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.     

多风道煤粉燃烧器拢焰罩的长度对窑内煤粉燃烧状况影响的研究

本文运用计算流体力学（CFD）技术，以大型流体工程计算软件CFX4．3为工具，模拟研究了喷煤燃烧器的扰焰罩长度对回转窑内煤粉燃烧过程规律的影响，结果表明，一定长度的扰焰罩有利于加强燃烧器出口附近的湍流混合和煤粉燃烧。     

Numerical modelling of the carrier gas phase in a laboratory-scale coal classifier model

The pneumatic transport of fine ideally combustible coal dust to the burner furnace is an important process in coal fired power plants. The strongly swirling air phase responsible for the particle separation and transport in a coal pulverising mill was characterised experimentally and numerically. Measurements of the swirl velocity component were taken in a scaled laboratory model of the device and compared to CFD model. In particular, an evaluation of the turbulence models used to describe the flow was performed. The modified isotropic k-epsilon turbulence models (RNG k-ε and Realizable k-ε) were compared to the anisotropic Reynolds stress model (RSM) and their ability to predict the bulk flow structure present in the classifier was assessed. The experiments showed that the swirling flow structure, responsible for coarse-fine particle classification, has several flow regimes which are governed by the areas it is bounded by. The numerical model predictions generally corroborate the results. However, a distinction in performance between the three models can be made based on accuracy, solution generation time and numerical stability. The RSM model predicted both the trends and magnitude the most accurately when compared to the isotropic models. However, the Realizable k-epsilon model, with its relatively low solution generation time, shows potential when using CFD as a classifier design optimization tool. The investigation has given some insight on single phase classifier flow and suggests a design improvement based on the results.     

Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a test furnace equipped with an advanced low-NO_x burner called CI-α burner, and the detailed combustion characteristics are investigated. In addition, the validities of the existing NO_x formation and reduction models are examined. The results show that a recirculation flow is formed in the high-gas-temperature region near the CI-α burner outlet, and this lengthens the residence time of coal particles in this high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O₂ region for effective NO reduction. It is also found that, by lessening the effect of NO reduction in Levy et al.'s model and taking the NO formation from char N into account, the accuracy of the NO prediction is improved. The efficiency factor of the conversion of char N to NO affects the total NO concentration downstream after the injection of staged combustion air.     

多通道烧嘴水煤浆气化过程数值模拟

针对气化炉烧嘴在使用过程中出现的雾化效果差的问题,采用了DPM颗粒离散多相流模型,研究了同心三通道烧嘴使用过程中的水煤浆管内流动和雾化规律。改变多个影响因素并正交分析后得出:水煤浆雾化受烧嘴内外两气道气体流速变化的影响,气体流速增大时,水煤浆SMD值降低;当增加水煤浆流道锥角,使水煤浆与气体射流充分预混时,SMD值减小明显。此外,增加烧嘴出口节流同样会减小SMD值。综合比较各因素的影响,发现节流尺寸和内喷嘴气体流速对水煤浆的雾化效果影响最大。     

A 1/4 scale test facility for PF transport in power station pipelines

Coal fired power station efficiency affects the environment and cost of power. Pulverised fuel (pf) is conveyed from the mill to the furnace through a convoluted network of steel pipes with several junctions and the coal can be badly distributed between the burners. In order to simulate the behaviour of pf in the network and to provide a solution to this problem, a test facility has been constructed at the University of Nottingham. The rig provides air drawn through a 154 mm pipe, which then splits two or three ways. A number of bends prior to the junction point ensure an unbalanced distribution of powder in the downstream pipes. Conventional and novel techniques have been applied to quantify the mal-distribution.     

