Kinetics Of Methane Oxidation Research Articles

Seasonality of water column methane oxidation and deoxygenation in a dynamic marine environment

Most of the methane input to the world’s oceans is intercepted by microorganisms in sediment and the overlying water column and oxidized before it has an opportunity to reach the atmosphere, where it acts as a greenhouse gas. The factors controlling methane consumption in the ocean are not well established and its biogeochemistry in dynamic marine environments is understudied in-part because of challenges in capturing spatial and temporal variability. Our study focused on the factors that structure methane’s biogeochemistry in a dynamic marine environment, the Santa Barbara Basin. The deep-water column of the Santa Barbara Basin experiences seasonal oxygen loss and episodic replenishment which we found to be major factors in structuring the accumulation of methane and the rate at which microorganisms consumed that methane. We found the gradual decline in oxygen that commonly occurs through the summer culminated with a pronounced accumulation of methane in the water column during the fall. Rates of methane oxidation remained low in summer, increased with the buildup of methane in fall, and remained elevated into spring, even after methane concentration had declined. However, results from methane oxidation kinetics experiments revealed a zero-order kinetic dependence on oxygen concentration, indicating that oxygen’s effect on methanotrophy at the ecosystem scale is likely indirect. We also captured an apparent mixing event during fall that drove spatial and temporal variability in oxygen, nitrate and methane concentrations in the Santa Barbara Basin, with stark variations at the investigated timescale of 8 days and along isobaths at a spatial scale of 7 km. Collectively, these results indicate the seasonal development and attenuation of a methanotrophic community associated with restricted circulation, but also of a spatiotemporal variability not previously appreciated for this environment.

Trait-Based Model Reproduces Patterns of Population Structure and Diversity of Methane Oxidizing Bacteria in a Stratified Lake

In stratified lakes, methane oxidizing bacteria are critical methane converters that significantly reduce emissions of this greenhouse gas to the atmosphere. Efforts to better understand their ecology uncovered a surprising diversity, vertical structure, and seasonal succession. It is an open question how this diversity has to be considered in models of microbial methane oxidation. Likewise, it is unclear to what extent simple microbial traits related to the kinetics of the oxidation process and temperature optimum, suggested by previous studies, suffice to understand the observed ecology of methane oxidizing bacteria. Here we incorporate niche partitioning in a mechanistic model of seasonal lake mixing and microbial methane oxidation in a stratified lake. Can we model MOB diversity and niche partitioning based on differences in methane oxidation kinetics and temperature adaptation? We found that our model approach can closely reproduce diversity and niche preference patterns of methanotrophs that were observed in seasonally stratified lakes. We show that the combination of trait values resulting in coexisting methanotroph communities is limited to very confined regions within the parameter space of potential trait combinations. However, our model also indicates that the sequence of community assembly, and variations in the stratification and mixing behavior of the lake result in different stable combinations. A scenario analysis introducing variable mixing conditions showed that annual weather conditions and the pre-existing species also affect the developing stable methanotrophic species composition of the lake. Both, effect of pre-existing species and the environmental impact suggest that the MOB community in lakes may differ from year to year, and a stable community may never truly occur. The model further shows that there are always better-adapted species in the trait parameter space that would destabilize and replace an existing stable community. Thus, natural selection may drive trait values into the specific configurations observed in nature based on physiological limits and tradeoffs between traits.

Adaptability of different mechanisms and kinetic study of methane combustion in steam diluted environments

The chemical kinetics of methane oxidation in a steam-diluted environment are studied in the present study. Various well-validated mechanisms for methane combustion are adopted and compared with experimental data. Ignition delay, laminar flame speed, and emissions for CH4 combustion with steam dilution are discussed. Cumulative relative error parameter was determined for all mechanisms considered in this study to evaluate the prediction level in quantifiable terms. Reaction pathways under no and steam-diluted environments are analyzed, and key elementary reactions and species are identified in these conditions. The analysis gives a relative idea of the applicability of some of the reduced mechanisms for the diluted steam conditions. This study aims to guide future computational fluid dynamics simulations to accurately predict combustion characteristics in these conditions. Computations of laminar flame speed from GRI-3.0, Aramco3.0, Curran, and San Diego mechanisms were the most precise under diluted steam conditions. Similarly, for the calculation of ignition delay of methane under the steam dilution, the Aramco mechanism and the Curran’s mechanism were able to predict the experimentally observed values most closely. Sensitivity study for the OH concentrations shows that the H-abstraction of methane from OH radicals has an opposing trend with dilution for Aramco and GRI-3.0 mechanism. On the other hand, CO and NO emissions were reduced significantly, with the dilution increased from 0 to 20%. The third-body effect of steam is observed to dominate the deviation observed between the detailed and reduced mechanism. For low operating pressure conditions, the GRI-3.0 mechanism gives an excellent prediction, whereas, for applications like gas turbines and furnaces, Aramco-3.0 and Curran mechanisms can be adopted to give good results. The San Diego mechanism can be chosen for low computational facility purposes as it shows very good predictions for ignition delay and laminar flame speed computations.

