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.     

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.     

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]     

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.     

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.     

A study of heat transfer in a circulating fluidized bed

An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 μm, 400 μm and 500 μm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m²K, 51 to 74 W/m²K, and 50 to 59 W/m² K for sand sizes of 200 μm, 400 μm and 500 μm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.     

A study on heat transfer for immersed tube in internally circulating fluidized bed

INTRODUCTIONThemethodhowtodealwiththedomesticandindustrialwasteswithoutfurthercontaminationisoneofthemostimportantenvironmentalissues.Fluidizedbedcombustor(FBC)hasadvantagesofhighcombustionefficiency)lowpollution,convenienceinpreprocessingbeforefedin...     

Operation control of circulating fluidized bed boilers

The heat balance of different circulating fluidized bed boiler designs is investigated for variations in input parameters, such as specific flue-gas amount and ash content of the fuel. A simple model and a corresponding diagram are developed to describe the boiler behaviour when conditions deviate from the nominal ones. The model shows how the furnace temperature is influenced in different ways in different boiler designs by changes of furnace heat transfer and of the mass flow of particles and flue gases. The model results are verified by measurements in one type of boiler. Finally, it is shown how the desired stable furnace temperature and stable heat flux distribution are in conflict during variations of the input parameters. As a consequence, a two-dimensional boiler control strategy is suggested.     

Combustion Model For Staged Circulating Fluidized Bed Boiler

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

Heat transfer characteristics in a small-scale fluidized bed boiler

Heat transfer characteristics in a small-scale fluidized bed boiler (2MW_th) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 75–55% and 25–45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 0·45 and 0·15 MWm⁻², respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100–300 W m⁻² K⁻¹, whereas that between flue gas and convective tube bank was 10–30 Wm⁻² K⁻¹. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300–800 W m⁻² K⁻¹ for in-bed tube and 30–150 W m⁻² K⁻¹ for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.     

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.     

3D modelling of radiative heat transfer in circulating fluidized bed combustors: influence of the particulate composition

A three-dimensional model is developed to predict the bed-to-wall radiative heat transfer coefficient in the upper dilute zone of circulating fluidized bed (CFB) combustors. The radiative transfer equation is solved by the discrete ordinates method and Mie scattering theory is applied to calculate the absorption and scattering efficiency factors of particles existing in CFB combustors. Empirical correlations calculate both spacial variation of solid volume fraction and temperature distribution at the wall. The model considers the influences of the particle properties (including particle size distribution, particle optical constants and solid composition) on the radiative heat transfer coefficient. Simulation results show that the particle properties have significant influences on the bed-to-wall radiative heat transfer coefficient in CFB combustors. A very good agreement of predicted results is shown with experimental data.     

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     

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     

Nitrogen oxides emission from a circulating fluidized bed combustor

Experiments were carried out in a pilot-scale circulating fluidized bed (CFB) coal combustor to investigate the mechanism of N₂O formation, nitrogen oxides (including NO_x and N₂O) emission and the effect of temperature, excess air ratio, recirculation ratio, etc. The concentrations of nitrous oxide and nitric oxide were measured along the height of the CFB furnace. N₂O concentration increased with height, and in the exit of the combustor N₂O reached the highest level. NO_x, however, decreased with height, showing the inverse trend compared with N₂O. The N₂O emission decreased sharply with the rise of temperature at the bottom of the combustor; at the same time, the NO_x concentration increased.     

循环流化床掺渣锅炉运行技术

循环流化床锅炉作为一种高效、节能、低污染的环保锅炉 ,越来越被中小型化肥厂所接受和认可。但其影响燃烧的因素较多 ,尚处在不断完善之中。本文就WZ - 35 3.82 -M9型循环流化床锅炉掺烧炉渣的运行情况 ,在点火及各参数的调整方面作了一定的技术总结。     

Selective non-catalytic reduction of flue gas in a circulating fluidized bed

Selective non-catalytic reduction can meet the requirements of the new National Emission Regulation due to the low NOx emission characteristic of circulating fluidized bed boilers. In this work, ammonia was injected into the simulated circulating fluidized bed flue gas as a reducing agent. Optimum reaction conditions were obtained as: temperature of 920°C, residence time of 0.6 s, and a normalized stoichiometric ratio (NSR₁) of 1.5. H₂, as an additive, made a shift of 170°C towards a lower temperature, while CH₄ made a shift of 80°C.     

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     

Experimental Study on an On—Line Measurement of High Temperature Circulating Ash Flux in a Circulating Fluidized Bed Boiler

A new kind of measuring method that may be used to measure high temperature circulating solid particles flux in a circulating fluidized bed boiler is studied in this paper. The measuring method is founded on the principle of thermal equilibrium. A series of cold tests and hot tests were carried to optimize the structure and collocation of water-cooling tubes and showed that the method had the advantage of simple, accurate, reliable and good applicability for on-line usage in a circulating fluidized bed boiler.     

循环流化床锅炉优点分析

循环流化床锅炉因具有燃料适应性广、负荷调节性强以及环保性能优良而日益得到人们的重视,并且是传统技术所无法实现的,正是由于这些技术优点,使循环流化床锅炉得以快速发展和广泛应用.