Experimental study of the structure of laminar premixed flames of ethanol/methane/oxygen/argon

The structures of three laminar premixed stoichiometric flames at low pressure (6.7 kPa): a pure methane flame, a pure ethanol flame, and a methane flame doped by 30% of ethanol, have been investigated and compared. The results consist of mole fraction profiles of CH₄, C₂H₅OH, O₂, Ar, CO, CO₂, H₂O, H₂, C₂H₆, C₂H₄, C₂H₂, C₃H₈, C₃H₆, CH₃-C CH (propyne), CH₂ C CH₂ (allene), CH₂O, and CH₃HCO, measured as a function of the height above the burner by probe sampling followed by on-line gas chromatography analyses. Flame temperature profiles have been also obtained by using a PtRh thermocouple. The similarities and differences between the three flames have been analyzed. The results show that, in these three flames, the mole fraction of the intermediates with two carbon atoms is much larger than that of the species with three carbon atoms. In general, the mole fraction of all intermediate species in the pure ethanol flame is the largest, followed by the doped flame, and finally the pure methane flame.     

Laminar burning velocities and Markstein numbers of syngas-air mixtures

In the currently reported work, three typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas-air flames. The maximum value of syngas-air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas-air flames are generally unstable. Karlovitz numbers indicates that typical syngas-air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas-air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H₂/95% CO and proved to be similar to methane, although somewhat slower than propane.     

Probe sampling measurements and modeling of nitric oxide formation in ethane + air flames

Burning velocity and probe sampling measurements of the concentrations of O₂, CO₂, CO and NO in the post-flame zone of ethane + air flames are reported. The heat flux method was used for stabilization of laminar, premixed, non-stretched flames on a perforated plate burner at 1 atm. Axial profiles of the concentrations of the major species were used to assess interaction of the flame with the burner surface and conversion of the sampling gases in the probe. Tests performed with the probes of different inlet diameters showed negligible CO–CO₂ and NO–NO₂ conversion within the experimental accuracy. Two kinetic models, the GRI-mech. 3.0 and in-house modified detailed reaction mechanism, were tested. Both kinetic mechanisms accurately reproduce laminar burning velocities and concentrations of the major species, CO, CO₂ and O₂, in these flames. Numerical predictions of the concentrations of NO in a post-flame zone of lean and stoichiometric flames are in good agreement with experiment when downstream heat losses to the environment were taken into account. The GRI-mech. 3.0 over-predicts the [NO] by about 30 ppm at the equivalence ratio of 1.4. The predictions of the in-house mechanism in rich flames are closer to the experimental data with an under-prediction of about 15 ppm. The influence of the assumed temperature gradient downstream the flame front on the calculated flame structure was also assessed.     

Effect of the addition of triphenylphosphine oxide,hexabromocyclododecane, and ethyl bromide on a CH<Subscript>4</Subscript>/O<Subscript>2</Subscript>/N<Subscript>2</Subscript> flame at atmospheric pressure

The chemical and thermal structure of a Mache-Hebra burner stabilized premixed rich CH₄/O₂/N₂ flame with additives of vapors of triphenylphosphine oxide [(C₆H₅)₃PO], hexabromocyclododecane (C₁₂H₁₈Br₆), and ethyl bromide (C₂H₅Br) was studied experimentally using molecular beam mass spectrometry (MBMS) and a microthermocouple method. The concentration profiles of stable and active species, including atoms and free radicals, and flame temperature pro.les were determined at a pressure of 1 atm. A comparison of the experimental and modeling results on the flame structure shows that MBMS is a suitable method for studying the structure of flames stabilized on a Mache-Hebra burner under near-adiabatic conditions. The relative flame inhibition effectiveness of the added compounds is estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the flame propagation velocity. The results of the investigation suggest that place of action of the examined flame retardants is the gas phase. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 12–20, September–October, 2007.     

Experimental and numerical study of CH4/CH3Cl/O2/N2 premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH₃Cl addition on CH₄/O₂/N₂ premixed flames under oxygen enrichment. The laminar flame speeds of CH₄/CH₃Cl/O₂/N₂ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH₃Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH₃Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH₃Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H₂ → H + H₂O reaction is reduced because of the reduction of OH concentration. However, the function of CH₃Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 103–111, November–December, 2006.     

