Numerical modeling of bed-to-wall heat transfer in a circulating fluidized bed combustor based on cluster energy balance

The bed-to-wall heat transfer in a circulating fluidized bed (CFB) combustor depends on the heat transfer contributions from particle clusters, dispersed/gas phase and radiation from both of them. From the available CFB literature, most of the theoretical investigations on cluster and bed-to-wall heat transfer are based on mechanistic models except a few based on mathematical and numerical approaches. In the current work a numerical model proposed to predict the bed-to-wall heat transfer based on thermal energy balance between the cluster/dispersed phase and the riser wall. The effect of cluster properties and the thermal boundary conditions on the cluster heat transfer coefficient are analyzed and discussed. The fully implicit finite volume method is used to solve the governing equations by generating a 2D temperature plot for the cluster and the dispersed phase control volumes. From this 2D temperature profile, space and time averaged heat transfer coefficients (for cluster, dispersed phase and radiation components) are estimated for different operating conditions. The results from the proposed numerical simulation are in general agreement with published experimental data for similar operating conditions. The results and the analysis from the current work give more information on the thermal behavior of the cluster and dispersed phases, which improves the understanding of particle and gas phase heat transfers under different operating conditions in CFB units.     

Effect of dilute and dense phase operating conditions on bed-to-wall heat transfer mechanism in a circulating fluidized bed combustor

In the present paper investigations are conducted on bed-to-wall heat transfer to water-wall surfaces in the upper region of the riser column of a circulating fluidized bed (CFB) combustor under dilute and dense phase conditions. The bed-to-wall heat transfer depends on the contributions of particle convection, gas convection and radiation heat transfer components. The percentage contribution of each of these components depends on the operating conditions i.e., dilute and dense phase bed conditions and bed temperature. The variation in contribution with operating conditions is estimated using the cluster renewal mechanistic model. The present results contribute some fundamental information on the contributions of particle convection, gas convection and radiation contributions in bed-to-wall heat transfer under dilute and dense phase conditions with bed temperature. This leads to better understanding of heat transfer mechanism to water-wall surfaces in the upper region of the riser column under varying load conditions i.e., when the combustor is operated under dilute and dense phase situations. The results will further contribute to understanding of heat transfer mechanism and will aid in the efficient design of heat transfer surfaces in the CFB unit.     

Effect of operating parameters on bed-to-wall heat transfer in a high temperature circulating fluidized bed

An experimental investigation was made to study the effect of some operating parameters on the bed-to-wall heat transfer in a 5.25 m-tall circulating fluidized bed having a 102 mm-square cross-section with a bed temperature varying from 350 to 1173 K, a superficial velocity from 4 to 8 m/s, and a bed inventory from 15 to 40 kg of sand with a mean particle diameter of 309 μm. Two heat flux probes were used at two different locations in the furnace for measuring the heat transfer coefficient. The experimental results were compared with those of other investigators and also with the data predicted from a proposed theoretical model.     

Numerical computation of a circulating fluidized bed combustor

A mathematical model to describe a circulating fluidized-bed combustor is presented. A modified two-phase model which was used in the bubbling fluidized-bed combustor is considered to simulate the dense zone of the bottom section. For the upper section of the bed the momentum and energy-balance equation are used to predict the temperature and velocity profiles for the gas and the particles. The model performs mass balances for the chemical gas species (O₂, H₂O, CO, CO₂ and SO₂) with consideration being given to the last for retention by limestone particles. The model is applied to typical conditions of a circulating atmospheric fluidized-bed boiler and the simulation results show the expected trends. © 1998 John Wiley & Sons, Ltd.     

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.     

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.     

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.     

Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.     

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.     

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.     

Experimental investigations on heat transfer from suspension to impact separators in the riser column of a circulating fluidized bed combustor

In the present work experiments are conducted to investigate the effect of operating parameters on heat transfer from bed to U-beam impact separators located in the top region of the riser column. The effect of suspension density and bed temperature on heat transfer from bed to the impact separators (test sections) are investigated. The experimental unit consists of a circulating fluidized bed riser column, which is 0.23 m×0.23 m in bed cross-section, 6.3 m in height with a return leg and back pass. The U-beam impact separators are located in the top region of the riser column. Furnace oil # 2 is burnt in the unit and the experimental investigations are conducted. Water is circulated through the U-beam impact separators. The presence of the impact separators in the top region of riser column helps in solids separation and also to absorb certain fraction of heat liberated in the furnace. The bed to U-beam impact separator heat transfer coefficient increases with suspension density due to increased particle concentration, which results in higher cluster and particle heat transfer. The heat transfer coefficient increases with bed temperature due to increased convection and radiation.     

