Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475?K, in six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O₂/N₂/H₂O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O₂/N₂/H₂O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.     

Ignition of dimethyl ether/air mixtures by hot particles: Impact of low temperature chemical reactions

Understanding and characterizing ignition of flammable mixtures by hot particles is important for assessing and reducing the risk of accidental ignition and explosion in industry and aviation. Recently, many studies have been conducted for ignition of gaseous mixtures by hot particles. However, the effects of low-temperature chemistry (LTC) on ignition by hot particles received little attention. LTC plays an important role in the ignition of most hydrocarbon fuels and may induce cool flames. The present study aims to numerically assess the effects of LTC on ignition by the hot particles. We consider the transient ignition processes induced by a hot spherical particle in quiescent and flowing stoichiometric dimethyl ether/air mixtures. 1D and 2D simulations, respectively, are conducted for the ignition process by hot-particles in quiescent and flowing mixtures. A detailed kinetic model including both LTC and high-temperature chemistry (HTC) is used in simulations. The results exhibit a premixed cool flame to be first initiated by the hot particle. Then a double-flame structure with both premixed cool and hot flames is observed at certain conditions. At zero or low inlet flow velocities, the hot flame catches up and merges with the leading cool flame. At high inlet flow velocities, the hot flame cannot be initiated due to the short residence time and large convective loss of heat and radicals. Comparing the results with and without considering LTC confirms that LTC accelerates substantially ignition via HTC in a certain range of hot particle temperatures. The mechanism of ignition promotion by LTC is interpreted by analyzing the radical pool produced by the LTC and HTC surrounding the hot particle. Moreover, the influence of inlet flow velocity on ignition by hot particles is assessed. Non-monotonic change of ignition delay time with flow velocity is observed and discussed.     

Synthesis,structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe₃O₄ oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.     

Laser ignition of flammable mixtures via a solid core optical fiber

To date no commercial fiber coupled laser systems have reached the irradiance and pulse energy required for flammable mixtures ignition. In this work we report preliminary results on the ignition of two-phase mixtures promoted by a laser delivering pulses through optical fiber. Experiments undertaken on free beam path configurations have allowed identification of the optical parameters required for laser ignition. The fiber coupled system used is based on a Q-switched nanosecond laser operating at 1064 nm. The fiber input angle and the focal length have been identified as the most important parameters. We demonstrated the possibility of delivering nanosecond pulses of 30 mJ focused onto a spot of 200 μm through a solid core optical fiber, and to promote ignition of n-heptane/air and JP4/air mixtures. PACS 42.62.-b; 42.81.-i; 47.55.-t; 82.33.Vx     

Synthesis of silicon carbide nanopowders in a two-jet plasma-chemical reactor

Results of experimental studies on production of nanostructured silicon carbide powders in a plasma-chemical reactor based on a two-jet plasmatorch are presented. The conditions of SiC formation as a function of temperature and composition of the initial components are determined by thermodynamic calculations. Possibility of silicon carbide synthesis with the size of particles of 5?20 nm is shown experimentally.     

Effects of fuel stratification on ignition kernel development and minimum ignition energy of n-decane/air mixtures

Fuel-stratified combustion has broad application due to its promising advantages in extension of lean flammability limit, improvement of flame stabilization, enhancement of lean combustion, etc. In the literature, there are many studies on flame propagation in fuel-stratified mixtures. However, there is little attention on ignition in fuel-stratified mixtures. In this study, one-dimensional numerical simulation is conducted to investigate the ignition and spherical flame kernel propagation in fuel-stratified n-decane/air mixtures. The emphasis is placed on assessing the effects of fuel stratification on the ignition kernel propagation and critical ignition condition. First, ignition and flame kernel propagation in homogeneous n-decane/air mixture are studied and different flame regimes are identified. The minimum ignition energy (MIE) of the homogeneous n-decane/air mixture is obtained and it is found to be very sensitive to the equivalence ratio under fuel-lean conditions. Then, ignition and flame kernel propagation in fuel-stratified n-decane/air mixture are investigated. The inner equivalence ratio and stratification radius are found to have great impact on ignition kernel propagation. The MIEs at different fuel-stratification conditions are calculated. The results indicate that for fuel-lean n-decane/air mixture, fuel stratification can greatly promote ignition and reduce the MIE. Six distinct flame regimes are observed for successful ignition in fuel-stratified mixture. It is shown that the ignition kernel propagation can be induced by not only the ignition energy deposition but also the fuel-stratification. Moreover, it is found that to achieve effective ignition enhancement though fuel stratification, one needs properly choose the values of stratification radius and inner equivalence ratio.     

