气固循环流化床全回路数值模拟研究进展

气固循环流化床具有良好的混合、传热、传质、反应特性,同时还具有处理量大、可连续生产等优点,在众多领域均有广泛应用。气固循环流化床是一个由多个单元连接组合形成的循环回路,各单元间相互耦合、相互影响。对整个循环流化床系统进行全回路数值模拟,不仅能够获得更全面和详实的结果,而且在揭示系统流动规律和探究各单元内、单元间、单元与系统间的相互作用上具有独特优势。近十年来,以气固循环流化床全系统为模拟对象的全回路数值模拟研究逐渐兴起。本文对气固循环流化床全回路数值模拟方法的研究进展进行综述,对该方法的应用情况进行详细介绍,并对方法中采用的模型及相应特点进行逐一分析。循环流化床全回路系统同时存在多种流态,有待于建立适用于全回路系统的多流态物理模型（气固曳力模型与固相应力模型）。随着计算能力的提高以及物理建模的不断发展,全回路模拟方法将不断完善并发挥出更大的作用。     

基于CPFD方法的流化床生物质气化数值模拟

基于计算颗粒流体动力学(CPFD)建立了三维鼓泡流化床水蒸气-空气混合气化的数值模型,并进行了模型验证,结果表明模拟和实验具有良好的一致性。在该模型的基础上,研究了气化炉内气体分布以及温度分布;同时探究了生物质属性(颗粒粒径、含水率、种类)以及操作条件(气化温度、床料高度)对气化特性的影响。结果表明,生物质颗粒粒径对气化性能的影响存在一个最优值,平均粒径为0.6 mm是最佳的;较高的含水率会降低可燃气体产量,不利于气化反应的进行;四种生物质中,锯末气化的效率最高、可燃气体产量最大、气体热值最高,稻壳仅次于锯末但其碳转化率高于锯末;提高气化温度可以增加可燃气体的比例、提高气化效率;而初始床层高度的变化可以改变H₂/CO的比例。本实验为生物质水蒸气/空气气化提供了理论参考,有助于生物质原料的选取和处理,也有助于气化炉的放大和优化。     

Numerical simulation of 3D fluidized bed biomass gasification based on CPFD

Based on computational particle fluid dynamics (CPFD), a three-dimensional bubbling fluidized bed steam-air mixed gasification numerical model was established, and it was verified with experiment trials. The results show that the simulation and experiment have good consistency. Based on the model, the gas distribution and temperature distribution in the gasifier were studied; meanwhile, the biomass properties (particle size, water content, types) and operating conditions (gasification temperature, bed height) were investigated. The results show that there is an optimal value for the impact of biomass particle size on gasification performance, with an average particle size of 0.6 mm being the best; a higher water content will reduce the output of combustible gas and is not conducive to the gasification reaction. Among the four types of biomass, sawdust gasification has the highest efficiency, the largest combustible gas production, and the highest gas calorific value. Rice husk is second only to sawdust but its carbon conversion rate is higher than that of sawdust; increasing the gasification temperature can increase the proportion of combustible gas and increase gasification efficiency; while the change of initial bed height can change the ratio of H₂/CO. This experiment provides a theoretical reference for biomass steam/air gasification, which is helpful for the selection and processing of biomass raw materials, and also facilitates the amplification and optimization of the gasifier.     

Modelling and simulation of a coal gasification process in pressurized fluidized bed

A coal gasification mathematical model that can predict temperature, converted fraction and particle size distribution for solids have been developed for a high pressure fluidized bed. For gases in both emulsion and bubble phase, it can predict temperature profiles, gas composition, velocities and other fluid-dynamic parameters. In the feed zone, it could be considered a Gaussian distribution or any other distribution for the solid particle size. Experimental data from literature have been used to validate the model. Finally, the model can be used to optimize the gasification process changing several parameters, such as excess of air, particle size distribution, coal type and reactor geometry.     

