Showing papers on "Methane published in 1997"
TL;DR: In this paper, the authors present an inverse modeling approach which can be used to deduce information on methane sources and sinks from the temporal and spatial variations of atmospheric methane mixing ratios, based on a three-dimensional atmospheric transport model combined with a tropospheric background chemistry module, is also employed to calculate the global distribution of OH radicals which provide the main sink for atmospheric methane.
Abstract: Estimates of the global magnitude of atmospheric methane sources are currently mainly based on direct flux measurements in source regions. Their extrapolation to the entire globe often involves large uncertainties. In this paper, we present an inverse modeling approach which can be used to deduce information on methane sources and sinks from the temporal and spatial variations of atmospheric methane mixing ratios. Our approach is based on a three-dimensional atmospheric transport model which, combined with a tropospheric background chemistry module, is also employed to calculate the global distribution of OH radicals which provide the main sink for atmospheric methane. The global mean concentration of OH radicals is validated with methyl chloroform (CH3CCl3) observations. The inverse modeling method optimizes the agreement between model-calculated and observed methane mixing ratios by adjusting the magnitudes of the various methane sources and sinks. The adjustment is constrained by specified a priori estimates and uncertainties of the source and sink magnitudes. We also include data on the 13C/12C isotope ratio of atmospheric methane and its sources in the model. Focusing on the 1980s, two scenarios of global methane sources are constructed which reproduce the main features seen in the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) methane observations. Differences between these two scenarios may probably be attributed to underestimated a priori uncertainties of wetland emissions. Applying the inverse model, the average uncertainty of methane source magnitudes could be reduced by at least one third. We also examined the decrease in the atmospheric methane growth rate during the early 1990s but could not uniquely associate it with changes in particular sources.
577 citations
01 Jan 1997
TL;DR: In this article, anaerobic digestion of energy crops and organic wastes is used to produce a clean fuel from renewable feedstocks, which would replace fossil fuel-derived energy and reduce environmental impacts including global warming and acid rain.
Abstract: Production of methane via anaerobic digestion of energy crops and organic wastes would benefit society by providing a clean fuel from renewable feedstocks. This would replace fossil fuel-derived energy and reduce environmental impacts including global warming and acid rain. Although biomass energy is more costly than fossil fuel-derived energy, trends to limit carbon dioxide and other emissions through emission regulations, carbon taxes, and subsidies of biomass energy would make it cost competitive. Methane derived from anaerobic digestion is competitive in eAciencies and costs to other biomass energy forms including heat, synthesis gases, and ethanol. 7 2000 Elsevier Science Ltd. All rights reserved.
501 citations
TL;DR: In this article, a detailed description of the formation and the gasification of filamentous carbon is given, and a thermodynamic basis for the different solubilities is provided, where the segregation of carbon, taking place at the gas side of the nickel particle is added as one of the steps in the global mechanism of carbon filament formation and gasification.
378 citations
TL;DR: In this paper, the interaction of methane with unsupported and supported molybdenum compounds (Mo, MoO2, Mo3, Mo2C, and MoC(1−x)) has been investigated at 973 K. ZSM-5 was used as a support.
370 citations
TL;DR: In this paper, the results from smoldering combustion during 24 fires are presented including production of carbon dioxide, carbon monoxide, methane, ethene, ethyne, propene, formaldehyde, 2-hydroxyethanal, methanol, phenol, acetic acid, formic acid, ammonia, hydrogen cyanide, and carbonyl sulfide.
Abstract: Biomass samples from a diverse range of ecosystems were burned in the Intermountain Fire Sciences Laboratory open combustion facility. Midinfrared spectra of the nascent emissions were acquired at several heights above the fires with a Fourier transform infrared spectrometer (FTIR) coupled to an open multipass cell. In this report, the results from smoldering combustion during 24 fires are presented including production of carbon dioxide, carbon monoxide, methane, ethene, ethyne, propene, formaldehyde, 2-hydroxyethanal, methanol, phenol, acetic acid, formic acid, ammonia, hydrogen cyanide, and carbonyl sulfide. These were the dominant products observed, and many have significant influence on atmospheric chemistry at the local, regional, and global scale. Included in these results are the first optical, in situ measurements of smoke composition from fires in grasses, hardwoods, and organic soils. About one half of the detected organic emissions arose from fuel pyrolysis which produces white smoke rich in oxygenated organic compounds. These compounds deserve more attention in the assessment of fire impacts on the atmosphere. The compound 2-hydroxyethanal is a significant component of the smoke, and it is reported here for the first time as a product of fires. Most of the observed alkane and ammonia production accompanied visible glowing combustion. NH3 is normally the major nitrogen-containing emission detected from smoldering combustion of biomass, but from some smoldering organic soils, HCN was dominant. Tar condensed on cool surfaces below the fires accounting for ∼1% of the biomass burned, but it was enriched in N by a factor of 6–7 over the parent material, and its possible role in postfire nutrient cycling should be further investigated.
