Showing papers on "Methane published in 2003"

A redox-stable efficient anode for solid-oxide fuel cells.

Abstract: Solid-oxide fuel cells (SOFCs) promise high efficiencies in a range of fuels. Unlike lower temperature variants, carbon monoxide is a fuel rather than a poison, and so hydrocarbon fuels can be used directly, through internal reforming or even direct oxidation. This provides a key entry strategy for fuel-cell technology into the current energy economy. Present development is mainly based on the yttria-stabilized zirconia (YSZ) electrolyte. The most commonly used anode materials are Ni/YSZ cermets, which display excellent catalytic properties for fuel oxidation and good current collection, but do exhibit disadvantages, such as low tolerance to sulphur and carbon deposition when using hydrocarbon fuels, and poor redox cycling causing volume instability. Here, we report a nickel-free SOFC anode, La0.75Sr0.25Cr0.5Mn0.5O3, with comparable electrochemical performance to Ni/YSZ cermets. The electrode polarization resistance approaches 0.2 Omega cm2 at 900 degrees C in 97% H2/3% H2O. Very good performance is achieved for methane oxidation without using excess steam. The anode is stable in both fuel and air conditions, and shows stable electrode performance in methane. Thus both redox stability and operation in low steam hydrocarbons have been demonstrated, overcoming two of the major limitations of the current generation of nickel zirconia cermet SOFC anodes.

Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes

Abstract: This review examines the interactions between soil physical factors and the biological processes responsible for the production and consumption in soils of greenhouse gases. The release of CO 2 by aerobic respiration is a non-linear function of temperature over a wide range of soil water contents, but becomes a function of water content as a soil dries out. Some of the reported variation in the temperature response may be attributable simply to measurement procedures. Lowering the water table in organic soils by drainage increases the release of soil carbon as CO 2 in some but not all environments, and reduces the quantity of CH 4 emitted to the atmosphere. Ebullition and diffusion through the aerenchyma of rice and plants in natural wetlands both contribute substantially to the emission of CH 4 ; the proportion of the emissions taking place by each pathway varies seasonally. Aerated soils are a sink for atmospheric CH 4 , through microbial oxidation. The main control on oxidation rate is gas diffusivity, and the temperature response is small. Nitrous oxide is the third greenhouse gas produced in soils, together with NO, a precursor of tropospheric ozone (a short-lived greenhouse gas). Emission of N 2 O increases markedly with increasing temperature, and this is attributed to increases in the anaerobic volume fraction, brought about by an increased respiratory sink for O 2 . Increases in water-filled pore space also result in increased anaerobic volume; again, the outcome is an exponential increase in N 2 O emission. The review draws substantially on sources from beyond the normal range of soil science literature, and is intended to promote integration of ideas, not only between soil biology and soil physics, but also over a wider range of interacting disciplines.

Atmospheric methane levels off: Temporary pause or a new steady-state?

Abstract: [1] The globally-averaged atmospheric methane abundance determined from an extensive network of surface air sampling sites was constant at ∼1751 ppb from 1999 through 2002. Assuming that the methane lifetime has been constant, this implies that during this 4-year period the global methane budget has been at steady state. We also observed a significant decrease in the difference between northern and southern polar zonal annual averages of CH4 from 1991 to 1992. Using a 3-D transport model, we show that this change is consistent with a decrease in CH4 emissions of ∼10 Tg CH4 from north of 50°N in the early-1990s. This decrease in emissions may have accelerated the global methane budget towards steady state. Based on current knowledge of the global methane budget and how it has changed with time, it is not possible to tell if the atmospheric methane burden has peaked, or if we are only observing a persistent, but temporary pause in its increase.

