Flame Reactor Research Articles

An extensive experimental and modeling study of 1-butene oxidation

In this study, a series of ignition delay time (IDT) experiments of 1-butene were performed in a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) under conditions of relevance to practical combustors. This is the first 1-butene IDT data taken at engine relevant conditions, and the combination of HPST and RCM results greatly expands the range of data available for the oxidation of 1-butene to higher pressures (10–50atm), lower temperatures (670–1350K) and to a wide range of equivalence ratios (0.5–2.0).A comprehensive chemical kinetic mechanism to describe the combustion of 1-butene has simultaneously been applied. It has been validated using the IDT data measured here in addition to a large variety of literature data: IDTs, speciation data from jet-stirred reactor (JSR), premixed flame, and flow reactor, and laminar flame speed data. Important reactions have been identified via flux and sensitivity analyses including: (a) H-atom abstraction from 1-butene by hydroxyl radicals and molecular oxygen from different carbon sites; (b) addition reactions, including hydrogen atom and hydroxyl radical addition to 1-butene; (c) allylic radical chemistry, including the addition reactions with methyl radical, hydroperoxy radical and self-recombination; (d) vinylic radical chemistry, including the addition reaction with molecular oxygen; (e) alcohol radical chemistry, including the Waddington type propagating reaction pathways and alkyl radical low-temperature branching chemical pathways.

Formation of Non-Aggregated Li4Ti5O12 Particles in a Flame Reactor

Formation of Non-Aggregated Li4Ti5O12 Particles in a Flame Reactor

Study of the high heating rate devolatilization of bituminous and subbituminous coals—Comparison of experimentally monitored devolatilization profiles with predictions issued from single rate, two-competing rate, distributed activation energy and chemical percolation devolatilization models

This paper deals with the measurement and modeling of devolatilization profiles obtained with 7 different coals. To do so, a specifically designed flat flame reactor (FFR) has been used to reproduce industrial heating rates (∼106Ks−1) with peak particle temperatures (PTp) ranging from ∼1000K to ∼1300K. The thermal history of the fuel particles has been characterized by coupling particle image velocimetry (PIV) and pyrometry measurements so as to be integrated in empirical and phenomenological models including single rate, two-competing rate, distributed activation energy (DAE) and chemical percolation devolatilization (CPD) ones. Final yields ranging from ∼70% to ∼89% with respect to the initial fuel volatiles content have been measured for PTp≈1000K. As expected, a decrease of the devolatilized fractions with an increase of the fuel rank has been observed while releases of volatiles exceeding the proximate analysis predictions have been recorded for temperatures higher than ∼1090K. Besides, the experimental conditions investigated in this work and especially the high heating rate delivered by the FFR used allowed testing the ability of devolatilization models from the literature to simulate data obtained in conditions relevant to coal power plant applications which should be of particular interest for future CFD calculations. The main findings highlighted through the comparison work proposed herein can be summed up as follows: 1) The single kinetic rate model integrating the original parameters proposed by Badzioch and Hawksley globally reproduces measured weight losses for bituminous coals while needing adjustments for lower rank fuels; 2) Different kinetic constants tabulated in the literature for two-competing rate models have been tested and turned out to be unsuitable to simulate the devolatilization profiles we monitored. Insights regarding most adapted parameters and optimization procedures that might be operated are, nonetheless, discussed; 3) Adjusting the standard deviation of the activation energy introduced into the DAE model consistently with the thermal gradient experienced by the fuel particles may lead to improved agreements between measured and simulated data; 4) The CPD model slightly overstimates devolatilization rates at low temperatures while showing the best performances to predict the final devolatilization yields for all the studied coals (including the subbituminous one) due to the structural approach integrated in such a modeling tool.

