Multiphase flow simulation of a simplified coal pulveriser

In coal-fired power plants, the first major component is pulveriser, whose performance dictates the total power station efficiency. Pulveriser is employed to grind the lumped coal and transport the fine coal powder to furnace chamber for an efficient combustion. In this study, we have simulated motion of air and coal particles inside a commercial-scale pulveriser. Multiphase flow simulation of a simplified pulveriser was carried out using a granular Eulerian–Eulerian approach. Due to inclined air-distributor vanes, the flow field within pulveriser was slightly asymmetric. Regions of exceptionally high velocities were predicted close to the outer walls of the pulveriser, indicating a strong probability of particles carryover within these regions. The 100 μm coal particles qualitatively followed the air path-lines. However, the velocity vectors for 500 μm particle deviated significantly from those of airflow. The results presented in the paper provide impetus for the development of a complex pulveriser model, which further could prove valuable to designers for optimisation of components within the mill.     

CFD based investigations into optimization of coal pulveriser performance: Effect of classifier vane settings

In coal-fired power plant, pulveriser is the first major component, whose performance dictates the total power plant efficiency. Uniform flow rate and desired size fraction at outlet pipes along with higher classifier efficiency are three important measures which decide the pulverizer performance. Optimization of pulverizer at its best operating conditions has been considered as a potential area that needs to be addressed for improving unit performance, emissions, operations, and maintenance. The best operating conditions are optimum air velocity and classifier vane settings. In this investigation, numerical simulations of a typical pf coal based pulveriser have been carried out for different classifier vane settings to evaluate uniform flow rate and desired size fraction at outlet pipes along with high optimum classifier efficiency. The optimum opening for the vanes has been determined based on the above measures, which not only reduces unburnt CO, SO_x and NO_x emissions at boiler end but also minimise energy consumption of mill (in terms of reductions in regrinding cost). Computational Fluid Dynamics (CFD) simulations of the coal classifier physical model indicate good agreement with the plant data, in terms of internal flow patterns, particle collection efficiency and desired cut size. From the simulation studies, optimum opening for the vanes is found to be 65% for selected utility which leads to closest uniformity with 60% classifying efficiency wherein 70% particles pass through 75 μm sieve.     

风速对骨料烘干煤粉燃烧器排放特性的影响

基于煤粉燃烧机理,结合骨料烘干工艺,建立了骨料烘干煤粉燃烧器内部场的控制模型,采用Fluent软件模拟煤粉燃烧器内部燃烧状况,考察了一、二、三次风的风速对煤粉燃烧器中心轴线处CO, CO2, NO和SO2浓度的影响。结果表明,在研究的风速范围内,一、二、三次风风速越大燃烧越充分,一、二、三次风风速越小,产生的NO越少;三次风风速为40 m/s时,SO2浓度最低;较合理的控制参数为一次风风速30~35 m/s,二次风风速45~50 m/s,三次风风速30~40 m/s。     

Influence of the adjustable vane position on the flow and combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler

Experiments with a small-scale cyclone burner used for burner enrichment in a down-fired pulverized-coal 300 MW_e utility have been conducted on an air/particle test facility. Particle separating efficiency was obtained with different positions of an adjustable vane. Industrial experiments were performed on a full-scale boiler. The gas temperature distribution along the primary air and coal mixture flow, gas temperature distribution of the furnace, and gas components such as O₂, CO, CO₂ and NO_X in the near-wall region were measured for the first time. The influence of the adjustable vane position on coal combustion in the furnace was determined. With the adjustable vanes at the nozzle, ignition of the primary air and pulverized-coal mixture was delayed and the gas temperature peak was above the burner arch, with high NO_X emission. Raising the vanes can bring forward the ignition point but results in the fuel-rich flow being up ahead of time, leading to a rise in carbon content in fly ash and NO_X emission.     

