Showing papers on "Methane published in 1996"

Carbon dioxide reforming of methane to produce synthesis gas over metal-supported catalysts: State of the art

Abstract: Carbon dioxide reforming of methane produces synthesis gas with a low hydrogen to carbon monoxide ratio, which is desirable for many industrial synthesis processes. This reaction also has very important environmental implications since both methane and carbon dioxide contribute to the greenhouse effect. Converting these gases into a valuable feedstock may significantly reduce the atmospheric emissions of CO2 and CH4. In this paper, we present a comprehensive review on the thermodynamics, catalyst selection and activity, reaction mechanism, and kinetics of this important reaction. Recently, research has centered on the development of catalysts and the feasible applications of this reaction in industry. Group VIII metals supported on oxides are found to be effective for this reason. However, carbon deposition causing catalyst deactivation is the major problem inhibiting the industrial application of the CO2/CH4 reaction. Ni-based catalysts impregnated on certain supports show carbon-free operation and thus attract much attention. To develop an effective catalyst for CO2 reforming of CH4 and accelerate the commercial application of the reaction, the following are identified to be the most important areas for future work: (1) selection of metal and support and studying the effect of their interaction on catalyst activity; (2) the effect of different promoter on catalyst activity; (3) the reaction mechanism and kinetics; and (4) pilot reactor performance and scale-up operation.

Marine pore-water sulfate profiles indicate in situ methane flux from underlying gas hydrate

Abstract: Marine pore-water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the Carolina Rise and Blake Ridge (U.S. Atlantic continental margin). Here, sulfate gradients are linear, implying that sulfate depletion is driven by methane flux from below, rather than by the flux of sedimentary organic matter from above. Thus, these linear sulfate gradients can be used to quantify and assess in situ methane flux, which is a function of the methane inventory below.

Reduction of CO2 during serpentinization of olivine at 300 °C and 500 bar

Abstract: CO 2 reduction processes occurring during experimental serpentinization of olivine at 300 °C and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas. Data reveal that conversion of Fe(II) in olivine to Fe(III) in magnetite during serpentinization leads to production of H 2 and conversion of dissolved CO 2 to reduced-C species including methane, ethane, propane, and an amorphous carbonaceous phase. Hydrocarbon gases generated in the process fit a Schulz-Flory distribution consistent with catalysis by mineral reactants or products. Magnetite is inferred to be the catalyst for methanization during serpentinization, because it has been previously shown to accelerate Fischer-Tropsch synthesis of methane in industrial applications involving mixtures of H 2 and CO 2 . The carbonaceous phase was predominantly aliphatic, but had a significant aromatic component. Although this phase should ultimately be converted to hydrocarbon gases and graphite, if full thermodynamic equilibrium were established, its formation in these experiments indicates that the pathway for reduction of CO 2 during serpentinization processes is complex and involves a series of metastable intermediates.

Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling

Abstract: Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.

Methane exploitation by carbon dioxide from gas hydrates - Phase equilibria for CO2-CH4 mixed hydrate system

Abstract: Natural-gas hydrate fields having a large amount of methane deposits have become the object of public attention as a potential natural-gas resource. An idea of methane exploitation in linkage with CO2 isolation has been presented elsewhere. In the present study, the isothermal phase equilibrium relations of pressure and compositions in the gas, liquid, and hydrate phases for the CO2-CH4 mixed hydrate system at 280 K are obtained in company with the apparent Henry constants for the methane-water system and the three-phase coexisting lines for the methane hydrate system. The averaged distribution coefficient of methane between gas phase and hydrate phase is about 2.5, that is, methane in the hydrate phase is replaced selectively by CO2. This is the first experimental evidence for the possibility of methane exploitation combined with CO2 isolation.

Modeling of Aromatic and Polycyclic Aromatic Hydrocarbon Formation in Premixed Methane and Ethane Flames

Abstract: Detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in rich, sooting, methane and ethane premixed flames. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph / mass spectrometer technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon formation primarily involve the combination of resonantly stabilized radicals. In particular, propargyl and I-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl and f...

