Desulfurization characteristics of slaked lime and regulation optimization of circulating fluidized bed flue gas desulfurization process--A combined experimental and numerical simulation study

Circulating fluidized bed flue gas desulfurization(CFB-FGD) process has been widely applied in recent years. However, high cost caused by the use of high-quality slaked lime and difficult operation due to the complex flow field are two issues which have received great attention. Accordingly, a laboratory-scale fluidized bed reactor was constructed to investigate the effects of physical properties and external conditions on desulfurization performance of slaked lime, and the conclusions were tried out in an industrial-scale CFB-FGD tower. After that, a numerical model of the tower was established based on computational particle fluid dynamics(CPFD) and two-film theory. After comparison and validation with actual operation data, the effects of operating parameters on gas-solid distribution and desulfurization characteristics were investigated. The results of experiments and industrial trials showed that the use of slaked lime with a calcium hydroxide content of approximately 80% and particle size greater than 40 μm could significantly reduce the cost of desulfurizer. Simulation results showed that the flow field in the desulfurization tower was skewed under the influence of circulating ash. We obtained optimal operating conditions of 7.5 kg·s^(-1)for the atomized water flow, 70 kg·s^(-1)for circulating ash flow, and 0.56 kg·s^(-1)for slaked lime flow, with desulfurization efficiency reaching 98.19% and the exit flue gas meeting the ultraclean emission and safety requirements. All parameters selected in the simulation were based on engineering examples and had certain application reference significance.

The Carbonation Behaviors of Limestone Particle in Oxygen-Fuel Circulating Fluidized Bed O₂/CO₂Flue Gas

Limestone powder is still applied as SO2 sorbent in emerging oxygen-fuel circulating fluidized bed boiler, but its carbonation in O2/CO2 flue gas is an unclear problem. For a better understanding of carbonation behaviors, the tube furnace heating system was built for simulating circulating fluidized bed boiler flue gas by regulating the supply of O, CO2, N2, SO2 and H2O, and Carbonation reaction was tested. Thermal gravimetric analysis and scanning electron microscopy were used. It was found that carbonation is closely related to temperature, CO2 concentration, impurities, water vapor, and cycle times;high temperature can promote carbonation process;high concentration of CO2 can inhibit the chemical reaction stage speed of carbonation process, but it has little effect on the final conversion rate;water vapor can increase the final conversion rate of carbonation;the cycle times will reduce the activity of carbonation. The presence of carbonation turns the traditional boiler flue gas indirect desulfurization model into indirect desulfurization mechanism which does not have a negative impact on SO2 removal efficiency.

A computational particle fluid-dynamics simulation of hydrodynamics in a three-dimensional full-loop circulating fluidized bed: Effects of particle-size distribution

A computational particle fluid-dynamics model coupled with an energy-minimization multi-scale(EMMS)drag model was applied to investigate the influence of particle-size distribution on the hydrodynamics of a three-dimensional full-loop circulating fluidized bed.Different particle systems,including one monodisperse and two polydisperse cases,were investigated.The numerical model was validated by comparing its results with the experimental axial voidage distribution and solid mass flux.The EMMS drag model had a high accuracy in the computational particle fluid-dynamics simulation of the three-dimensional full-loop circulating fluidized bed.The total number of parcels in the system(Np)influenced the axial voidage distribution in the riser,especially at the lower part of the riser.Additional numerical simulation results showed that axial segregation by size was predicted in the two polydisperse cases and the segregation size increased with an increase in the number of size classes.The axial voidage distribution at the lower portion of the riser was significantly influenced by particle-size distribution.However,radial segregation could only be correctly predicted in the upper region of the riser in the polydisperse case of three solid species.

