Characterization of moisture distribution in a fluidized bed

The determination of moisture distribution in a sample of wet solids is important as it can provide information on how well a liquid reagent has been sprayed in a fluidized bed reactor. The moisture is divided into three groups: (i) free moisture, or water coating individual particles; (ii) micro-agglomerates, which are fluidized; and (iii) macro-agglomerates, which are too large to remain fluidized and, consequently, settle to the bottom of the bed. This study evaluates various methods for the determination of the moisture distribution in a sample of wet solids, such as funnel flow, angle of repose, compressibility index, avalanche tests and electrical conductance. In addition, a different method is implemented to obtain directly the moisture distribution from the bed fluidization quality, based on image processing and on the variation with time of the conductance of the de-fluidized bed after a liquid injection.A solids sample was taken from the fluidized bed. Its free moisture was estimated through the abovementioned tests and its overall moisture was used to obtain the fraction of water trapped in micro-agglomerates. The amount of water in the macro-agglomerates was then obtained by subtracting the measured free and micro-agglomerates moistures from the total mass of liquid injected into the bed.     

An experimental study on combustion efficiency of low grade anthracite in AFBC

This is the basic study to develop a fluidized bed combustion boiler which can use low grade anthracite. In this study, the anthracite of about 3400 kcal/kg was burned in the bench scale non-recycling atmospheric fluidized bed combustor of 200mm diameter and 2215mm height with the static bed height of 250mm and the combustion temperature range of 800–950°C. During the combustion, the effect of factors such as the superficial gas velocity in bed, the air ratio, the coal supply location and the coal particle size on the combustion efficiency, the elutriation ratio and the unburned carbon content both in elutriated ash and in drained ash was thoroughly analyzed. When the superficial gas velocity in bed is 0.7～2.2m/s, the air ratio is 1.0～1.6 and coal supply locations are 300,500 and 700mm above the air distributor, the combustion efficiencies range from 66% to 83.5% for the mean coal particle size of 0.209mm, and from 71% to 88% for the case of 0.265mm. The combustion efficiency decreases as the superficial gas velocity in bed and the air ratio increase. The lower the coal supply location is, the better the combustion efficiency becomes in general.     

Comparison of the radial profiles of particles velocity between invasive and non-invasive measurement techniques

In this work, the radial profiles of the particles velocity in gas-solid fluidized beds were measured using two techniques: (1) an advanced optical fiber probe, an invasive technique, and (2) radioactive particle tracking (RPT), a noninvasive technique. Two gas-solid fluidized bed columns of 0.14 and 0.44 m inside diameters were used to estimate the radial profile of the particles velocity. The particle velocity was measured at different operating conditions and axial heights. It was found that the implementation of the noninvasive radioisotope particle tracking (RPT) technique was able to successfully guide the measurement of the radial profile of the particle velocity using the optical fiber probe technique. The average inversion point of the radial particle velocity, from positive to negative, was identified from the measurements using both techniques for different operating conditions and axial heights.     

A system to estimate coarse particle velocities at the pipe wall in settling slurry flow

Understanding and modelling of coarse particle (settling) slurries in laminar flow in pipes is still not complete. Better analytical procedures will enable more efficient pipeline operation which could result in significant power savings in the transport of coarse materials. The depth and velocity of the settled bed significantly affect the pressure gradients required to pump these slurries and knowledge of these parameters is of importance. Existing measurement techniques that can be used for monitoring of these complex fluid systems are either very expensive, not easy to implement or mostly used under controlled laboratory conditions. A new measurement system was developed that is capable of detecting the deposition of solids, the depth of a settled bed and the velocity of coarse particles at the pipe wall, around the pipe circumference. Particle velocities were determined by cross-correlating modulated signals from pairs of electrodes mounted flush with the pipe wall, in contact with the slurry. Tests were conducted using a mixture of acetal beads in water, at bulk velocities between 0.5 and 4 m/s. Estimated particle velocities from the cross-correlation analyses were compared with those obtained using a 30 fps video camera combined with visual inspection, and found to be within ±6%, thus validating the viability of the system. Particle velocity resolution using the cross-correlation technique is limited by block size, sample rate and the measurement distance between electrode pairs. Further test work with a range of real slurries (different particle sizes, solids concentrations and rheologies) needs to be conducted, along with more extensive verification of the results, to establish the limits of the system. Initial testing and evaluation of the system, which is capable of coarse particle flow monitoring in real time, showed significant potential for development of a new commercial sliding bed detector that can be used in a wide range of industrial applications in which particles are transported hydraulically.     

