W型火焰锅炉冷态空气动力特性的测试研究

针对引进的配备直接缝隙式燃烧器的法国Ｓtein公司３６０ＭＷ的Ｗ型火焰锅炉进行了冷态模型的空气动力特性研究。利用热线风速仪测量炉内的流场速度,得出了不同工况下炉内Ｗ型气流的流场图,并对不同工况流场的气流速度分布规律、炉内气流的充满度、气流行程、炉膛出口处的速度偏差等进行了分析,研究了炉内空气动力场的特性和变化规律,测试了Ｗ型火焰锅炉火焰短路与炉膛出口速度偏差的性能。     

负荷变化对900 MW超临界锅炉炉内燃烧影响的数值模拟

利用Fluent软件对1台900 MW四角切圆燃烧锅炉在不同负荷下炉内燃烧过程进行了数值模拟,分析了负荷变化对炉内流动和传热的影响规律.结果表明:在高负荷工况下运行时,炉内燃烧充分且稳定,但是炉内火焰更容易冲刷水冷壁,可能发生局部结渣现象;在低负荷工况下运行时,炉内火焰充满度较差,切圆燃烧的稳定性显著下降,炉膛水冷壁灰污表面温度也相应降低,水冷壁表面结渣的倾向弱化,沿高度方向水冷壁吸热不均匀性增大.由于该锅炉的低NOx燃烧器采用了分离燃尽风,使得高温区扩展,火焰中心高度比采用有关标准推荐的方法计算所得结果高4~5 m.     

高温低氧燃烧锅炉传热特性研究

介绍一种新型工业锅炉－高温低氧燃烧锅炉，研究其传热特性，实验研究表明，炉内燃烧火焰边界趋于消失，体积明显增大，火焰颜色变浅，无局部高温区，辐射传热得到强化，提高空气预热温度可加大炉气和水冷壁间传热，为开发推广新型工业锅炉奠定基础。     

2008t/h四角燃烧煤粉锅炉烟温偏差实炉测试分析

大型四角切圆燃烧锅炉的烟、汽温度偏差对其安全运行具有重要影响。在2008t/h锅炉上,对不同负荷工况下炉膛上部屏区烟温及屏式过热器壁温进行了测试。结果表明,汽温左高右低,大致成W型分布形态,而烟温呈M型分布,与汽温分布状态峰谷反向对应。认为炉膛上部空间残余旋转形成沿屏区宽度烟气流量的不均匀分布,因而烟气对受热面的冲刷强度不同,导致屏间烟气换热量存有差异,这是烟、汽温偏差产生的根本原因。     

Multimode heat transfer in cyclic flow reversal combustion in a porous medium

Experimental and numerical studies of combustion and multimode heat transfer in a porous medium, with and without a cyclic flow reversal of a mixture through a porous medium, were performed. Parametric studies were done in order to understand combustion characteristics such as maximum flame temperature and radiative heat flux using a one‐ dimensional conduction, convection, radiation and premixed flame model. The porous medium was assumed to emit and absorb radiant energy, while scattering is ignored. Non‐local thermodynamic equilibrium between the solid an d gas is taken into account by introducing separate energy equations for the gas and the solid phase. As a prelimina ry study, the combustion regime was described by a one‐step global mechanism with an internal heat source uniformly dist ributed along the reaction zone. The effects of the flame position, cyclic flow reversal, period of the cyclic flow rever sal, the optical thickness and the flow velocity on the burner performance were clarified by a rigorous radiation analysis. Th e model was validated by comparing the theoretical results with the experiments. It was shown that, for maximizing the fl ame temperature and the net radiative heat flux feedback, the flame should be stabilized near the centre of the po rous medium with a cyclic flow reversal, the period of which should be as small as possible. A high optical thickness prod uced a high flame temperature and a high net radiative feedback. Also, a high flow velocity at low period of the cyclic f low reversal of mixture yielded a high value of both the flame temperature and the net radiative feedback. Thermal structure predictions in terms of the gas‐phase and the solid‐phase temperature distributions along the axis of the combustor show good agreement with the experimental ones. Copyright © 1999 John Wiley & Sons, Ltd.     

Three-dimensional modeling of utility boiler pulverized coal tangentially fired furnace

This paper presents selected results of numerical simulations of processes in utility boiler pulverized coal tangentially fired dry-bottom furnace. The simulations have been performed by specially developed comprehensive mathematical model. The main features of the model are a three-dimensional geometry, k–ε gas turbulence model, Eulerian–Lagrangian approach, particles-to-turbulence interaction, diffusion model of particle dispersion, six-flux method for radiation modeling and pulverized coal combustion model based on the global particle kinetics and experimentally obtained kinetic parameters. Five operation regimes of 210 MW_e boiler furnace burning Serbian lignites, with different grinding fineness of coal and coal quality, have been simulated. The model successfully predicts the influence of the parameters on the furnace processes and operation characteristics (like the flue gas temperature and the furnace walls radiation fluxes). The predicted flame temperature and percentage combustibles in bottom ash are in good agreement with the measurements. The developed model can find different applications, both in research and practice.     