The stability of turbulent hydrogen jet flames with carbon dioxide and propane addition

In the study the lift-off, blow-out and blow-off stability limits of hydrogen/propane flames and hydrogen/carbon dioxide flames were tested in three different mixing arrangements. The first was to premix hydrogen with carbon dioxide or propane to form a jet flame. The second was to add the gas as an annular jet around the hydrogen flame. The third was to inject into the centre of the hydrogen flame. Propane and carbon dioxide have the same density but create very different chemical kinetic changes when added to hydrogen flames. The results showed that when premixed with hydrogen, propane is more effective in flame lift-off and blow-out. The analysis of kinetic mechanisms revealed that the propane is the dominating fuel in determining the burning rate of the hydrogen/propane while carbon dioxide mainly acted to dilute the hydrogen/CO₂ mixture. Comparing the three mixing arrangements, the experiments showed that hydrogen flame can be effectively lifted or blown out when gases were in annular flow around the hydrogen flame. The isothermal mixing process of the co-flow configuration was discussed.     

Fuel processing in direct propane solid oxide fuel cell and carbon dioxide reforming of propane over Ni-YSZ

This work is aimed at understanding the reaction mechanism of propane internal reforming in the solid oxide fuel cell (SOFC). This mechanism is important for the design and operation of SOFC internal processing of hydrocarbons. An anode-supported SOFC unit with Ni-YSZ anode operating at 800 °C was tested with direct feeding of 5% propane. CO₂ reforming of propane was carried out in a reactor with Ni-YSZ catalyst to simulate internal propane processing in SOFC. The performance of this direct propane SOFC is stable. The C specie formed over the anode functional layer of SOFC can be completely removed. The major gas products of SOFC are H₂, CO, CH₄, C₂H₄ and CO₂. Pseudo-steady-state internal processing of propane in the anode catalytic layer of SOFC is associated with a CO₂/C₃H₈ molar ratio of about 1.26 and basically CO₂ reforming of propane. CO₂ dissociation to produce the O species to oxidize the C species from dehydrogenation and dissociation of propane and its fragments should be the major reaction during CO₂ reforming of propane.     

Atomization and combustion of canola methyl ester biofuel spray

The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 μm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO₂ and O₂ concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames.     

NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H₂ as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H₂ and CH₄, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H₂/50%CO and 45%H₂/45%CO/10%CH₄ by volume, and three diluents, N₂, H₂O, and CO₂. The effectiveness of these diluents is characterized in terms of their ability to reduce NO_x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO_x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H₂/50%CO mixture, it is due to the prompt mechanism for the 45%H₂/45%CO/10%CH₄ mixture. For both mixtures, CO₂ and H₂O are more effective than N₂ in reducing NO_x in syngas flames. H₂O is the most effective diluent on a mass basis, while CO₂ is more effective than N₂. The effectiveness of H₂O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.     

Kinetics modelling of ethyl acetate oxidation in flame conditions

To optimize the good working of the common thermal oxidizers, a better understanding of the high temperature oxidation kinetic of volatile organic compounds (VOCs) in flame conditions is needed. So the experimental study of Ethyl Acetate (EA) oxidation in CH₄/EA/O₂/N₂ low pressure flames has been investigated. Molecular species concentration profiles of CH₄, O₂, CO, CO₂, H₂O, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, EA, CH₃OH, C₂H₅OH, CH₃CHO, C₂H₅CHO, CH₃COCH₃, CH₃OCH₃, CH₃COOCHCH₂ and CH₃COOH have been obtained by coupling microprobe sampling with gas chromatography-mass spectrometry (GC/MS) analysis. A detailed kinetic mechanism has been developed to model the EA oxidation in these conditions. The kinetic scheme includes 23 oxygenated species involved in 142 reversible reactions. It takes into account the first steps of the EA oxidation and the oxidation processes of all the measured oxygenated intermediate compounds. The proposed mechanism globally well predicts the experimental results obtained in the methane/air flames even if some discrepancies are pointed out. Sensitivity analysis allows the determination of the main reactional pathways involved in the thermal degradation of EA.     

Comparative study of the influence of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O on the chemical structure of lean and rich methane-air flames at atmospheric pressure

A comparative study of the influence of CO₂ and H₂O on both lean and rich CH₄-air laminar flames is performed. Six premixed flames are stabilized on a flat flame burner at atmospheric pressure: lean (with the equivalence ratio maintained constant at ? = 0.7) and rich (with the equivalence ratio maintained constant at ? = 1.4) CH₄-air, CH₄-CO₂-air, and CH₄-H₂O-air flames. These flames are studied experimentally and numerically. The [CO₂]/[CH₄] and [H₂O]/[CH₄] ratios are kept equal to 0.4 for both flames series. Species mole fraction profiles are measured by gas chromatography and Fourier transform infrared spectroscopy analyses of gas samples withdrawn along the vertical axis by a quartz microprobe. Flames structures are computed by using the ChemkinII/Premix code. Four detailed combustion mechanisms are used to calculate the laminar flame velocities and species mole fraction profiles: GRI-Mech 3.0, Dagaut, UCSD, and GDFkin®3.0.     

