Super-quadric CFD-DEM simulation of chip-like particles flow in a fluidized bed

Mixing and dispersion behaviours of ellipsoid particles in a bubbling fluidized bed

CFD-DEM investigation of flow and heat transfer characteristics in a directly irradiated fluidized bed

The effect of the electrostatic charge of particles was investigated in fluidized beds with conductive and nonconductive walls. For this purpose, changes in the wall surface charge density and particle contact time near the wall were examined. Computational fluid dynamics/discrete element method (CFD‐DEM) was utilized to simulate the accumulation of particle and wall‐induced electrostatic charges in a quasi‐two‐dimensional fluidized bed. Particles and walls were initially considered neutral and gained charge with time. The simulations showed that aggregated particles are formed due to the presence of the opposite electrostatic charges. Agglomerates are produced by joining the aggregated particles together. Agglomerates have a layered‐like structure and are made of positive and negative particles. Accumulation of these agglomerates and aggregated particles on the wall forms wall sheeting, which has a layered‐like structure. In other words, particles do not accumulate on the wall individually. A significant accumulation of particles near the wall was seen in the presence of electrostatic charges compared with the neutral bed. Also, the contact time of particles on the wall in the conductive case was more prolonged than in the nonconductive case. The results indicate that the CFD‐DEM approach is a helpful technique for predicting particles behaviour in the presence of the electrostatic charge.

Orientation simulation and image experiment for flexible biomass particle in wedge fluidization channel

Straw biomass particles, characterized by their complex non-spherical geometries, exhibit intricate motion and coupling mechanisms with combustion processes in gas flow fields, which remain largely unexplored. Current models, predominantly based on spherical particle assumptions, fail to accurately describe the dynamic behavior of biomass particles with high aspect ratios (Ar) and complex shapes, particularly when translational and rotational dynamics are coupled. This study focuses on the Motion characteristics of non-spherical, cylindrical straw biomass particles. By integrating visualization experiments and computational fluid dynamics-discrete element method numerical simulations, a novel non-spherical motion model applicable to particles with varying aspect ratios was developed and validated, building upon the findings from our previous Particle-resolved Direct numerical simulations research. The study systematically analyzed the translational and rotational behavior of particles with aspect ratios of 6, 10, and 15 in upward gas flows with a particular focus on the dynamic variations in drag coefficients (CD), lift coefficients (CL), and torque coefficients (CT) and their influence on particle motion. Experimental results demonstrate that the developed model effectively predicts the trajectories of particles with different aspect ratios. The prediction errors for vertical and lateral positions were both below 5%, while errors for incident angles and angular velocities remained within 8% under different aspect ratio geometries, indicating the model's high accuracy and applicability. Beyond its ability to accurately simulate the detailed motion of particles, the model also addresses the coupling of translational and rotational dynamics in non-spherical particles. In the future, this model can serve as a theoretical and optimization tool for studying the transportation and combustion of biomass particles in coal-fired boilers. Moreover, it lays a solid foundation for further research on the dynamics of complex-shaped particles and their applications in intricate flow fields and industrial environments.

Bubble dynamics and thermochemical characteristics of bubbling fluidized bed methanation

The purpose of the study is to investigate the effects of various air flow rate or superficial gas velocity in term of excess air to obtain the optimum distribution of solid phase in the Circulating Fluidized Bed (CFB). The Computational Fluid Dynamics (CFD) method used for simulations uses the Multi-fluid Eulerian approaches incorporating the Kinetic Theory of Granular to calculate the motion of particles and the standard k-ॉ turbulence model to simulate the fluid flows. The simulations were carried out using five different air mass flow rates, based on the air consumption in the combustion of coal used in real CFB boiler operation. Since this simulation is the preliminary study of the particle dynamics in CFB, the combustion process had not simulated yet. In this work, the complex flow has been resolved by applying the CFDSOF commercial CFD software. The pressure on bed, the distribution of pressure, the particle velocity and volume fraction in the CFB were investigated to obtain the optimum results of the fluidization. The values of the pressure found from experiments at the bottom of bed was around 4,500 Pa, and the results from simulations were not different significantly with this values. At the excess air of 90%, the velocity of particles at the surface of bed is highest that can lead to the erosion on the firewall in long term operations, meanwhile at the lower excess air values, their velocity were relatively small. The optimum of excess air that found to be safe for boiler to operate was around 60% - 75% for average particle size 1 mm.

