燃准东煤锅炉辐射受热面的灰沉积特性

建立了与气固两相流动燃烧模型相耦合的飞灰沉积模型,对一台660 MW超临界锅炉燃用准东五彩湾煤时的飞灰沉积特性进行了数值模拟,并且讨论了烟气流速和灰颗粒粒径对飞灰沉积速率的影响规律.结果表明:在BMCR工况下,炉膛辐射受热面上的飞灰颗粒沉积分布主要集中在高度为30～45 m处的区域,且各墙的沉积分布规律呈现出相似性;烟气流速为9.5～12.5 m/s时,辐射受热面上的飞灰沉积速率与烟气流速成正比;烟气流速大于12.5 m/s时,辐射受热面上的飞灰沉积速率与烟气流速成反比;辐射受热面上飞灰颗粒的沉积速率的整体变化趋势是随灰颗粒粒径的增大而增大的.     

硅微粉在余热锅炉受热面上积灰的实验研究

为研究硅铁余热锅炉受热面的积灰特性,在热态积灰实验台上用硅微粉和燃气烟气模拟余热锅炉内的实际烟气,研究了烟气温度、流速、飞灰浓度和积灰时间等对硅微粉积灰过程的影响规律。实验表明:随着烟气温度升高,捕获率和积灰速率均增大;烟气流速增大,捕获率减小,积灰速率先增大后减小,流速为8m/s时,积灰速率达到最大;飞灰浓度增大,捕获率减小,积灰速率增大;积灰时间延长,积灰总量增大,捕获率和积灰速率基本不变。研究成果可指导硅铁余热锅炉的设计和运行。     

三维变形管管内积灰特性数值模拟研究

烟气中的高灰分、高黏度、高腐蚀性成分不可避免地造成换热器烟气侧换热面积灰结垢的问题,如何有效解决这些问题一直是烟气换热器研究的焦点之一。三维变形管内的螺旋流增强了管内流体的湍动程度从而实现强化传热,基于其内部灰尘受气流携带而处于湍动便不易沉积的特点,本文通过数值模拟的方法探讨了三维变形管几何参数、粒径、气体流速对灰尘颗粒沉积率的影响。结果表明,在给定的粒径范围内,烟气流速和三维变形管的几何参数对灰分沉积率有重大影响。随着螺距和烟气流速的增大,灰尘颗粒沉积率降低,然而灰尘颗粒沉积率随着长短轴比和平均粒径的增大而增加。     

准东褐煤燃烧形成的颗粒在圆管上的沉积数值模拟

准东煤中的碱性化合物在燃烧时会形成熔点低的灰烬,在锅炉的运行中容易引起传热表面的结垢,烟气通道的堵塞,传热速率的降低以及过热器管的腐蚀等问题。文中提出了一个模型用以描述和预测高碱金属成分烟气飞灰沉积的形成。在目前的工作中,建立的数学模型包括了从燃料床中释放出飞灰颗粒和碱蒸气向换热表面运输过程以及飞灰颗粒和碱蒸气的沉积行为。结果表明,烟气速度主要通过影响飞灰颗粒的碰撞效率对沉积产生影响,而在高烟气速度条件下,炉温通过改变积灰表面的熔融组分来影响飞灰颗粒的黏附效率。惯性碰撞和热泳引起的颗粒沉积决定了高炉温下灰分的形成,碱蒸气直接冷凝引起的沉积量主要存在于前期。     

炼化废液焚烧飞灰粒子碰撞概率影响因素研究

利用CFD软件FLUENT对某石化企业废液焚烧余热锅炉内换热管壁上飞灰粒子碰撞及分布情况进行数值模拟研究。采用Eulerian-Largrangian法对余热锅炉内气-固两相流问题进行求解,重点分析粒子尺寸、烟气流速、纵向管间距、横向管间距等四种因素对飞灰粒子碰撞概率的影响情况。研究结果表明,除纵向管间距之外,其余三种因素对飞灰粒子的碰撞概率影响显著。飞灰粒子碰撞概率随粒子尺寸的增大而增大,其中大尺寸粒子主要在前两排换热管束的迎风侧发生碰撞,小尺寸粒子在迎风侧和背风侧均发生碰撞,且碰撞概率分布均匀。当烟气流速增大时,飞灰粒子碰撞概率随之增大,且20.0μm飞灰粒子比0.2μm飞灰粒子对烟气流速更为敏感。随着纵向管间距的增大,飞灰粒子碰撞概率仅出现轻微的下降趋势,而随着横向管间距的增大,飞灰粒子碰撞概率下降明显。     

