Spout Region Research Articles

Gas/particle flow and pressure variation characteristics of Ende pulverized coal gasifier equipped with ejector

AbstractEnde pulverized coal gasifier (EPCG), as a typical representative of circulating fluidized bed gasifier, has been widely used in gasification field. The return conduit of EPCG is generally not equipped with a feedback control device (FCD), which tends to result in a low solid circulation rate (Gs) and high unburned combustibles in fly ash. To address these issues, a new technique using the ejector as the FCD was designed to provide a stable pressure barrier and improve solid circulation. Preliminary thermal experiments demonstrated the feasibility of improving the EPCG with injector system. Currently, the specific mechanisms that contribute to further optimize this technology have not been systematically studied. Considering the large impact of ejector operation on solid circulation rate and stability as well as syngas production, this work adopts a gas/particle 1:15 cold EPCG bench to investigate the gas/particle flow and pressure variation characteristics before and after ejector operation and under different working medium pressures (Pp). When the ejector operates at the design pressure Ps = 0.187 MPa, the outlet pressure of the return conduit is slightly larger than the furnace bottom pressure, thus preventing the reverse gas flow at the furnace bottom. The particle velocity and concentration distribution in the furnace are basically consistent with that without the ejector, showing a similar central spout region and near‐wall annulus flow region. As Pp increases from 0.04 to 0.24 MPa, the average particle velocity in the return conduit increases from 0.95 to 2.4 m/s, the particle concentration decreases from 0.088 to 0.051, and the solid circulation rate increases from 0.37 to 0.53 kg/h. The ejector outlet pressure gradually increases, the inlet pressure decreases, and the pressure barrier provided by the ejector rises from 1052 to 3640 Pa. When Pp is 0.12 MPa, the ratio of GN2/Gair is 0.91%, which can achieve the purpose of continuous and stable pressurization with a small amount of nitrogen.

Numerical comparison of thermal energy performance between spouted, fluidized and fixed beds using supercritical CO2 as fluidizing agent

The different thermal energy performance between spouted, fluidized, and fixed beds when using supercritical CO2 as a fluidizing agent was numerically investigated by employing a combined approach of Computational Fluid Dynamics (CFD) and Coarse-Grained Particle (CGP) methods. The model was first validated against experimental data available in the literature, confirming that the simulated particle velocity and temperature agreed well with the experimental measurements. The numerical results reveal that for the spouted bed, gas velocity, gas temperature, and convective heat transfer were higher in the spout region. Hence, most heat was transported directly from the inlet to the outlet of this system. For the fluidized bed, the particles near the gas inlet were quickly heated up at first, and a uniform temperature distribution was achieved in the whole bed during the later stages of the process. Concerning the fixed bed, its behavior is well-known with three temperature zones differentiated, which are a hot region, a cold zone, and the intermediate thermocline. Besides, numerical simulations allow further analysis in detail of the different heat transfer mechanisms in the configurations tested. Convection is the dominant mechanism in all cases. Conduction is noticeable in the fixed bed regime compared to the spouted or fluidized regimes, but even in this case, it is still an order of magnitude lower than the convection mechanism. Finally, thermal energy and exergy were also analyzed for the three beds.

Fluidization characteristics of wet particles in a spouted bed using computational fluid dynamics and discrete element method

This study numerically investigated the effects of water content and contact angle on the fluidization characteristics of wet particles in a spouted bed via the combination of computational fluid dynamics (CFD) and the discrete element method (DEM). As the water content increased from 0.01 to 1.00%, particle agglomeration occurred. Moreover, as the water content increased and the contact angle decreased, the particle velocity initially increased before decreasing. The results showed that when the contact angle was 30°, the particle flow improved. Furthermore, the larger the contact angle (within 15–60°), the closer the central temperature region was to the outlet of the spout region, and the greater the particle velocity fluctuation. Particle agglomeration occurred at the outlet of the spouted region, resulting in the particles in the spouted channel not being completely fluidized, hence reducing the granulation efficiency. The simulation results provide theoretical guidance for future reactor design.

