The Thermochemical Conversion of Forestry Residues from Silver Fir ( Abies alba Mill.) by Torrefaction and Pyrolysis

Forestry residue is a renewable energy biomass whose valorization has increased due to the interest in replacing exhaustible and environmentally unfriendly fossil resources. Needles, cones and bark from silver fir were thermally processed by separated and combined torrefaction (250 °C) and pyrolysis (550 °C). The torrefaction removed the humidity and extractives and degraded the hemicelluloses, significantly decreasing the oxygen content to ~11 wt% and increasing the carbon content to ~80 wt%, while enhancing the calorific value of the solids (~32 MJ/kg). The pyrolysis produced solid materials with high amounts of fixed carbon (~60–70 wt%) and high heating values, of ~29 MJ/kg. The combined torrefaction + pyrolysis increased the energy yield of the process and decreased the O/C and H/C atomic ratios to about 0.1 and 0.5, respectively, which is close to those of coals. It also led to condensable products with more homogeneously distributed compounds, regardless of the initial biomass type. More than 110 chemical compounds were confirmed in the condensable products, in amounts that depended on the type of starting material and on the thermal treatment. These included the following: terpenes, from extractives; furans, acids and linear ketones, from hemicelluloses; cyclic ketones and saccharides, from cellulose; and aromatic hydrocarbons and phenol derivatives, from lignin. Clear distinctions between the thermal procedures and the sample origins were evidenced by an exploratory data analysis (PCA), which suggested the presence of different types of lignin in the three starting materials.