Showing papers on "Methane published in 1988"

Biogeochemical aspects of atmospheric methane

Abstract: Methane is the most abundant organic chemical in Earth's atmosphere, and its concentration is increasing with time, as a variety of independent measurements have shown. Photochemical reactions oxidize methane in the atmosphere; through these reactions, methane exerts strong influence over the chemistry of the troposphere and the stratosphere and many species including ozone, hydroxyl radicals, and carbon monoxide. Also, through its infrared absorption spectrum, methane is an important greenhouse gas in the climate system. We describe and enumerate key roles and reactions. Then we focus on two kinds of methane production: microbial and thermogenic. Microbial methanogenesis is described, and key organisms and substrates are identified along with their properties and habitats. Microbial methane oxidation limits the release of methane from certain methanogenic areas. Both aerobic and anaerobic oxidation are described here along with methods to measure rates of methane production and oxidation experimentally. Indicators of the origin of methane, including C and H isotopes, are reviewed. We identify and evaluate several constraints on the budget of atmospheric methane, its sources, sinks and residence time. From these constraints and other data on sources and sinks we construct a list of sources and sinks, identities, and sizes. The quasi-steady state (defined in the text) annual source (or sink) totals about 310(±60) × 1012 mol (500(±95) × 1012 g), but there are many remaining uncertainties in source and sink sizes and several types of data that could lead to stronger constraints and revised estimates in the future. It is particularly difficult to identify enough sources of radiocarbon-free methane.

Chemical Kinetic Data Base for Combustion Chemistry. Part 3: Propane

Abstract: This publication contains evaluated and estimated data on the kinetics of reactions involving propane, isopropyl radical, n‐propyl radical, and various small inorganic and organic species which are of importance for proper small inorganic and organic species which are of importance for proper understanding of propane pyrolysis and combustion. It is meant to be used in conjunction with the kinetic data given in earlier publications which are of direct pertinence to the understanding of methane pyrolysis and combustion, but which also contain a large volume of data that are applicable to the propane system. The temperature range covered is 300–2500 K and the density range 1×1016 to 1×1021 molecules cm−3.

Oxidative Coupling of Methane to Higher Hydrocarbons

Abstract: Natural gas is an abundant resource in various parts of the world. Methane is the major component of natural gas, often comprising over 90 mol% of the hydrocarbon fraction of the gas. Methane itself is primarily used as a fuel, while the nonmethane components can be separated and used as feedstocks for the production of chemicals or liquid fuels. In many cases, however, natural gas reserves are found in locations distant from their place of utilization. Since it is not generally economical to transport liquefied natural gas, efficient methods are needed to convert methane into transportable liquid products. A possible route is oxidative dimerization of methane followed by oligomer-ization of the C2 products.

Carbon and hydrogen isotope fractionation resulting from anaerobic methane oxidation

Abstract: Methane oxidation in the anoxic sediments of Skan Bay, Alaska resulted in fractionation of carbon and hydrogen isotopes in methane. Isotope fractionation factors were estimated by fitting methane concentration, delta C{sup 13}-CH{sub 4} and delta-D-CH{sub 4} data with depth distributions predicted by an open system, steady state model. Assuming that molecular diffusion coefficients for C{sup 12}-CH{sub 4}, C{sup 13}-CH{sub 4}, and C{sup 12}-CH{sub 4}D are identical, the predicted fractionation factors were 0.0088 + or{minus}0.0013 and 1.157 + or{minus}0.023 for carbon and hydrogen isotopes, respectively. If aqueous diffusion coefficients for the different isotopic species of methane differ significantly, the predicted fractionation factors are larger by an amount proportional to the diffusion isotope effect. 50 refs., 5 figs.