Artificial intelligence control of food freezer using rotating disk air distributor for uniform freezing

A rotating disk air distribution (RDAD) system consisting of a rotating disk with an air supply hole was fabricated to solve the nonuniform freezing problem of a freezer that supplies air through a duct at a fixed location. The RDAD system was installed in a freezer to supply freezing air in various directions by controlling the position of the air supply hole. An equation estimating air velocity in the freezer was developed based on heat balance between a brass block thermocouple (BBT) probe and ambient air stream. The equation was applied to the analysis of airflow patterns developed through the RDAD and the fixed duct system. For control of the RDAD system, a fuzzy matrix was constructed with the measured nodal temperature at 3D locations in the freezer and used to search for the optimum direction of air supply. Uniform cooling was accomplished with the RDAD system using fuzzy and expert algorithms leading to temperature deviation σ=0.666, compared with 0.289 of the fixed air supply system. Cooling rate of model food (BBT) was 1.75 times faster than that of the fixed system.     

ARTIFICIAL INTELLIGENCE CONTROL OF FOOD FREEZER USING ROTATING DISK AIR DISTRIBUTOR FOR UNIFORM FREEZING

A rotating disk air distribution (RDAD) system consisting of a rotating disk with an air supply hole was fabricated to solve the nonuniform freezing problem of a freezer that supplies air through a duct at a fixed location. The RDAD system was installed in a freezer to supply freezing air in various directions by controlling the position of the air supply hole. An equation estimating air velocity in the freezer was developed based on heat balance between a brass block thermocouple (BBT) probe and ambient air stream. The equation was applied to the analysis of airflow patterns developed through the RDAD and the fixed duct system. For control of the RDAD system, a fuzzy matrix was constructed with the measured nodal temperature at 3D locations in the freezer and used to search for the optimum direction of air supply. Uniform cooling was accomplished with the RDAD system using fuzzy and expert algorithms leading to temperature deviation σ=0.666, compared with 0.289 of the fixed air supply system. Cooling rate of model food (BBT) was 1.75 times faster than that of the fixed system.     

CFD modeling of MPS coal mill with moisture evaporation

Coal pulverizers play an important role in the functioning and performance of a PC-fired boiler. The main functions of a pulverizer are crushing, drying and separating the fine coal particles toward combustion in the furnace. It is a common experience that mill outlet pipes have unequal coal flow in each pipe and contain some coarse particles. Unequal coal flow translates into unequal air-to-fuel ratio in the burner, deviating from the design value and thus increasing unburned carbon in fly ash, NO_x and CO. Coarser particles at the mill outlet originate from poor separation and decrease the unit efficiency. In addition, coarser particles reduce burner stability at low load. Air flow distribution at the mill throat, as well as inside the mill, significantly influences the mill performance in terms of separation, drying, coal/air flow uniformity at the mill outlet, wear patterns and mill safety. In the present work, a three-dimensional computational fluid dynamics (CFD) model of the MPS Roll Wheel pulverizer at Alliant Energy's Edgewater Unit 5 has been developed. The Eulerian-Lagrangian simulation approach in conjunction with the coal drying model in Fluent, a commercial CFD software package, has been used to conduct the simulation. Coal drying not only changes the primary air temperature but it also increases the primary air flow rate due to mass transfer from coal. Results of the simulation showed that a non-uniform airflow distribution near the throat contributes significantly to non-uniform air-coal flow at the outlet. It was shown that uniform velocity at the throat improves the air and coal flow distribution at the outlet pipes. A newly developed coal mill model provides a valuable tool that can be used to improve the pulverizer design and optimize unit operation. For example, reject coal rate, which is controlled by the air flow near the mill throat, can be reduced. The model can also be used to further aid in identifying and reducing high temperature or coal-rich areas where mill fires are most likely to start.     

Fractal and turbulence characteristics of aerodynamic fields of swirl burners

Turbulence intensities at the exit of a centrally fuel rich (CFR) burner were measured employing a probe with hot-film sensors. In addition, using glycol as a smog tracer, optical images of the highlighted primary airflow were taken with a CCD camera and contrast-enhanced by computer. Profiles of the interface between primary and secondary air were obtained from which fractal dimensions (FDs) of the primary air boundary were estimated. Results indicate that FDs of this air boundary are in the range of 1.10–1.30 depending on burner conditions. As the FD and mean turbulence intensities of the primary air boundary have uniform distributions, they can be used to analyze coal combustion and NO_x formation characteristics of the burner. When either decreasing the outer secondary air vane angle or increasing either the inner secondary air vane angle or total mass flow rate of the secondary air, the burner swirl number increases which subsequently shows up as an increase in the FD and mean turbulence intensities of the primary air boundary. Increasing the ratio of the mass flow rate of inner to outer secondary air decreases the burner swirl number, but increases the FD and mean turbulence intensities of primary air boundary.     

