(793c) Temperature Dependence of Aspen Torrefaction Reaction Kinetics

Torrefaction of biomass
removes the most easily degraded components from wood, mainly from hemicellulose,
through the application of mild heat (200-300°C) in an oxygen free atmosphere. 
The end result is a solid fuel that has significantly increased energy density
and favorable fuel properties such as lower acidity and reduced size reduction
energy requirements.  The main evolved compounds include primarily CO, CO₂,
H₂O, organic acids, and small amounts of other oxygenated organics
such as furfural.  In order to predict the properties of the torrefied biomass
product, or fully appreciate torrefaction as a biomass pretreatment method, it
is essential to understand the kinetics of decomposition and volatile evolution
under torrefaction conditions.  This presentation will discuss the
temperature-dependent decomposition of aspen under torrefaction conditions
through the use of a 3-step sequential kinetic model, shown below.

Biomass samples around
1.0mg were processed in an inert, high purity helium atmosphere.  A Pyroprobe
5200 micropyrolysis reactor (CDS Analytical, Inc.) in-line with a Trace Gas
Chromatograph Ultra (Thermo-Fisher Scientific, Restek Rxi-5ms capillary column)
and Trace DSQ II Mass Spectrometer (Thermo-Fisher Scientific) were used to
generate and record the experimental data. The rates of evolution of these
products were studied from 250-300°C over 90 minutes of torrefaction and were
found to follow Arrhenius temperature activation. The stoichoimetric
coefficients in the 3-step sequential reaction model were found to increase linearly
with increasing temperature.  In addition, stoichiometric coefficients for the
first reaction were much greater than for the second and third reactions. This
observed increase in stoichoimetric coefficients with increasing torrefaction
temperature suggests that hemicellulose bonding with cellulose and lignin may
be disrupted at higher temperatures with net effect of accelerating
hemicellulose degradation. 

References

Klinger J., Klemetsrud
B., Shonnard D., Bar-Ziv E. 2013. Kinetic Study of Aspen during Torrefaction. 
Journal of Analytical and Applied Pyrolysis. Manuscript submitted for
publication.