Experimental investigation of pressure fluctuation propagation in two orthogonal directions using a clapboard-type internally circulating fluidized bed

In this work, the pressure wave propagation in the vertical and horizontal directions has been studied using a self-designed clapboard-type internally circulating fluidized bed. The effects of flow regimes and solid mixture on the pressure wave propagation characteristics were analyzed in terms of time-averaged pressure, standard deviation, power spectral density, dominant frequency, propagation time delay, and attenuation. The superficial velocity ratio had a remarkable effect on the pressure wave propagation time delay in the vertical direction but had less influence on the pressure wave propagation in the horizontal direction. The particle circulation provided an extra resistance force to the horizontal pressure wave propagation and resulted in an increase in the pressure wave decay factor in the horizontal direction. The addition of glass beads and salt greatly increased the time delay and decay factor both in vertical and horizontal directions. The addition of 15% corn powder (Geldart-C group) reduced the propagation time delay but increased the decay factor.     

Evaluation of sorption-enhanced reforming over catalyst-sorbent bi-functional particles in an internally circulating fluidized bed

An internally circulating fluidized bed (ICFB) has been applied in various industrial processes owing to its potential in the reduction of heat loss and compact size. In this work, the sorption-enhanced reforming process in the ICFB is investigated. The dense discrete phase model (DDPM) is employed to evaluate the performance of catalyst-sorbent bi-functional particles, considering the particle size distribution. The results demonstrate that the utilization of bi-functional particles can significantly increase hydrogen production. The impacts of operating parameters including solid loading and regenerator velocity on solid circulation rate and gas leakage are examined. It is found that the gas leakage between reactors is increased by 46.6% when the regenerator gas velocity varies from 1.8 m/s to 2.4 m/s so as to weaken the hydrogen yield.     

Evaluation of direct quadrature method of moment for the internally circulating fluidized bed simulation with ultrafine particles

In the framework of the Euler-Euler gas–solid two-fluid model, the particle population balance equation is solved by the direct quadrature method of moment. The dynamic process of ultrafine particle movement and aggregation in an internally circulating fluidized bed is simulated. The distribution of the concentration and velocity of the agglomerates in the flow process is given, and the changes of the moments in the bed are shown. The effects of different breakage coefficients and inlet gas rates on the concentration distribution of agglomerates are compared. The results show that the particle size decreases with the increase of breakage coefficient, and the time required to reach steady fluidization state increases; the higher the inlet velocity, the better the effect of circulating particles in the bed. When there is a certain gas velocity difference between the two sides, the effect of circulating particles in the bed is better.     

The impact of vertical internals array on the key hydrodynamic parameters in a gas-solid fluidized bed using an advance optical fiber probe

The effect of a circular configuration of intense vertical immersed tubes on the hydrodynamic parameters has been investigated in a gas-solid fluidized bed of 0.14?m inside diameter. The experiments were performed using glass beads solid particles of 365?μm average particle size, with a solid density of 2500?kg/m³ (Geldart B). An advanced optical fiber probe technique was used to study the behavior of six essential local hydrodynamic parameters (i.e., local solids holdup, particles velocity, bubble rise velocity, bubble frequency, and bubble mean chord length) in the presence of vertical immersed tubes. The experimental measurements were carried out at six radial positions and three axial heights, which represent the three key zones of the bed: near the distributor plate, the middle of the fluidizing bed, and near the freeboard of the column. Furthermore, four superficial gas velocities (u/u_mf?=?1.6, 1.76, 1.96, and 2.14) were employed to study the effect of operating conditions. The experimental results demonstrated that the vertical internals had a significant effect on all the studied local hydrodynamic characteristics such that when using internals, both the solids holdup and bubble mean chord length decreased, while the particles velocity, bubble rise velocity, and bubble frequency increased. The measured values of averaged bubble rise velocities and averaged bubble chord lengths at different axial heights and superficial gas velocities have been compared with most used correlations available in the literature. It was found that the measured values are in good agreement with values calculated using predicted correlation for the case without vertical internals. While, the absolute percentage relative error between the measured and calculated values of these two hydrodynamic parameters indicate large differences for the case of vertical internals.     

