Experimental study on secondary air mixing along the bed height in a circulating fluidized bed with a multitracer-gas method

A multitracer-gas method was proposed to study the secondary air(SA)mixing along the bed height in a circulating fluidized bed(CFB)using carbon monoxide(CO),oxygen(O_(2)),and carbon dioxide(CO_(2))as tracer gases.Experiments were carried out on a cold CFB test rig with a cross-section of 0.42 m×0.73 m and a height of 5.50 m.The effects of superficial velocity,SA ratio,bed inventory,and particle diameter on the SA mixing were investigated.The results indicate that there are some differences in the measurement results obtained using different tracer gases,wherein the deviation between CO and CO_(2) ranges from 42%to 66%and that between O_(2) and CO_(2) ranges from 45%to 71%in the lower part of the fluidized bed.However,these differences became less pronounced as the bed height increased.Besides,the high solid concentration and fine particle diameter in the CFB may weaken the difference.The measurement results of different tracer gases show the same trends under the variation of operating parameters.Increasing superficial velocity and SA ratio and decreasing particle diameter result in better mixing of the SA.The effect of bed inventory on SA mixing is not monotonic.

Simulation of gas-solid flow characteristics of the circulating fluidized bed boiler under pure-oxygen combustion conditions

Under the pressure of carbon neutrality,many carbon capture,utilization and storage technologies have witnessed rapid development in the recent years,including oxy-fuel combustion(OFC)technology.However,the conventional OFC technology usually depends on the flue gas recirculation system,which faces significant investment,high energy consumption,and potential low-temperature corrosion problem.Considering these deficiencies,the direct utilization of pure oxygen to achieve particle fluidization and fuel combustion may reduce the overall energy consumption and CO_(2)-capture costs.In this paper,the fundamental structure of a self-designed 130 t·h^(-1) pure-oxygen combustion circulating fluidized bed(CFB)boiler was provided,and the computational particle fluid dynamics method was used to analyze the gas-solid flow characteristics of this new-concept boiler under different working conditions.The results indicate that through the careful selection of design or operational parameters,such as average bed-material size and fluidization velocity,the pure-oxygen combustion CFB system can maintain the ideal fluidization state,namely significant internal and external particle circulation.Besides,the contraction section of the boiler leads to the particle backflow in the lower furnace,resulting in the particle suspension concentration near the wall region being higher than that in the center region.Conversely,the upper furnace still retains the classic core-annulus flow structure.In addition to increasing solid circulation rate by reducing the average bed-material size,altering primary gas ratio and bed inventory can also exert varying degrees of influence on the gas-solid flow characteristics of the pure-oxygen combustion CFB boiler.

Experimental Study on Gas-Solid Mass Transfer in Circulating Fluidized Beds

This study is devoted to gas-solid mass transfer behavior inheterogeneous two-phase flow. Experiments were carried out in a coldcirculating fluidized bed of 3.0 m in height and 72 mm in diameterwith naphthalene particles. Axial and radial distributions ofsublimated naphthalene concentration in air were measured with an on-line concentration monitoring system HP GC-MS. Mass transfercoefficients were obtained under various operating conditions,showing that heterogeneous flow structure strongly influences theaxial and radial profiles of mass transfer coefficients.

Simulation of the L-valve in the circulating fluidized bed with a coarse-grained discrete particle method

Stable and controllable solid flow is essential in circulating fluidized bed (CFB) systems. The L-valve is a typical non-mechanical valve that can provide flexible solid feeding. The investigation of the solid circulation rate and the hydrodynamic characteristics of the L-valve is crucial to its design and operation. The gas-solid flow in the L-valve of a full-loop CFB is studied with the coarse-grained discrete particle method (EMMS-DPM). Good agreements on the solid circulation rate and the pressure drop through the L-valve are achieved between the simulated and experimental data. The solid circulation rate increases linearly with the aeration velocity until the stable particle circulation of the CFB is destroyed. The flow patterns in the horizontal section of L-valve are gas-solid slug flow above the stationary solid layer and the moving solid layer, respectively. The effects of L-valve geometric parameters on the solid flow characteristics are also investigated. The results indicate that reducing the diameter and length of the horizontal section of L-valve can improve the solid transport efficiency, especially at low aeration velocity. Besides, the solid conveying capacity and flow stability are improved when the sharp bend of L-valve is modified to be a gradual bend.