Mechanism and Kinetics of Methane Oxidation to Methanol Catalyzed by AuPd Nanocatalysts at Low Temperature

The selective oxidation of methane to methanol at ambient conditions has the potential to enable the use of natural gas obtained in remote areas. We performed multivariate statistical analysis of 1...

Time and space resolved diagnostics for plasma thermal-chemical instability of fuel oxidation in nanosecond plasma discharges

An instability in a nanosecond pulsed dielectric barrier discharge plasma occurring in methane–oxygen–argon mixtures is experimentally observed and measured by 1D time-resolved in situ electric field measurements. This instability, which seems to be created by the positive feedback between plasma kinetics and plasma-assisted low temperature fuel oxidation, is studied using electric field induced second harmonic generation and direct ICCD imaging. The rapid formation of streamers from an originally uniform discharge appears to be caused by the chemical kinetics of plasma-assisted low temperature methane oxidation, which may be resulting in a new type of plasma instability: a thermal-chemical instability. The results also revealed that the occurrence of this possible thermal-chemical instability in a reactive flow drastically changes the plasma properties by forming multiple secondary discharges and possibly leads to micron-sized non-uniform electric distributions. Single shot uncalibrated measurements of the electric field of the micron sized streamers appears to show much greater strengths than the average electric field. Furthermore, one-dimensional data analysis shows the positive feedback loop between the streamers and the low temperature plasma assisted oxidation chemistry in the plasma thermal-chemical instability. The present finding advances the understanding plasma instability growth and provides a new way to control plasma uniformity in plasma-assisted combustion and plasma fuel reforming.

Lake mixing regime selects apparent methane oxidation kinetics of the methanotroph assemblage

Abstract. In lakes, large amounts of methane are produced in anoxic sediments. Methane-oxidizing bacteria effectively convert this potent greenhouse gas into biomass and carbon dioxide. These bacteria are present throughout the water column, where methane concentrations can range from nanomolar to millimolar. In this study, we tested the hypothesis that methanotroph assemblages in a seasonally stratified freshwater lake are adapted to the contrasting methane concentrations in the epi- and hypolimnion. We further hypothesized that lake overturn would change the apparent methane oxidation kinetics as more methane becomes available in the epilimnion. In addition to the change in the methane oxidation kinetics, we investigated changes in the transcription of genes encoding methane monooxygenase, the enzyme responsible for the first step of methane oxidation, with metatranscriptomics. Using laboratory incubations of the natural microbial communities, we show that the half-saturation constant (Km) for methane – the methane concentration at which half the maximum methane oxidation rate is reached – was 20 times higher in the hypolimnion than in the epilimnion during stable stratification. During lake overturn, however, the kinetic constants in the epi- and hypolimnion converged along with a change in the transcriptionally active methanotroph assemblage. Conventional particulate methane monooxygenase appeared to be responsible for methane oxidation under different methane concentrations. Our results suggest that methane availability is one important factor for creating niches for methanotroph assemblages with well-adapted methane oxidation kinetics. This rapid selection and succession of adapted lacustrine methanotroph assemblages allowed the previously reported high removal efficiency of methane transported to the epilimnion to be maintained – even under rapidly changing conditions during lake overturn. Consequently, only a small fraction of methane stored in the anoxic hypolimnion is emitted to the atmosphere.

Total oxidation of methane over Pd/Al2O3 at pressures from 1 to 10 atm

The kinetics of total methane oxidation over a 0.15 wt% Pd/Al2O3 monolith catalyst has been measured during temperature programmed methane oxidation experiments at total pressures from 1 to 10 atm and compared with multiscale simulations.

Palladium dispersion effects on wet methane oxidation kinetics

The catalytic activity for dry and wet methane oxidation over a series of palladium–alumina catalysts with systematically varied palladium loadings and PdO dispersions was measured and compared with conceptual multiscale simulations.

