60米高矩形截面循环流化床内物料分布冷态试验

为研究超高提升管内的气固流动特性,依托四川白马电厂600MW超临界循环流化床锅炉现有钢架,将原有60m高的提升管冷模试验台的上部20m改为矩形截面的循环流化床提升管试验台。本文重点研究了提升管流化风速对上部颗粒浓度的轴向/截面分布特性及其影响因素。试验结果表明：颗粒浓度和颗粒粒径的分布特性与流化风速和几何结构密切相关,在一定初始床料高度下,随着风速的增加,提升管上部的空隙率沿轴向先不变然后减少,并最终呈现倒C形分布;截面浓度从均匀分布逐渐变为近短边壁处的颗粒浓度要明显大于近长边壁处的不均匀分布;平均颗粒粒径则随风速的增加而增大,沿截面分布均匀,但是沿提升管高度方向平均颗粒粒径沿轴向会略微减小,且提升管上部近短边壁的颗粒粒径要稍小于近长边壁的。     

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。     

在具有锥形料腿的循环流化床中流化CaCO3超细颗粒

根据粘附性颗粒在流化过程中形成的聚团具有较宽粒径分布并因此导致大聚团在流化床中沉积和死床的问题,提出了循环流化床的锥形回料系统设计. 该回料系统包括两部分：锥形料腿和带辅助进气的V型阀. 实验证明,锥形料腿通过提供变化的表观流化气速,克服了流化聚团沉积死床等现象;而V型阀的辅助进气,对于保证V型阀顺利输送粘附性颗粒具有关键性作用. 借助这种回料系统,实现了高粘附性超细CaCO3颗粒在循环流化床的稳定快速流化. 从提升管内部拍摄的照片显示,尽管提升管采用较高的流化气体速度,但超细CaCO3颗粒仍然是以聚团的形式被流化. 对在提升管不同高度采集的聚团分析表明,处于快速流化状态的CaCO3聚团的直径远小于传统流化床中聚团的直径,并且在提升管高度方向聚团直径没有较大的变化. 同时实验还显示,提升管轴向空隙率呈S型分布,而径向则体现环-核结构,具有典型的快速床特征.     

高密度下行床相结构分析

在自行设计的一套高密度下行循环流化床中（80 mm ID, 高5.6 m）,对下行床内相结构进行了实验研究.实验结果显示：稀密两相时间分率的径向分布中心区域比较均匀,变化范围在10%～20%之间;轴向密相尺寸在十几毫米之间,密相内颗粒平均浓度为床层浓度的1.7倍,稀相颗粒浓度约为平均浓度的40%.在下行床中,稀密两相的转变是一渐变过程,而提升管内两相的转变是突变.通过比较高密度下行床与高密度提升管内密相特性,发现下行床内的密相因停留时间短、尺寸小和密度低而难以观察到.     

六回路环形炉膛循环流化床试验研究

针对大型循环流化床锅炉存在的二次风穿透不佳、受热面布置困难等问题,提出了适用于600 MW等级及以上超(超)临界循环流化床锅炉的炉型——六回路环形炉膛循环流化床,并进行冷态试验研究,考查环形炉膛内气固流动特性和六回路间循环流率分布特性。结果表明:颗粒浓度沿环形炉膛高度的分布与矩形单炉膛相似,呈下浓上稀的指数型分布;随着流化速度的增大,炉膛下部密相区颗粒浓度减小,炉膛中上部和出口区域的颗粒浓度增大,各回路的循环流率均明显增大;随着静止料层高度的增大,整个炉膛高度的颗粒浓度都增大且高度越高处增幅越小;流化速度较低时循环流率不因静止料层高度的增大而变化,流化速度较高时循环流率随静止料层高度增大而稍有增大;六回路间循环流率的分布较均匀,设计工况下循环流率的相对偏差为4.5%;环形炉膛内环长边壁面悬吊屏对循环流率的大小和分布影响较小。     