Combustion characteristics of low-quality lignite for different bed material sphericities in a circulating fluidized bed boiler:A numerical study

This study delves into the combustion behavior of various lignite types within a circulating fluidized bed boiler(CFBB),with a primary focus on the impact of different bed material sphericity ratios(0.5,0.7,and 0.9).Utilizing bed material with a sphericity ratio of 0.9 sourced from theÇan power plant and verified through experimentation,the research reveals several key findings.Notably,furnace temperatures tended to rise with higher sphericity ratios,albeit with variations between lignite types,particularly highlighting the complexity of this relationship in the case of GLI-Tunçbilek lignite.Pressure levels in the combustion chamber remained consistent across different sphericity ratios,indicating minimal influence on pressure dynamics.Improved combustion efficiency,especially at the bottom of the boiler,was observed at lower sphericity levels(0.5 and 0.7)forÇan lignite,as reflected in CO_(2) mole fractions.While NO_(x) emissions generally decreased with lower sphericity,the sensitivity to sphericity varied by lignite type,with Ilgın lignite showcasing low NO_(x) but high SO_(2) emissions,underscoring the intricate interplay between lignite properties,sphericity,and emissions.Overall,this study advances our understanding of CFBB combustion dynamics,offering insights valuable for optimizing performance and emissions control,particularly in lignite-based power.

Effects of riser geometry on gas-solid flow characteristics in circulating fluidized beds

The performance of a circulating fluidized bed strongly depends on its parameter settings,including that of riser geometry.In this study,a laboratory-scale circulating fluidized bed with three different riser geometries(circular,square,and rectangular)that had the same cross-sectional area and height was operated under two static bed heights(20,and 35 cm).Electrical capacitance tomography was combined with differential pressure transducers and an optical-fiber probe to measure the solids'volume fraction,differential pressure fluctuations,and radial particle concentration variations.Computational particle fluid dynamics simulations were also performed.The results showed that single bubbles appeared in the bottom region of the circular and square risers and double bubbles in the bottom region of the rectangular riser.The autocorrelation of capacitance signals was periodic for the circular and square risers and non-periodic for the rectangular riser.The radial particle concentration profiles showed a single-core annulus structure in the circular and square risers,but a double-core annulus structure along the long side and single-core annulus structure along the short side in the rectangular riser.Shannon entropy analysis showed that fluidization was less disordered and most predictable for the rectangular riser.

Experimental and numerical study of pseudo-2D circulating fluidized beds

We present experimental investigations and numerical simulations of a pseudo-2D riser. Experiments were performed for various airflow rates, particle types/diameters, and particle size distributions. Pres- sure distributions along the wall of the riser were measured, Additional measurements from a smaller pseudo-2D riser （Kallio et al., 2009; Shah et al., 2012） were used to analyze horizontal solids volume fraction profiles. The experimental data were compared with simulation results carried out using an Euler-Euler approach, A mesh sensitivity study was conducted for numerical simulations and effects associated with simplifying real 3D geometry to a 2D model were examined. In addition, the effect of using an algebraic equation to represent the granular temperature versus a full partial differential equation also was examined for numerical simulations. Results showed small but significant near-wall sensitivity of the flow variables to mesh size. Substantial differences in mean pressure, solids distribution, and solid velocities were obtained, when 2D and 3D simulation results were compared. Finally, applying the simplified granular temperature equation for turbulent fluidization and for dilute-phase transport can lead to incorrect predictions in models,

Modeling of gas-solid flow in a CFB riser based on computational particle fluid dynamics

A three-dimensional model for gas-solid flow in a circulating fluidized bed（CFB） riser was developed based on computational particle fluid dynamics（CPFD）.The model was used to simulate the gas-solid flow behavior inside a circulating fluidized bed riser operating at various superficial gas velocities and solids mass fluxes in two fluidization regimes,a dilute phase transport（DPT） regime and a fast fluidization（FF） regime.The simulation results were evaluated based on comparison with experimental data of solids velocity and holdup,obtained from non-invasive automated radioactive particle tracking and gamma-ray tomography techniques,respectively.The agreement of the predicted solids velocity and holdup with experimental data validated the CPFD model for the CFB riser.The model predicted the main features of the gas-solid flows in the two regimes;the uniform dilute phase in the DPT regime,and the coexistence of the dilute phase in the upper region and the dense phase in the lower region in the FF regime.The clustering and solids back mixing in the FF regime were stronger than those in the DPT regime.

