Showing papers on "Methane published in 1998"

Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative

Abstract: Platinum catalysts are reported for the direct, low-temperature, oxidative conversion of methane to a methanol derivative at greater than 70 percent one-pass yield based on methane. The catalysts are platinum complexes derived from the bidiazine ligand family that are stable, active, and selective for the oxidation of a carbon-hydrogen bond of methane to produce methyl esters. Mechanistic studies show that platinum(II) is the most active oxidation state of platinum for reaction with methane, and are consistent with reaction proceeding through carbon-hydrogen bond activation of methane to generate a platinum-methyl intermediate that is oxidized to generate the methyl ester product.

Methane production and methane consumption: a review of processes underlying wetland methane fluxes.

Abstract: Potential rates of both methane production and methane consumption vary over three orders of magnitude and their distribution is skew. These rates are weakly correlated with ecosystem type, incubation temperature, in situ aeration, latitude, depth and distance to oxic/anoxic interface. Anaerobic carbon mineralisation is a major control of methane pro- duction. The large range in anaerobic CH4:CO2 production rates indicate that a large part of the anaerobically mineralised carbon is used for reduction of electron acceptors, and, hence, is not available for methanogenesis. Consequently, cycling of electron acceptors needs to be studied to understand methane production. Methane and oxygen half saturation constants for methane oxidation vary about one order of magnitude. Potential methane oxidation seems to be correlated with methanotrophic biomass. Therefore, variation in potential methane oxidation could be related to site characteristics with a model of methanotrophic biomass.

Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability

Abstract: Atmospheric methane mixing ratios from 1000 A.D. to present are measured in three Antarctic ice cores, two Greenland ice cores, the Antarctic firn layer, and archived air from Tasmania, Australia. The record is unified by using the same measurement procedure and calibration scale for all samples and by ensuring high age resolution and accuracy of the ice core and firn air. In this way, methane mixing ratios, growth rates, and interpolar differences are accurately determined. From 1000 to 1800 A.D. the global mean methane mixing ratio averaged 695 ppb and varied about 40 ppb, contemporaneous with climatic variations. Interpolar (N-S) differences varied between 24 and 58 ppb. The industrial period is marked by high methane growth rates from 1945 to 1990, peaking at about 17 ppb yr−1 in 1981 and decreasing significantly since. We calculate an average total methane source of 250 Tg yr−1 for 1000–1800 A.D., reaching near stabilization at about 560 Tg yr−1 in the 1980s and 1990s. The isotopic ratio, δ13CH4, measured in the archived air and firn air, increased since 1978 but the rate of increase slowed in the mid-1980s. The combined CH4 and δ13CH4 trends support the stabilization of the total CH4 source.

Continuing decline in the growth rate of the atmospheric methane burden

Abstract: The global atmospheric methane burden has more than doubled since pre-industrial times1,2, and this increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse-gas emissions. Research into future climate change and the development of remedial environmental policies therefore require a reliable assessment of the long-term growth rate in the atmospheric methane load. Measurements have revealed that although the global atmospheric methane burden continues to increase2 with significant interannual variability3,4, the overall rate of increase has slowed2,5. Here we present an analysis of methane measurements from a global air sampling network that suggests that, assuming constant OH concentration, global annual methane emissions have remained nearly constant during the period 1984–96, and that the decreasing growth rate in atmospheric methane reflects the approach to a steady state on a timescale comparable to methane's atmospheric lifetime. If the global methane sources and OH concentration continue to remain constant, we expect average methane mixing ratios to increase slowly from today's 1,730 nmol mol−1 to ∼1,800 nmol mol−1, with little change in the contribution of methane to the greenhouse effect.

Porous adsorbents for vehicular natural gas storage: a review

Abstract: Methane adsorption data (both experimental and simulated) under conditions of direct relevance for vehicular natural gas storage, i.e., at 500 psig and ambient temperature, has been compiled from the literature for various microporous adsorbents and discussed in this work. Characterization of microporosity has been briefly reviewed, followed with a discussion on the porous structure of natural gas adsorbents. A common trend of gravimetric methane adsorption capacity scaling with surface area among the diverse microporous adsorbents (viz., coals, carbons, zeolites, silica gel and an MCM-41 type material) is demonstrated. Further, it is substantiated and emphasized that increasing the adsorbent surface area on a volumetric basis is very important for vehicular natural gas storage where the fuel storage volume is a constraint. The effect of other adsorbent properties such as heat of adsorption and heat capacity on the natural gas storage capacity is also discussed.

