Mechanism Of Methane Oxidation Research Articles

Mechanism for catalytic partial oxidation of methane to syngas over a Ni/Al 2O 3 catalyst

Mechanism for catalytic partial oxidation of methane to syngas over a Ni/Al 2O 3 catalyst

Geochemical characteristics of the carbonate constructions formed during microbial oxidation of methane under anaerobic conditions

The aragonite constructions of the Black Sea are formed in a stable anaerobic zone and are a perfect object to study the natural mechanism of anaerobic methane oxidation. The most probable pathway of methane oxidation is its methanogen-mediated reaction with bicarbonates, dissolved in seawater, with the formation of water and acetate, which is then consumed by other components of the anaerobic community. Comparison of the delta 13C values of carbonate minerals and organic matter once more demonstrated that the formation of the organic matter of biomass is accompanied by intense fractionation of carbon isotopes, as a result of which the total organic matter of biomass acquires an extremely light isotopic composition, characterized by delta 13C values as low as -83.8@1000.

Oxygen Isotope Effects in the Oxidation of Methane to Carbon Monoxide

A significant fraction (estimated to be as large as 30%) of the carbon monoxide in the atmosphere is produced by the oxidation of methane. One of the approaches that has been used to determine this fraction is an inversion of observed carbon and oxygen isotopic abundance ratios, 13C/12C and 18O/16O, together with estimates of the magnitudes of other sources of carbon monoxide, and isotope effects in each source. For this purpose, values of the kinetic isotope effects in the methane oxidation are required, and although these have been determined experimentally for 13C, they have not been measured for 18O. This article examines the kinetic mechanism of methane oxidation and shows that an oxygen isotope effect could take place in the addition reaction of methyl radicals with molecular oxygen to form methylperoxy. Subsequent reactions in the complicated mechanism for methane oxidation do not lead to isotopic fractionation of oxygen. The expected kinetic isotope effect is calculated using variational transitio...

Hydrogen production by methanogens under low-hydrogen conditions.

Hydrogen production was studied in four species of methanogens (Methanothermobacter marburgensis, Methanosaeta thermophila, Methanosarcina barkeri, and Methanosaeta concilii) under conditions of low (sub-nanomolar) ambient hydrogen concentration using a specially designed culture apparatus. Transient hydrogen production was observed and quantified for each species studied. Methane was excluded as the electron source, as was all organic material added during growth of the cultures (acetate, yeast extract, peptone). Hydrogen production showed a strong temperature dependence, and production ceased at temperatures below the growth range of the organisms. Addition of polysulfides to the cultures greatly decreased hydrogen production. The addition of bromoethanesulfonic acid had little influence on hydrogen production. These experiments demonstrate that some methanogens produce excess reducing equivalents during growth and convert them to hydrogen when the ambient hydrogen concentration becomes low. The lack of sustained hydrogen production by the cultures in the presence of methane provides evidence against "reverse methanogenesis" as the mechanism for anaerobic methane oxidation.

Oxidative catalytic removal of hydrocarbons over Pt/Al 2O 3 catalysts

The reaction kinetics, structure sensitivity, and in situ activation of cyclopentane and methane combustions were studied on Pt/Al 2O 3 catalysts of different platinum and chlorine loadings. The catalyst activities were evaluated through both conversion vs. temperature (light-off curves) and conversion vs. time catalytic tests. Cyclopentane oxidation turnover rates (TOF) increased dramatically with increasing Pt crystallite size while TOF values for methane oxidation increased only three times by diminishing the Pt dispersion from 65 to 15%. The reaction orders in oxygen were one (cyclopentane) and zero (CH 4). For both reactions, the orders and activation energies did not change by changing the Pt dispersion. Results are interpreted in basis of two different reaction mechanisms over the metallic Pt active sites. Cyclopentane oxidation proceeds via a surface redox mechanism, being the dissociative adsorption of oxygen the rate-determining step. The observed turnover rate increase with increasing Pt particle size reflects an increase in the density of reactive Pt–O species resulting from higher Pt oxidation rates. The methane oxidation mechanism is interpreted in terms of Mars–van Kravelen reduction–oxidation pathways which include the abstraction of the first hydrogen on the adsorbed methane molecule as the rate-determining step. Low-conversion catalytic tests performed at constant temperature showed that on well-dispersed Pt/Al 2O 3 catalysts the cyclopentane conversion increases with time on stream, while the methane activity decreases. Activating induction periods during the oxidation of cyclopentane are related to the sintering of the metallic phase in reaction conditions. Hot-spots on the metallic particles together with the presence of gaseous water cause the formation of larger, more reactive, Pt crystallites, even at mild reaction conditions. The activation phenomenon ab initio of the reaction is not verified for methane oxidation on Pt/Al 2O 3 catalysts. The different structure sensitivity of the slowest steps in the reaction–oxidation mechanisms explains the existence of induction periods on well-dispersed Pt catalysts only for cyclopentane oxidation.