Effect of particle ingestion on the fouling reduction and heat transfer enhancement of a No-Distributor-Fluidized heat exchanger

To overcome the fouling problem that is common in heat exchangers for waste heat recovery, a new type of fluidized heat exchanger was devised and tested. Fluidized bed heat exchangers are considered to be a good candidate for waste heat recovery flue gases due to their demonstrated ability to avoid fouling or to clean out deposition on heat transfer surfaces, but have a major drawback with significant pressure losses. These pressure drops typically associated with the distributor plate, which is a key component in constructing any conventional fluidized bed system, limit the applicability of fluidized bed heat exchangers for use as an energy saving device. In a new design, however, dilute gassolid particulate is maintained without having a distributor plate. The main feature of this no-distributor-fluidized (NDF) heat exchanger is the self-cleaning action by ingested circulating particles at minimal additional pressure loss. In the present study, a multi riser NDF heat exchanger of 7,000 kcal/hr capacity was built to evaluate its heat transfer performance and fouling reduction characteristics. To experimentally simulate the fouled condition, fuel rich combustion gas with soot was introduced to the heat exchanger, then a cleaning test was performed by introducing glass bead particles (600μm) inside the gas passage of the heat exchanger unit. Through the present experimental study, the performance degradation due to fouling was successfully demonstrated and the cleaning role of particle circulation was identified. It was also demonstrated that small amounts of circulating particles contribute not only to the fouling reduction on the gas side, but also to the heat transfer enhancement. Experimental operation data for 50 hours including accelerated fouling are obtained to simulate the long-term behavior of the system.     

Airborne particles detection and sizing at single particle level by a novel electrical current pulse sensor

A novel method for detecting and sizing airborne particles at single particle level is presented in this paper. When a particle hits a metal electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated in the measurement system. The number of the signal pulse represents the number of particles in the sample. To determine the effect of the particle size on the magnitude of the signal, the correlation between the magnitude of the signal and the size of particle was experimentally investigated. The results show that the magnitude of the measured signal is linearly proportional to the square of particle’s diameter. Such a correlation can be used to evaluate particle size from the measured signal. The airborne particle detection method presented in this paper can be used for counting and sizing airborne particles at single particle level.     

落球冲击破碎下的非限制料层夹持形态

为提高球磨粉碎的效率,构建了球形介质对非限制料层的冲击粉碎模型,开展了落球冲击破碎下的料层夹持形态研究。通过石英砂料层的落球冲击破碎实验,分析了初始料层厚度、落球冲击能量、冲击速度、落球大小以及料层颗粒大小对料层夹持形态的影响。实验结果表明,夹持颗粒完全破碎时,料层夹持厚度为2层颗粒的堆叠厚度情况下,夹持范围受落球大小和夹持角度的影响较大。根据夹持角度与安息角的关联性,建立了料层夹持半径与落球大小、安息角的数学模型。     

基于神经网解耦的循环流化床锅炉燃烧系统自适应模糊控制

电厂循环流化床锅炉的正常运行受到很多因素的影响,因此使其在实现自动控制方面遇到困难.根据循环流化床锅炉的特点,将自适应理论与模糊控制相结合,设计了一种基于BP神经网解耦的具有参数在线自调整功能的自适应模糊控制器,并应用于循环流化床锅炉燃烧控制系统中.该控制器不但可以适应被控对象的变参数运行工况,而且可以实现循环流化床锅炉燃烧过程主汽压力与床层温度的解耦控制.仿真试验和现场应用结果证明,本控制器对循环流化床锅炉燃烧过程具有良好的控制效果.     

Hydrodynamics and particle mixing/segregation measurements in an industrial gas phase olefin polymerization reactor using image processing technique and CFD-PBM model

Particle size distribution (PSD) has a significant impact on the performance of fluidized bed reactors due to uneven distribution in the segregation and mixing phenomena. This paper develops a new method of digital image processing that investigates the hydrodynamics of an industrial gas phase olefin polymerization reactor and studies the fluidization structure of a wide range of particle size distribution in an industrial gas phase polymerization reactor by means of a CFD-PBM coupled model, where the direct quadrature method of moments (DQMOM) was implemented to solve the population balance model. It was shown that the applied parameter assumptions and closure laws were appropriately chosen to satisfactorily predict the available operational data in terms of pressure drop and bed height. The transient CFD-PBM/DQMOM coupled model and image analysis technique are then implemented extensively to analyze bubble fluidization structure and segregation phenomena at different velocities. The particle segregation indicates that the small bubbles present in the bed are unable to induce vigorous mixing at low superficial gas velocity while particle mixing improves at a velocity above the minimum fluidization velocity. Further, the predicted results show higher axial segregation phenomena when compared to the radial direction.     

