Characteristics of a Float Nozzle Designed for CFB Boilers

A float nozzle was designed and developed for use with large‐scale industrial circulating fluidized bed (CFB) boilers. The gas distributor equipped with the float nozzle has a unique pressure drop characteristic. It has a higher resistance than other nozzles which results in the formation of an effective barrier against backflow at low boiler loads, which results from the pressure fluctuation caused by bubble burst and solids coming from the recycle system. In addition, it has a relatively low pressure drop at high or full boiler loads, which can greatly reduce the energy cost of the primary air fan.     

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.     

Predicting the pressure drop across the solids flow rate control device of a circulating fluidized bed

The control of the solids circulation rate in circulating fluidized beds (CFB) can be obtained by means of a mechanical valve located at the bottom of the return leg. The valve acts by provoking a pressure drop that depends on the degree of the opening. The aim of this work is to develop a predictive model for the pressure drop in a butterfly valve used as a control device for the solids circulation rate. A model has been developed and validated against experimental data obtained from a 0.1 m id, 6 m high CFB using a group B powder. The equations proposed by, Jones and Davidson [D.R.M. Jones, J.F. Davidson, The flow of particles from a fluidised bed through orifices, Rheologica Acta 4 (1965) 180] and Cheng et. al. [L. Cheng, P. Basu, Solids circulation rate prediction in a pressurized loop seal, in: K. Chen (Ed.), Chemical Engineering Research and Design, vol. 76, 1998, p. 761] to predict the discharge rate of granular solid through orifices have been modified to account for the shape of the openings in the valve. A corrective parameter, which is based on the dimensionless hydraulic diameter of the valve opening, has been introduced. Very good agreement with the experimental data was obtained.     

A model for the pressure balance of a low density circulating fluidized bed

The pressure balance along the solid circulation loop of a circulating fluidized bed equipped with a solid flux regulating device has been modelled and the influence of the pressure balance on the riser behaviour has been predicted.The solid circulation loop has been divided into many sections, where the pressure drop was calculated independently: riser, cyclone, standpipe, control device and return duct. A new theoretical model, that is able to predict the pressure losses in the return path of the solid from the standpipe to the riser, has been built. A new correlation for cyclone pressure loss with very high solid loads has been found on the basis of experimental data.The pressure loss in the riser has been calculated by imposing the closure of the pressure balance, ΣΔP = 0. Once the riser pressure drop had been calculated, the holdup distribution along the riser was obtained by imposing a particular shape of the profile, according to the different fluid-dynamics regimes (fast fluidization or pneumatic transport). In the first case, an exponential decay was imposed and the bottom holdup was adjusted to fit the total pressure drop, in the second case, the height of the dense zone was instead varied.The experimental data was used to develop the sub-models for the various loop sections have been obtained in a 100 mm i.d. riser, 6 m high, CFB. The solid was made of Geldart B group alumina particles. The tests were carried out with a gas velocity that ranged between 2 and 4 m/s and a solid flux that ranged between 20 and 170 kg/m²s. A good agreement was found between the model and experimental data.     

An Improved Cyclone Pressure Drop Model at High Inlet Solid Concentrations

Calculating the cyclone pressure drop, Δp_c, at a high inlet solid concentration, C_s,in, is important for the design and operation of circulating fluidized bed boilers. Three typical Δp_c models that consider the effect of C_s,in were selected and assessed. Compared with experimental results, it was found that only the model developed by Chen and Shi can correctly predict the variation trend of Δp_c with C_s,in at high values of C_s,in, but the predicted values are much greater than the measured values. It is suggested that the dissipative loss of swirling dynamic pressure should be excluded from the total dissipative loss of gas dynamic energy when the pressure tap is closely installed to the outlet and the swirl exponent could be assumed to be null for high C_s,in conditions. A set of equations modified from the original C‐S model is given. Compared with the experimental data obtained from the present study and some data from literature, the improved C‐S model can predict Δp_c at high C_s,in well. In addition, it was found that the correction of pressure tap position is more obvious than the correction of swirl exponent.     

