CH4 O2 Research Articles

Numerical study on flame structure and NO formation in CH4-O2-N2 counterflow diffusion flame diluted with H2O

SUMMARY Numerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H2O on either fuel- or oxidizer-side in CH4–O2–N2 counterflow diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties of added H2O, is introduced to feasibly isolate the chemical effects. Special concern is focused on the important role of remarkably produced OH radicals due to chemical effects of added H2O on flame structure and NO emission. The reason why the difference of behaviours between the principal chain branching reaction rate and flame temperature appear is attributed to the drastic change of reaction step (R120) from the production to the consumption of OH. It is also, however, seen that the most important contribution of produced OH due to chemical effects of added H2O is through reaction step (R127). The importantly contributing reaction steps to NO production are also examined. The production rates of thermal NO and prompt NO are suppressed by chemical effects of added H2O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed. Copyright # 2004 John Wiley & Sons, Ltd.

Possibility of Initiation of Combustion of CH4–O2(Air) Mixtures with Laser-Induced Excitation of O2Molecules

The possibility of initiation of combustion of CH4–O2 mixtures with excitation of O2 molecules to the states a1Δg and b1Σ+ g by laser radiation with wavelengths of 1.268 μm and 762 nm is considered. It is shown that excitation of O2 molecules significantly decreases the induction period and ignition temperature because of accelerated formation of active atoms and radicals and intensification of the chain mechanism of the process. Even if the specific energy of laser radiation absorbed by the gas is low (≈ 0.1 eV per molecule), the ignition temperature of the CH4–O2 mixture with a ratio of 1:2 can be reduced from 1000 to 300 K.

NO x -Catalyzed Gas-Phase Activation of Methane: In Situ IR and Mechanistic Studies

The products of reactions occurring in a CH4–O2–NO x mixture have been investigated by in situ FTIR spectroscopy. The results show that low temperatures favor the formation of HCHO and CH3OH, while high temperatures favor that of C2H4. Possible reaction mechanisms based on the in situ observations are briefly discussed.

Natural nuclear fission reactors: time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere

Abstract Knowledge of the formation conditions of Francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 Ma ago. The model explaining the formation of the Oklo uranium deposits suggests that at the time of sediment deposition in the Franceville basin 2150 million years ago, the oxygen deficient atmosphere would have inhibited uranium dissolution. Dissolution of uranium was only possible during later diagenesis, approximately 1950 Ma. Reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to U4+, forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions. Such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a ‘transition phase’ some 2450–2100 Ma ago. The manganese deposits were formed before the uranium deposits, during the deposition of the black shales and very early diagenesis, and thus at a time when oxygen content in atmosphere was very low. Carbon isotopes data of organic matter show decrease of δ13C upward in the Francevillian series (−20 to −46% PDB) reflecting the high CH4 and low O2 contents in the atmosphere during sediment deposition. This favoured anoxic conditions during deposition of the basinal FB black shales and likewise the migration of Mn over long distances. The manganese precipitated first as Mn-oxides at the shallow edges of the Franceville basin, in photic zones, where photosynthetic organisms flourished. Mn-oxides were then reduced in the black shales forming Mn-carbonates when conditions became more reducing during transgression episodes and/or the first stages of burial. In the black shales, reducing conditions prevailed until recent weathering, allowing the good preservation of organic matter and the Mn deposits. The present-day alteration is responsible for the dissolution of Mn-carbonates and precipitation of Mn-oxides at the water table to form the high grade Mn ore (45–50% Mn). Development of photosynthesizing organisms, a volcanic source of the Mn, and favourable palaeogeography of the Francevillian basins are all important parameters for the formation of the Mn deposits. For the occurrence of the natural nuclear reactors, the age of 2.0 Ga is the main parameter that controls the abundance of fissile 235U and the critical mass. Before 2.0 Ga the 235U/238U ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved. Then, after 2.0 Ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235U/238U ratio was too low to support criticality.

In Situ Fourier Transform Infrared Study of the Selective Reduction of NO with Propene over Ga2O3–Al2O3

The selective reduction of NO with propene has been investigated on Al2O3 and Ga2O3–Al2O3, using in situ diffuse reflectance Fourier transform infrared (FT–IR) spectroscopy combined with on-line mass spectrometry and IR gas analysis. During the NO–C3H6–O2 reaction, the main surface species detectable by IR were adsorbed nitrate, acetate, formate, cyanide (–CN), and isocyanate (–NCO). Ga2O3 was found to promote the formation of these surface species, especially nitrate, –CN, and –NCO. From the experiments focusing on the reactivity of surface NOx(ads) (nitrite and nitrate) species, we found a very high reactivity of those compounds toward C3H6, leading to the rapid formation of –CN and –NCO species. On the other hand, acetate and formate species were deduced to be spectator species in this reaction. An appearance of ν(NH) bands ascribed to the formation of the surface complexes including amido groups (–NH complexes) was observed simultaneously with a decrease of the –NCO band, suggesting that the –NCO species is hydrolyzed to the –NH complexes by the reaction with some traces of water. A reaction mechanism has been proposed: NOx(ads) species formed by NO–O2 reaction on the catalyst surface react with C3H6-derived species to generate the –NCO species via acrylic species and organic nitro compounds, and then the surface –NH complexes generated by hydrolysis of the –NCO species react with NOx(ads) species or NO2 to produce N2.

