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.     

高温流化床的流化特性及结焦非流化行为

在 8 0 mm× 30 mm和 80 mm× 10 mm石英流化床中 ,以低温粘结的高密度聚乙烯和聚丙烯 ,高温粘结的玻璃珠为实验物料 ,研究了高温流化床的流化特性及高温下物料结焦产生的非流化行为。结果表明 ,在本文实验条件下 ,Geldart A、B类高温表面粘结物料 ,床层温度小于其最小粘结温度时 ,床层温度增大 ,颗粒的最小流化速度减小 ;Geldart D类高温表面粘结物料的最小流化速度随温度增加而增大。得出了不同温度下颗粒最小流化速度预测式。床层温度大于最小粘结温度时 ,流化床需在较高的表观气速下才能保持流化 ,床层温度愈高床层流化所需的表观气速越大。研究同时发现 ,颗粒物料的粒径减小 ,流化颗粒的最小粘结温度减小。     

Heat transfer from an immersed surface in a magnetofluidized bed

Heat Transfer from an immersed horizontal cylindrical heat-transfer probe in an air-fluidized bed of iron shots exposed to an external magnetic field has been investigated. The probe is equipped to provide both total and local heat-transfer coefficients at different axial and angular positions. The fluidizing-air velocity and external magnetic-field intensities are varied to bring about different fluidization regimes starting from fixed-bed, magnetically stabilized-bed, partially stabilized-or bubbling-bed, and frozen-bed regimes. The nature of heat transfer is interpreted in all these regimes and conditions are identified where advantages of magnetic bed stabilization can be enjoyed without loss of heat-transfer rates.     

Investigations of pressure fluctuation history records of gas–solid fluidized beds

A transducer pressure probe is devised to measure the pressure‐history curves up to a maximum frequency of 200 Hz. It is installed on the wall of a 153 mm² fluidized bed and is employed to establish the solids mixing and movement pattern of an air fluidized bed comprising of glass beads of 2093 μm average diameter. The pressure variations recorded with a speed of about 11 Hz for a period of 92 s are employed to compute several statistical functions and analysed to infer the quality of fluidization. For the range of fluidization number 1.05–1.48, it is inferred that in this bubbling regime, the solids near the bed wall region descend down while the air bubbles drift to the central region and rise up in the fluidized bed. This bulk macroscopic hydrodynamic picture of solids movement is in conformity with the conclusions of other co‐workers. Copyright © 2000 John Wiley & Sons, Ltd.     

Two phase biomass air-steam gasification model for fluidized bed reactors: Part I—model development

A two-phase model capable of predicting the performance of fluidized bed biomass air-steam gasification reactor during dynamic and steady state operations was developed based on the two phase theory of fluidization. Material and energy balances were taken into consideration and the minimization of free energy technique was used to calculate the gas mole fractions. The fluidized bed was divided into three zones (jetting, bubbling and slugging) and the mass and heat transfer coefficients were calculated for each zone in both bubble and emulsion phases. The model includes the hydrodynamics, transport and thermodynamic properties of fluidized bed. The finite element method was used to solve the partial differential equations. The input variables of the computer program included fluidization velocity, steam flow rate and biomass to steam ratio. The model is capable of predicting the bed temperature, gas mole fractions, higher heating value and production rate.     

Heat transfer between a fluidized bed and an immersed vertical U-tube

The design details of a 0.254 m stainless steel cylindrical fluidized bed pilot plant facility, whose fabrication, installation and testing have been recently completed, are described. It primarily consists of a fluidized-bed reactor, fluidizing air-supply system, solids feeder, off-gas cleaning and exhaust system, and cooling water-supply system for heat-transfer tubes provided in the bed and in the freeboard sections. The plant is operated at ambient pressure in the temperature range 300–600 K, both in the batch and continuous modes for solids feed. Bed-pressure drop measurements as a function of fluidizing velocity for two different bed heights reveal that the quality of fluidization is good. Similar experiments have been conducted with the continuous solids feed. The heat-transfer coefficient between the bed and an immersed stainless steel U-tube is measured as a function of fluidizing air velocity at five different temperatures. The effects on bed-to-tube heat-transfer rate of solids feed rate, bed height, air-flow rate, and bed temperature are examined. All of these observed variations are interpreted in terms of the solids mixing and bubble mechanics in the bed.     

