Bed Of Particles Research Articles

Segregation of binary particles in gas-solid fluidized bed

Segregation of binary particles in gas-solid fluidized bed

Experimental investigation on shape segregation of 3D printed particles using an Freeman FT4 rheometer

This study examines how granular mixtures of differently shaped particles segregate in a Freeman (FT4) rheometer apparatus. The mixtures contained two sets of particles with varying shapes and relative sizes. While our focus was on the effect of particle shape on segregation, we recognized that even slight differences in size could lead to segregation. We specifically investigated when particles of different shapes have the same effective size, exploring three cases: 1) the largest sphere within a cubic particle (inscribed sphere), 2) the smallest sphere enclosing a cubic particle (circumscribed sphere), and 3) a sphere and cube with equal volume. Our key finding indicates that binary mixtures of cubical and spherical particles can significantly segregate either axially or radially in the particle bed with a circumscribed sphere configuration. We propose that the primary mechanism for this is percolation and additionally, in the case of radial segregation, ordering of cubes adjacent to the cylinder wall.

Determination of interparticle radiative heat transfer and radiation absorption for ellipsoidal particle beds using Monte Carlo ray tracing

The calculation of radiative heat transfer between particles in particle-based solar systems presents significant challenges in terms of computational cost and accuracy. This study addresses these challenges by employing the discrete element method to generate random particle packings and the Monte Carlo ray tracing method to investigate the influence of particle shape and volume fraction (ϕ) on interparticle radiation transfer and the absorption of incident radiation. The results reveal that, at low volume fractions, the shape and orientation of ellipsoidal particles have a pronounced impact on direct radiative transfer. Oblate particles exhibit the highest average radiation distribution factor (RDF), and the dispersion of RDF data is more prominent for oblate and elongated prolate particles compared to spheres. As ϕ increases to 0.35, the contribution of indirect transfer from neighboring particles becomes significant for oblate particles, resulting in a reduction in the dispersion of RDF data. Furthermore, when ϕ reaches 0.55, the decisive factor in determining RDF becomes the shielding effect of particles positioned between the emitting and receiving particles. The average RDF tends to converge among particles of different shapes, although oblate and elongated prolate particles still exhibit considerably larger RDF dispersion compared to spherical and near-spherical particles. Additionally, the effective absorptivity of the particle bed for incident radiation decreases as the aspect ratio of particles increases at all volume fractions. Moreover, the transmitted fraction demonstrates an exponential relationship with the depth, and beds composed of spherical particles exhibit better radiation transmittance compared to beds containing non-spherical particles.

Characterization of different darken sand particles and behavior under fluidized and irradiated conditions

The use of solid particles in a fluidized bed with concentrated solar irradiation from the top is a promising technology for the next generation of concentrated solar power (CSP) plants. Sand is an inexpensive and abundant material easy to fluidize, but it has a low absorptivity, which is around 0.5 according to different previous works. This optical property is a key parameter for CSP applications with solid particles. This work presents a novel methodology to induce darkening of the sand surface by inducing solid state diffusion of Mn in SiO2, rendering a stable material resistant to abrasion upon the fluidization process. For this study, two different samples considering different MnCO3:SiO2 weight ratios (1:30 and 1:50) were analyzed. The objective is to compare the two samples and optimize the concentration of MnCO3 to get the desired darkening of the sand and high absorptivity. First, the main properties of the particles were analyzed (particle size, morphology, color and absorptivity). Second, the samples were tested in a lab-scale fluidized bed directly irradiated from the top with a beam-down 4kW Xe lamp. Both samples were tested under three different fluidized velocities: 1.5, 2.0, and 2.5 times the minimum fluidization velocity (Umf). In both cases, there is a significant increase in the maximum temperature reached during the process, with temperatures exceeding 260 °C. This is clearly higher than the case of raw sand, which reaches 230 °C under the same conditions. Furthermore, these values exceed the highest temperature reached by SiC in the same facility, which is 250 °C, and it is considered as one of best tested materials for CSP applications.

