Optimization of the Internal Circulating Fluidized Bed Using Computational Fluid Dynamics Technology

The computational fluid dynamics(CFD)technology is analyzed and calculated utilizing the turbulence model and multiphase flow model to explore the performance of internal circulating fluidized beds(ICFB)based on CFD.The three-dimensional simulation method can study the hydrodynamic properties of the ICFB,and the performance of the fluidized bed is optimized.The fluidization performance of the ICFB is improved through the experimental study of the cross-shaped baffle.Then,through the cross-shaped baffle and funnel-shaped baffle placement,the fluidized bed reaches a coupled optimization.The results show that CFD simulation technology can effectively improve the mass transfer efficiency and performance of sewage treatment.The base gap crossshaped baffle can improve the hydraulic conditions of the fluidized bed and reduce the system energy consumption.The cross-shaped baffle and funnel-shaped baffle can perfect the performance of the reactor and effectively strengthen the treatment in the intense aerobic process of industrial sewage.

Boiling Heat Transfer in Circulating Fluidized Beds

A model is proposed to predict boiling heat transfer coefficient in a three-phase circulating fluidized bed (CFB), which is a new type of evaporation boiling means for enhancing heat transfer and preventing fouling. To verify the model, experiments are conducted in a stainless steel column with 39mm ID and 2.0m height, in which the heat transfer coefficient is measured for different superficial velocities, steam pressures, particle concentrations and materials of particle. As the steam pressure and particle concentrations increase, the heat transfer coefficient in the bed increases. The heat transfer coefficient increases with the liquid velocity but it exhibits a local minimum.The heat transfer coefficient is correlated with cluster renewed model and two-mechanism method. The prediction of the model is in good agreement with experimental data.

Hydrodynamics in commercial-scale internally circulating fluidized beds with different central downcomer outlets

Standpipes,or downcomers,are commonly used in fluidized beds to transport particles.The outlet structure of the downcomer greatly affects the performance of flow from it and even overall reactor performance.In this study,the hydrodynamics in commercial-scale internally circulating fluidized beds(ICFBs)with central downcomers having different outlet structures was investigated using computational fluid dynamics simulations with an energy minimization multi-scale drag model.The predicted results closely agreed with experimental data.Results showed that in an ICFB with a downcomer outlet directly open to the bed(model A),nearly 12.7%to 5.4%of the gas in the draft tube bypasses into the downcomer.In the ICFB models B and C with a conic baffle below the downcomer,the gas bypass is significantly weakened or even eliminated when the diameter of the conic baffle is 1.1 times that of the downcomer(model C).In addition,the solids circulation mass flux in ICFBs increased by about 62.5%,from 126.8 kg/(m2 s)in model A to 206 kg/(m2 s)in model C.

Hydrodynamic behavior of an internally circulating fluidized bed with tubular gas distributors

To better understand the hydrodynamic behavior of an internally circulating fluidized bed, solids holdup in the down-comer （Eso）, solids circulation rate （Gs） and gas bypassing fraction （from down-comer to riser y^R, and from riser to down-comer yRD） were experimentally studied. The effects of gas velocities in the riser and in the down-comer （UR and UD）, orifice diameter in the draft tube （dor）, and draft tube height （HR） were investigated. Experimental results showed that increase of gas velocities led to increase in Gs and yDR, and slight decrease in yeD. Larger orifice diameter on the draft tube led to higher 8sD, Gs and yDR, but had insignificant influence on YRD. with increasing draft tube height, both Gs and YDR first increased and then decreased, while yRD first decreased and then increased. Proposed correlations for predicting the hydrodynamic parameters agreed reasonably well with experimental values.

Solids flow characteristics and circulation rate in an internally circulating fluidized bed

Internally circulating fluidized beds(ICFBs)enable effective control of the reactions and heat distribution in reactors.The ICFB contains two or more connected fluidized regions with different gas velocities to promote controlled solid circulation.The control of solid circulation rate(G_(0))is a critical factor.We recorded single particle trajectories by tracing a fluorescent particle,based on which particle flow behaviors were analyzed in different regions.G_(0)was obtained for a wide range of operating parameters.An increase in gas velocity in the down-and upflow beds shortened the particle circulation time in both beds and G_(0)increased significantly.As the static bed height increased,the differential pressure on both sides of the circulation port increased,which resulted in an increase in the solid circulation rate.As the orifice area increased,the flow resistance through the orifice decreased and thus the solid circulation rate increased.G_(0)increased with the decrease in particle size.The gas velocity in the upflowing bed and orifice area was the most important parameter to control the solid circulation rate.G_(0)was compared with the experimental measurements in literature and predictions using the correlation based on Bernoulli’s equation,and they agreed well.

A Study on Heat Transfer for Immersed Tube inInternally Circulating Fluidized Bed

Heat transfer coefficients for horizontally immersed tubes have been studied in a model of ICFB (Internally Circulating Fluidized Bed). The characteristics in ICFB were found to be significantly differentfrom those in bubbling bed. There is a flowing zone with high velocity in the heat exchange zone.The heat transfer coefficients strongly depend on the fluidized velocity in the flowing zone. The heatexchange process and suitable bed temperature can be controlled according to this feature. Based onthe results of the experiments, a formulation for heat transfer has been developed.