Catalytic reforming of methane with carbon dioxide over nickel catalysts II. Reaction kinetics

Abstract: The reforming of methane with carbon dioxide was studied over nickel supported on SiO2, TiO2, MgO and activated carbon. Specific activities on a turnover frequency basis were in the order: Ni/TiO2 > Ni/C > Ni/SiO2 > Ni/MgO. Interestingly, a 2-fold increase in activation energy for this reaction was observed over Ni/TiO2 after several hours time on stream. The reverse water-gas shift reaction was found to be close to thermodynamic equilibrium over all catalysts. Partial pressure dependencies were obtained with the Ni/C and Ni/SiO2 catalysts at 723 K for comparative purposes only, but a more thorough kinetic analysis was made with the Ni/MgO and Ni/TiO2 catalysts, which were shown previously to strongly inhibit carbon deposition. Partial pressure dependencies were obtained at 673, 698, and 723 K for Ni/TiO2 and at 773, 798, and 823 K for Ni/MgO. In situ DRIFTS studies clearly showed the presence of both linear and bridged carbon monoxide adsorption on Ni/SiO2 under reaction conditions; however, adsorbed carbon monoxide could not be identified on Ni/TiO2. A reaction model for CH4—CO2 reforming, based on CH4 activation to form CHx and CHxO decomposition as the slow kinetic steps, successfully correlated the rate data.

Open-path Fourier transform infrared studies of large-scale laboratory biomass fires

Abstract: A series of nine large-scale, open fires was conducted in the Intermountain Fire Sciences Laboratory (IFSL) controlled-environment combustion facility. The fuels were pure pine needles or sagebrush or mixed fuels simulating forest-floor, ground fires; crown fires; broadcast burns; and slash pile burns. Mid-infrared spectra of the smoke were recorded throughout each fire by open path Fourier transform infrared (FTIR) spectroscopy at 0.12 cm−1 resolution over a 3 m cross-stack pathlength and analyzed to provide pseudocontinuous, simultaneous concentrations of up to 16 compounds. Simultaneous measurements were made of fuel mass loss, stack gas temperature, and total mass flow up the stack. The products detected are classified by the type of process that dominates in producing them. Carbon dioxide is the dominant emission of (and primarily produced by) flaming combustion, from which we also measure nitric oxide, nitrogen dioxide, sulfur dioxide, and most of the water vapor from combustion and fuel moisture. Carbon monoxide is the dominant emission formed primarily by smoldering combustion from which we also measure carbon dioxide, methane, ammonia, and ethane. A significant fraction of the total emissions is unoxidized pyrolysis products; examples are methanol, formaldehyde, acetic and formic acid, ethene (ethylene), ethyne (acetylene), and hydrogen cyanide. Relatively few previous data exist for many of these compounds and they are likely to have an important but as yet poorly understood role in plume chemistry. Large differences in emissions occur from different fire and fuel types, and the observed temporal behavior of the emissions is found to depend strongly on the fuel bed and product type.

Dietary, environmental and microbiological aspects of methane production in ruminants

Abstract: Methane gas is produced in the rumen by methanogenic bacteria as a metabolic end product. The energy released by bacteria in the process of methane formation can be used for bacterial cell formation. Methane formation acts as an electron sink into which the hydrogen from all ruminal microorganisms drains, allowing a higher yield of adenosine triphosphate. Factors such as the type of carbohydrate in the diet, level of feed intake, digesta passage rate presence of ionophores or lipids in the diet, and ambient temperature influence the emission of methane from ruminants. Methanobrevibacter spp. appear to be the major methanogens in the rumen, but it is likely that phytogenetic analyses will identify new species. The biochemical reduction of carbon dioxide to methane is well defined, and it has been shown that interspecies hydrogen transfer between methanogens and ruminal bacteria prevents the accumulation of reduced nucleotides and the inhibition of feed digestion. The development of strategies to mitigate m...