Pressurised flash drying of Yallourn lignite

A previously developed 1D model of lignite drying for pulverised lignite feeding into a conventional pulverised fuel boiler during the coal milling process is applied to lignite in an entrained flow configuration at elevated pressure. A combustor fired with diesel and air was used to produce a flue gas at 800 °C and 10 atm to flash dry Yallourn lignite at a nominal feed rate of 725 kg/h along a 50 m duct. A coal feeder arrangement was also developed which provides a simple positive feeding device for feeding against a back pressure. The feeder takes as an input coal which is nominally <50 mm and discharges it as a finely divided product with a mean particle size of approximately 1.0-1.5 mm. The comparison between model predictions and measured temperature profiles for the flue gas and final moisture content of the dried coal product showed excellent agreement. Coal moisture was reduced from 67 wt% to between 30 and 40 wt%, depending upon on the coal feed rate and particle size. The small variation in the final outlet temperature observed between the model and experimental results is due to heat losses from the exposed duct work to the environment.     

HP磨煤机磨制高水分印尼煤选型

根据高水分印尼煤的煤质特点,分析了HP磨煤机磨制高水分印尼煤时的技术特点,从研磨出力和干燥出力角度出发,详细论述了HP磨煤机的选型方法。通过实际工程HP磨煤机选型实践,得出HP磨煤机可以磨制收到基水分高达40%的印尼煤,研磨出力和干燥出力均能满足设计要求。     

基于EDM、PDF和FR/ED模型的煤粉燃烧过程适应性研究

还原气氛下煤粉反应过程兼具燃烧和气化的特点,是煤粉实现低氮燃烧的关键因素,合适的化学反应模型是准确模拟该过程的基础。笔者基于煤粉双锥燃烧器,对比了EDM模型、PDF模型和FR/ED模型在氧化气氛和还原气氛的模拟结果,通过与试验结果的对比验证,确定了不同反应气氛适合的化学反应模型。研究结果表明,3种模型在煤粉着火位置和逆喷传播距离的预测上存在差别,PDF模型起火点更靠近燃烧器逆喷喷口,EDM模型一次风和煤粉喷出经过一段升温过程后逐渐燃烧,FR/ED模型的升温过程更长,壁面低温区域接近前锥长度一半;使用FR/ED模型模拟时燃烧器内平均温度最高,EDM模型次之,PDF模型最低;EDM模型中未考虑CO反应,燃烧器内基本无CO存在,PDF模型高CO浓度区域集中在煤粉燃烧初期,燃烧器出口CO含量较低,FR/ED模型化学反应动力学参数在煤粉燃烧整个区域内均有较高CO浓度存在。由于考虑了焦炭与氧气气化反应的影响,FR/ED模型模拟燃烧器内氧含量明显低于EDM和PDF模型,当过量空气系数为1.2时,燃烧器出口处氧含量为7.0%,明显低于EDM模型的11.1%和PDF模型的12.0%,燃烧器出口处CO含量为3.5%,明显高于EDM(0)和PDF模型(0.8%);过量空气系数为0.5时,EDM模型对CO成分的预测结果偏差较大,燃烧器出口CO浓度为0.05%,采用PDF和FR/ED模型时燃烧器出口CO含量分别为5.73%和10.7%。从模拟结果与试验结果的对比来看,在氧化气氛下,主要发生煤粉的燃烧反应,EDM模型和PDF模型在温度、CO含量上的预测较为准确,与试验数据偏差较小,FR/ED模型偏差较大;在还原性气氛下,EDM模型模拟的结果几乎不生成CO和H2,并不适合还原性气氛,PDF模型和FR/ED模型有较合理的还原气氛模拟结果,两者的差别在于还原性气体的生成位置,PDF模型喷口位置CO浓度较高,出口浓度偏低,FR/ED模型随着煤粉反应流程的进行,CO浓度逐渐升高,出口浓度更接近试验结果。综合考虑,氧化气氛下双锥燃烧器适合采用EDM模型和PDF模型进行模拟,还原气氛下适合采用FR/ED模型进行模拟。     

Absolute measurement of pneumatically conveyed powder using a single long throat venturi