Further Validation of an Augmented Reduced Mechanism for Methane Oxidation: Comparison of Global Parameters and Detailed Structure

An augmented reduced mechanism (ARM) for methane oxidation, consisting of 16 species and 12 lumped reaction steps, was developed from GRI-Mech 1.2. It showed excellent performance in reproducing the global response of a wide varieties of combustion phenomena including auto-ignition, laminar flame propagation, and counterflowing non-premixed systems under extensive thermody-namical parametric variations including those of temperature, pressure, and composition. The present study extends the rigor of validation of this reduced mechanism to the next level by showing that good reproduction still exists for the detailed evolutionary and spatial structures of these phenomena. Validation comparisons have also been conducted for the two skeletal mechanisms developed by Kazakov and Frenklach, showing that DRM-22, which consists of 23 species and 102 elementary steps, appears to be the minimum size for a skeletal mechanism to approach the comprehensiveness of the 12-stepARM.

Reaction Mechanism of Oxidation of Methane with Hydrogen Peroxide Catalyzed by 11-Molybdo-1-vanadophosphoric Acid Catalyst Precursor

The reaction mechanism for the selective oxidation of methane with hydrogen peroxide was investigated for a H4PV1Mo11O40 catalyst precursor, which has been reported to be the most active among vari...

Mechanism for catalytic partial oxidation of methane to syngas over a Ni/Al 2O 3 catalyst

The mechanism of catalytic partial oxidation of methane to syngas (POM) over a Ni/α-A1 2O 3 catalyst was studied by using a pulse reactor and temperature-programmed surface reaction (TPSR) techniques. Over a reduced nickel catalyst (Ni 0/A1 2O 3), methane activation follows the dissociation mechanism; while on oxidic nickel catalysts (NiO/Al 2O 3), methane is first oxidized to carbon dioxide and water, and simultaneously, NiO is reduced to Ni 0. CH 4 dissociation occurs over Ni 0 active sites, generating hydrogen and surface C species. A transient process was observed during the CH 4/O 2 reaction. The nickel valence was transformed from NiO to Ni 0 at a certain critical temperature and simultaneously, the reaction was transformed rapidly from the complete oxidation of methane to the partial oxidation of methane. It has been found that the POM reaction takes place over a thin layer of the catalyst bed. This reaction zone is nearly isothermal, over which almost 100% of oxygen and more than 90% of methane are converted. The temperature drop in the downstream of the catalyst bed does not imply that the steam or carbon dioxide reforming reaction occurs in the lower part of the bed. Ni 0 species constitute the active sites for the partial oxidation of methane to syngas. Both methane and oxygen are activated on Ni 0 sites, generating surface Ni⋯C and Ni δ+ ⋯O δ− species. These two kinds of intermediates have been proposed to account for the mechanism of methane partial oxidation. The Ni δ+ ⋯O δ− species over Ni 0 catalyst surface is considered to be a kind of weakly bounded, mobile oxygen species. The reaction between Ni δ+ ⋯O δ− and Ni⋯C intermediates generate the primary product of CO. However, the presence of NiO over the catalyst surface significantly reduces the CO selectivity. Thus, the NiO species are not possible to be the intermediate for the POM reaction. The mechanism of partial oxidation of methane should follow the direct oxidation route.

Modelling steady state and ignition during catalytic methane oxidation in a monolith reactor

A one-dimensional two-phase reactor model for the oxidation of methane to synthesis gas over platinum in a monolith reactor is presented. The model incorporates a detailed elementary step reaction mechanism for methane oxidation, which is verified against experimental data. A good quantitative agreement with steady-state experiments and qualitative agreement with ignition experiments is achieved. The importance of individual reaction steps, homogeneous side reactions, and main reactor parameters are investigated. Essential steps in the catalytic reaction mechanism as well as crucial reactor parameters are identified. It is shown that the reaction system is strongly dominated by competition for oxygen on the catalyst surface. Conclusions about optimal reactor configurations are discussed.

