A method of determining thermal parameters of granular coal particles during devolatilization

Fluidized bed combustion offers great potential for the utilization of high-sulphur coal and low-grade coal in an environmentally acceptable manner. Utilization of fluidized bed technology, especially for the combustion of low quality lignites, enables pollutant emission control as well as efficient combustion. The most important stage during the combustion of coal particles is devolatilization, in which various factors such as heat transfer from the surroundings to the particle surface, heat conduction within the particle, the chemicals involved, the kinetics and the transport of volatile compounds within the particle play significant roles. The heat transfer coefficient, thermal diffusivity, thermal conductivity, heating coefficient and lag factor are the most significant thermal parameters in this process. In this study, a 1-D transient heat transfer analysis is carried out for a granular coal particle during devolatilization in a fluidized bed. The particle is idealized as a spherical solid body. Models are developed to determine the thermal parameters of such particles, and are verified using experimental centre temperatures of a 10 mm granular coal particle subjected to devolatilization at a medium temperature of 960 K. The data are taken from the literature. The results show that the thermal parameters determined here are in good agreement with experimental findings.     

流化床-煤粉复合燃烧锅炉的炉膛传热计算方法

针对流化床－煤粉复合燃烧锅炉的特点,在综合考虑流化床、火焰和受热面之间换热的基础上,推导了流化床－煤粉复合燃烧锅炉炉膛传热计算的基本方程,得到了复合燃烧锅炉炉膛传热计算的零维模型半径验法。以某７５ｔ／ｈ树皮流化床－煤粉复合燃烧锅炉为例,进行了炉膛传热计算。图１表６参５     

树皮在复合燃烧锅炉流化床内的燃烧份额

分析树皮流化床－粉复合燃烧锅炉中流化床的热平衡,得到了树皮在流化床中燃烧份额的计算公式。以某７５ｔ／ｈ树皮流化床－煤粉复合燃烧锅炉为例,分析了流化床温度,汉化床出口名义过量空气系数,热空气温度,擀 管吸热份额及流化床未燃对皮所含水分蒸发耗热对燃烧份额的影响。     

Direct‐contact heat exchange between fluidizing particles and a heat transfer surface in a fluidized bed: Temperature visualization of fluidizing particles

Heat conduction during contact between a heat transfer surface and fluidizing particles, a phenomenon which is one of the effective heat transfer mechanisms in a gas–solid fluidized bed, has been empirically investigated. The temperature profile of the fluidizing particles during the contact period is visualized with the aid of an infrared imager. The visualization reveals that the particles have been considerably heated in the thermal boundary layer on the heat transfer surface before contact. Based on the visualized temperature profile of the particles, the contact conductance between a fluidizing particle and the heat transfer surface is estimated by an in inverse analysis. Using the evaluated contact conductance, the contributions of the conductive heat transfer to the total heat transfer are also evaluated. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 165–181, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10027     

Heat transfer on tube bundles embedded horizontally in a liquid‐fluidized bed: 2nd report: On tube bundles of fundamental layouts including in‐line layouts

Heat transfer coefficients were measured on tube bundles of fundamental layouts including in‐line layouts embedded horizontally in a liquid‐fluidized bed. Tested tube layouts were single tubes, transverse single tube rows, longitudinal single tube rows, and in‐line arranged tube bundles. A total of 7 kinds of particles were used. Comparisons of the experimental data showed a good agreement with the heat transfer correlation developed for staggered layouts, when the average liquid velocity through each tube bundle was used as the reference velocity for the particle Reynolds number. Distribution of the local heat transfer coefficient was also investigated around tubes. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20245     

基于灰颗粒的物理特性为分类原则的试验研究

循环流化床(CFB)锅炉炉内的燃烧及传热与炉内床料的状态密切相关,而炉内床料主要是由燃煤含有的矿物组分经过燃烧、爆裂和磨耗过程形成的。文中对6种煤样在固定床燃烧后,使用可视化显微仪,获取了灰颗粒的微观形貌特征,根据灰颗粒的机械强度和耐磨性能的不同,将灰颗粒定义为3类不同性质的灰。以此为基点,采用固定床燃烧后冷态振动筛分和流化床实验台热态流化后筛分的方法,研究了不同燃烧温度下升温速率对灰颗粒粒径变化的影响,以及不同燃烧温度下燃烧时间对灰颗粒粒径变化的影响,推演了不同煤样在燃烧过程中的演化特征。结果表明:3类灰颗粒在不同的燃烧温度和时间的演化过程存在明显的不同,从而为预测循环流化床中的床料粒径分布提供了理论依据。     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.     