Peculiarities of Methane Clathrate Hydrate Formation and Solid-State Deformation, Including Possible Superheating of Water Ice

Abstract: Slow, constant-volume heating of water ice plus methane gas mixtures forms methane clathrate hydrate by a progressive reaction that occurs at the nascent ice/liquid water interface. As this reaction proceeds, the rate of melting of metastable water ice may be suppressed to allow short-lived superheating of ice to at least 276 kelvin. Plastic flow properties measured on clathrate test specimens are significantly different from those of water ice; under nonhydrostatic stress, methane clathrate undergoes extensive strain hardening and a process of solid-state disproportionation or exsolution at conditions well within its conventional hydrostatic stability field.

Vertical distribution of Titan's atmospheric neutral constituents

Abstract: The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N 2 , of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH 3 C 2 H and HC 3 N. The formation of CH 3 CN may involve the reaction of CN with either CH 4 or (preferably) C 2 H 6 . The observed CO 2 profile can be modeled by assuming an external source of water of ∼6 × 10 6 cm -2 s -1 . For a nominal CO mixing ratio of 5 × 10 -5 , the chemical loss of CO exceeds its production by ∼15%, and equilibrium is achieved for CO = 1 × 10 -5 .

Carbon dioxide reforming of methane to synthesis gas over Ni/La2O3 catalysts

Abstract: Carbon dioxide reforming of methane to synthesis gas was studied employing Ni/La2O3 catalysts. It was found that, in contrast to the performance of other nickel-based catalysts (e.g. Ni/Al2O3 and Ni/CaO) which exhibit continuous deactivation with time on stream, the rate over the Ni/La2O3 catalyst increases during the initial 2–5 h of reaction and then tends to be essentially invariable with time on stream, displaying very good stability. X-ray diffraction (XRD) studies reveal that a large CO2 pool, stored in the form of La2O2CO3, is accumulating on the Ni/La2O3 catalyst, following exposure to the CH4/CO2 mixture at reaction conditions. Results of H2- and CO-temperature-programmed desorption reveal that the HNi bond is weakened and CO disproportionation is unfavoured on the Ni/La2O3 catalyst, as compared to the Ni/Al2O3 catalyst. Comparison of H2 and CO uptake and Ni dispersion by XRD shows that H2 and CO uptakes are significantly suppressed, by ca. 3–10 times, suggesting that a large portion of the Ni surface is blocked by lanthanum species. It is proposed that the interaction between Ni crystallites and La2O3 support or La species which are decorating the Ni crystallites is responsible for the unusual chemisorptive and catalytic behaviour observed over the Ni/La2O3 catalyst.

Comparative Study of Carbon Dioxide Reforming of Methane to Synthesis Gas over Ni/La2O3 and Conventional Nickel-Based Catalysts

Abstract: Carbon dioxide reforming of methane to synthesis gas was studied by employing a Ni/La2O3 catalyst as well as conventional nickel-based catalysts, i.e., Ni/γ-Al2O3, Ni/CaO/γ-Al2O3, and Ni/CaO. It is observed that, in contrast to conventional nickel-based catalysts, which exhibit continuous deactivation with time on stream, the rate of reaction over the Ni/La2O3 catalyst increases during the initial 2−5 h and then tends to be essentially invariable with time on stream. X-ray photoelectron spectroscopy (XPS) studies show that the surface carbon on spent Ni/Al2O3 catalyst is dominated by −C−C− species that eventually block the entire Ni surface, leading to total loss of activity. The surface carbon on the working Ni/La2O3 catalyst is found to consist of −C−C− species and a large amount of oxidized carbon. Both XPS and secondary ion mass spectrometry results reveal that a large fraction of surface Ni on the working Ni/La2O3 catalyst is not shielded by carbon deposition. FTIR studies reveal that the enhancement...

Quantifying the effect of oxidation on landfill methane emissions

Abstract: Field, laboratory, and computer modeling methods were utilized to quantitatively assess the capability of aerobic microorganisms to oxidize landfill-derived methane (CH4) in cover soils. The investigated municipal landfill, located in Nashua, New Hampshire, was operating without gas controls of any type at the time of sample collection. Soil samples from locations of CH4 flux to the atmosphere were returned to the laboratory and subjected to incubation experiments to quantify the response of oxidation in these soils to temperature, soil moisture, in situ CH4 mixing ratio, soil depth, and oxygen. The mathematical representations of the observed oxidation reponses were combined with measured and predicted soil characteristics in a computer model to predict the rate of CH4 oxidation in the soils at the locations of the measured fluxes described by Czepiel et al. [this issue]. The estimated whole landfill oxidation rate at the time of the flux measurements in October 1994 was 20%. Local air temperature and precipitation data were then used in conjunction with an existing soil climate model to estimate an annual whole landfill oxidation rate in 1994 of 10%.