The role of molecular hydrogen and methane oxidation in the water vapour budget of the stratosphere

Abstract: The detailed photochemistry of methane oxidation has been studied in a coupled chemical/dynamical model of the middle atmosphere The photochemistry of formaldehyde plays an important role in determining the production of water vapor from methane oxidation At high latitudes, the production and transport of molecular hydrogen is particularly important in determining the water vapor distribution It is shown that the ratio of the methane vertical gradient to the water vapor vertical gradient at any particular latitude should not be expected to be precisely 2, due both to photochemical and dynamical effects Modeled H2O profiles are compared with measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment at various latitudes Molecular hydrogen is shown to be responsible for the formation of a secondary maximum displayed by the model water vapor profiles in high latitude summer, a feature also found in the LIMS data

A methane flux time series for tundra environments

Abstract: Seasonal measurements of net methane flux were made at permanent sites representing important components of arctic tundra. The sites include Eriophorum tussocks, intertussock depressions, moss-covered areas, and Carex stands. Methane fluxes showed high diel, seasonal, intra site, and between site variability. Eriophorum tussocks and Carex dominated methane release to the atmosphere, with mean annual net methane fluxes of 8.05 + or{minus}2.50 g CH{sub 4}/sq m and 4.88 + or{minus}0.73 g CH{sub 4}/sq m, respectively. Methane fluxes form the moss sites and intertussock depressions were much lower. Over 90% of the mean annual methane flux from the Eriophorum, intertussock depressions, and Carex sites occurred between thaw and freeze-up. Some 40% of the mean annual methane flux from the moss sites occurred during winter. Composite methane fluxes for tussock tundra and Carex-dominated wet meadow tundra environments were produced by weighting measured component fluxes according to areal coverage. Tussock and wet meadow tundra account for an estimated global methane emission of 19-33 Tg/yr. 39 refs., 7 figs., 2 tabs.

Measurement of stable species present during filament‐assisted diamond growth

Abstract: We have measured mole fractions of two of the major stable species at the surface of a silicon substrate during filament‐assisted diamond growth as a function of the filament‐to‐substrate distance. Input gases were methane and hydrogen. A quartz probe withdrew gases at the growing surface, and the gases were sampled with an on‐line mass spectrometer. Close to the filament the methane is largely consumed, with most of the remaining gas phase carbon in the form of acetylene. Mass spectral results are compared to compositions calculated with a detailed chemical kinetics model. Our initial analysis suggests that diamond growth comes mainly from reaction of acetylene, ethylene, methane, or methyl radical.

Trapping of gas mixtures by amorphous water ice

Abstract: Our studies on gas trapping in amorphous water ice at 24-100 K were extended, by using mixtures of CH4, CO, N2, and Ar, rather than single gases. In 1:1 gas:(water vapor) mixtures, the competition among these gases on the available sites in the ice showed that the trapping capacity for the various gases is determined not only by the structure and dynamics of the ice, but is also influenced by the gas itself. Whereas at 24-35 K all four gases are trapped in the ice indiscriminantly, at 50-75 K there is a clear enhancement, in the order of CH4 > CO > N2 > or approximately Ar. This order is influenced by the gas-water interaction energy, the size of the trapped gas atom or molecule, the type of clathrate-hydrate formed (I or II) and, possibly, other factors. It seems that the gas can be trapped in the amorphous ice in several different locations, each being affected in a different way by the deposition temperature and gas composition. Once a gas atom or molecule is trapped in a specific location, it is predestined to emerge in one of eight different temperature ranges, which are associated with changes in the ice. The experimentally observed enhancements, together with the findings on the gas composition of comet Halley, might enable an estimation of the gas composition in the region of comet formation.

Low-temperature cross-polarization/magic angle spinning carbon-13 NMR of solid methane hydrates: structure, cage occupancy, and hydration number

Abstract: /sup 13/C NMR spectra of methane trapped in solid type I hydrate and in a mixed methane/propane type II hydrate were studied at -80/sup 0/C under cross-polarization and magic angle spinning conditions. Type I could be distinguished from type II structures due to the chemical shift pattern of methane trapped in the large and small cages. In agreement with the statistical theory of clathrate hydrate formation, methane was found to occupy both large and small cages in each structure to a significant extent. In the case of type I methane hydrate, the distribution of methane over the two sites was obtained, and together with a previous determination of the ..mu.. potential of the empty lattice with respect to ice, a hydration number of 6.05 +/- 0.06 was determined.

Selective low-power plasma decomposition of silane-methane mixtures for the preparation of methylated amorphous silicon.

