气固流化床中声发射和流动模式关系

颗粒在气固流化床壁面区域（或局部空间区域）碰撞产生的声波能量反映了颗粒的碰撞速度和频率（活跃程度），从中可以揭示流化床内颗粒的流动混合模式。通过在φ150mm流化床冷模装置中，对聚乙烯颗粒－空气体系进行流态化实验，运用声发射技术测得声能量沿气固流化床的轴向分布，继而获得了颗粒的流动模式，并发现其与颗粒粒径、表观气速和分布板形式密切相关。对于颗粒粒径为460 μm的聚乙烯颗粒，当表观气速在0.3～0.7 m·s^-1内，其对应的流动模式为带有滞留区的双循环流动模式。如果气速增大到0.8 m·s^-1以上时，流动模式将转化为无滞留区的单循环流动模式。而当颗粒平均粒径降为365μm，对应的双循环流动模式蜕化为单循环模式，壁面不存在滞留区。进一步发现，滞留区位置与静床高无关。研究同时发现，颗粒的流动模式和分布板形式密切相关，对于在多孔平板分布板下为单循环流动模式的小粒径颗粒，在锥帽式分布板下，则在稍高气速时表现为双循环流动模式。     

Model development and validation: Co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasification system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and secondary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at the bottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone (modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomass char is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and the temperature profile predicted by the model are in good agreement with the measured value.     

Combustion characteristics of a two-stage swirl-flow fluidized bed combustor

In order to investigate the combustion characteristics of a two-stage swirl-flow fluidized bed combustor, combustion experiments of low-grade anthracite coal were performed. Experimental parameters were the fluidizing air velocity, coal feed rates, bed temperature, stoichiometric air ratio, swirl nozzle diameter and rotational diameter. The experimental results showed that, due to the swirl flow, the elutriation rates of fines were lower than those of the single-stage fluidized bed combustor. The combustible contents of the ash in the outflow streams were also reduced. Therefore, the combustion efficiency of the two-stage swirl-flow fluidized bed combustor was 20% greater than that of the single-stage fluidized bed combustor under the same operating conditions.     

Dynamic characteristics of the volatile cloudy zone in a gas–solid fluidized bed with hydrocarbon liquid spray

In a gas–solid fluidized bed with continuous hydrocarbon liquid spray, a volatile “cloudy zone” could be formed, defined as a dynamically steady liquid-affected zone, including droplets, wet particles, and the gas which passes through the zone. A new flow pattern with the dynamic coexistence of cloudy zone and non-cloudy zone (gas–solid zone), is accordingly established. The temperature, particle concentration, and particle velocity fields are measured in real-time via infrared thermography and particle imaging velocimetry, respectively. Results show that the area and range of central position of the cloudy zone illustrate a heavier fluctuant trend with the increasing velocity of liquid spray, and the main frequency of area fluctuation is close to that of the bubble rising. Moreover, the particle concentration and particle velocity in the cloudy zone are lower than those in the non-cloudy zone, breaking the conventional symmetrical distributions of hydrodynamic parameters of particles in a gas–solid fluidized bed.     

Combustion of liquid petroleum gas in a fluidized bed furnace with a jetting-mixing distributor

Liquid petroleum gas (LPG) fluidized beds have potential applications in metal heating or workpiece heat treatments. The combustion of LPG and the controls of the atmosphere inside the bed and the bed temperature are very concerned. The combustion of LPG has been investigated in a pilot-scale bubbling fluidized bed with a jetting-mixing nozzle distributor and hollow corundum sphere particles of 0.867-1.212 mm in diameter and 386-870 kg/m³ in bulk density at 800-1100°C. Experiments were carried out for fuel-rich mixtures to explore the possibility to obtain mild oxidizing, non-oxidizing or reducing atmosphere in the bed. Air factor (the ratio of the volume of air actually fed into the bed to that in a stoichiometric mixture) is in between 0.3 and 1.0 and U/U_mf 1.3-3.0. The distributor brings LPG and air into an intense contact sufficient to permit in-bed combustion without backfire problems. The experimental results show that the fluidized bed furnace offers excellent thermal uniformity and temperature control. The size of the combustion zone is usually larger than that of the temperature variation zone. Particle properties, initial bed height, air factor and U/U_mf all affect the bed temperature profile, whereas only the air factor and U/U_mf have significant effects on the combustion in the bed. The bed temperature can be adjusted by separate or combined adjusting of air factor and U/U_mf.     

