EFB, PMF and PKS were subjected to subcritical and supercritical hydrothermal liquefaction conditions, as shown in Table 2.
Temperature range of 330–390 C was chosen as high temperatures (above 400 C) will increase the dominance of gasification
reaction [14]. The pressure range of 25–35 MPa was selected as it
is above the critical pressure of water (22.1 MPa) to ensure water
is in the subcritical and supercritical state. Prior to the experiments, the densities of water at all experimental conditions were
determined using Water V3.3 software developed by Summit
Research Corporation (Santa Fe, USA), as shown in Table 2. The
details of the experimental procedures, apparatus and bio-oil
extraction steps have been described in our previous work [39].
Fig. 1 depicts the schematic diagram of the batch reactor and furnace used in this study. Mechanical stirring of the reactor is provided with a cyclic horizontal swing span of 2 cm at a frequency
of 60 cycles/min. The ratio of biomass to water was set at 1:10 in
all the experiments in this study, based on several works reported
in the literature, in which liquefaction experiments were optimized at this ratio. Mazaheri et al. liquefied fruit press fiber using
ratio of biomass to organic solvents of 1:10 in an autoclave reactor
[40]. Hydrothermal liquefaction of macro-alga Laminaria saccharina was also conducted at biomass to water ratio of 1:10 [41].
Similar ratio of biomass to solvent was also being utilized in the
liquefaction of microalgae [42] and sugar kelp [43]. To investigate
the effect of temperature and pressure, a fixed reaction time of 1 h
was chosen as a basis because this time frame was based on literature studies [44] in which completion of reaction were observed.
To investigate the effect of reaction time on the bio-oil yields,
the range of 30–240 min was selected for liquefaction at 330,
360 and 390 C and 30 MPa