CFD‐DEM analysis of the spouted fluidized bed with non‐spherical particles

AbstractHydrodynamics of spouted fluidized beds of spherical and elongated sphero‐cylindrical particles with an aspect ratio (AR) of 4 were studied by the CFD‐DEM technique. The multi‐sphere method was adopted to present the sphero‐cylindrical particles. Simulations were validated with the experimental data for both particle types. Bubble equivalent diameter, bubble shape factor, and leakage fraction were obtained in the simulations. Time‐averaged particle velocity and porosity profiles, solids circulation rate, particle exchange distribution at the spout‐annulus interface, were investigated in the beds filled with spherical and sphero‐cylindrical particles at gas mass flow rates of 0.005, 0.007, and 0.009 kg/s. The equivalent bubble diameter was found to be lower for spherical particles compared to sphero‐cylindrical particles. The leakage fraction of the bubble was lower for sphero‐cylindrical particles. Solid circulation rate and spout diameter were larger for the bed of spherical particles. A higher probability of particles transition into the spout was observed for spherical particles. Investigating the orientation showed that sphero‐cylindrical particles tend to orient almost vertically against the gas flow in the spout region, which lowers the exerted drag force, whereas the particles align nearly horizontally in the annulus region. The results revealed that the CFD‐DEM approach is a promising method for investigating the fluid and both spherical and non‐spherical particle behaviours in spouted fluidized beds.

Experimental investigation of spout incoherence in a spouted bed

Spout incoherence is a typical instability phenomenon in spouted beds, in which the particles in the spout region move upward as groups, leading to poor spouting stability and efficiency. In this paper, the moving particle blockage in the spout region is measured to quantify the spout incoherence for the first time, by means of depicting the regularity, frequency and particle blockage length of the spout incoherence phenomenon. Specifically, the particle blockage is quantified using the particle concentration obtained by image processing method based on intensity differences between gas and solid phase in the spout centreline. The centreline of the spout channel is located by the particle velocity vector field. The method is validated by visual observation and frequency analysis. Then, the effects of static bed height and spouting velocity on the spout incoherence are investigated, respectively. The results show that the main features of spout incoherence, including frequency, regularity and particle blockage length, remain constant with varying spouting velocity in the investigated range. The results reveal that the static bed height has significant effects on the spout incoherence, and the increasing bed height reduces the frequency and particle blockage length of spout incoherence. This work quantifies the spout incoherence and unveil many interesting mysteries of the spout incoherence in spouted beds.

Particle-scale evaluation of the pyrolysis process of biomass material in a reactive gas-solid spouted reactor

Biomass pyrolysis is numerically studied in a spouted bed reactor. Thermophysical processes with complex reactions are considered for gas and solid phases under Eulerian and Lagrangian frameworks. After thorough model validations with experimental measurements, particle-scale evaluation of sand and biomass species combined with the segregation phenomenon is presented. The results show that the axial segregation significantly affects the spatial distribution of particle-scale information in three distinct regions. Particle heat transfer coefficient (HTC) has the largest value in the spout region, and then fountain region. A continuous decrease of particle HTC along the bed height appears. Particles close to the spout-annulus interface have a comparatively smaller temperature. Different distribution patterns of the HTC for sand and biomass can be observed. The increase of pyrolysis temperature raises the biomass temperature and HTC, while the effect of gas velocity is negligible. The dispersion intensity of biomass species is obviously larger than that of sand particles. A smaller sand size results in a smaller temperature, but a larger HTC, and a reduced dispersion coefficient.

Numerical study of the biomass pyrolysis process in a spouted bed reactor through computational fluid dynamics

