Pressure drop and collection efficiency of cyclone and impact separators in a CFB

Experimental results of pressure drop and gas–solid separation of impact separators with three rows of impact blades with included angles (60°, 90° or 120°) in a cold CFB set‐up of 102 mm×102 mm cross section are presented for two different samples of sand and they are compared with the same of a conventional reverse flow cyclone. The pressure drop increases with higher superficial velocity for all the solid separators including cyclone at a constant solid circulation rate. ‘Overall collection efficiency’ (η_oc) remains practically unaltered for the cyclone within the range of superficial velocity of the experiment while it decreases with higher superficial velocity for the impact separators. ‘Fractional collection efficiency’ (η_fc) of cyclone increases monotonically with grain size while that of the impact separator becomes minimum for an intermediate grain size. The collection efficiency of impact separator is comparable to that of cyclone for either very small or very large particles. Copyright © 1999 John Wiley & Sons, Ltd.     

带加速段的卧式旋风分离器的数值模拟及工业应用

带加速段的卧式旋风分离器具有体积小、结构简单、造价低、分离效率高等优点,为了解其工作机理,应用Fluent软件,采用雷诺应力模型模拟气相流动、离散相模型模拟颗粒运动轨迹,同时计算并给出了加速段出口速度为25.46 m/s时,粒径为1μm、5μm、10μm和30μm的颗粒运动轨迹,结果表明该分离器对细小颗粒具有一定的分离效率,与实验吻合良好。最后应用Fluent软件对实际应用的卧式旋风分离器进行了数值模拟,在不考虑物料循环的条件下,该分离器分离效率为85.8%,分离效率较高。     

An experimental study of air-solid two-phase flow in a 90° bend using LDV system

The measurements of the mean streamwise and radial velocities, the associated turbulence and the relative particle densities were made in an air-solid two-phase flow in a square sectioned (30mm×30mm) 90° vertical to horizontal bend using laser Doppler velocimetry. The radius ratio of the bend was 2.0. Glass beads of 100μm in diameter were employed to form the solid phase. The measurements of air and solid phases were performed separately at the same bulk velocity 19.34m/s, corresponding to a Reynolds number of 3.87×10⁴. The mass ratio of solid to air was 1.6%. The results indicate that the particle trajectories are very close to straight lines. The streamwise velocity profiles for the gas and the solids cross over near the outer wall with the solids having the higher speed. At θ=30° and 45°, particle-wall collisions happen mostly in the region from θ=30° to θ=75°, and cause a sudden change in solid velocity. The particles tend to move towards the outer wall in 90° bend. The particle concentration near the outer wall is much higher than that near the inner wall in the bend, and there are few particles in the inside of the bend. The bend leads to apparent phase separation: at θ=45°, the solids concentrate in the half of the duct near the outer wall. After θ=60° the second peak concentration appears, and goes gradually towards the inner wall.     

Investigation of the Mechanisms of Heat Exchanger Corrosion in a Municipal Waste Incineration Plant by Analysis of the Raw Gas and Variation of Operating Parameters

The detailed mechanism of high temperature chlorine corrosion, the dominant cause of corrosion in a municipal solid waste incinerator (MSI), has still to be clarified. Upon its way through the boiler the raw gas is subject to various physical and chemical processes and interactions. Of these, sulfation of chlorides is supposed to have the major impact on chlorine corrosion. The physical and chemical mechanisms of corrosion were investigated at a municipal solid waste incinerator. Both the particulate and gas phases of the flue gas were chemically and physically analyzed during their way through the boiler, at temperatures from close to 1000°C down to 200°C. The raw gas composition was analyzed during normal operation and during the soot blowing cleaning routine. Additionally, operating parameters of the plant were varied, and deposition processes were evaluated with the aim to find out primary measures to reduce corrosion rates. The particle mass concentration exhibits a bimodal size distribution with maxima at approximately 0.5 μ m—growing by duration of travel—and 100 μ m. First results show that sulfation of the particles can be observed upon travel through the boiler and on the fouling. Sulfur-containing additives increased the sulfation of the particles during flight, though not to completion.     