Fully coupled discrete element method for graded particles transport in pipes

Hydraulic conveying of graded particles is much more complex than that of uniform particles but is not fully understood. A fully coupled computational fluid dynamics–discrete element method model is established for the hydraulic conveying of graded particles, which integrally considers the particle–fluid and particle–particle interactions and turbulence modulation from particles. The proposed model accounts for the stochastic motion of particles by the discrete random walk method and applies the diffusion averaging algorithm to obtain particle concentration in arbitrary cells for smooth and cell-independent data fields on unfavorable cells (fluid cell size ≤ particle size). The particle–fluid drag force is applied to slurry flows through densely packed particle beds due to the consideration of porosity modification. The proposed model well performs in simulating the hydraulic conveying of dense graded particles. Dynamics in slurry mixtures of bi-disperse particles are investigated regarding different particle size compositions. The results show obvious stratification between coarse and fine particles, e.g., fine particles settling at the pipe bottom elevate the coarse particles and form a “lubrication layer” with high velocity. The torque caused by particle–particle/wall contact is greater than the torque caused by the fluid. The pipe cross section is divided into four regions according to the particle angular velocity. The effect of particle concentration on liquid motion is small because the difference in local particle concentration is relatively small, but the maximum pressure drop corresponds to a critical particle size composition.

Dry Downhill Particle Motion on Mars

We recently flew a new setup on parabolic flights for the first time to study particle motion on Martian slopes under Martian gravity. Here, we describe the initial experiments. We used dust/sand beds at varying ambient pressure of a few hundred pascals. The inclination of the particle bed was varied from 0° to 45° and parts of the surface were illuminated under varying conditions. We could observe downhill motion of material related to the insolation at the lowest light flux used of 591 ± 11 W m−2 for JSC Martian simulant. Motion occurred at significantly lower inclinations under illumination than without illumination, i.e., down to about 10° compared to about 20°–30°, respectively. We attribute this reduction in slope to thermal creep gas flow in the subsoil. This induces a Knudsen compressor, which supports grains against gravity and leads to smaller angles of repose. This is applicable to recurring slope lineae and slopes on Mars in general.

Study of energy dissipation mechanisms and hotspot formation patterns during impact process in octogen explosives with circular cavities

Considerable focus has been given to hotspot generation and ignition phenomena in impact-induced explosives. Hotspot formation in explosives is typically attributed to internal dissipation and heat transfer occurring within them. This study refines the momentum and energy equations to illustrate the entire process of particle collision, temperature-rise evolution, and hotspot formation in octogen explosive bed under impact. By octogen, we mean the substance known as cyclotetramethylene tetranitramine, which is also commonly referred to as HMX. Dense particles are considered to have pseudo-fluid properties. During the impact of the explosive, we captured the propagation of the stress wave and compared its similarities and differences with the shock wave. The collision force model incorporates a combination of Hertz–Mindlin elastic and Thornton elastoplastic contact theories. The temperature-governing equation includes sliding friction, rolling resistance, and plastic dissipations as energy sources, taking into account the heat transfer processes between particles. Temperature evolution is a spatiotemporally correlated phenomenon divided into three processes: high-temperature bands formation, cavity collapse, and particle bed dispersion, all of which lead to hotspot formation near the cavity and near the wall. Plastic dissipation is the primary source for particle temperature-rise and hotspot formation. Furthermore, the effect of cavity size, impact velocity, and particle size on temperature evolution and hotspot formation patterns is analyzed. It was found that higher impact velocities and smaller cavity sizes are associated with increased hotspot temperatures near the wall, but the hotspot temperature near the cavity does not consistently vary with impact velocity and cavity size. This is attributed to the relationship between energy dissipation rate and void collapse time.