363 citations
TL;DR: In this paper, the effects of pH and moisture content on high-solids sludge digestion were investigated using a mesophilic batch digester fed with sludge cake, and the experiments were carried out by changing the initial moisture contents from 90 to 96% and the initial pH from 5.6-7.8 with an optimum of pH 6.3.
354 citations
TL;DR: In this article, the authors report the direct measurement of in situ methane abundances stored as gas hydrate and free gas in a sediment sequence from the Blake ridge, western Atlantic Ocean.
Abstract: Certain gases can combine with water to form solids—gas hydrates—that are stable at high pressures and low temperatures1,2. Conditions appropriate for gas-hydrate formation exist in many marine sediments where there is a supply of methane. Seismic reflection profiles across continental margins indicate the frequent occurrence of gas hydrate within the upper few hundred metres of sea-floor sediments, overlying deeper zones containing bubbles of free gas3–9. If large volumes of methane are stored in these reservoirs, outgassing may play an important role during climate change10–12. Gas hydrates in oceanic sediments may in fact comprise the Earth's largest fossil-fuel reservoir2,13. But the amount of methane stored in gas-hydrate and free-gas zones is poorly constrained2–9,13–18. Here we report the direct measurement of in situ methane abundances stored as gas hydrate and free gas in a sediment sequence from the Blake ridge, western Atlantic Ocean. Our results indicate the presence of substantial quantities of methane (˜15 GT of carbon) stored as solid gas hydrate, with an equivalent or greater amount occurring as bubbles of free gas in the sediments below the hydrate zone.
350 citations
TL;DR: The sizes of major sources and sinks of atmospheric methane (CH4), an important greenhouse gas, are poorly known as discussed by the authors, but it is known that CH4 from north Siberian lakes contributes ∼1.5 teragrams CH4 year−1 to observed winter increases in atmospheric CH4 concentration at high northern latitudes.
Abstract: The sizes of major sources and sinks of atmospheric methane (CH4), an important greenhouse gas, are poorly known. CH4 from north Siberian lakes contributes ∼1.5 teragrams CH4 year−1 to observed winter increases in atmospheric CH4 concentration at high northern latitudes. CH4 emitted from these lakes in winter had a radiocarbon age of 27,200 years and was derived largely from Pleistocene-aged carbon.
330 citations
TL;DR: In this paper, the coalbed methane capacity of Bowen Basin coal from the Bowen Basin of Queensland, Australia was investigated and related to other coal properties, including surface area and coal surface area.
309 citations
TL;DR: In this article, a mixed-conducting perovskite dense membrane reactor at 850°C was used for the partial oxidation of methane to synthesis gas (syngas, CO + H2).
Abstract: The partial oxidation of methane to synthesis gas (syngas, CO + H2) was performed in a mixed-conducting perovskite dense membrane reactor at 850°C, in which oxygen was separated from air and simultaneously fed into the methane stream. Steady-state oxygen permeation rates for La1-xA′xFe0.8 Co0.2O3-δ perovskite membranes in nonreacting air/helium experiments were in the order of A′x = Ba0.8 > Ba0.6 > Ca0.6 > Sr0.6. Deep oxidation products were obtained from a La0.2 Ba0.8 Fe0.8 Co0.2 O3–δ disk-shaped membrane reactor without catalyst, with a 4.6% CH4 inlet stream. These products were further reformed to syngas when a downstream catalytic bed was added. Packing the 5% Ni/Al2O3 catalyst directly on the membrane reaction-side surface resulted in a slow fivefold increase in O2 permeation, and a fourfold increase in CH4 conversion. XRD, EDS, and SEM analyses revealed structure and composition changes on the membrane surfaces. Oxygen continuously transported from the air side appeared to stabilize the membrane interior, and the reactor was operated for up to 850 h.