Numerical study of the effect of the gas and solids distributors on the uniformity of the radial solids concentration distribution in CFB risers

The non-uniform radial solids distribution usually has a negative effect on the performance of the circulating fluidized bed (CFB) riser since it may greatly decrease the reactor efficiency and controllability. In order to improve the performance of industrial CFB risers, the numerical study of the effects of the gas distributor and solids distributor at the CFB riser inlet on the uniformity of the radial solids distribution was carried out in this study. Two potential approaches to improve the uniformity of radial solids concentration profile were proposed: (1) the use of the center-sparse side-dense air jets arrangement for the gas distributor and (2) the use of the side-covered arrangement for the solids distributor. The Eulerian-Eulerian computational fluid dynamics (CFD) model with kinetic theory of granular flow was adopted to simulate the gas-solids two-phase flow in a CFB riser with FCC particles. The numerical results show that the patterns of the inlet gas distributor and solids distributor have significant effect on the flow structure in both the entrance region and the fully-developed region in the riser. The gas distributor with center-sparse side-dense air jet arrangement improves the uniformity of the radial solids distribution, while the center-dense side-sparse air jet arrangement steepens the non-uniformity of the solids radial profile. The core-annulus structure can be greatly flattened by applying a side-covered solids distributor, while it can be heavily steepened by employing the center-covered solids distributor.     

Comminution characteristics of Korean anthracite in a CFB reactor

Comminution characteristic of Korean anthracite has been determined with operation conditions in a laboratory scale circulating fluidized bed (CFB) combustor. The fragmentation of the anthracite occurs explosively, and generates lots of fine particles at an early stage of devolatilization. The fragmented particles continue to be reduced with generation of the fine particles during an attrition stage in the CFB combustor. With an increase of operation temperature, the coal shows a high degree of fragmentation and generation of fine particles in the CFB reactor. The particle fragmentation occurs actively as its size and Hard Grove Index (HGI) increase. The attrition is also affected with particle size and HGI of the coal. The initial surface crack and the fine clusters on the particle surface are found to be reasons for explosive fragmentation and for generation of fine particles during devolatilization and combustion in the CFB reactor.     

On the superficial gas velocity in deep gas–solids fluidized beds

The superficial gas velocity is one of the key parameters used to determine the flow hydrodynamics in gas–solids fluidized beds. However, the superficial velocity varies with height in practice, and there is no consistent basis for its specification. Different approaches to determine the superficial gas velocity in a deep gas–solids system are shown to cause difficulties in developing models and in comparing predictions with experimental results. In addition, the reference conditions for superficial gas velocity are important in modeling of deep gas–solids systems where there is a considerable pressure drop.     

Study of the influence of axial conduction in a boiling heated pipe

In this work a characterization of a horizontal heated pipe has been performed. This characterization consists in a steady state analysis of the thermohydraulic behavior of a boiling heated channel with subcooled liquid at the inlet. The temperature, velocities and pressure profiles along the heated section have been analyzed and the length of the single- and two-phase flow regions have been characterized. Without axial conduction and due to the big difference in the magnitude of the single- and two-phase heat transfer coefficient, steep temperature gradients were observed. By adding the effect of axial conduction, more heat is removed from the single phase region and added to the two-phase region through the wall. The net effect was a decrease in length of the two-phase flow region and consequent increase of the single-phase region. It was also observed that the axial conduction decreases the gradients in the wall temperature profile but it does not influence markedly its temperature differences with the fluid.     

Designing of primary air nozzles for large-scale CFB boilers in a combined numerical-experimental approach

The article presents a numerical-experimental approach for designing the primary air nozzles working with a large-scale Circulating Fluidized Bed (CFB) boiler. An analysis of the criteria that must be satisfied by CFB air grids has been made and it has been demonstrated that the existing correlations which are used in designing air grids are not possible to be utilized for determining the optimal nozzle design. On the example of a prototype nozzle design, based on the proposed design algorithm, the results of both numerical simulations and laboratory tests, whose aim was to determine the optimal geometry of a nozzle operating with a 235 MW_e asymmetric-design CFB boiler, are presented.     