NO reburning study based on species quantification obtained by coupling LIF and cavity ring-down spectroscopy.

NO reburning is studied in a low pressure (15 hPa) premixed flame of CH4-O2 seeded with 1.8% of NO. Measurements were carried out by using cavity ring-down spectroscopy (CRDS) and laser induced fluorescence (LIF) techniques. The temperature profile was obtained by OH-LIF thermometry in the A-X (0-0) band. The OH profile was determined by LIF and calibrated by single pass absorption. The NO concentration profile was obtained by LIF in the A-X (0-0) band and corrected for Boltzmann fraction and quantum yield variations. The absolute concentration profile was determined in the burned gases by CRDS allowing a direct experimental determination of the NO reburning amount. Finally CH and CN mole fraction profiles were obtained by CRDS by exciting rotational transitions in the B-X (0-0) bands of CH and CN around 387 nm. We found a peak mole fraction of 29 ppm for CH and 3.3 ppm for CN. This last result is in contrast with a previous study of W. Juchmann, H. Latzel, D. L. Shin, G. Peiter, T. Dreier, H. R. Volpp, J. Wolfrum, R. P. Lindstedt and K. M. Leung, XXVIIth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1998, p. 469, performed in a similar flame, which reported much lower levels of CN. In that study the absolute concentration of CN was indirectly obtained by LIF calibrated by Rayleigh scattering. In a second part, experimental species profiles are compared with predictions of the GRI 3.0 mechanism. Comparison between experimental and predicted profiles shows a good agreement particularly for CN and NO species. A qualitative analysis of NO reburning is then performed.

Improvement of the Catalytic Performance of Pd/WO3/ZrO2 in the Selective NO–CH4–O2 Reaction by the Addition of Water Vapor

Abstract The Pd/WO3/ZrO2 catalyst exhibited activity for the NO–CH4–O2 reaction in the presence of water vapor when WO3 monolayer covered the ZrO2 surface. The selectivity of methane consumption for NO reduction was considerably improved by the addition of 10% of water vapor.

Charge-transfer mediated photochemistry in alkene–O2 complexes

The photochemistry of a series of alkene–O2 complexes was studied in a supersonic expansion using a resonance enhanced multiphoton ionization probe of the O(3Pj) photoproduct at 226 nm. The relative yield of oxygen atoms from each complex was correlated to the ionization potential of the alkene species and indicates that initial excitation of an intermolecular charge-transfer state mediates the subsequent excited state chemistry. The behavior is similar to that observed previously for the C6H6–I2 system: a reverse electron-transfer step yields electronically excited O2 which subsequently dissociates. The kinetic energy release of the O(3Pj) fragment was also measured using a time-of-flight analysis and found to be small with an isotropic spatial distribution. No evidence for photo-oxidation of the alkenes was observed in the mass spectra. A comparison is made to the charge-transfer absorption spectra observed in cryogenic oxygen matrices of similar alkene complexes. Ab initio models were used to identify the stable ground state geometry of the C2H4–O2 complex and complete active space self-consistent-field calculations were performed to identify the energy of the charge-transfer state for several alkene–O2 complexes.

Enhancement of Methanol Selectivity in the Products of Direct Selective Oxidation of Methane in CH4–O2–NO with Cu–ZnO/Al2O3

Enhancement of methanol selectivity in the products of the direct selective oxidation of methane with CH4–O2–NO in a gas-phase reaction was examined using a Cu–ZnO/Al2O3 catalyst. Three distinct reaction paths over the Cu–ZnO/Al2O3 catalyst were detected in the gas-phase selective oxidation of methane in CH4–O2–NO. The formation of CH3OH from CH2O–H2 and the water–gas shift reaction of CO–H2O progressed chiefly at around 250°C over Cu–ZnO/Al2O3 catalyst. The steam reforming reaction of CH3OH progressed over the same Cu–ZnO/Al2O3 catalyst at around 350°C and higher. Both CH3OH and CH2O were observed as C1-oxygenates at 550°C in the gas-phase selective oxidation of methane in CH4–O2–NO, but only CH3OH was observed as a C1-oxygenate in the presence of Cu–ZnO/Al2O3 catalyst in addition to the gas-phase selective oxidation of methane. The complete exhaustion of oxygen in the gas-phase selective oxidation of methane in CH4–O2–NO was a key to the effective use of Cu–ZnO/Al2O3 catalyst. Of the two reactions, CH3OH formation and water–gas shift over Cu–ZnO catalyst, the water–gas shift reaction progressed more over the catalyst with a higher surface area and with a lower surface Cu/Zn atomic ratio.