110th Anniversary: Kinetics and X-ray Absorption Spectroscopy in Methane Total Oxidation over Alumina-Supported Pt, Pd, and Ag–Pd Catalysts

The kinetics of methane oxidation over Pt/Al2O3, Pd/Al2O3, and Ag–Pd/Al2O3 is investigated as a function of the O2/CH4 ratio. We show that as the oxygen fraction increases from low values, three distinct kinetic regimes exist that are nearly first order, negative order, and zero order with respect to the oxygen concentration. At O2/CH4 < 3, Pd and Ag–Pd are superior catalysts, whereas at higher oxygen concentrations, Pt is superior. Hysteresis with bistability is demonstrated over all three catalysts with two distinct activity regimes at identical feed compositions: higher activity upon increasing oxygen concentration from low values and lower activity upon decreasing oxygen concentration from high values. In situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) over the Ag–Pd catalyst suggest that the bistability stems from different oxidation states of the catalyst as well as a core–shell structure in the reduced state.

Kinetics of anaerobic methane oxidation coupled to denitrification in the membrane biofilm reactor.

Anaerobic oxidation of methane coupled to denitrification (AOM-D) in a membrane biofilm reactor (MBfR), a platform used for efficiently coupling gas delivery and biofilm development, has attracted attention in recent years due to the low cost and high availability of methane. However, experimental studies have shown that the nitrate-removal flux in the CH4 -based MBfR (<1.0 g N/m2 -day) is about one order of magnitude smaller than that in the H2 -based MBfR (1.1-6.7 g N/m2 -day). A one-dimensional multispecies biofilm model predicts that the nitrate-removal flux in the CH4 -based MBfR is limited to <1.7 g N/m2 -day, consistent with the experimental studies reported in the literature. The model also determines the two major limiting factors for the nitrate-removal flux: The methane half-maximum-rate concentration (K2 ) and the specific maximum methane utilization rate of the AOM-D syntrophic consortium (kmax2 ), with kmax2 being more important. Model simulations show that increasing kmax2 to >3 g chemical oxygen demand (COD)/g cell-day (from its current 1.8 g COD/g cell-day) and developing a new membrane with doubled methane-delivery capacity (Dm ) could bring the nitrate-removal flux to ≥4.0 g N/m2 -day, which is close to the nitrate-removal flux for the H2 -based MBfR. Further increase of the maximum nitrate-removal flux can be achieved when Dm and kmax2 increase together.

Methane oxidation kinetics in northern freshwater lakes

Understanding the drivers of aerobic methane (CH4) oxidation (MOX) is paramount in assessing the current and potential future CH4 emissions from freshwater aquatic systems. Regulation of MOX kinetics is a complex function of CH4 and oxygen (O2) concentrations. While MOX activity is usually proportional to the concentration of CH4 itself, the effects of O2 have been more conflicting, with maximum MOX rates often restricted to low O2 concentrations. Despite the complexity involved, MOX kinetics are often modelled as monotonic positive functions of both CH4 and O2 concentrations. We conducted a series of incubation experiments using natural and unamended water samples obtained from multiple depths in northern temperate lakes that vary widely and independently in their CH4 and O2 concentrations. Our results showed the expected positive effect of CH4 concentration and temperature but also demonstrated the strong inhibitory effects of O2 at high concentration. We then developed a general model describing the kinetics of MOX, simultaneously integrating the effects of CH4 concentration, temperature as well as the non-linear effect O2 on MOX activity. The model revealed an overall temperature dependency (activation energy = 0.49 ± 0.06 eV) much lower than reported for methanogenesis and an optimal O2 level around 15 μmol O2 L−1 where maximum MOX activity occurs, regardless of CH4 concentration and temperature. We further show that ignoring the inhibitory effect of O2 can lead to significant bias in calculating the expected MOX rates in different portions of the water column.

The impact of methane oxidation kinetics on a rocket nozzle flow

The impact of methane combustion kinetics on a rocket nozzle flow is theoretically studied in the work. To evaluate the effect of the kinetics, simulations of a rocket nozzle flow are carried out using the computational fluid dynamics solver Fluent. At first, a methane kinetic model is selected. The comparison of different kinetic models shows that the recent skeletal mechanism of Zhukov and Kong has a very good accuracy at a small size. For comparison, the flow simulations are performed both for hydrogen/oxygen and methane/oxygen propellant combinations, and for three different reaction models: non-reactive (“frozen”) flow, reactive (non-equilibrium) flow, and chemically equilibrium flow. Chemically non-equilibrium flow is modelled using the Zhukov–Kong model. Simulations results show that the recombination reactions of combustion products should be taken into account for modelling rocket nozzle flow. For hydrogen the difference in results between chemically non-equilibrium and equilibrium flows is negligible while in methane the difference is small but noticeable.

