The volatile matter (VM) content and composition of birch biochars produced at 320
°C under elevated pressure (0.1–11 MPa) and constant pressure or constant volume reactor
conditions were characterized by thermogravimetry/mass spectrometry (TG/MS) and pyrolysis–gas
chromatography/mass spectrometry (Py–GC/MS). Some of the thermal properties of the
biochars and the composition of the VMs varied as a function of the maximal pressure
applied during carbonization. The samples prepared at higher pressures released more
volatiles up to 320 °C, while the maximal rate of thermal decomposition at around
440 °C showed decreasing tendency with the carbonization pressure. In terms of VM
composition, the most apparent effect was the significant increase of the amounts
of apoallobetulins from biochars prepared at elevated pressures, which were formed
by dehydration, ring closure and rearrangement from the betulin content of birch.
The change in the ratio of the evolved guaiacol and 4-methylguaiacol as well as that
of syringol and 4-methylsyringol as a function of the maximal pressure of carbonization
indicated a modification of the lignin decomposition mechanism.
