Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 340

surface area is available. Of course, these conditions are not possible.
Figure 9.11 shows variations in heat-exchange efficiency with changes in
tube surface area and water pressure.
The specific heat of water rises sharply close to its critical point and then
drops equally as the temperature increases ( Figure 9.2 ). Thus, around the
critical point we may expect a modest temperature rise along the heat-

exchanger length.
Thermal conductivity in SCW is lower than that in subcritical water
because SCW's intermolecular space is greater than that in liquid. A slight
increase in conductivity is noticed as the fluid approaches the critical point.
This increase is due to an increase in the agitation of molecules when the
change from a liquid-like to a gas-like state SCW takes place. Above the
critical point, thermal conductivity decreases rapidly with temperature.
The heat-transfer coefficient varies with temperature near its pseudo-
critical value (see Section 9.2 ) because of variations in the thermophysical
properties of water. As the temperature approaches the pseudo-critical value,
conductivity and viscosity decrease but specific heat increases. The drop in
viscosity and the peak of specific heat at the pseudo-critical temperature
overcome the effect of decreased thermal conductivity so as to increase the
overall heat-transfer rate.
As the temperature further increases, beyond the pseudo-critical point, the
specific heat decreases sharply; the drop in thermal conductivity continues as
well, and therefore the heat-transfer coefficient reduces. For a given heat flux,
the wall temperature rises for the drop in heat-transfer coefficient. Generally,
for high heat flux and low mass flux, the heat transfer deteriorates, leading to

120

100

80

60

40

20 M Pa
22.5 M Pa
25 M Pa
30 M Pa

20

0

1 10 100 1000

Surface area (m 2 / kg.s of feed)

FIGURE 9.11 Calculated heat exchange efficiency for water water counter current heat

exchanger at different pressures when feed and product entered at 24 C and 600 C, respectively.
Source: Data taken from Knoef (2005).