Characterization of Nano‐Particles Using Laser‐Induced Incandescence

Laser‐induced incandescence (LII) is introduced as a valuable tool for the characterization of nanoparticles. This optical measurement technique is based on the heating of the particles by a short laser pulse and the subsequent detection of the thermal radiation. It has been applied successfully for the investigation of soot in different fields of application, which is described here in the form of an overview with a focus on work done at the LTT‐Erlangen during the last 10 years. In laboratory flames the soot primary particle size, volume concentration, and relative aggregate size have been determined in combination with the number density of primary particles. Furthermore, the primary particle sizes of carbon blacks have been measured in situ and online under laboratory conditions and also in production reactors. Measurements with different types of commercially available carbon black powders, which were dispersed in a measurement chamber yielded a good correlation between LII results and the specified product properties. Particle diameters determined by LII in a furnace black reactor correlate very well with the CTAB‐absorption number, which is a measure for the specific surface area. It turned out that the LII method is not affected by variations of the aggregate structure of the investigated carbon blacks. The LII signal also contains information on the primary particle size distribution, which can be reconstructed by the evaluation of the signal decay time at, at least, two different time intervals. Additionally, soot mass concentrations have been determined inside diesel engines and online measurements were performed in the exhaust gas of such engines for various engine conditions simultaneously providing information about primary particle size, soot volume, and number concentration. The LII results exhibit good correlation with traditional measurement techniques, e.g., filter smoke number measurements. In addition to the soot measurements, primarily tests with other nanoparticles like TiO₂ or metal particles are encouraging regarding the applicability of the technique for the characterization of such different types of nanoparticles.     

Heat and mass transfer at gas-phase ignition of grinded coal layer by several metal particles heated to a high temperature

Characteristics of gas-phase ignition of grinded brown coal (brand 2B, Shive-Ovoos deposit in Mongolia) layer by single and several metal particles heated to a high temperature (above 1000 K) have been investigated numerically. The developed mathematical model of the process takes into account the heating and thermal decomposition of coal at the expense of the heat supplied from local heat sources, release of volatiles, formation and heating of gas mixture and its ignition. The conditions of the joint effect of several hot particles on the main characteristic of the process–ignition delay time are determined. The relation of the ignition zone position in the vicinity of local heat sources and the intensity of combustible gas mixture warming has been elucidated. It has been found that when the distance between neighboring particles exceeds 1.5 hot particle size, an analysis of characteristics and regularities of coal ignition by several local heat sources can be carried out within the framework of the model of “single metal particle / grinded coal / air”. Besides, it has been shown with the use of this model that the increase in the hot particle height leads, along with the ignition delay time reduction, to a reduction of the source initial temperatures required for solid fuel ignition. At an imperfect thermal contact at the interface hot particle / grinded coal due to the natural porosity of the solid fuel structure, the intensity of ignition reduces due to a less significant effect of radiation in the area of pores on the heat transfer conditions compared to heat transfer by conduction in the near-surface coal layer without regard to its heterogeneous structure.     

Effect of small particle sizes on the measured density of nanocrystalline powders of nonstoichiometric tantalum carbide TaC<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript>

Nanocrystalline powders of the nonstoichiometric tantalum carbide TaC_y (0.81 ≤ y ≤ 0.96) with an average particle size in the range from 45 to 20 nm have been prepared using high-energy ball milling of coarse-grained powders. The density of the initial coarse-grained and prepared nanocrystalline powders of TaC_y has been measured by helium pycnometry. The sizes of particles in tantalum carbide powders have been estimated using the X-ray diffraction analysis and the Brunauer–Emmett–Teller (BET) method. The density of TaC_y nanopowders measured by helium pycnometry is underestimated as compared to the true density due to the adsorption of helium by the highly developed surface of the nanocrystalline powders. It has been shown that the difference between the true and measured densities is proportional to the specific surface area or is inversely proportional to the average particle size of the nanopowders. The large difference between the true and measured pycnometric densities indicates a superhydrophobicity of the tantalum carbide nanopowders.     