Surface reaction kinetics of methane oxidation over PdO

A two-site mean field extended microkinetic model was developed based on DFT data to investigate the methane oxidation reaction over PdO(1 0 1) for environmental applications at atmospheric to moderate pressures, fuel-lean and low-temperature model exhaust gas conditions. The mechanism includes various carbonaceous pathways for methane oxidation together with lattice oxygen vacancy formation via Mars-van-Krevelen steps. The mechanism was compared with catalytic light-off curves (573–823 K) on a Pd/Al2O3 coated on monolith for CH4/O2/H2O/N2 mixtures with 1000 ppm CH4, 10 vol% O2 at varying H2O feed concentration (0–12 vol%) and pressure (1–4 bar). The mechanism was demonstrated to quantitatively reproduce experimental light-off curves for dry and wet feeds and capture the water inhibition phenomena, when catalyst deactivation and/or particle size dependent kinetic effects are taken into account. A degree of rate control analysis reveals dissociative CH4 adsorption via hydrogen abstraction over Pdcus-Ocus site-pairs as the major rate controlling step during light-off. Supplementary in situ DRIFTS investigations analyzed for dry and wet reactive gas-mixtures containing different types of C1-fuels, namely methane, methanol and formic acid were conducted to identify surface species during catalytic methane oxidation and hydroxide formation.

A reactive molecular dynamics simulation study of methane oxidation assisted by platinum/graphene-based catalysts

Platinum-decorated functionalized graphene sheet (Pt@FGS) is a promising nanoparticle additive for catalytic fuel combustion. In this study, four cases involving pure methane oxidation and methane oxidation in the presence of various Pt/graphene-based nanoparticle catalysts are investigated using the reactive force field molecular dynamics (ReaxFF MD) simulations to reveal catalytic mechanisms and kinetics of methane oxidation. The results demonstrate that Pt@FGS is the most effective catalyst among all the nanoparticle candidates involved in this research. Compared with pure methane oxidation, the combination of Pt and FGS in the Pt@FGS reaction improves the catalytic activity by dramatically lowering the activation energy by approximately 73%. Additionally, the catalytic methane oxidation is initiated by the cleavage of CH bond and the production of hydroxyl. The observed H transfer process suggests that enhanced dehydrogenation of Pt@FGS and interatomic exchanges activate the catalytic cycle and dominate the catalytic process. Moreover, FGS can be further oxidized mostly at the edge of the sheet to increase the functionality. In summary, this research sheds light on the catalytic mechanisms for enhanced fuel combustion in the presence of Pt@FGS.

Automated Chemical Kinetic Modeling via Hybrid Reactive Molecular Dynamics and Quantum Chemistry Simulations.

An automated scheme for obtaining chemical kinetic models from scratch using reactive molecular dynamics and quantum chemistry simulations is presented. This methodology combines the phase space sampling of reactive molecular dynamics with the thermochemistry and kinetics prediction capabilities of quantum mechanics. This scheme provides the NASA polynomial and modified Arrhenius equation parameters for all species and reactions that are observed during the simulation and supplies them in the ChemKin format. The ab initio level of theory for predictions is easily exchangeable, and the presently used G3MP2 level of theory is found to reliably reproduce hydrogen and methane oxidation thermochemistry and kinetics data. Chemical kinetic models obtained with this approach are ready to use for, e.g., ignition delay time simulations, as shown for hydrogen combustion. The presented extension of the ChemTraYzer approach can be used as a basis for methodological advancement of chemical kinetic modeling schemes and as a black-box approach to generate chemical kinetic models.

Hypoxic methane oxidation coupled to denitrification in a membrane biofilm

Methane oxidation coupled to denitrification (MOD) was tested in a membrane biofilm reactor (MBfR) using methane gas as the sole electron donor. Nitrate reduction to nitrite was rate limiting, and CH4 was present in the effluent. Slow kinetics of methane oxidation by bacteria were the factors that led to slow kinetics and incomplete removals. Methylocystaceae contained the largest fraction (21%) of bacterial SSU rRNA genes, and Archaea were nearly absent. The functional metagenome included all the genes essential for aerobic methane oxidation (pmo, mdh, mtdB, folD, and fdh) and nitrate reduction to dinitrogen (nap/nar, nir, nor and nos), but not for reverse methanogenesis (mcr). The functional metagenome supports that Methylocystaceae conducted MOD in syntrophy with heterotrophic denitrifiers (e.g., Comamonadaceae and Brucellaceae), suggesting aerobic MOD. DO measurements, serum-bottle tests, and calculation of O2 permeation bolster hypoxically aerobic MOD would mainly account for denitrification in the MBfR.