The principle of using a long throated venturi meter to measure the flow rate of two mixed phases is widely published, yet there is no practicable application for accurate measurement in relatively lean particulate conveying. Coal fired power stations would benefit from such a meter. Pulverised coal is conveyed pneumatically through pipes from the mill to the furnace. Control devices that ensure delivery of coal to each burner is equal require an accurate on-line measurement of the coal flowing through each line. Relative comparison of coal flow rate between similar pipelines is possible on-line, but to date no absolute on-line measure has been achieved, and weigh hopper discharge measurement for total coal flow rate is crude. Using a program developed by Azzopardi et al. [B.J. Azzopardi, S.F.C.F Teixeira, C.I. Pulford, A quasi-one-dimensional model for gas/solids flow in venturis, Powder Technology 102 (1999) 281-288.] for calculation of the pressure drop in a two phase venturi, a venturi was designed with optimised geometry to give a measurable sensitivity to coal flow rate at the relatively lean conditions encountered in a power station. The venturi was inserted into a pneumatic conveying test rig designed to simulate a coal fired power station. The program predicted the pressure drops measured in the test rig venturi with good agreement. The pressure calibration graphs thus obtained provided a possible means of calculating the air and coal flow rates. Air flow rate was accurate to within 2%. Coal flow rate accuracy was within 20%. Error analysis showed this error to be related to experiment. The program accuracy was shown to be within 7%. This accuracy merits a full scale power station trial.     

Modelling coal combustion: the current position

At the present time, computer models for coal combustion are not sufficiently accurate to enable the design of combustion plant or the selection of a coal based on combustion behaviour. Most comprehensive combustion models can predict with reasonable accuracy flow fields and heat transfer, but usually with a much lesser degree of accuracy than the combustion of the coal particles through to char burnout. Many research programmes are aimed at developing a much more accurate predictive tool for assessing coals specially fired in burners or furnaces employing a range of NO_x abatement technologies. Some of the current developments in CFD coal combustion modelling are outlined here. Particular attention is paid to the first step, where the devolatilisation pre-processor code is used to compute the pyrolysis rate, the yields and the composition of volatiles and char. These parameters are used as inputs to the devolatilisation and volatile combustion sub-models, where various options can be used, and also the char burnout sub-models. The accuracy of the sub-models is examined using data from four well-studied coals, three from the UK and one from the US. The main network devolatilisation codes are compared with experimental data. Two char combustion models have also been investigated in order to compare char burnout predictions and the development of char morphology and surface area during burnout are considered. The applications of these sub-models to two combustion situations were considered. These involve reactions in a drop tube furnace and a low NO_x industrial burner and in both cases, the model predictions were compared with experimental measurements.     

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。     

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。     

Prediction of unburned carbon and NOx in a tangentially fired power station using single coals and blends

Coal blends are now widely used by the power generation industry and the general characteristics are well known. Attention is still directed to the emission of NO_x, which is subject to more stringent regulation, and to the amount of carbon in ash. The latter is increased when low NO_x burners are employed, which is the norm now. It is also increased as a result of additional air staging when over-fire air is added in furnaces, especially tangential fired systems. Such a furnace is studied here. Two approaches can be employed for prediction of NO_x and unburned carbon. The first approach uses global models such as the ‘slice’ model which requires the combustor reaction conditions as an input but which has a detailed coal combustion mechanism. The second involves a computational fluid dynamic model that in principle can give detailed information about all aspects of combustion, but usually is restricted in the detail of the combustion model because of the heavy computational demands. The slice model approach can be seen to be complimentary to the CFD approach since the NO_x and carbon burnout is computed using the slice model as a post-processor to the CFD model computation. The slice model that has been used previously by our group is applied to a commercial tangentially fired combustor operated in Spain and using a range of Spanish coals and imported coals, some of which are fired as blends. The computed results are compared with experimental measurements, and the accuracy of the approach assessed. The CFD model applied to this case is one of the commercial codes modified to use a number of coal combustion sub-models developed by our group. In particular it can use two independent streams of coal and as such it can be used for the combustion of coal blends. The results show that both model approaches can give good predictions of the NO_x and carbon in ash despite the fact that certain parts of the coal combustion models are not exactly the same. However, if a detailed insight into the combustor behaviour is required then the CFD model must be used.     

