预混合乙烯火焰生成物相对浓度的激光诊断

针对4种典型的预混合乙烯火焰,在不同火焰温度及不同当量比下,用激光诱导炽热法(LII)和激光诱导荧光法(LIF),对预混合燃烧过程中产生的碳烟颗粒及其前驱物的相对浓度分布进行了研究,结果表明,高温、缺氧是产生碳烟颗粒前驱物的重要原因,在相同的当量比下,随着火焰温度的升高,碳烟颗粒前驱物的相对浓度明显上升,导致碳烟颗粒物相对浓度也不断升高,碳烟颗粒前驱物的相对浓度随着燃料当量比的升高而显著上升,另外,碳烟颗粒物的相对浓度变化总是与其前驱物的相对浓度变化相一致,与小分子碳烟颗粒前驱物相比较,大分子碳烟颗粒前驱物在预混合火焰中的浓度高,分布范围广.     

温度对等离子体热解烃类生成炭黑形貌的影响

使用磁旋转电弧等离子体发生器产生均匀的高温热源,通过热解碳氢化合物制备了炭黑颗粒。利用透射电子显微镜(TEM)图像,表征了炭黑聚集体的形貌,计算了聚集体的分形维数、粒度分布和平均粒径,研究了反应温度和原料对炭黑聚集体形貌和微观物理特性的影响。研究结果显示,在1 500 K较低温度下所有热解产物均为球形碳颗粒;随着温度的升高,初级粒子粒径减小,分形维数变大,聚集体形态变复杂;甚至在约2 500 K高温下在甲烷和乙烯的热解产物中出现石墨烯片层。分析认为,温度和原料影响了初级粒子的成核速率、表面生长和碰撞团聚等过程,从而决定了聚集体的最终形貌。     

火焰参数对辐射图像法不同测温模型的影响

对火焰温度分布的实时测量能够了解燃烧过程、验证燃烧机理、预防工业事故、优化燃烧设备。图像法测温在工业现场的火焰三维温度测量上有明显的优势,但通常均考虑火焰为均匀折射率介质,给测温结果带来了不可避免的误差。本文建立了梯度折射率介质下火焰的辐射成像模型和图像法测温模型,验证了方法的正确性,分析了火焰尺寸对成像的影响及炭黑颗粒浓度对温度场重建的影响。得出随着火焰尺寸的增大,图像强度随火焰尺寸出现先增大后减小的趋势,梯度介质模型与均匀介质模型的差异逐渐增大,随着炭黑颗粒浓度的增加,两种模型的重建精度逐渐下降。     

基于BP神经网络的固体废弃物热解产物分布规律预测

选择4种不同种类的固体废弃物样品进行热解实验,探究在不同热解温度、停留时间、载气流量以及升温速率影响下固、液、气三态产物的分布规律.基于BP神经网络原理,利用Matlab神经网络工具箱,建立了针对不同种类废弃物在不同反应条件下的热解产物产率分布模型.模型的输入条件为反应工况和样品特性参数,输出结果为热解三态产物产率,预...     

对冲扩散火焰中单颗粒煤着火瞬态模型

建立了适用于分析对冲扩散火焰中单个煤粉颗粒穿焰过程的瞬态模型,考虑了流场、温度场及组分浓度分布对单颗粒煤的运动、升温及着火特性的影响.结果表明,计算条件下煤粉颗粒最大升温速率均超过1×105,K/s.煤颗粒在穿焰过程中的着火温度随粒径增长单调递减,同时着火延迟时间先增长后趋缓.在火焰面后发生均相着火的煤颗粒其着火延迟时间随粒径变化很小,可能是因为进入高浓度氧化性气氛使得析出的挥发分迅速着火.     

葡萄糖在超(近)临界水/甲醇中的稳定性研究

在超(近)临界水/甲醇中分别试验考察了温度、压力、停留时间等参数对葡萄糖稳定性的影响.结果表明:无论在水中还是在甲醇中,温度越高,葡萄糖的稳定性越低,降解速率越高;降解产物5-羟甲基糠醛(HMF)和5-甲基(甲氧基)糠醛(MMF)的选择性随温度的变化存在着最大值.压力对葡萄糖在临界水中转化率的影响不明显,但在超临界甲醇中,葡萄糖的转化率随压力升高而增大,当达到临界压力以后基本不变.HMF和MMF的最大收率都出现在临界压力附近.停留时间越长,葡萄糖在超临界水和甲醇中的转化率越高,HMF的收率和选择性越低,而MMF的最大收率和最高选择性却存在着一个适宜的停留时间.     