264 citations
TL;DR: In this paper, a reaction mechanism for carbon dioxide methanation is proposed, including the formation step of the formates through a carbonate species, which is fixed on the support, in equilibrium with a active formate species on the interface metal-support.
Abstract: Transient measurement techniques are applied for the kinetic investigation of the reaction mechanism of the carbon dioxide methanation, using diffuse reflectance infrared spectroscopy and mass spectrometry. The coupled information of the surface intermediates and the gas phase components time evolution leads to accurate identification of spectator species on the surface. Reaction intermediates, carbon monoxide and formates have been identified. The former is a key intermediate, and its hydrogenation leads to methane formation. The latter is fixed on the support, in equilibrium with a active formate species on the interface metal-support. A reaction mechanism is proposed including the formation step of the formates through a carbonate species.
TL;DR: In this paper, the kinetics of elementary surface reactions involved in the reforming of methane to synthesis gas over supported nickel were studied using transient isotopic methods, and an overall model that describes the reactions of methane with steam and CO2 in microkinetic terms was constructed based on these rate constants and on previously published steam reforming and CO 2methanation data.
TL;DR: In this article, the chemical phase behavior of the ceramic powders with varying stoichiometries was studied by high-temperature in-situ X-ray diffraction (XRD) as a function of oxygen partial pressure.
TL;DR: In this article, the authors used estimates of methane yields to estimate the amount of carbon released into the atmosphere from landfilled forest products and found that only 30% of the carbon from paper and 3% of wood are ever emitted as landfill gas.
TL;DR: In this article, two rice (Oryza sativa var. japonica) cultivars, Lido and Roma, were tested in the field for methane production, oxidation and emission.
Abstract: Two Italian rice (Oryza sativa var. japonica) cultivars, Lido and Roma, were tested in the field for methane production, oxidation and emission. In two consecutive years, fields planted with the rice cultivar Lido showed methane emissions 24-31% lower than fields planted with the cultivar Roma. This difference was observed irrespective of fertilizer treatment. In contrast to methane emissions, differences in methane production or oxidation were not observed between fields planted with the two cultivars. Plant-mediated transport of methane from the sediment to the atmosphere was the dominating pathway of methane emission. During the entire vegetation period, the contribution of this pathway to total methane emission amounted to c. 90%, whereas the contribution of gas bubble release and of diffusion through the water column to total methane emission was of minor significance. Results obtained from transport studies of tracer gas through the aerenchyma system of rice plants demonstrated that the root-shoot transition zone is the main site of resistance to plant-mediated gas exchange between the soil and the atmosphere. The cultivar Lido, showing relatively low methane emissions in the field, had a significantly lower gas transport capacity through the aerenchyma system than the cultivar Roma. Thus, the observed differences in methane emissions in the field between the cultivars Lido and Roma can be explained by different gas transport capacities. Apparently, these differences in gas transport capacities are a consequence of differences in morphology of the aerenchyma systems, especially in the root-shoot transition zone. It is, therefore, concluded that identification and use of high-yielding rice cultivars which have a low gas transport capacity represent an economically feasible, environmentally sound and promising approach to mitigating methane emissons from rice paddy fields.
TL;DR: In this article, a rigorous kinetic model for the formation of filamentous carbon on a nickel catalyst by methane cracking is derived, where the experimental study was performed in an electrobalance unit.
TL;DR: In this paper, a study of the hydrocarbon sealing efficiency of clastic sediments was carried out in the Haltenbanken area offshore Norway and red claystones from Carboniferous and Permian intervals of Northern Germany.
TL;DR: Methane can be highly activated, yielding high rates of conversion to higher hydrocarbons and aromatics by its reaction over H-galloaluminosilicate ZSM-5 type (MFI) zeolite in the presence of alkenes or higher alkanes.
Abstract: Conversion of methane to higher hydrocarbons by its low-temperature activation without forming undesirable carbon oxides is of great scientific and practical importance Methane can be highly activated, yielding high rates of conversion to higher hydrocarbons and aromatics (10 to 45 percent) at low temperatures (400° to 600°C), by its reaction over H-galloaluminosilicate ZSM-5 type (MFI) zeolite in the presence of alkenes or higher alkanes The methane activation results from its hydrogen-transfer reaction with alkenes
TL;DR: In this paper, the authors consider that both explanations for isotopically light methane (bacterial generation or migration fractionation) are realistic and use a simple diagram (ethane over methane versus the carbon isotopic ratio of methane), and modelling the two discussed processes (mixing of bacterial and thermal gas, or diffusion of thermal gases) to decipher the origin of the gas series.