Experimental Modeling of Gas–Solid Heat Transfer in a Pipe with Various Inclination Angles

Gas–solid flow in a pipe with different configurations (vertical, horizontal, and inclined positions) is studied experimentally. Air with temperature around 170°C and sand particles with mean diameter of 253 μm are used as gas and solid mediums, respectively. Effects of different parameters (pipe slope and solid particles feed rate) are studied on heat transfer rate between gas and solid particles. The Nusselt number decreases at lower solids feed rate in a dilute regime of the mixture; however, it increases at higher solids feed rates. Furthermore, results show that a higher Nusselt number takes place at the angles closer to 45 degrees.     

Numerical simulation of the combustion processes in a W‐shaped boiler furnace under different operating conditions

Numerical simulations of gas–solid flows, heat transfer and gas–particle turbulent combustion have been conducted for a three‐dimensional, W‐shaped boiler furnace. The gas–particle flow, distributions of temperature and concentrations of gaseous constituents, distributions of the rates of heat release, burnout rates of coal particles, and formations of volatiles have been predicted. The results indicate that a steady high‐temperature zone is formed under the arch of the W‐shaped flame boiler, this zone would be of benefit to the ignition and carbon burn‐out and suggest that the W‐shaped flame boiler is suitable for burning low‐quality coals and can operate well under different operating conditions for full and partial loads. Copyright © 1999 John Wiley & Sons, Ltd.     

利用电容层析测量煤粉浓度的实验研究

锅炉燃烧过程中,准确测量风管中的煤粉浓度并指导调节对锅炉的安全、经济运行非常重要。采用电容层析成像技术,对常温下稀相气力输送的固体浓度进行了测试研究。为克服电容传感器敏感场的不均匀性对成像造成的影响,实验系统中装置了一旋风分离装置,通过旋风分离作用将固体颗粒主要集中在壁面区域,传感器电极布置在分离器直管上。这种测试方法的优点在于不破坏原有系统的运行特性,属于非侵入式、在线的快速测试技术。通过建立实验台及相应的在线测量系统,实验结果表明该法是可行的。     

Suspension-to-wall heat transfer in a 12-MWth circulating fluidized-bed combustor

An experimental investigation was carried out to study the effects of operating parameters on the local suspension-to-wall heat transfer in the combustor of a 12-MW_th circulating fluidized-bed (CFB) boiler. The heat transfer coefficients were measured with a conduction-type heat flux meter at five different vertical levels. The measurements covered a range of superficial gas velocities from 4 to 6 m/s, a bulk bed temperature from 800 to 850 °C and a suspension density from 6 to 70 kg/m³ for 270-μm silica sand particles. The heat transfer coefficient for the membrane wall in the combustion chamber of the CFB boiler was in the range of 100 to 180 W/m² K for the range of operating conditions employed in this work. The heat transfer coefficient decreased with increasing height and increased with increasing bulk bed temperature, superficial gas velocity and suspension density. Based on the experimental data, a simple correlation is proposed for predicting the suspension-to-membrane wall heat transfer coefficient. The results were analysed and compared with the experimental data of other workers.     

Dimensioning a Cyclone Separator for Circulating Fluidized Bed Gasifier: Validation of a Procedure

Abstract

The transformation process of biomass through gasification may produce a large amount of solid particles of different granulometry. In order to avoid equipment damage, these particles must be separated from the producer gas before it is utilized in other processes. This separation can be performed through a cyclone separator. This equipment is part of the circulating fluidized bed gasifier and is responsible for particulate separation and producer gas partial cleanup. Cyclone dimensioning requires careful attention and must be done according to the conditions this device is going to be subject. Thus, this work aims to validate and analyze a dimensioning methodology and apply it to a gasifier. Three cyclone geometries were considered (Stairmand, Swift and Lapple) and they were compared in terms of collection efficiency and pressure drop. A parameter sensibility study was performed. Variations of dimension, efficiency and pressure drop for a producer gas volumetric flow range were tested. It was found that an increase of the volumetric flow rate provoked the rise of separation efficiency and pressure drop. From the tested geometries, Swift presented the highest collection efficiency and pressure drop.     

