Effect of temperature on gas bypassing and solids circulation rate in an internally circulating fluidized bed

The effect of bed temperature in the annulus region on gas bypassing from annulus to riser and solids circulation rate has been determined in an internally circulating fluidized bed. The bed voidage in the annulus region increases with increasing temperature. The average fractions of gas bypassing of annulus gas are 59.4% and 87% at room temperature and high temperature (400°C to 800°C), respectively. Solids circulation rate increases with increasing annulus gas velocity, but is nearly independent of riser velocity. Solids circulation rate increases with increasing temperature.     

Solid circulation characteristics in an internally circulating fluidized bed with orifice-type draft tube

Effects of superficial gas velocities to a draft tube, to an annulus section and particle size on the solid circulation rate (G,) have been determined in an internally circulating fluidized bed (0.28 m I.D. × 2m high) with an orifice type draft tube. The solid circulation rate from the draft tube to an annulus section increases with increasing gas velocities to the draft tube(U _d ) and annulus section (U_a) and consequent increase in pressure drop across the orifice (ΔP_or). However, the values ofG _s decrease by 7–21% with increasing particle size from 86 to 288 μm. The pressure drop across the orifice increases with increasingU _d andU _a . However, ΔP_or decreases by 5–23% with increasing particle size. To predictG _s in an internally circulating fluidized bed, a correlation is proposed as a function of ΔP_or This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement Korea University.     

Solid circulation and gas bypassing in an internally circulating fluidized bed with an orifice-type draft Tube

The effects of orifice diameter in the draft tube, particle size, gas velocities and bed height on the circulation rate of solids and gas bypassing between the draft tube and annulus have been determined in an internally circulating fluidized bed (i.d., 0.3 m ; height, 2.5 m) with an orifice-type draft tube. A conical shape gas separator has been employed above the draft tube to facilitate the separation of gases from the two beds. The circulation rate of solids and the quantity of gas bypass from the annulus to draft tube show their minimums when the static bed height is around the bottom of the separator. The circulation rate of solids increases with an increase in orifice diameter in the draft tube. At fixed aeration to the annulus, gas bypassing from the draft tube to annulus sections decreases, whereas reverse gas bypassing from the annulus to the draft tube increases with increasing the inlet gas velocity to the draft tube. The obtained solids circulation rate has been correlated by a relationship developed for the cocurrent flow of gas and solid through the orifice.     

Flue gas desulfurization in an internally circulating fluidized bed reactor

An internally circulating fluidized bed reactor (ICFBR) was used as a desulfurization apparatus in this study. The height of the bed was 2.5 m, and the inner diameter was 9 cm. The bed materials were calcium sorbent and silica sand. The effects of the operating parameters of the flue gas desulfurization including relative humidity, particle size of the calcium sorbent, inlet concentration of SO₂, difference between the superficial gas velocities in the draft tube and the annulus, and superficial gas velocity in the draft tube on SO₂ removal efficiency (RE) were investigated. It was found that when the relative humidity (RH) was varied from 40% to 80%, the steady state RE had a largest value of approximately 15% when the relative humidity was 60%. When RH = 50%, 60% and 70%, RE decreased initially and then increased. After that RE decreased again until a steady state was reached. In addition, RE decreased with increasing calcium particle size or inlet SO₂ concentration. A larger difference between the superficial gas velocities in the draft tube and the annulus had a higher RE resulting from increasing reactivity of the calcium sorbent caused by a higher attrition rate. Moreover, a higher attrition rate had a higher total volume of the flue gas treated. Finally, a model to predict the steady state RE in ICFBR was proposed. It assumed that the draft tube section was a bubbling fluidized bed while the annulus section was a moving bed. In addition, the effects of the calcium sorbent conversion, attrition rate and gas-bypassing fractions on RE were also taken into account in this model. It was found that the values of RE predicted by this model agreed with the experimental results.     

The internally circulating fluidized bed (ICFB): A novel solution to gas bypassing in spouted beds

The bypassing of a portion of the inlet spout gas to the annular region of spouted and spout-fluid beds can have a critical effect on processes requiring accurate control of the spout gas residence time even when a draft tube is utilized. A modification to the spout-fluid bed with draft tube called the Internally Circulating Fluidized Bed or ICFB is proposed which eliminates this bypassing problem. Experiments were conducted on a cold 20 cm diameter semi-cylindrical unit to define the operational characteristics of the ICFB with particular emphasis on the gas and solids flow behavior. The solids and gas flow rates in the unit were successfully related by a published correlation which was developed for the cocurrent flow of gases and solids through orifices.     