Time-series analysis of the characteristic pressure fluctuations in a conical fluidized bed with negative pressure

The negative pressure conical fluidized bed is widely used in the pharmaceutical industry. In this study, experiments based on the negative pressure conical fluidized bed are carried out by changing the material mass and particle size. The pressure fluctuation signals are analyzed by the time and the frequency domain methods. A method for absolutely characterizing the degree of the energy concentration at the main fre-quency is proposed, where the calculation is to divide the original power spectrum by the average signal power. A phenomenon where the gas velocity curve temporarily stops growing is observed when the mate-rial mass is light, and the particle size is small. The standard deviation and kurtosis both rapidly change at the minimum fluidization velocity and thus can be used to determine the flow regime, and the variation rule of the kurtosis is independent of both the material mass and particle size. In the initial fluidization stage, the dominant pressure signal comes from the material movement;with the increase in the gas veloc-ity, the power of a 2.5 Hz signal continues to increase. A method of dividing the main frequency by the aver-age cycle frequency can conveniently determine the fluidized state, and a novel concept called stable fluidized zone proposed in this paper can be obtained. Controlling the gas velocity within the stable flu-idized zone ensures that the fluidized bed consistently remains in a stable fluidized state.     

Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation

The two-phase combustion model for biomass char combustion in a riser of a dual fluidized bed gasification unit that has been presented in part I is validated using the data obtained from the 8 MWth dual fluidized bed reactor at Guessing/Austria. The model is capable of calculating the average temperatures in all zones, the gas phase composition, solid hold up, char feed rates and air ratio. The model predictions for the temperature profile along the riser and for the exiting gas composition are in good agreement with the measured values. The simulation results show that the residual char from the gasifier is only partly converted in the riser for char particles larger than 0.6 mm. Un-combusted char is circulated back into the gasification reactor. Parameter variations show that the exact location where additional liquid fuels are introduced in the middle zone of the riser does not affect the global behaviour of the combustion reactor. Based on the simulation results it is proposed that external supply of char (additional) may be a very effective method for reducing producer gas recycling to the riser, which is currently necessary to obtain the desired gasification temperatures.     

Demarcation of a new circulating turbulent fluidization regime

Transient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

EXPERIMENTAL AND NUMERICAL STUDY ON GAS FLOW IN THE GRID ZONE OF JETTING-FLUIDIZED BEDS

A grid model describing the gas flow and interchange in the grid zone of jetting fluidized beds is proposed. Based on this model, longitudinal gas concentration profiles in the jet and annulus are calculated. The longitudinal gas concentration distribution is also experimentally investigated in a jetting fluidized bed with an inside diameter of 50 mm at the ambient temperature, and a jetting fluidized bed with an inside diameter of 80 mm at high temperatures. Comparison between the calculated and experimental results has shown that the experimental profiles can be qualitatively predicted by the grid model. The results indicated that the concentration in the grid zone depends on the gas exchange between the jet and the annulus, and the net gas flow from the jet to the annulus. The gas exchange rate is mainly affected by the inlet gas velocity from the nozzle. The present study is thought to be helpful to understand the grid gas behavior in the jetting fluidized bed coal gasifier.     

