含灰湿空气外掠翅片管束积灰和阻力特性实验研究

本文针对以褐煤预干燥乏气为代表的含灰含湿气体余热回收问题,采用含灰湿空气模拟真实干燥乏气/烟气,搭建了混合气体外掠圆形翅片管束对流冷凝实验系统,获得了灰分浓度、水蒸气质量分数、主流气体入口温度等因素对其积灰和阻力特性的影响规律.实验结果表明,阻力系数随着灰分浓度和气体入口温度的增加而增加,随水蒸气质量分数的增加而减小,...     

Biomass fly ash deposition in an entrained flow reactor

Fly ash deposition on boiler surfaces is a major operational problem encountered in biomass-fired boilers. Understanding deposit formation, and developing modelling tools, will allow improvements in boiler efficiency and availability. In this study, deposit formation of a model biomass ash species (K₂Si₄O₉) on steel tubes, was investigated in a lab-scale Entrained Flow Reactor. K₂Si₄O₉ was injected into the reactor, to form deposits on an air-cooled probe, simulating deposit formation on superheater tubes in boilers. The influence of flue gas temperature (589 – 968°C), probe surface temperature (300 – 550°C), flue gas velocity (0.7 – 3.5?m/s), fly ash flux (10,000 – 40,000?g/m²h), and probe residence time (up to 60?min) was investigated. The results revealed that increasing flue gas temperature and probe surface temperature increased the sticking probability of the fly ash particles, thereby increasing the rate of deposit formation. However, increasing flue gas velocity resulted in a decrease in the deposit formation rate, due to increased particle rebound. Furthermore, the deposit formation rate increased with probe residence time and fly ash flux. Inertial impaction was the primary mechanism of deposit formation, forming deposits only on the upstream side of the steel tube. A mechanistic model was developed for predicting deposit formation in the reactor. Deposit formation by thermophoresis and inertial impaction was incorporated into the model, and the sticking probability of the ash particles was estimated by accounting for energy dissipation due to particle deformation. The model reasonably predicted the influence of flue gas temperature and fly ash flux on the deposit formation rate.     

粉煤灰微珠的气力分级原理与实验研究

按气固两相流理论,分析了粉煤灰颗粒在气流中的运动规律,按沉降和平移的原理,利用流幕式气力分级系统对沈阳热电厂粗飞灰进行了多次分级实验,得到了分级效果及分级产量随气速变化的初步规律。     

低品位烟气余热回收换热器热力学分析

低品位烟气余热回收过程存在冷凝现象,烟气的放热过程分为显热、潜热两部分。冷凝时,局部热流率和熵产率明显增大;增加水蒸气质量分数、冷却水质量流量和降低烟气入口温度都会导致烟气提前冷凝;存在最优冷却水质量流量使得热回收过程熵产数最小。另外,提出热回收效率评价烟气热回收程度,该指标受冷凝的影响很大。随着烟气中蒸汽质量分数的增加,冷凝过程的影响明显增强,因此,在低品位烟气的全热回收中必须考虑潜热的影响。     

减少二氧化碳对LIBS检测飞灰中未燃碳含量影响的研究

通常热力发电厂将飞灰中未燃碳的含量作为评价锅炉燃烧效率的重要指标,通过测量飞灰中未燃碳的含量来评价煤粉燃烧的充分程度,进而实现优化燃烧、提高机组效率。基于激光诱导击穿光谱技术(LIBS)无接触、快速响应、高灵敏度、可以在线测量等特点,备用来测量飞灰中未燃碳的含量。由于烟气中CO₂气体的存在,碳谱线强度会随CO₂浓度的变化而改变。为了减少CO₂气体对飞灰未燃碳测量结果的影响,提出并设计了具有二级旋风分离器的LIBS测量飞灰未燃碳含量实验系统,飞灰从给粉机流出后通过二级旋风分离器进入测量腔体,脉冲激光经过透镜作用于飞灰样品进而产生等离子体。LIBS系统采用双中心波长光谱仪,可测得飞灰中C,Si,Mg,Fe,Ca和Al等主要元素谱线,同时高分辨率通道可分辨出相邻C和Fe的元素谱线,可以在获得充分的飞灰光谱信息的同时保证了测量的精度。实验结果表明该系统可有效分离和收集飞灰颗粒,减少CO₂气体对测量结果的干扰,为LIBS技术的工程应用提供了更准确的依据。     