Geldart C类脱硫灰颗粒在环流耦合提升管内稳定流动特性

我国CO_(2)的排放中70%来自工业领域,故工业过程的碳捕集对实现“双碳目标”十分关键。工业烟气中往往含有的硫氧化物会腐蚀设备并使后续脱碳等过程使用的催化剂中毒。因此,工业烟气的深度脱硫技术对后续的CO_(2)捕集或提纯过程至关重要。循环流化床半干法烟气脱硫因具有脱硫效率高、无污染、停留时间可控等优点受到广泛关注。循环流化床脱硫工艺中作为脱硫剂的脱硫灰颗粒为典型Geldart C类颗粒。由于C类颗粒的强黏附性,其在循环流化床操作中容易结块,从而影响装置稳定运行。为了强化脱硫灰颗粒在循环流化床内的流动稳定性,提出了环流强化的耦合提升管反应器的概念,并自行设计搭建了一套导流筒内径100 mm、高度300 mm,外筒内径160 mm、高度760 mm,输送段内径75 mm、总高度12.6 m的环流耦合提升管。在U_(g)=4 m/s、G_(s)=45 kg/(m^(2)·s),U_(g)=7 m/s、G_(s)=25 kg/(m^(2)·s)的操作条件下,考察了环流段的压力分布、标准差以及功率谱密度。当环隙区气速为0.4 m/s时,环流流动能够实现稳定、连续的密相环流流动。在C类颗粒形成稳定流动基础上,讨论了环流耦合提升管内的流动特性分布规律,包括固含率和颗粒速度。实验发现,环流流动的设计可以强化C类颗粒的流动特性,大幅提高耦合反应器内C类颗粒脱硫灰固含率,并实现了C类颗粒循环流态化装置的稳定运行。同时,这些研究结果可以为C类颗粒新型循环流态化反应器设计提供参考。

低负荷下CFB锅炉二次风优化对NO_(x)排放影响的数值模拟

为控制循环流化床(circulating fluidized bed,CFB)锅炉低负荷下NO_(x)的原始排放,以某350 MW超临界CFB锅炉为研究对象,基于计算颗粒流体力学(computational particle fluid dynamics,CPFD)方法对40%负荷下燃烧过程进行数值模拟。分析了不同二次风角度、新增二次风量对NO_(x)排放的影响。结果表明:随着射流角度的减小,炉膛出口NO浓度逐渐降低,CO浓度无明显增加。部分二次风上移后炉膛密相区氧浓度降低,不完全燃烧增加,还原性氛围增强,抑制了NO的生成。当新增风量从10%增加到30%时,NO排放浓度降低了17.2%。但随着比例的进一步提高,炉膛密相区的缺氧环境造成燃烧效率下降,温度大幅降低。因此,在不影响燃烧的前提下可以通过提高新增二次风比例来降低NO的原始排放浓度。

多流程循环流化床气固流动和燃烧的计算颗粒流体力学数值模拟

多流程循环流化床与传统流化床相比,能有效降低炉膛高度,实现小型化。以多流程循环流化床锅炉为研究对象,基于计算颗粒流体力学,对气固流动和燃烧过程进行了数值模拟,计算结果与中试装置结果吻合较好。结果表明:炉膛内颗粒相浓度由高到低依次为主燃烧室、副燃烧室、燃尽室,炉内颗粒分布呈现“环-核”结构;炉膛温度范围为1 000~1 200 K,主燃烧室的温度大于副燃烧室,副燃烧室的温度大于燃尽室与旋风分离器;增大一、二次风的配比使炉膛密相区的氧气含量增大,加剧了燃烧,使床温升高;由副燃烧室进行分级给料,有利于焦炭还原NO,使出口NO浓度明显下降,有利于减少氮氧化物的排放。

