Effect of primary air flow types on particle distributions in the near swirl burner region

A three-component particle-dynamics anemometry is used to measure, in the near-burner region, the characteristics of gas-particle two phase flows with two swirl burners with different primary air flow types, on a gas-particle two phase test facility. One burner is the radial bias combustion swirl pulverized coal burner whose primary air is non-swirl, and the other is the swirl burner whose primary air is swirl. With the former one, particle volume fluxes, particle volume fractions and particle number concentrations are bigger near the edge of central recirculation zone, and the particle volume fractions and the particle number concentrations are also bigger in the central recirculation zone. With the latter one, the particle volume fluxes and particle number concentrations are less near the edge of the central recirculation zone, and they are bigger in the wall zone. The influence of gas-particle flow characteristics on combustion has been analyzed, and the theory of air-surrounding-coal combustion is given.     

煤粉工业锅炉燃烧器研究进展

燃烧器作为煤粉燃烧技术核心关键设备,主要作用是将燃料与空气合理混合,使燃料稳定着火并形成某种流场以利于后续完全燃烧。实现安全节能环保燃烧,必须根据燃料的特性,选择合理的燃烧器类型及布置形式。因此,本文介绍了国内外几种典型的旋流燃烧器的分类及其特点,并总结了旋流燃烧器的发展趋势。     

The influence of fuel bias in the primary air duct on the gas/particle flow characteristics near the swirl burner region

A three-component particle-dynamics anemometer is used to measure, in the near-burner region, the influence of the particle bias in the primary air duct on the gas/particle flow characteristics for a centrally fuel rich swirl coal combustion burner, in conjunction with a gas/particle two-phase test facility. Velocities, particle volume flux profiles and normalized particle number concentrations were obtained. Compared with a common burner (a centrally fuel rich burner without a particle concentrator), the degree of penetration for the centrally fuel rich burner is higher, the residence time of particles in the central recirculation zone is longer and the central recirculation zone is larger. The particle volume flux and normalized particle number concentration for the centrally fuel rich burner are much larger near the chamber axis. The influence of gas/particle flow characteristics on combustion has been analyzed.     

Prediction of ash deposition in ash hopper when tilting burners are used

This work presents a model of ash deposition growth in the ash hopper of utility boilers when the nozzles are tilted downwards. The model consists a comprehensive combustion code to predict the flow field, temperature field, the gas and solid combustion and the deposition growth behavior. The results show that burner tilting has a significant effect on the deposition in the ash hopper. The work indicates that the numerical model can be used to optimize the design and operation of pulverized coal-burning boiler equipped with tilting burner systems.     

旋流浓淡燃烧器中煤粉浓缩器对煤燃烧NOx生成的影响

应用数值模拟方法研究了旋流燃烧器中煤粉浓缩器对煤燃烧NO_x生成的影响，给出了热NO_x和燃料NO_x生成的详细信息.预报结果表明：采用作者研制的弱旋煤粉浓缩器后，可以大大地降低燃烧器出口区域燃料NO_x的生成，同时可以保持煤粉颗粒较高的燃尽率.模拟结果也表明，燃烧器出口区域的NO_x生成主要是由燃料NO_x的生成机理来控制，热NO_x在总NO_x的生成量中所占比例不大.因此，该旋流浓淡燃烧器具有高效低污染性能.     

Investigation of the effects of several porosity variation profiles on performance and pollutants emission of the porous media burners

This paper presents results of numerical investigation on the effect of using variable porosity porous media burner on its performance and pollutant emission. A two‐dimensional axisymmetric model for premixed methane/air combustion in porous media has been developed. This code solves the continuity, Navier Stokes, solid and gas energies, and chemical species transport equations using the finite volume method. The pressure and velocity have been coupled with the SIMPLE algorithm. In this paper, the results of applying two different profiles of porosity instead of constant porosity for two zones of burner have been presented. The results showed that by applying porosity variation along the burner, the peak temperature can be decreased about 4.5%, and subsequently, the amount of exhaust pollutants such as NO_x can also be decreased while increase in pressure losses along the burner is negligible. In addition, the effects of excess air ratio, volumetric heat transfer coefficient, inlet velocity, chemical kinetics, conductivity coefficient, and wall temperature on the porous media burners with variation of porosity are investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

The effect of the mixing characters of primary and secondary air on NOx formation in a swirling pulverized coal flame