How Particle Resuspension from Inner Surfaces of Ventilation Ducts Affects Indoor Air Quality—A Modeling Analysis

Dust particles deposited on the inner surfaces of the ventilation ducts can be resuspended by passing airflow. A physical-science-based model is developed to understand how particle resuspension affects the indoor air quality. This integrated model takes into consideration particle mass balance models for straight ventilation duct, duct bend, ventilation room, and air filter. The straight duct and room models have been validated using experimental data. With the integrated model, we find that in-duct resuspension of particles could lead to significant increase in exposure to airborne particles for indoor occupants. It is also found that indoor particle exposure is a linear function of dust mass loading. Greater ventilation rate, which means higher air speed above the dust particles, would lead to greater exposure ratio, while fresh air ratio has little influence. Possible control methods are discussed as well.     

Experimental and numerical study of the airflow distribution in mixed-flow grain dryers

The aim of this study was to investigate the airflow distribution in a mixed-flow dryer (MFD) and to study the effect of different bed materials and air duct arrangements. The results were used to validate the numerical model developed in a previous work based on Computational Fluid Dynamics (CFD). A series of experiments have been conducted at a semi-technical MFD test dryer with horizontal and diagonal air duct arrangement. Wheat and rapeseed were used as bed materials. The experiments were performed under isothermal conditions. Two experimental methods were selected and adapted to the measuring problem—the measurement of the isobar distribution within the grain bed and the residence time analysis using the tracer gas pulse method. As could be shown, the isobar distributions measured for wheat and rapeseed agreed well with the model predictions. The numerical model could calculate the influence of the bed material with its different particle characteristics (e.g., particle shape, particle size, bed porosity). The results obtained from the residence time analysis confirmed the known quartering of the air stream flowing from one inlet air duct to the four surrounding outlet air ducts for the horizontal air duct arrangement; in the diagonal air duct arrangement, the air stream from one inlet air duct was nearly halved flowing to the two adjacent diagonal outlet air ducts. These results were confirmed by investigations of the air velocity distribution within the grain bulk. Further experiments are necessary to refine the model. The residence time and isobar measurements will be extended to study the influence of different air properties under real drying conditions, the effect of structural elements, and dryer designs.     

供煤系统的改进和完善

回转窑喂煤系统喂煤量波动和跑煤的原因是:煤粉仓容量偏小;螺旋喂料机螺旋叶片磨损过大;螺 旋泵选型不合理;风量风压不足。根据以上原因实施相应的措施后,改造效果显著。     

高速煤粉燃烧器内燃烧特性数值模拟及结构优化

高速煤粉燃烧器火焰喷射速度高达60~200 m/s,炉膛内火焰较长,对流换热比例提高,使得炉膛内温度分布均匀,没有传统低速煤粉燃烧器火焰短,炉膛内局部过热和结焦等缺点。笔者以14 MW高速煤粉燃烧器为研究对象,采用数值模拟的方法,研究旋流强度、二次风温度等关键参数对燃烧器内煤粉燃烧的影响,针对燃烧器内煤粉燃烧特点进行结构优化设计。对旋流强度研究结果表明,当旋流强度S=2.2、2.8、3.2及3.7时,燃烧器内回流区形状变化不大,从一次风喷口开始到旋流叶片位置结束,回流区环绕一次风管;最大回流量在一次风喷口附近,距离一次风喷口越远,回流量越小;旋流强度对一次风喷口附近最大回流量影响不大,喷口附近最大回流量均在0.45 kg/s左右,当距喷口超过一定距离(L/H<0.35)时,旋流强度对回流量的影响开始变得明显,表现为旋流强度越大,回流区末端回流量越大,回流区末端回流量最大为0.30 kg/s,最小为0.17 kg/s。研究燃烧器喷口处燃烧状态表明,喷口处火焰旋流强度为0.10~0.28,与入口旋流强度正相关,火焰喷射速度150 m/s,为中等旋流强度的高速旋流火焰;喷口中心区可燃性组分富集,缺氧,燃料和氧气分层分布。当旋流强度提高,喷口中心区可燃性组分浓度降低,CO浓度从11%降低到10%,H2浓度从1.65%降低到1.40%,焦炭浓度从0. 14%降低到0. 11%,喷口边缘O2浓度从13%降低到10%。旋流强度S=3.2和S=3.7时可燃组分和氧气浓度分布变化较小,说明旋流强度提高对燃烧的影响减弱。考察0、100和200℃下二次风温度对燃烧的影响,结果表明,当二次风温度提高,煤粉在燃烧器内的反应时间有所降低,从0.15 s降低到0.11 s,但燃烧器内的煤粉碳转化率提高20%,达到65%。对燃烧器结构进行优化,加入中心风,对比中心风直流和旋流与不加中心风3种状态,结果表明,加入旋流中心风和直流中心风后喷口中心区半径r≤75 mm范围内可燃组分浓度降低,采用直流时由于气流刚性较强,喷口中心区氧气浓度升高,采用旋流中心风对中心区氧浓度影响弱,对可燃组分浓度降低效果优于直流中心风。     

