In hydrothermal liquefaction water simultaneously acts as,
reactant and catalyst, and this makes the process significantly
different from pyrolysis. At conditions close to the critical point,
water has several very interesting properties. Among them are low
viscosity and high solubility of organic substances, which make
subcritical water an excellent medium for fast, homogeneous and
efficient reactions [18e25]. Consequently, during the last decades,
there has been a strong interest in using subcritical water as
a solvent and reaction medium for biomass conversion. Table 1 lists
some properties of sub- and supercritical water.