Transient CFD modeling of wood
log combustion to predict the
emission rates
Oxidation of wood log is a highly transient phenomenon due to the onset of various
processes such as drying, devolatizing and char oxidation occuring randomly during
different stages of time period. They are initiated by the temperature patterns of the wood
log at different spatial locations during the process. For particles with small Biot number
Bi < 0.1 the temperature distribution can be approximated as isothermal conditions.
Nevertheless for wood log, the temperature gradients with in the wood are varying
spatially at all locations. So a novel CFD based model is developed to include the effects
of localized temperature variations of wood and the above various processes are governed
by first order Arrhenius expression. This model can also able to introduce the additional
heat flux caused by the flame to the surface of wood unlike the model developed by
Huttunen et.al. (2006). In this work, the code was originally developed by Andersen and
Heinsen (2004) and was implemented for bio mass particles conveyed in boilers. A few
alterations in the code and different solution strategies are adopted for this case to obtain
a proper numerical treatment for wood logs.
Basic working principle of CFD model
In this model, wood is specified as a porous region with high viscous resistance and is
discretized in to small control volume whose total sum represents mass 1.1 kg. A constant
heat flux of about 25 is supplied to one end of wood and it gradually diffuses
from outer surface to the inner core of the wood. During this process drying and
devolatizing of wood is performed. First order Arrhenius expression determines the
release rates of moisture, volatiles and char for their corresponding local cell
temperatures. The released volatiles undergo combustion in the gas phase and it further
heats up the wood by introducing an additional heat flux from the flame side. This
mechanism was studied by Spearpoint (1999) through experiments by monitoring the
increase in burning rate during that period. The onset of char combustion progresses with
respect to oxygen availability inside the wood and their kinetic rates are determined by
the local cell temperature. The minimum of the two determines char oxidation i.e.
although temperature of the wood is at high degree; the char oxidation is controlled by
the oxygen diffusion in to the wood. Initially oxygen inside the wood is patched as zero
to let oxygen diffusion happen over time. The geometry test case of specimen is
2
/ mkW
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