Analysis of heat and mass transfer in diffusion flame reactors coupled with aerodynamic lenses

In this paper, it is presented a numerical investigation of the main phenomena involved during the synthesis of nanoparticles in a diffusion flame reactor coupled with an aerodynamic lens system. The lenses are used to restrict the main flow to a small region, allowing the particles to be directed to a collector surface in order to create nanoparticles films with specific shapes. The performance of the system was evaluated by the analysis of the velocity, temperature and concentration fields in order to validate the experimental data and to provide information about the process. The model considered continuous gas phase under steady-state conditions and compressible turbulent flow. By changing the system geometry, we investigated various configurations to determine the optimum geometry and operation mode of cluster sources for particle focusing. The coupling techniques show very promising features in view of the particle focusing to nanostructured films manufacturing, enabling novel functional and structural properties. The results show that the aerodynamic lenses system can restrict the main flow to a much narrow region in comparison with the reactor with no lens, therefore helping the control of film deposition.

Atmospheric Pressure Nonequilibrium Plasma Jets for Synthesis, Deposition, and Modification of Various Oxides

Atmospheric pressure nonequilibrium plasma jets (APNP-Js) have garnered increasing interest over the last decade due to their simplicity, high excitation selectivity, and relatively low temperature operation.1 The utility of such plasmas has already been realized in areas ranging from medical applications (sterilization, intracellular treatments, and surface functionalization, among others) to industrial material processing (etching, cleaning, and surface treatments). However, not until recently have these microjet plasmas been explored as an alternative to costly and complex plasma sources (i.e. microwave, glow discharge, and electron cyclotron resonance, among others) for the deposition and synthesis of materials.2-4 Current efforts at the University of Louisville’s Conn Center for Renewable Energy Research are being directed toward employing various APNP-J configurations for this purpose. One promising application we are presently investigating entails the use of dielectric barrier discharge-like APNP-Js to deposit ceramic nanoparticles on a desired substrate. Here, a low frequency (<100kHz) RF plasma driver is used to ignite a helium plasma inside a quartz tube. An aerosol of the ceramic nanoparticles is then generated in a custom-built fluidization system and directed through the plasma via a second inlet. It has been found that successful deposition typically occurs at total flow rates less than 3 SLM. The plasma deposition of particles into thin films seems to offer an interesting way to form uniform, pinhole-free layers with thicknesses ranging from 50nm to 2µm. It is envisioned that this technique can serve as an intermediary between atomic layer deposition and spin coating. The sintering of particles during plasma deposition of particles is an interesting phenomenon, which needs to be understood further. Another promising application extends upon our group’s recently published plasma oxidation-based synthesis technique for the production of mixed metal oxide solid solutions.5 Favorable results in this study were obtained by exposing nickel, iron, and manganese nitrate solutions to an atmospheric microwave plasma flame reactor. As an alternative to this microwave driven plasma, we have recently observed that single electrode plasma jets can achieve similar results on comparable time scales. The plasma apparatus utilized for this application uses the same RF plasma driver described above to generate a helium plasma. However, a different configuration was designed to permit the introduction of oxygen to the plasma without extinguishing it. The resulting plasma jet is much more concentrated with characteristics that border on the arc regime of operation. Upon exposure of metal-nitrate solutions to this plasma, their colors were observed to change rapidly (<2 seconds) to black and dark grey. Results were verified by x-ray diffraction and electron microscopy. Work is currently on-going to create a wide ranging metastable mixed metal oxide alloys for future material development. Fig. 1 Scanning electron micrograph of metal oxide nanopowder deposited on silicon substrates via APNP. (a) Top view and (b) Cross-sectional view. Acknowledgements:Authors gratefully acknowledge the Conn Center for Renewable Energy Research for facilities and access to characterization equipment In addition to these two promising applications, we are currently exploring APNP-Js for use in a wide range of other applications including greenhouse gas reduction, nanowire deposition, and surface functionalization. It is our goal to further extend the capabilities of this technology in an effort to simplify and reduce the cost of plasma processing in mass-production.

Synthesis of powder uranium tetrafluoride from depleted uranium hexafluoride in hydrogen fluoride flame

The mechanism of uranium tetrafluoride formation during the reduction of depleted uranium hexafluoride in hydrogen fluoride flame has been studied. Based on the performed studies, it has been established that powder uranium tetrafluoride with characteristics that enable it to be used for both long-term storage (minimum specific surface area, 0.6 ± 0.1 m2/g; untapped density, 2.7 g/cm3) and metallic uranium production (maximum specific surface area, 1.45 m2/g) can be obtained by adjusting the operational regime of a flame reactor.