炉内喷钙脱硫过程数值模拟

建立了一套较为完整的对炉内喷钙脱硫过程进行模拟的方法，获得了炉内气固两相流动、温度分布、石灰石颗粒脱硫化学反应和SO2浓度分布的详细情况。石灰石颗粒的硫化反应主要发生在颗粒与烟气均匀混合之前。     

Prediction of particle sticking efficiency for fly ash deposition at high temperatures

The tendency of ash particles to stick under high temperatures is dictated by the ash chemistry, particle physical properties, deposit surface properties and furnace operation conditions. A model has been developed in order to predict the particle sticking efficiency for fly ash deposition at high temperatures. The model incorporates the particle properties relevant to the ash chemistry, particle kinetic energy and furnace operation conditions and takes into consideration the partial sticking behaviour and the deposit layer. To test the model, the sticking behaviours of synthetic ash in a drop tube furnace are evaluated and the slagging formation from coal combustion in a down-fired furnace is modelled. Compared with the measurements, the proposed model presents reasonable prediction performance on the particle sticking behaviour and the ash deposition formation. Through a sensitivity analysis, furnace operation conditions (velocity and temperature), contact angle and particle size have been found to be the significant factors in controlling the sticking behaviours for the synthetic ash particles. The ash chemistry and furnace temperature dictate the wetting potential of the ash particles and the melting ability of the deposit surface; particle size and density not only control the particle kinetic energy, but also affect the particle temperature. The furnace velocity condition has been identified as being able to influence the selective deposition behaviour, where the maximum deposition efficiency moves to smaller particles when increasing the gas velocity. In addition, the thermophoresis effect on the arrival rate of the particles reduces with increasing the gas velocity. Further, increasing the melting degree of the deposit layer could greatly enhance the predicted deposition formation, in particular for the high furnace velocity condition.     

数值模拟颗粒旋转对流化床内气固两相流动特性的影响

流化床内颗粒自旋转将影响颗粒相的流动特性.本文运用基于颗粒动理学理论的欧拉-欧拉气固多相流模型,考虑颗粒自旋转流动对颗粒碰撞能量交换和耗散的影响,数值模拟流化床内气体颗粒两相流动特性.计算结果表明颗粒的自旋转使得床内更容易形成气泡,颗粒浓度分布变化增大.颗粒自旋转运动将导致床内非均匀结构更明显.     

Experimental study on electrostatic removal of high-carbon particle in high temperature coal pyrolysis gas

A high-temperature electrostatic precipitator (ESP) presents a good solution for hot gas cleaning, which can remove fly ash from pyrolysis gas at temperatures higher than the tar dew point. In this paper, the characteristics of negative DC corona discharge in air and simulated coal pyrolysis gas were studied. The removal of coal pyrolysis furnace fly ash (ash A) was investigated and compared with that of coal-fired power plant fly ash (ash B) in ESP with a temperature ranging from 300?K to 900?K. The current density of simulated gas was higher than that of air under the same discharge voltage and at different temperatures. The simulated gas also had a higher spark voltage and a lower onset voltage compared with air. The fractional collection efficiency of ash A was lower for particles with diameters of larger than 0.1?µm at high temperature, compared with ash B. A lower collection efficiency in simulated gas was obtained for particles with diameters of less than 0.1?µm compared with air. The collection efficiency of submicron particles in simulated gas was usually higher than it in air, especially for particles with diameters of less than 0.04?µm. In simulated gas, the overall collection efficiency of ash A was obviously lower than that of ash B, especially at high temperature. From 300?K to 700?K, the collection efficiencies of both ash samples were as high as above 93%, but the collection efficiency of ash A in simulated gas decreased to 78.7% at 900?K.     

