750 research outputs found
Gasification of coal-derived chars in synthesis gas mixtures under intraparticle mass-transfer-controlled conditions
A model has been formulated to describe the quasi-steady-state gasification of coal-derived chars in gas mixtures where both the reactants carbon dioxide and steam, and the gasification products carbon monoxide and hydrogen are present. As such, these conditions reflect the situation found in most practical gasification systems.\ud
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The model presented is applied under conditions where intraparticle mass transfer is rate-controlling. Intraparticle heat transfer is neglected. In view of the non-equimolar gasification reactions, the mass flux equations are derived from the continuum limit of the dusty gas model. These flux equations are combined with strongly non-linear Langmuir-Hinshelwood kinetics for the gasification reactions. The model accounts for local variations of the diffusive and convective permeability parameters, as well as variations in the reactive surface area, during burnoff of a char particle.\ud
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The impact of the various relevant mass-transfer parameters, the gasification temperature and pressure, and the char particle size on the gasification behaviour is discussed. A comparison is made between the present model and existing models, and various modelling approaches are critically reviewed
Millimeter-Wave Aperture Synthesis Imaging of Vega: Evidence for a Ring Arc at 95 AU
We present the first millimeter-wave aperture synthesis map of dust around a
main sequence star. A 3'' resolution image of 1.3 mm continuum emission from
Vega reveals a clump of emission 12'' from the star at PA 45 deg, consistent
with the location of maximum 850 micron emission in a lower resolution
JCMT/SCUBA map. The flux density is 4.0+/-0.9 mJy. Adjacent 1.3 mm peaks with
flux densities 3.4+/-1.0 mJy and 2.8+/-0.9 mJy are located 14'' and 13'' from
the star at PA 67 deg and 18 deg, respectively. An arc-like bridge connects the
two strongest peaks. There is an additional 2.4 +/-0.8 mJy peak to the SW 11''
from the star at PA 215 deg and a marginal detection, 1.4+/-0.5 mJy, at the
stellar position, consistent with photospheric emission. An extrapolation from
the 850 micron flux, assuming F_{1.3mm-0.85mm} proportional to lambda^{-2.8},
agrees well with the total detected flux for Vega at 1.3 mm, and implies a dust
emissivity index, beta, of 0.8. We conclude that we have detected all but a
very small fraction of the dust imaged by SCUBA in our aperture synthesis map
and that these grains are largely confined to segments of a ring of radius 95
AU.Comment: 4 pages, 1 figure, 1 table, accepted for publication in Astrophysical
Journal Letter
Small industrial-scale producer gas units
The on-site generation of fuel gas from coal is one of the currently available technologies that may facilitate the reintroduction of indirect coal firing in industrial practice. In the present article the attention will be restricted to gasifiers with comparatively small unit capacities, producing low heating-value gas by gasification of coal with air. The article is intended to provide the reader with a broad review of the available technology, the implications of using coal-derived fuel gas a substitute for conventional fuels, and the economic viability of on-site fuel gas generation.\ud
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On the basis of cost estimates, framed within the Western European situation, the competitiveness of low heating-value gas are discussed. A comparison is made between low and medium heating-value gas, natural gas and direct coal firing. Potential markets for low heating-value fuel gas are identified and discussed
Characterization of Al2O3-Supported Manganese Oxides by Electron Spin Resonance and Diffuse Reflectance Spectroscopy
Alumina-supported manganese oxides, used as catalysts for the selective catalytic reduction of NO, were
characterized by combined electron spin resonance and diffuse reflectance spectroscopies. Upon impregnation
of the acetate precursor solution, the [Mn(H2O)6]^2+ complex interacts strongly with surface hydroxyls of the
y-Al2O3. Evidence was obtained that this anchoring reaction proceeds at a Mn/OH = 1/2 ratio up to 4.5 wt
% Mn loading, leading to a highly dispersed oxidic manganese layer. At higher loadings, the precursor
complex is deposited on the surface concurrently. Upon drying at 383 K, part of the manganese is oxidized
to higher oxidation states (Mn^3+ and Mn^4+ ), while a further increase in (average) oxidation state takes place
upon calcination at 573 K. After calcination, the manganese species are present as a mixture of Mn^2+ ,Mn^3+ ,
and Mn^4+ . At low loadings (<1 wt %), approximately equal amounts of these three oxidation states are
present, whereas Mn 3+ becomes the predominant species at higher loadings. ESR reveals that at low loadings,
almost all the manganese is present as isolated species, while at 4.5 wt % Mn loading, still more than 70%
of the manganese is isolated. The decrease of the fraction of isolated manganese species at higher loadings
is accompanied by a decreased selectivity toward N2 production in the selective catalytic reduction of NO.