Multi-scale investigation of heat and momentum transfer in packed-bed TES systems up to 800 K

With the rising cost of energy and the advancement of corporate social responsibility, there is a growing interest in addressing the challenge of recovering and storing high-temperature waste heat. Sensible heat storage in packed beds stands out as a cost-effective and seemingly straightforward solution for high-temperature Thermal Energy Storage (TES). Engineering models developed to design low-temperature TES systems were tentatively used to design this new generation of high-temperature systems. Delving into the physics of coupled heat and mass transfer reveals a lack of validation of this approach. This study seeks to establish a comprehensive bottom-up methodology - from the particle scale up to the system level - to provide informed and validated engineering models for the design of high-temperature TES systems. To achieve this goal, we developed a multi-scale numerical model to explore the physics of heat and momentum transfer in packed-bed TES systems. At the microscopic scale (pore/particle), we consider the flow of a compressible high-temperature gas between the particles, coupled to transient heat conduction within the particles, with particular attention given to incorporating accurate temperature-dependent viscosity for the gas phase and thermal conductivity and density for both solid and gas phases. At the macroscopic scale (engineering), we propose a high-temperature extension of state-of-the-art two-equation TES models. The governing equations considered are the volume-average conservation laws for gas-mass, gas-momentum and energy of both phases. The multi-scale strategy is applied to a randomly packed bed of spherical particles generated with the discrete element method (DEM) software LIGGGHTS. Numerical models for both scales were implemented in the Porous material Analysis Toolbox based on OpenFoam (PATO), which is made available in Open Source by NASA. Microscopic scale simulations were used to infer the effective parameters needed to inform the macroscopic model, namely, permeability, Forchheimer coefficient, effective thermal conductivities, and the heat transfer coefficient. The informed macroscopic model reproduces with excellent accuracy the average temperature fields of the physics-based microscopic model. Pore-scale analysis shows highly three-dimensional flow characterized by reverse flow and strong cross-flow in the packed bed system. Moreover, it indicates the coupling between temperature and velocity fields, where a nonuniform velocity field results in uneven temperature distributions across the fluid and the solid spheres within the packed bed, subsequently affecting the macroscopic heat transfer coefficient. The overall strategy is validated by comparison to available experimental data. This bottom-up methodology contributes to the understanding and opens new perspectives for a more precise design and monitoring of high-temperature TES systems.

Study on the anisotropic flow characteristics of wet particles in liquid-solid fluidized beds

In liquid-solid fluidized beds, the presence of a liquid film around the surface of particles influences their collision behavior, leading in turn to the restitution coefficient no longer a constant. The present study introduces a dynamic restitution coefficient for wet particles to describe its dynamic variation during the process of particles colliding. The liquid-solid two-phase flow is simulated with Eulerian-Eulerian two-fluid method incorporating the anisotropic kinetic theory of granular flow based on second-order moment (SOM) model, in view of the anisotropy of particle fluctuations. This study aims to probe into the influence of liquid film and anisotropy of particle fluctuation on the particle-particle interactions and collision mechanisms. The predicted particle velocity and solid volume fraction are verified by the Limtrakul's et al. experiments. The simulated results show that the particle velocity fluctuations exhibit significant anisotropy, and the SOM model is superior to the KTGF model in capturing inhomogeneity and anisotropy of the flow field. The existence of a liquid film enhances energy dissipation during particle collisions and diminishes the anisotropy of particle velocity fluctuations. Also, the enhancement of particle density leads to a thicker liquid film and higher restitution coefficient, while the second-order moments are weakened and the anisotropy is remarkable. Furthermore, the effect of liquid viscosity on the liquid film thickness and restitution coefficient is more significant, where the particle fluctuations are reduced and the anisotropy is intensified by enhancing the liquid viscosity. In brief, both the liquid film and the anisotropy have impact on liquid-solid two-phase flow, by comparison, the effect of anisotropy is greater than liquid film.