Simulation of the catalytic partial oxidation of methane to synthesis gas

Abstract: The modeling and simulation of reactors for the catalytic partial oxidation of natural gas to synthesis gas is complex and requires detailed kinetics if it is to be representative and reliable. Adiabatic fixed bed reactors with a catalytic combustion zone fed with methane/oxygen or methane/air mixtures were simulated based upon the kinetics of total combustion, steam reforming and water-gas shift on a Ni catalyst. The steam reforming reactions and water-gas shift reaction are parallel or more or less consecutive to the total combustion, depending upon the degree of reduction of the catalyst, which is determined by the temperature and the gas phase composition. The calculation of the net rates of coke formation was included in the simulation. The influence of carbon dioxide and steam was also investigated.

Development of highly stable nickel catalyst for methane-steam reaction under low steam to carbon ratio

Abstract: A NiMgO solid solution with low Ni content (Ni0.03Mg0.97O, atomic ratio) which was reduced at high temperature (> 1073 K) was found to be an active and stable catalyst for the steam reforming of methane in a steam to carbon ratio of 1.0. The reduced Ni0.03Mg0.97O catalyst showed higher activity and much higher stability than a commercial reforming catalyst (Ni/Al2O3MgO). The content of metallic nickel on the catalyst surface reduced by hydrogen was only 0.17 μmol/m2 and the turnover frequency of the methane-steam reaction at 1023 K was 62 s−1. The catalyst kept its activity for 60 h or more at 1123 K and a steam to carbon ratio of 1.0, giving little coke on the catalyst (

Coprecipitated Ni-alumina and NiCu-alumina catalysts of methane decomposition and carbon deposition. II. Evolution of the catalysts in reaction

Abstract: Using coprecipitated Ni-alumina and NiCu-alumina catalysts, up to 250 g/gcat of filamentous carbon was produced by methane decomposition at 823 K and a methane pressure of 100 kPa. The process evolved three general stages. In the induction period carbon was dissolved into the Ni particles as evidenced by in situ X-ray powder diffraction measurements. This promoted the formation of large metal particles pear-shaped in Ni catalysts and quasi-octahedral in NiCu ones. The steady-state growth of the filaments occurred on the second stage. Finally, the catalyst was deactivated producing porous granules composed of interlaced long carbon filaments. In deactivated Ni catalysts the nickel was found in metal state, while in NiCu samples about half of the nickel was atomically dispersed in carbon as revealed by extended X-ray absorption fine structure spectroscopy. The deactivation of the catalysts was suggested to include the fragmentation of the metal particles as well as an atomic erosion in NiCu samples. In addition, in both catalysts the filaments growth could also be limited when the close-packed structures of porous carbon were achieved.

Methane efflux from emergent vegetation in peatlands

Abstract: 1 Methane emissions from emergent plants in two peatlands were measured in plant enclosure experiments. During the summer growing season, methane emissions on a plant dry mass basis from Scheuchzeriapalustris (130 + 14 4umol g-' day-') were higher than from other peatland plants, such as Chamaedaphne calyculata (20 + 3 umol g-' day-') and Carex oligosperma (91 + 38 4umol g-' day-'), but were lower than from Calla palustris (280 + 37 umol g-' day-'). 2 The contribution to net methane efflux from S. palustris, the dominant emergent plant in one peatland, was 15.6 + 2.2 mmol m-2 day-', based on the plant enclosure experiments and areal biomass measurements. These results were not significantly different from the difference between measurements of methane efflux using chambers enclosing clipped and unclipped plots (12.0-27.9 mmol m-2 day-'). 3 We determined that methane transport through S. palustris contributed between 64 and 90% of the net methane efflux from the peatland, while the other peatland without plants capable of rapid methane transport had significantly lower methane efflux. The higher annual efflux from peat with aerenchymatous plants appears to be due to several factors, including increased substrate production which enhances methane production, reduction in methane oxidation, as well as easier methane transport from the site of production. 4 Plants capable of transporting methane regulated the methane pool in the saturated zone by depleting both dissolved methane in the pore-waters and the concentration of methane in gas bubbles trapped in the saturated peat. 5 Our experiments indicate that methane transport through S. palustris is due to molecular diffusion, and is not influenced by pressurization or stomatal aperture.