循环流化床脱硫塔内流场及气固分离特性数值模拟

应用气固两相双流体模型结合颗粒动力学理论,对内置分离器循环流化床脱硫塔内气固两相流中的气相湍流流动和颗粒相脉动流动进行了数值模拟。模拟得到脱硫塔内颗粒相、气相速度和浓度径向分布。利用FFT方法分析了颗粒质量流率振幅图,结果表明,利用数值模拟方法能够很好地预测不同内部构件条件下塔内分离效率。内置分离器有利于强化内部循环,减轻外部除尘设备负荷。气力输送条件下,脱硫塔内呈环核流动,有利于防止壁面结垢和提高脱硫塔的运行稳定性。模拟得到的结果与实验值基本吻合,为化学反应模拟奠定了基础。

粗糙颗粒动理学及稠密气固两相流动的数值模拟

考虑颗粒碰撞过程中摩擦作用,给出了粗糙颗粒碰撞动力学.引入颗粒相拟总温来表征颗粒平动和转动脉动能量的特征.基于气体分子运动论,建立颗粒碰撞中平动和旋转共同作用的粗糙颗粒动理学,给出了颗粒相压力和黏度等输运参数计算模型.运用基于颗粒动理学的欧拉-欧拉气固两相流模型,数值模拟了流化床内气体颗粒两相流动特性,分析了颗粒旋转流动对颗粒碰撞能量交换和耗散的影响.模拟得到的流化床内径向颗粒浓度和提升管内颗粒轴向速度与他人实验结果相吻合.模拟结果表明随着颗粒浓度的增加,颗粒相压力和能量耗散逐渐增加,而颗粒拟总温先增加后下降.随着颗粒粗糙度系数的增加,床内平均颗粒相拟总温和能量耗散增加,表明颗粒旋转产生的摩擦将导致颗粒旋转脉动能量的改变,影响床内气体-颗粒两相宏观流动特性.

循环流化床烟气脱硫系统数学模型研究

以烟气脱硫过程中的质量平衡方程为基础,结合可以描述脱硫剂颗粒反应的收缩未反应核模型(shrinking unreacted core model),同时考虑到再循环物料的影响,建立能够预测循环流化床反应器内烟气脱硫效率的数学模型,该模型可以分别量化新鲜脱硫剂和再循环颗粒的脱硫效率。模型仿真出脱硫效率在不同参数条件下的变化趋势与实际运行情况一致,并且模型经校正后能够比较准确地描述脱硫过程,因此该模型可以用来指导循环流化床烟气脱硫系统的工艺设计和参数优化。

循环流化床烟气脱硫工艺中颗粒增湿团聚现象的分析

在循环流化床烟气脱硫(CFB-FGD)试验台上进行了颗粒增湿团聚的试验研究.通过采集系统沿程不同位置的灰样并进行相应分析,研究了颗粒粒度和形貌变化特性,并分析了颗粒增湿团聚现象及其对脱硫系统、除尘系统稳定运行的影响.结果表明:系统内颗粒增湿团聚明显,而且增湿段上方灰样粘附较多超细颗粒物;增湿颗粒存在超细颗粒团聚体、以较大颗粒为载体的团聚体和包衣结构3种不同的团聚特征.合理利用CFB-FGD工艺作为预处理措施,能够实现超细颗粒物团聚长大,有利于超细颗粒物的脱除.

循环流化床多组分颗粒气固两相流动模型和数值模拟

基于稠密气体分子运动论和颗粒动力学 ,考虑多组分颗粒中颗粒组分与颗粒组分、颗粒组分内颗粒之间的相互作用以及气体与颗粒之间的相互作用 ,提出多组分颗粒非等温颗粒气固两相流动模型 .以颗粒压力、径向分布函数、黏度、颗粒碰撞耗散等耦合各颗粒组分间和颗粒间的相间作用 .采用大涡模拟方法模拟气相湍流流动 .提出了多组分颗粒的径向分布函数计算方法 .对循环流化床上升管中双组分颗粒气固两相流动特性进行了数值模拟 ,模拟结果揭示了上升管中双组分颗粒气固两相流动的环 核流动结构 ,得到了平均颗粒粒径的轴向和径向分布规律 ,计算结果与文献中实验结果相吻合 .

