Showing papers on "Methane published in 1989"

Methane steam reforming, methanation and water‐gas shift: I. Intrinsic kinetics

Abstract: Intrinsic rate equations were derived for the steam reforming of methane, accompanied by water-gas shift on a Ni/MgAl2O4 catalyst. A large number of detailed reaction mechanisms were considered. Thermodynamic analysis helped in reducing the number of possible mechanisms. Twenty one sets of three rate equations were retained and subjected to model discrimination and parameter estimation. The parameter estimates in the best model are statistically significant and thermodynamically consistent.

The influence of water table levels on methane and carbon dioxide emissions from peatland soils

Abstract: The evolution of carbon dioxide and methane was measured from laboratory columns packed with surface (0–30 cm) materials representing a fen, a bog and a swamp and with varying water tables and treated with water containing 10 mg L−1 dissolved organic carbon. Carbon dioxide evolution increased in a linear relationship as the water table was lowered, ranging from 0.3–0.5 g CO2 m−2 d−1 to 6.6–9.4 g CO2 m−2 d−1 for the water table at 10 cm above and 70 cm below the peat surface, respectively. Methane evolution decreased in a logarithmic relationship as the water table was lowered. The fen showed the highest rates of methane flux (28 mg CH4 m−2 d−1 when inundated) and the bog the lowest (0.7 mg CH4 m−2 d−1 when inundated). These differences appeared to be related to the acidity of the soils and their microbial characteristics. Molar ratios of carbon dioxide:methane evolution increased from 4 to 173 under inundated conditions to > 2500 when the water table was at a depth of 70 cm. Key words: Methane, carbon dio...

Bacterial population development and chemical characteristics of refuse decomposition in a simulated sanitary landfill.

Abstract: Population development of key groups of bacteria involved in municipal refuse conversion to methane was measured from the time of initial incubation through the onset of methane production. Hemicellulolytic bacteria, cellulolytic bacteria, hydrogen-producing acetogens, and acetate- and H2-plus-CO2-utilizing methanogens were enumerated by the most-probable-number technique with media containing oat spelt xylan, ball-milled cellulose, butyrate, acetate, and H2 plus CO2, respectively. Refuse decomposition was monitored in multiple replicate laboratory-scale sanitary landfills. A laboratory-scale landfill was dismantled weekly for microbial and chemical analysis. Leachate was neutralized and recycled to ensure methanogenesis. The methane concentration of the sampled containers increased to 64% by day 69, at which time the maximum methane production rate, 929 liters of CH4 per dry kg-year, was measured. Population increases of 2, 4, 5, 5, and 6 orders of magnitude were measured between fresh refuse and the methane production phase for the hemicellulolytic bacteria, cellulolytic bacteria, butyrate-catabolizing acetogens, and acetate- and H2-CO2-utilizing methanogens, respectively. The cellulolytic bacteria and acetogens increased more slowly than the methanogens and only after the onset of methane production. The initial decrease in the pH of the refuse ecosystem from 7.5 to 5.7 was attributed to the accumulation of acidic end products of sugar fermentation, to the low acid-consuming activity of the acetogenic and methanogenic bacteria, and to levels of oxygen and nitrate in the fresh refuse sufficient for oxidation of only 8% of the sugars to carbon dioxide and water. Cellulose and hemicellulose decomposition was most rapid after establishment of the methanogenic and acetogenic populations and a reduction in the initial accumulation of carboxylic acids. A total of 72% of these carbohydrates were degraded in the container sampled after 111 days. Initially acetate utilization, but ultimately polymer hydrolysis, limited the rate of refuse conversion to methane. Microbial and chemical composition data were combined to formulate an updated description of refuse decomposition in four phases: an aerobic phase, an anaerobic acid phase, an accelerated methane production phase, and a decelerated methane production phase.

Gas transport from methane‐saturated, tidal freshwater and wetland sediments

Abstract: Variations in hydrostatic pressure controlled by diurnal tides triggered ebullition from subtidal freshwater sediments dominated by methanogenesis in the White Oak River estuary, North Carolina. Pulses of gas consisting of 50-80% methane were released when the tidal cycle reached its nadir. In August, site-to-site variations in these fluxes ranged from 60 to 650 ml (39-425 mg) CH, m-2 d-l. At a single site, cbullition made up 50% of the total CH, flux out of the sediments; the remainder was transported across the sediment-water interface by molecular diffusion. The sedimentary gas bubble reservoir varied seasonally between 2.6 and 14.8 liters m-3 at two White Oak sites. These quantities represented lo-30% of the total sedimentary CH, inventory, the balance of which was dissolved in pore waters. Methane shifted between the two pools with seasonal changes in temperature as bubble methane partial pressure maintained equilibrium with dissolved CH,. Factors controlling the composition of sedimentary gas bubbles were investigated by collecting samples from scvcral environments. These bubbles consisted primarily of CH,, N2, and CO,. The ratio of CH, to N, was found to bc a useful indicator of mechanisms transporting gases from sediments and was controlled by both the ebullition rate and the presence of rooted emergent macrophytes.