Spontaneous ignition temperature for the compacted mixture of Ti–Al system: Theory and experimental comparisons

Spontaneous ignition of compacted mixture has been examined not only experimentally but also theoretically, relevant to materials synthesis by combustion for Ti–Al system. By varying compact and particle sizes, mixture ratio, and degree of dilution, spontaneous ignition temperature has been measured, which is determined from the inflection-point of the temporal variations of the surface temperature. It is found that the spontaneous ignition temperature decreases with increasing aspect ratio, defined as the ratio of the sample height and compact diameter, due to an establishment of the stationary temperature distribution in the radial direction in the compacted mixture, as the sample height becomes tall. It is also found that the spontaneous ignition temperature decreases with increasing size ratio, defined as the ratio of compact and particle diameters, due to an increase in the particle surface per unit spatial volume of the compacted mixture, caused by a decrease in the particle diameter and/or an increase in the compact diameter. By further examining its dependence on mixture ratio and/or degree of dilution, it is confirmed that the limit of flammability also exerts influences on the spontaneous ignition temperature. In addition, a fair degree of agreement, shown in experimental comparisons with theoretical results, indicates that the present formulation has captured the essential features of the spontaneous ignition of compacted mixture. Since this kind of particle size effect, especially, relevant to the spontaneous ignition of compacted mixtures, has not been captured in the previous studies, its elucidation can be considered not only notable but also useful, especially, in manipulating combustion process in materials synthesis.     

Microstructure of nanocrystalline nonstoichiometric vanadium carbide VC0.875

A nanocrystalline powder of nonstoichiometric vanadium carbide VC_0.875 has been prepared by the high-energy ball milling method. The crystal structure, microstructure, morphology, and size distribution of particles of the initial and milled powders have been investigated using X-ray diffraction, laser diffraction, and scanning electron microscopy. For vanadium carbide, the model calculation of the particle size of a VC_0.875 nanopowder as a function of the milling duration has been performed for the first time. A comparison of the experimental and theoretical results has demonstrated that a nanopowder with an average particle size of 40–80 nm can be obtained by a 10-h high-energy ball milling of the initial vanadium carbide powder with an average particle size of ～6 μm.     

Model of heating and ignition of conductive polydisperse powder in electrostatic discharge

Heating of a conductive polydisperse powder by electrostatic discharge (ESD) is modelled numerically. Powder packing is described using a discrete element model; powder resistance is defined by geometry of particle contacts and properties of plasma produced by electrical breakdown between neighbour particles. A set of parametric calculations in combination with experimental data is used to determine necessary adjustable model parameters. The model predicts the temperature for each powder particle resulting from its heating by the ESD current. Location and packing of individual particles within the powder affects greatly their achieved temperatures and thus the likelihood of ignition. Consistently with experiments, a trend showing that smaller particles are generally heated to higher temperatures at a given ESD energy is detected for coarser powders; this trend becomes less clear for finer powders with particle sizes less than the breakdown distance given by the Paschen curve in air. Comparison of the experimental data and calculations suggests that the transition from single particle to cloud combustion occurs when the distance between the particles ignited by ESD becomes close to the flame size for the individual burning particle. This distance, inversely proportional to the number of ignited particles, is primarily determined by the ESD energy.     

Silicon Carbide Nanoparticles Produced by CO2 Laser Pyrolysis of SiH4/C2H2 Gas Mixtures in a Flow Reactor

Pulsed CO₂-laser-induced decomposition of different mixtures of SiH₄ and C₂H₂ in a flow reactor has been employed to produce silicon carbide clusters and nanoparticles with varying content of carbon. The as-synthesized species were extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where, after having traversed a differential chamber, they were analyzed with a time-of-flight mass spectrometer. Thin films of silicon carbide nanoclusters were produced by depositing the clusters at low energy on potassium bromide and sapphire windows mounted into the differential chamber. At the same time, Si and SiC nanoparticles were collected in a filter placed into the exhaust line of the flow reactor. Both beam and powder samples were characterized by FTIR spectroscopy. The close resemblance of the spectra suggests that the composition of the beam and powder particles obtained during the same run is nearly identical. XRD spectroscopy could only be employed for the investigation of the powders. It was found that CO₂ laser pyrolysis is ideally suited to produce silicon carbide nanoparticles with a high degree of crystallinity. Nanopowders produced from the pyrolysis of a stoichiometric (2:1) mixture of SiH₄/C₂H₂ were found to contain particles or domains of pure silicon. The characteristic silicon features in the FTIR and XRD spectra, however, disappeared when C₂H₂ was applied in excess.     

