Showing papers on "Methane published in 1992"

Catalytic chemistry of supported palladium for combustion of methane

Abstract: The high-temperature catalytic chemistry of supported palladium for methane oxidation has been studied. Palladium oxide supported on alumina decomposes in two distinct steps in air at one atmosphere. The first step occurs between 750 and 800 ° C and is believed to be a decomposition of palladium-oxygen species dispersed on bulk palladium metal designated (PdOx/Pd). The second decomposition is between 800 and 850 ° C and behaves like crystalline palladium oxide designated (PdO). To reform the oxide, the temperature must be decreased well below 650 ° C. Thus, there is a significant hysteresis between decomposition to palladium and re-formation of the oxide. Above 500 ° C, methane oxidation occurs readily when the catalyst contains PdO. However, when only palladium metal is present no oxygen adsorption occurs and no methane activity exists. One may conclude that the high temperature (> 500 ° C) activity of a supported palladium containing catalyst is due to the ability of palladium oxide to chemisorb oxygen. Palladium, as a metal, does not chemisorb oxygen above 650 ° C and thus, is completely inactive toward methane oxidation.

Geographical distribution and seasonal variation of surface emissions and deposition velocities of atmospheric trace gases

Abstract: The geographical distributions on the global scale of trace gas surface emission and deposition are established on the basis of a variety of technical, geographic, and climatic data. The 5° × 5° resolution maps of the sources and deposition velocities are constructed, which can be used as surface boundary conditions in a three-dimensional chemical/transport model of the troposphere. Special attention is devoted to emissions of CO, NOx, and several nonmethane hydrocarbons (NMHC) and to the fossil fuel emissions of methane. Anthropogenic sources, i.e., the emissions produced or controlled by human activities, represent about 75% or more of the total surface emissions of CO, CH4, SOx and NOx and about two thirds of the total production of atmospheric CO (from surface sources and atmospheric oxidation of hydrocarbons). The possibility arises that methane releases from natural gas exploitation in the USSR are substantially larger than accounted for in previous studies, implying possible important consequences for the methane budget.

A mechanism for the formation of methane hydrate and seafloor bottom‐simulating reflectors by vertical fluid expulsion

Abstract: Bottom-simulating reflectors (BSR) are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation. They are believed to correspond to the deepest level at which methane hydrate (clathrate) is stable. We present a model in which BSR hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field. In this model, most of the methane is generated below the level of hydrate stability, but not at depths sufficient for significant thermogenic production; the methane is primarily biogenic in origin. The model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid. The model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field, for the source of the large amount of methane contained in the hydrate, and for BSRs being common only in special environments. Strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes, so BSRs are uncommon. Upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation. In these areas hydrate BSRs are common. The most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field. The critical boundary in the phase diagram between the fluid-plus-hydrate and fluid-only fields is not well constrained. However, the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep-sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production. In most fluid expulsion regimes, the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation. Thus, as observed in borehole data, there are no large chloride or isotope anomalies remaining in the local pore fluids. The differences in the concentration of methane and probably of CO2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions.

Thermodynamic properties and dissociation characteristics of methane and propane hydrates in 70-.ANG.-radius silica gel pores

Abstract: The pressure-temperature profiles for the hydrate-ice-gas and hydrate-liquid water-gas equilibria were measured for methane and propane hydrates in 70-{Angstrom}-radius silica gel pores In both cases, the equilibrium pressures were 20-100% higher than those for the bulk hydrates The dissociation characteristics of the gas hydrates in pores were also studied calorimetrically by heating the hydrates under about zero pressure from 100 K to room temperature It was found that after the initial dissociation into ice and gas the hydrate became totally encapsulated among the pore walls and the ice caps formed at the pore openings The hydrate thus trapped in the interior of the pore remained stable up to the melting point of pore ice These results are similar to those obtained in previous studies on the bulk hydrates which are also stabilized by a shielding layer of ice However, the apparent increase in the stability of the pore hydrates was found to be much larger than that of the bulk hydrates The composition of methane hydrate in 70-{Angstrom} pores was determined to be CH{sub 4}{center_dot}594H{sub 2}O, and its heat of dissociation into pore water and gas, obtained calorimetrically, was 4592 kJ mol{sup {minus}1}; The corresponding values in the bulk phasemore » are 600 and 5419 kJ mol{sup {minus}1}, respectively 30 refs, 4 figs, 2 tabs« less

