TY - BOOK
T1 - Melting and Sintering of Ashes
AU - Hansen, Lone Aslaug
PY - 1997
Y1 - 1997
N2 - The thesis contains an experimental study of the fusion and
sintering of ashes collected during straw and coal/straw
co-firing.A laboratory technique for quantitative determination of
ash fusion has been developed based on Simultaneous Thermal
Analysis (STA). By means of this method the fraction of melt in
the investigated ashes has been determined as a function of
temperature. Ash fusion results have been correlated to the
chemical and mineralogical composition of the ashes, to results
from a standard ash fusion test and to results from sintering
experiments. Furthermore, the ash fusion results have been
employed in a simple model for prediction of ash deposit
formation, the results of which have been compared to ash
deposition formation rates measured at the respective boilers.The
ash fusion results were found to directly reflect the ash
compositional data:a) Fly ashes and deposits from straw combustion
are characterised by a large fraction of KCl and a smaller
fraction of K-, Ca-, Al-silicates and quartz. The salt part of
these ashes melt in the temperature range from 600-750°C, whereas
the silicate part predominantly melts between 1000 and 1200°C.
Increasing salt (KCl) content in the ashes lead to increased melt
fractions in the temperature range 600-750°C.b) Bottom ashes from
straw combustion consist purely of silicates, with varying ratios
of the quite refractory Al-silicates and quartz to the less
refractory K- and Ca-silicates. Bottom ashes melt in the
temperature range 800-1300°C, and a trend of higher fusion
temperatures with increasing contents of Al-silicates and quartz
was found.c) Fly ashes, bottom ashes and deposits from coal/straw
co-firing were all found to consist mainly of metal-alumina and
alumina-silicates. These ashes all melt in the temperature range
1000-1400°C.Comparison of the obtained ash fusion results to
results from the commonly used standard ash fusion test revealed
that at the Initial Deformation Temperature, IDT, substantive melt
formation had occurred. The difference between the melting onset,
as determined by the STA, and the IDT varied typically between
-300 - -50°C for salt rich ashes and between -150 - +40°C for
silicate-rich ashes. Melt fractions present at the IDT varied
between 0 - 35% melt for silicate rich ashes and between 10 - 70%
melt for salt rich ashes. This emphasises that the IDT does not
denote the onset of ash melting, the biggest deviations being
found for salt rich (i.e. straw derived) ashes.A simple model
assuming proportionality between fly ash fusion and deposit
formation was found to be capable of ranking deposition rates for
the different straw derived fly ashes, whereas for the fly ashes
from coal/straw co-firing, the model only had a qualitative
agreement with the measured ash deposit formation rates.Sintering
measurements were carried out by means of compression strength
testing of ash pellets. This method showed to not be applicable
for the salt rich fly ash derived from straw combustion. For the
fly ashes obtained during coal/straw co-firing, substantive
sintering strength was observed to build up in the ashes below the
melting onset. The strength obtained was thus assumed to be due to
viscous flow sintering, and the sintering onset was for the four
ashes investigated simultaneous to a calculated ash viscosity of
1-3 · 106 Pa·s.In conclusion, the employed laboratory technique to
measure ash fusion gives a more detailed and precise melt
quantification compared to conventionally used techniques, and
since ash fusion is important for ash deposition propensities, the
employed method is judged to bring valuable new information. The
measurement principle is not especially designed for studying
ashes from combustion, so the method is as well applicable for
other types of materials, for which a detailed characterisation of
the melting behaviour is relevant.
AB - The thesis contains an experimental study of the fusion and
sintering of ashes collected during straw and coal/straw
co-firing.A laboratory technique for quantitative determination of
ash fusion has been developed based on Simultaneous Thermal
Analysis (STA). By means of this method the fraction of melt in
the investigated ashes has been determined as a function of
temperature. Ash fusion results have been correlated to the
chemical and mineralogical composition of the ashes, to results
from a standard ash fusion test and to results from sintering
experiments. Furthermore, the ash fusion results have been
employed in a simple model for prediction of ash deposit
formation, the results of which have been compared to ash
deposition formation rates measured at the respective boilers.The
ash fusion results were found to directly reflect the ash
compositional data:a) Fly ashes and deposits from straw combustion
are characterised by a large fraction of KCl and a smaller
fraction of K-, Ca-, Al-silicates and quartz. The salt part of
these ashes melt in the temperature range from 600-750°C, whereas
the silicate part predominantly melts between 1000 and 1200°C.
Increasing salt (KCl) content in the ashes lead to increased melt
fractions in the temperature range 600-750°C.b) Bottom ashes from
straw combustion consist purely of silicates, with varying ratios
of the quite refractory Al-silicates and quartz to the less
refractory K- and Ca-silicates. Bottom ashes melt in the
temperature range 800-1300°C, and a trend of higher fusion
temperatures with increasing contents of Al-silicates and quartz
was found.c) Fly ashes, bottom ashes and deposits from coal/straw
co-firing were all found to consist mainly of metal-alumina and
alumina-silicates. These ashes all melt in the temperature range
1000-1400°C.Comparison of the obtained ash fusion results to
results from the commonly used standard ash fusion test revealed
that at the Initial Deformation Temperature, IDT, substantive melt
formation had occurred. The difference between the melting onset,
as determined by the STA, and the IDT varied typically between
-300 - -50°C for salt rich ashes and between -150 - +40°C for
silicate-rich ashes. Melt fractions present at the IDT varied
between 0 - 35% melt for silicate rich ashes and between 10 - 70%
melt for salt rich ashes. This emphasises that the IDT does not
denote the onset of ash melting, the biggest deviations being
found for salt rich (i.e. straw derived) ashes.A simple model
assuming proportionality between fly ash fusion and deposit
formation was found to be capable of ranking deposition rates for
the different straw derived fly ashes, whereas for the fly ashes
from coal/straw co-firing, the model only had a qualitative
agreement with the measured ash deposit formation rates.Sintering
measurements were carried out by means of compression strength
testing of ash pellets. This method showed to not be applicable
for the salt rich fly ash derived from straw combustion. For the
fly ashes obtained during coal/straw co-firing, substantive
sintering strength was observed to build up in the ashes below the
melting onset. The strength obtained was thus assumed to be due to
viscous flow sintering, and the sintering onset was for the four
ashes investigated simultaneous to a calculated ash viscosity of
1-3 · 106 Pa·s.In conclusion, the employed laboratory technique to
measure ash fusion gives a more detailed and precise melt
quantification compared to conventionally used techniques, and
since ash fusion is important for ash deposition propensities, the
employed method is judged to bring valuable new information. The
measurement principle is not especially designed for studying
ashes from combustion, so the method is as well applicable for
other types of materials, for which a detailed characterisation of
the melting behaviour is relevant.
M3 - Book
BT - Melting and Sintering of Ashes
ER -