矿渣微粉填充树脂基复合材料力学性能和摩擦学性能研究

为了充分利用废弃材料研发绿色制动片,在树脂基复合材料中加入不同颗粒度的矿渣微粉作为无机填料,用热压成型的方法配制试样,并对试样的力学性能和摩擦磨损性能进行试验。研究结果表明：在复合材料中加入矿渣微粉作为无机填料,摩擦系数高于未掺样且更稳定,试样摩擦系数随掺量增加而增大。增加矿渣微粉后复合材料的磨损率均有明显降低,200目掺量为30％的试样具有较好的综合性能。两种粒径试样的硬度均高于未掺试样,冲击强度略有下降,仍高于标准较多。磨损形式由未掺样的严重的粘着磨损转变为有掺样轻微的磨粒磨损和粘着磨损。     

A Pin-on-Disc Study Focusing on How Different Load Levels Affect the Concentration and Size Distribution of Airborne Wear Particles from the Disc Brake Materials

Airborne wear particles originating from disc brakes are one important contributor to the concentration of airborne particles in urban environments. It is therefore of interest to improve the knowledge of these particles. The purpose of this article is to investigate the concentration and size distribution of the airborne wear particles generated from the contact between a low-metallic pad material and a grey cast iron disc at different load levels. This is done on model level with a pin-on-disc machine that allows the cleanliness of the air surrounding the test specimens to be controlled, and thus the airborne portion of the wear particles to be studied separately. The concentration and size of airborne wear particles were measured online during testing with four particle instruments. In addition, airborne wear particles were collected on filters during the tests and afterward analysed using SEM. Trimodal size distributions with peaks around 280, 350 and 550 nm were registered during running-in for all load levels. After running-in bimodal size distributions with peaks around 350 and 550 nm were registered for all load levels with the exception of the highest load level where multimodal size distributions were registered. At the two highest load levels the concentration of ultrafine/fine particles showed an increase up to a factor hundred indicating a change in wear mechanism. SEM images show ultrafine, fine and coarse airborne wear particles.     

A scanning force microscope study of detachment of nanometer‐sized particles from glass surfaces

The nominal shear stress required to detach nanometer‐scale, single‐crystal salt particles from a soda lime glass substrate is a strong function of particle size and relative humidity. We use the tip of an atomic force microscope to detach these particles from a glass substrate under controlled atmospheres of known humidity. The peak lateral force at detachment was divided by the nominal particle area to yield an effective interfacial shear strength. We describe the variation of shear strength with particle area and humidity in terms of detachment by chemically assisted crack growth along the salt–glass interface. This revised version was published online in September 2006 with corrections to the Cover Date.     

液固两相介质多频超声粒径分布测量

液固两相流动广泛存在于工业过程中,其粒径分布信息在线测量对生产优化与控制十分重要。超声法作为无扰动式多相流动参数测量方法,其衰减特性与固相颗粒粒径、体积分数等密切相关,可用于实现粒径分布在线测量。本文搭建液固两相介质超声衰减实验装置,装置采用石英砂作为固相颗粒,自来水为液相,利用线性调频超声信号激励研究液固两相介质的超声衰减特性。实验结果表明,随着激励频率和固相体积分数的升高,超声衰减系数逐渐增加,采用Twomey及遗传算法等对液固两相介质粒径分布进行反演,测试结果与马尔文激光粒度分析仪的粒径分布结果相比,相关系数为0.918。     

Influence of particle size on wear assessment by spectrometric oil analysis I: Atomic absorption spectrometry

The relationship between wear particle size and data from spectrometric analysis, e.g. for iron in used oil, is systematically investigated in this paper. The particle size to which the iron content measured by direct atomic absorption spectroscopy (AAS) (with dilution of the oil sample) corresponds to the true iron concentration in the used oil was determined.The absolute value of the iron content was determined by a colorimetric process after incineration. True wear particles from gear tests were added to particle-free oil and homogenized for sample oils.The iron content of these oil samples was determined by (1) direct AAS, (2) AAS after incineration and (3) the absolute colorimetric process. When wear particles larger than 2 μm are present the amount of iron measured by direct AAS is only about 50% of the amount measured by absolute colorimetry. Thus only for wear particles smaller than about 1 μm is the true iron content found by the direct AAS process.     