Demonstration of a multi-channel fluidized bed particle–supercritical carbon dioxide heat exchanger for concentrating solar applications

High-temperature thermal energy storage in oxide particles at temperatures above 600°C can couple concentrated solar energy with high-efficiency thermal power cycles to provide dispatchable solar-driven electricity. Challenges remain in developing cost-effective primary heat exchangers, which require expensive alloys, to extract the high-temperature thermal energy from the particles to power cycle fluids, such as supercritical CO2 (sCO2) in recuperated Brayton cycles. To explore one pathway for cost-effective, high-temperature particle heat exchangers, the current study demonstrates a shell-and-plate, particle–sCO2 heat exchanger with narrow-channel fluidized beds coupled with micro-channel sCO2 flows in the heat exchanger walls. This study evaluates the feasibility of multiple parallel, narrow-channel fluidized beds in shell-and-plate particle–sCO2 HXs, to achieve high bed-wall heat fluxes at elevated temperatures. A reduced-order model simulates the narrow-channel, fluidized-bed particle–sCO2 heat exchanger to design the fluidized bed geometry, in terms of depth, height, and number of channels,for a nominal 40-kWth heat exchanger at particle and sCO2 inlet temperatures up to 600°C and 400°C respectively. The resulting shell-and-plate heat exchanger design operates with bubbling fluidization of the downward-flowing oxide particles to enhance bed-wall heat transfer. The heat exchanger core is fabricated with etched sCO2 micro-channels in thin wall plates that are diffusion bonded to spacer frames to form the shell-and-plate structure with 12 parallel, fluidized bed channels, 10.4 mm deep. The heat exchanger is tested at the National Solar Thermal Test Facility at Sandia National Laboratories with CARBOBEAD HSP particles at design particle flow rates of 0.20 kg s−1 and inlet temperatures up to 525°C. Results show that fluidization across multiple parallel channel beds can maintain uniform particle inventory with a common freeboard zone above the heat exchanger core. Bubbling fluidization improves particle–wall heat transfer coefficients but also increases axial dispersion of particle thermal energy, which lowers the log-mean temperature difference such that total heat transfer remains relatively constant to within ±10% over a broad range of fluidization gas velocities. The axial dispersion required particle and sCO2 flow rates to be increased by 25% over model-designed conditions to achieve the targeted 40 kWth, which indicates the importance of incorporating axial dispersion into heat exchanger design models and of deploying bed structures to suppress it. This study demonstrates the feasibility and preferred fluidizing gas conditions for particle heat exchangers for releasing high-temperature thermal energy storage systems.

Spreading Behavior of Non-Spherical Particles with Reconstructed Shapes Using Discrete Element Method in Additive Manufacturing.

The spreading behavior of particles has a significant impact on the processing quality of additive manufacturing. Compared with spherical metal material, polymer particles are usually non-spherical in shape. However, the effects of particle shape and underlying mechanisms remain unclear. Here, the spreading process of particles with reconstructed shapes (non-spherical particles decomposed into several spherical shapes by stereo-lithography models) are simulated by integrating spherical particles with the discrete element method. The results show that more cavities form in the spreading beds of particles with reconstructed shapes than those of spheres with blade spreading. Correspondingly, particles with reconstructed shapes have lower packing densities, leading to more uniform packing patterns. Slow propagation speeds of velocity and angular velocity lead to "right-upwards" turning boundaries for particles with reconstructed shapes and "right-downwards" turning boundaries for spherical particles. Moreover, as the blade velocity increases, the packing density decreases. Our calculation results verify each other and are in good agreement with the experiment, providing more details of the behavior of non-spherical particles before additive manufacturing. The comprehensive comparison between polymer non-spherical particles and spherical particles helps develop a reasonable map for the appropriate choice of operating parameters in real processes.