内循环流化床光催化反应器的数值模拟与结构优化

设计了一种新型三相内循环流化床光催化反应器并采用气、液两相模拟方法对其进行了结构优化.依据模拟结果,明确了挡板的位置、长度、底部曝气面积等因素对反应器流体行为的影响.结果表明,最优反应器升、降流区横截面积比为1:1,挡板距反应器底部的距离为0.175m,挡板顶部与沉降区底部持平,底部曝气面积为升流区面积的1.2倍.将该反应器进行三相数值模拟,模拟结果表明,固相催化剂在反应器内能够很好地流化,反应器内无明显的死区,其固液分界面低于气液分界面,沉降区能够发挥很好的固液分离效果.

内旋流流化床床内颗粒运动特性的试验研究

流化床床内颗粒的大规模循环不仅可以加剧了颗粒横向扩散 ,提高燃烬速率 ,实现燃烧的均匀性和稳定性 ,还将有利于促使不燃物质的定向移动排出炉外。文中采用冷态流化床装置 ,应用分层取样技术对微倾斜布风板实现流化床内不均匀布风 ,床内物料混合与分层特性进行了系统的试验。研究床内颗粒扩散的动态特性和流化风速、颗粒密度以及颗粒粒度等因素对流化床内分层的影响 ,并通过求解扩散方程来分析布风不均匀性和布风倾角对风板附近物料扩散的影响。研究表明 :增大流化风速和布风板倾角能够强化颗粒的横向扩散和流化床的内旋流强度 ;对大而重的颗粒存在一个最佳速度比 ,既能实现床内旋流又能防止颗粒的严重分层 ,并针对低风速区的分层现象 ,提出了一个关联式 ,该式的计算值与实验值吻合较好。

三相内循环流化床光催化反应器及其光辐射传递规律

提出并建立了三相内循环流化床光催化反应器 ,对其内部光辐射传递规律进行了研究 .结果表明 ,在气固相共同作用下(通气量 >0 3m3 /h) ,光强的径向分布服从指数衰减规律 ,与Rose公式中通过均匀悬浊介质时的光强分布规律一致 .光辐射能的数值受初始光强 .催化剂颗粒浓度和液层厚度的共同影响 .通气量只影响床层流化状态 ,对光强分布影响可以忽略 .对罗丹明B的光催化降解实验表明 ,三相内循环流化床的光催化效率高于传统的悬浆式反应床 ,本系统的最佳催化剂浓度为 10～12 g/L .

内循环生物流化床反应器流体力学特性的数值模拟

首先采用气、液两相模型对7组结构参数的反应器进行模拟计算,然后根据计算结果选择了一组较优的结构参数进行气、液、固3相的数值模拟计算.结果表明,通过数值模拟方法对内循环生物流化床反应器流体力学特性进行计算可获得优化的反应器结构参数;明确了循环挡板的长度及位置对反应器内液相流场分布的影响较为显著的事实;同时指出固相颗粒的加入对反应器内液相流速的分布几乎没有影响,固相含率较高的区域在反应器升流区.

内循环生物流化床水力及生物硝化特性研究

采用一种新型轻质多孔性载体，研究了内循环生物流化床的水力特性。结果表明，在降流区接近于推流式，而升流区接近于完全混合式，整个反应器可以模拟为一个推流式和一个完全混合式的串联。随着下向流速增加，膨胀率和回流比呈正比增加。该系统对ＮＨ＋４－Ｎ去除率达９０％以上，容积负荷最大为２ｋｇＮ／（ｍ３·ｄ），出水稳定，抗冲击能力强。循环速度和通气量对处理效果有较大影响。

城市固体废物的焚烧实验

稳定均匀的燃烧温度是确保减少垃圾焚烧系统大气污染物排放量的一个重要因素.采用内旋流流化床(ICFB)进行了城市生活垃圾焚烧实验,探讨了不同的布风速度、垃圾焚烧量、流化床浓相区高度和不同种类垃圾对焚烧稳定性的影响,并给出了流化床内部温度和CO、NOX、SO2等大气污染物的浓度变化.内旋流流化床采用非均匀布风,低速风的移动区尚未流化时,浓相区温度存在一定的不均匀性,低速风区流速超过2倍初始流化速度后,浓相区温度是均匀一致的;流化床的床料具有较好的蓄热能力,较厚的床层有利于提高燃烧的稳定性,可减少垃圾给料和垃圾热值的波动对燃烧温度造成的不利影响;垃圾的焚烧效果与垃圾的热值有直接关系,焚烧低热值垃圾时,为了提高焚烧温度并达到较好的排放指标,需要增加一定量辅助燃料进行助燃;内旋流流化床在燃烧稳定性以及燃烧温度控制上具有一定优势.