北巴600MW“W”锅炉水冷壁结焦问题防治分析

北巴600MW超临界W型火焰锅炉运行中炉膛结焦严重,掉焦频繁发生,影响锅炉安全运行。本文介绍了北巴W火焰燃烧技术特点和卫燃带敷设情况,从炉膛温度、空气动力厂、还原性气氛等因素分析了结焦的原因,对炉膛具体结焦位置进行了描述。根据卫燃带改造后锅炉运行情况,提出了防治结焦对策。     

Analysis of flame gas velocity and temperature distribution from double-pipe nozzle jet in a production unit for spherical particles by flame spray method

The flame spray method for preparing spherical powders is a technique in which solid particles are forced to fuse into spheres by exposing them directly to the combustion flame, and has already been applied to producing ceramic (metal) spherical powders. With this technique, it is critical to analyze quantitatively the fusing and sphering characteristics of solid particles, and thus the distributions of gas velocity and temperature of the flame gas have to be calculated first. However, little research has been conducted regarding this subject. In this study, a combustion model is formulated to calculate the profiles of gas velocity and temperature for the fuel gas flame jetted from the commonly used coaxial double-pipe nozzle. The calculation agrees roughly with the flame shape observed in experiments, implying the possibility of using this model to calculate the heat transfer from flame gas to solid powders. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 201–213, 1996     

A numerical study on parametric sensitivity of the flow characteristics on pulverized coal gasification

In order to analyse the sensitivity on pulverized coal flames of variables such as initial turbulent intensity, steam addition, primary/secondary momentum ratio, and radiation heat transfer, a numerical study was conducted at the gasification process. Eulerian approach is used for the gas phase, whereas Lagrangian approach is used for the solid phase. Turbulence is modeled using the standard k–ϵ model. The turbulent combustion model incorporates the eddy dissipation model. The radiation heat transfer is solved using a Monte‐Carlo method. One‐step two‐reaction model is employed for the devolatilization of a Kideco coal. In pulverized flame of long liftoff height, the initial turbulent intensity is an important factor to predict the accurate flame front position. The radiation heat transfer and wall heat loss ratio distort the temperature distributions along the reactor wall, but do not affect the reactor performance such as coal burnout, residence time and flame front position. The primary/secondary momentum ratio only affects the position of flame front, but the coal burnout is slightly influenced. It is confirmed that the momentum ratio is a variable only associated with the flame stabilization. The addition of steam in the reactor has a detrimental effect on all the aspects, particularly in reactor temperature and coal burnout. The increase of liftoff height and dropped gas temperature mean the harm of flame stabilization in the reactor, and so the gasification reaction may be deactivated. Copyright © 2000 John Wiley & Sons, Ltd.     

Effects of primary soot particle size distribution on the temperature of soot particles heated by a nanosecond pulsed laser in an atmospheric laminar diffusion flame

Temperature histories of nanosecond pulsed laser heated soot particles of different primary particle size distributions were calculated using a single primary particle based heat and mass transfer model under conditions of a typical atmospheric laminar diffusion flame. The critical peak soot particle temperatures beyond which soot particle sublimation cannot be neglected were identified to be about 3300–3400 K. Knowledge of this critical soot particle temperature is required to conduct low-fluence laser-induced incandescence experiments in which soot sublimation is avoided. After the laser pulse, the temperature of smaller primary soot particles decreases faster than that of larger ones as a result of larger surface area-to-volume ratio. Unlike the common belief that the peak soot particle temperature is independent of the primary particle diameter, the numerical results indicate that this assumption is valid only when soot sublimation is negligible and for primary soot particle diameters greater than about 20 nm. The effective temperature of a soot particle ensemble having different primary particle diameters in the laser probe volume was calculated based on the ratio of the total thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. In the non-sublimation regime, the initial effective temperature decay rate after the peak soot temperature is related to the Sauter mean diameter of the primary soot particle diameter distribution. At longer times, the effective temperatures of soot particle ensembles start to display different decay rates for different soot primary particle diameter distributions. A simple approach was proposed in this study to infer the two parameters of lognormal distributed primary soot particle diameter. Application of this approach was demonstrated in an atmospheric laminar ethylene diffusion flame with the inferred primary soot particle diameter distribution compared with independent ex situ measurement.     

