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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