循环流化床脱硫塔直/旋流复合流化下的两相流场试验研究

循环流化床脱硫装置的文丘里管直流流化速度随锅炉负荷的变化而变化,这会影响脱硫效率。本文提出了适应锅炉负荷变化的直/旋流复合流化方式,并用PDA测量系统对这种流化方式的气固两相流场进行测试,得到了循环流化床内旋流风率和假想切圆半径改变时气固切向速度和浓度分布。试验表明,复合流化循环流化床的切向速度随着半径增大而升高,气固切向滑移速度比直流流化增大,脱硫塔内的浓度增加,内循环增强,脱硫效率随之提高。

三相内循环流化床光催化反应器及其性能

依据流态化原理,设计出一种三相内循环流化床光催化反应器,对反应器的水力特性做了推算,确定了反应器的操作影响因素.对罗丹明B的降解实验结果显示出优越的性能,依据催化剂投加量及初始光强的不同,反应半衰期在4—10.5min之间.

基于多变量内模结构的循环流化床锅炉床温自抗扰控制

循环流化床锅炉具有燃烧劣质煤和炉内高效脱硫的优势,已成为我国火力发电装机的重要组成部分。但是,相较于煤粉锅炉,循环流化床锅炉燃烧过程的迟延和惯性更大、被控变量更多且关系更为复杂。床温作为循环流化床锅炉燃烧过程中特有的参数,对锅炉运行的安全性、经济性和环保性均有重要影响,而其所具有的大时滞、强耦合、多干扰等特性又使之成为一大控制难题。为此,提出一种应用改进线性自抗扰原理的床温控制方法,用以提高循环流化床锅炉燃烧控制性能。通过引入多变量内模结构,可有效整定扩张状态观测器和反馈控制器参数,进而实现系统整体控制性能的优化。理论分析与仿真验证表明,该方法便于控制参数整定与动态性能分析,不依赖于精确的对象模型,具有良好的负荷适应性和抗干扰能力。

废弃物颗粒在非均匀布风流化床中的横向扩散特性

在试验基础上 ,分析了非均匀布风流化床中废弃物的横向扩散系数。表明配风方式对于燃料颗粒的扩散有很大影响 ,Λ字型布风板的横向扩散系数最大 ,V字型布风板较小 ,鼓泡床最小。同时发现内旋流床中沿床宽的扩散系数值有较大不同 。

内循环流化床气泡运动特性的可视化研究

城市固体废弃物焚烧过程中采用非均匀布风的内循环流化床 ,这种内循环流化床有其独有的特点 ,使用非均匀布风实现床料颗粒的大尺度回旋流动。内循环流化床中气泡的运动特性直接影响着床料的回旋流动及床料的混合。因此 ,有必要研究内循环流化床中气泡的运动。该文采用可视化的方法研究气泡的运动轨迹及运动速度。图 6参

内旋流流化床埋管传热研究

垃圾焚烧系统中 ,内旋流流化床存在不同布风速度的移动区、流动区和换热区 ,处于换热区的埋管的对流换热系数受附近流动区气流参数的影响 ,其变化趋势及数值大小与普通鼓泡型流化床之间有明显不同 :最大的对流换热系数明显高于鼓泡床 ;换热区尚未流化时 ,对流换热系数已经大幅提高 ;整条换热曲线的变化比较平缓 ,易于流化床浓相床内换热。

隔板式内循环流化床的流动特性研究

以石英砂和稻壳为实验床料,在隔板式内循环流化床气化炉冷态实验装置上对颗粒的内循环流动特性进行了研究,考察了高速区和低速区的流化速度、结构尺寸和侧风量等对颗粒内循环流动的影响。结果表明:在保持低速区流化速度一定的条件下,随着高速区风速增大,颗粒循环量先增大后减小;流化速度不变的条件下,颗粒循环量随孔口和侧风量的增大而增加。但增加趋势逐渐变缓。实验给出了合理的运行设计参数。通过实验数据回归,得到了石英砂和稻壳通过隔板式内循环流化床孔口的颗粒循环量关联式,计算结果与实验值误差分别小于6%和14%,能较好地预测孔口颗粒流动。

强化启动对内循环生物流化床硝化效果的影响

通过改变内循环生物流化床的启动水质,提高N/C组成以强化流化床后期的硝化作用.结果表明,高N/C和低进水COD强化启动后处理生活污水,在HRT为2h时,可以同时高效去除COD和氨氮,氨氮的平均去除率为74%.耗氧速率试验表明,强化启动后,流化床中生物膜的异养菌活性大幅度降低,氨氧化细菌活性明显提高,硝化细菌活性变化不大.对反应系统微生物醌进行的跟踪分析表明,强化启动后,生物膜中的硝化细菌数量明显增加,微生物种群的分布均匀性变化较小,以革兰氏阴性菌为主.扫描电镜观察显示,低N/C启动条件下,生物膜厚且致密,异养菌所占比例高;高N/C启动条件有利于硝化细菌的生长,生物膜相对稀薄.