循环流化床锅炉燃烧系统动态数学模型与仿真

对循环流化床锅炉燃烧系统建立了动态数学模型,仿真了在参数和变动下锅炉的动态特性,模型沿气体和因体的主要流动方向将流化床系统划分成连续的一系列小室,在小室内对各物质建立了质量和能量的非稳态方程。同时,模型考虑了气固两相流动,煤颗粒燃烧,ＳＯｘ和ＮＯｘ的生成与还原反应及受热面上的热传递。针对一３５ｔ／ｈ循环流化床锅炉进行了动态仿真计算,仿真了给煤量降低及突然断煤事故。计算结果表明所建立的模型可以准确反     

Investigation of the Dynamics of a High‐Flux CFB Riser Using Chaos Analysis of Pressure Fluctuations

The dynamics of a high‐flux circulating fluidized bed riser (10 m high with i.d. of 76.2 mm) were studied using a chaos analysis technique with differential pressure signals sampled at 400 Hz. Spent FCC particles (d_p = 67 μm) were used as the solid phase. The solids flux ranged between 50–400 kg/m² s while the gas velocity was varied between 4.0–10.0 m/s. Using the measured signals, the complexity and predictability of the gas‐solids flow were characterized using correlation dimension and Kolmogorov entropy. The axial profiles of the dimension and entropy revealed a more complex and less predictable gas‐solids flow in the transition section of the riser. Both dimension and entropy decreased with increasing solids holdup and solids flux. Moreover, the correlation dimension increased exponentially with decreasing average absolute deviation (AAD), at a power of 0.3. On the other hand, the entropy was observed to decrease exponentially with increasing average cycle time (ACT) of the pressure fluctuations at a characteristic power ranging between 0.95 and 1.10. Also, a comparison was made between low‐ and high‐flux conditions based on the dynamic properties of the riser.     

高硫石油焦用作CFB锅炉燃料的使用前景

高硫石油焦带来设备腐蚀和环境污染等严重问题,探索高硫焦脱硫方法及综合利用具有重要意义。采用延迟焦化/热电联产模式,对热电站改造采用循环流化床(CFB)锅炉,燃烧石油焦技术,给高硫石油焦的综合利用找到较好的出路。介绍了CFB锅炉的特点,国内外现状及应用前景,对比了燃煤锅炉与燃石油焦CFB锅炉的经济性。     

循环流化床锅炉SO2的生成与脱除

循环流化床锅炉在燃烧的过程中可脱除S、N等有害气体，其排放物可达到国家的排放标准。通过对云天化循环流化床锅炉的生产实践研究，分析循环流化床锅炉的Ca／S摩尔比、炉温、流化速度、脱硫剂等等因素CFB锅炉脱硫效率的影响。云天化循环流化床锅炉在Ca／S=2．5、床温时850～920℃、石灰石的平均粒度100～150um左右的脱硫效率达到90．6％、SO2平均排放量为1060mg／m^3,证明了循环流化床锅炉脱除SO2的能力。     

快速流化床压降之研究

本文介绍了在两个不同床径的快速流化床上,用5种固体物料所进行的床层轴向压降的实验工作。并用以前所进行的φ115快速床的压降关联式为基础,对三个不同床径的压降数据进行了多元拟线性回归,得出了一个比较好地预测压降值的经验关联式。式中包括气速、固体循环量、颗粒带出速度、固、气密度比和快速床的床径等参数。预测值和实验值的平均误差为15.7%,个别点大于±20%。     