The influence of airflow on fuel spray characteristics from a slit injector

Optimization of fuel spray, airflow, and their interaction with the cylinder and piston wall is crucial to achieve stable combustion of stratified charge with minimum emissions in direct-injection spark-ignition (DISI) engines. In this study, the interaction between air and fuel spray from slit injector was investigated in a steady airflow system generated by a wind tunnel under atmospheric conditions. Both Mie scattering images and phase Doppler anemometry (PDA) measurements of the spray were analyzed for different air velocities. Three-dimensional computational fluid dynamics (3-D CFD) have been employed to further explain the mutual interaction between air and spray. It was found that increasing the airflow velocity across the spray results in a significant change in the bottom part of the spray while a slight change was observed close to the nozzle exit. The variation in spray geometry, which is mainly attributed to an aerodynamic effect and to extracted droplets from the main spray by the airflow effect, was evaluated and presented for different air velocities. The spray droplet size redistribution within the spray plume was investigated, and regions with smaller and larger droplets were identified and discussed. The results indicate that the effect of airflow pattern on droplet size distribution within the spray is a considerable factor in the optimization of airflow and spray together. This could be considered in achieving a limited ignitable region without much diffusion of smaller droplets to the non burning zone during the part load operation of DISI engines.     

潮湿细煤气流分级机内气固两相流的CFD模拟

以CFD计算软件FLUENT为平台,采用Realizablek-着湍流模型和欧拉-拉格朗日方法的离散相模型对实验室研制的潮湿细煤气流分级机内的空气流场进行数值模拟,得到分级机中流场的气流速度、流场静压、流场湍动能的分布情况,以及不同粒径细粒煤在分级机中的运动轨迹。数值计算结果表明:分级机内多孔层的设置可造成压强和流速阶跃,增强多孔层上方区域的流速,提升气体对细粒煤的携带作用;导流板的设置使入料口到细料出口间出现了较强的流带,有利于细粒煤分离;导流板和倾斜多孔层的设置使分级机内压差最大且湍流较弱,有利于颗粒分散,实现小颗粒与大颗粒的分离,提高分级效率同时也有利于中等粒径团聚体的破碎、分散,但对大粒径团聚体的分裂破坏作用有限。     

Experimental investigation on NOx emission and carbon burnout from a radially biased pulverized coal whirl burner

Experiments have been performed on 1 MW pulverized coal (pc) furnace in order to investigate the characteristics of coal combustion and NO_x emission from a new type of radially biased dual register whirl burner. The burner is characterized by a primary air pipe with a continuously changing cross-section and an impact ring. The mixture of pulverized coal and air inside the primary pipe is split into two streams, which are the outer pc rich annular jet and the inner pc lean annular jet respectively. Three Chinese coals, which are high rank bituminous coal, low rank bituminous coal and meager coal respectively, are used in the experiments. We examine the influences of various parameters such as the relative position of the over-fire air (OFA) nozzle, over-fire air ratio (19.1%), primary air ratio, inner secondary air ratio, outer secondary air ratio, inner secondary air swirling intensity, and outer secondary air swirling intensity on NO_x formation and unburned carbon in fly ash. With the primary air ratio increasing from 13.4% to 23.4%, the value of the NO_x emission of the SH coal decreases by 26.7% at first, and then increases by 21.7%. In contrast, the value of the carbon in fly ash (C_FA) increases by 40.1% at first, and then decreases by 58.3%.According to the experimental results, the influence of each individual parameter on NO_x formation and unburned carbon in fly ash agrees well with the existing literature. In this study, the influences of various combinations of these parameters are also examined, thus providing some reference for the design of the radial biased whirl burner, the configuration of the furnace and the distribution of the air.     