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     

Flow pattern identification of fluidized beds using ECT

IntroduchonPeople's understanding of the solids distribution inthe bottom zone of ~ating lluldized beds (CFB) isvery limited. There are divided assUInptions for thebottom aeon Of CFBs, e.g. turbulent flow and fastfiuldization (such as' ~ and the']) as oPPosed tobubbling fiuldized bed (e.g. Werther and Hirschberglz]).Also, different models ealst for the reactions in thisaeon, such as Plug flow model (Van Sw~['], Wen['],Fane and benal), anally dispersed Plug flow of gas(Avidant'], Edward…     

Heat-transfer characteristics in a rocking fluidized bed

Aiming at the development of coal-fired fluidized boiler for marine use, characteristics of fluidization at pitching or rolling states were studied. A two-dimensional fluidized bed with a cross section of 100 × 5 cm was set on a rocking stand and swung sinusoidally. It was found that the rocking causes both the formation of dead zones in the bed and the vigorous fluctuation of heat-transfer coefficients at the surface of water tubes.     

Fundamentals of turbulent gas-solid flows applied to circulating fluidized bed combustion

A summary is made of the present state of knowledge of turbulent gas-solid flow modeling and in particular its application to circulating fluidized bed combustion chambers. Models are presented to close the set of equations describing isothermal non-reacting turbulent gas-particle flows applied to fluidization, and it is shown under which assumptions the models can be derived. With the kinetic theory of granular flow, transport equations for the velocity moments and closure laws for the stress tensor and the energy flux are derived for the particle phase. Closure equations for the drift velocity and for the fluid-particle velocity correlation tensor are presented, first based on algebraic models and, second, based on transport equations with the fluid-particle joint probability density function. An alternative derivation of the fluid-particle velocity covariance transport equation is compared to the formulation based on the fluid-particle joint probability density function. Two-way coupling is discussed, and a transport equation for the second-order velocity moments is used to derive a two-equation model accounting for the modulation of gas phase turbulence by particles. Boundary conditions for the set of equations describing a turbulent gas-solid flow are discussed. Provided that the domain of applicability of the models is known, a discussion on the usefulness of the models is given, as well as an application to fluidization and especially to circulating fluidized bed combustors. Prospects for improvement of the existing models are presented.     

Fundamental design of a continuous biomass gasification process using a supercritical water fluidized bed

A novel biomass gasification (first stage of hydrogen production from biomass) process using a supercritical water fluidized bed was proposed and the fundamental design of the process was conducted. The flow rate was determined by evaluating the minimum fluidization velocity and terminal velocity of alumina particles enabling fluidization with the thermodynamic properties of supercritical water. Three cases were examined: a bubbling fluidized bed in which water was used mainly as the fluidized medium and biomass were added for gasification, a bubbling fluidized bed fluidized by biomass slurry feed alone, and a fast fluidized bed fluidized by biomass slurry feed alone. According to calculations of the residence time and thermal efficiency assuming heat recovery with a heat exchanger efficiency of 0.75, the bubbling fluidized bed fluidized by biomass slurry alone was appropriate for continuous biomass gasification using a fluidized bed.     

Wall-to-bed heat transfer in circulating fluidized beds

Heat Transfer from the wall of a circulating fluidized bed to the fast bed suspension has been investigated for several materials. The range of investigation includes dense and dilute phase fast fluidization and pneumatic transport. The overall heat transfer coefficient was found to be a function mainly of cross-sectional average suspension density. Effects of superficial velocity and solids mass flux were obscured by their interrelationship to the suspension density. Two models from the literature are evaluated using present and published data.     

隔板式内循环流化床的流动特性研究

以石英砂和稻壳为实验床料,在隔板式内循环流化床气化炉冷态实验装置上对颗粒的内循环流动特性进行了研究,考察了高速区和低速区的流化速度、结构尺寸和侧风量等对颗粒内循环流动的影响.结果表明:在保持低速区流化速度一定的条件下,随着高速区风速增大,颗粒循环量先增大后减小;流化速度不变的条件下,颗粒循环量随孔口和侧风量的增大而增加,但增加趋势逐渐变缓.实验给出了合理的运行设计参数.通过实验数据回归,得到了石英砂和稻壳通过隔板式内循环流化床孔口的颗粒循环量关联式,计算结果与实验值误差分别小于6%和14%,能较好地预测孔口颗粒流动.     