State of Pt/SiO2 Catalysts After Reaction with a Lean NO–C3H6–O2 Mixture

During the selective reduction of NOx under lean-burn conditions with Pt/SiO2 solids, a particle size dependency has previously been observed. Furthermore, under the reactant mixture up to 773 K, the Pt particles sinter at the same time as the solids are activated. In fact, the size dependency is linked to the strength of the Pt–O bond and to the ease of NO dissociation.

Effect of Ba Addition on Catalytic Activity of Pt and Rh Catalysts Loaded on γ-Alumina

The catalytic activity of Pt catalyst loaded on γ-alumina was improved by Ba addition in simulated automotive exhaust gases. On the other hand, the result of Rh catalyst was the opposite. From the results of the partial reaction orders in C3H6–O2 reaction and TPR, it was concluded that the Ba addition to Pt catalyst suppressed the hydrocarbon chemisorption on the Pt catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, Ba addition to Rh catalyst caused such a strong oxygen adsorption on Rh that rejected the hydrocarbon adsorption and suppressed the reaction.

The enhancement effects of added methanol on methane-selective oxidation in the gas phase reaction of CH4–O2–NO2

The reactivity of the selective oxidation of methane in the gas phase reaction of CH4–O2–NO2 was enhanced by the addition of a small amount of methanol. The selectivity of methanol produced at the same level of methane conversion was enhanced by adding a small amount of methanol in the feed gas. The enhanced effect of methanol addition was restricted to methane conversion in the presence of NO2. The enhancement effect was obtained more effectively when the ratio of methanol to NO2 was near to 1, at which higher concentration of both methanol and NO2 a significant enhancement was shown. Theoretical calculations concluded as follows: CH3OH + NO2→CH2OH + HNO2 was the predominant step of hydrogen abstraction from methanol. HO2 was then produced by CH2OH + O2→CH2O + HO2. CH4 + HO2→CH3 + H2O2 was the key reaction to enhance the reactivity of methane. HO2 + OH→H2O + O2 brought about an OH radical dissipation reaction to stabilize the produced methanol from the following decomposition reaction: CH3OH + OH→CH2OH + HO2 .

Enhancement effects of methanol on the reactivity for methane partial oxidation in the gas phase reaction of CH4–O2–NO2

The partial oxidation of methane in the gas phase reaction of CH4–O2–NO2 was enhanced with the addition of a small amount of methanol; the selectivity of methanol at the same level of CH4 conversion was enhanced in the presence of methanol which showed this effect exclusively in the presence of NO2.

In Situ FTIR Studies of the Selective Catalytic Reduction of NO by C3H6 over Pt/Al2O3

The selective catalytic reduction (SCR) of nitric oxide (NO) by propylene (C3H6) over a 0.8 wt% Pt/Al2O3 catalyst was studied by the use of in situ Fourier transform infrared (FTIR) spectroscopy. Spectra of both the Pt/Al2O3 catalyst as well as the bare Al2O3 support were collected during the course of this study. NO adsorption resulted in the formation of nitrate species associated with the Al2O3 support, as well as surface NO species associated with Pt. Similarly, C3H6 adsorption resulted in the formation of carboxylate species associated with Al2O3, as well as surface CO associated with Pt. Finally, experiments conducted with the reactive NO–C3H6–O2 mixture resulted in the formation of surface cyanide (–CN) and isocyanate (–NCO) species. The origin and reactivity of these species is discussed in view of their potential involvement as intermediates in the hydrocarbon–SCR reaction.

Laboratory and modelling studies of tropospheric multiphase conversions involving some C1 and C2 peroxyl radicals

Abstract T-dependent rate constants for the self-reaction of selected peroxyl radicals in aqueous solution have been determined. For C2H5O2·, HOCH2O2· and C2H4(OH)O2· negative activation energies have been found. The redox potential of CH3O2·, HOCH2O2·, ·O2CH2COOH can be estimated to E0

Suppression of surface cracks on (111) homoepitaxial diamond through impurity limitation by oxygen addition

The use of oxygen in improving diamond quality has been investigated by comparing two (111) homoepitaxial diamond films deposited with H2–CH4 and H2–CH4–O2 mixtures by microwave assisted chemical vapor deposition. The (111) diamond deposited using a H2–CH4 mixture showed surface cracks due to the presence of nondiamond phases as well as a significant amount of hydrogen and silicon impurities. The (111) diamond deposited using a H2–CH4–O2 mixture showed an absence of hydrogen and silicon impurities and nondiamond phases, and exhibited a flat surface. The addition of oxygen is one of the suitable methods to produce high-quality (111) homoepitaxial diamond.