循环流化床气体返混及停留时间的分布

本文采用脉冲示踪技术,研究了直径140 mm循环流化床内气体的返混特性及其停留时间分布规律,并通过一维轴向扩散模型,得到了气体轴向扩散系数。实验结果表明,在循环流态化条件下,气体轴向流动明显偏离于平推流,并且随操作气速增大或颗粒循环速率的减小,气体轴向返混程度减小。     

大型射流流化床的流型转变与射流深度

在高 8m、直径 0 .5m的大型射流流化床中 ,通过摄像分析法 ,研究了射流、喷流两种流型的转变 .结果表明在低静床高、大射流气速下易出现喷流 ;喷流出现时 ,功率谱主频射流气速曲线上出现一转折点 .转折点射流气速与摄像法得出的流型转变射流气速相同 .提出了射流、喷流流型转变关系式 .同时发现射流深度随射流气速变大而增大 ,结合二维射流床射流深度的研究结果 ,得出了射流深度的预测式     

Flow development in a gas-solids downer fluidized bed

The development of gas and solids flow structure was studied in a 9.5 m high and 0.10 m diameter, gas-solids cocurrent downflow circulating fluidized bed (downer). Local solids concentration and particle velocity were measured using two separate optical fibre probes at different radial positions on several axial levels along the downer. The results show that the flow development is significantly influenced by the operating conditions. For most of the conditions under which the experiments were conducted, the gas-solids flow reaches its fully developed zone within 3 to 8 m away from the entrance. On the other hand, the development zone can extend as long as the downer itself, under certain conditions. When the solids circulation rate is over 100 kg/m²s, an increasing solids circulation rate largely extends the length of radial flow development. It is found that the flow developments in the core and at the wall are not quite simultaneous. For solids concentration, the core develops more quickly at low gas velocities and the wall region develops faster at high gas velocities. For particle velocity, higher gas velocity speeds up the development of the wall region but does not significantly affect the development of the core region. The wall region is much more sensitive to the change of superficial gas velocity than the core region. At high superficial gas velocities (> 7 m/s), a “semi-dead” region is observed in the fully developed zone adjacent to the wall where the dilute solids are moving at a very low velocity.     

流化床-提升管耦合反应器内沥青颗粒的流动特性

针对重油残渣(沥青颗粒)气化制氢工艺,在流化床-提升管耦合反应器大型冷模实验装置上,考察了不同操作条件下沥青颗粒体系在耦合反应器内截面平均密度的轴向分布. 结果表明,对单组分沥青颗粒体系,耦合反应器适宜的操作条件为：提升管表观气速ug,r=0.70~1.76 m/s;与A类颗粒相比,沥青颗粒在耦合反应器内的流动特性呈现不同的特点,整个反应器沿轴向可分为底部流化床密相区、提升管底部低密度区、提升管颗粒密度重整区、提升管加速区、充分发展区和出口约束区6个区域;反应器内截面平均密度随颗粒质量流率增大而增大,随表观气速增大而减小;确定了耦合反应器内提升管区域截面平均固含率的影响参数为ep', Fr及H/Dr,并利用实验数据回归了平均固含率的轴向分布经验模型,其计算值与实验值吻合较好.     

流化床-提升管耦合反应器内固含率的轴向分布及流动发展

在耦合流化床反应器大型冷模实验装置上,考察了不同表观气速下FCC颗粒在耦合流化床内截面平均密度的轴向分布. 结果表明,反应器轴向固含率可分为底部流化床区域和上部提升管区域. 前者的密相区平均固含率随表观气速增大而减小;后者的平均固含率随表观气速Ug增大而增大,Ug<0.58 m/s时固含率分布均匀,Ug=0.70~1.04 m/s时提升管出口出现约束返混区(>8.62 m),Ug>1.16 m/s时提升管底部出现密度重整区(3.82~4.57 m)、加速平稳区(4.57~8.62 m)和出口返混区(>8.62 m). 确定了耦合反应器内提升管区域截面平均固含率的影响参数,并利用实验数据回归了平均固含率的轴向分布经验模型,计算值与实验值吻合较好.     