Vacuum Ultraviolet Photoionization of Complex Chemical Systems.

Tunable vacuum ultraviolet (VUV) radiation coupled to mass spectrometry is applied to the study of complex chemical systems. The identification of novel reactive intermediates and radicals is revealed in flame, pulsed photolysis, and pyrolysis reactors, leading to the elucidation of spectroscopy, reaction mechanisms, and kinetics. Mass-resolved threshold photoelectron photoion coincidence measurements provide unprecedented access to vibrationally resolved spectra of free radicals present in high-temperature reactors. Photoionization measurements in water clusters, nucleic acid base dimers, and their complexes with water provide signatures of proton transfer in hydrogen-bonded and π-stacked systems. Experimental and theoretical methods to track ion-molecule reactions and fragmentation pathways in intermolecular and intramolecular hydrogen-bonded systems in sugars and alcohols are described. Photoionization of laser-ablated molecules, clusters, and their reaction products inform thermodynamics and spectroscopy that are relevant to astrochemistry and catalysis. New directions in coupling VUV radiation to interrogate complex chemical systems are discussed.

Effect of furanic biofuels on particles formation in premixed ethylene–air flames: An experimental study

Furanic fuels have been recently investigated because they are considered possible fossil fuel substituents due to their high energy density, their renewable origin and consequently low green-house gas emissions. Few data are available about the behaviors of these fuels in laboratory flame reactors. We have recently studied their sooting tendency in a counter-flow diffusion flame showing a different propensity to form particulate matter depending on the richness of the combustion conditions. In this paper, we explore their tendency in forming particulate matter in atmospheric pressure premixed flames burning mixtures of ethylene and furanic fuels at four different equivalence ratios, namely 2.01, 2.16, 2.31 and 2.46, thus covering the range from slightly sooting to fully sooting conditions. Liquid furan, 2-methylfuran and 2,5-dimethylfuran have been added to ethylene as 10% and 20% of the total carbon fed to the flame.In-situ spectroscopy, namely laser UV-induced emission, has been used as diagnostic tool to detected different classes of precursor nanoparticles by changing the detection wavelength from the UV to the visible. Laser induced incandescence has instead been used to detect soot particles. Equivalence ratios, the total carbon flow rate and cold gas velocity have been kept constant, allowing the combustion temperature to be almost the same also when furans were added.LII signal decreases significantly when furans are added showing a strong effect of furanic fuels in decreasing the amount of soot formed in atmospheric pressure premixed flames. Particularly the reduction increases as the equivalence ratio increases. On the other side, LIF UV and LIF VIS slightly decrease when furans are added. The absence of a significant reduction of the LIF UV signal when furans are added suggests that small particle formation is not effectively inhibited by these additives. This behavior is similar to what found for ethanol and dimethyl ether addition to ethylene premixed flames, showing a similarity in the behavior of these oxygenated fuels.

Development and numerical/experimental characterization of a lab-scale flat flame reactor allowing the analysis of pulverized solid fuel devolatilization and oxidation at high heating rates.

This paper deals with the thorough characterization of a new experimental test bench designed to study the devolatilization and oxidation of pulverized fuel particles in a wide range of operating conditions. This lab-scale facility is composed of a fuel feeding system, the functioning of which has been optimized by computational fluid dynamics. It allows delivering a constant and time-independent mass flow rate of fuel particles which are pneumatically transported to the central injector of a hybrid McKenna burner using a carrier gas stream that can be inert or oxidant depending on the targeted application. A premixed propane/air laminar flat flame stabilized on the porous part of the burner is used to generate the hot gases insuring the heating of the central coal/carrier-gas jet with a thermal gradient similar to those found in industrial combustors (>10(5) K/s). In the present work, results issued from numerical simulations performed a priori to characterize the velocity and temperature fields in the reaction chamber have been analyzed and confronted with experimental measurements carried out by coupling particle image velocimetry, thermocouple and two-color pyrometry measurements so as to validate the order of magnitude of the heating rate delivered by such a new test bench. Finally, the main features of the flat flame reactor we developed have been discussed with respect to those of another laboratory-scale system designed to study coal devolatilization at a high heating rate.

