耦合流化床提升管流动结构及边壁层厚度的研究

在射流流化床与提升管耦合的多段分级转化流化床冷态实验装置上,采用压力传感器和PV-6型颗粒速度测量仪,对提升管流动结构和边壁层厚度进行了系统研究。结果表明,一定的操作气速下,固体循环量增加使提升管中气固流动状态从稀相气力输送过渡到快速流态化区域。当提升管处于快速流态化区域时,一定固体循环量下,表观气速增加使提升管轴向各个位置的边壁层厚度减小;一定气速下,固体循环量增加使提升管各个截面的边壁层厚度增加,且低气速时提升管各个截面的边壁层厚度随固体循环量增加的程度明显高于高气速时。拟合得到了边壁层厚度与截面平均固体浓度的关系式,较好地预测了快速流态化区域内边壁层厚度随截面平均固体浓度的变化关系,该表达式的计算值和实验值吻合较好。     

上行气固两相流充分发展段颗粒浓度关联及预测

在高度分别为15.1m和10.5m的两套实验装置上,对快速流态化到稀相气力输送流型下提升管内的轴向压力梯度进行了系统测试,以研究提升管充分发展段内不同颗粒的浓度变化及其与操作参数的关系。实验在其中175组操作条件下展现出明显的充分发展段(>2.8m)。结果表明,表观气速在3~8m?s-1之间变化时,对充分发展段颗粒浓度随终端颗粒浓度的变化关系影响显著,但当表观气速>8m?s?1或<3m?s?1时,其对充分发展段颗粒浓度随终端颗粒浓度线性增加的关系影响极弱;在此基础上提出的预测关联式更明确地反映了操作条件等因素对充分发展段颗粒浓度的定量影响关系,其计算结果与本实验和相关文献的实验数据吻合良好。     

不同流型下组合约束型提升管出口段固含率分布

在一套组合约束型提升管冷态实验装置上,研究了气力输送和快速流态化两种流型下,出口段局部固含率分布规律及不同操作条件对固含率的影响。结果表明:局部固含率径向分布整体上呈中心小、边壁大的分布特征,并随分布器开孔率和表观气速的降低而增大,随上部流化床层压降和颗粒循环强度的降低而减小;在快速流态化操作下,局部固含率曲线分布形式与常规提升管类似,而在气力输送状态下,临近出口区域局部固含率最大值通常不出现在边壁处,其位置随表观气速和分布器开孔率增加以及颗粒循环强度和上部流化床层压降降低而远离边壁;两种流型下局部固含率径向分布的均匀性均随表观气速及分布器开孔率的增加而升高,随颗粒循环强度及流化床层压降的增加而降低。     

气固两相上行流动中颗粒加速行为的研究

根据空气－FCC颗粒在16m高循环床提升管内的压力梯度实验数据,对提高升管颗粒加速区的平衡颗粒浓度、颗粒加速区长度以及操作条件的影响进行了系统的分析研究。颗粒的加速导致了颗粒表观浓度沿提升管轴向的不均匀分布,加速区截面上颗粒表观浓度随操作参数的变化明显不同于充分发展段;颗粒加速区长度受操作条件影响非常著,增加颗粒循环量或减小表观气速,都将延长颗粒加速过程,颗粒表观浓度也随之增加;特别地,当提升管底部有大量颗粒聚集和絮状物形成时,颗粒加速区将显著增长,甚至扩展到整个提升管高度。     

组合约束型提升管出口气固分布器的压降

在一套组合约束型提升管冷态实验装置上,通过实验研究了不同操作条件下提升管出口气固分布器的压降,并与常规气体分布器压降进行了对比。实验结果表明,在零床层及有床层的操作模式下,气固分布器压降均随提升管内表观气速和颗粒循环强度的增加而增大,在颗粒循环强度较低时,气固分布器压降曲线变化的斜率随着表观气速的增加而增大,在颗粒循环强度较高时,气固分布器压降曲线变化的斜率随着表观气速的增加而减小;随着开孔率及上部流化床层压降增加,气固分布器压降呈降低趋势,当流化床层压降达到一定程度后,分布器各孔方可实现有效布气,此后气固分布器压降趋于近似不变;在相同表观气速及开孔率下,气固分布器压降大于常规气体分布器压降。     

组合约束型提升管出口气固分布器的压降

在一套组合约束型提升管冷态实验装置上,通过实验研究了不同操作条件下提升管出口气固分布器的压降,并与常规气体分布器压降进行了对比。实验结果表明,在零床层及有床层的操作模式下,气固分布器压降均随提升管内表观气速和颗粒循环强度的增加而增大,在颗粒循环强度较低时,气固分布器压降曲线变化的斜率随着表观气速的增加而增大,在颗粒循环强度较高时,气固分布器压降曲线变化的斜率随着表观气速的增加而减小;随着开孔率及上部流化床层压降增加,气固分布器压降呈降低趋势,当流化床层压降达到一定程度后,分布器各孔方可实现有效布气,此后气固分布器压降趋于近似不变;在相同表观气速及开孔率下,气固分布器压降大于常规气体分布器压降。     

