CFD-DEM simulation of nanoparticle agglomerates fluidization with a micro- jet

Nanoparticles can be fluidized as agglomerates, but for some materials this is cumbersome due to the cohesive nature. Micro-jets are shown to be effective for improving the fluidization in such cases (1). In this study, the mechanisms of micro-jet assistance are investigated by using an adhesive CFD-DEM (Computational Fluid Dynamics – Discrete Element Modelling) model. In previous studies, the complex agglomerates found in a fluidized bed are treated as the discrete elements (2). Here we use the simple agglomerates as the discrete elements, which are the building blocks of the larger complex agglomerates. The collision of the simple agglomerates are modeled by including collision mechanisms of elastic-plastic, cohesive and viscoelastic forces. Particles with =40 and =250 are used to represent the simple agglomerates. The cohesive force is expressed by the non-dimensional parameter , definded by the ratio of der Waals force over the particle gravity. A fluidized bed with dimension of 3 mm × 0.4 mm × 12 mm containing ~120,000 particles is simulated. At different cases, a micro-jet with horizontal cross-section size of 20 x 20 pointing downwards is turned ON or OFF (36 m/s) while the gas velocity to the bed is set as 2.8 cm/s or 4 cm/s, respectively. The schematic of the microjet in the bed is shown in Figure 1. In this way, like in our previous study, we keep the total amount of gas provided to the bed equal (2). Please click Additional Files below to see the full abstract

Bubble Behaviors of Large Cohesive Particles in a 2D Fluidized Bed

Fluidization hydrodynamics is greatly influenced by interparticle cohesive forces. In this paper, we study the bubbling behaviors of cohesive Geldart B particles in a 2D fluidized bed, using the “polymer coating” approach to introduce cohesive force. The effect of cohesive force on bubbles can be differentiated into two regimes: (i) by increasing the cohesive force within a low level, the bubble number increases, while the bubble fraction and bubble diameter decrease; (ii) when the force is large enough to cause the particles to adhere to the side walls of the bed, the bubble numbers and the bed expansion sharply decrease. With the increasing cohesive force, the bubble shape changes from roughly circular shape, to oblong shape, leading to the “short pass” of fluidizing gas through the bed. Finally, we analyzed the switching frequency and standard deviation of local pixel values to characterize the bubble dynamics

Investigation of CO₂ Capture in Three-Dimensional Full-Loop Integrated Bubbling-Transport Bed Adsorber

Carbon capture using solid sorbents in fluidized bed reactors has attracted increasing attention. However, optimized design and operation require excellent control over the gas–solid contact and sorbent circulation between the adsorber and desorber reactors. In this study, the hydrodynamics and CO2 capture process using potassium-based solid sorbents in a full-loop integrated system consisting of a bubbling-transport bed adsorber and a bubbling bed desorber are investigated by using a three-dimensional two-fluid model (TFM), in which the operating parameters can be realistically adjusted. Results show that the sorbent circulation rate increases with the gas velocity in the central pipe and with the static bed height in the bubbling section of the adsorber. The pressure distribution, sorbent concentration, and distributions of the gas and solid velocities are explored, and optimized gas velocities in the bubbling section and the central pipe are recommended. The CO2 capture efficiency increases as the water vapor concentration in the inlet flue gas is increased from 8 to 18%, and the optimal water vapor concentration is identified as 14%. The results of this study are useful for the design and optimization of CO2 capture reactors with solid sorbents

Devolatilization of a single fuel particle in a fluidized bed under oxy-combustion conditions. Part B: Modeling and comparison with measurements

A detailed one-dimensional transient model is developed to describe the conversion of a single fuel particle in O2/N2 and O2/CO2 atmospheres in a fluidized bed (FB). The model takes into account the main relevant phenomena occurring from the addition of a particle to the FB up to the instant when most of the volatiles have been released. The model accounts for the rates of drying, fuel devolatilization, homogeneous combustion of volatiles in a thin flame, heterogeneous combustion of char, and mass and heat transfer, the latter involving the heat transfer from the FB reactor and flame to the particle. The model is used to simulate and explain the experiments given in Part A of the present work, which includes tests with four ranks of coal (from anthracite to lignite) and one type of wood in O2/N2 and O2/CO2 atmospheres with the O2 volume concentration varying in the range of 0–40% at a fixed bed temperature of 1088 K. The predicted history of the temperature of the fuel particle and of the volatiles flame agrees well with the measurements. The simulated results indicate that the heat transfer processes at the particle scale are similar in pure N2 and CO2. The model reveals that only a small amount of heat from the flame is transferred to the fuel particle, explaining why the rate of particle heating is hardly affected by the flame. The decrease in the devolatilization time measured at higher O2 concentration is explained by heterogeneous (char) combustion, which is seen to be significant during the last stages of devolatilization. The model shows that the char combustion is limited by the rate of diffusion of O2 to the particle and justifies the lower heating rate observed in O2/CO2 compared to in O2/N2. A sensitivity analysis shows that the thermal capacity and conductivity of the fuel, as well as the convective heat transfer coefficient, are the most influencing parameters affecting the time of devolatilization

