Thermochemical conversion of birch bark by temperature-programmed slow pyrolysis with fractional condensation

Abstract Pyrolysis is a promising method for thermochemical conversion of forest biomass into liquid biofuels or platform/specialty chemicals. In this work, bark from silver birch (Betula pendula) was processed through slow pyrolysis with fractional condensation, and the obtained liquid fractions were analyzed by various bulk chemical analyses as well as high-resolution mass spectrometry. The process had three successive stages, drying, torrefaction, and pyrolysis, occurring at 135, 275, and 350 °C, respectively, and pyrolysis liquids were obtained at three condensing temperatures, 130, 70, and 5 °C, resulting in eight liquid fractions. Most fractions had high water content, especially those obtained from the drying stage. Only the semi-solid product, obtained from the pyrolysis stage at the condensing temperature of 70 °C, had a very low water content and a high carbon yield. There were major differences in the chemical compositions between the liquid fractions; the drying fractions contained mostly aliphatic and phenolic extractives as well as light hydrocarbons, while the torrefaction fractions were enriched with sugaric compounds, resulting from hemicellulose and cellulose degradation. In contrast, the pyrolysis fractions were mainly composed of suberinic fatty acids, terpenoids (e.g., betulin) and their condensation products, and phenolic compounds. Many terpenoids were dehydrated to form a plethora of condensed, water-insoluble alicyclic and aromatic hydrocarbons. Although the temperature-programmed slow pyrolysis shows potential for thermochemical processing of industrial bark residues, excessive dehydration reactions may limit its applicability especially for “in situ fractionation”.