Catalytic reduction of nitrogen oxides with methane in the presence of excess oxygen

Abstract: We discovered a family of catalysts that can effectively reduce NOx with methane in the presence of excess oxygen This new catalytic chemistry offers an alternative means for controlling NOx emissions Complete reduction of nitric oxide was obtained at 400°C over a Co-ZSM-5 catalyst The presence of oxygen in the feed greatly enhances the nitric oxide reduction activity on Co-ZSM-5, and the nitric oxide conversion is strongly related to the inlet methane level On the other hand, Cu-ZSM-5, which is a unique catalyst for the direct nitric oxide decomposition, is a poor catalyst for nitric oxide reduction by methane in the presence of excess of oxygen

Northern fens: methane flux and climatic change

Abstract: Methane flux from northern peatlands is believed to be an important contribution to the global methane budget. High latitude regions are predicted to experience significant changes in surface temperature and precipitation associated with the 2 × CO 2 climate scenarios, but the effects of these changes on methane emission are poorly understood. A peatland hydrologic model predicted June - August decreases in water storage of between 82 and 144 mm, using as inputs increases in temperatures of 3 °C and rainfall of 1 mm d - . These changes translate into a water table drop, relative to the peat surface, of between 14 and 22 cm, depending on whether the fen has a floating or non-floating surface. The 3 °C air temperature increase was predicted to raise peat temperature at 10 cm depth by 0.8 °C. These changes were then applied to relationships derived at a subarctic fen for water table: methane flux: temperature at 10 cm depth. Increased temperatures raise the methane flux by between 5 and 40%, but the lowered water table decreases methane flux by 74 and 81%, at the floating and nonfloating fen sites, respectively. These results suggest that methane emissions from northern peatlands are more sensitive to changes in moisture regime than temperature within the range of changes predicted for 2 × CO 2 scenarios. DOI: 10.1034/j.1600-0889.1992.t01-1-00002.x

Slowing down of the global accumulation of atmospheric methane during the 1980s

Abstract: MEASUREMENTS of methane in modern air1–8 and in air trapped in ice cores9–12 have shown convincingly that the abundance of atmospheric methane has been rising since the Industrial Revolution. This is a matter of concern because of the important role of methane in determining the radiative balance and chemical composition of the atmosphere13. The causes of this increase have not been identified unambiguously because of uncertainties in our understanding of the global budget of atmospheric methane14 and in how it is changing with time. Here we report on measurements of atmospheric methane from an extensive global network of flask sampling sites, which reveal that, although methane continues to accumulate in the atmosphere, there has been a substantial slowing of the global accumulation rate between 1983 and 1990. If this deceleration continues steadily, global methane concentrations will reach a maximum around the year 2006. Our results hint that changes in methane emissions in the latitude band 30–90° N may be of particular significance to this trend.

Methane emission from rice fields: The effect of floodwater management

Abstract: Rice fields emit methane and are important contributors to the increasing atmospheric CH4 concentration. Manipulation of rice floodwater may offer a means of mitigating methane emission from rice fields without reducing rice yields. To test methods for reducing methane emission, we applied four water management methods to rice fields planted on silty-clay soils near Beaumont, Texas. The four water treatments investigated were: normal permanent flood (46 days post planting), normal flood with mid- season drainage aeration, normal flood with multiple drainage aeration, and late flood (76 days post planting). Methane emission rates varied markedly with water regime, showing the lowest seasonal total emission (1.2 g m−2) with a multiple-aeration treatment and the highest (14.9 g m−2) with a late flood. Although the multiple- aeration water management treatment emitted 88% less methane than the normal irrigation treatment and did not reduce rice yields, the multiple-aeration treatment did require 2.7 times more water than the 202 mm required by the normal floodwater treatment. A comparison of measured methane emission and production rates obtained from incubated soil cores indicated that, depending on time of season and flood condition, from zero to over 90% of the methane produced was oxidized. The average amount of methane which was oxidized during times of high emission was 73.1 ± 13.7 percent of that produced.

