Environmental Engineering Reference
In-Depth Information
Fig. 5.6
Carbon dioxide energy system for recovering the cryogenic exergy from liquefied natural
gas (LNG) transport vessels with a transcritical carbon dioxide cycle. From
Point 1
to
2
, liquid
CO
2
at − 50 °C is feed into a pump where it is compressed to 15 MPa. It is heated to 600 °C in the
heat exchanger. The CO
2
is then expanded to generate electrical energy from
Point 3
to
4
. Then,
the CO
2
is liquefied from
Point 4
to
1
by using the cold energy source of the LNG that is at − 60 °C.
As the LNG is warmed to the desired conditions, the CO
2
cycle produces electrical energy. Besides
the electrical energy of the pump (
W
e
), some amount of fuel must be burned to heat the CO
2
in the
heat exchanger
the potential to recover much of the energy required in LNG liquefaction. Many
cryogenic energy recovery systems have been proposed based on nitrogen, organic
compounds or steam (Angelino and Invernizzi
2009
). However, carbon dioxide is
favored as a working fluid not only for its wide operating range (− 50 to 800 °C) that
allows for simple process systems, but also because CO
2
is noncorrosive, nonflam-
mable, safe and inexpensive.
Figure
5.6
shows an example of a carbon dioxide energy system for using cryo-
genic exergy of LNG transport ships. In the cycle, CO
2
begins from its liquid state
at Point 1 where it is pressurized. Then, the compressed liquid is heated to a high
temperature (ca. 600 °C). Because the pressure of the compressed CO
2
liquid is
greater than the critical pressure of CO
2
(
P
c
= 7.3 MPa), there is no phase change
during the heating. The supercritical CO
2
is expanded in an expander, where it cre-
ates electricity through shaft work. The expanded CO
2
is in a gas phase at
Point 4
.
The LNG provides the necessary energy to cool and liquefy the gaseous CO
2
at
Point 4
.
In the energy system of Fig.
5.6
, the LNG is warmed to the desired conditions
while the CO
2
is cycled in the loop to create electrical energy. Some amount of fuel