Prediction of heat transfer coefficient in the cyclone separator of a CFB

This paper deals with the study of heat transfer behaviour in the cyclone separator of a circulating fluidized bed (CFB) and how it is affected by relevant operating parameters. The experiments are conducted in a 140 mm diameter cyclone of a cold bed set‐up of 102 mm×102 mm bed cross‐section, 5.25 m height CFB. The cyclone separator is designed according to the high‐efficiency Lapple design and made to accommodate two identical heat transfer probes. From the experimental results, the heat transfer coefficient is found to increase with increase in solid circulation rate, as well as gas superficial velocity. The effect of bed inventory and heat flux on heat transfer coefficient has also been investigated. An empirical equation has been developed to predict the heat transfer coefficient in the cyclone separator based on dimensional analysis. The experimental results are compared with the predicted results and a good agreement has been observed. There is an optimal distance from the entry of the cyclone where the local heat transfer coefficient is maximum. The collection efficiency of the cyclone separator has been measured for all the operating conditions. Copyright © 2000 John Wiley & Sons, Ltd.     

Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers

The mercury emission was obtained by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and circulating fluidized bed (CFB) utility boilers. The relationship was obtained between the mercury emission and adsorption characteristics of fly ash. The parameters included unburned carbon content, particle size, and pore structure of fly ash. The results showed that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury was enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. As the coal factor increased, the mercury emission to the atmosphere decreased. It was also found that the mercury content of fly ash in the CFB boiler was ten times higher than that in the PC boiler. As the unburned carbon content increased, the mercury adsorbed increased. The capacity of adsorbing mercury by fly ash was directly related to the particle size. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range from 77.5 to 106 µm. The content of mesoporous (4–6 nm) of the fly ash in the particle size of 77.5–106 µm was the highest, which was beneficial to adsorbing the mercury. The specific surface area played a more significant role than specific pore volume in the mercury adsorption process.     

W型火焰炉旋风分离器分离特性的实验研究

W型火焰炉旋风分离器是一种利用旋风分离原理进行浓缩的煤粉浓缩器。在气固两相实验台上进行了分离器气固分配特性的实验研究。结果表明,在乏气挡板开度为100%时,乏气仅占总风量的35%,一次风浓侧颗粒浓度由入口处的0.58 kg/kg上升到0.80 kg/kg,煤粉浓缩效果较差。为此,将乏气管直径由80 mm提高到100 mm,实验测得:在乏气挡板开度100%时,乏气占总风量的65%,一次风颗粒浓度上升到1.58 kg/kg,取得了较好的浓缩效果。     

Numerical study of different gas–solid flow regimes effects on hydrodynamics and heat transfer performance of a fluidized bed reactor

In this study, a two‐?uid Eulerian–Eulerian model has been carried out applying the kinetic theory of granular flow (KTGF) to study the hydrodynamics and heat transfer behavior of a fluidized bed reactor simultaneously. The effects of different gas–solid flow regimes on the operating conditions and heat transfer rate between the hot air and two types of low and high‐density inert particles are investigated in a fluidized bed dryer. Different gas–solid flow regimes for wood and glass particles of groups A, B, and D of Geldart's classification are simulated to introduce the most optimal flow regime in terms of heat transfer rate and operating costs. The compromise between the heating rate, the height required for the reactor, and the ratio of the final mass to the initial mass of solid particles, which specifies the need for a cyclone separator showed that the bubbling regime of Geldart B powder for low‐density particles and the turbulent regime of Geldart D powder or bubbling regime of Geldart B powder for high‐density particles are the optimal operating conditions and flow regimes. Furthermore, it was concluded that the convective heat transfer is the dominant mechanism, which increases with increasing the air velocity and decreasing the particle diameter in each group.     