Effects of Temperature on Methane Consumption in a Forest Soil and in Pure Cultures of the Methanotroph Methylomonas rubra.

Abstract: Methane oxidation in soil cores from a mixed hardwood-coniferous forest varied relatively little as a function of incubation temperatures from −1 to 30°C. The increase in oxidation rate was proportional to T2.4 (in kelvins). This relationship was consistent with limitation of methane transport through a soil gas phase to a subsurface zone of consumption by diffusion. The Q10 for CO2 production, 3.4, was substantially higher than that for methane oxidation, 1.1, and indicated that the response of soil respiration to temperature was limited by enzymatic processes and not diffusion of either organic substrates or molecular oxygen. When grown under conditions of phase-transfer limitation, cultures of Methylomonas rubra showed a minimal response to temperature changes between 19 and 38°C, as indicated by methane oxidation rates; in the absence of phase-transfer limitations, M. rubra oxidized methane at rates strongly dependent on temperature.

Method of recovery of natural gases from underground coal formations

Abstract: Methane is produced from a coal seam penetrated by an injection well and a gas production well by first introducing liquefied or gaseous carbon dioxide through the injection well and into the coal seam and subsequently introducing a weakly adsorbable gas through the injection well and into the coal seam. As the weakly adsorbable gas passes through the coal seam, it forces the carbon dioxide through the seam. If the carbon dioxide is in liquefied form, it evaporates as it moves through the seam, and the carbon dioxide gas desorbs methane from the coal and sweeps it toward the production well. The methane is withdrawn from the seam through the production well.

Importance of methane-oxidizing bacteria in the methane budget as revealed by the use of a specific inhibitor

Abstract: METHANE is a greenhouse gas whose concentration in the atmosphere is increasing1–3. Much of this methane is derived from the metabolism of methane-generating (methanogenic) bacteria4,5 and over the past two decades much has been learned about the ecology of methanogens; specific inhibitors of methanogenesis, such as 2-bromoethanesulphonic acid, have proved useful in this regard6. In contrast, although much is known about the biochemistry of methane-oxidizing (methanotrophic) bacteria7, ecological investigations have been hampered by the lack of an analogous specific inhibitor6. Methanotrophs limit the flux of methane to the atmosphere from sediments8,9 and consume atmospheric methane10, but the quantitative importance of methanotrophy in the global methane budget is not well known5. Methylfluoride (CH3F) is known to inhibit oxygen consumption by Methylococcus capsu-latus11, and to inhibit the oxidation of 14CH4 to 14CO2 by endosymbionts in mussel gill tissues12. Here we report that methylfluoride (MF) inhibits the oxidation of methane by methane monooxy-genase, and by using methylfluoride in field investigations, we find that methanotrophic bacteria can consume more than 90% of the methane potentially available.

Kinetic isotopic fractionation during air‐water gas transfer of O2, N2, CH4, and H2

Abstract: The authors present experimental results that show that the kinetic isotopic fractionation during gas exchange is 0.9972 [plus minus] 0.0002 for oxygen, 0.9992 [plus minus] 0.0002 for methane, 0.9987 [plus minus] 0.0001 for nitrogen and 0.982 [plus minus] 0.002 for hydrogen, and that the equilibrium fractionation between water and gas phases is 1.037 for hydrogen. They show that the isotopic fractionation during gas transfer for these gases is not equal to the square root of their reduced mass in water, as would be predicted by an extension of the kinetic theory of ideal gases to dissolved gases. The use of isotopes as tracers of biogeochemical gases requires knowledge of the fractionation factor for air-water gas transfer; there have been few direct measurements of these factors. 31 refs., 11 figs., 1 tab.

