads to the asymmetric flow phenomenon of the gas–solid fluid at the top of the furnace. Compared with the previous furnace structure, the uniformity of gas–solid flow in the optimized ‘inverted U‐shaped’ structure has been significantly improved. The cold experimental results are in good agreement with the numerical simulation results, which verifies the accuracy of the proposed model.

A unified CFD methodology for Gas-Solid fluidized beds based on the Quasi-Three-Fluid approach

This work aims on developing a unified numerical methodology which can be utilized for addressing the challenge of accurately describing mesoscale structures in the simulation of gas–solid fluidization systems. To achieve this goal, a dependable and unified model is established to correct the simulation results under diverse fluidization through adjusting the drag models or initial emulsion phase state, and several correction methods that are applicable to a variety of fluidizations are proposed. The developed quasi-three-fluid method can be used to accurately predict the mesoscale structures of bubbles and particle clusters in gas–solid fluidized beds with particle type of Geldart A or B. Furthermore, the unified analysis capability for gas–solid fluidized beds is improved via reasonable application of quasi-three-fluid approach.

Optimal design of gas distributor in fluidized bed for synthesis of silicone monomer

The structure of the gas distributor is closely related to the production efficiency of organosilicon monomers. To improve the production efficiency of organosilicon monomers, this study uses Eulerian-Eulerian two fluid model and proposes a design formula for the gas distributor to optimize the gas distributor. It is proposed that the pressure drop of the gas distributor, the velocity nonuniformity coefficient, the relative standard deviation of the solid holdup, and the solid particle dispersion coefficient are used to evaluate the performance of the gas distributor. The results show that the performance of the gas distributor is significantly improved when the opening ratio Φ = 0.53% is optimized to Φ = 0.18%, in which the relative standard deviation of the solid holdup is reduced by 22%, and the solid particle dispersion coefficient is reduced by 40%. On this basis, this article studies the influence of different arrangements of vent holes on gas-solid fluidization characteristics. The results show that the circular arrangement of vent holes is helpful to the mixing of gas and solid.

Simulation of Fluidization Quality for Various Reduced−Gas Composition and Agitation Speed Circumstances in a Gas-Solid Fluidized Bed with an Inclined Agitator

Blast furnace ironmaking is one of the most serious carbon dioxide emission processes. To reduce energy consumption and CO2 emissions, fluidized bed ironmaking technology with hydrogen as a reducing agent has attracted more and more attention. An inclined agitator was added to the fluidized bed reactor to address the sticking issue in the conventional fluidized bed ironmaking process. In this research, numerical simulation was used to examine the impacts of reducing gas composition and agitation speed on the gas-solid fluidization quality in the cold fluidization of iron ore powder in the fluidized bed with an inclined agitator. The results indicate that the fluidization effect of iron ore powder is better when the volume ratios of H2 to CO and H2 to N2 are 1:1. Under the intensive shear action of the agitator, the standard deviation of pressure drop constantly decreases with the increase in agitation speed, and the decreasing range is smaller and smaller. The fluidization state of the iron ore powder particles in the bed stabilized when the agitation speed reached 160 rpm.

Effect of inlet gas velocity on gas-solid fluidization characteristics in fluidized bed

Abstract In this article, the Eulerian–Eulerian TFM model is used to simulate the fluidization of the synthesis process of organosilicon monomers. A new method for analyzing the gas-solid fluidization characteristics is proposed by combining the gas-solid two-phase flow evolution formula with the parameters such as particle concentration and bed voidage. On this basis, the fluidization characteristics of silicon powder particles at constant velocity and variable velocity are compared, and the fluidization characteristics of silicon powder particles with different particle sizes under five sets of variable velocity are discussed. The simulation results show that compared with constant velocity, the mean bed voidage is 0.55 when silicon particles adopt variable velocity, which can not only keep silicon particles fully fluidized but also improve the problem of poor gas-solid contact. For silicon particles with particle diameters of 300.1–515 μm, variable velocity fluidization has the advantages of uniform bed distribution and sufficient gas-solid fluidization. In the five groups of variable velocity function, when the inlet gas velocity and time are the quadratic functions of the opening upward, the fluctuation of pressure fluctuation is small, and the maximum fluctuation range of particle solid phase distribution is only 0.13, indicating that the heat and mass transfer efficiency between silicon particles is better, the gas-solid mixing is sufficient, and the gas-solid fluidization quality is better.