Characteristics of slurry transport regimes: Insights from experiments and interface-resolved Direct Numerical Simulations

A common way to transport solids in large quantities is by using a carrier fluid to transport the solids as a concentrated solid/liquid mixture or slurry through a pipeline. Typical examples are found in dredging, mining and drilling applications. Dependent on the slurry properties and flow conditions, horizontal slurry pipe flow is either in the fixed-bed, sliding-bed or fully-suspended regime. In terms of non-dimensional numbers, the flow is fully characterized by the bulk liquid Reynolds number (Re), the Galileo number (Ga, a measure for the tendency of particles to settle under gravity), the solid bulk concentration (ϕb), the particle/fluid density ratio (ρp/ρf), the particle/pipe diameter ratio (Dp/Dpipe), and parameters related to direct particle interactions such as the Coulomb coefficient of sliding friction (μc). To further our fundamental understanding of the flow dynamics, we performed experiments and interface-resolved Direct Numerical Simulations (DNS) of slurry flow in a horizontal pipe. The experiments were performed in a transparent flow loop with Dpipe=4 cm. We measured the pressure drop along the pipeline, the spatial solid concentration distribution in the cross-flow plane through Electrical Resistance Tomography (ERT), and used a high-speed camera for flow visualization. The slurry consisted of polystyrene beads in water with Dp=2mm, ρp/ρf=1.02, Ga between 40–45 and ϕb between 0.26–0.33. The different flow regimes were studied by varying the flow rate, with Re varying from 3272 till 13830. The simulations were performed for the same flow parameters as in the experiments. Taking the experimental uncertainty into account, the results from the DNS and the experiments are in reasonably good agreement. The results for the pressure drop agree also fairly well with popular empirical models from literature. In addition, we performed a parametric DNS study in which we solely varied Re and Ga. In all flow regimes, a secondary flow of Prandtl’s second kind is present, ascribed to the presence of internal flow corners and a ridge of densely packed particles at the pipe bottom during transition towards the fully-suspended regime. In the bulk of the turbulent flow above the bed, secondary flow transport of streamwise momentum dominates over turbulent diffusion in regions where the secondary flow is strong and vice versa where it is weak. The transition between flow regimes appears to be governed by the competition between the net gravity force on the particles and shear-induced particle migration from particle–particle interactions. This competition can be expressed by the Shields number, θ. For θ≲0.75, gravity is dominant and the flow is in the fixed-bed regime. For θ≳0.75, shear-induced migration becomes progressively more important for increasing θ. Low-concentration zones flanking the sliding bed start to form at the top corners of the bed, and gradually expand downwards along the pipe wall till the pipe bottom is reached. For θ≳1.5, shear-induced migration is responsible for lifting the particle bed away from the wall, associated with the onset of the suspended regime. For θ≫1, gravity is of minor importance and the mean flow eventually reaches axi-symmetry with a high-concentration particle core at the pipe center and negligible secondary flow.

Surfaces of coastal biogenic structures: exploiting advanced digital design and fabrication strategies for the manufacturing of oyster reef and mussel bed surrogates

Coastal biogenic structures, formed by ecosystem engineering species, often feature rough surfaces characterized by intricate topographies and highly three-dimensional reliefs. Their surfaces are shaped by waves and tidal currents and reciprocally influence the ambient hydrodynamics, reflecting an equilibrium. Despite their significance, the impact of these surfaces on the ambient hydrodynamics remains underexplored due to limited knowledge of accurately replicating their complex topographies in experimental setups. The recent advent of advanced digital manufacturing presents an efficient means to manufacture highly complex, three-dimensional surrogate models for experimental modeling. This work explores the accurate replication of rough coastal biogenic structures for experimental modeling on the examples of an oyster reef and a mussel bed, utilizing a flexible design methodology and, for the first time, particle bed 3D printing with Selective Cement Activation (SCA) as a fabrication and manufacturing method. A workflow is proposed, which includes an iterative surrogate model development based on in-situ topographical features, requirements of the experimental setup, and parameters of the particle bed 3D printer with SCA. The results demonstrate the effectiveness of the methodology in achieving highly accurate surrogate surfaces of complex coastal biogenic structures by validation against a set of topographical features relevant to hydraulic roughness. Particle bed 3D printing with SCA proved to be a suitable method to manufacture complex surrogate surfaces for experimental modeling, offering advantages such as independence of production time from surface complexity. However, challenges persist in achieving exact comparability between the manufactured surrogate surface and the real coastal biogenic structures, particularly for surfaces with very high complexity. Nonetheless, the manufactured generic surrogate surfaces enable detailed investigations into the influence of complex coastal biogenic structures on the ambient hydrodynamics, thereby enhancing the understanding of the processes governing wave energy dissipation attenuation, turbulence production, and vertical mixing – critical for efficient application as a nature-based solution on coastal protection or restoration efforts.