The characteristics of burning pulverized coal motion and the combustion process are investigated by numerical simulation to analyze the relations between the NO_x formation and the gas flow and burning pulverized coal moving in a swirl pulverized coal burner. The depth of particle penetration into the IRZ L_dav and the effective time of IRZ τ_eav are obtained under three swirl numbers s=0.5, 1.03 and 1.2, respectively. The results show that the minimum outlet NO_x emission appears at the maximum L_dav and τ_eav and the minimum outlet NO emission, the maximum L_dav and the maximum τ_eav occur at the same time (s=1.03). It is also found that, the diameter of pulverized coal first increases, but then decreases the particle residence time τ_int in IRZ.     

Numerical analysis of the aerodynamics and combustion of a turbulent pulverized-coal burner jet

A mathematical model and a computing technique are proposed for the motion, heat transfer, and combustion in the initial region of a pulverized-coal jet injected through a burner into the furnace volume. In the simulation, the dispersed phase is divided into two kinds of particles: particles that enter the region being studied from the portion of the furnace volume that is external with respect to the selected zone and the burner-jet particles. The continual approach and the assumption of equilibrium with respect to the gas phase are used to describe the motion of particles of the first kind, and the Lagrangian approach taking into account the dynamic and thermal lag of particles is used to simulate the processes in the medium of particles of the second kind. The model also describes the evolution of volatile substances and burnup of the coke residue. The results obtained using this numerical model make it possible to analyze in detail the combustion of a pulverized coal-air jet in the area near the burner. The degree of detail achieved allows one to make effective decisions for organization of an optimal jet aerodynamics to reduce nitric-oxide formation. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 1, pp. 51–59, January–February, 1997     

旋流浓淡燃烧器中煤粉浓缩器对煤燃烧NOx生成的影响

应用数值模拟方法研究了旋流燃烧器中煤粉浓缩器对煤燃烧NO_x生成的影响,给出了热NO_x和燃料NO_x生成的详细信息.预报结果表明：采用作者研制的弱旋煤粉浓缩器后,可以大大地降低燃烧器出口区域燃料NO_x的生成,同时可以保持煤粉颗粒较高的燃尽率.模拟结果也表明,燃烧器出口区域的NO_x生成主要是由燃料NO_x的生成机理来控制,热NO_x在总NO_x的生成量中所占比例不大.因此,该旋流浓淡燃烧器具有高效低污染性能.     

煤粉锅炉炉膛燃烧一维数学模型的研究

为了有效地进行直流煤粉多相流动与燃烧数值模拟,实现煤粉低NOx燃烧,本文在连续介质模型的框架中建立了综合考虑气—固两相流流动、燃烧与传热的直流煤粉燃烧一维数学模型。应用这一模型对一维煤粉炉炉膛内煤粉燃烧和气体燃烧的数值计算表明,该模型可快速有效地用于模拟直流煤粉多相流动与燃烧过程,给出炉内温度、NOx分布等主要参数。     

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。     

Experimental and numerical investigation of the hydroerosive grinding

This paper presents an Euler-Euler approach for the numerical simulation of the hydroerosive grinding (HE) process. It describes a two-phase slurry flow consisting of a liquid and a dispersed solid phase which causes wear at walls of devices. The continuous fluid phase is solved using a finite volume scheme in which the Large Eddy Simulation (LES) [1] model is applied to resolve large-scale turbulent structures. The solid phase is dispersed and treated as a second continuum in which drag and lift forces as well as added mass, pressure and history force are taken into account. Considering particle-particle interactions, the granular model from Gidaspow [2] is used for particle volume concentrations over 1%. Investigations of erosion processes proofed that non-spherically shaped particles as well as harder particles increase the wear on devices significantly. Consequently, non-spherical particles are utilised for the hydroerosive grinding. Their steady drag, unsteady drag and lift coefficients, depending on the particle Reynolds number, are determined by a direct numerical simulation via an in-house LES Lattice-Boltzmann solver. This Lattice-Boltzmann method was presented for laminar flows by Hölzer and Sommerfeld [3]. In this work, interpolating functions of these coefficients are implemented in the Euler-Euler approach which enables the simulation of non-spherical particle transport. Hydroerosive grinding experiments in 3D throttles and 3D planar geometries are carried out to determine an erosion model depending on particle impact velocity, particle size, particle concentration and wall hardness. Implementation of a mesh-morphing algorithm combined with the Euler-Euler scheme of the commercial solver ANSYS CFX11 [4] enables an online simulation of the hydroerosive grinding process. Additionally, the online simulation is used to validate the applied numerical methods. Very good agreements are achieved and will be presented in this paper.     