Gas-solid fluidization modification of calcium carbonate for high-performance poly (butylene adipate-co-terephthalate) (PBAT) composites.

Modifying biodegradable poly (butylene adipate-co-terephthalate) (PBAT) plastic with inorganic fillers is critical for improving its overall performance, lowering the costs, and expanding its application scope. The chemical modification method for the inorganic filler determines the application performance of PBAT composites. In this work, gas-solid fluidization method was developed as a simple, efficient, and scalable strategy for chemically modifying CaCO3 filler. The modified CaCO3 filler was mixed with PBAT and melt extruded to prepare biodegradable PBAT/CaCO3 composites. The characterization results show that gas-solid fluidization method combines the traditional wet modification method's excellent modification effect with the scalability of the traditional dry modification method. The effects of modification methods and amount of CaCO3 filling on the crystallinity, mechanical, and rheological properties of PBAT/CaCO3 composites were compared. The results demonstrated that PBAT/CaCO3 composites containing 30% gas-solid fluidization modified CaCO3 could still maintain excellent overall performance. As a result, this work provides a simple, efficient, and scalable method for chemically modifying inorganic fillers and preparing biodegradable composites.

A Review of the Continuum Theory-Based Stress and Drag Models in Gas-Solid Flows

The continuum theory-based models, which include solid stress models and gas-solid drag models, are required for the modeling of gas-solid flows in the framework of the Eulerian–Eulerian method. The interactions among particles are characterized by their diverse behaviors at different flow regimes, including kinetic motion, particle–particle collision and enduring friction. It is difficult to describe the particle behaviors at various regimes by mathematical methods accurately. Therefore, it is very important to develop proper solid stress models that can capture the inherent characteristics of the flow behaviors. In addition, the gas-solid fluidization system is a typical heterogeneous system, which exhibits locally inhomogeneous structures such as bubbles or particle clusters with different shapes and sizes. Due to these inhomogeneous characteristics, the gas-solid drag model has become one of the key challenges in the simulation of gas-solid flows. Various forms of constitutive relations for solid stress models and gas-solid drag models have been reported in the literature. In this paper, we reviewed the solid stress models crossing various flow regimes and drag models in both micro- and mesoscales, which provide a useful reference for model selection in simulating gas-solid flows.

Hydrodynamic CFD-DEM model validation in a gas–solid vortex unit

Process intensification in gas–solid fluidization processes can be achieved by working in a centrifugal rather than a gravitational field. In this regard, the gas–solid vortex unit (GSVU) is an ideal candidate for heterogeneously catalyzed processes. A four-way coupled CFD-DEM model describing the hydrodynamics in the GSVU with an unprecedented level of detail is validated using 2D particle image velocimetry (PIV) experimental data on both azimuthal and radial particle velocity components. It captures high and low velocity regions, both qualitatively and quantitatively. Gas-solid slip velocities several times higher than those obtainable in a gravitational field are achieved, greatly enhancing heat and mass transfer rates. Furthermore, the gas-phase residence time distribution in the GSVU is shown to be narrow. This developed model presents a powerful tool for a better understanding and a detailed design aimed at enhancing the non-reactive and reactive process intensification capabilities of the gas–solid vortex technology.