循环流化床二次风射流及床料粒径对流化特性的影响

NOx的生成控制与流化床流化特性及气固流动有关,而冷态试验能更直观地反映气固流动状态和流化效果,因此冷态试验的研究结果可为流化床脱硝反应热态试验的参数选取提供参考。前人研究大多使用窄筛分床料颗粒,且较多针对传统的墙式布置二次风,鲜有学者综合研究中心布置二次风的穿透性能及其对炉膛流化特性的影响。因此,在循环流化床冷态试验台上,研究了中心布置二次风和宽、窄筛分的床料对流化特性的影响,采用无量纲剩余温度、物料循环流率和表观颗粒体积分数来定量描述二次风穿透性能、物料循环效率和颗粒浓度的分布。结果表明:喷射高度附近的颗粒浓度会随二次风射流增大而增大。二次风喷射高度为15 cm时,在35 cm以下的密相区,二次风占比越大,颗粒浓度的增长越明显。增大二次风占比、提高二次风射流的速度、提高二次风射流的喷射位置等可以有效提高二次风射流的穿透性能。其中,当二次风喷射高度距炉膛底部5 cm处,射流穿透率为0.4;喷射高度为15 cm时,射流穿透率为0.84;当喷射高度继续上升10 cm后,射流穿透率达到1.0。造成这个现象的原因是越靠近炉膛底部,床料颗粒的浓度越大,二次风所受的阻力急剧增加。随着窄筛分床料的平均粒径减小,炉膛整体的压降上升,且压降会在炉膛更高位置趋于平稳,物料循环流率也随之提高。这说明更多的颗粒能够随流化风的扬析被带到炉膛外,进入分离器参与炉外循环。与窄筛分床料不同的是,床料组分中,细颗粒占比也决定了宽筛分床料的炉膛压降、颗粒浓度分布和物料循环流率等参数。细颗粒占比越高,炉膛压降和物料的循环流率越大。宽筛分中,平均粒径大的床料颗粒浓度分布和物料循环流率不一定小,这是由于试验风速下,300μm颗粒更易随流化风的扬折作用,被携带至炉膛出口。这部分细颗粒占比越高,造成颗粒浓度分布和物料循环流率越大,而粗颗粒更倾向于聚集在炉膛底部。     

流化床内自由移动石墨球表面的传热系数

密相区内自由移动的煤颗粒表面传热系数是循环流化床锅炉设计和运行的重要参数。利用石墨球模拟煤颗粒,在小型流化床实验台上对由粒度较小的石英砂颗粒组成的密相区内自由移动的石墨球表面传热系数进行了测量。测量结果显示,随着流化风速的增加,石墨球表面传热系数首先升高,当流化风速达到某一临界值时,继续增大流化风速,传热系数将保持不变,从传热的角度证明了流化床内煤颗粒基本停留在乳化相内。在多数情况下,石墨球表面传热系数随床料粒度的增大而减小。而在较低流化风速的情况下,随着床料粒度的增大,石墨球表面传热系数呈先下降后升高的趋势。当流化风速和床料粒径保持不变时,石墨球表面传热系数随着石墨球直径的增大而减小,且下降的趋势随石墨球直径的增大而减弱。而随着床层高度的增加,石墨球表面传热系数将会略有升高。     

鼓泡流化床离散模拟中的一个局部空隙率模型

气固流化床离散颗粒模拟中通常采用面积加权平均法计算局部空隙率,不能较好地反映流化床中显著的非均匀结构特性.为了考虑非均匀结构对局部空隙率的影响,文中提出一个适用鼓泡流化床的局部空隙率模型.将流场划分为稀区(气泡区)和密区(乳相区);非均匀结构对密区局部空隙率的影响通过引入局部空隙率下限和非均匀影响系数描述;将提升管流动...     

流化床内自由移动石墨球表面的传热系数

密相区内自由移动的煤颗粒表面传热系数是循环流化床锅炉设计和运行的重要参数。利用石墨球模拟煤颗粒,在小型流化床实验台上对由粒度较小的石英砂颗粒组成的密相区内自由移动的石墨球表面传热系数进行了测量。测量结果显示,随着流化风速的增加,石墨球表面传热系数首先升高,当流化风速达到某一临界值时,继续增大流化风速,传热系数将保持不变,从传热的角度证明了流化床内煤颗粒基本停留在乳化相内。在多数情况下,石墨球表面传热系数随床料粒度的增大而减小。而在较低流化风速的情况下,随着床料粒度的增大,石墨球表面传热系数呈先下降后升高的趋势。当流化风速和床料粒径保持不变时,石墨球表面传热系数随着石墨球直径的增大而减小,且下降的趋势随石墨球直径的增大而减弱。而随着床层高度的增加,石墨球表面传热系数将会略有 升高。     