Research on bimodal particle extinction coefficient during Brownian coagulation and condensation for the entire particle size regime

The extinction coefficient of atmospheric aerosol particles influences the earth’s radiation balance directly or indirectly, and it can be determined by the scattering and absorption characteristics of aerosol particles. The problem of estimating the change of extinction coefficient due to time evolution of bimodal particle size distribution is studied, and two improved methods for calculating the Brownian coagulation coefficient and the condensation growth rate are proposed, respectively. Through the improved method based on Otto kernel, the Brownian coagulation coefficient can be expressed simply in powers of particle volume for the entire particle size regime based on the fitted polynomials of the mean enhancement function. Meanwhile, the improved method based on Fuchs–Sutugin kernel is developed to obtain the condensation growth rate for the entire particle size regime. And then, the change of the overall extinction coefficient of bimodal distributions undergoing Brownian coagulation and condensation can be estimated comprehensively for the entire particle size regime. Simulation experiments indicate that the extinction coefficients obtained with the improved methods coincide fairly well with the true values, which provide a simple, reliable, and general method to estimate the change of extinction coefficient for the entire particle size regime during the bimodal particle dynamic processes.     

稠密可压缩气粒两相流动中的等熵声速计算建模及物理规律

气体相与颗粒相混合流场的声速研究, 由于具有重要的基础理论价值与广泛的工程应用背景, 逐渐受到人们重视. 针对稠密可压缩气粒两相流动, 综合考虑颗粒相所占空间体积以及颗粒间相互作用, 推导给出了新的等熵声速计算公式; 新公式包含了已有的纯气体、稀疏气粒两相流情形的计算公式作为其特例, 一方面验证了公式推导的正确性, 另一方面说明新公式更具有通用性; 分析了不同颗粒质量分数条件下的声速变化规律, 相应结果与普朗特的理论分析符合, 特别对于稠密气粒两相流动工况得到了一些新的物理认识; 开展了颗粒间相互作用建模参数的物理分析, 揭示了其对气粒两相流动声速的影响机理. 本文取得的成果为稠密可压缩气粒两相流动研究以及相关工程应用提供理论支撑.     

Influence of metal powder shape on drag coefficient in a spray jet

In plasma spraying, particle shape, size, distribution and density are the important factors to be considered in order to ensure high spray efficiency and better coating properties. In the present work, nickel, iron and aluminium irregular powders in the size range from 50 to 63 μm were spheroidized using thermal plasma processing. The spheroidization experiments have been carried out at different gas flow rates and plasma torch power levels. The sphericity was analyzed using shape factor. Drag coefficients of the powders were estimated using Reynolds number and sphericity of the powders in plasma. For irregular particles, the drag coefficient is higher than that of the spherical because of its large area of contact with plasma. The temperature dependent on drag coefficient is also discussed. Increasing temperature increases the drag coefficient of the powder particles injected in to the plasma jet. Increasing plasma jet temperature changes the density and viscosity of the plasma which affects the particle’s drag coefficient in the plasma. The results are reported and discussed.     

Ash characteristics in controlled diode laser pyrolysis of chlorinated rubber

This paper describes the effects of 60 W High Power Diode Laser (HPDL) beams on the removal of chlorinated rubber (CR) paint from concrete surfaces and the ash particles generated from this process. The physical characteristics, including shape and size distribution of the removed and collected airborne CR particles, down to a size of around 1 μm in diameter, were determined using optical microscopy and image analysis. The shape of the particles observed was highly irregular, displaying no symmetry. The size distribution of the collected particles was found to range between 1–2000 μm, with the maximum concentration being found between 29 and 60 μm. The chemical characteristics of the CR ash particles were investigated by means of ESEM and EDX techniques. From a comparative analysis, it was found that the concentration of chlorine within the CR material was significantly reduced after HPDL treatment. This, together with DTA/TGA results indicated a combustive degradation of the CR polymer through the interaction with the process gas, oxygen, and the laser irradiation. Also, a strong correlation between laser power and average particle sizes has been found, with higher powers generally producing larger particle sizes. Opposite effects have been found by changing the oxygen flow rate, with higher oxygen flow producing, on average, smaller particles. An interpretation of the combustion process, as well as a brief discussion on operational safety and environmental impact of the products is attempted.     