The present paper reports on the design of a pilot circulating fluidized bed combustor (CFB) for the conversion of lignin-rich residue derived from a second-generation bioethanol production plant. The designed combustor aims to improve the combustion efficiency of the lignin-rich residue, therefore increasing the heat recovery to partially cover the biorefinery’s heat consumption. Among the existing technologies, the fluidized bed combustion shows several advantages, in particular the fuel flexibility and the higher combustion efficiency mainly due to the presence of an inert material that uniforms the temperature and enhance the reactants mixing. The CFB differs from the bubbling fluidized bed (BFB) for the higher velocity of the oxidizer in the reactor, resulting in a better gas-solid mixing and higher burning rate (Basu 2015). Hence, by providing enhanced fuel conversion rate is and reduced reaction time, the characteristics of this apparatus meet the requirements for a more efficient combustion of the lignin-rich residue. The CFB combustor is mainly composed by a vertical combustion chamber (riser), a solid gas-particle separator (cyclone) and a solid particles recirculation system (recirculation valve or loop seal valve). The design process followed some consequential steps. The first step involved the theoretical 1D modeling of thereactor. Considering the project constraints of maximum plant capacity (5 kg h-1) and the consequentmaximum thermal power, the geometry was chosen correlating some literature data to obtain a riser diameter of DN100 and a riser height of 3500 mm. Besides, energy and mass balances were carried out to obtain air mass flowrate data, which has an impact on the hydrodynamic study of the fluidized bed. After the geometry selection and the stoichiometric calculation, a mono-dimensional empirical model was implemented to estimate the physical behavior of the bed particles (Basu 2015), (Kunii & Levenspiel 1991). The main output of the model is the voidage fraction profile along the riser. Further important outcomes of the model are the pressure drop along the riser and the solid recirculating rate. The latter is the main input parameter for the design of the cyclone and the loop seal valve. The second step was focused on the realization of the Piping and Instrumentation Diagram selecting the measurement instruments (thermocouples, pressure transducers and flowmeters) and the ancillary equipment (blower pumps, valves and pipes). In the third step, a three- dimensional CAD model was drawn for the design of the mechanical parts including the choice of the materials and the layout. The last step involved the project of the control logic and the hardware for the data acquisition system. The results of combustion tests on the circulating fluidized bed prototype will be used to gain insights into the combustion process of the lignin-rich residue, in the perspective of an industrial scale up to increase the efficiency of the energy recovery.

Pressure fluctuations at individual grid holes of a gas-solid fluidized bed

The overall objectives of this project are to develop experimental techniques for measuring the forces of fluidized particles, and to predict the solid-gas performance in fluidized beds by using data analysis system, and by elucidating the intrinsic mechanism of erosion and attrition phenomena in fluidized beds. The reduction of erosion and attrition rates is one of the critical engineering problems for the design and operation of fluidized bed combustors. Specifically, the objectives are to: (1) develop the experimental techniques to measure the forces of solid particles prevailing in fluidized beds: (2) measure and characterize the forces of solid particles in various types of fluidized beds with various configurations (conventional and spouted fluidized beds) and with different scales (10, 20, and 30cm) under various fluidization conditions (particle size, bed aspect ratio and gas velocity); (3) find and verify the mechanism of erosion rates of in-bed tubes and attrition rates of fluidized particles by forces of solid particles in fluidized beds. We developed three different kinds of measurement methods, i.e., fracture sensitive sensor, piezoelectric sensor and gas pressure fluctuation method. By using these methods the exact forces of solid particles, including the transient corporate in fluidized beds, were systematically measured. Simultaneously, the erosion rates of in-bed tubes and attrition rates of fluidized particles were measured. 69 figs., 9 tabs.

Gas leakage between reactors in a dual fluidized bed system

Experimental study on reactor performance of gas–solids low-velocity fluidized beds

Computational fluid dynamics (CFD) combined with the discrete element method (DEM) are powerful tools for analyzing dense gas–solid flows. However, the computational cost of CFD–DEM will be unfeasibly great when simulating large-scale engineering applications with billions of particles. Accordingly, the coarse-grained (CG) CFD–DEM method is applied to solve this problem. This investigated method replaces several smaller particles with larger ones called parcels, aiming to reduce the number of particles and fully consider the collision of particles between composition parcels and the collision of particles within composition parcels. First, high-speed photography verifies the numerical simulation's reliability. Then, the CG CFD–DEM was used to analyze the transient spatial distribution, transient average velocity, pressure drop, bed height, and the mixing state of particles in a dense gas–solid fluidized bed. The CG CFD–DEM was also compared with the CFD–DEM results, which showed a good agreement with the calculation results and proved the accuracy and applicability of the method. Finally, the computation time of the CG CFD–DEM was evaluated, showing a significant decrease in computation time with an increasing coarse ratio (k). This investigation can provide theoretical reference for the numerical simulation of the CG CFD–DEM method in dense gas–solid flow.

Comprehensive investigation and comparison of TFM, DenseDPM and CFD-DEM for dense fluidized beds

A comprehensive mathematical model based on the discrete particle model and computational fluid dynamics was utilized to investigate mixing and segregation of particles in fluidized beds at high pressure. To quantify the extent of mixing in the bed, the Lacey mixing index was used. Simulations were carried out with different mass fractions of small particles at various pressures ranging from 1 to 64 bar and at various superficial gas velocities. The results showed that the bed transforms from a segregated state to a fully mixed condition when the operating pressure is increased. Vertical segregation of particles at low pressure was replaced by horizontal segregation of particles at high pressures in which small particles were collected mainly near the walls and large particles at the center of the bed. At the same pressure, the rate of segregation decreased with increasing the mass fraction of small particles.