Competing Surface Oxidation Reactions During Diamond Synthesis in Low Pressure Flames

Carbon oxidation reactions affecting diamond growth in low pressure premixed flames are addressed. Modifications are made to the relatively simple yet robust Harris growth mechanism [S. J. Harris: Appl. Phys. Lett. 56 (1990) 2298], to better capture anomalous behavior seen in past experimental studies of films deposited from low pressure acetylene-oxygen (C2H2–O2), ethylene-oxygen (C2H4–O2), and methane-oxygen (CH4–O2) flames. With surface reactions involving oxygen, detailed simulations are shown to better predict the observed growth rates, generally to within a factor of two for C2H2–O2 and C2H4–O2 flames. More importantly, the simulations with oxidation reactions capture the sharp onset of growth as reactant mixture compositions are varied towards rich flame conditions. The model is less successful at capturing the growth rates seen in CH4–O2 flames. The diamond synthesized in CH4–O2 is seen to be more defective, with a seemingly greater fraction of sp2 bonds, the presence of which are not accounted for in the simple growth mechanism employed in this study.

Temperature measurements in low-pressure, diamond-forming, premixed flames

Measurements of temperature in low-pressure, diamond-forming, premixed flames are described. Optical emission spectroscopy was used to determine the rotational temperature of CH radicals in the flame by analyzing the spectra of the R-branch of the A 2Δ→X 2Π(0,0) CH electronic transition near 430 nm. Emission spectroscopic measurements were taken in stagnation-point flames of three different fuel types; CH4–O2, C2H4–O2, and C2H2–O2 flames, all of which have been previously used in the synthesis of diamond films. The emission-based rotational temperatures are found to be in good agreement with temperature measurements from thermocouples for the CH4–O2 flame, when a correction is made to the thermocouple measurements for radiation losses. The measurements show that peak flame temperatures are higher than the adiabatic flame temperatures for both C2H4–O2, and C2H2–O2 flames; a result that is also predicted by a numerical simulation of the stagnation-point strained flame that accounts for detailed finite-rate gas-phase chemistry and heterogeneous surface reactions.

Reactivity of surface nitrate species in the selective reduction of NO with propene over Na[ndash ]H-mordenite as investigated by dynamic FTIR spectroscopy

The reactivity of surface adsorbed species over Na–H-mordenite has been examined by using dynamic insitu IR spectroscopy and the mechanism of the selective reduction of NO with C3H6 discussed. When Na–H-mordenite was exposed to a flow of NO–C3H6–O2 at 573 K, a strong absorption band assignable to NO3− was observed at 1394 cm−1. The intensity of the band significantly decreased when the flowing gas was switched to C3H6–O2. The rate of decrease in NO3− was ca. 3.0 × 10−8 mol g−1 s−1 which was equivalent to the rate of NO conversion to N2 in the NO–C3H6–O2 flow reaction. Thus, it was indicated that NO3− is a reaction intermediate for the formation of N2 and the surface reaction of NO3− is the rate determining step of the formation of N2 in the selective reduction of NO with C3H6. When the flowing gas was switched to C3H6, the rate of decrease in NO3− species was twice as fast as that in C3H6–O2, i.e., the reactivity of NO3− species strongly depended on the presence of O2. Since NO was also formed simultaneously with N2 in the case of NO2–C3H6 reaction, it was suggested that NO3− species oxidize C3H6 accompanying not only the conversion into N2 but also the dissociative decomposition into NO. Weak bands assignable to adsorbed hydrocarbon, carbonyl, nitrite and isocyanate were also observed. On the basis of the changes in the intensity of these bands, the consecutive reaction of surface organic species with NO3− species over Na–H-mordenite was suggested.

Direct methanol synthesis using non-thermal pulsed plasma generated by a solid state pulse generator

Conversion of hydrocarbon fuels to methanol will promote their efficient utilization since methanol can easily be converted to hydrogen using low temperature heat energy, which would otherwise be disposed. Conventional process for methanol synthesis requires large amount of energy, therefore it can not be applied in the energy utilization system. The non-thermal plasma process can induce chemical reactions that do not occur under usual condition, and attempts have been made to apply a pulsed discharge plasma for direct methanol synthesis. A solid state pulse generator has been developed to increase frequency of pulse. A pulsed voltage of 1 ∼ 2 μ s rise time and 4 ∼ 5 μ s pulse width can be generated with frequency of 10 kHz. It was found that voltage and current characteristics was different with type of the reactor. The pulse generator was used for the methanol synthesis. Methanol with concentration of up to 0.7% could be obtained at 5 kHz pulse frequency and 6.7 sec gas residence time, when CH4 O2 (96:4) mixture was used at room temperature.