Facile growth of 1‐D nanowire‐based WO3 thin films with enhanced photoelectrochemical performance

A flame reactor embedded with a constant tungsten wire feeding system to prepare one‐dimensional (1‐D) nanostructured tungsten oxide thin film for photoelectrochemical (PEC) water splitting was developed. Photoactive vertically‐aligned nanowire‐based WO3 thin films could be obtained with a controlled thickness via a flame vapor deposition process followed by air‐annealing. The PEC performances of WO3 photoelectrodes for different thin film thicknesses were examined. The optimum thickness of WO3 thin film was found to be about 7.2 μm for PEC water splitting based on incident photon‐to‐current efficiency plots and I–V curves. The WO3 prepared with optimum thickness showed better PEC performance than those of recently reported nanostructured WO3 photoanodes. © 2015 American Institute of Chemical Engineers AIChE J, 62: 421–428, 2016

Synthesis of Titanium Dioxide Aerosol Gels in a Buoyancy-Opposed Flame Reactor

Aerosol gels are a novel class of materials with potential to serve in various energy and environmental applications. In this work, we demonstrate the synthesis of titanium dioxide (TiO2) aerosol gels using a methane-oxygen coflow diffusion flame reactor operated in down-fired configuration (fuel flow in the direction opposite to buoyancy forces). Titanium tetraisopropoxide was fed as a precursor to the flame under different operating conditions. Control of the monomer size and crystalline phase of TiO2 gel particles was achieved by adjusting the flame operating conditions, specifically the flame temperature, which was shown to significantly influence the phase transformation and rate of particle growth and sintering. The resulting materials were characterized for their physical and optical properties. Results showed that the TiO2 aerosol gels had effective densities in the range 0.021–0.025 g/cm3, which is 2 orders of magnitude less than the theoretical mass density of TiO2. The monomer size distribution, crystalline phase, and UV-Vis absorbance spectra of the gels showed distinct characteristics as a function of flame temperature.Copyright © 2015 American Association for Aerosol Research

Experimental study on elemental behaviors in fluorination of nuclear spent fuel with flame reactor

Our proposed spent nuclear fuel reprocessing technology named FLUOREX is a hybrid system based on reprocessing technologies of fluorination and solvent extraction for light water reactor fuel. In the current research, we experimentally clarified solid–gas transfer behaviours of the fluorides in the FLUOREX process and identified the volatile and non-volatile compounds in the fluorination. We carried out a fluorination experiment for simulated spent nuclear fuel and solid separation from the UF6 gas stream. The distribution ratios of fission product elements in the experimental apparatus were evaluated. Molybdenum, Te, Nb, and Ru were volatilized by fluorination and they accompanied the UF6 gas. However, 22.9% of the Ru and 3.4% of the Nb were retained as solids in the experimental apparatus, contrary to the fact that their partial pressures in the experiment were lower than their vapor pressures. Rubidium, Sr, Zr, Ce, and Nd were completely recovered as solid fluorides, and these results agreed with the prediction based on boiling points of their fluorides. Antimony was completely recovered as a solid; nevertheless, the boiling point of antimony pentafluoride was lower than the process temperature, and that was attributed to the formation of a non-volatile antimony oxyfluoride.

Effect of fluctuations on time-averaged multi-line NO-LIF thermometry measurements of the gas-phase temperature

Multi-line NO laser-induced fluorescence (LIF) thermometry enables accurate gas-phase temperature imaging in combustion systems through least-squares fitting of excitation spectra. The required excitation wavelength scan takes several minutes which systematic biases the results in case of temperature fluctuations. In this work, the effect of various types (linear, Gaussian and bimodal) and amplitudes of temperature fluctuations is quantified based on simulated NO-LIF excitation spectra. Temperature fluctuations of less than ±5 % result in a negligible error of less than ±1 % in temperature for all cases. Bimodal temperature distributions have the largest effect on the determined temperature. Symmetric temperature fluctuations around 900 K have a negligible effect. At lower mean temperatures, fluctuations cause a positive bias leading to over-predicted mean temperatures, while at higher temperatures the bias is negative. The results of the theoretical analysis were applied as a guide for interpreting experimental multi-line NO-LIF temperature measurements in a mildly turbulent pilot-plant scale flame reactor dedicated for nanoparticle synthesis.