Numerical studies on velocity, temperature history and heat transfer to the particles injected into the argon plasma

A new approach to study the particle velocity in a thermal plasma in relation to input parameters (power, gas flow rate, injection velocity of the particle and particle size) and nozzle dimensions (nozzle length and diameter) has been made. Injected particle's temperature and thermal history were calculated for particles of three different materials (alumina, tungsten and graphite) in argon plasma. Allowable powder feed rate was calculated for the particles. Heat transfer per particle injected in to the plasma is reported. Liquid fraction of the particle after it reached the melting point is also reported. Particle velocity is found to increase with increase in power, gas flow rate and injection velocity and decrease with increase in particle size, nozzle length and nozzle diameter. Thermal histories of the particles in relation to the plasma temperature and particle diameter are presented. Particle's residence time is found to increase with increase in diameter of the particle. Allowable powder feed rate for complete melting of the particle is higher at higher percentage utilisation of the plasma power. Powder feed rate is seen to decrease with increase in particle size and it is higher for tungsten and lower for graphite particle. Heat transfer rate from plasma to particle is seen to decrease with increase in time and the same is higher for plasmas of higher temperature and smaller sized particle. Received 4 May 2000 and Received in final form 15 March 2001     

Particle Dynamics in an Electrohydrodynamic Flow Field investigated with a two-component laser-doppler velocimeter

A laser-Doppler instrument has been used to measure the migration velocity of NaCl particles in an electrohydrodynamic flow field of an electrical precipitator. The measured average migration velocity of 1.40-μm particles (number distribution median with a geometric standard devitation of 1.46) is approximately five to six times higher than the calculated steady-state velocity for a 1.40-μm particle, provided there is a saturation charge of at least 90f%. Further, the particle velocities in the main flow direction are also influenced by the electrical operation conditions. Both observations demonstrate the important role of the state of the electrohydrodynamic flow field (superposition of moving gas ions and neutral gas molecules) on the particle transport, characterized by the dimensionless electrohydrodynamic number N_EHD. A comparison between six different electrohydrodynamic states revealed that N_EHD ≈? 1 is a critical value for the mutual interactions between the gas ions and the neutral gas phase. Whereas for N_EHD values > 1 the stochastic particle motion is chiefly determined by the nonsteady-state character of the negative corona, for N_EHD values < 1 the particle velocity fluctuations are governed by the turbulence level of the neutral fluid. These finding might be helpful in adjusting the operating conditions in electrical precipitators for and optimized particle separation.     

Electrostatic Charge Measurement and Charge Neutralization of Fine Aerosol Particles during the Generation Process

An aerosol charge analyzer has been constructed to measure the charge distribution of NaCl particles generated in the laboratory. A radioactive electrostatic charge neutralizer utilizing Po‐210 was used to neutralize the electrostatic charge of the particles. The atomization technique was used to generate NaCl particles with diameters of 0.2 to 0.8 μm, while the evaporation and condensation method was adopted to generate particles of 0.01 to 0.2 μm in diameter. The experimental data demonstrates that the absolute average particle charge depends on the particle diameter, and is higher than that calculated by the Boltzmann charge equilibrium for particles within the range of 0.2 to 0.8 μm. The charge increases with decreasing NaCl concentration. When these particles are neutralized using the Po‐210 neutralizer, it is found that the electrostatic charge reaches the Boltzmann charge equilibrium. For 0.01 to 0.2 μm NaCl particles generated using the evaporation and condensation method, test results show that the absolute average particle charge is higher than that calculated by the Boltzmann charge equilibrium for particles larger than 0.03 to 0.05 μm in diameter, while it is lower than that predicted by the Fuchs theory [1], for particles smaller than 0.03 to 0.05 μm. However, after charge neutralization, particles with diameter above 0.05 μm reach the Boltzmann charge equilibrium condition, and the charges for particles with diameters of 0.010 to 0.05 μm, agree well with Fuchs' theory.     