TL;DR: The phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures is described in this paper, where the depth extent and stability of the hydrate zone are governed by the phase diagram.
Abstract: Methane hydrates are solid, nonstoichiometric mixtures of water and the gas methane. They occur worldwide in sediment beneath the sea floor, and estimates of the total mass available there exceed 1016Kg. Since each volume of hydrate can yield up to 164 volumes of gas, offshore methane hydrate is recognized as a very important natural energy resource. The depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures. In sea depths greater than about 300 m, the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor. The fraction of hydrate in the sediment usually increases with increasing depth. The base of the hydrate zone is a phase boundary between solid hydrate and free gas and water. Its depth is determined principally by the value of the geothermal gradient. It stands out on seismic sections as a bright reflection. The diffuse upper boundary is not as well mar...
TL;DR: In the absence of oxygen, methane is a significant greenhouse gas as discussed by the authors, and it is produced in soils as the end product of the anaerobic decomposition of organic matter.
Abstract: Methane is considered to be a significant greenhouse gas. Methane is produced in soils as the end product of the anaerobic decomposition of organic matter. In the absence of oxygen, methane is very...
TL;DR: In this article, a 10-step kinetic model of the oxidative coupling of methane to C2+ hydrocarbons over a La2O3/CaO catalyst was developed on the basis of kinetic measurements in a microcatalytic fixed-bed reactor covering a wide range of reaction conditions.
Abstract: A comprehensive 10-step kinetic model of the oxidative coupling of methane to C2+ hydrocarbons over a La2O3/CaO catalyst was developed on the basis of kinetic measurements in a microcatalytic fixed-bed reactor covering a wide range of reaction conditions (1 < pO2 < 20 kPa, 10 < pCH4 < 95 kPa, 700 < T < 955 °C, 0.76 mCat/VSTP ≤ 250 kg·s/m3). The reaction scheme contains three primary and seven consecutive steps. The conversion of hydrocarbons and of carbon monoxide with oxygen were described by applying Hougen−Watson type rate equations. For the other reactions power-law rate equations were used. From the experimental data kinetic parameters, i.e. frequency factors, apparent activation energies, and adsorption enthalpies, were estimated. With the kinetic model the experimentally determined conversions of methane and oxygen, as well as yields to C2 hydrocarbons and carbon oxides, could be predicted with an average accuracy of ±20%.
TL;DR: In this paper, a stepwise regression model was used to investigate the role of soil temperature in methane emissions in Swedish landfills, finding that soil temperature was negatively correlated with biological methane oxidation, which strongly suggests that biological oxidation is an important regulating factor.
TL;DR: In this article, a static enclosure (closed chamber) technique supplemented by soil gas concentration profiles and field incubations was used to simulate methanotrophic oxidation of methane at a northeastern Illinois landfill with pumped gas recovery.
Abstract: Rates and controlling variables for methanotrophic oxidation of methane at a northeastern Illinois landfill with pumped gas recovery were examined in a field study from June to December 1995. Cover materials consisted of a simple clay-topsoil sequence without geomembranes. Through use of a static enclosure (closed chamber) technique supplemented by soil gas concentration profiles and field incubations, the study concentrated on proximal (near gas recovery well) and distal (between well) sites established in 1994. A personal computer-based three-dimensional finite−difference model was also developed which includes both gaseous mass transfer (CH4, CO2, O2) and microbial CH4 oxidation. Mass transfer is modeled through a modified chemical potential gradient within a cubic network of nodes; a strict mass balance for each gas is maintained through successive timesteps. Methane-oxidizing conditions with no net CH4 emissions to the atmosphere persisted into full winter conditions in December, 1995. Rates of CH4 o...
TL;DR: In this article, the Gibbs energy of a mixture of CH4, C2H4, CH2H2, H2, C6H6 and H2 was calculated at thermodynamic equilibrium.
TL;DR: In this article, an analytical model is derived for the idealized problem of hydrate growth in a porous half-space which is cooled on its boundary, and the growth rate of a hydrate layer is strongly dependent on the two-phase equilibrium between hydrate and seawater.