Physical and thermochemical characteristics of biomass pyrolysis in a lab-scale spouted bed reactor are studied by means of a reactive multiphase particle-in-cell approach in which gas and solid motions are respectively resolved under the Eulerian and Lagrangian frameworks. Model validation is first conducted, followed by exploring the reactor hydrodynamics and spatial distributions of gas thermochemical properties. The results show that the preferential distribution of biomass particles at the annulus top affects the distribution pattern of gas thermochemical properties. The vertical flux of solid phase in the spout region is at the scale of 100 kg/(m2s), which is nearly one order of magnitude larger than the horizontal one. The turbulent viscosity of gas phase is at the scale of 10−4 m2/s, and the temperature difference within the apparatus is nearly 100 K. The intensively turbulent flow of gas phase exists in the spout region, which will be enhanced as gas inlet velocity and pyrolysis temperature increase. The preferential distribution of biomass particles gives rise to the large content of non-condensable gas and tar in the primary reaction, low temperature, and large density of gas phase in this region. The large content of non-condensable gas in the secondary reaction exists in the upper part of the bed. The results obtained provide a deep understanding of the hydrodynamics and thermochemical properties of gas-solid flows in spouting apparatuses for biomass pyrolysis.

Particle-scale evaluation of the biomass steam-gasification process in a conical spouted bed gasifier

Based on the multiphase particle-in-cell approach, the steam gasification of biomass in a lab-scale spouted bed gasifier is simulated to explore the particle-scale transport behavior in three distinct regions of the bed. The numerical results are firstly validated with the experimental data, followed by assessing the particle-scale features of sand and biomass species in the gasifier. The results demonstrate that biomass particles in three regions of spouted bed behave with different constituent content, heat transfer coefficient and temperature. The intensive heat transfer occurs in the spout region and fountain region. Biomass particles in the spout have a small temperature, large mass, small carbon fraction, and large volatile fraction. Heterogeneous reactions of the biomass particles mainly occur in the lower part of the fountain region but can be negligible in the spout and annulus region. The water-gas reaction is two-order of magnitude faster than the methanation reaction and Boudouard reaction. The spatial distribution of particle-scale level information of both particle species is nonuniform due to the presence of three regions and biomass accumulation. The horizontal and vertical dispersion coefficient of both sand and biomass species are at the scale of 10 −4 m 2 /s and 10 −2 m 2 /s, respectively. Spatial distribution of biomass particles colored with the carbon and volatile content in the spouted bed • Particle-scale transport behavior of biomass in spouted bed gasifier revealed. • Most intensive heat transfer occurs in the spout region. • Heterogeneous reactions of biomass mainly occur in the lower part of the fountain. • Operating parameters influence the biomass particles released from the bed outlet.

Effects of liquid content and surface tension on fluidization characteristics in a liquid-containing gas–solid fluidized bed: A CFD–DEM study

The fluidization process within gas–solid fluidized bed reactor can be intensified by adding liquid, as it can enhance mass and heat transfer between gas phase and solid phase. With the help of computational fluid dynamics(CFD) and discrete element methods(DEM), fluidization characteristics are investigated at different liquid contents(V^) and surface tension(Bog). To be specific, the bed expansion ratio, particle concentration and particle velocity profiles are compared and analyzed. Numerical simulations show that Bog should be less than 5 at V^ = 0.013. At this recommended range, liquid addition makes interparticle force increase and has little effect on fluidization performance. From transient particle agglomeration behavior, the Bog increase is more likely to enhance interparticle force, resulting in obvious particle agglomerations during fluidization process. The variation of stagnation zone height can be reflected from the abrupt increase of particle velocity in annulus region. It can be found that V^ increase reduces particle velocity at critical height but Bog increase is more likely to cause larger stagnation region. By analyzing the distribution of spout, annulus and fountain regions at different V^ and Bog, the annulus region expands with V^ increase below the initial bed height while the spout region expands with Bog increase.

Eulerian-Eulerian Numerical Study of the Flue Gas Desulfurization Process in a Semidry Spouted Bed Reactor.

The Eulerian–Eulerian two-fluid model (TFM) in conjunction with kinetic theory of granular flows (KTGF) was used for analyzing water vaporization and the semidry flue gas desulfurization process in a two-dimensional powder–particle spouted bed (PPSB). In an environment with high-temperature gas, desulfurization slurry is wrapped on the surface of moving particles and evaporated, along with the application of the user defined function (UDF) method to accomplish water heat and mass transfer by considering evaporation in the simulation process. The simulation results revealed that the best mass- and heat-transfer effect of each phase can be found in the outer annulus and the near spout region, both of which are also the main areas where water vaporization occurs. The rate of desulfurization products decreases with the increase in inlet gas temperature as the water vaporization rate increases. The volume fraction of desulfurization reaction products decreases with the increase in inlet flue gas temperature. Compared with other working conditions, the highest desulfurization efficiency reaches 84% when the inlet flue gas temperature is 480 K. The change of the desulfurization product rate with the radial distance is the same under different superficial gas velocities, with the peak desulfurization efficiency appearing in the annulus. The optimal operating parameter for the desulfurization process is available in PPSB, and the desulfurization efficiency and gas handling capacity reach the best result when the superficial gas velocity equals 1.2 Ums.

