高温鼓泡流化床的流化行为

床层温度在20－1000℃范围内,以4种粒径的煤灰为实验物料,研究了不同表观气速下最小流化速度,床层平均空隙率,压力波动标准偏差和主频的变化规律,最小流化速度随床层温度升高而减小;相同床温下,平均空隙率随表观气速升高而增大,不同床温下,压力波动偏差随流化数增加而增大。相同流化数时,B类颗粒的压力波动标准偏差受床层温度变化影响较小,而D类粒子随床层温度升高压力波动标准偏差减小,胡着流化数增加,压力波动主频减小。     

增压流化床最小流化速度的实验研究

在80mm的铸铁流化床实验台上,以窄筛分和宽筛分两类颗粒为实验物料,在压力(101kPa~6 000kPa)和温度(20℃~800℃)范围内,研究了两类颗粒的最小流化速度。实验结果表明:在相同的温度下,随压力的增大最小流化速度减小。然而,在压力不变的情况下,温度对最小流化速度的影响与床料的平均粒径有关。最后,根据上述实验结果和空气在不同压力和温度下的密度,研究了增压流化床在临界流化状态下,风机送风量随流化床压力和温度的变化规律。风机送风量在温度不变的情况下,随压力的增大而明显增加。然而,温度对其的影响随床料的平均粒径的不同而略有不同。     

压力对流化床最小流化速度影响的冷态试验研究

为进一步探明压力对流化床最小流化速度的影响,在内径90 mm的冷模加压鼓泡流化床试验装置上测量了不同粒径的石英砂和陶粒砂两类试验物料在压力0.1～0.6 MPa内的最小流化速度Umf.结果表明:对于陶粒砂颗粒,Umf随压力的增加明显减小,但减小的幅度逐渐变小;而对石英砂颗粒,压力对Umf影响较小.基于厄贡方程和床层受力分析,得到了最小流化速度的计算式,并给出了确定增压流化床最小流化速度的合理计算步骤.计算得出的预测值与试验值吻合较好,相对误差在10%以内.     

高温鼓泡流化床的压力波动

在内径82mm，高1500mm的不锈钢流化床中，采用4种不同粒径加压流化床灰为实验物料，通过统计分析，功率谱分析和小波分析研究了流化床操作温度由室温至1000℃变化时压力波动行为。研究显示，灰在不同温度下随着流化数增加压力波动偏差增大。B类颗粒，流化数相同时，床层温度升高，压力波动标准偏差减小不明显。D类灰，当流化数相同时，床层温度升高使得压力波动标准偏差减小。在同样的温度下，随着流化数的增加，主频减小。高温鼓泡流化床压力波动信号包含低频成份和高频分份，高频成份较小。压力信号通过离散小皮变换可分解为5尺度的近似信号和1到5尺度的细节信号，5尺度细节信号图上幅值大于平均幅值的尖峰数代表了气泡生成数。     

油页岩热解飞灰颗粒流化夹带特性

在一套准φ80mm×4090mm有机玻璃冷模试验装置中,以油页岩热解飞灰为试验物料,使用在线压力采集系统,测定了流化床轴向平均颗粒密度分布,根据稀相空间平均颗粒密度获得了输送分离高度(TDH)和气体饱和夹带量(εp∞),并与FCC催化剂颗粒的流化夹带特性进行了对比。试验结果表明,对于密度相近的两种物料,尽管油页岩热解飞灰的平均粒径小于FCC催化剂,但同一表观气速下流化床内各截面平均颗粒密度均低于FCC催化剂。表观气速较低时(u=0.22m/s),两种物料的TDH和气体饱和夹带量基本相等;随着气速的提高,FCC催化剂的TDH和气体饱和夹带量逐渐高于油页岩热解飞灰;当表观气速大于0.88m/s时,油页岩热解飞灰的TDH逐渐接近于FCC催化剂。对试验结果进行关联分析后,建立了油页岩热解飞灰TDH和气体饱和夹带量的经验模型,计算值与试验值吻合较好。     

