Heat transfer in a large-scale circulating fluidized bed boiler

Heat transfer of a furnace in a large-scale circulating fluidized bed (CFB) boiler was studied based on the analysis of available heat transfer coefficient data from typical industrial CFB boilers and measured data from a 12 MW_e, a 50 MW_e and a 135 MW_e CFB boiler. The heat transfer of heat exchanger surfaces in a furnace, in a steam/water cooled cyclone, in an external heat exchanger and in the backpass was also reviewed. Empirical correlation of heat transfer coefficient was suggested after calculating the two key parameters, solids suspension density and furnace temperature. The correlation approach agrees well with the data from the large-scale CFB boilers. __________ Translated from Journal of Power Engineering, 2006, 26(3): 305–310 (in Chinese) [译自: 动力工程]     

Heat transfer characteristics in a pulsating fluidized bed in relation to bubble characteristics

A pulsating fluidized bed is operated with two sequential durations designated as an on‐period with injecting fluidization gas and an off‐period without it. The heat transfer coefficient between a vertically immersed heater and bed in a pulsating fluidized bed is measured under various pulse cycles and fluidized particles. The obtained results are compared with those in a normal fluidized bed with continuous fluidization air injection. The relationship between heat transfer coefficients and bubble characteristics, evaluated using a digital video camera, has also been investigated. For certain fluidized particles and operating pulse cycles, the fluidization of particles and the increment of heat transfer coefficients can be obtained under a mean air velocity based on a pulse cycle duration smaller than the minimum fluidization air velocity in a normal fluidized bed. Under the pulse cycles where a static bed through the whole bed is formed in the off‐period duration, the improved heat transfer rate over that in a normal fluidized bed can be measured. This may be attributed to large bubble formation. As heat transfer in the pulsating fluidized bed is obstructed with increasing time to keep a static bed due to the excessive off‐period duration, it is indicated that there is an optimum off‐period duration based on the heat transfer rate. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 307–319, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10038     

Heat transfer to a horizontal tube bundle located in the freeboard of a bubbling fluidized bed combustor

The characteristics of heat transfer from bubbling gas-fired fluidized bed to a horizontal staggered water-tube bundle located in the freeboard region is experimentally investigated. The purpose is to demonstrate the effect of bed temperature on the coefficients of heat transfer by the different modes to each of the four rows of the bundle, which experiences heat transfer by convection from flue gases, luminous radiation from bed material and non-luminous radiation from gases. The bed temperature itself is varied and controlled through the fuel–air mass ratio. Sixteen runs have been conducted with bed temperature ranging from 1114 to 1429 K, resulting in an overall heat transfer coefficient in the range 74·0–105·0 W m⁻² K⁻¹ for the first row and 58·0–65·0 W m⁻² K⁻¹ for the last. An overall convective heat transfer coefficient from gases, and possible carried over sand particles, to the bundle is formulated. © 1997 by John Wiley & Sons, Ltd.     

循环流化床锅炉设计方法探讨

阐述了目前国内外循环流化床锅炉的最新发展趋势 ,重点分析了循环流化床锅炉燃烧室和飞灰循环系统的设计 ,综合考虑了商用循环流化床锅炉的运行数据和前人的一些设计方法 ,在此基础上提出了作者认为合理的一套设计方案     

The Imaginary Plane Mothod of Radiation Heat Transfer in The Freeboard of Atmospheric Bubbling Fluidized Bed Boiler

An imaginary plane method for calculation of radiative heat trensfer and its application in the freeboard of AFBC boiler is presented in this paper. The combustion reaction and particle concentration are taken into account in this method. With is method, one-dimensional freeboard model for radiative heat transfer has been made. Results from this model have been compared with the experimental results of a 130 t/h AFBC boiler. The distribution of flue gas temperature and heat flux at the waterwell are obtained. It is shown that this model has the advantage of good accuracy and requiring less computation time. The applicability of the predicted results in the AFBC boiler design and operation was also discussed.     

Modelling heat transfer in a circulating fluidized bed

A model for the bed-to-wall heat transfer under low temperature condition in a circulating fluidized bed (CFB) was developed based upon a simplified cluster renewal concept. The age of clusters in contact with the wall at different locations along the height of the CFB was estimated as the weighted average age considering their formation and disintegration. One set of experimental data on heat transfer in a 4.5-metre high, 0.15-metre diameter CFB under low temperature condition (67–77°C) was chosen for comparison with prediction of local heat transfer coefficient. The experimental observation and prediction have shown a qualitative agreement.     

循环流化床锅炉燃烧系统动态特性分析

根据循环流化床锅炉(CFBB)的工作特点以及燃烧系统输入和输出过程变量间的耦合关系,讨论了CFBB的蒸汽压力和床温的动态特性。认为引起蒸汽压力变动的主要原因在于燃料量(内扰)和汽轮机调门的变化(外扰);而影响床温变化的主要因素是给煤量、风量、物料循环量的变动,并从传热和燃烧过程分析了这些因素间的相互耦合关系。这对CFBB燃烧自动控制系统的设计与调试,以及整个控制系统的可靠运行都至关重要。     

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s⁻¹. Local sand of mean size (d_p) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.     

流化床锅炉点火启动方式的分析与比较

介绍了流化床锅炉的点火方式,分析比较了几种常见的流化床锅炉点火方式和流化床锅炉点火的影响因素。     

Modeling Study of a New Circulating Fluidized Bi—Bed Boiler Combustion System

INTRODUCTIONThecirculatingfluidizedbed(CFB)boileriscurrentlybeingactivelydevelopedinChina.AnalysisofthecharacteristicsofCFBismadetoeffectivelydesign,operateandcontroltheCFB.Mechanismanalysisandaccumulatedexperiencewereusedtodevelopasetofgeneraldynami...     