变负荷下空气预热器内硫酸氢铵粘结性积灰特性研究

针对SCR脱硝氨逃逸造成的空气预热器堵塞问题,构建了可反映蓄热板表面飞灰碰撞、粘附与脱离特性的硫酸氢铵（ABS）粘结性积灰计算模型,并对比分析了锅炉在50%~100%最大连续蒸发量工况（BMCR）之间的6种负荷下蓄热板间烟气流速分布及ABS粘结性和松散性积灰的积灰强度、积灰概率等积灰特征量,同时对30% BMCR负荷下蓄热板表面积灰情况进行了预测。结果表明：各负荷下ABS粘结性积灰强度均为松散性积灰强度的4.8倍以上;负荷每增加10%,ABS粘结性积灰与松散性积灰强度分别增加约9.90和2.15 μg/mm2,ABS粘结性积灰强度增加量较大;ABS粘结性积灰区域随负荷增大逐渐减小,当负荷由50%增大到100%时,积灰上限向冷端移动0.07 m;ABS粘结性积灰概率受负荷影响较小,各负荷下均为0.9,而松散性积灰概率为0.2~0.4,且随着飞灰粒径增大而降低。     

飞灰作用下的SCR系统催化剂磨损特性研究

采用实验与数值模拟结合的方法研究飞灰颗粒与催化剂的磨损问题,利用Lagrange法追踪飞灰颗粒在催化剂近壁区的运动轨迹,并结合催化剂磨损关联式计算催化剂的磨损率。结果表明:在研究工况范围内,催化剂前端磨损率为催化剂孔道磨损率的266～304倍,沿催化剂孔道出口方向,催化剂孔道磨损率在0～30 mm时下降较快, 30 mm之后时下降减缓;受催化剂钢网处飞灰的二次反弹和局部烟气速度高的影响,小飞灰粒径对催化剂前端产生两次撞击角度波峰,钢网边缘波峰处的磨损率较大,导致催化剂变薄,大飞灰粒径惯性大对催化剂前端中部只产生一次撞击角度波峰,引起磨损区域变大;催化剂前端磨损率随烟气速度呈指数增长,随飞灰粒径增大,飞灰粒径对催化剂前端磨损率的影响减弱。     

硅灰在对流管束表面沉积特性的试验研究

硅冶炼炉余热烟气中的硅灰在换热器表面沉积影响换热效率,研究了硅灰的基本粘结特性,及其在对流管束表面的沉积特性和清除方式。试验结果表明：硅灰的粘结性不强,主要在管壁迎风侧形成单侧楔形干松灰,虽然温度影响灰样的粒径分布,但管壁温度对硅灰的沉积和清除特性影响不大;越往烟气下游的受热面积灰越严重;存在临界烟气流速12m／s,当烟气流速超过该值,增大流速不再有利于减轻积灰;钢珠清灰可高效地清除管壁积灰,而机械振打效果不佳。     

低温烟气换热器传热特性的试验研究

ND钢作为优良耐硫酸腐蚀钢已被广泛用于低温烟气余热回收,而目前尚缺乏其传热相关数据,故搭建小型传热试验台,研究了工质与烟气流速、进口烟温和管材质对光管、外螺纹管、螺旋翅片管传热特性的影响。结果表明:工质流速对传热影响不大;烟气流速从4.1增大到7.4 m/s时,总换热系数分别增大了1.86倍、1.7倍、1.44倍,且外螺纹管、螺旋翅片管的总换热系数分别是光管的1.05~1.15倍和1.52~2倍;进口烟温从75℃升高到190℃时,总换热系数分别增大了31%、33%、20%;不同管材质通过接触热阻影响换热性能。     

烟气飞灰对SCR脱硝催化剂的磨损性能试验研究

利用自制的冷态飞灰磨损试验台,进行了飞灰对SCR脱硝催化剂磨损性能影响的试验研究。结果表明:催化剂磨损主要发生在端面,烟气流速是催化剂磨损的主要因素,催化剂磨损率以速度指数基本为4的规律变化,且速度指数随飞灰粒径的增大而减小;飞灰粒径和飞灰质量浓度是催化剂磨损的次要因素,粒径指数在1附近变化,且随烟气流速的增大而减小,催化剂磨损率正比于飞灰质量浓度;在实际运行过程中,应尽可能保证流场均匀,避免局部高速,以减轻催化剂磨损,保证脱硝系统经济安全地运行。     