加压喷动流化床最小喷动速度的试验研究

在内径100 mm的有机玻璃冷模装置上进行了加压喷动流化床试验.床料直径为1.6 mm、2.3 mm的小米.研究了压力、静止床高及流化风对最小喷动速度的影响.试验结果表明:喷动流化床的最小喷动速度随压力的增大而减小,但减小幅度逐渐变小;静止床高增大,最小喷动速度增大,但床高的增加对最小喷动速度的影响随着压力的增大而减弱.流化风风量增加导致最小喷动速度降低.根据试验数据进行了线性回归,分别得出了uf=0和uf＞0(uf为流化风床内表观气速)时最小喷动速度的关联式,相关系数分别为0.964和0.920,关联式和试验值吻合较好.     

流化床内不同密度颗粒流动特性的数值模拟

将计算流体力学和离散单元法相结合,在自行开发程序上对流化床内随机生成的密度服从正态分布的颗粒的流动特性进行数值模拟,得到了不同床层表观气速下床内颗粒的流化过程图、气相速度场以及不同密度颗粒的速度场、体积分数沿床高的分布和分离率.结果表明:在较小床层表观气速下,床内不同密度的颗粒会出现分离现象,并且颗粒的密度差越大,颗粒分离越明显,当增大床层表观气速后,颗粒间的分离程度减弱;小密度颗粒的速度大于大密度颗粒,颗粒间的密度差越大,颗粒间的速度差也越大.     

高温气固流化床的流化特性

气固流化床中的压力脉动信号是研究流化床内流体动力特性的一个重要途径，本文在２０℃～６４０℃的温度范围内，测试了３种颗粒在不同流化速度下气固流化床中的瞬态压力脉动信号，通过分析压力脉动进一步研究了温度对床内流化特性的影响。研究结果表明：高温下气固流化床内的流化特性与常温下有显著的差异，随温度的升高，床内的流化质量有所改善；对于同一种颗粒，不能将常温下所得到的流化特性推广到高温的条件下。     

二维喷动流化床内气体混合的实验研究

在300 mm×30 mm×2 000 mm的喷动流化床冷态试验台上,在喷动区和环形区分别采用CO和SO2作为示踪气体的方法,获得了不同喷动气流速和流化气流率这两个重要的操作参数下,不同床层高度上示踪气体的径向分布,考察了床内喷动区与环形区之间气体的混合特性。结果表明,示踪气体在不同床层高度的径向分布呈现较大的差异,并沿床层高度依次降低。在稳定的流动状态下,喷动气速度的增大,使喷动气向环形区的传质加强,也促进了流化气在环形区的混合;流化气流率的增大,气体由环形区向喷动区的传质大于由喷动区向环形区的传质,且流化气在环形区混合加强。在不稳定的流动状态下,气体的径向浓度分布曲线两边出现不对称。     

造纸污泥流化特性试验研究

在一床截面积为０４×０．４ｍ２的冷态流化床试验台上对一全水份为１２７５％的造纸污泥进行了流化特性试验研究。试验表明，只用该种物料作床料，床层无法正常流化。它必须与异比重的惰性物料配制成双组份床料后才能获得理想的流化效果。通过对比试验确定了合适的惰性物料粒径；研究了床料配比对临界流化速度的影响并给出一回归公式。还通过试验测定了造纸污泥的终端速度。上述研究结果对该类物料的流化干燥装置及燃用该类物料的流化床锅炉（或焚烧炉）的设计和运行均有重要的参考价值。     

生物质内循环流化床气化炉结焦特性

分析了近年来生物质流化床气化炉的结焦研究的状况,总结了影响生物质流化床气化过程中结焦的因素和确定结焦时最小流化速度的几类计算方法。分析了辽宁营口160kW气化装置结焦过程中运行参数的变化情况,给出防止结焦发生的一些措施。     

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     

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.     

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.     

Experimental study on cotton stalk combustion in a circulating fluidized bed

This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO₂ and NO) in flue (stack) gas were measured.     

