156
5 Chemistry of HTL
Subcritical water acts as a hot pressurized liquid solvent in HTL. Subcritical water
is maintained at a pressure where the operating temperature becomes higher than its
boiling point of 100 °C up to its critical point of 374 °C (i.e., similar to superheated
water in a pressure cooker). At subcritical condition, the solubility of hydrophobic
organic compounds in water is improved due to water’s low permittivity (78 F m
−1
(25 °C, 0.1 MPa) to 14.07 F m
−1
(350 °C, 20 MPa)) [18]. Thus, hydrolysis is promoted yielding higher concentrations of H
+
and OH
−
ions (greater water ion product) than those in water at STP. Conversely, supercritical water is ideal for gasification
where radical reactions are pronounced. HTL is affected by temperature, reaction
0
100
200
300
400
500
(supercritical fluid)
(vapor)
Temperature, °C
600
700
0
100
200
300
400
500
600
700
40
35
30
Catalytic
gasification
liquefaction
Carbonization
High-temperature
gasification
25
20
15
10
5
0
40
35
30
25
(liquid)
Pressure, Mpa
20
15
10
5
0
Fig. 5 Hydrothermal processing regimes superimposed with the phase diagram of water. To the
left of the triple point are liquefaction and carbonization. To the upper right of the triple point are
catalytic gasification and high-temperature gasification, both within the supercritical region
(printed with permission from [17])
E. P. Resurreccion and S. Kumar