entrained-bed携带床.ppt

5 COMPREHENSIVE MODELING,5.1 INTRODUCTION 5.1.1 Background,(i) Key Model Premises,Smoot and Smith suggested a number of key premises as a foundation for the development of comprehensive combustion models. These premises are summarized below with illustrative examples. premise 前提、假设 assume 假设 assumption 假设,Premise 1,The only steps that need to be considered are those that control the progress. Example In entrained-bed combustion, homogeneous chemical kinetics are often ignored and turbulent mixing is assumed to be rate-limiting. entrained-bed 携带床 homogeneous 同相的、均相的 rate-limiting 影响速率的 kinetics 动力学,Premise 2,Model components can be independently investigated and modelled with competing processes suppressed or absent. competing process 同时发生影响的 suppress 抑制,Example,The rate of particle weight loss due to coal devolatilization can be measured in inert atmospheres where the resulting char does not oxidize. Thus, all of the weight loss can be attributed to devolatilization, and the kinetics for devolatilization in inert atmospheres is assumed to apply in oxidative atmospheres also. char 焦炭、炭 devolatilization 析出挥发分 inert atmosphere 惰性气氛 oxidative atmosphere 氧化性气氛,Premise 3,Individual model components (i.e. submodels) can be combined to give a total description of the overall process. Example Fig. 5.1 shows the required components for a generalized model of large-scale, entrained-bed furnaces and gasifiers that is being developed at ACERC. submodel 子模型 generalized model 通用模型 gasifier 气化器,Premise 4,Basic rate measurements made in laminar systems can be applied to turbulent flows when the effects of turbulence on kinetic processes are taken into account. Laminar 层流 turbulent 湍流,Example,Kinetic measurements made in laminar systems for nitric oxide formation are applied to entrained-bed coal combustion by convolving instantaneous reaction rates over the statistics (e.g. mixture fraction probabilities) of the turbulence nitric oxide NO2 convolve 卷 instantaneous reaction rate 瞬时反应速率 mixture fraction 混合分数 probability 概率,Premise 5,The behavior of individual particles or droplets can be used to predict the behavior of clusters or clouds of particles or droplets in entrained-beds. cluster 颗粒团 cloud of particles 颗粒云 droplet 液滴,The Lagrangian approach is often used for particles in entrained combustion. Individual particle trajectories are independently calculated without any particle-particle effects. Particle cloud properties are currently a subject of research and have not yet been incorporated into comprehensive combustion models. Lagrangian approach 拉格朗日法 Eulerian approach 欧拉法 trajectory 轨迹,Premise 6,Fundamental, steady-state data can be used to predict transient or pseudo-steady-state behavior. Example Devolatilizing and reacting coal particles represent a pseudo-steady-state situation where the particle mass and composition are changing. However, Steady-state correlations for external heat, mass, and momentum transfer are used to predict reaction rate, temperature, and particle motion. pseudo-steady-state 准稳态 momentum transfer 动量传递,Premise 7,Numerical methods validated for simple systems can be applied to complex systems that defy rigorous validation. validate 使证实 validation 验证 defy 使成为不可能 rigorous 严格的,Example,The governing equations of a comprehensive combustion model, for which there is no known analytical solution, can be modified so as to be exactly satisfied by a simple, analytical solution. The analytical solution can then be used to validate the finite difference approximations used in the model. governing equation 控制方程 analytical solution 分析解 finite difference approximation 有限差分法 approximation 近似,The above premises help make the development of comprehensive models for complex coal combustion processes feasible with todays technology. feasible 可行的,(ii) Key Model Variables,Key variables in comprehensive models include independent variables dependent variables, input data for each inlet stream, reactor parameters,Independent variables,physical coordinates (x, y, z) (x, r,) time (t) cylindrical coordinate 柱坐标,Dependent variables-1,gas species composition gas temperature gas velocity pressure turbulent kinetic energy(湍流动能) turbulent energy dissipation rate(湍流动能耗散率) mixture fraction (mean and variance) species 组分； composition 成分 variance 方差,Dependent variables-2,particle/droplet composition particle/droplet temperature particle/droplet velocity particle/droplet diameter gas density gas viscosity,Input