Further study on the influence of particle coating on fluidized bed heat transfer

Heat transfer to an immersed sphere from fluidized uncoated sand particles of different mean size and size distribution is compared with that from coated sand particles of equal size extracted from two full-scale fluidized bed boilers for different superficial gas velocities and mean particle diameters from 350 to 646 μm. The thin coating on the sand bed particles from full-scale boilers was found to have a significant effect on the heat transfer coefficient, while the particle size distributions, as well as coating thickness, had little or no influence on the heat transfer coefficients for the conditions investigated.     

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     

Study of immersed heat exchange surface for high-efficiency heat recovery from wire-rim tires in a fluidized-bed boiler

In an internally circulating fluidized-bed (ICFE) boiler, the fluidized bed is separated by a partition into main combustion and heat recovery chambers. The flows in these chambers are generated by using silica sand as the fluidizing medium. To determine the overall heat transfer coefficient (HTC) of the boiler's panel-type immersed heat transfer tubes, combustion tests were performed with wire-rim tires. The overall (HTC) of a panel tube array was lower than that of a zigzag tube arrangement. In practice, the heat absorbed by the fins makes the coefficients of either type of tube array almost identical. The air flowrate in the circulating bed at the bottom of the heat recovery chamber can be changed to control the overall HTC to a value virtually identical with that of a zigzag tube array. The combustion of wire-rim tires leads to a buildup of wires in the zigzag array, hampering the transfer of heat. Yet, the panel-type array showed no buildup so that it was possible to maintain steady operation with this type of tube arrangement. © 1997 Scripta Technica. Inc. Heat Trans Jpn Res. 25 (2): 120–134, 1996     

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.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

Bubble behavior in vertical tube banks installed in a fluidized bed

Fluidized bed combustion is one of the advantageous technologies for coal and/or incineration firing especially with respect to the environmental protection of emissions, such as NO_x/SO_x. Bed material movement in such a fluidized bed has a prime importance in the heat transfer process. Thus, quantitative measurement of the bed material movement and the void fraction are indispensable for better understanding of the fluidized bed. In this investigation, neutron radiography is applied to visualize the bed material movement in a simulated fluidized bed heat exchanger installed with vertical tubes. Bubble behavior and void fraction profile are obtained by the image processing technique. Bubble movement is highly restrained by these vertical tubes, so that the bubbles rise up along the tube. The bubble diameter is well correlated by the modified Mori and Wen's correlation taking into account the pitch of the tube arrangement. The bubble rise velocity and void fraction are well correlated by applying the drift‐flux model. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 727–739, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10127     

Augmentation of boiling heat transfer from horizontal cylinder to liquid by movable particles

This paper presents a series of experimental results on a passive augmentation technique of boiling heat transfer by supplying solid particles in liquid. A cylindrical heater 0.88 mm in diameter is placed in saturated water, in which a lot of mobile particles exist, and the nucleate and film boiling heat transfer characteristics are measured. Particle materials used were alumina, glass, and porous alumina, and the diameter ranged from 0.3 mm to 2.5 mm. Particles are fluidized by the occurrence of boiling without any additive power, and the heat transfer is augmented. The maximum augmentation ratio obtained in this experiment reaches about ten times the heat transfer coefficient obtained in liquid alone. The augmentation ratio is mainly affected by the particle material, diameter, and the height of the particle bed set at no boiling condition. The augmentation mechanism is discussed on the basis of the experimental results. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 28–41, 2002     

Fouling Reduction Characteristics of a No-Distributor-Fluidized-Bed Heat Exchanger for Flue Gas Heat Recovery