Emission factors of hydrocarbons, halocarbons, trace gases and particles from biomass burning in Brazil

Abstract: Airborne measurements of the emissions of gases and particles from 19 individual forest, cerrado, and pasture fires in Brazil were obtained during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) study in August-September 1995. Emission factors were determined for a number of major and minor gaseous and particulate species, including carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, methane, nonmethane hydrocarbons, halocarbons, particulate (black and organic) carbon, and particulate ionic species. The magnitude of the emission factors for gaseous species were determined primarily by the relative amounts of flaming and smoldering combustion, rather than differences in vegetation type. Hydrocarbons and halocarbons were well correlated with CO, which is indicative of emissions primarily associated with smoldering combustion. Although there was large variability between fires, higher emission factors for SO2 and NOχ were associated with an increased ratio of flaming to smoldering combustion; this could be due to variations in the amounts of sulfur and nitrogen in the fuels. Emission factors for particles were not so clearly associated with smoldering combustion as those for hydrocarbons. The emission factors measured in this study are similar to those measured previously in Brazil and Africa. However, particle emission factors from fires in Brazil appear to be roughly 20 to 40% lower than those from North American boreal forest fires.

Air-water gas exchange

Abstract: ▪ Abstract The exchange of inert and sparingly soluble gases—including carbon dioxide, methane, and oxygen—between the atmosphere and oceans is controlled by a thin 20- to 200-μm-thick boundary layer at the top of the ocean. The hydrodynamics in this layer are significantly different from boundary layers at rigid walls, since the orbital motion of the waves is of the same order as the velocities in the viscous boundary layer. Laboratory and field measurements show that wind waves significantly increase the gas transfer rate and that it is significantly influenced in this way by surfactants. Because of limited experimental techniques, the mechanisms for this enhancement and the structure of the turbulence in the boundary layer at a wavy water surface are still not known. A number of new imaging techniques are described that give direct insight into the processes and promise to trigger substantial theoretical progress in the near future.

CO2 reforming of methane on Ni catalysts: Effects of the support phase and preparation technique

Abstract: The effects of the support phase and catalyst preparation methods on catalytic activity and carbon deposition were systematically investigated over nickel catalysts supported on Al2O3, SiO2 and MgO for the reforming reaction of methane with carbon dioxide. It is found that the pore structure of the support and metal-support interaction significantly affected the catalytic activity and coking resistance. Catalyst with well-developed porosity exhibited higher catalytic activity. Strong interaction between metal and the support made the catalyst more resistant to sintering and coking, thus resulting in a longer time of catalyst stability. (C) 1998 Elsevier Science B.V.

CO2-Free Production of Hydrogen by Catalytic Pyrolysis of Hydrocarbon Fuel

Abstract: All conventional options of hydrogen production from hydrocarbon fuel (primarily natural gas, NG), e.g., steam reforming (SR), partial oxidation, and autothermal reforming, involve CO2 production at some point in the technological chain of the process. Therefore, the main problem remains: how to produce hydrogen from hydrocarbon fuels without CO2 emission. The capture of CO2 from the SR process streams and its sequestration (underground or ocean disposal) are actively discussed in the literature. However, this method is energy intensive, poses uncertain ecological consequences, and still does not completely eliminate CO2 emission. Another approach is to decompose hydrocarbon fuels into hydrogen and carbon. The thermal decomposition of NG is a technologically simple one-step process without energy and material intensive gas separation stages and shows the potential to be a CO2-free hydrogen production process. The experimental results of the thermocatalytic decomposition of gaseous (methane and propane) a...

Modeling the partial oxidation of methane in a short‐contact‐time reactor

Abstract: Partial oxidation of methane in monolithic catalysts at very short contact times offers a promising route to convert natural gas into syngas (H2 and CO), which can then be converted to higher alkanes or methanol. Detailed modeling is needed to understand their complex interaction of transport and kinetics in these systems and for their industrial application. In this work, the partial oxidation of methane in noble-metal (Rh and Pt)-coated monoliths was studied numerically as an example of short-contact-time reactor modeling. A tube wall catalytic reactor was simulated as a model for a single pore of the monolithic catalyst using a 2-D flow field description coupled with detailed reaction mechanisms for surface and gas-phase chemistry. The catalytic surface coverages of adsorbed species are calculated vs. position. The reactor is characterized by competition between complete and partial oxidation of methane. At atmospheric pressure, CO2 and H2O are formed on the catalytic surface at the entrance of the catalytic reactor. At higher pressure, gas-phase chemistry becomes important, forming more complete oxidation products downstream and decreasing syngas selectivity by about 2% at 10 bar. Temperature (from 300 to ∼ 1,200 K), velocity, and transport coefficients change very rapidly at the catalyst entrance. The dependence of conversion and selectivity on reactor conditions was examined.