Effect of fluidization/gasification parameters on hydrogen generation in syngas during fluidized-bed gasification process

This study discusses the influence of fluidization and gasification parameters on the hydrogen composition in syngas. For gasification conditions, when Stage 1 and Stage 2 gasifier temperature is 900 °C, the hydrogen content in syngas is 35.59 mol.% when the activated carbon is used as bed material. For using zeolite as bed material, the hydrogen content is 38.25 mol.%. The hydrogen content is higher than that under other conditions, but if the Steam/Biomass ratio is increased to 0.6, the hydrogen content resulted from zeolite as bed material is the highest 39.38 mol.%. For fluidization parameters, when Stage 2 bed material size is changed to 0.46 mm, no matter the bed material is activated carbon or zeolite, the hydrogen content in syngas is the best among three particle sizes. In terms of operating gas velocity, when gas velocity is 1.5 U_mf, the hydrogen content is higher. For fluidization parameters, the two bed materials can increase hydrogen content in syngas effectively in Stage 2 fluidized bed, and their effects are similar to each other. However, considering the fluidization parameters, the hydrogen content in syngas when activated carbon is used as bed material is better than that when the zeolite is used.     

Hydrodynamics of air–sand flow in a conical swirling fluidized bed: A comparative study between tangential and axial air entries

Hydrodynamic regimes and characteristics of air–sand flow were studied in a cone-shape swirling fluidized bed for two types of air entry, or swirl generator: through a four-nozzle system (tangential entry) and using an annular-spiral air distributor (axial entry). Quartz sand of four particle sizes, 180–300, 300–500, 500–600 and 850–1180 μm, was used as the bed material in experimental tests on a cold model of a conical swirling fluidized bed combustor. During each test run, the pressure drop across the bed (Δp) was measured versus superficial velocity at the lower bed basis (u) for three static bed heights (20, 30 and 40 cm). Four regimes were found in the bed behavior for both swirl generators. The Δp–u diagrams were compared between tangential and axial air entries for different operating conditions. Mathematical models for predicting major hydrodynamic characteristics, the minimum fluidization velocity (u_mf) and corresponding pressure drop across the bed (Δp_mf), were empirically developed. The dimensionless dependency of Δp/Δp_mf on u/u_mf showed the apparent common trends and similarity for most of the test trials. For the two air injection systems, a Nomograph for assessment of Δp at any arbitrary superficial velocity and bed height was proposed as well.     

双室内循环流化床煤气化系统的冷态实验研究

本文发展并建了双室并列流化床煤气化系统的冷态模型。在此基础上,对影响气化系统操作的两个主要因素－两流化床之间颗粒物料的循环技术及流化床床层的膨胀特性进行了实验研究。     

循环流化床中气固两相流动特性的可视化研究

在自行设计并搭建的试验台上,利用CCD的图像快速采集功能,获得了不同气速下流化床内的流态图像,并用图像处理方法对这些原始图像进行了有效的处理。研究了贴壁回探流、气体局部扩散系数、局部空隙率随横向位置和高度的分布、气泡的上升速度,给出了实验中气泡边界曲线的分维数。实现了循环流化床内气固两相流体动力学特性的可视化,为图像处理技术应用到循环流化床作了有益的尝试。     

流化床锅炉燃烧室的流体动力学模拟

流化床的放大效应十分显著,使得流化床的试验研究在大型工业流化床设计中的应用发生问题。近年来在国外发展起来的流化床模拟放大技术是从流态化现象的基本控制方程出发,根据相似原理导出一组无量纲相似准则数。这些准则数建立了不同条件下(结构尺寸、温度、压力和床料特性等)的流化性能间相似的内在关系,对流化床的试验研究及设计实践有重要的指导意义及应用前景。     

多孔球三相流化床流动特性的研究

对多孔球为填料的三相流化床流动特性进行初步研究,得出多孔球三相流化床初始流化速度的计算公式;分别研究在无喷淋和有喷淋两种情况下床层的阻力特性,推导这两种情况下床层阻力的计算公式.试验气体流速、填料静止高度、喷淋密度等因素对流化床阻力特性的影响,并与空心球为填料的流化床阻力特性进行了比较.     

双循环流化床冷态实验研究

由主循环流化床(主床)和副循环流化床(副床)组成的双循环流化床冷态实验系统中,测量了不同工况下的压力分布以及主床和副床的物料循环率,分析了装料量、主床一次风流化速度和副床二次风流化速度对压力分布以及物料循环率的影响,为双循环流化床的工艺设计提供实验依据.