烟气脱硫循环流化床内气固流动的PDA试验研究

提出了由两级分离系统组成的循环流化床烟气脱硫工艺。用粒子动态分析仪(PDA)对床内不同高度方向上颗粒横向、轴向速度、颗粒粒径及浓度分布进行了测量,得到了床内气固流动的瞬时脉动特性以及总体气固流动行为,试验结果为烟气脱硫新工艺的结构设计及优化提供了依据。

Onset velocity of circulating fluidization and particle residence time distribution:A CFD-DEM study

Until now, the onset velocity of circulating fluidization in liquid-solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superfcial liquid velocity, and is reported to be only dependent on the liquid and particle properties. This study presents a new approach to calculate the onset velocity using CFD-DEM simulation of the particle residence time distribution （RTD）. The onset velocity is identified from the intersection of the fitted lines of the particle mean residence time as a function of superficial liquid velocity. Our results are in reasonable agreement with experimental data. The simulation indicates that the onset velocity is infuenced by the density and size of particles and weakly affected by riser height and diameter, A power-law function is proposed to correlate the mean particle residence time with the superficial liquid velocity. The collisional parameters have a minor effect on the mean residence time of particles and the onset velocity, but influence the particle RTD, showing some humps and trailing. The particle RTD is found to be related to the particle trajectories, which may indicate the complex flow structure and underlying mechanisms of the particle RTD.

CFB密相区床料粒径分布对其横向扩散影响的CPFD模拟

该文运用基于计算颗粒流体力学（computational particle fluid dynamics,CPFD）方法的barracuda 17.0软件,对尺寸为900mm×100mm×1200mm的准三维流化床的密相区大颗粒扩散行为进行模拟试验。模拟中,主要考虑在不同流化风速与不同示踪颗粒粒径的条件下,床料粒径分布对密相区中颗粒横向扩散系数的影响。模拟结果显示,宽筛分床内不同粒径颗粒分布十分均匀,无明显分层现象。流化风速及平均粒径相同条件下,单一床料粒径条件下得到的颗粒横向扩散系数略大于宽筛分条件下得到的结果;流化风速相同但床料平均粒径不同条件下,床料平均粒径为1 050μm床内获得的颗粒横向扩散系数要小于床料平均粒径为600μm床内得到的结果。

液-液循环流化床制取流体冰过程的颗粒形成与运动特性

采用水与另一种非相溶载冷液体直接接触换热制取流体冰的液-液循环流化床是一种全新的动态制冰方法,在所建立的循环流化床试验台上针对制冰过程的关键问题——颗粒(液滴、冰颗粒)形成及其运动特性展开研究,研究过程中采用快速摄像与图像处理相结合的方法,获得颗粒形成及其沿程运动过程的图像以及循环流化床不同高度上颗粒的粒径信息。研究发现射流形状、射流长度及其波动的变化规律直接影响形成液滴的粒径大小与均匀性;颗粒的沿程粒径分布具有向粒径增大方向倾斜与移动特征,并随射流液体流量的增大,颗粒的粒径分布范围与倾斜程度增大;颗粒在流化床内沿程运动过程中具有明显的颗粒聚团、聚并与分散现象,且颗粒的聚并现象不可避免,主要出现在流化床0.50 m高度以上,并随射流液体流量增大出现颗粒聚并现象的高度在增高。

循环流化床收集器内颗粒团聚流动特性的研究

基于气固两相流理论和气溶胶动力学原理,建立流化床收集器(CFBA)内气体细颗粒聚团气固两相双流体模型。对不同入口气体速度、初始颗粒尺寸分布和不同颗粒团聚形成机理下收集器内颗粒聚团流动的流体动力特性进行数值模拟。研究结果表明湍流运动和剪切作用对颗粒聚团的形成起主要作用,布朗运动对颗粒团聚形成的影响可忽略不计。吸收颗粒可有效提高捕获细颗粒和颗粒聚团形成的能力。