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.

	Contribution From
		(reaction 1)
		(reaction 2)
		(reaction 3)

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.