Abstract: We report on a systematic study of the deposition and the physical properties of amorphous-silicon\char21{}carbon alloys produced by glow discharge from silane-methane mixtures. We define a ``low-power'' regime of preparation in which the chemistry of deposition is separated from the chemistry of the plasma. In this regime there is no primary decay of the methane, which then acts as a buffer gas. The carbon is incorporated in the solid only by reaction of the methane gas with the active species produced by the plasma decomposition of the silane. In this mode of preparation, we find the following. (1) The physical properties (in particular, the optical properties) are insensitive to the preparation conditions, depending mostly on the methane-to-silane ratio. (2) There is a smooth ``chemical'' incorporation of carbon in the amorphous silicon network, giving an alloy with good semiconducting properties. (3) The amount of carbon that can be incorporated is limited to less than 40 at. %, whereas in the high-power regime alloys can be produced with arbitrary [C]/[Si] proportions. (4) In the low-power regime the carbon is incorporated mostly in the form of methyl groups ${\mathrm{CH}}_{3}$, so that the material produced in this condition should be labeled as ``methylated amorphous silicon.''

Tropospheric methane from an Amazonian floodplain lake

Abstract: The sources of methane and its flux to the troposphere from the Amazonian floodplain were investigated during the dry season of July and August 1985, using measurements of methane concentration gradients obtained aboard a houseboat laboratory anchored in Lago Calado, a stratified dendritic lake of about 6-sq km area located near the center of the Amazon Basin. Methane concentrations in the mixed layer of the lake were found to vary from 0.0001 to 0.0055 mM, with no consistent temporal trend. The measured methane flux from the surface of the open lake to the atmosphere averaged 27 mg CH4/sq m per day, consistent with the buildup in ambient methane in the nocturnal surface mixed layer of the troposphere. Ebullition contributed 70 percent to the average total flux. The source of methane to the lake and, ultimately, to the troposphere is the benthic sediments.

Methane cycling in the sediments of Lake Washington

Abstract: Aerobic oxidation is important in the cycling of methane in the sediments of Lake Washington. About half of the methane flux from depth is oxidized to CO, in the upper 0.7 cm of the sediments and the remainder escapes into the water column. In terms of the total carbon budget of the lake, the upward flux of methane is insignificant with only about 2% of the carbon fixed by primary production being returned as methane. The upward flux of methane, however, does represent about 20% of the organic carbon decomposed within the sediments. In addition, methane oxidation consumes 7-10% of the total oxygen

Chemical model for the origin of minor limestone-shale cycles by anaerobic methane oxidation

Abstract: Chemical and isotopic studies of Jurassic carbonate concretions indicate an origin by anaerobic methane oxidation. Concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late, rejuvenated phase of sulfate reduction. Similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone-shale sequences, and an origin by anaerobic methane oxidation is proposed for these sequences also. The evolution of isolated concretions into diagenetic limestones depends on the extent of, and depth variations in, carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus. Diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive /sup 34/S values than the adjacent shales.

Recovery of methane from land fill gas

Abstract: OF THE DISCLOSURE High purity methane and carbon dioxide are recovered from landfill gas in an integrated multi-column adsorption system having a temperature swing adsorption section (TSA) for pretreatment of the crude landfill gas to remove trace impurities therefrom the thus cleaned gas being fed to a pressure swing adsorption section (PSA) for bulk separation of CO2 from methane. Regeneration of the impurity-laden adsorbent bed of the TSA section is carried out using part of the CO2 product gas recovered from the PSA section which gas is heated to thermal regeneration temperature.

Pressure and Temperature Effects on Growth and Methane Production of the Extreme Thermophile Methanococcus jannaschii

Abstract: The marine archaebacterium Methanococcus jannaschii was studied at high temperatures and hyperbaric pressures of helium to investigate the effect of pressure on the behavior of a deep-sea thermophile. Methanogenesis and growth (as measured by protein production) at both 86 and 90°C were accelerated by pressure up to 750 atm (1 atm = 101.29kPa), but growth was not observed above 90°C at either 7.8 or 250 atm. However, growth and methanogenesis were uncoupled above 90°C, and the high-temperature limit for methanogenesis was increased by pressure. Substantial methane formation was evident at 98°C and 250 atm, whereas no methane formation was observed at 94°C and 7.8 atm. In contrast, when argon was substituted for helium as the pressurizing gas at 250 atm, no methane was produced at 86°C. Methanogenesis was also suppressed at 86°C and 250 atm when the culture was pressurized with a 4:1 mix of H2 and CO2, although limited methanogenesis did occur when the culture was pressurized with H2.