电站锅炉炉膛内火焰刷墙时对流换热系数的计算方法

分析了电站锅炉炉膛内火焰刷墙时的对流换热,提出了火焰刷墙时的对流换热由烟气对流换热和粒子对流换热组成的换热模型。按此模型的计算结果与试验数据符合良好。     

Charge fraction distribution of nucleation mode particles: New insight on the particle formation mechanism

We measured particle size distributions of total and singly charged nanoparticles in premixed flames with different flame stoichiometry and temperature to investigate particle inception.Particle charging in flames occurs by diffusion charging involving ions formed by chemi-ionization reactions in the flame front. It can be described by a Boltzmann charge fraction distribution evaluated at the local flame temperature where the particles interact with the chemi-ions. As the particles coagulate in the post flame zone, their charge fraction is reduced. The charge distribution of the coagulated aerosol again results in a Boltzmann curve, this time evaluated at the local post flame gas temperature where the particles had their last coagulation event. Particle nucleation in the post flame zone, where chemi-ions are drastically reduced, produces uncharged particles.Considering the above charging processes, the charge fraction of the nucleation mode contains information on the location within the flame these particles were formed. The results show that in flames near the particle inception threshold, particles are charged close to the flame front and remain charged even late in the post flame zone. Furthermore, smaller particles undergo less charge neutralization by coagulation as they travel through the post flame zone than larger particles. A different scenario is observed in richer flames; the smaller particles eventually become uncharged, indicating that significant amounts of freshly nucleated particles in these flames are formed in the post flame zone. Whether nucleation preferentially occurs close to the flame front or persists into the post flame zone also depends on flame temperature.     

煤焦颗粒燃烧模拟在W型火焰锅炉炉膛设计热力计算中的应用

分析了炉膛设计热力计算中引入煤焦颗粒燃烧模拟的可能性和必要性,探讨了采用早期的锅炉机组热力计算标准进行新型的W型火焰锅炉炉膛热力计算的可行性及经验系数的取值问题,分析、比较了煤焦颗粒在实验室和炉膛内燃烧环境的差异,并引入氧量分布不均系数、区段充满度系数来考虑炉膛内颗粒表面氧浓度、停留时间等方面与实验环境的差异.在此基础上,建立了适合于炉膛分区段热力计算的煤焦燃烧模型,并最终将煤焦非均相反应的实验室数据应用于炉膛分区段热力计算,克服了目前炉膛设计热力计算仅考虑传热所带来的不足.在一台W型火焰锅炉上的应用表明,颗粒燃烧和炉膛传热相耦合的分区段热力计算模型能够揭示过量空气量、煤粉细度、煤种、负荷等因素对燃尽和传热的影响,适合工程应用.     

Condensation heat transfer on tubes in actual flue gas

In order to improve boiler efficiency, latent heat recovery from flue gas is a very important concept. Condensation heat transfer on horizontal stainless‐steel tubes was investigated experimentally by using an actual flue gas from a natural gas boiler. The experiment was conducted at different air ratios of the flue gas and a wide range of tube wall temperatures. The condensation pattern was similar to a dropwise condensation near the dew point. By decreasing the wall temperature, the wall region covered with a thin liquid film increased. The heat and mass transfer behavior was well predicted with the analogy correlation at the high‐wall‐temperature region. At the low‐wall‐temperature region, the total heat transfer was higher than that predicted by the analogy correlation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 139–151, 2001     

Structure and temperature distribution of a stagnation-point Diesel spray premixed flame

We experimentally examine the flow and flame characteristics of a stagnation point premixed flame influenced by Diesel sprays. In the experiment, distributions of drop size, drop axial velocity and its fluctuation as well as the gas phase temperature are measured by using the phase-doppler particle analyzer and a thin thermocouple. As might be expected, similar to the gasoline spray flame, the partially prevaporized Diesel spray flame is composed of a weak blue flame zone, indicating the burning of methane fuel, and a strongly luminous zone containing many bright yellow lines showing the passages of burning Diesel drops. It is found that the axial temperature profiles at various radial positions consist of an upstream preheat region, a maximum temperature downstream of the blue flame and a downstream region with a declined temperature curve because of the heat loss to the quartz plate. The SMD of the drops increases from the upstream preheat region to a maximum near the blue flame and then decreases in the downstream burning zone. Along the axial position, the drops are decelerated in front of the flame but accelerated when passing through the blue flame. It is also interesting to note that the radial distributions of SMD and number density of drops in the upstream region are mainly influenced by small drops flowing outward, since the upstream vaporization of Diesel drops is very limited; while those in the downstream region should be influenced by both small drops flowing outward and Diesel drops burning. From the experimental observations, there are impinging and bouncing of Diesel drops downstream of the spray flame near the quartz plate, resulting in a small amount of soot and carbon deposits on the wall. These interesting phenomena will be reported in the near future.     