炉顶结构对循环流化床锅炉颗粒内循环影响的实验研究

针对循环流化床锅炉内沿高度方向颗粒流态呈现湍流床、快速床,颗粒浓度沿高度方向呈上稀下浓分布,沿床横截面方向呈环形分布的特点,在分析总结国内外学者主要研究成果的基础上,阐述炉顶结构对循环流化床锅炉颗粒内循环的影响机理,并在不同安装高度下对比研究了多种炉顶结构对颗粒内循环的强化作用.     

In-situ repair of composite palladium membranes with macro defects: A case study

A new method was developed for repairing Pd/Al₂O₃ membranes with macro defects without the need of disassembling the membrane from the module. In order to target and fill the membrane defect automatically with solid particles, a TiO₂ powder was firstly tested by flowing high-pressure nitrogen as a carrier gas, followed by a heat treatment. A filter cake was found on the membrane defect but still porous. A glass powder was selected instead of TiO₂, and the membrane defect was successfully sealed by glazing. The in-situ repair of a waste commercial Pd/Al₂O₃ membrane separator was carried out with the glass powder, and the hydrogen flux and H₂/N₂ selectivity of the membrane separator at 450 °C under 100 kPa reached 12.6 m³m⁻²h⁻¹ and 1600, respectively.     

循环流化床锅炉两种返料立管水冷装置及比较

在以绝热的高温旋风分离器作为分离元件的循环流化床锅炉中 ,因旋风分离器及返料立管内物料的2次燃烧 ,导致物料超温甚至结焦 ,影响锅炉的正常运行。本文介绍了目前常用的一种返料立管水冷装置和新开发的一种水冷装置 ,对两种水冷装置进行了结构、热力计算和防磨措施的比较 ,指出了两种水冷装置的煤种适应性。     

两相流动对流化床燃烧行为的影响

循环床锅炉沿床高的烟气浓度及燃烧份额分布测试结果证明,鼓泡流化床和循环流化床的重要差异表现为密相区燃烧行为的根本不同,由于床料平均粒径较低,循环床密相区的流动不同于鼓泡床,导致气固两相之间的传质阻力增加,从而影响燃烧反应,密相区的燃烧行为表现为欠氧。循环床锅炉沿床高乃至分离器都有燃烧反应发生,建立了考虑气固相间传质阻力的流化床密相区燃烧模型,并与实际循环流化床锅炉的测试数据比较,计算结果与测试值比较吻合。     

热水循环流化床锅炉高、低速混合床节能改造及效果

针对1台29MW热水锅炉热效率低和飞灰含量高等问题,改两级分离的高速循环流化床锅炉为带高温旋风水冷分离器的高、低速混合循环流化床锅炉;将分离器中心筒插入深度从原来的30％增大到100％,并在分离器中心筒底部增设缩口使烟气流速增大15m／s。实践表明：锅炉热效率提高19个百分点;节煤量远大于分离器电力消耗的增加;分离器飞灰含量降低28个百分点。     

新型扩散式下排气气固分离器实验研究

通过实验方法（分离效率的测定和三维粒子动态分析实验）研究了扩散式下排气气固分离器的分离流阻、流场分布和分离效率等，从而降低扩散式气固分离器的阻力，提高分离效率。分离器内部的各种设计（上导流锥高度的确定、下导流锥的倾角和排气管高度等）均为提高气固分离器的效率和降低阻力，从实验可以看出该气固分离器对于30μm粒径粗灰的分离效率很高，能达到95％以上。     

方形下排气分离器气固两相流动的PDA研究

应用激光相多谱勒三维颗粒动态分析仪（3D-PDA）对方形下排气分离内部气固两相流场进行了测试,得到了分离器内不同截面的气相和颗粒相的流场矢量图,时均速度等基本运行规律。研究结果发现方形分离器内横截面上的流场旋转中心偏离其几何中心,在边角位置存在局部小旋涡,在分离器左右两个壁面附近流运有向壁面流动的趋势,研究结果有助于对分离器的分离机理的认识和指导其结构的优化设计。图14参6