Combustion Model For Staged Circulating Fluidized Bed Boiler

INTRODUCTIONNumericalsimulationofcirculatingfluidizedbedcombustion(CFBC)isoneoftheimportantmeth-odsforthe0ptimizationofdesignandoperationandtheperformancepredictionofcirculatingfluidizedbedboiler.Researcheffortshavemademuchpr0gressinmodellingCFBfluiddynamics,heattransferandcom-bustioninthepastdecades.Duet0thecomplexity0fgassolidsfiowbehavi0rsinCFBcombustor,mostofthemodelsaresteadyone-dimensi0nal.Weissetal.(1987)[']devel0pedaCFBCcellmodel.ItdividestheCFBsystemintoaseriesofblockseach…     

生物质燃料循环流化床锅炉的模型化设计

生物质燃料在循环流化床（CFB）中的燃烧正在得到进一步的应用和开发。在燃煤CFB锅炉模型化设计的基础上，针对生物质燃料的特点和现有的CFB研究成果，进行了生物质燃料CFB锅炉的建模、程序设计和锅炉设计，同时对一些新的模型内容进行了介绍。模型的计算结果同已报道的试验结果较为吻合。     

Heat transfer on tube bundles embedded horizontally in a liquid‐fluidized bed

Heat transfer on tube bundles embedded horizontally in a liquid‐fluidized bed was investigated experimentally. In the experiment, a total of 5 kinds of tube bundles in an equilateral triangular staggered arrangement, including a single tube, was used. Tested particles were of glass and ceramics, and their diameter range was from 2.1 to 6.0 mm. It was found that the distribution of local heat transfer coefficients around a tube depends not on the kind of particles, but on the tube pitch only, when a good fluidizing condition is maintained. Based on the experimental data, a new method was proposed to predict average heat transfer coefficient, which can be applicable for tube bundles having a tube pitch to diameter ratio of 1.2 to infinity (single tube). © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 85–98, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20048     

An experimental and theoretical investigation into the heat transfer of a finned water wall tube in a circulating fluidized bed boiler

Heat transfer improvement in a water wall tube with fins was investigated in a circulating fluidized bed (CFB) boiler. Experiments were first conducted in a 6 MWth CFB boiler then a model was developed to analyse and interpolate the results. Temperatures at some discrete points within the wall cross‐section of the tube were measured by burying 0.8 mm thermocouples within a tube. Experimental data showed an increase in heat absorption up to 45 per cent. A good agreement between measured and predicted values was noted. The distribution of temperature in the metal wall and of heat flux around the outer wall of a tube with longitudinal and lateral fins was analysed by numerical solution of a two‐dimensional heat conduction equation. Effects of bed‐to‐wall heat transfer coefficient, water‐to‐tube inside heat transfer coefficient, bed temperature, water temperature and thermal conductivity of the tube material on the heat flux around the water tube are discussed. The present work also examines the influence of the length of the longitudinal fin and the water tube thickness. Heat flux was highest at the tip of the longitudinal fin. It dropped, but increased again near the root of the lateral fin. Copyright © 2000 John Wiley & Sons, Ltd.     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

循环流化床锅炉的现状及发展

循环流化床燃烧技术是一种清洁煤燃烧技术,以这种燃烧技术设计的锅炉具有较好的可靠性、经济性和环保性。分析了循环流化床锅炉旋风分离器的形式和整体布置的发展与改进,指出第三代采用方形分离器、冷却型紧凑布置的循环床燃烧技术将成为将来的发展方向。     

循环流化床锅炉的磨损及处理

以WZ - 35 3.82 -M9循环流化床锅炉的运行为例 ,浅谈其浓相区、分离器、燃尽室、回料口及省煤器等部位的磨损情况及处理方式     

Process simulation of a lignite-fired circulating fluidized bed boiler integrated with a dryer and a pyrolyzer

A process model using Aspen Plus was established to simulate the performance of a lignite-fired 1,025 t/h circulating fluidized bed boiler integrated with a dryer and a pyrolyzer for improving the energy efficiency. The combustion module of the proposed model was first verified by design data and the system characteristics of the circulating fluidized bed boiler combined with the coal dryer and the pyrolyzer were then investigated. The most suitable case for the retrofit of existing 1,025 t/h CFB boilers is that 35% of the coal was dried and then pyrolyzed with the energy efficiency of 95.3% and exergy efficiency of 50.2%. The results demonstrate the lignite-fired circulating fluidized bed boiler integrated with a dryer and a pyrolyzer facilitates the system performances effectively.     

Heat transfer characteristics and particle behavior around circular tubes immersed in fluidized beds

In this study, the particle velocity, particle volume fraction (attenuation of transmission light), and heat transfer coefficient were simultaneously measured for a horizontal tube bundle immersed in a fluidized bed of 0.42 mm diameter glass beads. To improve the accuracy of the particle velocity measurements, an optical fiber probe composed of a combination of transmission and reflection types was designed. The particle velocities were calculated by using the cross-correlation method and the particle passing-time method, respectively. It was shown by comparison of particle velocities obtained by the two methods that the particle passing-time method gives more stable results. The particle velocities at the bottom and at the side of the tube were relatively large and appeared to have periodicity while those near the top of the tube were low. This particle contacting feature can be used to explain the heat transfer characteristics. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(5): 306–318, 1997     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.