水泥预分解窑用一次风辨析

回转窑三风道或四风道煤粉燃烧器供风分为"煤风"和"净风"两种,国外以前将"净风"称为"一次风",国内也有不少人认为将"煤风"和"净风"之和称为"一次风"更合理。针对"一次风"概念两种不同定义并行的混乱情况,从"一次风"的由来和回转窑用风的发展历程进行分析,提出将"煤风"与"净风"之和称为"一次风"更科学合理,建议应摒弃只将"净风"作为"一次风"的概念。     

Transport and deposition of inertial aerosols in bifurcated tubes under oscillatory flow

The transport and deposition of aerosol particles in a single bifurcation tube under oscillatory airflow condition are investigated via experiments and numerical simulations. In the experimental study, a novel stabilized laser-photodiode measurement technique is used to quantify the effects of frequency of oscillatory airflow and airflow rate on surface deposition of particles within the bifurcation tube. Surface deposition in the parent and daughter tubes is measured by orientating the laser-photodiode device parallel and perpendicular to the bifurcation plane and calculated using a transmission loss model. In the numerical simulation study, the same bifurcation tube is constructed using a two- and three-dimensional computer mesh in COMSOL^® to model particle mobility and deposition characteristics, considering the simultaneous effects of inertial impaction, gravitational settling and interception. The effects of frequency of oscillatory airflow, airflow rate and particle size on the particle trajectory and spatial deposition pattern are examined.     

循环流化床锅炉的节能技改

介绍了循环流化床锅炉的使用状况以及生产运行中存在的问题,对一次风风道、钟罩式风帽和旋风分离器进行了改造,对比了改造前后主要运行参数和技术性能,总结了改造后运行效果。结果表明,改造后循环流化床锅炉蒸汽产量由65 t/h提高到82 t/h,吨蒸汽煤耗比改造前的2006年下降13kg。     

SCX分级机进料管流场数值分析与结构优化

以FLUENT软件为计算平台,气相采用欧拉模型,颗粒相采用DPM模型对SCX超细分级机进料管内流场进行了数值模拟,进料管内部流场不均匀,存在明显的偏流现象。在弯管处添加导流片进行优化,对比分析了导流片数量、尺寸和间距分布形式对压力、速度和颗粒质量浓度分布的影响。结果表明:导流片的添加提高了进料管流场的品质;当导流片数量为5,尺寸为1/4圆弧并在前后加长150 mm(半径的1.2倍),并按等间距分布时,进料管内压力、速度和颗粒质量浓度分布较为均匀,为后续的分级提供了良好的前提。     

A simplified model of nitric oxide emission from a circulating fluidized bed combustor

A simplified mathematical model leading to a closed form of solution is developed for estimation of nitric oxide emission from a coal fired circulating fluidized bed (CFB) furnace. The furnace is divided into two sections: a lower section below and an upper section above the secondary air injection level. Reactions in the cyclone and the return leg are neglected. Furnace dimensions, coal feed rate, coal composition and furnace temperature are inputs to the model which was validated against several pilot scale and commercial units. Experimental results from two pilot plants and two commercial power plants agree with model predictions. A sensitivity analysis was carried out using the model to examine the effect of different operating parameters and coal properties on the overall NO emission from the furnace. It was found that excess air and furnace temperature are most important factors influencing the NO emission level. The primary to secondary air ratio influences the NO emission level reasonably. Properties of coal are other factors which affect the NO emission to a large extent. The model, though it invovles some simplification, predicts the overall emission of NO with a level of accuracy accepted in commercial operation.