The effect of an electric field on the shape of co-flowing and candle-type methane–air flames

The effects, which an electric field exerts on flames, have been observed and reported in the literature for a long time. Burning velocity, flame stability, flame shape, flame luminosity, extinction limit, and soot formation, are among the effects that have been observed. Most of the studies in this field were experimental observations. There is fairly limited information in the literature on numerical studies in the area of electric field and flame interaction. Therefore, our fundamental understanding of the process and our ability to use electric field as a means to control the combustion process, are restricted. In the present work, co-flowing diffusion methane/air flames and candle-type methane/air flames under the electric field effect have been observed experimentally. A numerical model, which considers the more important physical and chemical phenomena associated with the flame–field interaction process, has been developed to explain the experimental observations. The model employs a two-dimensional cylindrical coordinate system and assumes axial symmetry. A simplified chemical reaction scheme for a methane–air mixture, which contains 19 chemical species and 31 reactions is employed. It combines existing methane oxidation mechanisms with a series of chemiionization, ion-molecule, and dissociative-recombination reactions, which are important for the ionic species. The mass, momentum, species and energy conservation equations are solved numerically by an integrated version of the PHOENICS and CHEMKIN computer codes. It is concluded that the effects of an electric field on the flame behavior are mainly due to ionic wind effects.

Probability density function calculations of local extinction and no production in piloted-jet turbulent methane/air flames

The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause local extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of local extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of local extinction with increasing jet velocity is accurately represented by the calculations.

Numerical Studies of Methane Catalytic Combustion Inside a Monolith Honeycomb Reactor Using Multi-Step Surface Reactions

Abstract The heterogeneous oxidation of methane-air mixture in a honeycomb catalytic reactor is investigated numerically in the present study. An improved multi-step surface reaction mechanism for methane oxidation on platinum is proposed so that surface ignition of lean methane-air mixtures is better modeled. First, this surface mechanism is used to determine the apparent activation energy of methane-air catalytic combustion. The predicted activation energies are found to agree well with the experimental data by Trimm and Lam (1980) and by Griffin and Pfefferle (1990). The chemical model indicates that, depending on the surface temperature, the surface reaction rate is dominated by either the oxygen desorption rate or by the methane adsorption rate. Second, the surface chemistry model is used to model a methane-air catalytic reactor with a two-dimensional flow code. The substrate surface temperatures are solved directly with a thermal boundary condition derived by balancing the energy fluxes at the gas-c...

Catalytic and inhibitory effects of Pt surfaces on the oxidation of CH4/O2/N2 mixtures

The catalytic oxidation of lean CH4/O2/N2 mixtures was studied by injecting a stream of the mixture onto a polycrystalline platinum foil whose temperatures could be varied by resistance heating between 300 and 1800 K. Experiments were performed in the stagnation point flow configuration under steady-state conditions in order to derive the fuel conversion efficiencies and the product selectivities to CO and CO2. These were compared with a numerical model. With increasing foil temperature, the first appearance of CO in the products signalled the onset of homogeneous oxidation. The surface temperatures of the transition from catalytic to homogeneous oxidations were higher than those of the ‘reversed’ transition from homogeneous to catalytic, causing hysteresis in the fuel conversion and product selectivities curves. For the conditions investigated, the number of thermal cycles applied to the foils were found not to affect the fuel conversion with surface temperature in the catalytic regime. In contrast, they increased the peak of CO selectivity by a factor of two and enhanced the hysteresis behaviour by lowering the temperature of the transition from homogeneous to catalytic oxidation. Decreasing fuel concentrations in the fuel-lean mixture feed did not change appreciably the catalytic oxidation conversion curve, but reduced the temperature of the transition from catalytic to homogeneous oxidation, and eliminated the hysteresis. The peak of CO selectivity was found not to vary significantly in the range of fuel mixture strengths studied. The fuel conversion efficiency curves with surface temperature were successfully modelled over the whole catalytic oxidation regime using one global chemical reaction of methane oxidation. A combination of existing detailed heterogeneous and homogeneous chemical mechanisms of methane oxidation offered the advantage of predicting the CO and CO2 products selectivities, but was found to predict larger conversions in the kinetically controlled catalytic oxidation regime. The agreement with the experimental conversions was restored in the mass transport rate limited regime and permitted a discussion of the inhibitory role of the Pt surface in the transitions between catalytic and homogeneous oxidation regimes. Both the global and detailed chemical mechanisms yielded higher fuel conversions than those measured once the homogeneous oxidation regime was established, showing the need for further validation in this regime, in particular with respect to the formation of CO. Copyright © 2000 John Wiley & Sons, Ltd.