Deep sea NMR: Methane hydrate growth habit in porous media and its relationship to hydraulic permeability, deposit accumulation, and submarine slope stability

Abstract: [1] Review of the literature reveals that the nature of pore-scale interactions between gas hydrates and porous media remains a matter of controversy. To clarify the situation, nuclear magnetic resonance (NMR) measurements have been made on methane hydrate-bearing sandstones. The samples were synthetically prepared within the gas hydrate stability zone, at or near the seafloor in Monterey Bay, California. The method simulated natural hydrate deposition by gas flows that are not in thermodynamic equilibrium with the surrounding earth. The efficiency of hydrate production was variable, as has been observed elsewhere. When substantial hydrate saturations were achieved, NMR relaxation time measurements indicated that hydrate tended to replace water in the largest pore spaces. The relative permeability to water, as determined by an NMR-based correlation, was significantly reduced. The magnitude of this reduction was also consistent with formation of hydrate in the centers of pores, rather than with hydrate coating the grains. The growth habit suggested by these results is consistent with creation of hydrate nodules and lenses in coarse, unconsolidated sediments. It is also consistent with scenarios in which methane gas is delivered efficiently to the atmosphere as a result of seafloor slope failure, thereby strengthening global warming feedback mechanisms.

Mechanistic studies on the hydroxylation of methane by methane monooxygenase.

Abstract: Methanotrophs are bacteria that live on methane as their only source of carbon.1 The first step in their utilization of this simplest of all hydrocarbons is its selective conversion to methanol. Subsequent biochemical pathways transform methanol to formaldehyde, which in turn is processed into biomass. Further oxidation of formaldehyde to carbon dioxide provides energy that is stored for later use as NADH.2 The conversion of methane to methanol is catalyzed at the active site of a metalloenzyme known as methane monooxygenase, or MMO.3-9

Factors controlling large scale variations in methane emissions from wetlands

Abstract: [1] Global wetlands are, at estimate ranging 115-237 Tg CH4/yr, the largest single atmospheric source of the greenhouse gas methane (CH4). We present a dataset on CH4 flux rates totaling 12 measurement years at sites from Greenland, Iceland, Scandinavia and Siberia. We find that temperature and microbial substrate availability (expressed as the organic acid concentration in peat water) combined explain almost 100% of the variations in mean annual CH4 emissions. The temperature sensitivity of the CH4 emissions shown suggests a feedback mechanism on climate change that could validate incorporation in further developments of global circulation models.

Reactivity of some metal oxides supported on alumina with alternating methane and oxygen - Application for chemical-looping combustion

Abstract: Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from the combustion air to the fuel. Two reactors are used in the process: (i) a fuel reactor where the metal oxide is reduced by reaction with the fuel, and (ii) an air reactor where the reduced metal oxide from the fuel reactor is oxidized with air. The possibility of using oxides of Cu, Co, Mn, and Ni as oxygen carriers was investigated. Particles were prepared by deposition of the metal oxides on γ-Al2O3 particles by so-called dry impregnation. The reactivity of the oxygen carrier particles was evaluated in a thermogravimetric analyzer (TGA), where the alternating atmosphere which an oxygen carrier encounters in a CLC system was simulated by exposing the sample to alternating reducing (10% CH4, 5% CO2, 10% H2O) and oxidizing (10% O2) conditions at temperatures between 750 and 950 °C. The particles ...

Anaerobic oxidation of methane above gas hydrates at Hydrate Ridge, NE Pacific Ocean

Abstract: At Hydrate Ridge (HR), Cascadia convergent margin, surface sediments contain massive gas hydrates formed from methane that ascends together with fluids along faults from deeper reservoirs. Anaerobic oxidation of methane (AOM), mediated by a microbial consortium of archaea and sulfate-reducing bacteria, generates high concentrations of hydrogen sulfide in the surface sediments. The production of sulfide supports chemosynthetic communities that gain energy from sulfide oxidation. Depending on fluid flow, the surface communities are dominated either by the filamentous sulfur bacteria Beggiatoa (high advective flow), the clam Calyptogena (low advective flow), or the bivalve Acharax (diffusive flow). We analyzed surface sediments (0 to 10 cm) populated by chemosynthetic communities for AOM, sulfate reduction (SR) and the distribution of the microbial consortium mediating AOM. Highest AOM rates were found at the Beggiatoa field with an average rate of 99 mmol m-2 d-1 integrated over 0 to 10 cm. These rates are among the highest AOM rates ever observed in methane-bearing marine sediments. At the Calyptogena field, AOM rates were lower (56 mmol m-2 d-1). At the Acharax field, methane oxidation was extremely low (2.1 mmol m-2 d-1) and was probably due to aerobic methane oxidation. SR was fueled largely by methane at flow-impacted sites, but exceeded AOM in some cases, most likely due to sediment heterogeneity. At the Acharax field, SR was decoupled from methane oxidation and showed low activity. Aggregates of the AOM consortium were abundant at the fluid-impacted sites (between 5.1 × 1012 and 7.9 × 1012 aggregates m-2) but showed low numbers at the Acharax field (0.4 × 1012 aggregates m-2). A transport-reaction model was applied to estimate AOM at Beggiatoa fields. The model agreed with the measured depth integrated AOM rates and the vertical distribution. AOM represents an important methane sink in the surface sediments of HR, consuming between 50 and 100% of the methane transported by advection.

