Hydrodynamic and heat-transfer studies in fine particle gas-fluidized beds

A 0.152 m internal diameter fluidized-bed pilot-plant facility designed to operate up to a maximum temperature of 1500 K at ambient pressures has been fabricated, installed and tested. High temperatures are obtained by the combustion of propane-air mixtures in the bed. A specially designed bubble cap distributor plate ensures good gas dispersion and solids mixing in the bed.Preliminary measurements dealing with hydrodynamic and heat-transfer characteristics in the moderate temperature range 330–500 K at ambient pressures in beds of fine powders of sand, glass beads and alumina particles are reported here. These include the bed pressure drop and bed expansion as a function of fluidizing velocity and bed temperature. The characteristics of the bubble cap air distributor plate are also investigated in view of its role in controlling the quality of fluidization. The heat transfer between the bed and an immersed water-cooled vertical tube is analyzed to compute the heat-transfer coefficient and its dependence on the superficial air-fluidizing velocity and on bed temperature. These data are critically examined in the light of mechanistic processes taking place in the bed.     

Fluidization characteristics of gas-fluidized beds: Air and glass beads system

Good gas-solids contacting and solids mixing are essential features, both for efficient coal combustion and desulfurization of coal in a fluidized-bed combustor. It will therefore be very useful if suitable procedures could be developed to characterize the quality of fluidization in gas-solid systems. Three different procedures are discussed in this article with reference to air and glass beads of four different average sizes:

1. (i) in one scheme, the bed voidage is measured at minimum fluidization and the variation of this quantity with Reynolds number at minimum fluidization is correlated with the powder classification scheme of Saxena and Ganzha and hence with the corresponding fluidization characteristics. A major effort is directed at substantiation and confirmation of this characterization scheme, which relates the fluidization behavior to the Reynolds number at incipient fluidization.

2. (ii) The local heat-transfer coefficient of a probe element immersed in the bed is also measured as a function of the superficial fluidizing velocity and the nature of variation is taken as representing the quality of bed fluidization.

3. (iii) A technique is developed to measure the time-history of the temperature fluctuation of a probe element and to relate this parameter to the bed-fluidization characteristics.

Additional experimental data with other solids will be generated in the future to develop these proposed three schemes further for the characterization of bed-fluidization behavior.     

循环流化床布风方式对颗粒循环流率的影响

在带中心提升管的内循环流化床冷态试验台上,针对锥形和平板形2种布风板布置方式下各控制参数对循环流率的影响进行了试验研究．结果表明：锥形布风板上的临界全部流化速度明显大于物料临界流化速度;随着气化室和提升管内风速的增加,循环流率增大,但增大的速率逐渐降低;气化室和提升管内风速以及料层高度对锥形布风板下循环流率的影响大于平板形布风板,但是物料粒径对锥形布风板循环流化床循环流率的影响却小于平板形布风板．     

Heat Transfer to Kraft Black Liquor in a Liquid?Solid Fluidized Bed

Wall-to-bed heat transfer coefficients to pulp mill Kraft black liquor in a nominal 25-mm tubular, liquid?solid fluidized bed have been determined. The fluidizing particles were stainless steel balls of 4.0, 4.76, and 6.35 mm diameter. The heat transfer coefficients were determined for conditions where porosity, bulk temperature, and liquor concentration were varied. The best correlation was obtained when the data were split into two groups of low and high liquor concentrations, i.e., 30?40% and 50?65% total dissolved solids. The data were also compared with the predictions obtained from 26 published correlations, of which the best 10 are presented and discussed in this article. The results of this investigation are relevant to other fluidized bed systems with high liquid viscosity, such as sugar concentrations, hydrocarbons, and polymers.     

高温肋板式蓄热器蓄/放热特性的数值模拟

采用计算流体动力学方法对高温不锈钢肋板式相变蓄热器的蓄/放热特性进行了数值模拟。分析了多孔肋片和锯齿肋片对蓄热器蓄/放热特性的影响以及载热体入口温度和流量对相变材料熔化和凝固速度的影响,计算结果表明:在该新型肋板式相变蓄热器中,多孔翅片的性能优于锯齿肋片;随着蓄热器传热温差的增大和载热体流量的增加,蓄热器的蓄/放热性能越好;肋片作为换热元件可以很好的提高蓄热器的蓄/放热性能。所得结论可为高温肋板式蓄热器的优化设计提供有益的参考。     

流化床启动过程中底料磨损行为研究

以国内首台自主研发的65t/h低倍率油页岩循环流化床锅炉底料灰为研究对象,在小型流化床试验台上考察了流化风速、流化时间和床层温度等因素对底料磨损程度的影响,据此建立了二阶磨损模型.结果表明：流化状态下页岩灰的磨损先后经历快速衰减和稳定磨损2个阶段,页岩灰在流化床内的质量保持率呈指数衰减趋势;随着流化风速增大,床层温度升高,磨损相对加剧;二阶磨损模型能够很好地描述流化床内页岩灰质量的变化规律.     