The fraction Mn^2+ is present in an axially distorted octahedral coordination
High-resolution imaging at the SOAR telescope
Bright single and binary stars were observed at the 4.1-m telescope with a
fast electron-multiplication camera in the regime of partial turbulence
correction by the visible-light adaptive optics system. We compare the angular
resolution achieved by simple averaging of AO-corrected images (long-exposure),
selection and re-centering (shift-and-add or "lucky" imaging) and speckle
interferometry. The effect of partial AO correction, vibrations, and image
post-processing on the attained resolution is shown. Potential usefulness of
these techniques is evaluated for reaching the diffraction limit in
ground-based optical imaging. Measurements of 75 binary stars obtained during
these tests are given and objects of special interest are discussed. We report
tentative resolution of the astrometric companion to Zeta Aqr B. A concept of
advanced high-resolution camera is outlined.Comment: Accepted for publication in PASP. 14 pages, 9 figures, 2 tabl
The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 Ultracool Dwarfs
We report parallax measurements for 70 ultracool dwarfs (UCDs). Using both
literature values and our sample, we report new polynomial relations between
spectral type and M. Including resolved L/T transition binaries in the
relations, we find no reason to differentiate between a "bright" (unresolved
binary) and "faint" (single source) sample across the L/T boundary. Isolating
early T dwarfs, we find that the brightening of T0-T4 sources is prominent in
M where there is a [1.2 - 1.4] magnitude difference. A similar yet
dampened brightening of [0.3 - 0.5] magnitude happens at M and a plateau
or dimming of [-0.2 - -0.3] magnitude is seen in M. Comparing with
evolutionary models that vary gravity, metallicity, and cloud thickness we find
that a near constant temperature of 1200 100 K along a narrow spectral
subtype of T0-T4 is required to account for the brightening and color magnitude
diagram of the L-dwarf/T-dwarf transition. Furthermore, there is a significant
population of both L and T dwarfs which are red or potentially "ultra-cloudy"
compared to the models, many of which are known to be young indicating a
correlation between enhanced photospheric dust and youth. For the low
surface-gravity or young companion L dwarfs we find that 8 out of 10 are at
least [0.2-1.0] magnitude underluminous in M and/or M compared to
equivalent spectral type objects. We speculate that this is a consequence of
increased dust opacity and conclude that low-surface gravity L dwarfs require a
completely new spectral-type/absolute magnitude polynomial for analysis.Comment: 65 pages, Accepted for publication to Ap
The Vela Pulsar in the Near-Infrared
We report on the first detection of the Vela pulsar in the near-infrared with
the VLT/ISAAC in the Js and H bands. The pulsar magnitudes are Js=22.71 +/-
0.10 and H=22.04 +/- 0.16. We compare our results with the available
multiwavelength data and show that the dereddened phase-averaged optical
spectrum of the pulsar can be fitted with a power law F_nu propto
nu^(-alpha_nu) with alpha_nu = 0.12 +/- 0.05, assuming the color excess
E(B-V)=0.055 +/-0.005 based on recent spectral fits of the emission of the Vela
pulsar and its supernova remnant in X-rays. The negative slope of the pulsar
spectrum is different from the positive slope observed over a wide optical
range in the young Crab pulsar spectrum. The near-infrared part of the Vela
spectrum appears to have the same slope as the phase-averaged spectrum in the
high energy X-ray tail, obtained in the 2-10 keV range with the RXTE. Both of
these spectra can be fitted with a single power law suggesting their common
origin. Because the phase-averaged RXTE spectrum in this range is dominated by
the second X-ray peak of the pulsar light curve, coinciding with the second