Axial holdup distributions of FCC catalysts in a small-diameter riser with abrupt exit in circulating fluidized bed

Effect of exit geometry on solid holdup distributions in a small-diameter riser (0.01 m i.d. × 3.07 m high) has been determined with three fluid catalytic cracking (FCC) catalysts of different physical properties. The axial holdup distributions of the catalysts showed the C-shape with relatively dense bottom and upper regions of the riser. The solids holdups (ε_s) in dilute phase increased with decreasing gas velocity and increasing solids circulation rate. Higher ε_s in the upper region of the riser were observed with larger catalyst diameter and density. As the exit diameter decreased, the ε_s in the upper region were increased. The T-shaped exit with a projected roof increased the solids holdups near the riser top region compared to the L-shaped exit. No significant change in solid holdups was observed with increasing the projected roof height above a certain height. Interestingly, decay constants affected by the riser exit (a_e) increased with increasing solid circulation rate in the small-diameter riser due to the increased ratio of perimeter to unit area of the riser. Empirical correlations on a_e and reflux constant (C_e) for the small-diameter riser were proposed. Finally, a model was proposed for prediction of the axial ε_s distribution in the small-diameter riser with abrupt exit.     

Investigation of gas-solid flow characteristics in a novel internal fluidized bed combustor by experiment and CPFD simulation

Based on the coal self-preheating combustion technology, this research proposed a novel internal fluidized bed combustor (IFBC) with an internal separator for stable preheating of fuel. In order to verify feasibility and operation stability of IFBC, cold experiment, electrical capacitance tomography (ECT) and computational particle fluid dynamic (CPFD) simulation were performed in a laboratory-scale IFBC. The effects of superficial air velocity (U_g) and return valve structure on the operation and gas-solid flow characteristics were investigated. The results revealed that the CPFD prediction agreed well with the experiment values. The pressure balance curve presented an “8″ shape distribution, and the particle volume fraction (PVF) showed ‘core-annular’ distribution features. With the increase of U_g, the PVF in the standpipe increased, and the discharge pattern of the return valve changed from continuous discharge to intermittent discharge, and the solid circulation flux showed a trend of increasing first and then decreasing. With the decrease of the outlet opening of return valve (Φ), the gas–solid flow behavior in standpipe experienced a transition from gas leakage, stabilizing material seal, and blocking state. For U_g = 2 m/s, Φ = 50 %, an effective solid seal in the return valve was established and IFBC has a stable circulation and operation.     

Solids inventory and external solids circulation rate in risers of circulating fluidized beds

This study was conducted to provide improved relationships on the external solids circulation rate and solids inventory in the riser of the circulating fluidized bed (CFB) through increasing the basic understanding on how the solids flow rate in the riser exit establishes. The relation between the solids inventory in the riser and the solids flow rate in the riser exit was investigated in batch elutriation and CFBs, using air as fluidizing gas at atmospheric pressure and temperature. A batchwise riser (0.1 m-i.d., 2.75 m-height) and the riser of a CFB system (0.1 m-i.d., 2.5 m-height) were used to measure the relation for different gas velocities (1.5–2.4 m/s) and groups of solids (0.064–0.201 mm in diameter, 2045–4080 kg/m³ in apparent density).The batch elutriation indicated that the amount of solids inventory in the riser determined the solids flow rate in the riser exit, increasing with the amount of the solids inventory in the riser. The solids flow rate in the exit of the riser strongly depended on the elutriation rate constant combined with the amount of the solids inventory in the riser. When the riser was same in size, the relationship on the solids flow rate in the riser exit, derived in the batch elutriation, was valid in the CFB. Relationships on the external solids circulation rate and solids inventory in the riser of the CFB were proposed based on the data from this study and the literature. It was found that the external solids circulation rate per cross-sectional area of the riser decreased with increasing the riser diameter, however, the static bed height of solids in the riser (h_st,c) increased with the riser diameter. The dependency of h_st,c on riser diameter was greater in the fast bed than in the bed of pneumatic conveying.     

循环流化床锅炉风帽合金的组织及抗高温氧化性能研究

针对循环流化床锅炉风帽的严重高温氧化腐蚀问题,研制出适用于风帽的高温合金AN1(Cr23Ni32MoNb),结合金相(OM)、X射线衍射(XRD)、场发射扫描电镜(FE-SEM)、能谱(EDS)等分析方法对风帽试样的微观组织和高温氧化性能进行了研究,并与25-20钢和316钢进行对比分析。结果表明,AN1铸态组织为典型的树枝状晶结构,由奥氏体基体和Cr23C6、NbC等晶间碳化物组成,固溶处理后,枝晶结构基本消失;1000℃氧化100h,AN1的高温氧化抗力分别为25-20钢和316钢的2.2和3.8倍;AN1氧化实验后的表层相结构主要由γ奥氏体、Cr2O3和Cr与Mn的复合氧化膜组成。     