循环流化床煤气化技术新进展

介绍国内外几种主要的循环流化床煤气化方法、流程和煤气化技术发展趋势。     

Numerical simulation of coal gasification at circulating fluidised bed conditions

This paper presents modelling results for a new pressurised fluidised bed gasifier concept, called the Power High-Temperature Winkler gasifier (PHTW gasifier). The numerical simulation of the steam/oxygen blown and lignite-fuelled power plant gasifier is performed on the 4800 t/day (1000 MW) at a pressure of 33 bar. The formation of flow pattern, turbulence, product gas composition, temperature, and radiation heat transfer were investigated. Influence of diameter variation on the flow patterns at constant operating conditions is presented. A comparison between the calculation and literature data of similar fluidised bed systems shows good conformance. To anticipate the solid's behaviour, particle concentration, particle size change due to pyrolysis and surface reactions, and particle tracks were modelled using an Eulerian–Lagrangian approach. While varying the total particle mass flow, the pressure drop as a function of reactor height was observed.     

CFD based combustion model for sewage sludge gasification in a fluidized bed

Gasification is one potential way to use sewage sludge as renewable energy and solve the environmental problems caused by the huge amount of sewage sludge. In this paper, a three-dimensional Computational Fluid Dynamics (CFD) model has been developed to simulate the sewage sludge gasification process in a fluidized bed. The model describes the complex physical and chemical phenomena in the gasifier including turbulent flow, heat and mass transfer, and chemical reactions. The model is based on the Eulerian-Lagrangian concept using the nonpremixed combustion modeling approach. In terms of the CFD software FLUENT, which represents a powerful tool for gasifier analysis, the simulations provide detailed information on the gas products and temperature distribution in the gasifier. The model sensitivity is analyzed by performing the model in a laboratory-scale fluidized bed in the literature, and the model validation is carried out by comparing with experimental data from the literature. Results show that reasonably good agreement was achieved. Effects of temperature and Equivalence Ratio (ER) on the quality of product syngas (H₂ + CO) are also studied.     

射流流化床体积效应对煤气化特性的影响

根据射流流化床内流体流动特性,在流体动力学行为相似的基础上,利用化学动力学软件Chemkin构建了射流床煤气化炉的动力学模型,通过体积放大引起物料停留时间及流体力学相似性的改变,研究其对射流床中物料特性的影响.分析了射流流化床几何相似放大与高径比减小放大对反应器特性的影响,认为等高径比放大停留时间较长,反应器内流体力学相似性较好,在后续工艺对出口组成要求较高时,采用此放大原则能满足要求.     

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。     

串行流化床煤气化试验

针对串行流化床煤气化技术特点,以水蒸气为气化剂,在串行流化床试验装置上进行煤气化特性的试验研究,考察了气化反应器温度、蒸汽煤比对煤气组成、热值、冷煤气效率和碳转化率的影响。结果表明,燃烧反应器内燃烧烟气不会串混至气化反应器,该煤气化技术能够稳定连续地从气化反应器获得不含N₂的高品质合成气。随着气化反应器温度的升高、蒸汽煤比的增加,煤气热值和冷煤气效率均会提高,但对碳转化率影响有所不同。在试验阶段获得的最高煤气热值为6.9 MJ•m^-3,冷煤气效率为68％,碳转化率为92％。     

Fundamentals of coal topping gasification: Characterization of pyrolysis topping in a fluidized bed reactor

Coal topping gasification refers to a process that extracts the volatiles contained in coal into gas and tar rich in chemical structures in advance of gasification. The technology can be implemented in a reactor system coupling a fluidized bed pyrolyzer and a transport bed gasifier in which coal is first pyrolyzed in the fluidized bed before being forwarded into the transport bed for gasification. The present article is devoted to investigating the pyrolysis of lignite and bituminite in a fluidized bed reactor. The results showed that the highest tar yield appeared at 823 to 923 K for both coals. When coal ash from CFB boiler was used as the bed material, obvious decreases in the yields of tar and pyrolysis gas were observed. Pyrolysis in a reaction atmosphere simulating the pyrolysis gas composition of coal resulted in a higher production of tar. Under the conditions of using CFB boiler ash as the bed material and the simulated pyrolysis gas as the reaction atmosphere, the tar yields for pyrolytic topping in a fluidized bed reactor was about 11.4 wt.% for bituminite and 6.5 wt.% for lignite in dry ash-free coal base.     