掺混甲烷对预混合丙烯火焰碳烟生成的影响

基于当量比为1.79且最高火焰温度为1829 K的预混合丙烯火焰,研究了燃料掺混对碳烟生成的影响以及协同效应.分别将5%、20%、40%摩尔分数的甲烷混合到丙烯中,形成具有相同当量比和最高火焰温度的预混火焰.使用微孔探针采样技术和扫描电迁移率粒径谱仪,在燃烧器稳定滞止火焰中测量了碳烟粒径分布.研究发现,掺混甲烷后的火焰生成碳烟颗粒粒径仍旧呈现双峰性分布;但随甲烷掺混率增加,双峰性减弱,颗粒生长速率减弱,碳烟体积分数也迅速减小.在所研究的试验工况下,丙烯与甲烷在碳烟生成方面不仅没有协同效应,而且甲烷的添加降低了碳烟颗粒的生长速度和碳烟生成总量.     

炭黑非催化燃烧特性的固定床实验研究

含碳燃料在还原气氛下燃烧会生成炭黑，在动力设备的燃烧装置中，炭黑的后期氧化对污染控制是非常重要的。利用石英管固定床反应器对天然气扩散火焰中生成的炭黑在不同氧浓度下（20％、15％、10％和5％）的燃烧特性进行了研究，并选用了蜡烛炭黑、丁烷炭黑和煤焦作为对比。根据实验中得出的燃烧特性，与煤焦相比，炭黑的着火温度较低，但是炭黑的燃烧活化能相对更高。氧浓度对各试样着火温度影响不大，而却影响各试样燃烧过程。还进行了水蒸汽对天然气炭黑燃烧的影响研究，水蒸汽能引起炭黑燃烧速率的显著增大。图9参12     

板式换热器颗粒污垢特性的实验研究

以纳米氧化镁颗粒溶液为实验工质,进行了板式换热器颗粒污垢特性的实验研究,分析了颗粒质量浓度、颗粒粒径、流速和低温介质温度对颗粒污垢热阻的影响.结果表明:板式换热器颗粒污垢无明显诱导期存在,结垢速率和污垢热阻渐进值均随颗粒质量浓度的增大而增大,且增大幅度逐渐减小;颗粒粒径对污垢热阻的影响较明显,在相同质量浓度下,颗粒粒径越小,结垢速率越快,且污垢热阻越大;流速对污垢热阻的影响较为复杂,高流速下的结垢速率略大于低流速下,且高流速下达到稳定时的污垢热阻渐进值小于低流速下;低温介质温度对颗粒污垢热阻的影响不明显.     

可溶与不可溶混合PM_(2.5)的蒸汽相变凝结增长

建立了蒸汽相变凝结增长的动力学模型,利用数值模拟方法研究可溶与不可溶颗粒组成的混合PM2.5的凝结增长特性,分析了操作参数对相变凝结增长效果的影响.结果表明:PM2.5中可溶颗粒所占比例越大,凝结增长后数目浓度峰值粒径和中位粒径越小,粒径分布越分散;与单一组分的不可溶颗粒相比,可溶颗粒的混入极大地改变了凝结增长后的粒径分布;粒径越小的可溶与不可溶颗粒的凝结增长速率越接近,粒径大于0.3μm时,可溶颗粒的凝结增长速率和最终粒径明显大于不可溶颗粒;较高的初始饱和度和温度能够促进混合PM2.5的凝结增长;随颗粒数目浓度的增大,凝结增长后数目浓度峰值粒径减小,粒径分布更为分散.     

Copper oxide nanoparticle made by flame spray pyrolysis for photoelectrochemical water splitting – Part I. CuO nanoparticle preparation

Copper oxide (CuO) semiconductor nanoparticles are of interest because of their promising use for electronic and optoelectronic devices, and the size of the CuO particles for these applications is important. In this work, near spherical CuO nanoparticles with aspect ratio of 1.2–1.3 were made by a flame spray pyrolysis (FSP) method. In FPS, flame temperature, residence time, precursor concentration can be used to control particle size. As the precursor concentration increased from 0.5% to 35% w/w, primary particle diameter increased from 7 ± 2 to 20 ± 11 nm. Larger primary particle diameters were observed in the low gas flow system (set B) due to the long residence time in the high temperature zone. For the dependence of temperature on particle diameter, particles grew to similar diameter, i.e. ∼11 nm, in both flame conditions within the hot temperature zone (80% of melting point of CuO) but for particles having longer residence time, i.e. 550 ms in set B, the standard deviation of particle diameter is 45% larger than for particles with 66 ms as residence time in set A. Modeling gave a result for CuO final particle diameter, based on collision/sintering theory with sintering by solid state diffusion, of 6.7 and 9.0 nm for set A and set B, respectively, with surface tension assumed to be 0.5 J/m².Comparison with the experiment results, 11 ± 4 nm diameter for both flame conditions, indicates the simulations were reasonable.     