Mass transfer in the core-annular and fast fluidization flow regimes of a CFB

In gas-solid reactors, particularly circulating fluidized beds (CFB) it is becoming increasingly more important to be able to predict the conversion and yield of reactant species given the ever rising cost of the reactants and the ever decreasing acceptable level of effluent contaminants. As such, the development and use of predictive models for the reactors is necessary for most processes today. These models all take into account, in some manner, the interphase mass transfer. The model developer, unless equipped with specific experimentally based empirical correlations for the reactor system under consideration, is required to go to the open literature to obtain correlations for the mass transfer coefficient between the solid and gas phases. This is a difficult task at present, since these literature values differ by up to 7 orders of magnitude. The wide variation in the prediction of mass transfer coefficients in the existing literature is credited to flow regime differences that can be identified in the cited literature upon careful inspection.A new theory is developed herein that takes into account the local hydrodynamics. The resulting model is compared with data generated in the NETL cold flow test facility and with values from the literature. The new theory and the experimental data agree quite well, providing a fundamentally based mass transfer model for predictive reactor simulation codes.     

油页岩循环流化床燃烧特性研究

摘要:在循环流化床燃烧试验台上对油页岩的着火、稳定燃烧、燃尽特性等燃烧特性进行详细地试验方案分析,同时提出了油页岩循环流化床锅炉的设计与运行的特点。根据拟做试验方案进行试验研究,可以测试出油页岩的燃烧特性,为油页岩CFB锅炉燃烧特性的试验研究,以及大型化油页岩循环流化床锅炉的设计提供了借鉴。     

循环流化床提升管中固体颗粒停留时间的分布

在内径１４０ｍｍ，高１０ｍ的循环流化床提升管中，采用磷光颗粒示踪法对床内固体颗粒的停留时间分布进行了测定。在气速１．５～９．０ｍ／ｓ，固体循环量１０～１４０ｋｇ／ｍ２ｓ的范围内，实验测得的停留时间分布曲线均有明显的双峰分布。这种双峰分布是由于提升管中弥散颗粒和颗粒团共同作用的结果。本文提出的一维两组分扩散叠加模型可较好地描述提升管中固体的混合行为。考察了在实验条件下，操作条件对固体混合行为的影响。发现：气速及固含对颗粒的轴向Ｐｅｃｌｅｔ数影响不大，提升管中颗粒的返混主要是由于颗粒团引起的。将研究结果与近期文献报道的其他研究进行了对比     

COMPREHENSIVE STUDY OF PRESSURE-DROP BALANCE AND PERFORMANCE OF AN L-VALVE IN A CIRCULATING FLUIDIZED BED

Pressure-drop balance and L-valve performance in a circulating fluidized bed (CFB) were investigated for variation in gas superficial velocity (U_g), secondary air fraction (SA), total solids inventory (I_s), and external aeration flow rate (Q_A), at ambient temperature. The study shows that the solids discharge rate of an L-valve (G_s) changes with the system control variables. Apparently, a wider range of G_s is achieved at increasing U_g, decreasing SA, increasing I_s, and increasing Q_A. The pressure-drop analysis indicates that these characteristics are directly governed by the loop pressure balance. Standpipe bottom pressure is found to be an important indicator of the operating status of the riser, standpipe, and L-valve. The experimental results confirm that an L-valve can be operated effectively by regulating the standpipe bottom pressure. The knowledge obtained from this study is essential for maintaining the performance of a CFB system for combustion operation, as well as for operating circulating fluidized beds on a practical scale.     

CFB密相区内颗粒横向扩散对燃烧的影响

循环流化床(CFB)床内燃料颗粒的扩散、混和，特别是复杂的密相区内的混和特性在很大程度上影响了燃烧状况，密相区颗粒横向扩散的规律，对于循环流化床的设计具有重要意义，在循环流化床密相区颗粒横向扩散实验研究的基础上，总结了密相区内颗粒横向扩散系数的经验公式，以此为基础，研究了密相区内碳的分布规律，并建立了相应的燃烧模型，模型包括两个子模型，即密相区二维流动及燃烧子模型、稀相区一维流动及燃烧子模型。通过模型定性模拟了流化风速、给料点布置对床内燃烧的影响，有效地反映了实际情况，并确认了将密相区颗粒横向扩散规律引入现有循环流化床燃烧模型的重要意义。