Waste water combustion in a confined swirl flame

Water with dissolved N-compounds ammonia, aminoethanol and ε-caprolactam (NH₃, C₂H₇NO and C₆H₁₁NO) was sprayed into a confined swirl flame operating under various conditions. The model waste water and the atomizing fluid influence the visible flame structure. Droplet evaporatio takes place in the recirculation zone of the flow. Measurements of the NO concentrations admit an evaluation of the conversion efficiency of the water dissolved N compounds under the present conditions. It can be shown that Fenimore's universal curve of fuel-N conversion in premixed ideal flames is applicable to describe and predict the NO formation in this very complex technical combustion systems.     

Investigation of the effect of ethanol additives on the structure of low-pressure ethylene flames by photoionization mass spectrometry

Molecular beam mass spectrometry with photoionizationby synchrotron radiation was used to study the structure of flat premixed flames of C₂H₄/O₂/Ar and C₂H₄/C₂H₅OH/O₂/Ar at a pressure of 30 torr (4 kPa) with equivalence ratios ϕ = 1 and 2. It was shown that the replacement of part of ethylene with ethanol in the starting mixture lowered the concentration of a number of intermediate flame species, including soot precursors C₃H₃ and C₆H₆. These data are important for the development, analysis, and further improvement of detailed kinetic mechanisms for combustion of mixtures of hydrocarbon and oxygenated fuels.     

Exploring formation pathways of aromatic compounds in laboratory-based model flames of aliphatic fuels

This presentation summarizes our recent experimental and flame modeling studies focusing on understanding of the formation of small aromatic species, which potentially grow to polycyclic aromatic hydrocarbons (PAHs) and soot. In particular, we study premixed flames, which are stabilized on a flat-flame burner under a reduced pressure of ≈15–30 torr, to unravel the important chemical pathways to aromatics formation in flames fueled by small C₃–C₆ hydrocarbons. Flames of allene, propyne, 1,3-butadiene, cyclopentene, and C₆H₁₂ isomers 1-hexene, cyclohexane, 3,3-dimethyl-1-butene, and methylcyclopentane are analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry. Isomer-specific experimental data and detailed modeling results reveal the dominant fuel-destruction pathways and the influence of different fuel structures on the formation of aromatic compounds and their commonly considered precursors. As a specific aspect, the role of resonance-stabilized free radical reactions is addressed for this large number of similar flames of structurally different fuels. While propargyl and allyl radicals dominate aromatics formation in most flames, contributions from reactions involving other resonance-stabilized radicals like i-C₄H₅ and C₅H₅ are revealed in flames of 1,3-butadiene, 3,3-dimethyl-1-butene, and methylcyclopentane. Dehydrogenation processes of the fuel are found to be important benzene formation steps in the cyclohexane flame and are likely to also contribute in methylcyclopentane flames.     

Effect of ethanol on the chemistry of formation of precursors of polyaromatic hydrocarbons in a fuel-rich ethylene flame at atmospheric pressure

The effect of the addition of ethanol (EtOH) to the initial combustible mixture on the concentration of various compounds, in particular, those preceding the formation of polyaromatic hydrocarbons in a fuel-rich (equivalence ratio of fuel ? = 1.7) flat premixed ethylene/oxygen/argon flame at atmospheric pressure was studied experimentally and by numerical modeling using a detailed mechanism of chemical reactions. Concentrations of various stable and labile species, including reactants, major combustion products, and intermediates in C₂H₄/O₂/Ar and C₂H₄/EtOH/O₂/Ar flames were measured along the height above the burner using molecular beam mass spectrometry. Experimental mole fraction profiles were compared with those calculated using the previously proposed mechanisms of chemical reactions. This mechanism was analyzed to determine the cause of the ethanol effect on the flame concentration of propargyl, the main precursor of polyaromatic hydrocarbons.     

Experimental and modeling study of a lean premixed iso-butene/hydrogen/oxygen/argon flame

The experimental structure of a lean iso-butene/hydrogen/oxygen/argon flame (2.7% iC₄H₈, 4.5% H₂, 83.0% O₂, 9.8% Ar, ? = 0.225) has been determined by molecular beam mass spectrometry at low pressure (40 mbar). The detected species throughout the flame thickness were: H₂, CH₃, O, OH, H₂O, C₂H₂, CO, C₂H₄, CH₂O, O₂, HO₂, Ar, C₃H₄, C₃H₆, CO₂, C₂H₄O, C₄H₆, iC₄H₈, C₃H₆O, C₄H₆O and C₄H₈O. An original model, validated against premixed rich C₂H₄, has been extended by building a sub-mechanism taking into account the formation and the consumption of species involved in iso-butene combustion. This mechanism contains 520 reactions and 99 chemical species. A good agreement appears between calculated mole fraction profiles predicted by this mechanism, compared to experimental results.     