Numerical simulation of the devolatilization of a moving coal particle

The devolatilization of an isolated coal particle moving relative to the surrounding gas is numerically simulated using a competing reaction model of the pyrolysis and assuming that the released volatiles burn in an infinitely thin diffusion flame around the particle or not at all. The temperature of the particle is assumed to be uniform and the effects of the heat of pyrolysis, the intraparticle mass transfer resistance, and the variation of the particle radius are neglected. The effects of the size and velocity of the particle and of the temperature and oxygen mass fraction of the gas on the particle and flame temperature histories, the devolatilization time and the yield of light and heavy volatiles are investigated. The motion of the particle may have an important effect on the shape and position of the flame of volatiles, but it has only a mild effect on the devolatilization process for the particle sizes typical of pulverized coal combustion. This effect increases for large particles or in the absence of radiation. The relative motion enhances the heat transfer between the particle and the gas, causing the devolatilization time to decrease at high gas temperatures and to increase at low gas temperatures. The numerical results are compared with a blowing-corrected Nusselt number correlation often used in heat transfer models of the process.     

Aerosol-based method for investigating biomass char reactivity at high temperatures

An aerosol-based method was proposed and developed to characterize particles fragmented from biomass chars during oxidation. The chars were prepared from both wood and miscanthus pellets under various pyrolysis conditions. Char fragments with aerodynamic diameters in the range of 0.5–10 μm were suspended and transported in a reactive gas through an aerosol reactor, which was heated by an electric oven. The oxidation of char particles in the reactor was investigated by determining on-line the particle size distributions before and after passage through the reactor using an aerodynamic particle sizer (APS) spectrometer. The interpretation of APS data was evaluated by both experiments and models in which the fine char particles were assumed to keep either constant density or constant diameter during the oxidation process. The results indicate that the aerosol-based method can be used to determine reaction kinetics of char particles in the high-temperature range, where oxidation is normally controlled by diffusion limitation if measuring with the conventional techniques. The application of the aerosol method indicated that high pyrolysis temperature and prolonged retention time will reduce the char reactivity.     

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.     

Fast reactions with nano- and micrometer aluminum: A study on oxidation versus fluorination

The use of fluorine as an oxidizing agent in aluminum (Al)-based thermite reactions yields higher peak pressures and an increase in gas production compared with oxygen-containing oxidizers, such as molybdenum trioxide (MoO₃). Thus fluorination reactions have the potential to excel in situations that require high pressures and flame speeds. This study compares the combustion behaviors of Al/Teflon, Al/MoO₃/Teflon, and Al/MoO₃ in an effort to determine the effects that the replacement of oxygen with fluorine has on the reaction dynamics in both open and confined burning configurations. Data were collected from pressure sensors and high-speed imaging. The mass percent of Al was varied from 10 to 90% to study the effects of composition. The composites were then further tested at the optimum stoichiometry using either 50 nm or 1-3 μm Al to examine the effect of Al particle size. The addition of Teflon in an open configuration hinders the reaction due to a loss of liberated gas. Confining the reaction enables the trapped gases to enhance convection, yielding increased flame speeds. For confined conditions, the reactions containing Teflon exhibit higher peak pressures but lower flame speeds than the reactions with MoO₃. These results imply that a direct relationship between generated gas pressures and flame speeds does not generally exist when comparing different oxidizers. The theoretically predicted relationship for the relative flame speed versus relative particle size based on the melt-dispersion mechanism agrees with experimental data for all Al particle sizes and for the fluorination reaction. Particle synthesis parameters are suggested that could be controlled to enable micrometer-scale Al particles to achieve the performance of nanoscale Al particles. This is of significant practical importance, because nanoparticles are 30 to 50 times more expensive than the micrometer particles.     

Fuelnitrogen transformations in one-dimensional coal-dust flames

Experiments on the distribution of fuel nitrogen in pulverized-coal/O₂/Ar flames stabilized on a flat-flame burner are reported. The gas-phase concentration of HCN, NO, NH₃, and N₂, and the solid-phase nitrogen content were measured as a function of reaction time. A complete inventory of the major nitrogen-containing species is attained for two fuel-rich stoichiometries. Coal-nitrogen release is found to be more complete in the leaner flame. HCN and NO account for most of the gas-phase nitrogen early in the flames, while N₂ predominates later. Particle and gas temperature profiles were obtained with an infrared pyrometer. Early in the flame the particle and gas temperatures do not differ much, but the gas may eventually be as much as several hundred Kelvin hotter than the particles. The experimental results are used to determine overall rate constants for coal-nitrogen devolatilization and for N₂ formation.     

