Assessment of the temporal release of atomic sodium during a burning black liquor droplet using quantitative planar laser-induced fluorescene (PLIF).
Date
2009
Authors
Saw, Woei Lean
Editors
Advisors
Nathan, Graham
Ashman, Peter John
Alwahabi, Zeyad T.
Ashman, Peter John
Alwahabi, Zeyad T.
Journal Title
Journal ISSN
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Thesis
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Abstract
The release of sodium during the combustion of black liquor is a significant source of
fume formation in a kraft recovery boiler, affecting efficiency in a pulp and paper mill.
The fume is deposited on the surface of heat exchanger tubes in the upper furnace,
causing fouling and corrosion, especially to the superheaters. This thesis reports on
work done to develop improved understanding of fume formation.
The mechanisms of sodium release during each stage of black liquor combustion are
influenced by the surface temperature. The addition of boron to the black liquor, which
debottlenecks the recausticizing plant by a reduction in lime usage, also influences the
characteristics of black liquor combustion, such as combustion time and swelling.
Previously, no effective measurement technique has been available to quantify sodium
concentration in the plume of a burning black liquor droplet with or without boron, or to
record the distribution of surface temperature through the time history of a burning
droplet. This thesis reports on the adaptation of two techniques for the measurement of
the release of atomic sodium and the temperature history, and their application to
investigate several aspects of the release of atomic sodium during combustion of black
liquor in a flat flame environment.
The simultaneous employment of a planar laser-induced fluorescence (PLIF) technique
with an absorption technique has been adapted to allow quantitative measurement of the
release of atomic sodium. The absorption technique has been employed to correct for
both fluorescence trapping due to absorption and attenuation by high concentration of
the atomic sodium in the plume, and for collisional quenching by the other major gas
components present in the flat flame. An independent assessment was performed using
kinetic calculations, based on measured total sodium that is residual in a particle
obtained at different stages in the combustion process. These independent assessments
were used to provide greater insight in to the release process and to cross-check. The influence of both the initial diameter of the droplet and addition of boron to the black
liquor on the temporal release and the release rate of atomic sodium during the
combustion have been performed using the present PLIF technique.
The second technique, two-dimensional two-colour optical pyrometry, has been adapted
to measure the distribution of surface temperature and the swelling (change in surface
area) of a burning black liquor droplet. The influence of surface temperature or the
change in the external surface area of the droplet on the release of atomic sodium during
the combustion of black liquor has been assessed through concurrent use of both
adapted techniques.
The highest concentration of atomic sodium was measured in the final stage of
combustion that of smelt coalescence, where it is an order of magnitude greater than in
the other stages combined. While the extensive release of atomic sodium at high
temperature in this final combustion stage occurs in only a relatively small percentage
of droplets in a kraft recovery boiler, the effect could still be significant in fume
formation. This is because the extensive release is expected to occur in the very small
droplets, predominantly generated by splitting or physical ejection. Small droplets will
have a very short combustion time and so could remain in suspension within hot gases
for sufficient time for extensive release of sodium. These measurements outcomes can
be used to support the future development of sub-models for computational fluid
dynamics (CFD) models in order to better understand and optimise fume formation in a
kraft recovery boiler.
School/Discipline
School of Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) - University of Adelaide, School of Mechanical Engineering, 2009
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.