Numerical Study on the Gas-Solid Flow in a Spouted Bed Installed with a Controllable Nozzle and a Swirling Flow Generator.

The swirling flow technology is adopted on a nozzle of the spouted bed in order to enhance the radial movement of the particles. The hydrodynamic characteristics in a spouted bed with a swirling flow generator installed on the nozzle are numerically investigated based on the two-fluid model (TFM). The traditional spouted bed and spouted bed with an integral swirling blade nozzle (ISBN) are simulated and analyzed. Numerical results show that the dead zone at the cone region of the annulus can be effectively eliminated by using the ISBN. The maximum decrease in particle concentration near the cone region is 72%, and the ISBN structure can significantly improve the comprehensive fluidization degree of the spouted bed when γ equals 86°. The turbulent kinetic energy of gas can be significantly increased by the swirling flow along the radial direction in the spouted bed, especially in the spout region. Also, the swirling flow can promote the radial velocity and granular temperature of the particles in the spouted bed, which is helpful to the radial mixing of particles and gas phase between the central spout and the annulus in the spouted bed. There exists a value of ξ (equals 0.526), which brings the greatest elimination effect of the flow dead zone in the annulus of the limited spouted bed space, and the overall fluidization of the spouted bed has the best performance when ξ = 0.316.

Particle-scale characteristics of the three distinct regions in the multi-chamber slot-rectangular spouted bed

The particle-scale characteristics of the solid phase in the three distinct regions of the dual-column slot-rectangular spouted bed are studied, along with the effects of operating parameters and scaling up, via the computational fluid dynamics coupled with discrete element method (CFD-DEM). Results demonstrate that: (i) the spout-annulus interface expands from the bottom initially and then shrinks to a neck; (ii) the majority of the particles are in the annulus and the solid residence time (SRT) patterns are distinctly different in the three regions; (iii) the collision force is much larger than the fluid force, with the greatest magnitudes in the spout region and bed surface; (iv) significantly different solid dispersion patterns are observed among three directions; and (v) scaling up the capacity of the multi-chamber spouting system by horizontally aligning more chambers negligibly alters the flow pattern or the particle-scale information (e.g., SRT) during steady spouting.

Numerical simulation of reactive gas-particle flow in a solar jet spouted bed reactor for continuous biomass gasification

Solar steam gasification of beech wood biomass in a novel solar conical jet spouted bed reactor was studied. A 3D numerical model of the reactive two-phase gas-particle flow was developed and the model was experimentally validated. The cavity-type solar reactor was operated in both direct and indirect heating configurations at a constant wall temperature of 1200 °C with a biomass feeding rate that varied from 1.2 g/min to 2 g/min. Experiments showed that directly heating the particles increases the H2 yield and the carbon conversion efficiency. The Computational Fluid Dynamics (CFD) model was developed to thoroughly investigate the biomass conversion process inside the conical jet spouted bed subjected to direct or indirect solar irradiations. The desired vigorous cyclic flow pattern of the biomass particles was accurately predicted by the model with peak particles velocity of 2 m/s in the spout region. The maximum temperature reached by the indirectly-irradiated particles is about 1200 °C while it is above 1500 °C when considering directly-irradiated particles. Direct heating thus increases both particles gasification rates and carbon conversion efficiency. Furthermore, simulations reveal that the reaction zone temperature is 100 °C higher for the direct heating configuration. This promotes the H2 formation at the expense of CH4. The steam oxidant concentration, gas products distribution and particles trajectories inside the cavity were also investigated to identify improvement strategies for enhancing the phase mixing and the gas/solid residence time.