Simulation analysis of the effect of a small amount of fine particle added to a slurry on the liquid behavior and forces acting on the main particles

The viscosity of highly concentrated slurries is reduced by adding a small amount of fine particle with the same material of main particles (high concentration ratio particles per total concentration) in the slurries. We investigated how added fine particles affect the liquid behavior and forces acting on the main particles in the slurry by using direct numerical simulations, and we examined one factor that contributes to the viscosity reduction mechanism in highly concentrated slurries containing a small amount of fine particle. The results indicated that whether fine particles are coated (they contact) on the surface or not have little influence on the change in forces acting on the main particles. In highly concentrated slurries containing a small amount of fine particle, there was a large change in the liquid velocity between and around the main particles. From these results, we proposed a possible mechanism to assist in reducing the viscosity of highly concentrated slurries containing a small amount of fine particle. The magnitude and direction of the forces acting on each main particle were non-uniform by adding fine particles, resulting in non-uniform strength within the main particle bed in the slurry, which could facilitate particle bed deformation and assist in reducing the viscosity.

Active intruder motion in a two-dimensional granular medium

We studied an active intruder traversing a two-dimensional granular medium. First, we conducted experiments to analyze the motion of the active intruder within a bed of soft particles distributed throughout a horizontal container. Our observations revealed that the intruder preferentially moved along regions characterized by higher disorder. As a consequence of its motion, and as the intruder repeatedly traversed specific regions, the structure in the vicinity of the path, but also in more distant regions, gradually became more ordered. Additionally, the motion of the intruder tended to be localized in closed paths, with this effect becoming more pronounced at higher particle concentrations. Secondly, we analyzed the behavior of the active intruder within a slightly tilted container and under vibration and shearing conditions, which served to fluidize the granular bed. Our findings indicated that the trajectory of the intruder tended to intersect regions of the granular medium with lower structural order. The overall order and fluidization degrees were found to be the primary influences on the intruder’s penetration length. We used the sixth-bond-orientational order parameter to characterize the local structural aspects of the medium and the mean squared displacement to describe the dynamics of both the intruder and the medium. We conducted an experimental analysis of the ordering of the medium and motion of the intruder using network contact forces in a similar system based on soft gelatin particles. The results of this experimental analysis are consistent with the general results, showing that media with an ordered configuration exhibit more resistance than disordered media.

Combustion characteristics of millimeter-sized aluminum particles in fluidized bed under various O2/CO2/H2O atmospheres

Aluminum is an attractive alternative fuel, but it burns very inefficiently due to the formation of a dense Al2O3 layer which prevents O2 from diffusion to the surface of Al particles. In previous experiments, the combustion of millimeter-sized Al (mAl) particles in the fluidized bed has achieved a substantial increase in the combustion efficiency, but further improvements are still needed. In this study, the effects of reaction atmosphere on the fluidized combustion of mAl particles were investigated. The experiments with different O2/H2O/CO2 concentrations were conducted. The experimental results indicate that the combustion efficiency of mAl particles in fluidized bed increases as the mole fraction of O2, H2O or CO2 increases, and the highest combustion efficiency can reach 38.7%. After the analysis of the oxide film on the surface of aluminum particles, it was found that it is easier to generate the unstable θ-Al2O3 under CO2 atmosphere, and it is easier to generate the unstable γ-Al2O3 and θ-Al2O3 under H2O atmosphere. The unstable Al2O3 film is more likely to be abraded in the fluidized bed, which leads to the effective improvement of the combustion efficiency.