Investigation on Horizontal Mixing of Particles in DenseBed in Circulating Fluidized Bed (CFB)

A two dimensional cold CFB test rig has been established, investigation on horizontal mixing of particles in dense bed has been carried out on this test rig. Mixing model has been used in data reduction,the horizontal miring coefficients of particles in different experimental conditions and in different structures of dense bed have been obtained and compared. By using dimensional analysis, non-dimensionalexpression of experimental condition and mixing coefficient have been obtained.

EXPERIMENTAL RESEARCH OF FLOW STRUCTURE IN A GAS-SOLID CIRCULATING FLUIDIZED BED RISER BY PIV

Particle Imaging Velocimetry （PIV） techniques were applied to investigate the particle motion and cluster properties in a gas-solid two-phase flow in a circulating fluidized bed riser. Visual images and micro-structure of various clusters were captured. After the boundary of clusters was determined by the gray level threshold method, clusters were classified by the distance between particles and the shape and position of clusters. In addition, the process of clusters forming and breaking up was described, and the sizes of clusters were also obtained. With the Minimum Quadric Difference （MQD） cross-correlation algorithm suitable for high-density particles, the axial velocities of the particles were obtained in the dilute phase section. The features of particle motion were revealed by investigating statistically the magnitude and distribution of particle axial velocity in the radial direction. At most radial cross-sections, there exists a parabola-shaped distribution of upward axial velocity of particles, namely, the magnitude of axial velocity in the core region is higher than that near the wall region of the riser.

Comparative study of two fluid model and dense discrete phase model for simulations of gas-solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics(CFD)has become a valuable tool to study the complex gas-solid hydrodynamics in the circulating fluidized bed(CFB).Based on the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,this work conducts the comparative study of the gas-solid hydrodynamics in a CFB riser by these two different models.Results show that DDPM could be used to predict gas-solid hydrodynamics in the circulating fluidized bed,and there are differences between TFM and DDPM,especially in the radial distribution profiles of solid phase.Sensitivity analysis results show that the gas-solid drag model exhibits significant effects on the results for both the two models.The specularity coefficient and the restitution coefficient in the TFM,as well as the reflection coefficient and the parcel number in the DDPM,exhibit less impact on the simulated results.

Anti-wear beam effects on gas-solid hydrodynamics in a circulating fluidized bed

Anti-wear beams installed on water walls of circulating fluidized bed （CFB） boilers are one of the most effective ways to protect against water-wall erosion. Beam effects from, for example, beam size and superficial gas velocity were investigated on gas-solid hydrodynamics in a CFB test rig using CFD simulations and experimental methods. The downward flow of the wall layer solids is observed to be disrupted by the beam but is then restored some distance further downstream. When falling solids from the wall layer hit the anti-wear beam, the velocity of the falling solids decreases rapidly. A fraction of the solids accumulates on the beam. Below the beams, the falling solids have reduced velocities but upward-moving solids were observed on the wall. The effect of the beam increases with width and superficial gas velocity. Wear occurs mainly above the beam and its variation with width is different above to below the beam. There is an optimum width that, when combined with beam height, results in less erosion.

Particle clustering(mesoscale structure)of high-flux gas-solid circulating fluidized bed

Particle clustering is an important dynamic phenomenon in circulating-fluidized-bed(CFBs)systems,and has been suggested as a key contributing factor to the non-uniform hydrodynamics of CFBs.Studies show that particle clusters can be affected by solids flux,in terms of frequency,duration,and solids holdup.To understand the characteristics of particle clusters under high-solids-flux conditions,experimental and modeling studies in high-solids-flux gas-solids CFBs were reviewed and summarized.Optical and electrical measurements and imaging methods were used to monitor the particle-clustering phenomenon in CFBs.Particles were found to cluster in high-flux CFBs,and were characterized by a denser cluster-solids holdup and a shorter time fraction,which was different from the behavior in low-flux CFBs.Particle properties affected particle clustering in high-flux CFBs significantly.In modeling work,Eulerian-Eulerian and Eulerian-Lagrangian methods were used to study the particle-cluster characteristics.Good results can be obtained by using the Eulerian-Eulerian method to simulate the CFB system,especially the high-flux CFBs,and by considering the effects of particle clusters.The Eulerian-Lagrangian method is used to obtain detailed cluster characteristics.Because of limits in computing power,no obvious results exist to model particle clusters under high-solids-flux conditions.Because high-solids-flux conditions are used extensively in industrial applications,further experimental and numerical investigations on the clustering behavior in HF/DCFBs are required.