Effect of operating conditions on the drying of fine and ultrafine sand in a fountain confined conical spouted bed

A study has been carried out on the influence process conditions and design parameters have on the drying in a conical spouted bed. Optimum parameters have been determined for the draft tube and the fountain confiner. Drying time is reduced when the spout region is enlarged. A distance between the bed surface and the lower end of the confiner in the 0.10–0.15 m range and a confiner of 0.5–0.9 m length minimize the drying time. Longer distances do not avoid entrainment, and shorter distances lead to higher pressure drop and minimum spouting velocity. Longer fountain confiners do not reduce significantly the drying time. Draft tubes with a reduced aperture ratio lead to longer drying times due to their adverse effect on the gas–solid contact. Aperture ratios above 60% cause a decrease in the efficiency of the process due to the higher gas flow rates required to achieve spouting regime.

Comparison of Euler-Euler and Euler-Lagrange Approaches for Simulating Gas-Solid Flows in a Multiple-Spouted Bed

Abstract In this work, a comparative study of Euler-Euler and Euler-Lagrange approaches for modeling gas-solid flows in the multiple-spouted bed has been carried out to investigate the hydrodynamics of gas-solid flows. The influence of inlet gas velocity on the hydrodynamics of gas-solid flows in the multiple-spouted bed is investigated as well. Hydrodynamic characteristics of gas-solid flows such as flow behaviors, solid volume fraction, particle velocity and particle trajectory are analyzed and discussed in detail, providing some basic mechanism analysis of the gas-solids in the multiple-spouted bed. It is found that the central spout gas jet is a little confined by the auxiliary gas jets, and the hole-to-hole synergy is quite obvious when the auxiliary spout gas velocity is higher than the central spout gas velocity. When central/auxiliary gas velocity is 10/20 m/s, the maximum vertical particle velocities predicted by Euler-Euler and Euler-Lagrange approaches are 452 mm/s and 721 mm/s at the height of 10 mm respectively. A typical cycle period of a single particle is about 1.25 s, and the residence time in the spout regions is about 0.14 s in one cycle period in auxiliary dominant pattern. The curves of bed expansion height versus time calculated by Euler-Lagrange approach rise and fall periodically, while the curves calculated by Euler-Euler approach keep steady with little change. It is much easier for particles to be blew in the multiple-spouted bed using the Euler-Lagrange approach. The simulation results obtained from two models can provide some guidance for modifying the multiple-spouted bed to optimize physical operations such as drying and coating in the multiple-spouted bed.

CFD Simulation and Optimization of Mixing Behaviors in a Spouted Bed with a Longitudinal Vortex.

The radial mixing characteristics of gas and solid phases in a cylindrical spouted bed with a pair of longitudinal vortex generators (LVGs) are numerically studied by a two-fluid model. The influence of the distance between the centers of two spheres (L) and the shape of LVGs on hydrodynamics in a spouted bed are discussed. The results show that the value of the coefficient of variation (CV) of the particle concentration can be significantly reduced by the longitudinal vortex produced in spouted beds. The particle concentration near the spout decreases first and then increases with the increase of L. When the shape of LVGs is cylindrical, a maximum increase of 188 % for particle volume fraction is found near the spout region of the spouted bed. The strengthening effect on particles’ velocity achieves the best in annulus of the spouted bed, and CV of the particle concentration in the spouted bed reaches its minimum value with a pair of cylindrical LVGs.

Evaluating the new mechanistic scale-up methodology of gas-solid spouted beds using gamma ray computed tomography (CT)