Measurement of solid circulation rate in a circulating fluidized bed

A method using an optical mouse sensor was developed to monitor the moving velocity of a solid mass. Calibration was carried out using a rotating plate. Results clarified that the developed method is useful to monitor the velocity up to some limit (v < 0.3 m/s), which depends on the optical mouse sensor used. A solid circulation rate in a circulating fluidized bed (CFB) was measured using this method. Results obtained using this method show agreement with those of visual observations.     

导向管喷动流化床的喷动气旁路分率

在内径为182 mm的喷动流化床中安装内径80 mm的导向管,以平均粒径为2.2 mm的尿素颗粒为物料,对喷动气旁路特性进行了实验研究,分别考察了夹带区高度、导向管长度、喷嘴内径、床层高度、喷动气速和流化气速对喷动气旁路分率的影响,结果表明随着喷动气速的增大,喷动气体旁路分率先增后减。导向管安装高度越高,气体旁路分率越大。床层高度增大喷动气体旁路分率略有降低。而喷嘴直径小于50 mm时气体旁路分率随喷嘴直径增大而提高,在大于50 mm时气体旁路分率随喷嘴直径增大维持不变。当气速较小时,导向管高度增大会引起气体旁路分率增大,引入少量流化气能有效地抑制喷动气旁路。     

Effect of solid circulation rate on coating efficiency and agglomeration in circulating fluidized bed type coater

Circulating fluidized bed was proposed to be used as a coater, and coating experiments of glass beads with silica powder were performed in a circulating fluidized bed. Glass beads and silica powder were chosen as model particles, because their shape was almost spherical. The respective effects of gas flow rates supplied from a distributor and from an air nozzle for solid circulation, feed rate of powder suspension and particle content in the bed on coating efficiency and agglomeration are mainly discussed. Coating efficiency in circulating fluidized bed coater was correlated well with solid circulation time rather than with gas flow rates or solid circulation rate, while the agglomeration among core particles was mainly governed by solid circulation rate.     

内循环流化床烟气脱硫装置研究

开发研究了一种内循环流化床烟气脱硫装置,用40—80目河沙做粗颗粒床料,可实现固体颗粒在脱硫塔内的内循环,床内物料浓度高于传统结构的流化床烟气脱硫反应器,强化热质传递的同时可克服粘壁现象。通过热态烟气脱硫试验表明,系统能在接近绝热饱和温度的条件下连续稳定运行,在钙硫摩尔比x=1.3,粗颗粒质量浓度ρs=10 kg/m3,趋近绝热饱和温度Δt=8—10℃的条件下,脱硫效率达90%。     

Pressure drop and gas bypassing in recirculating fluidized beds

The effect of different operating and design parameters on the pressure drop profile for a recirculating fluidized bed has been studied. A mathematical model was developed for the pressure drop in the recirculating fluidized bed. The different parameters considered were flow rate, inventory of solids and spacing between the draft-tube bottom and the distribution plate. Geldart D and B particles were used for the study. The gas bypassing from the jet towards the downcomer was calculated on the basis of the mathematical model and the effect of various parameters on gas bypassing were analyzed.     

循环流化床中颗粒振荡循环现象的实验研究

颗粒的循环特性是循环流化床研究的重点及热点之一,在建立颗粒循环时,颗粒循环由非稳态向稳态过渡。本文在循环流化床实验装置中,采用摄像法和压力检测法研究了不稳定颗粒循环条件下颗粒流动结构和各段压降的变化情况。首次发现了不稳定颗粒循环状态下的颗粒振荡循环现象及其具有的两个特征:下降段、提升段气固流动结构呈周期性变化;下降段压降p_d和提升段压降p_r周期性波动且p_d>p_r和p_dr交替出现。进一步对颗粒振荡循环过程进行受力分析,建立了颗粒振荡周期的计算公式。研究还发现在颗粒藏量一定的条件下,循环气流量或颗粒循环段阀门开度大于某一临界值时才能诱发颗粒振荡循环;颗粒振荡循环周期随着循环气流量的增大而增大,与阀门开度无关。研究结果为动态颗粒循环建立过程的研究奠定了基础,有助于快速地建立颗粒的稳定循环。     

