The tree bark represents
an abundant but currently underutilized
forest biomass side stream. In this work, temperature-programmed slow
pyrolysis with fractional condensation was used for thermochemical
conversion of the bark obtained from three short rotation tree species,
aspen, goat willow, and rowan. Heating was performed in three stages,
drying (135 °C), torrefaction (275 °C), and pyrolysis (350
°C), and the resulting vapors were condensed at 120, 70, and
5 °C, producing nine liquid fractions. An additional fraction
was collected in the pyrolysis stage at 0 °C. The obtained liquid
fractions were characterized in terms of their yields and bulk chemistry
(i.e., CHNOS content, water content, pH, and total acid number) as
well as their molecular level chemistry by high-resolution mass spectrometry.
The highest liquid yields were obtained for the fractions condensed
at 70 °C. The water content varied considerably, being the highest
for the drying fractions (>96%) and the lowest for the pyrolysis
fractions
obtained at 120 °C (0.1–2%). Considerable compositional
differences were observed between the liquid fractions. While the
drying fractions contained mostly some dissolved phenolics, the torrefaction
fractions contained more sugaric compounds. In contrast, the pyrolysis
fractions were enriched lipids (e.g., suberinic fatty acids and their
derivatives) and alicyclic/aromatic hydrocarbons. These fractions
could be further refined into different platforms and/or specialty
chemicals. Thus, slow pyrolysis with fractional condensation offers
a potential route for the valorization of tree bark residues from
forest industry.