Selective adsorption of simple mixtures in slit pores: a model of methane-ethane mixtures in carbon

Abstract: The authors report a study of Lennard-Jones mixtures in model carbon pores having parallel walls. The fluid potential parameters were chosen to model methane and ethane, and the 10-4-3 model was used for the solid-fluid potential. A density-functional theory with the nonlocal density approximation was used for the calculations. The authors focused on the selectivity of ethane relative to methane for a wide range of system parameters. Different types of selectivity isotherm were found, which can be explained microscopically in terms of intermolecular and surface forces. Macroscopically, each type of isotherm corresponds to a certain range of temperature in relation to the capillary critical temperatures. At very low temperatures where layering transitions occur, the selectivity isotherm is steplike. The density profiles show a strong surface segregation of ethane from methane. 48 refs., 23 figs.

Mechanisms for the reactions between methane and the neutral transition metal atoms from yttrium to palladium

Abstract: Calculations including electron correlations have been performed for the oxidative addition reactions between methane and the whole sequence of second row transition metal atoms from yttrium to palladium. The lowest barrier for the C-H insertion reaction is found for the rhodium atom. Palladium has the lowest methane elimination barrier. The barrier height is governed by two factors. In the reactant channel low repulsion favors a low barrier, and in the product channel strong bond formation is important. The atomic state with lowest repulsion toward methane is the d{sup n+2} state and the strongest bonds are formed to the d{sup n+1} s state. For rhodium both these states are energetically low lying. Only palladium has a bound {eta}{sup 2} precursor state on the ground state potential surface. Another interesting result is that the potential surface for the reaction between methane and the rhodium atom is remarkably similar to the potential surface for the reaction between methane and ClRhL{sub 2}, which has been studied experimentally. 44 refs., 5 figs., 5 tabs.

Bubbling Reefs In The Kattegat - Submarine Landscapes Of Carbonate-Cemented Rocks Support A Diverse Ecosystem At Methane Seeps

Abstract: Methane seeps in shallow waters In the northern Kattegat off the Danish coast form spectacular submarine landscapes the 'bubbling reefs' due to carbonate-cemented sandstone structures which are colonized by brightly coloured animals and plants. These structures may be 100 m2 in area and consist of pavements, complex formations of overlying slab-type layers, and pillars up to 4 m high. The carbonate cement (high-magnesium calcite, dolomite or aragonite) is 13C-depleted, indicating that it originated as a result of microbial methane oxidation. It is believed that the cementation occurred in the subsurface and that the rocks were exposed by subsequent erosion of the surrounding unconsolidated sediment. The formations are interspersed with gas vents that intermittently release gas, primarily methane, at up to 25 1 h-' The methane most likely originated from the microbial decomposition of plant material deposited during the Eemian and early Weichselian periods, i.e. l00 000 to 125 000 years B.P. Aerobic methane oxidation in the sediment was restricted Lo the upper 4 cm in muddy sand and to the upper 13 cm In coarse sand. Maximum aerobic methane oxidation rates ranged from 4.8 to 45.6 pm01 dm-3 d". The rock surfaces and epifauna around the seeps were also sites of methane-oxidizing activity. Integrated sulphate reduction rates for the upper 10 cm of muddy sand gave 4.2 to 26.6 mm01 m-2 d-' These rates are higher than those previously reported from similar water depths in the Kattegat but did not relate to the sediment methane content. Since gas venting occurs over several km2 of the sea floor in the Kattegat it is likely to make a significant local contribution to the cycling of elements in the sediment and the water column. The rocks support a diverse ecosystem ranging from bacteria to macroalgae and anthozoans. Many animals live within the rocks in holes bored by sponges, polychaetes and bivalves. Stable carbon isotope composition (6'") of tissues of invertebrates from the rocks were in the range -17 to -24 'A, indicating that methane-derived carbon makes little direct contribution to their nutrition. Within the sediments surrounding the seeps there is a poor metazoan fauna, in terms of abundance, diversity and biomass. This may be a result of toxicity due to hydrogen sulphide input from the gas.