方形气固密相床中局部颗粒浓度分布的实验研究

在368mm×368mm方形气固流化床中采用FCC颗粒研究了局部颗粒浓度分布的基本行为,实验测试了不同高度床层截面上的局部颗粒浓度分布。结果表明:局部颗粒浓度在床层中心最小,向外逐渐增加,边壁处颗粒浓度急剧增加到最大;表观气速(Ug)较低时,床层截面内不同方向上颗粒浓度分布的差异较大;随表观气速增大,床层截面内不同方向上的颗粒浓度分布规律趋于一致。局部流动结构的转变首先发生于床层中心,然后再向边壁逐渐扩展。颗粒浓度概率密度分布曲线(PDD)表明在湍动流态化下稳定的两相流动结构已被打破。     

循环床提升管内轴向压力梯度和物料浓度的分布特性

以3.6m高循环床提升管内轴向不同高度测点的压力数据为基础,分析了管内轴向压力梯度的变化规律,并通过压差法对提升管内轴向物料浓度的分布特性进行了研究,得出了表观气速和物料颗粒粒径的变化对二者的影响。结果显示,提升管内轴向压力梯度沿管上升方向逐渐减小,物料浓度在轴向方向上呈上疏下密的不均匀分布趋势。增加表观气速,可以减小管内上下压力梯度的差异,改善颗粒浓度分布的不均匀程度,减小颗粒流动过程中消耗的总压降。而相对于粗大粒径颗粒,细小颗粒在提升管内流动时轴向压力梯度和浓度分布都更加均匀,整个流动过程所造成气体的总压降也相对更小。     

循环流化床压力波动信号的间歇性分析

用小波变换模极大值的方法对循环流化床靠近床层底部和顶部压力波动时间序列进行多重分形分析,确定相应的压力波动信号的间歇性指数.结果表明循环流化床内气-固流动具有多重分形的特性,由间歇性参数随操作气速的变化可判断出由快速流化床向密相气力输送的改变,提出了新的判断循环流化床流型转变的方法,指出流型的转变沿提升管轴向并不是均匀一致同时改变,床层底部的转变要滞后于床层顶部.     

提升管内颗粒运动分析及稀相输送过程浓度检测

为了研究提升管底部区域内颗粒的运动行为及浓度分布情况,采用电荷耦合(CCD)高速摄像机对提升管底部距离入风口20~30 cm轴向区域内的流动过程进行拍摄,根据流动图像对颗粒分布规律和流动结构进行了分析,并基于图像法实现了对稀相气力输送过程颗粒浓度的检测。结果显示:受送风方式的影响,在快速床阶段,颗粒在整体上呈现环核流动结构,同时在环隙区呈现左疏右密的分布形式;在节涌床和湍动床阶段,颗粒依次呈现柱塞式流动结构以及顺时针内循环流动结构。结合压差法测定的实验结果发现,图像法对稀相输送过程颗粒浓度的检测比较准确,其相对误差小于20%,且相对误差超过5%的情况只出现在表观气速相对较低的阶段。     

Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions

A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas-solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian-Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas-solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas-solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes.     

Identification and characteristics of different flow regimes in a circulating fluidized bed

A series of experiments was conducted in a 0.3-m diameter circulating fluidized bed (CFB) cold model to evaluate the operating flow regimes and their transitions. A single unambiguous experimental method was developed to identify the transitions between CFB operating regimes. Experiments were conducted at riser gas velocities ranging from dense phase turbulent, through fast fluidization (S-shape riser pressure profile), and up to dilute-phase flow regimes. A transient method was applied to a low density, Geldart Type B, cork bed material. Two distinct transition velocities were found by analyzing the time required to empty out all solids from the riser of the CFB after cutting off solids flow. The lowest transition velocity marked the transition between the dense-phase turbulent and the fast fluidization flow regimes, while a higher or second transition represented the transition between the fast fluidization and the dilute-phase flow regimes. Based on the experimental results, the axial pressures and its fluctuations along the riser exhibited markedly distinct profiles in each of the three different operating flow regime regions as defined by these two transport velocities.     

循环型高通量输送床的建立与控制

密相输送床气化和双流化床气化是基于循环型流化床反应器发展起来的两种新型煤和生物质气化技术,根据这两种技术对流动的要求,提出了在循环流化床的下行床底部耦合一段移动床,为输送床内的流动提供足够高的驱动压力而提高颗粒循环量的技术思想。在根据该思想而建立的直径90 mm的输送床实验装置上的实验研究表明,利用所提出的床型构造可在表观气速9.6 m&#8226;s^-1下实现400 kg&#8226;m^-2&#8226;s^-1的颗粒循环量。输送床的一次风速和移动床松动风速是影响颗粒循环量和输送床内颗粒浓度的主要因素,但循环量随输送床一次风速的增大而增加的走势弱于普通循环流化床。移动床松动风速在小于颗粒最小流化速度的范围内轻微变动即可显著改变颗粒循环量和输送床内颗粒浓度。在保持输送床总气速不变的前提下,通过二次风可在40%的比例范围内调节颗粒循环量,且调节作用随二次风位置的增高而减弱。     