data for each inlet stream - 1,gas velocity gas composition gas temperature gas turbulence intensity gas mass flowrate pressure turbulence intensity 湍流强度 mass flowrate 质量流率,Input data for each inlet stream - 2,particle/droplet slip velocity particle/droplet composition particle/droplet temperature particle/droplet size distribution particle/droplet loading particle/droplet bulk density slip velocity 滑移速度 size distribution 粒度分布 particle loading 颗粒浓度 bulk density 堆积密度,Reactor parameters,configuration (e.g. upflow, downflow) inlet configurations (e.g. location, presence of quarl) inlet locations dimensions wall properties wall thickness configuration 布置，构造 quarl 扩口,Decision of the Key Model Variables,Reactor parameters and input data for each inlet stream specified by the user Dependent variables calculated by the model as functions of the independent variables Physical coordinates either Cartesian (x, y, z) or cylindrical (x, r,) Cartesian 迪卡尔坐标的 cylindrical 柱坐标的,(iii) Model Classification,携带床 固定(移动)床 流化床 (磁流体) 柱塞流 良搅拌 回流 预混 扩散,Flame type,Flame type is a special criterion which applies to entrained beds. In a premixed flame, the particles are entrained in the combustion air and fed to the furnace. In a diffusion flame, the particles and combustion air are fed separately and mixed in the furnace. Most practical pulverized coal flames are of the diffusion type.,Current status,computer-aided design and engineering have become commonly accepted practice in many advanced technical areas. the application of comprehensive modeling in combustor design has been rather limited. Combustors have been largely designed and optimized by gradual scale-up of experimental prototypes. scale-up 放大 prototypes 原型，样机,factors limit application of computer-aided design in combustion - 1,include processes, the complexity of fuel feedstocks, the variety and complexity of combustion equipment (e.g. fixed-bed, fluidized-bed, entrained-bed, cyclones, spray combustors, turbines) the variety and complexity of operating conditions feedstock 填料 cyclone 旋风子 spray 液雾,factors limit application of computer-aided design in combustion - 2,the substantial computational times and associated costs required to achieve converged solutions the lack of experienced individuals in industry with expertise to apply the developing technologies the specialized and non-robust nature of existing comprehensive models, which require a broad range of experience and competence from users. converged 收敛的 expertise 经验 robust 稳定的 competence 能力,factors limit application of computer-aided design in combustion - 3,the lack of suitable hardware and software for graphically viewing model predictions, once they are obtained Comprehensive, three-dimensional models for gaseous or entrained-flow coal combustion, for example, produce values for 25-30 variables at each of 104-106 nodes. Such enormous quantities of data are impractical to analyze without the aid of suitable visualization graphics. Variable 变量 node 节点 visualization 可视化 graphics 图,recent improvements,recent developments are helping to improve the accuracy the generality the reliability of comprehensive models the ease of interpreting results.,recent developments,Improved diagnostic methods providing more accurate and more complete data for model evaluation. More powerful and less costly computers providing quicker turnaround at lower cost. Sophisticated graphics programs providing increased capability for visualization of data in three dimensions. evaluation 评价 turnaround 完成 Sophisticated 复杂的,With these and other advancements, comprehensive modeling is becoming accepted as an important tool in coal combustor design.,5.1.2 Scope and Objectives,provide a general description of the state-of-the-art of comprehensive modeling of coal combustion processes. Entrained, fixed (slowly moving), and fluidized beds are considered. The material is oriented strongly towards ACERC experiences in modeling entrained and fixed beds the state-of-the-art 现状,5.2 ENTRAINED-BED MODELS 5.2.1 Basic Model Element,Entrained-bed coal combustion and gasification processes,These processes are characterized by dilute suspensions of small (1-400 m) particles entrained in turbulent, flowing gas. Secondary air streams are often swirled to promote particle ignition and flame stability, causing complex, recirculating flow patterns. dilute 稀的 swirl 旋流，使旋转 ignition 着火 recirculate 回流 flame stability 火焰稳定性 flow pattern 流型,The particles are dispersed by