Role of mesoscale structure in gas–solid fluidization: Comparison between continuum and discrete approaches

The coarse-grained discrete particle model (DPM) is fast growing into a powerful tool and a useful counterpart of the widely used two-fluid model (TFM) in simulation of large-scale reactors. This work aims to study the role of mesoscale modeling in both TFM and DPM approaches to understand the advantage and disadvantage of each approach for further development. Both simulation approaches with and without considering mesoscale structures in drag modeling are systematically investigated through simulations of an industrial diameter-transformed fluidized bed reactor with complex reactions. It is found that considering mesoscale drag can obviously improve the prediction in solid concentration for both approaches, and the effect of mesoscale drag for TFM modeling is more significant than for DPM approach. Besides, the DPM approach can reveal local heterogeneous structures without using mesoscale drag because it can distinguish different parcels in each fluid cell, but it overestimates the accumulation of solid particles below the distributor, as the large coarse-grain ratio may over-enhance the particle collision. For reaction, the coke content can be better predicted by both approaches with mesoscale drag, and the DPM simulation can capture more heterogeneous distribution of coke content than TFM modeling. The predicted temperature and product distribution still have obvious deviation from industrial data, suggesting a need of mesoscale heat and mass transfer modeling. The underlying mechanisms are further analyzed together with proposing future work.

Exploration on the stability conditions in bubble columns by noncooperative game theory

• Different pathways of compromise of dominant mechanisms in bubbly flow. • Formulation of different stability conditions for gas–iquid bubbly flow. • Game theory and non-inferior solution to unravel the compromising mechanisms. • Understand regime transition and structure of bubble classes via stability condition. The energy-minimization multiscale (EMMS) model, originally proposed for gas–solid fluidization, features a stability condition to close the simplified conservation equations. It was put forward to physically reflect the compromise of two dominant mechanisms, i.e. , the particle-dominated with minimal potential energy of particles, and the gas-dominated with the least resistance for gas to penetrate through the particle bed. The stability condition was then formulated as the minimization of the ratio of these two physical quantities. Analogously, the EMMS approach was later extended to the gas–liquid flow in bubble columns, termed dual-bubble-size model. It considers the compromise of two dominant mechanisms, i.e ., the liquid-dominated regime with small bubbles, and the gas-dominated regime with large bubbles. The stability condition was then formulated as the minimization of the sum of these two physical quantities. Obviously, the two stability conditions were expressed in different manner, though gas–solid and gas–liquid systems bear some analogy. In addition, both the conditions transform the original multi-objective variational problem into a single-objective problem. The mathematical formulation of stability condition remains therefore an open question. This study utilizes noncooperative game theory and noninferior solutions to directly solve the multi-objective variational problem, aiming to explore the different pathways of compromise of dominant mechanisms. The results show that only keeping the single dominant mechanism cannot capture the jump change of gas holdup, which is associated with flow regime transition. Hybrid of dominant mechanisms, noninferior solutions and noncooperative game theory can predict the flow regime transition. However, the game between the two mechanisms makes the two-bubble structure degenerate and reduce to the single-bubble structure. The game of the three mechanisms restores the two-bubble structure. The exploration on the formulation of stability conditions may help to understand the roles and interactions of different dominant mechanisms in the origin of complexity in multiphase flow systems.

On the fluidization of cohesive powders: Differences and similarities between micro‐ and nano‐sized particle gas–solid fluidization

AbstractThe fluidization of cohesive powders has been extensively researched over the years. When looking at literature on the fluidization of cohesive particles, one will often find papers concerned with only micro‐ or only nano‐sized powders. It is, however, unclear whether they should be treated differently at all. In this paper, we look at differences and similarities between cohesive powders across the size range of several nanometres to 10s of micrometres. Classification of fluidization behaviour based on particle size was found to be troublesome since cohesive powders form agglomerates and using the properties of these agglomerates introduces new problems. When looking at inter‐particle forces, it is found that van der Waals forces dominate across the entire size range that is considered. Furthermore, when looking into agglomeration and modelling thereof, it was found that there is a fundamental difference between the size ranges in the way they agglomerate. Where the transition between the types of agglomeration is located is, however, unknown. Finally, how models are made and agglomerate sizes are measured is currently insufficient to accurately predict or measure their sizes consistently.