基于ICP-MS的二氧化硅亚微米粒子特征参数分析

将电感耦合等离子体质谱仪（ICP-MS）配合外部质谱信号采集存储装置,用于研究表征亚微米颗粒粒径和浓度参数。在模拟采集模式下,以300~2 000 nm粒径的SiO₂粒子为例,通过优化进样系统及仪器的工作参数,分析了样品提取速率和雾化气流速对单颗粒质谱信号强度的影响。在优化的实验条件下,SiO₂颗粒粒径部分检测限为233 nm,对300~900 nm粒径粒子测量的线性相关系数大于0.99,但对1 500、2 000 nm粒径粒子的检测结果出现明显偏差。论证了利用样品传输效率测量悬浮液粒子浓度的可行性,并将ICP-MS的粒径测量结果与扫描电镜法（SEM）、光子相关光谱法（PCS）的测量结果进行比较,3种方法对于粒径小于900 nm粒子的测定结果基本一致,且具有相似的测量精度。该方法分析速度快、结果准确,可用于SiO₂亚微米粒子粒径、浓度参数的测量。     

Tracing the motion of a large object in a fluidized bed using electrical capacitance tomography

An innovative method using electrical capacitance tomography (ECT) to trace a large object's motion on an air distributor in a fluidized bed is described here. The method fills the large object to be traced with a high permittivity material, and then a recalibration process is applied to reduce the nonlinearity caused by the large permittivity difference between the tracer particle and other fine particles in the measurement zone. The local dynamic threshold selection method is performed on the reconstructed image to locate the tracer particle's position.Static simulations and dynamic experiments testify that tracer particles with a diameter of one ninth of the measured cross-section and a fluidization velocity v/v_cr≤4.2 can be accurately located and traced. Employing this method to trace the motion of a spherical object in a bed shows that the fluidization velocity significantly influences the directional motion of a large, heavy object on an inclined air distributor.     

ECT measurement and CFD–DEM simulation of particle distribution in a down-flow fluidized bed

In the present study, a combined model of computational fluid dynamics and the discrete element method (CFD–DEM) was used to simulate the particle distributions in a down-flow fluidized bed (DFB) with a newly designed particle–air distributor. In the simulation model, particle motion is calculated by solving Newton’s equations and the flow field of air is predicted by the Navier–Stokes equations. The calculation was made for the same geometric and operating conditions as the experiment which was carried out for comparison with the simulation using electrical capacitance tomography (ECT). The numerical predictions for the axial and radial profiles of the particle distribution agreed well with the experimental results.     

Triboelectrostatic separation system for separation of PVC and PS materials using fluidized bed tribocharger

A triboelectrostatic separation system using a fluidized bed tribocharger for the removal of PVC material in the mixture of PVC/PS plastics is designed and evaluated as a function of electric field strength, air flow rate, and the mixing ratio of two- component mixed plastics. It consists of a fluidized-bed tribocharger, a separation chamber, a collection chamber and a controller. PVC and PS particles can be imparted negative and positive surface charges, respectively, due to the difference in the work function values of plastics suspended in the fluidized-bed tribocharger, and can be separated by passing them through an external electric field. Experimental results show that separation efficiency is strongly dependent on the electric field strength and particle mixing ratio. In the optimum conditions of 150 lpm air flow rate and 2.6 kV/cm electric field strength a highly concentrated PVC (99.1%) can be recovered with a yield of more than 99.2% from the mixture of PVC and PS materials for a single stage of processing.     

介质球间非限制料层夹持形态研究

颗粒的球磨粉碎以介质球间非限制料层破碎为主,料层的夹持特性与颗粒破碎密切相关。通过石英颗粒料层挤压实验和离散元仿真发现,介质球径和颗粒粒度对夹持料层厚度存在显著的影响：介质球径小则料层薄、颗粒粒度小则料层厚。通过对实验数据的回归分析,构建了石英颗粒的夹持料层高度计算模型;依据料层高度与料层自锁条件给出了料层夹持范围计算公式,计算模型和公式与料层挤压实验结果符合较好。     

The effect of the composition of fluidized bed combustor bed materials on the erosion-corrosion of carbon steel

The relative amounts of SiO₂, CaO, CaCO₃, CaSO₄, and alkali compounds in the bed materials of circulating fluidized bed combustors determine the metal wastage of heat exchanger tubing alloys exposed to the particles. The weight ratio of SiO₂ to CaO and to CaCO₃ is especially important in determining the erosivity of the bed materials. It was found that increasing this ratio increased the metal wastage. A sharp transition ratio occurred, on either side of which the metal wastages were linear. The slope of the linear curve was low for the lower sand content mixtures and higher for the higher sand content mixtures. The transition ratio required a higher CaO content for higher velocity particles.

The transition ratio of SiO₂ to CaCO₃ is related to the formation or lack of formation of a protective layer on the metal surface. The transition ratio of SiO₂ to CaO is related to the formation of a thick protective layer or a thin, less protective layer on the metal surface. In the thick layer, CaO mechanically mixed with iron oxide to form an intimate mixture whose CaO content decreased inward from the gas-scale layer surface.