Study of the performances of an oxygen carrier: Experimental investigation of the binder's contribution and characterization of its structural modifications

The aim of this work is to investigate the contribution of the binder (NiAl₂O₄) on the performances of the oxygen carrier NiO/NiAl₂O₄. To this purpose, oxidation/reduction cycles have been performed in a fixed bed reactor using CO as a fuel. The results reveal that the binder can react with the fuel to form CO₂, and that its total reduction capacity increases with temperature. XRD characterizations performed on the binder (on the fresh and after several cycles) show a shift of the diffraction peaks of NiAl₂O₄ toward the ones of γ-alumina, which can be attributed to a progressive decomposition of NiAl₂O₄ to alumina and NiO.     

Anti-wear beam effects on gas–solid hydrodynamics in a circulating fluidized bed

Anti-wear beams installed on water walls of circulating fluidized bed (CFB) boilers are one of the most effective ways to protect against water-wall erosion. Beam effects from, for example, beam size and superficial gas velocity were investigated on gas–solid hydrodynamics in a CFB test rig using CFD simulations and experimental methods. The downward flow of the wall layer solids is observed to be disrupted by the beam but is then restored some distance further downstream. When falling solids from the wall layer hit the anti-wear beam, the velocity of the falling solids decreases rapidly. A fraction of the solids accumulates on the beam. Below the beams, the falling solids have reduced velocities but upward-moving solids were observed on the wall. The effect of the beam increases with width and superficial gas velocity. Wear occurs mainly above the beam and its variation with width is different above to below the beam. There is an optimum width that, when combined with beam height, results in less erosion.     

床面固体颗粒圆柱绕流的试验现象观察

本文介绍了床面固体颗粒随水流绕过圆柱体时，将在圆柱周围的床面上形成一个无粒子运动区的试验现象。水槽试验结果表明，当固体颗粒的粒径减小时，无粒子区的范围将增大；无粒子区的范围随圆柱直径的增大而增大；水流条件的变化直接影响着床面固体颗粒的运动情况，同无粒子区的形成、消失及范围大小有密切的关系。根据试验资料，结合量纲分析，建立了无粒子区的无量纲经验关系式     

Modeling fluid and heat transport in the reactive, porous bed of downdraft (biomass) gasifier

A fluid flow and heat transfer model has been developed for the reactive, porous bed of the biomass gasifier to simulate pressure drop, temperature profile in the bed and flow rates. The conservation equations, momentum equation and energy equation are used to describe fluid and heat transport in porous gasifier bed. The model accounted for drag at wall, and the effect of radial as well as axial variation in bed porosity to predict pressure drop in bed. Heat transfer has been modeled using effective thermal conductivity approach. Model predictions are validated against the experiments, while effective thermal conductivity values are tested qualitatively using models available in literature. Parametric analysis has been carried out to investigate the effect of various parameters on bed temperature profile and pressure drop through the gasifier. The temperature profile is found to be very sensitive to gas flow rate, and heat generation in oxidation zone, while high bed temperature, gas flow rate and the reduction in feedstock particle size are found to cause a marked increase in pressure drop through the gasifier. The temperatures of the down stream zones are more sensitive to any change in heat generation in the bed as compared to upstream zone. Author recommends that the size of preheating zone may be extended up to pyrolysis zone in order to enhance preheating of input air, while thermal insulation should not be less than 15 cm.     

Investigation of operational parameters for an industrial CFB combustor of coal, biomass and sludge

The combustion of coal and/or biomass （sludge, wood waste, RDF, etc.） in a circulating fluidized bed has been a commercial topper for over 20 years, and references to principles and applications are numerous and widespread although few data are presented concerning the operation of large scale CFB-units. The authors studied the CFB-combustion at UPM-Kymmene （Ayr）, a major paper mill relying for its steam production upon the combustion of coal （80-85 %）, wood bark （5-10%） and wastewater treatment sludge （5-10%）. The maximum capacity of the CFB is 58 MWth. A complete diagnostic of the operation was made, and additional tests were performed to assess the operating mode. The plant schematics, relevant dimensions and process data are given. To assess the operation of the UPM-CFB, it is important to review essential design parameters and principles of CFB combustors, which will be discussed in detail to include required data, heat balance and flowrates, operating versus transport velocity, kinetics and conversion （including the possible effect of the Bouduard reaction if carbon is present）. Since the residence time in the riser and the cyclone efficiency determine the burnout of circulating fuel-particles, the UPM-CFB was subjected to a stimulus response technique using nickel oxide as tracer. Results illustrate the efficiency of the cyclone separation and the number of recycle loops for particles of a given size. Results will also be used to assess the cyclone operation and efficiency and to comment upon expected and measured carbon conversion.     

改进蚁群PID-神经元解耦的CFB锅炉燃烧系统控制

床温和主汽压都是循环流化床锅炉生产运行中的重要参数,会直接影响机组的安全性和经济性。但由于这两者对象存在非线性、大时延、强耦合等特点,其现场控制效果一直不太理想。本文首先采用自适应神经元将床温和主汽压解耦,再利用具有分工特征的蚁群算法优化参数的PID控制器对两者进行独立控制。采用该算法优化常规PID控制参数,能够实现控制参数的快速寻优。该方案应用于循环流化床锅炉燃烧系统仿真,结果表明能有效实现系统解耦,且具有响应快、超调量小等优点,有效地提高了控制品质。     

增殖剂球床载气体流动特性

增殖剂球床是聚变堆或混合堆产氚包层可选结构之一,准确把握增殖剂球床中载带气体的流动特性有助于提高对球床载氚过程的认识并优化包层设计。采用离散元程序PFC3D模拟增殖剂小球的填充行为,在球床内不同位置随机截取不同尺寸的控制体,利用布尔运算中的"差集"得到孔隙范围,建立孔隙分布的三维几何模型,进一步划分网格并用计算流体力学(CFD)方法求解,得出控制体上单位长度的压降以及单元体内的速度分布特征,计算结果发现载带气体速度分布与γ分布很类似,且只要选取恰当的控制体,通过计算流体力学方法可以较好地分析整个球床孔隙内流体的流动,有利于进一步研究载氚及相关过程。     

Open-channel turbulent flow over non-uniform gravel beds

the measurements of flow over non-uniform gravel open channel have been conducted with Laser Doppler Velocimetry (LDV). The experimental results indicate that the distribution of mean velocity agrees well with the Nikuradse's law. From the distributions of resistance coefficient, reference level and turbulence intensity, the classification of small scale roughness case is obtained.Project supported by the National Science Foundation of China     

Waves in a Bed under Periodic Tangential Loading

The far-zone structure of the wave field in an elastic bed on a rigid foundation is considered. The wave field is generated by a tangential periodic force applied to the bed surface. The amplitude- frequency characteristics of surface vibrations for propagating modes are found. The partition of energy between different modes is considered.__________Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 4, pp. 109–115, July– August, 2005.     

STUDY ON THE OVERALL PRESSURE BALANCE OF A DOWNFLOW CIRCULATING FLUIDIZED BED SYSTEM

A pressure balance model for a circulating fluidized bed unit that incorporates a downer has been proposed. The model predictions were validated with the experimental data obtained from a special cold-model circulating fluidized bed. Comparison of the operation stability between a CFB downer and a CFB riser has been carried out. Only one critical gas velocity exists in the CFB-riser for a given riser solids flux, while there can be many critical gas velocities for the operation of a CFB downer. Therefore, it is possible to achieve high solids concentration in a CFB downer if appropriate operating conditions are used.