As water is naturally contained in biomass, liquefaction of biomass
for bio-oil production using water (hydrothermal liquefaction)
has been widely investigated. Effect of temperature on
hydrothermal liquefaction of barley straw was conducted by Zhu
et al. [17]. Tungal and Shende [18] studied the effect of catalyst
on the hydrothermal liquefaction of pinewood for biogas and biocrude
production. The properties of bio-oil produced from
hydrothermal liquefaction of microalgae with varying biochemical
contents were reported by Biller and Ross [19]. Mazaheri et al.
compared the efficiency of catalysts in the subcritical hydrothermal
liquefaction of Malaysian oil palm fruit press fiber [20]. The
feasibility of liquefying undried macroalgae using water was
reported by Yang et al. [21]. The work of Hafez and Hassan had
shown the synergetic effect of ethanol in the hydrothermal
liquefaction of giant miscanthus [22]. Besides, water is a green
and environmentally benign solvent, non-toxic and non-flammable
[23]. It is relatively cheap and abundant, and hence safe to be
employed in larger bio-oil production scale [24]. In this study,
water in subcritical and supercritical conditions is used as a solvent
to liquefy oil palm biomass. Subcritical water is water below the
critical temperature (374 C) and above the vapor pressure at that
temperature, whereas supercritical water is water above the critical
temperature (374 C) and critical pressure (22.1 MPa) [25]. Due
to the low dielectric constant, weak hydrogen bond and high
isothermal compressibility of subcritical water, it is suitable to be
used to degrade and decompose biomass into gaseous, liquid and
solid products [20]. Supercritical water possesses liquid-like densities
and gas-like high compressibilities and diffusivities, making
supercritical water a powerful solvent in the extraction, degradation
and separation of organic compounds [26].