main peak of its optical pulse profile, we suggest that this optical peak can
be redder than the first one. We also detect two faint extended structures in
the 1.5''-3.1'' vicinity of the pulsar, projected on and aligned with the
south-east jet and the inner arc of the pulsar wind nebula, detected in X-rays
with Chandra. We discuss their possible association with the nebula.Comment: 12 pages, 8 figures, accepted for publication in A&A, the associated
near-infrared images in the fits format are available at
http://www.ioffe.ru/astro/NSG/obs/vela-ir
L- and M-band imaging observations of the Galactic Center region
We present near-infrared H-, K-, L- and M-band photometry of the Galactic
Center from images obtained at the ESO VLT in May and August 2002, using the
NAOS/CONICA (H and K) and the ISAAC (L and M) instruments. The large field of
view (70" x 70") of the ISAAC instrument and the large number of sources
identified (L-M data for 541 sources) allows us to investigate colors, infrared
excesses and extended dust emission. Our new L-band magnitude calibration
reveals an offset to the traditionally used calibrations, which we attribute to
the use of the variable star IRS7 as a flux calibrator. Together with new
results on the extinction towards the Galactic Center (Scoville et al. 2003;
Raab 2000), our magnitude calibration results in stellar color properties
expected from standard stars and removes any necessity to modify the K-band
extinction. The large number of sources for which we have obtained L-M colors
allows us to measure the M-band extinction to A_M=(0.056+-0.006)A_V
(approximately =A_L), a considerably higher value than what has so far been
assumed. L-M color data has not been investigated previously, due to lack of
useful M-band data. We find that this color is a useful diagnostic tool for the
preliminary identification of stellar types, since hot and cool stars show a
fairly clear L-M color separation. This is especially important if visual
colors are not available, as in the Galactic Center. For one of the most
prominent dust embedded sources, IRS3, we find extended L- and M-band continuum
emission with a characteristic bow-shock shape. An explanation for this
appearance is that IRS3 consists of a massive, hot, young mass-losing star
surrounded by an optically thick, extended dust shell, which is pushed
northwest by wind from the direction of the IRS16 cluster and SgrA*.Comment: 24 pages, 7 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
A Model for the Voltage Steps in the Breakdown of the Integer Quantum Hall Effect
In samples used to maintain the US resistance standard the breakdown of the
dissipationless integer quantum Hall effect occurs as a series of dissipative
voltage steps. A mechanism for this type of breakdown is proposed, based on the
generation of magneto-excitons when the quantum Hall fluid flows past an
ionised impurity above a critical velocity. The calculated generation rate
gives a voltage step height in good agreement with measurements on both
electron and hole gases. We also compare this model to a hydrodynamic
description of breakdown.Comment: 4 pages including 3 figure
Color Transformations for the 2MASS Second Incremental Data Release
Transformation equations are presented to convert colors and magnitudes
measured in the AAO, ARNICA, CIT, DENIS, ESO, LCO (Persson standards), MSSSO,
SAAO, and UKIRT photometric systems to the photometric system inherent to the
2MASS Second Incremental Data Release. The transformations have been derived by
comparing 2MASS photometry with published magnitudes and colors for stars
observed in these systems. Transformation equations have also been derived
indirectly for the Bessell & Brett (1988) and Koornneef (1983) homogenized
photometric systems.Comment: To appear in AJ, May 200
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