Oxy-fuel conversion of sub-bituminous coal particles in fluidized bed and pulverized combustors

Oxy-fuel combustion in pulverized coal (PC) and fluidized bed (FB) boilers is being increasingly investigated due to its potential use for carbon dioxide capture. The combustion conditions in the two types of unit differ significantly because of fuel size, furnace temperature, and fluid dynamics. These differences affect the change of combustion characteristics from air (O2/N2)to oxy-fuel (O2/CO2) conditions in PC and FB in different ways. In this paper, the oxy-fuel combustion behavior of a single sub-bituminous coal particle in PC is compared with that in FB conditions. The FB data were measured in our bench-scale FB test rig, whereas the PC data were collected from literature. The FB tests were performed at 1088 K with 0%vol<O2<40%vol,using sub-bituminous coal with a diameter of 6 mm. An extensive list of parameters is compared, including the ignition-delay time, volatiles’ flame temperature, devolatilization time, burnout time and peak temperature of the coal particle. Results indicate that the impact of shifting from air-firing to oxy-firing affects the devolatilization, burnout times, and peak temperature in PC and FB differently: PC particles require higher concentration of oxygen than coarse particles for FB to attain similar results as in air-firing. However, the impact on the volatile flame temperature of shifting to oxy-fuel flame is similar in PC and FB. In general, the CO2 atmosphere delays ignition compared to air-firing, particularly for coarse particles at low O2 concentration

Oxy-fuel combustion of a single fuel particle in a fluidized bed: Char combustion characteristics, an experimental study

The effect of an oxy-fuel atmosphere on char conversion in a fluidized bed (FB) was examined by comparing measurements of single fuel particles exposed to O2/N2 and O2/CO2 atmospheres. The experiments were carried out in a transparent and electrically heated FB at 1088 K and five O2 inlet concentrations (ranging from 0 up to 40%vol) using four ranks of coal (from anthracite to lignite) and one type of wood, all with 6 mm (spherical) diameter. The evolution of temperature with time of the various fuels at different gas atmospheres and the microstructure of the char resulting after devolatilization are presented, from which the effect of the oxy-fuel atmosphere (changing CO2 by N2 at different fixed O2 concentrations) on char combustion characteristics is determined and analyzed. Results show that the apparent average combustion rate of a fuel particle decreases (and consequently the burnout time increases) when changing O2/N2 by O2/CO2. This effect was more significant at high O2 concentrations and most notable for anthracite. Consistently, the time to reach the peak temperature was longer when shifting from O2/N2 to O2/CO2. However, the char specific surface area, pore volume, and average pore diameter of char were not significantly modified by replacing N2 with CO2

Elutriation and agglomerate size distribution in a silica nanoparticle vibro-fluidized bed

Fluidization of nanoparticle agglomerates is a promising technique to process nanoparticles. However, possible elutriation of small agglomerates may cause significant loss of bed material. To obtain the elutriation behavior under stable operation, in this study the elutriation fraction of silica nanoparticle agglomerates is measured in a vibro-fluidized bed, which is operated for several hours. Among conditions with different fluidizing gas velocities, Ug, and vibration strength, Λ, the lowest elutriation fraction measured is around 5% after 7-hour fluidization. The elutriation fraction increases significantly with Ug, while varies slightly with Λ. To help elucidating the elutriation behavior, the agglomerates at three different locations (bed surface, splash zone, and bed outlet) are sampled and their size distributions are determined. The elutriation rate constant is found to be much smaller than the literature results for ordinary particles, and the reasons are discussed in detail. Finally, an empirical correlation considering size distribution is proposed to fit the elutriation rate constant for the conditions in this study.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ChemE/Product and Process Engineerin