高颗粒通量循环流化床的构型作用

In order to achieve high solids circulation rate (G_s),an idea of coupling a moving bed to the bottom section of the riser of a circulating fluidized bed (CFB) was proposed and tested.The results from the preliminary study demonstrated that the solids circulation rate in the new-structure bed approached 370 kg·m^-2·s^-1 at superficial gas velocities around 10.5 m·s^-1 for sand particles with an average Sauter mean size of 378 μm.This study was devoted to further justifying the effects of the coupled moving bed by performing comparative studies in two CFBs with conventional configurations.It was shown that the pressure at the riser bottom and the realized solid circulation rate were only about 15 kPa and 230 kg·m^-2·s^-1 in the two conventionally configured CFBs,obviously lower than 25 kPa and 370 kg·m^-2·s^-1 in the moving bed coupled CFB.These verified that the coupled moving bed increased the force driving particles form the particle recycling side into the riser.The study further tested the effect of a few specially designed riser exit configurations,revealing that a smooth riser exit could facilitate solids circulation to increase the solids circulation rate.     

Effect of operating parameters inside circulating fluidized bed reactor riser with ring baffles using CFD simulation and experimental design analysis

The effect of the operating parameters on the system hydrodynamics and mixing inside two circulating fluidized bed reactor (CFBR) risers with different ring baffle configurations were investigated using computational fluid dynamics simulations and a 2⁴ factorial experimental design analysis. The operating parameters varied were the gas inlet velocity, and the mass flux, diameter and density of the solid particles, while the response variables were the standard deviation of the solid volume fraction (SVF) in the radial direction (SDSVF-RD) and the average SVF (ASVF). The results from the two CFBR risers with different ring baffle configurations showed a similar trend. The operating parameters that significantly affected the ASVF in both modified CFBR risers were the inlet gas velocity and solid particle mass flux, while those that significantly affected the SDSVF-RD were the inlet gas velocity and the inlet gas velocity–solid particle diameter–solid particle density interaction. For these systems, the lowest and highest ASVF was approximately 0.07 and 0.20, respectively, while the lowest and highest SDSVF-RD was 0.01 and 0.04, respectively. The low variability of the solid particle distribution and the high solid particle concentration will be suitable for chemical reactions. All the obtained results could be explained in terms of the system hydrodynamics. Finally, regression models to predict the mean solid particle concentration and variability of solid particle distribution in the system were obtained.     

Hydrodynamic studies on three-phase fluidized bed using CFD analysis

Three-phase fluidization refers to fluidization of solid particles by co-current, upward flow of gas and liquid-phases for the purpose of bringing three-phases in contact in a single operation. Due to complications in understanding hydrodynamics of three-phase fluidized bed, CFD analysis is used to predict the hydrodynamics of it. In this study, liquid-phase is water which flows continuously, where as the gas phase is air which is distributed discretely throughout the bed. Ceramic particle of 1 mm diameter, density of 2650 kg/m³ is used as a solid phase. Excellent mixing, heat and mass transfer rates are the unique features of three-phase fluidized bed. The selection of distributor plays an important role in the quality of fluidization [1]. CFD model is created as the realistic representation of actual fluidized bed. The liquid and solid flow is represented by the mixture model. The air is injected from the bottom of the fluidized by means of discrete phase method (DPM). Simulation results are obtained by using porous jump and porous zone model to represent the distributor. It is found that porous zone model is best applicable in the industries, since stability of operating conditions is achieved even with non-uniform air, water flowrates and with different bed heights(100 mm, 200 mm, 300 mm, 400 mm and 500 mm).Simulated Pressure drop values of the fluidized bed have good agreement with the experimental findings. As the gas flowrate increases, the pressure drop in the column is decreases, provided the initial bed height, diameter of the column, and liquid flowrate are constant. This is due to decrease in density of the fluid medium in the bed by means of more gas hold up. The approach of the simulated values to the experimental values can be reduced with better understanding the nature of the fluidized bed.     