Structure and extinction of a counterflow partially premixed,diffusion flame

The fundamental heat and mass transport processes in a partially premixed, diffusion flame stabilized between counterflowing streams of fuel A, and fuel B premixed with an oxidizer C and an inert gas are analysed. The gas phase chemical reaction between fuel A and the oxidizer C and between fuel B and the oxidizer C is approximated as a one step process. Asymptotic analysis is performed in the limit of a large value for the ratio of the activation energy characterizing the chemical reactions to the thermal energy in the flame. Guided by experimental results it is presumed that two distinct, thin reaction zones are present, a premixed flame and a diffusion flame. The outer structure and the inner structure of the reaction zones are analysed. It is shown that for the flame to extinguish the reaction zones must merge. The outer structure and inner structure of the merged reaction zone is analysed. An explicit algebraic relation is obtained relating the Damköhler number at extinction to the ambient conditions in the counterflowing streams, thermophysical properties of the reactants, and the overall chemical kinetic rate parameters characterizing the gas phase oxidation of the fuels.The results of the analysis are then extended to a merged flame that is stabilized in a stagnation point boundary layer over the surface of a liquid fuel when a premixed stream of gaseous fuel and oxidizer flows over its surface. To test the predictions of the theory, extinction experiments are performed on a partially premixed, diffusion flame stabilized between a vaporizing surface of heptane and a gaseous stream consisting of methane, oxygen and nitrogen. The results are used to deduce the overall chemical kinetic rate parameters characterizing the gas phase oxidation of methane in a premixed flame.     

Analysis of flame structure by molecular-beam mass spectrometry using electron-impact and synchrotron-photon ionization

Molecular-beam mass spectrometry (MBMS) has proven to be a powerful tool for the general analysis of flame structure, providing concentrations of radical and stable species for low-pressure flat flames since the work of Homann and Wagner in the 1960’s. In this paper, we will describe complementary measurements using electron-impact ionization with a high-mass-resolution quadrupole mass spectrometer and vacuum-ultraviolet photoionization in a time-of-flight mass spectrometer. Isomers are resolved that have not been separately detectable before in MBMS studies of flames, including C₃H₂, C₃H₄, C₄H₃, C₄H₄, C₄H₅, C₆H₆, and C₂H₄O. The qualitative and quantitative results of MBMS have led to advances in modeling and applying flame chemistry. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 58–63, November–December, 2006.     

Study on flame structures and emissions of CO and NO in Various CH<Subscript>4</Subscript>/O<Subscript>2</Subscript>/N<Subscript>2</Subscript>–O<Subscript>2</Subscript>/N<Subscript>2</Subscript> counterflow premixed flames

An oxygen-diluted partially premixed/oxygen-enriched supplemental combustion (ODPP/OESC) counterflow flame is studied in this paper. Flame images are obtained through experiments and numerical simulations with the GRI-Mech 3.0 chemistry. The oxygen dilution effects are revealed by comparing the flame structures and emissions with those of a premixed flame and partially premixed flame (PPF) at the same equivalence ratio (?_Σ = 0.95 and ? _f = 1.4). The results show that both PPF and ODPP/OESC flames have distinct double flame structures; however, the location of the premixed combustion zone and the distance between premixed/nonpremixed combustion zone are significantly different for these two cases. For the ODPP/OESC flame, the temperature in the premixed combustion zone is lower and the premixed zone itself is located farther downstream from the fuel nozzle, which leads to reduction of NO and CO emissions, as compared to those of the PPF. Therefore, by adjusting the distribution of the oxygen concentration in the premixed and nonpremixed combustion zones, the ODPP/OESC can effectively balance the chemical reaction rate in the entire combustion zone and, consequently, reduce emissions.     

The effects of bimetallic interactions for CO2-assisted oxidative dehydrogenation and dry reforming of propane

The catalytic reduction of CO₂ by propane may occur via dry reforming to produce syngas (CO + H₂) or oxidative dehydrogenation to yield propylene. Utilizing propane and CO₂ as coreactants presents several advantages over conventional methane dry reforming or direct propane dehydrogenation, including lower operating temperatures and less coke formation. Thus, it is of great interest to identify catalytic systems that can either effectively break the C C bond to generate syngas or selectively break C H bonds to produce propylene. In this study, several precious and nonprecious bimetallic catalysts supported on reducible CeO₂ were investigated using flow reactor studies at 823 K to identify selective catalysts for CO₂-assisted reforming and dehydrogenation of propane.