Preparation and improvement of nickel catalyst supported ordered mesoporous spherical silica for thermocatalytic decomposition of methane

The thermocatalytic alteration of CH₄ into highly pure hydrogen and filaments of carbon was investigated on a series of Ni-catalysts with various contents (25, 40, 55, and 70 wt%) supported mesoporous spherical SiO₂. The silica with ordered structure and high specific surface area (1136 m²/g) was synthesized using the Stöber technique with TEOS as a silica precursor and CTAB as the template in a simple synthesis system of aqueous-phase. This technique led to the preparation of mesoporous spherical silica. The prepared samples were characterized using BET, TPR, XRD, TPO, and SEM analyses. The prepared catalysts with different nickel loading showed the BET surface area ranging from 225.0 to 725.7 m²/g. These results indicated that an increase in nickel content decreases the surface area and leads to a subsequent collapse of a pore structure. SEM analysis confirmed a spherical nanostructure of catalysts and revealed that with the increase in loading of Ni, the particle size enlarged, because of the agglomeration of the particles. The results implied that the high methane conversion of 54% obtained over the 55 wt% Ni/SiO₂ at 575 °C and this sample had higher stability at lower reaction temperature than the other prepared catalysts, slowly deactivation was observed for this catalyst at a period of 300 min of time on stream.     

H2 production by methane decomposition: Catalytic and technological aspects

Ni and Co supported on SiO₂ and Al₂O₃ silica cloth thin layer catalysts have been investigated in the catalytic decomposition of natural gas (CDNG) reaction. The influence of carrier nature and reaction temperature was evaluated with the aim to individuate the key factors affecting coke formation. Both Ni and Co silica supported catalysts, due to the low metal support interaction (MSI), promotes the formation of carbon filament with particles at tip. On the contrary, in case alumina was used as support, metals strongly interact with surface thus depressing both the metal sintering and the detachment of particles from catalyst surface. In such cases, carbon grows on metal particle with a “base mechanism” while particles remain well anchored on the catalyst surface. This allowed to realize a cyclic dual-step process based on methane decomposition and catalyst oxygen regeneration without deactivation of catalyst. Technological considerations have led to conclude that the implement of a process based on decomposition and regeneration of catalyst by oxidation requires the development of a robust catalytic system characterized by both a strong MSI and a well defined particle size distribution. In particular, the catalyst should be able to operate at high temperature, necessary to reach high methane conversion values (> 90%), avoiding at the same time the formation of both the carbon filaments with metal at tip or the encapsulating carbon which drastically deactivate the catalyst.     

Determination of particle temperatures in a silica-generating counterflow flame via flame emission measurements

We propose a simple technique to measure particle temperatures in a particle generating counterflow flame. The silica particle temperature was derived from flame light emission measurements. This technique allows the non-intrusive measurement of particle temperatures over 2000 K. In addition, the OH concentration distribution in the hydrogen–oxygen flame was estimated from flame emission spectra in the ultraviolet region. A numerical model of the combustion processes, which included the reactions of SiCl₄ leading to the formation of silica particles, verified that the measured particle temperatures and OH concentration were close to the theoretical values.     

A Comparative Exploration of Enhancing Thermal Characteristics of Natural Gas Flame by Synchronous Combustion Technique

This article is a comparative study of how the injection of micro kerosene droplets and pulverized anthracite coal particles affects soot particle nucleation inside natural gas flame and, subsequently, radiation. To this end, the yellow chemiluminescence of soot particles and IR photography were used to locate radiative soot particles and discover their qualitative distribution. The IR filter was tested with a Thermo Nicolet Avatar 370 FTIR Spectrometer for its spectral transmittance to be specified. Also, the spectral absorbance of soot particles, which are formed in flame, was measured by BOMEM FTIR. Furthermore, the variations of flame temperature, transient heat transfer, and thermal efficiency were investigated. The results indicate that, for equal heating values, kerosene droplets are more effective than coal particles in improving the radiation and thermal characteristics of natural gas flame. Also, kerosene droplets cause a higher rise in the temperature in flame downstream and make the axial flame temperature more uniform than coal particles do. In quantitative terms, when kerosene droplets were injected, the radiative heat transfer and thermal efficiency of flame were 93% and 35% higher than the corresponding values for the coal particles injection mode.