CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

Hydrodynamics of a gas-driven gas-liquid-solid spouted bed with a draft tube

A cylindrical gas-liquid-solid spouted bed, driven exclusively by gas flow, has been developed with a high potential for use in biochemical processes, such as a biological wastewater treatment. A plexiglass column with a 152 mm inner diameter was used in combination with a 53 mm inner diameter plexiglass draft tube. Three particle types were studied with densities ranging from 1044 kg/m³-1485 kg/m³ and average particle sizes ranging from 0.7-2.5 mm. Four flow regimes were observed when increasing the gas velocity, including fixed bed, semispouted bed, full spouted bed, and internal circulating fluidized bed. The transition gas velocities between those regimes were experimentally measured and termed as minimum spouting velocity, full spouting velocity, and minimum circulating velocity, respectively. A measurement of the downward particle flux in the annulus was used to identify the minimum spouting velocity, while the particle velocity and dense phase retraction in the annulus were monitored for the full spouting and minimum circulating velocities. All regime transition velocities increased with more dense particles and longer draft tubes. The minimum spouting velocity and full spouting velocity were not affected when varying the nozzle-tube gap, while the minimum circulating velocity increased with longer nozzle-tube gaps. Experiments without a draft tube were carried, though the spouting stability was significantly reduced without the draft tube.     

Particle mixing/segregation in gas-solid fluidized bed of ternary mixtures

The particle concentration profiles and minimum fluidizing velocity of ternary mixtures were investigated experimentally. All the experiments were carried out in an 88 mm I.D. transparent acrylic column containing fluidized beds of ternary particles of different sizes, densities and shape. Mixing/segregation patterns were visualized under different operating conditions. The experimental results are compared with empirical relationships for mixing index and take-off velocity. A new definition of the mixing index, including particle minimum fluidizing velocity and density predicted the mixing/segregation behaviour reliably. The proposed correlation of take-off velocity agrees with the experimental results very well.     

Simulation of bubbling fluidized bed of fine particles using CFD

Computational fluid dynamics (CFD) simulation for bubbling fluidized bed of fine particles was carried out. The reliability and accuracy of CFD simulation was investigated by comparison with experimental data. The experimental facility of the fluidized bed was 6 cm in diameter and 70 cm in height and an agitator of pitched-blade turbine type was installed to prevent severe agglomeration of fine particles. Phosphor particles were employed as the bed material. Particle size was 22 μm and particle density was 3,938 kg/m³. CFD simulation was carried by two-fluid module which was composed of viscosity input model and fan model. CFD simulation and experiment were carried out by changing the fluidizing gas velocity and agitation velocity. The results showed that CFD simulation results in this study showed good agreement with experimental data. From results of CFD simulation, it was observed that the agitation prevents agglomeration of fine particles in a fluidized bed.     

Segregation in a stirred fluidized bed

A bed of particulate solids supported by an upward current of gas can be stirred at moderate power input per unit volume using thin horizontal rods mounted on a vertical shaft. The bed is fluidized, but bubbles are suppressed below a critical value of the fluidizing velocity. Stirring increases bed bulk density and reduces the minimum fluidizing velocity. Segregation in a stirred fluidized bed is enhanced with both flotsam and jetsam tracer particles, but the rate of segregation is reduced with flotsam tracers. The stirred fluidized bed may be useful as a device for dry separation of solids.     

Experimental studies and empirical models for the prediction of bed expansion in gas–solid tapered fluidized beds

Studies in the expansion behaviour of tapered fluidized bed systems are important for specifying the height of the bed. Data have been obtained on the expanded heights of tapered fluidized beds and bed expansion ratios for spherical and non-spherical particles have been calculated. Based on dimensional analysis, models have been developed as a function of geometry of tapered bed, static bed height, particle diameter, density of solid and gas and superficial velocity of the fluidizing medium. The data used to derive the models cover a wide range of operating conditions, with varying fluidization velocities. Effects of static bed height, particle diameter, density, tapered angle and superficial gas velocity over minimum fluidization velocity on bed expansion ratios have been investigated experimentally. A comparison has been made between the calculated values of bed expansion ratios using proposed models and the experimental data. It has been seen that calculated values by models agree well with the experimental values. Models have also been compared with literature data of conventional bed and found its applicability at higher gas velocities with good accuracy.     

CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

Abstract

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

振动流化床双组分颗粒的混合与分离

研究了振动流化床中颗粒的轴向浓度分布及最小流化速度,根据不同操作条件下最小流化速度的实验值,定义了最小流化速度的计算式;同时考察双组分颗粒混合与分离情况,根据实验结果,修正了Ｎｉｅｎｏｗ定义的双组分颗粒的转变气速,并与实验值相符。     

Effect of temperature on slug properties in a gas fluidized bed

In order to understand the effect of temperature on slug properties, the onset velocity of slugging, slug rising velocity and slug frequency have been measured by a differential pressure method in an electrically heated gas fluidized bed of 0.1 m i.d. and height of 1.97 m and, with respect to variations in gas velocity (U_mf∼0.15 m/s) and temperature (25-400 °C). Air was used as fluidizing gas and fluid catalytic cracking (FCC) catalyst particle (Geldart's group-A particle, d_p=0.071 mm, ρ_p=1600 kg/m³) as bed material. The minimum slugging velocity was found to increase a little with bed temperature. The qualitative change in minimum slugging velocity was found to agree with the inverse of minimum fluidizing velocity as temperature was varied. As the bed temperature increased, slug frequency was found to decrease a little, whereas slug rising velocity increased. A correlation between slug rising velocity and bed temperature was proposed.     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3～20 mm的4个气-固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响.结果 表明,气-固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异.在固定床阶段,与B类颗粒相比,A类颗粒与...     

Pressure drop,bed expansion and liquid holdup in a three phase spouted bed contactor

Data on the pressure drop, bed expansion and liquid holdup in a three phase spouted bed contactor with an initial bed height of 243 mm. were obtained as a function of the gas and liquid flowrate. Polyethylene spheres 10 mm. in diameter with a density of 320 kg/m³ were spouted in a 194 mm. column using a 30 mm. nozzle. The spouted bed contactor with gas and liquid mass flow-rates of 2.18 and 1.88 kg/m² sec, respectively had similar pressure drop per unit area of particle surface, total liquid holdup per unit volume of operating bed, and “active” holdup, as a fluidized contactor.     

液固流化床的混合离析模型

本文从颗粒的微观运动出发，采用随机过程模型描述了定常态下液固流化床内二元颗粒的混合和离析现象。在φ１０４×１５００ｍｍ的流化床内，以水为流化介质，对两种不同大小的同种颗粒二元体系在不同流速、不同组成、不同粒径比情形下进行了测定，确定了模型参数关联式，获得了颗粒沿床层的浓度分布关系。将模型计算值与实验值及文献值作了比较，证实了该模型的可行性     

流化床内自由移动石墨球表面的传热系数

密相区内自由移动的煤颗粒表面传热系数是循环流化床锅炉设计和运行的重要参数。利用石墨球模拟煤颗粒,在小型流化床实验台上对由粒度较小的石英砂颗粒组成的密相区内自由移动的石墨球表面传热系数进行了测量。测量结果显示,随着流化风速的增加,石墨球表面传热系数首先升高,当流化风速达到某一临界值时,继续增大流化风速,传热系数将保持不变,从传热的角度证明了流化床内煤颗粒基本停留在乳化相内。在多数情况下,石墨球表面传热系数随床料粒度的增大而减小。而在较低流化风速的情况下,随着床料粒度的增大,石墨球表面传热系数呈先下降后升高的趋势。当流化风速和床料粒径保持不变时,石墨球表面传热系数随着石墨球直径的增大而减小,且下降的趋势随石墨球直径的增大而减弱。而随着床层高度的增加,石墨球表面传热系数将会略有 升高。     

气固流化床气泡发生频率的研究

在单孔二维气固流化床中(292mm×16mm)用高灵敏度电容探针研究气泡发生频率.以频谱分析仪分析气泡频率分布曲线.考察了一系列参数对气泡频率功率分布密度曲线的影响,其中包括颗粒直径(0.105—0.590mm),颗粒重度(590—2990kg/m~3),颗粒最小流化速度(0.0072—0.481m/s),床层初始高度(205—565mm),探针离孔口垂直距离,孔口气体流率(0.5—35×10~(-4)m~3/s)以及床层辅助流化气速(0—3倍最小流化速度)等.对于重度低的小颗粒流化床,单孔气泡发生频率符合Davidson和Harrison早先推导的模型.随着颗粒直径和重度的增大,实验数据与上述模型呈有规律的偏差.本文提出气体从形成中气泡的顶半球以最小流化速度值向乳浊相泄漏的模型,推导了气泡发生频率的基本方程.以本研究的泄漏模型,用数值计算方法在计算机上计算的气泡发生频率与实验数据相吻合.     