Numerical investigations on gas-solid flow in circulating fluidized bed risers using a new cluster-based drag model

A cluster-based drag model is proposed for the gas-solid circulating fluidized bed(CFB)riser by including the cluster information collected from image processing and wavelet analysis into the calculation of system drag.The performance of the proposed drag model is compared with some commonly used drag models.A good agreement with the experimental data is achieved by the proposed cluster-based drag model.Error analysis of the proposed cluster-based drag model based on the local distributions of solids holdup and particle velocity is conducted.The clustering phenomenon in the low-density and high-density CFB risers and the effect of the cluster size on the simulation accuracy are also numerically studied by the proposed drag model.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.

Experimental study of the solid circulation rate in a pressurized circulating fluidized bed

The solid circulation rate is essential for design of pressurized circulating fluidized beds(PCFBs).With increasing pressure from atmospheric pressure to a few bars,the gas density linearly increases with the pressure,which affects the gas-solid flow characteristics.In this work,experiments were performed at room temperature in a cold PCFB apparatus with a riser of 3.3 m in height and 0.05 m in diameter.The solid circulation rate was studied from 20 to 80 kg/(m^(2)·s)under various conditions with increasing pressure from 0.1 to 0.6 MPa and fluidizing gas velocity from around 1.5 to 8.0 m/s for different Geldart B group particles.Most of the conditions were in the flow regimes of core-annulus flow(CAF)only and CAF with a turbulent fluidized bed at the bottom.The trend of the apparent slip factor with the dimensionless slip velocity was similar at different pressures and for different average particle sizes,and it converged to an exponential function.An empirical equation was obtained by fitting the solid circulation rate with the operating parameters(particle transport velocity,particle volume fraction,Archimedes number,and Froude number),which is helpful for design and operation of PCFBs.

Evaluation of the effect of wall boundary conditions on numerical simulations of circulating fluidized beds

A computational fluid dynamics （CFD） modeling of the gas-solids two-phase flow in a circulating fluidized bed （CFB） riser is carried out. The Eularian-Eularian method with the kinetic theory of granular flow is used to solve the gas-solids two-phase flow in the CFB riser. The wall boundary condition of the riser is defined based on the Johnson and Jackson wall boundary theory （Johnson ＆ Jackson, 1987） with specularity coefficient and particle-wall restitution coefficient.The numerical results show that these two coefficients in the wall boundary condition play a major role in the predicted solids lateral velocity, which affects the solid particle distribution in the CFB riser. And the effect of each of the two coefficients on the solids distribution also depends on the other one. The generality of the CFD model is further validated under different operatin~ conditions of the CFB riser.

FUNDAMENTAL STUDIES ON THE HYDRODYNAMICS AND MIXING OF GAS AND SOLID IN A DOWNER REACTOR

The effect of flow direction on hydrodynamics and mixing in the upflow and downflowcirculating fluidized beds is discussed in details.Similar profiles of gas and solids velocities andsolids concentration are found in both risers and downers.When the flow is in the direction ofgravity(downer),the radial profiles of gas and particle velocity are more uniform than that inthe riser,the solids mixing is very small and the flow pattern approaches plug flow,while theflow is against gravity(riser),the solids backmixing significantly increase and the flow pattern isfar from plug flow.Among many of factors the flow direction has the largest influence onhydrodynamics and axial mixing of gas and solids.

Modeling of gas-solid flow in a CFB riser based on computational particle fluid dynamics

A three-dimensional model for gas-solid flow in a circulating fluidized bed（CFB） riser was developed based on computational particle fluid dynamics（CPFD）.The model was used to simulate the gas-solid flow behavior inside a circulating fluidized bed riser operating at various superficial gas velocities and solids mass fluxes in two fluidization regimes,a dilute phase transport（DPT） regime and a fast fluidization（FF） regime.The simulation results were evaluated based on comparison with experimental data of solids velocity and holdup,obtained from non-invasive automated radioactive particle tracking and gamma-ray tomography techniques,respectively.The agreement of the predicted solids velocity and holdup with experimental data validated the CPFD model for the CFB riser.The model predicted the main features of the gas-solid flows in the two regimes;the uniform dilute phase in the DPT regime,and the coexistence of the dilute phase in the upper region and the dense phase in the lower region in the FF regime.The clustering and solids back mixing in the FF regime were stronger than those in the DPT regime.