Swelling properties and intrinsic reactivities of coal chars produced at elevated pressures and high heating rates

A high-temperature, high-pressure flat-flame burner reactor was developed to prepare char at different pressures. This system achieves particle heating rates of 10⁵ K/s, which better mimics industrial conditions than conventional drop tube or radiative flow reactors. Previous data at atmospheric pressure demonstrated a significant decrease in particle swelling during devolatilization as heating rates increased from 10⁴ K/s (the typical drop tube heating rate) to 10⁵ K/s. Pyrolysis experiments were performed at pressures from 1 to 15 atm at 1300 °C for two bituminous coals and a lignite. Average swelling was determined from a combination of the mass release and the average density. The results indicate significantly lower swelling ratios at elevated pressures than reported in the literature. Scanning electron micrographs show that the bubbles in the bituminous coal particles popped before significant swelling at these elevated heating rates. Lignite particles exhibited shrinkage rather than swelling, but still showed a small effect of pressure. TGA oxidation reactivities were determined for the Pitt #8 and Knife River lignite char samples at their respective char preparation pressures. The oxidation reactivities of both the bituminous and lignite chars decreased with increasing pressure.     

气粒两相流传热问题的光滑离散颗粒流体动力学方法数值模拟

在气粒两相流动问题中,颗粒间以及气体与颗粒间的传热问题不可忽略.光滑离散颗粒流体动力学(SDPH)模型作为一种新的求解气粒两相流动问题的方法,已经成功应用于模拟风沙运动等问题.在此基础上,提出了SDPH方法的热传导模型,模拟了气粒两相流动问题中的热传导过程以及颗粒蒸发过程.首先引入各相的能量方程,利用有限差分与光滑粒子流体动力学一阶导数相结合的方法,处理各相内部热传导项中的二阶导数问题,基于气粒两相间温度差及对流换热系数计算颗粒与气体间的热传导量,推导得到了含热传导模型的气粒两相流SDPH计算方程组,模拟计算了圆盘形颗粒团算例及鼓泡流化床内部热传导算例,并与双流体模型计算结果进行对比,结果基本符合;其次利用离散液滴模型中的颗粒蒸发传质传热定律计算颗粒的蒸发过程,数值模拟了颗粒射流蒸发过程,并与离散颗粒模型结果进行对比,两者符合得较好,验证了该方法的准确性及实用性.     

Synthesis of zinc ultrafine powders via the Guen–Miller flow-levitation method

Zinc ultrafine powders (UFPs) with the average particle size of 0.175 to 1.24 μm are synthesized via the flow-levitation method. The peculiarities of the formation of zinc UFPs are considered with respect to the carrier gas properties (heat capacity, thermal conductivity, and diffusion coefficient), as well as the gas flow parameters (pressure and flow rate). The obtained zinc particles are studied via scanning electron microscopy and X-ray diffraction. The factors determining the crystal structure of zinc particles and their size distribution are discussed as well. The data on oxidation of zinc stored in unsealed containers under normal conditions are also presented.     

Dependence of the condensation coefficient of bismuth on the energy of particles deposited from an ion beam on silicon substrates

The dependence of the condensation coefficient of bismuth ions on the energy of particles deposited from an ion beam on silicon substrates is investigated experimentally. It is found that, as an accelerating voltage is applied to the substrate, the condensation coefficient of bismuth ions drastically increases at voltages ranging from 0 to 10 V and monotonically decreases at voltages higher than 60 V. The critical temperature of condensation of bismuth particles deposited from the ion beam is 100 K higher than that from the vapor phase.