Reduced Chemical Kinetics for the Modeling of TiO2 Nanoparticle Synthesis in Flame Reactors

Flame synthesis represents a viable technique for large-scale production of titanium dioxide (TiO2) nanoparticles. A key ingredient in the modeling of this process is the description of the chemical kinetics, which include Ti oxidation, hydrocarbon fuel combustion, and chlorination. While detailed chemical mechanisms have been developed for predicting TiO2 nanoparticle properties by West et al. (e.g., Combust. Flame 2009, 156, 1764), their use in turbulent reacting flow simulations is limited to very simple configurations or requires significant modeling assumptions to bring their computational cost down to an acceptable level. In this work, a reduced kinetic scheme describing the oxidation of TiCl4 in a methane flame is derived from and validated against the predictions of a detailed mechanism from the literature. The reduction procedure uses graph-based methods for unimportant kinetic pathways elimination and quasi-steady-state species selection. Reduction targets are chosen in accordance with previous modeling results that showed the importance of temperature and overall concentration of titanium-containing species in both nucleation and surface growth rates. The resulting reduced scheme is thoroughly evaluated over a wide range of conditions relevant to flame-based synthesis, and the capability of the reduced model to adequately capture the process dynamics at a much lower computational cost is demonstrated.

Study of the high heating rate devolatilization of a pulverized bituminous coal under oxygen-containing atmospheres

This study deals with the analysis of pulverized coal devolatilization in oxygen-containing atmospheres. To do so, a newly developed flat flame reactor allowing coal jet flames to be stabilized with fuel heating rates of the order of 106Ks−1 has been used to mimic operating conditions close to those met in practical combustors. A high volatile bituminous coal milled in an industrial grinder has been fluidized by carrier gases containing various amounts of oxygen (air and pure oxygen) and then injected in hot gases (temperatures ranging up to ∼1240K) generated by propane/air flat flames. The thermal history of coal particles has been monitored by coupling particle image velocimetry (PIV) and pyrometric measurements to be integrated in constant-rate, single-kinetic-rate, two-competing-rate and DAEM devolatilization models. Devolatilization profiles derived from the analysis of the char collected at different residence times have then been compared with simulated data and a simplified fitting procedure has been implemented to find kinetic parameters leading to reproduce experimental data so as to evaluate the relative influence of the surrounding atmosphere on the devolatilization kinetics. Obtained results confirm that devolatilization is faster and more complete under oxygen enriched environments which has been related to an enhanced combustion of volatiles inducing a rise of the temperature of fuel particles. On the other hand, apparent devolatilization rates appeared to be only indirectly affected by the surrounding atmosphere through the way this one influences the heating of the coal. Furthermore, results obtained in this work tend to indicate the absence of overlapping between devolatilization and char oxidation stages. Measurements of gaseous species released during or formed after the devolatilization process finally confirm that the combustion of volatiles is more complete under oxygen enriched combustion (OEC). More CO2 is thus produced while CO concentrations significantly decrease. An enhanced fuel-N conversion has moreover been related to the higher temperatures and oxygen partial pressures met under OEC while SO2 concentrations ∼50% higher have been measured due the higher quantities of fuel-sulfur and oxygen available in the medium.

Nanoscale mixing during double-flame spray synthesis of heterostructured nanoparticles

The combination of two nanoparticle-producing flame reactors to a double-flame (DF) spray pyrolysis process is an attractive method for the high-temperature gas-phase synthesis of multicompound materials and heterostructures. It allows separate control of particle growth in the individual flames up to the intersection or mixing point where the formation of heterostructures takes place. The effect of mixing of the aerosol streams on the process temperature and product characteristics is investigated based on the example of Pt on TiO2. Temperatures were determined by Fourier-transform infrared spectroscopy and thermocouple measurements along with computational fluid dynamics, while the degree of mixing was investigated based on surface area, Pt-dispersion measurements, and transmission electron microscopy image analyses. The quadrat method in combination with the variation coefficient was used to quantify the uniformity of the Pt cluster distribution on the TiO2 support. For high intersection distances of the two flame jets and small intersection angles, nonuniform mixing of the compounds and the formation of large Pt particles are observed. For small intersection distances and large angles, a uniform Pt distribution was achieved. Based on these findings, process design rules were established which can be transferred to other material systems.

