Valorization of forestry residues by thermal methods. The effect of temperature on gradual degradation of structural components in bark from silver fir (Abies alba Mill.)

Abstract

Thermal treatments are largely used for valorization of biomass resources. However, thorough knowledge on gradual degradation of chemical and structural components of biomass are still needed to allow selection of best procedures and operation conditions that lead to desired properties according to target applications. In this paper we studied thermal degradation of silver fir bark by torrefaction at 240–310 °C and pyrolysis at 550 °C. Detailed characterization of products and hierarchical clustering analysis was performed to obtain an overview on the relation between the severity of thermal treatment, the compositional and structural changes of material and the properties of final products. Hemicelluloses started to degrade around 250 °C into furans, acids and linear ketones. Part of volatile terpenes remained embedded in the unaffected lignin and cellulose. Degradation of lignin became significant around 270 °C, producing phenol derivatives, especially at higher temperatures. Cellulose was apparently not affected by torrefaction but was degraded by pyrolysis. Color gradually changed towards black with increasing temperature, O/C and H/C atomic ratios decreasing close to those of coals. Compressibility was maintained after torrefaction but was lost by pyrolysis while wettability changed to the hydrophobic side, even reaching highly hydrophobic character. Porosity increased, solids having complex structured morphology due to slow release of volatiles during torrefaction pretreatment step.