A simple fuel bed model for predicting particulate emissions from a wood-waste boiler

Two simple physical and chemical models of a burning wood-waste fuel bed have been developed in order to predict the influence of fuel and operating parameters on particulate emissions from this source. First, a quiescent overfeed fuel bed is considered. Mass and energy balances along with assumptions for off-gas compositions are used to establish the bed off-gas temperature, composition, and bulk flow velocity. These have been used to predict entrainment of fuel particles. Second, the potential for fluidization of the fuel bed under typical operating conditions is considered. A simple comparison of bulk flow velocity with the minimum fluidization velocity for the bed is used as a criterion for fixed bed/fluidized bed operation. The ejected char and ash particles from the fluidized bed are included as part of the entrained particle stream in order to predict particulate emissions with a fluidized fuel bed. These two models are then used to predict the influence of undergrate airflow rate, fuel feed rate, and fuel size distribution on particulate emission from a wood-waste boiler.     

Energy analysis in fluidized‐bed drying of large wet particles

Energy analysis of a fluidized‐bed drying system is undertaken to optimize the fluidized‐bed drying conditions for large wet particles (Group D) using energy models. Three critical factors; the inlet air temperature, the fluidization velocity, and the initial moisture contents of the material (e.g., wheat) are studied to determine their effects on the overall energy efficiency to optimize the fluidized bed drying process. In order to verify the model, different experimental data sets for wheat material taken from the literature are used. The results show that the energy efficiencies of the fluidized‐bed dryer decrease with increasing drying time and become the lowest at the end of the drying process. It is observed that the inlet air temperature has an important effect on energy efficiency for the material where the diffusion coefficient depends on both the temperature and the moisture content of the particle. Furthermore, the energy efficiencies showed higher values for particles with high initial moisture content while the effect of gas velocity varied depending on the material properties. A good agreement is achieved between the model predictions and the available experimental results. Copyright © 2002 John Wiley & Sons, Ltd.     

Experimental research on agglomeration in straw-fired fluidized beds

Agglomeration is a major problem in straw combustion in a fluidized bed. This paper presents the results of rice straw combustion experiments carried out under different operating conditions in a bubble-fluidized bed (BFB). The influences of bed material size and type, feeding mode, temperature, and fluidization number on agglomeration were discussed, and the mode of alkali accumulation in the bed was analyzed. The results indicated that low bed temperature, short fuel in-bed residence time, high fluidization number, and small bed material particles are conducive to agglomeration prevention. Successful extended combustion in a small BFB and in a bench-scale circulating fluidized bed was carried out without agglomeration under selected parameters. This work suggests the possibility for combusting high-alkali straw using fluidization technology.     

Fluidization analysis for catalytic decomposition of methane over carbon blacks for solar hydrogen production

A series of bed collapse tests were conducted for determining the dense fluidization flow rate of a gas-solid mixture in a micro-channel fluidized bed reactor, and a separate simulation was created for calculating the reactor conversion and temperature of the catalytic methane pyrolysis. The minimum fluidization and minimum bubbling flow rates were determined to be 3.04 and 8.07 sccm for a 2 × 4 mm² reactor channel with an average voidage of 0.57; 6.21 and 15.9 sccm for a 4 × 6 mm² channel with an average voidage of 0.42, respectively. By building a correlation between these critical velocities and the cross-sectional area of the fluidized bed reactor channel, the dense fluidization flow rate at the micro-/mini-channel level with an internal diameter range from 0.3 to 1 mm is predicted between 1.47 to 4.21 sccm. In the simulation, an internal diameter of 0.6 mm, a 10-kW solar input rate, and an initial gas flow rate from 0.08 to 0.23 sccm that expands to 1.5–4.3 ccm at the reaction temperature, are considered as the optimal conditions to maintain a reasonable conversion of methane pyrolysis and to keep the mixed fluid in the dense fluidization within the laminar flow range. The conversion of 79% under these conditions was calculated numerically and found to be promising compared to literature reports. An additional force analysis on a single carbon black particle is shown with different reactor orientations to validate the experimental data and simulation results.