Development of a Five-Step Global Methane Oxidation-NO Formation Mechanism for Lean-Premixed Gas Turbine Combustion

It is known that many of the previously published global methane oxidation mechanisms used in conjunction with computational fluid dynamics (CFD) codes do not accurately predict CH4 and CO concentrations under typical lean-premixed combustion turbine operating conditions. In an effort to improve the accuracy of the global oxidation mechanism under these conditions, an optimization method for selectively adjusting the reaction rate parameters of the global mechanisms (e.g., pre-exponential factor, activation temperature, and species concentration exponents) using chemical reactor modeling is developed herein. Traditional global mechanisms involve only hydrocarbon oxidation; that is, they do not allow for the prediction of NO directly from the kinetic mechanism. In this work, a two-step global mechanism for NO formation is proposed to be used in combination with a three-step oxidation mechanism. The resulting five-step global mechanism can be used with CFD codes to predict CO, CO2, and NO emission directly. Results of the global mechanism optimization method are shown for a pressure of 1 atmosphere and for pressures of interest for gas turbine engines. CFD results showing predicted CO and NO emissions using the five-step global mechanism developed for elevated pressures are presented and compared to measured data.

Chemometric analysis of a detailed chemical reaction mechanism for methane oxidation

A widely used detailed reaction mechanism for methane oxidation (Gas Research Institute (GRI) mechanism 2.11) has been analysed, in order to evaluate if reactions were to be added to the mechanism. This mechanism consists of 279 reversible elementary reactions between 48 different species, each with a temperature dependence described by a modified Arrhenius expression. The mechanism was transformed to 558 irreversible reactions, and the rate constants were analysed at a fixed temperature, to reduce the complexity of the analysis. A partial least squares (PLS) model was generated, which estimated reaction rate constants as a function of a reaction descriptor vector. This vector characterized the different chemical bonds in the reactants and products of a chemical reaction. The model was validated through full cross validation. The original mechanism was unable to correctly predict oxidation of methane in a natural gas engine exhaust manifold: Oxidation of 2300 ppm methane in the presence of 300 ppm NO, 9% oxygen and 2% water. Therefore, these conditions were used for evaluation of the reaction mechanism. The potential reactions for expanding the mechanism were selected among reactions with one or two reactants and one or two products. A stepwise analysis combining rate of production (ROP) analysis with sensitivity analysis was used to reduce 2138 potential elementary reactions to nine important reactions, which were added to the mechanism. The analysis was based on PLS estimates of the reaction rate constants, but in the final model, literature values were included where available. This modification of the mechanism improved model predictions.

An experimental and numerical study of a laminar triple flame

A lifted laminar axisymmetric diffusion flame is stabilized in the downstream region of a diluted methane jet that is surrounded by a lean methane-air co-flow and an outer co-flow of air. The flame shows a distinct triple flame structure in the stabilization region. It is investigated experimentally by PIV for the velocity field, OH-LIPF imaging, C 2H x -LIF imaging, and a 1D-Raman technique for major species concentrations, combined with a Rayleigh technique for temperature. This is complemented by numerical simulations solving the two-dimensional axisymmetric Navier-Stokes equations in the zero Mach number limit on an adapted mesh, coupled with balance equations for temperature and species. A simplified model for molecular transport properties was used with constant, but non-unity, Lewis numbers for all species. Chemistry is represented by a ten-step reduced mechanism for methane oxidation, which was derived starting from a 61-step elementary mechanism that includes the C 1 and C 2 chains. The agreement between the measured and the predicted flow field is very satisfactory. Owing to gas expansion, the velocity decreases immediately ahead of the flame and increases strongly at the flame front. Further downstream acceleration due to buoyancy is dominant and is predicted accurately. There is a good agreement between measurements and computations for flame shape and flame length. The measured OH-LIPF image and the computed OH concentrations indicate that OH is concentrated in the vicinity of stoichiometric mixture. The results from a newly developed C 2H x -LIF method are also supported by calculations. While these measurements were only qualitative, the temperature and mole fractions of the major species could be measured quantitatively with the combined Raman-/Rayleigh technique along a line and were found to agree well with the numerical predictions. It is found that the structure is a triple flame and is influenced essentially by two external parameters: heat exchange between the branches and heat loss at the curved flame front near the triple point.