Study on flow fields of centrally fuel rich swirl burner and its applications

Experiments on a single-phase test facility were done to optimize primary air outlet cones of a centrally fuel rich swirl coal combustion burner. On the basis of optimized results from the single-phase test, a three-component particle-dynamics anemometer was used to measure, in the near-burner region, the characteristics of gas/particle two-phase flows for the burner with two primary air outlet cones, on a gas/particle two-phase test facility. Velocities, RMS velocities and particle volume flux profiles were obtained. According to the results, the primary air outlet cone structure of the centrally fuel rich burner was matching a 670 ton per hour boiler. The performance of the burner on a 670 ton per hour boiler was studied.     

Pulverized coal combustion characteristics of high-fuel-ratio coals

It is strongly desired for coal-fired power plants in Japan to utilize not only low-rank coals with high moisture and high ash contents, but also high-rank coals with high fuel ratio for diversifying fuel sources and lowering cost. In this study, pulverized coal combustion characteristics of high-fuel-ratio coals are experimentally investigated using an approximately 100 kg-coal/h pulverized coal combustion test furnace. The combustion characteristics are compared to those for bituminous coal. The coals tested are six kinds of coal with fuel ratios ranging from 1.46 to 7.10. The results show that under the non-staged combustion condition, the minimum burner load for stable combustion rises as fuel ratio increases. To improve the stability, it is effective to lengthen the residence time of coal particles in the high gas temperature region close to the burner outlet by using a recirculation flow. The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increases with increasing the fuel ratio. In addition, as the fuel ratio increases, NOx reduction owing to the staged combustion becomes small, and unburned carbon fraction increment becomes significant. The numerical simulations conducted under the staged combustion condition show that although the numerical results are in general agreement with the experimental ones, there remains room for improvement in NOx reduction model for high-fuel-ratio coals.     

预燃室对逆喷旋流煤粉燃烧器流场特性作用研究

煤粉高效低氮燃烧技术是煤炭高效利用领域持续关注的热点。煤粉燃烧器作为煤粉锅炉的核心设备,研究适合多煤种、宽负荷条件的煤粉燃烧器设计原理及技术至关重要。逆喷射流稳燃机理大都应用在航空发动机和燃气轮机领域,在煤粉燃烧领域应用极少。前人大量研究了预燃室对旋流燃烧器流场特性的影响,但鲜见预燃室对逆喷旋流燃烧器流场影响的相关研究。为了探究预燃室对逆喷旋流煤粉燃烧器流场特性的影响规律,笔者针对一款20 t/h逆喷旋流燃烧器,基于等温模化原理建立冷态燃烧器模型,利用热线风速仪和飘带法进行了流场测试和分析,结果表明:预燃室的存在不改变逆喷旋流煤粉燃烧器回流区环形的形状,但在逆喷旋流煤粉燃烧器内形成一个有利于煤粉着火的轴向速度低和湍流强度大的回流区。在X/D<1.3区域内,由于圆锥形预燃室对气流的挤压作用,预燃室的存在对回流区的面积起到抑制作用;在1.32.3区域内,预燃室对燃烧器内部流场的作用减弱,可忽略不计。在预燃室的作用下,回流区最宽处的直径从0.97D降至0.86D,最大相对回流率位置从截面X/D=1后移到截面X/D=1.6处,相对回流率从1.17减小至0.99。预燃室的存在对二次风区域内的轴向平均速度和湍流度分布规律影响较大。无预燃室工况下,在X/D<0.6区域内,速度和湍流度均出现峰值,在X/D>1.6区域内峰值消失,内外二次风完全混合;有预燃室工况下,在X/D<0.6区域速度沿着径向方向逐渐增大,湍流度沿着径向方向逐渐减小,在X/D>1.6区域,速度和湍流度沿着径向方向分布均匀。预燃室的存在有利于回流区煤粉的稳定燃烧,工程应用中起到煤粉迅速着火以及难燃煤稳定燃烧的作用。另外预燃室壁面气流速度较大,刚性强,避免预燃室壁面超温或结焦现象的发生,延长了煤粉燃烧器无故障运行时间和整体的使用寿命。     

