With a 1 h retention time, the
carbon, hydrogen, and nitrogen contents as well as the HHV of
bio-crude oils first slightly decreased and then increased as the
temperature increased from 260 to 320 C. In contrast, the toluene
solubility of raw oils showed the opposite trend. The elemental
analysis of bio-crude oils implies that the hydrolysis of proteins
may begin before 260 C; the deamination and decarboxylation
of proteins became dominant from 260 to 300 C; and repolymerization
governed beyond 300 C. Yet, there was a trade-off between
HHV and toluene solubility of raw oil, implying that two competitive
reactions – producing volatile matter and char – may take
place as the reaction temperature increased (Zhang, 2010). At
300 C, the carbon, hydrogen, and nitrogen contents as well as
the HHV of bio-crude oils first increased with retention time and
then decreased as the retention time exceeded 0.5 h (Table 3).
When the retention time exceeded 1.5 h, these values increased
again. On the contrary, the oxygen contents presented a reverse
trend; the toluene solubility of raw oils increased with retention
time and then leveled off after 1 h. The above results indicate that
the biomass may be depolymerized to small compounds initially
and then these compounds began to rearrange through condensation,
cyclization, and repolymerization to form new compounds
(Peterson et al., 2008).