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。     

Investigations of temperature fluctuation history records of gas-solid fluidized beds

An experimental technique has been developed to delineate the different hydrodynamic regimes of a gas-solid fluidized bed, each regime signifying a characteristic type of fluidization. The technique involves the measurement and statistical analysis of temperature-history records of an element of the heat-transfer surface immersed in a fluidized bed in which different regimes are brought about by changing gas velocity. The experiments were conducted in a 0.153-m square fluidized bed of 2093-μm average diameter spherical glass beads at ambient conditions. The statistical functions computed at each gas velocity were the standard deviation, probability density function, autocorrelation function, and power spectral density function. The standard deviation and the major frequency defined from the power spectral density function were found to represent characteristically the bubbling, transitional and turbulent hydrodynamic regimes. The bed voidage and the local heat-transfer coefficient were also measured as functions of gas velocity. Their appropriateness to represent the quality of bed fluidization has been examined.     

Heat transfer investigations for in-bed and freeboard vertical U-tubes in a fluidized bed at moderate temperatures

The heat transfer coefficients for a single U-tube immersed vertically in fluidized beds of silica sands ($\text{d}\text{?}{}_{\mathrm{p}}\text{= 222, 488, and 778 μ}\text{m}$) and for a bundle of six hair-pin shaped nested vertical tubes mounted in the freeboard have been measured as a function of fluidizing velocity at bed temperatures ranging from 365 to 535 K. Simultaneous measurements of bed voidage were also made in each case. The effects of bed temperature, fluidizing velocity and particle size on in-bed and freeboard heat transfer coefficients and voidage are examined and explained.     

Devolatilization of wood particles in a hot fluidized bed: Product yields and conversion rates

Spherical wood particles of various well-defined diameters were fed into a bed of fluidized olivine maintained at temperatures in the range of 700–900 °C. The fluidizing gas was either nitrogen or a steam–nitrogen mixture, and the wooden spheres were introduced in two different ways: on to the bed surface and deep into the bed itself. The resulting gas, tar and char yields were found to be influenced by the method of feed introduction as well as by the bed temperature, fluidizing gas composition and wood particle diameter. The total devolatilization time was correlated by means of a power law expression similar to that used for coal devolatilization and having the advantage of being able to fit well the experimental data over the full temperature range in both nitrogen and steam–nitrogen atmospheres.     

Attrition of lignite char in a spouted bed combustor

The generation of carbon fines by attrition during burning of Thai lignite char has been studied experimentally by means of a 92 mm i.d. continuous spouted bed combustor at different values of spouting gas velocity, bed temperature, and char feed size. Both inert particle size and static bed depth were fixed for all experimental runs. The collected data were used to analyse size distributions of both in-bed particles and elutriated fines, and to generate the suitable correlations for carbon attrition rate. Results obtained showed that attrition rate in the spouted bed is proportional to the excess of gas velocity above the minimum spouting gas velocity and the overall bed carbon surface exposed to attrition. The attrition rate constant is slightly dependent on operating bed temperature. Its values for the char studied were 1.6511 × 10^?6 for 707°C operating bed temperature, and 1.1222 × 10^?6 for 850°C, with the average for all tested runs being 1.224 × 10^?6.     

二维喷动流化床内气体混合的实验研究

在300 mm×30 mm×2 000 mm的喷动流化床冷态试验台上,在喷动区和环形区分别采用CO和SO2作为示踪气体的方法,获得了不同喷动气流速和流化气流率这两个重要的操作参数下,不同床层高度上示踪气体的径向分布,考察了床内喷动区与环形区之间气体的混合特性。结果表明,示踪气体在不同床层高度的径向分布呈现较大的差异,并沿床层高度依次降低。在稳定的流动状态下,喷动气速度的增大,使喷动气向环形区的传质加强,也促进了流化气在环形区的混合;流化气流率的增大,气体由环形区向喷动区的传质大于由喷动区向环形区的传质,且流化气在环形区混合加强。在不稳定的流动状态下,气体的径向浓度分布曲线两边出现不对称。     

Modeling of a continuous fluidized bed dryer using artificial neural networks

The work involves experimentation on drying of solids in a continuous fluidized bed dryer covering different variables like bed temperature, gas flow rate, solids flow rate and initial moisture content of solids. The data are modeled using artificial neural networks. The results obtained from artificial neural networks are compared with those obtained using Tanks-in-series model. It was found that results obtained from ANN fit the experimental data more accurately compared to the RTD model with less percentage error. This indicates a better fit of artificial neural networks to experimental data compared to various mathematical models.     

