Environmental Engineering Reference
In-Depth Information
Table 9.3.1
Energy requirements of electrolysis and hybrid thermochemical Cu-Cl
cycle.
Energy requirement
Solar thermal energy-to
thermal
H
2
production
hydrogen efficiency
MJ
th
/kg
electricity
method
%
(MJ
th
/kmol)
MJ
e
/kg
Water electrolysis
40%
0
161
(0)
Cu-Cl cycle
50%
222
32
(444)
by many factors. They have been actively studied by others for the capture of carbon
dioxide from flue gases or air (Finkenrath, 2011; Von Zedtwitz-Nikulshyna, 2009;
Stolaroff, 2006). The focus of this section is to examine the heat requirements by sorp-
tion processes. The energy requirements include thermal energy for CO
2
release from
sorbents and electricity for capturing and transporting emissions comprising CO
2
and
other gases to CO
2
absorption equipment. As the electricity requirement is influenced
by many parameters such as the emission composition, distance between the solar ther-
mal energy capture site and CO
2
capturing plant, and the flow type for CO
2
absorption
and desorption processes, this section will focus on the thermal energy requirement of
aCO
2
capture method, which is characterized by both quality and quantity influencing
the feasibility of linking the CO
2
capture process with a nuclear reactor.
Table 9.3.2 summarizes the heat requirements of CO
2
absorption and desorption
processes of some CO
2
capture cycles currently under active investigation, including
Na
2
CO
3
-based (Nikulshina et al., 2008; Liang et al., 2004; Lee et al., 2008), K
2
CO
3
-
based (Zhao et al., 2010; Lee et al., 2004, 2008), CaO-based (Blamey et al., 2010;
MacKenzie et al., 2007; Salvador et al., 2003), CaO-NaOH-based (Mahmoudkhani
et al., 2009; Siriwardane, 2007), and MEA-based methods (Han et al., 2011; Yeh et al.,
2001). The adsorption heat is evaluated with data of the National Institute of Standards
and Technology (NIST, 2012), and other investigators (Han et al., 2011; Yeh et al.,
2001). The table shows that all adsorption processes are exothermic but occur below
100
◦
C, so heat is not easily recovered. As for the CO
2
desorption processes, they are
all endothermic. Also, CaO-based and NaOH-CaO-based cycles may directly provide
high purity CO
2
because of no need of separation of CO
2
and water vapour. However,
these two processes have a temperature threshold of 900
◦
C, which can only be satisfied
by a large intensity of concentrated solar irradiance. The temperature requirements of
CO
2
desorption processes of other cycles are below 200
◦
C, which can be satisfied by
current industrial solar concentrators. This provides a good flexibility for the linkage
of a solar thermal power plant and a CO
2
capture plant.
The enthalpy changes of different desorption processes may differ significantly in
the range of 135-180 kJ/mol CO
2
, as shown in Table 9.3.2. Except for CaO-based and
NaOH-CaO-based cycles, other carbon capture cycles require a thermal energy range
of 135-165 kJ/mol CO
2
below 200
◦
C. To have an approximation of the CO
2
capture
capacity with off-peak hours of a nuclear plant, the thermal efficiency of the CO
2
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