Hydrogen Adsorption in Flame Synthesized and Lithium Intercalated Carbon Nanofibers—A Comparative Study

Carbon nanofibers (CNF) have been synthesized under partial combustion conditions in a flame reactor using different mixtures of hydrocarbon gases in the presence and absence of precursors. The hydrogen (H2) adsorption studies have been carried out using a high pressure Sievert's apparatus maintained at a constant temperature (24 degrees C). The flame synthesized CNFs showed high degree of H2 adsorption capacities at 100 atm pressure. The highest H2 capacities recorded have been 4.1 wt% [for CNF produced by liquefied petroleum gas (LPG)-Air (E-17)], 3.7 wt% [for nano carbons produced by Methane-Acetylene-Air (EMAC-4)] and 5.04 wt% for [Lithium intercalated sample (Li-EMAC-4)] respectively.

New Perspectives in Monitoring the Flame Synthesis of Iron Oxide Nanoparticles: Addressing Solid and Gas-Phase Diagnostics Challenges

ABSTRACTSeveral techniques for in-situ monitoring and characterization of flame synthesized nanoparticles are described with the goal of gaining further insight into the mechanisms governing nanoparticle (NP) formation in flame reactors. These include: a combined particle mass spectrometer - quartz crystal microbalance apparatus (PMS-QCM); The Light Induced Detuning –Quartz Crystal Microbalance (LID-QCM) method; Application of Intra Cavity Laser Absorption Spectroscopy (ICLAS) for monitoring gas phase intermediates in a particle laden environment.

Laser-based in situ measurement and simulation of gas-phase temperature and iron atom concentration in a pilot-plant nanoparticle synthesis reactor

A scaled-up flame reactor for nanoparticle synthesis was investigated through a combination of in-situ laser-induced fluorescence (LIF) measurements and computational fluid dynamics (CFD) simulations with detailed chemistry. Multi-line NO-LIF was used for imaging gas-temperature and Fe-LIF for measurement of iron atom concentration. Despite the challenging environment of production reactors in an industrial environment, various conditions for stable flames with different gas flows with and without adding Fe(CO)5 as precursor for the synthesis of iron-oxide nanoparticles were investigated. In contrast to previous measurements in laminar lab-scale flames, a second mechanism for forming iron oxide nanoparticles was found via intermediate formation of iron clusters and elemental iron particles in hot, oxygen-free gas streams followed by subsequent oxidation.

Numerical investigation of the process steps in a spray flame reactor for nanoparticle synthesis

The synthesis of titanium dioxide nanoparticles from titanium tetraisopropoxide (TTIP) in a nanoparticle spray flame reactor was investigated. The nanoparticle properties are affected by different processes: (a) the break-up of the liquid jet from the spray nozzle, (b) the combustion of the spray and in the pilot flame and (c) the formation and growth of the nanoparticles. The spray process of the injected liquid was analyzed by volume of fluid (VOF) calculations and validated by shadowgraphy imaging which provided the size distribution and the mean velocity of the droplets. The spray angle was determined by a side illuminated long exposure image of the spray. The resulting spray properties (droplet sizes, velocity, and spray angle) served as injector boundary conditions for the downstream combustion simulations. Spray and gas phase of the flame were simulated using an Euler–Lagrange approach, turbulence was modeled by the RNG k-epsilon model, and turbulent combustion was described as a partially stirred reactor (PaSR). For the formation and growth of the nanoparticles within the synthesis reactor, the population balance equation was solved coupled to the spray combustion using a monodisperse model. The findings from experiment and simulation are discussed in terms of flow, species, temperature, and nanoparticle formation inside the reactor. The effect of the spray droplet properties as droplet size, angle, mean velocity and the dispersion behavior on the nanoparticle synthesis process are investigated and discussed, confirming the observation that this type of spray reactor is a robust design overall.