内循环微型流化床流动特性

针对开发适用于化学气相沉积反应动力学研究的微型流化床反应分析仪的应用需求,研究了外径为30 mm的内循环微型流化床中气固流动特性,具体考察了中心射流管伸入高度、内导流管直径和颗粒装载量对实现固体物料内循环的最小操作气速和导流管与环隙区间窜气的影响。结果表明,随着射流管伸入高度的增大,实现颗粒内循环流动的最小操作气速变大;存在最优的导流管直径（20 mm）,使得实现颗粒环流的最小操作气速较小;增大颗粒装载量有利于降低颗粒内循环的最小操作气速。通过检测示踪气体在环隙区内的质谱信号,发现在所考察的参数范围内,反应器底部不存在导流管区向环隙区的窜气;在反应器上部,由于颗粒对气体的夹带,环隙区上部总能检测到示踪气体,且窜气特性随操作气速的增大而增强。研究结果可为设计适用于化学气相沉积反应的内循环微型流化床反应器提供参考。     

循环流化床回流物料循环的特性

为了研究循环物料在流化床内回流并参与循环的过程,进一步揭示流化床内物料的循环特性,文中搭建了冷态循环流化床顶部回流试验台。利用激光多普勒粒子动态分析仪(PDA)对塔内颗粒回流时床内气固二相流场和颗粒浓度分布进行了测量。同时对物料回流量和床内表观气速等操作条件对回流过程的影响进行了考察。研究表明,回流颗粒在流化床内的返混可以分为3个区域:回流区、返混加速区和主流区,回流颗粒造成床内流场的不均匀性。回流长度主要受物料回流量和床内表观气速影响。     

The characteristics of cluster in a high density circulating fluidized bed

The cluster images in a high density circulating fluidized bed (HDCFB) measured by a one-dimensional optical fiber image analysis system are studied. The experimental results show that the characteristics of cluster in the riser vary greatly at different operating conditions. The radial cluster size, the cluster interception time and the probability of cluster appearance increase with bed density and decrease with gas velocity. Based on the analysis of the cluster image at various operating conditions, correlations of the radial cluster size, the cluster interception time and the probability of cluster appearance in a HDCFB are proposed. It is found that they are related to local solids concentration and gas velocity.     

Solid circulation rate in a circulating fluidized bed in the presence of fine powders

Fine powders (Geldart's group C) are added to a circulating fluidized bed (CFB) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of as the coarse particles and cohesive aluminum hydroxide powders of 0.5- as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated.The SCR strongly depended on the hold-up of fine powders of 0.5- in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than . Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified.     

The control of bed height and solids circulation rate in the standpipe of a cold flow circulating fluidized bed

Circulating fluidized beds (CFBs) are used widely in the chemical industry. Knowing or estimating the bed height in the standpipe and the solids circulation rate are essential for effective control of the system. This paper incorporates a 2-region model to calculate the bed height in the standpipe with a Kalman filter algorithm to estimate the solids circulation rate (SCR). Simulations of both the standpipe bed height and SCR were compared with experimental data and shown to give good agreement.

In addition, a neural network method was applied to model the entire cold flow CFB system and measured data sets were used to train the neurons of the network. Finally, a linear controller was applied to control both the bed height and solids circulation rate to desired set points. Simulations were performed for both positive and negative step inputs for both variables and satisfactory control was demonstrated using this controller in combination with the neutral network and Kalman estimator.     

Gas and particle circulation in an internally circulating fluidized bed membrane reactor cold model

A cold model study was carried out of an internally circulating fluidized bed membrane reactor for hydrogen production by steam reforming. This configuration facilitates a novel process in which air is introduced for autothermal operation, while avoiding diluting the hydrogen in the gas stream by nitrogen. A column of diameter 230 mm and height 2.4 m contained a membrane box of square cross-section. Gas circulation was investigated by injecting helium tracer into the top of the bed above the membrane box and detecting its concentration in the annulus and lower membrane box by a thermal conductivity detector. Solids circulation was determined from the downward velocity of particles in the annulus. The major resistance to solids circulation was found to be at the passage connecting the outer downflow to the inner upflow compartment. A ring distributor helped overcome this resistance.     

隔板式内循环流化床压力脉动信号递归分析

隔板式内循环流化床中流化态及颗粒循环特性对压力脉动信号特征具有重要影响,其作用机制尚未完全清楚。测量了隔板式内循环流化床在不同气速比条件下的压力脉动信号,通过时域及递归分析,获得了压力脉动信号的标准差、递归率、确定性及香农熵等特征参数。结果表明,随着表观气速比的增加,内循环中颗粒循环状态存在未循环、鼓泡循环、过渡循环和湍动循环四个阶段;通过压力脉动信号的标准差、递归图黑白结构占比和递归特征参数可识别这四种循环状态,递归特征参数在不同循环区域内显示出良好的线性关系,可用于识别隔板式内循环流化床系统的循环状态。