CH-Radical Concentration Measurements in Fuel-Rich CH 4/O 2/Ar and CH 4/O 2/NO/Ar Mixtures Behind Shock Waves

CH radicals are important precursors in prompt-NO formation and they also play a role in NO removal via reburning. Little information exists, however, on the absolute concentrations of CH during fuel-rich methane oxidation and their dependence on NO concentrations. Here the authors report results of CH-radical measurements for such conditions, using a shock tube in combination with a sensitive CH absorption diagnostic. These measurements provide useful data for the further development of large-scale reaction mechanisms, such as the recently developed Gas Research Institute methane oxidation and NO reburning mechanism (GRI-Mech 2.11). CH-radical profiles were measured at high temperatures for fuel-rich methane oxidation with and without addition of NO. Comparison of these data with predictions based on the GRI-Mech 2.11 mechanism shows significant deviations, suggesting the need for additional high-temperature rate data on important CH reactions and for further optimization of the detailed mechanism.

Evaluation of Ignition Improvers for Methane Autoignition

The objective of this study was to estimate the efficiency of methane autoignition promotion by testing different ignition improvers including hydrogen, H2, hydrogen peroxide, H2O2, ozone,O3 and dimethyl ether, DME, (CH3)2O. This was accomplished by computing ignition delays for CH4/O2/Ar or N2 mixtures of various compositions, concentrations of the promoters, pressures and temperatures. Ignition delay times for additive-free mixtures were used for tuning a methane oxidation mechanism consisting of 185 reversible elementary reactions between 32 species. A selection of the reaction rate parameters available in the standard databases was made to optimize the agreement between simulation and experimental results for one particular set of test conditions (reference mixture) by refining the rate parameters of the most sensitive stages revealed by sensitivity analysis. The agreement achieved between model predictions and shock tube experimental data is very good. To investigate the effect of dimethyl ether on methane autoignition, the mechanism was extended up to 301 reactions and 53 species to predict the autoignition delay at high pressures. Methane autoignition promotion is proven to be much stronger for H2O2, O3 and (CH3)2O additives than for H2. The mechanism of ignition acceleration is attributed mainly to the dominant role of the O and OH-radicals generated by the rapid decomposition of hydrogen peroxide and ozone, and to the peroxide intermediates and low temperature branching, in the case of dimethyl ether.

Kinetics and mechanism of methane oxidation in supercritical water

We oxidized methane in supercritical water at 250 atm and at temperatures between 525 and 587 °C. The methane conversions ranged from 3 to 70%. CO was the product present in the highest yields at low conversions (< 10%), but CO 2 became the most abundant product at higher conversions. These experimental results were used to test the predictions of a detailed chemical kinetics model, which is based on gas-phase oxidation mechanisms and kinetics and comprised 150 elementary reaction steps. The model predicted methane disappearance rates that were about 30–50% faster than those observed experimentally. This behavior led to consistently high predictions of the methane conversion and the CO 2 yield. However, the model accurately predicted the yields of CO and CO 2 as a function of the methane conversion. The predicted activation energy for the pseudo-first-order rate constants of 36 ± 3 kcal/mol is similar to the experimental value of 44 ± 6 kcal/mol. Overall, the ability of the model to predict several of the experimental observations demonstrates that the analogy between gas-phase oxidation and oxidation in supercritical water is a good one. A sensitivity analysis revealed that the calculated methane concentration is most sensitive to the kinetics of OH + H 2O 2 = HO 2 + H 2O, OH + HO 2 = H 2O + O 2, H 2O 2 = OH + OH and HO 2 + HO 2 = O 2 + H 2O 2. These reactions control the concentration of OH radical, which is the main oxidant under SCWO conditions.

An augmented reduced mechanism for methane oxidation with comprehensive global parametric validation

Using a computer algorithm for automatic generation of reduced chemistry, an augmented reduced mechanism, consisting of 16 species and 12 lumped reaction steps, has been developed for methane oxidation from GRI-Mech 1.2. Because the present mechanism consists of a larger number of non-steady-state intermediates than the conventional four- or five-step reduced mechanisms, it exhibits good to excellent performance in predicting a wide range of combustion phenomena under extensive thermodynamical parametric variations. Specifically, the phenomena tested include perfectly stirred reactor responses, autoignition and shock-tube ignition delay times, laminar flame propagation speeds, and ignition-extinction limits of counterflowing systems, whereas the thermodynamical parametric variations include those of temperature, pressure, and composition. It is recognized that, with the anticipated increase in computing capability in the foreseeable future, use of the present four- to five-step mechanisms will be unnecessarily limiting. Consequently, it is suggested that efforts should be expended toward development of augmented reduced mechanisms for more comprehensive description of combustion phenomena and for their potential implementation in the computational simulation of complex flows and systems.