In this work, we implemented the gamma-ray computed tomography (CT) technique to evaluate the new mechanistic scale-up methodology of gas-solid spouted beds by locally measuring the time-averaged cross-sectional distribution of solids and gas holdups, as well as their radial profiles along the bed height. The new scale-up methodology of geometrically similar beds is based on maintaining a similar radial profile of the gas holdup at a height within the bed, to achieve local and global similarity in dimensionless hydrodynamic parameters since the gas dynamic dictates the hydrodynamics of the gas-solid spouted bed. Two sizes of spouted beds (with diameters of 0.076 m and 0.152 m) were used with conditions that provided close magnitudes and trends of radial profiles of gas holdup; and with conditions that provide mismatches in magnitudes of the gas holdup radial profiles. The results clearly show the validity of the new method for scale-up, where the solids and gas holdup cross-sectional distributions and profiles are close to each other at all the heights and in the three regions (spout, annulus and fountain) of spouted beds, when the radial profiles of the gas holdup are close at one height between the two studied beds. In addition, the CT results clearly identified three regions mentioned above (spout, annulus, and fountain regions). In the spout region, the solids holdup increases along the spout height. When the radial profiles of gas holdups are different in two spouted beds, the differences are noticeable in local solids and gas holdups in all the heights, which further endorse the new mechanistic scale-up methodology.

Effect of a draft tube on oil shale particle drying process of a spouted bed: CPFD simulation studies

Material drying pretreatment plays an important role in the oil shale retorting process. In this study, the drying process of particulate oil shale in a spouted bed was simulated using computational particle fluid dynamics numerical schemes. This study analyzes the effect of the draft tube and inlet air temperature on the drying rate and drying uniformity, and establishes a new insight into the drying process and multiphase flow characteristics inside the spouted bed. The results indicate that using an ordinary cylindrical spouted bed results in a poor drying uniformity, which causing by a large number of the annulus-region particles fall into the spout region from the height of approximately 150 mm and resulting in a stirless area usually observed at the height of 50–150 mm near the wall. Inserting the draft tube and reducing the drying temperature can improve the uniformity of the drying process, and the former is more efficient than the latter.

Gas distribution characteristics for heterogeneous flows in the slender particle-containing scrubbing–cooling chamber of an entrained-flow gasifier

Dual-tip conductivity probes and a high-speed camera were used to study the gas distribution characteristics in a cold model apparatus of the slender particle-containing scrubbing–cooling chamber. The three-dimension column was 200mm in internal diameter and 1353mm in height. The effects of superficial gas velocities, fiber volume fractions and fiber aspect ratios on global gas holdups were determined. The results showed that under the influence of reverse buoyancy and negative pressure gradients, plume gas flow went up along the outside of the downcomer in downcomer spout region. Local radial gas holdup which was closely related with the internals and flow patterns presented obviously proximate core peak distribution in bubble-breaking plate region. The foam region consisted of bubble coalescence and breakup as well as droplets formation and splashing. At higher gas velocities, the increasing rate of global gas holdup gradually reduced. With the increase of fiber volume fraction, global gas holdup slightly increased at low gas velocities due to suppressed bed turbulence and bubble loading, but decreased at high velocities for increased liquid viscosity and vortex-shedding. Due to suppressed small-scale velocity fluctuation, global gas holdup increased with increased fiber aspect ratio. A modified drift-flux model containing crowding factor and fiber number density for global gas holdup correlations was proposed and all of the data could be calculated with a relative deviation less than 10%.

Numerical analysis of gas-solid flow in a novel spouted bed structure under the longitudinal vortex effects

The longitudinal vortex technology is adopted in the spouted bed so as to strengthen the particles radial mixing between spout and annulus zones. Three dimensional gas-solid two phase flow characteristics in a novel spouted bed structure under the longitudinal vortex effects was numerically investigated by two-fluid model (TFM) with kinetic theory for granular flow. Simulations and analyses have been conducted on three types of spouted beds: without disturbance units, with a pair of balls and with a pair of longitudinal vortex generators (LVGs). The simulation results show that: the fountain height decreases significantly when LVGs are installed in spouted bed, causing the secondary vortex flow. A pair of symmetrical distribution longitudinal vortex appear in the cross section of spouted bed under the action of LVGs, both for the gas phase and particle phase. The particle volume fraction near the central spout region, the radial velocity and granular temperature of particle phase in spouted bed can be promoted significantly by longitudinal vortex. Also, the value of turbulent kinetic energy of gas phase can be obviously enhanced by longitudinal vortex along the axial direction in spouted bed. As a result, the radial mixing of particles and gas phase between central spout and annulus in spouted bed can be significantly enhanced by the longitudinal vortex.