Carbon-14 in methane sources and in atmospheric methane: the contribution from fossil carbon.

Abstract: Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4).

Activation of methane dissociation on a Pt(111) surface

Abstract: This paper reports detailed molecular beam measurements of the dissociative chemisorption probability for methane on a Pt(111) surface. We find large increases in the dissociative chemisorption probability S0 with increases in Ei cos2 θi (the normal component of translational energy), Ev (the vibrational energy of the incident methane), and Ts (surface thermal energy). The comparable activation of the reaction by addition of any of these three forms of energy cannot be accounted for by any single model for C–H bond activation proposed to date. A large kinetic isotope effect is also observed, with S0 decreasing significantly for CD4 relative to CH4.

The role of hydrogen in vapor deposition of diamond

Abstract: The effect of hydrogen addition on growth of diamond films under the conditions of chemical vapor deposition was investigated computationally. A detailed chemical kinetic mechanism was composed to describe the evolution of reaction species in pyrolysis of hydrogen‐ and argon‐diluted methane mixtures with imposed temperature profiles, simulating the gas‐phase conditions of diamond film growth in an idealized hot‐filament reactor. The reaction mechanism was comprised of two basic parts: decomposition of methane, and formation and growth of polycyclic aromatic hydrocarbons; it contained a total of 120 elementary reactions and 45 chemical species. The reaction rate coefficients included temperature and pressure dependencies. The computations were performed for a variety of initial conditions, elucidating the effects of critical parameters on the product composition in the regime of diamond deposition. Analysis of the computational results indicated that the key role of the hydrogen addition in the diamond deposition process is to suppress the formation of aromatic species by H2 in the gas phase and thereby to prevent the formation and growth of nondiamond, graphitic phases on the deposition surface.

Experimental and numerical determination of laminar flame speeds of methane/(Ar, N2, CO2)-air mixtures as function of stoichiometry, pressure, and flame temperature

Abstract: By using the counterflow method, the laminar flame speeds of methane/(Ar, N 2 , CO 2 )-air mixtures have been accurately and extensively determined over the stoichiometric range from very lean to very rich, the pressure range from 0.25 to 2 atm, and flame temperature range from 1,550 to 2,250 K; independent variation in the flame temperature is achieved by substituting nitrogen in the air by an equal amount of argon or carbon dioxide. These data are expected to be useful for the partial validation of proposed kinetic mechanisms. In the present study numerical simulation of the experimental flame speeds has been conducted by using a C 1 mechanism and a full C 2 mechanism. The calculated results agree well with the experimental data, for both the C 1 and C 2 mechanisms, except for very rich mixtures for which there is substantial overprediction by the C 2 mechanism. Sensitivity analyses have also been performed where appropriate for enhanced insight into the controlling elementary reactions.

Seasonal variation of methane emissions from a temperate swamp

Abstract: Methane flux measurements were made at four sites in a freshwater temperate swamp over the 13 month period of April 1985 through May 1986. Emissions were highly variable both between sites and over time at any one site. Ebullition from sediments was an important component of methane release. Although release of methane through bubbling occurred in only 19% of the measurements made between April and June 1985, when instrumentation allowed us to separate diffusive and bubble fluxes, ebullition accounted for 34% of the total flux during this period. Methane release rates showed a strong seasonal variation, with highest emission rates observed in early spring and again in late summer, which was associated with changes in plant growth and physiology. Emission rates were partially correlated with sediment temperature, but the relationship was not straightforward, and resembled a step function. Emissions responded strongly to temperature change through the range of 10–16°C. At winter sediment temperatures between 4–9°C, CH4 flux continued at low rates (0–28 mg CH4 m−2d−1; average = 7.9 mg CH4m−2d−1) and appeared insensitive to changes in sediment temperature. Annual methane emission from three constantly flooded sites (mean water depth = 35 cm) was 43.7 +/- 7.8 gm−2 (standard error); annual flux from a bank site was 41.4 +/- 20.5 gm−2. A comparison of flux measurements from fresh and saline wetlands in the immediate area of Newport News Swamp emphasizes the importance of edaphic factors in controlling flux.