Air pollution and climate-forcing impacts of a global hydrogen economy

Abstract: If today's surface traffic fleet were powered entirely by hydrogen fuel cell technology, anthropogenic emissions of the ozone precursors nitrogen oxide (NOx) and carbon monoxide could be reduced by up to 50%, leading to significant improvements in air quality throughout the Northern Hemisphere Model simulations of such a scenario predict a decrease in global OH and an increased lifetime of methane, caused primarily by the reduction of the NOx emissions The sign of the change in climate forcing caused by carbon dioxide and methane depends on the technology used to generate the molecular hydrogen A possible rise in atmospheric hydrogen concentrations is unlikely to cause significant perturbations of the climate system

Catalytic, Oxidative Condensation of CH4 to CH3COOH in One Step via CH Activation

Abstract: Acetic acid is an important petrochemical that is currently produced from methane (or coal) in a three-step process based on carbonylation of methanol. We report a direct, selective, oxidative condensation of two methane molecules to acetic acid at 180°C in liquid sulfuric acid. Carbon-13 isotopic labeling studies show that both carbons of acetic acid originate from methane. The reaction is catalyzed by palladium, and the results are consistent with the reaction occurring by tandem catalysis, involving methane C-H activation to generate Pd-CH 3 species, followed by efficient oxidative carbonylation with methanol, generated in situ from methane, to produce acetic acid.

Adsorption of Methane, Nitrogen, Carbon Dioxide, and Their Binary Mixtures on Dry Activated Carbon at 318.2 K and Pressures up to 13.6 MPa

Abstract: A detailed experimental study has been made of the adsorption of pure methane, nitrogen, carbon dioxide, and their binary mixtures on dry activated carbon (Filtrasorb 400, 12 × 40 mesh, Calgon Carbon) at 318.2 K and pressures up to 13.6 MPa. The mixture measurements were made at nominal feed-gas compositions of 20, 40, 60, and 80 mol %. The mixture data clearly elucidate the competitive nature of the individual-component adsorption from the mixtures. Measurements were made using a volumetric technique, coupled with gas chromatographic analysis of the equilibrium gas-phase compositions. Error propagation analysis reveals the expected average experimental uncertainties in the amount adsorbed of 2% for pure methane and nitrogen and 6% for CO2. For the mixture measurements, the uncertainties are estimated to be about 3% for the total adsorption, while the individual-component uncertainties vary from 0.02 to 0.2 mmol/g activated carbon, depending on the mixture composition. The data were correlated using the t...

Catalytic autothermal reforming of methane and propane over supported metal catalysts

Abstract: Catalytic autothermal reforming of methane and propane over supported metal catalysts has been investigated in the present study. The carbon deposition region and the heat balance of the reaction have been determined from the equilibrium calculations. The sequence of the activities of the 2 wt.% metal on alumina support for autothermal reforming of methane was Rh>Pd>Ni>Pt>Co. The catalytic activity of 10 wt.% Ni/Al 2 O 3 was higher than that of the 2 wt.% Rh/Al 2 O 3 . The activity of Ni was significantly lowered by the preferential oxidation of the catalyst in the reactant gas at low temperatures. Although little carbon deposition was observed for the autothermal reforming of methane in the deposition-free region expected from the equilibrium, a large amount of carbon deposition was observed for the propane autothermal reforming even in the steam-rich conditions. The deposited carbon possessed fibrous morphology. The catalytic autothermal reforming appears to be initiated by decomposition of hydrocarbon at the inlet zone; then the reforming reaction subsequently proceeded in the catalyst bed.