鸡粪和农林生物质混燃发电的探索

为克服鸡粪燃料的限制问题,从燃料收储与加工、锅炉技术改造及运行管理优化等方面,对鸡粪与农作物秸秆、稻壳及林木废弃物混合燃料在高温超高压30MW生物质直燃发电机组中的应用进行探索研究。针对混合燃料入炉质量与给料量不稳定的问题,改进混合燃料制备方案,优化燃料给料机结构和输送系统运行频率。通过热力学计算分析,增大锅炉水冷壁蒸发受热面积,从而提高锅炉出力。根据积灰形成机理,重点改造锅炉吹灰方式、重新布置吹灰器,以消除高、中温过热器处的严重积灰和结渣现象。把锅炉部分烟冷器更换为光管式管束,使烟冷器积灰堵塞现象得到解决。由此,基本消除了设备长周期运行的制约因素。同时根据不同的燃料结构,形成多套有针对性的入炉燃料掺配与运行参数调整方案,锅炉运行调整及各参数控制达到优化组合状态。经过一系列技术改造与运行优化调整,在机组负荷率99%以上的工况下,单炉膛120t/h高温超高压循环流化床锅炉实现了持续96天的稳定运行。

双级料腿循环流化床中颗粒停留时间分布的研究

双级料腿循环流化床垃圾焚烧炉采用分级转化技术,燃烧前脱氯,有效解决了高温腐蚀和二噁英造成的二次污染。该文采用不同的大颗粒物料来模拟垃圾中的不同组分,在双料腿循环流化床试验台中研究了颗粒的停留时间分布。结果表明,影响大颗粒停留时间的主要因素是循环流 率、表观流化气速、颗粒的密度和尺寸。循环流率增大,停留时间急剧降低。表观气速对停留时间影响较小。颗粒密度对停留时间的影响比尺寸要大。颗粒越轻,停留时间越短。根据实验结果,建立了两并联全混釜停留时间分布模型,模型计算结果与实验结果吻合较好。

磷光颗粒示踪技术在循环流态化中的应用

1 引言 固体颗粒的停留时间分布(Residence Time Distribution简称RTD)在循环流态化研究中十分重要,它对于了解其两相流动特性、反应器的模拟计算和工程设计是必不可少的,对于流态化干燥过程及传热行为的研究也很重要。流化床中RTD的研究已有大量文献报道,发展了诸如染色颗粒、盐颗粒、磁性颗粒、放射性颗粒及热(冷)颗粒等众多的示踪方法,但是由于固体颗粒示踪本身存在着示踪剂的注入、在线检测、残留及示踪颗粒与床体物料一致性、示踪过程对床内流场的干扰等一系列技术上的困难,不但使实验操作繁琐,而且实验结果的可靠性、重复性均不理想,特别是在颗粒运动速度较快的循环流化床中,其颗粒示踪的难度更大。为了解决上述难题,本文参考了Jin Yong等和Yu Zhiqing及M.Kwauk在常规流化床中曾采用的磷光示踪方法,在循环流化床条件下进行了新的探索。

棉秆在循环流化床中燃烧特性的实验研究

该文介绍了长度为10—100mm棉秆与弱酸性床料的混合流化特性，主要研究流化风速和棉秆与床料不同配比对混合均匀度的影响。实验结果表明，10—100mm棉秆与一定粒径分布的床料，质量配比为1-2％，流化数N〉3时，能较均匀地混合流化，但流化数N〉7时，混合均匀度有所降低。棉秆与床料配比对混合均匀度也有影响，棉秆的质量配比越小，混合得越均匀，所以设计循环流化床锅炉时流化风速、静止床高要选择合理。在0．5MWCFB实验装置上研究了纯棉秆燃烧时，流化速度、二次风率和棉秆给料量对炉内温度场分布的影响，结果表明，根据CFB实际运行参数，当流化速度为4-4．5m／s时，尽管此时混合均匀度有所降低，但并没有影响到密相区稳定燃烧，其温度能够维持在830-870℃。实验后放出的底渣没有出现烧结现象，基本保持原来的形貌，说明弱酸性床料能够适合棉秆循环流化床燃烧。