Modeling and Numerical Analysis of an Atypical Convective Coal Drying Process

This work presents modeling and numerical simulation of batch convective coal drying in a deep packed bed after a high-pressure steam treatment (a part of the Fleissner coal drying process). The process is atypical, because ambient air is used to dry and cool hot particles, while usually, e.g., in the deep packed bed drying of biomaterials, hot air is contacting cold particles. Product-specific data (intraparticle mass transfer, gas-solids moisture equilibrium) for coal (here lignite) are taken over from literature. Available data on coal drying in packed beds of medium height are used for model validation. Then, the model is applied to the considered industrial process. The design point of the process is critically reviewed, and alternatives are developed by systematically simulating the influence of inlet air conditions (temperature, humidity, flow-rate) and coal particle size. This type of analysis is necessary for efficiently scheduling plant dryers, since coal particle size may change, and air inlet temperature and humidity are changing with the ambient conditions.     

燃烧器出口磨损数学模型的建立及其应用

燃烧器出口磨损速度决定着其使用寿命。在研究了燃烧器内高速煤粉流对壁面磨损情况后,建立了具有普遍意义的燃烧器磨损预测模型。进一步研究模型得出:燃烧器出口磨损主要与煤粉流量、煤风速度、煤粉粒径、壁面温度、壁面煤粉浓度、出口收缩角、壁面温度等参数及出口材质等因素有关,降低上述参数值及增加出口材质耐磨性等方法能有效减少磨损。由该模型计算出的回转窑燃烧器寿命与实际使用寿命基本吻合,这证明了模型的可靠性。某厂用该模型指导现有燃烧器工作参数的优化,使平均寿命增加了300多天。     

NUMERICAL STUDY OF CLASSIFICATION OF ULTRAFINE PARTICLES IN A GAS-SOLID FIELD OF ELBOW-JET CLASSIFIER

Computational fluid dynamics (CFD) is applied to develop a novel submicron air classifier. Based on various sizes and positions of the inner structure of the elbow-jet classifier, its two-dimensional airflow field has been simulated by the Fluent software. For this typical cross-flow field, the standard k ? ? turbulence model is applied. The Coanda effect plays a paramount role in separating ultrafine particles in the high-speed flow field of the elbow-jet classifier. The factors that influence the performance of the Coanda block, i.e., a quarter-cylinder centered in the classifier, are analyzed and discussed. The trajectories of moving particles with different diameters in the channels and chamber of the classifier have been calculated through the velocity field obtained from the CFD simulation. The cut sizes of three products from three outlets of the classifier are obtained based on the calculations of the trajectories of particles and are compared with corresponding experimental results. The ground and classified experiments have been studied simultaneously where the product outlet of a vortex jet mill is used as the feed in an elbow-jet classifier. The combination of the vortex jet mill with the external classifier provides an alternative to grinding equipment for multiple-size products of fine/medium/coarse powder. A centrifugal channel is added between the vortex jet mill and elbow-jet classifier to improve the performance of the air classifier. Both numerical and experimental results show that the pre-distributed feed particles at the exit of the centrifugal channel have a considerable effect on the separation of fine powder and a lesser effect on the separation of coarse powder.     

CFD Evaluation of Droplet Drying Models in a Spray Dryer Fitted with a Rotary Atomizer

Computational fluid dynamics (CFD) modeling of spray dryers requires a simple but sufficiently realistic drying model. This work evaluates two such models that are currently in discussion; reaction engineering approach (REA) and characteristic drying curve (CDC). Two versions of the CDC, linear and convex, drop in drying rate were included. Simulation results were compared to the overall outlet conditions obtained from our pilot-scale experiments. The REA and CDC with a linear drop in drying rate predicted the outlet conditions reasonably well. This is contrary to the kinetics determined previously. Analysis shows that the models exhibit different responses to changes in the initial feed moisture content. Utilizing different models did not result in significantly different particle trajectories. This is due to the low relaxation time of the particles. Despite the slight differences in the drying curves, both models predicted similar particle rigidity depositing the wall. For the first time in a CFD simulation, the REA model was extended to calculate the particle surface moisture, which showed promising results for wet particles. Room for improvement was identified when applying this concept for relatively dry particles.     