Calibrating the frictional-pressure model from two-fluid simulation of fluidised beds in the defluidisation regime

The two-fluid simulation of a fluidised bed of non-adhesive particles, in which particles are represented as a continuous medium, requires a model for the pressure associated with particle contacts. While closures may be derived in the framework of the kinetic theory of granular flow for short-duration contacts, the frictional pressure associated with enduring particle contacts is generally modelled with an empirical closure involving several free parameters. This article proposes to use the experimental transition between the fluidised-bed and fixed-bed regimes in the determination of these parameters. The prediction of the minimum fluidisation velocity is shown to be governed by the frictional parameters, with either a discrete-element method or a two-fluid model. The shape of the particles, proxied by a sphericity coefficient, also influences the minimum fluidisation velocity. Such dependences of the model prediction can be used to select relevant frictional parameters, by comparison with experimental data or CFD-DEM simulation results.

Impact of flow direction on the isothermal pressure drop through a packed bed of crushed rock – A DEM-CFD approach

Pressure drop plays a crucial role in the design of a packed bed of crushed rock, necessitating a deep understanding of flow behaviour for precise determination of pumping power. This study employs ellipsoidal particles to represent crushed rock, matching the volume and aspect ratio of the sampled rocks. A discrete element model was employed to generate packed beds of ellipsoidal particles, while computational fluid dynamics simulated the flow through the voids in different directions. The model's findings were validated against experimental results from both ellipsoidal particles and crushed rocks. While the ellipsoids effectively capture general flow trends, they underestimate pressure drop through crushed rocks by 5% horizontally and overestimate it by 20% vertically. Consequently, the study proposed a correlation to predict pressure drop in a packed bed using the porous media approach, expressing inertial resistance as a tensor to capture flow direction effects.

Micromechanical origin of heat transfer to granular flow.

Heat transfer across a granular flow is comprised of two resistances in series : near the wall and within the bulk particle bed, neither of which is well understood due to the lack of experimental probes to separate their respective contribution. Here, we use a frequency modulated photothermal technique to separately quantify the thermal resistances in the near-wall and the bulk bed regions of particles in flowing states. Compared to the stationary state, the flowing leads to a higher near-wall resistance and a lower thermal conductivity of bulk beds. Coupled with discrete element method simulation, we show that the near-wall resistance can be explained by particle diffusion in granular flows.

Agglomeration behaviour of various biomass fuels under different air staging conditions in fluidised bed technology for renewable energy applications

The most common technology for thermochemical conversion of solid fuel through the process of gasification, such as coal, varieties of residues from biomass and agriculture are the bubbling fluidised bed or fluidised bed systems. However, there are some disadvantages within these systems including the formation of eutectic mixtures, increased process of melting and bed agglomeration. Hence, the current study aimed to evaluate the agglomeration behaviour of various biomass fuels. Characteristics of five biomass fuels (miscanthus, peanut, straw, domestic wood and industrial wood) in a bubbling fluidised bed (BFB) combustor were studied by SEM-EDX, XRD and XRF analysis of bed materials and ash samples. The influence of bed temperatures on the fuel ash composition and bed particles was evaluated, as well as fuel ash components, their interaction with the particles from the bed sand and morphology of the bed material. The combustion efficiency ranged from 95.5 to 99.5% with the highest value obtained for miscanthus under SA conditions. The peanut and straw had the highest K content, whereas the industrial wood ash contained the highest Ca content compared to other biomasses. The XRF results showed that most of the potassium (K) from the wheat straw biomass was converted to K2O with 19%. No bed agglomeration was noticed without staging-air (WSA) and with staging-air (SA) combustion conditions, even with temperatures up to 850 °C during the testing period. Overall obtained results demonstrated that agglomeration is unlikely expected to be a major problem during fluidised bed combustion of tested biomass fuels.