鼓泡流化床内粉煤气化的数值模拟

将应用欧拉双流体模型对鼓泡气化炉内的气化过程进行研究。摒弃传统颗粒动理学理论中颗粒光滑无旋转的假设,引入颗粒的旋转运动,构建粗糙颗粒动理学理论来封闭双流体模型。基于燃烧理论建立粉煤热解、气化模型以及鼓泡床内气固之间以及气体和气体之间的传热、传质模型。采用该模型进行数值模拟计算,分析床内的气固反应过程,对比实验结果表明粗糙颗粒动理学理论适用于模拟鼓泡床气化炉内的反应。     

榆林煤作湿法加压气流床气化原料的分析

榆林煤属于高发热量、低灰、低灰熔点、高还原率的优质煤;通过成浆性试验,确定了原煤的粒度分布、添加剂加入量,并测定了料浆的流变特性;分析了原料煤反应活性和煤灰特性对气化操作的影响,并估计了原料煤的气化指标;指出榆林煤是适合湿法加压气化的优良煤种。     

玉米秸秆循环流化床气化中试试验

玉米秸秆是农业生产过程中产生的剩余物,其热解气化是秸秆类生物质处理应用的重要选择方向。为此,采用循环流化床气化中试装置对玉米秸秆进行了气化试验,研究空气当量比ER、原料含水率对反应温度、气化燃气组分与热值、气化效率及燃气中的焦油含量等气化特性影响规律,并通过改变进料量试验得到了在不同负荷运行条件下的优化工作参数。结果表明：①随着ER的增大,循环流化床气化炉内的反应温度升高,气化燃气中的CO₂含量增加,焦油与CO含量及燃气热值降低,气化效率随ER的增大呈现先增大后减小趋势,较理想的ER为0.26,此时的气化效率达到70.2%、燃气热值为5.1MJ/m³;②原料含水率的增大降低了气化炉内的反应温度,当原料含水率在5%～15%之间逐渐增大时,燃气中的H₂含量、燃气热值及气化效率均有提升,当含水率由15%继续增大到25%过程中,燃气热值与气化效率均出现了快速下降;③根据气化炉额定进料量设计值,改变进料负荷在66%～120%范围内,调节ER在0.26～0.3时均可得到较好的运行工况,对应得到的燃气热值为4.8～5.1MJ/m³、气化效率为69%～72%。     

CFD simulation of entrained-flow coal gasification: Coal particle density/sizefraction effects

Computational Fluid Dynamics (CFD) simulation of commercial-scale two-stage upflow and single-stage downflow entrained-flow gasifiers was conducted to study effects of simulating both the coal particle density and size variations. A previously-developed gasification CFD model was modified to account for coal particle density and size distributions as produced from a typical rod mill. Postprocessing tools were developed for analysis of particle-wall impact properties.For the two-stage upflow gasifier, three different simulations are presented: two (Case 1 and Case 2) used the same devolatilization and char conversion models from the literature, while Case 3 used a different devolatilization model. The Case 1 and Case 3 solutions used average properties of a Pittsburgh #8 seam coal (d = 108 μm, SG = 1.373), while Case 2 was obtained by injecting and tracking all of the series of 28 different coal particle density and size mass fractions obtained by colleagues at PSU as a part of the current work, for this same coal. Simulations using the two devolatilization models (Case 1 and Case 3) were generally in reasonable agreement. Differences were observed between the single-density solution and the density/size partitioned solution (Case 1 and Case 2). The density/size partitioned solution predicted nominally 10% less CO and over 5% more H₂ by volume in the product gas stream. Particle residence times and trajectories differed between these two solutions for the larger density/size fractions. Fixed carbon conversion was 4.3% higher for the partitioned solution. Particle-wall impact velocities did not vary greatly.Grid independence studies for the two-stage upflow gasifier geometry showed that the grid used in the comparison studies was adequate for predicting exit gas composition and wall impact velocities. Validation studies using experimental data for the Pittsburgh #8 coal from the SRI International pressurized coal flow reactor (PCFR) at 30 atmospheres indicated adequate agreement for gasification and combustion cases, but poor agreement for a pyrolysis case. Simulation of a single-stage downflow gasifier yielded an exit gas composition that was in reasonable agreement with published data.