DEVELOPMENT AND VALIDATION OF A PROCESS SIMULATOR FOR DRYING SUBBITUMINOUS COAL

Drying subbituminous coal has never been practiced commercially. The commercial dryers built to date have been designed for drying surface moisture in conjunction with upstream coal preparation facilities. This type of drying is mainly controlled by input energy and the basis of the design is an energy balance. In drying inherent moisture from subbituminous coal, the thermal conductivity of the coal and the diffusion of molecular water within coal particles impose limitations on the process conditions. Energy input and solids residence time in the dryer have to be controlled properly for simultaneously balancing the heat and mass transfer within the coal particles. Improper control of either parameter can cause fires and explosions during the key steps of the drying process—drying and cooling

In parallel to the Anaconda coal drying pilot plant program, a cross-flow, fluid-bed coal drying/cooling process simulator was developed for: (1) understanding the drying phenomena on an individual particle basis; (2) analyzing potential risks and safety limits, and (3) designing the Anaconda pilot plant program

The development of the process simulator was based on both first principles and laboratory data and can be divided into two phases:

1 Development of a semi-mechanistic drying model for Powder River Basin subbituminous coal employing an analytical solution of the diffusion equation

2.Formulation of a fluid-bed cross-bed cross-flow dryer/cooler simulator employing simultaneous heat and mass transfer

This model was validated against process variables data taken on a 4 tph pilot plant. An operable range, or process envelope, has been developed through the pilot plant experience and the process simulation study. Based on the model predictions, an uncertainly range was defined in the design recommendations of a pioneer coal drying plant in scale-up.     

CFD simulation of entrained-flow coal gasification: Coal particle density/sizefraction effects

Computational Fluid Dynamics (CFD) simulation of commercial-scale two-stage upflow and single-stage downflow entrained-flow gasifiers was conducted to study effects of simulating both the coal particle density and size variations. A previously-developed gasification CFD model was modified to account for coal particle density and size distributions as produced from a typical rod mill. Postprocessing tools were developed for analysis of particle-wall impact properties.For the two-stage upflow gasifier, three different simulations are presented: two (Case 1 and Case 2) used the same devolatilization and char conversion models from the literature, while Case 3 used a different devolatilization model. The Case 1 and Case 3 solutions used average properties of a Pittsburgh #8 seam coal (d = 108 μm, SG = 1.373), while Case 2 was obtained by injecting and tracking all of the series of 28 different coal particle density and size mass fractions obtained by colleagues at PSU as a part of the current work, for this same coal. Simulations using the two devolatilization models (Case 1 and Case 3) were generally in reasonable agreement. Differences were observed between the single-density solution and the density/size partitioned solution (Case 1 and Case 2). The density/size partitioned solution predicted nominally 10% less CO and over 5% more H₂ by volume in the product gas stream. Particle residence times and trajectories differed between these two solutions for the larger density/size fractions. Fixed carbon conversion was 4.3% higher for the partitioned solution. Particle-wall impact velocities did not vary greatly.Grid independence studies for the two-stage upflow gasifier geometry showed that the grid used in the comparison studies was adequate for predicting exit gas composition and wall impact velocities. Validation studies using experimental data for the Pittsburgh #8 coal from the SRI International pressurized coal flow reactor (PCFR) at 30 atmospheres indicated adequate agreement for gasification and combustion cases, but poor agreement for a pyrolysis case. Simulation of a single-stage downflow gasifier yielded an exit gas composition that was in reasonable agreement with published data.