Abstract: Vast quantities of clathrate hydrate are found in the Arctic and in marine sediments along continental margins. The clathrate structure traps enormous volumes of methane gas, which is both a possible source of global climate change and a potential energy resource. The growth rate and spatial distribution of gas hydrate in the shallow sediments are influenced by a variety of interacting physical processes. In order to quantify these processes, we develop mathematical models for hydrate formation in porous media. An analytical model is derived for the idealized problem of hydrate growth in a porous half-space which is cooled on its boundary. Our calculations predict the growth rate of a hydrate layer for a given rate of cooling and show that the volume of hydrate is strongly dependent on the two-phase equilibrium between hydrate and seawater. For a representative phase diagram we find that the volume of hydrate in the layer is less than 1% of the pore volume. Larger volumes of hydrate observed in some locations demand a sustained supply of gas and a long accumulation time. Numerical calculations are used to investigate situations that are more representative of conditions in marine sediments. A simple theoretical expression is derived for the rate of hydrate accumulation due to advection of methane gas from depth. Using typical estimates of fluid velocities in accretionary environments, we obtain an accumulation rate of 1% of the pore volume in 105 years. The predicted vertical distribution of hydrate is consistent with geophysical inferences from observed hydrate occurrences along the Cascadia margin. Similar distributions can arise from the combined effects of in situ methane production and warming due to ongoing sedimentation. Predicted differences between these two formation models may be detectable in geophysical and geochemical measurements.
TL;DR: In this article, the reforming of methane by carbon dioxide is studied over a Ni/La2O3 catalyst which presents an unusual catalytic stability compared to other nickel-based catalysts.
TL;DR: In this article, the authors calculate the solubility of methane gas over a range of pressure and temperature, and find that the gas solubilities decreases sharply with decreasing temperature.
Abstract: We calculate the solubility of methane gas over a range of pressure and temperature. The gas is dissolved in liquid water, which coexists with free gas at high temperature or solid hydrate at low temperature and high pressure. We show that solubility is significantly altered by the presence or absence of the hydrate phase. When hydrate is absent at high temperatures, our calculations reproduce experimentally observed increases in solubility with decreasing temperature. When hydrate is present, however, we find that the gas solubility decreases sharply with decreasing temperature. Such an abrupt decrease in solubility permits hydrate to crystallize directly from the aqueous solution, without the need of any free gas. This result has important implications for the formation of gas hydrate in marine environments, where the gas supply may not be sufficient to provide free gas. We apply our calculations at typical pressure and temperature conditions in marine sediments to establish the gas concentration needed to stabilize hydrate. Estimates of the vertical distribution of hydrate in marine sediments and the rate of accumulation are obtained using simple models of hydrate formation.
TL;DR: In this paper, the authors measured landfill methane emissions and found that the rate of landfill methane oxidation from field and laboratory incubation studies ranges up to 166 g m{sub -2} day{sup -1} among the highest for any natural setting.
Abstract: Field measurement of landfill methane emissions indicates natural variability spanning more than 2 seven orders of magnitude, from approximately 0.0004 to more than 4000 g m{sub -2} day{sup -1}. This wide range reflects net emissions resulting from production (methanogenesis), consumption (methanotrophic oxidation), and gaseous transport processes. The determination of an {open_quotes}average{close_quotes} emission rate for a given field site requires sampling designs and statistical techniques which consider spatial and temporal variability. Moreover, particularly at sites with pumped gas recovery systems, it is possible for methanotrophic microorganisms in aerated cover soils to oxidize all of the methane from landfill sources below and, additionally, to oxidize methane diffusing into cover soils from atmospheric sources above. In such cases, a reversed soil gas concentration gradient is observed in shallow cover soils, indicating bidirectional diffusional transport to the depth of optimum methane oxidation. Rates of landfill methane oxidation from field and laboratory incubation studies range up to 166 g m{sup -2} day{sup -1} among the highest for any natural setting, providing an effective natural control on net emissions. Estimates of worldwide landfill methane emissions to the atmosphere have ranged from 9 to 70 Tg yr{sup -1}, differing mainly in assumed methane yields from estimated quantities of landfilled refuse. At highly controlled landfill sites in developed countries, landfill methane is often collected via vertical wells or horizontal collectors. Recovery of landfill methane through engineered systems can provide both environmental and energy benefits by mitigating subsurface migration, reducing surface emissions, and providing an alternative energy resource for industrial boiler use, on-site electrical generation, or upgrading to a substitute natural gas.
TL;DR: In this paper, the authors analyzed the different correlations found in the literature between methane storage capacity and parameters related with the porous texture and found that the best correlation was found with the total micropore volume.