Heat exchangers and heat exchanger networks are extensively used for the purpose of recovering energy. In conventional flue gas heat recovery systems, the fouling by fly ashes and the related problems such as corrosion and cleaning are known to be major drawbacks. To overcome these problems, a single-riser no-distributor-fluidized-bed heat exchanger is devised and studied. Fouling and cleaning tests are performed for a uniquely designed fluidized bed-type heat exchanger to demonstrate the effect of particles on the fouling reduction and heat transfer enhancement. The tested heat exchanger model (1 m high and 54 mm internal diameter) is a gas-to-water type and composed of a main vertical tube and four auxiliary tubes through which particles circulate and transfer heat. Through the present study, the fouling on the heat transfer surface could successfully be simulated by controlling air-to-fuel ratios rather than introducing particles through an external feeder, which produced soft deposit layers with 1 to 1.5 mm thickness on the inside pipe wall. Flue gas temperature at the inlet of heat exchanger was maintained at 450°C at the gas volume rate of 0.738 to 0.768 CMM (0.0123 to 0.0128 m³/sec). From the analyses of the measured data, heat transfer performances of the heat exchanger before and after fouling and with and without particles were evaluated. Results showed that soft deposits were easily removed by introducing glass bead particles, and also heat transfer performance increased two times by the particle circulation. In addition, it was found that this type of heat exchanger had high potential to recover heat of waste gases from furnaces, boilers, and incinerators effectively and to reduce fouling related problems.     

Combustion Model For Staged Circulating Fluidized Bed Boiler

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

Unsteady thermal flow analysis in a heat regenerator with spherical particles

Heat regenerator occupied by regenerative materials improves thermal efficiency of regenerative combustion system through the recovery of sensible heat of exhaust gases. By using one-dimensional two-phase fluid dynamics model, the unsteady thermal flow of regenerator with spherical particles, were numerically analysed to evaluate the heat transfer and pressure drop and to suggest the parameter for designing heat regenerator. It takes about 7 h for the steady state in the thermal flow of regenerator, where heat absorption of regenerative particle is concurrent with heat desorption. The regenerative particle experiences small temperature fluctuation below 10 K during the reversing process. The thermal flow in heat regenerator varies with inlet velocity of exhaust gas and air, configuration of regenerator and diameter of regenerative particle. As the gas velocity increases with decreasing the cross-sectional area of the regenerator, the heat transfer between gas and particle enhances and pressure losses increase. As particle diameter decreases, the air is preheated higher and the exhaust gases are cooled lower with the increase of pressure losses. At the same exhaust gases temperature at the regenerator outlet, the regenerator length need to be linearly increased with inlet Reynolds number of exhaust gases. It is confirmed that inlet Reynolds number of exhaust gases should be introduced as a regenerator design parameter. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat transfer in a crater bed

The aim of this work is to study heat transfer in a laboratory scale crater bed, which was set up from a cylindrical acrylic/quartz tube, using sand as the bed particle. The bed employs a downward gas jet from a nozzle which causes the particles to ascend fountain-like into the freebroad, leaving a crater on the bed surface. After reaching a certain height, these particles will descend again to the bed surface and move into the crater, where the cycle or circulation pattern starts again. The study had been separated into three parts. Firstly, the void fraction of the bed fountain zone was studied by direct measurement of the ascending sand weight within the specific volume. Secondly, the convection heat transfer coefficients between the fountain zone and the external surface of the gas inlet tube were determined by measuring the quantity of heat loss from an electrical heater that was wrapped on the outside surface at desired positions of the gas inlet tube. Thirdly, the radiation heat transfer coefficients were evaluated by heat balance of LPG combustion in the crater bed. From experimental results, the void fraction of the fountain zone could be approximated as a dilute bed (>0.98). For convective heat transfer coefficients, the value found experimentally varied from 80–260 W/m² K depending on the experimental conditions, showing an increase when the gas velocity increases, and a decrease along the height of the gas inlet tube. Radiation heat transfer coefficients, the values of which are (within the experimental temperature range), the same order as the convective mode, increase when the bed temperature is increased and when the bed particle diameter is decreased. Empirical correlations for both bed voidage and heat transfer coefficients are proposed. The combined model, gas and particle convection and the published data on radiation heat transfer, showed good prediction when compared with experimental data.     

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.