Experimental methods of estimating heat transfer in circulating fluidized bed boilers

Four different experimental methods have been used for the estimation of the bed-to-membrane wall heat transfer in a 12 MW_th circulating fluidized bed boiler. The methods are compared for a case of normal operating conditions and the measured heat transfer coefficients are presented. In the central part of the combustion chamber where most of the cooling surface is located, the cross-sectional average suspension density normally varies in the range of 10–20 kg m⁻³ and the heat transfer coefficient is around 130 W m⁻² K⁻¹ with a scatter of ±15% due to the different methods. The methods are critically analyzed and the heat transfer data are compared with relevant literature data.     

CFD modeling of hydrodynamic and heat transfer in fluidized bed reactors

In this study, a gas–solid fluidized bed reactor has been simulated applying CFD techniques in order to investigate hydrodynamic and heat transfer phenomena. Reactor model predictions were compared with corresponding experimental data reported in the literature to validate the model. The results indicate that considering two solid phases, particles with smaller diameters have lower volume fraction at the bottom of the bed and higher volume fraction at the top of the bed. In addition, it was revealed that bed expansion was larger when a bimodal particle mixture was applied compared with the case of mono-dispersed particles. Gas and solid phase temperature distributions in the reactor were also computed, considering the hydrodynamic of the fluidized bed and the heat generated by the solid particles. The results showed that gas temperature increases as it moves upward in the reactor due to the heat of polymerization reaction leading to the higher temperatures at the top of the bed.     

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]     

Augmentation of heat transfer by deflectors on circulating fluidized bed membrane walls

It is shown experimentally that the addition of angled deflectors to the fin region of membrane water-wall heat exchanger surfaces in circulating fluidized beds can lead to a significant increase in local and overall suspension-to-wall heat transfer. The experiments were carried out in the 12 MWth circulating fluidized bed (CFB) boiler at Chalmers University. The results are consistent with calculations based on renewal of packets traveling along the fin.     

Effect of riser exit geometry on bed hydrodynamics and heat transfer in a circulating fluidized bed riser column

This paper reports the variation of suspension density along the riser column and the effect of riser exit geometry on bed hydrodynamics and heat transfer in the upper region of a circulating fluidized bed (CFB) riser column. The experiments are conducted in a CFB riser column which is 102 mm × 102 mm in bed cross‐section (square), 5.25 m height, with a return leg of the same dimension. The unit is made up of interchangeable plexiglass columns. The superficial primary air velocity is varied between 4.2 and 6.4 m/s. The suspension density profile along the riser height is influenced by the exit geometry. With a 90° riser exit geometry, the suspension density profile in the upper region of the CFB riser column increases towards the riser exit. This particular trend has been observed for about 2 m length in the top region of the riser. The change in suspension density profile in the top region influences the variation of heat transfer coefficient. With a 90° riser exit geometry, the suspension density increases towards the riser exit, which in turn increases the heat transfer coefficient. The effect of riser exit geometry on hydrodynamics and heat transfer is significant for about 2 m length in the upper region of the riser column. Copyright © 2001 John Wiley & Sons, Ltd.     

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

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

Numerical investigation of gas-to-particle cluster convective heat transfer in circulating fluidized beds

Convective heat transfer between gas and cluster in a circulating fluidized bed (CFB) riser is numerically studied using a three-dimensional computational fluid dynamic (CFD) model. Distributions of gas velocity and temperature as well as the gas-cluster heat flux and convective heat transfer coefficient are obtained. Variations of heat flux of each particle in the cluster are predicted. The heat flux of the individual particle inside the cluster is smaller than that of an isolated particle. The convective heat transfer coefficients increase with the increase of cluster porosity and Reynolds number. The convective heat transfer coefficients of the downward moving cluster are larger than that of upward moving cluster. Numerical results of an isolated particle are in agreement with data from previously published correlations.     

Suspension-to-wall heat transfer in a 12-MWth circulating fluidized-bed combustor

An experimental investigation was carried out to study the effects of operating parameters on the local suspension-to-wall heat transfer in the combustor of a 12-MW_th circulating fluidized-bed (CFB) boiler. The heat transfer coefficients were measured with a conduction-type heat flux meter at five different vertical levels. The measurements covered a range of superficial gas velocities from 4 to 6 m/s, a bulk bed temperature from 800 to 850 °C and a suspension density from 6 to 70 kg/m³ for 270-μm silica sand particles. The heat transfer coefficient for the membrane wall in the combustion chamber of the CFB boiler was in the range of 100 to 180 W/m² K for the range of operating conditions employed in this work. The heat transfer coefficient decreased with increasing height and increased with increasing bulk bed temperature, superficial gas velocity and suspension density. Based on the experimental data, a simple correlation is proposed for predicting the suspension-to-membrane wall heat transfer coefficient. The results were analysed and compared with the experimental data of other workers.