炸药爆轰法制备纳米石墨粉及其在高压合成金刚石中的应用

介绍了一种制备纳米石墨粉的新方法——负氧平衡炸药爆轰法. 对合成的黑粉产物进行x射线衍射分析，确认其为石墨结构，平均晶粒度为2.58 nm. 透射电子显微分析的结果表明，炸药爆轰法制备的黑粉为六方结构的纳米石墨粉，颗粒呈球形或椭球形. 用小角x射线散射法测定纳米石墨粉的粒度分布在1—50 nm，有92.6wt%的粉末粒度小于16 nm. 平均粒径为8.9 nm. 纳米石墨粉的比表面积约为500—650 m²/g. 在六面顶压机中用纳米石墨粉在Fe粉触媒的作用下进行金刚石的高压合成实验 关键词： 纳米石墨粉 爆轰 金刚石 合成     

pH-Dependent shape changes of water-soluble CdS nanoparticles

Annealing of silicon-carbon nanoparticles was performed in argon at atmospheric pressure to enable formation of silicon carbide nanomaterials and/or carbon structures. Three precursor powders with increasing crystallinity and annealing temperatures from 1,900 to 2,600 °C were used to gain information about the effect of precursor properties (e.g. amorphous vs. nanocrystalline, carbon content) and annealing temperature on the produced materials. Three structures were found after annealing, i.e. silicon carbide crystals, carbon sheets and spherical carbon particles. The produced SiC crystals consisted of several polytypes. Low annealing temperature and increasing crystallinity of the precursor promoted the formation of the 3C-SiC polytype. Raman analysis indicated the presence of single-layer, undoped graphene in the sheets. The spherical carbon particles consisted of curved carbon layers growing from the amorphous Si–C core and forming a ‘nanoflower’ with a diameter below 60 nm. To our knowledge, the formation of this kind of structures has not been reported previously. The core was visible in transmission electron microscopy analysis at the annealing temperature of 1,900 °C, decreased in size with increasing temperature and disappeared above an annealing temperature of 2,200 °C. With increasing crystallinity of the precursor material, fewer layers (~5 with the most crystalline precursor) were detected in the carbon nanoflowers. The method presented opens up the possibility to produce new carbon nanostructures whose properties can be controlled by changing the properties of the precursor material or by adjusting an annealing temperature.     

Synthesis of carbides in the arc plasma

A method of synthesis of iron, silicon, titanium, and tungsten carbides in an argon arc between graphite electrodes is developed. It is found that iron, silicon, and titanium atoms introduced into the interelectrode gap cause the voltage drop across the gap to decrease, whereas tungsten atoms increase this voltage. The X-ray diffraction analysis of synthesized powders reveals carbide and graphite crystallites. Their relative concentrations are estimated from the intensities of the respective reflection peaks. The morphological examination of the synthesized particles shows that titanium carbide particles are the largest and iron carbide ones are the finest. Raman spectra taken of the powders confirm the presence of graphite and carbide crystallites.     

Phase identification of microfeatures using EPMA methods,especially high-resolution X-ray spectroscopy

Methods of electron-probe microanalysis (EPMA), with some input from scanning and transmission electron microscopy (SEM/TEM), are applied for the identification of micro-scale constituents in a solid matrix. The subject of the study is a magnesium alloy composite, which contains silicon carbide-based fibres made by a liquid metal infiltration process. Backscattered electron imaging of the composite in the SEM showed that during composite manufacture, fibres were chemically attacked by the metal, many of the fibres exhibiting three distinct grey levels, indicative of different reaction zones, and others appearing uniformly black. EPMA measurements showed that each region contained ∼12 wt.% oxygen and that penetration of the fibre by magnesium was accompanied by a reduction in the concentration of silicon and carbon. From studying the position and shape of specific X-ray lines it was shown that magnesium penetration involved a chemical reaction with silicon oxycarbide, established in earlier EPMA studies as one of the fibre constituents. Also, in the outermost region, aluminium in the alloy reacted with free carbon in the fibre to form aluminium carbide and with magnesium to produce a Mg–Al intermetallic. The composition of black fibres was quite different from the grey ones, with negligible silicon and only a small amount of aluminium. Oxygen levels in black fibres were consistent with complete oxidation, indicating these fibres were subjected, locally, to severe oxidising conditions during composite manufacture. In the metal matrix itself, particles of a mixed magnesium/aluminium oxide, silicon carbide and magnesium silicide were observed, the latter two phases forming as silicon and carbon were ejected from fibres.     