稻壳的流态化燃烧实验

在高为6m,内径为φ0.3 m的冷态流化床装置上,以0～0.6 mm的河砂为床料,进行了高速循环流化床(CFB)和低速鼓泡床流化床(BFB)不同工况的冷态流态化实验研究,当流化风速达到2.8m/s时,流化床就能实现循环;鼓泡流化床压力分布主要集中在底部的密相区,循环流化床压力分布更趋均匀。以稻壳为原料,在相同尺寸的流化床热态装置上进行了流态化燃烧实验,稳定燃烧阶段,循环流化床和鼓泡流化床沿炉膛的温度分布情况较为类似,循环流化床燃烧效率达到93.36%,鼓泡流化床达到93.01%,循环流化床和鼓泡床燃烧排放烟气中烟尘、SO2、NOx的含量都能满足国家排放标准。     

Hydrodynamic modeling of a circulating fluidized bed

Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB). The numerical simulation of CFBs is very important in the prediction of its flow behavior. From this point of view, in the present study a dynamic two dimensional model is developed considering the hydrodynamic behavior of CFB. In the modeling, the CFB riser is analyzed in two regions: The bottom zone in turbulent fluidization regime is modeled in detail as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase. In the upper zone core-annulus solids flow structure is established. Simulation model takes into account the axial and radial distribution of voidage, velocity and pressure drop for gas and solid phase, and solids volume fraction and particle size distribution for solid phase. The model results are compared with and validated against atmospheric cold bed CFB units' experimental data given in the literature for axial and radial distribution of void fraction, solids volume fraction and particle velocity, total pressure drop along the bed height and radial solids flux. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05-0.418 m, bed height from 5-18 m, mean particle diameter from 67-520 μm, particle density from 1398 to 2620 kg/m³, mass fluxes from 21.3 to 300 kg/m²s and gas superficial velocities from 2.52-9.1 m/s.As a result of sensitivity analysis, the variation in mean particle diameter and superficial velocity, does affect the pressure especially in the core region and it does not affect considerably the pressure in the annulus region. Radial pressure profile is getting flatter in the core region as the mean particle diameter increases. Similar results can be obtained for lower superficial velocities. It has also been found that the contribution to the total pressure drop by gas and solids friction components is negligibly small when compared to the acceleration and solids hydrodynamic head components. At the bottom of the riser, in the core region the acceleration component of the pressure drop in total pressure drop changes from 0.65% to 0.28% from the riser center to the core-annulus interface, respectively; within the annulus region the acceleration component in total pressure drop changes from 0.22% to 0.11% radially from the core-annulus interface to the riser wall. On the other hand, the acceleration component weakens as it moves upwards in the riser decreasing to 1% in both regions at the top of the riser which is an important indicator of the fact that hydrodynamic head of solids is the most important factor in the total pressure drop.     

Analysis of Regime Transitions and Flow Instabilities in Vertical Conveying of Coarse Particles Using Different Solids Feeding Systems

The transition between dense and dilute flow in vertical conveying of Geldart D particles were investigated for risers of different diameters using a spouted bed as a solid feeding system. The transition and choking velocities were identified by combining analyses of pressure gradient versus air velocity diagrams, pressure fluctuation signals and voidage values. Experimental data were used to evaluate the effect of particle and riser diameters on the pressure gradient, mean mixture voidage, the regime transition and choking velocities. The transition velocity from dilute to dense phase could be identified, as well as the onset of the choking condition, which appeared as the air velocity was further reduced. Data obtained in the same experimental apparatus facility using a screw conveyor and a gravitational system as solid feeding devices have been used as a reference to be compared to those obtained using the spouted bed feeder.     

湍动流化床过渡段固含率分布特征的实验及数值模拟

在湍动流化床中,过渡段对于包括甲醇制烯烃在内的气固催化快反应有着重要的作用。采用PV6D反射型光纤探针对内径95 mm的湍动流化床内过渡段的固含率分布和脉动参数进行了测量,分别考察了表观气速和静床高的影响,并采用修正的基于颗粒动力学的三段曳力双流体模型进行模拟。实验表明,湍动流化床过渡段中固含率的轴向分布呈现S型和指数型两种类型,固含率轴向与径向分布都在过渡段内出现最大梯度,表明过渡段中固体浓度分布比稀相段和密相段更不均匀。表观气速和静床高的变化将导致S型和指数型分布的相互转变,并且对过渡段底部与壁面附近的固体高浓度区影响最为显著。局部固含率脉动概率密度分布表明,在静床高较小时,随着气速的增大,床层下部气含率最大值位置将从中心区移动至环隙区,呈现气含率的双峰型分布。本文提出的修正三段曳力模型考虑了颗粒团聚的影响,对过渡段中分布板影响区之外的固含率分布均能较好地模拟。     

Hydrodynamic modelling of the axial density profile in the riser of a low‐density circulating fluidized bed

The axial density profile is an important characteristic of the CFB riser and a key parameter in the CFB design. A simple, but reliable model is needed to predict the density profiles. Elutriation‐based models treat the dense phase at the bottom of the riser as a dense bubbling bed whereas the dilute phase higher up can be looked upon as the entrainment zone above the dense bed. The elutriation model, as originally presented by Rhodes et al. (1986) and based on Wen et al. (1982) is extensively studied and modified. In spite of the modifications, the use of entrainment models has certain clear limitations due to a wide range of predictions as evident from Table 1. Elutriation rates are calculated based on the hydrodynamic phenomena in the dense bed and a fitting procedure for the entrainment decay constant (σ) was performed.     

