Experimental and modeling study of the effect of torrefaction on the rapid devolatilization of biomass

Abstract

In the present work, experimental and computational fluid dynamics (CFD) approaches were proposed and applied to assess rapid devolatilization behaviors of four types of biomass (forest residue, torrefied forest residue, Norwegian spruce, and torrefied Norwegian spruce). Biomass particles were subjected to devolatilization experiments at 1073 and 1473 K in a drop-tube reactor. Torrefaction was found to have consistent effects on the size reduction of studied biomass. In addition, similar behaviors of char fragmentation were observed for tested torrefied biomass after rapid devolatilization at 1473 K. Mass loss during devolatilization of biomass was highly dependent on heating condition. Both rates and extents of devolatilization of biomass were increased at elevated temperatures and heating rates. In comparison with raw feedstock, high char yields were realized with the torrefied biomass after devolatilization experiments. Evolution of elemental composition of studied biomass was found to be insensitive to tested conditions. However, organic composition of char was strongly affected by elemental composition of fuel, thus also influenced by torrefaction. CFD simulation showed that sizes of fuel particles had decisive effects on residence time of them in the reactor, especially particles with diameter larger than 355 μm. Particle temperature, in contrast, depended on both particle diameter and particle density. A modified two-competing-rates devolatilization model was also presented in the present work. On the basis of experimental data, one optimal set of kinetic parameters was obtained following a proposed procedure. The model predicted well the mass loss of all tested fuel and the evolution of each organic element in char at all operation conditions. © 2015 American Chemical Society.