Studies of the fischer-tropsch synthesis on a cobalt catalyst II. Kinetics of carbon monoxide conversion to methane and to higher hydrocarbons

Abstract: Six Langmuir-Hinshelwood-Hougen-Watson models have been derived for the kinetics of conversion of carbon monoxide to hydrocarbons in the Fischer-Tropsch synthesis The models were fitted to experimental data obtained in an internal recycle reactor over a wide range of operating conditions Two models, one based on the hydrogenation of surface carbon and the other on a hydrogen-assisted dissociation of carbon monoxide as rate limiting steps were both able to provide a satisfactory fit to the experimental rate data A general model was also developed for the rate of methanation in the presence of higher hydrocarbons The same two rate limiting assumptions as those used in formulating the rate of total CO conversion are used in these models The two models were fitted to experimental data for methane formation It was the model assuming CH formation as rate limiting that showed the best fit for both CO conversion for CH4 formation

Evaluation of Data on Solubility of Simple Apolar Gases in Light and Heavy Water at High Temperature

Abstract: The solubility data of apolar gases in light and heavy water over the temperature range covered experimentally have been evaluated, laying particular emphasis to the region above the normal boiling points of the solvents. The systems that have been included in this work are the inert gases and CH4 in light water and heavy water, H2, O2, N2, and C2H6 in light water and D2 in heavy water. Data in the original sources have been brought to the same footing by calculating from the raw experimental data P, T, and x when they were not reported by the author. This step is considered necessary to assess critically the available sets of data. The temperature dependence of Henry’s constants for all the binary systems have been expressed in terms of two different polynomial equations. The formulations presented are discussed and the limits of application given.

The 13C/12C kinetic isotope effect for soil oxidation of methane at ambient atmospheric concentrations

Abstract: During a survey of the Alaskan North Slope to estimate the isotopic composition and fluxes of methane (CH4) from the tundra, two sites were encountered that showed net methane consumption within flux chambers. Methane concentrations decreased from ambient (1.78 ppmv) by up to 50 percent, and the delta C-13 increased by up to 10 percent in the two chamber deployments showing CH4 consumption. CH4 consumption rates were measured to be 1.2 and 0.6 mg CH4/sq m per day; the corresponding carbon kinetic isotope effects (k13/k12) were 0.974 and 0.984, respectively.

Bacterial production of methane and its influence on ground-water chemistry in east-central Texas aquifers

Abstract: Geochemical and isotopic data for methane and ground water indicate that gaseous hydrocarbons in Eocene aquifers in east-central Texas form by bacterial processes. The δ13C values of methane from five wells in the clay-rich Yegua and Cook Mountain Formations range from -71‰ to -62‰. Methane from ten wells in the cleaner sands of the Sparta and Queen City Formations have δ13C values between -57‰ and -53‰. The carbon isotopic difference between methanes from the Yegua and Sparta aquifers is comparable to the isotopic difference in sedimentary organic matter from outcrops of the units, suggesting substrate control on the δ13C of bacterial methane. Hydrogen isotopic compositions of methane from the aquifers are similar, averaging -181‰. This high value suggests methane production predominantly by CO2 reduction. The δ13C of dissolved inorganic carbon (DIC) in high bicarbonate waters increases from about -20‰ to 0‰ with increasing DIC. Mass-balance calculations indicate that the DIC added to the ground water has δ;13C values as high as 10‰. This 13C-enriched carbon is predominantly derived from CO2 production by fermentation and anaerobic oxidation reactions combined with CO2 consumption by CO2 reduction. This process is responsible for high bicarbonate contents in these and probably other Gulf Coast ground waters.

Direct conversion of methane into methanol

Abstract: The direct oxidation of methane into methanol has been studied in a flow microreactor The effects, on the selectivity to methanol, of the type of reactor, temperature, pressure, residence time, oxygen content, gas mixing and dilution, have been investigated The results show that at relatively low pressures the use of metallic reactors and/or catalysts leads to the formation of complete oxidation products However, at higher pressures none of the experimental parameters studied has a significant influence on the selectivity to methanol, which in all cases is ca 40 %, much lower than claimed in recent publications Investigations of the stability of methanol at high temperatures and pressures in the presence of oxygen show that loss of methanol through further oxidation is unimportant except in the presence of metals such as Cu In the presence of methane and oxygen some methanol is destroyed, and this is attributed to reactions involving methyl radicals produced during the activation of methane by oxygen However, this loss of methanol is insufficient to account for the low overall selectivity observed It is concluded that the detailed design of the reactor may be of critical importance in obtaining high methanol yields

Methane production from carbonaceous subterranean formations

Abstract: A method of producing methane by injecting inert gas, such as nitrogen, through an injection well into a solid carbonaceous subterranean formation (e.g., coal) and recovering methane from a production well(s). Methane desorption is achieved by reduction in methane partial pressure rather than by reduction in total pressure alone.