Gas-solid hydrodynamics in a stator-rotor vortex chamber reactor

The gas–solid vortex reactor (GSVR) has enormous process intensification potential. However the huge gas consumption can be a serious disadvantage for the GSVR in some applications such as fast pyrolysis. In this work, we demonstrate a recent novel design, where a stator-rotor vortex chamber (STARVOC) is driven by the fluid’s kinetic energy, to decouple the solids bed rotation and gas. Gas-solid fluidization by using air and monosized aluminum balls was performed to investigate the hydrodynamics. A constructed fluidization flow regime map for a fixed solids loading of 100 g shows that the bed can only be fluidized for a rotation speed between 200 and 400 RPM. Below 200 RPM, particles settle down on the bottom plate and cannot form a stable bed due to inertia and friction. Above 400 RPM, the bed cannot be fluidized with superficial velocities up to 1.8 m/s (air flow rate of 90 Nm3/h). The bed thickness shows some non-uniformities, being smaller at the top of the bed than at the bottom counterpart. However by increasing the air flow rate or rotation speed the axial nonuniformity can be resolved. The bed pressure drop first increases with increasing gas flow rate and then levels off, showing similar characteristics as conventional fluidized beds. Theoretical pressure drops calculated from mathematical models such as Kao et al. model agree well with experimental measurements. Particle velocity discrepancies between the top and bottom particles reveal that the impact of gravity cannot be completely neglected. Design guidelines and possible applications for further development of STARVOC concept are proposed based on fundamental data provided in this work.

Bubble dynamics in a binary Gas-Solid fluidization system of Geldart B and Geldart D particles

In order to gain a more fundamental understanding of binary particle systems, a two-dimensional (2D) fluidized bed was used to study the bubble dynamics and dense phase composition. Digital image analysis (DIA) was employed to measure bubble diameter, bubble rise velocity, bed expansion, and the distribution of the two types of particles in the bed. Magnetite and sand particles as the single components and binary mixtures were used as the bed materials. The bubble diameter and bubble rise velocity both increased with the distance above the gas distributor and the excess gas velocity. The bubble rise velocity also increased while bubbles ascending in the bed due to an increase in bubble size and buoyancy. Both mono particle systems had higher average bubble sizes than that of the system with nearly identical particle composition, which had the smallest bubble size and the lowest bubble rise velocity. The bubble coalescence and splitting processes were also imaged and the total bubble volume was found to increase after coalescence and decrease after splitting due to some uncaptured smaller bubbles. In addition, a preliminary experiment was carried out to trace dense the phase composition and the overall bed density was estimated based on the dense phase composition and bed expansion.

Hydrodynamics of gas-solid fluidization at ultra-high temperatures

The interest in processing granular materials in fluidized beds at ultra-high temperatures has grown recently to reduce energy consumption and CO2 emissions in relation to numerous high-temperature solid-state reactions. The present research focuses on understanding the hydrodynamics of gas-solid fluidization at temperatures up to 1650 °C to aid the development of ultra-high temperature fluidized bed reactors. Experiments are conducted in a fluidized bed of 30 mm diameter at temperatures up to 1650 °C with corundum and magnesite oxide particles. The fluidization stability is examined based on bed pressure drop measurements using a non-isothermal experimental method. The results demonstrate that stable fluidization of these particles at ultra-high temperatures can be attained by using particles of large and broad size distribution and operating at sufficiently high gas velocities. This article also discusses the bed pressure drop fluctuation, minimum fluidization velocity, and pressure drop offset at ultra-high temperatures.

A scaled MP-PIC method for bubbling fluidized beds

Coarse-graining is widely used in simulations of gas-solid fluidization. However, as a key parameter, the coarse-graining ratio, and its relevant scaling law is still far from reaching a consensus. In this work, a scaling law is developed based on a similarity analysis, and then it is used to scale the multi-phase particle-in-cell (MP-PIC) method. The approach is then validated by simulating two bubbling fluidized beds. The simulation results show that scaled MP-PIC reduces the errors of solids volume fraction and velocity distributions over a wide range of coarse-graining ratios for simulated conditions. In future, a scaling law with consideration of the heterogeneity inside a parcel or numerical particle will be developed which is expected to further improve the performance of coarse-grained modeling in simulation of fluidized beds.