Experimental study on fluidization characteristics of vinegar residue in a vibrated fluidized bed

As a by-product in the vinegar brewing process, vinegar residue always has a high moisture content, which is detrimental to the storage and recycle process. The vibrated fluidized bed can be used to dry the vinegar residue. In present work, inert particles were added to a vibrated fluidized bed to improve the fluidization of vinegar residue. Experimental studies were carried out to investigate the fluidization behaviors of the binary mixtures. Flow pattern maps indicated that there was an upper limit to the vinegar residue mass concentration c_w at which stable fluidization could be achieved. The minimum fluidization velocity u_mf of the binary mixture increased as the vinegar residue mass concentration c_w increased and decreased with the increase of the vibration intensity Λ. As increasing vibration intensity Λ or decreasing vinegar residue mass concentration c_w, the drying rate of vinegar residue increased.     

Study on hydrodynamic characteristics of the liquid-solid circulating fluidized bed with a central pulsating nozzle by numerical approach

Pulsating fluidization is gaining increasing attention due to its superiority in efficiency improvement. A novel type of liquid–solid circulating fluidized bed with a central pulsating nozzle was proposed to improve the reaction efficiency in this work. Based on Eulerian-Eulerian two-fluid model and the kinetic theory of granular flow, the hydrodynamic characteristics in the riser was investigated by three-dimensional numerical simulation. The effects of pulsating liquid flow and particle properties on the distributions of solids holdup, the radial velocity of the particles, the slip velocity and the radial mixing degree were studied. The simulation results showed that with the addition of the central pulsating liquid flow, the particles displayed apparent radial motion, and the interphase mixing degree as well as the interphase mass transfer efficiency were enhanced. Based on the numerical results, the regression formulas of the axial and radial slip velocities were obtained respectively.     

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO₂ and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.     

Clusters identification and meso-scale structures in a circulating fluidized bed based on image processing

To understand the behaviors of particle clusters in the circulating fluidized bed (CFB), experiments were conducted with glass beads to acquire the image sequences of gas-solid flow on a CFB riser with a 100 mm × 25 mm cross-section and 3.2 m in length by adopting high-speed photography. An image multilevel thresholding approach using k-means algorithm was applied to perform image segmentation to identify clusters as well as core clusters in the riser, automatically. Cluster characteristics, such as the density and the number of clusters were obtained subsequently. The results show the image segmentation method based on k-means algorithm has made some improvement in terms of precision and systematicness for cluster identification. In addition, the internal structure of the cluster was analysed. Collectively, the cluster always consists of a dense core with highest solids holdup surrounded by a relatively dilute cloud with no clear boundary. High solids holdup enhances the cluster formation. On the contrary, the core cluster disappears at low solids holdup condition, indicating the cluster is only composed of cluster cloud in this case. Furthermore, based on the present experimental data, the correlations between the cluster density and the local time-mean solids holdup are presented.     

Experimental and theoretical study on hydrodynamic characteristics of tapered fluidized beds

A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity u_mf and critical velocity for full fluidization u_mff were particularly studied. The effects of taper angle α, static bed height H, air inlet section width δ and particle size d_p on the u_mf and u_mff were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the u_mf and u_mff, respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the u_mf and u_mff increase with the increase of taper angle α, static bed height H and particle size d_p, but decrease with the increase of air inlet section width δ, respectively. Additionally, the predicted values of u_mf and u_mff compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the u_mf of the modified bed.     

Elimination of stagnant particles from a N-valve with side aeration in circulating fluidized bed

The existence of stagnant particle layer in the conventional non-mechanical valves limits their utilization in CFB with the feedstock of caking particles. A new N-valve consisted of a fluidized weir chamber with bottom aeration and a moving-bed angled standpipe with side aeration was developed to eliminate the stagnant particle layer and reach high solids circulation rate G_s in CFBs. The particle flow behavior and its control in the N-valve were studied experimentally. By combining the bottom aeration for weir chamber and the side aeration for angled standpipe the G_s over 270 kg/(m² s) was achieved, and the stagnant particle layer completely disappeared. The G_s increased with increasing the side aeration gas flow rate Q_sa, and this loosing gas flow was optimally injected from the bend between the downcomer and the angled standpipe. At a constant but enough high Q_sa, the increase in the bottom aeration gas flow rate Q_ba elevated G_s linearly.     