Prediction improvements of ignition characteristics of isolated coal particles with a one-dimensional transient model

A modified 1-D transient model considering intra-particle thermal conduction is adopted to improve the predictions of the ignition characteristics of isolated coal particles. The study aims at resolving the incorrect prediction on the variation trend of ignition temperature T_i with the change of oxygen concentration X_O2, interpreting the contradictory dependencies on coal particle size and furnace temperature and clarifying the conditions when the intra-particle thermal conduction should be considered. The predictions are compared with microgravity data in which the buoyancy effect is minimized. The results reveal that the previous ignition model with transient adiabatic criterion fails to predict the T_i variation with X_O2, since it cannot accurately predict T_i and delay time in the low X_O2 region. Instead, the ignition model with flammability limit ignition criterion can well predict T_i in a wide range of X_O2. Intra-particle thermal conduction causes remarkable temperature differences for large coal particles, and moreover, the variation trends of surface and center temperatures with particle size are very different. The center temperature at ignition drops remarkably with increasing particle size, while the surface temperature barely changes or slightly increases with particle size. At the same particle size, the variation trends of surface and center temperatures with furnace temperature are also opposite. The ignition mode and variation trend of ignition surface temperature with particle size depends on the heating rate and particle size itself. The contradictory experimental results reported by different researchers are attributed to the particle size and temperature measurement location. The conditions necessary to consider the intra-particle thermal conduction are discussed. Lastly, the effect of the intraparticle thermal conduction is shown on an ignition mode diagram.     

High-pressure shock tube studies on graphite oxidation reactions with carbon dioxide and water

Heterogeneous reactions involving graphite particles reacting with gaseous carbon dioxide and steam were studied under high-pressure and high-temperature conditions. The high-pressure shock tube facility at the University of Illinois at Chicago was utilized to perform the heterogeneous experiments with post-shock pressures ranging from 209 to 363 atm, temperatures ranging between 1546 and 2692 K, and nominal reaction times of 1 ms. Graphite particles were injected into the shock tube, by means of a particle injector, containing mixtures of varying concentrations of H₂O or CO₂ in Ar as balance. The overall reaction rate coefficients from these graphite experiments have been calculated and compared to previous work performed with carbon black particles as well as theoretical and experimental values from prior investigators.     

Convective burning of fine ammonium nitrate–aluminum mixtures in a closed volume bomb

It is commonly assumed that the burning of ammonium nitrate–aluminum mixtures is much less prone to undergo a transition to explosion and detonation than similar mixtures based on ammonium perchlorate. However, this conclusion has been made for mixtures based on commercial-grade ammonium nitrate with large particles. In this study, the combustion of fine loose-packed mixtures of ammonium nitrate and aluminum in a closed-volume bomb has been examined. It has been shown that fine mixtures (ammonium nitrate with a particle size of less than 40 µm and an ASD-4 aluminum powder with spherical particles with a size of about 4 µm) undergo high-intensity combustion; in experiments with a stoichiometric mixture, explosions are observed. The explosions occur in the initial phase of convective combustion and lead to abrupt pressure pulsations with an amplitude of a few kilobars and to the destruction of the cup in which the sample is placed. The dynamics of development of the explosion has been analyzed in detail using numerical simulation. According to the results of experiments with varied parameters—the degree of dispersion of the ammonium nitrate powders, the aluminum content in the mixture, the length and diameter of the charge, and the level of pressure generated by the combustion of the igniter,—threshold conditions have been determined to separate the following modes: the absence of ignition, layer-by-layer combustion, or convective combustion with a transition into an explosion in experiments with a stoichiometric mixture.