the turbulence of the gas and heated by radiation and convection, causing devolatilization and heterogeneous reaction. Particle reaction rate is influenced by internal and external diffusion, heat transfer processes, and intrinsic kinetics. heterogeneous 异相的 diffusion 扩散 intrinsic 内在的,Volatiles and oxidizers continue to react in the gas phase, while mineral matter collects on walls and heat transfer surfaces. mineral matter 矿物质,Non-comprehensive models,Computational fluid dynamics (CFD) models have been used to predict the fluid flowfield for non-reacting conditions. CFD models have also been used with specified, local heat input to predict the effects of heat of reaction. Models for gaseous combustion have been applied to predict the reacting flowfield in the absence of particles. predict 预报，模拟,Heat transfer models have been used to calculate the effects of radiation on wall heat flux, based on a user-supplied, turbulent gas flowfield. Some of these calculations have been extended to include particle burnout and ash deposition. heat flux 热流(密度) burnout 燃尽 deposition 沉积,I the sense of treating the overall process in standalone fashion. They each focus on various decoupled subprocesses and require significant input from other models or experimental data. standalone 孤立的 decouple 解藕的 subprocess 子过程,Components or submodels in a comprehensive entrained-bed model -1,1. Turbulent fluid-mechanics 2. Gaseous, turbulent combustion 3. Particle dispersion 4. Coal devolatilization. 5. Char oxidation 6. Radiation dispersion 弥散,Components or submodels in a comprehensive entrained-bed model -2,7. Pollutant formation 8. Fouling/slag formation 9. Other (e.g. particle fragmentation) Particle-particle interactions are usually neglected in entrained-bed modeling due to the lightly loaded nature of these systems fouling 沾污 slag 结渣 fragmentation 破碎,5.2.2 Review of Existing Comprehensive Models,Summary of the models,All of the models have been applied to coal combustion. 3 models (PCGC-2, the Imperial College model, and the University of Newcastle model) appeared during the first half 1980s and continue to evolve. FLUENT (Sheffield Univ. and the IFRF)is an extended CFD model and was applied it to coal combustion. The rest of the models were developed specifically for coal combustion.,Similarities among the models - General,All of the models are steady-state. Eight are 2-D, and six are 3-D. The FLUENT model is capable of 3-D, transient CFD calculations, but only 2-D, Steady-state results have been reported for coal combustion.,Similarities among the models - Turbulence,Most of the models use a k- submodel for turbulence. Only a few has incorporated a Reynolds stress closure. closure (模型的)封闭,Similarities among the models - chemistry/turbulence interaction 1,All fourteen assume fast chemistry (ref. 4) for the gas phase. One incorporates kinetics for CO oxidation. Nine use the less rigorous eddy breakup (EBU) method for modeling the chemistry/turbulence interaction. Four use the statistical probability density function (pdf) method. fast chemistry 快速化学反应 probability density function 概率密度函数(pdf),Similarities among the models - chemistry/turbulence interaction 2,One neglects the interaction altogether and calculates time-mean gas properties based on local equilibrium. Prof. Chou at Tsinghua University employed correlation modeling to avoid the fast chemistry assumption and model the cross-correlation terms in the time-mean reaction rate with algebraic expressions involving time-mean properties. equilibrium 化学平衡 correlation 关联(式) cross-correlation 互关联,Similarities among the models - particle dispersion,Thirteen of the models perform Lagrangian calculations for particles. Only one uses a strictly Eulerian approach. Four of the models ignore turbulent particle dispersion.,Similarities among the models - devolatilization,Most of the models use a competing-reactions submodel for devolatilization which predicts weight loss as a function of heating rate. One uses a distributed activation energy. Three models use a single-reaction devolatilization submodel with specified products. Several include an empirical swelling factor. competing-reactions submodel 平行反应子模型 activation energy 活化能 empirical 经验的,Similarities among the models char reaction,Most models account for external diffusion of oxidizing species to the particle surface. Only two (PCGC-2/-3) are reported to account for the blowing effect of volatiles on mass transfer. All of the models