烟气循环流化床脱硫数值模拟进展及现状分析

介绍了循环流化床半干法烟气脱硫过程气固两相流动和反应特征及其数值模拟方法。对两相流常用的离散颗粒模型、双流体模型的研究进展进行论述,对于四步骤的脱硫模型进行说明,并介绍了耦合流动模型与脱硫模型过程模拟的优缺点。提出对循环流化床脱硫进一步数值模拟的工作重点应集中在颗粒动力学、曳力模型的修正和反应过程细化模拟方面。     

基于连续方法的气固循环流化床全回路模拟及稳定性分析

相比对单个操作单元的模拟，气固循环流化床的全回路模拟能全面揭示各单元之间的联系、诊断操作突变等现象，对实际工业生产更具指导意义。本研究在连续介质模型结合颗粒动理论的框架下，对一套虚拟过程工程(VPE)的气固循环流化床装置进行了全回路模拟和稳定性分析。模拟发现了提升管中的颗粒浓度及压降发生大幅度的周期性震荡现象，两种完全不同的操作状态，即稀相输送和浓相输送，交替式地出现。为分析该现象产生的原因，考察了模型因素(主要是气固相间曳力)和操作因素(颗粒藏料量和提升管表观气速)对周期性震荡现象的影响。研究发现，将考虑非均匀结构影响的曳力替换成均匀曳力，仍不能消除周期震荡现象，其颗粒输送返回装置(Loop-seal)压头不足以保证颗粒从下降管平稳输送到提升管，而降低气速和增大藏料量都有利于颗粒循环输送的稳定性，防止“窜气”现象的发生。结合上述现象，进一步聚焦影响颗粒输送的关键点，即Loop-seal气动阀，采用引入虚拟阀门的方式提高Loop-seal输送管中的输送阻力，从而有效改进了全回路模拟的稳定性，其预测得到的提升管轴向压降分布与实验值基本吻合。     

网格尺寸对循环流化床内气固两相流动的影响

将基于能量最小多尺度方法(EMMS)的曳力模型耦合到双流体模型中,并针对循环流化床内的气固两流动进行了模拟研究。采用全滑移壁面边界条件处理颗粒相,考察了3种网格尺度对轴向空隙率和出口颗粒循环量等气固流动特性的影响。计算结果表明,应用EMMS曳力模型处理相间作用力,同时在采用全滑移壁面边界条件处理颗粒相时,双流体模型能够正确预测轴向空隙率分布。采用网格尺寸为2.325 mm×20 mm时,模拟结果和实测数据吻合较好,表明在循环流化床的数值模拟中选择恰当的网格尺度是极为重要的。     

基于EMMS的循环流化床流域研究

流化床的设计、放大和优化需要对流域有基础的认识，然而气固系统的流域划分至今仍存在诸多争议。总结了气固流化系统流域划分的研究现状，并分析了流域划分的主要争议，发现文献中对快速床的界定存在分歧。通过耦合基于稳态EMMS的曳力模型开展双流体模拟，对不同气速和颗粒浓度下的循环流化床进行了数值研究。模拟结果捕捉到了颗粒回流、节涌等现象，据此确定了快速床的操作边界并绘制了流域图，该流域图能够展示循环床中的各流域形态。     

65 t/h高低差速循环流化床流动特性模拟

对一台65 t/h高低差速循环流化床炉内流动特性进行二维数值模拟。采用基于颗粒动力学理论的欧拉双流体模型来描述气固流动,湍流模型、气固曳力模型和不同粒径颗粒间曳力模型分别采用RNG k-ε per phase模型、Gidaspow模型和Schiller-naumann模型,并应用商业计算流体力学软件Fluent进行数值计算,得到炉内颗粒速度分布、压力分布和颗粒浓度分布,并将压力分布与实测值进行对比。在欧拉双流体模型中分别采用单粒径固相模型和多粒径固相模型,并对模拟结果进行对比分析。结果表明,单粒径固相模型能够较好预测高低差速循环流化床炉内流动特性,为其优化设计、运行及大型化提供了理论依据。     

气固流化系统多尺度跨流域EMMS建模

气固流化床反应器是典型的具有多尺度非均匀动态结构的复杂系统。实现对该类反应器定量描述和定向调控的关键是深入了解系统内介尺度结构的形成和演化特征。能量最小多尺度（EMMS）方法为气固非均匀系统的量化表征提供了一种通用的建模思路。首先回顾了EMMS理论在构建曳力本构关系方面的应用,重点介绍了本课题组在EMMS曳力模型普适化方面所做的部分工作;随后对介尺度结构时空动态演化行为的群平衡建模方法进行了论述,并给出了群平衡和结构曳力模型相耦合的连续介质模拟框架;最后讨论了EMMS原理在预测反应器宏尺度动力学方面的应用,包括模型在不同流域的拓展、操作相图的绘制以及循环流化床的全回路稳态建模方法等。     