Modelling a circulating fluidized bed riser reactor with gas—solids downflow at the wall

A predictive model was developed for the fully developed zone of a circulating fluidized bed (CFB) riser reactor operating in the fast fluidization regime that overcomes limitations of existing models. The model accounts for the upward flow of gas and solids in the core and downward flow of the two phases in the annulus. Additionally, a numerical solution methodology for the simulation of a riser reactor employing the hydrodynamic model was devised. A simulation was performed using the fast, main Claus reaction to demonstrate the effects of backmixing in the fast fluidization regime. It was found that the molar flow rates of the reactants leaving a fast fluidized CFB riser reactor were significantly higher than those leaving an identical reactor operating in the pneumatic transport regime.     

Full‐Loop Simulation of Gas‐Solids Flow in a Pilot‐Scale Circulating Fluidized Bed

In order to study the system hydrodynamics in a circulating fluidized bed (CFB), a 3D full‐loop simulation was conducted for a pilot‐scale CFB. The Eulerian‐Eulerian two‐fluid model with the kinetic theory of granular theory helped to simulate the gas‐solids flow in the CFB. The system hydrodynamics including pressure balance, vectors of gas and solids, distribution of solids holdup, and instantaneous circulating rates were obtained to get a comprehensive understanding of the system. It was predicted that the main driving force was the pressure drop of the storage tank. The storage height and valve opening were critical operating factors to control the riser operation. The effects of operating conditions including solids circulating rates and superficial gas velocity on the hydrodynamics were investigated to provide guidance for the stable operation of the CFB system.     

Identification of the flow structures and regime transition in gas–solid fluidized beds through moment analysis

Using statistic parameters of solids holdup signals, a moment consistency data processing method (MCDPM) was proposed. Experiments were carried out using FCC particles of 76 μm under different operating conditions, and MCDPM was used to successfully obtain solids holdups of the dense and dilute phases and the phase fractions over five fluidization regimes, bubbling (BFB), turbulent (TFB), circulating turbulent (CTFB), high‐density circulating (HDCFB), and circulating (CFB) fluidized bed systems. In BFB, TFB, and CTFB regimes, only dense phase fraction decreased with increasing air velocity, while the transition from HDCFB to CFB experienced appreciable change in the solids holdup of the dense phase. From the low‐velocity to the high‐velocity regimes, both the solids holdup and the fraction of the dense phase experienced a drastic decrease, suggesting that this transition corresponded to a profound change in flow structure and further suggesting that CTFB is in reality still a TFB. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1479–1490, 2013     

Transition from turbulent to fast fluidization

Based on eight transition criteria, at least two types of transition velocities are identified for the demarcation of the transition from turbulent to fast fluidization. The "critical velocity," U se , corresponds to the significant entrainment of particles from the bed, beyond which a circulating fluidized bed operation becomes essential. The "transport velocity," U tr , defines the transition to fast fluidization based on the axial solids concentration profiles. Below this velocity, a distinct interface exists between a top dilute region and a bottom dense region. Above U tr , the variation of voidage with height becomes relatively smooth. U tr is found to be a function of measurement location and riser height, as well as gas and particle properties.     

TRANSITION FROM TURBULENT TO FAST FLUIDIZATION

Based on eight transition criteria, at least two types of transition velocities are identified for the demarcation of the transition from turbulent to fast fluidization. The "critical velocity," U se , corresponds to the significant entrainment of particles from the bed, beyond which a circulating fluidized bed operation becomes essential. The "transport velocity," U tr , defines the transition to fast fluidization based on the axial solids concentration profiles. Below this velocity, a distinct interface exists between a top dilute region and a bottom dense region. Above U tr , the variation of voidage with height becomes relatively smooth. U tr is found to be a function of measurement location and riser height, as well as gas and particle properties.     

下行气固两相流与管内壁间的摩擦压降

在较宽的操作条件范围内系统测试了下行床床层压力降，获得气固两相流与管内壁间的摩擦压降，提出了下行气固两相流与管壁间摩擦压降的计算模型。结果表明，在下行床的充分发展段，气固两相流与管壁间的摩擦导致表观颗粒浓度显著小于真实颗粒浓度；当表观气速大于8 m·s^-1时，气固两相流与管壁间的摩擦压降接近甚至超过气固两相流重力产生的静压降。在采用压差法测试下行床中的平均颗粒浓度时，如忽略气固两相流与管壁间的摩擦，则可能导致显著的偏差。下行气固两相流与管内壁间的摩擦压降主要来自于颗粒与管壁间的摩擦。颗粒直径对气固两相流与管壁间摩擦压降的影响随着操作气速的提高逐渐减弱。采用提出的摩擦压降模型对表观颗粒浓度进行修正后，预测值与实验值吻合较好。