一种新型受热面飞灰颗粒的沉积特性

以一种余热锅炉中新型的受热面为研究对象,采用实验研究和数值模拟的方法研究其飞灰沉积特性。建立了菱形受热面飞灰颗粒的沉积模型,对飞灰颗粒的反弹、黏附及脱落过程进行预测,并与叉排管束和顺排管束的含灰烟气流的速度场、温度场和飞灰颗粒沉积率进行比较。结果表明,菱形受热面在换热和飞灰沉积方面优势明显。沉积主要集中于受热面左上部,颗粒由于惯性碰撞在迎风侧沉积。相同速度下,随颗粒粒径增加沉积率先增大后减小,在3 m·s~(-1)的烟气流速下颗粒直径为5μm时飞灰颗粒沉积率最高,为9.49%。保持粒径不变,随速度增大沉积率逐渐降低。     

基于椭圆管束菱形受热面的流动与传热性能分析

圆形管束菱形受热面在流动与传热方面表现出传热能力强、流动阻力大、吹灰方便等特点。以此为基础,提出了一种基于椭圆管束的菱形受热面。椭圆管束菱形受热面保持了圆形管束菱形受热面传热能力强和吹灰方便的特点,同时显著降低了烟气侧流动阻力。采用数值模拟方法,分析了叉排椭圆管束不同迎风角下的流动传热特性,与叉排圆形管束进行了对比,给出了各特征参数随Reynolds数的变化规律。模拟结果表明,迎风角为0°的叉排椭圆管束菱形受热面较圆形管束传热效果好,流动阻力小,综合性能高。     

基于椭圆管束菱形受热面的流动与传热性能分析

圆形管束菱形受热面在流动与传热方面表现出传热能力强、流动阻力大、吹灰方便等特点。以此为基础,提出了一种基于椭圆管束的菱形受热面。椭圆管束菱形受热面保持了圆形管束菱形受热面传热能力强和吹灰方便的特点,同时显著降低了烟气侧流动阻力。采用数值模拟方法,分析了叉排椭圆管束不同迎风角下的流动传热特性,与叉排圆形管束进行了对比,给出了各特征参数随Reynolds数的变化规律。模拟结果表明,迎风角为0°的叉排椭圆管束菱形受热面较圆形管束传热效果好,流动阻力小,综合性能高。     

飞灰颗粒与平板表面撞击过程的实验研究

以煤粉锅炉积灰过程为研究背景,在常温常压环境下对飞灰颗粒与平板表面撞击过程进行实验研究,分析飞灰颗粒入射法向速度对颗粒法向反弹速度的影响,以及飞灰颗粒粒径对临界捕集速度的影响。实验结果表明,飞灰颗粒粒径相同时,法向恢复系数随入射法向速度的增大先增大后平缓最后减小,在增大区域具有相当陡的斜率,反映了不同形式的力在不同阶段所占的比重不同;颗粒入射法向速度相同时,法向恢复系数随着颗粒粒径的增大而增大;而临界捕集速度随颗粒粒径的增大而减小。     

液固循环流化床换热器管束的固含率

在二维中试循环流化床中使用组合式固液分布器进行实验,考察下管箱表观液速,主分布器内径、主分布器下插深度,颗粒加入量和粒径对径向管束固含率的影响.结果表明:固含率随主分布器内径、颗粒加入量和下管箱表观液速的增加而增大,随粒径的增大而减小,随主分布器下插深度几乎没有变化;固含率不均匀度随颗粒粒径的增大而增大,随下管箱表观液速、颗粒加入量,主分布器下插深度增加而减小,但达到一定深度后,不均匀度不再随之减小,随主分布器内径几乎没有变化.在综合考虑各影响因素的基础上,提出计算固含率的经验公式,并用实验数据拟合了公式中的参数.     