基于图像法喷动床内空隙率研究

利用图像二值化方法处理图片,并采用Matlab编程实现空隙率的测量,从微观层次分析空隙率对喷动流化床内流动状态的影响。研究了颗粒粒径、喷口数量和表观气速对空隙率分布的影响。结果表明,增大表观气速使床层底部的稀相区增大,床层膨胀高度增加,空隙率也随之增加。增大颗粒粒径会增大最小临界流化速率,所以在其达到流化状态后流化床内颗粒粒径增大,床内喷动区空隙率减小比较明显。在相同条件下,与单喷口相比双喷口在床层底部附近存在合并射流,同时颗粒能到达的高度显著增加,底部两侧停滞区面积减小。结合对空隙率的分析,提出一种新的表征流化床内流化状态的参数(流化指数),可以直观地表示流化床内颗粒的流化状态。     

气-固循环流化床的流动特性

循环流化床是一种新型高效无气泡气-固反应器,应用广泛。文章首先介绍了气-固循环流化床的特点,总结了循环流化床上部区域的流动特性,并讨论了循环流化床底部区域的流动特性,最后展望了循环流化床流动特性研究的发展趋势。     

Fluid dynamics of a fluidized-moving bed (flumov)

The fluid dynamics and other features of a gas-solid contactor based on a combination of FLUidized and MOVing beds (flumov) were studied. The fluidized zone is below the moving one; the mov/flu diameter ratio ranges between 2 and 3. The two zones are joined by means of a conical part and no mechanical elements are used between the bottom of the moving bed and the top of the fluidized bed. The gas is fed at the bottom of the fluidized bed; the solid is fed at the top of the moving bed and, consequently, it moves in countercurrent to the gas and in a continuous mode. The adequate regime is reached when a stable vault between the bottom of the moving bed and the top of the fluidized bed is formed and maintained. The solid circulation from the moving to the fluidized bed is controlled by injecting air pulses from the bottom of the moving bed, at different points of the conical zone. The study comprises the influence of both the contactor geometry and the solid type on the pressure drop profiles and the general flumov performance. The angle of the conical device joining the two zones is an important geometrical parameter and, in the range studied, the flumov performance does not depend on the tested solid size and type.     

基于复杂性分析的流化床中流型的声发射检测

运用声发射检测技术,对Unipol气相流化床中声发射信号沿床高的规律性变化进行考察,对声信号进行复杂性分析,提取信号的涨落复杂性及Lempel-Ziv复杂性作为特征参数,成功获得了工业流化床中的颗粒流动模式。对声发射信号进行能量分析和复杂性分析均发现,Unipol工业气相流化床中的流型是带有滞留区的双循环流动模式。研究结果表明,基于复杂性分析和声发射检测的流化床中流型的测量是可行的。     

Analysis of gamma‐ray imaging data to describe hydrodynamic properties of gas–polyethylene fluidized beds

A scintillation gamma camera was used for measuring the instantaneous velocity profile and average velocity as well as the trajectories of a radioactive particle in small laboratory scale air–polyethylene–fluidized beds. A large number of frames, with frequencies between 1 and 50 Hz, were analyzed to investigate the effects of bed height, gas velocity and particle morphology. Subsequently: (1) horizontal and axial variations of the particle velocity were found; (2) three different flow zones along the axis (i.e., entrance zone, circulation zone and free board) were quantified; and (3) a characteristic time and a new definition of circulation (cycle) period were introduced. It was also found that the particle sometimes oscillates in the free board or gets trapped in the entrance zone.     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3~20 mm的4个气?固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响。结果表明,气?固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异。在固定床阶段,与B类颗粒相比,A类颗粒与壁面间的相互作用更强,导致实验压降值偏离计算值更大;在流化床阶段,较大颗粒粒径和密度的B类颗粒在床层内表现出了更高的气泡聚并和破裂程度,加剧了颗粒间的碰撞,增加了能量损失,从而形成了较高的实验压降。气?固微型流化床的最小流化速度除了与操作条件和物相性质有关外,床内径与静态床层高度对其也会产生显著影响。随着床径减小及静态床高增加,最小流化速度逐渐增加。综合考察各影响的因素,提出了适用于实验考察范围内预测微型流化床最小流化速度的经验关联式。