氢氧催化燃烧流化床反应器流体力学特征的数值模拟

针对氢氧催化燃烧流化床反应器尺寸小、气体密度低并具有放射性、气流量小、催化剂粒度小等特点,改进了欧拉一欧拉多相流模型中的Syamlal-OBrien曳力模型,从而实现了均匀布气氢氧催化燃烧流化床反应器的流体力学模型的模拟,得到了该气固流化床的流化形态,计算结果与文献结果吻合性好,所采用的模型和数值方法具有较高可靠性.     

提升管内细长颗粒流化运动的气固多向耦合模型的构建

细长颗粒气固两相流研究已成为气固两相流研究的重点问题之一。而各种细长颗粒在流场中的受力、运动模型构建以及细长颗粒/流场间的耦合关系的构建是细长颗粒两相流数值模拟研究的难点之一。在已经基于刚体动力学原理构建了细长粒子气固两相流单向耦合模型的基础上,结合基于κ-ε模型的球形粒子-流场气固间耦合关联式,并改进细长颗粒间碰撞模型,从而构建了细长颗粒气固两相流动多向耦合数值模拟平台。并且采用此平台对某一实际流化床内的细长粒子气固两相流场进行了数值模拟研究。研究表明,不同时间细长颗粒在流场内的停留位置具有较强的随机性;细长颗粒在流化运动过程中伴随有较明显的取向选择性;细长颗粒的存在会导致当地流场的速度、压强均有较为明显的下降。     

一个气固两相流动阻力的新模型

为了解决现有经验气固阻力模型的普适性问题,合理描述颗粒团聚现象对气固阻力的严重影响,从理论分析入手,将传统的CFD方法与系统能量分析方法相结合,建立了计及颗粒团聚效应的气固阻力分析模型.与现有模型相比,新模型不仅合理地描述了气固两相相互作用的物理过程,而且避免了以往采用经验系数所导致的误差和局限性.经循环流化床数值模拟的检验证明,新模型的计算结果与实验数据吻合,从而在稠密气固两相流动的数值模拟中具有相当的优越性.     

耦合EMMS曳力与简化双流体模型的气固流动模拟

提出了一种耦合EMMS曳力的简化双流体模型,该模型忽略固相黏度,用简单的经验关联式来计算固相压力,并且耦合考虑了介尺度结构的EMMS曳力模型来计算气固相间作用力。采用简化双流体模型成功模拟一个三维实验室尺度鼓泡流化床,数值模拟结果与完整双流体模型以及实验测量结果进行了比较,结果表明耦合EMMS曳力的简化双流体模型模拟结果与完整双流体模型耦合EMMS曳力的模拟结果基本相当,并且都与实验结果吻合良好,然而简化双流体模型的计算速度是完整双流体模型的两倍以上。这表明曳力模型在气固模拟中起着主导作用,而固相应力的作用是其次的,耦合EMMS曳力的简化双流体模型在实现工业规模气固反应器快速模拟中具有巨大潜力。     

Particle Residence Time Determination with More Than 400 Tracer Particles in a Circulating Fluidized-Bed Full-Loop System

The residence time of particles in a circulating fluidized-bed (CFB) reactor is the essential parameter for design and performance of a CFB system. In this work, a noninvasive measurement method of positing tracer particles in a full-loop CFB system is proposed. Based on the radio frequency identification technology, more than 400 tracer particles with individual code were made and moved freely in the CFB full loop system. With detecting the tracer particles' trajectory by high-frequency readers located outside of the system, the effects of operating parameters including fluidization number and solid inventory on the movement and residence time of the tracer particles in the CFB system were investigated. The particle residence time is distributed in a certain range, influenced by superficial gas velocity and solid inventory.     

Full‐Loop Simulation of Gas‐Solids Flow in a Pilot‐Scale Circulating Fluidized Bed

In order to study the system hydrodynamics in a circulating fluidized bed (CFB), a 3D full‐loop simulation was conducted for a pilot‐scale CFB. The Eulerian‐Eulerian two‐fluid model with the kinetic theory of granular theory helped to simulate the gas‐solids flow in the CFB. The system hydrodynamics including pressure balance, vectors of gas and solids, distribution of solids holdup, and instantaneous circulating rates were obtained to get a comprehensive understanding of the system. It was predicted that the main driving force was the pressure drop of the storage tank. The storage height and valve opening were critical operating factors to control the riser operation. The effects of operating conditions including solids circulating rates and superficial gas velocity on the hydrodynamics were investigated to provide guidance for the stable operation of the CFB system.     