Numerical simulation of pulverized coal combustion and NO formation

A pure two-fluid model, instead of the Eulerian gas-Lagrangian particle models (particle trajectory models), is used for simulating three-dimensional (3-D) turbulent reactive gas–particle flows and coal combustion. To improve the simulation of the flow field and NO_X formation, a modified k–ε–k_p two-phase turbulence model and a second-order-moment (SOM) reactive rate model are proposed. The proposed models are used to simulate NO formation (fuel NO produced by NH₃) of methane–air combustion, and the prediction results are compared with those using the pure presumed PDF-finite-reaction-rate model and experimental data. The modified k–ε model and SOM model are more reasonable than the standard k–ε model and the pure presumed PDF-finite-reaction-rate model. The proposed models are also used to predict the coal combustion and NO formation at the exit of a double air register swirl pulverized-coal burner. The predicted results indicate that a pulverized-coal concentrator installed in the primary-air tube of burner has a strong effect on the coal combustion and NO_X formation.     

3-D modelling of kerosene-fuelled HVOF thermal spray gun

Liquid-fuelled high-velocity oxy-fuel (HVOF) thermal spraying systems are capable of generating more momentum output to powder particles in comparison with gas-fuelled systems. The use of low-cost fuel such as kerosene makes this technology particular attractive. High-quality coating requires thermal spraying systems delivering consistent performance as a result of the combustion during HVOF spraying. The combustion of kerosene is very complicated due to the variation of fuel composition and subsequently makes it extremely challenging for process control. This paper describes a 3-D simulation using mathematical models available in a commercial finite volume CFD code. The combustion and discrete particle models within the numerical code are applied to solve the combustion of kerosene and couple the motion of fuel droplets with the gas flow dynamics in a Lagrangian fashion. The effects of liquid fuel droplets on the thermodynamics of the combusting gas flow are examined thoroughly.     

Simulation of gas-particle turbulent combustion in a pulverized coal-fired swirl combustor

A particle stochastic trajectory model for turbulence-particle reaction interactions is proposed and formulated in the present paper. This model provides the basis for a comprehensive model of pulverized coal combustion. It is applied to the simulation of gas-particle turbulent flow and combustion in a pulverized coal-fired swirl combustor. The results are compared with the measured test data and those obtained by the particle stochastic trajectory model without considering turbulence-particle reaction interactions. The predicted gas temperature and species concentrations in the upstream region of the combustor are improved by utilizing the model with turbulence-particle reaction interactions.     

回转窑二次风富氧燃烧的数值模拟

针对一实际尺寸的回转窑建立模型,分别进行了空气助燃(21% O₂)和二次风富氧(23% O₂)燃烧的数值模拟研究。结果表明,二次风富氧后,高温区覆盖形状没有明显变化,仍呈“棒槌状”;在回转窑前端,煤粉挥发分与焦炭燃烧速度加快,整体温度有所提升,最高温度由2386 K增至2427 K,壁面所接收的辐射量得到了提升;但NO_x的生成量也大幅度提高,其中出口处NO_x由247 mg/m³增至367 mg/m³。考虑到制氧成本问题及NO_x排放问题,在二次风中进行富氧燃烧的总体效果不够理想。     

Computational modeling of a utility boiler tangentially-fired furnace retrofitted with swirl burners

The paper presents 3D numerical investigation of OP-380 boiler tangentially-fired furnace utilizing bituminous coal. The boiler was retrofitted by replacing traditional jet burners with RI-JET2 (Rapid Ignition — JET) swirl burners. This kind of solution is unique in power generation systems. The purpose of this work is to show how the flow, combustion performance and heat exchange in the furnace are affected by introducing rapid ignition phenomena in RI-JET2 burners instead of delayed ignition associated with the traditional jet burners. Results were compared to simulations of similarly designed boiler equipped with traditional jet burners. Furnace simulation was preceded with a single RI-JET2 burner simulation at the inlet to a virtual combustion chamber. The results have shown that pulverized coal (PC) concentrator separates the PC into two streams: concentric with fine particles and axial with coarse particles. Stable flame operation was noticed even without secondary and tertiary air swirl. 3D simulations of combustion chamber have shown that in a burner zone a visibly isolated, concentrated flame exists in the furnace axis. This kind of flame shape reduces corrosion risk and furnace walls slagging as a result of RI-JET2 burner's long range.