Conversion of methane to benzene over Mo2C and Mo2C/ZSM-5 catalysts

Abstract: The activation and dehydrogenation of CH2 on Mo2C and MO2C/ZSM-5 have been investigated under non-oxidizing conditions. Unsupported Mo2C exhibited very little activity towards methane decomposition at 973 K. The main reaction pathway was the decomposition of methane to give hydrogen and carbon with a trace amount of ethane. Mixing Mo2C with ZSM-5 support somewhat enhanced its catalytic activity, but did not change the products of the reaction. A dramatic change in the product formation occurred on partially oxidized Mo2C/ZSM-5 catalyst; besides some hydrocarbons benzene was produced with a selectivity of 70–80% at a conversion of 5–7%. Carburization of highly dispersed MoO3 on ZSM-5 also led to a very active catalyst: the conversion of methane at the steady state was 5–6% and the selectivity of benzene formation was 85%.

Biomass of termites and their emissions of methane and carbon dioxide: A global database

Abstract: A global database describing the geographical distribution of the biomass of termites and their emissions of methane and carbon dioxide has been constructed. Termite biomasses were assigned to various ecosystems using published measurements and a recent high-resolution (10' x 10') database of vegetation categories. The assigned biomasses were then combined with literature measurements of fluxes of methane and carbon dioxide from termites and extrapolated to give global emission estimates for each gas. The global emissions of methane and carbon dioxide are 19.7 ± 1.5 and 3500 ± 700 Mt yr -1 , respectively (1 Mt = 10 12 g). These emissions contribute approximately 4% and 2%, respectively, to the total global fluxes of these gases. This database gives an accurate distribution of the biomasses and gaseous emissions by termites and may be incorporated into global models of the atmosphere.

Trapped methane volume and potential effects on methane ebullition in a northern peatland

Abstract: A novel way of estimating the gas bubble volume in the floating mat sediment of a peatland was developed at Thoreau's Bog in Concord, Massachusetts. Statistically significant relationships between the buoyancy of the floating Sphagnum mat and atmospheric pressure were observed, and these relationships were used to estimate the gas bubble volume. The mass of CH 4 stored in gas bubbles is estimated to be as much as 3 times the mass of dissolved CH 4 , depending on the time of year. The gas bubble volume is frequently large enough to serve as a significant buffer between microbial production of CH 4 and the release of CH 4 to the atmosphere. Changes in atmospheric pressure, temperature, and water-table elevation may result in modulation of the ebullitive CH 4 flux. Periods of rapidly rising atmospheric pressure or equivalent pressure changes due to water-table elevation are capable of arresting bubble volume growth, thereby halting CH 4 ebullition. Periods of rapid cooling of the bog could also temporarily halt ebullition, as thermally induced contraction of bubbles and dissolution of CH 4 offset bubble volume growth due to methanogenesis.

Design of stable catalysts for methane-carbon dioxide reforming

Abstract: Pt/ZrO2 is an active and stable catalyst for methane- carbon dioxide reforming reaction. The reaction between CO2 and CH4 to yield synthesis gas might proceed via two different pathways. At high temperatures (>1075K) CO2 can be dissociated on Pt to CO and adsorbed oxygen. Methane can be dissociated to H2 and carbon at temperatures above 775K. Recombination of the adsorbed oxygen and carbon yields a second CO molecule. At low temperatures i.r. studies of CO2 adsorption on Pt/ZrO2 showed that CO was formed and in addition carbonates were formed on the support. A linear relationship between the activity and the Pt-ZrO2 perimeter length indicates that the support might be important for activating carbon dioxide and subsequent reaction with carbon.