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By Mikko Hupa, Christian Mueller, and Anders Brink,
Åbo Akademi University, Finland
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Over the years, CFD has been
widely used for optimizing the
design and operation of coal
combustion systems, and a number of
useful submodels have been developed
to describe the various combustion details
of coal particles. In addition to coal, biofuels
have become increasingly utilized
in large-scale combustion facilities as
well. They differ from coal in several ways,
and submodels specifically designed for
coal combustion cannot be applied to
biofuels. At the Åbo Akademi University
in Finland, researchers are working to
develop improved CFD models and submodels
for a variety of biofuel combustion
systems, such as fluidized beds and black
liquor recovery boilers. These models
are in the form of user-defined functions
(UDFs) for FLUENT 6.1.
Nitrogen oxide (NO) chemistry in biofuel systems differs from that in
coal systems due to the fact that the principle volatile fuel nitrogen
compound in biofuels is ammonia (NH3), rather than hydrogen cyanide (HCN),
which is the dominant source in coal combustion. Åbo Akademi engineers
have developed a novel, fast submodel to describe the conversion of the
biofuel’s ammonia into NO. The model is derived from comprehensive
kinetic reaction schemes, and has given realistic predictions (within
15%) of the NO formation in the free board of a fluidized bed combustor
(FBC).
Furnace free board NOx distribution calculated with FLUENT and the ÅA
biomass NOx submodel in a 270 MWth bubbling fluidized bed boiler fired
with wood waste and peat
Impaction maps of fly ash particles hitting furnace walls; a photograph
of one of the walls indicates that wall fouling takes place in the area
of the hottest particle impactions
Ash particles from biofuel combustion
are chemically very reactive and
may cause severe fouling of heat
exchanger surfaces. Åbo Akademi’s
novel routine for biomass ash particle
behavior can be used to study the consequences
of ash particles impacting on
different boiler surfaces, and to establish
whether the particles are in the necessary
chemical and physical states to
cause them to either stick on the surface
or bounce back into the gas flow.
The biomass combustion research
is part of the four-year research program
CODE, which was recently completed
in Finland. This program consisted of
some fifty research projects in various
research laboratories and universities
throughout the country. The program
was aimed at developing improved CFD
methodologies for large-scale combustion
systems. The total expenditure for the
program in 1999-2002 was 12.6 million
Euros, of which roughly half came
from the National Technology Agency
of Finland, Tekes, and half from the participating
companies. Fluent provided
support to the program in the form of
software and technical exchange.
Editor’s note: Professor Mikko Hupa was the scientific advisor
of the CODE research program in Finland.
References:
- A. Brink, P. Kilpinen, and M. Hupa, Energy
& Fuels, 15(5), p. 1094-1099, 2001.
- C. Mueller, D. Lundmark, B.-J. Skrifvars,
R. Backman, M. Zevenhoven, and
M. Hupa, CFD Based Ash Deposition
Prediction in a BFB Firing Mixtures of Peat
and Forest Residue, 17th Int. Conf. on
Fluidized Bed Combustion, Jacksonville,
2003.
More Information
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