Simulation of particle deposition on the tube in ash-laden flow using the lattice Boltzmann method

The LBM-Lagrange tracking method with multiple relaxation time (MRT) model has been developed to predict the flow field and particle deposition a circular or elliptical tube in ash-laden gas turbulent flow with Re of 10,229. The model can be used for predict particle deposition effect on thermal resistance or fouling factor of heat exchangers mostly operating in turbulent flow.Particle deposition morphology on the circular and the elliptical tubes were obtained with the lattice Boltzmann method (LBM). The particle deposition mechanism has been investigated. The dominating mechanism of particle deposition on the circular tube is Brownian diffusion for the Stokes number of 0.002, whereas the dominating mechanism of particle deposition is drag inertia for the Stokes number larger than 0.031. When the long axis of the elliptical tube is parallel to the flow, both the collision efficiency and the deposition efficiency for the elliptical tube are fewer than those of the circular tube which means less particle deposition. It also can be concluded that both ratios of the collision efficiency and the deposition efficiency decrease with increasing axial length ratio of the elliptical tube. The elliptical tube is better than the circular tube as heat transfer surface in the aspect of preventing ash particle deposition.     

Numerical evaluation of a heat exchanger with inline tubes of different size for reduced fouling rates

This paper describes the numerical evaluation of a novel cross flow tube bundle heat exchanger that combines tubes of different diameter in an inline arrangement for the purpose of reducing gas side particulate fouling rates while preserving acceptable levels of heat transfer and pressure drop performance. Three arrangements are compared: a common inline tube bundle heat exchanger with cylinders of equal diameter and two other arrangements that consist of alternately placed cylinders with a diameter ratio of d/D = 0.5, at two different transverse spacings. Numerical calculations are performed in order to study heat transfer, pressure drop and fouling rates from flue gases with suspended ash particles. The alternating tube sizes achieve a suppression of the vortex shedding mechanism that has previously been shown to enhance downstream particle deposition. Results show that, compared to the standard arrangement, the tube bundle with unequal cylinders placed at the largest transverse spacing achieves a significant (～30%) reduction in particle deposition rate without sacrificing acceptable values of heat transfer per unit volume and low pressure drop.     

Unsteady CFD simulation on ash particle deposition and removal characteristics in tube banks: Focusing on particle diameter,flow velocity,and temperature

This study establishes a numerical deposition and removal model coupled with unsteady gas–solid turbulent flow to predict transport, impaction, sticking, and removal (or rebounding) characteristics of ash particles with high alkali metals based on Ansys Fluent software extended by user–defined functions. Dynamic mesh analytical strategy is employed to adjust the grid on the interface of flue gas and deposition layer to illustrate ash deposit growth characteristics. In this study, the effects of particle size, flow velocity, and inlet gas temperature on formation and distribution of ash deposits are studied. The results indicate that ash deposit distribution has significant particle size dependence. Larger particles tend to deposit on the windward side of first–row tubes, and account for the greatest share of total deposited mass. Smaller particles may deposit on the leeward side of the tube surfaces. Variation in impaction rates with time are influenced by the thermophoretic force and enlarged deposition area. With the increase in flow velocity, total deposited mass increases gradually. The influence of flow temperature on impacting rates is mainly embodied in the effect of flow temperature on thermophoretic force. However, the increase in inlet gas temperature has little effect on the sticking efficiency and ash deposit distribution. Nevertheless, the total deposited mass shows an increasing trend with inlet flue gas temperature.     

Effect of Condensable Species on Particulate Fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprise particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger, such as the superheater, evaporator, and so on. If the temperatures of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles that are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness, and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a suitable model for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with detailed impaction experiments.     