Experimental measurement of methane and carbon dioxide clathrate hydrate equilibria in mesoporous silica

Abstract: We present experimental structure-I clathrate hydrate (methane, carbon dioxide, and methane−carbon dioxide) equilibrium and ice-melting data for mesoporous silica glass. In both cases, high capillary pressures result in depressed solid decomposition temperatures (clathrate dissociation and ice melting), as a function of pore diameter. Clathrate dissociation data show a significant improvement over existing literature data, which is attributed to the improved experimental techniques and interpretative methods used. Through application of a melting (or clathrate dissociation) modified Gibbs−Thomson relationship to experimental data, we determine similar values of 32 ± 2, 32 ± 3, and 30 ± 3 mJ/m2 for ice−water, methane clathrate−water, and carbon dioxide clathrate−water interfacial tensions, respectively. The data are important for the accurate thermodynamic modeling of clathrate systems, particularly with respect to subsea sedimentary environments, and should prove useful in the simulation of potential meth...

Nonoxidative Activation of Methane

Abstract: Effective utilization of methane remains one of the long-standing problems in catalysis. Over the past several years, various routes, both direct and indirect, have been considered for the conversion of methane to value-added products such as higher hydrocarbons and oxygenates. This review will focus on the range of issues dealing with thermal and catalytic decomposition of methane that have been addressed in the last few years. Surface science studies (molecular beam methods and elevated-pressure reaction studies) involving methane activation on model catalyst systems are extensively reviewed. These studies have contributed significantly to our understanding of the fundamental dynamics of methane decomposition. Various aspects of the nonoxidative methane to higher hydrocarbon conversion processes such as high-temperature coupling and two-step low-temperature methane homologation have been discussed. Decomposition of methane results in the production of COx-free hydrogen (which is of great interest to sta...

Methane-rich Proterozoic atmosphere?

Abstract: Methane mixing ratios of 100–300 ppm in the Proterozoic atmosphere (0.75–2.3 Ga) would have been sufficient to offset the climatic effects of the faint early sun and maintain the warm climate during those ∼1.5 b.y. The major argument against this type of the atmosphere is the short atmospheric oxidation time of methane after the first oxygenation event ca. 2.3 Ga. Here we argue that the net methane flux from the oxygen-poor Pro tero zo ic ocean could have been 10–20 times higher than the present total biological methane flux. We demonstrate that increased methane production would have been sufficient to maintain methane concentrations at 100–300 ppm, which would keep the surface warm throughout the Proterozoic without invoking high CO 2 levels (although the CO 2 abundance could have been higher as well). A second oxygenation event at the end of the Proterozoic would have resulted in a decrease of methane flux and could have caused the first Neoproterozoic “snowball” glaciation.

Formation of Porous Gas Hydrates from Ice Powders: Diffraction Experiments and Multistage Model

Abstract: Gas hydrates grown at gas−ice interfaces were examined by electron microscopy and found to have a submicron porous structure. In situ observations of the formation of porous CH4- and CO2-gas hydrates from deuterated ice Ih powders were made, using time-resolved neutron diffraction on the high-flux diffractometer D20 (ILL, Grenoble) at different pressures and temperatures. For the first time neutron diffraction experiments were also performed with methane in hydrogenated samples. The isotopic differences between H2O and D2O are found insignificant concerning the clathrate formation kinetics. At similar excess fugacities, the reaction of CO2 was distinctly faster than that of CH4. The transient formation of the CO2-hydrate crystal structure II was also observed in coexistence with the usual type-I hydrate reaching a maximum of 5% after 5 h of the reaction at 272 K. A phenomenological model for the kinetics of the gas hydrate formation from ice powders is developed with special account of sample consolidatio...