27 research outputs found
Oxy-fuel combustion of coal and biomass blends
The ignition temperature, burnout and NO emissions of blends of a semi-anthracite and a high-volatile bituminous coal with 10 and 20 wt.% of olive waste were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under several oxy-fuel atmospheres (21%O2–79%CO2, 30%O2–70%CO2 and 35%O2–65%CO2) were compared with those attained in air. The results indicated that replacing N2 by CO2 in the combustion atmosphere with 21% of O2 caused an increase in the temperature of ignition and a decrease in the burnout value. When the O2 concentration was increased to 30 and 35%, the temperature of ignition was lower and the burnout value was higher than in air conditions. A significant reduction in ignition temperature and a slight increase in the burnout value was observed after the addition of biomass, this trend becoming more noticeable as the biomass concentration was increased. The emissions of NO during oxy-fuel combustion were lower than under air-firing. However, they remained similar under all the oxy-fuel atmospheres with increasing O2 concentrations. Emissions of NO were significantly reduced by the addition of biomass to the bituminous coal, although this effect was less noticeable in the case of the semi-anthracite.This work was carried out with financial support from the Spanish MICINN (Project
PS-120000-2005-2) co-financed by the European Regional Development Fund. M.V.G.
and L.A. acknowledge funding from the CSIC JAE-Doc and CSIC JAE-Pre programs,
respectively, co-financed by the European Social Fund. J.R. acknowledges funding
from the Government of the Principado de Asturias (Severo Ochoa program).Peer reviewe
A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
Massive stars are very rare, but their extreme luminosities make them both
the only type of young star we can observe in distant galaxies and the dominant
energy sources in the universe today. They form rarely because efficient
radiative cooling keeps most star-forming gas clouds close to isothermal as
they collapse, and this favors fragmentation into stars <~1 Msun. Heating of a
cloud by accreting low-mass stars within it can prevent fragmentation and allow
formation of massive stars, but what properties a cloud must have to form
massive stars, and thus where massive stars form in a galaxy, has not yet been
determined. Here we show that only clouds with column densities >~ 1 g cm^-2
can avoid fragmentation and form massive stars. This threshold, and the
environmental variation of the stellar initial mass function (IMF) that it
implies, naturally explain the characteristic column densities of massive star
clusters and the difference between the radial profiles of Halpha and UV
emission in galactic disks. The existence of a threshold also implies that
there should be detectable variations in the IMF with environment within the
Galaxy and in the characteristic column densities of massive star clusters
between galaxies, and that star formation rates in some galactic environments
may have been systematically underestimated.Comment: Accepted for publication in Nature; Nature manuscript style; main
text: 14 pages, 3 figures; supplementary text: 8 pages, 1 figur