Clarification of an improved cluster renewal model (ICRM) for heat transfer characteristics in a rolling circulating fluidized bed (RCFB)

This paper presents a novel method to predict the gas layer thickness δ′ in an improved cluster renewal model (ICRM) to calculate the heat transfer coefficient h′_c for a rolling circulating fluidized bed (RCFB). Eulerian-Eulerian two-fluid model (TFM) with kinetic theory of granular flow is used to perform a numerical simulation. After comparing the pressure gradient −△p/△z′ and the simulated heat transfer coefficient between multiphase and the wall h_gs with previously published experiment data, the correctness and reliability of the simulation are able to be verified. In conclusion, firstly, the variation of h′_c calculated from δ′ in the ICRM is almost the same with h_gs in the case that the RCFB undergoes the rolling motion. In order to quantitatively evaluate the proposed novel method, an error percentage α between h′_c calculated from δ′ and h_gs is 2.043% which is less than 5%. This certified that the novel method to predict δ′ in the ICRM has higher accuracy to calculate h′_c. Secondly, h′_c calculated from δ′ is mainly influenced by rolling amplitude Θ rather than by rolling period T. Specifically, with the increase of Θ, amplitudes of T_s are decreased, which is caused by the increased heat transfer and the decreased δ′ between cluster and wall. The decreased amplitudes of T_s are able to increase heat transfer efficiency, which eventually increases h′_c. Thirdly, in the case that normalized rolling period t/T is changed from 0 to 0.5, higher h′_c calculated from δ′ in Region Ⅰ results from the large local cross-sectional particle volume fraction c_local in Region Ⅰ with a smaller δ′, while lower h′_c calculated from δ′ in Region III results from the smaller c_local in Region III with a larger δ′. Therefore, it is concluded that h′_c calculated from δ′ is able to well predict the bed-to-wall heat transfer coefficient of the RCFB even if the rolling motion is considered.     

Experimental and CFD-DEM numerical evaluation of flow and heat transfer characteristics in mixed pulsed fluidized beds

Pulsed fluidized beds can make gas-solid mix and contact more uniform, therefore obviously improving heat transfer efficiency. The mixed pulsed fluidized bed, whose total gas flow is composed of stable gas flow and pulsed gas flow, is proposed in this research. Firstly, the experimental device for drying particles in a mixed pulsed fluidized bed is established. Pressure signals with different frequencies and gas flow ratios are collected, and flow pattern diagrams are obtained through a high-speed camera. Secondly, the CFD-DEM parallel numerical simulation method is constructed to research the mixed pulsed fluidized bed performance. Particle mixing, motion and heat transfer characteristics under different pulse frequencies and flow ratios are studied. Results show that particles in the mixed pulsed fluidized bed exhibit regular periodic motion, thereby promoting the mixing effect of particles. Moreover the bubble nucleation point moves to the bottom of the bed with the increasing pulse frequency. When the total gas velocity is relatively low, particle mixing effect can be enhanced by increasing the proportion of pulsed gas. However, when the velocity is relatively high, particle mixing effect will be enhanced by decreasing the proportion.     

Study on scale-up characteristics of oxy-fuel combustion in circulating fluidized bed boiler by 3D CFD simulation

The oxy-fuel combustion CFB technology as a promising carbon capture technologies needs to study the scale-up process for the commercial diffusion. Numerical simulation would be a rational tool to investigate the gas-solid flow and oxy-fuel combustion process before constructing an expensive and complicated industry-scale plant. A three-dimensional (3D) CFD simulation according to the Eulerian-Lagrangian approach was applied to simulate the hydrodynamics of gas-solid flow and oxy-fuel combustion process in lab-scale, pilot-scale and industry-scale CFB boiler (from 0.1 MW_th to 330 MW_e). Results present the differences of the boiler configuration, the gas-solid flow and the oxy-fuel combustion characteristics between lab-scale, pilot-scale and industry-scale CFB boilers. The cross-section thermal load gradually decreased, while the cross-section area increased with the thermal inputs increased. In the lab-scale and pilot-scale oxy-fuel CFB, the particle velocity field was more uniform than that in the industry-scale CFB. The carbon conversion ratio increased with an increase in the thermal input. The emission of CO, NO and SO₂ in the industry-scale oxy-fuel CFB boilers was lower than those in the lab-scale and pilot-scale. A larger oxy-fuel combustion power plant is beneficial to carbon capture and low pollutant emission. The results would be beneficial to the design and operation of industry-scale oxy-fuel CFB.