The ignition temperature of lignite char in a fluidized bed combustor

This paper summarises the experimental and modelling work carried out for the variation of bed ignition temperature of a fluidized bed combustor with the char particle diameter and the fluidizing velocity. A lignite char was used and its reactivity was represented using data from Field (1967) and Turnbull and Davidson (1984). The modelling involved solving the steady state heat balance around the fluidized bed combustor at the ignition temperature. A correlation of the total area of char ignited per unit bed mass was determined as a function of the char particle diameter and the fluidizing velocity. This correlation was used to determine the ignition temperature of the fluidized bed combustor operating at different conditions. The fluidized bed combustor heat balance was then solved for the bed ignition temperature which was influenced by both the rate of heat loss from the bed and the reactivity of the char. A sensitivity analysis suggests that the chemical rate reaction coefficient is the most prominent variable when determining the ignition temperature of a fluidized bed combustor.     

颗粒与分布板的碰撞声信号分析及其流化状态的识别

引言 在气固流态化过程中,流型的不同对反应装置内气固接触、传热、传质都有重要影响,并直接关系着反应器的生产能力、收率和选择性.     

Transition from Low Velocity to High Velocity in a Three Phase Fluidized Bed

The onset liquid velocity demarcating the conventional and the circulating fluidization regimes of three‐phase fluidized beds was determined by measuring the time required to empty all particles in a batch fluidized bed at various liquid and gas velocities. Experiments were performed in a gas‐liquid‐solid circulating fluidized bed of 2.7 m in height using glass beads of 0.508 mm in diameter as solid phase and air and tap water as the fluidizing gas and liquid, respectively. The results show that gas velocity is a strong factor on the onset liquid velocity. Higher gas velocity yields a lower onset liquid velocity. It is also demonstrated that the onset liquid velocity has the same value as particle terminal velocity in a gas‐liquid mixture. Within the gas‐liquid‐solid circulating fluidization regime, the solids circulation rate is increased with the total liquid velocity and the auxiliary liquid velocity.     

Effect of Scattering by Fluidization of Electrically Conductive Beads on Electrical Field Intensity Profile in Microwave Dryers

Abstract:

A technology to apply a fluidized bed of electrically conductive beads is proposed to improve uniformity of the electric field intensity in microwave dryers, which are required for uniform heating of wet media. The principle of this effectiveness lies in a dynamic random scattering of microwave due to motion of the conductive beads in the bed. The electrically conductive beads were prepared by wrapping aluminum foil around styrene foam balls with cellophane tape. The diameter and density were 13 mm and 123 kg/m³, respectively. The minimum fluidizing velocity of these beads agreed with the one predicted by the Wen–Yu equation when the distributor was a porous plate. However, the fluidization of the beads took place at a lower gas rate than the minimum fluidizing velocity due to a jet flow from the small pores for the distributor of a perforate plate. The intensity in the applicator of a commercial microwave oven became the most uniform when the beads were fluidized with a uniform holdup profile along the height of the bed placed in front of the applicator walls. The dynamic effect making the intensity uniform by the fluidization was advanced by increasing the area of apparent reflection by the fluidized bed and the holdup of beads and was superior to a conventional stirring blade.     

空气重介质流化床流化特性及密度梯度分布研究

为实现物料的有效分选,以磁铁矿粉和玻璃微粉为混合加重质,研究了混合加重质的流化特性及空气重介质流化床床层密度梯度分布情况。结果表明:空气重介质流化床形成了均匀稳定的流化状态,当流化气速大于7.10 cm/s后,床层压降基本维持在510 Pa,床层密度基本不变,为1.71~1.74 g/cm3。当流化气速为7.95 cm/s时,流化床内气泡直径为15~25 mm,且分布均匀,流化床各层平均密度从上至下依次为1.72、1.74、1.74、1.74、1.73 g/cm3。流化床上部区域,超微细玻璃微粉被气流带到床层表面,使表面床层密度较小;流化床底部区域,气体分布相对均匀,并未形成大气泡,使该区域流化床床层平均密度偏小;而床层大部分区域床层平均密度均为1.74 g/cm3,比较稳定。因此,当流化气速为7.95 cm/s时,流化床内并未形成明显的分层和分级现象,说明加重质混合比较均匀,为空气重介质流化床分选物料创造良好条件。     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.