Fluxes of Carbon Dioxide, Nitrous Oxide, and Methane in Grass Sod and Winter Wheat‐Fallow Tillage Management

Abstract: Cropping and tillage management can increase atmospheric CO{sub 2}, N{sub 2}O, and CH{sub 4} concentrations, and contribute to global warming and destruction of the ozone layer. Fluxes of these gases in vented surface chambers, and water-filled pore space (WFPS) and temperature of surface soil were measured weekly from a long-term winter wheat (Triticum aestivum L.)-fallow rotation system under chemical and mechanical tillage follow management and compared with those from native grass sod at Sidney, NE, from March 1993 to July 1995. Cropping, tillage, within-field location, time of year, soil temperature, and WFPS influenced net greenhouse gas fluxes. Mean annual interrow CO{sub 2} emissions from wheat-fallow ranged from 6.0 to 20.1 kg C ha{sup {minus}1} d{sup {minus}1} and generally increased with intensity and degree of tillage. Nitrous oxide flux averaged autumn > winter. Winter periods accounted for 4 to 10% and 3 to 47% of the annual CO{sub 2} and N{sub 2}O flux, respectively, and 12 to 21% of the annual CH{sub 4} uptake. Fluxes of CO{sub 2} and N{sub 2}O, and CH{sub 4} uptake increased linearly with soil temperature. No-till fallow exhibited the least threat to deterioration of atmospheric or soil quality as reflected by greater CH{sub 4} uptake, decreased N{sub 2}O and CO{sub w} emissions, and less loss of soil organic C than tilled soils. However, potential for increased C sequestration in this wheat-fallow system is limited due to reduced C input from intermittent cropping.« less

Quantitative Production of H2 by Pyrolysis of Gas Chromatographic Effluents

Abstract: Hydrogen gas can be produced quantitatively from nanomole amounts of organic H in continuously flowing gas streams. The system described here is suitable for use in isotope-ratio-monitoring mass spectrometric systems and is based on a pyrolysis reactor consisting of a graphitized alumina tube heated to 1450 °C. Methane forms as an intermediate product at temperatures above 750 °C, but, for all tested analytes, yields of H2 were quantitative at temperatures between 1430 and 1460 °C, provided residence times in the reactor were greater than 300 ms. Quantitative yields of H2 were obtained for all components of a homologous series of n-alkanes (C15 to C30). Analyses of low-molecular-weight alcohols demonstrated that O-bound H was also quantitatively converted to H2 and, thus, that H2O, if formed, was quantitatively reduced to H2.

Comparative study at low and medium reaction temperatures of syngas production by methane reforming with carbon dioxide over silica and alumina supported catalysts

Abstract: Two series of transition metals (Co, Ni, Ru, Rh, Ir, Pt) based catalysts have been prepared using silica and γ-alumina. Their activity and stability for the dry reforming of methane in the temperature interval from 673 up to 1023 K have been examined and compared. The obtained results show that the support exerts a great influence on the turnover frequency of a given metal but deactivation occurring under reaction conditions mainly depends on the nature of the active metallic phase. Some of the catalysts suffer deactivation processes at temperatures close to 1023 K, that can be ascribed to sintering, and in only some cases, also to carbon deposition. At low reaction temperatures, i.e. 723 K, deactivation was only observed over supported iridium. Among all tested catalysts, those based on nickel, cobalt and rhodium appear to be the most resistant to deactivation processes under our experimental conditions and for the whole temperature range up to 1023 K. Among them, supported rhodium catalysts show an excellent stability, though alumina supported rhodium exhibits a much higher turnover frequency.

Use of stable isotopes to determine methane oxidation in landfill cover soils

Abstract: The mean isotopic composition of CH4 emitted from six New England (United States) landfills was 13C and D enriched (−48.1 to −50.4‰ and −273 to −281‰) relative to anoxic zone landfill CH4 (mean values of −55.9 to −56.2‰ and −296 to −300‰) owing to the oxidation of methane as it was transported from the landfill to the atmosphere through the soil cap. The fraction of methane oxidized f0 during its passage through the soil cap was calculated from the degree of 13C enrichment in emitted CH4 relative to anoxic zone CH4 in conjunction with values determined for the preference of soil methane oxidizing bacteria for 12CH4 over 13CH4 (α = 1.022 ± 0.008). Mean values for methane oxidation in six landfills were from 24 to 35% of the total flux through the soil during the warm season, depending upon how the data were grouped. Our results bracket recent estimates of methane oxidation of about 30% in the warm summer period produced using a model with the input terms of soil temperature, moisture, depth, and oxygen concentration. Because of variations in the response of methane oxidation to temperature at these New England sites, our study is consistent with the modeling results of Czepiel et al. [1996b] that the best estimate for the annual value for methane oxidation in the landfills considered is about 10%.

Methane combustion on some perovskite-like mixed oxides

Abstract: Some perovskite-like mixed oxides of general formula La1−xA′xBO3 were prepared by the amorphous citrate method and tested for methane combustion within the 300–600°C temperature range. Substitution at A-site with a bivalent (Eu, Sr) or tetravalent (Ce) metal cation led to a decrease or increase of catalytic activity, respectively. La0.9Ce0.1CoO3 proved to be the most active catalyst, showing complete conversion at 500°C. The nature of the metal cation introduced modifies the oxidation state of cobalt, which leads to the formation of cationic or anionic vacancies. TPD-MS analysis confirmed that the catalytic activity is related to the oxygen storage properties of the catalyst. The substitution at B-site (B=Fe, Co, Ni) allowed to find interesting correlations between catalytic activity and the temperature Tmax of maximum oxygen desorption rate.