超临界“W”火焰炉关键技术应用

针对燃用高硫高灰无烟煤超临界"W"火焰锅炉投产后暴露出来的燃烧稳定性差、锅炉受热面热偏差严重、NO_x排放浓度高、锅炉受热面结焦腐蚀、启动油耗偏大等影响机组安全稳定运行、高效清洁燃烧等问题,开展超临界"W"火焰锅炉设计优化、燃烧及制粉系统制造运行优化、锅炉热面在线监测技术研究、锅炉受热面防结焦防高温腐蚀技术材料研究、无烟煤节油点火技术开发等关键技术研究与应用,有效解决了超临界"W"火焰锅炉安全稳定、高效清洁燃烧的技术难题。     

Investigating the Effects of Natural Gas Preheating on Soot Formation,Flame Luminosity,and NOX Emissions: A Combined Experimental and Numerical Approach

One of the most problematic aspects of gas‐fired furnaces is their poor luminosity and radiative characteristics which are directly affected by the carbon percent of the fuel molecules. This study investigates the soot content, flame temperature, and efficiency of a 120 kW boiler resulting from inlet gas (natural gas) preheating. A probability density function (PDF) being parameterized by the mean and variance of mixture fraction was used to model chemical reactions. To describe the effects of turbulences on soot formation, a Moss–Brooks model and a β‐PDF in terms of normalized temperature is employed. Using Nickel–Chrome electrical heaters, the fuel was preheated to more than 573 K. Radiations emitting from the flame were measured by a laboratory pyranometer with a photovoltaic sensor; moreover, a Testo 350 XL gas analyzer was used for recording the temperature and combustion species. Results revealed that gas preheating up to about 510 K has no considerable effect on the flame luminosity. On the other hand, preheating the inlet gas up to 700 K increases the soot content of the flame up to 300% resulting in a serious augmentation of flame luminosity. This increase causes a significant reduction in flame temperature (150 K) and NO emission. The predicted results have good agreement with measurement results.     

Population balance model of heat transfer in gas–solid particulate systems

A population balance model is derived for heat transfer processes in gas–solid systems with intensive motion of particles in order to describe the temperature distribution of particulate phase. The model involves collisional particle–particle and particle–wall heat transfers, and continuous gas–particle, gas–wall and wall–liquid environment heat transfer processes. Collisional heat transfers are characterised by collision frequencies and random heat exchange parameters with general probability distributions with support [0, 1], describing the heat transfer efficiency between the colliding solid bodies. An infinite hierarchy of moment equations, describing the time evolution of moments of the temperature of particle population is derived from the population balance equation, which can be closed at any order of moments. The properties of the model and the effects of parameters are examined by numerical experiments using the second order moment equation model of a spatially homogeneous fluidized bed.     

Assessment of a novel numerical model for combustion and in-flight heating of particles in an industrial furnace

The key factors for efficient in-flight particle heating in a combusting flow were investigated within this paper for the development of a novel boiler slag bead production furnace. A natural gas fired industrial burner with a thermal input of 1.2?MW was thus evaluated using Computational Fluid Dynamics (CFD). The steady laminar flamelet model (SFM) and a detailed chemical reaction mechanism, considering 25 reversible chemical reactions and 17 species were used to account for the steady-state gas phase combustion. Measurements of gas temperature and flow velocity within the furnace were found to be in good accordance with the numerical results. In the second step, sintered bauxite beads were injected into the furnace as an experimental material and heated up in flight. The particle heating characteristics were investigated using the Discrete Phase Model (DPM). The computational results of the particle laden flow raised the issue that convective heat transfer is a key factor for efficient particle heating. At the burner chamber outlet, the temperature of a particle which had been injected into the burner flame was 178?K higher compared to a particle, which trajectory led through zones with lower gas temperatures.     

Experimental‐ and numerical‐simulation research on the inner–secondary‐air ratio in a 600‐MWe down‐fired boiler

Using a phase Doppler‐anemometer measurement system, the cold gas/particle‐airflow behavior in a 1:40 scale‐model furnace was assessed to study the influences of adjusting the inner–secondary‐air ratio in a 600‐MW_e multi‐injection and multistaging down‐fired boiler. Numerical simulations were also conducted to verify the results of the modeling trials and to provide heat‐state information. The results demonstrate that reducing the inner–secondary‐air ratio from 19.66% to 7.66% gradually enhances the downward velocity decay of the gas/particle airflow, while the inner secondary‐air downward‐entraining effect on the fuel‐rich flow is weakened. Lowering the inner–secondary‐air ratio greatly inhibits the decay of the near burner–particle volume flux. In addition, the fuel rich–flow ignition distance is reduced, from 1.02 to 0.87 m. A lower inner–secondary‐air ratio is harmful to restrain early NO_x formation. Reducing the ratio also causes the fuel‐rich flow to turn upwards ahead, while the penetration depth of this flow gradually decreases and the maximum temperature in the hopper region falls from 1900 to 1800 K. On the basis of these data, an optimal inner–secondary‐air ratio of 13.66% is recommended.