Kinetic study of aspen during torrefaction

Using torrefied biomass, or biocoal, as a solid combustion fuel provides opportunity to introduce a sustainable feedstock into the energy market. The decomposition of biomass to biocoal is a very complex chemical process, yet detailed understanding of the mechanistic changes that occur can allow for system control and optimization. Torrefaction of lignocellulosic biomass has been studied extensively, and kinetic models have been developed to attempt to describe the observed behavior during Thermal Gravimetric Analysis (TGA) experiments, using very slow heating rates. The goal of this paper was to develop a baseline semi-empirical model for torrefaction of aspen wood using very fast heating rates (1000 C/s) and GC-MS analysis. This fast heating rate was adopted in order to uncouple heat transfer and chemical kinetics. Having direct information about the specific chemical evolution during torrefaction affords mechanistic insights into torrefaction, along with opportunities for improved process design and control. The proposed kinetics model involves three sequential reactions in which initially the parent hemicellulose material produces gaseous products (CO and CO₂), water, and organic acids (formic and acetic) and a solid intermediate. The next reaction further degrades the solid intermediate into the stated products. The final reaction further generates the products along with the final bio-coal product. We found the sequential reactions to have characteristic times of 0.99 min, 2.14 min and 39.45 min respectively at 300 C. Improvements to the models and routes for future investigation are also discussed.