Development of mathematical model of methane oxidation on fibrous catalyst

Using the experimental and numerical methods, the kinetics of deep methane oxidation on a fiber-like oxide catalyst with honeycomb structure is studied. ALSIFLEX-KT1600 fibers are used as a carrier, and spinel (% of mass) is used as a catalytic contact: MgAl 2 O 4 – 22.07 %; NiO – 2.02 %; Cr 2 O 3 – 3.49 %. Si+SiO2 adhesive – 18.97 % and organic glue – 1.72 are used to impart a stable honeycomb structure. To complete the numerical analysis of kinetic data, a complete two-phase mathematical model is developed. The microchannel model is adopted as a basis. This model is characterized by the equality of the total area of the cross-sections of the microchannels and the area of the free section of the catalyst, and the total surface area of heat and mass transfer is equal to the specific surface area of the catalyst. As a result of the work, the values of similarity, mass and heat transfer coefficients, the effective working surface of the catalyst, participating in the heterogeneously catalytic reaction of complete oxidation of methane S w = 7640±150 m²/m³ are calculated. The experimental kinetic data of methane oxidation on the catalyst have been confirmed by mathematical calculations, a refined microkinetic equation has been derived, the flow areas of the process have been established, and the performance of the synthesized catalyst has been established

Modeling of termokinetic oscillations at partial oxidation of methane

Partial oxidation of natural gas at moderate temperatures below 1500 K has significant interest for a number of industrial applications. But such processes can proceed at different unstable regimes including oscillating modes. Nonlinear phenomena at partial oxidation of methane were observed at different conditions. The investigation of the complex nonlinear system of equations that describes this process is a real method to insure its stability at industrial conditions and, at the same time, is an effective tool for its further enhancement. Numerical analysis of methane oxidation kinetics in the continuous stirred-tank reactor, with the use of detailed kinetic model has shown the possibility of the appearance of oscillating modes in the appropriate range of reaction parameters that characterize the composition, pressure, reagents flow, thermophysical features of the system, and geometry of the reactor. The appearance of oscillating modes is connected both with the reaction kinetics, heat release and sink and reagents introduction and removing. At that, oscillations appear only at a limited range of parameters, but can be accompanied by significant change in the yield of products. We have determined the range of initial temperature and pressure at which oscillations can be observed, if all other parameters remained fixed. The boundaries of existence of oscillations on the phase plane were calculated. It was shown that depending on the position inside the oscillation region the oscillations have different frequency and amplitude. It was reviled the role of heat exchange with the environment: at the absence of heat exchange the oscillating modes are impossible. In the vicinity of the boundary of phase range, where oscillations exist, significant change of concentration of some products were observed, for example, that of CO2, which in this case one of the principal products is. At that, insignificant increase in pressure not only change the character of CO2 behaving with time, but as well lead to significant increase of its mole fraction simultaneously twice decreasing the mole fraction of CO.

Thermokinetic oscillations in the partial oxidation of methane

The occurrence of oscillating modes at specific process parameters was demonstrated by the numerical analysis of the kinetics of methane oxidation in a continuous stirred tank reactor. These parameters characterize the composition and pressure of a reaction mixture, the feed rate of reagents, the thermal properties of the system, and the reactor geometry. The oscillating process is related to both the reaction kinetics and the evolution and removal of heat and reactants. In this case, oscillations occur only in a relatively small range of changes in the parameters that characterize the properties of the system; however, they can be accompanied by a considerable change in the yield of products.

Structure and kinetic investigations of surface-stepped CeO2-supported Pd catalysts for low-concentration methane oxidation

Lamellar octahedral ceria (LOC) was synthesized by a facile hydrothermal route. The catalyst properties of Pd-supported on the as-synthesized LOC material for low-concentration methane oxidation and reaction kinetics were also investigated. The structure and morphology of samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen ad/desorption, X-ray photoelectron spectroscopy and hydrogen temperature-programmed reduction techniques. As-synthesized ceria has octahedral morphology and surface-stepped microstructures, and this unique structural CeO2 exposes (110) crystal planes superior to general (111) facet in catalyst. The results show that LOC-supported Pd catalyst exhibits excellent low-temperature activity and stability by optimizing Pd loadings. The properties are further enhanced by introducing guest metal into the LOC material and the advantage of the Zr-modified LOC-supported Pd catalyst was confirmed by kinetic study. The optimum catalyst maintains high low-temperature activity and stability under various gas hourly space velocities, suggesting it has promising application in complete oxidation of low-concentration methane for different flow rates of ventilation air.