neglect the effect of turbulence on the solid phase and heterogeneous reactions. blowing effect,Similarities among the models radiation,Multiflux radiation models (including discrete ordinates) are popular because of their reasonable predictions at moderate cost. A few models use the more accurate and more costly zone and Monte Carlo methods. multiflux radiation model 多(4/6)通量模型 zone method 区域法 Monte Carlo methods 蒙特卡罗法,Similarities among the models fouling, slagging and pollutant formation,Most models do not currently predict fouling, slagging and pollutant formation, although interest in these areas is high and model development is on-going.,Similarities among the models algorithm,Most solve the fluid mechanics equations using a method based on the well-known SIMPLE algorithm. Only one model (PCGC-2) has significantly widespread application or evaluation for a number of different processes, including both combustion and gasification. algorithm 算法,Major differences - method of modeling turbulent particle dispersion 1,Several incorporate the effects of gas turbulence on particle motion through an empirical turbulent diffusion velocity, which is expressed as a gradient of either the mean velocity or particle bulk density. The use of empirical constants makes it possible to perform the particle phase calculations with relatively few particle trajectories (e.g. 50-1200), each representing not a single particle, but an ensemble of particles with a particle number flowrate. gradient 梯度 ensembl 组，群,Major differences - method of modeling turbulent particle dispersion 2a,A second approach uses the more rigorous Monte Carlo method, wherein a statistically significant number (e.g. 5-10 times the number required for the deterministic approach) of individual particle trajectories are calculated. Particle velocity and motion are calculated using the instantaneous gas velocity, including turbulent fluctuations. deterministic approach 确定轨道模型 deterministic 宿命的 fluctuation 脉动，波动 instantaneous velocity 瞬时速度 fluctuating velocity 脉动速度,Major differences - method of modeling turbulent particle dispersion 2b,During each particles flight, the fluctuating velocity is randomly sampled at appropriate time intervals and added to the mean velocity. random 随机的 sample 取样 interval 间隔 mean velocity 平均速度,Major differences - method of modeling turbulent particle dispersion 3,The difference in computational burden between the two methods is substantial, given the difference in number of required trajectories and the need to compute fluctuating velocity at appropriate intervals in the stochastic method. The stochastic approach is more realistic, and an important consideration is that of correctly specifying the instantaneous particle conditions at the inlet. stochastic method 随机轨道模型,Major differences - gas chemistry/turbulence interaction 1,Treating the gas chemistry/turbulence interaction (i.e. local equilibrium, eddy dissipation, full pdf) is important. Ignoring the effects of turbulence resulted in a very localized flame with steep temperature and concentration gradients. The smoothing caused by including turbulence effects is evident. localized 狭小的,Major differences - gas chemistry/turbulence interaction 2a,The degree of smoothing is even more pronounced with the eddy dissipation model. The eddy dissipation and full pdf methods are equivalent if the probability density function for the pdf method is made to consist of three delta functions centered at stoichiometries of pure oxidizer, fuel, and full stoichiometric equilibrium product. stoichiometry 化学当量比,Major differences - gas chemistry/turbulence interaction 2b,The eddy dissipation model has a significant drawback in that it “cannot predict intermediate species in fuel-rich regions where the volatile products may form species such as CO“ intermediate species 中间组分 fuel-rich regions 富燃料区，浓燃料区,Grid technique,The FURMO model employs a rather interesting approach to achieve computational efficiency with Eulerian particles. A fine grid is used to solve the fluid mechanics equations, and a coarse grid is used to solve the equations for the combustion model (e.g. mixture fraction). fine grid 密网格，细网格 coarse grid 疏网格，粗网格,Fine and coarse grids for a 560MW boiler(FURMO),5.2.3 Description of a Comprehensive Coal Combustion/Gasification Model (PCGC-2/PCGC-3),Background,PCGC(Pulverized Coal Gasification or Combustion) PCGC-2 was developed at BYU in the early 1980s and has continued to evolve. The curre