大型循环流化床锅炉整体动态模型的构成及其求解方法

介绍了构成大型商用循环流化床锅炉整体模型的燃烧系统、汽水系统以及烟风系统模型的建立及综合方法 ,并提出了一种求解整体模型的“双桢速”算法 .采用这种整体模型的构成及综合方法能够有效地求解具有复杂物理化学特性和结构的大型复杂系统模型     

Multi‐scale CFD simulation of operating diagram for gas–solid risers

Our recently presented multi‐scale computational fluid dynamics (CFD) approach has proven to be able to capture the choking phenomena in a circulating fluidized bed (CFB). However, how to transfer this capability to assist industrial operation remains to be explored. To this end, this paper presents further simulation results over the intrinsic flow regime diagram and the operating diagram for gas–solid risers, showing the variation of flow regimes with gas velocity and solids flux as well as riser height. It is confirmed that the choking in CFB risers, characterized by the saturation carrying capacity and the coexistence of both dense and dilute flows, holds clear‐cut definition in hydrodynamics. In physics, both the choking, non‐choking transitions, and the critical point in‐between are intrinsic nature of gas–solid riser flows; they initiate as functions of gas velocity and solids flux. In engineering operation, however, their appearances vary with the riser height used. As a result, the intrinsic flow regime diagram can be defined by the combination of gas velocity and solids flux, although it is hard to obtain in practice owing to the limitation of riser height. The operating diagram of a CFB should be, accordingly, height‐dependent in practice, demanding the riser height as a parameter besides commonly believed gas velocity and solids flux.     

A multiscale mass transfer model for gas-solid riser flows: Part II—Sub-grid simulation of ozone decomposition

This article is to test the EMMS-based multiscale mass transfer model through computational fluid dynamics (CFD) simulation of ozone decomposition in a circulating fluidized bed (CFB) reactor. Three modeling approaches, namely types A, B and C, are classified according to their drag coefficient closure and mass transfer equations. Simulation results show that the routine approach (type C) with assumption of homogeneous flow and concentration overestimates the ozone conversion rate, introduction of structure-dependent drag force will improve the model prediction (type B), while the best fit to experimental data is obtained by the multiscale mass transfer approach (type A), which takes into account the sub-grid heterogeneity of both flow and concentration. In general, multiscale behavior of mass transfer is more distinct especially for the dense riser flow. The fair agreement between our new model with literature data suggests a fresh paradigm for the CFB related reaction simulation.     

Experiment and 3D simulation of slugging regime in a circulating fluidized bed

A circulating fluidized bed (CFB) is widely applied in many industries because it has high efficiency. To develop and improve the process, an understanding of the hydrodynamics inside the CFB is very important. Computational fluid dynamics (CFD) represents a powerful tool for helping to understand the phenomena involved in the process. In this study, a CFD model was developed to represent a cold model of the laboratory scale CFB which was designed to study the hydrodynamics of a CFB using commercial CFD software. The Eulerian approach with kinetic theory of granular flow was used for simulating the hydrodynamics inside the system. After proper tuning of relevant parameters, the pressure profile along the equipment from the simulation was well agreed with that from the experiment. The simulation result expresses the hydrodynamic parameters of the slug flow such as solid volume fraction, gas and solid velocities and granular temperature in the riser.     

超低排放燃煤机组硒的迁移转化及飞灰对其富集特性

采用微波消解法和氢化物发生-原子荧光光谱法考察了9台超低排放在役机组硒迁移转化规律,探究了循环流化床（CFB）和煤粉炉（PC）机组飞灰特性差异对硒吸附能力的影响。燃烧后煤中硒几乎全部呈现挥发态,底渣中残留量极低。与浓度归一化和质量分布法相比较,相对富集系数法可以客观地评价燃煤副产物中硒的富集能力,两类机组中硒均主要富集于飞灰中。CFB较低炉膛温度和添加CaO可以降低入炉煤中硒释放比例并增强飞灰对硒的吸附能力,故其底渣和飞灰中硒的富集程度均高于PC,导致脱硫石膏中硒富集程度低于PC。飞灰对硒的吸附量随比表面积或孔容积增大而增大,但随粒径或孔径增大而减小。CFB飞灰中未燃尽碳含量高、形状不规则、表面粗糙且存在较多蜂窝状孔隙,导致其对硒的富集程度高于PC飞灰。