燃煤电厂飞灰PM_(2.5)中Cu、Zn等重金属物质

选取国内四个焚烧不同煤种的发电厂机组中的飞灰作为研究对象,通过粒度分析仪筛选出了粒径低于2.5、2.5～10、10～30μm共三种粒径下的灰样,对4个电厂飞灰迚行了质量分布频率的分析以及痕量元素富集规律的研究,结果表明:PM2.5的质量频率均在10%以下,2.5μm dp10μm的质量频率占比在40%～50%、dp10μm颗粒的质量频率占比在50%左右;燃煤电厂所产生颗粒的真密度与粒径大小密切有关,颗粒真密度随粒径增大而减小,随粒径减小而增大;燃煤电厂所产生的痕量元素(重金属物质)存在形态较为稳定,其存在形式与燃烧方式和煤的种类无关;燃煤电厂所产生的飞灰中各元素的富集系数大致均为随粒径增大而减小,尤其以Cu表现最为明显。     

错流微滤制备动态膜过程中细微颗粒沉积机理

以煤基炭膜为基膜,以ZrO₂为涂膜颗粒,在实验研究的基础上对错流微滤过程中沉积颗粒进行受力分析,分别建立径向颗粒的可能沉积临界粒径模型、轴向上颗粒的可能移动临界粒径模型和圆周方向的颗粒可能滚动临界粒径模型。并以温度、错流速率、渗透通量为主要影响参数,分别对3种临界粒径模型进行模拟研究,探讨颗粒沉积机理。结果表明,压力增大(渗透通量提高)、温度降低以及错流速度降低,都可增大动态膜层厚度;由于净重力的影响,在水平膜管内圆周方向动态膜膜层厚度分布不均匀通过增加错流速率可明显减小此种差异;模拟结果与实验结果的一致性证实了所建模型的可靠性。     

垃圾焚烧锅炉省煤器冲蚀模拟及优化研究

以某600 t/d垃圾焚烧锅炉省煤器管束及烟道为研究对象，针对其在长期运行中因受到烟气冲蚀磨损而导致失效问题，基于FLUENT软件和粒子运动学，采用SST k-ω模型和DPM模型对省煤器管束间烟气流与烟灰颗粒运动进行模拟研究，提出用一种波浪导流板进行优化，并在烟气流速、烟灰粒径、质量流量三因素下进行了对优化前后省煤器冲蚀影响的对比分析。结果表明：省煤器管束的平均冲蚀速率随烟气速度、烟灰粒径和质量流量的增大而增加，优化后的平均冲蚀速率降幅，三因素中烟灰粒径最大；在其入口设置波浪导流板后，省煤器管束的冲蚀现象明显改善，为其防护和优化改造进而提高使用寿命提供了重要参考与数据支持。     

内置管束布置对换热器内固体颗粒流动的影响

换热器内设的水平换热管束是影响换热器内固体颗粒流动均匀性的重要因素,基于离散单元法构建了内置横管式固体颗粒换热器的流动计算模型,研究了管束排列方式和水平管间距对换热器内固体颗粒流动的影响。结果表明,颗粒流过错排管束的均匀性优于顺排管束,颗粒层的形变和停留时间波动性均较小,颗粒流动不均匀度为0.00721,比顺排管束的减少了42.3%,错排管束的颗粒停留时间标准偏差为0.029,比顺排管束的减少了39.1%。当颗粒流过错排管束时,随着水平管间距的减小,颗粒层的形变和停留时间波动性均增大,颗粒流动均匀性显著恶化,颗粒流动不均匀度由0.00721增大到0.00996,增大38.1%,颗粒停留时间标准偏差由0.029增大到0.039,增大33.8%,剪切区内颗粒速度差异增大,颗粒平均速度梯度由5.97×10^-4s^-1增大到7.85×10^-4s^-1,增大了31.4%。     

一种新型受热面传热和流动特性的数值模拟及实验研究

针对余热利用过程中低温热源的含尘量高、不连续及不稳定等特点, 提出了一种新型菱形受热面结构。在传热过程中, 该受热面表现出管束叉排布置的特征, 传热过能力较强, 流动阻力较大, 壳侧对流换热表面传热系数较高。在实施吹灰过程中, 该受热面呈现出管束顺排布置的特征, 易清洗, 吹灰效率高。采用数值模拟和实验方法研究了新型受热面结构的传热和流动特性, 给出了壳侧的Nusselt数和摩擦因子随Reynolds数的变化规律。实验结果和数值分析均表明, 该受热面能够适应现有余热利用过程的基本要求, 在便于清灰和除垢的同时实现高效传热。     

