Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 318

Subcritical water (T , T sat ; P , P c ): When the pressure is below its criti-
cal value, P c , and the temperature is below its critical value, T c , the fluid
is called subcritical. If the temperature is below its saturation value, the
fluid is known as subcritical water, as shown in the lower left block of
Figure 9.1 .
Subcritical steam (T P c . Note: T may be above T c ): When

water (below critical pressure) is heated, it experiences a drop in density
and an increase in enthalpy; this change is very sharp when the tempera-
ture of the water just exceeds it saturation value, T sat . Above the satura-
tion temperature, but below the critical value, the fluid (H 2 O) is called
subcritical steam. This regime is shown below the saturation line in
Figure 9.1 .
Supercritical water (T

T sat ; P

. ,

P c ): When heated above its critical pres-
sure, P c , water experiences a continuous transition from a liquid-like state
to a vapor-like state. The vapor-like, supercritical, state is shown in the
upper right block in Figure 9.1 . Unlike in the subcritical stage, no heat of
vaporization is needed for the transition from liquid-like to vapor-like.
Above the critical pressure, there is no saturation temperature separating
the liquid and vapor states. However,

T c ; P

. .

there is a temperature, called

pseudo-critical P c )

above which the transition from liquid-like to vapor-like takes place. The
pseudo-critical temperature is characterized by a sharp rise in the specific
heat of the fluid.

temperature, that corresponds to each pressure ( .

The pseudo-critical temperature depends on the pressure of the water. It
can be estimated within 1% accuracy by the following empirical equation
(Malhotra, 2006):

F

T 5 ð

P Þ

2

F 0 : 1248 0 : 01424P 2 0 : 0026 ð P Þ

5 1

(9.2)

T sc
T c ;

P
P c

T 5

P 5

where T sat is the saturation temperature at pressure P; P sat is the saturation
pressure at temperature T; P c is the critical pressure of water, 22.089 MPa;
T c is the critical temperature of water, 374.29 C; and T sc is the pseudo-
critical temperature at pressure P (P . P c ).
9.2.1 Properties of SCW
The critical point marks a significant change in the thermophysical properties
of water ( Figure 9.2 ). There is a sharp rise in the specific heat (less than 5 to
higher than 90 kJ/kg K) near the critical temperature followed by a similar
drop ( Figure 9.2 ). The peak value of specific heat decreases with system
pressure. The thermal conductivity of water also drops from 0.330 at 400 C
to 0.176 W/m K at 425 C. The drop in molecular viscosity is also significant,