水滴形管换热与阻力特性的数值研究

为提高换热管的综合性能,设计并研究了一种新型水滴形换热管,采用CFD软件,在相同管横截面积的前提下,对圆形、椭圆形及水滴形换热管单管和管束的换热与阻力特性进行了模拟计算。研究表明:水滴形管可以明显减小换热管背流面的回流;在相同的进口流速下,水滴形管管束努塞尔数Nu较圆管和椭圆管分别增大了28%和18. 5%,而进出口压降相较于椭圆形及圆形管分别减小了45. 5%及90%,相较于圆形管及椭圆形管,水滴型管具有更好的换热流动特性。     

Fouling Reduction Characteristics of a No-Distributor-Fluidized-Bed Heat Exchanger for Flue Gas Heat Recovery

Heat exchangers and heat exchanger networks are extensively used for the purpose of recovering energy. In conventional flue gas heat recovery systems, the fouling by fly ashes and the related problems such as corrosion and cleaning are known to be major drawbacks. To overcome these problems, a single-riser no-distributor-fluidized-bed heat exchanger is devised and studied. Fouling and cleaning tests are performed for a uniquely designed fluidized bed-type heat exchanger to demonstrate the effect of particles on the fouling reduction and heat transfer enhancement. The tested heat exchanger model (1 m high and 54 mm internal diameter) is a gas-to-water type and composed of a main vertical tube and four auxiliary tubes through which particles circulate and transfer heat. Through the present study, the fouling on the heat transfer surface could successfully be simulated by controlling air-to-fuel ratios rather than introducing particles through an external feeder, which produced soft deposit layers with 1 to 1.5 mm thickness on the inside pipe wall. Flue gas temperature at the inlet of heat exchanger was maintained at 450°C at the gas volume rate of 0.738 to 0.768 CMM (0.0123 to 0.0128 m³/sec). From the analyses of the measured data, heat transfer performances of the heat exchanger before and after fouling and with and without particles were evaluated. Results showed that soft deposits were easily removed by introducing glass bead particles, and also heat transfer performance increased two times by the particle circulation. In addition, it was found that this type of heat exchanger had high potential to recover heat of waste gases from furnaces, boilers, and incinerators effectively and to reduce fouling related problems.     

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

Ash deposition always brings boilers some trouble due to fouling or slagging. In this paper, a completely controlled system was developed to study the growth of ash deposit. A novel sampling probe was designed to online measure the heat flux through ash deposit. Additionally, the thickness of ash deposit can be obtained by an online figure collecting system. The results of this research showed that as the thickness of ash deposit increased, the heat flux decreased. It was also found that at the initial stage of ash deposition when the thickness of ash deposit is approximately 1 mm, the heat flux through ash deposit had a sharp reduction. An effective method was attempted to situ measure the effective thermal conductivity of the ash deposit in the simulated combustion flue gas. It was found that temperature of the ash deposit layer had no obvious effect on its value. It was concluded that the structure of ash deposit had no obvious change in a short deposition time of 30 min with varied surface temperatures of the probe head between 400 °C and 600 °C.     

Experimental study on titanium heat exchanger used in a gas fired water heater for latent heat recovery

In general, latent heat recovery is usually accompanied by the corrosion of the heat exchanger, which is caused by the strongly acidic condensate when the temperature of the flue gas is lowered below the acid dew point. The present study has been conducted to investigate the heat and mass transfer characteristics in a titanium heat exchanger with excellent corrosion resistance used for waste heat recovery with the condensation arranged in a gas fired water heater. In addition, the thermal efficiency of the gas fired water heater was evaluated based on the net calorific value at the maximum rated output during latent heat recovery from the exhaust flue gas. Parametric studies were conducted for the flue gas flow rate, inlet temperature and mass flow rate of the supplied water, respectively. Different arrangements of the tubes of the heat exchanger including in-line and staggered configurations were investigated. The experimental results indicate that the thermal efficiency of the gas fired water heater with a latent heat recovery (LHR) heat exchanger was enhanced by about 10% compared with conventional instantaneous water heaters, i.e., water heaters without heat recovery. In addition, in terms of the Nusselt number and the Sherwood number, the heat and mass transfer performance of the staggered tube bank type were approximately 50% and 10% higher than that on the in-line tube bank type when the Reynolds number of the flue gas was 10³.     

复合相变换热器在锅炉烟气余热回收中的应用

为了降低锅炉排烟温度,回收锅炉尾部烟气热量,提高机组效率,降低机组发电煤耗,确保电除尘、引风机安全运行,采用复合相变换热器加热同机组的凝结水,在避免换热器结露积灰的前提下,能大幅度降低锅炉排烟温度,回收锅炉尾部烟气的余热,减少汽轮机的抽汽量,增加机组发电能力,可获得较好的经济效益。