烟气横掠管束外表面的积灰特性

针对石灰线生产中烟尘易沉积,烟气换热器表面积灰的问题,为开发具有自清灰功能和扩展受热面的高效传热元件,对粉尘物理、化学和外部工作过程特性进行研究,通过采用扫描电子显微镜及XRD等仪器,研究灰的粒径分布、外观形貌及化学成分等物理化学特性,并对管束外表面积灰特性进行实验研究,结果表明：石灰线粉尘粒径较大,颗粒结构紧密,表面有大小不一的孔,易产生物理沉积形成松散积灰;灰样浓度的增大会导致积灰量增加,相同时间内灰样浓度越高积灰量越大,顺风面更易产生积灰现象;烟气流速较大时,积灰量很快达到稳定,烟气流速的增加会导致迎风面积灰量减少,背风面积灰量增加,存在最佳的烟气流速使飞灰颗粒在管道表面的沉积量最小;增加横向节距、减少纵向节距有利于减少积灰量。     

基于离散元方法的高碱煤灰沉积过程数值模拟研究

针对锅炉燃用高碱煤产生的受热面积灰问题,以流化床锅炉对流过热器为研究对象,采用离散元方法（discrete element method）建立黏性颗粒碰撞黏附模型,对过热器对流受热面的积灰过程进行数值模拟。分别研究了烟气流速、颗粒粒径、烟气温度以及壁面温度对积灰特性的影响规律。模拟结果表明：烟气流速增大、颗粒粒径减小会导致颗粒的碰撞率上升、捕集率下降;烟气温度和壁面温度提升时,颗粒表面能上升,导致颗粒捕集率上升,且壁面温度的影响更为显著;不同工况下最大积灰厚度、沉积物形状变化较小。     

Two-dimensional modelling of deposits formation on platen superheaters in pulverized coal boilers

The 1D modeling of deposits formation is not able to describe properly the gas and particles flow in the vicinity of heat transfer tubes in boilers. Especially, the recirculation zones in the down-stream side of tubes are not possible to reproduce by the 1D models. Known applications of the CFD codes to deposition modelling were able to reproduce only the rate of particles deposition on surfaces, while for practical reasons the most important information is the shape and size of deposits, because it enables prediction of heat transfer abatement with boiler operation time. Paper presents 2D modelling of deposits on platen superheaters in pulverized coal boilers. Particularly, the observed in industrial boilers bridging of the tubes spacing was reproduced by the model in which the particle size dependence of the ash sticking probability was proposed. It has been demonstrated that the ash particles size play important role in the deposits growth mechanism.     

生物质锅炉对流受热面积灰冷态模拟实验研究

燃用生物质的CFB锅炉的尾部受热面上容易出现严重的积灰问题,严重影响换热并可能导致停炉等问题。惯性碰撞是引起生物质锅炉积灰的主要机理,而温度通过影响灰中熔融质所占的比例,进而影响积灰程度。采用加热熔融的石蜡与循环灰的混合物来模拟真实的高黏性飞灰,并搭建了冷态积灰实验台。发现石蜡与循环灰的熔融物可以快速地黏附在受热面上,大大缩短了实验时间。通过图像处理得到沉积厚度随时间的变化情况,沉积过程的生长趋势与真实生物质积灰实验一致。在冷态下实验发现,随着熔融质比例、烟气速度、颗粒粒径的增加,积灰程度呈上升趋势,为生物质锅炉的设计和运行提供了一定的参考依据。     

生物质锅炉对流受热面积灰冷态模拟实验研究

燃用生物质的CFB锅炉的尾部受热面上容易出现严重的积灰问题,严重影响换热并可能导致停炉等问题。惯性碰撞是引起生物质锅炉积灰的主要机理,而温度通过影响灰中熔融质所占的比例,进而影响积灰程度。采用加热熔融的石蜡与循环灰的混合物来模拟真实的高黏性飞灰,并搭建了冷态积灰实验台。发现石蜡与循环灰的熔融物可以快速地黏附在受热面上,大大缩短了实验时间。通过图像处理得到沉积厚度随时间的变化情况,沉积过程的生长趋势与真实生物质积灰实验一致。在冷态下实验发现,随着熔融质比例、烟气速度、颗粒粒径的增加,积灰程度呈上升趋势,为生物质锅炉的设计和运行提供了一定的参考依据。     