A hydrodynamic model of a circulating fluidised bed with low‐density particles

In this study, a two‐dimensional mathematical model was developed considering the hydrodynamic behaviour of a circulating fluidised bed biomass gasifier (CFBBG), which is also applicable for other low‐density particles. In the modelling, the CFB riser was divided into two regions: a dense region at the bottom and a dilute region at the top of the riser. Kunii and Levenspiel's [Kunii and Levenspiel, Powder Technol. 61, 193‐206 (1990)] model was adopted to express the vertical solids distribution with some other assumptions. Radial distributions of bed voidage were taken into account in the upper zone by using Zhang et al.'s [Zhang et al., Chem. Eng. Sci. 46(12), 3045‐3052 (1991)] correlation. For model validation purposes, a cold model CFB was employed, in which sawdust was transported with air as the fluidising agent. The column is 10 m in height and 280 mm in diameter, and is equipped with pressure transducers to measure axial pressure profile and with a reflective optical fibre probe to measure local solids holdup. A satisfactory agreement between the model predictions and experimental data was found. © 2011 Canadian Society for Chemical Engineering     

Flow patterns in high-velocity fluidized beds and pneumatic conveying

Four flow patterns are identified for gas-solids vertical upward flows. Homogeneous dilute phase flow is characterized by the absence of both radial and axial solids segregation. Heterogeneous dilute phase flow (also called core-annulus flow) is characterized by the absence of axial solids segregation, with solids carried upward in the core and travelling downward near the outer wall due to the formation of particle streamers. Collapsed flow with a lower dense region and an upper dilute region is also referred to as the fast fluidization regime. In this case, the flow structure in the upper dilute region is similar to that in heterogeneous dilute phase flow, while the lower dense region resembles that in a turbulent fluidized bed. Dense phase flow can be reached when the riser is completely occupied by a relatively dense suspension with little axial density variation and no net solids downflow near the riser wall. The transition from fast fluidization to dense phase flow is still not clearly defined.     

16m高气固提升管中的压力梯度与流动行为研究

在较宽操作条件范围对16m高提升管中气－固两相流（空气－FCC颗粒）的压力梯度进行了实验测试,进一步揭示了快速流态化和密相气力输送这两种流动形态的动力学特征及其与操作参数的关系。结果表明,在表观气速增大的过程中气固提升管中的轴向压力梯度并非总是不断趋于均匀分布;提升管高度对快速流态化到密相气力输送状态的过渡有重要影响,对于给定的表观气速,提升管高度增加将使过渡点所应的颗粒循环量和床层颗粒浓度都减小。本实验条件下所有过滤点对应的床层颗粒浓度较为一致,平均为0．0104,并由此得到过渡点操作参数Ug与Gs的关联式。本文研究表明,在以往工作基础上进一步研究提升管高度对流动行为的影响极有必要。     

Estimating radial voidage profiles for all fluidization regimes in circulating fluidized bed risers

In terms of variation characteristics of radial voidage profiles, we had identified that all fluidization regimes from minimum fluidization to pneumatic transport prevailing in circulating fluidized bed (CFB) risers can be grossly divided into two distinctive variational regions, the dense phase continuous bubbling flow region and the dilute phase continuous clustering flow region, at a cross-sectionally averaged voidage of 0.75 [Powder Technol. 101 (1999) 91]. Succeeding that work, this article is devoted to further estimating the radial voidage profiles in two such distinctive variational regions by concerning the case of circular circulating fluidized bed risers. A new correlation was proposed for the dense phase continuous bubbling flow region, while the equation of Zhang et al. [Chem. Eng. Sci. 46 (1991) 3045] was considered applicable to the dilute phase continuous clustering flow region. Further improving the equation of Zhang et al. according to the result obtained in our last report ensured the continuity between the two correlations. In addition, the article also highlighted the formation mechanics of the two distinctive variational regions for radial voidage profiles in terms of the different gas/particle hydrodynamic behaviors in bubbling and clustering flows.