Analysis of cold flow fluidization test results for various biomass fuels

A systematic theoretical and experimental study was conducted to obtain hydrodynamic properties such as particle size diameter, bulk density, fluidizing velocity, etc. for locally available biomass residue fuels in Malaysia like rice husk, sawdust, peanut shell, coconut shell, palm fiber as well as coal and bottom ash. The tests were carried out in a cold flow fluidization bed chamber of internal diameter 60 mm with air as fluidizing medium. The height of the chamber could be raised up to 630 mm by five separate cylindrical rings. Bed-pressure drop was measured as a function of superficial air velocity over a range of bed heights for each individual type of particle. The data were used to determine minimum fluidization velocity, which could be used to compare with theoretical values. The particle size of biomass residue fuel was classified according to Gildart's distribution diagram. The results show that Gildart's particle size (B) for sawdust, coal bottom ash, coconut shell have good fluidizing properties compared to rice husk, type (D) or palm fiber, type (A). The bulk density and voidage are found to be main factors contributing to fluidizing quality of the bed.     

Simultaneous estimation of heat-transfer rate and coolant fluid velocity in a transpiration cooling process

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to estimate the unknown time-dependent heat-transfer rate on the heated surface and coolant fluid velocity in a transpiration cooling process using temperature measurements of the porous medium and coolant fluid. It is assumed that no prior information is available on the functional forms of the unknown heat-transfer rate and coolant fluid velocity; hence the procedure is classified as the function estimation inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements. The accuracy of the inverse analysis is examined by using simulated exact and inexact temperature measurements. Results show that an excellent estimation on the time-dependent heat-transfer rate and coolant fluid velocity can be obtained for the test case considered in this study.     

运行参数对粉煤流化床_PC_FB_密相燃烧区燃烧特性的影响

在一座0．3MW热输入的煤粉流化床燃烧（PC－FBC）试验台上,研究了煤粉在其密相燃烧区内的燃烧特性,文中用密相燃烧区内煤粉挥发份的释放率V^τ（％）,及其出口处煤灰的碳含量C（％）,来描述此特性,并研究了床温、氧浓度、流化速度、二次风率时V^τ（％）,C（％）的影响规律。研究表明,煤粉在密相燃烧区内释放57．4％－84％的挥发分和燃烧20％－30％左右的固定碳,释放40％－50％的发热量。     

A study on heat transfer in a conical fluidized-bed reactor with an immersed cylindrical heater

In this paper, numerical and experimental analyses of the heat transfer between an immersed heater and a cone bed of sand particles were carried out. A three-dimensional (3D) model using the Eulerian–Eulerian model coupled with the kinetic theory for granular flow was used to simulate heat transfer and the related bed flow characteristics. The effects of different inlet gas velocities, represented by the fluidizing number (the ratio between inlet gas velocity to minimum fluidizing velocity), and different particle-wall boundary conditions on heat transfer and hydrodynamics were investigated. Both the experiments and numerical simulation results showed that the heat transfer coefficient and the bed expansion ratio increased with increasing the inlet gas velocity. For the particle-wall boundary condition, applying the no-slip condition showed the best agreement in the heat transfer coefficient and the bed expansion ratio to the experimental results.     

循环流化床冷态流动特性的试验研究

研究了循环流化床气固两相流动在轴向的宏观流体动力特性,采用计算机数据采集系统测量不同工况下沿主床高度上的压力分布,进而计算出空隙率沿循环流化床主床高度方向上的分布情况,研究不同操作条件(流化风速、存料量)对空隙率分布、物料循环量和压力平衡的影响。     

An iterative regularization method in simultaneously estimating the inlet temperature and heat-transfer rate in a forced-convection pipe

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to estimate the unknown space- and time-dependent inlet temperature and heat-transfer rate on the external wall of a pipe system using temperature measurements at two different locations. It is assumed that no prior information is available on the functional form of the unknown inlet temperature and heat-transfer rate; hence the procedure is classified as the function estimation in inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements. The accuracy of the inverse analysis is examined by using simulated exact and inexact temperature measurements. Results show that an excellent estimation on the space- and time-dependent inlet temperature and heat-transfer rate can be obtained for the test case considered in this study.     

Experimental study of pressurized gas-fluidized bed heat transfer

This paper reports on an experimental study of the influence of operating pressure, in the range 150-1100 kPa, on wall-to-bed heat transfer coefficient in a bubbling fluidized bed. Both Geldart Group A and B solids were studied and the fluidizing gases were air and superheated steam. Fluidizing velocities were in the range 1-33 U_mf and wall temperatures in the range 125-275°C. Wall-to-bed heat transfer coefficients were found to increase steadily with increasing fluidizing gas velocity and not to pass through a maximum. Increase in operating pressure was found generally to result in an increase in wall-to-bed heat transfer coefficient, although the effect is probably non-linear. In the bubbling regime, the wall-to-bed heat transfer coefficient was found to change with vertical position in the bed. Wall-to-bed heat transfer coefficients decreased when the bed entered the slugging regime.