流化床垃圾焚烧炉飞灰沉积实验

在自行搭建的小型流化床积灰试验台上选用流化床垃圾焚烧炉对流管束浮灰，模拟焚烧炉烟气环境下开展飞灰沉积实验。首先分析浮灰理化特性，结果表明：随着粒径增大，浮灰中CaO和SO₃含量逐渐降低，而SiO₂和Al₂O₃的含量逐渐增加；碱金属Na和K以及卤素Cl的含量在粒径较小的浮灰中含量最高。其次重点研究了浮灰粒径、烟气温度和换热管表面温度对飞灰沉积特性的影响规律，结果表明：积灰中主要富含Ca、S、Si和Al等元素，积灰中CaO和SO₃含量比浮灰高，而Al₂O₃和SiO₂含量比浮灰低；积灰量随着烟气温度升高而增加；粒径对灰沉积影响作用显著，大粒径灰颗粒难沉积而小粒径灰粒易沉积；管壁温度在600℃时，积灰量最小。积灰中CaO和SO₃的含量随着管壁温度升高而减少，而难熔Al₂O₃和SiO₂的含量随着管壁温度升高而增加。     

Mechanistic prediction of ash deposition in a pilot-scale test facility

A mechanistic approach has been used for assessing the ash deposition tendency and has been used for predicting the ash deposition behavior of Australian bituminous coals in a pilot-scale test furnace. The detailed analysis of mineral matter in coal determined by QemSCAN analytical technique was incorporated into an ash formation model to estimate the character of ash particles. The detailed chemical composition and particle size distribution data of ash particles from the ash formation model were incorporated into a computational fluid dynamics (CFD) code to get the arrival rate and retention ability of each ash particle onto the heat transfer surfaces.The CFD code along with the ash character from the ash formation model was able to simulate trends in ash deposition along the furnace length similar to those in the test furnace. The approach was also able to distinguish coals with high ash deposition potential from low ash deposition problems and the results agreed with those in test furnace.     

Total and gas convective heat transfer from a vertical tube to a mixed particle gas—solid fluidized bed

Experimental data were obtained for the average gas convective and total heat transfer coefficients for a vertical tube immersed in an air-fluidized bed of narrowly as well as widely distributed particle size mixtures. The gas convective heat transfer coefficient was determined by measuring the rate of mass loss from a vertical naphthalene tube 0.0262 m in diameter and 0.1012 m in length and using a heat and mass transfer analogy. These data were obtained at a bed temperature of about 330 K and superficial velocity of 0.1 to 1.1 m/s. The total heat transfer coefficients were measured under identical conditions using an electrically heated vertical tube. The total heat transfer coefficient decreased with an increase in particle diameter from 0.237 to 1.35 mm. The addition of fines was found to increase the total heat transfer coefficient. The gas convective heat transfer coefficient increased with increase in particle size and fluidizing velocity. The dependence of the gas convective heat transfer coefficient on gas velocity was more pronounced for large particles. The addition of fines resulted in decrease in gas convective coefficient. The relative contribution of the gas convective component of heat transfer coefficient was found to increase with increase in particle diameter. Its dependency on fluidizing velocity was found to be more complex. The experimental data were compared with the existing heat transfer models and correlations.     

Dynamic mechanistic model of superheater deposit growth and shedding in a biomass fired grate boiler

A dynamic mathematical model of ash deposit growth and shedding on a horizontal cooled probe in a straw fired grate boiler has been developed and validated. The model includes submodels of deposition (impaction, thermophoresis, Brownian and eddy diffusion, and condensation) and shedding by deposit surface melting. Three new submodels describing deposit formation on the downstream side of the probe, ash droplet formation and detachment, and a correlation of ash melt fraction as functions of temperature and ash chemical composition have been developed. Measured flue gas temperature, ash particle size distribution as well as chemical composition of the fly ash were used as input parameters to the model calculations. The model predicted reasonable well the measured evolutions of the deposit weight, the heat uptake, and the deposit shape. The model predicted an elliptical deposit shape at the upstream side of the probe and a very small initial amount of deposit on the downstream side of the probe, and these are in agreement with the probe measurements at an early stage of the deposit formation process. KCl condensation was found to initiate the deposit formation and particle impaction is the main contribution to the deposit growth. The deposit weight and the heat uptake reach a steady state condition after 285 h when the deposit formation rate is balanced by the shedding rate. High peak flue gas temperature events lead to higher shedding rates and to enhancements of the heat uptake. A parametric study quantified the influence of changed local conditions and ash material properties on the deposit formation process.